Process and apparatus for separation of hydrocarbons and nitrogen

Abstract

There is provided a process for the separation of a gaseous feed comprising a mixture of nitrogen, hydrocarbons and at least 0.005 mol % carbon dioxide, the process comprising: (i) cooling and at least partially condensing the gaseous feed, and (ii) separating in one or more stages the cooled and at least partially condensed gaseous feed into a hydrocarbon rich product stream low in nitrogen and a nitrogen rich reject stream low in hydrocarbons, and wherein refrigeration is provided to one or more stages of the separation process by a heat pump system in which a heat pump refrigerant fluid is compressed and subsequently expanded at one or more pressure levels below the condensing pressure, and subsequently heated in heat exchange with the gaseous feed and/or one or more streams generated by the separation process to provide refrigeration thereto; and further wherein at least part of the heated refrigerant is recycled through the heat pump system. There is also provided an apparatus for the separation of a gaseous feed comprising a mixture of nitrogen, hydrocarbons and at least 0.005 mol % carbon dioxide.

1 . A process for the separation of a gaseous feed comprising a mixture of nitrogen, hydrocarbons and at least 0.005 mol % carbon dioxide, the process comprising:
 (i) cooling and at least partially condensing the gaseous feed, and   (ii) separating in one or more stages the cooled and at least partially condensed gaseous feed into a hydrocarbon rich product stream low in nitrogen and a nitrogen rich reject stream low in hydrocarbons,   
       and wherein refrigeration is provided to one or more stages of the separation process by a heat pump system in which a heat pump refrigerant fluid is compressed and subsequently expanded at one or more pressure levels below the condensing pressure, and subsequently heated in heat exchange with the gaseous feed and/or one or more streams generated by the separation process to provide refrigeration thereto; 
       and further wherein at least part of the heated refrigerant is recycled through the heat pump system. 
     
     
         2 . A process according to  claim 1 , wherein the hydrocarbons in the gaseous feed comprise or consist of methane. 
     
     
         3 . A process according to  claim 1 , wherein the gaseous feed comprises or consists of natural gas. 
     
     
         4 . A process according to any preceding claim, wherein the gaseous feed comprises less than 35 mol % nitrogen gas. 
     
     
         5 . A process according to any preceding claim, wherein the gaseous feed comprises from 5 mol % to 25 mol % nitrogen gas. 
     
     
         6 . A process according to any preceding claim, wherein the gaseous feed comprises from 0.01 mol % to 4.0 mol % carbon dioxide. 
     
     
         7 . A process according to any preceding claim, wherein the heat pump refrigerant fluid comprises less than 0.02 mol % carbon dioxide. 
     
     
         8 . A process according to any preceding claim, wherein, after expansion, the heat pump refrigerant is at a temperature of −100° C. or lower. 
     
     
         9 . A process according to any preceding claim, wherein the heat pump refrigerant fluid is sub-cooled prior to expansion to reduce evolution of flash vapour. 
     
     
         10 . A process according to any preceding claim, comprising an additional, intermediate stage, in which the cooled and at least partially condensed gaseous feed is separated into a first intermediate stream with a nitrogen content lower than that of the gaseous feed and a second intermediate stream with a nitrogen content higher than that of the gaseous feed. 
     
     
         11 . A process according to  claim 10 , wherein the cooled and at least partially condensed gaseous feed is separated into the first intermediate stream and the second intermediate stream in a fractionation column. 
     
     
         12 . A process according to  claim 11 , further comprising reducing the levels of components having limited solubility at lower operating temperatures, such as carbon dioxide, in the second intermediate stream. 
     
     
         13 . A process according to any of  claims 10  to  12 , wherein the first intermediate stream, containing substantially all of the carbon dioxide removed from the gaseous feed, is evaporated at a pressure and temperature selected to avoid solidification of carbon dioxide in the stream. 
     
     
         14 . A process according to  claim 13 , wherein at least a portion of the first intermediate stream is pumped and evaporated at a pressure above the operating pressure of the fractionation column. 
     
     
         15 . A process according to any of  claims 10  to  14 , wherein the gaseous feed undergoes one or more stages of partial condensation and vapour/liquid separation before separation into the first and second intermediate streams. 
     
     
         16 . A process according to any of  claims 10  to  15 , wherein the second intermediate stream is subjected to downstream separation to produce a hydrocarbon rich stream very low in nitrogen and the nitrogen rich reject stream low in hydrocarbons. 
     
     
         17 . A process according to any preceding claim, wherein the heat pump system operates in a closed cycle, so that substantially all of the heated refrigerant fluid is recycled through the heat pump system. 
     
     
         18 . A process according to any of  claims 1  to  16 , wherein the heat pump refrigerant fluid comprises at least a portion of one or more streams obtained from the gaseous feed in the separation process comprising less than 0.02 mol % carbon dioxide. 
     
     
         19 . A process according to any of  claims 10  to  16 , wherein the heat pump refrigerant fluid comprises at least a portion of the second intermediate stream. 
     
     
         20 . A process according to  claim 16 , wherein the heat pump refrigerant fluid comprises at least a portion of the hydrocarbon rich stream very low in nitrogen. 
     
     
         21 . An apparatus for the separation of a gaseous feed comprising a mixture of nitrogen, hydrocarbons and at least 0.005 mol % carbon dioxide, the apparatus comprising:
 (i) means for cooling and at least partially condensing the gaseous feed,   (ii) means for separating in one or more stages the cooled and at least partially condensed gaseous feed into a hydrocarbon rich product stream low in nitrogen and a nitrogen rich reject stream low in hydrocarbons, and   (iii) a heat pump system in which a heat pump refrigerant fluid is compressed and subsequently expanded at one or more pressure levels below the condensing pressure and subsequently heated in heat exchange with the gaseous feed and/or one or more streams generated by the separation of the gaseous feed, to provide refrigeration to one or more stages in the separation process, and wherein at least part of the heated refrigerant fluid is recycled through the heat pump system.   
     
     
         22 . An apparatus according to  claim 21 , wherein the means to expand the heat pump refrigerant fluid comprises one or more liquid or two-phase expansion turbines. 
     
     
         23 . An apparatus according to  claim 21  or  claim 22 , wherein the means to compress the heat pump refrigerant fluid comprises a multistage compressor with inter-cooling and after-cooling. 
     
     
         24 . An apparatus according to any of  claims 21  to  23 , wherein the apparatus comprises means for separating the gaseous feed into a first intermediate stream with a nitrogen content lower than that of the gaseous feed and a second intermediate stream with a nitrogen content higher than that of the gaseous feed. 
     
     
         25 . An apparatus according to  claim 24 , wherein the means for separating the gaseous feed into the first intermediate stream and the second intermediate stream comprises a fractionation column. 
     
     
         26 . An apparatus according to  claim 25 , wherein the fractionation column includes a reboil heat exchanger. 
     
     
         27 . An apparatus according to  claim 25  or  claim 26 , wherein the fractionation column further incorporates an overhead condenser and reflux drum above the column feed. 
     
     
         28 . An apparatus according to any of  claims 25  to  27 , further comprising means for partial condensation and vapour/liquid separation upstream of the fractionation column. 
     
     
         29 . An apparatus according to any of  claims 24  to  28 , further comprising means to process the second intermediate stream to produce a hydrocarbon rich stream very low in nitrogen and the nitrogen rich reject stream low in hydrocarbons. 
     
     
         30 . An apparatus according to any of  claims 21  to  29 , wherein the heat pump system is a closed cycle, so that substantially all of the heated refrigerant fluid is recycled through the heat pump system. 
     
     
         31 . An apparatus according to any of  claims 21  to  29 , wherein the heat pump refrigerant fluid comprises at least a portion of one or more streams obtained from the gaseous feed in the separation process comprising less than 0.02 mol % carbon dioxide. 
     
     
         32 . An apparatus according to any of  claims 24  to  29 , wherein the heat pump refrigerant fluid comprises at least a portion of the second intermediate stream. 
     
     
         33 . An apparatus according to  claim 29 , wherein the heat pump refrigerant fluid comprises at least a portion of the hydrocarbon rich stream very low in nitrogen.