Production of biomethane using multiple types of membrane

Abstract

The invention relates to a process for recovering methane from digester biogas or landfill gas. More specifically, the invention pertains to a method for producing biomethane that removes impurities from a compressed digester biogas with staged membrane modules of at least two different types, to produce a biomethane having at least 94% CH 4 , below 3% of CO 2 , and below 4 ppm of H 2 S.

1 . A system for producing biomethane, comprising:
 a source of a biogas feed comprising 40-75% of methane, 20-55% of carbon dioxide, up to 5,000 ppm of hydrogen sulfide (“H 2 S”), an amount of water, and up to 2,000 ppm of volatile organic compounds (“VOC's”);   a compressor fluidly communicating with said source and being adapted and configured to receive and compress said biogas feed;   a first separation stage in fluid communication with said compressed feed, comprising one or more gas separation membranes for processing said compressed feed being selective for VOCs over CH 4  and having a selectivity of at least 10 for H 2 S over CH 4 ;   a second separation stage in fluid communication with said first separation stage, comprising one or more polymeric gas separation membranes for processing a retentate from said first separation stage of membrane to produce biomethane, wherein said one or more polymeric gas separation membranes of said second separation stage has a selectivity of at least 20 for CO 2  over CH 4 , and   an inlet for recycling a permeate from said second separation stage back to said compressor inlet, thereby commingling said biogas feed with said second stage permeate stream as a mixed gas feed to said compressor so that said compressor compresses and discharges said compressed feed to said first separation stage of membrane, wherein no regenerable adsorbent beds are in fluid communication between said compressor and said second membrane stage.   
     
     
         2 . The system of  claim 1 , further comprising a water removal apparatus adapted and configured to remove water from said compressed biogas feed prior to said feeding said feed into said first separation stage of membrane. 
     
     
         3 . The system of  claim 1 , wherein each of said at least one membrane of said first separation stage exhibits a pressure drop between a pressure of the feed gas and a pressure of the retentate gas is less than 50 psi (3.45 bar). 
     
     
         4 . The system of  claim 1 , wherein said at least one membrane of said first separation stage has a separation layer made of a copolymer or block polymer of the formula: 
       
         
           
           
               
               
           
         
         where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) poly(tetramethylene oxide). 
       
     
     
         5 . The system of  claim 1 , wherein said at least one membrane of said first membrane stage has a separation layer made of repeating units of the following monomers: 
       
         
           
           
               
               
           
         
       
     
     
         6 . The system of  claim 1 , wherein each of said at least one membrane of said first separation stage has a separation layer that is supported by a support layer. 
     
     
         7 . The system of  claim 6 , wherein each of said support layers is made of a polyimide, polysulfone, polyether ether ketone, or mixtures thereof. 
     
     
         8 . The system of  claim 7 , wherein each of said support layers is porous and is made of polyether ether ketone. 
     
     
         9 . The system of  claim 1 , wherein each of said membranes of said second separation stage is made of cellulose acetate, a polysulfone, a polyimide, or mixtures thereof. 
     
     
         10 . The system of  claim 1 , further comprising a H 2 S scavenger media in fluid communication between said first and second separation stages that is adapted and configured to remove at least some H 2 S from a retentate from said first separation stage before said retentate is sent to said second separation stage. 
     
     
         11 . The system of  claim 1 , further comprising a H 2 S scavenger media located on a high-pressure retentate side of said second separation stage, said media being in fluid communication with said second separation stage and being adapted and configured to remove at least some H 2 S from a retentate from said second separation stage to produce biomethane.