GB2162172A - Process for the production of ethanol - Google Patents

Abstract

A process for the production of ethanol comprises: (a) producing first and second streams of synthesis gas, each containing H2, CO and CO2; (b) separating CO2 from the first synthesis gas stream to give a CO2-rich gas and a CO2-depleted gas; (c) incorporating resulting CO2-rich gas into the second synthesis gas stream to give a synthesis gas of enriched carbon content; (d) passing resulting synthesis gas of enriched carbon content as make-up gas to a methanol synthesis plant containing a charge of a methanol synthesis catalyst and maintained under methanol synthesis conditions; (e) converting methanol from step (d) in one or more stages, including a carbonylation stage, into an alkyl acetate; (f) separating CO2-depleted gas of step (b) to form a CO-rich gas and an H-rich gas; (g) passing CO-rich gas from step (f) to the carbonylation stage of step (e); (h) subjecting alkyl acetate from step (e) to hydrogenolysis by reaction with H2-rich gas of step (f), in the presence of a hydrogenolysis catalyst maintained under hydrogenolysis conditions; (i) recovering product ethanol; and (j) recycling to step (e) an alkyl alcohol. t

Description

SPECIFICATION Process This invention relates to a process for the production of ethanol.

International Patent Publication No. W083/03409 describes a process for for the production of ethanol in which methanol is carbonylated by reaction with carbon monoxide in the presence of a suitable carbonylation catalyst, typically a nickel-or rhodium-based carbonylation catalyst, to form acetic acid which is then esterified to give an acetic ester which is, in turn, subjected to hydrogenolysis in the vapour phase using, as hydrogenolysis cata lyst, a catalyst comprising a reduced mixture of copper and zinc oxides. In the plant of Figure 1 of that prior proposal a synthesis gas plant 1 is ar ranged to supply a synthesis gas, desirably an ap proximately 2:1 H2:CO molar mixture, to methanol synthesis plant 3 and also to provide the CO required for carbonylation of methanol in carbonylation stage 5.Hydrogen for hydrogenolysis of ethyl acetate in zone 8 is supplied by way of line 7 as an off gas from the carbonylation stage 5. Figure 2 il lustrates a plant in which synthesis gas is fed in line 111 to ester hydrogenolysis zone 109; the same synthesis gas plant can supply the methanol synthesis plant (not shown) used to produce the methanol supplied in line 101. In the plant of Figure 3 a synthesis gas plant supplies synthesis gas both to the methanol synthesis plant 205 and also to the ester hydrogenolysis zone 211.

In a methanol synthesis plant hydrogen and car bon oxides react together at elevated temperature and pressure in the presence of a methanol synthesis catalyst. The major reactions occurring are: CO + 2H2 = CH30H CO2 + 3H2 = 0H20H + H2O.

Hence the presence of CO2 in the synthesis gas is not disadvantageous. For optimum operation of the methanol synthesis plant it is best to supply as make-up gas a stoichiometric mixture of H2, CO and CO2 of the following composition: xCO + y CO2 + (2x + 3y)H2, where x and y are integers. In practice the compo sition of a synthesis gas depends upon the feedstock and the process used and may vary considerably. Thus a typcial dry gas composition obtained by steam reforming of natural gas may have the following composition: CO 15.5%. H2 75.7%, CO2 8.1%, N2 t A 0.2%, CHa 0.5%. On the other hand partial oxidation of a heavy fuel oil may yield a gas of the composition: CO 47.3%, H2 46.7%, CO2 4.3%. N2 + A 1.4%, CH4 0.3%.Although the composition of the synthesis gas may be adjusted by use of the water gas shift reaction, i.e.: CO + H20 = CO2 + H2, by absorption of CO2 in a solvent liquor, such as hot potassium carbonate or an amine wash liquor (e.g. monoethanolamine or diethanolamine), or by adsorption of H2 using the so-called pressure swing technique, it is often difficult, if not impossible, to achieve the desired stoichiometric synthesis gas without having effectively to discard either H2 or carbon in the form of CO2. This has the result of decreasing the overall efficiency of conversion of the synthesis gas feedstock to methanol.

Similarly, the optimum efficiency of operation of the methanol carbonylation plant it is best to use substantially pure CO and it is also best to operate the ester hydrogenolysis step using substantially pure H2. Again, although separate CO and H2 streams can be produced from a synthesis gas by CO2 removal followed by dehydration and then CO/ H2 separation, for example by the so-called "Cosorb" process, this sequence of steps results in loss of CO2 in the CO2 removal step. Moreover, the carbon oxides and H2 may not be available in the desired molar ratio so that one or other component of the synthesis gas may have to be wasted, thus increasing feedstock usage.

These losses inevitably increase the cost of producing ethanol by this route.

It would accordingly be desirable to improve the process efficiency of this route to synthetic ethanol.

The present invention accordingly seeks to provide an improved ethanol synthesis process in which the components of a synthesis gas are used to optimum advantage so as to improve correspondingly the yield of ethanol from a given quantity of synthesis gas feedstock and to reduce its cost of production.

According to the present invention there is provided a process for the production of ethanol which comprises: (a) producing first and second streams of synthesis gas, each containing H2, CO, Cho2; (b) separating CO2 from the first synthesis gas stream to give a 002-rich gas and a CO2-depleted gas; (c) incorporating resulting 002-rich gas into the second synthesis gas stream to give a synthesis gas of enriched carbon content; (d) passing resulting synthesis gas of enriched carbon content as make-up gas to a methanol synthesis plant containing a charge of a methanol synthesis catalyst and maintained under methanol synthesis conditions; (e) converting methanol from step (d) in one or more stages, including a carbonylation stage, into an alkyl acetate;; (f) separating CO2-depleted gas of step (b) to form a CO-rich gas and an H2-rich gas; (g) passing CO-rich gas of step (f) to the carbonylation stage of step (e); (h) subjecting alkyl acetate from step (e) to hydrogenolysis by reaction with H2-rich gas of step (f) in the presence of a hydrogenolysis catalyst maintained under hydrogenolysis conditions; (i) recovering resulting ethanol; and (j) recycling to step (e) an alkyl alcohol.

The first and second synthesis gas streams of step (a) can be produced by dividing the output gas stream from a single synthesis gas plant. Alternatively they can be produced in separate synthesis gas plants or in different sections of the same synthesis gas plant. Any known process for generation of synthesis gas can be used. For example, the synthesis gas can be generated by steam reforming of natural gas or light hydrocarbon fractions (e.g. naphtha), by partial oxidation of a wide range of hydrocarbons from light hydrocarbons (e.g. natural gas) through to heavy fuel oils, or by gasification of coal using air to oxygen and steam. When the synthesis gas is generated by steam reforming of natural gas, the principal reaction is H20 + OH = CO a 3H2.

The resulting product mixture is in practice a mixture of H2O, H2, CO, CO2 and CH4 in dynamic equilibrium. Further description of the production of synthesis gas can be found, for example, in a survey article by J.H. Carvie, "Synthesis Gas Manufacture", Chemical and Process Engineering, November 1967, page 55 et seq.

The CO2 separation step (b) preferably comprises contacting the first synthesis gas stream in a CO2 absorption zone with a CO2 absorption liquor, subjecting resulting CO2 loaded liquor to CO2 desorption conditions in a CO2 desorption zone, and recovering resulting 0O2-rich gas. Typical CO2 absorption liquors include hot alkaline liquors, such as those based on hot potassium carbonate, or on amines such as mono-and diethanolamine.

In step (c) of the process of the invention CO,- rich gas from step (b) can be directly admixed with the second synthesis gas stream in order to enhance its carbon content. Alternatively, when the two synthesis gas streams are produced separately, either in separate plants or in different sections of the same plant, 0O2-rich gas from step (b) can be admixed with the carbonaceous feedstock, such as natural gas, used to generate the second synthesis gas stream; in this way the 002-rich gas is incorporated into the second synthesis gas stream upstream from the synthesis gas production step itself.Whichever method is adopted, the quantity of CO2-rich gas thus incorporated into the second synthesis gas stream is desirably selected so as to yield a synthesis gas of enriched carbon content which has a composition which is approximately stoichiometric for methanol synthesis. In this way the methanol synthesis plant can be operated under optimum conditions with enhanced efficiency of usage of the carbon content of the carbonaceous feedstock used to form the second synthesis gas stream. This is particularly beneficial when using "carbon deficient" feedstocks, such as natural gas, to generate the synthesis gas for the second synthesis gas stream.

Any 0O2-rich gas from step (b) in excess of that required to form the synthesis gas of enriched carbon content of the second synthesis gas stream can be recycled to the steam reforming step which is used to produce the first stream of synthesis gas. This has the benefit that the overall efficiency of use of carbon in the feedstock is maximised.

The methanol synthesis plant of step (d) can be of conventional design. Its design and operation do not form any part of the present invention. For a description of a typical methanol synthesis plant reference should be made to a paper by J.B. Horsley, P.L. Rogerson and R.H. Scott entitled "The Design and Performance of the ICI 100 atmosphere Methanol Plant", A. I. Chem. E., 74th National Meeting, March 11-15, 1973, New Orleans.

In step (e) of the process of the invention methanol is converted by a procedure involving one or more steps, including a carbonylation step, to form an alkyl acetate. Carbonylation of methanol has been described on many occasions and the reaction products usually include both acetic acid and methyl acetate. If the chosen carbonylation procedure yields methyl acetate as a major product, this can be passed on to the hydrogenolysis step and any acetic acid produced as by-product can either be recycled to the carbonylation step or can be esterified to give methyl acetate, or another alkyl acetate, which can also be passed forward for hydrogenolysis. On the other hand, it is alternatively possible to select in step (e) a carbonylation procedure that yields acetic acid as a major product, with possibly one a minor amount of methyl acetate as a by-product.In this case the acetic acid formed can be esterified with methanol, or with another alkyl alcohol, to form in a second step an alkyl acetate for use in the hydrogenolysis step (h) of the process of the invention. Any methyl acetate by-product from the carbonylation step can be passed also to the hydrogenolysis step (h).

As examples of descriptions of the carbonylation of methanol there can be mentioned GB-A-628509, GB-A-1233121, GB-A-1277242, GB-A-2089803, US A-4134912 and US-A-4356320.

In the case that conversion of methanol to an alkyl acetate in step (e) of the process of the invention proceeds in two stages, with acetic acid being formed as an intermediate product, the alkyl alochol is preferably methanol or ethanol so that the alkyl acetate is methyl or ethyl acetate. However it is also possible to use a 0, alkanol having, for example, up to about 8 carbon atoms. Thus the alkyl alcohol may also be selected from n-propanol, ipropanol, n-butanol, sec-butanol, n-amyl alcohol, iso-amyl alcohol, n-hexanol, and 2-ethylhexanol whereupon the alkyl acetate corresponds to the alkyl alcohol selected.

Esterification can be conducted in known manner, typically by reaction of acetic acid with ethanol or another alkyl alcohol in the liquid or vapour phase in the presence of a suitable, typically acidic, esterification catalyst at normal or elevated temperature and under normal, elevated or reduced pressure.

Separation of CO and H2 in step (f) of the process of the invention can also be carried out in conventional manner. Typical techniques include absorption of CO in a copper-containing liquor (the so-called "Cosorb" process), methane wash cry ogenic procedures, and pressure swing adsorption using molecular sieves. Again, the precise design of the CO/H2 separation equipment and operating conditions to be used therein form no part of the present invention.

Hydrogenolysis in step (h) can be effected in known manner. Although conventional liquid phase, high pressure processes can be used, in which the catalyst is, for example, a copper chromite or barium promoted copper chromite catalyst, it is preferred to utilise the vapour phase process described in International Patent Publication No.

W082/03854. This describes use of a catalyst comprising a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 75"C to about 300"C and at a pressure in the range of from about 0.1 bar to about 100 bar. Preferred reaction conditions include use of, for example, a pressure of from about 3 bar to about 60 bar, e.g.

about 5 bar to about 50 bar, and a tempearture of from about 150"C to about 250"C, e.g. about 180"C to about 240 C. In such a hydrogenolysis step the vapour pressure of the alkyl acetate preferably ranges from about 0.05 bar up to about 10 bar.

Preferably the hydrogenolysis step is conducted in the presence of excess hydrogen. Thus the hydrogen:alkyl acetate molar ratio may lie in the range of from about 2:1 up to about 100:1 or more.

Hydrogenolysis of each mole of alkyl acetate yields, in the case of ethyl acetate, 2 moles of ethanol or, in the case of methyl acetate or another alkyl acetate, 1 mole of ethanol and 1 mole of methanol or the corresponding alkyl alcohol. In the former case approximately 1 mole of ethanol is recovered and passed forward for product recovery, whilst the remainder is recycled for esterification of further acetic acid. In the latter case, the alcohol mixture is separated, the ethanol being passed forward for product recovery whilst the other alkyl alcohol is recycled to esterification. Alternatively, if a single stage carbonylation procedure is used to produce methyl acetate, then the methanol recovered from the hydrogenolysis step can be recycled for further carbonylation.When methyl or ethyl acetate is selected as the ester for hydrogenolysis, any make up quantities of methanol or ethanol required for alkyl acetate production can be produced within the plant itself. When another alkyl acetate is selected, it may be necessary to import from beyond battery limits any make up alkyl alcohol required.

In order that the invention may be clearly understood and readily carried into effect, three forms of ethanol synthesis plant, each designed to operate according to a preferred process in accordance with the invention, will now be described, by way of example only, with reference to Figures 1 to 3 of the accompanying drawings, each of which is a flow diagram of a respective form of plant.

Referring to Figure 1 of the drawings, natural gas from line 1 is supplied by way of lines 2, 3 to steam reformer sections 4, 5 of conventional design. These reformer sections 4, 5 may comprise separate steam reformer furnaces or may comprise different sections of the same steam reformer furnace. Steam is supplied by way of lines 6,7 to each of the steam reformer sections 4,5.

From steam reformer section 4 there issues in line 8 a first synthesis gas stream containing typically 15.5% CO, 75.7% H2, 8.1% CO,, 0.2% (N, + A), and 0.5% CH4. This is passed to a conventional CO, recovery zone 9 which is arranged to absorb CO, in a suitable liquor, such as activated potassium carbonate or an alkanolamine, from which the CO, is subsequently desorbed by pressure reduction and heating in a desorption zone, the absorbent solution thereby regenerated being recycled for further absorption. The CO, separated from the first synthesis gas stream is recovered from zone 9 in line 10, whilst the residual gas mixture, which now largely consists of CO and H,, is passed on in line 11 to a COIH2 separation zone 12.

A second synthesis gas stream of a similar composition to that of the first stream is produced in reformer section 5 and exists therefrom in line 13.

It is then compressed by means of compressor 14 to a pressure suitable for methanol synthesis, e.g.

about 50 bar to 200 bar, and passes on in line 15 to a methanol synthesis plant 16.

The CO, recovered in line 10 is compressed by means of compressor 17; a part is passed by way of line 18 and is admixed with the second synthesis gas stream in line 13, whilst any CO, surplus to the methanol synthesis requirements is recycled to the inlet of reformer section 4 in line 19. The proportion of CO, in line 10 that is passed through line 18 is preferably selected so as to produce a stoichiometric or near stoichiometric mixture of H2, CO and CO, for methanol synthesis, e.g. a mixture containing 70.9% H,, 14.5% CO, 13.9% CO,, 0.2% (N,+A), and 0.5% CH4.

Methanol synthesis plant 16 contains a conventional "low pressure" methanol synthesis catalyst, such as, for example, "ICI 51-2" catalyst manufactured by ICI Limited. It is operated under conventional conditions, e.g. about 50 bar to about 200 bar at a temperature of from about 200"C to about 300 C. Methanol synthesis occurs by the following reactions: CO + 2H, = CH,OH CO, * 3H, = CH,OH + H,O.

The crude product is separated from unreacted gas which is recycled in line 20 using recycle gas compressor 21, and is passed to a refining plant 22 of conventional design, water produced as byproduct being discarded in line 23, whilst pure methanol is passed in line 24 to an acetic acid synthesis plant 25.

In acetic acid synthesis plant 25 methanol is carbonylated by reaction with carbon monoxide which is supplied from CO/H, separation plant 12 in line 26 with the aid of compressor 27. This reaction can be carried out using a suitable nickel-or rhodiumbased carbonylation catalyst. Typical rhodiumbased catalysts and relevant operating conditions therefor are described, for example, in GB-A1233121 and GB-A-1277242, whilst typical nickel carbonylation catalysts and relevant operating conditions are described, for example, in US-A4134912, US-A-4356320 and GB-A-2089803. A COcontaining purge gas is recycled in line 28 and is mixed with the gas in line 11. Part or all of this purge gas can, if desired, be taken in line 29 for use, for example, as fuel in one of the steam reformer sections 4 and 5.

Acetic acid from plant 25 is then esterified in esterification section 30 with an alcohol, such as ethanol, supplied in line 31. Esterification section 30 can be operated using conventional techniques.

For example, esterification can be carried out in the liquid phase by contacting a mixture of ethanol or other alkyl alcohol and acetic acid with a conventional esterification catalyst such as sulphuric acid or an acidic ion exchange resin. Such a liquid phase esterification process can be conducted at elevated temperature, e.g. at the boiling point of the mixture, at normal or elevated pressure and the ester recovered by distillation in a refining zone 32. Alternatively esterification may be carried out as a vapour phase process by passing a mixture of acetic acid and ethanol, or another alcohol, at normal or elevated pressure and appropriate temperature over a suitable acidic catalyst, such as silica gel, activated charcoal, potassium hydrogen sulphate, phosphoric acid-treated coal, calcium phosphate, or an acidic ion exchange resin.In such a vapour phase process an inert carrier gas may be included in the esterification mixture. In the esterification step it is preferred to use an approximately stoichiometric mixture of acetic acid and alcohol, e.g. ethanol. However it is also possible to operate with either the acid or the alcohol in excess.

In refining zone 32 the water formed during the esterification reaction is separated by distillation in one or more stages from alkyl acetate and any unreacted acetic acid andlor alkyl alcohol. If the ester forms an azeotrope with water and or the alkyl alochol, then this azeotrope is broken and separated in conventional manner. The water formed in the esterification reaction is recovered in line 33, whilst the ester, now essentially free from water, passes on in line 34. Untreated alkyl alcohol (e.g. ethanol) andíor acetic acid is or are recycled internally to the esterification zone.

Alkyl acetate (e.g. ethyl acetate) in line 34 is fed to a vapour phase hydrogenolysis zone 35, to which is also fed the H,-rich stream in line 36 from COIN2 separation plant 11. This H2-rich stream is first compressed using compressor 37.

CO/H2 separation plant 12 may be, for example, a pressure swing adsorption plant, a plant using the so-called "Cosorb" process, or a methane wash cryogenic plant. A description of pressure swing adsorption is given in an article by J.L. Heck entitled "First U.S. Polybed PSA unit proves its reliability", Oil & Gas Journal, February 1980. The "Cosorb" process and the methane wash cryogenic process are both described by K.W. Foo and I. Shortland in an article "Compare CO production methods", Hydrocarbon Processing, May 1976, pages 149 to 152. Another description of the "Cosorb" process can be found in "CO recovery and purification" by Donald J. Haase, Paul M. Duke and John W. Cater, Hydrocarbon Processing, March 1982, pages 103 to 106.

Hydrogenolysis zone 35 uses the vapour phase hydrogenolysis process described, for example in International Patent Publication No. W082/03854.

In this process a vaporous mixture of alkyl acetate (e.g. ethyl acetate) from line 34 and hydrogen rich gas from line 36 is contacted with a catalyst comprising a reduced mixture of copper oxide and zinc oxide, typically at a pressure of from about 5 bar to about 40 bar and at a temperature of from about 150"C to about 240"C. Usually it will be preferred to use an excess of hydrogen, i.e. in excess of the two moles of hydrogen demanded by the stoichiometry of the hydrogenolysis reaction; e.g.: CH3.CO.O.CH2.CH3 + 2H2 = 2CH2.CH2.OH.

Thus the hydrogen:ester molar ratio in the vaporous mixture preferably is in excess of about 5:1 up to about 20:1 or more. Zone 35 can comprise a tubular or multi-tubular cooled reactor or a single bed or multi-bed adiabatic hydrogenolysis reactor or reactors. At the downstream end of zone 35 the condensible components are separated from the gaseous components by condensation. the liquid condensate, which consists predominantly of ethanol, when the alkyl acetate is ethyl acetate, or a mixture of ethanol and another alcohol such as methanol, if another alkyl acetate (e.g. methyl acetate) is used, is passed to a refining section 38 of conventional design, whilst excess hydrogen is recycled in line 39 by means of gas recycle compressor 40.Ethanol product is recovered in line 41, whilst alkyl alcohol required for esterification of acetic acid in esterification section 30 (which is ethyl alcohol if the ester is ethyl acetate or another alcohol, e.g. methanol, if a different alkyl acetate, such as methyl acetate, is used) is recycled in line 31, together with any unreacted alkyl acetate. If an alkyl alcohol other than ethyl alcohol is used, then any necessary make up alkyl alcohol is supplied via line 42 in the case of methanol (or via line 43 in the case of a C, alkanol) and is admixed with the recycled material in line 31.

A purge stream to control the level of inert materials (e.g. methane) in the methanol synthesis loop is taken from line 20 by way of line 44 and used as fuel. Any hydrogen in excess of that required for hydrogenolysis is recovered in line 45 for export beyond battery limits or for use as fuel, e.g. for the reformer sections 4, 5. The level of inert gases andl or excess hydrogen in the hydrogenolysis recycle loop is controlled by taking a purge stream in line 46 for use as a fuel. Conveniently the purge streams in lines 44 and 46 are used in the steam reformer sections 4 and 5, thus reducing the amount of natural gas or other fuel required for this purpose.

The plant of Figure 2 is generally similar to that of Figure 1 and accordingly the same reference numerals have been used to denote like parts to those present in the plant of Figure 1. In the plant of Figure 2 line 18 is replaced by a line 47 so that part of the CO2 recovered in line 10 is admixed with the natural gas feedstock supplied in line 3 to steam reformer section 5. Hence part of the CO2 recovered from the first synthesis gas stream is incorporated into the second synthesis gas stream in line 13 upstream from the steam reformer section 5, whilst the remainder is recycled in line 19 to the upstream end of steam reformer section 4.As with the plant of Figure 1, the quantity of CO, passed by way of line 47 is preferably sufficient to produce in the second synthesis gas stream in line 13 the desired CO:CO2H2 molar ratio that is stoichiometric, or near stoichiometric, for methanol production.

Whereas a two step process is used in the plant of Figure 1 to produce an alkyl acetate, such as ethyl acetate, from methanol in line 24, carbonylation stage 25 and esterification section 30 of the plant of Figure 1 are replaced by a single stage methyl acetate production plant 48 in the plant of Figure 2. Methyl acetate from refining zone 32 is passed by line 34 to hydrogenolysis plant 35 whilst methanol is separated from ethanol in refining section 38 and recycled to the carbonylation step in line 31.

The plant of Figure 3 is similar to that of Figure 1 except that, in Figure 3, a single synthesis gas plant 49 replaces the two steam reformer sections 4 and 5 of the plant of Figure 1. The resulting synthesis gas stream in line 50 is divided to give the first and second synthesis gas streams in lines 8 and 13 respectively. Any CO2-rich gas in line 10 in excess of that required to give the approximately stoichiometric mixture in line 13 required for methanol synthesis is recycled in line 19 to the upstream end of reformer furnace 49.

Claims (9)

1. A process for the production of ethanol which comprises: (a) producing first and second streams of synthesis gas, each containing H2, CO and CO2; (b) separating CO, from the first synthesis gas stream to give a CO,-rich gas and a CO,-depleted gas; (c) incorporating resulting CO,-rich gas into the second synthesis gas stream to give a synthesis gas of enriched carbon content; (d) passing resulting synthesis gas of enriched carbon content as make-up gas to a methanol synthesis plant containing a charge of a methanol synthesis catalyst and maintained under methanol synthesis conditions; (e) converting methanol from step (d) in one or more stages, including a carbonylation stage, into an alkyl acetate; (f) separating CO,-depleted gas of step (b) to form a CO-rich gas and an H,-rich gas; (g) passing CO-rich gas from step (f) to the carbonylation stage of step (e);; (h) subjecting alkyl acetate from step (e) to hydrogenolysis by reaction with H,-rich gas of step (f), in the presence of a hydrogenolysis catalyst maintained under hydrogenolysis conditions; (i) recovering product ethanol; and (j) recycling to step (e) an alkyl alcohol.

2. A process according to claim 1, in which the alkyl acetate is ethyl acetate and the alkyl alcohol recycled in step (j) is ethanol.

3. A process according to claim 1, in which the alkyl acetate is methyl acetate and the alkyl alcohol recycled in step (j) is methanol.

4. A process according to claim 3, in which methanol is carbonylated in step (e) to yield methyl acetate directly.

5. A process according to any one of claims 1 to 4, in which the CO, separation step (b) comprises contacting the first synthesis gas stream in a CO, absorption zone with a CO, absorption liquor, recovering resulting CO,-depleted gas stream, subjecting resulting CO,-loaded liquor to CO, desorption conditions in a CO, desorption zone, and recovering resulting CO,-rich gas.

6. A process according to any one of claims 1 to 5, in which the synthesis gas of enriched carbon content of step (c) has a composition which is approximately stoichiometric for methanol synthesis.

7. A process according to claim 6, in which the first synthesis gas stream is produced by steam reforming of natural gas and CO,-rich gas in excess of that required to form the synthesis gas of enriched carbon content is recycled to the steam reforming step.

8. A process according to any one of claims 1 to 7, in which hydrogenolysis is effected in step (h) by contacting a vaporous mixture comprising alkyl acetate and hydrogen with a catalyst comprising a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 75"C to about 300"C and at a pressure in the range of from about 0.1 bar to about 100 bar.

9. A process for the production of ethanol conducted substantially as herein described with particular reference to any one of Figures 1 to 3 of the accompanying drawings.