Process and apparatus for separation of hydrocarbons and nitrogen

Abstract

The invention provides a process and apparatus for the separation of nitrogen from a gaseous feed comprising a mixture of hydrocarbons and nitrogen gas, the process comprising the steps of: (i) cooling and at least partially condensing the gaseous feed; (ii) feeding the cooled and at least partially condensed feed from step (i) to a first fractionation to produce an overhead vapour stream having an enriched nitrogen content and a condensed product having a reduced nitrogen content which is subjected to a second fractionation, which comprises reboil, at a lower pressure than the first fractionation; (iii) partially condensing the overhead vapour stream, and separating to provide a liquid stream, which is used to provide reflux to the first fractionation, and a separated vapour stream, which is condensed to provide reflux to the second fractionation; and (iv) sub-cooling the condensed product of the first fractionation and dividing the resulting sub-cooled product into at least two streams: a first stream being expanded and fed to the second fractionation, and a second stream being expanded and reheated in heat exchange with the separated vapour stream from step (ii) before being fed to the second fractionation; (v) removing a hydrocarbon product stream low in nitrogen from the second fractionation; and (vi) removing a nitrogen rich stream from the second fractionation.

1 . A process for the separation of nitrogen from a gaseous feed comprising a mixture of hydrocarbons and nitrogen gas, the process comprising the steps of:
 (i) cooling and at least partially condensing the gaseous feed;   (ii) feeding the cooled and at least partially condensed feed from step (i) to a first fractionation to produce an overhead vapour stream having an enriched nitrogen content and a condensed product having a reduced nitrogen content which is subjected to a second fractionation, which comprises reboil, at a lower pressure than the first fractionation;   (iii) partially condensing the overhead vapour stream, and separating to provide a liquid stream, which is used to provide reflux to the first fractionation, and a separated vapour stream, which is condensed to provide reflux to the second fractionation; and   (iv) sub-cooling the condensed product of the first fractionation and dividing the resulting sub-cooled product into at least two streams: a first stream being expanded and fed to the second fractionation, and a second stream being expanded and reheated in heat exchange with the separated vapour stream from step (ii) before being fed to the second fractionation;   (v) removing a hydrocarbon product stream low in nitrogen from the second fractionation; and   (vi) removing a nitrogen rich stream from the second fractionation.   
     
     
         2 . A process according to  claim 1 , wherein the hydrocarbons in the gaseous feed comprises methane. 
     
     
         3 . A process according to  claim 1 , wherein the gaseous feed comprises natural gas. 
     
     
         4 . A process according to  claim 1 , wherein the gaseous feed comprises less than 40 mol % nitrogen. 
     
     
         5 . A process according to  claim 4 , wherein the gaseous feed comprises less than 35 mol % nitrogen. 
     
     
         6 . A process according to  claim 5 , wherein the gaseous feed comprises less than 30 mol % nitrogen. 
     
     
         7 . A process according to  claim 1 , wherein the gaseous feed comprises at least 20 mol % nitrogen. 
     
     
         8 . A process according to  claim 1 , wherein reboil in the second fractionation is provided at least in part by heat exchange during cooling of the gaseous feed. 
     
     
         9 . A process according to  claim 1 , wherein the gaseous feed is cooled and at least partially condensed via heat exchange with the hydrocarbon product stream low in nitrogen from the second fractionation and/or the nitrogen rich stream from the second fractionation. 
     
     
         10 . A process according to  claim 9 , wherein the hydrocarbon product stream low in nitrogen is pumped to elevated pressure and evaporated to provide cooling for the gaseous feed. 
     
     
         11 . A process according to  claim 1 , wherein the gaseous feed is expanded prior to the first fractionation. 
     
     
         12 . A process according to  claim 1 , wherein the gaseous feed is split into at least two streams, and each stream is independently processed. 
     
     
         13 . A process according to  claim 12 , wherein the at least two streams are directed to the first fractionation. 
     
     
         14 . A process according to  claim 12 , wherein reboil in the second fractionation is provided at least in part by heat exchange with at least one of the at least two streams. 
     
     
         15 . A process according to  claim 14 , wherein the at least one stream used for heat exchange is fed to an intermediate stage of the first fractionation. 
     
     
         16 . A process according to  claim 15 , wherein at least one of the at least two streams is not used for heat exchange in the second fractionation and is fed to a bottom stage of the first fractionation. 
     
     
         17 . A process according to  claim 1 , wherein reboil in the second fractionation is provided at least in part by heat exchange during the partial condensing of the overhead vapour stream. 
     
     
         18 . A process according to  claim 17 , wherein the heat exchange is between the overhead vapour stream and a liquid product of the second fractionation. 
     
     
         19 . A process according to  claim 1 , wherein the first fractionation is at a pressure in the range of 5 to 30 bar (0.5 to 3.0 MPa). 
     
     
         20 . A process according to  claim 1 , wherein in step (iv) the first stream is expanded to form a two-phase feed for feeding to the second fractionation. 
     
     
         21 . A process according to  claim 1 , wherein in step (iv) the second stream is expanded to form a two-phase feed and reheated via heat exchange for feeding to the second fractionation. 
     
     
         22 . A process according to  claim 21 , wherein the second stream is reheated via heat exchange with the condensed product from the first fractionation and/or the overhead vapour stream from the first fractionation. 
     
     
         23 . A process according to  claim 1 , wherein the first stream in step (iv) is fed to the second fractionation at a higher stage than the second stream. 
     
     
         24 . A process according to  claim 1 , wherein the nitrogen rich stream from the second fractionation is reheated by heat exchange with the overhead vapour stream from step (ii) and/or the separated vapour stream from step (iii). 
     
     
         25 . A process according to  claim 1 , wherein the hydrocarbon product stream is removed as an at least partly liquefied product. 
     
     
         26 . A process according to  claim 1 , wherein the gaseous feed additionally comprises further inert gases. 
     
     
         27 . A process according to  claim 26 , wherein the gaseous feed comprises helium. 
     
     
         28 . A process according to  claim 1 , wherein the gaseous hydrocarbon feed is pre-treated to remove impurities and/or other unwanted components which solidify in the first and second fractionations. 
     
     
         29 . An apparatus for the separation of nitrogen from a gaseous feed comprising a mixture of hydrocarbons and nitrogen gas, the apparatus comprising of:
 (i) means for cooling and at least partially condensing the gaseous feed;   (ii) a first fractionator for producing an overhead vapour stream and a condensed product and a second fractionator operable at a lower pressure than the first fractionator;   (iii) means for conveying the cooled and at least partially condensed feed from step (i) to the first fractionator;   (iv) means for conveying the condensed product from the first fractionator to the second fractionator;   (v) means for partially condensing the overhead vapour stream, and means for separating the partially condensed vapour stream to provide a liquid stream, and a separated vapour stream;   (vi) means for conveying the liquid stream to the first fractionator which is used to provide reflux to the first fractionation, and means for conveying and condensing the separated vapour stream to provide reflux to the second fractionator; and   (vii) means for dividing the condensed product of the first fractionator, prior to entry into the second fractionator, into at least two streams;   (viii) means for expanding a first stream prior to entry thereof into the second fractionator, and means for expanding and reheating a second stream prior to entry thereof into the second fractionator;   (ix) means for conveying a hydrocarbon product low in nitrogen from the second fractionator; and   (x) means for conveying a nitrogen rich stream from the second fractionator.   
     
     
         30 . An apparatus according to  claim 29 , wherein the first and second fractionators are in a stacked configuration. 
     
     
         31 . An apparatus according to  claim 29 , wherein the first and second fractionators are in a non-stacked configuration, allowing reduction on the overall height of the apparatus. 
     
     
         32 . An apparatus according to  claim 29 , wherein the second fractionator comprises a reboil heat exchanger. 
     
     
         33 . An apparatus according to  claim 29 , wherein the means for expanding the streams comprises liquid or two-phase expansion turbines. 
     
     
         34 . An apparatus according to  claim 29  wherein the second stream in step (viii) is reheated in heat exchange with the separated vapour stream from step (v).