Natural gas liquefaction method with enhanced propane recovery

Abstract

The liquefaction method provides fractionation of the natural gas with ethane recycle in order to enhance propane recovery and to increase the critical pressure of the gas to be liquefied. The natural gas is partly liquefied by cooling in E 1 , then separated in fractionating column 2 into a methane-rich stream and a stream rich in hydrocarbons heavier than methane. The methane-rich stream is partly liquefied in 3 , then the condensates separated in drum 4 are recycled to the top of column 2 through line 7 . The gas fraction from drum 4 is liquefied in exchanger E 2 to produce the liquefied natural gas. The stream rich in hydrocarbons heavier than methane is separated in the deethanizer into an ethane-enriched fraction and heavier hydrocarbons. According to the invention, the ethane-enriched fraction is at least partly liquefied, then part of the liquefied ethane is recycled to separating drum 4 or reflux line 7.

1 ) A natural gas liquefaction method wherein the following stages are carried out:
 a) partly liquefying the natural gas by cooling,   b) feeding the partly liquefied natural gas into a fractionating column so as to obtain a methane-enriched gas fraction and a methane-depleted liquid fraction,   c) cooling the gas fraction up to partial liquefaction, then feeding the cooled gas fraction into a separating drum so as to separate a gas phase and a liquid phase,   d) recycling at least part of the liquid phase to the fractionating column as reflux,   e) separating the liquid fraction so as to obtain an ethane-enriched fraction and at least one fraction enriched in compounds heavier than ethane,   f) recycling at least part of the ethane-enriched fraction by carrying out at least one of the following operations:   feeding said at least part of the ethane-enriched fraction into said separating drum,   prior to stage d), mixing said at least part of the ethane-enriched fraction with said liquid phase,   g) liquefying the gas phase obtained in stage c) by cooling, then by expansion, so as to produce a liquid natural gas.   
     
     
         2 ) A method as claimed in  claim 1 , wherein said ethane-enriched fraction obtained in stage e) comprises at least 90% by mole of ethane. 
     
     
         3 ) A method as claimed in  claim 1 , wherein, in stage f), said at least part of the ethane-enriched fraction is recycled at a flow rate ranging between 5% and 20% of the ethane flow rate of ethane contained in said natural gas. 
     
     
         4 ) A method as claimed in  claim 1 , wherein:
 the fractionating column works at a pressure ranging between 40 bars and 60 bars,   in stage a), the natural gas is cooled to a temperature ranging between 0° C. and −60° C., and   in stage c), the gas fraction is cooled to a temperature ranging between −45° C. and −70° C.   
     
     
         5 ) A method as claimed in  claim 1  wherein, in stage e), the liquid fraction is separated in a deethanization column, said ethane-enriched fraction being obtained at the top of the deethanization column, the fraction enriched in compounds heavier than ethane being obtained at the bottom of the deethanization column. 
     
     
         6 ) A method as claimed in  claim 5 , wherein the ethane-enriched fraction is at least partly liquefied, part of the ethane-enriched liquid fraction being introduced at the top of the deethanization column as reflux, another part of the ethane-enriched liquid fraction being recycled according to stage f). 
     
     
         7 ) A method as claimed in  claim 6 , wherein:
 the deethanization column works at a pressure ranging between 20 and 35 bars, and   said ethane-enriched fraction is at least partly liquefied by cooling to a temperature ranging between −5° C. and 10° C.   
     
     
         8 ) A method as claimed in  claim 1  wherein, in stage e), the liquid fraction is separated in a demethanization column so as to obtain a methane-enriched gas stream and a liquid stream enriched in compounds heavier than methane, then the liquid stream is separated in a deethanization column, said ethane-enriched fraction being obtained at the top of the deethanization column, the fraction enriched in compounds heavier than ethane being obtained in the bottom of the deethanization column. 
     
     
         9 ) A method as claimed in  claim 8 , wherein the ethane-enriched fraction is at least partly liquefied, part of the ethane-enriched liquid fraction being introduced at the top of the deethanization column as reflux, another part of the ethane-enriched liquid fraction being recycled according to stage f). 
     
     
         10 ) A method as claimed in  claim 8 , wherein a portion of the liquid phase obtained in stage c) is fed to the top of the demethanization column as reflux. 
     
     
         11 ) A method as claimed in  claim 8 , wherein:
 the demethanization column works at a pressure ranging between 25 and 40 bars,   the deethanization column works at a pressure ranging between 20 and 35 bars,   said ethane-enriched fraction is at least partly liquefied by cooling to a temperature ranging between −5° C. and 10° C.   
     
     
         12 ) A method as claimed in  claim 9  wherein a portion of the liquid phase obtained in stage c) is fed to the tip of the demethanization column as reflux.