| CPC C10L 3/104 (2013.01) [B01D 53/02 (2013.01); B01D 53/226 (2013.01); B01D 53/228 (2013.01); B01D 53/229 (2013.01); B01D 53/265 (2013.01); B01D 53/526 (2013.01); B01D 53/62 (2013.01); B01D 53/75 (2013.01); B01D 53/84 (2013.01); B01D 53/8618 (2013.01); C10L 3/103 (2013.01); C10L 3/106 (2013.01); B01D 2053/224 (2013.01); B01D 2253/102 (2013.01); B01D 2253/25 (2013.01); B01D 2256/245 (2013.01); B01D 2257/104 (2013.01); B01D 2257/304 (2013.01); B01D 2257/504 (2013.01); B01D 2257/80 (2013.01); B01D 2258/05 (2013.01); C10L 2290/26 (2013.01); C10L 2290/542 (2013.01); C10L 2290/548 (2013.01)] | 26 Claims |
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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.
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