acetic acid

In 1996 a new process for the carbonylation ofmethanol to acetic acid was announced by BPChemicals, based on a promoted iridium catalystpackage, named CativaTM. The new process offersboth significant improvements over the conven-tional rhodium-based Monsanto technology andsignificant savings on the capital required to buildnew plants or to expand existing methanol car-bonylation units. Small-scale batch testing of thenew CativaTM process began in 1990, and inNovember 1995 the process was first used com-mercially, in Texas City, U.S.A., see Table I.

The new technology was able to increase plantthroughput significantly by removing previousprocess restrictions (debottlenecking), for instanceat Hull, see Figure 1. The final throughputachieved has so far been determined by local avail-

ability of carbon monoxide, CO, feedstock ratherthan any limitation imposed by the CativaTM sys-tem. In 2000 the first plant to use this newtechnology will be brought on-stream in Malaysia.The rapid deployment of this new iridium-basedtechnology is due to these successes and its manyadvantages over rhodium-based technology. Thebackground to this industrial method of producingacetic acid is explained below.

The Rhodium-BasedMonsanto Process

The production of acetic acid by the Monsantoprocess utilises a rhodium catalyst and operates ata pressure of 30 to 60 atmospheres and at temper-atures of 150 to 200ºC. The process givesselectivity of over 99 per cent for the major feed-

Platinum Metals Rev., 2000, 44, (3), 94–105 94

The CativaTM Process for the Manufactureof Acetic AcidIRIDIUM CATALYST IMPROVES PRODUCTIVITY IN AN ESTABLISHED INDUSTRIAL PROCESS

By Jane H. JonesBP Chemicals Ltd., Hull Research & Technology Centre, Salt End, Hull HU12 8DS, U.K.

Acetic acid is an important industrial commodity chemical, with a world demand of about 6

million tonnes per year and many industrial uses. The preferred industrial method for its

manufacture is by the carbonylation of methanol and this accounts for approximately 60 per

cent of the total world acetic acid manufacturing capacity. The carbonylation of methanol,

catalysed by rhodium, was invented by Monsanto in the 1960s and for 25 years was the leading

technology. In 1996 a new, more efficient, process for the carbonylation of methanol was

announced by BP Chemicals, this time using an iridium catalyst. This article describes the

new process and looks at the ways in which it improves upon the prior technology.

Table I

Plants Producing Acetic Acid Using the New CativaTM Promoted Iridium Catalyst Package

Plant Location Year Debottlenecking orincreased throughput achieved, %

Sterling Chemicals Texas City, U.S.A. 1995 20Samsung-BP Ulsan, South Korea 1997 75BP Chemicals Hull, U.K. 1998 25Sterling Chemicals Texas City, U.S.A. 1999 25BP Petronas Kertih, Malaysia 2000 Output 500,000 tonnes per annum

Platinum Metals Rev., 2000, 44, (3) 95

Scheme I

The reaction cycle for the Monsantorhodium-catalysed carbonylation of

methanol to acetic acid

Fig. 1 The CativaTM acetic acid plant which is now operating at Hull. The plant uses a promoted iridium catalyst package for the carbonylation of methanol. The new combined light ends and drying column can be seen

stock, methanol (1). This reaction hasbeen investigated in great detail byForster and his co-workers at Monsanto and the accepted mecha-nism is shown in Scheme I (2). Thecycle is a classic example of a homoge-neous catalytic process and is made upof six discrete but interlinked reactions.

During the methanol carbonylation,methyl iodide is generated by the reac-tion of added methanol with hydrogeniodide. Infrared spectroscopic studieshave shown that the major rhodiumcatalyst species present is [Rh(CO)2I2]–,A. The methyl iodide adds oxidativelyto this rhodium species to give a rhodi-um-methyl complex, B. The key to theprocess is that this rhodium-methylcomplex undergoes a rapid change inwhich the methyl is shifted to a neigh-bouring carbonyl group, C. After thesubsequent addition of CO, the rhodi-um complex becomes locked into thisacyl form, D. Reductive elimination ofthe acyl species and attack by water canthen occur to liberate the original rhodium dicarbonyl diiodide complex


and to form acetic acid and hydrogen iodide, HI.When the water content is high (> 8 wt.%), the

rate determining step in the process is the oxida-tive addition of methyl iodide to the rhodiumcentre. The reaction rate is then essentially firstorder in both catalyst and methyl iodide concen-trations, and under commercial reaction conditionsit is largely independent of any other parameters:

Rate µ [catalyst] × [CH3I] (i)

However, if the water content is less than 8wt.%, the rate determining step becomes thereductive elimination of the acyl species, from cat-alyst species D.

Although rhodium-catalysed carbonylation ofmethanol is highly selective and efficient, it suffersfrom some disadvantageous side reactions. Forexample, rhodium will also catalyse the water gasshift reaction. This reaction occurs via the compet-ing oxidative addition of HI to [Rh(CO)2I2]– andgenerates low levels of carbon dioxide, CO2, andhydrogen, H2, from CO and water feed.

[Rh(CO)2I2]– + 2HI ® [Rh(CO)2I4]– + H2 (ii)

[Rh(CO)2I4]– + H2O + CO ®[Rh(CO)2I2]– + CO2 + 2 HI (iii)

Overall: CO + H2O ® CO2 + H2 (iv)

This side reaction represents a loss of selectivi-ty with respect to the CO raw material. Also, thegaseous byproducts dilute the CO present in thereactor, lowering its partial pressure – which wouldeventually starve the system of CO. Significant vol-umes of gas are thus vented – with further loss ofyield as the reaction is dependent upon a minimumCO partial pressure. However, the yield on CO isgood (> 85 per cent), but there is room forimprovement (3, 4).

Propionic acid is the major liquid byproductfrom this process and may be produced by the car-bonylation of ethanol, present as an impurity in themethanol feed. However, much more propionicacid is observed than is accounted for by thisroute. As this rhodium catalysed system can gener-ate acetaldehyde, it is proposed that thisacetaldehyde, or its rhodium-bound precursor,undergoes reduction by hydrogen present in the

system to give ethanol which subsequently yieldspropionic acid.

One possible precursor for the generation ofacetaldehyde is the rhodium-acyl species, D,shown in Scheme I. Reaction of this species withhydrogen iodide would yield acetaldehyde and[RhI4CO]–, the latter being well known in this sys-tem and proposed to be the principal cause ofcatalyst loss by precipitation of inactive rhodiumtriiodide. The precipitation is observed in CO-deficient areas of the plant.

[RhI3(CO)(COCH 3)]– + HI ®[RhI4(CO)]– + CH3CHO (v)

[RhI4(CO)]– ® RhI3 + I– + CO (vi)

In addition to propionic acid, very small amountsof acetaldehyde condensation products, theirderivatives and iodide derivatives are alsoobserved. However, under the commercial operat-ing conditions of the original Monsanto process,these trace compounds do not present a problemto either product yield or product purity. Themajor units comprising a commercial-scaleMonsanto methanol carbonylation plant areshown in Figure 2.

The Monsanto Industrial ConfigurationThe carbonylation reaction is carried out in a

stirred tank reactor on a continuous basis. Liquid isremoved from the reactor through a pressurereduction valve. This then enters an adiabatic flashtank, where the light components of methylacetate, methyl iodide, some water and the productacetic acid are removed as a vapour from the topof the vessel. These are fed forward to the distilla-tion train for further purification. The remainingliquid in the flash tank, which contains the dis-solved catalyst, is recycled to the reactor. A majorlimitation of the standard rhodium-catalysedmethanol carbonylation technology is the instabili-ty of the catalyst in the CO-deficient areas of theplant, especially in the flash tank. Here, loss of COfrom the rhodium complexes formed can lead tothe formation of inactive species, such as[Rh(CO)2I4]–, and eventually loss of rhodium as theinsoluble RhI3, see Equations (v) and (vi).

Conditions in the reactor have to be maintained


within certain limits to prevent precipitation of thecatalyst. This imposes limits on the water, methylacetate, methyl iodide and rhodium concentra-tions. A minimum CO partial pressure is alsorequired. To prevent catalyst precipitation andachieve high reaction rates, high water concentra-tions in excess of 10 wt.% are desirable. Theserestrictions place a limit on plant productivity andincrease operating costs since the distillation sec-tion of the plant has to remove all the water fromthe acetic acid product for recycling to the reactor.(The water is recycled to maintain the correctstanding concentration.)

Significant capital and operational costs are alsoincurred by the necessity of operating a large dis-tillation column (the “Heavies” column) toremove low levels of high boiling point impurities,with propionic acid being the major component.

The CativaTM Iridium Catalyst forMethanol Carbonylation

Due to the limitations described above and alsobecause of the very attractive price differencebetween rhodium ($5200 per troy oz) and iridium($300 per troy oz) which existed in 1990, researchinto the use of iridium as a catalyst was resumed by

BP in 1990, after earlier work by Monsanto. Theinitial batch autoclave experiments showed signif-icant promise, and the development rapidlyrequired the coordinated effort of several diverseteams.

One early finding from the investigations wasof the extreme robustness of the iridium catalystspecies (5). Its robustness at extremely low waterconcentrations (0.5 wt.%) is particularly significantand ideal for optimisation of the methanol car-bonylation process. The iridium catalyst was alsofound to remain stable under a wide range of con-ditions that would cause the rhodium analogues todecompose completely to inactive and largelyirrecoverable rhodium salts. Besides this stability,iridium is also much more soluble than rhodium inthe reaction medium and thus higher catalyst con-centrations can be obtained, making much higherreaction rates achievable.

The unique differences between the rhodiumand iridium catalytic cycles for methanol carbony-lation have been investigated in a close partnershipbetween researchers from BP Chemicals in Hulland a research group at the University of Sheffield(6). The anionic iridium cycle, shown in Scheme II,is similar to the rhodium cycle, but contains

Fig. 2 The major units comprising a commercial-scale Monsanto methanol operating plant, which uses a rhodium-based catalyst. The technology uses three distillation columns to sequentially remove low boilers (methyl iodide andmethyl acetate), water, and high boilers (propionic acid) and deliver high purity acetic acid product


sufficient key differences to produce the majoradvantages seen with the iridium process.

Model studies have shown that the oxidativeaddition of methyl iodide to the iridium centre isabout 150 times faster than the equivalent reactionwith rhodium (6). This represents a dramaticimprovement in the available reaction rates, as thisstep is now no longer rate determining (as in thecase of rhodium). The slowest step in the cycle isthe subsequent migratory insertion of CO to formthe iridium-acyl species, F, which involves theelimination of ionic iodide and the coordination ofan additional CO ligand. This would suggest atotally different form of rate law:

Rate µ [catalyst] × [CO] (vii)¾¾¾¾¾¾

[I–]

or, taking the organic equilibria into account:

Rate µ [catalyst] × [CO] × [MeOAc] (viii)

The implied inverse dependence on ionic iodideconcentration suggests that very high reaction ratesshould be achievable by operating at low iodideconcentrations. It also suggests that the inclusionof species capable of assisting in removing iodideshould promote this new rate limiting step.Promoters for this system fall within two distinctgroups:

� simple iodide complexes of zinc, cadmium, mercury, gallium and indium (7), and

� carbonyl-iodide complexes of tungsten, rhenium,ruthenium and osmium (8, 9).

Batch Autoclave StudiesThe effect on the reaction rate of adding five

molar equivalents of promoter to one of the iridi-um catalyst is shown in Table II. A combination ofpromoters may also be used, see runs 13 and 14.None of these metals are effective as carbonylationcatalysts in their own right, but all are effectivewhen used in conjunction with iridium.

The presence of a promoter leads to a substan-tial increase in the proportion of “active anionic”species [Ir(CO)2I3Me]–, E, and a substantialdecrease in the “inactive” [Ir(CO)2I4]–. A suggestedmechanism for the promotion of iridium catalysisby a metal promoter [M(CO)xIy], is given inScheme III. The promotion is thought to occur viadirect interaction of promoter and iridium speciesas shown. The rate of reaction is dependent uponthe loss of iodide from [Ir(CO)2I3Me]–. These metalpromoters are believed to reduce the standing con-centration of I– thus facilitating the loss of iodidefrom the catalytic species. It is also postulated thatcarbonyl-based promoters may then go on todonate CO in further steps of the catalytic cycle.

Scheme II

Catalytic cycle for thecarbonylation of methanol

using iridium


Another key role of the promoter appears to bein the prevention of the build up of “inactive”forms of the catalyst, such as [Ir(CO)2I4]– and[Ir(CO)3I3]. These species are formed as intermedi-ates in the water gas shift reaction.

For the rhodium system the rate of the carbony-lation reaction is dependent only upon theconcentrations of rhodium and methyl iodide.However, the situation is more complex for the pro-moted iridium system. Table III illustrates the effect

of the system parameters on the rate of reaction.The effect of water concentration on the car-

bonylation rates of a rhodium system and aniridium/ruthenium system is illustrated in Figure3. For rhodium, a decline in carbonylation rate isobserved as the water content is reduced belowabout 8 wt.%. There are a number of possible the-ories for this, including a possible build up of the“inactive” [Rh(CO)2I4]– species formed in thewater gas shift cycle at lower water concentrations,

Table II

Effect of Various Additives on the Rate for the Iridium-Catalysed Carbonylation of Methanola fromBatch Autoclave Data

Experimental Additive Additive:iridium, Carbonylation rate,run molar ratio mol dm–3 h–1

1 None – 8.22 LiI 1:1 4.33 Bu4NI 1:1 2.74 Ru(CO)4I2 5:1 21.65 Os(CO)4I2 5:1 18.66 Re(CO)5Cl 5:1 9.77 W(CO)6 5:1 9.08 ZnI2 5:1 11.59 CdI2 5:1 14.7

10 HgI2 5:1 11.811 GaI3 5:1 12.712 InI3 5:1 14.813 InI3 /Ru(CO)4I2 5:1:1 19.414 ZnI2 /Ru(CO)4I2 5:1:1 13.115 Ru(CO)4I2 Control: no iridiumb 0c

aReaction conditions: 190ºC, 22 barg, and 1500 rpm. Autoclave charge: methyl acetate (648 mmol), water (943 mmol), acetic acid(1258 mmol), methyl iodide (62 mmol), and H2IrCl6 (1.56 mmol) plus additive as required. Carbonylation rate, in mol dm–3 h–1,measured at 50 per cent conversion of methyl acetate.

bControl experiment conducted in the absence of iridium. Amount of the ruthenium complex used is the same as in run 4.

cNo CO uptake observed

Scheme III

A proposed mechanism for the promotion of iridium catalysis by a metal promoter, [M(CO)xIy(solv)].The solvent could be water or methanol


which is a precursor for the formation of insolubleRhI3.

Another theory for the decline in the carbony-lation rate is that the rate determining step in thecatalytic cycle changes to the reductive elimination(attack by water) instead of oxidative addition. Thisis consistent with the increased amount ofacetaldehyde-derived byproducts in a low waterconcentration rhodium system, as the rhodium-

acyl species, D, is longer lived.At lower water concentrations, the addition of

ionic iodides, especially Group I metal iodides, tothe process has been found to stabilise the rhodi-um catalysts and sustain the reaction rate byinhibiting the water gas shift cycle, inhibiting theformation of [Rh(CO)2I4]– and its degradation toRhI3 and promoting the oxidative addition step ofthe catalytic cycle (10–13).

Table III

The Rate Dependence Differences between the Rhodium and Iridium Systems

Rhodium Iridium/promoter

Water 1st order below 8 wt.% Increases with increasing waterIndependent above 8 wt.% up to ~ 5 wt.%, then decreases

with increasing water

Methyl acetate Independent above ~ 1 wt.% Increases with increasing methyl acetate

Methyl iodide 1st order Increases with increasing methyl iodideup to ~ 6 wt.%, then independent

CO partial pressure A minimum CO partial pressure is Increases with increasing CO partial pressure.required; above this, independent As the CO partial pressure falls below ~ 8 bara

the rate decreases more rapidly

Corrosion metals Independent As the corrosion metals increase in concentration, the rate decreases

Rhodium 1st order Non applicable

Iridium Non applicable 1st order, effect tails off at highcatalyst concentrations

Promoter Non applicable Increases with increasing promoter,effect tails off at higher concentrations

bara is bar absolute; atmospheric pressure = 1 bar absolute (= 0 bar gauge, barg)

Fig. 3 A comparison of carbonylation ratesfor iridium/ruthenium and rhodium processesdepending on water concentration. Thesebatch autoclave data were taken underconditions of ~ 30 % w/w methyl acetate,8.4 % w/w methyl iodide, 28 barg totalpressure and 190ºC; (barg is a bar gauge,referenced to atmospheric pressure, withatmospheric pressure = 0 bar gauge)


However, there is also a downside, in the lithium-promoted rhodium system, the acetalde-hyde is not scavenged sufficiently by the catalystsystem to form propionic acid and therefore theconcentration of acetaldehyde increases, conden-sation reactions occur and higher non-acidiccompounds and iodide derivatives are formed, forexample hexyl iodide. Further purification stepsare then required (14).

For a CativaTM system, in contrast to rhodium,the reaction rate increases with decreasing watercontent, see Figure 3. A maximum value is reachedat around 5 % w/w (under the conditions shown).Throughout this region of the curve the iridiumspecies observed are [Ir(CO)2I4]– (the “inactive”species which is formed in the water gas shiftcycle) and [Ir(CO)2I3Me]– (the “active” species inthe anionic cycle). When the water concentrationfalls below 5 % w/w the carbonylation rate declinesand the neutral “active” species [Ir(CO)3I] and thecorresponding “inactive” water gas shift species[Ir(CO)3I3] are observed.

Other Factors Affecting the Reaction Rate(i) Methyl acetate concentration

In the rhodium system, the rate is independentof the methyl acetate concentration across a rangeof reactor compositions and process conditions(1). In contrast, the CativaTM system displays astrong rate dependence on methyl acetate concen-tration, and methyl acetate concentrations can beincreased to far higher levels than in the rhodiumsystem, leading to high reaction rates. High methylacetate concentrations may not be used in the

rhodium process because of catalyst precipitationin downstream areas of the plant.(ii) Methyl iodide concentration

The reaction rate for CativaTM has a reduceddependency on the methyl iodide concentrationcompared with the rhodium system. This is con-sistent with the fast rate of oxidative addition ofmethyl iodide to [Ir(CO)2I2]– giving [Ir(CO)2I3Me]–.(iii) CO partial pressure

The effect of CO partial pressure in theCativaTM process is more significant than for therhodium process with the rate being suppressedbelow 8 bara when operating in the ionic cycle.(iii) Poisoning the CativaTM system

Corrosion metals, primarily iron and nickel,poison the CativaTM process. However, it is not thecorrosion metals themselves that poison theprocess, but rather the ionic iodide which theysupport that inhibits the iodide loss step in thecarbonylation cycle, see Scheme II.(iv) Catalyst concentration

The effects of catalyst concentrations on thecarbonylation rate for an unpromoted and for aruthenium-promoted iridium catalyst are shown inFigure 4. The ruthenium promoter is effectiveover a wide range of catalyst concentrations. Ashigh catalyst concentrations and high reactionrates are approached a deviation from first orderbehaviour is noted, and a small but significant lossin reaction selectivity is observed.(v) Promoters

The addition of further promoters, to the onesalready present, for example iridium/ruthenium,can have positive effects. For instance, a synergy is

Fig. 4 The effect ofcatalyst concentration onthe carbonylation ratefor an unpromoted and aruthenium-promotediridium catalyst. Theruthenium promoter iseffective over a widerange of catalystconcentrations. Batchautoclave data weretaken at ~ 20 % w/wmethyl acetate, 8 % w/wmethyl iodide, 5.7 % w/wwater, 28 barg totalpressure and 190ºC


observed between the promoters and iodide salts,such as lithium iodide (15). Iodides usually poisonthe iridium catalyst, for example, if lithium iodideis added to an iridium-only catalyst at low water (~ 2 % w/w) and high methyl acetate (30 % w/w),there is a markedly reduced carbonylation rate. Aratio of one molar equivalent of lithium iodide:iridium reduces the reaction rate by 50 per cent,see run 2 in Table IV but, under the same reactionconditions two molar equivalents of ruthenium:iridium increases the carbonylation rate by 25 percent. Remarkably, adding lithium iodide to theruthenium-promoted catalyst under these condi-tions further doubles the carbonylation rate (run4). The net effect is that ruthenium and lithiumiodide in combination under certain conditionsincrease the reaction rate by 250 per cent withrespect to an unpromoted iridium catalyst. Thus,adding low levels of iodide salts to a promoted irid-ium catalyst allows the position of the ratemaximum, with respect to the water concentration,

to be moved to even lower water.The effect of the lithium iodide:iridium molar

ratio on the carbonylation rate is shown in Figure5 for a ruthenium-promoted iridium catalyst, hav-ing iridium:ruthenium molar ratios of 1:2 and 1:5.Under these conditions an exceptionally high rateof 47 mol dm–3 h–1 can be achieved with a molarratio for iridium:ruthenium:lithium of 1:5:1.

Interdependence of Process VariablesThe CativaTM process thus displays a complex

interdependence between all the major processvariables, notably between [methyl acetate],[water], [methyl iodide], [iridium], CO partial pres-sure, temperature and the promoter package used.For example, the methyl iodide concentration,above a low threshold value, has only a small influ-ence on the reaction rate under certain conditions.However, when the reaction rate is declining withreducing water concentration, as shown for aruthenium-promoted iridium catalyst in Figure 3,

Fig. 5 The effect of addinga second promoter oflithium iodide to ruthenium-promoted iridium catalystson the methanolcarbonylation rates. Batchautoclave data taken at 2 % w/w water and30 % w/w methyl acetate

Table IV

Effect of Lithium Iodide Additions on the Carbonylation Rate for Iridium and Iridium/RutheniumCatalysed Methanol Carbonylationa from Batch Autoclave Data

Experimental Catalyst system Water, Carbonylation rate,run % w/w mol dm–3 h–1

1 Iridium only 2.1 12.12 Iridium/lithium 1:1 molar ratio 2.0 6.33 Iridium/ruthenium 1:2 molar ratio 2.0 15.14 Iridium/ruthenium/lithium 1:2:1 molar ratio 2.0 30.8

aReaction conditions: 190ºC, 28 barg total pressure, and 30 % w/w methyl acetate, 8.4 % w/w methyl iodide and 1950 ppm iridium


increasing the methyl iodide concentration from8.4 to 12.6 % w/w doubles the reaction rate.Increasing the methyl iodide concentration underthese conditions also increases the effectiveness ofthe ruthenium promoter (16). In the CativaTM

process these interactions are optimised to max-imise reactor productivity and reaction selectivityand minimise processing costs.

In addition to the batch autoclave studies, apilot plant unit operating under steady state condi-tions was used to optimise the CativaTM process.The unit provided data on the carbonylation rate,the byproducts, catalyst stability, corrosion ratesand product quality under continuous steady stateoperation.

PurificationThe quality of the acetic acid produced in the

CativaTM process is exceptional. It is inherently lowin organic iodide impurities, which trouble otherlow water, rhodium-based, processes (14).Acetaldehyde is responsible for the formation ofthe higher organic iodide compounds via a seriesof condensation steps and other reactions. Thesehigher iodides are difficult to remove by conven-tional distillation techniques and further treatmentsteps are sometimes necessary to ensure that theacetic acid is pure enough for all end uses.

In particular ethylene-based vinyl acetate man-ufacturers or those using palladium catalystsrequire the iodide concentration in the acetic acidto be at a low ppb level (14). In the CativaTM

process the levels of acetaldehyde in the reactorare very low, typically less than 30 ppm, comparedto a few hundred ppm in the conventionalMonsanto process and several hundred ppm in thelithium-promoted rhodium process. Further treat-ment steps are not therefore necessary to give aproduct that can be used directly in the manufac-ture of vinyl acetate.

The levels of propionic acid in the acetic acidfrom the CativaTM process are substantially lessthan those from the rhodium process. In the con-ventional high water content rhodium process, thepropionic acid present in the acetic acid productprior to the “Heavies” removal column is between

1200 and 2000 ppm. In the CativaTM process theseconcentrations are reduced to about one third ofthese levels.

The Environmental Impact of CativaTM

As the CativaTM process produces substantiallylower amounts of propionic acid compared to therhodium process, much less energy is required topurify the product. As mentioned previously, theCativaTM system can be operated at much lowerwater concentrations, thus reducing the amount ofenergy required to dry the product in the distilla-tion train. Steam and cooling water requirementsare reduced by 30 per cent compared to the rhodi-um system. The water gas shift reaction doesoccur with CativaTM, as with rhodium, but at alower rate, resulting in ~ 70 per cent lower directCO2 emissions. Overall, including indirect CO2

emissions, the CativaTM process releases about 30per cent less CO2 per tonne of product than doesthe rhodium process. The comparative insensitivi-ty of the system to the partial pressure of COallows operation with lower reactor vent rates thanin the rhodium system. This results in the com-bined benefits of less purge gas released to theatmosphere via the flare system and also greaterCO utilisation, leading to decreased variable costs.In practice, total direct gaseous emissions can bereduced by much more than 50 per cent.

Cost ReductionsAs discussed before there are a number of fac-

tors which have lead to substantial variable costreductions for the CativaTM process compared tothe rhodium process. In particular, steam usage isreduced by 30 per cent, while CO utilisation isincreased from ~ 85 per cent to > 94 per cent.

The CativaTM process also allows simplificationof the production plant, which reduces the cost ofa new core acetic acid plant by ~ 30 per cent. Asthe CativaTM catalyst system remains stable downto very low water concentrations, the purificationsystem can be reconfigured to remove one of thedistillation columns completely and to combinethe light ends and drying columns into a single col-umn. The lower production rates of higher acids,


compared to the Monsanto process, allows the sizeand operating cost of the final distillation columnto be reduced. The major units of a commercialscale CativaTM methanol carbonylation plant areshown in Figure 6.

The reactor in the CativaTM system does notrequire a traditional agitator to stir the reactor con-tents. Eliminating this leads to further operationaland maintenance cost savings. The reactor con-tents are mixed by the jet mixing effect providedby the reactor cooling loop, in which materialleaves the base of the reactor and passes through acooler before being returned to the top of the reac-tor. A secondary reactor after the main reactor andbefore the flash tank further increases CO utilisa-tion by providing extra residence time under plugflow conditions for residual CO to react and formacetic acid.

ConclusionsThe new CativaTM iridium-based system delivers

many benefits over the conventional Monsantorhodium-based methanol carbonylation process.The technology has been successfully proven on acommercial scale at three acetic acid plants world-wide having a combined annual production of 1.2million tonnes. These benefits include:

� an inherently stable catalyst system

� less dependence on CO partial pressure

� the reactor can run with a lower vent rate, whichresults in a higher utilisation of CO, which can befurther improved by the addition of selected pro-moters. These effectively remove the dependenceof reaction rate on the CO partial pressure.

� plants can operate with a higher reactor produc-tivity, and higher rates still have beendemonstrated at pilot plant scale

� the production of byproduct propionic acid isreduced, leading to reduced purification costs

� the water concentration in the reactor can bereduced as the system has a high tolerance to lowwater conditions. As the reactor contains lesswater, less has to be removed in the purificationstages, again reducing processing costs.

� the level of acetaldehyde in the CativaTM processis lower than in the rhodium process, giving a fun-damentally purer product. Hydrogenation of anyunsaturated species present is catalysed by the iridium species, resulting in almost complete elim-ination of unsaturated condensation products andiodide derivatives.

Thus, the reduced environmental impact of theCativaTM system along with the cost reductionshave allowed substantial benefits to be gained fromthis new industrial process for the production ofacetic acid.

Fig. 6 Simplified process flowsheet for a commercial scale CativaTM methanol carbonylation plant. The low boiler andwater removal duties are combined into one, smaller, distillation column. The size of the high boiler removal columnhas also been reduced


AcknowledgementsSpecial thanks are due to all colleagues, both past and pre-

sent, in BP Chemicals who have made innumerable contributionsto this work. In particular I would like to thank the members ofthe Acetyls technology teams at our Hull and Sunbury onThames research facilities. Special acknowledgement is also dueto the external parties that have participated in this development.In particular to Professor Peter M. Maitlis and Anthony Haynesand co-workers at the University of Sheffield (mechanistic stud-ies), Simon Collard and team at Johnson Matthey (catalystdevelopment) and Joe A. Stal and team at Sterling Chemicals(process implementation).

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FootnotesIn September 1999, the Royal Society of Chemistry gave the

CativaTM process the “Clean and Efficient Chemical Processing”award in recognition of its positive environmental impact.

BP commissioned their first plant using the rhodium-basedprocess in 1982 licensed from Monsanto and acquired the rightsto this process in 1986.

The AuthorJane H. Jones is a Close Plant Support Technologist with BPChemicals. She is responsible for delivering technical support toplants operating the CativaTM process and will be a member of thecommissioning team for the Malaysian plant start-up later this year.

The enormous saucer-shaped Bushveld Complexin South Africa is the world’s largest layered intru-sion and the major world platinum resource (1). Itcomprises layers rich in platinum group metals(pgms): the Merensky Reef (the traditional mainsource of platinum), the underlying UG-2 Reef andthe Platreef in the north. The Merensky Reef hasbecome less important recently as fewer high grademineral-bearing deposits remain near the surface (2).

In the 1970s mining was begun on the UG-2Reef (typically 1 m thick) where it breaks throughthe surface (2). Recently, in the Rustenburg area atKroondal, Aquarius Exploration began explorationwork. Here the reef has two distinct layers, allowinggreater mechanisation and some open-cast mining.At Kroondal the total resource is estimated at 20.4million tonnes (t), of grade of 5.5 g t–1 with a life of14 years (3). Laboratory work on drill core samplesindicated that a concentrate containing the bulk ofthe pgms could be produced by flotation at a coarsegrind. The concentrate grade was high at ~ 400 g t–1

but chromium content was higher than desired. Afeasibility study was then undertaken with a smallshaft sunk to access ore below the oxidised zone,

and Mintek executed pilot plant runs to aid designof a concentration plant. This design, unique to theplatinum industry, uses a DMS (dense media sepa-ration) plant as the first step before the flotationprocess. The DMS upgrades the pgm-content andrejects barren waste (chromite mining technology).A single-stage rod mill is the only mill. An attrition-er to treat the rougher concentrate prior to cleaningand open-circuiting of the cleaner tails enabled pro-duction of very high concentrate grade withacceptable chromium grades. Concentrate gradesof over 600 g t–1 were predicted at a maintainedrecovery at over 85 per cent (4).

Each platinum mine has some unique process-ing, but this new process and other technologiescould help to optimise pgm operations on themore accessible UG-2 deposits and aid smallermines to exploit pgm deposits effectively.

References1 R. P. Schouwstra, E. D. Kinloch and C. A. Lee,

Platinum Metals Rev., 2000, 44, (1), 33 2 “Platinum 2000”, Johnson Matthey, London, p. 203 “Platinum 1998”, Johnson Matthey, London, p. 164 Mintek press release and figures; www.mintek.co.za

Platinum Excavation on the UG-2 Reef in South Africa

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