Nitrogen recovery apparatus and method of recovering nitrogen

Abstract

A nitrogen recovery apparatus for recovering nitrogen from natural gas comprises a separator having a liquid fraction port and a vapour fraction port in fluid communication with a split flow arrangement, the split flow arrangement having a sub-cooled fluid path and an expanded fluid path. A fractionating column has a reflux inlet port in fluid communication with the subcooled fluid path above a middle feed port thereof, the middle feed port being in fluid communication with the expanded fluid path. A bottom feed port of the fractionating column is in fluid communication with the liquid fraction port of the separator. A side reboiler circuit and a reboiler circuit are operably coupled to the fractionating column below the bottom feed port. A bottom hydrocarbon product stream path is in fluid communication with a bottom hydrocarbon port of the fractionating column.

1 . A nitrogen recovery apparatus ( 100 ) for recovering nitrogen from natural gas, the apparatus ( 100 ) comprising:
 a separator ( 110 ) having a liquid fraction port and a vapour fraction port in fluid communication with a split flow arrangement ( 112 ), the split flow arrangement ( 112 ) having a sub-cooled fluid path ( 118 ) and an expanded fluid path ( 120 );   a fractionating column ( 116 ) having a reflux inlet port ( 124 ) in fluid communication with the subcooled fluid path ( 118 ) above a middle feed port ( 130 ) thereof, the middle feed port ( 130 ) being in fluid communication with the expanded fluid path ( 120 );   a bottom feed port ( 114 ) of the fractionating column ( 116 ) in fluid communication with the liquid fraction port of the separator ( 110 );   a side reboiler circuit ( 132 ) operably coupled to the fractionating column ( 116 ) below the bottom feed port ( 114 );   a reboiler circuit ( 140 ) operably coupled to the fractionating column ( 116 ) below the side boiler circuit ( 132 ); and   a bottom hydrocarbon product stream path ( 160 ) in fluid communication with a bottom hydrocarbon port ( 162 ) of the fractionating column ( 116 ).   
     
     
         2 . An apparatus as claimed in  claim 1 , wherein the separator ( 110 ) comprises a feed inlet port for receiving partially liquefied natural gas. 
     
     
         3 . An apparatus as claimed in  claim 2 , further comprising:
 a heat exchanger ( 108 ) having a natural gas feed inlet port, the heat exchanger ( 108 ) also having an output port; wherein   the output port is in fluid communication with the feed inlet port of the separator ( 110 ).   
     
     
         4 . An apparatus as claimed in  claim 1 , wherein the fractionating column ( 116 ) further comprises:
 an overhead gas stream path ( 148 ) arranged to support sub-cooling in the subcooled fluid path ( 118 ).   
     
     
         5 . An apparatus as claimed in  claim 4 , wherein the overhead gas stream path ( 148 ) is also in fluid communication with the heat exchanger ( 108 ). 
     
     
         6 . An apparatus as claimed in  claim 1 , further comprising:
 a reflux condenser stream circuit ( 184 ) which is fed by a drawn off from the fractionating column ( 116 ) from below the middle feed ( 130 ), the reflux condenser stream circuit ( 184 ) being configured to provide cooling in the subcooled fluid path ( 118 ).   
     
     
         7 . An apparatus as claimed in  claim 3 , further comprising:
 a sub-cooler ( 122 ); wherein   the sub-cooled fluid path ( 118 ) is arranged to pass through the sub-cooler ( 122 ).   
     
     
         8 . An apparatus as claimed in  claim 1 , further comprising:
 an expander ( 128 ); wherein   the expanded fluid path ( 120 ) is arranged to pass though the expander ( 128 ).   
     
     
         9 . An apparatus as claimed in  claim 8 , wherein the expander ( 128 ) is arranged to drive a first compressor ( 129 ). 
     
     
         10 . An apparatus as claimed in  claim 9 , wherein the fractionating column further comprises:
 a nitrogen product fraction tapping point ( 190 ) disposed above the reflux inlet port ( 124 ) and operably coupled to a reflux circuit comprising the first compressor ( 128 ), the reflux circuit returning to the fractionating column ( 116 ) above the nitrogen product fraction tapping point ( 190 ).   
     
     
         11 . An apparatus as claimed in  claim 10 , further comprising the sub-cooler ( 122 ) in the reflux circuit. 
     
     
         12 . An apparatus as claimed in  claim 4 , further comprising:
 a nitrogen return compression and cooling arrangement ( 151 ) disposed in the overhead gas stream path ( 148 ).   
     
     
         13 . An apparatus as claimed in  claim 12 , further comprising:
 an expander ( 128 ), wherein the expanded fluid path ( 120 ) is arranged to pass through the expander ( 128 ); and   wherein the expander ( 128 ) is arranged to drive at least part of the nitrogen return compression and cooling arrangement ( 151 ).   
     
     
         14 . A hydrocarbon capture and recovery system comprising:
 the nitrogen recovery apparatus ( 100 ) as claimed in  claim 1 ; and   a return fluid path fluidly coupling an output port of a nitrogen return compression and cooling arrangement ( 151 ) to a wellbore for returning nitrogen recovered from a natural gas feed and the wellbore under pressure.   
     
     
         15 . A method of recovering nitrogen from natural gas, the method comprising:
 separating ( 202 ) a partially liquefied natural gas feed into a vapour fraction and a liquid fraction;   splitting ( 206 ) the flow of the vapour fraction to form a first portion and a second portion of vapour fraction;   subcooling ( 208 ) the first portion of the vapour fraction using an overhead gas stream ( 148 ) of a fractionating column ( 116 ) to form a reflux stream, the reflux stream being applied to the fractionating column ( 116 );   expanding ( 210 ) the second portion of the vapour fraction to form a middle feed to the fractionating column ( 116 ) below the reflux stream;   feeding ( 204 ) the liquid fraction as a bottom feed to the fractionating column ( 116 );   a side-reboiler circuit ( 132 ) feeding the fractionating column ( 116 ) below the bottom feed; and   a reboiler circuit ( 140 ) feeding the fractionating column ( 116 ) below the side reboiler circuit.