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% CH4, below 3% of CO2, and below 4 ppm of H2S.

1 . 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 (“H 2 S”), 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 H 2 S and VOC's over CH 4 ,   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 CO 2 , below 4 ppm of H 2 S, below 100 ppm of VOC's, and below 0.01 wt. % of H 2 O.   
     
     
         2 . The method of  claim 1 , wherein said biogas feed contains 40-75% of methane (“CH 4 ”), and impurities of:
 up to 10% nitrogen (“N 2 ”); 
 up to 1% oxygen (“O 2 ”), 
 20-55% of carbon dioxide (“CO 2 ”), 
 up to 5,000 ppm of hydrogen sulfide (“H 2 S”); 
 siloxanes, 
 up to 2,000 ppm of VOC's; and 
 water. 
 
     
     
         3 . The method of  claim 1 , further comprising a step of passing said compressed feed stream to a water removal apparatus to remove water, after said step of compressing a biogas feed and prior to introducing said feed into said first separation stage. 
     
     
         4 . The method of  claim 1 , wherein said compressed feed stream has a water content of less than 0.5 wt. %. 
     
     
         5 . The method of  claim 1 , wherein said at least one membrane of said first separation stage is comprised of a porous polymeric substrate having at least one separation layer, wherein said substrate is selected from the group consisting of polyimides, poly sulfones, polyether ether ketones (“PEEK”), and mixtures thereof. 
     
     
         6 . The method of  claim 5 , wherein said substrate is polyether ether ketones (“PEEK”). 
     
     
         7 . The method of  claim 6 , wherein said separation layer is 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). 
       
     
     
         8 . The method of  claim 6 , wherein said separation layer is made of repeating units of the following monomers: 
       
         
           
           
               
               
           
         
       
     
     
         9 . The method of  claim 6 , wherein said separation layer is made of a copolymer or block polymer of tetramethylene oxide, propylene oxide, and/or ethylene oxide. 
     
     
         10 . The method of  claim 1 , wherein said at least one membrane of said first separation stage has a selectivity of at least 10 for H 2 S over CH 4 . 
     
     
         11 . The method of  claim 1 , wherein said stream of said first gas mixture has a pressure drop of less than 50 psi from said feed gas. 
     
     
         12 . The method of  claim 1 , wherein said step of passing said compressed feed stream to a first separation stage further includes removing an amount of H 2 S from said first gas mixture using a H 2 S scavenger media to produce a low-H 2 S output stream having less than 4 ppm of H 2 S from said scavenger media, and sending said low-H 2 S output stream into said second separation stage polymeric membrane in said step of sending a stream of said first gas mixture into a second separation stage. 
     
     
         13 . The method of  claim 1 , wherein said second separation stage has a selectivity of at least 20 for CO 2  over CH 4 . 
     
     
         14 . The method of  claim 1 , wherein said biomethane is comprised of at least 94% methane, below 3% of CO 2 , below 4 ppm of H 2 S, below 100 ppm of VOC's, and below 0.01 wt. % of H 2 O. 
     
     
         15 . The method of  claim 1 , wherein said second permeate gas mixture is comprised of about 30-70% methane, at least 30% CO 2 , below 1,000 ppm of H 2 S, below 100 ppm of VOC's, and below 0.05 wt. % of H 2 O. 
     
     
         16 . The method of  claim 1 , wherein said step of sending a stream of said first gas mixture into a second separation stage further includes sending said second gas mixture to a H 2 S scavenger media, to produce a low-H 2 S product stream having less than 4 ppm of H 2 S from said media. 
     
     
         17 . The method of  claim 1 , wherein the permeate of said second stage is compressed in a second compressor, and said compressed gas is fed to said first stage. 
     
     
         18 . The method of  claim 1 , wherein a selectivity of H 2 S over CH 4  of the polymeric gas separation membranes of the first separation stage is higher than that of the polymeric gas separation membranes of the second separation stage. 
     
     
         19 . 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 CF 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.   
     
     
         20 . The system of  claim 19 , 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. 
     
     
         21 . The system of  claim 19 , 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). 
     
     
         22 . The system of  claim 19 , 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). 
     
     
         23 . The system of  claim 19 , wherein said at least one membrane of said first membrane stage has a separation layer made of repeating units of the following monomers: 
       
         
           
           
               
               
           
         
       
     
     
         24 . The system of  claim 19 , wherein each of said at least one membrane of said first separation stage has a separation layer that is supported by a support layer. 
     
     
         25 . The system of  claim 24 , wherein each of said support layers is made of a polyimide, polysulfone, polyether ether ketone, or mixtures thereof. 
     
     
         26 . The system of  claim 25 , wherein each of said support layers is porous and is made of polyether ether ketone. 
     
     
         27 . The system of  claim 19 , wherein each of said membranes of said second separation stage is made of cellulose acetate, a polysulfone, a polyimide, or mixtures thereof. 
     
     
         28 . The system of  claim 19 , 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. 
     
     
         29 . The system of  claim 19 , 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.