Cryogenic purification of biogas with pre-separation and external solidification of carbon dioxide

A combined facility for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, comprising:

• a mixing means for mixing a biogas stream with a recycle gas stream, thereby producing a mixed biogas stream,
• a compressor for compressing the mixed biogas stream to a pressure suitable for distillation, thereby producing a compressed stream,
• a first exchanger for cooling the compressed stream, thereby producing a cooled stream,
• a first separator vessel for receiving the cooled stream and for recovering an overhead vapour stream and a CO2-enriched liquid stream,
• a cooling means for cooling the overhead vapour stream, thereby producing a cooled overhead vapor stream,
• a cryogenic separation unit supplied with the cooled overhead vapour stream, comprising a distillation column, the distillation column comprising a top and a bottom, and producing a methane stream at the top of the distillation column and a CO2-enriched liquid stream at the bottom of the distillation column,
• a second exchanger for liquefying the methane stream produced at the top of the distillation column, thereby producing a liquefied methane stream,
• a separation means for separating the liquefied methane stream into two portions:
• a “reflux” portion and a “product” portion,
• an expanding means for expanding the CO-enriched liquid stream, thereby producing an expanded CO2-enriched liquid stream, and the first heat exchanger configured to heat the expanded CO2-enriched liquid stream for recovering cold from the expanded CO2-enriched liquid stream, thereby producing an expanded CO2 enriched stream
• a second separator vessel for receiving the expanded CO2-enriched stream from the expanding means and the first heat exchanger for producing an overhead vapour stream and liquid CO2 stream,
• wherein
• the mixing means is configured such that the recycle gas stream corresponds to the overhead vapour stream recovered at the outlet of the second separator vessel,
• and
• the cryogenic separation unit comprising:
• the distillation column comprising a cold section at the top of the distillation column and a hot section at the bottom of the distillation column,
• at least two containers external to the distillation column, for placing in contact a liquid from the cold section and a vapour rising from the hot section and for trapping all solid CO2, and
• a device for regenerating the at least two external containers, comprising a means for extracting a fluid from the distillation column which is configured to liquefy the solid CO2, a means for introducing the fluid into the at least two external containers in regeneration thereby bringing about the melting of the solid CO2 and a means for reintroducing the resulting liquid CO2-vapour stream into the distillation column.