Production of biomethane using a high recovery module

Abstract

The invention relates to a process for recovering methane from digester biogas or landfill gas. More specifically, the invention pertains to biomethane production that substantially removes carbon dioxide from a digester biogas or landfill gas using first, second, and third purification stages each comprising one or more membranes selective for carbon dioxide over methane. A retentate from the first stage is separated by the one more membranes of the second stage into a second state retentate, forming a biomethane product gas. A permeate from the first stage is separated by the one or more membranes of the third stage into a third stage retentate and a third stage permeate. Recovery of methane from the the biogas is boosted by feeding the third stage retentate to the first purification stage. The recovery may be optionally further boosted by compressing the second stage permeate with the biogas at a main compressor.

What is claimed is: 
     
         1 . A installation for producing biomethane containing at least 94% methane, comprising:
 a source of a biogas;   a main compressor, an inlet of which is in fluid communicating with the source;   a first purification stage comprising at least one membrane selective for carbon dioxide over methane, an inlet of the first purification stage being in fluid communication with an outlet of the main compressor;   a second purification stage comprising at least one membrane selective for carbon dioxide over methane, an inlet of the second purification stage being in fluid communication with a retentate outlet of the first purification stage; and   a high recovery module comprising a third purification stage comprising one or more membranes in parallel or in series that are selective for carbon dioxide over methane and a secondary compressor in fluid communication between a permeate outlet of the first purification stage and an inlet of the third purification stage, wherein a retentate outlet of the third purification stage is in fluid communication with the first purification stage inlet so as to allow a retentate stream from the third purification stage to be recycled to the first purification stage.   
     
     
         2 . The installation of  claim 1 , wherein: a permeate outlet of the second purification stage is in fluid communication with the main compressor inlet so as to allow a permeate stream from the second purification stage to be recycled to the main compressor. 
     
     
         3 . The installation of  claim 1 , further comprising a dehydration unit in fluid communication between the main compressor outlet and the first purification stage inlet that is adapted and configured to remove amounts of water from a combination stream received from the compressor outlet prior to the combination stream being fed to the first purification stage inlet. 
     
     
         4 . The installation of  claim 3 , wherein said dehydration unit is a condenser or dehydration media. 
     
     
         5 . The installation of  claim 1 , wherein the at least one membrane of the first purification stage comprises a porous polymeric substrate and at least one separation layer, the polymeric substrate being selected from the group consisting of polyimides, polysulfones, polyether ether ketones, and mixtures thereof. 
     
     
         6 . The installation of  claim 5 , wherein said polymeric substrate is a polyether ether ketone. 
     
     
         7 . The installation of  claim 5 , wherein the 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) or poly(tetramethylene oxide). 
       
     
     
         8 . The installation of  claim 5 , wherein the separation layer is made of repeating units of the following monomers: 
       
         
           
           
               
               
           
         
       
     
     
         9 . The installation of  claim 5 , wherein the separation layer is made of a copolymer or block polymer of tetramethylene oxide, propylene oxide, and/or ethylene oxide. 
     
     
         10 . The installation of  claim 1 , wherein the at least one membrane of the first purification stage is in the form of flat films or a plurality of hollow fibers. 
     
     
         11 . The installation of  claim 1 , wherein the at least one membrane of the first purification stage is selectivity for H 2 S, CO 6+  hydrocarbons, carbon dioxide, siloxane and water over methane. 
     
     
         12 . The installation of  claim 1 , wherein said stream of said first gas mixture has a pressure drop of less than 50 psi from said feed gas. 
     
     
         13 . The installation of  claim 1 , wherein each of said membranes of said second stage is made of cellulose acetate, a polysulfone, a polyimide, polyamide, or mixtures thereof. 
     
     
         14 . The installation of  claim 1 , further comprising a H 2 S removal unit comprising a H 2 S scavenger media in fluid communication between the main compressor and the first purification stage, the H 2 S removal unit being adapted and configured to remove amount of H 2 S from a combination stream received from the compressor. 
     
     
         15 . The installation of  claim 1 , further comprising a H 2 S removal unit comprising a H 2 S scavenger media in fluid communication downstream of a retentate outlet of the second stage purification unit, the H 2 S unit being adapted and configured to remove amounts of H 2 S from a second stage retentate stream produced by the second purification stage. 
     
     
         16 . The installation of  claim 1 , wherein said at least one membrane of said third purification stage is the same as the membrane of the first purification stage. 
     
     
         17 . The installation of  claim 1 , wherein said at least one membrane of the third purification stage is the same as the membrane of the second purification stage. 
     
     
         18 . The installation of  claim 1 , wherein: the high recovery module further comprises a fourth purification stage comprising at least one membrane selective for carbon dioxide over methane; an inlet of the fourth purification being in downstream flow communication with a retentate outlet of the third purification stage; a permeate outlet of the second purification stage is in fluid communication with the main compressor inlet so as to allow a permeate stream from the second purification stage to be recycled to the main compressor; and a retentate outlet of the fourth purification stage is in fluid communication with the first purification stage inlet so as to allow a retentate stream from the fourth purification stage to be recycled to the first purification stage. 
     
     
         19 . A method for increasing a recovery of methane in biomethane product gas produced by an existing installation for producing biomethane comprising a source of a biogas, a main compressor, a first purification stage comprising at least one membrane selective for carbon dioxide over methane, a second purification stage comprising at least one membrane selective for carbon dioxide over methane, an inlet of the main compressor being in fluid communication downstream of the source, an inlet of the first purification stage being in fluid communication downstream of an outlet of the main compressor, an inlet of the second purification stage being in fluid communication downstream of a retentate outlet of the first purification stage, a permeate outlet of the second purification stage being in fluid communication with the inlet of the main compressor, said method comprising the steps of:
 providing a high recovery module comprising a secondary compressor and a third purification stage comprising one or more membranes in series or in parallel that are selective for carbon dioxide over methane;   placing an inlet of the secondary compressor in downstream flow communication with a permeate outlet of the first purification stage;   placing an inlet of the third purification stage in downstream flow communication with an outlet of the secondary compressor; and   placing a retentate outlet of the third purification stage in fluid communication with the inlet of the first purification stage.