Process for producing a purified synthesis gas stream

Abstract

A process for producing a purified synthesis gas stream from a feed synthesis gas stream comprising besides the main constituents carbon monoxide and hydrogen also hydrogen sulphide,HCN and/or COS, the process comprising the steps of: (a) removing HCN and/or COS by contacting the feed synthesis gas stream with a water gas shift catalyst in a shift reactor in the presence of steam/water to react at least part of the carbon monoxide to carbon dioxide, and to obtain a synthesis gas stream depleted in HCN and/or in COS; (b) removing hydrogen sulphide in the synthesis gas stream depleted in HCN and/or in COS by contacting this gas stream in a H 2 S-removal zone with an aqueous alkaline washing liquid to obtain a H 2 S-depleted synthesis gas stream and a sulphide-comprising aqueous stream; (c) contacting the sulphide-comprising aqueous stream with ulphide-oxidizing bacteria in the presence of oxygen in a bioreactor to obtain a sulphur slurry and a regenerated aqueous alkaline washing liquid; (d) removing carbon dioxide from the H 2 S-depleted synthesis gas stream, to obtain the purified synthesis gas stream and a gas stream enriched in CO2.

1 . A process for producing a purified synthesis gas stream from a feed synthesis gas stream, comprising besides the main constituents carbon monoxide and hydrogen also hydrogen, sulphide, HCN and/or COS, the process comprising the steps of:
 (a) removing HCN and/or COS from the feed synthesis gas stream by contacting the feed synthesis gas stream with a water gas shift catalyst in a shift reactor in the presence of steam/water to react at least part of the carbon monoxide to carbon dioxide, and to obtain a synthesis gas stream depleted in HCN and/or in COS;   (b) removing hydrogen sulphide in the synthesis gas stream depleted in HCN and/or in COS by contacting it in a H 2 S-removal zone with an aqueous alkaline washing liquid to obtain a H 2 S-depleted synthesis gas stream and a sulphide-comprising aqueous stream;   (c) contacting the sulphide-comprising aqueous stream with sulphide-oxidizing bacteria in the presence of oxygen in a bioreactor to obtain a sulphur slurry and a regenerated aqueous alkaline washing liquid; and   (d) removing carbon dioxide from the H 2 S-depleted synthesis gas stream, to obtain the purified synthesis gas stream and a gas stream enriched in CO 2 .   
     
     
         2 . A process according to  claim 1 , wherein the sulphide-oxidising bacteria are selected from the group of autotropic aerobic cultures consisting of the genera  Thiobacillus  and  Thiomicrospira.    
     
     
         3 . A process according to  claim 1 , wherein the steam/water to carbon monoxide molar ratio in the feed synthesis gas stream as it enters the shift reactor is is in the range of from 0.2:1 and 0.9:1 and wherein the temperature of the feed synthesis gas stream as it enters the shift reactor is in the range of from 190 to 230° C. and wherein the feed synthesis gas stream comprises at least 50 volume % of carbon monoxide, on a dry basis. 
     
     
         4 . A process according to  claim 1 , wherein in step (d) carbon dioxide is removed by contacting the H 2 S-depleted synthesis gas stream with an absorbing liquid at low temperature and at elevated pressure, thereby transferring carbon dioxide from the H 2 S-depleted gas streams to the absorbing liquid to obtain absorbing liquid enriched in carbon dioxide and the purified gas stream. 
     
     
         5 . A process according to  claim 1 , wherein in step (d) carbon dioxide is removed using a membrane. 
     
     
         6 . A process according to  claim 1 , wherein in step (d) carbon dioxide is removed by cooling the synthesis gas stream depleted in hydrogen sulphide to a temperature at which carbon dioxide will separate from the gas streams. 
     
     
         7 . A process according to  claim 1 , wherein the purified synthesis gas stream is used in a combustion turbine to produce electricity. 
     
     
         8 . A process according to  claim 7 , wherein hot exhaust gas is emitted from the combustion turbine and is introduced into a heat recovery steam generator unit to produce a steam, which steam is used to produce additional electricity. 
     
     
         9 . A process according to  claim 1 , wherein the further purified synthesis gas is used in catalytic processes selected from the group consisting of Fischer-Tropsch synthesis, methanol synthesis, di-methyl ether synthesis, acetic acid synthesis, ammonia synthesis, methanation to make substitute natural gas (SNG) and processes involving carbonylation or hydroformylation reactions. 
     
     
         10 . A process according to  claim 1 , wherein the gas stream enriched in CO 2  is at a pressure in the range of from 5 to 50 bara. 
     
     
         11 . A process according to  claim 10 , wherein the gas stream enriched in CO2 is further pressurised and injected into a subterranean formation for use in enhanced oil recovery or for storage into an aquifer reservoir or for storage into an empty oil reservoir.