Production of biomethane using multiple types of membrane

A method for producing biomethane, comprising:

• compressing a biogas feed in a compressor to produce a compressed feed stream;
• passing said compressed feed stream to a first separation stage comprising at least one polymeric gas separation membrane to retain a first gas mixture comprising at least 60% of methane, and to permeate a first low-quality gas mixture comprising impurities present in said biogas feed stream and less than 20% of methane, said biogas feed comprising 40-75% of methane, 20-55% of carbon dioxide, up to 5,000 ppm of hydrogen sulfide (“H2S”), an amount of siloxanes, an amount of water, and up to 2,000 ppm of volatile organic compounds (“VOC's”), said at least one membrane of said first separation stage being selective for each of H2S, siloxanes, and VOC's over CH4,
• sending a stream of said first gas mixture into a second separation stage comprising at least one polymeric gas separation membrane, to retain a second gas mixture containing at least 94% of methane, and to permeate a second permeate gas mixture comprising impurities present in said biogas feed stream and less than 70% of methane;
• recycling a stream of said second permeate gas mixture to said compressor;
• and
• withdrawing said second gas mixture from said second separation stage as biomethane, wherein said membrane of said first separation stage is substantially different from said membrane of said second separation stage, and said method excludes the use of regenerable adsorbent, said biomethane containing at least 94% methane, below 3% of CO2, below 4 ppm of H2S, below 100 ppm of VOC's, and below 0.01 wt. % of H2O.