GB2334955A - Carbonylation process for the production of acetic acid - Google Patents

Abstract

In a process for the production of acetic acid by the continuous liquid-phase carbonylation of methanol and/or a reaction derivative thereof in the presence of iridium carbonylation catalyst a concentration of by-product propionic acid in the recovered acetic acid of less than 275 ppm is achieved by maintaining (a) a partial pressure of hydrogen in the reactor of less than 0.1 bar absolute and (b) an iridium catalyst concentration in the liquid reaction composition of less than 1300 ppm measured as iridium metal.

Description

CARBONYLATION PROCESS FOR THE PRODUCTION OF ACETIC ACID The present invention relates in general to a carbonylation process for the production of acetic acid and in particular to a process for the production of acetic acid by the carbonylation of methanol and/or a reactive derivative thereof in the presence of an iridium catalyst and methyl iodide co-catalyst.

Acetic acid is a well-known commodity chemical which is manufactured by carbonylation throughout the world.

Processes for the production of acetic acid by liquid phase, iridium catalysed carbonylation reactions are described in, for example, EP-A-0616997; EP-A-0618184; US-3,772,380; US1234641 and GB-A-1234642.

Howard et al in Catalysis Today, 18 (1993) 325-354, describe the rhodium and iridium-catalysed carbonylation of methanol to acetic acid. The continuous rhodiumcatalysed, homogeneous methanol carbonylation process is said to consist of three basic sections; reaction, purification and off-gas treatment. The iridium-catalysed process is essentially similar to the rhodium-catalysed process with respect to these sections. The reaction section comprises a stirred tank reactor, operated at elevated temperature and pressure, and a flash vessel. Liquid reaction composition is withdrawn from the reactor and is passed through a flashing valve to the flash tank where the majority of the lighter components of the liquid reaction composition (methyl iodide, methyl acetate and water) together with product acetic acid are vaporised. The vapour fraction is then passed to the purification section whilst the liquid fraction (comprising the rhodium catalyst in acetic acid) is recycled to the reactor (cf Figure 2 of Howard et al). The purification section is said to comprise a first distillation column (the light ends column), a second distillation column (the drying column) and a third distillation column (the heavy ends column) (cf Figure 3 of Howard et al). In the light ends column methyl iodide and methyl acetate are removed overhead along with some water and acetic acid. The vapour is condensed and allowed to separate into two phases in a decanter, both phases being returned to the reactor. Wet acetic acid is removed from the light ends column as a sidedraw and is fed to the drying column where water is removed overhead and an essentially dry acetic acid stream is removed from the base of the distillation column.

From Figure 3 of Howard et al it can be seen that the overhead water stream from the drying column is recycled to the reaction section. Heavy liquid by-products are removed from the base of the heavy ends column with product acetic acid being taken as a sidestream.

The construction and operation of carbonylation plant for the production of acetic acid is a competitive business and clearly any saving in capital expenditure and operating costs by eliminating plant is an economically desirable objective. One method of achieving this would be to produce acetic acid having so low a concentration of heavy end impurities, chief amongst which is propionic acid, that the third distillation column of Howard et al, i. e. the heavy ends column, can be eliminated.

The technical problem to be overcome by the process of the present invention is that of restricting the propionic acid concentration to less than 275 ppm, preferably less than 250 ppm, in the acetic acid product withdrawn from the purification section in the absence of a heavy ends column.

A process for the production of an acetic acid process stream comprising less than 400 ppm propionic acid and less than 1500 ppm water is described in, for example EP-A-0849250 (BP Case No. 8644). This process comprises the steps : (a) feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor in which there is maintained during the course of the process a liquid reaction composition comprising: (i) an iridium carbonylation catalyst; (ii) methyl iodide co-catalyst; (iii) optionally one or more promoters selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, indium and tungsten; (iv) a finite amount of water at a concentration of less than about 8% byweight; (v) methyl acetate; (vi) acetic acid; and (vii) propionic acid by-product and its precursors; (b) withdrawing liquid reaction composition from the carbonylation reactor and introducing at least part of the withdrawn liquid reaction composition, with or without the addition of heat, to a flash zone to form a vapour fraction comprising water, acetic acid product, propionic acid by-product, methyl acetate, methyl iodide and propionic acid precursors, and a liquid fraction comprising involatile iridium catalyst, involatile optional promoter or promoters, acetic acid and water; (c) recycling the liquid fraction from the flash zone to the carbonylation reactor; (d) introducing the vapour fraction from the flash zone into a first distillation zone; (e) removing from the first distillation zone at a point above the introduction point of the flash zone vapour fraction a light ends recycle stream comprising water, methyl acetate, methyl iodide, acetic acid and propionic acid precursors which stream is recycled in whole or in part to the carbonylation reactor, and (f) removing from the first distillation zone at a point below the introduction point of the flash zone vapour fraction, a process stream comprising acetic acid product, propionic acid by-product, and less than 1500 ppm water and, (g) if the process stream removed in step (f) comprises greater than 400 ppm propionic acid introducing said stream into a second distillation column, removing from a point below the introduction point of the stream from (f) propionic acid by-product and from a point above the introduction point of the stream from (f) an acetic acid process stream containing less than 400 ppm propionic acid and less than 1500 ppm water.

We have now found that the solution to the technical problem as described hereinbefore is the control of two key parameters in the process, namely the hydrogen partial pressure in the reactor, and the iridium catalyst concentration in the liquid reaction composition.

Accordingly, the present invention provides a process for the production of acetic acid comprising (1) continuously feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor which contains a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, optionally at least one promoter, a finite concentration of water, methyl acetate and acetic acid; (2) contacting the methanol and/or reactive derivative thereof with the carbon monoxide in the liquid reaction composition to produce acetic acid and byproduct propionic acid; (3) removing from the reactor liquid reaction composition containing acetic acid and by-product propionic acid; and (4) recovering acetic acid from the liquid reaction composition characterised in that by maintaining a partial pressure of hydrogen in the reactor of less than 0.1 bar absolute, and (ii) an iridium catalyst concentration in the liquid reaction composition of less than 1300 ppm measured as iridium metal, the concentration of by-product propionic acid in the recovered acetic acid is less than 275 ppm.

A hydrogen partial pressure of less than 0.1 bar absolute is maintained in the reactor. Preferably, the hydrogen partial pressure is maintained below 0.05 bar, even more preferably below 0. 01 bar, or below. A number of factors contribute to the hydrogen partial pressure in the reactor, foremost amongst these being the hydrogen content of the carbon monoxide gas fed to the process. Suitably the hydrogen content of the carbon monoxide gas is less than 0.1 mole %, preferably less than 0.05 mole %, even more preferably less than 0.01 mole %, or less. Desirably the hydrogen content is as low as can be practically achieved. Since the most commonly operated processes for the production of carbon monoxide entail the co-production of hydrogen, purification of carbon monoxide produced thereby by removal of hydrogen will generally be necessary.

Means for purifying carbon monoxide by removing hydrogen are well-known in the art and any such means may be used in the process.

In addition to hydrogen introduced to the reactor as an impurity in the carbon monoxide gas feed to the reactor it is also generally formed in the reactor by the Water Gas Shift reaction which may be represented as follows: CO + Hz0 > C02 + H2 (I) It is therefore preferred to employ means for suppressing or inhibiting the Water Gas Shift reaction. One such means by which hydrogen is generally consumed in the reactor is by the Methanation reaction which may be represented as follows : CH30H + H2- > CH4 + H20 (I) It is therefore preferred to promote the methanation reaction as a supplementary means of maintaining the hydrogen partial pressure below 0.1 bar.

An iridium catalyst concentration in the liquid reaction composition of less than 1300ppm, preferably less than 1000 ppm, typically less than 800 ppm, for example less than 700 ppm, is necessary for the fulfillment of the process of the invention. In order to achieve a viable carbonylation rate using lower catalyst concentrations than typically employed hitherto it is very much preferred to employ at least one and preferably all of the following expedients: (a) a high reaction temperature, (b) at least one promoter, (c) when a promoter is employed, a high ratio of promoter to iridium catalyst, and (d) operate at a lower carbonylation rate.

With regard to the carbonylation temperature, in the literature carbonylation temperatures in the range from 100 to 300 C, preferably from 150 to 220 C are generally quoted. For the purpose of the present invention the temperature is suitably the highest temperature which is compatible with an acceptable rate of plant corrosion and mechanical integrity. Clearly this temperature depends on the nature of the metal used in the construction of the relevant areas of the plant. Using conventional construction materials, however, the carbonylation temperature is suitably in the range from 175 to 220 C, preferably from 185 to 200 C, for example from 188 to 195 C. The use of higher temperatures has the additional advantages that a deeper cut into the flash resulting in lower catalyst recycle rates and reductions in the recycling and flashing flows are facilitated.

There is preferably employed at least one promoter selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, indium, tungsten and mixtures thereof. Preferably the promoter is ruthenium and/or osmium, more preferably ruthenium. The promoter is suitably present in the liquid reaction composition 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. A high ratio of promoter to catalyst is preferably employed. Using ruthenium as promoter, for example, the molar ratio of ruthenium: iridium is suitably from [2.5 to 15]: 1, preferably from [4 to 12] : 1.

The promoter may comprise any suitable promoter metal-containing compound which is soluble in the liquid reaction composition. The promoter may be added to the liquid reaction composition for the carbonylation reaction in any suitable form which dissolves in the liquid reaction composition or is convertible to soluble form. Examples of suitable ruthenium-containing compounds which may be used as sources of promoter include ruthenium (III) chloride, ruthenium (III) chloride trihydrate, ruthenium (IV) chloride, ruthenium (III) bromide, ruthenium metal, ruthenium oxides, (M) formate, [Ru (CO) 3I3]1H+, [Ru (CO) 2I2] CRu (C) 4I2 yCO) 3I2] 2 tetra (aceto) chlororuthenium (II, III), ruthenium (III) acetate, ruthenium (III) propionate, ruthenium (III) butyrate, ruthenium pentacarbonyl, trirutheniumdodecacarbonyl and mixed ruthenium halocarbonyls such as dichlorotricarbonylruthenium (II) dimer, dibromotricarbonylruthenium (II) dimer, and other organoruthenium complexes such as tetrachlorobis (4-cymene) diruthenium (II), tetrachlorobis (benzene) diruthenium (II), dichloro (cycloocta-1,5-diene) ruthenium (II) polymer and tris (acetylacetonate) ruthenium (III).

Examples of suitable osmium-containing compounds which may be used as sources of promoter include osmium (III) chloride hydrate and anhydrous, osmium metal, osmium tetraoxide, triosmiumdodecacarbonyl, [Os (CO) 4I2], [Os (CO) 3I2] 2, [Os (CO) 3I3]-H+, and mixed osmium halocarbonyls such as tricarbonyldichloroosmium (II) dimer and other organoosmium complexes.

Examples of suitable rhenium-containing compounds which may be used as sources of promoter include Re2 (CO) 1o, Re (CO) 5CI, Re (CO) 5Br, Re (CO) sI, ReCl3. xH20 [Re (CO) 4I] 2, [Re (CO) 4I2]-H+ and ReCl5. yH20.

Examples of suitable cadmium-containing compounds which may be used include Cd (OAc) 2, CdI2, CdBr2, CdCl2, Cd (ou2, and cadmium acetylacetonate.

Examples of suitable mercury-containing compounds which may be used as sources of promoter include Hg (OAc) 2, HgI2, HgBr2, HgCl2, Hg2I2, and Hg2C12 Examples of suitable zinc-containing compounds which may be used as sources of promoter include Zn (OAc) 2, Zn (OH) 2, ZnI2, ZnBr2, ZnCl2, and zinc acetylacetonate.

Examples of suitable gallium-containing compounds which may be used as sources of promoter include gallium acetylacetonate, gallium acetate, GaCl3, GaBr3, GaI3, Ga2C14 and Ga (OH) 3.

Examples of suitable indium-containing compounds which may be used as sources of promoter include indium acetylacetonate, indium acetate, InCIg, InBr3, InI3, InI and In (OH) 3. Examples of suitable tungsten-containing compounds which may be used as sources of promoter include W (CO) 6, WCl4, WC16, WBrs, WI2, or CgH12 W (CO) 3 Preferably, the iridium-and promoter-containing compounds are free of impurities which provide or generate in situ ionic iodides which may inhibit the reaction, for example, alkali or alkaline earth metal or other metal salts.

Methanol and/or a reactive derivative thereof is fed to the carbonylation reactor.

Suitable reactive derivatives of methanol include methyl acetate, dimethyl ether and methyl iodide. A mixture of methanol and reactive derivatives, for example dimethyl ether, may be used as reactants in the process of the invention. Preferably methanol and/or methyl acetate is fed to the reactor. At least some of the methanol and/or reactive derivative thereof will be converted to, and hence present as, methyl acetate in the liquid reaction composition by reaction with acetic acid present therein.

Carbon monoxide is also continuously fed to the carbonylation reactor. The carbon monoxide may contain minor amounts of one or more inert impurities selected from carbon dioxide, nitrogen, noble gases, water and Cl to C4 paraffinic hydrocarbons.

The partial pressure of carbon monoxide in the carbonylation reactor is suitably in the range from 1 to 70 bar, preferably from 1 to 35 bar, more preferably from 1 to 20 bar.

The carbonylation reactor contains a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, optionally at least one promoter, a finite concentration of water, methyl acetate and acetic acid.

The iridium catalyst in the liquid reaction composition may comprise any iridiumcontaining 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 dissolves in the liquid reaction composition or is convertible to a soluble form. Examples of suitable iridium-containing compounds which 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]-FF, [Ir (CH3) I3 (CO) 2] H+, Ir4 (CO) 12, IrCI3. 3H20, IrBr3.3H20, iridium metal, Ir203, IrO2, Ir (acac) (CO) 2, Ir (acac) 3, iridium acetate, [Ir30 (OAc) 6 (H20) 3] [OAc], and hexachloroiridic acid [H21rCI61, preferably, chloride-free complexes of iridium such as acetates, oxalates and acetoacetates which are soluble in one or more of the carbonylation reaction components such as water, alcohol and/or carboxylic acid. Particularly preferred is green iridium acetate which may be used in an acetic acid or aqueous acetic acid solution.

As co-catalyst there is used methyl iodide. Suitably, the concentration of methyl iodide co-catalyst in the liquid reaction composition is greater than 4% by weight, typically from 4 to 30% by weight, preferably from 4 to 20% by weight. Generally, as the methyl iodide concentration in the liquid reaction composition increases the amount of propionic acid by-product decreases. It is therefore preferred to maximise the methyl iodide concentration within the above ranges.

As regards the promoter in the liquid reaction composition reference may be made to the foregoing.

There is present in the liquid reaction composition a finite concentration of water.

By a finite amount in this context is meant at least 0.1% by weight based on the weight of the composition. In general as the concentration of water in the liquid reaction composition increases, the amount of propionic acid by-product decreases. However, the greater the concentration of water employed the greater is the drying duty required in the drying column of the purification section. A suitable compromise is to employ a water concentration in the liquid reaction composition in the range from 2 to 10% by weight, preferably from 4 to 6% by weight, at which concentration the carbonylation rate passes through a maximum. Water may be found in situ in the liquid reaction composition by, for example, the esterification reaction between methanol reactant and acetic acid product. Water may be introduced to the carbonylation reactor together with or separately from other components of the liquid reaction composition. Water may be separated from other components of the reaction composition withdrawn from the carbonylation reactor and may be recycled in controlled amounts to maintain its required concentration in the liquid reaction composition.

Methyl acetate is a component of the liquid reaction composition in the reactor.

As a general rule propionic acid production decreases with increasing concentration of methyl acetate in the liquid reaction composition. It is therefore preferred to maximise the concentration of methyl acetate in the liquid reaction composition. Thus, the methyl acetate concentration may be up to 35% by weight, preferably in the range from 10 to 35% by weight.

Acetic acid comprises the remainder of the liquid reaction composition. Acetic acid is present by reason of being a product of the process and as a solvent for the carbonylation reaction.

Ionic contaminants such as, for example, (a) corrosion metals, particularly nickel, iron and chromium and (b) phosphines or nitrogen-containing compounds or ligands which may quaternise in situ should be kept to a minimum in the liquid reaction composition as these may generally have an adverse effect on the reaction by generating I'in the liquid reaction composition which may have an adverse effect on the reaction rate. Some corrosion metal contaminants such as for example molybdenum have been found to be less susceptible to the generation of I-Corrosion metals which have an adverse affect on the reaction rate may be minimised by using suitable corrosion resistant materials of construction. Similarly, contaminants such as alkali metal iodides, for example lithium iodide, may be kept to a minimum. Corrosion metal and other ionic impurities may be reduced by the use of a suitable ion exchange resin bed to treat the reaction composition, or preferably a catalyst recycle stream. Such a process is described in US 4007130. Ionic contaminants may be kept below a concentration at which they would generate less than 500 ppm I-, preferably less than 250 ppm I-in the liquid reaction composition.

The carbonylation reactor is suitably maintained at a total pressure in the range from 10 to 200 barg, preferably from 15 to 100 barg, more preferably from 15 to 50 barg, There is removed from the reactor liquid reaction composition containing less than 275 ppm, preferably less than 250 ppm, propionic acid.

Acetic acid is recovered from the liquid reaction composition in step (4) of the process. This may suitably be achieved by the steps of :- (5) introducing at least part of the withdrawn liquid reaction composition, with or without the addition of heat, to a flash zone to form a vapour fraction comprising water, acetic acid product, propionic acid by-product, methyl acetate, methyl iodide and propionic acid precursors, and a liquid fraction comprising involatile iridium catalyst, involatile optional promoter or promoters, acetic acid and water; (6) recycling the liquid fraction from the flash zone to the carbonylation reactor; (7) introducing the vapour fraction from the flash zone into a first distillation zone ; (8) removing from the first distillation zone at a point above the introduction point of the flash zone vapour fraction a light ends recycle stream comprising water, methyl acetate, methyl iodide, acetic acid and propionic acid precursors, which stream is recycled in whole or in part to the carbonylation reactor; and (9) removing from the first distillation zone at a point below the introduction point of the flash zone vapour fraction a fraction comprising acetic acid product containing less than 275 ppm propionic acid.

Advantageously the process of the invention allows the production of acetic acid containing less than 275 ppm, preferably less than 250 ppm, more preferably less than 200 ppm, propionic acid using a single distillation column for the purification as opposed to the three generally employed. It may be desirable in some circumstances to introduce the fraction from step (9) to a second distillation zone wherein water is removed overhead and dry acetic acid is removed below the introduction point of the fraction from step (9).

The flash zone is preferably maintained at a pressure below that of the reactor, typically at a pressure of 0 to 10 barg. The flash zone is preferably maintained at a temperature of 100 to 160 C.

The vapour fraction from the flash zone may be introduced to the first distillation zone as a vapour or the condensable components therein may be partially or fully condensed and the vapour fraction may be introduced as a mixed vapour/liquid or as a liquid with non-condensables.

The first distillation zone preferably has up to 40 theoretical stages. Since distillation zones may have differing efficiencies this may be equivalent to 57 actual stages with an efficiency of about 0.7 or 80 actual stages with an efficiency of about 0.5.

Preferably the fraction removed in step (9) is removed at the base of the first distillation zone or at a point one or more stages above the base. It may be withdrawn as a liquid or as a vapour. When withdrawn as a vapour, preferably a small liquid bleed is also taken from the base of the distillation zone.

It will often be the case that the vapour stream passing overhead from the first distillation zone will be two phase when it is cooled. When the overhead stream is two phase it is preferred that the reflux to the distillation zone be provided by separating the phases and using only the light, aqueous phase; the heavy, methyl iodide-rich phase being recycled to the carbonylation reactor. At least a portion of the aqueous phase may be recycled to the carbonylation reactor.

In a preferred embodiment of the present invention, liquid reaction composition may be withdrawn from the carbonylation reactor and introduced, with or without the addition of heat to a preliminary flash zone. The use of a preliminary flash zone has the advantage that it allows higher methyl acetate concentrations to be employed. Higher methyl acetate concentrations in the liquid reaction composition, as mentioned hereinbefore, are favourable for reducing the propionic acid content of the product acetic acid. A further advantage of using a preliminary flash zone is that the decanter may be dispensed with because the overhead from the first distillation zone will generally be single phased. Not only does this result in a capital saving but it also offers operational advantage in that it avoids any problems associated with consistently obtaining and maintaining two phases. In this preliminary flash zone, a preliminary flash vapour fraction comprising some of the methyl acetate, methyl iodide, acetic acid, water, methanol and propionic acid precursors in the introduced liquid reaction composition, is separated from a preliminary flash liquid fraction comprising the remaining components.

The preliminary flash vapour fraction is recycled to the carbonylation reactor. The preliminary flash liquid fraction is introduced to the flash zone of the present invention with or without the addition of heat, in the same way as if the preliminary flash zone had not been used. In this embodiment, the preliminary flash zone is preferably operated at a pressure below that of the reactor, typically at a pressure of 3 to 9 bara and the flash zone is operated at a pressure below that of the preliminary flash zone, typically at a pressure of 1 to 4 bara. Preferably,. the preliminary flash zone is maintained at a temperature of 120 to 160 C and the flash zone is maintained at a temperature of 100 to 140 C.

It is important that any process stream containing iridium carbonylation catalyst which is to be recycled to the carbonylation reactor contains a water concentration of at least 0.5% by weight to stabilise the iridium catalyst.

The invention will now be illustrated by reference to the following examples and Figure in which Figure 1 represents in schematic form apparatus for performing an embodiment of the process of the present invention having a single flash zone.

Referring to Figure 1 a carbonylation reactor (1) is provided with a stirrer (2), an inlet for carbon monoxide (3) and an inlet for methanol and/or a reactive derivative thereof (4). The reactor is also provided with an outlet (5) for withdrawing liquid reaction composition from the reactor and an outlet (6) for withdrawing gas from the head of the reactor. The outlet (5) is connected by line (7) through flashing valve (8) directly to flash zone (9).

The flash zone (9) is an adiabatic flash zone without heat input and is provided with an outlet (10) for a vapour fraction and an outlet (11) for a liquid fraction formed in use therein. In an alternative embodiment, heat can be supplied to the flash zone (9) to alter the ratio of vapour and liquid fractions. The flash zone is also provided with a scrubbing section (12) and optional wash through line (13). The liquid outlet (11) from the flash zone is connected to recycle pump (14) for recycling the liquid fraction to the reactor. At least part of the flash zone liquid fraction may be passed through an ion exchange resin bed (15) to remove corrosion metals therefrom and maintain the concentration of corrosion metals in the liquid reaction composition at less than that which would generate less than 500 ppm I'.< zone is provided with a base liquid take-off (20) for removing a process stream comprising acetic acid containing less than 275ppm propionic acid. Alternatively, the distillation zone (16) may be provided below the feed point with a take-off for a vapour stream comprising acetic acid product containing less than 275 ppm propionic acid and with a base liquid take-off suitably for recycle to the reactor.

In the apparatus shown in Figure 1, the flash zone and distillation zone may be operated at a pressure of 1 to 3 bar gauge.

Example 1-6 The apparatus illustrated in Figure 1 was used to produce acetic acid employing the conditions shown in the following Table.

TABLE

Reactor conditions CT1 CT2 Example 1 Example 2 CT 3 CT 4 CT 5 CT 6 Reactor Temperature ( C) 190.3 190.1 190.0 190.0 175.0 195.0 185.0 185.0 Reactor Pressure (barg) 27.1 27.4 29.6 30.0 23.8 27.7 25.0 26.2 CO paritial pressure (bara) 8.9 9.2 9.1 8.9 8.9 8.9 9.1 8.9 H2 partial pressure (bara) 0.21 0.15 0.05 0.04 0.27 0.16 0.18 0.24 Liquid reaction composition Water (% by weight) 4.9 5.2 6.0 5.7 4.8 5.0 5.2 5.0 Methyl iodide (% by weight) 6.8 6.7 9.8 9.9 7.4 6.7 6.7 6.5 Methyl acetate (% by weith) 13.9 13.9 18.8 19.5 15.4 14.5 15.5 15.2 Ir (ppm) 1360 1570 1270 880 2900 960 990 1620 Ru (ppm) 2130 2080 1800 2730 3720 1410 1380 2210 Ru:Ir(molar) 3.0 2.5 2.7 5.9 2.4 2.8 2.7 2.6 H2 in feed (% v/v) 0.29 0.06 0.05 0.05 0.28 0.30 0.29 0.29 Carbonylation rate (mol/l/h) 20.1 20.6 20.4 20.1 17.3 17.9 10.8 17.7 CO2 rate (% of carbonylation) 0.93 1.40 0.79 0.68 1.19 0.97 0.84 0.97 CH4 rate (% of carbonylation) 1.06 0.89 0.72 0.65 1.31 0.95 0.90 1.11 Product Water (ppm) 840 880 780 730 810 890 700 880 Propioin acid (ppm) 550 420 210 180 730 530 520 630

Reactor conditions Example 3 Example 4 Example 5 Example 6 Reactor Temperature ( C) 190.0 190.0 195.0 190.0 Reactor Pressure (barg) 30.0 28.4 30.0 29.2 CO partial pressure (bara) 8.9 8.7 8.8 9.0 H2 partial pressur (bara) 0.05 0.05 0.04 0.04 Liquid reaction composition Water (% by weight) 5.9 6.1 5.9 5.7 Methyl iodide (% by weight) 9.5 9.7 9.8 10.0 Methyl acetate (% by weight) 19.3 15.7 15.0 15.4 Ir (ppm) 990 1080 820 790 Ru (ppm) 2510 3390 2590 6160 Ru;Ir (molar ) 4.8 6.0 6.0 14.9 H2in feed(% v/v) 0.05 0.04 0.05 0.05 Carbonylation rate (mol/l/h) 20.4 20.1 20.0 20.1 CO2 rate (% of carbonylation) 0.71 0.80 0.79 0.75 CH4 rate (% of carbonylation) 0.67 0.76 0.74 0.73 Product Water (ppm) 660 680 860 650 Propionic acid (ppm) 190 230 240 240 With reference to the Table it can be seen from Examples 1-6 that acetic acid product having less than 275 ppm propionic acid content can be achieved by using conditions of the present invention, namely an iridium catalyst concentration of less than 1300 ppm and a hydrogen partial pressure in the carbon monoxide reactant gas of less than 0.1 bar absolute, as well as using only one column : a combined light ends/drying column in the purification section.

Comparison Tests 1-6, which are not according to the invention because one or both of the catalyst concentration or hydrogen partial pressure conditions is not fulfilled, are provided for the purpose of demonstrating the following points: (i) the effect of reducing the H2 content of the feed gas and hence the partial pressure of hydrogen under otherwise similar conditions (Comparison Tests 1 and 2). It can be seen from the Table that the propionic acid content in the acetic acid product falls.

(ii) the effect of operating at a higher temperature for the same carbonylation rate, the iridium concentration being lower for the higher reaction temperature (Comparison Tests 3 and 4). It can be seen that at the higher reaction temperature (lower iridium concentration) the propionic acid content in the acetic acid product falls.

(iii) the effect of operating at a lower carbonylation rate at the same reaction temperature, the iridium concentration being lower for the lower carbonylation rate (Comparison Tests 5 and 6). It can be seen that at the lower carbonylation rate (lower iridium concentration) the propionic acid content of the acetic acid product falls.

Claims (8)

1. Claims : 1. A process for the production of acetic acid comprising (1) continuously feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor which contains a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, optionally at least one promoter, a finite concentration of water, methyl acetate and acetic acid ; (2) contacting the methanol and/or reactive derivative thereof with the carbon monoxide in the liquid reaction composition to produce acetic acid and by-product propionic acid; (3) removing from the reactor liquid reaction composition containing acetic acid and by-product propionic acid; and (4) recovering acetic acid from the liquid reaction composition characterised in that by maintaining a partial pressure of hydrogen in the reactor of less than 0.1 bar absolute, and (ii) an iridium catalyst concentration in the liquid reaction composition of less than 1300 ppm measured as iridium metal, the concentration of by-product propionic acid in the recovered acetic acid is less than 275 ppm.
2. 2. A process as claimed in claim 1 in which the hydrogen partial pressure is maintained below 0.05 bar, preferably below 0.01 bar.
3. 3. A process as claimed in claim 1 or claim 2 in which the carbon monoxide fed to the carbonylation reactor contains less than 0.1 mole% hydrogen, preferably less than 0.05 mole%, even more preferably less than 0.01 mole%.
4. 4. A process as claimed in any one of the preceding claims in which the iridium catalyst concentration in the liquid reaction compostion is less than 1000 ppm, preferably less than 800 ppm, more preferably about 700 ppm.
5. 5. A process as claimed in any one of the preceding claims in which the reaction temperature is in the range 175 to 220 C, preferably 185 to 200 C, more preferably 188 to 195 C.
6. 6. A process as claimed in any one of the preceding claims in which there is present in the reaction composition at least one promoter selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, indium, tungsten and mixtures thereof.
7. 7. A process as claimed in any one of the preceding claims in which acetic acid is recovered from the liquid reaction compostion in step (4) by the steps of :- (5) introducing at least part of the withdrawn liquid reaction composition, with or without the addition of heat, to a flash zone to form a vapour fraction comprising water, acetic acid product, propionic acid by-product, methyl acetate, methyl iodide and propionic acid precursors, and a liquid fraction comprising involatile iridium catalyst, involatile optional promoter or promoters, acetic acid and water; (6) recycling the liquid fraction from the flash zone to the carbonylation reactor; (7) introducing the vapour fraction from the flash zone into a first distillation zone; (8) removing from the first distillation zone at a point above the introduction point of the flash zone vapour fraction a light ends recycle stream comprising water, methyl acetate, methyl iodide, acetic acid and propionic acid precursors, which stream is recycled in whole or in part to the carbonylation reactor; and (9) removing from the first distillation zone at a point below the introduction point of the flash zone vapour fraction a fraction comprising acetic acid product containing less than 275 ppm propionic acid.
8. 8. A process as claimed in claim 7 in which the fraction from step (9) is introduced to a second distillation zone wherein water is removed overhead and dry acetic acid is removed below the introduction point of the fraction from step (9).