Iridium-catalyzed carbonylation process for the production of acetic acid

A process as provided for producing an acetic acid process stream having less than 400 ppm propionic acid and less than 1500 ppm water. Methanol or a reactive derivative thereof and carbon monoxide is fed to a carbonylation reactor in which there is maintained during the course of the process a liquid reaction composition containing an iridium carbonylation catalyst, methyl iodide co-catalyst, a promoter, water at a concentration of less than about 8% by weight, methyl acetate, acetic acid, and propionic acid by-product and its precursors. Liquid reaction composition is withdrawn from the carbonylation reactor and introduced to a flash zone to form a vapor fraction comprising water, acetic acid product, propionic acid by-product, methyl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. The liquid fraction is recycled from the flash zone to the carbonylation reactor and introduced into a first distillation zone where a light ends recycle stream is removed and recycled to the carbonylation reactor, A process stream is removed from the first distillation zone comprising acetic acid, propionic acid by-product and less than 1500 ppm water. If the process stream comprises greater than 400 ppm propionic acid, the process stream is introduced into a second distillation zone where propionic acid by-product is removed together with an acetic acid process stream containing less than 400 ppm propionic acid and less than 1500 ppm water.

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 
the carbonylation of methanol and/or a reactive derivative thereof in the 
presence of an iridium catalyst. 
Acetic acid is a well-known commodity chemical which has many industrial 
uses. 
Processes for the production of acetic acid by liquid phase 
iridium-catalyzed carbonylation reactions are known and are described in 
for example, EP-A-0616997, EP-A-06 18184; EP-A-0643034; U.S. Pat. No. 
3,772,380; GB-A-1234641 and (GB-A-1234642. 
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. The technical problem to be overcome by the process 
of the present invention is that of reducing the capital expenditure 
and/or operating costs of a plant for the production of acetic acid by the 
liquid phase carbonylation of methanol and/or a reactive derivative 
thereof using, an iridium catalyst. We have found that by operating with a 
defined liquid reaction composition it is possible to produce acetic acid 
of a quality sufficient in terms of water and propionic acid content for 
its ultimate industrial applications using a single distillation column to 
separate and recycle the light ends from the acetic acid product. 
Accordingly,y the present invention provides a process for the production 
of an acetic acid process stream comprising less than 400 ppm propionic 
acid and less than 1500 ppm water which 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% by 
weight; 
(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. 
Advantageously, the process of the invention allows the production of 
acetic acid containing less than 400 ppm , for example less than 300 ppm 
propionic acid and less than 1500 ppm water, for example less than 1000 
ppm, using two or less distillation zones for the basic purification 
rather than the three generally employed in carbonylation purification 
systems. 
Suitably, hydrogen present in the carbonylation reactor, present for 
example, as a result of the water gas shift reaction and optionally as 
part of the gas feed, is maintained at as low a partial pressure as 
possible, typically a partial pressure of less than 0.5 bar, preferably 
less than 0.3 bar. By maintaining as low a partial pressure of hydrogen as 
possible in the carbonylation reactor the amount of hydrogenation 
by-products (methane and propionic acid) is reduced. Preferably, hydrogen 
in the carbon monoxide feed gas is maintained at less than 0.5 mol %, more 
preferably less than 0.3 mol % and most preferably less than 0.1 mol %. 
Suitably, the concentration of methyl iodide co-catalyst in the liquid 
reaction composition is greater than 4% by weight, typically from 4 to 20% 
by weight, preferably from 4 to 16% by weight. As the methyl iodide 
concentration in the liquid reaction composition is increased, the amount 
of propionic acid by-product decreases. 
Suitably, the molar ratio of methyl iodide: iridium in the liquid reaction 
composition is [greater than 20]:1, preferably [up to 400]:1, more 
preferably [from 20 to 200]:1. As the molar ratio of methyl iodide: 
iridium catalyst in the liquid reaction composition is increased, the 
amount of propionic acid by-product decreases. 
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.degree. 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 product acid stream may be removed at the base of the first 
distillation zone or at a point one or more stages above the base of the 
distillation zone. The process stream containing acetic acid may be 
withdrawn as a liquid or as a vapour. When the process stream is 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 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 also 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. The methyl 
acetate concentration in the liquid reaction composition is suitably in 
the range from 1 to 70% by weight, preferably from 2 to 50% by weight and 
more preferably from 5 to 40% by weight. 
The carbon monoxide fed to the carbonylation reactor may be essentially 
pure or may contain inert impurities such as carbon dioxide, methane, 
nitrogen, noble gases, water and C.sub.1 to C.sub.4 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 is suitably maintained at a pressure in the range 
from 10 to 200 barg, preferably from 15 to 100 barg, more preferably from 
15 to 50 barg. 
The carbonylation reactor is suitably maintained at a temperature in the 
range from 100 to 300.degree. C., preferably in the range from 150 to 
220.degree. C. 
The process of the present invention is preferably performed as a 
continuous process but may be performed as a batch process. 
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 IrCl.sub.3, IrI.sub.3, 
IrBr.sub.3, [Ir(CO).sub.2 I].sub.2, [Ir(CO).sub.2 Cl].sub.2, [Ir(CO).sub.2 
Br].sub.2, [Ir(CO).sub.2 I.sub.2 ].sup.- H.sup.+, [Ir(CO).sub.2 Br.sub.2 
].sup.- H.sup.+, [Ir(CO).sub.2 I.sub.4 ].sup.-H.sup.+, 
[Ir(CH.sub.3)I.sub.3 (CO).sub.2 ].sup.- H.sup.+, Ir.sub.4 (CO).sub.12, 
IrCl.sub.3.3H.sub.2 O, IrBr.sub.3.3H.sub.2 O, Ir.sub.4 (CO).sub.12, 
iridium metal, Ir.sub.2 O.sub.3, IrO.sub.2, Ir(acac)(CO).sub.2, 
Ir(acac).sub.3, iridium acetate, [Ir.sub.3 O(OAc).sub.6 (H.sub.2 O).sub.3 
][OAc], and hexachloroiridic acid [H.sub.2 IrCl.sub.6 ], 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. The concentration of iridium 
is suitably less than 2500 ppm, preferably from 400 to 2000 ppm. 
In the process of the present invention optionally one or more promoters 
may be present in the reaction composition. Suitable promoters are 
preferably selected from the group consisting, of ruthenium, osmium, 
rhenium, cadmium, mercury, zinc, gallium, indium and tungsten, and are 
more preferably selected from ruthenium and osmium and most preferably is 
ruthenium. Preferably, the promoter is present in an effective amount up 
to the limit of its solubility in the liquid reaction composition aid/or 
any liquid process streams recycled to the carbonylation reactor from the 
acetic acid recovery stage. The promoter is suitably present in the liquid 
reaction composition at a molar ratio of promoter: iridium of [from 0.5 to 
15]: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, ruthenium 
(III) formate, [Ru(CO).sub.3 I.sub.3 ].sup.- H.sup.+, [Ru(CO).sub.2 
I.sub.2 ].sub.n, [Ru(CO).sub.4 I.sub.2 ], [Ru(CO).sub.3 I.sub.2 ].sub.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)diruthienium(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).sub.4 
I.sub.2 ], [Os(CO).sub.3 I.sub.2 ].sub.2, [Os(CO).sub.3 I.sub.3 ].sup.- 
H.sup.+, 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 Re.sub.2 (CO).sub.10, Re(CO).sub.5 Cl, 
Re(CO).sub.5 Br, Re(CO).sub.5 I, ReCl.sub.3.xH.sub.2 O [Re(CO).sub.4 
I].sub.2, [Re(CO).sub.4 I.sub.2 ].sup.- H.sup.+ and ReCl.sub.5.yH.sub.2 
O. 
Examples of suitable cadmium-containing compounds which may be used include 
Cd(OAc).sub.2, CdI.sub.2, CdBr.sub.2, CdCl.sub.2, Cd(OH).sub.2, and 
cadmium acetylacetonate. 
Examples of suitable mercury-containing compounds which may be used as 
sources of promoter include Hg(OAc).sub.2, HgI.sub.2, HgBr.sub.2, 
HgCl.sub.2, Hg.sub.2 I.sub.2, and Hg.sub.2 Cl.sub.2. 
Examples of suitable zinc-containing compounds which may be used as sources 
of promoter include Zn(OAc).sub.2, Zn(OH).sub.2, ZnI.sub.2, ZnBr.sub.2, 
ZnCl.sub.2, and zinc acetylacetonate. 
Examples of suitable gallium-containing compounds which may be used as 
sources of promoter include gallium acetylacetonate, gallium acetate, 
GaCl.sub.3, GaBr.sub.3, GaI.sub.3, Ga.sub.2 Cl.sub.4 and Ga(OH).sub.3. 
Examples of suitable indium-containing compounds which may be used as 
sources of promoter include indium acetylacetonate, indium acetate, 
InCl.sub.3, InBr.sub.3, InI.sub.3, InI and In(OH).sub.3. 
Examples of suitable tungsten-containing compounds which may be used as 
sources of promoter include W(CO).sub.6, WCl.sub.4, WCl.sub.6, WBr.sub.5, 
WI.sub.2, or C.sub.9 H.sub.12 W(CO).sub.3. 
Preferably, the iridium- and promoter-containing compounds are free of 
impurities which provide or generate insitu ionic iodides which may 
inhibit the reaction, for example, alkali or alkaline earth metal or other 
metal salts. 
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 quaternize 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.sup.- 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.sup.- 
Corrosion metals which have an adverse affect on the reaction rate may be 
minimized 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 U.S. Pat. No. 4,007,130. Ionic 
contaminants may be kept below a concentration at which they would 
generate less than 500 ppm I.sup.-, preferably less than 250 ppm I.sup.- 
in the liquid reaction composition. 
Water may be formed in situ in the liquid reaction composition, for 
example, by 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 the required concentration of the water 
in the liquid reaction composition. Suitably, the concentration of water 
in the liquid reaction composition is in the range from 0.5 to 8% by 
weight. 
In a further 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. 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.degree. C. and the flash zone is maintained at a 
temperature of 100 to 140.degree. 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 stabilize the iridium 
catalyst. 
In a preferred embodiment of the present invention the reaction conditions 
are selected to give an acetic acid process stream from step (f) 
containing less than 400 ppm propionic acid and less than 1500 ppm water.