U.S. patent application number 12/737767 was filed with the patent office on 2011-06-09 for process and apparatus for recovery of acetic acid from a feed stream containing the corresponding ester.
Invention is credited to Polly Pai-Yu Chiang, Tze-Tang Karl Chuang, Ji-young Jang, Wu Kuang-Yeu.
Application Number | 20110137080 12/737767 |
Document ID | / |
Family ID | 41668602 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110137080 |
Kind Code |
A1 |
Kuang-Yeu; Wu ; et
al. |
June 9, 2011 |
PROCESS AND APPARATUS FOR RECOVERY OF ACETIC ACID FROM A FEED
STREAM CONTAINING THE CORRESPONDING ESTER
Abstract
An apparatus and process for recovery of a carboxylic acid e.g.
acetic acid, from an aqueous feed stream containing the
corresponding ester, an alcohol and a small amount of water, and in
some cases the carboxylic acid includes a catalytic distillation
column containing an acidic catalyst and a distillation column. The
alcohol is catalytically dehydrated to the corresponding ether and
water, and the water reacts with the ester to generate a liquid
carboxylic acid rich product stream. The acid is recovered by
distillation in the distillation column. In a second embodiment,
additional methanol and/or water are co-fed with the feed or fed
directly to the catalytic distillation column, resulting in a
liquid bottoms product stream of substantially pure acetic acid and
a tops distillate stream of substantially pure ether.
Inventors: |
Kuang-Yeu; Wu; (Plano,
TX) ; Chiang; Polly Pai-Yu; (Dallas, TX) ;
Jang; Ji-young; (McKinney, TX) ; Chuang; Tze-Tang
Karl; (Edmonton, CA) |
Family ID: |
41668602 |
Appl. No.: |
12/737767 |
Filed: |
August 11, 2009 |
PCT Filed: |
August 11, 2009 |
PCT NO: |
PCT/CA2009/001108 |
371 Date: |
February 14, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61088468 |
Aug 13, 2008 |
|
|
|
Current U.S.
Class: |
562/608 ;
202/154 |
Current CPC
Class: |
Y02P 20/10 20151101;
B01D 3/009 20130101; C07C 51/44 20130101; C07C 51/42 20130101; Y02P
20/127 20151101; C07C 51/09 20130101; C07C 51/09 20130101; C07C
53/08 20130101; C07C 51/44 20130101; C07C 53/08 20130101 |
Class at
Publication: |
562/608 ;
202/154 |
International
Class: |
C07C 51/44 20060101
C07C051/44; B01D 3/14 20060101 B01D003/14 |
Claims
1. An apparatus for recovery of acetic acid from an aqueous feed
stream containing methyl acetate and methyl alcohol and a small
amount of water, and in some cases acetic acid, by catalytic
distillation of the aqueous stream to form acetic acid and dimethyl
ether, the apparatus comprising a catalytic distillation column
containing an acidic catalyst for acid catalyzed chemical
reactions, and a recovery distillation column for recovery of
acetic acid as a liquid bottoms product stream.
2. An apparatus according to claim 1, wherein the catalytic
distillation column includes a top portion, a middle portion
including a reactive zone containing the acidic catalyst and a
bottom portion.
3. An apparatus according to claim 2, wherein the acidic catalyst
is any one of several acidic resin catalysts suitable for use in
the catalytic hydration of methyl acetate, selected from the acidic
forms of commercially available resins, containing SO.sub.3H+
reactive sites attached to a styrene divinyl benzene copolymer.
4. An apparatus according to claim 3, additionally comprising a
first condenser for condensing a portion of vapors exiting the top
portion, from which a fraction is recycled to the catalytic
distillation column and another fraction comprising substantially
pure dimethyl ether is recovered as first volatiles product.
5. An apparatus according to claim 4, additionally comprising a
first reboiler to reboil a first liquid bottoms product stream
exiting the bottom portion of the catalytic distillation column to
produce a first fraction for recycle thereto, and another fraction
comprising acetic acid and methyl acetate for removal and directing
to the recovery distillation column.
6. An apparatus according to claim 5, wherein the recovery
distillation column includes a top portion, a middle portion and a
bottom portion, and additionally comprising a second condenser for
condensing vapors exiting the top portion, from which a fraction is
recycled to the recovery distillation column and another fraction
comprising dimethyl acetate is recovered as second volatiles
product, and a second reboiler to reboil a second liquid bottoms
product stream exiting the bottom portion of the recovery
distillation column, to produce a fraction for recycle thereto, and
another fraction is recovered as substantially pure acetic
acid.
7. An apparatus for recovery of acetic acid from an aqueous feed
stream containing methyl acetate and methyl alcohol and a small
amount of water, and in some cases acetic acid, by catalytic
distillation of the aqueous stream to form acetic acid and dimethyl
ether, the apparatus comprising a catalytic distillation column
containing an acidic catalyst for acid catalyzed chemical
reactions, and means for addition of a supplemental amount of
methanol and/or water, either to the feed stream before it is fed
into the catalytic distillation column, or directly into the column
at an appropriate location on the column, to maintain optimum
operation independent of the composition of the ester containing
feed stream from which the acetic acid is to be recovered, to
provide a volatile tops product stream that is substantially pure
dimethyl ether, and a liquid bottoms product stream that is
substantially pure acetic acid.
8. An apparatus according to claim 7, additionally comprising a
feed line for supplying the feed stream to the catalytic
distillation column at a location on the column, closely below the
middle portion.
9. A process for the recovery of acetic acid from an aqueous feed
stream containing methyl acetate, methyl alcohol and a small amount
of water and in some cases acetic acid, comprising (a) supplying
the aqueous feed stream to a catalytic distillation column
containing an acidic catalyst, wherein in one reaction the alcohol
is dehydrated to produce dimethyl ether and water, and in another
reaction the water is used to hydrolyze the methyl acetate to
methyl alcohol and acetic acid, wherein both reactions proceed
concurrently in reversible equilibrium, to produce a liquid bottoms
product stream containing the acid exiting the catalytic
distillation column, and (b) transferring the liquid bottoms
product stream to a recovery distillation column from which
substantially pure acetic acid is recovered as a liquid bottoms
product stream.
10. A process for the recovery of acetic acid from an aqueous feed
stream containing methyl acetate, methyl alcohol and a small amount
of water and in some cases acetic acid, comprising (a) supplying
the aqueous feed stream to a catalytic distillation column
containing an acidic catalyst, wherein in one reaction the alcohol
is dehydrated to produce dimethyl ether and water, and in another
reaction this water is used to hydrolyze the methyl acetate to
methyl alcohol and acetic acid, wherein both reactions proceed
concurrently in reversible equilibrium, and (b) adding a
supplemental amount of methanol and/or water, either to the feed
stream before it is fed into the catalytic distillation column, or
directly into the column at an appropriate location on the column,
to maintain optimum operation of the process independent of the
composition of the ester containing feed stream from which the
acetic acid, is to be recovered, to provide a volatile tops product
stream that is substantially pure dimethyl ether, and a liquid
bottoms product stream that is substantially pure acetic acid.
11. A process according to claim 10, wherein the acidic catalyst is
any one of several acidic resin catalysts suitable for use in the
catalytic hydration of methyl acetate, selected from the acidic
forms of commercially available resins, containing SO.sub.3H+
reactive sites attached to a styrene divinyl benzene copolymer.
12. A process according to claim 11, wherein the aqueous feed
stream is a stream produced at a facility for manufacturing at
least one product from among polyvinyl alcohol and derivatives of
polyvinyl alcohol.
13. A process according to claim 10, wherein dimethyl ether is
continuously removed from the system so that substantially all
methyl species in the feed stream are converted to dimethyl ether
and thereby removed from the reaction mixture, and the liquid
product stream is substantially pure acetic acid.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to recovery of a carboxylic
acid from an aqueous feed stream containing the corresponding ester
and alcohol, and in particular to recovery of acetic acid from an
aqueous feed stream containing methyl acetate and methanol, from a
polyvinyl alcohol production process.
BACKGROUND OF THE INVENTION
[0002] Several industrial processes produce streams containing one
or more esters of carboxylic acids, RO.CO.R', where R and R' are
selected from among hydrocarbyl radicals or substituted hydrocarbyl
radicals. Examples of such industrial processes include those for
production of vinyl alcohol polymers or terephthalic acid.
Carboxylic acid esters can be hydrolyzed to generate the
corresponding acid and alcohol, as shown in Equation 1. Equation 1
is an equilibrium reaction and requires an excess of water to drive
the reaction well to the right hand side.
RO.CO.R'+H.sub.2OROH+HO.CO.R' [1]
[0003] Processes for production of polyvinyl alcohol (PVA) and its
derivatives are described by Marten in "Vinyl Alcohol Polymers" in
Kirk-Othmer Encyclopedia of Technology, John Wiley & Sons, Inc.
A variety of vinyl ester monomers can be polymerized to form a
polymer, of which polyvinyl acetate (PVAc) is preferred. PVAc then
is further reacted to manufacture PVA. Commonly, PVAc is reacted
with methanol (MeOH) to form PVA and methyl acetate (MeOAc). The
components of the polymerization reaction mixture are continuously
separated. Unreacted monomer can be stripped from the reactor
using, for example, methanol vapor. The overhead fraction from the
stripper comprises a mixture of vinyl ester monomer and at least
one solvent such as methanol. The vinyl ester monomer is then
extracted for recycle to the polymerization reactor. In the
production of polyvinyl alcohol (PVA) MeOAc is produced as a
by-product at a ratio of 1.68 tons of MeOAc per ton of PVA.
[0004] One outlet stream typically comprises a mixture including
MeOAc, MeOH and a small amount of water. The weight ratio of these
components varies over a range of relative concentrations, among
which a typical composition is approximately 75% MeOAc, 23% MeOH
and 2% water. Among these components MeOH and water have relatively
low value when compared to the values of MeOAc and acetic acid
(HOAc).
[0005] The MeOH and MeOAc can be distilled off and, at the same
time, water can be added in order to obtain an aqueous PVA
solution. However, there are disadvantages to this approach. The
resulting PVA suspension is fine, difficult to filter, and so the
process is uneconomical. Further, this approach requires
time-consuming, energy intensive and hence expensive distillation
of large amounts of solvents requiring a plurality of distillation
columns. Several approaches have been undertaken to improve the
chemical efficiency and economics of processes for production of
PVA. In particular, efforts have been directed to recycle of
solvents and processing of the outlet streams to recover valuable
by-products.
[0006] Kowaka et al. in U.S. Pat. No. 6,743,859 issued in 2004
describe a method for production of high-strength PVA with a high
degree of saponification. The apparatus for the process of '859
includes an outlet line for recovery of MeOH and MeOAc identified
in FIGS. 1 through 3 by the reference numeral 7, however no details
are presented for the process for the separation and recovery of
those components.
[0007] Bauer et al. in U.S. Pat. No. 6,576,720 issued in 2003
describe an alternative approach in which a liquid phase comprising
MeOH, MeOAc and HOAc is recycled for use in further polyvinyl ester
transesterification. The process of '720 can be used for other
alcohols and esters. The mixture of alcohol and corresponding ester
is recycled for use as the polymerization reaction medium. Make-up
comprising one or both of MeOH and HOAc is provided to maintain the
composition of the recycle mixture. The PVA is produced and
isolated using saponification with KOH and then neutralization,
preferably with a strong acid such as HCl. Thus water, less than 1%
by weight in the initial reaction mixture, is produced by both the
reaction of MeOH and HOAc and the neutralization process.
[0008] MeOAc may be sold or further hydrolysed to recover HOAc.
[0009] Kim et al. in U.S. Pat. No. 5,770,770 issued in 1998
describe a reactive distillation process for the well known
equilibrium reaction 2 for the recovery of MeOH and HOAc from
catalytic hydrolysis of MeOAc. Reaction 2 is a specific example of
the type of reaction shown in Equation 1.
MeOAc+H.sub.2OMeOH+HOAc [2]
[0010] It has long been recognized that this reaction could be used
to recover HOAc from MeOAc from a PVA manufacturing process as
described, by example, by Adelman et al. in U.S. Pat. No. 4,352,940
issued in 1982. It also was recognized in '940 that it was
necessary to minimize the amount of water used in the process to
reduce the costs of recovery and re-use of the products from the
reaction in the PVA manufacture process. However, when a minimum
amount of water is used, the equilibrium reaction 2 lies to the
left hand side. Reaction 2 can be driven to the right hand side
only by continuous removal of at least one of the products.
[0011] In an alternative approach for treatment of the PVA
manufacture outlet stream, MeOAc can be separated from the mixture
using extractive distillation. One example of this method is
described by Xiao et al. in Chemical Engineering Science, volume
56, pages 6553-6562 (2001). In the first column, water is added to
the liquid stream from the PVA plant. The volatiles from the first
column are then hydrolyzed in a fixed bed reactor containing a bed
of an acidic catalyst which catalyzes the hydrolysis of MeOAc to
MeOH and HOAc. The effluent stream from the fixed bed reactor is
distilled in a second distillation column to provide a volatiles
stream and a bottoms stream. The volatiles from the second column
are recycled for mixing with further MeOAc feed to the first
extractive distillation column. The bottoms from the second column
are separated into a water rich stream and a HOAc rich stream by
distillation in a third column. The bottoms from the first column
are separated by distillation in a fourth column into a water rich
stream and a MeOH rich stream. Thus the overall process for
recovery of HOAc requires four distillation columns and a fixed bed
reactor. Further, to drive well to the right hand side the well
known catalytic MeOAc hydrolysis equilibrium reaction shown as
Equation 2, it is necessary to use a large amount of water. Thus
the process is energy intensive as that water must be volatolized
in both the second and third columns.
[0012] Each of the above processes requires use of a plurality of
columns and reactors to react, separate and recover the components
of the stream from the PVA manufacturing reactor. Consequently,
capital and operating costs are high. Further, when water is added,
either as reagent or for extractive distillation, that water must
also be separated, which is a costly and time consuming
feature.
[0013] Hoyme et al. in U.S. Pat. No. 6,518,465 issued in 2003
describe another concept based process, derived from simulations
using the commercial available program Aspen Plus, in which the
stream containing MeOAc from PVA manufacture is reacted in a
reactive distillation column to produce DME and HOAc. Water was
added to hydrolyze MeOAc and thereby generate HOAc which is
recovered. The molar ratio of water in the process stream is
between 0.05% and 20%, and preferably is between 0.3% and 3%. In
this process it is recognized that methanol also may react to
generate dimethyl ether (DME) and water in the also well known acid
catalyzed equilibrium reaction shown in Equation 3. The process of
Hoyme et al. in '465 is basically hydrolysis of MeOAc to HOAc by
addition of water.
2MeOHMe.sub.2O+H.sub.2O [3]
SUMMARY OF THE INVENTION
[0014] It is an object of the present invention to provide an
apparatus and process for separation and recovery of a carboxylic
acid e.g. acetic acid (HOAc) produced by hydrolysis of the
corresponding ester, in the exemplary case methyl acetate (MeOAc),
in an aqueous feed stream containing the ester and alcohol e.g.
methyl alcohol, from a reactor in which polyvinyl alcohol (PVA) is
produced.
[0015] According to a first embodiment of the invention, an
apparatus is provided for recovery of a carboxylic acid e.g. HOAc,
from an aqueous feed stream containing the corresponding ester e.g.
MeOAc and alcohol e.g. MeOH and a small amount of water, and in
some cases the acid e.g. HOAc, by catalytic distillation of the
aqueous stream to form a carboxylic acid e.g. HOAC and the
corresponding ether e.g. DME, the apparatus comprising a catalytic
distillation column containing an acidic catalyst and a
distillation column for carboxylic acid e.g. HOAc recovery as a
liquid bottoms product stream.
[0016] In a second embodiment of the apparatus, means is provided
for addition of a supplemental amount of methanol and/or water,
either to the feed stream before it is fed into the catalytic
distillation column, or directly into the column at an appropriate
location on the column, to maintain optimum operation of the
process independent of the composition of the ester containing feed
stream from which the carboxylic acid e.g. acetic acid, is to be
recovered, to provide a volatile tops product stream that is
substantially pure DME, and a liquid bottoms product stream that is
substantially pure acetic acid. In this case, there is no need for
a distillation column to separate acetic acid from the liquid
bottoms product stream.
[0017] According to another aspect of the first embodiment of the
present invention a process is provided for recovery of a
carboxylic acid e.g. HOAc, from an aqueous feed stream containing
the corresponding ester e.g. MeOAc and alcohol e.g. MeOH and a
small amount of water and in some cases the acid e.g. HOAc,
comprising
(a) supplying the aqueous feed stream to a catalytic distillation
column containing an acidic catalyst, wherein, in one reaction the
alcohol is dehydrated to produce the corresponding ether and water,
and in another reaction this water is used to hydrolyze the ester
to the corresponding alcohol and acid, wherein both reactions
proceed concurrently in reversible equilibrium, to produce a
bottoms liquid product stream containing the acid exiting the
catalytic distillation column, and (b) transferring the bottoms
product stream to a distillation column from which substantially
pure acid is recovered as a liquid bottoms product stream.
[0018] According to the process aspect of the second embodiment of
the invention, a supplemental amount of methanol and/or water is
added, either to the feed stream before it is fed into the
catalytic distillation column, or directly into the column at an
appropriate location on the column, to maintain optimum operation
of the process independent of the composition of the ester
containing feed stream from which the carboxylic acid e.g. acetic
acid, is to be recovered, to provide a volatile tops product stream
that is substantially pure DME, and a liquid bottoms product stream
that is substantially pure acetic acid.
[0019] By way of further explanation, in the process according to
the invention, in the catalytic distillation column, reactive
distillation in the presence of an acidic catalyst, is used for
conversion of an aqueous feed stream containing MeOAc-MeOH and a
small amount of water, and in some cases HOAc, respectively to HOAc
and DME, through a two-step concurrent reaction process. One
reaction involves the reversible equilibrium dehydration of MeOH to
form DME and water. The other reaction employs this water to react
with MeOAc in the reversible equilibrium hydrolysis reaction that
produces MeOH and HOAc. The MeOH produced during hydrolysis of
MeOAc is then able to participate further in MeOH dehydration to
produce more water. In effect, the two reactions are "self-feeding"
and can be visually represented as follows:
##STR00001##
[0020] A significant benefit of the reactive distillation process
of the first embodiment of the invention is that no additional
water is added as feed and the process continues as long as there
is water and/or MeOH in the column to react. Once the MeOAc-MeOH
azeotrope has been broken, any further separation of MeOAc and HOAc
in a liquid bottoms product stream becomes a matter of simple
distillation.
[0021] A significant benefit of the second embodiment is that a
distillation column is not required to separate the HOAc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] For a more complete understanding of the present invention
and for further description of objects and advantages thereof,
reference is made to the following accompanying drawings in
which.
[0023] FIG. 1 is a schematic diagram of the apparatus according to
the invention including a catalytic distillation column and a
distillation column.
[0024] FIG. 2 is a graph showing the effect of changing pressure on
the temperature profile in the catalytic distillation column.
[0025] FIG. 3 is a graph showing the effect of excess water on
product distribution in the CD column.
[0026] FIG. 4 is a graph showing the effect on product distribution
in the CD column when there is no water content in the feed.
[0027] FIG. 5 is a graph showing the effect on the composition
profile when the MeOAc-MeOH feed is in molar ratio 0.55; RR=1.8,
D/F=0.475, T=120.degree. C. and P=150 psi.
[0028] FIG. 6 is a schematic diagram showing the apparatus
according to the invention for experiments to determine the effect
of mixture composition on production of DME.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The following description comprises data obtained through
laboratory experiments and simulations using ASPEN PLUS.RTM.
software. However, in order to perform the simulations and thereby
correlate experimental data with theory, it was necessary to obtain
data on the physical and chemical equilibria occurring in the
respective reaction mixtures. The data for individual reactions 2
and 3 are available in the open literature. However, no physical
and chemical kinetic data were available for mixtures comprising
all five components involved. To obtain accurate simulation
results, experiments were performed to obtain kinetic data for
reactions among combinations of all five components. The following
brief descriptions provide background data available and the
measures taken to determine the equilibria parameters.
[0030] As a consequence of having experimental data on physical and
chemical equilibria that were hitherto unavailable to those skilled
in the art, the construction, experimental demonstration and
modeling of the process comprising the invention described herein
have higher reliability than those described elsewhere. For the
first time, we show accurately a process comprising the two
concurrent continuous reversible equilibrium reactions occurring in
a single reactor, one generating water and DME from MeOH and
another consuming said water by reaction with MeOAc to generate
both MeOH and HOAc. Water generated in one reaction is consumed in
the other, and MeOH generated in the latter reaction is consumed in
the former.
[0031] We are for the first time able to compute fully the roles
and impacts of each of the components in the reaction mixture,
experimentally verify the computed data, and so establish operating
parameters so that in the first embodiment of the invention: [0032]
there is essentially no water generated in the overall process, and
substantially all water is consumed when there is excess MeOAc to
undergo hydrolysis; [0033] DME is recovered as a substantially pure
volatile distillate in the overhead; [0034] a liquid bottoms stream
that is rich in HOAc is generated in the catalytric distillation
column for transfer to a distillation column for recovery of
essentially anhydrous substantially pure HOAc, separately from
recovery of DME; and [0035] the process has high overall energy
efficiency, as there is no requirement for consumption of energy to
remove added high concentrations of water.
[0036] Accordingly, the process of the present invention offers
advantages over those described in the prior art.
[0037] We will now show that in the first embodiment of the
invention, components of the liquid bottoms product stream from a
single catalytic distillation column reactor in which the two
reversible equilibrium reactions are occurring, include a much
higher concentration of HOAc than the feed stream, along with MeOAc
and smaller amounts of additional components. For the overall
process to operate with high energy efficiency and generate
substantially pure HOAc, it is necessary that the liquid bottoms
from the catalytic distillation column are transferred to a
separate simple distillation column.
[0038] In the second embodiment of the invention, using either a
batch reactor or a continuous reaction system, if the feed
composition is adjusted through addition of a supplemental amount
of methanol and/or water, either to the feed stream or to the
catalytic distillation column, substantially pure HOAc can be
produced in a single catalytic distillation column because DME is
very volatile and can be completely removed from the feed stream
reaction mixture. Accordingly, it is a feature of the second
embodiment of the invention that all methyl species are converted
when DME is completely removed as a distillate in a tops product
stream by catalytic distillation, with the consequence that the
liquid bottoms stream is substantially pure HOAc. In this case, a
distillation column is not required.
Physical Equilibria:
[0039] An important consideration in any reactive distillation
simulation is the choice of physical equilibrium model and the
ability to reliably predict multicomponent vapour-liquid equilibria
(VLE) and liquid-liquid equilibria (LLE). Reliable VLE and LLE are
needed to establish distillation boundaries and to determine if and
where azeotropes and phase separations occur. There exists an
abundance of patchwork/piecemeal phase equilibrium data in the open
literature on the multicomponent mixture of
MeOAc-MeOH-water-HOAc-DME and the respective subsystems. The
various equations used to model these systems take into account the
non-ideality of the vapor phase (due to dimerization of HOAc).
Chemical Equilibria:
[0040] The two concurrent reversible reactions considered in the
process are the hydrolysis of MeOAc and the dehydration of MeOH.
The hydrolysis of MeOAc with water over an acid catalyst produces
HOAc and MeOH (Equation 2). Reaction 2 is equilibrium limited with
a reported equilibrium constant of 0.13 at 25.degree. C. (Ge et
al., "Kinetics of Heterogeneous Hydrolysis of Methyl Acetate",
Chemical Reaction and Engineering and Technology (Chinese Journal),
Vol. 14, No. 2, 1998, pp 138-144). The dehydration of methanol,
consumes two moles of methanol to produce one mole of DME and one
mole of water (Equation 3).
[0041] The equilibrium constant has been reported to range from
68.4 to 52.3 in the temperature range from 85.degree. C. to
115.degree. C. (Nisoli et al., "Attainable Regions for Reaction
with Separation" in AIChE Journal, Vol. 43, No. 3 (2), 1997, pp
374-387). The overall reaction is given by:
MeOAc+MeOHDME+HOAc (4)
[0042] Because water is both consumed and produced
stoichiometrically, it drops out of the overall reaction
equation.
The Invention:
[0043] An exemplary embodiment of the invention, which is
non-limiting, will now be described with reference to FIG. 1
through FIG. 5. By way of example, the apparatus and process
parameters described for the first embodiment will be presented
with reference to recovery of HOAc by hydrolysis of MeOAc from a
feed stream mixture fed to the apparatus from a PVA manufacturing
facility. The feed stream (mixture) typically includes MeOAc, MeOH
and water, although it could also contain some HOAc. If and when
some HOAc is included in the feed stream, some impact in the
equilibrium reactions described above would occur. It is noted that
the spec of feed streams at PVC plants does not include HOAc.
However, since HOAc is used extensively at these plants, it is
therefore possible that some streams may be contaminated with
HOAc.
[0044] It will be recognized that the method and the principles of
operation of the apparatus will apply to recovery of other lower
carboxylic acids from other mixtures, when the operating parameters
are amended according to the properties of those carboxylic acids
and the esters from which they are recovered.
[0045] Referring to FIG. 1, an apparatus 10 includes a catalytic
distillation column 12 and a distillation column 14. The term
"catalytic distillation" as used herein refers to the concurrent
reversible equilibrium chemical reactions of the components within
a feed stream mixture, at least one reaction occurring in the
column being catalyzed by a catalyst.
[0046] Catalytic distillation column 12 has a top portion
("rectifying zone") 18, a middle portion ("reactive zone") 20 and a
bottom portion ("stripping zone") 22. A fixed bed 24 containing an
acidic catalyst 26 is situated within middle portion 20 of
catalytic distillation column 12.
[0047] When, for example, a feed 30 is an MeOAc rich stream from a
chemicals manufacturing facility (not illustrated) such as a PVA
manufacturing facility, a feed line 28 carries feed 30 to a
location 32 on the catalytic distillation column, closely below
fixed bed 24. A first condenser 34 is used to condense a portion of
vapors in top portion 18, from which a fraction is recycled to
distillation column 12 and another fraction is recovered as first
volatiles product 44 via a first volatiles outlet line 36. A first
reboiler 38 is used to reboil a portion of first liquid bottoms 40
exiting bottom portion 22 for recycle to catalytic distillation
column 12, and another portion of first liquid bottoms 40 is
removed via a first liquid bottoms outlet line 42.
[0048] Acidic catalyst 26 can be one or more of several different
acidic catalysts. It has been found through experiments that any
one of several acidic resin catalysts is suitable for use in the
catalytic hydration of MeOAc according to Equation 2, including but
not limited to use of the acidic forms of commercially available
resins, Amberlyst.RTM.15, 35 or 37. Amberlyst.RTM. catalysts are
widely used for acid catalyzed reactions. They contain SO.sub.3H+
reactive sites attached to a styrene divinyl benzene copolymer.
[0049] Distillation column 14 has a top portion 50, a middle
portion 52, a bottom portion 54, a second condenser 56 and a second
reboiler 58. The another portion of first liquid bottoms 40 from
catalytic distillation column 12 is fed via first liquid bottoms
outlet line 42 to a position 60 approximately midway up middle
portion 52 of distillation column 14. The optimum position 60 for
connection of first liquid bottoms outlet line 42 and distillation
column 14 has been determined experimentally using modeling and
predicted operating performance data have been confirmed
experimentally, as described in the EXAMPLES below.
[0050] Second condenser 56 condenses a portion of vapors in top
portion 50 for recycle to distillation column 14, and another
portion is recovered as a second volatiles product 61 via a second
volatiles outlet line 62. Second reboiler 58 reboils a portion of a
second liquid bottoms 64 in bottom portion 54 of distillation
column 14, and another portion of second liquid bottoms 64 is
recovered via a second liquid bottoms outlet line 66.
[0051] When the chemicals processing facility manufactures one or
more of PVA and derivatives of PVA, feed 30 is a stream from the
manufacturing facility typically comprising MeOAc, MeOH and water.
It is desirable to recover acetic acid, which may be recycled for
use in manufacture of vinyl acetate which in turn can be
polymerized for manufacture of PVA. The ether formed from
dehydration of MeOH is DME and it is recovered as first volatiles
product 44. DME can be recovered for sale or for other use.
[0052] The second volatiles product 61 from distillation column 14
is much richer in MeOAc than was feed 30, and this stream can be
recovered for sale or recycled to distillation column 12.
[0053] The method of use of apparatus 10 will now be illustrated
with reference to FIGS. 3 through 5, using as the example reaction
hydrolysis of MeOAc for recovery of HOAc.
[0054] The composition of feed 30 from a PVA manufacturing facility
is shown in Table 3. Also in Table 3 are the compositions, for one
exemplary set of operating parameters, of that portion of first
volatile products 44 recovered via first volatiles outlet line 36
and that portion of first liquid bottoms 40 removed via first
liquid bottoms outlet line 42.
[0055] Laboratory experiments, described in EXAMPLE 1 below, showed
that the rate of dehydration of MeOH to form DME (Equation 3) is
affected by the concentrations of both water and MeOH in the
aqueous feed stream mixture 30 initially comprising
MeOH-water-HOAc-MeOAc. Thus, to maintain efficient operation, it is
desirable that the mixture fed to catalytic distillation column 12
has an optimum composition profile.
[0056] The second embodiment of the present invention enables more
efficient operation of the process independent of variations that
may occur from time to time in the composition of feed 30 before it
is fed via feed line 28 into catalytic distillation column 12,
illustrated in FIG. 1. At least one additional feed line (not
illustrated) is provided that independently can feed additional
amounts of one or both of methanol and water to catalytic
distillation column 12. Depending on the composition of feed 30,
additional methanol and/or water may be added into feed line 28
before feed 30 or at appropriate locations on catalytic
distillation column 12. We found that the consequence of ensuring a
substantially optimum concentration profile of both methanol and
water along the catalytic distillation column 12 is that the
operation of the process occurring therein is production of
substantially pure DME and HOAc, without the need for a
distillation column, so the operating cost for the process is
minimized.
[0057] A benefit from use of the second embodiment of the present
invention is that the apparatus is more versatile. The capability
to provide a supplemental amount of one or both of methanol and
water in addition to feed 30 can be used for recovery of
substantially pure HOAc from a variety of different industrial
streams containing the corresponding hydrolysable ester.
[0058] Another benefit arising from use of apparatus 10 for the
second embodiment of the present invention is that the process can
be operated so as to produce a stream of substantially pure DME and
HOAc as products. In this embodiment, DME is continuously removed
as a volatile tops stream from apparatus 10 so that substantially
all methyl species are converted to DME and are thus removed from
the system, with the consequence that first liquid bottoms 40
comprises substantially pure HOAc.
[0059] The following EXAMPLES will illustrate applications of the
apparatus and method for recovery of HOAc according to the
invention. Each set of conditions has been modeled using ASPEN.RTM.
software and results from modeling have been confirmed through
experiment.
EXAMPLES
Example 1
Batch Distillation
[0060] Laboratory experiments were conducted to confirm literature
data and to obtain additional insights into the two catalytic
reactions for the formation of DME and HOAc, using an apparatus 100
illustrated in FIG. 6. Apparatus 100 for conducting batch
non-equilibrium experimental reactions comprised a reaction vessel
102, a condenser 104 and a line 106 for recovery and analysis of a
DME containing stream 108. The feed 110 comprised several different
mixtures of MeOH, water and MeOAc in different ratios. Each
reaction mixture 110 and catalyst 112 were mixed in reaction vessel
102 and refluxed. The amount of the DME containing stream 108
exiting via line 106 was determined gravimetrically and its
composition was determined using gas chromatography. Using
apparatus 100 it was found that the rate of DME and HOAc formation
depended upon the composition of reaction mixture 110 as well as
the concentration of catalyst 112 therein. In particular, it was
found that the formation of DME depended on the amounts of water
and MeOH present in reaction mixture 110. The data so obtained were
then used to design and simulation of the process.
[0061] Using the system demonstrated in these laboratory
experiments and illustrated in FIG. 6, that all methyl groups from
MeOAc can be removed as DME, thus producing a liquid product rich
in HOAc, which remains in the reaction vessel 102. In this further
embodiment of the process, sufficient MeOH and/or water are added
to reaction mixture 110 so as to hydrolytically cleave all methyl
groups from MeOAc. The methyl groups are removed as DME by
volatilization. The products formed are a liquid product rich in
HOAc and a volatile distillate product rich in DME. In this case, a
distillation column is not needed to recover acetic acid.
Example 2
Model Batch Catalytic Distillation
[0062] To examine the reliability and effectiveness of the physical
and reaction equilibrium models, a batch catalytic distillation
simulation was performed and compared to experimental data
collected in the lab.
[0063] For the experimental test, a batch Parr reactor was loaded
with a 150 g mixture of MeOAc/MeOH feed in a 75:25 weight ratio,
along with 30 g of previously dried acid catalyst (Amberlyst
35.RTM.). The top of the batch reactor was modified with a
stainless steel condenser, which allowed the most volatile
component (i.e., DME) to escape. The condenser was cooled with cold
water; GC analysis of the off-gas from the condenser confirmed that
>99% DME was leaving the reactor. The reactor was operated at
120.degree. C. and 150 psig over a period of 13 hours until
equilibrium had been reached. It was determined that equilibrium
was established based on vapor and liquid samples from the reactor
which showed constant concentration over several samples. Vapor and
liquid samples were measured on an HP.RTM. 6890 GC equipped with
capillary column and TCD. A two-stage equilibrium reactor with
distillation column was sufficient to model the batch distillation
process in Aspen Plus.RTM.. Table 1 shows the liquid composition
for the batch catalytic distillation simulation and compares it to
experimental batch catalytic distillation results.
TABLE-US-00001 TABLE 1 Liquid product composition for batch
catalytic distillation experiment and simulation at T = 120.degree.
C. and P = 150 psi for MeOAc/MeOH mass feed ratio of 75:25.
Simulation Experimental "Product" stream Product stream Component
Weight % Weight % MeOAc 58.5 69.25 MeOH 2.1 2.87 DME 9.8 3.5 Water
4.0 3.31 HOAc 25.7 21.06
[0064] Comparison of data from the simulation and experimental
results confirms that the predictions from our model are accurate
in terms of reaction rate and equilibrium constants.
Example 3
Simulation of Continuous Catalytic Distillation Column
[0065] The continuous catalytic distillation process consists of a
20 stage CD column with total condenser and partial reboiler. The
parameters for the base case simulation are given in Table 2. All
feed streams enter at 25.degree. C.
TABLE-US-00002 TABLE 2 Parameter values for the base case used in
Aspen Plus .RTM. simulations Parameter Value Column pressure (atm)
5 Distillate to feed ratio (D/F) 0.45 Reflux ratio (RR) 2.1 Total
number of stages 20 Reaction stage location 3 to 16 (inclusive)
Feed stage location (above stage) 12 Feed composition (mass basis):
MeOAc 0.75 MeOH 0.23 H.sub.2O 0.02
Example 4
Effect of Pressure
[0066] Operating pressure is one of the key elements in the design
of a CD column. The choice of operating pressure for a catalytic
distillation column depends on many considerations such as the
overhead temperature, bottom temperature, reaction temperature and
relative volatilities of the components in the system. The column
pressure sets the lower and upper bounds of the temperature within
the column. For the present process the operating pressure is
chosen within a range such that water can be used as a coolant for
the overhead condenser and steam can be used as a heating medium
for the reboiler. Within this range, the reaction temperature
mainly determines the operating pressure. Because the catalytic
reactions take place within the liquid phase, the reaction
temperature is close to the boiling point of liquid flowing around
the catalyst. As a result the reaction temperature increases with
column pressure. FIG. 2 shows the effect of column pressure on the
temperature profile in the column. In these simulations the
parameters are those as in the base case, except for the reflux
ratio. As column pressure was increased, the reflux ratio was also
increased concurrently in order to achieve mathematical convergence
in the simulation. The reflux ratio was increased by the minimum
amount that would satisfy convergence of the CD column, with all
other parameters being held constant.
Example 5
Effect of Water
[0067] The amount of water in the feed is an important variable in
the CD process because water is a necessary reactant in the
hydrolysis reaction. However, water is also produced via MeOH
dehydration. Therefore, there should be an optimum amount of water
that will allow both reactions to proceed readily without
accumulating a large amount of water in the reaction mixture. The
optimum process achieves maximum conversion of reactants while at
the same time completely removing water from any of the product
streams. FIG. 3 shows the effect of excess water content of the
feed stream (relative ratio of MeOAc:MeOH in the feed remains
constant). When excess water enters as a component of the feed the
hydrolysis reaction is limited by the concentration of MeOAc. MeOAc
is completely consumed and excess water exits with HOAc in the
bottoms product.
[0068] Conversely, when no water enters in the feed, only the water
that is produced through MeOH dehydration is available for the
hydrolysis reaction. In this case DME is again the distillate
product, and a mixture of HOAc and MeOAc is now the liquid bottoms
product. All of the water and MeOH is reacted away. The resulting
profile of concentrations of all CD column reaction mixture
components is shown in FIG. 4.
Example 6
Effect of MeOAc/MeOH Ratio
[0069] Based on the overall reaction equation given by equation
(4), one might predict that the there should be very little
influence on the process arising from changes in the ratio of MeOAc
to MeOH. Increasing either MeOH or MeOAc in the feed should favour
DME and HOAc product formation. However, based on the stoichiometry
of the individual reactions, this is not necessarily true. The
effect of increasing the MeOAc/MeOH feed ratio results in complete
conversion of water and methanol in the CD column with a profile
similar to that shown in FIG. 4. When considering only the
hydrolysis reaction, one would expect that by increasing the amount
of MeOAc in the feed, water could be more easily reacted to
completion. When water is consumed and MeOH is produced, the
equilibrium for the dehydration consequently shifts to favor the
production of DME. When the MeOAc/MeOH ratio decreases below a
specific stoichiometric amount, the MeOAc reactant becomes the
limiting reactant.
[0070] Simulation results show that column parameters can be varied
to completely remove both MeOAc and MeOH from the column mixture,
and a mixture of water and HOAc remains as the bottoms product.
FIG. 5 illustrates this effect when using a MeOAc/MeOH molar feed
ratio of 0.55.
Example 7
Simulation of the Process of the First Embodiment
[0071] Catalytic distillation column 12 has 20 stages. Feed 30
comprises a mixture containing about 75% MeOAc, 23% MeOH and 2%
water, and is fed at 100 kgh.sup.-1 into stage 12 into catalytic
distillation column 12 at 100.degree. C. The pressure under which
feed 30 is supplied is 10 atm. The column operates at 5 atm. and so
feed 30 is supplied via a back-pressure regulator (not
illustrated). The reflux ratio is 2 and the distillate-to-feed
ratio is 0.45. At steady state, the temperature at stage 3, at the
top of the reaction zone is 31.degree. C. and at the bottom of the
reaction zone, stage 16, is 127.degree. C.
[0072] First volatile products 44 exiting top portion 18 of
catalytic distillation column 12 comprises almost entirely DME
(over 99%; Table 3). First liquid bottoms 40 exiting bottom portion
22 of catalytic distillation column 12 comprises about 91% HOAc and
the balance is less than 9% MeOAc, with only trace amounts of MeOH,
water and DME (Table 3).
[0073] More specifically, first volatiles product 44 comprises
close to 100% DME. First liquid bottoms 40 comprises a mixture of
about 91% HOAc, 8.9% MeOAc, 0.04% water, and traces of MeOH and
DME.
[0074] Distillation column 14 has 10 stages and has a reflux ratio
of 7 and a distillate-to-feed ratio of 0.07. First liquid bottoms
40 from catalytic distillation column 12 is fed at stage 5 into
distillation column 14 at a temperature of 170.5.degree. C. and a
pressure of 5 atm.
[0075] Second liquid bottoms 64 from distillation column 14 is
substantially pure HOAc (see Table 4). Second volatiles product 60
is rich in MeOAc and is recyclable.
TABLE-US-00003 TABLE 3 Composition of feed and product streams for
catalytic distillation column 12 in Example 7. Flow rates (kg
h.sup.-1) Feed First volatiles First liquid bottoms Component 30
product 44 product 40 MeOAc 75 0.093 5.50 MeOH 23 <0.001 0.0025
HOAc <0.001 56.26 Water 2 <0.001 0.026 DME 3.81 <0.001
TABLE-US-00004 TABLE 4 Composition of feed and product streams for
distillation column 14 in Example 7. Flow rates (kg h.sup.-1) Feed
(first Second Second liquid bottoms volatiles liquid bottoms
Component product 40) product 61 product 64 MeOAc 5.50 5.20 0.31
MeOH 0.0025 <0.002 <0.001 HOAc 56.26 0.025 56.24 Water 0.026
0.005 0.021 DME <0.001 <0.001 <0.001
REFERENCES CITED
TABLE-US-00005 [0076] U.S. Patent Documents 4,352,940 October 1982
Adelman et al. 562/607 5,770,770 June 1998 Kim et al. 562/608
6,518,465 February 2003 Hoyme et al. 568/698 6,576,720 June 2003
Bauer et al. 526/70 6,743,859 June 2004 Kowaka et al. 525/62
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