Process for increasing hydrogen content of synthesis gas
Process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds, the synthesis gas including hydrogen, carbon oxides and steam, and having a ratio defined as R═(H 2 —C0 2 )/(CO+C0 2 )≦0.6 and a steam to carbon monoxide ratio ≦1.8, includes the steps of (i) adjusting the temperature of the synthesis gas; (ii) passing at least a portion of the heated synthesis gas adiabatically through a first bed of sulphur-tolerant water-gas shift catalyst disposed in a first shift vessel at a space velocity ≧12,500 hour −1 to form a pre-shifted gas stream; and (iii) forming a shifted gas stream by subjecting at least a portion of the pre-shifted gas stream to a second stage of water-gas shift in a second shift vessel containing a second bed of sulphur-tolerant water-gas shift catalyst that is cooled in heat exchange with a gas stream including the synthesis gas.
1. A process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds, said synthesis gas comprising hydrogen, carbon oxides and steam, and having a ratio, R, defined as R═(H 2 —CO 2 )/(CO+CO 2 )<0.6 and a steam to carbon monoxide ratio ≦1.8, comprising the steps of (i) heating the synthesis gas, (ii) passing at least a portion of the heated synthesis gas adiabatically through a first bed of sulphur-tolerant water-gas shift catalyst disposed in a first shift vessel at a gas hourly space velocity ≧12,500 hour −1 to form a pre-shifted gas stream to convert 10 to 40% by moles of the carbon monoxide present in the synthesis gas to carbon dioxide, and (iii) forming a shifted gas stream by subjecting at least a portion of the pre-shifted gas stream to a second stage of water-gas shift in a second shift vessel containing a second bed of sulphur-tolerant water-gas shift catalyst that is cooled in heat exchange with a gas stream comprising the synthesis gas.
2. The process according to claim 1 wherein the synthesis gas containing one or more sulphur compounds is formed by gasification of a carbonaceous feedstock at elevated temperature and pressure, followed by cooling and washing the resulting gas stream to remove particulate material.
3. The process according to claim 2 wherein the gasification is performed on a coal powder or aqueous slurry in a gasifier using oxygen or air and in the presence of steam at a pressure up to about 85 bar abs and an exit temperature up to 1450° C.
4. The process according to claim 2 wherein the steam to carbon monoxide ratio is in the range 0.2 to 1.8.
5. The process according to claim 2 wherein the R ratio is in the range 0.1 to 0.6.
6. The process according to claim 2 wherein the R ratio is in the range 0.2 to 0.6.
7. The process according to claim 1 wherein the heated synthesis gas is divided into first and second streams, with the first stream passed over the first bed of shift catalyst and the second stream by-passing the first bed of shift catalyst, thereby forming a catalyst by-pass stream.
8. The process according to claim 7 wherein the by-pass stream is fed to one or more of the pre-shifted gas stream, the shifted gas stream, or separately to downstream processes.
9. The process according to claim 1 wherein the space velocity of the synthesis gas flowing through the first bed of sulphur-tolerant water-gas shift catalyst is ≧15,500 hour −1 .
10. The process according to claim 1 wherein the inlet temperature for the first bed of water-gas shift catalyst is in the range 190 to 350° C.
11. The process according to claim 1 wherein the synthesis gas is heated by passing it through a plurality of tubes, coils or plates disposed within the second catalyst bed.
12. The process according to claim 1 wherein the synthesis gas is subjected to the water-gas shift reaction over a supported cobalt-molybdenum water-gas shift catalyst.
13. The process according to claim 1 wherein the water-gas shift reaction over the first bed of water-gas shift catalyst is performed at a temperature in the range 190 to 420° C.
14. The process according to claim 1 wherein the second stage of water-gas shift is performed in a vessel containing a supported cobalt-molybdenum water-gas shift catalyst.
15. The process according to claim 1 wherein the second stage of water gas shift is performed at a temperature in the range 250 to 420° C.
16. The process according to claim 1 wherein the space velocity in the second bed of sulphur-tolerant water-gas shift catalyst is ≧6000 h −1 .
17. The process according to claim 1 wherein the second shift vessel comprises tubes, and the synthesis gas is passed through the tubes in the second shift vessel in a direction that is counter-current or co-current to the flow of the pre-shifted gas stream through the vessel.
18. The process according to claim 1 wherein the shifted gas stream is subjected to one or more further water-gas shift stages.
19. The process according to claim 1 further comprising the steps of:
(i) cooling a shifted gas stream obtained from the one or more further stages of water-gas shift, or a mixture of the shifted gas stream and a bypass stream, to below the dew point to condense water,
(ii) separating the resulting condensate therefrom to form a dry shifted gas stream,
(iii) feeding the dry shifted gas stream to a gas-washing unit operating by means of counter-current solvent flow, to produce a product synthesis gas enriched in hydrogen and
(iv) collecting the product synthesis gas from the washing unit.
20. The process according to claim 1 wherein the space velocity of the synthesis gas flowing through the first bed of sulphur-tolerant water-gas shift catalyst is ≧17,500 hour −1 .
21. The process according to claim 1 wherein the space velocity of the synthesis gas flowing through the first bed of sulphur-tolerant water-gas shift catalyst is ≧20,000 hour −1 .
22. The process according to claim 1 wherein the space velocity in the second bed of sulphur-tolerant water-gas shift catalyst is in the range 6000 h −1 to 12000 h −1 .