System and method for cleaning of biogas
An improved process for removing acid gases from raw biogas streams, such as biogas from landfills or biogas from controlled anaerobic digestion, provides for efficient H 2 S removal combined with carbon dioxide removal. The biogas is treated in a biological hydrogen sulfide removal system integrated with a multi-stage membrane gas separation system. The combined system provides for efficient acid gas removal while simultaneously limiting oxygen carryover into the treated product stream by beneficially utilizing oxygen in the biological desulfurization system.
1. A process for removing acid gases from a raw biogas feed stream comprised of methane, carbon dioxide, nitrogen, oxygen and hydrogen sulfide, said process comprising the steps of:
(i) processing a combined feed gas stream comprised of the raw biogas feed stream and at least one recycled gas stream generated by a membrane separation unit through a biological hydrogen sulfide removal system at a substantially atmospheric pressure, thus generating a first effluent stream with reduced hydrogen sulfide and oxygen content as compared to the combined feed gas stream, wherein said biological hydrogen sulfide removal system comprises bacteria that oxidizes hydrogen sulfide to elemental sulfur and sulfuric acid;
(ii) compressing the first effluent stream in a compressor to a pressure from 6 to 25 barg followed by aftercooling and condensate separation, thereby forming a second effluent stream;
(iii) passing the second effluent stream into a first membrane separation unit, wherein a first membrane removes a portion of oxygen and carbon dioxide from the second effluent stream, thereby forming a retentate third effluent stream having a reduced content of carbon dioxide as compared to the second effluent stream and a permeate fourth effluent stream enriched in oxygen and carbon dioxide as compared to the second effluent stream;
(iv) passing the third effluent stream to a second membrane separation unit, wherein a second membrane removes a portion of carbon dioxide and oxygen from the third effluent stream, thereby forming a retentate fifth effluent stream having a lower carbon dioxide and oxygen content as compared to the third effluent stream and a permeate sixth effluent stream enriched in carbon dioxide and oxygen content as compared to the third effluent stream;
(v) passing the fourth effluent stream to a third membrane separation unit wherein a third membrane removes a portion of carbon dioxide and oxygen from the fourth effluent stream, thereby forming a retentate seventh effluent stream having a lower carbon dioxide content as compared to the fourth effluent stream and a higher oxygen content as compared to the raw biogas feed stream and a permeate eighth effluent stream with a carbon dioxide concentration above 95 volumetric percent, wherein the eighth effluent stream is at substantially atmospheric pressure or a sub-atmospheric pressure;
(vi) wherein the sixth effluent stream is recycled and comprises a portion of the recycled gas stream combined with the raw biogas feed stream in step (i) or wherein the sixth effluent stream is recycled and combined with the first effluent stream prior to compression in step (ii);
(vii) wherein the seventh effluent stream is recycled and comprises at least a portion of the recycled gas stream combined with the raw biogas feed stream in step (i); and
(viii) collecting the fifth effluent stream generated by the second membrane generation unit as a product, wherein the fifth effluent stream has an increased methane content and reduced hydrogen sulfide, oxygen and carbon dioxide content as compared to the raw biogas feed stream.
2. The process of claim 1 further comprising the step of collecting the eighth effluent stream generated by the third membrane separation unit as a product.
3. The process of claim 2 wherein the eighth effluent stream is additionally treated in a desulfurization unit to remove remaining hydrogen sulfide.
4. The process of claim 1 wherein at least a portion of the sixth effluent stream is combined with the raw biogas feed stream in step (i).
5. The process of claim 1 wherein the sixth effluent stream is combined with the first effluent stream prior to the combined stream being subjected to compression.
6. The process of claim 1 wherein at least one of the first effluent stream and the second effluent stream is treated in an adsorption unit to remove additional hydrogen sulfide not removed in the biological hydrogen sulfide removal system.
7. The process of claim 6 wherein an adsorbent in the adsorption unit comprises an activated carbon or activated carbon impregnated with a substance that is characterized by catalytic activity with respect to the oxidation of hydrogen sulfide in the presence of oxygen.
8. The process of claim 6 wherein at least a portion of the sixth effluent stream is combined with the first effluent stream upstream of the adsorption unit provided for hydrogen sulfide removal before the combined stream is subjected to compression.
9. The process of claim 1 wherein a supplemental oxygen-containing gas is injected into the biological hydrogen sulfide removal system to facilitate hydrogen sulfide removal.
10. The process of claim 1 wherein the combined feed gas stream is further comprised of a supplemental oxygen source stream.
11. The process of claim 1 wherein the first, second and third membrane separation units are equipped with semipermeable polymeric hollow fiber membranes exhibiting a CO 2 /CH 4 gas separation factor above 20 and an oxygen/methane separation factor above 5.
12. The process of claim 11 wherein the first, second and third membrane separation units are equipped with semipermeable polymeric hollow fiber membranes exhibiting a CO 2 /CH 4 gas separation factor above 30 and an oxygen/methane separation factor above 6.
13. The process of claim 12 wherein the first, second and third membrane separation units are equipped with semipermeable polymeric hollow fiber membranes exhibiting a CO 2 /CH 4 gas separation factor above 40 and an oxygen/methane separation factor above 7.
14. The process of claim 11 wherein the first, second and third membrane separation units in at least one stage are equipped with semipermeable polymeric hollow fiber membranes that differ in gas separation characteristics.
15. The process of claim 1 wherein the third effluent stream is treated in a deoxygenation unit followed by aftercooling and condensate separation.
16. The process of claim 1 wherein the fifth effluent stream is treated in a deoxygenation unit followed by aftercooling, condensate separation and dehydration.
17. The process of claim 1 wherein a pressure boosting device is disposed between the first membrane separation unit and the third membrane separation unit.
18. The process of claim 1 wherein a vacuum pump is disposed on the eighth effluent stream to collect the eighth effluent stream at a sub-atmospheric pressure.
19. The process of claim 1 wherein the fifth effluent stream is treated in a nitrogen rejection unit.
20. The process of claim 1 wherein the fifth effluent stream is directed to a membrane unit or an adsorption unit to remove remaining acid gases with recycling of resulting tail gases enriched in acid components being combined with the first effluent stream.
21. The process of claim 1 wherein the first effluent stream or the second effluent stream or both are treated to reduce water vapor content upstream of the first membrane separation unit.
22. The process of claim 1 wherein an oxygen concentration in the raw biogas feed is below 5% by volume, a carbon dioxide concentration in the raw biogas feed is above 25 volumetric percent and a hydrogen sulfide concentration in the raw biogas feed is from 100 to 30,000 ppmv.
23. The process of claim 1 wherein the bacteria is a chemoautotrophic bacteria or a sulfur oxidizing bacteria.
24. The process of claim 23 wherein the bacteria is of a genus selected from the group consisting of Thiobacillus, Acidithiobacillus, Achromatium, Beggiatoa, Thiothrix, Thioplaca, Thiomicrospira, Thiosphaera , and Thermothrix.
25. The process of claim 24 wherein the bacteria is of the genus Thiobacillus or Acidithiobacillus.
26. The process of claim 1 wherein the bacteria consumes oxygen passed into said biological hydrogen sulfide removal system.