Process for purifying synthesis gas by washing with aqueous solutions of amines

Abstract

The invention relates to a process for purifying synthesis gas, comprising at least one stage for separating the crude synthesis gas to be treated into at least two effluents, namely a first part and a complementary part, in which the said first part is subjected to a carbon monoxide conversion stage with steam and the said complementary part is subjected to a COS and HCN catalytic hydrolysis stage, the two gas flows, namely the first part and complementary part, are then each treated separately in two stages intended to remove acid gases such as CO.sub.2 and H.sub.2S, by washing with aqueous solutions of specific amines, before a recombination stage of the two treated effluents.

Claims

1. A process for purification of synthesis gas, comprising at least the following stages: a) a stage for dividing the synthesis gas into at least a first synthesis gas flow and a second synthesis gas flow of the same composition, b1) a stage for the steam conversion of carbon monoxide of the first synthesis gas flow leaving stage a), in order to produce a stage b1) gaseous effluent containing at least hydrogen H.sub.2 and carbon dioxide CO.sub.2, less than 15% volume (vol. %) of carbon monoxide CO and acid gases including H.sub.2S and CO.sub.2, b2) a stage for the removal of acid gases including H.sub.2S and CO.sub.2 from the stage b1) gaseous effluent by contacting said stage b1) gaseous effluent with a solvent which is a first aqueous solution of amines comprising at least one secondary amine, said first aqueous solution of amines further comprising at least one tertiary amine or a sterically hindered secondary amine different from the secondary amine and containing at least one quaternary carbon atom in the ? or alpha position of the nitrogen atom or two tertiary carbon atoms in the ? and ? positions, so as to remove acid gases including H.sub.2S and CO.sub.2 and produce at least one stage b2) gaseous effluent containing less than 5 vol. % of carbon dioxide CO.sub.2, and less than 50 ppm by volume of H.sub.2S, c1) a stage for catalytic hydrolysis of COS and HCN present in the second synthesis gas flow that has not undergone the conversion reaction of carbon monoxide with steam, in order to produce a stage c1) gaseous effluent containing less than 25 ppm by volume of COS, and less than 5 ppm by volume of HCN, and containing acid gases including H.sub.2S and CO.sub.2, c2) a stage for removal of acid gases including H.sub.2S and CO.sub.2 from said stage c1) gaseous effluent by contacting said stage c1) gaseous effluent with a solvent which is a second aqueous solution of amines containing at least one tertiary amine, so as to produce at least one stage c2) gaseous effluent containing less than 10 vol. % of carbon dioxide CO.sub.2, and less than 50 ppmv of H.sub.2S, wherein the second aqueous solution of amines contains between 25 and 50 wt. % of a tertiary amine and contains between 50 and 75 wt. % of water, and d) recombination of at least a part of said stage b2) effluent and at least part of said stage c2) to obtain a purified synthesis gas, wherein said first aqueous solution of amines and said second aqueous solution of amines are different.

2. The process according to claim 1, wherein stage b1) is carried out at an absolute pressure between 20 and 120 bar, at a gas hourly space velocity between 1,000 and 10,000 h.sup.1, and at a temperature between 150 and 550? C.

3. The process according to claim 1, wherein stage b1) is conducted in the presence of a catalyst containing at least one element from group VIII and/or at least one element from group VIB of the Periodic Table, and a support chosen from aluminas, silica, and silica-aluminas.

4. The process according to claim 1, wherein said stage b1) gaseous effluent is routed to a stage b1) for catalytic hydrolysis of COS and HCN to H.sub.2S and NH.sub.3 to obtain a stage b1) gaseous effluent which is sent to said stage b2).

5. The process according to claim 1, wherein the secondary amine or amines used in stage b2) are chosen from diethanolamine, diisopropanolamine, piperazine and derivatives thereof containing at least a non substituted nitrogen atom, morpholine and non N-substituted derivatives thereof, piperidine and non N-substituted derivatives thereof, N-(2-hydroxyethyl)-2-amino-2-methyl-1-propanol, N-(2-hydroxypropyl)-2-amino-2-methyl-1-propanol, and N-(2-hydroxybutyl)-2-amino-2-methyl-1-propanol, individually or as a mixture.

6. The process according to claim 1, wherein said stage b2) is carried out in the presence of an aqueous solution of amines containing at least one secondary amine, mixed with at least one tertiary amine, wherein said tertiary amine or amines are chosen from methyldiethanolamine, triethanolamine, ethyldiethanolamine, diethylethanolamine, dimethylethanolamine, 1-methyl-4-(3-dimethylaminopropyl)-piperazine, 1-ethyl-4-(diethylaminoethyl)-piperazine, 1-methyl-4-hydroxy-piperidine, 1-methyl-2-hydroxymethyl-piperidine, tert-butyldiethanolamine, 1,2-bis-(2-dimethylaminoethoxy)-ethane bis(dimethylamino-3-propyl)ether, bis(diethylamino-3-propyl)ether, (dimethylamino-2-ethyl)-(dimethylamino-3-propyl)-ether, (diethylamino-2-ethyl)-(dimethylamino-3-propyl)-ether, (dimethylamino-2-ethyl)-(diethylamino-3-propyl)-ether, (diethylamino-2-ethyl)-(diethylamino-3-propyl)-ether, N-methyl-N-(3-methoxypropyl)-2-aminoethanol, N-methyl-N-(3-methoxypropyl)-1-amino-2-propanol, N-methyl-N-(3-methoxypropyl)-1-amino-2-butanol, N-ethyl-N-(3-methoxypropyl)-2-aminoethanol, N-ethyl-N-(3-methoxypropyl)-1-amino-2-propanol, N-ethyl-N-(3-methoxypropyl)-1-amino-2-butanol, N-isopropyl-N-(3-methoxypropyl)-2-aminoethanol, N-isopropyl-N-(3-methoxypropyl)-1-amino-2-propanol, N-isopropyl-N-(3-methoxypropyl)-1-amino-2-butanol, 1-(4-morpholino)-2-(methylisopropylamino)-ethane, 1-(4-morpholino)-2-(methyltertiobutylamino)-ethane, 1-(4-morpholino)-2-(diisopropylamino)-ethane, and 1-(4-morpholino)-2-(1-piperidinyl)-ethane, individually or as a mixture.

7. The process according to claim 1, wherein stage b2) is carried out in the presence of an aqueous solution of amines containing at least one secondary amine, mixed with at least one sterically hindered secondary amine containing at least one quaternary carbon atom in the ? or alpha position of the nitrogen atom, or two tertiary carbon atoms in the ? and ? position, and wherein the sterically hindered secondary amine or amines are chosen from N-(2-hydroxyethyl)-2-amino-2-methyl-1-propanol, N-(2-hydroxypropyl)-2-amino-2-methyl-1-propanol, N-(2-hydroxybutyl)-2-amino-2-methyl-1-propanol.

8. The process according to claim 1, wherein stage b2) is carried out in the presence of an aqueous solution of amines containing at least one secondary amine, mixed with at least one tertiary amine or a sterically hindered secondary amine containing at least one quaternary carbon atom in the ? or alpha position of the nitrogen atom or two tertiary carbon atoms in the ? and ? position, and wherein said primary and/or secondary amines are selected from diethanolamine, N-butylethanolamine, piperazine, 1-methylpiperazine, 2-methylpiperazine, N-(2-hydroxyethyl)piperazine, N-(2-aminoethyl)piperazine, morpholine, 3-(methylamino)propylamine, and 1,6-hexanediamine and N-alkylated derivatives thereof, individually or as a mixture.

9. The process according to claim 1, wherein stage c1) is carried out in the presence of a catalyst containing a compound based on platinum, or an oxide of an element selected from titanium, zirconium, aluminium, chromium and zinc, individually or as a mixture, said stage c1) being carried out at a temperature between 100 and 400? C., and at an absolute pressure between 20 and 120 bar.

10. The process according to claim 1, wherein the tertiary amine or amines used in the stage c2) are chosen from methyldiethanolamine, triethanolamine, ethyldiethanolamine, diethylethanolamine, dimethylethanolamine, 1-methyl-4-(3-dimethylaminopropyl)-piperazine, 1-ethyl-4-(diethylaminoethyl)-piperazine, 1-methyl-4-hydroxy-piperidine, 1-methyl-2-hydroxymethyl-piperidine, tert-butyldiethanolamine, 1,2-bis-(2-dimethylaminoethoxy)-ethane, bis(dimethylamino-3-propyl)ether, bis(diethylamino-3-propyl)ether, (dimethylamino-2-ethyl)-(dimethylamino-3-propyl)-ether, (diethylamino-2-ethyl)-(dimethylamino-3-propyl)-ether, (dimethylamino-2-ethyl)-(diethylamino-3-propyl)-ether, (diethylamino-2-ethyl)-(diethylamino-3-propyl)-ether, N-methyl-N-(3-methoxypropyl)-2-aminoethanol, N-methyl-N-(3-methoxypropyl)-1-amino-2-propanol, N-methyl-N-(3-methoxypropyl)-1-amino-2-butanol, N-ethyl-N-(3-methoxypropyl)-2-aminoethanol, N-ethyl-N-(3-methoxypropyl)-1-amino-2-propanol, N-ethyl-N-(3-methoxypropyl)-1-amino-2-butanol, N-isopropyl-N-(3-methoxypropyl)-2-aminoethanol, N-isopropyl-N-(3-methoxypropyl)-1-amino-2-propanol, N-isopropyl-N-(3-methoxypropyl)-1-amino-2-butanol, 1-(4-morpholino)-2-(methylisopropylamino)-ethane, 1-(4-morpholino)-2-(methyltertiobutylamino)-ethane, 1-(4-morpholino)-2-(diisopropylamino)-ethane, and 1-(4-morpholino)-2-(1-piperidinyl)-ethane, individually or as a mixture.

11. The process according to claim 1, wherein stages b2) and c2) each comprise a first absorption stage for absorption of the acid compounds of the synthesis gas by the solvent followed by a regeneration stage for regeneration of the solvent.

12. The process according to claim 11, wherein the first and second aqueous solutions of amines used in the absorption stage of stages b2) and c2), respectively, are in each case already partially loaded with acid gases and are derived from the absorption stage of an Acid Gas Enrichment Unit or from a Tail Gas Treating Unit downstream of a Claus Unit.

13. The process according to claim 12, wherein each of the regeneration stages of stages b2) and c2) is combined with a regeneration stage of an Acid Gas Enrichment Unit or a Tail Gas Treating Unit.

14. The process according to claim 1, further comprising subjecting said purified synthesis gas from stage d) to a final purification stage e).

15. The process according to claim 14, wherein purified synthesis gas leaving final purification step e) is routed to a Fischer-Tropsch synthesis stage f).

16. The process according to claim 14, wherein in said final purification stage e) said purified synthesis gas from stage d) flows through at least one guard bed and/or a catalytic reactor, and said final purification stage e) is performed at an absolute pressure of between 20 and 120 bar.

17. The process according to claim 1, wherein said first synthesis gas constitutes 20 to 80 vol. % of the synthesis gas flow entering said stage a).

18. The process according to claim 1, wherein said first synthesis gas constitutes 40 to 60 vol. % of the synthesis gas flow entering said stage a).

19. The process according to claim 1, wherein stage b1) is carried out at an absolute pressure between 30 and 50 bar, at a gas hourly space velocity between 1500 and 8500 h.sup.?1, and at a temperature between 250 and 500? C.

20. The process according to claim 1, wherein the gaseous effluent leaving stage b1) is sent directly to acid gases removal stage b2).

21. The process according to claim 1, wherein the synthesis gas flow leaving the COS and HCN catalytic hydrolysis stage c1) is sent directly to acid gases removal stage c2).

22. The process according to claim 1, wherein said second aqueous solution of amines does not contain a secondary amine.

23. The process according to claim 1, wherein said first aqueous solution of amines consists of one or more secondary amines, one or more tertiary amine, and water, and said second aqueous solution of amines consists of one or more tertiary amine and water.

24. A process for purification of synthesis gas, comprising at least the following stages: a) a stage for dividing the synthesis gas into at least a first synthesis gas flow and a second synthesis gas flow of the same composition, b1) a stage for the steam conversion of carbon monoxide of the first synthesis gas flow leaving stage a), in order to produce a stage b1) gaseous effluent containing at least hydrogen H.sub.2 and carbon dioxide CO.sub.2 and acid gases including H.sub.2S and CO.sub.2, b2) a stage for the removal of acid gases including H.sub.2S and CO.sub.2 from the stage b1) gaseous effluent by contacting said stage b1) gaseous effluent with a solvent which is a first aqueous solution of amines comprising at least one secondary amine, said first aqueous solution of amines further comprising at least one tertiary amine or a sterically hindered secondary amine different from the secondary amine and containing at least one quaternary carbon atom in the ? or alpha position of the nitrogen atom or two tertiary carbon atoms in the ? and ? positions, so as to produce at least one stage b2) gaseous effluent, c1) a stage for catalytic hydrolysis of COS and HCN present in the second synthesis gas flow that has not undergone the conversion reaction of carbon monoxide with steam, in order to produce a stage c1) gaseous effluent containing acid gases COS and HCN, c2) a stage for removal of acid gases including H.sub.2S and CO.sub.2 from said stage c1) gaseous effluent by contacting said stage c1) gaseous effluent with a solvent which is a second aqueous solution of amines containing at least one tertiary amine, so as to produce at least one stage c2) gaseous effluent, wherein the second aqueous solution of amines contains between 25 and 50 wt. % of a tertiary amine and contains between 50 and 75 wt. % of water, and d) recombination of at least a part of said stage b2) effluent and at least part of said stage c2) to obtain a purified synthesis gas, wherein said first aqueous solution of amines is sensitive to the presence of CO and said second aqueous solution of amines is insensitive to the presence of CO.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 represents the general reaction scheme of the process according to the invention, in which a synthesis gas (1) is divided in a separation zone (a) into two effluents (2) and (5), the effluent (2) undergoing a stage of conversion of carbon monoxide with steam in unit b1), and the effluent (5) undergoing a catalytic hydrolysis stage of COS and HCN in unit c1). The effluents (3) and (6) leaving respectively the units b1) and c1) undergo respectively a stage involving the removal of acid gases b2) and c2), and the effluents leaving the units b2) and c2) are recombined in unit d) to produce a purified synthesis gas (8).

(2) FIG. 2 represents the general reaction scheme of the process according to the invention, in another embodiment in which the effluent leaving the unit b1) is routed via line (3) to unit b1) for the hydrolysis of COS and HCN, the effluent leaving unit b1) is next routed via line (3) to the stage b2) for removing acid gases.

(3) FIG. 3 represents the general reaction scheme of the process according to the invention, in another embodiment according to which the effluent (8) leaving the recombination unit d) of the synthesis gas flows is routed to a final purification unit e) in order to produce a purified synthesis gas via the line (9) and then to a Fischer-Tropsch hydrocarbon synthesis unit f).

(4) FIG. 4 represents a scheme involving a sequence of processes according to the prior art, according to which a removal stage of the acid gases is carried out on the gas flows after the recombination stage d). FIG. 4 is described in detail in Example 1.

(5) Other advantages, details and characteristics of the invention will appear more clearly in the description of three embodiments illustrated in FIGS. 1, 2 and 3. These embodiments are given by way of example and are not intended to restrict the invention. This illustration of the process of the invention does not include all the components necessary for its implementation. Only the elements necessary for the understanding of the invention are included here, the person skilled in the art being able to supplement this representation in order to implement the invention.

EXAMPLES

Example 1: Purification of a Synthesis Gas According to the Prior Art

(6) FIG. 4 describes a process sequence according to the prior art. In FIG. 4 the stage a) corresponds to a separation stage of the synthesis gas no. 1, into two gas flows of identical composition no. 2 and no. 4, the gas flow no. 2 being routed to a stage b1) for converting carbon monoxide CO with steam to give a gas flow no. 3, the gas flows no. 3 and no. 4 being recombined in the recombination stage d) to give the gas flow no. 5, the gas flow no. 5 then being subjected to a stage b1) for removing acid gases to give a gas flow no. 6.

(7) Total gas flow rate (flow no. 1): 453 000 Nm.sup.3/h

(8) Gas flow rate to stage b1) (flow no. 2): 232 000 Nm.sup.3/h

(9) Gas flow rate bypassing stage b1) (flow no. 4): 221 000 Nm.sup.3/h

(10) Stage b1) for Conversion of CO with Steam

(11) The catalyst C1 that is used is an industrial catalyst based on cobalt and molybdenum. This industrial catalyst has a content of metallic cobalt of 2.2 wt. % and of metallic molybdenum of 8.3 wt. %. Its specific surface determined by the BET method is 196 m.sup.2/g. This catalyst is used in its sulfided form. The catalyst is available in the form of extrudates of about 3 mm in diameter.

(12) The gaseous feedstock having the composition described in Table 1 (flow no. 2) is injected into a fixed bed reactor charged with catalyst C1. The selected operating conditions are the following: gas hourly space velocity GHSV (feedstock volume/catalyst volume/hour)=3000 h.sup.?1 absolute operating pressure: 26 bar inlet temperature of the catalytic bed: 250? C. temperature of the catalytic bed: 350? C.

(13) The analysis follow-up at the reactor outlet enabled the carbon monoxide conversion to be determined. The experimental results are given in Table 1 (composition of the outlet flow no. 3).

(14) Stage b2) Removal of Acid Gases

(15) According to the prior art, the synthesis gas is purified by washing with an aqueous solution of amines in order to remove CO.sub.2 and H.sub.2S. By using an aqueous solution of activated methyldiethanolamine (MDEA) containing 38 wt. % of methyldiethanolamine (MDEA), 8 wt. % of diethanolamine (DEA) and 54 wt. % of water, the flow rate of solvent required to purify 270 000 Nm.sup.3/h of gas is 1640 Sm.sup.3/h under the following operating conditions in the absorber: Temperature: 45? C. Pressure: 26 bar

(16) The residual amounts of acid compounds in the gas leaving the acid gas removal stage b1) according to the prior art are: for H.sub.2S 1 ppmv, for CO.sub.2 10 ppmv and for COS 50 ppmv.

(17) The evolution of the composition of the gas is detailed in Table 1 below:

(18) The nos. of the flows refer to FIG. 4.

(19) TABLE-US-00001 TABLE 1 composition of the synthesis gas to be purified. Flow no 1, no 2, no 4 Flow no 3 Flow no 5 Flow no 6 H.sub.2 (% v) 14.6 39.6 27.4 63.4 CO (% v) 25.7 2.0 13.6 31.5 CO.sub.2 (% v) 6.6 31.5 19.4 0.001 H.sub.2O (% v) 50.8 24.6 37.4 0.25 H.sub.2S (ppm vol) 4300 4750 4530 1 COS (ppm vol) 480 30 250 145 HCN (ppm vol) 200 1 100 15 NH.sub.3 (ppm vol) 650 850 750 10 H.sub.2/CO 0.6 19.8 2.0 2.0

Example 2: Purification of a Synthesis Gas According to the Invention

(20) FIG. 1 describes a sequence of processes according to the invention.

(21) Total gas flow rate (flow no. 1): 453 000 Nm.sup.3/h

(22) Flow rate of gas to stage b1) (flow no. 2): 232 000 Nm.sup.3/h

(23) Flow rate of gas to stage c1) (flow no. 5): 221 000 Nm.sup.3/h

(24) Stage b1) for Conversion of CO with Steam

(25) The catalyst C1 used is an industrial catalyst based on cobalt and molybdenum. This industrial catalyst has a content of metallic cobalt of 2.2 wt. % and of metallic molybdenum of 8.3 wt. %. Its specific surface determined by the BET method is 196 m.sup.2/g. This catalyst is used in its sulfided form. The catalyst exists in the form of extrudates about 3 mm in diameter.

(26) The gaseous feedstock with the composition described in Table 2 (flow n? 2) is injected into a fixed bed reactor charged with catalyst C1. The selected operating conditions are: gas hourly space velocity GHSV (feedstock volume/catalyst volume/hour)=3000 h.sup.?1 absolute operating pressure: 26 bar inlet temperature of the catalyst bed: 250? C. temperature of the catalyst bed: 350? C.

(27) The analysis follow-up at the reactor outlet enabled the carbon monoxide conversion to be determined. The experimental results are given in Table 2 (composition of the outlet flow no. 3).

(28) Stage b2) Removal of Acid Gases

(29) According to the invention, the synthesis gas leaving the stage b1) is purified by washing with an aqueous solution of activated methyldiethanolamine (MDEA) containing 38 wt. % of methyldiethanolamine (MDEA), 8 wt. % of diethanolamine (DEA) and 54 wt. % of water, and the flow rate of solvent required to purify 168 000 Nm.sup.3/h of gas is 1340 Sm.sup.3/h under the following operating conditions in the absorber: temperature: 45? C. pressure: 26 bar
Stage c1) Hydrolysis of COS and HCN

(30) The catalyst C2 used is an industrial catalyst based on titanium oxide. This industrial catalyst has a content of titanium oxide of 85 wt. %. Its specific surface determined by the BET method is 120 m.sup.2/g. The catalyst exists in the form of extruded pellets about 3 mm in diameter.

(31) The gaseous feedstock having the composition described in Table 2 (flow no. 5) is injected into a fixed bed reactor charged with catalyst C2. The selected operating conditions are: gas hourly space velocity GHSV (feedstock volume/catalyst volume/hour)=1500 h.sup.?1 absolute operating pressure: 26 bar temperature of the catalyst bed: 250? C.

(32) The analysis follow-up at the reactor outlet enabled the conversions of COS and HCN to be determined. The experimental results are given in Table 2 (composition of the outlet flow no. 6).

(33) Stage c2) Removal of Acid Gases

(34) According to the invention, the synthesis gas leaving the stage c1) is purified by washing with an aqueous solution of methyldiethanolamine (MDEA) containing 45 wt. % of methyldiethanolamine (MDEA) and 55 wt. % of water, and the flow rate of solvent required to purify 102 000 Nm.sup.3/h of gas is 200 Sm.sup.3/h under the following operating conditions in the absorber: temperature: 50? C. pressure: 26 bar

In Table 2 the Flow Nos. Refer to FIG. 1

(35) TABLE-US-00002 TABLE 2 Composition of the synthesis gas to be purified. Flow n ?1, n ?2, Flow Flow Flow Flow Flow n ?5 n ?3 n ?4 n ?6 n ?7 n ?8 H.sub.2 (% v) 14.6 39.6 90.2 14.6 30.7 60.0 CO (% v) 25.7 2.0 4.6 25.7 54.0 29.7 CO.sub.2 (% v) 6.6 31.5 0.001 6.6 5.0 2.5 H.sub.2O (% v) 50.8 24.6 0.25 50.8 0.25 0.25 H.sub.2S 4300 4750 1 4775 1 1 (ppm vol) COS 480 30 17 3 3 10 (ppm vol) HCN 200 1 1 1 1 1 (ppm vol) NH.sub.3 650 850 10 850 10 10 (ppm vol) H.sub.2/CO 0.6 19.8 19.8 0.6 0.6 2.0

(36) TABLE-US-00003 TABLE 3 Flow rates of solvents used for Example 1 according to the prior art, and for Example 2 according to the invention. Example 1 according Example 2 according to the prior art to the invention Flow rate of solvent 1640 Sm.sup.3/h 1340 Sm.sup.3/h stage b2) Flow rate of solvent 200 Sm.sup.3/h stage c2)

(37) For the same overall flow rate of synthesis gas of 270,000 Nm.sup.3/h, the purification scheme according to the invention requires a flow rate of 200 Sm.sup.3/h of MDEA in stage c2) and a flow rate of 1340 Sm.sup.3/h of activated MDEA in stage b2). The process according to the invention thus enables the overall flow rate of solvent used to be reduced by 6%. The energy consumptions required for the regeneration of the aqueous solutions of amines are also reduced in the same proportions. The residual amounts of acid components in the gas leaving the wash stage are:

(38) For H.sub.2S 1 ppmv, for COS 10 ppmv, and for HCN 1 ppmv, i.e. levels of impurities that are well below those obtained in Example 1 according to the prior art.

(39) The obtained synthesis gas has a H.sub.2/CO ratio equal to 2.

Example 3: Purification of a Synthesis Gas According to the Invention

(40) FIG. 2 describes a sequence of processes according to the invention.

(41) Total Flow rate of gas (flow no. 1): 453 000 Nm.sup.3/h

(42) Flow rate of gas to stage b1) (flow no. 2): 232 000 Nm.sup.3/h

(43) Flow rate of gas to stage c1) (flow no. 5): 221 000 Nm.sup.3/h

(44) Stage b1) Conversion of CO with Steam

(45) The catalyst C1 that is used is an industrial catalyst based on cobalt and molybdenum. This industrial catalyst has a content of metallic cobalt of 2.2 wt. % and of metallic molybdenum of 8.3 wt. %. Its specific surface determined by the BET method is 196 m.sup.2/g. This catalyst is used in its sulfided form. The catalyst exists in the form of extrudates about 3 mm in diameter.

(46) The gaseous feedstock having the composition described in Table 4 (flow no. 2) is injected into a fixed bed reactor charged with catalyst C1. The selected operating conditions are: gas hourly space velocity GHSV (feedstock volume/catalyst volume/hour)=3000 h.sup.?1 absolute operating pressure: 26 bar inlet temperature of the catalyst bed: 250? C. temperature of the catalyst bed: 350? C.

(47) The analysis follow-up at the reactor outlet enabled the carbon monoxide conversion to be determined. The experimental results are given in Table 4 (composition of the outlet flow n? 3).

(48) Stage b1) Hydrolysis of COS and HCN

(49) The catalyst C2 that is used is an industrial catalyst based on titanium oxide. This industrial catalyst has a titanium oxide content of 85 wt. %. Its specific surface determined by the BET method is 120 m.sup.2/g. The catalyst exists in the form of extrudates about 3 mm in diameter.

(50) The gaseous feedstock having the composition described in Table 4 (flow no. 3) leaving the stage b1) of CO conversion with steam is injected into a fixed bed reactor charged with catalyst C2. The selected operating conditions are: gas hourly space velocity GHSV (feedstock volume/catalyst volume/hour)=1500 h.sup.?1 absolute operating pressure: 26 bar temperature of the catalyst bed: 250? C.

(51) The analysis follow-up at the reactor outlet enabled the conversions of COS and HCN to be determined. The experimental results are given in Table 4 (composition of outlet flow no. 3)

(52) Stage b2) Removal of Acid Gases

(53) According to the invention, the synthesis gas leaving the stage b1) is purified by washing with an aqueous solution of activated methyldiethanolamine (MDEA) containing 38 wt. % of methyldiethanolamine (MDEA), 8 wt. % of diethanolamine (DEA) and 54 wt. % of water, and the flow rate of solvent required to purify 168 000 Nm.sup.3/h of gas is 1340 Sm.sup.3/h under the following operating conditions in the absorber: Temperature: 45? C. Pressure: 26 bar
Stage c1) Hydrolysis of COS and HCN

(54) The catalyst C2 that is used is an industrial catalyst based on titanium oxide. This industrial catalyst has a titanium oxide content 85 wt %. Its specific surface determined by the BET method is 120 m.sup.2/g. The catalyst exists in the form of extrudates about 3 mm in diameter.

(55) The gaseous feedstock having the composition described in Table 4 (flow no. 5) is injected into a fixed bed reactor charged with catalyst C2. The selected operating conditions are: gas hourly space velocity GHSV (feedstock volume/catalyst volume/hour)=1500 h.sup.?1 absolute operating pressure: 26 bar temperature of the catalyst bed: 250? C.

(56) The analysis follow-up at the reactor outlet enabled the conversions of COS and HCN to be determined. The experimental results are given in Table 4 (composition of the outlet flow n? 3).

(57) Stage c2) Removal of Acid Gases

(58) According to the invention the synthesis gas leaving the stage c1) is purified by washing with an aqueous solution of MDEA containing 45 wt. % of MDEA and 55 wt. % of water, and the flow rate of solvent required to purify 102 000 Nm.sup.3/h of gas is 200 Sm.sup.3/h under the following operating conditions in the absorber: temperature: 50? C. pressure: 26 bar
Table 4 Refers to FIG. 2.

(59) TABLE-US-00004 TABLE 4 composition of the synthesis gas to be purified. Flow n ?1, n ?2, Flow Flow Flow Flow Flow Flow n ?5 n ?3 n ?3 n ?4 n ?6 n ?7 n ?8 H.sub.2 (% v) 14.6 39.6 39.6 90.2 14.6 30.7 60.0 CO (% v) 25.7 2.0 2.0 4.6 25.7 54.0 29.7 CO.sub.2 (% v) 6.6 31.5 31.5 0.001 6.6 5.0 2.5 H.sub.2O (% v) 50.8 24.6 24.6 0.25 50.8 0.25 0.25 H.sub.2S 4300 4750 4770 1 4775 1 1 (ppm vol) COS 480 30 8 5 3 3 4 (ppm vol) HCN 200 1 1 1 1 1 1 (ppm vol) NH.sub.3 650 850 850 10 850 10 10 (ppm vol) H.sub.2/CO 0.6 19.8 19.8 19.8 0.6 0.6 2.0

(60) TABLE-US-00005 TABLE 5 Flow rates of solvents used for Example 1 according to the prior art, and for Example 3 according to the invention. Example 1 according Example 3 according to the prior art to the invention Flow rate of solvent 1640 Sm.sup.3/h 1340 Sm.sup.3/h stage b2) Flow rate of solvent 200 Sm.sup.3/h stage c2)

(61) For the same overall flow rate of synthesis gas of 270 000 Nm.sup.3/h, the purification scheme according to the invention requires a flow rate of 200 Sm.sup.3/h of MDEA in stage c2) and a flow rate of 1340 Sm.sup.3/h of activated MDEA in stage b2). The process according to the invention thus enables the overall flow rate of solvent to be reduced by 6%. The energy consumptions required for the regeneration of the aqueous solutions of amines are also reduced in the same proportions.

(62) In the variant of the process according to the invention described in example 3, the residual amounts of acid compounds in the gas leaving the wash stage are: for H.sub.2S 1 ppmv, for COS 4 ppmv, and for HCN 1 ppmv, i.e. levels of impurities that are well below those obtained in Example 1 according to the prior art.

(63) The synthesis gas obtained has a H.sub.2/CO ratio of 2.