Patent ID: 7604681

Claim:
A process for separating carbon dioxide from methane in a gas stream comprising the steps of: (a) providing a first membrane having a first feed side and a first permeate side and being selective for carbon dioxide over methane; (b) passing the gas stream as a first feed stream at a first pressure of at least about 400 psia across the first feed side; (c) withdrawing from the first feed side a first residue stream depleted in carbon dioxide compared with the gas stream; (d) withdrawing from the first permeate side a first permeate stream enriched in carbon dioxide compared with the gas stream; (e) compressing the first permeate stream; (f) providing a second membrane having a second feed side and a second permeate side and being selective for carbon dioxide over methane; (g) passing the compressed first permeate stream as a second feed stream across the second feed side; (h) withdrawing from the second feed side a second residue stream depleted in carbon dioxide compared with the second feed stream; (i) withdrawing from the second permeate side a second permeate stream enriched in carbon dioxide compared with the second feed stream; (j) recirculating the second residue stream to step (b); (k) providing a third membrane having a third feed side and a third permeate side and being selective for carbon dioxide over methane; (l) passing the second permeate stream as a third feed stream without compression across the third feed side; (m) withdrawing from the third feed side a third residue stream depleted in carbon dioxide compared with the third feed stream; (n) withdrawing from the third permeate side a third permeate stream enriched in carbon dioxide compared with the third feed stream; (o) recirculating the third residue stream to step (e).