Hybrid Membrane/Distillation Method and System for Removing Nitrogen from Methane

Abstract

A hybrid gas separation membrane/cryogenic distillation method and system produces high purity gaseous methane from a gas mixture containing a majority of methane and a minority of nitrogen.

1 . A method of purifying a gas mixture having a majority of methane and a minority of nitrogen, comprising the steps of:
 cooling the gas mixture;   feeding the cooled gas mixture to a gas separation membrane to provide a permeate stream further enriched in methane and a residue stream further enriched in nitrogen;   cooling the residue stream to form a cooled residue stream;   reducing the pressure of the cooled residue stream to provide a nitrogen-enriched vapor and a methane-rich liquid;   condensing the nitrogen-enriched vapor;   feeding the condensed nitrogen-enriched vapor and the methane-rich liquid to a distillation column;   warming gaseous nitrogen withdrawn from a top of the distillation column to provide a gaseous nitrogen product stream;   pressurizing liquid methane withdrawn from a bottom of the distillation column;   vaporizing the pressurized liquid methane to provide a stream of vaporized methane;   warming the stream of vaporized methane;   combining the permeate stream and the stream of warmed vaporized methane to provide a gaseous methane product stream.   
     
     
         2 . The method of  claim 1 , wherein said step of cooling the gas mixture, said step of warming gaseous nitrogen and said step of warming the vaporized liquid methane are performed at a first heat exchanger. 
     
     
         3 . The method of  claim 2 , wherein the gaseous nitrogen withdrawn from the top of the distillation column is further warmed at a second heat exchanger disposed in fluid communication between the distillation column and the first heat exchanger and said step of vaporizing liquid methane is performed at the second heat exchanger. 
     
     
         4 . The method of  claim 3 , further comprising the step of warming the liquid methane withdrawn from a bottom of the distillation column at a third heat exchanger before vaporization thereof, wherein:
 the gaseous nitrogen withdrawn from the top of the distillation column is further warmed at the third heat exchanger before being warmed at the second heat exchanger; and   the condensed nitrogen-enriched vapor and the methane-rich liquid are cooled at the third heat exchanger before being fed to the distillation column.   
     
     
         5 . The method of  claim 1 , wherein said step of condensing the nitrogen-enriched vapor is conducted in a condenser-reboiler operatively associated with the distillation column. 
     
     
         6 . The method of  claim 1 , wherein the gas mixture is natural gas obtained from a subterranean formation. 
     
     
         7 . The method of  claim 6 , wherein the natural gas comprises from about 60 to about 90 mol % methane, up to about 25 mol % nitrogen, and from about 0 to about 10 mol % carbon dioxide. 
     
     
         8 . The method of  claim 7 , wherein amounts of CO 2  and H 2 S are removed from the natural gas prior to feeding it to the gas separation membrane. 
     
     
         9 . The method of  claim 1 , wherein the liquid methane from the distillation column is pressurized with a pump. 
     
     
         10 . The method of  claim 1 , wherein the gas separation membrane is maintained at a temperature lower than −20° C. 
     
     
         11 . The method of  claim 1 , wherein the gas separation membrane is maintained at a temperature of −50 to −90° C. 
     
     
         12 . The method of  claim 1 , wherein the gas separation membrane is made of a material selected from the group consisting of polypropylene oxide allyl glycidyl ether) and silicone rubber [poly(dimethyl siloxane). 
     
     
         13 . The method of  claim 1 , wherein the gas separation membrane is made of a material that has a methane to nitrogen selectivity of at least 5. 
     
     
         14 . The method of  claim 1 , wherein the gaseous methane product stream contains less than 6 mol % N 2  and greater than 94 mol % methane. 
     
     
         15 . The method of  claim 1 , further comprising the step of expanding the gaseous nitrogen product stream and compressing the methane product stream with a turbo expander. 
     
     
         16 . The method of  claim 1 , wherein the gas mixture is landfill gas from a landfill. 
     
     
         17 . A system for purifying a gas mixture having a majority of methane and a minority of nitrogen, comprising:
 a source of a gas mixture comprising a majority of methane and a minority of nitrogen;   a first heat exchanger adapted to cool a stream of said gas mixture;   a gas separation membrane having a feed inlet, a permeate gas outlet, and a residue gas outlet, said feed inlet being in fluid communication with said source via said first heat exchanger;   a distillation column having a top and a bottom, a plurality of inlets, a gaseous nitrogen outlet disposed at said column top, and a liquid methane outlet disposed at said column bottom, said plurality of column inlets being in fluid communication with said residue gas outlet; and   a second heat exchanger adapted to cool a stream of residue gas from said residue gas outlet, warm a stream of gaseous nitrogen withdrawn from said column top, and vaporize a stream of liquid methane withdrawn from said column bottom, wherein said first heat exchanger is further adapted to:
 further warm the stream of gaseous nitrogen warmed at said second heat exchanger; 
 warm a stream of gaseous methane produced by vaporization at said second heat exchanger; and 
 warm a stream of permeate gas from said permeate gas outlet. 
   
     
     
         18 . The system of  claim 17 , further comprising:
 a Joule-Thomson valve in fluid communication between said residue gas outlet and said plurality of column inlets; and   a phase separator comprising an inlet in fluid communication with said Joule-Thomson valve, a vapor outlet, and a liquid outlet, said vapor and liquid outlets being in fluid communication with said plurality of column inlets, said phase separator being adapted to separate a stream of residue gas expanded at said valve into a stream of nitrogen-enriched vapor and a stream of methane-rich liquid.   
     
     
         19 . The system of  claim 18 , further comprising:
 a condenser-reboiler adapted to condense the stream of nitrogen-enriched vapor from said phase separator vapor outlet and vaporize a stream of liquid methane from said column bottom.   
     
     
         20 . The system of  claim 17 , further comprising a third heat exchanger adapted to warm the stream of gaseous nitrogen withdrawn from said column top before warming at said second heat exchanger and warm the stream of liquid methane withdrawn from said column bottom before warming at said second heat exchanger. 
     
     
         21 . The system of  claim 17 , further comprising a gaseous methane product conduit receiving a stream of the permeate gas warmed at said first heat exchanger and a stream of gaseous methane warmed at said first exchanger to provide a stream of gaseous methane product. 
     
     
         22 . The system of  claim 21 , further comprising a turbo expander adapted to expand the stream of gaseous nitrogen warmed at said first heat exchanger and compress the stream of gaseous methane product. 
     
     
         23 . The system of  claim 17 , wherein said gas mixture is natural gas and said source is disposed within a subterranean formation. 
     
     
         24 . The system of  claim 23 , further comprising a purification unit in fluid communication between said source and said gas separation membrane, said purification unit being adapted to remove at least a portion of CO 2  and H 2 S from a stream of said natural gas from said source using adsorption and/or membrane purification techniques. 
     
     
         25 . The system of  claim 17 , further comprising a pump adapted to pump a stream of liquid methane from said column bottom. 
     
     
         26 . The system of  claim 17 , wherein said gas separation membrane is made of a material selected from the group consisting of poly(propylene oxide allyl glycidyl ether) and silicone rubber [poly(dimethyl siloxane). 
     
     
         27 . The system of  claim 17 , wherein said gas separation membrane is made of a material that has a methane to nitrogen selectivity of at least 5. 
     
     
         28 . The system of  claim 17 , wherein said gas mixture is landfill gas and said source is a landfill.