(19)
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`European
`Patent Offke
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`Office eurapéen (11)
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`EP 1 737 808 B1
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`(12)
`
`EUROPEAN PATENT SPECIFICATION
`
`(45) Date of publication and mention
`of the grant of the patent:
`04.06.2008 Bulletin 2008/23
`
`(51) Int CI.:
`C07C 51/48 (“"5-"V
`
`C07C 53/08 (“"5-”"
`
`(86) International application number:
`
`(21) Application number: 05723599.6
`
`PCT/U32005/005785
`
`(22) Date of filing: 24.02.2005
`
`(87) International publication number:
`WO 2005/087699 (22.09.2005 Gazette 2005/38)
`
`(54) REMOVAL OF PERMANGANATE REDUCING COMPOUNDS FROM METHANOL
`CARBONYLATION PROCESS STREAM
`
`ENTFERNUNG VON PERMANGANATREDUZIERENDEN VERBINDUNGEN AUS EINEM
`METHANOLCARBONYLIERUNGSVERFAHRENSSTROM
`
`ENLEVEMENT DE COMPOSES REDUCTEURS DU PERMANGANATE DU CIRCUIT DE
`CARBONYLATION DE METHANOL
`
`(84) Designated Contracting States:
`AT BE BG CH CY CZ DE DK EE ES FI FR GB GR
`HU IE IS IT LI LT LU MC NL PL PT RO SE SI SKTR
`
`(30) Priority: 02.03.2004 US 708421
`
`(43) Date of publication of application:
`03.01.2007 Bulletin 2007/01
`
`0 SCATES, Mark, 0.
`Houston, TX 77058 (US)
`0 TALANCON, Jose, J., Arturo
`Av. Universidad Km 7.5
`
`Mexico 96539 (MX)
`0 TRUEBA, David, A.
`Houston, TX 77062 (US)
`0 ZINOBILE, Raymond, J.
`Houston, TX 77058 (US)
`
`(73) Proprietor: Celanese International Corporation
`Dallas, TX 75234 (US)
`
`(72) Inventors:
`0 PICARD, Wayne, D.
`Houston, TX 77006 (US)
`
`(74) Representative: Kador & Partner
`Corneliusstrasse 15
`
`80469 Miinchen (DE)
`
`(56) References cited:
`WO-A-98/17619
`
`US-A- 6 143 930
`
`EP1737808B1
`
`Note: Within nine months of the publication of the mention of the grant of the European patent in the European Patent
`Bulletin, any person may give notice to the European Patent Office of opposition to that patent, in accordance with the
`Implementing Regulations. Notice of opposition shall not be deemed to have been filed until the opposition fee has been
`paid. (Art. 99(1) European Patent Convention).
`
`Printed by Jouve, 75001 PARIS (FR)
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`Description
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`[0001] This invention relates to an improved process
`for the removal of permanganate reducing compounds
`and alkyl iodidesformed bythe carbonylation of methanol
`in the presence of a Group VIII metal carbonylation cat-
`alyst. More specifically, this invention relates to an im-
`proved process for reducing and/or removing precursors
`of permanganate reducing compounds and alkyl iodides
`from intermediate streams during the formation of acetic
`acid by said carbonylation processes.
`
`2. Technical Background
`
`[0002] Among currently employed processes for syn-
`thesizing acetic acid, one ofthe 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
`comprises rhodium, either dissolved or othenNise dis-
`persed in a liquid reaction medium or supported on an
`inert solid, along with a ha|ogen—containing catalyst pro-
`moter as exemplified by methyl iodide. The rhodium can
`be introduced into the reaction system in any of many
`forms, and the exact nature ofthe rhodium moiety within
`the active catalyst complex is uncertain. Likewise, the
`nature ofthe halide promoteris notcritical. The patentees
`disclose avery large number of suitable promoters, most
`of which are organic iodides. Most typically and usefully,
`the reaction is conducted by continuously bubbling car-
`bon monoxide gas through a liquid reaction medium in
`which the catalyst is dissolved.
`[0003] WO 98/17619 disclosesthe removal of perman-
`ganate compounds from a carbonylation process stream.
`[0004] 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. Patent Nos. 5,001,259, issued Mar. 19,
`1991; 5,026,908, issued Jun. 25, 1991; and 5,144,068,
`issued Sep. 1, 1992; and European Patent No. EP 0161
`874 B2, published Jul. 1,1992. As disclosed therein, ace-
`tic acid is produced from methanol in a reaction medium
`containing methyl acetate, methyl halide, especially me-
`thyl iodide, and rhodium present in acatalytically effective
`concentration. These patents disclose that catalyst sta-
`bility and the productivity ofthe 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 cat-
`alytically effective amount of rhodium and at least afinite
`concentration of water, a specified concentration of io-
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`dide ions over and above the iodide content which is
`
`present as methyl iodide or other organic iodide. The
`iodide ion is present as a simple salt, with lithium iodide
`being preferred. The patents teach thatthe concentration
`of methyl acetate and iodide salts are significant param-
`eters in affecting the rate of carbonylation of methanol to
`produce acetic acid, especially at low reactor water con-
`centrations. By using relatively high concentrations ofthe
`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 con-
`centrations 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 im-
`proves the stability ofthe rhodium catalyst, i.e. resistance
`to catalyst precipitation, especially during the product re-
`covery 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 ves-
`
`sel, is a ligand with stabilizing effect on the rhodium.
`[0005]
`It has been found that although a low water car-
`bonylation process for producing acetic acid reduces
`such by—products as carbon dioxide, hydrogen, and pro-
`pionic acid, the amount of other impurities, present gen-
`erally in trace amounts, is also increased, and the quality
`of acetic acid sometimes suffers when attempts are made
`to increase the production rate by improving catalysts,
`or modifying reaction conditions.
`[0006] These trace impurities affect quality of acetic
`acid, especially when they are recirculated through the
`reaction process. The impurities that decrease the per-
`manganate time of the acetic acid include carbonyl com-
`pounds and unsaturated carbonyl compounds. As used
`herein, the phrase "carbony|" is intended to mean com-
`pounds that contain aldehyde or ketone functional
`groups, which compounds may or may not possess un-
`saturation. See Catalysis of Organic Reaction, 75,
`369-380 (1998), forfurther discussion on impurities in a
`carbonylation process.
`[0007] The present invention is directed to reducing
`and/or removing permanganate reducing compounds
`(PRC’s) such as acetaldehyde, acetone, methyl ethyl ke-
`tone, butyraldehyde, crotonaldehyde, 2—ethyl crotonal-
`dehyde, and 2—ethyl butyraldehyde and the like, and the
`aldol condensation products thereof. The present inven-
`tion also leads to reduction of propionic acid.
`[0008] The carbonyl impurities described above, such
`as acetaldehyde, may react with iodide catalyst promot-
`ers 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 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, the prod-
`uct most commonly produced from acetic acid. The
`present invention is thus also directed to removal of alkyl
`iodides, in particular C242 alkyl iodide compounds. Ac-
`
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`cordingly, because many impurities originate with acetal-
`dehyde, it is a primary objective to remove or reduce the
`acetaldehyde and alkyl iodide content in the process.
`[0009] Conventional techniques to remove impurities
`include treating the acetic acid product with oxidizers,
`ozone, water, methanol, activated—carbon, amines, and
`the like, which treatment may or may not be combined
`with distillation of the acetic acid. The most typical puri-
`fication treatment involves a series of distillations of the
`
`final product. It is known, for example, from U.S. Patent
`No. 5,783,731 to remove carbonyl impurities from organ-
`ic streams by treating the organic streams with an amine
`compound such as hydroxylamine, which reacts with the
`carbonyl compounds to form oximes, followed by distil-
`lation to separate the purified organic product from the
`oxime reaction products. However, the additional treat-
`ment of the final product adds cost to the process, and
`distillation of the treated acetic acid product can result in
`additional impurities being formed.
`[0010] While it is possible to obtain acetic acid of rel-
`atively high purity, the acetic acid product formed by the
`|ow—water carbonylation process and purification treat-
`ment described above frequently remains somewhat de-
`ficient with respect to the permanganate time due to the
`presence of small proportions of residual
`impurities.
`Since a sufficient permanganate time is an important
`commercial test, which the acid product must meet to be
`suitable for many uses, the presence of impurities that
`decrease permanganate time is objectionable. Moreo-
`ver, it is not economically or commercially feasible to re-
`move minute quantities ofthese impurities from the acetic
`acid by distillation because some of the impurities have
`boiling points close to that of the acetic acid product.
`[0011]
`It has thus become important to identify eco-
`nomically viable methods of removing impurities else-
`where in the carbonylation process without contaminat-
`ing the final product or adding unnecessary costs. U.S.
`Patent No. 5,756,836, incorporated herein by reference,
`discloses a method for manufacturing high purity acetic
`acid by adjusting the acetaldehyde concentration of the
`reaction solution below 1500 ppm.
`It is stated that by
`maintaining the acetaldehyde concentration below this
`threshold, it is possible to suppress the formation of im-
`purities such that one need only distill the crude acetic
`acid product to obtain high purity acetic acid.
`[0012] European Patent No. EP 0 487 284 B1, pub-
`lished April 12, 1995, discloses that carbonyl impurities
`present in the acetic acid product generally concentrate
`in the overhead from the light ends column. Accordingly,
`the light ends column overhead is 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 distil-
`lation, resulting in an acetic acid product with improved
`permanganate time.
`[0013]
`European Patent Application No. EP 0 687 662
`A2 and U.S. Patent No. 5,625,095, describe a process
`for producing high purity acetic acid in which an acetal-
`
`dehyde concentration of 400 ppm or less is maintained
`in the reactor by using a single or multi—stage distillation
`process to remove acetaldehyde. Streams suggested for
`processing to remove acetaldehyde include a light phase
`containing primarily water, acetic acid and methyl ace-
`tate; a heavy phase containing primarily methyl iodide,
`methyl acetate and acetic acid; an overhead stream con-
`taining primarily methyl iodide and methyl acetate; or a
`recirculating stream formed by combining the light and
`heavy phase. These references do not identify which of
`these streams possesses the greatest concentration of
`acetaldehyde.
`[0014]
`EP 0 687 662 A2 and U.S. Patent No. 5,625,095
`also disclose management of reaction conditions to con-
`trol the formation of acetaldehyde in the reactor. Although
`it is stated that formation of by—products such as croton—
`aldehyde, 2—ethylcrotonaldehyde, and alkyl iodides is re-
`duced by controlling the formation of acetaldehyde, it is
`also pointed out that management of reaction conditions
`as proposed increases the formation of propionic acid,
`an undesirable byproduct.
`[0015] More recently,
`it has been disclosed in com-
`monly assigned U.S. Patent Nos. 6,143,930 and
`6,339,171 that it is possible to significantly reduce the
`undesirable impurities in the acetic acid product by per-
`forming a multi—stage purification on the light ends col-
`umn overhead. These patents disclose a purification
`process in which the light ends overhead is distilled twice,
`in each case taking the acetaldehyde overhead and re-
`turning a methyl iodide rich residuum to the reactor. The
`acetaldehyde—rich distillate is extracted with waterto re-
`move the majority ofthe acetaldehyde for disposal, leav-
`ing a significantly lower acetaldehyde concentration in
`the raffinate that is recycled to the reactor.
`[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 al-
`ternative processes to improve the efficiency of acetal-
`dehyde removal. The present invention provides one
`such alternative solution.
`
`SUMMARY OF THE INVENTION
`
`In one aspect, the present invention provides a
`[0017]
`process for producing acetic acid that includes the fol-
`lowing steps:
`
`(a) reacting methanol and carbon dioxide in a suita-
`ble reaction medium that includes a catalyst and an
`organic iodide;
`(b) separating the products of the reaction into avol—
`atile product phase that contains acetic acid, organic
`iodide, and at least one permanganate reducing
`compound (PRC), and a less volatile phase contain-
`ing the catalyst and acetic acid;
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`(c) distilling the volatile product phase to yield a pu-
`rified product and a first overhead that contains or-
`ganic iodide, water, acetic acid, and unreacted meth-
`anol;
`(d) distilling at least a portion of the first overhead to
`produce a PRC enriched second overhead;
`(e) extracting the second overhead with water and
`separating therefrom an aqueous extract containing
`concentrated PRC’s for disposal; and
`(f) distilling at least a portion ofthe extracted second
`overhead together with the first overhead portion.
`
`Preferably, another portion of the extracted second over-
`head is recycled to the reactor.
`[0018]
`In another aspect, the present invention pro-
`vides an improved method for separating a mixture con-
`taining water, acetic acid, methyl iodide, methyl acetate,
`methanol, at least one C242 alkyl iodide and at least one
`permanganate reducing compound (PRC). The im-
`proved method includes the following steps: (a) distilling
`the mixture to form a PRC enriched overhead stream;
`(b) extracting the PRC enriched overhead stream with
`water and separating therefrom an aqueous stream con-
`taining at leastone PRC; and (c) distilling at least a portion
`of the extracted PRC enriched overhead together with
`the mixture.
`
`In still another aspect, the present invention pro-
`[0019]
`vides an improved method for reduction and/or removal
`of permanganate—reducing compounds (PRC’s) and
`C242 alkyl iodide compoundsformed in the carbonylation
`of methanol to a product of acetic acid. In the improved
`method, the methanol is carbonylated in a reaction me-
`dium containing a catalyst, and an organic iodide; the
`products ofthe carbonylation reaction are separated into
`(1) a volatile phase containing acetic acid product, or-
`ganic iodide, water, and at least one PRC, and (2) a less
`volatile phase containing the catalyst; the volatile phase
`is distilled to yield a purified product and a first overhead
`containing organic iodide, water, acetic acid, and PRC.
`The improvement includes the steps of (a) distilling the
`first overhead to form a PRC enriched second overhead
`
`stream; (b) extracting the second overhead stream with
`water and separating therefrom an aqueous stream con-
`taining PRC’s; and (c) distilling at least a portion of the
`extracted second overhead together with the first over-
`head.
`
`In particularly preferred embodiments ofthe in-
`[0020]
`vention, the second overhead or PRC enriched overhead
`contains dimethyl ether in an amount effective to reduce
`the solubility of methyl
`iodide in the aqueous extract
`stream.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0021]
`
`FIG. 1 illustrates the prior art process, as disclosed
`in U.S. Patent No. 6,339,171, forthe removal of car-
`
`bonyl impurities from an intermediate stream of the
`carbonylation process for the production of acetic
`acid by a carbonylation reaction.
`
`FIG. 2 illustrates a preferred embodiment of the
`present invention.
`
`[0022] While the invention is susceptible to various
`modifications and alternative forms, specific embodi-
`ments have been shown by way of example in the draw-
`ings 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 intendedto cover all modifications, equiv-
`alents and alternatives falling within the scope of the in-
`vention as defined by the appended claims.
`
`DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
`
`Illustrative embodiments of the invention are
`[0023]
`described below. In the interest of clarity, not all features
`of an actual implementation are described in this speci-
`fication. ltwill of course be appreciated that in the devel-
`opment of any such actual embodiment, numerous im-
`plementation—specific decisions must be made to
`achieve the developers’ specific goals, such as compli-
`ance with system—related and business—related con-
`straints, which will vary from one implementation to an-
`other. Moreover, it will be appreciated that such a devel-
`opment effort might be complex and time—consuming, but
`would nevertheless be a routine undertaking forthose of
`ordinary skill in the art having the benefit of this disclo-
`sure.
`
`[0024] The purification process of the present inven-
`tion is useful in any process usedto carbonylate methanol
`(or another carbonylatable reactant such as methyl ac-
`etate, methyl formate, or dimethyl ether) to acetic acid in
`the presence of a Group VIII metal catalyst such as rho-
`dium and an iodide promoter. A particularly useful proc-
`ess is the low water rhodium—catalyzed carbonylation of
`methanol to acetic acid as exemplified in U.S. Patent No.
`5,001 ,259. Generally, the rhodium component ofthe cat-
`alyst system is believed to be present in the form of a
`coordination compound of rhodium with a halogen com-
`ponent providing at least one of the ligands of such co-
`ordination compound. In addition to the coordination of
`rhodium and halogen, it is also believed that carbon mon-
`oxide coordinates 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, io-
`dides, etc., or other coordination compounds of rhodium,
`and the like.
`
`[0025] The halogen—promoting component of the cat-
`alyst system consists of a halogen compound comprising
`an organic halide. Thus, alkyl, aryl, and substituted alkyl
`or aryl halides can be used. Preferably, the halide pro-
`moter is present in the form of an alkyl halide in which
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`the alkyl radical corresponds to the alkyl radical of the
`feed alcohol, which is carbonylated. Thus, in the carbo-
`nylation of methanol to acetic acid, the halide promoter
`will contain methyl halide, and more preferably methyl
`iodide.
`
`[0026] The liquid reaction medium employed may in-
`clude any solvent compatible with the catalyst system
`and may include pure alcohols, or mixtures ofthe alcohol
`feedstock and/or the desired carboxylic acid and/or es-
`ters of these two compounds. The preferred solvent and
`liquid reaction medium for the low water carbonylation
`process is the carboxylic acid product. Thus, in the car-
`bonylation of methanol to acetic acid, the preferred sol-
`vent is acetic acid.
`
`[0027] Water is contained in the reaction medium but
`at concentrations well below that which has heretofore
`
`been thought practical for achieving sufficient reaction
`rates. It has previously been taught that in rhodium—cat—
`alyzed carbonylation reactions ofthe type set forth in this
`invention, the addition of water exerts a beneficial effect
`upon the reaction rate (U.S. Patent No. 3,769,329). Thus
`most commercial operations run at water concentrations
`of at least about 14 wt %. Accordingly, it is quite unex-
`pected that reaction rates substantially equal to and
`above reaction rates obtained with such high levels of
`water concentration can be achieved with water concen-
`trations below 14 wt % and as low as about 0.1 1 wt %.
`
`In accordance with the carbonylation process
`[0028]
`most useful to manufacture acetic acid according to the
`present invention, the desired reaction rates are obtained
`even at low water concentrations by including in the re-
`action medium methyl acetate and an additional iodide
`ion which is over and above the iodide which is present
`as a catalyst promoter such as methyl iodide or other
`organic iodide. The additional iodide promoter is an io-
`dide salt, with Iithium iodide being preferred. It has been
`found that under low water concentrations, methyl ace-
`tate and lithium iodide act as rate promoters only when
`relatively high concentrations of each of these compo-
`nents are present and that the promotion is higher when
`both of these components are present simultaneously
`(U.S. PatentNo. 5,001 ,259). The concentration of lithium
`iodide used in the reaction medium of the preferred car-
`bonylation 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.
`[0029] The carbonylation reaction of methanol to ace-
`tic acid product may be carried out by contacting the
`methanol feed, which is in the liquid phase, with gaseous
`carbon monoxide bubbled through a liquid acetic acid
`solvent reaction medium containing the rhodium catalyst,
`methyl iodide promoter, methyl acetate, and additional
`soluble iodide salt, atconditions of temperature and pres-
`sure 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 mo-
`lar 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 used provided
`that the salt is sufficiently soluble in the reaction medium
`to provide the desired level ofthe iodide. When the iodide
`is added as a metal salt, preferably it is an iodide salt of
`a member of the group consisting ofthe 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, 1975-76 (56th edition).
`In par-
`ticular, alkali metal iodides are useful, with lithium iodide
`being preferred. In the low water carbonylation process
`most useful in this invention, the additional iodide over
`and above the organic iodide promoter is present in the
`catalyst solution in amounts of from about 2 to about 20
`wt %, the methyl acetate is present in amounts of from
`about 0.5 to about 30 wt %, and the lithium iodide is
`present in amounts of from about 5 to about 20 wt %.
`The rhodium catalyst is present in amounts offrom about
`200 to about 2000 parts per million (ppm).
`[0030] Typical reaction temperatures for carbonylation
`will be approximately 150 to about 250°C, with the tem-
`perature range of about 180 to about 220°C being the
`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 ofthe contained liquids,
`the total reactor pressure will range from about 15 to
`about 40 atmospheres.
`[0031] A typical reaction and acetic acid recovery sys-
`tem that is used for the iodide—promoted rhodium cata-
`lyzed carbonylation of methanol to acetic acid is shown
`in FIG. 1 and includes a liquid phase carbonylation reac-
`tor, flasher, and a methyl iodide acetic acid light ends
`column 14which has an acetic acid side stream 17 which
`
`proceeds to further purification. 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 a bubble column reactor, within which the re-
`acting Iiquid or slurry contents are maintained automat-
`ically at a constant level. Into this reactorthere are con-
`tinuously introduced fresh methanol, carbon monoxide,
`sufficient water as needed to maintain at least a finite
`
`concentration of water in the reaction medium, recycled
`catalyst solution from the flasher base, a recycled methyl
`iodide and methyl acetate phase, and a recycled aque-
`ous acetic acid phase from an overhead receiver decant-
`er of the methyl iodide acetic acid light ends or splitter
`column 14. Distillation systems are employed that pro-
`vide means for recovering the crude acetic acid and re-
`cycling catalyst solution, methyl iodide, and methyl ace-
`tate to the reactor. In a preferred process, carbon mon-
`oxide is continuously introduced into the carbonylation
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`reactorjust 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 gas-
`eous purge stream is 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.
`[0032]
`Liquid product is drawn off from the carbonyla—
`tion reactor at a rate sufficientto maintain a constant level
`therein and is introduced to the flasher. In the flasherthe
`
`catalyst solution is withdrawn as a base stream (predom-
`inantly acetic acid containing the rhodium and the iodide
`salt along with lesser quantities of methyl acetate, methyl
`iodide, and water), while the vapor overhead stream of
`the flasher contains largely the product acetic acid along
`with methyl iodide, methyl acetate, and water. Dissolved
`gases exiting the reactor and entering the flasher consist
`of a portion of the carbon monoxide along with gaseous
`by products such as methane, hydrogen, and carbon di-
`oxide and exit the flasher as part of the overhead stream.
`The overhead stream is directed to the light ends or split-
`ter column 14 as stream 26.
`
`It has been disclosed in U.S. Patent Nos.
`[0033]
`6,143,930 and 6,339,171 that there is a higher concen-
`tration, about 3 times, of the PRC’s and in particular
`acetaldehyde content in the light phase than in the heavy
`phase stream exiting column 14. Thus, in accordance
`with the present invention, PRC—containing stream 28 is
`directed to an overhead receiver decanter 16 whence
`
`the light ends phase, stream 30, is directed to distillation
`column 18.
`
`[0034] The present invention may broadly be consid-
`ered as an improved process for distilling PRC’s, prima-
`rily aldehydes and alkyl iodides, from avapor phase ace-
`tic acid stream. The vapor phase stream is distilled and
`extracted to remove PRC’s. An especially preferred
`method of removing aldehydes and alkyl iodides from a
`first vapor phase acetic acid stream and reducing levels
`of propionic acid in the acetic acid product, includes the
`following steps:
`
`a) condensing the first vapor phase acetic acid
`stream in a first condenser and biphasically separat-
`ing it to form a first heavy liquid phase product and
`a first light liquid phase product wherein said first
`heavy liquid phase contains the larger proportion of
`catalytic components than said first light liquid phase
`product;
`b) distilling the light liquid phase product in a first
`distillation column toform asecond vapor phase ace-
`tic acid product stream which is enriched with alde-
`hydes and alkyl iodides with respect to said first va-
`por phase acetic acid stream;
`c) condensing the second vapor phase stream in a
`second condenser to form a second liquid phase
`product;
`
`d) distilling the second liquid phase product in a sec-
`ond distillation column to form a third vapor phase
`stream;
`e) condensing the third vapor phase stream and ex-
`tracting the condensed stream with waterto remove
`residual acetaldehyde therefrom; and
`f) recycling at least a portion of the extracted third
`vapor phase stream to the second distillation col-
`umn.
`
`[0035] An embodiment of the prior art as disclosed in
`U.S. Patent No. 6,339,171 is shown in FIG. 1. Referring
`to FIG. 1, the first vapor phase acetic acid stream (28)
`contains methyl
`iodide, methyl acetate, acetaldehyde
`and other carbonyl components. This stream is then con-
`densed and separated (in vessel 16) to separate the
`heavy phase product containing the larger proportion of
`catalytic components——which is recirculated to the reactor
`(not shown in FIG. 1), and a light phase (30) containing
`acetaldehyde, water, and acetic acid.
`[0036]
`Either phase of the light ends overhead may be
`subsequently distilled to remove the PRC’s and primarily
`the acetaldehyde component of the stream, although it
`is preferred to remove PRC’s from the light phase (30)
`because it has been found that the concentration of
`
`acetaldehyde is somewhat greater in that phase. In the
`embodiment depicted and described herein, the distilla-
`tion is carried out in two stages; but it will be appreciated
`that the distillation may be performed in a single column
`as well.
`
`[0037] The light phase (30) is directed to column 18,
`which serves to form a second vapor phase (36) enriched
`in aldehydes and alkyl iodides with respect to stream 28.
`Steam 36 is condensed (vessel 20) and biphasically sep-
`arated to form a second heavy liquid phase product and
`a second light phase liquid product. This second heavy
`liquid phase contains a higher proportion ofcatalytic com-
`ponents than the second Iight liquid phase and is subse-
`quently recirculatedtothe reactor. The second liquid light
`phase (40) containing acetaldehyde, methyl
`iodide,
`methanol, and methyl acetate is directed to a second
`distillation column (22) wherein the acetaldehyde is sep-
`arated from the other components. This inventive proc-
`ess has been found to reduce and/or remove at least
`
`50% of the alkyl iodide impurities found in an acetic acid
`stream. It has also been shown that acetaldehyde and
`its derivatives is reduced and/or removed by at |east50%,
`most often greater than 60%. As a result, it is possible
`to keep the concentration of propionic acid in the acetic
`acid product below about 400 parts per million by weight,
`and preferably below 250 parts per million.
`[0038]
`From the top of the light ends or splitter column
`14, vapors are removed via stream 28, condensed, and
`directed to vessel 16. The vapors are chilled to a tem-
`perature sufficient to condense and separate the con-
`densable methyl iodide, methyl acetate, acetaldehyde
`and other carbonyl components, and waterintotwo phas-
`es. A portion of stream 28 contains noncondensable gas-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`6
`
`000006
`
`Exh. 1023
`
`000006
`
`Exh. 1023
`
`

`

`11
`
`EP1 737 808 B1
`
`12
`
`es such as carbon dioxide, hydrogen, and the like and
`can be vented as shown in stream 29 on FIG. 1. Also
`
`leaving overhead receiver decanter 1 6, but not illustrated
`in FIG. 1 , is the heavy phase of stream 28. Ordinarily this
`heavy phase is recirculated to the reactor, but a slip
`stream, generally a small amount, e.g., 25 vol. %, pref-
`erably less than about20 vol. %, of the heavy phase may
`also be directed to a c