Plant for the membrane permeation treatment of a biogas stream with a membrane separation unit containing two modules

A process for a membrane permeation treatment of a methane and carbon dioxide-containing biogas stream, comprising the steps of:

• providing a plant comprising a compressor and a membrane separation unit that comprises first and second modules each comprising at least one membrane that is more permeable to carbon dioxide than methane, wherein:
• the first module is adapted and configured to receive a compressed biogas stream and separate it into a first permeate, a second permeate, a third permeate, and a first retentate, and
• the second module is adapted and configured to receive the first permeate from the first module and separate it into a fourth permeate and a second retentate;
• compressing the biogas stream, the compressor producing the compressed biogas stream;
• supplying the first module with the compressed biogas stream and separating the supplied compressed biogas stream into a first retentate that is enriched in methane relative to the biogas stream and first, second, and third permeates each of which is enriched in carbon dioxide relative to the biogas stream, the first permeate being richer in methane than the second or third permeates;
• supplying the second module with the first permeate and separating the supplied first permeate into a second retentate that is enriched in methane relative to the first permeate and a fourth permeate that is enriched in carbon dioxide relative to the first permeate;
• and
• recycling the second retentate and the third permeate to the inlet of the compressor where they are compressed with the biogas stream to produce the compressed biogas stream.