U.S. patent application number 12/014186 was filed with the patent office on 2008-10-09 for method for purifying (meth)acrylic acid.
This patent application is currently assigned to MITSUBISHI CHEMICAL CORPORATION. Invention is credited to Yasushi Ogawa, Yoshiro Suzuki, Kenji Takasaki, Shuhei Yada.
Application Number | 20080245652 12/014186 |
Document ID | / |
Family ID | 27555009 |
Filed Date | 2008-10-09 |
United States Patent
Application |
20080245652 |
Kind Code |
A1 |
Yada; Shuhei ; et
al. |
October 9, 2008 |
METHOD FOR PURIFYING (METH)ACRYLIC ACID
Abstract
A method for purifying a crude (meth)acrylic acid obtained by a
vapor phase catalytic oxidation method, characterized in that the
crude (meth)acrylic acid having most parts of water and acetic acid
removed therefrom, is fed to and distilled in a first distillation
column of a purification system comprising first to third three
distillation columns, the top fraction from the first distillation
column is fed to and distilled in the second distillation column,
the resulting top fraction is recovered as a high purity
(meth)acrylic acid product, the bottoms from the first and second
distillation columns are fed to and distilled in the third
distillation column, and the resulting top fraction is fed to the
first distillation column.
Inventors: |
Yada; Shuhei; (Mie, JP)
; Ogawa; Yasushi; (Mie, JP) ; Suzuki; Yoshiro;
(Mie, JP) ; Takasaki; Kenji; (Mie, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
MITSUBISHI CHEMICAL
CORPORATION
Minato-ku
JP
|
Family ID: |
27555009 |
Appl. No.: |
12/014186 |
Filed: |
January 15, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10834075 |
Apr 29, 2004 |
7414150 |
|
|
12014186 |
|
|
|
|
PCT/JP02/11308 |
Oct 30, 2002 |
|
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10834075 |
|
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Current U.S.
Class: |
203/6 |
Current CPC
Class: |
C07C 57/04 20130101;
B01D 3/146 20130101; C07C 51/44 20130101; B01D 1/225 20130101; C07C
51/44 20130101 |
Class at
Publication: |
203/6 |
International
Class: |
B01D 3/34 20060101
B01D003/34 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2001 |
JP |
2001-332008 |
Nov 27, 2001 |
JP |
2001-360437 |
Dec 3, 2001 |
JP |
2001-368858 |
Dec 7, 2001 |
JP |
2001-373671 |
Jan 10, 2002 |
JP |
2002-003590 |
May 7, 2002 |
JP |
2002-131675 |
Claims
1. A method for producing (meth)acrylic acid, comprising: feeding a
crude (meth)acrylic acid obtained by vapor phase catalytic
oxidation to a distillation column to continuously distil and
purify it in the presence of a hydrazine, wherein the hydrazine is
added to the crude (meth)acrylic acid prior to feeding to the
distillation column, and the crude (meth)acrylic acid having the
hydrazine added thereto is heated to a feeding temperature of from
60.degree. C. to 80.degree. C. and then fed to the distillation
column at the feeding temperature.
2. The method according to claim 1, wherein the feeding temperature
is from 60.degree. C. to 75.degree. C.
3. The method according to claim 1, wherein the feeding temperature
is from 62.degree. C. to 80.degree. C.
4. The method according to claim 1, wherein the feeding temperature
is from 62.degree. C. to 75.degree. C.
Description
[0001] This is a continuation application of U.S. application Ser.
No. 10/834,075, filed Apr. 29, 2004, which is a continuation of
PCT/JP02/11308 filed on Oct. 30, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method for purifying
(meth)acrylic acid, particularly to a method for purifying a crude
(meth)acrylic acid obtained by vapor phase catalytic oxidation, by
distillation to obtain highly pure (meth)acrylic acid which is
useful for the production of a highly water absorptive resin and
for the production of a (meth)acrylic ester. In this specification,
(meth)acrylic acid means acrylic acid or methacrylic acid.
[0004] 2. Discussion of Background
[0005] a. As a method for producing (meth)acrylic acid, a method of
hydrolyzing the corresponding nitrile compound may, for example, be
mentioned. However, at present, a vapor phase catalytic oxidation
method of the corresponding hydrocarbon such as propylene or
isobutylene, is mainly employed. Recently, a study has been made
also on a vapor phase catalytic oxidation method using an
inexpensive corresponding alkane as the starting material instead
of an olefin.
[0006] In the production of (meth)acrylic acid by a vapor phase
catalytic oxidation method, firstly the reaction product gas
containing (meth)acrylic acid is contacted with an absorbing
solvent such as water to recover (meth)acrylic acid in the gas in
the form of a (meth)acrylic acid solution. This solution contains,
in addition to (meth)acrylic acid, various impurities formed as
by-products during the vapor phase catalytic oxidation, such as
acetic acid, maleic acid, acrolein, furfural, benzaldehyde,
acetone, etc. in the case of acrylic acid. Many methods have been
proposed for recovering purified (meth)acrylic acid from such a
(meth)acrylic acid solution. However, the principal ones are such
that the absorbing solvent and a part of impurities are removed
from the (meth)acrylic acid solution in a preliminary purification
step to obtain a crude (meth)acrylic acid substantially comprising
(meth)acrylic acid, dimers thereof and other heavy components, and
then such a crude (meth)acrylic acid is purified in a purification
step to obtain a product having a desired quality.
[0007] b. For example, in recent years, acrylic acid has found an
increase in its demand as a starting material for e.g. a food
additive or a highly water absorptive resin for e.g. paper diapers.
In such applications, highly pure acrylic acid is required. Namely,
if crude acrylic acid is used as a starting material for a polymer
of acrylic acid without removing impurities, there will be a
problem such as a delay in the reaction during the polymerization
reaction, a decrease in the polymerization degree or coloring of
the polymerized product.
[0008] Accordingly, industrially, purification of acrylic acid is
carried out by distillation. However, it is not easy to remove by
distillation impurities in crude acrylic acid obtained by vapor
phase catalytic oxidation.
[0009] Heretofore, as a method for producing highly pure acrylic
acid by separating and removing impurities from crude acrylic acid
obtained by vapor phase catalytic oxidation, a method of carrying
out distillation in the presence of a hydrazine, has, for example,
been known (JP-A-49-30312, JP-B-58-37290, etc.). However, such a
method is primarily intended to remove an aldehyde in the crude
acrylic acid, whereby removal of maleic acid and/or maleic
anhydride (these will be together hereinafter referred to as
"maleic acids") tends to be inadequate.
[0010] Further, JP-A-7-330659 discloses a method of carrying out
distillation in the co-presence of hydrazine and ammonia. This
method is effective for removal of maleic acids, but has a problem
such that the added ammonia will be distilled from the top, such
being not suitable for the production of highly pure acrylic acid.
Further, this application discloses batch treatment only, and
discloses nothing about a method for continuously obtaining highly
pure acrylic acid on a commercial scale.
[0011] Accordingly, it has been considered that with these
techniques, it is not easy to continuously produce high purity
acrylic acid by sufficiently removing impurities containing maleic
acids from the crude acrylic acid.
[0012] On the other hand, JP-A-2001-316326 discloses a method for
continuously producing high purity acrylic acid by preventing
sludge formation in the distillation column, wherein crude acrylic
acid having a concentration of maleic acids of at most 2000 ppm, is
used as a starting material for high purity acrylic acid. However,
in order to reduce the concentration of maleic acids in the
starting material crude acrylic acid, it is necessary to remove
maleic acids in the step of obtaining the crude acrylic acid, and
such does not provide a substantial solution to the problem.
Further, in the process for producing acrylic acid, for example, in
a step of recovering acrylic acid from the bottom residue of the
acrylic acid distillation column, maleic acids, will be distilled
in acrylic acid, and consequently, accumulation of maleic acids
will take place within the acrylic acid production process, and
accordingly, from the industrial view point, it is desired to
develop an economically excellent method whereby crude acrylic acid
containing at least 2000 ppm of maleic acids can be used as the
starting material, and yet, high purity acrylic acid can be
continuously produced constantly.
[0013] c. On the other hand, as a purification method for
(meth)acrylic acid obtained by vapor phase catalytic oxidation of
propylene or isobutylene, a distillation method is common, but
(meth)acrylic acid is extremely susceptible to polymerization, and
its handling was problematic.
[0014] d. As one of distillation apparatus, a vertical thin film
evaporator is known which comprises an evaporator main body with
its principal portion being cylindrical, which has a heating means
on its exterior surface, a liquid inlet and a vapor outlet at its
upper portion and a residue discharge port at its lower portion, a
rotary shaft set in the main body, and stirring vanes attached to
the shaft and being movable in a peripheral direction along the
inner wall surface of the evaporator main body. With this thin film
evaporator, the interior surface corresponding to the exterior
surface on which the heating means is provided, is a heat transfer
surface, and a liquid to be treated, which is supplied from the
upper liquid inlet will be pressed and spread in a film form on the
cylindrical inner wall surface by the rotating stirring vanes, and
in the process where this liquid film falls by gravity, low boiling
point components in the liquid to be treated are permitted to
evaporate by the heat supplied from the heating means. This
apparatus is capable of evaporating low boiling point components in
the liquid to be treated, in a short time, and thus, it is suitable
for treating a liquid containing a substance sensitive to heat,
such as a readily-polymerizable compound. Further, the treated
liquid is forcibly stirred by the rotating stirring vanes, whereby
the liquid in contact with the heat transfer surface is always
renewed by a fresh liquid, whereby there is a merit such that local
overheating of the liquid to be treated can be prevented, and
baking or scaling of the liquid tends to scarcely occur.
[0015] Many methods are available for attaching stirring vanes to
the rotary shaft. For example, in a movable vane system, the
stirring vanes are attached to the rotary shaft via fulcrums or
springs, so that they can be moved in a circumferential direction
about the rotary shaft, whereby by rotation of the rotary shaft,
they rotate while contacting with the cylindrical inner wall
surface or while maintaining a slight distance therefrom, by a
centrifugal force.
[0016] e. On the other hand, heretofore, it has been common to
employ a method for producing an acrylic ester by an esterification
reaction of acrylic acid with an alcohol. As the acrylic acid to be
used, one obtained by a vapor phase oxidation reaction of
propylene, followed by dehydration, removal of low boiling point
impurities and further purification treatment for removal of e.g.
high boiling point impurities, may be used. However, it has been
regarded advantageous to employ one not subjected to treatment for
removal of high boiling point impurities, since purification costs
of acrylic acid can thereby be made low (JP-A-9-157213,
JP-A-10-237012, JP-A-10-306052, JP-A-2001-213839).
[0017] However, if acrylic acid containing high boiling point
impurities, is used as the starting material, there have been
problems such that undesirable polymerization reactions or side
reactions are likely to take place, thus leading to clogging of
apparatus such as pipes by polymerized products, deterioration of
unit consumption of main materials such as acrylic acid and an
alcohol, and a decrease in the quality of the product.
[0018] f. The acrylic acid-containing gas obtained by vapor phase
oxidation will then be contacted with water in a collection column
to obtain an aqueous acrylic acid solution, and an azeotropic agent
is added to this aqueous acrylic acid solution, whereupon in an
azeotropic agent dehydration distillation column, an azeotropic
mixture comprising water and the azeotropic agent, is distilled,
while crude acrylic acid containing acetic acid is recovered from
the bottom of the column. Then, this crude acrylic acid is
subjected to a distillation column for separating low boiling point
components thereby to separate low boiling point impurities such as
acetic acid, and further, high boiling point impurities are removed
in a distillation column for separating high boiling components, to
obtain purified acrylic acid. Further, there may be a case where
acrylic acid is collected by contacting it with a high boiling
point solvent in a collection column.
[0019] Acrylic acid thus produced, may be used as a starting
material for various acrylic esters. In recent years, its demand as
a starting material for a highly water absorptive resin has
increased. Such acrylic acid as a starting material for a highly
water absorptive resin is required to be acrylic acid purified to a
high purity, and especially, aldehydes are required to be highly
removed, since they tend to hinder a polymerization reaction or
they tend to color the product polymer.
[0020] Heretofore, as a method for removing aldehydes simply and
efficiently from purified acrylic acid, a method is known wherein
aldehydes are converted to heavy substances by means of an
aldehyde-removing agent of e.g. an amine system including a
hydrazine system or an amino acid system (JP-A-49-30312,
JP-A-49-95920, JP-B-50-14, JP-A-10-204024), a hydrogen sulfite
system (JP-A-7-330672), a mercaptan system (JP-A-60-6635) or a
combined system of a hydrazine system and a dithiocarbamate system
(JP-A-7-228548), followed by distillation in a distillation column
for purification, to obtain high purity acrylic acid from the top
of the distillation column for purification.
[0021] The bottom fraction containing high boiling point compounds
formed at the time of this distillation, contains, together with
reaction products of aldehydes with an aldehyde-removing agent, a
polymerization inhibitor such as hydroquinone added at the time of
the distillation, and further high boiling point substances formed
during the distillation, such as many heavy substances, such as an
acrylic acid dimer (.beta.-acryloxypropionic acid) or oligomers
being Michael adducts of acrylic acid, polymers, etc.
[0022] Heretofore, such bottom fraction was disposed, or recovered
for the production process for acrylic acid. This bottom fraction
contains acrylic acid dimer, etc. being Michael adducts, and if it
is recovered for the production process for acrylic acid, it is
considered preferred to treat it in a thermal decomposition
treatment step before recovery in the purification step for acrylic
acid (JP-A-2001-213839).
[0023] g. It is an object of the present invention to solve the
above-mentioned conventional problems and to provide a method for
producing high purity (meth)acrylic acid by sufficiently removing
impurities such as aldehydes, ketones, dicarboxylic acids such as
maleic acids from crude (meth)acrylic acid obtained by a vapor
phase catalytic oxidation method, which is an economically
excellent method for producing (meth)acrylic acid, whereby
continuous operation for a long period of time is possible while
suppressing formation of sludge in the distillation column.
[0024] h. Further, as mentioned above, a high purity (meth)acrylic
acid product is required as the starting material for a water
absorptive resin such as paper diapers. The reason is that if an
impurity, particularly furfural, is contained in the
above-mentioned (meth)acrylic acid obtained by vapor phase
catalytic oxidation, there will be a problem such as delay in the
reaction, deterioration of the polymerization degree, coloration of
the polymerized product, etc. at the time of the polymerization
reaction for a water absorptive resin. Therefore, industrially,
purification of (meth)acrylic acid is carried out by distillation
or crystallization. Crystallization usually requires a large
initial investment, and from the economical viewpoint, a method by
distillation is employed in many cases, but it is difficult to
remove the above impurity, particularly furfural, by usual
distillation.
[0025] In order to solve this problem, a method has been proposed
wherein a hydrazine compound is added at the time of purification
of (meth)acrylic acid. This method is effective from the viewpoint
of removal of the above-mentioned impurity, but has had a problem
that it causes polymerization of (meth)acrylic acid during the
rectification.
[0026] Formation of a polymer causes clogging in the distillation
column, whereby the performance of the distillation column
decreases, or it will be required to stop the operation.
Accordingly, a method for suppressing formation of such a polymer,
is desired.
[0027] JP-A-7-228548 proposes to suppress the formation by adding
copper dithiocarbamate. In an operation for a short time, the
effect of this method is confirmed, but in a continuous operation
for a long time for a usual industrial operation, the effect has
been still inadequate.
[0028] i. Further, in the purification of (meth)acrylic acid or its
ester by distillation, if it is attempted to recover (meth)acrylic
acid or its ester by means of a thin film evaporator from a heavy
component containing (meth)acrylic acid or its ester discharged
from the bottom of the distillation column, there is a problem such
that clogging frequently occurs at a liquid-withdrawal tube or at
an outlet portion of a liquid collection part of the thin film
evaporator.
[0029] The present inventors have sought to find out the causes and
as a result, have found them to be such that the liquid flowing
down on the inner wall surface of the thin film evaporator will
polymerize on a lower inner wall surface rather than at the lower
end of the stirring vanes, and as the liquid introduced into the
same film evaporator is concentrated, a polymerization inhibitor
preliminarily added for (meth)acrylic acid or its ester, will
precipitate.
[0030] Namely, in the thin film evaporator, stirring vanes are
disposed to stir the liquid film on the heat transfer surface where
evaporation takes place, and no stirring vanes are disposed at the
inner wall surface portion below the heat transfer surface,
particularly at the inverted corn-shaped liquid collection portion
following the cylindrical portion, or at the funnel-shaped liquid
collection portion being a combination of the inverted corn-shape
and a cylindrical shape. Accordingly, at such a portion, the liquid
flowing down, has the majority of low boiling point components
removed and thus essentially has bad fluidity, and besides, no
stirring by stirring vanes takes place, whereby the liquid in
contact with the inner wall surface tends to be hardly renewed by a
fresh liquid. Consequently, the retention time of the liquid in
contact with the inner wall surface tends to be abnormally long,
and (meth)acrylic acid or its ester remaining in the liquid, tends
to gradually polymerize to change the liquid to be heavy, whereby
the fluidity of this liquid further decreases, and a polymer tends
to accumulate on the inner wall surface. Further, as the liquid is
concentrated, the polymerization inhibitor preliminarily added for
(meth)acrylic acid or its ester tends to be precipitated. The
accumulated polymer and precipitates not only hinder the flow of
the liquid, but also clog the outlet portion of the liquid
collection portion or the following liquid withdrawal tube, if they
are peeled off from the inner wall surface. Accordingly, it is an
object of the present invention to provide a thin film evaporator
free from such clogging.
[0031] j. It is an object of the present invention to avoid
conventional problems such as clogging of apparatus such as pipes
by a polymer, deterioration of the unit consumption of the starting
materials, deterioration of the quality of the product, etc. and to
provide a method for producing an acrylic ester which is
economically excellent and industrially advantageous.
[0032] k. Further, it is not desirable to obtain high purity
acrylic acid from the top of the above-mentioned distillation
column for purification while subjecting the bottom fraction of the
distillation column for purification to thermal decomposition
treatment to recover it for an acrylic acid purification step,
because as the thermal decomposition treatment is carried out at a
high temperature, many side-reactions or decomposition reactions
will take place, whereby formation of undesirable by-products which
cause to accelerate polymerization of acrylic acid, to contaminate
an acrylic acid product or to present coloration to the product, or
regeneration of aldehydes, takes place, and such compounds are
likely to be recycled to the purification step of acrylic acid.
[0033] To avoid such problems, a method has also been proposed
wherein distillation is carried out under such a distillation
condition that the acrylic acid concentration in this bottom
fraction will be sufficiently low, and the bottom fraction is
subjected to disposal treatment. However, if the concentration of
acrylic acid in the bottom fraction is lowered, the viscosity of
the bottom fraction will increase, whereby precipitation of a
polymer, etc. tends to readily take place, thus leading to a
trouble of clogging at the withdrawal pipe. Accordingly, the
lowering of the acrylic acid concentration is limited, whereby it
has been impossible to avoid a loss of acrylic acid in an amount
corresponding to the one disposed as contained in the bottom
fraction.
[0034] It is an object of the present invention to solve the above
described conventional problems and to provide a method for
producing high purity (meth)acrylic acid, whereby a highly purified
high purity (meth)acrylic acid is produced by simply and
efficiently removing aldehydes contained in (meth)acrylic acid, and
at the same time, a waste liquid other than high purity
(meth)acrylic acid fraction formed by this treatment of aldehydes,
is recovered industrially advantageously.
SUMMARY OF THE INVENTION
[0035] The present inventors have conducted various studies to
solve the above-mentioned problems and as a result, they have found
it possible to accomplish the above-mentioned objects and have
arrived at the invention having the following characteristics.
[0036] (1) A method for purifying a crude (meth)acrylic acid
obtained by a vapor phase catalytic oxidation method, characterized
in that the crude (meth)acrylic acid having most parts of water and
acetic acid removed therefrom, is fed to and distilled in a first
distillation column of a purification system comprising first to
third three distillation columns, the top fraction from the first
distillation column is fed to and distilled in the second
distillation column, the resulting top fraction is recovered as a
high purity (meth)acrylic acid product, the bottoms from the first
and second distillation columns are fed to and distilled in the
third distillation column, and the resulting top fraction is fed to
the first distillation column.
[0037] (2) The method according to the above (1), wherein the top
fraction from the first distillation column is, after applying an
aldehyde removal treatment thereto or after adding an aldehyde
removing agent thereto, fed to the second distillation column.
[0038] (3) The method according to the above (1) or (2), wherein
the aldehyde removal treatment comprises adding a hydrazine as an
aldehyde removing agent and heating to a temperature of lower than
80.degree. C.
[0039] (4) The method according to any one of the above (1) to (3),
wherein in the second distillation column, distillation is carried
out at a column bottom temperature of at most 110.degree. C. in the
presence of a hydrazine compound and a polymerization inhibitor
comprising copper (meth)acrylate and copper dithiocarbamate.
[0040] (5) The method according to any one of the above (1) to (4),
wherein as the third distillation column, a vertical thin film
evaporator is employed which comprises an evaporator main body with
its principal portion being cylindrical, which has a heating means
on its exterior surface, a liquid inlet and a vapor outlet at its
upper portion and a residue discharge port at its lower portion, a
rotary shaft set in the main body, and stirring vanes attached to
the shaft and being movable in a peripheral direction along the
inner wall surface of the evaporator main body, and which has
wipers movable in a peripheral direction in contact with the inner
wall surface between the lower end of the stirring vanes and the
residue discharge port; and the bottoms are fed to the vertical
thin film evaporator and permitted to flow down on the inner wall
surface, and the resulting vapor of (meth)acrylic acid is recovered
from the vapor outlet at the upper portion.
[0041] (6) The method according to any one of the above (1) to (5),
wherein at least a part of the top fraction from the first
distillation column and/or at least a part of the bottoms from the
second distillation column, is used as a material for a
(meth)acrylic ester.
[0042] (7) The method according to the above (6), wherein the
(meth)acrylic acid to be used as the material for a (meth)acrylic
ester, contains at most 1,000 weight ppm of
.beta.-acryloxypropionic acid, at most 500 weight ppm in a total
amount of furfural and benzaldehyde, and at most 2,000 weight ppm
of maleic anhydride.
[0043] (8) A method for purifying a (meth)acrylic acid obtained by
a vapor phase catalytic oxidation method, characterized in that a
crude (meth)acrylic acid having impurities tentatively removed via
a preliminary purification step, and a top fraction from a third
distillation column, are fed to and distilled in a first
distillation column of a purification system comprising first to
third three distillation columns, the top fraction from the first
distillation column is, after applying an aldehyde removal
treatment thereto or after adding an aldehyde removing agent, fed
to and distilled in the second distillation column, the resulting
top fraction is recovered as a product, the bottoms from the first
and second distillation columns are fed to and distilled in the
third distillation column, the resulting top fraction is fed to the
first distillation column, and the bottom fraction is discharged
out of the purification system.
[0044] (9) The method according to the above (8), wherein as the
third distillation column, a thin film evaporator is used.
[0045] (10) A method for producing an acrylic ester, which
comprises reacting an acrylic acid with an alcohol, wherein as the
acrylic acid, an acrylic acid is used which contains at most 1,000
weight ppm of .beta.-acryloxypropionic acid, at most 500 weight ppm
in a total amount of furfural and benzaldehyde, and at most 2,000
weight ppm of maleic anhydride.
[0046] (11) The method according to the above (10), wherein the
acrylic acid is an acrylic acid obtained by a vapor phase catalytic
oxidation reaction of propylene.
[0047] (12) A method for producing a high purity (meth)acrylic
acid, which comprises extracting and/or distilling a reaction
product containing a (meth)acrylic acid obtained by vapor phase
catalytic oxidation to remove low boiling point impurities and high
boiling point impurities from the reaction product thereby to
obtain a purified (meth)acrylic acid, treating the purified
(meth)acrylic acid with an aldehyde removing agent, and then
distilling it in a distillation column to obtain a high purity
(meth)acrylic acid from the top of the distillation column,
characterized in that the bottom fraction from the distillation
column is used as a material for producing a (meth)acrylic
ester.
[0048] (13) The method according to the above (12), wherein the
(meth)acrylic ester is methyl (meth)acrylate and/or ethyl
(meth)acrylate.
[0049] (14) A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical, which has a heating means on
its exterior surface, a liquid inlet and a vapor outlet at its
upper portion and a residue discharge port at its lower portion, a
rotary shaft set in the main body, and stirring vanes attached to
the shaft and being movable in a peripheral direction along the
inner wall surface of the evaporator main body, characterized in
that it has wipers movable in a peripheral direction in contact
with the inner wall surface between the lower end of the stirring
vanes and the residue discharge port.
[0050] (15) A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical and its lower portion
constituting an inverted cone-shaped liquid collection portion,
which has a heating means on its exterior surface, a liquid inlet
and a vapor outlet at its upper portion and a residue discharge
port at its lower portion, a rotary shaft set in the main body, and
stirring vanes attached to the shaft and being movable in a
peripheral direction along the inner wall surface of the evaporator
main body, characterized in that it has wipers movable in a
peripheral direction in contact with the inner wall surface of the
inverted cone-shaped liquid collection portion.
[0051] (16) A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical and its lower portion
constituting a funnel-shaped liquid collection portion with a
combination of an inverted cone-shape and a cylindrical shape,
which has a heating means on its exterior surface, a liquid inlet
and a vapor outlet at its upper portion and a residue discharge
port at its lower portion, a rotary shaft set in the main body, and
stirring vanes attached to the shaft and being movable in a
peripheral direction along the inner wall surface of the evaporator
main body, characterized in that it has wipers movable in a
peripheral direction in contact with the inner wall surface of the
funnel-shaped liquid collection portion.
[0052] (17) A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical, which has a heating means on
its exterior surface while the corresponding interior surface
constitutes a heat transfer surface, and has a liquid inlet and a
vapor outlet at its upper portion and a residue discharge port at
its lower portion, a rotary shaft set in the main body, and
stirring vanes attached to the shaft and being movable in a
peripheral direction along the inner wall surface of the evaporator
main body, characterized in that it has wipers movable in a
peripheral direction in contact with the inner wall surface being a
non-heat transfer surface at the lower portion of the evaporator
main body.
[0053] (18) The thin film evaporator according to any one of the
above (14) to (17), wherein the wipers are attached to the same
rotary shaft as the rotary shaft to which the stirring vanes are
attached.
[0054] (19) The thin film evaporator according to any one of the
above (14) to (17), wherein the wipers are attached to the same
rotary shaft as the rotary shaft to which the stirring vanes are
attached, and they are movable vanes.
[0055] (20) A method for recovering (meth)acrylic acid or its ester
from a distillation residue of (meth)acrylic acid or its ester,
characterized by feeding a liquid containing (meth)acrylic acid or
its ester to the thin film evaporator as defined in any one of the
above (14) to (19) from the liquid inlet at its upper portion to
let it flow down on the inner wall surface, wherein a vapor of the
(meth)acrylic acid or its ester formed, is withdrawn from the vapor
outlet at its upper portion to outside, and the distillation
residue is withdrawn from the residue discharge port to
outside.
[0056] (21) A method for producing (meth)acrylic acid, which
comprises feeding a crude (meth)acrylic acid obtained by vapor
phase catalytic oxidation to a distillation column to continuously
distil and purify it in the presence of a hydrazine, characterized
in that the hydrazine is added to the crude (meth)acrylic acid
prior to feeding to the distillation column, and the crude
(meth)acrylic acid having the hydrazine added thereto is heated to
a temperature lower than 80.degree. C. and then fed to the
distillation column.
[0057] (22) A method for purifying (meth)acrylic acid, which
comprises distilling and purifying acrylic acid or methacrylic acid
obtained by a vapor phase catalytic oxidation method (hereinafter
referred to as a crude (meth)acrylic acid), characterized in that
the distillation is carried out at a bottom temperature of not
higher than 110.degree. C. in the presence of a polymerization
inhibitor comprising copper (meth)acrylate and/or copper
dithiocarbamate, and a hydrazine compound.
[0058] (23) The method according to the above (22), wherein the
copper (meth)acrylate is mixed to the crude (meth)acrylic acid
and/or the top liquid.
[0059] (24) The method according to the above (22) or (23), wherein
the copper dithiocarbamate is mixed to the crude (meth)acrylic acid
and/or the top liquid.
[0060] (25) The method according to any one of the above (22) to
(24), wherein the copper (meth)acrylate is a solution obtained by
dissolving at least one compound selected from copper powder,
cupric carbonate, cuprous hydroxide, cupric hydroxide and copper
acetate, in acrylic acid.
[0061] (26) The method according to any one of the above (22) to
(25), wherein the copper dithiocarbamate is copper
dimethyldithiocarbamate, copper diethyldithiocarbamate, copper
dipropyldithiocarbamate, copper dibutyldithiocarbamate, copper
ethylenedithiocarbamate, copper tetramethylenedithiocarbamate,
copper pentamethylenedithiocarbamate, copper
hexamethylenedithiocarbamate or copper
oxydiethylenedithiocarbamate.
[0062] (27) The method according to any one of the above (22) to
(26), wherein the hydrazine compound is hydrazine, hydrazine
hydrate, phenyl hydrazine, hydrazine sulfate or hydrazine
hydrochloride.
[0063] (28) The method according to any one of the above (22) to
(27), wherein the crude (meth)acrylic acid is distilled in the
presence of a phenol compound.
[0064] (29) The method according to any one of the above (22) to
(28), wherein the crude (meth)acrylic acid is distilled in the
presence of a phenothiazine compound.
[0065] (30) The method according to any one of the above (22) to
(29), wherein the distillation is carried out continuously by
maintaining the temperature at a bottom temperature of at least
80.degree. C.
[0066] The present invention has the following preferred
embodiments (a) to (f).
[0067] a1. A method for purifying a (meth)acrylic acid obtained by
a vapor phase catalytic oxidation method, characterized in that a
crude (meth)acrylic acid having impurities tentatively removed via
a preliminary purification step, and a top fraction from a third
distillation column, are fed to and distilled in a first
distillation column of a purification system comprising first to
third three distillation columns, the top fraction from the first
distillation column is, after applying an aldehyde removal
treatment thereto or after adding an aldehyde removing agent, fed
to and distilled in the second distillation column, the resulting
top fraction is recovered as a product, the bottoms from the first
and second distillation columns are fed to and distilled in the
third distillation column, the resulting top fraction is fed to the
first distillation column, and the bottom fraction is discharged
out of the purification system.
[0068] a2. The method according to a1, wherein the bottoms from the
third distillation column are decomposed by subjecting them to a
thermal decomposition apparatus, and a low boiling point component
containing formed (meth)acrylic acid, is supplied to a preliminary
purification step, while a heavy component is discharged out of the
system.
[0069] a3. The method according to a1 or a2, wherein as the third
distillation column, a thin film evaporator is used.
[0070] a4. The method according to any one of a1 to a3, wherein the
crude (meth)acrylic acid supplied to the first distillation column
contains at least 85 wt % of (meth)acrylic acid, and the rest is a
higher boiling point component than (meth)acrylic acid.
[0071] b1. A method for producing (meth)acrylic acid, which
comprises feeding a crude (meth)acrylic acid obtained by vapor
phase catalytic oxidation to a distillation column to continuously
distil and purify it in the presence of a hydrazine, characterized
in that the hydrazine is added to the crude (meth)acrylic acid
prior to feeding to the distillation column, and the crude
(meth)acrylic acid having the hydrazine added thereto is heated to
a temperature lower than 80.degree. C. and then fed to the
distillation column.
[0072] b2. The method according to b1, wherein the crude
(meth)acrylic acid having the hydrazine added thereto is heated to
a temperature of at least 60.degree. C. and less than 80.degree. C.
and then fed to the distillation column.
[0073] c1. A method for purifying (meth)acrylic acid, which
comprises distilling and purifying acrylic acid or methacrylic acid
obtained by a vapor phase catalytic oxidation method (hereinafter
referred to as a crude (meth)acrylic acid), characterized in that
the distillation is carried out at a bottom temperature of not
higher than 110.degree. C. in the presence of a polymerization
inhibitor comprising copper (meth)acrylate and/or copper
dithiocarbamate, and a hydrazine compound.
[0074] c2. The method according to c1, wherein a packed column, a
perforated plate column or a distillation column consisting of a
combination thereof, is used, and continuous distillation is
carried out while maintaining the bottom temperature to a level of
at most 110.degree. C.
[0075] c3. The method according to c2, wherein the copper
(meth)acrylate is mixed to the crude (meth)acrylic acid and/or the
top liquid.
[0076] c4. The method according to c2 or c3, wherein the copper
dithiocarbamate is mixed to the crude (meth)acrylic acid and/or the
top liquid.
[0077] c5. The method according to any one of c1 to c4, wherein the
copper (meth)acrylate is a solution obtained by dissolving at least
one compound selected from copper powder, cupric carbonate, cuprous
hydroxide, cupric hydroxide and copper acetate, in acrylic
acid.
[0078] c6. The method according to any one of c1 to c5, wherein the
copper dithiocarbamate is copper dimethyldithiocarbamate, copper
diethyldithiocarbamate, copper dipropyldithiocarbamate, copper
dibutyldithiocarbamate, copper ethylenedithiocarbamate, copper
tetramethylenedithiocarbamate, copper
pentamethylenedithiocarbamate, copper hexamethylenedithiocarbamate
or copper oxydiethylenedithiocarbamate.
[0079] c7. The method according to any one of c1 to c6, wherein the
hydrazine compound is hydrazine, hydrazine hydrate, phenyl
hydrazine, hydrazine sulfate or hydrazine hydrochloride.
[0080] c8. The method according to any one of c1 to c7, wherein the
crude (meth)acrylic acid is distilled in the presence of a phenol
compound.
[0081] c9. The method according to any one of c1 to c8, wherein the
crude (meth)acrylic acid is distilled in the presence of a
phenothiazine compound.
[0082] c10. The method according to any one of c1 to c9, wherein
the distillation is carried out continuously by maintaining the
temperature at a bottom temperature of at least 80.degree. C.
[0083] d1. A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical, which has a heating means on
its exterior surface, a liquid inlet and a vapor outlet at its
upper portion and a residue discharge port at its lower portion, a
rotary shaft set therein, and stirring vanes attached to the shaft
and being movable in a peripheral direction along the inner wall
surface of the evaporator main body, characterized in that it has
wipers movable in a peripheral direction in contact with the inner
wall surface between the lower end of the stirring vanes and the
residue discharge port.
[0084] d2. A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical and its lower portion
constituting an inverted cone-shaped liquid collection portion,
which has a heating means on its exterior surface, a liquid inlet
and a vapor outlet at its upper portion and a residue discharge
port at its lower portion, a rotary shaft set in the main body, and
stirring vanes attached to the shaft and being movable in a
peripheral direction along the inner wall surface of the evaporator
main body, characterized in that it has wipers movable in a
peripheral direction in contact with the inner wall surface of the
inverted cone-shaped liquid collection portion.
[0085] d3. A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical and its lower portion
constituting a funnel-shaped liquid collection portion with a
combination of an inverted cone-shape and a cylindrical shape,
which has a heating means on its exterior surface, a liquid inlet
and a vapor outlet at its upper portion and a residue discharge
port at its lower portion, a rotary shaft set in the main body, and
stirring vanes attached to the shaft and being movable in a
peripheral direction along the inner wall surface of the evaporator
main body, characterized in that it has wipers movable in a
peripheral direction in contact with the inner wall surface of the
funnel-shaped liquid collection portion.
[0086] d4. A thin film evaporator being a vertical thin film
evaporator which comprises an evaporator main body with its
principal portion being cylindrical, which has a heating means on
its exterior surface while the corresponding interior surface
constitutes a heat transfer surface, and has a liquid inlet and a
vapor outlet at its upper portion and a residue discharge port at
its lower portion, a rotary shaft set in the main body, and
stirring vanes attached to the shaft and being movable in a
peripheral direction along the inner wall surface of the evaporator
main body, characterized in that it has wipers movable in a
peripheral direction in contact with the inner wall surface being a
non-heat transfer surface at the lower portion of the evaporator
main body.
[0087] d5. The thin film evaporator according to any one of d1 to
d4, wherein the wipers are attached to the same rotary shaft as the
rotary shaft to which the stirring vanes are attached.
[0088] d6. The thin film evaporator according to any one of d1 to
d5, wherein the wipers are of a movable vane type.
[0089] d7. A method for separating a liquid comprising a readily
polymerizable component into a vapor and an evaporation residue,
characterized in that into the thin film evaporator as defined in
any one of d1 to d6, a liquid containing a readily polymerizable
component, is supplied from the upper liquid inlet and permitted to
flow on an inner wall surface, a vapor generated is withdrawn from
the upper vapor outlet to the exterior, and the evaporation residue
is withdrawn from the lower residue discharge port to the
exterior.
[0090] d8. A method for recovering (meth)acrylic acid or its ester
from a distillation residue of (meth)acrylic acid or its ester,
characterized by feeding a liquid containing (meth)acrylic acid or
its ester to the thin film evaporator as defined in any one of d1
to d6 from the liquid inlet at its upper portion to let it flow
down on the inner wall surface, wherein a vapor of the
(meth)acrylic acid or its ester formed, is withdrawn from the vapor
outlet at its upper portion to outside, and the distillation
residue is withdrawn from the residue discharge port to
outside.
[0091] e1. A method for producing an acrylic ester, which comprises
reacting an acrylic acid with an alcohol, wherein as the acrylic
acid, an acrylic acid is used which contains at most 1,000 weight
ppm of .beta.-acryloxypropionic acid, at most 500 weight ppm in a
total amount of furfural and benzaldehyde, and at most 2,000 weight
ppm of maleic anhydride.
[0092] e2. The method according to e1, wherein the content of
.beta.-acryloxypropionic acid is at most 500 weight ppm.
[0093] e3. The method according to e1 or e2, wherein the acrylic
acid is an acrylic acid obtained by a vapor phase catalytic
oxidation reaction of propylene.
[0094] f1. A method for producing a high purity (meth)acrylic acid,
which comprises extracting and/or distilling a reaction product
containing a (meth)acrylic acid obtained by vapor phase catalytic
oxidation to remove low boiling point impurities and high boiling
point impurities from the reaction product thereby to obtain a
purified (meth)acrylic acid, treating the purified (meth)acrylic
acid with an aldehyde removing agent, and then distilling it in a
distillation column to obtain a high purity (meth)acrylic acid from
the top of the distillation column, characterized in that the
bottom fraction from the distillation column is used as a material
for producing a (meth)acrylic ester.
[0095] f2. The method according to f1, wherein the purified
(meth)acrylic acid contains aldehydes having boiling points close
to (meth)acrylic acid.
[0096] f3. The method according to f1 or f2, wherein the
(meth)acrylic ester is methyl (meth)acrylate and/or ethyl
(meth)acrylate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0097] FIG. 1 is an example of a flow sheet to put the present
invention in practice.
[0098] FIG. 2 is a schematic view of an embodiment of the thin film
evaporator according to the present invention.
[0099] FIG. 3 is an enlarged view of the bottom portion of the thin
film evaporator in FIG. 2.
[0100] FIG. 4 is a view showing the state in which the wipers are
attached to correspond to the inverted corn-shaped portion in FIG.
3.
[0101] FIG. 5 is a view showing the state in which the wipers are
attached to correspond to the cylindrical portion in FIG. 3.
[0102] FIG. 6 is a flow chart showing an embodiment of the process
for producing acrylic acid, to which the method for producing high
purity (meth)acrylic acid of the present invention can be
applied.
[0103] In the drawings, reference numeral 1 indicates a first
distillation column, 2 a second distillation column, 3 a third
distillation column, 4, an ion exchange resin column, 5 a thermal
decomposition column, 6 a supply tube for crude acrylic acid, 7 a
supply tube for an aldehyde-removing agent, 8 a withdrawing tube
for purified acrylic acid (for ester), 9 a withdrawing tube for
purified acrylic acid (for a highly water-absorptive resin), 10 a
withdrawing tube for thermal decomposition fraction, 11 a
withdrawing tube for thermal decomposition residue, 21 a heating
jacket, 22 a principal portion, 23 a rotational shaft, 24 a motor,
25 a stirring vane, 26 a liquid inlet, 27 a vapor outlet, 28 a
residue discharge port, 29 a liquid collection portion, 30 a wiper,
31 a wiper, 32 a wiper supporting arm, 33 a spring, 41 an acrylic
acid collection column, 42 a distillation column for dehydration,
43 a decanter, 44 a distillation column for separating a low
boiling point component, 44A, 45A, 47A reflux tanks, 45 a
distillation column for separating high boiling point component, 46
a reactor to change aldehyde to a heavy substance, and 47 a
distillation column for purification.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment a
[0104] In the present invention, the vapor phase catalytic
oxidation and the subsequent preliminary purification may be
carried out by conventional methods. For example, in the case of
acrylic acid, a method for obtaining acrylic acid by a one step
oxidation method of propane by means of a Mo--V--Te double oxide
catalyst or a Mo--V--Sb double oxide catalyst, or a one step
oxidation method of oxidizing propylene directly to acrylic acid,
and a two step oxidation method of converting propylene to acrolein
and then oxidizing acrolein to acrylic acid, are known, and any one
of the methods may be employed. Further, acrylic acid formed by the
vapor phase catalytic oxidation is usually absorbed in water to
form an aqueous acrylic acid solution, and recovery of crude
acrylic acid from this aqueous acrylic acid solution may also be
carried out by a conventional method. For example, a method may be
employed wherein after dehydration by azeotropic distillation,
distillation is further carried out to remove acetic acid and other
low boiling point components. The purity of crude (meth)acrylic
acid thus obtained is usually at least 85 wt %, in many cases at
least 90 wt %. As a matter of course, the higher the purity of this
crude (meth)acrylic acid, the better. Impurities contained in this
crude (meth)acrylic acid are a dimer of (meth)acrylic acid and
other heavy components, and low boiling point components are not
substantially contained.
[0105] In the present invention, such crude (meth)acrylic acid is
distilled and purified by a purification system comprising first to
third three distillation columns, to recover high purity
(meth)acrylic acid suitable for application to a highly water
absorptive resin. Firstly, the crude acrylic acid and the top
fraction from the third distillation column are fed to and
distilled in the first distillation column. The ratio of the two
fed to the first distillation column varies depending upon the
operation conditions of the purification system, particularly on
how much of the top fraction from the first distillation column
will be fed to the second distillation column. As the first
distillation column, it is common to use one having a theoretical
plate number of from 5 to 20 plates, and it is operated under
reduced pressure, whereby (meth)acrylic acid is distilled from the
top of the column. This (meth)acrylic acid usually has a purity of
at least 99.5 wt %, in many cases at least 99.7 wt %, and thus has
a sufficient purity for a (meth)acrylic ester. However, it still
contains an aldehyde component such as furfural or benzaldehyde,
and as such, is not adequate as a starting material for a highly
water absorptive resin.
[0106] In a preferred embodiment of the present invention, the top
fraction from the first distillation column is treated with an
aldehyde-removing agent, or it is fed together with an
aldehyde-removing agent to the second distillation column, followed
by distillation. As disclosed in JP-A-2001-58970 or
JP-A-2001-213839, it is known to remove an aldehyde component by
treating (meth)acrylic acid containing the aldehyde component with
an aldehyde-removing agent. As the aldehyde-removing agent, in
addition to a primary amine or a hydrazine disclosed in these
publications, a mercaptan such as n-butylmercaptan,
n-octylmercaptan or n-dodecylmercaptan may, for example, be used.
In such a case, after adding such a mercaptan, the top fraction is
then treated with a sulfonic acid type cation exchange resin.
Removal of the aldehyde component by the primary amine or the
hydrazine may be carried out before feeding the top fraction from
the first distillation column to the second distillation column, or
the aldehyde-removing agent may be supplied to the second
distillation column together with or separately from the top
fraction, so that the aldehyde removal reaction is carried out in
the column. Further, in a case where a mercaptan is to be used, one
having a mercaptan added to the top fraction from the first
distillation column is passed through a resin column packed with a
sulfonic acid type cation exchange resin at a temperature of from
20 to 90.degree. C. at SV=0.1 to 10 hr.sup.-1 to carry out removal
of the aldehyde component. Passing of the liquid may be a down flow
system or an up flow system. The aldehyde-removing agent is used
usually in an amount of from 1 to 8 times by mol relative to the
aldehyde component.
[0107] As the second distillation column, it is common to employ
one having a theoretical plate number of from 1 to 5 plates, and it
is operated under reduced pressure, and (meth)acrylic acid is
distilled from the top of the column. For example, in the case of
acrylic acid, it is preferred to adjust such that the bottoms will
be from 50 to 100.degree. C., and the retention time in the column
will be from about 1 to 2 hours. Further, the concentration rate of
the bottoms, i.e. the weight ratio of the top fraction from the
first distillation column to be fed, to the liquid permitted to
flow out from the bottom of the column, is preferably from 2 to 25.
The top fraction from the second distillation column is of
extremely high purity, usually at least 99.8 wt %, in many cases at
least 99.9 wt %, and contains no aldehydes, whereby it is suitable
as the starting material for a highly water absorptive resin.
[0108] The bottoms from the first and second distillation columns
still contain a large amount of (meth)acrylic acid, and therefore,
they are fed to and distilled in the third distillation column, and
from the top thereof, (meth)acrylic acid is distilled and supplied
to the first distillation column, whereby the amount of
(meth)acrylic acid discharged out of the purification system can be
reduced, and the recovery rate of (meth)acrylic acid can be
improved. The top fraction from the third distillation column is
further distilled in the first distillation column, whereby even if
a heavy component other than (meth)acrylic acid is contained as
accompanied by splash in the top fraction, such will not be
problematic.
[0109] As the third distillation column, it is preferred to employ
a thin film evaporating apparatus. It is well is known that for
this apparatus, there are a vertical type and a horizontal type.
Typically, in either type, in the interior of a cylinder having a
jacket, rotary stirring vanes or wipers are installed, so that a
thin film of a supplied liquid is formed on the inner surface of
the cylinder so as to be evaporated. It is particularly preferred
to employ a vertical type such as a Smith type thin film evaporator
or a Luwa type thin film evaporator. Also the third distillation
column is preferably operated under reduced pressure, for example,
in the case of acrylic acid, under a pressure at a level of from 67
Pa to 40 KPa. It is thereby possible to lower the operation
temperature and thereby to suppress polymerization, etc. of
(meth)acrylic acid.
[0110] In a preferred embodiment of the present invention, the
bottoms from the third distillation column are fed to and thermally
decomposed in a thermal decomposition apparatus. Such bottoms
comprise non-evaporated (meth)acrylic acid, its dimer, the
aldehyde-removing agent, maleic acids and other impurities, and
accordingly, (meth)acrylic acid can be recovered by this thermal
decomposition. In the purification by distillation of (meth)acrylic
acid, it is known to thermally decompose the bottoms to recover
(meth)acrylic acid, and also in the present invention, the recovery
may be carried out in accordance with such a known method. For
example, the temperature is usually preferably from 110 to
250.degree. C., particularly preferably from 120 to 230.degree. C.,
and the time required for the decomposition is, in the case of a
low temperature, usually from 10 to 50 hours, and in the case of a
high temperature, from 0.5 to 10 hours. The pressure may be
atmospheric pressure or reduced pressure. A low boiling fraction
containing (meth)acrylic acid obtained by thermal decomposition
contains low boiling point components, etc., and therefore, it is
supplied to a stage prior to the stage for removal of low boiling
point components in the preliminary purification step. The heavy
component is discharged out of the system and incinerated.
Embodiment b
[0111] In order to solve the conventional problems in continuous
production of high purity acrylic acid on an industrial scale, the
present inventors have conducted extensive studies on the relation,
etc. of the aldehyde-removing agent, various additives and their
amounts, formation of sludge and its thermal stability, and the
amounts of impurities remaining in purified acrylic acid, and as a
result, have found the following facts. Namely, usually, when crude
acrylic acid having a concentration of maleic acids being at least
2000 ppm, was used and reacted with a hydrazine, solid would
precipitate, and if such a starting material was fed to the side of
a distillation column, continuous distillation was impossible due
to clogging by the precipitated solid. Whereas, when the starting
material is reacted with hydrazine prior to feeding it to the side
of the distillation column, followed by heat treatment at a
temperature lower than 80.degree. C., it becomes possible to
suppress re-formation of maleic acid once removed by the reaction
with hydrazine and to have precipitated solid formed into a uniform
solution (namely, a state where no formation of precipitate is
observed even when the solution is left to stand for 30 minutes),
and it has been found possible to suppress formation of sludge in
the distillation column even by continuous distillation on a
commercial scale. The present invention has been made based on such
a finding.
[0112] Further, here, the method for purifying (meth)acrylic acid
of the present invention, will be described with respect to acrylic
acid, but the present invention can be applied to methacrylic acid
in the same manner. When the present invention is to be applied to
the production of methacrylic acid, crude methacrylic acid may be
obtained by vapor phase catalytic oxidation of isobutylene and/or
t-butyl alcohol, and in such crude methacrylic acid, aldehydes,
ketones, maleic acids as well as citraconic acids, are contained as
impurities, in the same manner as in the case of crude acrylic
acid.
[0113] The crude acrylic acid to be purified by the present
invention is one obtainable by vapor phase catalytic oxidation,
which contains maleic acids, etc. as impurities, and it is usually
produced industrially by the following methods.
[0114] Namely, it is produced by a one step oxidation method
wherein propane, propylene and/or acrolein is reacted with a
molecular oxygen-containing gas in the presence of e.g. a
molybdenum oxide type solid oxidized catalyst as a solid catalyst,
or a two step oxidation method wherein in the presence of a solid
catalyst such as a molybdenum oxide type solid oxidized catalyst,
firstly, in the first reaction zone, acrolein is obtained by the
reaction of propylene with a molecular oxygen-containing gas, and
in the subsequent second reaction zone, the acrolein is reacted
with molecular oxygen in the presence of a solid catalyst such as a
molybdenum oxide solid oxidized catalyst to obtain acrylic acid.
Or, by a method of obtaining an acrylic acid by oxidizing propane
by means of a Mo--V--Te type double oxide catalyst or a Mo--V--Sb
type double oxide catalyst, a formed gas of a vapor phase catalytic
oxidation reaction, is obtained, and this formed gas is
countercurrently contacted with water in an absorption column to
obtain an aqueous crude acrylic acid. This aqueous crude acrylic
acid solution is extracted with an organic solvent such as methyl
isobutyl ketone or diisobutyl ketone, followed by distillation, or
an azeotropic agent such as toluene, butyl acetate or octane is
added thereto, followed by direct azeotropic dehydration under such
conditions as a bottom temperature of from 80 to 100.degree. C. and
a pressure of from 6.67 to 20 kPa to obtain an acrylic
acid-containing liquid. From the obtained acrylic acid-containing
liquid, a low boiling point component such as acetic acid, is
removed, and the bottoms are further distilled to obtain crude
acrylic acid as the top fraction, and a high boiling point
component such as a dimer, is withdrawn from the bottom of the
column.
[0115] The crude acrylic acid to be used as a starting material for
high purity acrylic acid, in the present invention, is the top
fraction in the distillation step after removing such a low boiling
point component, and if recovery of acrylic acid from the dimer,
etc., is taken into consideration, this crude acrylic acid usually
contains, as impurities, carboxylic acids such as maleic acids and
acetic acid, aldehydes such as furfural and benzaldehyde, water,
etc.
[0116] As the crude acrylic acid to be used in the process for
producing high purity acrylic acid in the present invention, it is
preferred to employ one having a concentration of maleic acids
being at least 2000 ppm. Further, the upper limit of the
concentration of maleic acids is preferably 10000 ppm, more
preferably 5000 ppm. In order to treat one having maleic acids more
than this, the amount of the required hydrazine increases, such
being uneconomical. Whereas, in order to employ crude acrylic acid
having a concentration of maleic acids less than 2000 ppm, it is
necessary to increase the plate number of the distillation column
in order to increase the precision for separation of acrylic acid
and maleic acids in the process for producing crude acrylic acid,
or it is necessary to stop recovery of acrylic acid from a high
boiling point product containing a dimer of acrylic acid in order
to reduce the amount of maleic acids distilled from the top of the
column at the same time as carrying out the recovery of acrylic
acid from the dimer of acrylic acid and to dispose the entire
amount, such being undesirable as the economical loss is
substantial.
[0117] In the present invention, a hydrazine is added to crude
acrylic acid prior to feeding to the side of the distillation
column, to preliminarily react the hydrazine with maleic acids in
the crude acrylic acid, and then purification by distillation is
carried out. As the reaction apparatus to be used for the reaction
of the hydrazine with maleic acids in the crude acrylic acid, any
one may be used so long as the necessary temperature and the
retention time can be secured. For example, a reaction tank
equipped with a stirrer or a tubular reaction tank may be employed.
The reaction temperature is preferably as low as possible.
Specifically, it is selected within a range of at least the melting
point of acrylic acid and at most 50.degree. C. As the reaction
time, it is preferred to retain at least 10 minutes, usually from
30 minutes to 3 hours.
[0118] As the hydrazine to be added to the crude acrylic acid, it
is preferred to add hydrazine and/or hydrazine hydrate as it is.
The amount of the hydrazine is usually from 0.1 to 2 times by mol,
preferably from 0.5 to 2 times by mol, more preferably from 0.5 to
1 time by mol, to the total amount of maleic acids and aldehydes
such as furfural and benzaldehyde, in the crude acrylic acid.
[0119] After the above reaction, the reaction mixture of the crude
acrylic acid and the hydrazine, is heated before it is fed to a
distillation column. The upper limit of this heating temperature
(hereinafter sometimes referred to as "the feeding temperature") is
lower than 80.degree. C., but a preferred upper limit is 75.degree.
C. Further, a preferred lower limit of the feeding temperature is
60.degree. C., but more preferred lower limit is 62.degree. C. If
the feeding temperature is lower than 60.degree. C., solid formed
by the reaction of maleic acids and the hydrazine will be
precipitated and slurried, and if such a slurry is fed into a
distillation column as it is, such will cause deposition or
formation of sludge in the distillation column, such being
undesirable. On the other hand, if the feeding temperature is
80.degree. C. or higher, from the adduct once formed by the
reaction of the hydrazine and maleic acids, maleic acid will be
re-produced by a reverse reaction, and such maleic acid will be
distilled from the top of the distillation column, and besides,
there will be a problem of polymerization of thermally unstable
acrylic acid by heating at a high temperature, such being
undesirable.
[0120] The method for heating the reaction solution of the
hydrazine and the crude acrylic acid is not particularly limited so
long as the internal temperature can be set at the above-mentioned
temperature. For example, this reaction solution may be heated by
means of a heat exchanger and then fed directly to the side of a
distillation column.
[0121] The heating time of this reaction solution may depend on the
content of maleic acids in the crude acrylic acid. However, once
the internal temperature of the reaction solution reaches the
prescribed temperature, the yellow precipitated solid by the
reaction of maleic acids and the hydrazine, will disappear, and a
uniform solution will be formed, whereby the end of the heating
time can easily be ascertained. Accordingly, the time for this
disappearance may be taken as the end point of the heating. In a
usual case, one hour is sufficient for such a heating time. A
heating time of more than that is not desirable, since aldehydes
removed by the reaction with the hydrazine are likely to undergo a
reverse reaction.
[0122] The operation conditions of the distillation column into
which this heated reaction solution is fed, vary depending upon the
composition of the material to be distilled, the recovery rate, the
purity of acrylic acid distillate, etc. However, as acrylic acid is
a readily polymerizable compound, the distillation temperature and
pressure are preferably set so that they will be a low temperature
and low pressure as far as possible. Specifically, usually, the
bottom temperature is from 60 to 100.degree. C., and the top
pressure is selected within a range of from 1.33 to 26.7 kPa.
[0123] In the present invention, at the time of distillation, in
addition to a hydrazine as an agent for treating impurities, a
conventional known polymerization-preventing agent i.e. a
polymerization inhibitor and/or a polymerization controlling agent
may be added. As such a polymerization preventing agent, various
studies have already been made. The following ones may be mentioned
as examples of the polymerization-preventing agent. Namely, an
N-oxyl compound may, for example, be tertiary butyl nitrooxide,
2,2,6,6-tetramethyl-4-hydroxypiperidyl-1-oxyl,
2,2,6,6-tetramethylpiperidyl-1-oxyl,
2,2,6,6-tetramethylpiperidinooxyl,
4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl or
4,4',4''-tris1-(2,2,6,6-tetramethylpiperidinooxyl)phosphite, a
phenol compound may, for example, be hydroquinone, methoquinone,
pyrogallol, catechol, or resorcinol; a phenothiazine compound may,
for example, be phenothiazine,
bis-(.alpha.-methylbenzyl)phenothiazine, 3,7-dioctylphenothiazine,
or bis(.alpha.-dimethylbenzyl)phenothiazine; and a copper compound
may, for example, be cupric chloride, copper acetate, copper
carbonate, copper acrylate, copper dimethyldithiocarbamate, copper
diethyldithiocarbamate, or copper dibutyldithiocarbamate. These
polymerization-preventing agents may be used alone or in
combination as a mixture of two or more of them. The amount of such
a polymerization-preventing agent is not particularly limited, but
is preferably at a level of from 1 to 1000 ppm.
[0124] In the present invention, the method for distillation is not
particularly limited. For example, various methods such as simple
distillation, precision distillation, etc. may be employed. Such
distillation may be carried out either in a batch system or in a
continuous system. However, from the industrial point of view, it
is preferred to carry out the distillation in a continuous system.
Further, also with respect to the distillation apparatus, there is
no particular restriction.
[0125] As a distillation column, a perforated plate column, a
bubble-cap column, a packed column or a combination thereof (such
as a combination of a perforated plate column and a packed column)
may, for example, be available, and any one may be used in the
present invention without distinguishing the presence or absence of
an overflow gate or a down comer. As specific trays, bubble cap
trays, perforated plate trays, bubble trays, super flash trays, max
flux trays, or dual trays may, for example, be mentioned.
[0126] As packing materials, in addition to those which are
heretofore been used, such as columnar, cylindrical, saddle-type,
spherical, cubic or pyramid-shaped ones, regular or irregular
packing materials having specific shapes have been commercially
available as high performance packing materials in recent years.
These packing materials may suitably be used in the present
invention.
[0127] Examples of such commercial products include, as a regular
packing material, a gauze type regular packing material such as
Sulzer Packing (manufactured by Sulzer Brothers Company), Sumitomo
Sulzer Packing (manufactured by Sumitomo Heavy Industries, Ltd.) or
Tecknopack (manufactured by Mitsui & Co., Ltd.), a sheet type
regular packing material such as Mellapack (manufactured by
Sumitomo Heavy Industries, Ltd.), Tecknopack (manufactured by
Mitsui & Co., Ltd.), or MC Pack (manufactured by Mitsubishi
Chemical Engineering Corporation), and a grid type regular packing
material such as Flexigrid (manufactured by Koch Company). Further,
GEMPAK (manufactured by Glitsch Company), Montz Pack (manufactured
by Montz Company), Goodroll Packing (manufactured by Tokyo Tokushu
Kanaami K.K.), Honeycomb Pack (Manufactured by NGK Insulators,
Ltd.) and Impulse Packing (Manufactured by Nagaoka Corporation)
may, for example, be mentioned.
[0128] Further, an irregular packing material may, for example be
Raschig ring, Pall ring (manufactured by BASF), Cascade Miniring
(manufactured by Mass Transfer Company), IMTP (manufactured by
Norton Company), Intalox Saddle (manufactured by Norton Company),
Tellerette (manufactured by Nittetsu Chemical Engineering Ltd.) or
Flexiring (manufactured by JGC Corporation).
[0129] The material for the apparatus constituting the distillation
column, is not particularly limited, but since (meth)acrylic acid
is corrosive, it is preferred to use a stainless steel such as
SUS304, SUS304L, SUS316, SUS316L, SUS317, SUS317L, SUS329J1 or
SUS329J2L, or a nickel alloy such as hastelloy or inconel.
Embodiment c
[0130] The (meth)acrylic acid to be used here, is one obtained by
vapor phase catalytic oxidation of propane, propylene and/or
acrolein, or isobutylene and/or methacrolein. For example, it may
be acrylic acid which is obtained by vapor phase oxidation of
propane by means of a Mo--V--Te double oxide catalyst or a
Mo--V--Sb double oxide catalyst, or acrylic acid or methacrylic
acid which is obtained by vapor phase catalytic oxidation of
propylene or isobutylene in the presence of a Mo--Bi double oxide
catalyst to form acrolein or methacrolein, which is further
subjected to vapor phase catalytic oxidation in the presence of a
Mo--V double oxide catalyst. Here, the preliminary reaction of
oxidizing propylene to form mainly acrolein and the subsequent
reaction of oxidizing acrolein to form mainly acrylic acid, may be
carried out in separate reactors, respectively, or the catalyst for
the preliminary reaction and the catalyst for the subsequent
reaction may simultaneously be packed into one reactor to carry out
the reactions.
[0131] Such crude (meth)acrylic acid is one containing impurities
formed as by-products in the production process. Such impurities
may, for example, be low boiling point impurities such as water,
furfural, benzaldehyde, acetic acid, etc., and high boiling point
impurities such as a dimer or trimer of (meth)acrylic acid, maleic
anhydride, .beta.-hydroxypropionic acid, .beta.-alkoxypropionic
acid, etc.
[0132] The hydrazine compound to be used in the present invention
is one which acts to convert a compound having a boiling point
close to (meth)acrylic acid, such as furfural, to a component which
can be easily distilled and separated.
[0133] Such a hydrazine compound may, for example, be hydrazine,
hydrated hydrazine, phenyl hydrazine, hydrazine sulfate or
hydrazine hydrochloride. They may be used alone or in combination
as a mixture of two or more of them. The amount of the hydrazine
compound to be added, is suitably selected depending upon the
amount of impurities to be removed and the concentration of
impurities allowed to be contained in high purity acrylic acid
obtainable after the distillation.
[0134] In the present invention, it is used usually in an amount of
from 1 to 10 times by weight, preferably from 2 to 5 times by
weight, based on the weight of impurities to be removed, contained
in the starting material (meth)acrylic acid. It is used usually in
an amount of from 50 to 5000 ppm, preferably from 200 to 4000 ppm,
as represented based on the crude (meth)acrylic acid. If its amount
is small, impurities to be removed, will be contained in a large
amount in purified (meth)acrylic acid, such being undesirable. If
the amount is large, such will not be problematic for the removal
of impurities, but the effect by addition will be saturated, and
such is not economically desired.
[0135] The copper dithiocarbamate to be used in the present
invention is one which acts as a polymerization inhibitor (a
polymerization-preventing agent) for (meth)acrylic acid.
[0136] Such a copper dithiocarbamate may, for example, be a copper
dialkyldithiocarbamate such as copper dimethyldithiocarbamate,
copper diethyldithiocarbamate, copper dipropyldithiocarbamate or
copper dibutyldithiocarbamate, a copper cyclic alkylene
dithiocarbamate such as copper ethylenedithiocarbamate, copper
tetramethylenedithiocarbamate, copper pentamethylenedithiocarbamate
or copper hexamethylenedithiocarbamate, or a copper cyclic
oxydialkylenedithiocarbamate such as copper
oxydiethylenedithiocarbamate. They may be used alone or in
combination as a mixture of one or more of them.
[0137] The amount of the copper dithiocarbamate is from 1 to 100
weight ppm, preferably from 10 to 80 weight ppm, based on the
(meth)acrylic acid to be fed to the distillation column. If the
amount is small, the effect for suppressing polymerization tends to
be inadequate. If the amount is large, corrosion of the apparatus
at the bottom of the distillation column tends to take place, such
being undesirable. It is considered that in the distillation system
of the present invention, the copper dithiocarbamate has a larger
effect for suppressing polymerization of the bottoms than
suppression of the polymerization of the liquid which flows down
the interior of the distillation column. Accordingly, with respect
to the position for the addition of the copper dithiocarbamate, it
is preferred to add it to the crude (meth)acrylic acid as the
starting material, or to the bottoms of the distillation
column.
[0138] Copper (meth)acrylate to be used in the present invention
acts as a polymerization inhibitor (a polymerization preventing
agent) for (meth)acrylic acid, like the copper dithiocarbamate. By
a combined use of the two, a remarkable effect can be obtained for
the first time. The copper (meth)acrylate can be prepared by
dissolving a carbonate, a chloride, an organic salt or a hydroxide
of copper, or a copper powder in (meth)acrylic acid. Copper
carbonate is particularly preferred. The chloride is not preferred,
since stress corrosion cracking is likely to take place, as the
distillation column for (meth)acrylic acid, is usually made of a
stainless steel material. With respect to specific materials to be
dissolved in (meth)acrylic acid in order to obtain copper
(meth)acrylate to be used in the present invention, the carbonate
may, for example, be cupric carbonate; the salt of an organic acid
may, for example, be copper formate, copper acetate or copper
salicylate; and the hydroxide may, for example, be cuprous
hydroxide or cupric hydroxide. Further, copper powder may directly
be dissolved in (meth)acrylic acid. They may be used alone or in
combination as a mixture of two or more of them.
[0139] The copper (meth)acrylate may be obtained also by dissolving
such a material in a solvent containing (meth)acrylic acid. As the
solvent in such a case, it is preferred to employ a solvent having
a boiling point higher than (meth)acrylic acid, so that the solvent
will not be included in high purity (meth)acrylic acid obtained
from the top of the distillation column. Specifically, diphenyl
ether, an o-phthalic acid ester, an oleic acid ester, an adipic
acid ester, a hydrocarbon in a medium oil fraction, a heat
conductive oil having a boiling point of at least 170.degree. C.,
or a mixed solvent thereof, may be used.
[0140] In a case where water is contained in the crude
(meth)acrylic acid as the starting material to be distilled, water
may also be used as the solvent having a boiling point lower than
(meth)acrylic acid. The concentration of water may be determined
taking into consideration the value allowable for high purity
(meth)acrylic acid to be obtained and the necessary amount of the
copper (meth)acrylate. In a case where no water is contained in the
crude (meth)acrylic acid, a due care will be required, since
dehydration may again be required depending upon the specification
for the product.
[0141] The amount of the copper (meth)acrylate may be calculated on
the assumption that the copper dissolved is all converted to copper
(meth)acrylate, and it is from 1 to 100 weight ppm, preferably from
5 to 80 weight ppm, based on the crude (meth)acrylic acid to be fed
to the distillation column. If the amount is small, the effect for
suppressing polymerization tends to be inadequate. If the amount is
large, corrosion of the apparatus at the bottom of the distillation
column is likely to take place, such being undesirable.
[0142] As is different from the copper dithiocarbamate, the copper
(meth)acrylate provides a substantial effect to the liquid in such
a state that it flows down in the interior of the distillation
column. Accordingly, with respect to the position for addition of
the copper (meth)acrylate, it is preferred to add it to the crude
(meth)acrylic acid as the starting material, or to the liquid at
the top of the distillation column.
[0143] In the present invention, as mentioned above, two types of
polymerization inhibitors having different actions, are used. Even
if dissolved in (meth)acrylic acid, the copper dithiocarbamate can
hardly be converted to copper (meth)acrylate as a feature of the
present invention, and accordingly, the latter is required to be
added afresh as in the present invention.
[0144] Further, in the present invention, it is preferred to add a
phenol compound and/or a phenothiazine compound in addition to the
hydrazine compound, the copper dithiocarbamate and the copper
(meth)acrylate, whereby the effects of the present invention can be
further improved. If necessary, in some cases, an N-oxyl compound
such as tertiary butyl nitroxide,
2,2,6,6-tetramethyl-4-hydroxypiperidyl-1-oxyl,
2,2,6,6-tetramethylpiperidyl-1-oxyl,
2,2,6,6-tetramethylpiperidinooxyl,
4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl, or
4,4',4''-tris-(2,2,6,6-tetramethylpiperidinooxyl)phosphite; a
phenylene diamine such as p-phenylene diamine; a nitroso compound
such as N-nitrosodiphenylamine; a urea such as urea; and a thiourea
such as thiourea, may be used in combination.
[0145] The phenol compound may, for example, be hydroquinone,
methoquinone (methoxyhydroquinone), pyrogallol, catechol,
resorcinol, phenol or cresol, and such may be used alone or in
combination as a mixture of two or more of them. The amount of the
phenol compound is from 0 to 800 weight ppm, preferably from 50 to
600 weight ppm, based on the crude (meth)acrylic acid to be fed to
the distillation column. If the amount is small, the effect for
controlling polymerization may sometimes be inadequate. If the
amount is too much, such being economically undesirable, although
there will be no adverse effect to the effect for suppressing
polymerization.
[0146] The phenothiazine compound may, for example, be
phenothiazine, bis-(.alpha.-methylbenzyl)phenothiazine,
3,7-dioctylphenothiazine or
bis-(.alpha.-dimethylbenzyl)phenothiazine, and such may be used
alone or in combination as a mixture of two or more of them. The
amount of the phenothiazine compound is from 0 to 400 weight ppm,
preferably from 50 to 300 weight ppm, based on the (meth)acrylic
acid to be fed to the distillation column. If the amount is small,
the effect for suppressing polymerization may sometimes be
inadequate. If the amount is too large, such being economically
undesirable, although there may be no adverse effect to the effect
for suppressing polymerization.
[0147] The method for adding the copper (meth)acrylate, the copper
dithiocarbamate, the phenol compound and the phenothiazine
compound, which are effective for suppressing polymerization, is
not particularly limited. For example, there may be a method
wherein they are, respectively, directly added to the (meth)acrylic
acid to be fed to the distillation column or to the (meth)acrylic
acid liquid refluxed as a distillate liquid, or a method wherein
they are dissolved and added by means of a suitable solvent. The
temperature for the addition may also be suitably determined.
[0148] The hydrazine compound to remove impurities and the method
of its addition are also not particularly limited. However, the
hydrazine compound is required to react with impurities to be
removed, and a retention time of preferably from 10 minutes to 5
hours, more preferably from 20 minutes to 3 hours, is preferred
after adding the hydrazine compound to the crude (meth)acrylic acid
until purified (meth)acrylic acid will be obtained as a distillate
from the top of the distillation column. If the time for the
reaction is short, impurities will not be sufficiently reacted. If
the time for the reaction is too long, impurities may increase by a
decomposition reaction of the reacted products. Accordingly, the
time is selected within the above range.
[0149] The crude (meth)acrylic acid having the hydrazine compound,
the copper acrylate and the copper dithiocarbamate added thereto,
is subjected to distillation treatment, whereby impurities to be
removed, will be removed. The distillation method is not
particularly limited, and various distillation methods such as
simple distillation, precision distillation, etc. may be employed.
Further, the distillation may be either in a continuous system or
in a batch system. However, an embodiment whereby the effects of
the present invention can most remarkably be obtained, is such that
a constant operation for a long period of time can be accomplished
in an industrial and economical continuous distillation.
[0150] The distillation column to be used here, the type of the
packing material to be packed into the packing column, the material
for the apparatus constituting the distillation column, etc. are as
described in the foregoing. With respect to the distillation
temperature, the bottom temperature is at most 110.degree. C.,
preferably at most 100.degree. C., in order to improve the effect
for suppressing polymerization by the present invention.
Conventional distillation of (meth)acrylic acid used to be carried
out at a temperature of at most 100.degree. C., particularly at
most 70.degree. C. (e.g. JP-A-7-228548k) in many cases. Whereas,
according to the present invention, suppression of polymerization
can remarkably be made, whereby the operation range of the bottom
temperature can be increased. Accordingly, it is possible to carry
out the operation at a bottom temperature of preferably from 80 to
110.degree. C., particularly preferably from 90 to 105.degree. C.
The economical effect due to a reduction of the heat transfer area
of the reboiler for distillation column, is extremely large.
Embodiment d
[0151] The thin film evaporator of the present invention to be used
for the above-mentioned third distillation column, is characterized
in that it has wipers movable in a peripheral direction in contact
with the inner wall surface also at an inner wall surface portion
further lower than the lower end of the stirring vanes. It is
thereby possible to prevent formation of a deposition on the inner
wall surface further lower than the lower end of the stirring vanes
of the thin film evaporator and to operate the thin film evaporator
constantly over a long period of time.
[0152] Like a known thin film evaporator, the thin film evaporator
of the present invention comprises an evaporator main body with its
principal portion being cylindrical, which has a heating means on
its exterior surface, a liquid inlet and a vapor outlet at its
upper portion and a residue discharge port at its lower portion, a
rotary shaft set in the main body, and stirring vanes attached to
the shaft and being movable in a peripheral direction along the
inner wall surface of the evaporator main body. As a typical
example of such a thin film evaporator, a Smith system thin film
evaporator or a Luwa thin film evaporator may, for example, be
mentioned. The shape of a liquid collection portion at a lower
portion of the main body of the thin film evaporator is considered
to be preferably such a shape that it has an inclination to the
evaporation surface so that the evaporation residue will smoothly
flow into the liquid withdrawal tube from the residue discharge
port, and one wherein the shape of the liquid collection portion is
an inverted corn-shape or a funnel shape being a combination of an
inverted corn-shape and a cylindrical shape, is practically
used.
[0153] The thin film evaporator according to the present invention
is characterized in that in such a known thin film evaporator,
wipers are provided which are movable in a peripheral direction in
contact with an inner wall surface located further lower than the
inner wall surface corresponding to the lower end of the stirring
vanes attached to the rotary shaft. Stirring vanes are to
accelerate evaporation from the liquid film and accordingly usually
located at a position corresponding to the heat transfer surface,
whereby the lower end of the stirring vanes is substantially the
same position as the lower end of the heat transfer surface.
[0154] Thus, in the present invention, the wipers are provided to
correspond to a non-heat transfer surface below the heat transfer
surface. For example, in a case where the lower portion of the
cylindrical main body of the thin film evaporator constitutes a
non-heat transfer surface, the wipers are provided to correspond to
such a portion. In a case where a liquid collection portion of an
inverted-corn shape or a funnel shape being a combination of an
inverted corn shape and a cylindrical shape, is connected to the
cylindrical main body of the thin film evaporator, the wipers are
provided to correspond to such a liquid collection portion. The
wiper to be provided may be one or may be divided into a plurality.
It is usually preferred to provide the wiper(s) to correspond to
the entire surface of such a portion. However, so long as the
purpose of the wiper(s) to prevent deposition of an evaporation
residue on the non-heat transfer surface, it is not necessary to
provide the wiper(s) to cover the entire surface. The wipers are
usually attached to the rotary shaft to which the stirring vanes
are attached to form thin film or to a shaft extended downwardly
from the rotary shaft. It is thereby possible to have a common
driving source for the stirring vanes and the wipers, and the
structure of the apparatus can be simplified. Further, the
attaching method of the wipers is optional, but it is preferred to
attach them on the rotary shaft via fulcrums or springs in the same
manner as for the stirring vanes and to attach them in a movable
vane system so that they are movable in a peripheral direction
about the rotary shaft.
[0155] As the material to constitute the wipers, a material
suitable for the physical properties of the liquid to be treated by
the thin film evaporator, may be selected for use. For example, in
a case where the liquid to be treated is a highly corrosive liquid
such as acrylic acid, a stainless steel such as SUS304, SUS316,
SUS316L, SUS317, SUS317L, SUS329JL or SUS329J2L, or a nickel alloy
such as hastelloy or inconel, may, for example, be mentioned.
However, from the viewpoint of corrosion resistance and economical
efficiency, SUS304, SUS316 or SUS316L is preferred. Further, as the
material of a portion of the wipers, which will be physically in
contact with the inner surface of the thin film evaporator, it is
desired to employ a material which will not damage the inner wall
surface of the thin film evaporator. Preferably, one which is
highly corrosion resistant and will not damage the inner wall
surface of the thin film evaporator, such as Teflon, or a hybrid
carbon having a resin or metal vacuum-press impregnated to carbon
to have the mechanical strength and anti-sealing property improved
and being useful even under a high temperature condition, such as
Sliding Composite Carbon NC-07E manufactured by Nippon Carbon Co.,
Ltd., is used. Particularly preferred is the hybrid carbon which is
excellent in the dimensional stability for a long period of time
and which is useful even under a high temperature condition.
[0156] Like a conventional thin film evaporator, the thin film
evaporator of the present invention can be used to evaporate low
boiling point components from various liquids to be treated, but is
particularly suitable for evaporating low boiling point components
from a liquid to be treated, which contains components readily
polymerizable by heat. As such a liquid to be treated, a heavy
component may be mentioned which is discharged from the bottom of
the column at the time of purification by distillation of
(meth)acrylic acid or its ester and wherein the (meth)acrylic acid
or its ester still remains. An example of the (meth)acrylic ester,
methyl, ethyl, butyl, isobutyl, tertiary butyl, 2-ethylhexyl,
2-hydroxyethyl, 2-hydroxypropyl or methoxyethyl may, for example,
be mentioned.
[0157] Further, at the time of purification by distillation of such
a polymerizable one, it is common to add a polymerization
preventing agent to the liquid. For example, at the time of
purification by distillation of acrylic acid, it is common to
employ a polymerization preventing agent of a phenol type, such as
hydroquinone or hydroquinonemonomethyl ether, an organic substance
such as phenothiazine or N-oxyl compound, or a copper salt such as
copper dialkyldithiocarbamate, copper acrylate or copper acetate.
Accordingly, in the bottoms discharged from the bottom of the
distillation column, such a polymerization inhibitor is
concentrated. If the bottoms in which the polymerization preventing
agent is concentrated, is treated by a thin film evaporator, the
polymerization preventing agent will be further concentrated and
precipitated, whereupon it may deposit on the wall surface, or the
fluidity of the distillation residue tends to be deteriorated. By a
usual thin film evaporator, smooth treatment is difficult, while by
the thin film evaporator of the present invention, such can be
easily treated.
[0158] FIGS. 2 to 5 show schematic views of one embodiment of a
rotary slider type thin film evaporator according to the present
invention, but the present invention is by no means restricted to
such an embodiment.
[0159] This apparatus is a thin film evaporator having a
cylindrical main body portion (22) having a heating jacket (21) on
its exterior surface, and it has an interior rotary shaft (23) and
a motor (24) to rotate the shaft. Stirring vanes (25) are attached
to the rotary shaft (23) and the stirring vanes will rotate while
maintaining a slight distance from the inner wall of the main body
portion, and the liquid to be treated, fed from a liquid inlet (26)
at an upper portion of the thin film evaporator will flow down by
gravity along the inner wall of the main body portion while being
spread in a film form by the rotating stirring vanes. In the
process of this flowing down, a low boiling point component in the
liquid to be treated will be evaporated by heat from the heating
jacket. The evaporated low boiling point component will be led out
of the system from a vapor outlet (27) located at an upper portion
of the thin film evaporator, and the distillation residue having
the majority of such a low boiling point component removed to have
poor fluidity, will be led to a liquid collection portion (29).
And, wipers (30) and (31) attached to the rotary shaft will rotate
in contact with the wall surface of the liquid collecting portion,
whereby the distillation residue flowing down to this liquid
collection portion will constantly be removed from the inner wall
surface and withdrawn from the residue discharge port (28) located
at a lower portion of the thin film evaporator. It is thereby
possible to prevent clogging by the evaporation residue at the
outlet portion of the liquid collection portion or the subsequent
liquid withdrawal tube, whereby safe operation of the thin film
evaporator for a long period of time will be made possible.
[0160] An example will be shown in which a liquid to be treated,
containing a readily polymerizable component, was treated by means
of the thin film evaporator of the present invention. Using the
thin film evaporator of the present invention having wipers at
positions corresponding to the inversed corn-shaped portion and the
cylindrical portion, respectively, of the liquid collection
portion, as shown in FIGS. 2 to 5, an operation was carried out to
recover acrylic acid from the bottoms (acrylic acid: 69.4 wt %, a
dimer of acrylic acid: 20.9 wt %, maleic anhydride: 6.9 wt %,
others: 2.8 wt %) discharged from the bottom of the distillation
column in a process for purifying by distillation of acrylic acid,
whereby continuous operation for 10 months was accomplished.
Whereas, the same operation was carried out by means of the same
thin film evaporator except that no wiper was provided, whereby
upon expiration of 5 months, the pipeline at the residue discharge
port was clogged, and the operation was obliged to be stopped.
Embodiment e
[0161] The present invention provides a method for producing an
acrylic ester, wherein acrylic acid is produced by a vapor phase
catalytic oxidation reaction of propylene, and the acrylic acid is
used as the starting material, wherein as the starting material for
producing the acrylic ester, acrylic acid is used wherein the
concentration of a certain specific high boiling point impurity is
adjusted to be at most a specific concentration.
Acrylic Ester
[0162] The acrylic ester for the present invention may, for
example, be methyl acrylate, ethyl acrylate, n-butyl acrylate,
isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate,
isononyl acrylate or methoxyethyl acrylate.
[0163] The process for producing an acrylic ester comprises an
esterification reaction step of reacting acrylic acid with an
alcohol such as methyl alcohol, ethyl alcohol, n-butyl alcohol,
isobutyl alcohol, tertiary butyl alcohol, 2-ethylhexyl alcohol,
isononyl alcohol or methoxyethyl alcohol corresponding to each of
the above-mentioned acrylic esters, by using, as a catalyst, an
inorganic acid such as sulfuric acid, an organic acid such as
p-toluene sulfonic acid or methane sulfonic acid, or a solid acid
such as a cationic ion exchange resin, and a purification step of
carrying out washing, liquid-liquid separation, extraction,
evaporation, distillation, etc., as unit operations to carry out
separation of the catalyst, concentration, purification, etc. of
the crude acrylic ester obtained by the reaction. The starting
material molar ratio of the acrylic acid to the alcohol in the
esterification reaction, the type and amount of the catalyst to be
used, the reaction system, the reaction conditions, etc., are
optionally set depending upon the type of the alcohol material.
Further, a decomposition step may be provided wherein
.beta.-acryloxypropionic acid and its esters,
.beta.-hydroxypropionic acid and its esters, etc., as high boiling
point impurities produced as by-products of the esterification
reaction, are thermally decomposed or catalytically decomposed by
means of a catalyst, whereby starting material acrylic acid and
alcohol may be recovered.
Acrylic Acid
[0164] The acrylic acid to be treated by the present invention is
acrylic acid produced by a vapor phase oxidation reaction of
propane or propylene. In order to obtain acrylic acid useful as the
starting material for an acrylic ester from a gas product of the
vapor phase catalytic oxidation reaction of propylene, the acrylic
acid-containing gas is contacted with water to collect acrylic acid
in the form of an aqueous acrylic acid solution. Several methods
are available as methods for separating water from such an aqueous
acrylic acid solution, but a typical method will be shown as
follows. As an example, a method is available wherein acrylic acid
is extracted from water by means of an extracting solvent, followed
by separating acrylic acid and the extracting solvent by
distillation. As another example, a method may be mentioned wherein
the aqueous acrylic acid solution is subjected to an azeotropic
distillation employing an azeotropic agent with water, whereby
acrylic acid is separated from water and the azeotropic agent.
Further, at that time, by adding an azeotropic agent which is
azeotropically distilled with acetic acid as an impurity having a
boiling point lower than acrylic acid, water and acetic acid can
simultaneously be separated. In a case where water is separated and
acetic acid is not simultaneously separated, the low boiling point
component may substantially be removed by subjecting the product to
a distillation step to separate low boiling point impurities
composed mainly of acetic acid. As the starting material for a
highly water absorptive resin, high purity acrylic acid is
required. Accordingly, a distillation step to remove high boiling
point impurities will further be required to obtain high purity
acrylic acid. On the other hand, as the starting material for an
acrylic ester, highly pure acrylic acid is not necessarily
required, and it is regarded as economically advantageous to use
acrylic acid having high boiling point impurities not separated,
whereby such a distillation step is unnecessary, and the
construction costs and the operation costs such as a cost for
distillation can be saved.
Details of the Problems to be Solved
[0165] If acrylic acid containing high boiling point impurities, is
used as a starting material, undesirable polymerization reactions
or side reactions take place, whereby there have been problems,
such as clogging of apparatus such as pipings by a polymer,
deterioration of the unit consumption of main materials such as
acrylic acid and an alcohol, deterioration in the quality of the
product, etc. A solid polymer formed by an undesirable
polymerization reaction tends to accumulate on a filter, a pump
strainer, a nozzle, etc., and there has been a problem that
replacement or cleaning is often required, and finally, the
operation is obliged to be stopped due to clogging. Further, a
soluble polymer formed by the polymerization reaction tends to
bring about a trouble such that an emulsion will be formed in the
step of cleaning or separation of the catalyst, or a problem which
leads to a loss of the starting material acrylic acid.
[0166] The undesirable side-reactions include, for example, an
acetal-forming reaction, an esterification or ester exchange
reaction and an oxidation reaction, and consequently, they mean
side-reactions, whereby acrylic acid or an alcohol is consumed
uselessly, impurities to contaminate an acrylic ester as the
product, are formed, a substance to promote the polymerization
reaction is formed, impurities to create a trouble in operation
will be formed. The present inventors have found that certain
specific impurities are the main factor for the above problems, and
on this basis, have arrived at the present invention.
Specific Impurities
[0167] The specific high boiling point impurities which acrylic
acid of the present invention contains, are four i.e. benzaldehyde,
furfural, maleic anhydride and .beta.-acryloxypropionic acid. The
concentrations of such impurities which are usually contained in
acrylic acid prior to removal of the high boiling point impurities,
are from 300 to 1000 weight ppm of benzaldehyde, from 200 to 500
weight ppm of furfural, from 0.3 to 1.0 wt % of maleic anhydride,
and from 1.0 to 3.0 wt % of .beta.-acryloxypropionic acid.
[0168] Benzaldehyde and furfural have been found not only to
undergo acetal reactions with alcohols in the esterification
reaction step thereby to consume the starting material alcohol
uselessly, but also to adversely affect the polymerization behavior
of the product as the formed acetals or non-reacted aldehydes are
included in the ester product. Further, it has been found that
benzaldehyde and furfural, and their acetals formed by the
esterification reaction, are susceptible to oxidation by oxygen
added to prevent polymerization in the purification system, to form
a peroxide thereby to accelerate an undesirable polymerization
reaction.
[0169] It has been found that maleic anhydride not only undergoes
an esterification reaction with an alcohol in the esterification
reaction step to form a half ester or a diester thereby to consume
the starting material alcohol uselessly, but also be recycled and
accumulate in the system not only as maleic anhydride, maleic acid
and its ester, but also as fumaric acid and its ester as isomers
thereof, in the process including a step of decomposing a high
boiling point component. This isomerization reaction from maleic
acids to fumaric acids, is a reaction which takes place since the
step of decomposing the high boiling point component is carried out
at a relatively high temperature of from 150 to 250.degree. C., and
it is a characteristic behavior in a process having a step of
decomposing a high boiling point component. Accumulation of maleic
anhydride and fumaric acid, or their esters, has been found to
bring about a decrease in the treating capacity in the step of
decomposing the high boiling point component, a precipitation
trouble of maleic acid or fumaric acid or a problem such that it
leads to contamination of the ester product.
[0170] It has been found that .beta.-acryloxypropionic acid not
only undergoes an esterification reaction and an ester exchange
reaction with an alcohol in the esterification reaction step
thereby to consume the alcohol uselessly, but also
.beta.-acryloxypropionic acid itself accelerates polymerization of
acrylic acid.
[0171] The upper limit values of the amounts of the high boiling
point impurities contained in acrylic acid of the present invention
are such that the total amount of benzaldehyde and furfural is 500
weight ppm, maleic anhydride is 2000 weight ppm, and
.beta.-acryloxypropionic acid is 1000 weight ppm, preferably 500
weight ppm. Further, the respective components can be reduced by
increasing the precision in distillation (increasing the reflux
ratio, or increasing the theoretical plate number) but practical
conditions may be determined taking into consideration the balance
between the cost required and the effect by such reduction.
[0172] The lower limit values are considered to be such that the
total amount of benzaldehyde and furfural is 50 ppm, maleic
anhydride is 50 ppm and .beta.-acryloxypropionic acid is about 10
ppm.
Method for Producing Specific Acrylic Acid
[0173] Several methods may be mentioned as methods for producing
acrylic acid of the present invention which contains the specific
high boiling point impurities at specific concentrations such that
the total concentration of benzaldehyde and furfural is at most 500
weight ppm, the concentration of maleic anhydride is at most 2000
weight ppm, and the concentration of .beta.-acryloxypropionic acid
is at most 1000 weight ppm, preferably at most 500 weight ppm.
[0174] The conventional method for producing acrylic acid without
removing the high boiling point impurities for the production of an
ester, which has heretofore been commonly employed, is as described
above. By using such acrylic acid as the starting material, the
specific acrylic acid of the present invention can be produced by
employing a unit operation such as crystallization, distillation or
evaporation, or by combining an aldehyde-removing reaction
employing an amine or hydrazine. However, it is economically
preferred to employ a one step flash distillation or a very simple
distillation method having a low theoretical plate number and a low
reflux ratio.
Overall Economical Efficiency from Propylene to an Acrylic
Ester
[0175] When the cost for production of an acrylic ester from
propylene as the starting material, is compared as between a case
where the acrylic ester is produced by using acrylic acid as
prescribed by the present invention and a case where the acrylic
ester is produced by using acrylic acid containing a large amount
of high boiling point impurities according to the conventional
technique, an increase in the production cost including the
installation cost to lower the specific high boiling point
impurities in acrylic acid to the specific concentration, is not
substantial, while visible or invisible effects such as improvement
in the unit consumption of the starting materials in the process
for producing the acrylic ester, increase of the production amount
of the product, improvement of the quality of the product, decrease
of frequency of stopping the operation, etc., are much more
substantial, whereby the method of the present invention is far
advantageous also from the economical efficiency.
Embodiment f
[0176] In the present invention, purified (meth)acrylic acid
obtained by removing low boiling point impurities and high boiling
point impurities from a reaction product containing (meth)acrylic
acid obtained by vapor phase catalytic oxidation, particularly
preferably contains aldehydes having boiling points close to
(meth)acrylic acid. Further, the bottom fraction from the
distillation column is particularly preferably used as a starting
material for producing a light (meth)acrylic ester having a
standard boiling point lower than (meth)acrylic acid, such as
methyl (meth)acrylate or ethyl (meth)acrylate.
Process for Producing Acrylic Acid
[0177] The process for producing high purity acrylic acid as an
object of the present invention, comprises an oxidation step of
carrying out a vapor phase catalytic oxidation reaction using
propylene and/or propane and/or acrolein as the starting material,
a collecting step of contacting an acrylic acid-containing gas from
the oxidation step with an absorbing solvent such as water to
collect acrylic acid in the form of an acrylic acid solution, a
step of distilling and separating acrylic acid and the absorbing
solvent such as water from this acrylic acid solution, if
necessary, by means of a suitable azeotropic solvent, a step of
continuously distilling and separating acetic acid as a low boiling
point impurity from acrylic acid, and further a step of distilling
and separating high boiling point impurities, as the basic
construction. Here, as the absorbing solvent useful other than
water, diphenyl ether, biphenyl or a mixture of diphenyl ether and
biphenyl, may be mentioned as a typical example.
[0178] Further, also included in the present invention is a method
which comprises a step of distilling and separating water, acetic
acid and the solvent all at once from the aqueous acrylic acid
solution obtained in a case where water is used as the absorbing
solvent, or a step of extracting acrylic acid from the aqueous
acrylic acid solution by means of an extracting solvent such as
methyl isobutyl ketone, isopropyl acetate, methyl ethyl ketone or
toluene and distilling and separating the extracting solvent and
the remaining water in the extracted acrylic acid. Further, also
included in the present invention is a method which comprises a
step of decomposing a Michael adduct (one having water or acrylic
acid added to the double bond of acrylic acid or acrolein) formed
as a by-product in the process for production of acrylic acid, a
step of further distilling and purifying acetic acid separated by
distillation, or a step of recovering the solvent, etc. by further
distilling the aqueous fraction separated by distillation.
[0179] Now, as an example of a case where water is used as the
absorbing solvent, an embodiment of the process for purification of
an acrylic acid will be described with reference to FIG. 6.
[0180] An oxidation reaction gas containing acrylic acid, obtained
by vapor phase catalytic oxidation of propylene, propane and/or
acrolein by means of molecular oxygen-containing gas, is introduced
into an acrylic acid-collecting column 41 and contacted with water
to form a crude acrylic acid aqueous solution. Here, the oxidation
reaction gas contains N.sub.2, CO.sub.2, acetic acid, water, etc.,
and a part of acetic acid, N.sub.2 and CO.sub.2 will be withdrawn
as a vent gas from the top of the collecting column 41.
[0181] The crude acrylic acid aqueous solution from this collecting
column 41, is supplied together with an azeotropic agent to a
distillation column 42 for dehydration, and from the top of the
column, an azeotropic mixture comprising water and the azeotropic
agent will be distilled, and from the bottom of the column, a crude
acrylic acid containing acetic acid will be obtained. The
azeotropic mixture comprising water and the azeotropic agent
distilled from the top of the distillation column 42 for
dehydration, will be introduced into a decanter 43, wherein it is
separated into an organic phase composed mainly of the azeotropic
agent and an aqueous phase composed mainly of water. The organic
phase composed mainly of the azeotropic agent is, after an addition
of a polymerization preventing agent (not shown), returned to the
distillation column 42 for dehydration. On the other hand, the
aqueous phase is returned to the acrylic acid-collecting column 41
and used as collecting water to be contacted with the oxidation
reaction gas. Further, a part may be discharged as waste water out
of the system, as the case requires, and water may be supplemented
to the water-returning line. There may be a case where in order to
recover the azeotropic agent from the water in the water-returning
line, the water is passed through an azeotropic agent-recovery
column (not shown) and then returned to the acrylic acid-collecting
column 41.
[0182] The crude acrylic acid containing acetic acid, withdrawn
from the bottom of the distillation column 42 for dehydration, is
introduced into a distillation column 44 for separating a low
boiling fraction in order to remove a low boiling point fraction
(low boiling point impurities) such as remaining acetic acid, and
from the top of the column, a low boiling point fraction such as
acetic acid is separated and removed. The acetic acid from the top
of the column contains acrylic acid. Therefore, a part is returned
from a reflux tank 44A to the distillation column 44 for a low
boiling point fraction, and the rest is returned to the inlet side
of the distillation column 42 for dehydration. Such a low boiling
point fraction containing acetic acid, is separated in the
distillation column 42 for dehydration and finally discharged as a
vent gas out of the system via an acrylic acid-collecting column
41.
[0183] From the bottom of the distillation column for separating a
low boiling point fraction 44, acrylic acid containing
substantially no acetic acid, will be obtained. Such acrylic acid
is introduced into a distillation column for separating a high
boiling fraction 45, whereupon heavy substances (high boiling point
impurities) are separated and removed to obtain purified acrylic
acid. The bottoms (high boiling point substances) of the
distillation column for separating a high boiling point fraction
45, are sent to a decomposition reactor (not shown), whereupon
acrylic acid, etc. formed by the decomposition reaction will be
recycled for use.
[0184] The acrylic acid obtained in the distillation column for
separating a high boiling fraction 45 is sent to a reflux tank 45A,
whereupon a part is returned to the distillation column 45 for
separating a high boiling fraction, and the rest is sent to a
reactor for converting aldehydes to heavy substances 46, in order
to separate aldehydes still contained in a very small amount in
this purified acrylic acid by converting them to heavy substances,
and an aldehyde-removing agent is added to convert the aldehydes to
heavy substances, whereupon such heavy substances will be further
separated and removed in the distillation column for purification
47. High purity acrylic acid having heavy substances of aldehydes
removed by the distillation column for purification 47, is sent to
a reflux tank 47A whereupon a part is returned to the distillation
column for purification 47, and the rest is taken out as a
product.
[0185] In the present invention, in the production of such high
purity acrylic acid, the bottom fraction (the bottoms) withdrawn
from the distillation column for purification 47, i.e. the bottom
fraction from the distillation column for purification 47 to remove
aldehydes from the purified acrylic acid obtained by removing low
boiling point impurities and high boiling point impurities from the
reaction product containing acrylic acid, obtained by vapor phase
catalytic oxidation, is sent to a process for producing an acrylic
ester and is used as the material for producing an acrylic
ester.
[0186] Usually, the purified acrylic acid obtained by removing low
boiling point impurities and high boiling point impurities from a
reaction product containing acrylic acid, obtained by the vapor
phase catalytic oxidation, contains benzaldehyde and furfural
having boiling points close to acrylic acid, mainly as an aldehyde
component, and their contents are usually such that each of
benzaldehyde and furfural is from 20 to 300 weight ppm.
Treating Method for Removal of Aldehydes
[0187] The aldehyde-removing agent to remove aldehydes from
purified acrylic acid, to be used in the present invention, is not
particularly limited, and it may be any conventional removing
agent. A typical example of the aldehyde removing agent may, for
example, be a hydrazine such as anhydrous hydrazine, hydrated
hydrazine or phenyl hydrazine, an amino acid such as glycine, an
amine such as aniline or ethanol amine, a hydrogen sulfite such as
sodium hydrogen sulfite, a mercaptan such as octyl mercaptan,
dodecyl mercaptan, hexadecyl mercaptan, octadecyl mercaptan or
2-mercaptobenzothiazole, or a combination of a hydrazine and a
copper dithiocarbamate.
[0188] Such an aldehyde-removing agent will react with aldehydes
contained in purified acrylic acid to form heavy compounds. The
reaction conditions at that time are not particularly limited.
After such treatment for heavy compounds, distillation is carried
out, whereby high purity acrylic acid which is highly purified and
which contains substantially no aldehyde component, can be obtained
as the top fraction.
[0189] In the present invention, the bottom fraction formed by this
distillation is supplied to a process for producing an acrylic
ester. As will be mentioned hereinafter, in the process for
producing an acrylic ester, an acid catalyst is used as a catalyst
for the esterification reaction. Depending upon the type of the
above aldehyde-removing agent or the recycling place of the bottom
fraction, such an agent or fraction may sometimes poison or react
with the acid catalyst for the esterification reaction.
Accordingly, as the aldehyde-removing agent, it may sometimes be
preferred to avoid using a compound which is highly likely to
poison or react with the acid catalyst and which contains a
nitrogen atom having a basicity (such as an amine or hydrazine) or
a compound containing a metal atom (copper or sodium) having a
cation exchange ability.
[0190] Accordingly, a preferred aldehyde-removing agent for the
method of the present invention, is one not having a nitrogen atom
and a metal atom simultaneously. Specifically, it may, for example,
be an alkylmercaptan, an alkanediol, a mercapto alcohol or a
mercapto propionic acid. In a case where such an aldehyde-removing
agent is used, the reaction with aldehydes can be effectively
accelerated, whereby an acid catalyst may be employed. As the acid
catalyst to be used here, either a solid acid catalyst such as a
strongly acidic ion exchange resin or zeolite, or a homogeneous
acid catalyst such as sulfuric acid or p-toluene sulfonic acid, may
be employed.
Acrylic Esters
[0191] In the present invention, the acrylic ester for which the
bottom fraction from the above-mentioned distillation column for
purification is used as the starting material, may be any acrylic
ester obtainable by an esterification reaction of acrylic acid with
an alcohol, without any limitation as to the type of the alcohol.
For example, it may be methyl acrylate, ethyl acrylate, n-butyl
acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, isononyl
acrylate or isodecyl acrylate. However, from the after-mentioned
reason, an acrylic ester having a standard boiling point lower than
acrylic acid, such as methyl acrylate or ethyl acrylate, is
preferred.
Process for Producing an Acrylic Ester
[0192] As the process for producing an acrylic ester to which the
present invention is applied, one wherein an alcohol is reacted to
acrylic acid for esterification reaction, is common, and it may be
either a batch system or a continuous system. As the catalyst for
the esterification reaction, it is common to use an acid catalyst.
The process for producing an acrylic ester comprises mainly an
esterification reaction step and a purification step of carrying
out washing, extraction, evaporation, distillation or the like as a
unit operation to carry out separation of the catalyst or
concentration and purification or the like of the crude acrylic
ester solution obtained by the reaction. The starting material
molar ratio of acrylic acid to the alcohol in the esterification
reaction, the type and amount of the catalyst to be used for the
reaction, the reaction system, the reaction conditions, etc. are
suitably selected depending upon the type of the alcohol to be
used. Further, Michael adducts formed as by-products by a reaction
of acrylic acid, an alcohol, water, etc. to acrylic acid or an
acrylic ester, will be concentrated at the bottom of the
distillation column for separating a heavy fraction. Accordingly,
such bottoms are subjected to thermal decomposition or
decomposition by means of a Lewis acid or a Lewis base catalyst,
and the obtained useful components are recycled to and recovered in
the reaction step or the purification step. The distillation column
for separating the heavy fraction may vary depending upon the type
of the acrylic ester to be produced or the process employed.
Usually, it may be one to separate acrylic acid from the heavy
fraction, one to separate an acrylic ester from the heavy fraction,
or one to separate acrylic acid, an alcohol and an acrylic ester
from the heavy fraction. The present invention is applicable to any
one of them.
Recycling of the Bottom Fraction from the Distillation Column for
Purification to the Process for Producing an Acrylic Ester
[0193] In the present invention, the waste liquid obtained by the
treatment for removal of aldehydes from the purified acrylic acid,
specifically the bottoms obtained from the distillation column for
purification which separates by distillation high purity acrylic
acid containing no aldehydes and the bottom fraction containing
heavy substances converted from aldehydes, after treating purified
acrylic acid obtained by removing low boiling point impurities and
high boiling point impurities from a reaction product containing
acrylic acid obtained by vapor phase catalytic oxidation, with an
aldehyde-removing agent to convert aldehydes to heavy compounds, is
recycled to the process for producing an acrylic ester. The place
to which the bottoms will be recycled, may be suitably selected
depending upon the type of the acrylic ester and its production
process. However, since the main component of the bottoms is
acrylic acid, it is preferred to recycle the bottoms to the
esterification reaction step. However, in a case where a compound
containing nitrogen or a metal atom, is used as the
aldehyde-removing agent, such a compound adversely affects the
catalyst for the esterification reaction, and accordingly, it is
preferred to recycle the bottoms to the distillation system or to
the step of decomposing the heavy fraction.
[0194] Components other than acrylic acid in the bottoms from the
distillation column for purification, are, for example, a
polymerization inhibitor such as hydroquinone or methoquinone
(methoxyhydroquinone), a by-product such as a dimer
(.beta.-acryloxypropionic acid), oligomer or polymer of acrylic
acid and an excessively added remaining aldehyde-removing agent,
and a high boiling point compound formed by the reaction of the
aldehyde-removing agent with an aldehyde. Among such components in
the bottoms, the polymerization inhibitor can effectively be used
in the production process for an acrylic ester, whereby the amount
of the polymerization inhibitor to be used in the acrylic ester
system can be reduced. The dimer, oligomer or polymer of acrylic
acid may partially be converted to the corresponding ester by an
alcohol in the reaction step, but the major portion of the heavy
fraction is decomposed in the decomposition step, and can be
recovered as acrylic acid or an alcohol. The aldehyde-removing
agent and a reaction product of the aldehyde-removing agent with an
aldehyde, undergo substantially no chemical reaction in the
esterification reaction step, and finally subjected to the step of
decomposing the heavy fraction. In a case where the decomposition
reaction of this heavy fraction is carried out at a very high
temperature or by means of a Lewis acid or a Lewis base catalyst,
such an aldehyde-removing agent or the reaction product of the
aldehyde-removing agent with an aldehyde, may cause various
reactions such as a decomposition reaction, and therefore, a
suitable aldehyde-removing agent is selected depending upon the
type of the ester or the process.
[0195] According to the present invention, it is possible to easily
separate such side reaction products from the product ester thereby
to avoid contamination of the product ester by such side reaction
products. Accordingly, it is preferably applied to a light acrylic
ester having a standard boiling point lower than acrylic acid, such
as methyl acrylate or ethyl acrylate.
[0196] As mentioned above, in a case where the bottom fraction is
recycled to the process for producing acrylic acid, there have been
problems such as acceleration of polymerization of acrylic acid by
e.g. by-products formed by the heat decomposition treatment,
contamination of the product and coloring. Whereas, according to
the present invention, even if the aldehyde-removing agent or the
reaction product of the aldehyde-removing agent with an aldehyde,
contained in the bottom fraction, may produce a substance to
accelerate polymerization of acrylic acid at the time of
decomposition reaction of the heavy fraction in the process for
producing an acrylic ester, in the process for producing an acrylic
ester, the relative concentration of acrylic acid in the system is
low, whereby problems such as acceleration of polymerization of
acrylic acid, contamination of the product and coloration, by
by-products, etc., can be substantially reduced.
[0197] In the present invention, it is possible to send the waste
liquid obtained in the distillation column for purification of high
purity acrylic acid, as it is, to the process for producing an
acrylic ester, without treatment thereof, thereby to effectively
use acrylic acid contained therein, as a material for production of
an acrylic ester, whereby it is not necessary to set the conditions
to bring the acrylic acid concentration in the bottoms to be
sufficiently low, as the distillation condition for the
distillation column for purification for separation by distillation
of high purity acrylic acid, and distillation can be carried out
even under such a condition that the acrylic acid concentration in
the bottoms is at least 70 wt %, such as from 70 to 95 wt %. And,
in such an acrylic acid concentration, there will be no clogging of
the piping system of the distillation column for purification, and
continuous operation can be continued for a long period of
time.
[0198] In the foregoing, the production of high purity acrylic acid
has been described, but the present invention can be applied in the
same manner also in the process for producing high purity
methacrylic acid by a vapor phase catalytic oxidation reaction of
isobutylene and/or t-butyl alcohol.
Examples
[0199] Now, the present invention will be described in further
detail with reference to Examples, but it should be understood that
the present invention is by no means restricted to the following
Examples. Further, in the following Examples, analyses of the
respective compositions were carried out by gas chromatography
(GC14A, manufactured by Shimadzu Corporation). However, maleic acid
is converted to maleic anhydride in the step of gas chromatography
analysis, and the contents of the two cannot be specified.
Accordingly, in the following, the total content of maleic acid and
maleic anhydride is represented by the content of maleic acids.
Example a1
[0200] In accordance with the flow sheet shown in FIG. 1, acrylic
acid of the purity for an ester, and acrylic acid of the purity for
a highly water absorptive resin are produced. To the first
distillation column, crude acrylic acid (purity: 93.8 wt %) is
supplied at a rate of 11053 kg/hr, and the top fraction from the
third distillation column is supplied at a rate of 2390 kg/hr. As
the first distillation column, a distillation column equipped with
dual flow trays having a theoretical plate number of 7 plates is
used, and it is operated at a reflux ratio of 0.7 at a bottom
temperature of 80.degree. C. under a top pressure of 20 Torr. From
the top of the first distillation column, 10460 kg/hr (purity: 99.8
wt %) of the top fraction is obtained, and 6160 kg/hr is used for
an ester and 4300 kg/hr of the rest is, after mixed with 10 kg/hr
of n-dodecylmercaptan as an aldehyde-removing agent, passed through
a packed column of a sulfonic acid type cation exchange resin
(DIAION PK-216H, DIAION is a registered trademark of Mitsubishi
Chemical Corporation) and then supplied to the second distillation
column. As the second distillation column, a packed column having a
theoretical plate number of 9 plates is used, and it is operated at
a reflux ratio of 1 at a bottom temperature of 70.degree. C. under
a top pressure of 16 Torr, to obtain 3897 kg/hr of acrylic acid
having a purity of 99.94 wt % from the top. This acrylic acid
adequately satisfies the quality required for acrylic acid for a
highly water absorptive resin.
[0201] 2983 kg/hr of the bottoms from the first distillation column
and 413 kg/hr of the bottoms from the second distillation column
are put together and supplied to the third distillation column. As
the third distillation column, a vertical thin film evaporator is
used, and it is operated under a pressure of 70 Torr at a flow out
gas temperature of 110.degree. C. From the top, 2390 kg/hr of
acrylic acid having a purity of 89.0 wt % is recovered and as
mentioned above, supplied to the first distillation column. 1006
kg/hr of the bottoms from the third distillation column is supplied
to a thermal decomposition column and thermally decomposed at a
bottom temperature of 180.degree. C. under a top pressure of 500
Torr for a retention time of 3 hours, whereby from the top, 664
kg/hr of a distillate having an acrylic acid purity of 91.1 wt % is
taken out and returned to a low boiling point component-removing
stage in the preliminary purification step. 342 kg/hr of the
bottoms from the thermal decomposition column is supplied to an
incineration apparatus. In this manner, purification of acrylic
acid can be carried out constantly over a long period of time.
Example b1
[0202] To a crude acrylic acid obtained by vapor phase catalytic
oxidation and containing as impurities 239 pm (weight) of furfural,
238 ppm (weight) of benzaldehyde and 3300 ppm (weight) of maleic
anhydride, hydrazine hydrate was added in an amount equal by mol to
the total molar amount of aldehydes and maleic acids, and the
mixture was passed through a tubular reactor at a flow rate of 5000
kg/hr as the total liquid amount at a reaction temperature of
20.degree. C. for a retention time of 2 hours. After the treatment
for removal of aldehydes and maleic acids, the reaction solution
was taken out from the piping and found to be in a slightly yellow
slurry state. This slurry was heated by means of a heat exchanger
so that the internal temperature became 65.degree. C. The reaction
solution prior to the supply to the distillation apparatus was a
yellowish brown transparent liquid, and no precipitation of solid
was observed. This yellowish brown transparent liquid was sent, as
it was, to the packed column distillation apparatus and subjected
to continuous distillation. Here, the heating time in the heat
exchanger was about 1 minute which corresponds to the flowing time
of the reaction solution.
[0203] The continuous distillation was carried out at a bottom
temperature of 74.degree. C., whereby 99 wt % of the supplied
liquid was continuously distilled, and part of the distillate was,
as a reflux liquid, introduced into the column from the top at a
reflux ratio of 1.0. Further, at the time of the continuous
distillation, as a polymerization inhibitor, methoquinone (methoxy
hydroquinone) corresponding to 10 weight ppm to the liquid amount
introduced into the distillation column, was introduced into the
column, as dissolved in the reflux liquid.
[0204] The concentrations of maleic acids and aldehydes such as
furfural, benzaldehyde, etc. in the purified acrylic acid obtained
as a distillate from the top of this distillation column, were not
more than 1 ppm, respectively, and under this condition, it was
possible to carry out continuous distillation constantly for 5
months.
Comparative Example b1
[0205] Distillation was carried out under the same conditions as in
Example b1 except that the feeding temperature of the reaction
solution to the distillation column was changed to 50.degree. C. A
part of the reaction solution prior to feeding into the
distillation column was withdrawn and found to be still a slightly
yellow slurry, and it was sent, as it was, to the continuous
distillation apparatus, whereby upon expiration of 3 months, it
became impossible to continue the distillation due to an increase
in the pressure difference in the column.
Comparative Example b2
[0206] Distillation was carried out under the same conditions as in
Example b1 except that the feeding temperature of the reaction
solution to the distillation column was changed to 85.degree. C. As
a result, in the purified acrylic acid obtained as a distillate
from the top of the distillation column, the concentration of
furfural was 5 ppm, the concentration of benzaldehyde was 10 ppm
and the concentration of maleic acids was 160 ppm, and it was
impossible to use it as high purity acrylic acid.
Examples c1 to c4
[0207] A distillation column made of glass was used wherein in the
interior of the column having an inner diameter of 50 mm and a
length of 650 mm, a coil pack made of SUS316 and having a diameter
of 3 mm, was packed at a height of 300 mm at the enriching section
and to a height of 300 mm at the stripping section, and a three
necked flask of 1000 cc was provided at the bottom of the column.
The main body of the column was covered by an electric heater to
avoid condensation on the wall surface of the column, and also at a
lower portion of the three necked flask, an electric heater was
provided for heating, whereby distillation of crude acrylic acid
was carried out. As the crude acrylic acid monomer, a mixture was
used which contained 98.5 wt % of acrylic acid, 0.3 wt % of maleic
acid, 0.276 wt % of a dimer of acrylic acid, 0.02 wt % of furfural
and 0.004 wt % of benzaldehyde.
[0208] Prior to feeding the crude acrylic acid monomer into the
distillation column, hydrazine hydrate, copper
dibutyldithiocarbamate, copper acrylate, etc. were mixed in the
ratio as identified in Table 1. Here, the copper acrylate was one
prepared by dissolving cupric carbonate in acrylic acid, and the
mixing was carried out at 20.degree. C. for 30 minutes.
[0209] Prior to initiation of the operation, 800 g of acrylic acid
having a purity of 99.8 wt % containing 200 weight ppm of
methoquinone was supplied to the distillation column to wet the
surface of the packing material in the column. The distillation
feed liquid (the crude acrylic acid) containing the hydrazine
compound and the copper compounds in the above identified ratio,
was supplied from the center portion of the column, while
methoquinone was supplied to the reflux tank at the top so that its
concentration in the highly concentrated acrylic acid distilled
from the top would be 200 weight ppm. The liquid flowed down to the
bottom, was withdrawn out of the system. After supplying the
distillation feed liquid to the distillation column, when a liquid
surface was confirmed at the bottom, heating from the bottom of the
column was initiated.
[0210] Continuous operation was carried out under the respective
conditions as identified in Table 1. From the top of the column,
high purity acrylic acid having an acrylic acid purity of at least
99.5 wt % and furfural and benzaldehyde being not more than 1
weight ppm, respectively, was obtained. The feeding rate of the
distillation feed liquid during the steady operation was 265 g/hr,
the withdrawn amount of the high purity acrylic acid was 95 wt % of
the feeding amount of the distillation feed liquid, and further,
from the bottom, withdrawal of the bottoms was continuously carried
out so that the amount of the liquid in the three necked flask
would be constant.
[0211] Upon expiration of 48 hours, the operation was terminated,
and the interior of the distillation column was inspected, whereby
no formation of a polymer was observed at any place.
Examples c5 to c9
[0212] In Examples c1 to c4, without mixing copper compounds to the
crude acrylic acid monomer, only hydrazine hydrate was mixed in the
same amount to prepare a distillation feed liquid. Instead, copper
dibutyldithiocarbamate and copper acrylate were mixed to the top
liquid and supplied via a reflux line into the distillation column.
Then, distillation was carried out in the same manner as Examples
c1 to c4. Upon expiration of 48 hours, the operation was
terminated, and the interior of the distillation column was
inspected, whereby no formation of a polymer was observed at any
place. The results are shown in Table 1.
Comparative Examples c1 to c7
[0213] In Examples c1 to c4, without adding both copper
dibutyldithiocarbamate and copper acrylate to the crude acrylic
acid monomer, only either one was mixed to prepare a distillation
feed liquid. Further, in one example, no hydrazine compound was
added. Then, distillation was carried out in the same manner as in
Examples c1 to c4, but turbidity formed in the bottoms (Comparative
Examples c1, c5, c6 and c7), precipitation of a polymer was
observed in the three necked flask at the bottom (Comparative
Examples c1, c2, c4, c5, c6 and c7), and precipitation of polymer
was observed also at the stripping section of the distillation
column (Comparative Examples c2, c3, c4 and c7). The results are
shown in Table 2.
TABLE-US-00001 TABLE 1 Example Nos. 1 2 3 4 5 6 7 8 9 Hydrazine
Concentration 1650 1650 1650 1650 1650 1650 1650 1650 1650 hydrate
in distillation feed liquid: weight ppm Copper Same as above 40 20
40 40 20 60 40 40 40 dibutyldithio- carbamate Copper Same as above
40 20 40 40 20 60 40 40 40 acrylate Hydroquinone Same as above --
-- 300 -- -- -- 300 300 -- Phenothiazine Same as above -- -- -- 150
-- -- 150 150 Top pressure kPa 8.1 2.8 10.1 10.1 2.8 11.3 12.7 10.1
10.1 Bottom .degree. C. 90 80 95 95 80 100 105 95 95 temperature
Bottom kPa 10.1 6.5 12.1 12.1 6.5 13.3 14.4 12.1 12.1 pressure
Furfural in Weight ppm 1> 1> 1> 1> 1> 1> 1>
1> 1> top liquid Benzaldehyde Weight ppm 1> 1> 1>
1> 1> 1> 1> 1> 1> in top liquid Results of -- No
No No No No No No No No inspection of Polymer Polymer Polymer
Polymer Polymer Polymer Polymer Polymer Polymer distillation column
after continuous operation for 48 hours
TABLE-US-00002 TABLE 2 Comparative Example Nos. 1 2 3 4 Hydrazine
Concentration 1650 1650 1650 1650 hydrate in distillation feed
liquid: weight ppm Copper Same as above -- 40 40 40 dibutyl-
dithio- carbamate Copper Same as above 40 -- -- -- acrylate
Hydroquinone Same as above -- -- 300 -- Phenothiazine Same as above
-- -- -- 150 Top pressure kPa 8.1 8.1 10.1 10.1 Bottom .degree. C.
90 90 95 95 temperature Bottom kPa 10.1 10.1 12.1 12.1 pressure
Furfural in Weight ppm 1> 1> 1> 1> top liquid
Benzaldehyde Weight ppm 1> 1> 1> 1> in top liquid
Results of Operation 48 Terminated Terminated Terminated inspection
of time hours after after after distillation 5 hrs 9 hrs 10 hrs
column Main body No Polymer Polymer Polymer of distillation Polymer
observed observed observed column at at at stripping stripping
stripping section section section Bottom of Polymer Polymer No
Polymer column observed observed Polymer observed (three-necked
flask) Bottoms Turbidity No No No observed Polymer Polymer Polymer
Comparative Example Nos. 5 6 7 Hydrazine Concentration 1650 1650 0
hydrate in distillation feed liquid: weight ppm Copper Same as
above -- -- 40 dibutyl- dithio- carbamate Copper Same as above 40
40 -- acrylate Hydroquinone Same as above 300 -- 300 Phenothiazine
Same as above -- 150 -- Top pressure kPa 10.1 10.1 10.1 Bottom
.degree. C. 95 95 95 temperature Bottom kPa 12.1 12.1 12.1 pressure
Furfural in Weight ppm 1> 1> 100 top liquid Benzaldehyde
Weight ppm 1> 1> 4 in top liquid Results of Operation 48
hours 48 hours 48 hours inspection of time distillation Main body
No No Polymer column of distillation polymer polymer observed
column at Stripping section Bottom of Polymer Polymer Polymer
column observed observed observed (three-necked flask) Bottoms
Turbidity Turbidity Turbidity observed observed observed
Example c10
[0214] In Example c8, instead of the distillation column made of
glass and equipped with a three necked flask, a distillation column
made of stainless steel (SUS316) and having an irregular packing
material (IMTP) manufactured by Norton Company packed in 8 m in the
interior having an inner diameter of 1100 mm and a length of 20000
mm and having 9 perforated plates beneath the packing material, was
used, and the distillation feed liquid was supplied at a rate of
1300 kg/hr. Then, the operation was initiated, and the continuous
operation was carried out, in the same manner as in Example 8.
[0215] Constant operation was carried out without any change in the
pressure difference in the distillation column, whereby from the
top of the column, 95% of the supplied amount of the distillation
feed liquid, was withdrawn to obtain high purity acrylic acid
having an acrylic acid purity of at least 99.5 wt % and furfural
and benzaldehyde being not more than 1 weight ppm, respectively.
Upon expiration of 3 months, the operation was terminated, and the
interior of the distillation column was inspected, whereby no
formation of a polymer was observed at any place.
Example e1
Preparation of Acrylic Acid
[0216] Using as a feed material acrylic acid from the bottom of an
azeotropic distillation column having an azeotropic agent, water
and acetic acid removed in a plant for producing acrylic acid, the
first stage continuous flash distillation was carried out to obtain
acrylic acid of the present invention as a distillate. The
compositions of the feed material and the distillate were analyzed
by gas chromatography, and the results are shown in Table 4. The
flash distillation column was operated under a pressure of 10 kPa
at a temperature of 80.degree. C., and the operation was carried
out so that the flash ratio to the feed liquid became 40%.
TABLE-US-00003 TABLE 4 Compositions of the feed material for
production of acrylic acid and the product acrylic acid of the
present invention Feed material Distillate Purity of acrylic acid
96.00 wt % 99.70 wt % High boiling point impurities Benzaldehyde
421 weight ppm 279 weight ppm Furfural 242 weight ppm 203 weight
ppm Maleic anhydride 0.69 wt % 0.157 wt % .beta.-acryloxypropionic
2.26 wt % 290 weight ppm acid Total of other 0.96 wt % 760 weight
ppm impurities
[0217] Using the distillate in the above Table as a starting
material (material A) of the present invention and the feed
material in the above Table as a starting material (material B) of
Comparative Examples, production of the following esters was
carried out.
Example e2
Production of Methyl Acrylate
[0218] Acrylic acid recovered via an acid separation column and a
heavy fraction separation column using acrylic acid of material A
as the starting material, and fresh methyl alcohol and methyl
alcohol recovered from an alcohol recovery column were put together
and continuously fed at a rate of 1300 kg/hr to and reacted in an
esterification reactor packed with 1400 l of H type strongly acidic
ion exchange resin DIA-ionPK-208 (manufactured by Mitsubishi
Chemical Corporation). The reaction conditions were such that
methyl alcohol:acrylic acid=1.0:1.0 (molar ratio), and the
temperature was 80.degree. C. The reaction product was continuously
supplied to an acid separation column operated at the top pressure
of 27 kPa at a bottom temperature of 93.degree. C. at a top
temperature of 41.degree. C. in a reflux ratio (R/D) of 1.0. The
crude methyl acrylate obtained from the top of the column was sent
to a product column via an alcohol extraction column and a low
boiling component separation column, to obtain 860 kg/hr of methyl
acrylate. The fraction rich in acrylic acid at the bottom was
subjected to a heavy fraction separation column, whereby acrylic
acid was recovered from the top, and a heavy fraction at the bottom
was subjected to a thermal decomposition reaction at a temperature
of 200.degree. C. in a high boiling point component decomposition
reactor, to recover useful components.
[0219] As a result of continuous operation for 3 months, the
productivity of the product was constantly maintained at 20.6
ton/day. Further, it was not necessary to clean or replace the
strainer of the withdrawal pump at the bottom of the acid
separation column. Further, during this period, the conversion in
the heavy substance decomposition step (the conversion of methyl
.beta.-acryloxypropionate) was maintained to be 62%.
Comparative Example e1
[0220] Using acrylic acid of material B as the starting material,
operation was continued for 3 months under the same conditions as
in Example e2. The productivity of the product at the initial stage
of the operation was 20.4 ton/day, but 3 months later, it decreased
to 19.9 ton/day. Further, it was necessary to switch, disassemble
and clean the strainer of the pump at the bottom of the acid
separation column twice because of an increase in the pressure
difference due to clogging by a solid substance. Further, the
conversion in the heavy substance decomposition step decreased from
60% at the initial stage to 53% upon expiration of 3 months.
Comparative Example e2
PRODUCTION of 2-ETHYLHEXYL ACRYLATE
[0221] Using material B, the product 2-ethylhexyl acrylate was
produced by esterification with 2-ethylhexyl alcohol using
p-toluene sulfonic acid as a catalyst. A polymerizability test (*)
of the product was carried out, whereby the introduction period for
polymerization was 18 minutes on average. (*) Tests for judging the
polymerizability
Example e3
[0222] Using material A, 2-ethylhexyl acrylate was produced in the
same manner. A polymerizability test of the product was carried
out, whereby the introduction period for polymerization was 15
minutes on average.
[0223] 10 ml of the product 2-ethylhexyl acrylate (containing 15
ppm of p-methoxyphenol as the polymerization inhibitor) was put
into a container having an internal capacity of 30 ml and equipped
with a thermocouple and a gas-supply tube and immersed in an oil
bath while blowing air into the container at a rate of 30 ml/min
and the internal temperature of the container was raised to
150.degree. C. The internal temperature of the container was once
reached equilibrium at 150.degree. C., then, when polymerization
started, the temperature started to rise by the polymerization
heat, whereby the time from the start of rising of the internal
temperature to a point when it reached 155.degree. C., was taken as
the introduction period.
Example f1
A Case Wherein the Bottoms from the Distillation Column for
Purification were Recycled to the Process for Producing Methyl
Acrylate
[0224] 2.1 kg/hr of 1,2-ethanedithiol as an aldehyde-removing
agent, was mixed to 1200 kg/hr of acrylic acid (purity: 99.8 wt %)
containing 210 weight ppm of furfural and 100 weight ppm of
benzaldehyde, and the mixture was passed through a reaction column
packed with 600 l of H type strongly acidic ion exchange resin
(SK-104, manufactured by Mitsubishi Chemical Corporation) at a
temperature of 90.degree. C. This aldehyde removal treated liquid
was distilled by a distillation column for purification operated at
a plate number of 10 plates at a reflux ratio of 1 under a top
pressure of 27 kPa. At that time, to the feed liquid to the
distillation column for purification, an acrylic acid solution
containing hydroquinone at a concentration of 10 wt % as a
polymerization inhibitor, was added at a rate of 12 kg/hr, and to
the top of the distillation column, hydroquinone monomethyl ether
(methoquinone) as a polymerization inhibitor was injected so that
the concentration in the distilled high purity acrylic acid would
be 200 weight ppm.
[0225] As a result, from the bottom of the distillation column for
purification, the bottoms having a composition comprising 78 wt %
of acrylic acid, 12 wt % of .beta.-acryloxypropionic acid and 10 wt
% of other heavy substances, was obtained at a rate of 25 kg/hr.
Such bottoms were supplied in the entire amount to an
esterification reactor in the process for producing methyl
acrylate.
[0226] From the top of the distillation column for purification,
high purity acrylic acid having a purity of 99.92 wt % was obtained
at a rate of 1190 kg/hr. The concentrations of furfural and
benzaldehyde in this high purity acrylic acid were measured by gas
chromatography, whereby each was not more than the detectable limit
(1 weight ppm).
[0227] The operation of supplying the entire amount (25 kg/hr on
average) of the bottoms obtained to the esterification reactor in
the process for producing methyl acrylate, while maintaining the
above conditions, was continued for 2 months, whereby there was no
trouble of clogging in the withdrawal pipe of the bottoms or in the
feeding pipe to the reactor for methyl acrylate.
[0228] Further, also in the process for producing methyl acrylate,
no abnormality such as polymerization trouble or clogging trouble
was observed, and no change in the quality of the product of methyl
acrylate was observed, by the operation by supplying the bottoms,
as compared with the case of the operation carried out without
supplying the bottoms.
[0229] Further, unit consumption of the starting materials in the
process for producing methyl methacrylate for this period of 2
months is compared with the unit consumption at the time when no
bottoms were supplied like in the conventional process, whereby it
was calculated that the decrease of acrylic acid was about 19
kg/hr. Here, during this period, it was possible to carry out the
operation by reducing the feeding amount of hydroquinone in the
purification process for methyl acrylate by 1 kg/hr as compared
with a case where no bottoms were supplied.
Comparative Example f1
A Case Wherein the Bottoms from the Distillation Column for
Purification were Disposed
[0230] The aldehyde treatment and distillation of high purity
acrylic acid were carried out in the same manner as in Example f1
except that operation was carried out by reducing the withdrawal
amount of the bottoms from the distillation column for
purification. Namely, in this Comparative Example, as the bottoms
from the distillation column for purification were disposed, the
operation was carried out so that acrylic acid was recovered as far
as possible, and distillation was carried out so that the
withdrawal amount of the bottoms would be 15 kg/hr, but clogging
occurred in the withdrawal pipe of the bottoms, and it was
impossible to continue the operation. The composition of the
bottoms at that time was 51 wt % of acrylic acid, 20 wt % of
.beta.-acryloxypropionic acid and 29 wt % of other heavy
substances, etc.
Comparative Example f2
A Case Wherein the Bottoms from the Distillation Column for
Purification was Recycled to the Process for Producing Acrylic
Acid
[0231] The aldehyde treatment and distillation of the high purity
acrylic acid were carried out in the same manner as in Example f1.
However, 25 kg/hr of the bottoms from the distillation column for
purification were supplied to the decomposition reactor for the
heavy fraction in the process for producing acrylic acid, and
continuous operation for 1 month was carried out. The decomposition
reaction in the decomposition reactor for a heavy fraction was
carried out in the absence of a catalyst and conducted in a
reactive distillation system by connecting a distillation column to
the upper part of the decomposition reactor. The conditions for the
decomposition reaction were such that the retention time based on
the withdrawal liquid was 1 hour, the temperature was 190.degree.
C., and the pressure was 100 kPa. The distillate obtained from the
decomposition reaction distillation was supplied to a dehydration
column, but in this dehydration column, the pressure difference
between the top and the bottom of the column increased by 0.5 kPa
upon expiration of 1 month.
[0232] Further, in a case where the operation was carried out in
the same manner as in Example f1, no increase in the pressure
difference in the dehydration column was observed after one month
of operation, and in this Comparative Example, it is considered
that polymerization of acrylic acid was accelerated due to
decomposition by-products of the bottoms, whereby an increase in
the pressure difference in the dehydration column was brought
about.
INDUSTRIAL APPLICABILITY
[0233] a. According to the present invention, from crude
(meth)acrylic acid, both (meth)acrylic acid of the purity for an
ester and (meth)acrylic acid of the purity for a highly water
absorptive resin can efficiently be produced. In the present
invention, only (meth)acrylic acid directed to the purity for a
highly water absorptive resin, is treated by an aldehyde-removing
agent, whereby the removing agent can be saved, and as the amount
of the liquid to be treated is small, the removal operation is
easy. Further, also in a case where only (meth)acrylic acid for a
highly water absorptive resin is to be produced, the amount of the
liquid to be treated in the second distillation column is small as
compared with in the first distillation column, whereby the merit
of the present invention to supply an aldehyde-removing agent to
the second distillation column will continuously be maintained.
Further, in the present invention, the bottoms from the first
distillation column and the second distillation column are not
discharged out of the system as they are, but they are distilled in
the third distillation column, so that acrylic acid in these
bottoms is recovered as much as possible and recycled to the first
distillation column, whereby the obtainable ratio of the purified
acrylic acid from the supplied crude acrylic acid can be maintained
at a high level.
[0234] b. According to the method for producing (meth)acrylic acid
according to the present invention, even if impurities such as
maleic acid and/or citraconic acid are contained in a relatively
large amount in the crude (meth)acrylic acid obtained by vapor
phase catalytic oxidation, it becomes possible to produce high
purity (meth)acrylic acid having an extremely small content of
impurities constantly for a long period of time by suppressing
formation of sludge during the purification by continuous
distillation, and thus, its industrial value is significant.
[0235] c. Impurities contained in the crude (meth)acrylic acid
obtained by vapor phase catalytic oxidation can easily be removed
by the distillation method. By suppressing polymerization of a
(meth)acrylic acid monomer during the distillation, constant
operation for a long period of time can be carried out, and thus,
its industrial value is significant.
[0236] d. According to a thin film evaporator according to the
present invention, it is possible to suppress formation of a
polymer in the interior of the evaporator, and even if a polymer or
precipitate is formed, it is possible to prevent deposition
thereof, whereby constant operation for a long period of time of
the thin film evaporator is possible. It is thereby possible to
stabilize the production. Thus, the thin film evaporator according
to the present invention can be said to be an instrument very
useful for industrial purpose.
[0237] e. By using, as the starting material, acrylic acid having
the specific impurities controlled to the specific concentration
according to the present invention, it is possible to avoid
conventional problems such as clogging of apparatus such as pipings
by a polymer, deterioration of unit consumption of the starting
materials, deterioration of the quality of the product, etc., and
at the same time, it is possible to provide an industrially
advantageous method for producing an acrylic ester, which is
excellent also in the economical efficiency.
[0238] f. According to the present invention, it is possible to
produce high purity (meth)acrylic acid purified to a high level by
efficiently and simply removing aldehydes contained in
(meth)acrylic acid, and at the same time, it is possible to recycle
the waste liquid other than the high purity (meth)acrylic acid
fraction, which forms by such treatment of aldehydes without any
special treatment, to a process for producing a (meth)acrylic
ester, in the entire amount as it is, and to effectively use it,
whereby the following excellent effects can be obtained.
[0239] {circle around (1)} It is possible to avoid conventional
problems such as a polymerization trouble in the purification step
for (meth)acrylic acid caused by an aldehyde-removing agent, or
contamination or coloration of the product, etc. which used to be
caused by recycling of the waste liquid to the process for
producing (meth)acrylic acid.
[0240] {circle around (2)} Treatment of the waste liquid which used
to be required, will be unnecessary.
[0241] {circle around (3)} Useful components such as (meth)acrylic
acid and a dimer of (meth)acrylic acid which used to be disposed,
can be recovered and reused, whereby unit consumption of the
starting material will be improved.
[0242] {circle around (4)} (Meth)acrylic acid in the waste liquid
can be recovered without being lost, whereby it is possible to
increase the concentration of (meth)acrylic acid in the bottoms in
the distillation column for purification to separate high purity
(meth)acrylic acid and to avoid a trouble such as clogging in the
distillation system.
[0243] {circle around (5)} It is possible to effectively use a
polymerization inhibitor in the waste liquid, which used to be
disposed, whereby the amount of the polymerization inhibitor to be
used in the process for producing a (meth)acrylic ester, can be
reduced.
[0244] The entire disclosures of Japanese Patent Application No.
2001-332008 filed on Oct. 30, 2001, Japanese Patent Application No.
2001-360437 filed on Nov. 27, 2001, Japanese Patent Application No.
2001-368858 filed on Dec. 3, 2001, Japanese Patent Application No.
2001-373671 filed on Dec. 7, 2001, Japanese Patent Application No.
2002-003590 filed on Jan. 10, 2002 and Japanese Patent Application
No. 2002-131675 filed on May 7, 2002 including specifications,
claims, drawings and summaries are incorporated herein by reference
in their entireties.
* * * * *