Method of rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream and an apparatus therefor

Abstract

Method of, and apparatus for, rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream. A hydrocarbon stream is at least partially liquefied and subsequently expanded. The expanded hydrocarbon stream is fractionated in a fractionation column to provide an nitrogen-rich hydrocarbon stream and a nitrogen-lean hydrocarbon stream. The nitrogen-rich hydrocarbon stream is partially condensed in a condenser by cooling against a refrigerant circulated in a dedicated first refrigerant circuit, and phase-separated to provide a nitrogen-rejection stream and a nitrogen-lean reflux stream which is returned to the fractionation column. The nitrogen-lean hydrocarbon stream is partially vaporized and phase-separated to provide a vapour stream that is returned to the fractionation column and a liquefied nitrogen-lean hydrocarbon stream that is subjected to sub-cooling. The fuel gas stream is generated from the sub-cooled nitrogen-lean hydrocarbon stream.

1 . A method of rejecting nitrogen from a hydrocarbon stream to provide a fuel gas stream comprising at least the steps of:
 (a) at least partly liquefying a hydrocarbon stream in a heat exchanger to provide a cooled hydrocarbon stream;   (b) expanding at least a part of the cooled hydrocarbon stream in a first expansion device to provide an expanded hydrocarbon stream;   (c) fractionating the expanded hydrocarbon stream in a fractionation column to provide an upper nitrogen-rich hydrocarbon stream and a lower nitrogen-lean hydrocarbon stream;   (d) condensing the upper nitrogen-rich hydrocarbon stream in a condenser by cooling against an expanded first refrigerant stream in a dedicated first refrigerant circuit to provide a partially condensed nitrogen-rich hydrocarbon stream and a heated first refrigerant stream;   (e) separating the partially condensed nitrogen-rich hydrocarbon stream in a first separator to provide an upper nitrogen-rejection stream and a lower nitrogen-lean reflux stream which is returned to the fractionation column;   (f) heating the lower nitrogen-lean hydrocarbon stream from the fractionation column in a reboiler against a first refrigerant feed stream in the first refrigerant circuit to provide a partially vapourised nitrogen-lean hydrocarbon stream and a cooled first refrigerant stream;   (g) separating the partially vapourised nitrogen-lean hydrocarbon stream in a second separator to provide an upper second separator vapour stream which is returned to the fractionation column, and a lower liquefied nitrogen-lean hydrocarbon stream;   (h) sub-cooling the lower liquefied nitrogen-lean hydrocarbon stream in a heat exchanger to provide a sub-cooled nitrogen-lean hydrocarbon stream; and   (i) generating a fuel gas stream from the sub-cooled nitrogen-lean hydrocarbon stream.   
     
     
         2 . The method of  claim 1  wherein step (a) comprises fully liquefying the hydrocarbon stream in a first heat exchanger to provide a cooled hydrocarbon stream. 
     
     
         3 . The method of  claim 1  wherein in step (i) comprises:
 (1) expanding the sub-cooled nitrogen-lean hydrocarbon steam in a second expansion device to provide an expanded nitrogen-lean hydrocarbon stream; and 
 (2) separating the expanded nitrogen-lean hydrocarbon stream in a third separator to provide the fuel gas stream and a liquefied hydrocarbon steam. 
 
     
     
         4 . The method of  claim 1  wherein step (i) comprises:
 (1) cooling the sub-cooled nitrogen-lean hydrocarbon stream in a third heat exchanger to provide a pre-cooled nitrogen-lean hydrocarbon stream; 
 (2) expanding the pre-cooled nitrogen-lean hydrocarbon stream in a third expansion device to provide an expanded nitrogen-lean hydrocarbon stream; and 
 (3) separating the expanded nitrogen-lean hydrocarbon stream in an endflash unit to provide the fuel gas stream and a liquefied hydrocarbon stream, preferably a LNG stream. 
 
     
     
         5 . The method of  claim 4  wherein step (i) further comprises:
 (4) withdrawing an intermediate fraction of the nitrogen-lean hydrocarbon stream from the endflash unit and passing this to the third heat exchanger; 
 (5) heating the intermediate fraction of the nitrogen-lean hydrocarbon stream against the sub-cooled nitrogen-lean hydrocarbon stream to provide the pre-cooled nitrogen-lean hydrocarbon stream and a heated intermediate fraction nitrogen-lean hydrocarbon stream; and 
 (6) passing the heated intermediate fraction nitrogen-lean hydrocarbon stream to the end flash unit. 
 
     
     
         6 . The method of  claim 1  comprising the further step of heat exchanging the nitrogen-rejection stream from the first separator against one or more of the group consisting of: the sub-cooled nitrogen-lean hydrocarbon stream, the hydrocarbon stream and a refrigerant stream from the first or second heat exchanger. 
     
     
         7 . The method of  claim 1  wherein the first refrigerant stream in the first refrigerant circuit comprises a nitrogen. 
     
     
         8 . The method of  claim 1  wherein the first refrigerant circuit is a partially open circuit comprising a first refrigerant which is drawn from the upper nitrogen-rejection stream. 
     
     
         9 . The method of  claim 1  further comprising the steps of: heat exchanging the cooled first refrigerant stream against the heated first refrigerant stream in a fourth heat exchanger to provide an expander refrigerant feed stream and a compressor refrigerant feed stream; and expanding an expander refrigerant feed stream in a fourth expansion device to provide the expanded first refrigerant stream. 
     
     
         10 . The method of  claim 9  further comprising the steps of: compressing the compressor refrigerant feed stream in a first compressor to provide a compressed refrigerant steam. 
     
     
         11 . The method of  claim 10  further comprising cooling the compressed refrigerant stream in a cooling device to provide the first refrigerant feed stream. 
     
     
         12 . The method of  claim 10  further comprising cooling the compressed refrigerant stream in a cooling device to provide a cooled compressed refrigerant stream; and heat exchanging the cooled compressed refrigerant stream in a fifth heat exchanger to provide the first refrigerant feed stream. 
     
     
         13 . The method of  claim 12  wherein the cooled compressed refrigerant stream is heat exchanged against a second refrigerant in a second refrigerant circuit. 
     
     
         14 . The method of  claim 12  further comprising providing a fifth heat exchanger bypass line from the cooled compressed refrigerant stream to the first refrigerant feed stream, the fifth heat exchanger bypass line containing a fifth heat exchanger bypass line valve. 
     
     
         15 . A method of controlling the nitrogen concentration present in a fuel gas stream, the method comprising at least the steps of:
 bypassing the fifth heat exchanger in the method of  claim 14 ; and   controlling the fifth heat exchanger bypass valve to effect the relative proportion of the cooled compressed refrigerant stream being cooled in the fifth heat exchanger.   
     
     
         16 . An apparatus for the rejection of nitrogen from a hydrocarbon stream to provide a fuel gas stream, the apparatus comprising at least:
 a first heat exchanger having a first inlet for a hydrocarbon stream and a first outlet for a cooled hydrocarbon stream, the first outlet of the heat exchanger;   a first expansion device having an inlet connected to the first outlet of the first heat exchanger, and an outlet for an expanded hydrocarbon stream;   a fractionation column having a first inlet connected to the outlet of the first expansion device, and a first outlet for an upper nitrogen-rich hydrocarbon stream, a second outlet for a lower nitrogen-lean hydrocarbon stream, a second inlet for a lower nitrogen-lean reflux stream and a third inlet for an upper second separator vapour stream;   a condenser having a first inlet connected to the first outlet of the fractionation column, and a first outlet for a partially condensed nitrogen-rich hydrocarbon stream and a second inlet for an expanded first refrigerant stream and a second outlet for a heated first refrigerant stream;   a first separator having a first inlet connected to the first outlet of the condenser and a first outlet for an upper nitrogen-rejection stream and a second outlet for a lower nitrogen-lean reflux stream, said second outlet connected to the second inlet of the fractionation column;   a reboiler having a first inlet connected to the second outlet of the fractionation column, a first outlet for a partially vapourised nitrogen-lean hydrocarbon stream, a second inlet for a first refrigerant feed stream and a second outlet for a cooled first refrigerant stream;   a second separator having a first inlet connected to the first outlet of the reboiler, a first outlet for an upper second separator vapour stream and a second outlet for a lower liquefied nitrogen-lean hydrocarbon stream, said second outlet connected to the third inlet of the fractionation column; and   a second heat exchanger, which can be the first heat exchanger or a different heat exchanger, said second heat exchanger having a first inlet connected to the second outlet of the second separator and a first outlet for a sub-cooled nitrogen-lean hydrocarbon stream.   
     
     
         17 . The apparatus of  claim 16  further comprising:
 a fourth heat exchanger having a first inlet connected to the second outlet of the reboiler, a first outlet for an expander refrigerant feed stream, a second inlet connected to the second outlet of the condenser, and a second outlet for a compressor refrigerant feed stream; 
 a first compressor having an inlet connected to the second outlet of the fourth heat exchanger and an outlet for a compressed refrigerant stream; 
 a cooling device having an inlet connected to the outlet of the first compressor and outlet for a cooled compressed refrigerant stream; 
 a fifth heat exchanger having an inlet connected to the outlet of the cooling device and an outlet connected to the second inlet of the reboiler; and 
 a fourth expansion device having an inlet connected to the first outlet of the fourth heat exchanger and an outlet connected to the second inlet of the condenser.