Cryogenic purification of biogas with withdrawal at an intermediate stage and external solidification of carbon dioxide

A combined plant 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 mixture, thereby producing a cooled stream,
• a distillation column, comprising a top and a bottom, configured to be supplied with the cooled stream and configured to produce a methane stream at the top and a CO2-enriched liquid stream at the bottom,
• a second exchanger for liquefying the methane stream, thereby producing a liquefied methane stream,
• a separating means for separating the liquefied methane stream into two portions:
• a “reflux” portion and a “product” portion,
• an expanding means for expanding the CO2-enriched liquid stream thereby producing an expanded CO2-enriched liquid stream, and a heating means for heating the expanded CO2-enriched liquid stream, thereby producing a heated CO2-enriched stream, and for recovering the cold from the expanded CO2-enriched liquid stream, and
• a separator vessel for receiving the heated CO2-enriched stream and for producing an overhead vapour stream and a liquid CO2 stream,
• a means for withdrawing a vapour stream from an intermediate stage of the distillation column,
• a condensing means for partially condensing the vapour stream and producing a two-phase 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 separator vessel, and
• a cryogenic separation unit comprises:
• 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, configured to place in contact a liquid stream from the cold section and a vapour stream rising from the hot section and for trapping solid CO2, and
• a device for regenerating the at least two external containers, comprising an extracting means for extracting a fluid from the distillation column which is configured to liquefy the solid CO2, a means for introducing this fluid into the at least two external containers in regeneration thereby melting the solid CO2 and a means for reintroducing the resulting liquid CO2-vapour mixture into the distillation column.