GB2337751A - Iridium-catalysed carbonylation process for the production of acetic acid - Google Patents

Abstract

A process for the production of acetic acid by carbonylation with carbon monoxide of methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of copromoter and comprising an iridium carbonylation catalyst, methyl iodide cocatalyst, a finite concentration of water, acetic acid, methyl acetate and an aluminium-containing compound wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the aluminium-containing compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium : iridium of 0.1 :1 to 1.5:1.

Description

2337751 HUDIUM-CATALYSED CARBONYLATION PROCESS FOR THE PRODUCTION OF

ACETIC ACID The present invention relates to a process for the production of acetic acid and in particular to a process for the production of acetic acid by carbonylation in the presence of an iridium catalyst and a methyl iodide co-catalyst.

Preparation of carboxylic acids by iridium-catalysed carbonylation processes is known and is described, for example in GB-A-1234121, US-A 3772380, DE-A-1767150, EP-A-0616997, EP-A-0618184, EP-A-0618183, EP-A-0657386 and WO-A-95/31426 WO-A95131.426 discloses a process for the production of carboxylic acids or their esters having (n+l) carbon atoms by the liquid phase reaction of carbon monoxide with at least one alcohol having (n) carbon atoms in the presence of a catalytic system based on a compound of iridium and a halogen co-catalyst. The process is characterised by maintaining in the reaction medium water in a volume between greater than 0 and 10%, typically between 0.5 and 8%, preferably between 2 and 8%. the ester corresponding to the carboxylic acid and the alcohol in a volume varying between 2 and 40%; and iodides in soluble form of such a nature, that the atomic ratio of the iodides to iridium is between greater than 0 and 10, typically between greater than 0 and 3, preferably between greater than 0 and 1.5.

The volume of halogen co-catalyst in the reaction medium is between greater than 0 and 10%. typically between 0.5 and 8%, and preferably between 1 and 6%.

Suitable iodides include alkaline earth metal and alkali metal iodides, and specifically lithium iodide. The process of WO-A-95131426 is otherwise unpromoted.

EP-A-0643034 describes a process for the carbonylatioE of methanol andlor a reactive derivative thereof in the presence of acetic acid, an iridium catalyst, methyl iodide, at least a finite concentration of water, methyl acetate and a 1 promoter selected from ruthenium and osmium. Batch and continuous experiments are described therein. In the continuous experiments the water concentration is as low as 6.8 % by weight.

Our published European Patent Application No. 0752406 filed on 18.04.96 discloses 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, a finite concentration of water, acetic acid, methyl acetate and at least one promoter; (2) contacting the methanol andlor reactive derivative thereof with the carbon monoxide in the liquid reaction composition to produce acetic acid; and (3) recovering acetic acid from the liquid reaction composition characterised in that there is continuously maintained in the liquid reaction composition throughout the course of the reaction (a) water at a concentration of no greater than 6.5 % by weight, (b) methyl acetate at a concentration in the range 1 to 3 5 % by weight and (c) methyl iodide at a concentration in the range 4 to 20 % by weight.

In the promoted processes of EP-A-0643034 and European Application No. 96302734.7 it is said that ionic contan-dnants such as, for example, (a) corrosion metals, particularly nickel, iron and chron-dum 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 will have an adverse effect on the reaction by generating I- in the liquid reaction composition which has an adverse effect on the reaction rate. Similarly, it is said, contan- dnants such as alkali metal iodides, for example lithium iodide, should be kept to a minimum.

In WO-A-96/237757 which is directed to the preparation of iridium carboxylates and their use in inter ali carbonylation reactions, the use of promoters not being mentioned, it is stated in contrast to WO-A-951314326 that alkaline or alkaline earth ions are preferably eliminated, since their presence may have a harmful influence on the kinetics and selectivity of subsequent reactions in which the iridium carboxylate will be used as catalyst.

European Patent Application No. 973 10012.6 filed December 1997 directed to a carbonylation process of a liquid composition containing a promoter and a co-promoter wherein the co-promoter is selected from alkali metal iodides, alkaline earth metals, metal complexes and salts capable of generating F. The use of a promoter and co-promoter provides a process with relatively high yields and is 2 selectivity.

We have now found that the carbonylation process can be carried out without the need for a co-promoter by using an aluminium-containing compound as promoter.

Accordingly, the present invention provides a process for the production of acetic acid by carbonylation with carbon monoxide of methanol andlor a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of co-promoter and comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and an aluminium-containing compound wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the aluminium- containing compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium: iridium of 0. 1 is : 1 to L5:1.

The present invention utilises a liquid reaction composition comprising an alun-inium-containing compound. The composition does not require the presence of conventional promoters and/or co-promoters used in the carbonylation process. By co-promoter is meant alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating r and/or salts capable of generating I'. By conventional promoters is meant ruthenium, osn-dum, tungsten, rheniun. zinc, cadmium, indium, gallium, and mercury.

The increased carbonylation rate at the low water concentration of the present invention may allow operation at a reduced iridium catalyst concentration whilst maintaining the rate of carbonylation. This has benefits of reduced production rate of by-products such as propionic acid.

The rate of production of by-products propionic acid, methane, hydrogen and carbon dioxide may be reduced.

Water may be formed in situ in the liquid reaction composition, for example, by the esterification reaction between methanol reactant and acetic acid product. Small amounts of water may also be produced by hydrogenation of methanol to produce methane and water. 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 reaction composition withdrawn from the reactor and may be recycled in controlled amounts to maintain the required concentration of water in the liquid reaction 3 1 composition.

With reference to the aforesaid published European Application No. 0752406 the rate of the carbonylation reaction is said to increase as the water concentration in the liquid reaction composition is reduced from a concentration of greater than 6.5% by weight, passes through a maximum at a water concentration of no greater than 6.5% by weight and then declines as very low water concentrations are approached. In Figure 8 of the aforesaid application there is a plot of reaction rate versus water concentration which clearly shows a maximum. The water concentration at which the carbonylation rate is a maximum is said to increase as the concentration of methyl acetate in the liquid reaction composition is increased. It is believed that the water concentration at which the carbonylation rate is a maximum decreases as the concentration of methyl iodide in the liquid reaction composition is increased. For the purpose of the present invention the water concentration in the liquid reaction composition is preferably maintained below 6%, more preferably below 4.5% by weight. Operating at such a low water concentration according to the present invention gives rise to the advantage that recovery of acetic acid from the reaction composition withdrawn from the carbonylation reactor is facilitated because the amount of water which has to be separated from the acetic acid is reduced; separation of water from the acetic acid is an energy-intensive part of the recovery process and reduced water concentration results in reduced processing difficulty andlor costs.

In the process of the present invention, suitable reactive derivatives of methanol include methyl acetate, dimethyl ether and methyl iodide. A mixture 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. If methyl acetate or dimethyl ether are used, water co-reactant is required to produce acetic acid. 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 product or solvent. In the process of the present invention the concentration of methyl acetate in the liquid reaction composition is suitably in the range 1 to 70% by weight, preferably 2 to 50 % by weight, more preferably 5 to 40% by weight.

In the process of the present invention, the concentration of methyl iodide co-catalyst in the liquid reaction composition is suitably in the range from 1 to 30% by weight, preferably in the range from 1 to 20% by weight.

4 An advantage of achieving high carbonylation rates at low methyl iodide and water concentrations by the addition of co-promoters according to the present invention may be reduced corrosion, an alternative method of increasing the rate being to increase the methyl iodide concentration which can cause increased corrosion.

In the process of the present invention, the iridium carbonylation catalyst is suitably present in the liquid reaction composition at a concentration in the range 400 to 5000 ppm measured as iridium, preferably in the range 500 to 3000 ppm measured as iridium, more preferably in the range 700 to 3000 ppm measured as iridium. In the process of the present invention, the rate of the carbonylation reaction increases as the concentration of iridium is increased.

The iridium catalyst in the liquid 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 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 IrCI3, W3, IrBr3, [Ir(C0)2I]2, [lr(C0)2C112, [Ir(C0)2B12, [Ir(C0)2I2] I-I+, Rr(C0)2Br21-1-I+, Rr(C0)241-H+, [Ir(CH3)13(C0)21-R'-, Ir4(C0)12, IrCI3.3H20, IrBr3.3H20, Ir4(C0)12, iridium metal, Ir203, IrO2, Ir(acac)(C0b, Ir(aca03, iridium acetate, [Ir30(0Ac)6(H20)31[0Acl, 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.

In the process of the present invention an alun-linium-containing compound is present as promoter in the reaction composition. Preferably, the aluminiumcontaining promoter is 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. The aluminiumcontaining compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium to iridium of 0. 1: 1 to 1. 5: 1.

The aluminium-containing promoter may be added to the liquid reaction 35 composition for the carbonylation reaction in any suitable form which dissolves in the liquid reaction composition or is convertible to soluble form. Suitable aluminium-containing compounds which may be added to the reaction composition alone or as mixtures include aluminium oxides, for example A1203 -, aluminium hydroxides, for example AI(M)3; aluminium halides for example A1Br3, AIC13, AII3and A1F3.nH20; aluminium alkoxides; aluminium acetates, for example [AI(OAC)20111; mixed iodide acetate aluminium compounds; and aluminium hydrides, for example A1H3and boron aluminium hydride. Preferred aluminium containing compounds which may be added to the reaction composition are [AIfflAc)20M, A113and mixed iodide acetate aluminium compounds.

The carbon monoxide reactant may be essentially pure or may contain inert impurities such as carbon dioxide, methane, nitrogen, noble gases, water and Cl to C4 paraffinic hydrocarbons. The presence of hydrogen in the carbon monoxide feed and generated in situ by the water gas shift reaction is preferably kept low as its presence may result in the formation of hydrogenation products, Thus, the amount of hydrogen in the carbon monoxide reactant is preferably less than 1 mol %, more preferably less than 0.5 mol % and yet more preferably less than 0.3 mol % and/or the partial pressure of hydrogen in the carbonylation reactor is preferably less than 1 bar partial pressure, more preferably less than 0.5 bar and yet more preferably less than 0.3 bar. The partial pressure of carbon monoxide in the reactor is in the range greater than 0 to 40 bar, typically from 4 to 30 bar.

The total pressure of the carbonylation reaction is suitably in the range 10 to 200 barg, preferably 15 to 100 barg, more preferably 15 to 50 barg. The temperature of the carbonylation reaction is suitably in the range 100 to 3 00 'C, preferably in the range 15 0 to 220 'C.

The process of the present invention may be performed as a batch or as a continuous process, preferably as a continuous process.

The acetic acid product may be recovered from the liquid reaction composition by withdrawing vapour and/or liquid from the carbonylation reactor and recovering acetic acid from the withdrawn material. Preferably, acetic acid is recovered from the liquid reaction composition by continuously withdrawing liquid reaction composition from the carbonylation reactor and recovering acetic acid from the withdrawn liquid reaction composition by one or more flash andlor fractional distillation stages in which the acetic acid is separated-from the other components of the liquid reaction composition such as iridium catalyst. methyl iodide co-catalyst, promoter, methyl acetate, unreacted methanol and/or reactive 6 derivative thereo water and acetic acid solvent which may be recycled to the reactor to maintain their concentrations in the liquid reaction composition. To maintain stability of the iridium catalyst during the acetic acid product recovery stage, water in process streams containing iridium carbonylation catalyst for recycle to the carbonylation reactor should be maintained at a concentration of at least 0.5 % by weight.

The invention will now be illustrated by way of example only and with reference to the following examples. General Description of the Carbonylation Experiments

All experiments were performed using a 300 mI zirconium autoclave equipped with a magnetically driven stirrer with gas dispersion impellers, liquid catalyst injection facility and cooling coils. A gas supply to the autoclave was provided from a ballast vessel, feed gas being provided to maintain the autoclave at a constant pressure. The rate of gas uptake at a certain point in a reaction run was used to calculate the carbonylation rate, as number of moles of reactant consumed per litre of cold degassed reactor composition per hour {moVVhr}, at a particular reactor composition (reactor composition based on a cold degassed volume).

The methyl acetate concentration was calculated during the course of the reaction from the starting composition, assuming that one mole of methyl acetate is consumed for every mole of carbon monoxide that is consumed. No allowance was made for organic components in the autoclave headspace.

For each batch carbonylation experiment the catalyst, H21rCI6, dissolved in a portion of the acetic acid / water liquid reactor charge, was charged to the liquid injection facility. The reactor was then pressure tested with nitrogen, vented via a gas sampling system, and flushed with carbon monoxide several times (3 x 3 barg). If a promoter or additive was used this was placed in the autoclave and covered with a portion of the acetic acid charge (ca. 10g) prior to the pressure test.

The remaining liquid components of the reaction composition were charged to the autoclave via a liquid addition port. The autoclave was then optionally pressurised with 5 barg of carbon monoxide and slowly vented. The autoclave was then pressurised with carbon monoxide (typically 6 barg) and heated with stirring (1500 rpm) to reaction temperature, 190'C. The total pressure was then raised to approximately 3 barg below the desired operating pressure by feding forward carbon monoxide from the ballast vessel. Once stable at temperature (about minutes) the catalyst was injected using an over pressure of carbon monoxide. The 7 catalyst injection facility has an efficiency of > 90%. The reactor pressure was maintained at a constant value (:E 0.5 barg) by feeding gas from the ballast vessel throughout the experiment. Gas uptake from the ballast vessel was measured using datalogging facifities throughout the course of the experiment. The reaction temperature was maintained within +_ 1'C of the desired reaction temperature by means of a heating mantle connected to a Eurotherm (Trade Mark) control system.

In addition, excess heat of reaction was removed by means of cooling coils. Each run was conducted until the gas uptake had ceased, i.e. until less than 0. 1 bar per minute of gas was consumed from the ballast vessel. The ballast vessel was then isolated and the reactor crash cooled by use of the cooling coils.

H21rC16 (aqueous solution) was supplied by Johnson Matthey. The acetic acid was obtained from carbonylation of a mixed methanol/methyl acetate feedstock and contained very low amounts of propionic acid and its precursors. Methyl acetate, water, methyl iodide and aluminium acetate were supplied by Aldrich. Charge compositions are given in Table 1.

EXAlvTLES 1 to 4 and EXPERIMENTS A to F.

The general procedure described hereinabove was employed. The charge compositions are given in Table 1.

Experiments A to C are not according to the present invention for the reason that either no aluminium-containing compound was used or a copromoter was present in the reaction composition.

Experiments D and E show the effect of adding AI(OAC)20H at water concentrations where the carbonylation rate is declining with increasing water concentrations - that is at water concentrations which are higher than that at which the maximum in the graph of carbonylation rate versus water concentration occurs.

Under these conditions, addition of the aluminium compound caused a reduction in the carbonylation rate. A reduction was observed in the amount of propionic acid and its precursors (ethyl iodide and ethyl acetate) formed.

Example F illustrates the effect on carbonylation rate of adding AAfflAC)20H at 2.0: 1.0 molar ratio to iridium. The rate was less than that observed in the absence of any promoter; see Table 2. However, a reduction in the amount of propionic acid and its precursors (ethyl iodide and ethyl acetate) formed was observed.

Examples 1 and 2 demonstrate the effect on carbonylation rate of the adding an aluminium-containing compound in the absence of a co-promoter, using 8 an iridium catalyst at 19WC and 28 barg total pressure. Rate data, at various methyl acetate (MeOAc) and water concentrations, are given in Table 2.

Example 3 illustrates that the aluminium promoter can be added as A113.

Example 4 illustrates the promotional efflect on carbonylation rate of adding AI(OM)20H at 0.5: 1.0 molar ratio to iridium.

9 11 Table 1 Charge compositions for iridium catalysed reactions in a 300 mI zirconium batch autoclave Experiment/Example MeOAc (g) AcOH (g) Mel (g) Water (g) H21r06 (9') Additive Amount (g) A 60.02 68.99 13.96 6.46 0.642 B 60.02 69.00 13.96 6.40 0.636 c 60,00 68.80 13.96 6.41 0.637 LiI 0.209 D 60.00 61.89 13.97 13.71 0.643 - E 60.00 61.44 13,97 13.71 0.635 AIfflAc)20H 0.253 F 59.99 68.48 13,95 6,41 0.634 AIfflAc)2(Offi 0.500 1 60.00 68.75 13.96 6.40 0.636 AI(OM)2(0H) 0.250 2 60.04 68.76 13.96 6,40 0.634 A.IfflAC)2(0H) 0.250 3 60.06 68.40 13.97 6,39 0.635 AII3 0.640 4 60.01 68.90 13.97 6.40 0.635 AI(OM)2(Offi 0.120 a) Weight expressed as pure H21rC16 Table 2 Carbojiylation rate data for iridium catalysed reactions in a 300 ml zirconium autoclave Experiment/Example Catalyst System Water Rate/mo @ Water Rate /molXhr Molar ratio % w/w 30% MeOM % w/w @ 25% Me0Ac A Ir only 2.1 12.1 0.9 5.9 B Ir only 2.0 13.3 0.9 6.0 c ir/Li 1 2.0 6.3 0.9 4.4 D Ir only 6.7 19.6 5.5 17.4 E Ir/Al 1 6.7 15.1 5.5 13.2 Ir/Al 1:2 2,0 7.9 0.9 5.8 1 Ir/Al 1 2.0 16.4 0.9 11.1 2 Ir/Al 1 2.0 17.1 0.9 9.9 3 Ir/Al 1 A 2.0 15.6 0.9 10.6 4 Ir/Al 1 A.5 2.0 17,3 0.9 11.0 a) All reactions at 28 barg total pressure and 190'T with a stirrer speed of 1500 rpm. MeOAc.

ca. 8.4% w/w MeI at 3 0% w1w MeI concentration is adjusted slightly downwards based upon the approximation that each mole of iridium can consume a maximum of 4 moles of methyl iodide'to give [lr(C0)2141_.

11 claims: 1. A process for the production of acetic acid by carbonylation with carbon monoxide of methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of copromoter and comprising an iridium carbonylation catalyst, methyl iodide co- catalyst, a finite concentration of water, acetic acid, methyl acetate and an alun-dnium-containing compound wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the aluminium-containing compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium: iridium of 0.1: 1 to 1.5A.

2. A process as claimed in claim 1 in which there is absent from the reaction composition any promoter consisting of ruthenium, osmium, tungsten, rhenium, zinc, cadmium, iridium, gallium and/or mercury. 3. A process as claimed in any one of the preceding claims in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 400 to 5000 ppm measured as iridium, preferably in the range 500 to 3000 ppm measured as iridium, more preferably in the range 700 to 3000 ppm measured as iridium, the methyl acetate concentration in the reaction composition is in the range 1 to 70 % by weight, preferably 2 to 50 % by weight and more preferably 5 to 40 % by weight and the methyl iodide concentration in the reaction composition is in the range from 1 to 30 % by weight, preferably in the range from 1 to 20 % by weight. 4. A process for the production of acetic acid by carbonylation with carbon monoxide of methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of co- 12 promoter consisting of alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating F and/or salts capable of generating I" and in the absence in the reaction composition of any promoter consisting of ruthenium, osmium, tungsten, rhenium, zinc, cadmium, iridium, gallium and/or mercury said 5 reaction composition comprising an iridium carbonylation catalyst at a concentration in the range 500 to 3000 ppm measured as iridium, methyl iodide cocatalyst at a concentration in the range from 1 to 30% by weight, a finite concentration of water, acetic acid, methyl acetate at a concentration in the range 1 to 70 % and an aluminium-containing compound wherein the water concentration 10 is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the alun-dnium-containing compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium: iridium of 0.1: 1 to 1.5:1. 5. A process as claimed in any one of the preceding claims in which the water 15 concentration in the reaction composition is maintained at below 6% by weight. 6. A process as claimed in claim 5 in which the water concentration in the reaction composition is maintained at below 4.5 % by weight. 7. A process as claimed in any one of the preceding claims in which the aluminium-containing promoter is added to the reaction composition as an alun-dnium-containing compound selected from the group consisting of aluminium oxides, aluminium hydroxides, aluminium halides, aluminium alkoxides, aluminium acetate, iodide acetate aluminium compounds, aluminium hydrides and mixtures thereof 8. A process as claimed in claim 7 in which the aluminium oxide is A1.203; the 25 aluminium hydroxide is AI(OH)3; the aluminium halide is A1Br3, AIC13, A113 or A1F3.nH20; the aluminium acetate is [AI(OAC)20111; or the alun- dnium hydride is A1H3 or boron aluminium hydride. 9. A process as claimed in any one of the preceding claims in which the aluminium-containing promoter is added to the reaction composition as an aluminium-containing compound selected from the group consisting of [AI(OAC)20111, A113 and mixed iodide acetate aluminium compounds. 10. A process substantially as hereindescribed and with reference to Examples 1 to 4.

11. Acetic acid whenever produced by any one of the preceding claims.

13 Amendments to the claims have been filed " follows

Claims (14)

1. Claims: 1. A process for the production of acetic acid by carbonylation

   with carbon monoxide of methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of copromoter and comprising an iridium carbonylation catalyst, methyl iodide cocatalyst, a finite concentration of water, acetic acid, methyl acetate and an alurninium-containing compound wherein there is absent from the reaction composition any promoter consisting of ruthenium, osmium, tungsten, rhenium, zinc, cadmium, indium, gallium andlor mercury, and wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the aluminiumcontaining compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium: iridium of 0. 1: 1 to 1.5: 1.
2. 2. A process as claimed in any one of the preceding claims in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 400 to 5000 ppm measured as iridium, the methyl acetate concentration in the reaction composition is in the range 1 to 70 % by weight and the methyl iodide concentration in the reaction composition is in the range from 1 to 30 % by weight.
3. 3. A process as claimed in any one of the preceding claims in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 500 to 3000 ppm measured as iridium.
4. 4. A process as claimed in any one of the preceding claims in which the methyl acetate concentration in the reaction composition is in the range 2 to 50 % by weight.

   0
5. 5. A process as claimed in claim 4 in which the methyl acetate concentration in the reaction composition is in the ran-ae 5 to 40 % by weight.

   0 1-5
6. 6. A process as claimed in any one of the preceding claims in which the methyl iodide concentration in the reaction composition is in the range 1 to 20 % by weight.
7. 7. A process for the production of acetic acid by carbonylation with carbon monoxide of methanol andlor a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of copromoter consisting of alkali metal iodides, alkaline earth metal iodides, metal corn lexes capable of generating F andlor salts capable of generating F and in the p 0 absence in the reaction composition of any promoter consisting of ruthenium, osmium, tungsten, rhenium, zinc, cadmium, iridium, gallium andlor mercury said reaction composition comprising an iridium carbonylation catalyst at a concentration in the range 500 to 3000 ppm measured as iridium, methyl iodide co-catalyst at a concentration in the range from 1 to 30% by weight, a finite 0 0 11.

   concentration of water, acetic acid, methyl acetate at a concentration in the range 1 to 70 % and an aluminium-containing compound wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the alurninium-containing compound is present as promoter in the liquid reaction composition at a molar ratio of aluminium iridium of 0.1: 1 to 1. 5A.
8. 8. A process as claimed in any one of the preceding claims in which the water concentration in the reaction composition is maintained at below 6% by weight.
9. 9. A process as claimed in claim 8 in which the water concentration in the reaction composition is maintained at below 4.5 % by weight.
10. 10. A process as claimed in any one of the preceding claims in which the aluminium-containing promoter is added to the reaction composition as an aluminium-containing compound selected from the group consisting of aluminium oxides, aluminium hydroxides, aluminium halides, aluminium alkoxides, aluminium acetate, iodide acetate aluminium compounds, alurninium hydrides and mixtures thereof.
11. A process as claimed in claim 10 in which the alun-dnium oxide is A1203; the aluminium hydroxide is AI(OHh; the aluminium halide is A1Br3, AIC13, A113 or A1F3.nffiO; the alun-dnium acetate is [AI(OAc)20H1; or the aluminium hydride is A1H3 or boron aluminium hydride.
12. 12. A process as claimed in any one of the preceding claims in which the aluminium-containing promoter is added to the reaction composition as an 0 1 fcrw aluminium-containing compound selected from the group consisting of 0 c) [AI(OAc)20HI, A113 and mixed iodide acetate aluminium compounds.
13. 13. A process substantially as hereindescribed and with reference to Examples 1 to 4.
14. 14. Acetic acid whenever produced by any one of the preceding claims.