Purification of methane containing gas streams using selective membrane separation

A system for separating a gas mixture, the system comprising:

• a) at least one system inlet configured to receive a pressurized gas mixture from a pressurized vessel, the pressurized gas mixture comprising biogas;
• b) a feed system, the feed system in fluid communication with the at least one system inlet and configured to provide a plurality of pressurized streams of the biogas at successively lower pressures;
• c) a plurality of membrane systems having successively lower feed pressures, each membrane system comprising at least one stage and configured to receive gas from a respective one of the pressurized gas streams, each membrane system in the plurality of membrane systems configured to provide at least two streams, the at least two streams chosen from a product stream, an off-gas stream, and a partially purified stream;
• d) a pipe system for providing feed for each membrane system in the plurality of membrane systems, the pipe system configured to provide gas from at least one partially purified stream produced from the plurality of membrane systems for processing in one of the membrane systems in (c) having a lower feed pressure than the membrane system that produced that partially purified stream;
• and,
• e) at least one system outlet for providing a product gas from at least one product stream produced from the plurality of membrane systems.

A method of upgrading biogas, the method comprising:

• a) providing at least one pressure vessel, each pressure vessel containing biogas at a pressure of at least 1000 psig, the biogas comprising methane and a non-methane component;
• b) withdrawing portions of the biogas from the at least one pressure vessel, thereby depressurizing the at least one pressure vessel, and providing each portion as a pressurized gas stream at a different pressure, thereby providing a plurality of pressurized gas streams at successively decreasing pressures;
• c) providing a plurality of membrane systems, each membrane system in the plurality of membrane systems at a different pressure level and configured to provide at least two streams, the at least two streams chosen from a product stream, an off-gas stream, and a partially purified stream;
• d) feeding a first pressurized gas stream from the plurality of pressurized gas streams to a first a membrane system from the plurality of membrane systems, the first membrane system at a first pressure level and producing a first permeate;
• e) feeding a combined stream to a second membrane system in the plurality of membrane systems, the second membrane system at a second pressure level and producing a second permeate, the second pressure level lower than the first pressure level, the combined stream produced by combining a second pressurized gas stream from the plurality of pressurized gas streams with the first permeate or a stream derived from the first permeate, wherein the second pressurized gas stream is at a lower pressure than the first pressurized gas stream;
• and
• f) providing a product gas from at least one product stream produced from the plurality of membrane systems.