(19)
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`0>
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`European Patent Office
`Office européen des brevets
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`llllllllllllllllllllllllllllllllIllllllllllllllllllllllllllllllllllllllllll
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`(11)
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`EP 0 687 662 B1
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`(12)
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`EUROPEAN PATENT SPECIFICATION
`
`(45) Date of publication and mention
`of the grant of the patent:
`07.10.1998 Bulletin 1998/41
`
`(21) Application number: 95109122.2
`
`(22) Date of filing: 13.06.1995
`
`(51) Int. Cl.6: 0070 53/08, 0070 51/12,
`CO7C 51 /44
`
`(54) Process for producing high purity acetic acid
`
`Verfahren zur Herstellung von hochreiner Essigsaure
`
`Procéde de production d'acide acétique de haute pureté
`
`(84) Designated Contracting States:
`DE FR GB IT
`
`(30) Priority: 15.06.1994 JP 132724/94
`20.06.1994 JP 137213/94
`30.06.1994 JP 149652/94
`06.07.1994 JP 154401 I94
`22.08.1994 JP 196524/94
`
`(43) Date 01 publication of application:
`20.12.1995 Bulletin 1995/51
`
`(73) Proprietor:
`DAICEL CHEMICAL INDUSTRIES, LTD.
`Sakai-shi, Osaka (JP)
`
`(72) Inventors:
`- Miura, Hiroyuki
`Himeji-shi, Hyogo (JP)
`
`- Shimizu, Masahiko
`Himeji-shi, Hyogo (JP)
`- Sato, Takashi
`Ohtake-shi, Hiroshima (JP)
`- Morimoto,Yoshiaki
`Arai-shi, Niigata (JP)
`- Kagotani, Masahiro
`Kakogawa-shi, Hyogo (JP)
`
`(74) Representative:
`Griinecker, Kinkeldey,
`Stockmair & Schwanhausser
`Anwaltssozietat
`Maximilianstrasse 58
`
`80538 Miinchen (DE)
`
`(56) References cited:
`EP-A- 0 265 140
`EP-A- 0 638 538
`US-A- 4 008 131
`
`EP-A- 0 497 521
`GB—A— 1 063 133
`
`
`
`Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give
`notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in
`a written reasoned statement.
`It shall not be deemed to have been filed until the opposition fee has been paid. (Art.
`99(1) European Patent Convention).
`
`Printed by Xerox (UK) Business Seerces
`2.16.3/3 4
`1
`
`CE Ex. 2015
`Daicel v. Celanese
`IPR2015-00171
`
`EP0687662B1
`
`1
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`Description
`
`EP 0 687 662 B1
`
`The present invention relates to a process for producing high purity acetic acid formed by carbonylating methanol
`in the presence of a rhodium catalyst.
`Various processes are known as an industrial process for producing acetic acid. Among them, a process for pro-
`ducing acetic acid by continuously reacting methanol with carbon monoxide in the presence of water using a rhodium
`catalyst and methyl iodide is the industrially most excellent process.
`Recently, reaction conditions and catalysts improvement have been investigated, and processes for industrially
`producing acetic acid at high productivity are disclosed, wherein catalyst stabilizers such as iodide salts are added and
`the reaction is carried outwith lower water content than in conventional processes (US-A 5214203 and US-A 5001259).
`If is disclosed therein that the water content in a reaction liquid is reduced to decrease by-products such as carbon diox-
`ide and propionic acid. However, there is the problem that the other trace impurities increase in amount as the produc-
`tivity of acetic acid grows and deteriorate the quality of produced acetic acid. In particular, in a quality test by which the
`amounts of very minute reducing impurities present in acetic acid are checked, which is called a permanganate reduc-
`ing substance test (permanganate time), minute increases of impurities having minute concentrations, which are hard
`to quantitatively determine even with high-grade instrumental analysis, can be detected, and these impurities lead to
`deterioration of product quality.
`Impurities which particularly exert influences to some kind of applications are contained as well in these trace impu-
`rities. For example, it is known that in a process for producing vinyl acetate from ethylene and acetic acid, they deterio-
`rate a palladium series catalyst used. These impurities include carbonyl compounds and organic iodides. To be
`concrete, it is known that they include carbonyl compounds such as acetaldehyde, butylaldehyde, crotonaldehyde, and
`2-ethylcrotonaldehyde, aldol condensation products thereof, and alkyl iodides such as ethyl iodide, butyl iodide, and
`hexyl iodide (EP-A 487284).
`However, these carbonyl impurities deteriorating permanganate time have boiling points tightly close to those of
`iodide catalyst accelerators, and it is difficult to sufficiently remove alkyl iodides which deactivate catalysts for producing
`vinyl acetate by ordinary means such as, for example, distillation.
`In view of the forgoing, there are disclosed conventional techniques such as treatment of crude acetic acid contain-
`ing these minute reducing impurities with ozone and oxidazing agents. However, these treatments with ozone and oxi-
`dazing agents have limits in the concentrations of the impurities to be treated. For example, compounds generated by
`decomposing unsaturated compounds such as crotonaldehyde and 2-ethylcrotoaldehyde by ozone processing are sat-
`urated aldehydes. Aldehydes themselves have reducing properties and are nothing but compounds deteriorating per-
`manganate time. Accordingly, refining such as distillation and treatment with active carbon is required after treatment
`with ozone in order to remove saturated aldehydes (US-A 5155265).
`If is known as well to treat crude acetic acid with Macro reticulated strong acid cation exchange resins, or strongly
`acidic cation exchange resins, substituted with silver to remove organic iodides (US-A 4615806). While this method is
`effective for removing alkyl iodides, hydrogen iodide, and inorganic iodide salts, it is insufficient for removing the unsatu-
`rated carbonyl impurities described above.
`While in every method described above, crude acetic acid is processed, it is attempted as well to remove carbonyl
`impurities contained in a process circulating liquid in a continuous reaction process. That is, a method for removing car-
`bonyl impurities is disclosed, wherein a methyl iodide recirculating stream to a carbonylation reactor is reacted with
`amino compounds which react with carbonyl impurities to form water soluble nitrogen-containing derivatives, and an
`organic methyl iodide phase is then separated from an aqueous derivative phase, followed by distilling the methyl iodide
`phase to remove carbonyl impurities (EP-A 487284). However, the concentration of carbonyl impurities contained in an
`organic stream recirculated into the carbonylation reactor described above is still high, and therefore it is not clear if the
`carbonyl impurities have been able to sufficiently be removed. Further, a new problem of removing nitrogen-containing
`compounds is involved.
`Fig. 1 shows a flow diagram of a reaction used for the carbonylation of methanol to acetic acid - acetic acid recovery
`system.
`Fig. 2 shows one example of a distillation system for separating methyl iodide from acetaldehyde.
`In the drawings,
`
`10:
`12:
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`14:
`30'
`40, 60:
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`Carbonylation reactor.
`Flasher.
`
`Methyl iodide - acetic acid splitter column.
`Lower phase in liquid separator.
`Distillation columns.
`
`The object of the present invention is to provide a process for producing high purity acetic acid, wherein carbonyl
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`EP 0 687 662 B1
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`compounds or organic iodides which are impurities of acetic acid as described above are reduced by controlling condi-
`tions of the reactor in which they are generated.
`This object has been achieved by a process for producing high purity acetic acid, comprising the steps of continu-
`ously reacting methanol with carbon monoxide in the presence of a rhodium catalyst, an iodide salt, and methyl iodide,
`wherein reaction is carried out while maintaining an acetaldehyde concentration in the reaction liquid at 400 ppm or
`lower.
`
`It is preferable that the above defined reaction is carried out by removing acetaldehyde from the process liquid
`being circulated into the reactor to maintain the acetaldehyde concentration in the reaction liquid at 400 ppm or lower.
`Removing acetaldehyde from the process liquid being circulated into the reactor can be carried out by separating
`the reaction liquid into a volatile phase containing acetic acid, methyl acetate and methyl iodide and a low volatile phase
`containing the rhodium catalyst, distilling the volatile phase to obtain a product mixture containing acetic acid and the
`overhead containing methyl acetate and methyl iodide, and recirculating said overhead into the reactor, wherein the
`overhead or a codensaye of the carbonyl impurities of said overhead is contacted with water to separate it into an
`organic phase containing methyl acetate and methyl iodide and an aqueous phase containing the carbonyl impurities
`containing acetaldehyde, and said organic phase is recirculated into the reactor.
`Said overhead containing acetaldehyde and methyl iodide can as well be distilled at a top temperature of 55°C or
`higher, at a reflux tank's temperature of 25°C or higher, at a pressure of 98 kPa (1 kg/cm2) or more, whereby acetalde-
`hyde is separated and removed before the overhead is recirculated into the reactor.
`This destillation can also be carried out at a top temperature of less than 55°C and a reflux tank's temperature of
`less than 25°C in the presence of an alcohol whereby acetaldehyde is separated and removed before the overhead is
`recirculated into the reactor. It is preferable that methanol is introduced at a lower position than a stage charged with
`the overhead containing acetaldehyde and methyl iodide.
`A preferable embodiment of the invention is to maintain an acetaldehyde's concentration in the reaction liquid at
`400 ppm or lower by separating the resulting reaction liquid into a volatile phase containing acetic acid, methyl acetate
`and methyl iodide and a low volatile phase containing the rhodium catalyst, distilling the volatile phase to obtain a prod-
`uct mixture containing acetic acid and the overhead containing methyl acetate and methyl iodide, and recirculating said
`overhead into the reactor, wherein the overhead or a codensaye of the carbonyl impurities of said overhead is contacted
`with water to separate it into an organic phase containing methyl acetate and methyl iodide and an aqueous phase con-
`taining the carbonyl impurities containing acetaldehyde, and said organic phase is recirculated into the reactor.
`Another preferable embodiment of the invention is to remove acetoaldehyde and methyl iodide by that the overhead
`containing acetaldehyde and methyl iodide is distilled at a top temperature of 55°C or higher, at a reflux tank's temper-
`ature of 25°C or higher, at a pressure of 98 kPa (1 kg/cm2) or more, and acetaldehyde is separated and removed before
`the overhead is recirculated into the reactor. Alternatively, the overhead containing acetaldehyde and methyl iodide is
`distilled at a top temperature of less than 55°C and a reflux tank's temperature of less than 25°C in the presence of an
`alcohol and acetaldehyde is separated and removed before the overhead is recirculated into the reactor.
`As above shown, the overhead is distilled under specified conditions to separate and remove acetaldehyde, and
`thereafter recirculated into the reactor.
`
`The term, the low volitile, includes the non-volatile.
`First, the process for producing acetic acid according to the present invention will be explained.
`The rhodium catalyst used in the present invention is present in a reaction liquid in the form of a rhodium complex.
`Accordingly, the rhodium catalyst may be used in any form as long as it is changed to a complex which is dissolved in
`the reaction liquid. To be concrete, rhodium iodine complexes and rhodium carbonyl complexes such as Rhls and
`[Flh(CO)2|2]' are effectively used. The amount used thereof is 200 to 1,000 ppm, preferably 300 to 600 ppm in terms of
`concentration in the reaction liquid.
`In the present invention, an iodide salt is added particularly for stabilizing the rhodium catalyst under low water con-
`ditions and for suppressing side reactions. This iodide salt may be any one as long as it generates iodine ions in a reac-
`tion liquid. Examples thereof include alkaline metal iodide salts such as Lil, Nal, Kl, Rbl, and Csl, alkaline earth metal
`
`iodide salts such as Belg, Mglg, and Cat; and aluminum group metal iodide salts such as Big and All3. Organic iodide
`salts can be used besides the metal iodide salts and include, for example, quaternary phosphonium iodide salts (methyl
`iodide adducts or hydrogen iodide adducts of tributyl phosphine and triphenyl phosphine), and quaternary ammonium
`iodide salts (methyl iodide adducts or hydrogen iodide adducts of tertiary amine, pyridines, imidazoles, and imides). In
`particular, the alkaline metal iodide salts such as Lil are preferred. The amount used of the iodide salts is 0.07 to 2.5
`mole/liter, preferably 0.25 to 1.5 mole/liter in terms of iodide ions in a reaction liquid.
`In the present invention, methyl iodide is used as a catalyst accelerator and is present in a reaction liquid in 5 to 20
`weight %, preferably 12 to 16 weight %.
`A water content in a reaction liquid in the present invention is 15 weight % or less, preferably 10 weight % or less,
`and more preferably 1 to 5 weight %.
`As the reaction in the present invention is a continuous reaction, methyl acetate formed by reacting raw material
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`EP 0 687 662 B1
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`methanol with acetic acid is present in 0.1 to 30 weight %, preferably 0.5 to 5 weight %, and the balance of principal
`components in the reaction liquid is acetic acid which is a product as well as a reaction solvent.
`In the present invention, the typical temperature in the carbonylation of methanol is about 150 to 250°C, and tem-
`perature ranges of about 180 to 220°C are preferred. A partial pressure of carbon monoxide can be changed in a wide
`range and is typically about 202,65 kPa to 3039,75 kPa (230 atm), preferably 405,3 to 1519,875 kPa (4-15 atm). The
`whole reactor pressure resides within a range of about 1519,875 to 4053 kPa (15-40 atm) because of partial pressures
`of by-products and a vapor pressure of a liquid contained.
`The process of the present invention will be explained below based on a drawing.
`Fig. 1 is a flow diagram showing a reaction - acetic acid recovery system used for rhodium-catalyzed carbonylation
`of methanol to acetic acid.
`
`The reaction from methanol to acetic acid - acetic acid recovery system shown in Fig. 1 includes a carbonylation
`reactor 10, a flasher 12, and a methyl iodide - acetic acid splitter column 14. Usually, reaction liquid contents are auto-
`matically maintained at a fixed level in the carbonylation reactor 10. Fresh methanol and a sufficient amount of water
`are continuously introduced into this reactor according to necessity, and at least a measurable concentration of water
`is maintained in a reaction solvent. An alternative distillation system can also be used as long as it is equipped with
`means for recovering crude acetic acid, and means for recirculating a catalyst liquid, methyl iodide, and methyl acetate
`into the reactor.
`
`In a preferred process, carbon monoxide is continuously introduced into an immediate lower part of a stirrer used
`for stirring contents in the carbonylation reactor 10. A gaseous supplying material is dispersed all over the reaction liq-
`uid by this mean. A gaseous purge stream is discharged from the reactor to prevent the accumulation of gaseous by-
`products and to maintain a set partial pressure of carbon monoxide in the fixed overall reactor pressure. The reactor
`temperature is automatically controlled, and a carbon monoxide-supplying material is introduced at a reaction rate suf-
`ficient for maintaining the preferred overall reactor pressure. Liquid products are withdrawn from the carbonylation reac-
`tor 10 at a speed sufficient for maintaining a fixed level and introduced into an intermediate point between the top and
`bottom of the flasher 12 via a line 11.
`
`A catalyst liquid is withdrawn from the flasher 12 as a bottom stream 13 (acetic acid containing mainly the rhodium
`catalyst and iodide salts together with small amounts of methyl acetate, methyl iodide, and water) and returned to the
`carbonylation reactor 10. An overhead 15 from the flasher 12 contains mainly product acetic acid together with methyl
`iodide, methyl acetate, and water.
`Product acetic acid (can be withdrawn as a bottom stream) withdrawn from a side face close to the bottom of the
`methyl iodide - acetic acid splitter column 14 is further refined by methods known by persons having ordinary skill in the
`art. An overhead 20 from the methyl iodide - acetic acid splitter column 14 containing mainly methyl iodide and methyl
`acetate as well as small amounts of water and acetic acid is recirculated into the carbonylation reactor 10 via a line 21.
`The overhead 20 is typically separated into two liquid phases by condensing when a sufficient amount of water is
`present. A lower phase 30 comprises mainly methyl iodide and small amounts of methyl acetate and acetic acid, and
`an upper phase 32 comprises mainly water, acetic acid, and a small amount of methyl acetate.
`In the present invention, it is important in such reaction - acetic acid recovery system to carry out the reaction while
`keeping an acetaldehyde concentration in a reaction liquid at 400 ppm or less. The acetaldehyde concentration exceed-
`ing 400 ppm is not preferred because impurity concentrations in the acetic acid produced increase, and a complicated
`refining processing step is required. A method in which reaction conditions are managed and a method in which acetal-
`dehyde is removed from a process liquid circulated into a reactor are available in order to maintain the acetaldehyde
`concentration in the reaction liquid at 400 ppm or less.
`The mesures taken in order to manage the reaction conditions include increasing hydrogen partial pressure, water
`concentration, and rhodium catalyst concentration. These operations lower mainly the acetaldehyde concentration in
`the reaction liquid in the carbonylation reactor 10, which results in controlling the aldol condensation of acetaldehyde
`and decreasing the rate of the production of reducing by-products such as crotonaldehyde and 2-ethylcrotonaldehyde,
`and alkyl iodides such as hexyl iodide. However, in some cases, these methods have the disadvantage to increase the
`rate of the production of propionic acid.
`In view of the forgoing, in order to control the acetaldehyde concentration in the reaction liquid in the carbonylation
`reactor 10 to 400 ppm or less, it is preferrable to remove acetaldehyde from the process liquid circulated into the carb-
`onylation reactor 10.
`The method in which acetaldehyde is removed and the method in which the reaction conditions are controlled can
`be used in combination.
`
`Hydrogen partial pressure in the carbonylation reactor 10 originates in hydrogen generated in the system by water
`gas shift in the present reaction and, in some cases, originates in hydrogen introduced into the reactor together with
`raw material carbon monoxide.
`
`A method for removing acetaldehyde from the process liquid circulated into the carbonylation reactor 10 includes
`methods such as distillation and extraction, or the combination thereof, and distillation/extraction.
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`EP 0 687 662 B1
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`Preferred as the process liquid which is a target for removing carbonyl impurities containing acetaldehyde are the
`upper phase 32 of the condensate of the overhead 20, the lower phase 30 which is rich in methyl iodide, a homogene-
`ous liquid of the overhead 20 if the overhead 20 is not separated into two layers, an absorbing liquid for vent gas in a
`waste gas absorbing system, and a low boiling liquid obtained by further distilling crude acetic acid liquid withdrawn
`from the line 17 close to the bottom of the splitter column 14, because the concentrations of acetaldehyde are high. Of
`them, further preferred are the upper phase 32, the lower phase 30, the homogeneous liquid of the overhead 20 if the
`overhead 20 is not separated into two layers, or the carbonyl impurities concentrate thereof. Crude acetic acid liquid
`withdrawn from the line 17 is usually turned to product acetic acid after the crude acetic acid liquid is dehydrated in the
`subsequent distillation column and then introduced into an acetic acid product column for distilling to separate high boil-
`ing and low boiling matters.
`
`The process liquid which is a target for removing carbonyl impurities containing acetaldehyde as described above
`usually contains methyl iodide of 5 to 90 weight %, acetaldehyde 0.05 to 50 weight %, methyl acetate of 0 to 15 weight
`%, acetic acid of 0 to 80 weight %, moisture of 0.1 to 40 weight %, and other carbonyl impurities.
`The process liquid containing acetaldehyde and others contains useful components such as methyl iodide, methyl
`acetate and the like and therefore is circulated to the carbonylation reactor 10 for reuse. Accordingly, the acetaldehyde
`concentration in the reactor can be reduced by separating and removing acetaldehyde from these circulating liquids.
`A method for separating carbonyl impurities containing acetaldehyde includes a method in which a process liquid
`containing acetaldehyde is distilled and separated in one distillation column, a method in which low boiling components
`comprising acetaldehyde and methyl iodide are first separated from other components by distillation, and then acetal-
`dehyde is further separated from methyl iodide by distillation. Utilizing the fact that acetaldehyde is well miscible with
`water and methyl iodide is scarcely miscible with water, extraction with water may as well be employed for separating
`methyl iodide from acetaldehyde.
`When acetaldehyde is separated directly from the process liquid in a single distillation column, it is pretty difficult to
`concentrate acetaldehyde because the boiling point of methyl iodide is close to that of acetaldehyde. The concentration
`of acetaldehyde by distillation in a nonaqueous system such as methyl iodide not only generates paraldehyde and met-
`aldehyde and prevents acetaldehyde from concentrating but also deposites metaldehyde in the process and prevents
`stable operation. In view of the foregoing, the method in which extraction with water is used for separating methyl iodide
`from acetaldehyde is preferred, and particularly preferred is a method in which after an acetaldehyde liquid containing
`methyl iodide is separated from a process liquid by distillation, acetaldehyde is selectively extracted with water, and this
`is further separated by a distillation/separation process. According to this method, acetaldehyde can be very efficiently
`concentrated and removed because distillation temperature is high in the concentration of a water extract by distillation,
`and an increase in hydrogen ion concentration in a distillate due to the decomposition of ester can suppress the gener-
`ation of paraldehyde and metaldehyde. When distillation for separation is carried out in one distillation column, water
`may be charged into the distillation column, and/or the distillation temperature and pressure may be elevated to control
`the generation of paraldehyde and metaldehyde. Further, distillation conditions may be varied to positively generate
`paraldehyde and metaldehyde, and acetaldehyde may be separated and removed from bottom products in the forms of
`paraldehyde and metaldehyde. In this case, solvents dissolving metaldehyde, such as methanol have to be charged
`into the column to prevent clogging caused by the crystallization of metaldehyde.
`The method in which extraction with water is used for separating methyl iodide from acetaldehyde will be explained
`below in detail.
`
`In this water extraction method, carbonyl impurities contained in the lower phase 30 in a liquid separator containing
`carbonyl compounds such as, for example, acetaldehyde, crotonaldehyde, and butylaldehyde are separated from a
`reaction product by extracting them with water to form a recirculating stream containing no carbonyl impurities. Accord-
`ing to a preferred embodiment, the lower phase 30 in the liquid separator bath is separated into an organic phase recir-
`culating stream containing methyl iodide and an aqueous phase stream containing carbonyl impurities, particularly
`acetaldehyde by extraction with water, thus carbonyl impurities are removed from the organic phase recirculating
`stream to the reactor.
`
`At the first step in the preferred method, the lower phase 30 in the liquid separator bath containing carbonyl impu-
`rities such as, for example, acetaldehyde, crotonaldehyde and butylaldehyde is contacted to water to extract the carb-
`onyl impurities into an aqueous phase. The carbonyl impurities can be determined by an analysis before processing.
`The extraction is carried out at temperatures of 0 to 100°C for 1 second to 1 hour. Any pressure can be employed. Pres-
`sure is not essential in this method, and advantageous conditions can be selected in terms of cost. There can be used
`as an extractor, every suitable apparatus which is known in terms of technique, such as a combination of mixers and
`settlers, a combination of static mixers and decanters, RDC (rotated disk contactor), a Karr column, a spray column, a
`packed column, a perforated plate column, a baffle column, and a pulsation column.
`After passing through an extractor to a decanter, the aqueous phase stream containing carbonyl impurities and the
`organic phase stream containing no carbonyl impurities are obtained. The organic phase stream is recirculated to the
`carbonylation reactor. The aqueous phase stream is sent to a distillation column to separate the carbonyl impurities
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`EP 0 687 662 B1
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`from water, and water is recirculated to the extractor. The value of the carbonyl impurities removed can be determined
`by an analysis method.
`Next, the distilling method under particular conditions for separating methyl iodide and acetaldehyde is explained
`in detail.
`
`Investigations intensively made by the present inventors have resulted in finding that the generation and deposition
`of paraldehyde and metaldehyde which are the condensation products of acetaldehyde can be controlled, and methyl
`iodide can efficiently be separated from acetaldehyde by controlling top temperature, reflux tank temperature, and pres-
`sure or controlling top temperature and reflux tank temperature in the presence of an alcohol in distilling a mixed liquid
`containing acetaldehyde and methyl iodide, and completing the present invention.
`Efficiently separating acetaldehyde and methyl iodide by this method can be carried out by distilling a mixed liquid
`containing acetaldehyde and methyl iodide, for example the overhead described above, at top temperatures of 55°C or
`higher, reflux tanktemperatures of 25°C or higher, and pressures of 98 kPa (1 kg/cm2) or more, or distilling it at top tem-
`peratures of less than 55°C and circulating tank temperatures of less than 25°C in the presence of alcohol.
`Followings, Fig. 1 and Fig. 2 are used to illustrate.
`if they are not separated,
`A recirculating stream 21 can be formed by the lower phase 30, the upper phase 32 or,
`the whole overhead 20, or combining these phases and overhead withdrawn from the methyl iodide - acetic acid splitter
`column 14 with other recirculated products containing methyl iodide, methyl acetate, water, and impurities.
`The lower phase 30, the upper phase 32 or the whole overhead 20 withdrawn from the methyl iodide - acetic acid
`splitter column 14, or the recirculating stream 21 is introduced into a distillation column 40 and subjected to the process-
`ing as described below. Every suitable equipment which is known in terms of techniques can be used for distillation col-
`umns and separation. The stage number of distillation columns may be any stages. Two or more distillation columns
`may be used to carry out the processing if it can not be carried out in a single distillation column for reasons of facilities
`cost
`
`The case where the processing is carried out in two distillation columns will be explained below referring to Fig. 2.
`The lower phase 30, the upper phase 32 or the whole overhead 20 withdrawn from the methyl iodide - acetic acid
`splitter column 14, or the recirculating stream 21 is introduced into the distillation column 40, and a methyl iodide recir-
`culating stream withdrawn from the bottom of the column is recirculated into the reactor via a line 46. A distillate 44 is
`obtained from the top.
`The distillate 44 from the distillation column 40 is introduced into a distillation column 60 and subjected to further
`processing. A methyl iodide recirculating stream from which most of acetaldehyde has been removed is recirculated
`into the upper part of the distillation column 40 via a line 66. Or, in the case where a liquid from which most of acetal-
`dehyde has been removed and which is rich in methyl iodide is obtained from the top, a top distillate is recirculated into
`the distillation column 40.
`
`Usually, the process liquid of the whole overhead 20 withdrawn from the methyl iodide - acetic acid splitter column
`14 contains methyl iodide of 5 to 90 weight %, acetaldehyde of 0.05 to 50 weight %, methyl acetate of 0 to 15 weight
`%, acetic acid of 0 to 80 weight %, moisture of 0.1 to 40 weight %, and other carbonyl impurities.
`Because the process liquid containing acetaldehyde described above contains useful components such as methyl
`iodide and methyl acetate, it is circulated into the carbonylation reactor 10 for reuse. Accordingly, after separating and
`removing acetaldehyde as much as possible from these process liquids, they are preferably circulated into the reactor.
`If acetaldehyde is not sufficiently removed, acetaldehyde is accumulated in the process liquid, and the aldol con-
`densation of acetaldehyde is promoted, which result in an increase of the rate of production of reductive by-products
`such as crotonaldehyde and 2-ethylcrotonaldehyde and alkyl iodides such as hexyl iodide and therefore lead to obtain-
`ing a product acetic acid containing these impurities in a large amount.
`The separation of acetaldehyde and methyl iodide involves difficulties because the boiling points of acetaldehyde
`and methyl iodide are close to each other, and in addition, the concentration of methyl iodide by distillation in a non-
`aqueous system not only generates paraldehyde and metaldehyde and prevents acetaldehyde from concentrating but
`also deposits metaldehyde in the process and prevents stable operation.
`Paraldehyde is a trimer of acetaldehyde and is a liquid having a boiling point of 124°C and a melting point of 10°C.
`In general, paraldehyde is liable to be generated from acetaldehyde at low temperatures of 0 to -10°C, and critical gen-
`eration temperature is 55°C. It was confirmed in a laboratory that paraldehyde was generated at 20°C.
`Metaldehyde is a tetramer to hexamer of acetaldehyde and is a white acicular crystal having melting points of 140
`to 246°C. Metaldehyde is formed at lower temperatures than paraldehyde and is generally generated at -10 to -40°C. It
`was confirmed in a laboratory that metaldehyde was generated at 5°C. Temperature lowered down to -40°C or less
`causes polymerization. Paraldehyde and metaldehyde have stereoisomers, and it was confirmed that they have differ-
`ent melting points and solubilities.
`As shown here, the generation of paraldehyde and metaldehyde is influenced by temperature. That is, controlling
`operation pressure and operation temperature in a distillation column has made it possible to separate and remove
`acetaldehyde.
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`30
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`35
`
`4o
`
`45
`
`50
`
`55
`
`6
`
`

`

`EP 0 687 662 B1
`
`That is, it has been found that distilling at top temperatures of 5°C or higher, reflux tank temperatures of 25°C or
`higher, and a pressure of 98 kPa (1 kg/cmz) or more in a distillation column can control the generation of paraldehyde
`and metaldehyde and improves the separation efficiency of methyl iodide from acetaldehyde. Further, shortening resi-
`dence time for returning to the distillation column from a top condenser through a reflux tank is effective as well for sup-
`pressing the generation of paraldehyde and metaldehyde.
`
`Further, it has been found that since distilling at top temperatures of less than 55°C and reflux tank temperatures
`of less than 25°C converts acetaldehyde to paraldehyde and metaldehyde at the top, which have higher boiling points
`and therefore are moved to a bottom side, acetaldehyde can be removed from bottom products in the forms of paralde-
`hyde and metaldehyde. However, because metaldehyde is a solid having low solubility particularly in methyl iodide and
`is deposited, it clogs not only the perforated plates and packing of the distillation column