HK1166306B - Acetic acid production by way of carbonylation with enhanced reaction and flashing - Google Patents

Description

Acetic acid production by carbonylation with enhanced reaction and flash

Priority claim

This application claims priority from U.S. patent application serial No. 12/459,725, filed on 7/2009, which is incorporated by reference in its entirety.

Technical Field

This invention relates to acetic acid production and in particular to a methanol carbonylation system having a medium pressure pre-flash/post reaction vessel for the removal of methyl iodide and the consumption of methyl acetate prior to flashing in a low pressure flash vessel. The low pressure absorber eliminates the bottleneck of the light ends column of the purification train.

Background

Among the currently used processes for the synthesis of acetic acid, the most commercially used one is the catalytic carbonylation of methanol with carbon monoxide. Preferred methods of practicing this technology include the so-called "low water" methods catalyzed with rhodium or iridium in the following patents: U.S. Pat. No.5,001,259 issued 3/19 1991; U.S. Pat. No.5,026,908 issued on 25.6.1991; and U.S. Pat. No.5,144,068 issued on 9/1 1992; and european patent No. ep 0161874B 2 published on 7/1 of 1992. Features involved in practicing the low water carbonylation process may include elevated concentrations of inorganic iodide anion in addition to iodide ion present due to hydrogen iodide in the system, as well as a catalytically effective amount of rhodium and at least a limited concentration of water maintained in the reaction medium. This iodide ion may be a simple salt, with lithium iodide being preferred in most cases. U.S. Pat. Nos. 5,001,259, 5,026,908, 5,144,068 and European patent No. EP 0161874B 2 are incorporated herein by reference.

Generally, a methanol carbonylation production line includes a reaction section, a purification section, light ends recovery, and a catalyst storage system. In the reaction zone, methanol and carbon monoxide are contacted in a reactor with a rhodium or iridium catalyst in a uniformly stirred liquid phase reaction medium to produce acetic acid. Methanol was pumped from the methanol surge tank to the reactor. The process is efficient, with methanol to acetic acid conversions typically greater than 99%. The reaction section also typically includes a flash vessel connected to the reactor and that flashes an extract stream to remove crude product from the reaction section. The crude product is fed to a purification section, which typically includes a light ends or stripping column, a drying column, an auxiliary purification, and optionally a finishing column. In the process, various non-condensable vent streams containing light ends, particularly methyl iodide, carbon monoxide and methyl acetate, are produced and fed to the light ends recovery section. These vent streams are washed with a solvent to remove light ends which are returned to the system or discarded.

Despite advances in the art, catalyst deactivation and vent losses, especially carbon monoxide losses, remain persistent inefficiencies in methanol carbonylation systems. There is also always a need to reduce the capital and operating costs associated with vent scrubbing and product purification.

In conventional methanol carbonylation units, including high pressure and low pressure absorbers, acetic acid is used as the scrubber solvent. The acetic acid solvent must then be stripped of light ends, usually in another purification column so that the acid is not wasted. Such towers are expensive because they must be made of highly corrosion resistant materials such as zirconium alloys and the like. In addition, stripping the light ends from the acid requires steam and results in operating expenses. Methanol has been proposed for use as a scrubber solvent in connection with methanol carbonylation processes. See, in this regard, U.S. Pat. No.5,416,237, Aubigne et al, entitled "Process for the Production of Acetic Acid". It should be noted that' 237 claims that non-condensables from the flash tank overhead vapor can be washed counter-currently with cooled methanol. The methanol scrubber solvent residue stream was added to pure methanol and then used as the feed to the reactor. See column 9, lines 30-42. Chinese patent application No.200410016120.7 discloses a method for recovering light components in vent gas from the production of acetic acid/acetic anhydride by washing with methanol and acetic acid. Another system is found in the Industrial publication entitled "Process of 200ktpa methane Low pressure Oxo Synthesis AA" (SWRDICI 2006) (China) (hereinafter SWRDICI). In this research publication, a vent gas treatment system is shown that includes a high pressure absorber and a low pressure absorber. Both absorbers of this system are described as operating with methanol as the wash liquid.

European patent No. ep 0759419 suggests reducing the vent losses by injecting methanol into the reactor vent stream and catalytically producing more product in a secondary reactor optionally containing a heterogeneous catalyst.

Catalyst deactivation and loss are generally believed to be due to carbon monoxide depletion or low pressure environment in the carbonylation system as seen in the flasher. As the carbon monoxide level in the catalyst solution decreases, the rhodium increasingly takes the form of precipitated rhodium triiodide. Various improvements have been proposed in the art to address this aspect of the conventional process, perhaps the most successful being the use of lithium iodide under low water conditions to enhance catalyst stability and reaction rate. Other proposed improvements are discussed below.

U.S. patent 5,770,768, Denis et al, discloses a carbonylation system in which the catalyst solution recycled from the flasher is treated with additional carbon monoxide to improve catalyst stability before being returned to the reactor.

Chinese patent No. zl92108244.4 and SWRDICI (noted above) propose a high pressure "conversion" reactor. The conversion reactor demonstrated by SWRDICI was connected to a high pressure vent scrubber and reported to allow the reaction to proceed to a greater extent before flashing.

In accordance with the present invention, an improved carbonylation system is provided having staged reactions and pre-flash removal of light ends to increase production capacity and operating efficiency.

Summary of The Invention

According to the present invention, there is provided a process for producing acetic acid, which comprises: (a) catalytically reacting methanol or a reactive derivative thereof with carbon monoxide in the presence of a homogeneous group VIII metal catalyst and a methyl iodide promoter in a reaction vessel, wherein the reaction vessel contains a liquid reaction mixture comprising acetic acid, water, methyl acetate, methyl iodide and the homogeneous catalyst, the reaction vessel being operated at a reactor pressure; (b) withdrawing the reaction mixture from the reaction vessel and feeding the withdrawn reaction mixture together with additional carbon monoxide to a pre-flasher/post reaction vessel operated at a reduced pressure below the pressure of the reaction vessel; (c) the light ends were discharged in the pre-flasher and methyl acetate was consumed in the pre-flasher/post reactor vessel simultaneously. The reaction conditions, residence time and composition in the pre-flasher/post reactor vessel are controlled such that the pre-flash mixture in the pre-flasher/post reactor vessel is enriched in acetic acid and depleted in methyl iodide and methyl acetate as compared to the reaction mixture. (d) Withdrawing an acetic acid-rich mixture from the pre-flasher/post reactor vessel and feeding it to a flash vessel; from there (e) a crude acetic acid stream is flashed off from the reaction mixture. The flash vessel is operated at a pressure below that of the pre-flasher/post reactor vessel. The process further comprises (f) recycling residue from the flash vessel to the reaction vessel; and (g) purifying the crude product stream.

Advantages of the present system include increased production capacity, elimination of the light ends column bottleneck and optionally increased carbon monoxide efficiency and enhanced catalyst stability.

The pre-flasher/post reactor vessel is suitably operated at a pressure of at least 5 or 10psi lower than the pressure of the reactor vessel, preferably at least 15psi lower than the pressure of the reactor vessel. In some embodiments, the pre-flasher/post reactor vessel is operated at a pressure at least 20psi, 25psi, or 30psi lower than the pressure of the reactor vessel.

Preferably, additional carbon monoxide is injected in the pre-flasher/post reactor vessel to consume the methyl acetate.

Other details and advantages of the present invention will become apparent from the following discussion.

Brief Description of Drawings

The present invention is described in detail below with reference to the attached drawing figures, wherein like numerals represent like parts. In the figure:

FIG. 1 is a schematic diagram illustrating a carbonylation system for producing acetic acid in accordance with the present invention.

Detailed Description

The invention is described in detail below with reference to a number of embodiments which are intended to be exemplary and illustrative only. Modifications of the specific embodiments within the spirit and scope of the invention as described in the appended claims will readily occur to those skilled in the art.

Unless specifically defined below, the terms as used herein are given their ordinary meanings. Unless otherwise indicated,% percentages and like terms refer to% by weight.

"iodide salt stabilizer/co-promoter" and like terms refer to a component that produces and maintains an increased iodide anion content, i.e., above the level attributable to hydroiodonium acid. The iodide salt stabilizer/co-promoter may be a simple salt or any compound or component that generates and maintains iodide anion in the reaction mixture as discussed further herein.

"light ends" refers to components having a boiling point lower than that of acetic acid. Thus, for the purposes of this invention, methyl iodide, methyl acetate and dissolved carbon monoxide are the "light ends".

"Low pressure" and like terms refer to a pressure that is less than the pressure maintained in the carbonylation reactor discussed herein. The "reduced" pressure is typically at least 5psi lower than the reference pressure, preferably at least 10psi or 20psi lower than the reference pressure. By "low pressure" absorber is meant an absorber that operates at a pressure substantially below the reactor pressure, preferably greater than 25psi below the pressure maintained in the carbonylation reactor.

When referring to a reduction in methyl acetate due to its consumption at a particular level in the pre-flasher/post reactor vessel, the percentage reduction is relative to the amount of methyl acetate in the reaction mixture in the reactor. Thus, a 25% reduction in methyl acetate in the pre-flasher/post reactor refers to a level that is 25% lower in the outlet stream of the pre-flasher/post reactor compared to the level maintained in the reaction vessel. Thus, a 25% reduction was achieved when the level of methyl acetate was 4 wt% in the reaction vessel and methyl acetate was consumed to a level of 3% in the pre-flasher/post reaction vessel. In some preferred aspects of the invention, methyl acetate is consumed to a level of less than 1.5 wt% or less than 1 wt% in the reaction mixture exiting the pre-flasher/post reactor. In other instances, the concentration of methyl acetate in the stream exiting the pre-flasher/post reactor may be less than 0.5 wt% or less than 0.25 wt%.

In a conventional carbonylation reactor, a vent gas comprising hydrogen, carbon dioxide and carbon monoxide is fed from the reactor to a high pressure absorber operating at a pressure similar to that of the reactor to recover reactants and/or products. Acetic acid was separated from the catalyst solution in a flash evaporator. Methyl iodide and methyl acetate associated with the crude acetic acid product are removed in a light ends column and vent gases are removed by condensation or scrubbing with an absorber.

In the process of the present invention, the vent gas from the primary reactor may be fed directly to the pre-flasher/post reactor vessel, thereby preserving the carbon monoxide reactant while reducing or eliminating the need for a high pressure absorber. Additional carbon monoxide fed to the reaction mixture stabilizes the catalyst and reacts with methyl acetate to increase the acetic acid production capacity of the system.

The pre-flasher/post reactor vessel is operated at an intermediate pressure between the operating pressures of the primary reactor and the subsequent flasher, thereby keeping most of the product acetic acid in solution while flashing off methyl iodide and methyl acetate. The methyl iodide and methyl acetate flashed from the pre-flasher/post reactor vessel may be fed to a condenser or may be sent directly to a low pressure absorber, thereby reducing the duty of the subsequent light ends column. The operation of the absorber is generally more expensive than the operation of the condensing unit. Thus, minimizing the need for absorption results in reduced operating costs.

The reaction liquid is typically withdrawn from the reactor and flashed in a staged or multi-step process using a pre-flasher/post reactor vessel and a conventional flash vessel as described below. The crude vapor process stream from the flasher is passed to a purification section, which typically comprises at least a light ends column and a dehydration column as is known in the art.

The invention will be further understood by reference to fig. 1, which is a schematic diagram illustrating an exemplary carbonylation process and apparatus according to one embodiment of the present invention.

Figure 1 shows a carbonylation apparatus 10 constructed in accordance with the present invention. The plant 10 generally comprises a carbonylation reactor 12, a pre-flasher/post reactor vessel 14, a flasher 16, and additional purification such as a light ends stripper 18, and the like, as will be appreciated by those skilled in the art.

In operation, methanol and carbon monoxide are fed to reactor vessel 12 via lines 20, 22, respectively, to react in the catalytic reaction medium contained in reactor vessel 12. The carbonylation reaction is carried out in a homogeneous catalytic reaction system comprising a reaction solvent (typically acetic acid), methanol and/or a reactive derivative thereof, a soluble rhodium catalyst, methyl iodide, methyl acetate and at least a finite concentration of water. Methanol and carbon monoxide efficiencies are typically greater than about 98% and 90%, respectively, as exemplified by U.S. Pat. nos. 5,001,259 to Smith et al; 5,026,908 and 5,144,068, the disclosures of which are incorporated herein by reference.

From reactor 12, a portion of the reaction medium is fed onward via line 24 through pressure reduction valve 24a to pre-flasher/post reactor 14. Carbon monoxide is also fed via line 26 through the vent gas from the reaction vessel 12 to the pre-flasher 14 as shown. The preferred source of CO is vent gas 26 from through pressure relief valve 26a as this reduces the need to feed additional fresh carbon monoxide into the pre-flasher/post reactor 14, which may be done, for example, via line 28 as shown towards the lower part of the figure. Note that carbon monoxide is injected into vessel 14 at a separation height H at the bottom of vessel 14 and above line 34 to prevent (or reduce the amount of) carbon monoxide from traveling into line 34. The height H may be at least 0.25 meter or more, preferably at least 0.5 meter, or at least 1 meter.

In the pre-flasher/post reactor 14, the reaction medium is maintained at an intermediate pressure while CO is allowed to interact with the reaction mixture and consume methyl acetate. In a preferred embodiment, the amount of carbon monoxide added to vessel 14 and the reaction conditions are controlled such that the methyl acetate in the reaction mixture is substantially consumed before further processing. The pre-flasher/post reactor 14 has a vent at 30 to remove gases comprising non-condensables as well as methyl iodide and optionally some methyl acetate from the system to a low pressure scrubbing system 32 as shown. The pressure in the vent stream 30 is reduced by passing the stream through a pressure reducing valve shown at 30a before being fed to the low pressure absorption system 32.

The reaction mixture is thus modified and preconditioned before flashing. In particular, a portion of the methyl iodide and optionally a portion of the methyl acetate are removed from the reaction mixture and fed to a low pressure vent gas scrubbing system before flashing at low pressure. Thus, the purification requirements with respect to the crude product will be reduced, as will be appreciated from the discussion below. After reaction in the pre-flasher/post reactor vessel 14, the now light ends depleted conditioned reaction mixture is fed onwards via line 34 through a pressure reducing valve 34a to the flasher 16. Pressure is reduced in flasher 16 relative to pre-flasher 14, which in turn reduces pre-flasher 14 relative to reactor 12. In flasher 16, crude acetic acid is flashed from the reaction mixture and withdrawn as an overhead product as shown at 36 and fed to light ends column 18 as is known in the art.

From flasher 16, the catalyst is recycled to reactor 12 via lines 38, 40 as is also known in the art.

The crude product fed via line 36 to light ends column 18 has a much lower methyl iodide and methyl acetate content compared to conventional carbonylation systems because methyl acetate has been consumed in the pre-flasher/post reactor vessel 14 and methyl iodide and optionally methyl acetate has been pre-flashed to the low pressure vent gas scrubbing system 32 as shown. From light ends column 18, the product is fed forward to a purified stream 42, where most of the methyl iodide and methyl acetate are removed from the product. Stream 42 is fed forward to a dehydration column to remove water from the product stream and then optionally treated to remove other impurities such as heavy ends, organic iodides prior to storage and transport. The residue from column 18 is recycled via line 38a to lines 38 and 40 and finally to reactor 12.

The overhead from column 18 is condensed and discharged via 44 to receiver 46 and may be recycled as is known in the art. The non-condensables, i.e., at 48, are fed to a low pressure vent gas scrubbing system which may use methanol and/or acetic acid and/or methyl acetate as shown at 50. In this regard, an absorber 52 is provided. When methanol is used as the scrubbing fluid in the low pressure scrubber, the spent scrubbing fluid may be fed directly to reactor 12 via line 50a as shown. Preferably greater than 90% or 95% of the methyl iodide is removed from the vent gas by the absorbent fluid prior to additional treatment. The scrubber fluid is typically cooled to a temperature of about 5 to about 25 ℃ prior to use in the column, provided that when acetic acid is used as the scrubber solvent, the temperature of the solvent is maintained at 17 ℃ or more to prevent freezing.

Non-condensables, including carbon monoxide from column 52, are withdrawn via line 54 and may be further purified by pressure swing adsorption or vacuum pressure swing adsorption as is known in the art. In this regard, descriptions of these methods are provided in U.S. Pat. No.5,529,970 to Peng, and U.S. Pat. No.6,322,612 to Sircar et al, the disclosures of which are incorporated herein by reference.

A high pressure absorber is not required in the embodiment shown in figure 1, saving capital and operating costs. In another embodiment, the use of high pressure absorbers can be minimized, reducing operating costs.

It should be appreciated from the foregoing that the lower methyl iodide and methyl acetate content in the resulting flash crude product stream 36 eliminates the light ends column bottleneck. Because of the carbon monoxide consumption in the pre-flasher/post reactor 14, high gas injection rates can be achieved without loss of carbon monoxide.

The Carbonylation system 10 optionally employs only two main purification columns and preferably operates as described in more detail in U.S. Pat. No.6,657,078 to Scates et al, entitled "Low Energy Carbonylation Process", the disclosure of which is incorporated herein by reference. Depending on the system, additional columns are typically used as needed.

The group VIII catalyst metal used in connection with the present invention may be a rhodium and/or iridium catalyst. The choice of catalyst is not critical to the operation of the present invention. If a rhodium-based catalyst is selected, the rhodium metal catalyst may be added in any suitable form such that the rhodium is present as comprising [ Rh (CO ]₂I₂]^-An equilibrium mixture of anions in a catalyst solution, as is well known in the art. The solubility of rhodium is generally maintained when the rhodium solution is in the carbon monoxide rich environment of the reactor, since the rhodium/carbonyl iodide anionic species are generally soluble in water and acetic acid. However, when transferred to a carbon monoxide-depleted environment as typically found in flashers, light ends columns, and the like, the stability of the rhodium/catalyst composition is reduced because less carbon monoxide is available. For example, significant amounts of rhodium precipitates such as RhI₃Loss in conventional systems; details regarding the form of rhodium entrained downstream of the reactor are not fully understood. Such as those skilled in the artIt will be appreciated by the skilled person that the iodide salt stabiliser/co-promoter helps to mitigate precipitation in the flash vessel under so-called "low water" conditions. The rhodium catalyst may be present in a concentration ranging from 1ppm to solubility, preferably from 10 to 2000 ppm by weight of rhodium.

The iodide salt stabilizers/co-promoters used in connection with the present invention may be soluble salts or quaternary ammonium salts of alkali metals or alkaline earth metals orIn the form of a salt. In certain embodiments, the catalyst stabilizer/co-promoter is lithium iodide, lithium acetate, or mixtures thereof. The iodide salt may be added as a mixture of salts, for example a mixture of lithium iodide and sodium iodide and/or potassium iodide. See U.S. patent nos.' 259 to Smith et al, referred to above; ' 908; and' 068. Alternatively, the iodide salt stabilizer/co-promoter may be added as a salt precursor that generates iodide anion in situ due to the operating conditions of the reaction system. A wide range of non-iodinated salts useful as precursors include alkali metal acetates and carboxylates, which react with methyl iodide and/or HI to produce the corresponding iodide salt stabilizers. Suitable iodide salts may likewise be generated in situ from non-ionic precursors such as phosphine oxides, arsenic, phosphines, amines, amino acids, sulfides, sulfoxides or, if desired, one or more of any suitable organic ligands. Phosphine oxides, phosphines, amines, amino acids or other nitrogen or phosphorus containing compounds and suitable organic ligands are typically subjected to quaternization at elevated temperatures in the presence of methyl iodide to give salts which maintain an elevated iodide anion concentration in the reaction mixture. Thus, iodide salt stabilizers/co-promoters are limited by their ability to maintain high iodide anion levels, not the form in which they are added to the system. One method of introducing the iodide salt co-promoter is by introducing a suitable moiety into the rhodium catalyst system or complex as a cation or ligand (usually a monodentate or bidentate ligand) associated with the rhodium added to the mixture. These complexes decompose and/or quaternize under carbonylation conditions in the presence of methyl iodide to provide increased levels of iodide anion. In this regard, the following Chinese referencesThe literature is of particular interest: chinese publication CN 1345631; application No. 00124639.9; chinese publication No. cn1105603; application No. 94100505.4; and Chinese publication No. CN1349855; application No. 00130033.4. Suitable rhodium catalyst complexes that provide iodide salt co-promoters include complexes having the structure:

wherein R is H, or a carboxyl-containing hydrocarbon derivative; (X)^-) Is BPh₄ ^-、BF₄ ^-Or CH₃COO^-(ii) a X is I, Cl or Br; and n is 0, 1 or 2. Other compounds useful as iodide salt co-promoters include pyridine derivatives, such as:

wherein R is H, or a carboxyl group-containing hydrocarbon derivative, and n is 0, 1 or 2. Preferably R is H, or for example lithium pyridine-2-formate, lithium pyridine-3-formate, lithium pyridine-4-formate, lithium pyridine-2-acetate, lithium pyridine-3-acetate, lithium pyridine-4-acetate or lithium pyridine-3-propionate. One skilled in the art will appreciate that many other components may be used as iodide salt co-promoters.

The iridium catalyst in the liquid carbonylation reaction composition may comprise any iridium-containing compound which is soluble in the liquid reaction composition. The iridium catalyst may be added to the liquid reaction composition for the carbonylation reaction in any suitable form which is soluble in the liquid reaction composition or convertible to a soluble form. Examples of suitable iridium-containing compounds that may be added to the liquid reaction composition include: IrCl₃、IrI₃、IrBr₃、[Ir(CO)₂I]₂、[Ir(CO)₂Cl]₂、[Ir(CO)₂Br]₂、[Ir(CO)₂I₂]^-H⁺、[Ir(CO)₂Br₂]^-H⁺、[Ir(CO)₂I₄]^-H⁺、[Ir(CH₃)I₃(CO₂]^-H⁺、Ir₄(CO)₁₂、IrCl₃·3H₂O、IrBr₃·3H₂O、Ir₄(CO)₁₂Iridium metal, Ir₂O₃、Ir(acac)(CO)₂、Ir(acac)₃Iridium acetate, [ Ir ]₃O(OAc)₆(H₂O)₃][OAc]And hexachloroiridic acid [ H ]₂IrCl₆]. Chlorine-free complexes of iridium, such as acetates, oxalates and acetoacetates, are commonly used as starting materials. The iridium catalyst concentration in the liquid reaction composition may be 100-6000 ppm. Methanol carbonylation using iridium catalysts is well known and is generally described in the following U.S. patents: 5,942,460, respectively; 5,932,764; 5,883,295; 5,877,348, respectively; 5,877,347 and 5,696,284, the disclosures of which are incorporated herein by reference in their entirety.

Methyl iodide is used as a promoter, but the choice of promoter is not critical to the operation of the present invention. Preferably, the concentration of methyl iodide in the liquid reaction composition is in the range of from 1 to 50 wt.%, preferably from 2 to 30 wt.%.

The promoter may be combined with a salt stabilizer/co-promoter compound, especially in conjunction with a rhodium catalytic system. These promoters may comprise salts of group IA or IIA metals, or quaternary ammonium salts orSalts or their precursors, as described above. Particularly preferred are iodide salts or acetate salts, such as lithium iodide or lithium acetate.

Other promoters and co-promoters may be used as part of the catalytic system of the present invention, as described in European patent publication EP 0849248, the disclosure of which is incorporated herein by reference. Suitable promoters are selected from ruthenium, osmium, tungsten, rhenium, zinc, cadmium, indium, gallium, mercury, nickel, platinum, vanadium, titanium, copper, aluminum, tin, antimony, more preferably from ruthenium and osmium. Specific co-promoters are described in U.S. Pat. No.6,627,770, which is incorporated herein by reference in its entirety.

The promoter may be present in an effective amount up to the limit of its solubility in the liquid reaction composition and/or any liquid process streams recycled to the carbonylation reactor from the acetic acid recovery stage. When used, the promoter is suitably present in the liquid reaction composition in a promoter to metal catalyst molar ratio of [0.5-15] to 1, preferably [2-10] to 1, more preferably [2-7.5] to 1. A suitable promoter concentration is 400-5000 ppm.

The carbon monoxide reactant may be substantially pure or may contain inert impurities such as carbon dioxide, methane, nitrogen, noble gases, water and C₁-C₄An alkane. The presence of carbon monoxide to neutralise the hydrogen generated in situ by the water gas shift reaction is preferably kept low, for example less than 1 bar partial pressure, as its presence may lead to the formation of hydrogenation products. The partial pressure of carbon monoxide in the reaction is suitably in the range 1 to 70 bar, preferably 1 to 35 bar, most preferably 1 to 15 bar.

Acetic acid is typically included in the reaction mixture as a solvent for the reaction.

Suitable reactive derivatives of methanol include methyl acetate, dimethyl ether, methyl formate and methyl iodide. Mixtures of methanol and reactive derivatives thereof may be used as reactants in the process of the present invention. Preferably, methanol and/or methyl acetate are used as reactants. At least some of the methanol and/or reactive derivative thereof will be converted by reaction with the acetic acid product or solvent and will therefore be present as methyl acetate in the liquid reaction composition. The concentration of methyl acetate in the liquid reaction composition is suitably in the range 0.5 to 70 wt%, preferably 0.5 to 50 wt%, more preferably 1 to 35 wt%, most preferably 1 to 20 wt%, in the case of a rhodium catalytic system 1 to 10 wt%.

The carbonylation process in the main reactor and the pre-flash/post-reactor vessel may be based on batch or semi-continuous, but is preferably operated in a continuous mode. The pressure of the carbonylation reaction in the main reactor is typically 145-2900psi (10-200 bar), preferably 145-1450psi (10-100 bar), most preferably 217-725psi (15-50 bar), for example about 400psi (28 bar). The pressure in the pre-flash/post reactor vessel is in many cases reduced by 10-40% from the main reactor pressure, corresponding to a pressure reduction of about 40 psi. The pre-flash/post-reaction vessel is typically operated at a pressure of about 160psig to about 400 psig. The flash vessel is typically operated at a pressure of about 14 to about 100 psig. The main and pre-flash/post reactor vessels are operated at comparable temperatures. The temperature of the carbonylation reaction is suitably 212 ℃ and 572 ℃ F. (100 ℃ C. and 300 ℃ C.), preferably 302 ℃ and 428 ℃ F. (150 ℃ C. and 220 ℃ C.), for example about 370 ℃ F. (188 ℃ C.). With reference to fig. 1, suitable pressures and compositions in the various components of the apparatus and streams are as follows:

the device comprises the following steps:

the 12-carbonylation reaction pressure is 300-

14-Preflash/postreactor pressure 200-

16-flasher pressure of 0-100psig, preferably 15-45psig

The 52-vent scrubber pressure is 5-500psig, preferably 5-100psig, more preferably 10-50psig

Flow of material

30-containing MeI, MeAc, CO

34-contains HAc, Rh, H₂O, dissolved gas (CO/CO)₂) And a lower concentration of MeAc and MeI than stream 24

26-containing CO, H₂、CO₂、CH₄

48-containing non-condensable gases and MeI

54-essentially comprising non-condensable gases and having a lower concentration of MeI than stream 30

Water may be formed in situ in the liquid reaction composition, for example, by an esterification reaction between methanol reactant and acetic acid product. Water may be introduced into the carbonylation reactor together with or separately from the other components of the liquid reaction composition. The water may be separated from the other components of the reaction composition withdrawn from the reactor and may be recycled in controlled amounts to maintain the desired water concentration in the liquid reaction composition.

It is thus seen that in various embodiments, the pre-flasher/post reactor vessel operates at a pressure that is at least 5, 10, 15, 20, 25, or 30psi lower than the pressure of the reactor vessel. The group VIII metal catalyst is also a homogeneous rhodium catalyst and is present in the reaction mixture at a concentration of from about 300 to about 5,000 ppm by weight of the reaction mixture while the amount of water in the reaction mixture in the reaction vessel is maintained at a level of from 0.1 to 10% by weight of the reaction mixture and the reaction mixture also contains an iodide salt stabilizer/co-promoter. Alternatively, the amount of water in the reaction mixture in the reaction vessel is maintained at a level of 0.5 to 8% by weight of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter, or the amount of water in the reaction mixture in the reaction vessel is maintained at a level of 0.5 to 5% by weight of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter. In some preferred cases, the amount of water in the reaction mixture in the reaction vessel is maintained at a level of from 0.5 to 3% by weight of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter, with the iodide salt stabilizer/co-promoter being present in an amount to produce and maintain an iodide anion concentration of from about 2 to about 20% by weight of the reaction mixture in the reaction vessel, e.g., wherein the iodide salt stabilizer/co-promoter is present in an amount to produce and maintain an iodide anion concentration of from about 5 to about 17.5% by weight of the reaction mixture in the reaction vessel.

The iodide salt stabilizer/co-promoter is sometimes a mixture of iodide salts and/or is provided to the reaction mixture in a non-ionic form.

The group VIII metal catalyst may be a homogeneous iridium catalyst and the amount of water in the reaction mixture in the reaction vessel may be maintained at a level of 3 to 8 wt.% of the reaction mixture, while the amount of methyl iodide in the reaction mixture in the reaction vessel is maintained at a level of 2 to 8 wt.% of the reaction mixture and the amount of methyl acetate in the reaction mixture is maintained at a level of 10 to 20 wt.% of the reaction mixture in the reaction vessel.

In a preferred aspect, carbon monoxide is injected into the pre-flasher/post reactor vessel through the vent stream from the reactor. In another preferred aspect, the light ends from the pre-flasher/post reactor vessel are discharged to a low pressure scrubber.

Typically, the methyl acetate in the reaction mixture is consumed in the pre-flasher/post reactor vessel to a level at least 25% lower than the methyl acetate concentration in the reaction mixture in the reactor vessel; sometimes the methyl acetate in the reaction mixture is consumed in the pre-flasher/post reactor vessel to a level at least 50% lower than the methyl acetate concentration in the reaction mixture in the reactor vessel.

In another aspect of the present invention, there is provided a carbonylation system for producing acetic acid comprising: (a) a reaction vessel suitable for carbonylating methanol or a reactive derivative thereof with carbon monoxide in a liquid reaction mixture comprising acetic acid, water, methyl acetate and methyl iodide in the presence of a group VIII metal catalyst and a methyl iodide promoter, wherein the reaction vessel is operated at a reaction pressure of 300-500 psig; (b) a pre-flasher/post reactor vessel coupled to the reactor and adapted to receive the liquid reaction mixture conveyed thereto by the reactor, the pre-flasher/post reactor vessel operating at a pressure of 200 psig to 450psig provided that the pressure in the pre-flasher/post reactor is at least 5psi less than the pressure in the reactor vessel and wherein the composition and conditions in the pre-flasher/post reactor vessel are such that the light fraction is fed to the pre-flasher/post reactor vessel vent and forms a pre-flashed mixture that is enriched in acetic acid and depleted in methyl iodide and methyl acetate as compared to the reaction mixture; (c) a scrubber connected to the pre-flasher/post reactor vessel vent and adapted to recover light ends therefrom; (d) a flash vessel connected to the pre-flasher/post reactor vessel and adapted to receive the liquid pre-flash mixture delivered thereto from the pre-flasher/post reactor vessel, the flash vessel operating at a pressure substantially lower than the pressure of the pre-flasher/post reactor vessel, the flash vessel further adapted to flash a crude product stream from the pre-flash mixture and provide a recycled reaction mixture to the reactor; and (e) a purification section coupled to the flash vessel and adapted to purify the crude product stream. For example, the reactor can be operated at a pressure of 350-450psig, while the pre-flasher/post reactor is operated at a pressure of 300-400 psig. Suitably, the pre-flasher/post-reactor operates at a pressure at least 15psi lower than the reaction vessel, while the flash vessel operates at a pressure of from 0 to 100psig, for example wherein the flash vessel operates at a pressure of from 15 to 45 psig. The vent gas scrubber operates at a pressure of 5-450psig or greater; suitably, the vent gas scrubber operates at a pressure of from 5 to 100psig, for example wherein the vent gas scrubber operates at a pressure of from 10 to 50 psig.

In another preferred aspect of the invention, the pre-flasher/post reactor vessel is connected to a carbon monoxide source, for example wherein the carbon monoxide source comprises an exhaust stream from the reactor vessel.

Still further improvements include a system further comprising a pressure relief valve coupled to the reaction vessel and the vent stream of the pre-flasher/post reaction vessel and/or further comprising a pressure relief valve coupled to the pre-flasher/post reaction vessel and the scrubber and/or further comprising a pressure relief valve coupled to the pre-flasher/post reaction vessel and the flash vessel. In some cases, the reaction vessel is vented only to the pre-flasher/post reaction vessel and the system has a single low pressure vent scrubber.

Although the present invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those skilled in the art. In view of the foregoing discussion, the relevant knowledge in the art, and the above-mentioned references in connection with the background and detailed description, the disclosure of which is incorporated herein by reference in its entirety, further description is deemed unnecessary. In addition, it should be understood that aspects of the invention and portions of the various embodiments may be combined or interchanged either in whole or in part. Furthermore, those skilled in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.

Claims (24)

1. A process for producing acetic acid comprising:

(a) catalytically reacting methanol or a reactive derivative thereof with carbon monoxide in a reaction vessel in the presence of a homogeneous group VIII metal catalyst and a methyl iodide promoter, wherein the reaction vessel contains a liquid reaction mixture comprising acetic acid, water, methyl acetate, methyl iodide and the homogeneous catalyst, the reaction vessel operating at a reactor pressure;

(b) withdrawing the reaction mixture from the reaction vessel and feeding the withdrawn reaction mixture together with additional carbon monoxide to a pre-flasher/post reaction vessel operated at a reduced pressure below the pressure of the reaction vessel;

(c) withdrawing light ends in the pre-flasher and simultaneously consuming methyl acetate in the pre-flasher/post reactor vessel to produce a pre-flashed mixture that is enriched in acetic acid and depleted in methyl iodide and methyl acetate as compared to the reaction mixture;

(d) withdrawing the pre-flash reaction mixture from the pre-flasher/post reaction vessel and feeding the pre-flash mixture to a flash vessel;

(e) flashing a crude acetic acid stream from the mixture in a flash vessel operating at a pressure below the pre-flasher/post reactor vessel pressure;

(f) recycling the post-flash residue from the flash vessel to the reaction vessel; and

(g) purifying the crude product stream.

2. The process according to claim 1, wherein the pre-flasher/post reactor vessel is operated at a pressure at least 5psi lower than the pressure of the reactor vessel.

3. The process according to claim 1, wherein the pre-flasher/post reactor vessel is operated at a pressure at least 10psi lower than the pressure of the reactor vessel.

4. The process according to claim 1, wherein the pre-flasher/post reactor vessel is operated at a pressure at least 15psi lower than the pressure of the reactor vessel.

5. The process according to claim 1, wherein the pre-flasher/post reactor vessel is operated at a pressure at least 20psi lower than the pressure of the reactor vessel.

6. The process according to claim 1, wherein the pre-flasher/post reactor vessel is operated at a pressure at least 25psi lower than the pressure of the reactor vessel.

7. The process according to claim 1, wherein the pre-flasher/post reactor vessel is operated at a pressure at least 30psi lower than the pressure of the reactor vessel.

8. The carbonylation process according to claim 1 wherein said group VIII metal catalyst is a homogeneous rhodium catalyst and is present in the reaction mixture at a concentration of 300-5,000 ppm by weight of the reaction mixture.

9. The carbonylation process according to claim 8, wherein the amount of water in the reaction mixture in the reaction vessel is maintained at a level of 0.1 to 10 weight percent of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter.

10. The carbonylation process according to claim 9, wherein the amount of water in the reaction mixture in the reaction vessel is maintained at a level of 0.5 to 8 weight percent of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter.

11. The carbonylation process according to claim 9, wherein the amount of water in the reaction mixture in the reaction vessel is maintained at a level of 0.5 to 5 weight percent of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter.

12. The carbonylation process according to claim 9, wherein the amount of water in the reaction mixture in the reaction vessel is maintained at a level of 0.5 to 3 weight percent of the reaction mixture and the reaction mixture further comprises an iodide salt stabilizer/co-promoter.

13. The carbonylation process according to claim 9, wherein the iodide salt stabilizer/co-promoter is present in an amount to produce and maintain an iodide anion concentration of 2 to 20 weight percent of the reaction mixture in the reaction vessel.

14. The carbonylation process according to claim 9, wherein the iodide salt stabilizer/co-promoter is present in an amount to produce and maintain an iodide anion concentration of 5 to 17.5 weight percent of the reaction mixture in the reaction vessel.

15. The carbonylation process according to claim 9 wherein the iodide salt stabilizer/co-promoter is a mixture of iodide salts.

16. The carbonylation process according to claim 9, wherein the iodide salt stabilizer/co-promoter is provided to the reaction mixture in a non-ionic form.

17. The carbonylation process according to claim 1, wherein the group VIII metal catalyst is a homogeneous iridium catalyst.

18. The carbonylation process according to claim 17, wherein the amount of water in the reaction mixture is maintained at a level of from 3 to 8 weight percent of the reaction mixture in the reaction vessel.

19. The carbonylation process according to claim 17, wherein the amount of methyl iodide in the reaction mixture is maintained at a level of 2 to 8 weight percent of the reaction mixture in the reaction vessel.

20. The carbonylation process according to claim 17, wherein the amount of methyl acetate in the reaction mixture is maintained at a level of from 10 to 20 weight percent of the reaction mixture in the reaction vessel.

21. The process of claim 1 wherein carbon monoxide is injected into the pre-flasher/post reactor vessel through the vent stream from the reactor.

22. The process according to claim 1, wherein the light ends from the pre-flasher/post reactor vessel are discharged to a low pressure scrubber.

23. A process according to claim 1 wherein the methyl acetate in the reaction mixture is consumed in the pre-flasher/post reactor vessel to a level at least 25% lower than the methyl acetate concentration in the reaction mixture in the reactor vessel.

24. The carbonylation process according to claim 1, wherein the methyl acetate in the reaction mixture is consumed in the pre-flasher/post reactor vessel to a level at least 50% less than the methyl acetate concentration in the reaction mixture in the reactor vessel.