MXPA06010007A - Process for producing acetic acid - Google Patents

Abstract

An improved process is disclosed for producing acetic acid, including the following steps:reacting a carbonylatable reactant such as methanol, methyl acetate, methyl formate or dimethyl ether with carbon monoxide in a reaction medium containing water, methyl iodide, and a catalyst to produce a reaction product that contains acetic acid;separating the reaction product to provide a volatile phase containing acetic acid, water, and methyl iodide and a less volatile phase;distilling the volatile phase to produce a purified acetic acid product and a first overhead containing water, methyl acetate, and methyl iodide;phase separating the first overhead to provide a first liquid phase containing water and a second liquid phase containing methyl iodide;and adding dimethyl ether to the process in an amount effective to enhance separation of the first overhead to form the first and second liquid phases.

Description

PROCEDURE TO PRODUCE ACETIC ACID BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to an improved process for producing acetic acid by carbonylation of methanol.

PREVIOUS TECHNIQUE Among the methods currently employed to synthesize acetic acid, of the most commercially useful is a catalyzed carbonylation of methanol with carbon monoxide, as taught in the U.S. patent. 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 on an inert solid, together with a halogen-containing catalyst promoter, such as methyl iodide. 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. Also, the nature of the halide promoter is not critical. The patent holders exhibit a very large number of suitable promoters, most of which are organic iodides. In a very typical and useful manner, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved. A major improvement in 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 set forth in U.S. Patents. No. 5,001, 259 (issued March 19, 1991); 5,026,908 (issued June 25, 1991); and 5,144,068 (issued September 1, 1992) and European Patent No. EP 0 161 874 B2, published July 1, 1992. These patents disclose a process in which acetic acid is produced from methanol in a medium of reaction containing methyl acetate, methyl halide, especially methyl iodide, and a catalytically effective concentration of rhodium. The inventors of these patents found that the stability of the catalyst and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even very low water concentrations, ie 4% by weight (p) or less, in the reaction medium (despite the general Industrial practice of maintaining approximately 14% by weight or 15% by weight of water), maintaining in the reaction medium, together with a catalytically effective amount of rhodium, at least a finite concentration of water, acetate of methyl and methyl iodide, a specified concentration of iodide ions in addition to the content of iodide that 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 that the concentration of acetate salts and methyl iodide are significant parameters in influencing the rate of methanol carbonylation to produce acetic acid especially at low concentrations of water in the reactor. By using relatively high concentrations of the acetate salt and methyl iodide, a surprising degree of catalyst stability and reactor productivity is obtained, even when the liquid reaction medium contains water and concentrations as low as about 0.1% by weight, so low that they can be defined simply as "a finite concentration" of water. In addition, the reaction medium used improves the stability of the rhodium catalyst, ie its resistance to catalyst precipitation, especially during the product recovery steps of the process. The distillations carried out in the process to recover the acetic acid product tend to remove the carbon monoxide ligands from the catalyst. These ligands have a stabilizing effect on rhodium in the environment maintained in the reaction vessel. The patents of E.U.A. No. 5, 001, 259, 5,026,908 and 5,144, 068 are incorporated herein by reference. It has also been found that, although a carbonylation process with low water content for the production of acetic acid produces such byproducts as carbon dioxide, hydrogen and propionic acid, the amount of other impurities, generally present in trace amounts, is also increased. , and the quality of the acetic acid is sometimes reduced when attempts are made to increase the production rate, improving the catalysts or modifying the reaction conditions. These vestigial impurities affect the quality of the acetic acid product, especially when recirculated through the reaction procedure. See Catalysis of Organic Reactions, 75, 369-380 (1998), for further discussion on impurities in a carbonylation reaction system. The crude acetic acid product is typically distilled in one or more distillation columns to remove the reaction components of light fractions (typically methyl acetate and methyl iodide), water and heavy fraction impurities. It has been previously observed that it is particularly important to avoid refluxing large quantities of methyl iodide back to the light-distillation column, because the separation of the reaction components from light fractions of the acetic acid product is significantly degraded, if Methyl iodide is allowed to reflux back into the column of light reactions. Ordinarily, methyl iodide is prevented from entering reflux, separating most of the methyl iodide from the effluent vapor of light fractions as a distinct phase, but under certain conditions the effluent vapor from light fractions can form a single liquid phase that includes iodide of methyl. The present invention provides a method for avoiding this single-phase condition in the column of light fractions.

BRIEF DESCRIPTION OF THE INVENTION One aspect of the present invention is a process for producing acetic acid, which includes the following steps: reacting carbon monoxide with a carbonatable material, such as methanol, methyl acetate, methyl formate, dimethyl ether or mixtures thereof , in a reaction medium containing water, methyl iodide and a catalyst to produce a reaction product containing acetic acid; perform a separation of vapors and liquids on the reaction product to provide a volatile phase containing acetic acid, water and methyl iodide and a less volatile phase containing the catalyst; distilling the volatile phase to produce a purified product of acetic acid and a first effluent vapor containing water and methyl iodide; phasing the first effluent vapor to provide a first liquid phase containing water and a second liquid phase containing methyl iodide; and adding methyl ether to at least one of the reaction product, the volatile phase, the first effluent vapor or a stream associated with the distillation, to improve the separation of the first effluent vapor to form the first and second liquid phases. Another aspect of the invention is an improved method for distilling a mixture containing acetic acid, methyl iodide and water to provide a purified product of acetic acid, a first liquid phase containing water and a second liquid phase containing methyl iodide. . In this method, an effluent vapor fraction in the distillation is separated to form the first and second liquid phases and a portion of the first liquid phase in the distillation is refluxed. The improvement involves adding dimethyl ether to the mixture. To the effluent vapor fraction of the portion subjected to reflux of the first liquid phase in an amount effective to improve the phase separation of the first and second liquid phases.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a process flow diagram for a method according to the present invention. Although the invention is susceptible to various modifications and attractive forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms set forth. Rather, it is intended that the invention cover all modifications, equivalents and alternatives that fall within the scope of the invention, as defined in the appended claims.

DESCRIPTION OF THE ILLUSTRATIVE MODALITIES An illustrative embodiment of the invention is described below. For purposes of clarity, not all the characteristics of a real implementation in this specification are described. It will be appreciated, of course, that in the development of any real mode, numerous specific decisions must be made regarding implementation to achieve the specific purposes of the inventors, such as compliance with system-related and trade-related restrictions, which they will vary from one implementation to another. Furthermore, it will be appreciated that such a development effort could be complex and time-consuming, but would nonetheless be a routine enterprise for those skilled in the art who have the benefit of this exposure. The present invention is useful in any process used to carbonylate methanol to acetic acid in the presence of a metal catalyst of group Vll1, such as rhodium, and an iodide promoter. A particularly useful process is a rhodium catalyzed carbonylation with a low water content of methanol to acetic acid, as exemplified in the patent of E.U.A. No. 5,001, 259 mentioned above. The rhodium component of the catalyst system can be provided by introducing rhodium into the reaction zone in the form of rhodium metal, rhodium salts, such as oxides, acetates, iodides, etc., or other rhodium coordination compounds.

The halogen promoter component of the catalyst system includes an organic halide. Thus, alkyl, aryl, and substituted alkyl and 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 carbonylation of methanol to ac acid, the halide promoter will be a 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 alcohol supply material and / or the carboxylic acid and / or the desired esters of these two compounds. The preferred solvent and liquid reaction medium for the carbonylation process with low water content is the carboxylic acid product itself. Thus, in the carbonylation of methanol to ac acid, the preferred solvent is ac acid. Water is present in the reaction medium at concentrations much lower than what was originally thought to be practical for achieving sufficient reaction regimes. It had previously been taught that, in rhodium-catalyzed carbonylation reactions of the type disclosed in this invention, the addition of water exerts a beneficial effect on the reaction regime (U.S. Patent No. 3,769,329). Thus, most commercial operations are carried out at water concentrations of at least about 14% by weight. Accordingly, it was quite unexpected that reaction regimes substantially equal to or higher than the reaction regimes obtained with such high water concentration levels could be achieved, with water concentrations below 14% by weight and as low as about 0.1% by weight. weight. According to the carbonylation process it is not useful to manufacture ac acid according to the present invention, the desired reaction regimes are obtained even at low concentrations of water, including in the reaction medium methyl ate and an additional iodide ion is in addition to iodide which is present as a catalyst promoter, such as methyl iodide or other organic iodide. The additional iodide promoter is an iodide salt, with lithium iodide being preferred. It has been found that at low concentrations of water methyl ate and lithium iodide act as regime promoters, only when relatively high concentrations of each of its components are present, and that the promotion is greater, when these two components are present. present simultaneously (U.S. Patent No. 5,001, 259). The reaction of carbonylation of methanol to ac acid product can be carried out by contacting the supply of methanol, which is typically in the liquid phase, with gaseous carbon monoxide subjected to bubbling through a liquid reaction medium. of ac acid solvent containing the rhodium catalyst, the methyl iodide promoter, methyl ate and the additional soluble iodide salt, at suitable temperature and pressure to form the carbonylation product. It will generally be recognized that it is the concentration of the iodide ion in the catalyst system that is important and not the cation associated with the iodide and that at a given molar concentration of iodide the nature of the cation is not as significant as the effect of the concentration of iodide. I last. Accordingly, any metal iodide salt or any iodide salt of any organic cation or quaternary cation, such as a quaternary amine or a phosphine or an inorganic cation, can be used, provided the salt is sufficiently soluble in the reaction medium to provide the desired level of iodide. When the iodide is added as a metal salt, it is preferably an iodide salt of a group level consisting of metals of group IA and of the HA group of the periodic table as set forth in the Handbook of Chemistry and Physics., published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). In particular, alkali metal iodides are useful, with lithium iodide being preferred. In the most useful low water content carbonylation process in this invention, additional iodide in addition to the organic iodide promoter is present in the catalyst solution at about 2 and up to about 20% by weight, methyl acetate is present at about 0.5 and up to 30% by weight and the lithium iodide is present at about 5 and up to about 20% by weight. The rhodium catalyst is present at about 200 and up to about 2000 parts per million by weight (ppm).

Typical reaction temperatures for carbonylation are from about 150 ° to about 250 ° C, preferably from about 180 ° to about 220 ° C. The partial pressure of the 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. Because of the partial pressure of the byproducts and the vapor pressure of the contained liquids, the total pressure of the reactor will vary from about 15 to about 40 atmospheres. A typical acetic acid reaction and recovery system used for rhodium catalyzed carbonylation and promoted with methanol iodide to acetic acid is shown in Figure 1. The reaction system includes a carbonylation reaction 10, a rapid vaporization boiler. 12 and a column of light fractions 14 of methyl iodide and acetic acid having a side stream 17 of acetic acid which proceeds to the greatest purification. As set forth in the patent of E.U.A. No. 5,416,237, incorporated herein by reference, the column of light fractions 14 may also incorporate additional steps that facilitate the separation of acetic acid and water, thus obviating the need for a separate drying column to effect this separation. The carbonylation reactor 10 is typically a stirred vessel or bubble column type within which the liquid content is automatically maintained at a constant level. To this reactor fresh methanol is continuously introduced through stream 6, carbon monoxide through stream 8, enough water as needed to maintain at least a finite concentration of water in the reaction medium, recirculated catalyst solution to through the stream 13 from the base of the rapid vaporization boiler 12, a recirculated phase 21 and the methyl iodide and methyl acetate, and a recirculated phase 36 of aqueous acetic acid from a decanter steam receiver effluent from the separating column to the light fractions 14 of methyl iodide and acetic acid. Distillation systems are employed that provide recovery of the crude acetic acid and recirculation of the catalyst solution, methyl iodide and methyl acetate to the reactor. In a preferred process, carbon monoxide is continuously introduced into a stirred carbonylation reactor immediately below the stirrer, thus completely dispersing the carbon monoxide through the reaction liquid. The gaseous purge stream from the reactor is vented to prevent the formation of gaseous side products and to control the partial pressure of the carbon monoxide at a given total reactor pressure. The temperature of the reactor is controlled and the supply of carbon monoxide is introduced at a rate sufficient to maintain the desired total reactor pressure. The liquid product is extracted from the carbonylation reactor 10 at a rate sufficient to maintain a constant level therein and is introduced into the rapid vaporization boiler 12. In the rapid vaporization boiler, the catalyst solution is removed as a base current ( predominantly acetic acid containing the rhodium catalyst and the iodide salt together with minor amounts of methyl acetate, methyl iodide and water), while the vapor stream effluent from the rapid vaporization boiler contains the crude product of acetic acid together with some methyl iodide, methyl acetate and water. The stream 11 leaving the reactor and entering the rapid vaporization boiler also contains dissolved gases which include a portion of the carbon monoxide together with gaseous side products, such as methane, hydrogen and carbon dioxide. They leave the rapid vaporization boiler as part of the effluent vapor stream 26 which is directed to the separating column or to light fractions 14. From the top of the separating column or from light fractions 14, the vapors are removed through stream 28, condensed and directed to the decanter 16. Stream 28 contains condensable water, methyl iodide, methyl acetate, acetaldehyde and other carbonyl components, as well as non-condensable gases, such as dioxide of carbon, hydrogen and the like that can be vented, as shown in stream 29 of Figure 1. The condensable vapors are preferably cooled to a temperature sufficient to condense and separate the condensable components of methyl iodide, methyl acetate, acetaldehyde and other carbonyl components and water to two liquid phases. At least a portion of the stream 30 returned to the column of light fractions 14 is directed as reflux stream 34.; in a preferred embodiment of the invention, another portion of the stream 30 is diverted as sidestream 32 and processed to remove the aldehyde and other permanganate reducing compounds, before being returned to the reaction system or the light fraction column. Various methods of treatment for removing acetaldehyde and other PRC are known in the art; some examples of such methods are set forth in the U.S.A. 5,625,095; 5,783,731; 6,143,930; and 6,339,171, each of which is incorporated herein by reference in its entirety. To help maintain the water balance within the process, still another portion 41 of the light phase 30 of the system can be purged or treated to remove the excess water before being returned to the reaction system. The heavy phase 21 of the stream 28 leaving the effluent vapor receiver decanter 16 is ordinarily recirculated to the reactor, but a slipstream can also be directed, generally a small amount, eg 25% by volume, preferably less than about 20% by volume. % by volume of the heavy phase, to a procedure for removing PRC and recirculating the rest to the reactor or reaction system. This slip current of the heavy phase can be treated individually or combined with the light phase current 30 for further distillation and distraction of carbonyl impurities. As explained previously, it is highly desirable to maintain a low concentration of water, for example less than 8% and preferably much lower in the carbonylation reaction medium for at least two reasons: first, maintaining a low concentration of water helps to control the amount of carbon dioxide formed as a secondary product in the reactor by the water and gas exchange reaction; second, and more significantly, the low concentrations of water also help control the amount of propionic acid formed as a byproduct. As the concentration of water in the reaction medium decreases, however, the vapor load in column 14 increases. The increased vapor load results in an unacceptably high excess of acetic acid to the decanter 16 at the top of the column of light fractions 14. The solubility of acetic acid in both the methyl and aqueous iodide phase causes the separation to deteriorate of phases, finally resulting in a single liquid phase in the decanter. When this condition occurs, the reflux of column 14 includes a high concentration of methyl iodide. The presence of this additional methyl iodide significantly interferes with the ability of the column 14 to completely remove materials from light fractions, such as methyl acetate, from the acetic acid product 17. This frequently requires that the entire system be interrupted. reaction, until the problem is corrected. (For this reason, only light phase 30, which has relatively little methyl iodide, is typically used as reflux in column 14). In view of this potential problem, it is extremely important to maintain the phase separation in the decanter 16, although this is made more difficult by the reaction conditions with low water content and by the tendency of the high concentrations of methyl acetate to create high vapor loads in the column of light fractions, which promotes the formation of a single phase, as mentioned above. Although this problem has been recognized to some extent in the US patent. No. 5,723,660, the disclosure of which is incorporated herein by reference, the solutions proposed therein involve costly steps, such as distilling the effluent vapor of light fractions to remove the methyl acetate or significantly reducing the temperature at which the effluent vapor of light fractions is cooled, before it enters the decanter. The third proposed solution, intermittently feeding water to the column of light fractions to ensure that the concentration of methyl acetate remains below 40% by weight, is likely to significantly alter the water balance throughout the process each time water is added. The present applicants have discovered another effective method of ensuring phase separation in the steam decanter effluent of light fractions 16 without any of the complicated steps proposed in the US patent. No. 5,723,660 and without significantly altering the water balance in the process. In simple terms, applicants have discovered that proper separation of phases in the decanter can be ensured by adding a component that a) is immiscible in water; b) is compatible with the chemistry of processes and c) counteracts the effect of acetic acid in the promotion of a single phase. Specifically, the applicants have found that, by adding dimethyl ether (DME) to the steam effluent of light fractions, the column supply of light fractions or other stream associated with the column of light fractions 14, the liquid content of the decanter can be prevented. form a single phase. In addition to being almost immiscible with water, the DME is compatible with the chemistry of procedures. As explained above, the heavy organic phase (rich in methyl iodide) formed in the decanter 16 is returned to the carbonylation reactor 10. The DME reacts with water and carbon monoxide under carbonylation reaction conditions to produce acetic acid . In addition, as set forth in the patent of E.U.A. No. 5,831, 120, since the carbonylation of DME consumes water, the DME is also useful for controlling the accumulation of water in the process. For example, the additional water consumed in the carbonylation of DME may make it unnecessary to purge or treat the portion 36 of the light phase 30 that returns to the reactor to remove excess water. Finally, the presence of DME in the lateral stream 32 of the light phase 30 that is further processed to remove the acetaldehyde has certain beneficial effects. Very notably, as set forth in detail in the patent application of E.U.A. Nos. 10 / 708,420 and 10 / 708,421, assigned in common, presented concurrently with the present, when sufficient DME is present in the light phase side stream 32 or is formed at the same sites in the acetaldehyde removal system, are reduced significantly undesirable losses of methyl iodide during the acetaldehyde removal process.

It will be appreciated that in processes for obtaining acetic acid such as the process described above, several process streams are recirculated within the purification area or from the purification area to the reaction system. Consequently, the DME can be added at any place in the process, provided that sufficient amount of DME is accumulated in the decanter of light fractions 16 to achieve the desired effect of improving the phase separation therein. For example, the DME (via stream 37) can be injected to the steam from fast vaporization boiler effluent 26 which supplies the column with light fractions 14 or can be supplied separately to the column (via stream 38). ). Alternatively, the DME can be injected into the column of light reactions through the reflux stream 34. It is currently believed, however, that the supply of additional DME through the column of light fractions 14 can contribute excessively to the load of steam in the column. Accordingly, it is preferred to add DME directly to the decanter of light fractions 16 through a stream or series of streams that does not pass through the column of light fractions 14. For example, DME can be added directly to the vapor stream. Effluent of light fractions 28 (like stream 35). Alternatively, in certain embodiments of the acetaldehyde removal technology, set forth in the U.S.A. No. 6,143,930 and in the patent applications of E.U.A. copending Nos. 10 / 708,420 and 10 / 708,421, presented concurrently with the present, all or a portion of the return stream from the acetaldehyde removal system returns to the decanter 16 or to the column of light fractions 14. The DME could be added to such a return stream also (for example, stream 46 in Figure 1 of US Patent No. 6,143,930) or a stream elsewhere in the acetaldehyde removal system, such that the return stream contains sufficient DME to improve phase separation in the decanter 16. Although the invention has been described with reference to the preferred embodiments, modifications and alternations obvious to those skilled in the art are possible. Therefore, it is intended that the invention include all those modifications and alterations to the full extent that are included in the scope of the following claims or the equivalents thereof.

Claims (14)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for producing acetic acid, characterized in that it comprises the steps of: (a) reacting carbon monoxide with at least one reagent selected from the group consisting of methanol, methyl acetate, methyl formate, dimethyl ether and mixtures thereof. same, in a reaction medium comprising water, methyl iodide and a catalyst, to produce a reaction product comprising acetic acid; (b) conducting a separation of vapors and liquids in said reaction product to provide a volatile phase comprising acetic acid, water and methyl iodide and a less volatile phase comprising said catalyst; (c) distilling said volatile phase to produce a purified product of acetic acid and a first effluent vapor comprising water, methyl acetate and methyl iodide; (d) phase separating said first effluent vapor to provide a first liquid phase comprising water and a second liquid phase comprising methyl iodide; and (e) adding dimethyl ether to the process in an amount effective to improve the separation of the first effluent vapor to form the first and second liquid phases.

2. The process according to claim 1, further characterized in that the dimethyl ether is added to at least one of said reaction product, said volatile phase, said first effluent vapor or a stream or column associated with said distillation.

3. The process according to claim 2, further characterized in that the dimethyl ether is added to said first effluent vapor.

4. The process according to claim 1, further characterized in that it further comprises the step of removing the acetaldehyde from at least one of said first and second liquid phases and wherein the dimethyl ether is added to a stream associated with the step of acetaldehyde removal.

5. The process according to claim 4, further characterized in that the dimethyl ether is added to a return stream from an acetaldehyde removal system.

6. The process according to claim 4, further characterized in that the step of removing the acetaldehyde comprises extracting the acetaldehyde from a mixture comprising methyl iodide and wherein a portion of the dimethyl ether is effective to reduce the amount of iodide of methyl extracted from said mixture with acetaldehyde.

7. The process according to claim 1, further characterized in that at least a portion of the first liquid phase is used as a reflux stream in the distillation of the volatile phase.

8. - The method according to claim 1, further characterized in that the second liquid phase is recirculated to provide a portion of the reaction medium.

9. The process according to claim 8, further characterized in that a greater part of the added dimethyl ether is recirculated to the reaction medium in the second liquid phase.

10. The process according to claim 9, further characterized in that at least part of the recirculated dimethyl ether is converted to acetic acid in the reaction medium.

11. A method for phasing a mixture comprising acetic acid, methyl acetate, methyl iodide and water to provide a first liquid phase comprising water and methyl acetate, and a second liquid phase comprising methyl iodide, characterized in that it additionally comprises adding dimethyl ether to the mixture to facilitate separation.

12. A method for separating a mixture comprising acetic acid, methyl iodide and water to provide a purified product of acetic acid, a first liquid phase comprising water and a second liquid phase comprising methyl iodide, characterized in that it comprises the steps of: distilling the mixture to provide a fraction of effluent vapor in said purified acetic acid product; phase separation of the effluent vapor fraction to provide said first and second liquid phases; refluxing a portion of the first liquid phase in the distillation; and adding dimethyl ether to the mixture, to the effluent vapor fraction by the portion refluxing the first liquid phase in an effective amount to improve the phase separation of the first and second liquid phases.

13. The method according to claim 12, further characterized in that the dimethyl ether is added to the effluent vapor fraction.

14. The method according to claim 12, further characterized in that the mixture is provided as a volatile phase of a reaction product of a carbonylation reactor.