U.S. patent application number 11/116771 was filed with the patent office on 2006-11-02 for process for the production of acetic acid.
Invention is credited to Jonathan A. Makelki, Manuel Salado, Mark O. Scates, Raymond J. Zinobile.
Application Number | 20060247466 11/116771 |
Document ID | / |
Family ID | 36954738 |
Filed Date | 2006-11-02 |
United States Patent
Application |
20060247466 |
Kind Code |
A1 |
Zinobile; Raymond J. ; et
al. |
November 2, 2006 |
Process for the production of acetic acid
Abstract
A process for the reduction and/or removal of permanganate
reducing compounds formed by the carbonylation of methanol in the
presence of a Group VIII metal carbonylation catalyst to produce
acetic acid is disclosed. More specifically, a process for reducing
and/or removing permanganate reducing compounds or their precursors
from intermediate streams during the formation of acetic acid by
said carbonylation processes is disclosed. In particular, a process
in which a low boiling overhead vapor stream from a light ends
column is subjected to a single distillation to obtain an overhead
that is subjected to an extraction to selectively remove and/or
reduce PRC's from the process is disclosed.
Inventors: |
Zinobile; Raymond J.;
(Houston, TX) ; Scates; Mark O.; (Houston, TX)
; Makelki; Jonathan A.; (Bay City, TX) ; Salado;
Manuel; (Seabrook, TX) |
Correspondence
Address: |
HOWREY LLP
C/O IP DOCKETING DEPARTMENT
2941 FAIRVIEW PARK DRIVE, SUITE 200
FALLS CHURCH
VA
22042-7195
US
|
Family ID: |
36954738 |
Appl. No.: |
11/116771 |
Filed: |
April 28, 2005 |
Current U.S.
Class: |
562/517 ;
568/492 |
Current CPC
Class: |
C07C 51/48 20130101;
C07C 51/44 20130101; C07C 51/44 20130101; C07C 53/08 20130101; C07C
51/48 20130101; C07C 53/08 20130101 |
Class at
Publication: |
562/517 ;
568/492 |
International
Class: |
C07C 51/14 20060101
C07C051/14; C07C 45/90 20060101 C07C045/90 |
Claims
1. A process for the reduction and/or removal of permanganate
reducing compounds (PRC's) formed in the carbonylation of a
carbonylatable reactant to produce a carbonylation product
comprising acetic acid, comprising the steps of: (a) separating the
carbonylation product to provide a vapor overhead stream comprising
acetic acid and a less volatile catalyst phase; (b) distilling the
vapor overhead stream to yield a purified acetic acid product and a
low boiling overhead vapor stream comprising methyl iodide, water,
acetic acid, methyl acetate, and at least one PRC; (c) condensing
the low boiling overhead vapor stream and biphasically separating
it to form a condensed heavy liquid phase and a condensed light
liquid phase; (d) distilling the condensed light liquid phase in a
single distillation column to form a second vapor phase stream
overhead and a higher boiling liquid phase residuum, wherein the
second vapor phase stream is enriched with PRC's with respect to
the condensed light liquid phase; and (e) condensing the second
vapor phase stream and extracting the condensed stream with water
to obtain an aqueous acetaldehyde stream comprising PRC and a
raffinate comprising methyl iodide.
2. The process of claim 1, wherein the total concentration of
methyl acetate and methanol in the second vapor stream is less than
about 5 wt. %.
3. The process of claim 1, wherein the concentration of
acetaldehyde in the higher boiling liquid phase residuum is less
than about 0.3 wt %.
4. The process of claim 1, wherein the second vapor phase stream is
extracted in the presence of dimethyl ether.
5. The process of claim 1 wherein step (d) is performed without
removing a sidestream comprising methyl acetate from the
distillation column and wherein the higher boiling liquid phase
residuum is enriched with methyl acetate with respect to said
second vapor phase stream.
6. The process of claim 5, wherein the total concentration of
methyl acetate and methanol in the second vapor stream is less than
about 5 wt. %.
7. The process of claim 5, wherein the concentration of
acetaldehyde in the higher boiling liquid phase residuum is less
than about 0.3 wt %.
8. The process of claim 1, further comprising removing a sidestream
comprising methyl acetate from the distillation column of step (d),
wherein the higher boiling liquid phase residuum and the sidestream
are cumulatively enriched with methyl acetate with respect to said
second vapor phase stream.
9. The process of claim 8, wherein the total concentration of
methyl acetate and methanol in the second vapor stream is less than
about 5 wt. %.
10. The process of claim 8, wherein the concentration of
acetaldehyde in the higher boiling liquid phase residuum is less
than about 0.3 wt %.
11. A process for the reduction and/or removal of permanganate
reducing compounds (PRC's) formed in the carbonylation of a
carbonylatable reactant to produce a carbonylation product
comprising acetic acid, comprising the steps of: (a) separating the
carbonylation product to provide a vapor overhead stream comprising
acetic acid and a less volatile catalyst phase; (b) distilling the
vapor overhead stream to yield a purified acetic acid product and a
low boiling overhead vapor stream comprising methyl iodide, water,
acetic acid, methyl acetate, and at least one PRC; (c) condensing
the low boiling overhead vapor stream and biphasically separating
it to form a condensed heavy liquid phase and a condensed light
liquid phase; (d) distilling the condensed light liquid phase in a
single distillation column to form a second vapor phase stream
overhead and a higher boiling liquid phase residuum, wherein the
second vapor phase stream is enriched with PRC's with respect to
the condensed light liquid phase and wherein the higher boiling
liquid phase residuum is enriched with methyl acetate with respect
to said second vapor phase stream overhead; and (e) condensing the
second vapor phase stream and extracting the condensed stream with
water to obtain an aqueous acetaldehyde stream comprising PRC and a
raffinate comprising methyl iodide.
12. The process of claim 11, wherein step (d) is performed without
removing a sidestream comprising methyl acetate from the
distillation column.
13. The process of claim 12, wherein the total concentration of
methyl acetate and methanol in the second vapor stream is less than
about 5 wt. %.
14. The process of claim 12, wherein the concentration of
acetaldehyde in the higher boiling liquid phase residuum is less
than about 0.3 wt %.
15. The process of claim 11, comprising an additional step of
removing a sidestream comprising methyl acetate from the
distillation column of step (d), wherein the higher boiling liquid
phase residuum and the sidestream are cumulatively enriched with
methyl acetate with respect to the second vapor phase stream.
16. The process of claim 15, wherein the total concentration of
methyl acetate and methanol in the second vapor stream is less than
about 5 wt. %.
17. The process of claim 15, wherein the concentration of
acetaldehyde in the higher boiling liquid phase residuum is less
than about 0.3 wt %.
18. A process for the reduction and/or removal of permanganate
reducing compounds (PRC's) formed in the carbonylation of a
carbonylatable reactant to produce a carbonylation product
comprising acetic acid, comprising the steps of: (a) separating the
carbonylation product to provide a vapor overhead stream comprising
acetic acid and a less volatile catalyst phase; (b) distilling the
vapor overhead stream to yield a purified acetic acid product and a
low boiling overhead vapor stream comprising methyl iodide, water,
acetic acid, methyl acetate, and at least one PRC; (c) condensing
the low boiling overhead vapor stream and biphasically separating
it to form a condensed heavy liquid phase and a condensed light
liquid phase; (d) distilling the condensed light liquid phase in a
single distillation column to form a second vapor phase stream
overhead and a higher boiling liquid phase residuum, wherein the
second vapor phase stream is enriched with PRC's with respect to
the condensed light liquid phase; (e) removing a sidestream
comprising methyl acetate from the distillation column of step (d),
wherein the higher boiling liquid phase residuum and the sidestream
are cumulatively enriched with methyl acetate with respect to said
second vapor phase stream; and (f) condensing the second vapor
phase stream and extracting the condensed stream with water to
obtain an aqueous acetaldehyde stream comprising PRC and a
raffinate comprising methyl iodide.
19. The process of claim 18, wherein the total concentration of
methyl acetate and methanol in the second vapor stream is less than
about 5 wt. %.
20. The process of claim 18, wherein the concentration of
acetaldehyde in the higher boiling liquid phase residuum is less
than about 0.3 wt %.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a process for the production of
acetic acid and, in particular, to an improved process for the
reduction and/or removal of permanganate reducing compounds formed
by the carbonylation of methanol in the presence of a Group VIII
metal carbonylation catalyst to produce acetic acid. More
specifically, this invention relates to an improved process for
reducing and/or removing permanganate reducing compounds or their
precursors from intermediate streams during the formation of acetic
acid by said carbonylation processes.
[0003] 2. Technical Background
[0004] Among currently employed processes for synthesizing acetic
acid, one of the most useful commercially is the catalyzed
carbonylation of methanol with carbon monoxide as taught in U.S.
Pat. No. 3,769,329, issued to Paulik et al. on Oct. 30, 1973. The
carbonylation catalyst contains rhodium, either dissolved or
otherwise dispersed in a liquid reaction medium or supported on an
inert solid, along with a halogen-containing catalyst promoter as
exemplified by methyl iodide. The rhodium can be introduced into
the reaction system in any of many forms. Likewise, the nature of
the halide promoter is not generally critical. The patentees
disclose a very large number of suitable promoters, most of which
are organic iodides. Most typically and usefully, the reaction is
conducted by continuously bubbling carbon monoxide gas through a
liquid reaction medium in which the catalyst is dissolved.
[0005] An improvement in the prior art process for the
carbonylation of an alcohol to produce the carboxylic acid having
one carbon atom more than the alcohol in the presence of a rhodium
catalyst is disclosed in commonly assigned U.S. Pat. No. 5,001,259,
issued Mar. 19, 1991; U.S. Pat. No. 5,026,908, issued Jun. 25,
1991; and U.S. Pat. No. 5,144,068, issued Sep. 1, 1992; and
European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. As
disclosed therein, acetic acid is produced from methanol in a
reaction medium containing methyl acetate, methyl halide,
especially methyl iodide, and rhodium present in a catalytically
effective concentration. These patents disclose that catalyst
stability and the productivity of the carbonylation reactor can be
maintained at surprisingly high levels, even at very low water
concentrations, i.e., 4 weight percent or less, in the reaction
medium (despite the general industrial practice of maintaining
approximately 14-15 wt % water) by maintaining in the reaction
medium, along with a catalytically effective amount of rhodium and
at least a finite concentration of water, a specified concentration
of iodide ions over and above the iodide ion that is present as
hydrogen iodide. This iodide ion is a simple salt, with lithium
iodide being preferred. The patents teach that the concentration of
methyl acetate and iodide salts are significant parameters in
affecting the rate of carbonylation of methanol to produce acetic
acid, especially at low reactor water concentrations. By using
relatively high concentrations of the methyl acetate and iodide
salt, one obtains a surprising degree of catalyst stability and
reactor productivity even when the liquid reaction medium contains
water in concentrations as low as about 0.1 wt %, so low that it
can broadly be defined simply as "a finite concentration" of water.
Furthermore, the reaction medium employed improves the stability of
the rhodium catalyst, i.e., resistance to catalyst precipitation,
especially during the product recovery steps of the process. In
these steps, distillation for the purpose of recovering the acetic
acid product tends to remove from the catalyst the carbon monoxide,
which in the environment maintained in the reaction vessel, is a
ligand with stabilizing effect on the rhodium. U.S. Pat. Nos.
5,001,259, 5,026,908 and 5,144,068 are herein incorporated by
reference.
[0006] It has been found that although a low water carbonylation
process for producing acetic acid reduces such by-products as
carbon dioxide, hydrogen, and propionic acid, the amount of other
impurities, present generally in trace amounts, can be increased by
a low water carbonylation process, and the quality of acetic acid
sometimes suffers when attempts are made to increase the production
rate by improving catalysts, or modifying reaction conditions.
[0007] These trace impurities affect quality of acetic acid,
especially when they are recirculated through the reaction process,
which, among other things, can result in the build up over time of
these impurities. The impurities that decrease the permanganate
time of the acetic acid, a quality test commonly used in the acetic
acid industry, include carbonyl compounds and unsaturated carbonyl
compounds. As used herein, the phrase "carbonyl" is intended to
mean compounds that contain aldehyde or ketone functional groups,
which compounds may or may not possess unsaturation. See Catalysis
of Organic Reaction, 75, 369-380 (1998), for further discussion on
impurities in a carbonylation process.
[0008] The present invention is directed to reducing and/or
removing permanganate reducing compounds (PRC's) such as
acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde,
crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde
and the like, and the aldol condensation products thereof. The
present invention may also lead to reduction of propionic acid.
[0009] The carbonyl impurities described above, such as
acetaldehyde, may react with iodide catalyst promoters to form
multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide,
butyl iodide, pentyl iodide, hexyl iodide, and the like. It is
desirable to remove multi-carbon alkyl iodides from the reaction
product because even small amounts of these impurities in the
acetic acid product tend to poison the catalyst used in the
production of vinyl acetate, a product commonly produced from
acetic acid. Thus, the present invention may also lead to reduction
or removal of multi-carbon alkyl iodides, in particular C.sub.2-12
alkyl iodide compounds. Accordingly, because many impurities
originate with acetaldehyde, it is a primary objective to remove
carbonyl impurities, notably acetaldehyde, from the process so as
to reduce the multi-carbon alkyl iodide content.
[0010] Conventional techniques to remove such impurities include
treating the acetic acid product streams with oxidizers, ozone,
water, methanol, activated-carbon, amines, and the like. Such
treatments may or may not be combined with distillation of the
acetic acid. The most typical purification treatment involves a
series of distillations of the final product. It is also known to
remove carbonyl impurities from organic streams by treating the
organic streams with an amine compound such as hydroxylamine, which
reacts with the carbonyl compounds to form oximes, followed by
distillation to separate the purified organic product from the
oxime reaction products. However, the additional treatment of the
final product adds cost to the process, and distillation of the
treated acetic acid product can result in additional impurities
being formed.
[0011] While it is possible to obtain acetic acid of relatively
high purity, the acetic acid product formed by the low-water
carbonylation process and purification treatment described above
frequently remains somewhat deficient with respect to the
permanganate time due to the presence of small proportions of
residual impurities. Because a sufficient permanganate time is an
important commercial test, which the acid product may be required
to meet to be suitable for many uses, the presence of impurities
that decrease permanganate time is objectionable. Moreover, it has
not been economically or commercially feasible to remove minute
quantities of these impurities from the acetic acid by distillation
because some of the impurities have boiling points close to that of
the acetic acid product or halogen-containing catalyst promoters,
such as methyl iodide.
[0012] It has thus become important to identify economically viable
methods of removing impurities elsewhere in the carbonylation
process without contaminating the final product or adding
unnecessary costs. For example, a method for manufacturing high
purity acetic acid by adjusting the acetaldehyde concentration of
the reaction solution below a certain amount, such as 1500 ppm, has
been disclosed. It is stated that by maintaining the acetaldehyde
concentration below this threshold, it is possible to suppress the
formation of impurities such that one need only distill the crude
acetic acid product to obtain high purity acetic acid.
[0013] The art has also disclosed that carbonyl impurities present
in the acetic acid product streams generally concentrate in the
overhead from the light ends column. Accordingly, the light ends
column overhead has been treated with an amine compound (such as
hydroxylamine), which reacts with the carbonyl compounds to form
oxime derivatives that can be separated from the remaining overhead
by distillation, resulting in an acetic acid product with improved
permanganate time.
[0014] Other processes have been described for producing high
purity acetic acid in which it is stated that an acetaldehyde
concentration of 400 ppm or less is maintained in the reactor by
using distillation to remove acetaldehyde. Streams suggested for
processing to remove acetaldehyde include a light phase containing
primarily water, acetic acid and methyl acetate; a heavy phase
containing primarily methyl iodide, methyl acetate and acetic acid;
an overhead stream containing primarily methyl iodide and methyl
acetate; or a recirculating stream formed by combining the light
and heavy phase.
[0015] It has been disclosed in commonly assigned U.S. Pat. Nos.
6,143,930 and 6,339,171 that it is possible to significantly reduce
the undesirable impurities in the acetic acid product by performing
a multi-stage purification on the light ends column overhead. These
patents disclose a purification process in which the light ends
overhead is distilled twice, in each case taking the acetaldehyde
overhead and returning a methyl iodide rich residuum to the
reactor. The acetaldehyde-rich distillate obtained after the two
distillation steps is optionally extracted with water to remove the
majority of the acetaldehyde for disposal, leaving a significantly
lower acetaldehyde concentration in the raffinate that is recycled
to the reactor. U.S. Pat. Nos. 6,143,930 and 6,339,171 are
incorporated herein by reference in their entirety.
[0016] While the above-described processes have been successful in
removing carbonyl impurities from the carbonylation system and for
the most part controlling acetaldehyde levels and permanganate time
problems in the final acetic acid product, further improvements can
still be made. Accordingly, there remains a need for alternative
processes to improve the efficiency of acetaldehyde removal. The
present invention provides one such alternative solution.
SUMMARY OF THE INVENTION
[0017] This invention relates to a process for the production of
acetic acid and, in particular, an improved process for the
reduction and/or removal of permanganate reducing compounds and
alkyl iodides formed by the carbonylation of methanol in the
presence of a Group VIII metal carbonylation catalyst to produce
acetic acid. More specifically, this invention relates to an
improved process for reducing and/or removing permanganate reducing
compounds or their precursors from intermediate streams during the
formation of acetic acid by said carbonylation processes.
[0018] In one aspect, the present invention provides a process for
the reduction and/or removal of permanganate reducing compounds
(PRC's) formed in the carbonylation of a carbonylatable reactant to
produce a carbonylation product comprising acetic acid. The process
comprises the steps of: (a) separating the carbonylation product to
provide a vapor overhead stream comprising acetic acid and a less
volatile catalyst phase; (b) distilling the vapor overhead stream
to yield a purified acetic acid product and a low boiling overhead
vapor stream comprising methyl iodide, water, acetic acid, methyl
acetate, and at least one PRC; (c) condensing the low boiling
overhead vapor stream and biphasically separating it to form a
condensed heavy liquid phase and a condensed light liquid phase;
(d) distilling the condensed light liquid phase in a single
distillation column to form a second vapor phase stream overhead
and a higher boiling liquid phase residuum, wherein the second
vapor phase stream is enriched with PRC's with respect to the
condensed light liquid phase; and (e) condensing the second vapor
phase stream and extracting the condensed stream with water to
obtain an aqueous acetaldehyde stream comprising PRC and a
raffinate comprising methyl iodide. In certain variations, the
process can be operated with or without a sidestream comprising
methyl acetate being taken from the distillation column of step
(d).
[0019] In another aspect, the present invention provides a process
for the reduction and/or removal of permanganate reducing compounds
(PRC's) formed in the carbonylation of a carbonylatable reactant to
produce a carbonylation product comprising acetic acid, comprising
the steps of: (a) separating the carbonylation product to provide a
vapor overhead stream comprising acetic acid and a less volatile
catalyst phase; (b) distilling the vapor overhead stream to yield a
purified acetic acid product and a low boiling overhead vapor
stream comprising methyl iodide, water, acetic acid, methyl
acetate, and at least one PRC; (c) condensing the low boiling
overhead vapor stream and biphasically separating it to form a
condensed heavy liquid phase and a condensed light liquid phase;
(d) distilling the condensed light liquid phase in a single
distillation column to form a second vapor phase stream overhead
and a higher boiling liquid phase residuum, wherein the second
vapor phase stream is enriched with PRC's with respect to the
condensed light liquid phase and wherein the higher boiling liquid
phase residuum is enriched with methyl acetate with respect to said
second vapor phase stream overhead; and (e) condensing the second
vapor phase stream and extracting the condensed stream with water
to obtain an aqueous acetaldehyde stream comprising PRC and a
raffinate comprising methyl iodide. In certain variations, the
process can be operated with or without a sidestream comprising
methyl acetate being taken from the distillation column of step
(d)
[0020] In a third aspect, the present invention provides a process
for the reduction and/or removal of permanganate reducing compounds
(PRC's) formed in the carbonylation of a carbonylatable reactant to
produce a carbonylation product comprising acetic acid, comprising
the steps of: (a) separating the carbonylation product to provide a
vapor overhead stream comprising acetic acid and a less volatile
catalyst phase; (b) distilling the vapor overhead stream to yield a
purified acetic acid product and a low boiling overhead vapor
stream comprising methyl iodide, water, acetic acid, methyl
acetate, and at least one PRC; (c) condensing the low boiling
overhead vapor stream and biphasically separating it to form a
condensed heavy liquid phase and a condensed light liquid phase;
(d) distilling the condensed light liquid phase in a single
distillation column to form a second vapor phase stream overhead
and a higher boiling liquid phase residuum, wherein the second
vapor phase stream is enriched with PRC's with respect to the
condensed light liquid phase; (e) removing a sidestream comprising
methyl acetate from the distillation column of step (d), wherein
the higher boiling liquid phase residuum and the sidestream are
cumulatively enriched with methyl acetate with respect to said
second vapor phase stream; and (f) condensing the second vapor
phase stream and extracting the condensed stream with water to
obtain an aqueous acetaldehyde stream comprising PRC and a
raffinate comprising methyl iodide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 illustrates various embodiments of the present
invention.
[0022] While the invention is susceptible to various modifications
and alternative forms, specific embodiments have been shown by way
of example in the drawings and will be described in detail herein.
It should be understood, however, that the invention is not
intended to be limited to the particular forms disclosed. Rather,
the invention is intended to cover all modifications, equivalents
and alternatives falling within the scope of the invention as
defined by the appended claims.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0023] This invention relates to a process for the production of
acetic acid and, in particular, an improved process for the
reduction and/or removal of permanganate reducing compounds formed
by the carbonylation of methanol in the presence of a Group VIII
metal carbonylation catalyst to produce acetic acid. More
specifically, this invention relates to an improved process for
reducing and/or removing permanganate reducing compounds or their
precursors from intermediate streams during the formation of acetic
acid by said carbonylation processes.
[0024] In particular, the present invention relates to a process in
which a condensed light phase from a light ends column overhead is
subjected to a single distillation to obtain an overhead that is
subjected to an extraction to selectively reduce and/or remove
PRC's from the process. Among other advantages, the present
invention is able to reduce and/or remove PRC's from the process
using a single distillation column and extraction combination as
compared to previous process that utilized more than one
distillation column with (or without) extraction to reduce and/or
remove PRC's from a condensed light phase from a light ends column
overhead. Additional advantages include, but are not limited to,
lower energy usage and reduced equipment and associated costs.
[0025] Illustrative embodiments of the invention are described
below. In the interest of clarity, not all features of an actual
implementation are described in this specification. It will of
course be appreciated that in the development of any such actual
embodiment, numerous implementation-specific decisions must be made
to achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which will vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure.
[0026] The purification process of the present invention is useful
in any process used to carbonylate methanol (or another
carbonylatable reactant, including, but not limited to, methyl
acetate, methyl formate or dimethyl ether, or mixtures thereof) to
acetic acid in the presence of a Group VIII metal catalyst, such as
rhodium, and a halogen-containing catalyst promoter. A particularly
useful process is the low water rhodium-catalyzed carbonylation of
methanol to acetic acid as exemplified in U.S. Pat. No.
5,001,259.
[0027] Generally, the rhodium component of the catalyst system is
believed to be present in the form of a coordination compound of
rhodium with a halogen component providing at least one of the
ligands of such coordination compound. In addition to the
coordination of rhodium and halogen, it is also believed that
carbon monoxide will coordinate with rhodium. The rhodium component
of the catalyst system may be provided by introducing into the
reaction zone rhodium in the form of rhodium metal, rhodium salts
such as the oxides, acetates, iodides, carbonates, hydroxides,
chlorides, etc., or other compounds that result in the formation of
a coordination compound of rhodium in the reaction environment.
[0028] The halogen-containing catalyst promoter of the catalyst
system consists of a halogen compound comprising an organic halide.
Thus, alkyl, aryl, and substituted alkyl or aryl halides can be
used. Preferably, the halogen-containing catalyst promoter is
present in the form of an alkyl halide. Even more preferably, the
halogen-containing catalyst promoter is present in the form of an
alkyl halide in which the alkyl radical corresponds to the alkyl
radical of the feed alcohol, which is being carbonylated. Thus, in
the carbonylation of methanol to acetic acid, the halide promoter
will include methyl halide, and more preferably methyl iodide.
[0029] The liquid reaction medium employed may include any solvent
compatible with the catalyst system and may include pure alcohols,
or mixtures of the alcohol feedstock and/or the desired carboxylic
acid and/or esters of these two compounds. A preferred solvent and
liquid reaction medium for the low water carbonylation process
contains the desired carboxylic acid product. Thus, in the
carbonylation of methanol to acetic acid, a preferred solvent
system contains acetic acid.
[0030] Water is contained in the reaction medium but desirably at
concentrations well below that which has heretofore been thought
practical for achieving sufficient reaction rates. It has
previously been taught, e.g., U.S. Pat. No. 3,769,329, that in
rhodium-catalyzed carbonylation reactions of the type set forth in
this invention, the addition of water exerts a beneficial effect
upon the reaction rate. Thus, commercial operations are commonly
run at water concentrations of at least about 14 wt %. Accordingly,
it has been quite unexpected that reaction rates substantially
equal to and above reaction rates obtained with such comparatively
high levels of water concentration can be achieved with water
concentrations below 14 wt % and as low as about 0.1 wt %.
[0031] In accordance with the carbonylation process most useful to
manufacture acetic acid according to the present invention, the
desired reaction rates are obtained even at low water
concentrations by maintaining in the reaction medium an ester of
the desired carboxylic acid and an alcohol, desirably the alcohol
used in the carbonylation, and an additional iodide ion that is
over and above the iodide ion that is present as hydrogen iodide. A
desired ester is methyl acetate. The additional iodide ion is
desirably an iodide salt, with lithium iodide being preferred. It
has been found, e.g., U.S. Pat. No. 5,001,259, that under low water
concentrations, methyl acetate and lithium iodide act as rate
promoters only when relatively high concentrations of each of these
components are present and that the promotion is higher when both
of these components are present simultaneously. The concentration
of iodide ion maintained in the reaction medium of the preferred
carbonylation reaction system is believed to be quite high as
compared with what little prior art there is dealing with the use
of halide salts in reaction systems of this sort. The absolute
concentration of iodide ion content is not a limitation on the
usefulness of the present invention.
[0032] The carbonylation reaction of methanol to acetic acid
product may be carried out by contacting the methanol feed with
gaseous carbon monoxide bubbled through a acetic acid solvent
reaction medium containing the rhodium catalyst, methyl iodide
promoter, methyl acetate, and additional soluble iodide salt, at
conditions of temperature and pressure suitable to form the
carbonylation product. It will be generally recognized that it is
the concentration of iodide ion in the catalyst system that is
important and not the cation associated with the iodide, and that
at a given molar concentration of iodide the nature of the cation
is not as significant as the effect of the iodide concentration.
Any metal iodide salt, or any iodide salt of any organic cation, or
quaternary cation such as a quaternary amine or phosphine or
inorganic cation can be maintained in the reaction medium provided
that the salt is sufficiently soluble in the reaction medium to
provide the desired level of the iodide. When the iodide is a metal
salt, preferably it is an iodide salt of a member of the group
consisting of the metals of Group IA and Group IIA of the periodic
table as set forth in the "Handbook of Chemistry and Physics"
published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). In
particular, alkali metal iodides are useful, with lithium iodide
being particularly suitable. In the low water carbonylation process
most useful in this invention, the additional iodide ion over and
above the iodide ion present as hydrogen iodide is generally
present in the catalyst solution in amounts such that the total
iodide ion concentration is from about 2 to about 20 wt % and the
methyl acetate is generally present in amounts of from about 0.5 to
about 30 wt %, and the methyl iodide is generally present in
amounts of from about 5 to about 20 wt %. The rhodium catalyst is
generally present in amounts of from about 200 to about 2000 parts
per million (ppm).
[0033] Typical reaction temperatures for carbonylation will be from
about 150 to about 250.degree. C., with the temperature range of
about 180 to about 220.degree. C. being a preferred range. The
carbon monoxide partial pressure in the reactor can vary widely but
is typically about 2 to about 30 atmospheres, and preferably, about
3 to about 10 atmospheres. Because of the partial pressure of
by-products and the vapor pressure of the contained liquids, the
total reactor pressure will range from about 15 to about 40
atmospheres.
[0034] A typical reaction and acetic acid recovery system that is
used for the iodide-promoted rhodium catalyzed carbonylation of
methanol to acetic acid in accordance with the present invention is
shown in FIG. 1 and includes a liquid phase carbonylation reactor,
flasher, and a methyl iodide acetic acid light ends column ("light
ends column") 14. In the process, carbonylation product obtained in
the reactor is provided to the flasher where a volatile ("vapor")
overhead stream comprising acetic acid and a less volatile catalyst
phase (catalyst-containing solution) are obtained. The volatile
overhead stream comprising acetic acid is provided by stream 26 to
the light ends column 14 where distillation yields a purified
acetic acid product that is removed via sidestream 17 and an
overhead distillate stream 28 (hereafter "low-boiling overhead
vapor stream"). Acetic acid removed via sidestream 17 can be
subjected to further purification, such as to a drying column for
selective separation of acetic acid from water.
[0035] The reactor and flasher are not shown in FIG. 1. These are
considered standard equipment now well known in the carbonylation
process art. The carbonylation reactor is typically either a
stirred vessel or bubble-column type within which the reacting
liquid or slurry contents are maintained automatically at a
constant level. Into this reactor, there are continuously
introduced fresh methanol, carbon monoxide, sufficient water as
needed to maintain at least a finite concentration of water in the
reaction medium. Also introduced into the reactor is a recycled
catalyst solution, such as from the flasher base, a recycled methyl
iodide phase, a recycled methyl acetate phase, and a recycled
aqueous acetic acid phase. A recycled phase may contain one or more
of the foregoing components.
[0036] Distillation systems are employed that provide means for
recovering the crude acetic acid and recycling catalyst solution,
methyl iodide, methyl acetate, and other system components within
the process. In a typical carbonylation process, carbon monoxide is
continuously introduced into the carbonylation reactor, desirably
below the agitator, which is used to stir the contents. The gaseous
feed is thoroughly dispersed through the reacting liquid by this
stirring means. A gaseous purge stream is desirably vented from the
reactor to prevent buildup of gaseous by-products and to maintain a
set carbon monoxide partial pressure at a given total reactor
pressure. The temperature of the reactor is controlled and the
carbon monoxide feed is introduced at a rate sufficient to maintain
the desired total reactor pressure.
[0037] Liquid product is drawn off from the carbonylation reactor
at a rate sufficient to maintain a constant level therein and is
introduced to the flasher. In the flasher, a catalyst-containing
solution (catalyst phase) is withdrawn as a base stream
(predominantly acetic acid containing the rhodium and the iodide
salt along with lesser quantities of methyl acetate, methyl iodide,
and water), while a vapor overhead stream comprising acetic acid is
withdrawn overhead. The vapor overhead stream comprising acetic
acid also contains methyl iodide, methyl acetate, and water.
Dissolved gases exiting the reactor and entering the flasher
comprise a portion of the carbon monoxide and may also contain
gaseous by-products such as methane, hydrogen, and carbon dioxide.
Such dissolved gases exit the flasher as part of the overhead
stream. The overhead stream is directed to the light ends column 14
as stream 26.
[0038] It has been disclosed in U.S. Pat. Nos. 6,143,930 and
6,339,171 that there is generally a higher concentration of the
PRC's and in particular acetaldehyde content in the low-boiling
overhead vapor stream 28 exiting column 14 than in the high-boiling
residue stream exiting column 14. Thus, in accordance with the
present invention, low-boiling overhead vapor stream 28, containing
PRC's is subjected to additional processing to reduce and/or remove
the amount of PRC's present. Low-boiling overhead vapor stream 28,
therefore, is condensed and directed to an overhead receiver
decanter 16. In addition to PRC's, low-boiling overhead vapor
stream 28 will typically contain methyl iodide, methyl acetate,
acetic acid, and water. Conditions are desirably maintained in the
process such that low-boiling overhead vapor stream 28, once in
decanter 16, will separate into a light phase and a heavy phase.
Generally, low-boiling overhead vapor stream 28 is chilled to a
temperature sufficient to condense and separate the condensable
methyl iodide, methyl acetate, acetaldehyde and other carbonyl
components, and water into two phases. A portion of stream 28 may
include noncondensable gases such as carbon dioxide, hydrogen, and
the like that can be vented as shown in stream 29 on FIG. 1.
[0039] The condensed light phase in decanter 16 will generally
comprise water, acetic acid, and PRC's, as well as quantities of
methyl iodide and methyl acetate. The condensed heavy phase in
decanter 16 will generally comprise methyl iodide, methyl acetate,
and PRC's.
[0040] The present invention may broadly be considered as an
improved process for distilling PRC's, primarily aldehydes such as
acetaldehyde, from a low-boiling overhead vapor stream,
particularly the condensed light phase of a low-boiling overhead
vapor stream 28 from a light ends distillation column 14. In
accordance with the present invention, a condensed light phase of a
low-boiling overhead vapor stream 28 from a light ends distillation
column 14 is distilled once and then subjected to single- or
multi-stage extraction to reduce and/or remove PRC's.
[0041] This process, such as that in the embodiments disclosed in
FIG. 1, is distinct from prior processes, such as that disclosed in
U.S. Pat. No. 6,339,171, including that illustrated in FIG. 1 of
U.S. Pat. No. 6,339,171.
[0042] In accordance with the present invention, disclosed in FIG.
1, the low-boiling overhead vapor stream 28 contains methyl iodide,
methyl acetate, PRC's such as acetaldehyde and optionally other
carbonyl components. The low-boiling overhead vapor stream 28 also
contains water and some quantity of acetic acid.
[0043] The low-boiling overhead vapor stream 28 is then condensed
and separated (in vessel 16) to form a condensed heavy liquid phase
containing the larger proportion of methyl iodide, but also
containing PRC's, and a condensed light liquid phase (taken of as
stream 30), notably containing PRC's, water, and acetic acid but
also generally containing some quantity of both methyl iodide and
methyl acetate.
[0044] While either phase of the light ends overhead, i.e.,
low-boiling overhead vapor stream 28, may be subsequently processed
to remove the PRC's and primarily the acetaldehyde component of the
stream, in the present invention, the PRC's are removed from the
condensed light liquid phase 30.
[0045] Thus, the condensed heavy liquid phase in the decanter 16
can be conveniently recirculated, either directly or indirectly, to
the reactor (not shown in FIG. 1). For example, a portion of this
condensed heavy liquid phase can be recirculated to the reactor,
with a slip stream, generally a small amount, e.g., 25 vol. %,
preferably less than about 20 vol. %, of the heavy liquid phase
being directed to a carbonyl treatment process. This slip stream of
the heavy liquid phase may be treated individually or may be
combined with the condensed light liquid phase 30 for further
distillation and extraction of carbonyl impurities in accordance
with the present invention.
[0046] In accordance with the present invention, condensed light
liquid phase 30 is directed to distillation column 18, which serves
to form a second vapor phase 36 enriched in PRC's, notably
acetaldehyde, but also containing methyl iodide due to the similar
boiling points of methyl iodide and acetaldehyde. Second vapor
phase 36 is condensed and then extracted with water to reduce
and/or remove PRC's, notably acetaldehyde. In a preferred
embodiment a portion of the condensed stream 36 is provided as
reflux to distillation column 18. This can be accomplished, as
shown in FIG. 1, by provided the condensed stream 36 to an overhead
receiver 20, from which a portion of condensed stream 36 can be
provided to the extraction step (generally indicated as 70) by
stream 40 and another portion of condensed stream 36 can be
provided as reflux to distillation column 18 by stream 42.
[0047] Acetaldehyde is extracted by water to obtain an aqueous
acetaldehyde stream 72, which will generally be treated as a waste.
The raffinate from the extraction, notably containing methyl iodide
is desirably returned to the carbonylation process by stream 74. In
various embodiments, aqueous acetaldehyde stream 72 can be stripped
of aldehyde for treatment as waste with water being recirculated
for use in the process, such as for the water used in extraction
70.
[0048] In the present invention, a primary concern is in the
extraction step that separates acetaldehyde from methyl iodide. The
efficiency of this separation is primarily affected by the relative
solubility of acetaldehyde and methyl iodide in water. While
acetaldehyde is miscible in water, methyl iodide is not. However,
the solubility of methyl iodide in water increases, with a
concomitant loss of methyl iodide from the process system, with
increasing levels of methyl acetate and/or methanol. At high enough
methyl acetate and/or methanol levels, phase separation of methyl
iodide in the water extraction may not occur. Similarly, phase
separation of methyl iodide in the water extraction may not occur
if acetic acid concentrations are sufficiently high. Thus, it is
desirable that the distillate that is condensed and provided for
extraction contain methanol and methyl acetate at a combined
concentration of less than about 10 wt. %, more desirably less than
about 5 wt. %, even more desirably less than about 2 wt. %, and
even more desirably less than about 1.5 wt. %. It is desirable that
the distillate that is condensed and provided for extraction
contain less than about 3 wt. % acetic acid, more desirably less
than about 1 wt. %, and even more desirably less than about 0.5 wt.
%. Particularly desired would be acetic acid concentrations
approaching zero wt. %.
[0049] Thus, in the process of the present invention, a single
distillation is conducted in distillation column 18 under
conditions designed to control, notably minimize, the quantities of
methyl acetate and acetic acid in second vapor phase stream 36.
Desirably, minimization of quantities of methyl acetate and acetic
acid in second vapor phase stream 36 is achieved while
simultaneously maintaining higher acetaldehyde levels in second
vapor phase stream 36 than in the residuum of distillation column
18. It is desirable that the residuum of distillation column 18
contain less than about 0.3 wt. % acetaldehyde, more desirably less
than about 0.2 wt. %, and even more desirably less than about 0.1
wt. %. Particularly desired would be acetaldehyde concentrations
approaching zero wt. %.
[0050] In prior art processes, such as that illustrated in FIG. 1
of U.S. Pat. No. 6,339,171, two distillation steps are conducted in
order to obtain a final vapor distillate containing sufficiently
low quantities of methyl acetate, methanol, and acetic acid such
that the distillate can be subjected to extraction with water to
selectively separate acetaldehyde from methyl iodide. In such prior
art processes, water and acetic acid are preferentially separated
as a residuum in the first column with methyl acetate in the
distillate. Subsequently, methyl acetate is preferentially
separated as a residuum in the second column. The prior art did not
teach that methyl acetate, acetic acid, and water could be
effectively removed as residuum using a single distillation column
without having an undesirable concentration of acetaldehyde in the
residuum. Similarly, the prior art did not teach that acetaldehyde
could be effectively concentrated in the distillate of a single
column without obtaining an undesirable concentration of methyl
acetate in the distillate. As a result of their efforts, the
present inventors have found that such separations can be achieved
using a single distillation step, resulting in improved process
efficiency.
[0051] Thus, in accordance with one embodiment of the present
invention, illustrated in FIG. 1, low-boiling overhead vapor stream
28 is condensed in overhead receiver decanter 16 where it is
biphasically separated to form a condensed heavy liquid phase and a
light condensed liquid phase 30. The light condensed liquid phase
30 is provided to distillation column 18 via stream 30/32. In this
and other embodiments of the present invention, a portion of stream
30 can be directed back to the light ends column 14 as reflux
stream 34.
[0052] In distillation column 18, a second vapor phase stream 36
overhead and a higher boiling liquid phase residuum stream 38 are
formed. The second vapor phase stream 36 overhead is enriched with
PRC, notably acetaldehyde, with respect to the light condensed
liquid phase 30. The second vapor phase stream 36 overhead is
deficient with methyl acetate, methanol, and/or acetic acid
(desirably all three) with respect to said light condensed liquid
phase 30. The higher boiling liquid phase residuum stream 38 is
enriched with methyl acetate, methanol, and/or acetic acid
(desirably all three) with respect to said second vapor phase
stream 36. Desirably, the second vapor phase stream 36 overhead is
enriched with PRC, notably acetaldehyde, with respect to the higher
boiling liquid phase residuum stream 38. The higher boiling liquid
phase residuum stream 38 can be, and preferably is, retained in the
process.
[0053] One of ordinary skill in the art having the benefit of this
disclosure can design and operate a distillation column to achieve
the desired results of the present invention. Such efforts,
although possibly time-consuming and complex, would nevertheless be
routine for one of ordinary skill in the art having the benefit of
this disclosure. Accordingly, the practice of this invention is not
necessarily limited to specific characteristic of a particular
distillation column or the operation characteristics thereof, such
as the total number of stages, the feed point, reflux ratio, feed
temperature, reflux temperature, column temperature profile, and
the like.
[0054] Further in accordance with this first embodiment of the
present invention, second vapor phase stream 36 is extracted with
water (generally indicated by 70) to remove and/or reduce PRC's,
notably acetaldehyde. Acetaldehyde is extracted by the water to
obtain aqueous acetaldehyde stream 72, which is PRC-rich, and in
particular acetaldehyde-rich. Aqueous acetaldehyde stream 72 will
generally be treated as a waste, although in some embodiments
acetaldehyde may be stripped, with the water being recirculated to
the process. The raffinate, notably containing methyl iodide is
desirably returned to the carbonylation process by stream 74. The
efficiency of the extraction will depend on such things as the
number of extraction stages and the water to feed ratio.
[0055] Extraction with water 70, in accordance with this first or
other embodiments of the present invention, can be either a
singlestage or multistage extraction and any equipment used to
conduct such extractions can be used in the practice of the present
invention. Multistage extraction is preferred. For example,
extraction 70 can be accomplished by combining stream 40 with water
and providing the combination successively to a mixer and then a
separator. Multiple such combinations of mixer and separator can be
operated in series to obtain a multistage extraction. Optionally,
and desirably, multistage extraction is accomplished in a single
vessel having a series of trays. The vessel may be equipped with
paddle(s) or other mechanisms for agitation to increase extraction
efficiency. In such a multistage extraction vessel, stream 40 is
desirably provided proximate to one end of the vessel with water
being provided proximate to the other end of the vessel or such
other location to obtain a countercurrent flow.
[0056] The mutual solubility between the two phases in the
extraction can increase with temperature. Accordingly, it is
desirable that the extraction be conducted at a combination of
temperature and pressure such that the extractor contents can be
maintained in the liquid state. Moreover, it is desirable to
minimize the temperatures to which stream 40 is exposed to minimize
the likelihood of polymerization and condensation reactions
involving acetaldehyde. Water used in the extraction 70 is
desirably from an internal stream so as to maintain water balance
within the reaction system. Dimethyl ether (DME) can be introduced
to the extraction to improve the separation of methyl iodide in the
extraction, i.e., to reduce the loss of methyl iodide into the
aqueous acetaldehyde stream 72. The DME can be introduced to the
process or formed in situ.
[0057] In accordance with a second embodiment of the present
invention, also illustrated in FIG. 1, low-boiling overhead vapor
stream 28 is condensed in decanter 16 where it is biphasically
separated to form a condensed heavy liquid phase and a condensed
light liquid phase 30. The condensed light liquid phase 30 is
provided to distillation column 18 via stream 30/32. Again, in this
and other embodiments of the present invention, a portion of stream
30 can be directed back to the light ends column 14 as reflux
stream 34. In distillation column 18, a second vapor phase stream
36 overhead and a higher boiling liquid phase residuum stream 38
are formed. A sidestream 80, comprising methyl acetate, is also
taken.
[0058] The sidestream 80 allows the distillation column 18 to be
operated under conditions desirable for obtaining a higher
concentration of acetaldehyde in second vapor phase stream 36 while
providing a mechanism for removing methyl acetate that might
otherwise build up in the center of distillation column 18 or be
pushed into the second vapor phase stream 36 overhead. The
sidestream 80, comprising methyl acetate, is preferably retained in
the process.
[0059] In this second embodiment, the second vapor phase stream 36
overhead is enriched with PRC, notably acetaldehyde, with respect
to light condensed liquid phase 30. The second vapor phase stream
36 overhead is deficient with methyl acetate, methanol, and/or
acetic acid (desirably all three) with respect to light condensed
liquid phase 30. The second vapor phase stream 36 overhead is
deficient with methyl acetate, methanol, and/or acetic acid
(desirably all three) with respect to said sidestream 80 and,
desirably, also with respect to the higher boiling liquid phase
residuum stream 38. Desirably, the second vapor phase stream 36
overhead is enriched with PRC, notably acetaldehyde, with respect
to both the sidestream 80 and the higher boiling liquid phase
residuum stream 38.
[0060] Further in accordance with this second embodiment of the
present invention, second vapor phase stream 36 is extracted with
water (generally indicated by 70) to remove residual PRC's, notably
acetaldehyde. Extraction in accordance with this second embodiment
is conducted in accordance with the extraction procedures disclosed
for the first embodiment.
[0061] Operating in accordance with the first embodiment without a
sidestream, the process has been found to achieve the following
results respecting the separation capabilities of distillation
column 18: TABLE-US-00001 Weight Percent in Weight Percent in
Weight Percent in Component Stream 30/32 Stream 36 Stream 38 Methyl
iodide 1.5 74.5 <0.1 Methyl acetate 6.0 1.4 6.1 Methanol 4.0 0.2
4.1 Acetic acid 15 <0.1 15.3 Water 73 1.6 74.5 Acetaldehyde 0.5
22.2 0.1
[0062] Operating in accordance with the second embodiment with a
sidestream, it is expected that the following results respecting
the separation capabilities of distillation column 18 can be
achieved: TABLE-US-00002 Weight Weight Weight Weight Percent in
Percent in Percent in Percent in Component Stream 30/32 Stream 36
Stream 38 Stream 80 Methyl iodide 1.5 46.8 <0.1 28.7 Methyl 4.0
0.4 1.7 60.4 acetate Methanol 1.0 <0.1 1.0 0.5 Acetic acid 15
<0.1 15.7 0.5 Water 78 0.8 81.6 7.4 Acetaldehyde 0.5 52 <0.1
2.5
[0063] This inventive process has been found to reduce and/or
remove PRC's and their precursors, multi-carbon alkyl iodide
impurities, and propionic and higher carboxylic acids from the
carbonylation process. It has also been shown that acetaldehyde and
its derivatives are reduced and/or removed by sufficient amounts
such that it is possible to keep the concentration of propionic
acid in the acetic acid product below about 500 parts per million
by weight, preferably below about 300 parts per million, and most
preferably below 250 parts per million.
[0064] In variations of the embodiments of the present invention,
it is important to inhibit the formation of various aldehyde
related polymers and condensation products in distillation column
18. Acetaldehyde polymerizes to form metaldehyde and paraldehyde.
These polymers generally are low molecular weight, less than about
200. Higher molecular weight polymers of acetaldehyde can also
form. These higher molecular weight polymers (molecular weight
greater than about 1000) are believed to form during processing of
the light phase and are viscous and thixotropic. Acetaldehyde can
also undergo undesirable aldol condensation reactions.
[0065] The formation of these impurities, i.e., metaldehyde and
paraldehyde and higher molecular weight polymers of acetaldehyde,
can be suppressed by introducing into distillation column 18 a
flush stream containing at least water and/or acetic acid.
[0066] While the invention has been described with reference to the
preferred embodiments, obvious modifications and alterations are
possible by those skilled in the related art having the benefits of
this disclosure. Therefore, it is intended that the invention
include all such modifications and alterations to the full extent
that they come within the scope of the following claims or the
equivalents thereof.
* * * * *