Process for Producing Liquefied Natural Gas

Abstract

A process for liquefying methane-rich gases comprising: (a) providing a stream of methane-rich feed gas containing higher hydrocarbons comprising C 5+ hydrocarbons and/or aromatic compounds at a feed gas pressure of from 40 bar to 120 bar; (b) providing a stream of methane-rich recycle gas at a recycle gas pressure of from 40 bar to 120 bar; (c) mixing the feed gas with a first part of the recycle gas to form a mixture; (d) passing the resulting mixture to a first gas expander having an outlet, the first expander outlet having a first gas expander outlet pressure of between 3 bar and 50 bar and less than the feed gas and recycle gas pressures, to form a first gas expander outlet stream comprising a mixture of vapor and a condensed liquid containing the higher hydrocarbons; (e) separating the first gas expander outlet stream into a liquid stream and a vapor stream; (f) reheating and compressing the first vapor stream to a first vapor stream pressure of from 40 bar to 120 bar to form a first constituent of the recycle gas; (g) cooling a second part of the recycle gas to a temperature higher than an outlet temperature of the first gas expander; and, (h) liquefying said cooled second part of the recycle gas to form liquefied methane, wherein a content of C5+ hydrocarbons is about 0.1 mol % or less in the liquefied methane and a content of aromatic compounds is below 1 mol ppm in the liquefied methane. In an embodiment, the cooled second part of the recycle gas is completely or substantially liquefied in step (h). In another embodiment, the second part of the recycle gas is liquefied to form liquefied methane and a second vapor stream in step (h), and the second vapor stream is reheated and compress in a step (i) to a second vapor stream pressure of from 40 bar to 120 bar to form a second constituent of the recycle gas.

1 . A process for liquefying methane-rich gases, the process comprising:
 (a) providing a stream of methane-rich feed gas containing higher hydrocarbons comprising C 5+  hydrocarbons and/or aromatic compounds at a feed gas pressure of from 40 bar to 120 bar;   (b) providing a stream of methane-rich recycle gas at a recycle gas pressure of from 40 bar to 120 bar;   (c) mixing the feed gas with a first part of the recycle gas to form a mixture;   (d) passing the resulting mixture to a first gas expander having an outlet, the first gas expander outlet having a first gas expander outlet pressure of between 3 bar and 50 bar and less than said feed gas and recycle gas pressures, to form a first gas expander outlet stream comprising a mixture of vapor and a condensed liquid containing said higher hydrocarbons;   (e) separating the first gas expander outlet stream into a liquid stream and a first vapor stream;   (f) reheating and compressing said first vapor stream to a first vapor stream pressure of from 40 bar to 120 bar to form a first constituent of said recycle gas;   (g) cooling a second part of said recycle gas to a temperature higher than an outlet temperature of said first gas expander;   (h) liquefying said cooled second part of the recycle gas to form liquefied methane and a second vapor stream, wherein a content of C 5+  hydrocarbons is about 0.1 mol % or less in the liquefied methane and a content of aromatic compounds is below 1 mol ppm in the liquefied methane; and,   (i) reheating and compressing said second vapor stream to a second vapor stream pressure of from 40 bar to 120 bar to form a second constituent of said recycle gas.   
     
     
         2 . The process according to  claim 1 , comprising cooling the mixture of feed gas and the first part of the recycle gas in a heat exchanger before passing the mixture to the first gas expander. 
     
     
         3 . The process according to  claim 1 , comprising heating or cooling the first gas expander outlet stream in a heat exchanger prior to separation to modify the quantity of said higher hydrocarbons in the liquid. 
     
     
         4 . The process as claimed in  claim 1 , wherein the methane-rich feed gas pressure and the methane-rich recycle gas pressure are each from 50 bar to 100 bar and/or the first gas expander pressure is from 5 bar to 30 bar. 
     
     
         5 . The process as claimed in  claim 1 , comprising at least partially cooling the second part of the recycle gas by heat exchange with the second vapor stream prior to compressing said second vapor stream. 
     
     
         6 . The process as claimed in  claim 1 , wherein the methane-rich gas is natural gas. 
     
     
         7 . A process for liquefying methane-rich gases, the process comprising:
 (a) providing a stream of methane-rich feed gas containing higher hydrocarbons comprising C 5+  hydrocarbons and/or aromatic compounds at a feed gas pressure of from 40 bar to 120 bar;   (b) providing a stream of methane-rich recycle gas at a recycle gas pressure of from 40 bar to 120 bar;   (c) mixing the feed gas with a first part of the recycle gas to form a mixture;   (d) passing the resulting mixture to a first gas expander having an outlet, the first gas expander outlet having a first gas expander outlet pressure of between 3 bar and 50 bar and less than the feed gas and recycle gas pressures, to form a first gas expander outlet stream comprising a mixture of vapor and a condensed liquid containing said higher hydrocarbons;   (e) separating the first gas expander outlet stream into a liquid stream and a first vapor stream;   (f) reheating and compressing said first vapor stream to a vapor stream pressure of from 40 bar to 120 bar to form a first constituent of said recycle gas;   (g) cooling a second part of said recycle gas to a temperature higher than an outlet temperature of said first gas expander; and,   (h) completely or substantially liquefying said cooled second part of the recycle gas to form liquefied methane, wherein a content of C 5+  hydrocarbons is about 0.1 mol % or less in the liquefied methane and a content of aromatic compounds is below 1 mol ppm in the liquefied methane.   
     
     
         8 . The process according to  claim 7 , comprising cooling the mixture of feed gas and the first part of the recycle gas in a heat exchanger before passing the mixture to the first gas expander. 
     
     
         9 . The process according to  claim 7 , comprising heating or cooling the first gas expander outlet stream in a heat exchanger prior to separation to modify the quantity of said higher hydrocarbons in the liquid. 
     
     
         10 . The process as claimed in  claim 7 , wherein the methane-rich feed gas pressure and the methane-rich recycle gas pressure are each from 50 bar to 100 bar and/or the first gas expander pressure is from 5 bar to 30 bar. 
     
     
         11 . The process as claimed in  claim 7 , comprising at least partially cooling the second part of the recycle gas by heat exchange with the second vapor stream prior to compressing said second vapor stream. 
     
     
         12 . The process as claimed in  claim 7 , wherein the methane-rich feed gas is natural gas.