Combined removal of heavies and lights from natural gas

Abstract

A process is described for removing heavies and lights from a hydrocarbon-rich feed fraction. A heavies-rich liquid is rectificatorily removed (1st removal stage) from a partially condensed feed fraction. Heavies-depleted gas is partially condensed and rectificatorily separated into a methane-rich liquid fraction and a lights-rich gas fraction (2nd removal stage). The 1st removal stage is operated at a pressure of at least 25 bar. The heavies-depleted gas fraction does not undergo pressure elevation before being fed into the 2nd removal stage. Reflux for the 2nd removal stage is produced via an open loop refrigeration cycle. Refrigerant circulating in the open loop refrigeration cycle is vaporized to two different temperature levels against the reflux streams in a head condenser and a side condenser of the 2nd removal stage. The pressure of refrigerant vaporized in the side condenser is at least three times the pressure of refrigerant vaporized in the head condenser.

1 . A process for removing heavies and lights from a hydrocarbon-rich feed fraction, preferably from natural gas, said process comprising:
 a) partially condensing (E 1 , E 2 ) said feed fraction ( 1 ),   b) rectificatorily removing a heavies-rich liquid fraction ( 8 ) in a 1st removal stage (T 1 ),   c) partially condensing (E 4 ) the resultant, heavies-depleted gas fraction ( 10 ), and   d) rectificatorily separating said heavies-depleted gas fraction ( 10 ) into a methane-rich liquid fraction ( 11 ) and a lights-rich gas fraction ( 12 ) in a 2nd removal stage (T 2 ),   e) wherein the 1st removal stage (T 1 ) is operated at a pressure of at least 25 bar,   f) wherein said heavies-depleted gas fraction ( 10 ) does not undergo pressure elevation before being fed into the 2nd removal stage (T 2 ),   g) wherein reflux for the 2nd removal stage (T 2 ) is produced via an open loop refrigeration cycle,   h) wherein the refrigerant circulating in said open loop refrigeration cycle is vaporized to two different temperature levels against reflux streams ( 14 ,  15 ) in head condenser (E 7 ) and a side condenser (E 8 ) of the 2nd removal stage (T 2 ), and   i) wherein the pressure of the refrigerant ( 25 ) vaporized in said side condenser (E 8 ) is at least three times the pressure of the refrigerant ( 22 ) vaporized in said head condenser (E 7 ).   
     
     
         2 . The process according to  claim 1 , wherein said 2nd removal stage (T 2 ) is realized in a column having a dividing wall (T), wherein the dividing wall (T) is at least disposed in that region of the column where the column is fed with said heavies-depleted gas fraction ( 10 ) and where a carbon dioxide-lean fraction ( 25 ), which is admixed to the open loop refrigeration cycle, is withdrawn. 
     
     
         3 . The process according to  claim 1 , wherein the refrigerant of the open loop refrigeration cycle has a methane content of at least 80 mol %, preferably at least 85 mol %. 
     
     
         4 . The process according to  claim 2 , wherein the composition of the refrigerant of the open loop cooling cycle corresponds essentially to the composition of the carbon dioxide-lean fraction ( 25 ) withdrawn from the 2nd removal stage (T 2 ). 
     
     
         5 . The process according to  claim 1 , wherein said 1st removal stage is operated at a pressure of at least 28 bar. 
     
     
         6 . The process according to  claim 1 , wherein the pressure of refrigerant ( 25 ) vaporized in said side condenser (E 8 ) is at least five times the pressure of refrigerant ( 22 ) vaporized in said head condenser (E 7 ). 
     
     
         7 . The process according to  claim 1 , wherein the pressure of the hydrocarbon-rich feed fraction ( 1 ) is more than 50 bar, and said feed fraction ( 1 ) is expanded in at least two stages (X 1 , X 2 ) before being fed into said 1st removal stage (T 1 ). 
     
     
         8 . The process according to  claim 1 , wherein a substream of said heavies-depleted gas fraction ( 10 ) obtained in said 1st removal stage (T 1 ) is at least temporarily admixed ( 50 ) with the methane-rich liquid fraction ( 11 ) obtained in said 2nd removal stage (T 2 ). 
     
     
         9 . A process according to  claim 1 , wherein said hydrocarbon-rich feed fraction is natural gas. 
     
     
         10 . The process according to  claim 1 , wherein the refrigerant of the open loop refrigeration cycle has a methane content of at least 85 mol %. 
     
     
         11 . The process according to  claim 1 , wherein the pressure of the hydrocarbon-rich feed fraction ( 1 ) is between 35 and 50 bar, and said feed fraction ( 1 ) is expanded in a single stage (X 1 ) before being fed into said 1st removal stage (T 1 ). 
     
     
         12 . The process according to  claim 1 , wherein said methane-rich liquid fraction ( 11 ) is expanded (V 7 ) to a pressure from 3 to 15 bar below the operating pressure of said 1st removal stage (T 1 ). 
     
     
         13 . The process according to  claim 12 , wherein said methane-rich liquid fraction ( 11 ) is expanded (V 7 ) to a pressure from 5 to 10 bar below the operating pressure of said 1st removal stage (T 1 ). 
     
     
         14 . The process according to  claim 2 , wherein the carbon dioxide concentration in said carbon dioxide-lean fraction ( 25 ) is below 50 vppm. 
     
     
         15 . The process according to  claim 2 , wherein the carbon dioxide concentration in said carbon dioxide-lean fraction ( 25 ) is below 5 vppm. 
     
     
         16 . The process according to  claim 4 , wherein the carbon dioxide concentration in said carbon dioxide-lean fraction ( 25 ) is below 50 vppm. 
     
     
         17 . The process according to  claim 4 , wherein the carbon dioxide concentration in said carbon dioxide-lean fraction ( 25 ) is below 5 vppm. 
     
     
         18 . The process according to  claim 1 , wherein said lights-rich gas fraction ( 12 ) has a nitrogen content of more than 90 mol %. 
     
     
         19 . The process according to  claim 18 , wherein said lights-rich gas fraction ( 12 ) has a nitrogen content of more than 95 mol %.