MXPA06010005A - Removal of permanganate reducing compounds from methanol carbonylation process stream - Google Patents

Abstract

An improvement of the methanol carbonylation process for manufacturing acetic acid is disclosed. Specifically disclosed is a method for reducing the formation of alkyl iodides and C3-8 carboxylic acids by removing permanganate reducing compounds ("PRC's") from the light phase of the condensed light ends overhead stream, including (a) distilling the light phase to yield a PRC enriched overhead stream;and (b) extracting the third overhead stream with water in at least two consecutive stages and separating therefrom one or more aqueous streams containing PRC's.

Description

REMOVAL OF PERMANGANATE REDUCING COMPOUNDS FROM THE CURRENT OF A PROCEDURE OF METHANOL CARBONILATION BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to an improved process for the removal of permanganate reducing compounds and alkyl iodides formed by the carbonylation of methanol in the presence of a metal carbonylation catalyst of group VIII. More specifically, this invention relates to an improved process for reducing and / or removing precursors of permanganate reducing compounds and alkyl iodides from intermediate streams during the formation of acetic acid during said carbonylation processes.

TECHNICAL BACKGROUND Among the methods currently employed to synthesize acetic acid, one of the most commercially useful is the catalyzed carbonylation of methanol with carbon monoxide as shown in the patent of E.U.A. No. 3,769,329 issued to Paulik et al. on October 30, 1973. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported in an inert solid, together with a halogen-containing catalyst promoter as exemplified by iodide. of methyl. Rhodium can be introduced into the reaction system in any of many ways, and the exact nature of the rhodium portion within the active catalyst complex is uncertain. Likewise, the nature of the halide promoter is not critical. Patent holders disclose a very large number of suitable promoters, many of which are organic iodides. In a very typical and useful way, the reaction is performed by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved. An improvement of the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having a carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in the U.S. Patents. commonly assigned No. 5,001, 259, published March 19, 1991; 5,026,908, published June 25, 1991; and 5,144,068, published September 1, 1992; and European Patent No. EP 0 161 874 B2, published on July 1, 1992. As described therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate, methyl halide. , especially methyl iodide, and rhodium present in a catalytically effective concentration. These patents disclose that the stability of the catalyst and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low concentrations of water, ie 4 weight percent or less, in the reaction medium (notwithstanding the general practice of the industry to maintain 14-15% by weight of water) by maintaining in the reaction medium, together with a catalytically effective amount of rhodium and at least a finite concentration of water, a specified concentration of iodide ions in addition of the iodide content that is present as methyl iodide or other organic iodide. The iodide is present as a simple salt, with lithium iodide being preferred. The patents show that the concentration of methyl acetate and iodide salts are important parameters that affect the percentage of carbonylation of methanol to produce acetic acid, especially at low concentrations of water in the reactor. By using relatively high concentrations of the methyl acetate and iodide salt, a surprising degree of catalyst stability and productivity is obtained in the reactor even when the liquid reaction medium contains water at concentrations as low as about 0.1% by weight, so low that it can define itself broadly and simply as a "finite concentration" of water. Even more, the reaction medium used improves the stability of the rhodium catalyst, ie the resistance to precipitation of the catalyst, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove carbon monoxide from the catalyst which, in the environment maintained in the reaction vessel, is a ligand with a stabilizing effect on the rhodium. The patents of E.U.A. Nos. 5,001, 259, 5,026,908 and 5,144,068 are incorporated herein by reference. It has been found that, although a carbonylation process with little water to produce acetic acid reduces such byproducts as carbon dioxide, hydrogen and propionic acid, the amount of other impurities generally present in vestigial amounts, also increases, and the quality of the acid Acetic acid sometimes decreases when attempts are made to increase the percentage of production by improving the catalysts or modifying the reaction conditions. These vestigial impurities affect the quality of the acetic acid, especially when they are recirculated through the reaction procedure. Impurities that decrease acetic acid permanganate time include carbonyl compounds and unsaturated carbonyl compounds. As used herein the phrase "carbonyl" is intended to encompass compounds containing aldehyde or ketone functional groups, which compounds may or may not possess unsaturation. See Catalysis of Organic Reaction, 75, 369, 380 (1998), for further discussion on impurities in a carbonylation process. The present invention is directed to reducing and / or removing permanganate reducing compounds (PRC) such as acetaldehyde, acetone, methyl ethyl ketone, butyl aldehyde, crotonaldehyde, 2-ethyl-crotonaldehyde, and 2-ethyl-butyraldehyde and the like and the aldol condensation products thereof. The present invention also leads to the reduction of propionic acid. The carbonyl impurities described above, such as acetaldehyde, can react with the iodide catalysing promoters to form alkyl iodides with multiple carbons, for example ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide and Similar. It is convenient to remove alkyl iodides from the reaction product because even small amounts of these impurities in the acetic acid product tend to deplete the catalyst used in the production of vinyl acetate, the most commonly used product from acetic acid. The present invention is also directed to the removal of alkyl iodide, in particular C2-12 alkyl iodide compounds. Consequently, since many impurities originate with acetaldehyde, it is a primary objective to remove the acetaldehyde from the process to reduce the alkyl iodide content. Conventional techniques for removing impurities include treating the acetic acid product with oxidants, ozone, water, methanol, activated carbon, amines, and the like, which treatment may or may not combine with the distillation of acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is also known, for example, from the patent of E.U.A. No. 5,783,731, how to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the products of oxime reaction. However, additional treatment of the final product adds costs to the process and the distillation of the acetic acid product can result in the formation of additional impurities. Although it is possible to have acetic acid of relatively high purity, the acetic acid product formed by the low water level carbonylation process and the purification treatment described above is frequently somewhat deficient with respect to the permanganate time due to the presence of small proportions of residual impurities. Since enough time for permanganate is an important commercial test, which the acid product must meet to be suitable for many uses, the presence of impurities that decrease the permanganate time is objectionable. Furthermore, it is not economically or commercially feasible to remove minute amounts of these impurities from acetic acid by distillation because some of the impurities have boiling points close to those of the acetic acid product. Therefore, it has become important to identify economically viable methods to remove impurities in any part of the carbonylation process without contaminating the final product or adding unnecessary costs. U.S. Patent No. 5,756,836 incorporated herein by reference discloses a method for making acetic acid with high purity to adjust the acetaldehyde concentration of the reaction solution below 1500 ppm. It is stated that by maintaining the concentration of acetaldehyde below this threshold, it is possible to suppress the formation of impurities such that one only needs to distill the crude acetic acid product to obtain an acetic acid with high purity. European Patent No. EP 0 487 284 B1, published on April 12, 1995, discloses that the carbonyl impurities present in the acetic acid product are generally concentrated in the upper part of the column of light fractions. Accordingly, the effluent vapor from the light fraction column 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 effluent vapor by distillation, which results in an acetic acid product with better permanganate time. European Patent Application No. EP 0 687 662 A2 and U.S. Patent No. 5,625,095 describe a process for producing acetic acid with high purity in which it is stated that an acetaldehyde concentration of 400 ppm or less is maintained in a reactor using a single-stage or multi-stage distillation process to remove acetaldehyde. The currents suggested for processing to remove acetaldehyde include a light phase containing mainly water, acetic acid and methyl acetate, a heavy phase containing mainly methyl iodide, methyl acetate and acetic acid; an effluent vapor stream containing mainly methyl iodide and methyl acetate, or a recirculation stream formed by combining the light and heavy phase. These references do not identify which of these currents has the highest concentration of acetaldehyde. EP 0 687 662 A2 and U.S. Patent No. 5,625,095 also describe the handling of reaction conditions to control the acetaldehyde formulation in the reactor. Although it is stated that the formation of byproducts such as crotonaldehyde, 2-ethylcrotonaldehyde, and alkyl iodide is reduced by controlling the formation of acetaldehyde, it is also noted that the handling of reaction conditions as proposed increases the formation of propionic acid, a inconvenient secondary product. More recently, it has been described in commonly assigned US patents No. 6,143,930 and 6,339,171 that it is possible to significantly reduce the unwanted impurities in the acetic acid product by performing a multi-step purification in the effluent vapor of the fraction column. light. These patents describe a purification process in which the effluent vapor of light fractions is distilled twice, in each case taking the effluent vapor of acetaldehyde and returning a residue rich in methyl iodide to the reactor. The acetaldehyde-rich distillate is optionally extracted with water to remove most of the acetaldehyde for disposal, leaving a significantly lower concentration of acetaldehyde in the refined residue that is recycled to the reactor. U.S. Patent Nos. 6,143,930 and 6,339,171 are hereby incorporated by reference in their entirety. Although the processes described above have succeeded in removing the carbonyl impurities from the carbonylation system and for the most part controlling the acetaldehyde levels and the time problems of the permanganate in the final acetic acid product, further improvements can be made. Consequently, there remains a need for alternative procedures to improve the efficiency of acetaldehyde removal. The present invention provides one of these alternative solutions.

BRIEF DESCRIPTION OF THE INVENTION In one aspect, the present invention provides a process for producing acetic acid which includes the following steps: (a) reacting methanol, methyl acetate, methyl formate or dimethyl ether with carbon monoxide in a suitable reaction medium including a catalyst and an organic iodide; (b) separating the products of the reaction in a volatile product phase containing acetic acid, methyl iodide, water and permanganate reducing compounds (PRC) and a less volatile phase containing the catalyst and acetic acid, (c) distilling the volatile product phase to generate a purified product and a first effluent vapor containing organic iodide, water, acetic acid and unreacted methanol; (d) distilling at least a portion of the first effluent vapor to produce a second effluent vapor containing methyl iodide, water, C2-? 2 alkyl iodides, PRC and dimethyl ether; (e) extracting the second effluent vapor with water to provide a first aqueous extract and a first refined residue; and (f) extracting with water the first refined residue to provide a second refined residue and a second aqueous extract containing concentrated PRC for its disposal. Preferably, at least a portion of the second refined residue is recirculated directly or indirectly to the reactor, as the bottoms of the distillation steps. Most preferably, the second effluent vapor contains sufficient dimethyl ether to reduce the solubility of the methyl iodide in the aqueous extracts, as will be further explained below. In another aspect, the present invention provides an improved method for separating a mixture containing water, acetic acid, methyl iodide, methyl acetate, methanol, at least one C2-? 2 alkyl iodide and at least one compound permanganate reducer (PRC). The improved method includes the following steps: (a) distilling the mixture to form an effluent vapor stream enriched in PRC containing dimethyl ether; (b) extracting the effluent vapor stream with water and separating therefrom a first aqueous stream containing at least one PRC; and (c) extracting the effluent vapor stream extracted with water and separating therefrom a second aqueous stream containing at least one PRC. Most preferably, the effluent vapor stream contains sufficient dimethyl ether to reduce the solubility of methyl iodide in the aqueous extracts. In still another aspect, the present invention provides an improved method for reducing and / or removing PRC permanganate reducing compounds and C2-? 2 alkyl iodide compound formed in the carbonylation of a carbonylation material such as methanol, methyl acetate , methyl formate or dimethyl ether to an acetic acid product. In the improved method, the methanol is carbonylated in a reaction medium containing a catalyst and an organic iodide; the products of the carbonylation reaction are separated into phases in (1) a volatile phase containing product of acetic acid, organic iodide, water and at least one PRC; (2) a less volatile phase, and the volatile phase is distilled to generate a purified product and an effluent vapor containing organic iodide, water, acetic acid and PRC. The improvement includes the steps of (a) distilling at least a portion of the effluent vapor to provide an effluent vapor stream enriched in PRC containing dimethyl ether; (b) extracting with water the effluent vapor stream enriched in PRC and separating from this an aqueous waste stream containing PRC; and (c) extracting the extracted effluent vapor stream with water and separating therefrom a second aqueous waste stream which also contains at least one PRC. Most preferably, the effluent vapor stream contains sufficient dimethyl ether to reduce the solubility of methyl iodide in the aqueous extracts.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the prior art process, as described in U.S. Patent No. 6,339,171, for the removal of carbonyl impurities from the intermediate stream of the carbonylation process for the production of acetic acid by a reaction of carbonization. Figure 2 illustrates a preferred embodiment of the present invention. Although the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown as examples in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited by the particular forms described. Rather, it is intended that the invention cover all modifications, equivalents and alternatives that fall within the scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION Illustrative embodiments of the invention are described below. For clarity, not all features of a current implementation are described in the specification. Of course it will be understood that in the development of any current modality, numerous specific decisions can be made regarding implementation to achieve the specific objectives of the inventors, such as compliance with the restrictions related to the system and to the business, which they will vary from one implementation to another. Furthermore, it will be noted that such a development effort could be complex and time-consuming, but that would nevertheless be a routine enterprise for those skilled in the art who have the benefit of this description. The purification process of the present invention is useful in any process for carbonylating methane (or other carbonatable material such as methyl acetate, methyl formate or dimethyl ether, or mixtures thereof) to acetic acid in the presence of a metal catalyst. Group VIII as rhodium or an iodide promoter. A particularly useful method is rhodium-catalyzed carbonization with low water level of methanol to acetic acid as exemplified in U.S. Patent No. 5,001,259. In general, the rhodium component of the catalyst system is considered to be present in the form of a coordination compound of rhodium with a halogen component that offers at least one of the ligands of said coordination compound. In addition to the coordination of rhodium and halogen, it is also believed that carbon monoxide coordinates with rhodium. The rhodium component of the catalyst system can be provided by introducing rhodium salts such as oxides, acetates, iodides, etc. into the rhodium metal reaction zone. or other rhodium coordination compounds, and the like. The halogen promoter component of the catalyst system consists of a halogen system comprising an organic halide. Thus, alkyl, aryl, and substituted alkyl or aryl halides can be used. Preferably, the halide promoter is present in the form of an alkyl halide in which the alkyl radical corresponds to the alkyl radical of the supply alcohol, which is carbonylated. Thus, in the carbonization of methanol to acetic acid, the halide promoter will include methyl halide and more preferably methyl iodide. The liquid reaction medium employed can include any solvent compatible with the catalyst system and can include pure alcohols, or mixtures of the desired alcohol and / or carboxylic acid supply material and / or esters of these two compounds. The preferred solvent and liquid reaction medium for the carbonylation process with low water level is the carboxylic acid product. Thus, in the carbonylation of methanol to acetic acid, the preferred solvent is acetic acid. The water is contained in the reaction medium, but at concentrations well below what is thought until now that it is practical to achieve sufficient reaction percentages. It has previously been shown that in rhodium catalyzed carbonization reactions of the type established in this invention, the addition of water exerts a beneficial effect on the reaction percentage (U.S. Patent No. 3,769,329). Thus, most commercial operations operate with water concentrations of at least about 14% by weight. Accordingly, reaction rates substantially equal to and higher than the reaction rates obtained with such high levels of water concentration are not expected to be achievable with water concentrations below 14% by weight and as low as about 0.1% by weight. weight. In accordance with the most useful carbonylation process for making acetic acid according to the present invention, the desired reaction percentages obtain even low concentrations of water by including in the reaction medium methyl acetate and an additional iodide ion which is higher than of the iodide that is present as a catalyst promoter such as a methyl iodide or other organic iodide. The additional iodide promoter is an iodide salt, with lithium iodide being preferred. It has been found that under low water concentrations, methyl acetate and lithium iodide act as promoters of the percentage only when relatively high concentrations of each of these components are present and that the promotion is greater when both components are present simultaneously (patent of US No. 5,001, 259). It is thought that the concentration of lithium iodide used in the reaction medium of the preferred carbonization reaction system is quite high compared to how little the prior art handles regarding the use of halide salts in reaction systems of this type. The absolute concentration of iodide ion is not a limitation of the utility of the present invention. The reaction of carbonization of methanol to the acetic acid product can be carried out by contacting the methanol supply, which is in the liquid phase, with gaseous carbon monoxide bubbled through a solvent reaction medium with liquid acetic acid containing to the rhodium catalyst, methyl iodide promoter, methyl acetate and additional soluble iodide salt, under conditions of suitable temperature and pressure to form the carbonization product. It will generally be recognized that it is the concentration of the iodide ion to the catalyst system that is important and not the cation associated with the iodide and that, at a given molar concentration of iodide, the nature of the cation is not as important as the effect of the iodide concentration. Any metal iodide salt, or any iodide salt of any organic cation, or quaternary cation such as quaternary amine or phosphine or inorganic cation can be used with the proviso that the salt is sufficiently soluble in the reaction medium to provide the desired level of I last. When the iodide is added as a metal salt, it is preferably an iodide salt of a member of the group consisting of the metals of group IA and group HA of the periodic table as set out in the "Handbook of Chemistry and Physics", published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). In particular, alkali metal iodides are useful, with lithium iodide being preferred. In the low water content carbonization process which is most useful in this invention, the additional iodide in addition to the organic iodide promoter is present in the catalyst solution in amounts of about 2 to about 20% by weight, methyl acetate is present in amounts of about 0.5 to about 30% by weight and lithium iodide is present in amounts of about 5 to about 20% by weight. The rhodium catalyst is present in amounts of about 200 to about 2000 parts per million (ppm). Typical reaction temperatures for carbonylation will be from about 150 to about 250 ° C, preferring the temperature scale from about 180 to about 220 ° C. The partial pressure of carbon monoxide in the reactor can vary widely, but is typically from about 2 to about 30 atmospheres, and preferably from about 3 to about 10 atmospheres. Due to the partial pressure of byproducts and the vapor pressure of the contained liquids, the total pressure of the reactor will be in the range of about 15 to about 40 atmospheres. A typical acetic acid reaction and recovery system used for the rhodium catalyzed carbonylation promoted by methanol iodide to acetic acid is shown in Figure 1 and includes a liquid phase carbonylation reactor, rapid vaporization boiler and a column of light fractions of acetic acid with methyl iodide 14 having a side stream of acetic acid 17 which proceeds to further purification. The reactor and the rapid vaporization boiler are not shown in figure 1. They are considered standard equipment very well known at the present time in the technique of carbonylation processes. The carbonylation reactor is typically either a stirred vessel or a bubble column type within which the reaction liquid or suspension content is automatically maintained at a constant level. Fresh methanol, carbon monoxide, sufficient water as needed to maintain at least a finite concentration of water in the reaction medium, a recirculated catalyst solution from the base of the rapid vaporization boiler, a constant flow of fresh methanol, carbon monoxide, and a constant flow rate are continuously introduced into this reactor. recirculated phase of methyl iodide and methyl acetate and a recirculated phase of aqueous acetic acid from a steam receiver decanter effluent from the separating column or from light fractions 14 of acetic acid with methyl iodide. Distillation systems are employed which offer means for recovering the crude acetic acid and the recirculating catalyst solution, methyl iodide and methyl acetate to the reactor. In a preferred process, carbon monoxide is continuously introduced into the carbonylation reactor immediately below the agitator, which is used to stir the contents. The gaseous supply is dispersed profusely through the reaction liquid by means of this agitation medium. A gaseous purging stream is vented from the reactor to prevent the accumulation of gaseous by-products and to maintain a fixed partial pressure of carbon monoxide at a given total reactor pressure. The temperature of the reactor is controlled and the carbon monoxide supply is introduced at a sufficient percentage to maintain the total desired reactor pressure. The liquid product is extracted from the carbonylation reactor in a sufficient percentage to maintain a constant level inside it and is introduced to the rapid vaporization boiler. In the rapid vaporization boiler the catalyst solution is extracted as a base current (predominantly acetic acid containing rhodium and iodide salt together with minor amounts of methyl acetate, methyl iodide and water), while the steam stream Effluent from the rapid vaporization boiler contains mainly acetic acid together with methyl iodide, methyl acetate and water. Dissolved gases that leave the reactor and enter the rapid vaporization boiler consist of a portion of the carbon monoxide together with gaseous byproducts such as methane, hydrogen and carbon dioxide and leave the rapid vaporization boiler as part of the current of steam effluent. The effluent vapor stream is directed to the separating column or to light fractions 14 as the stream 26. It has been described in the patents of E.U.A. Nos. 6,143,930 and 6,339,171 that there is a higher concentration, approximately 3 times, of the PRC and in particular of the acetaldehyde content in the light phase current than in the heavy phase stream leaving the column 14., according to the present invention, the stream 28 containing the PRC is directed to a effluent vapor receiver decanter 16, in which the phase of light fractions, stream 30, is directed to the distillation column 18. It can be considered the present invention in general terms as an improved process for distilling the PRC, mainly aldehydes and alkyl iodides, from a stream of acetic acid in the vapor phase, such as the effluent vapor from a light-fraction distillation column or a column combined drying / light fractions. The steam phase is distilled and then extracted twice to remove the PRC. An especially preferred method of removing alkyl aldehydes and iodides from a first stream of acetic acid in the vapor phase and reducing the levels of propionic acid in the product includes the following steps: a) condensing the first stream of acetic acid in the vapor phase in a first condenser and separating it biphasically to form a first heavy product in liquid phase and a first light product in liquid phase; b) distilling the first light product in liquid phase in a first distillation column to form a second product stream of acetic acid in vapor phase which is enriched in aldehydes and alkyl iodide with respect to said first in-phase acetic acid stream steam; c) condensing the second stream in vapor phase in a second condenser to form a second product in liquid phase; d) distilling the second product in liquid phase in a second distillation column to form a third stream in vapor phase; e) condensing the third current in the vapor phase and extracting the condensed current with water to remove the residual acetaldehyde from it; and f) extracting the extracted condensed stream with water to remove additional residual acetaldehyde therefrom. One embodiment of the prior art as described in the U.S.A. No. 6,339,171 is shown in Figure 1. With reference to Figure 1, the first vapor phase acetic acid stream (28) contains methyl iodide, methyl acetate, acetaldehyde and other carbonyl components. It is then condensed and tapped (in vessel 16) to separate the heavy phase product containing the highest proportion of catalyst components - which is recirculated to the reactor (not shown in Figure 1) and a light phase (30). ) containing acetaldehyde, water and acetic acid. Any of the two phases of the effluent vapor of light fractions can be subsequently distilled to remove the PRC and mainly the acetaldehyde component of the stream, although it is preferred to remove the PRC from the light phase (30), because it has been found that the concentration of acetaldehyde is somewhat greater in that phase. In the embodiment shown and described herein, distillation is carried out in two stages; but it will be appreciated that the distillation can also be carried out in a single column. The light phase (30) is directed to column 18, which serves to form a second vapor phase (36) enriched in alkyl aldehydes and iodides with respect to stream 28. Stream 36 (vessel 20) is condensed to form a second product in liquid phase. The second liquid phase (40) containing acetaldehyde, methyl iodide, methanol and methyl acetate is directed to a second distillation column (22), in which the acetaldehyde is separated from the other components. It has been found that this method of the invention reduces and / or removes at least 50% of the alkyl iodide impurities found in a stream of acetic acid. It has also been shown that acetaldehyde and its derivatives are reduced and / or removed by at least 50%, more frequently by more than 60%. As a result, it is possible to maintain the concentration of propionic acid in the product of acetic acid less than about 400 parts per million by weight, preferably less than about 250 parts per million. From the upper part of the separating column or light fractions 14, the vapors are removed through stream 28, condensed and directed to vessel 16. The vapors are cooled to a temperature sufficient to condense and separate the condensable iodide components of methyl, methyl acetate, acetaldehyde and other carbonyl, and water in two phases. A portion of the stream 28 includes non-condensable gases, such as carbon dioxide, hydrogen and the like and can be vented, as shown in stream 29 of Figure 1. Also leaving the steam receiver decanter effluent 16, but not illustrated in Figure 1, there is the heavy phase of stream 28. Ordinarily this heavy phase is recirculated to the reactor, but a small amount, for example 25% by volume, preferably less than about 20% by volume, of the heavy phase to a carbonyl treatment process and recirculate the remainder to the reactor or reaction system. This slippery current of the heavy phase must be treated individually or combined with the light phase (stream 30) for subsequent distillation and extraction of carbonyl impurities. The light phase (stream 30) is directed to the distillation column 18. A portion of the stream 30 is returned to the column of light fractions 14 as reflux stream 34. The remainder of the column 30 enters the column 18 as the current 32 in approximately the middle part of the column. Column 18 serves to concentrate the aldehyde components of stream 32 to stream 36 of effluent vapor, separating water and acetic acid from lighter components. The first distillation column 18 preferably contains about 40 trays and the temperature varies therefrom from about 139.4 ° C in the lower part to about 88.3 ° C in the upper part of the column. Leaving the bottom of the column 18 is the stream 38 containing about 70% water and 30% acetic acid. Stream 38 is processed, cooled generally using a heat exchanger, recirculated to the decanter 16 in front of column vapor of light fractions through stream 46, 48 and finally the reactor or reaction system. It has been found that recirculation of a portion of stream 38 identified as current 46 back through decanter 16 increases the efficiency of the process of the invention and allows more acetaldehyde to be present in light phase stream 32. It has been found that stream 36 has approximately seven times more aldehyde content, when stream 38 is recirculated through decanter 16 in this manner. Exiting the top of column 18 is stream 36 containing PRC and in particular acetaldehyde, methyl iodide, methyl acetate and methanol, and alkyl iodides. The stream 36 is then directed to an effluent vapor receiver 20, after it has been cooled to start any condensable gases present. Exiting the effluent vapor receiver 20 is the stream 40 containing acetaldehyde, methyl iodide, methyl acetate and methanol. A portion of the stream 40 is returned to the column 18 as the reflux stream 42. the remainder of the stream 40 enters the second distillation column 22 near the bottom of the column. Column 22 serves to separate most of acetaldehyde from methyl iodide, methyl acetate and methanol in stream 40. In one embodiment, column 22 contains approximately 100 trays and overcomes a temperature ranging from about 106.6 ° C in the bottom approximately 79.4 ° C at the top. In an alternative preferred embodiment, column 22 contains structured packing in place of trays. The preferred packing is a structured packing with an interfacial area of approximately 2.13 m2 / m3, preferably made of a metal alloy such as 2205 or other suitable packaging material, so long as it is compatible with the compositions to be purified in the column. It was observed during the experimentation that the uniform loading of the column, which is required for good separation, was better with the structured packing than with the trays. Alternatively, ceramic packing can be employed. The residue from column 22, stream 44, leaves the bottom of the column and is recycled to the carbonization process. Acetaldehyde is polymerized in the presence of methyl iodide to form metaldehyde and paraldehyde. These polymers are generally of low molecular weight, less than about 200. It has been found that paraldehyde is relatively soluble in the reaction liquid and mainly in acetic acid. The metaldehyde, after its precipitation, is a sandy granular polymer that is not soluble in the reaction liquid beyond about 3% by weight of concentration. As described in the patent of E.U.A. 6,339,171, however, it has been discovered that, during the reaction and with the heating of column 22, polymers of higher molecular weight acetaldehyde are formed. It is believed that these higher molecular weight polymers (molecular weight greater than about 1000) are formed during the processing of the light phase and are viscous and thixotropic. As heat is applied to the system, they tend to harden and adhere to the walls of the tower, where their removal is uncomfortable. Once polymerized, they are only slightly soluble in organic or aqueous solvents and can be removed from the system only by mechanical means. Then an inhibitor is needed, preferably to column 22, to reduce the formation of these impurities, ie metaldehyde and paraldehyde and polymers of acetaldehyde (AcH) of higher molecular weight. The inhibitors generally consist of alkanoles of C? -10, preferably methanol; water, acetic acid and the like, used individually in a combination with one another or with one more of other inhibitors. Stream 46, which is a portion of the residue in column 18 and a slippery stream of stream 38, contains water and acetic acid and therefore serves as an inhibitor. As shown in Figure 1, stream 46 is separated to form streams 48 and 50. Stream 50 is added to column 22 to inhibit the formation of metaldehyde and paraldehyde impurities and higher molecular weight polymers. Since the residue of the second column 22 is recycled to the reactor, any added inhibitors must be compatible with the reaction chemistry. It has been found that small amounts of water, methanol, acetic acid or a combination thereof do not interfere with the reaction chemistry and virtually eliminate the formation of acetaldehyde polymers. The stream 50 is also preferably used as an inhibitor, since this material does not change the water balance in the reactor. Although water is not particularly preferred as an inhibitor, other important advantages are obtained by adding water to the column 22, as will be explained later. Leaving the top of column 22 is the current 52 containing PRC. The stream 52 is directed to a condenser and then to the effluent vapor receiver 24. After condensation, any non-condensable materials of the receiver 24 are vented; the condensed materials leave the receiver 24 as the stream 54. The stream 56, a slipstream of the stream 54, is used as a reflux liquid for the column 22. Exiting the bottom of the column 22 is the stream 44 containing Methyl iodide, methanol, methyl acetate, methanol and water. Current is combined with stream 72, which will be described later, and directed to the reactor. It is important for the extraction mechanism that the effluent vapor stream from column 22 remain cool, generally at a temperature of about 13 ° C. This current can be obtained or maintained at about 13 ° C by conventional methods known to those skilled in the art or any mechanism generally accepted by the industry. After leaving the receiver 24, the current 58 is measured preferably through a condenser / cooler (now the stream 62) and then to a first extractor 27. In the extractor 27, the PRCs of the alkyl iodides are extracted with water, preferably water from an internal stream in order to maintain the water balance within the reaction system. As a result of this extraction, the methyl iodide is separated from the aqueous phase of PRC and of alkyl iodide. In a preferred embodiment, a mixer-settler with a water ratio to the supply material of about 2 is employed. Stream 64 of aqueous extract leaves the extractor 27 from the top thereof. This aqueous phase rich in PCR and, in particular, rich in acetaldehyde is directed to the waste treatment. Also coming out of the extractor is stream 76 of refined residue containing methyl iodide. The stream 66 of additional refined residue is extracted with water in a second extractor 25. In the extractor 25, as in the extractor 27, the PRC and the alkyl iodides are extracted with water, preferably water coming from an internal stream in order to maintain the water balance within the reaction system. As a result of this extraction, the methyl iodide is separated from the aqueous phase of PRC and alkyl iodide. In a preferred embodiment, a mixer-settler with a water ratio to the supply material of about 1 is employed. The stream 70 of aqueous extract leaves the extractor from the top thereof. This aqueous phase is rich in PRC and, in particular, rich in acetaldehyde to the waste treatment. Also coming out of the extractor is stream 72 of refined residue containing methyl iodide. Normally recirculated is current to the reaction system and finally to the reactor. It will be readily apparent to one skilled in the art that additional extraction steps may be added, as desired, to further increase the fraction of methyl iodide recovered from the effluent vapor rich in acetaldehyde from column 22. It will be apparent also that variations are possible. additional, in which pass a single stream of water through the stages of extraction in series, instead of using pure water in each stage. Finally, it is evident that multiple stage extraction can also be performed as described herein, using a compact bed extractor (continuous contact) having an adequate number of theoretical stages in place of the equipment having individual steps. A potential problem with the multistage extraction described hereinabove is that each water extraction removes not only the acetaldehyde, but also a measurable amount of methyl iodide. As explained hereinabove, since methyl iodide is an especially expensive component of the reaction systemIt is highly desirable to minimize the amount of methyl iodide that is removed from the process as waste in order to reduce the amount of new methyl iodide that must be supplied to the reactor. The present applicants have discovered, however, that the addition of dimethyl ether (DME) to the supply material to extract 27 limits the loss of methyl iodide in the extraction steps. The presence of DME produces the solubility of methyl iodide in water, thus reducing the amount of methyl iodide extracted to streams 64 and 70 of aqueous extract and lost in the treatment of waste water. By way of example, applicants observed that the concentration of methyl iodide in stream 64 dropped from about 1.8%, when DME was not present, to about 0.5%, when DME was present. Accordingly, a further aspect of the present invention includes the step of injecting DME into the upstream of extractor process 27, for example to stream 62, to reduce the loss of methyl iodide to streams 64 and 70 of aqueous extract. . The required amount of DME can be achieved in stream 62 by adding water to column 22, for example to supply material 40 or reflux liquid 50. Although it is not necessary to understand the precise mechanism of DME formation in the column 22 to practice the present invention, it is believed that water reacts with methyl acetate and / or methyl iodide in column 22 to form methanol, which is then dehydrated in the presence of an acid catalyst (such as Hl). ) to form DME. Any DME that is not extracted to streams 64 and 70 of aqueous extract is recirculated directly or indirectly to the reaction system, where it reacts with carbon monoxide and water to form acetic acid.

Although the invention has been described with reference to the preferred embodiments, modifications and obvious alterations are possible by those skilled in the art. In particular, although the present invention has been described above in general using the light-fraction phase of column 14, any stream can be treated in the carbonylation process having a high concentration of PRC and alkyl iodides according to the present invention. Therefore, it is intended that the invention include all modifications and alterations to the full extent that fall within the scope of the following claims or equivalents thereof.

Claims (41)

NOVELTY OF THE INVENTION CLAIMS

1. - An improved method for the reduction and / or removal of permanganate reducing compounds (PRC), C3-8 carboxylic acids and C2-? 2 alkyl iodide, compounds formed in the carbonization of a carbonylatable reagent selected from the group consists of methanol, methyl acetate, methyl formate and dimethyl ester and mixtures thereof to an acetic acid product, the products of said carbonylation including a volatile phase which is distilled to produce a purified product of acetic acid and a first vapor of effluent comprising methyl iodide, water and at least one PRC, characterized in that the improvement comprises the steps of: (a) distilling at least a portion of the first effluent vapor to produce a second stream of effluent vapor comprising iodide of methyl, dimethyl ether and said at least one PRC; (b) extracting with water the second stream of effluent vapor to form a first refined residue and a first stream of aqueous extract containing said at least one PRC; and (c) extracting with water the first refined residue to form a second refined residue and a second stream of aqueous extract containing said at least one PRC.

2. - The method according to claim 1, further characterized in that said at least one PRC comprises acetaldehyde.

3. The method according to claim 2, further characterized in that sufficient acetaldehyde is removed from said volatile phase to maintain in said purified product a propionic acid concentration of less than about 400 parts per million by weight.

4. The method according to claim 2, further characterized in that sufficient acetaldehyde is removed from said volatile phase to maintain in said purified product a concentration of propionic acid of less than about 450 parts per million by weight.

5. The method according to claim 1, further characterized in that the improvement further comprises introducing at least a portion of the second refined residue directly or indirectly into the reaction medium.

6. The method according to claim 1, further characterized in that the extraction steps (b) and (c) are carried out in separate containers.

7. The method according to claim 1, further characterized in that the extraction steps (b) and (c) are carried out in at least one compact bed extractor.

8. - The method according to claim 1, further characterized in that the extraction steps (b) and (c) are carried out on trays within a single extraction vessel.

9. The method according to claim 1, further characterized in that the water, for one of the extraction steps (b) and (c), constitutes at least a portion of one of the aqueous extract streams.

10. The method according to claim 1, further characterized in that it additionally comprises at least one additional step of extracting with water the second refined residue to provide a third aqueous extract the third refined residue.

11. The method according to claim 10, further characterized in that the water, for said at least one additional extraction step, constitutes at least a portion of one or more of said first, second and third streams of aqueous extracts. .

12. The method according to claim 1, further characterized in that said first effluent vapor comprises dimethyl ether.

13. The method according to claim 1, further characterized in that it further comprises the step of adding dimethyl ether to at least one current associated with said distillation step (a).

14. - The method according to claim 1, further characterized in that said distillation step (a) further comprises the step of forming dimethyl ether during the distillation.

15. The method according to claim 1, further characterized in that said distillation step (a) comprises at least two consecutive distillation steps.

16. The method according to claim 1, further characterized in that said effluent vapor comprises an amount of dimethyl ether effective to reduce the solubility of methyl iodide in at least one of said aqueous extract streams.

17. A process for producing acetic acid, characterized in that it comprises the steps of: (a) carbonylating at least one reagent selected from the group consisting of methanol, methyl acetate, methyl formate and dimethyl ether in a reactor containing a suitable reaction medium comprising organic iodide; (b) separating the products of said carbonylation into a phase of volatile products comprising acetic acid and a less volatile phase; (c) distilling said phase of volatile products to produce a purified product of acetic acid and a first effluent vapor comprising said organic iodide and at least one permanganate reducing compound (PRC); (d) distilling at least a portion of the first effluent vapor to produce a second effluent vapor enriched in PRC, said second effluent vapor further comprising dimethyl ether; and (e) extracting the second effluent vapor with water; wherein step (e) comprises at least two consecutive extraction steps, each extraction step comprising contacting the second effluent vapor with water and separating therefrom an aqueous stream comprising said at least one PRC.

18. The process according to claim 17, further characterized in that said at least one PRC comprises acetaldehyde.

19. The process according to claim 18, further characterized in that sufficient acetaldehyde is removed from said volatile phase to maintain in said purified product a concentration of propionic acid of less than about 400 parts per million by weight.

20. The process according to claim 18, further characterized in that sufficient acetaldehyde is removed from said volatile phase to maintain in said purified product a concentration of propionic acid of less than about 450 parts per million by weight.

21. The process according to claim 17, further characterized in that it further comprises recirculating at least a portion of the effluent vapor extracted directly or indirectly to the reaction medium.

22. The method according to claim 17, further characterized in that said at least two extraction steps are carried out in separate containers.

23. - The method according to claim 17, further characterized in that the extraction steps (b) and (c) are carried out on trays within a single extraction vessel.

24. The method according to claim 17, further characterized in that the extraction steps (b) and (c) are carried out in at least one compact bed extractor.

25. The process according to claim 17, further characterized in that the water, at least for one of the extraction steps, constitutes at least a portion of one of the aqueous extract streams.

26. The process according to claim 17, further characterized in that said second effluent vapor comprises an amount of dimethyl ether effective to reduce the solubility of methyl iodide in at least one of said aqueous extract streams.

27. The method according to claim 17, further characterized in that said first effluent vapor comprises dimethyl ether.

28. The method according to claim 17, further characterized in that it further comprises the step of adding dimethyl ether to at least one current associated with said distillation step (d).

29. - The process according to claim 17, further characterized in that said distillation step (d) further comprises the step of forming dimethyl ether during the distillation.

30. A process for separating a mixture comprising water, acetic acid, methyl iodide, methyl acetate methanol and at least one permanganate reducing compound (PRC), said process comprising the steps of: (a) distilling the mixture for separating the mixture into a plurality of streams, at least one of said streams being an effluent vapor stream enriched in PRC comprising dimethyl ether; and (b) extracting the effluent vapor stream enriched in PRC with water; wherein step (b) comprises at least two consecutive extraction steps, each extraction step comprising contacting with water the effluent vapor stream enriched in PRC and separating therefrom an aqueous stream comprising said at least one PRC.

31. The process according to claim 30, further characterized in that said effluent vapor stream enriched in PRC comprises an amount of dimethyl ether effective to reduce the solubility of methyl iodide in said aqueous extract streams.

32. The method according to claim 30, further characterized in that it additionally comprises the step of adding dimethyl ether to the effluent vapor stream enriched in PRC, before extracting with water the effluent vapor stream enriched in PRC.

33. - The method according to claim 30, further characterized in that said at least one PRC comprises acetaldehyde.

34. The process according to claim 30, further characterized in that said distillation step (a) further comprises the step of forming dimethyl ether during the distillation.

The method according to claim 30, further characterized in that it further comprises the step of providing said mixture by separating a liquid composition into a light phase and a heavy phase, said liquid composition comprising water, acetic acid, iodide of methyl, methyl acetate, methanol and said at least one PRC, wherein the light phase comprises said mixture and the heavy phase comprises methyl iodide.

36. The method according to claim 30, further characterized in that said at least one PRC comprises acetaldehyde.

37.- The method according to claim 30, further characterized in that it additionally comprises the steps of: carrying out a separation of the liquid and vapor phases on the effluent of a methanol carbonization reactor to form a vapor phase and a phase liquid; distilling the vapor phase to form a first effluent vapor and a liquid product; and condensing at least a portion of the first effluent vapor to provide said liquid composition.

38. - The method according to claim 37, further characterized in that it comprises additionally directly or indirectly recirculating to the reactor at least a portion of the vapor effluent enriched in PRC, extracted.

39.- The method according to claim 30, further characterized in that said at least one PRC comprises acetaldehyde.

40.- The method according to claim 39, further characterized in that sufficient acetaldehyde is removed from said volatile phase to maintain a propionic acid concentration of less than about 400 parts per million by weight in said purified product.

41. The method according to claim 39, further characterized in that sufficient acetaldehyde is removed from said volatile phase to maintain a propyonic acid concentration of less than about 250 parts per million by weight in said purified product.