System and method for liquefaction of natural gas

Abstract

By using the power generated by an expander by an expansion of material gas, the outlet pressure of a compressor is increased, and a requirement on the cooling capacity of a cooler is reduced. The liquefaction system ( 1 ) for natural gas comprises a first expander ( 3 ) for generating power by using natural gas under pressure as material gas; a first cooling unit ( 11, 12 ) for cooling the material gas depressurized by expansion in the first expander; a distillation unit ( 15 ) for reducing or eliminating a heavy component in the material gas by distilling the material gas cooled by the first cooling unit; a first compressor ( 4 ) for compressing the material gas from which the heavy component was reduced or eliminated by the distillation unit by using power generated in the first expander; and a liquefaction unit ( 21 ) for liquefying the material gas compressed by the first compressor by exchanging heat with a refrigerant.

1 . A system for the liquefaction of natural gas that cools the natural gas to produce liquefied natural gas, comprising:
 a first expander for generating power by expanding natural gas under pressure as material gas;   a first cooling unit for cooling the material gas depressurized by expansion in the first expander;   a distillation unit for reducing or eliminating a heavy component in the material gas by distilling the material gas cooled by the first cooling unit;   a first compressor for compressing the material gas from which the heavy component was reduced or eliminated by the distillation unit by using the power generated in the first expander; and   a liquefaction unit for liquefying the material gas compressed by the first compressor by exchanging heat with a refrigerant.   
     
     
         2 . The system for the liquefaction of natural gas according to  claim 1 , further comprising a second cooling unit placed between the first compressor and the liquefaction unit to cool the material gas compressed by the first compressor. 
     
     
         3 . The system for the liquefaction of natural gas according to  claim 1 , wherein the liquefaction unit comprises a spool-wound heat exchanger, and the material gas expelled from the first compressor is introduced into a warm region of the spool-wound heat exchanger located on a hot side of the spool-wound heat exchanger. 
     
     
         4 . The system for the liquefaction of natural gas according to  claim 1 , further comprising a second compressor placed between the first compressor and the liquefaction unit for compressing the material gas expelled from the first compressor. 
     
     
         5 . The system for the liquefaction of natural gas according to  claim 4 , further comprising a first electric motor powered by an external electric power and controlled in dependence on a pressure value of the material gas introduced into the liquefaction unit, and the second compressor is driven by the first motor. 
     
     
         6 . The system for the liquefaction of natural gas according to  claim 4 , further comprising a second cooling unit placed between the second compressor and the liquefaction unit to cool the material gas. 
     
     
         7 . The system for the liquefaction of natural gas according to  claim 1 , further comprising an electric generator unit for converting the power generated by the first expander into electric power and a second electric motor for driving the first compressor, the second electric motor being powered by electric power generated by the generator unit. 
     
     
         8 . The system for the liquefaction of natural gas according to  claim 1 , further comprising a second electric motor mechanically coupling the first expander and the first compressor to each other and powered by external electric power, wherein the first compressor is configured to compress the material gas by using the power generated by the first expander and power generated by the second electric motor. 
     
     
         9 . The system for the liquefaction of natural gas according to  claim 1 ,
 wherein the material gas from which the heavy component is reduced or eliminated by the distillation unit is directly introduced into the first compressor, and the system further comprises a first gas-liquid separation vessel for receiving the material gas compressed by the first compressor via the liquefaction unit; and   wherein a gas phase component of the material gas separated in the first gas-liquid separation vessel is introduced into the liquefaction unit once again, and a liquid phase component of the material gas is recirculated to the distillation unit.   
     
     
         10 . The system for the liquefaction of natural gas according to  claim 9 , further comprising a second cooling unit placed between the first compressor and the first gas-liquid separation vessel to cool the material gas. 
     
     
         11 . The system for the liquefaction of natural gas according to  claim 1 , further comprising: a second expander placed between the first expander and the distillation unit to generate power by expanding the material gas; and
 a third compressor placed between the distillation unit and the first compressor to compress the material gas distilled by the distillation unit by using the power generated by the second expander.   
     
     
         12 . The system for the liquefaction of natural gas according to  claim 1 , further comprising:
 a second expander placed in parallel with the first expander to generate power by expanding the material gas; and   a third compressor placed between the distillation unit and the first compressor to compress the material gas distilled by the distillation unit by using the power generated by the second expander.   
     
     
         13 . The system for the liquefaction of natural gas according to  claim 1 , wherein the liquefaction unit comprises a plate-fin heat exchanger. 
     
     
         14 . The system for the liquefaction of natural gas according to  claim 1 , wherein the material gas compressed by the first expander has a pressure higher than 5,171 kPaA. 
     
     
         15 . The system for the liquefaction of natural gas according to  claim 4 , wherein the material gas compressed by the second expander has a pressure higher than 5,171 kPaA. 
     
     
         16 . The system for the liquefaction of natural gas according to  claim 1 , further comprising a heat exchanger for exchanging heat between the material gas introduced into the distillation unit and a top fraction from the distillation unit. 
     
     
         17 . The system for the liquefaction of natural gas according to  claim 1 , further comprising a first gas-liquid separation vessel for receiving a top fraction from the distillation unit, and a third cooling unit placed between the distillation unit and the first gas-liquid separation vessel to cool the top fraction from the distillation unit. 
     
     
         18 . The system for the liquefaction of natural gas according to  claim 1 , further comprising a second heat exchanger for exchanging heat between the material gas to be introduced into the first compressor and the material gas compressed by the first compressor. 
     
     
         19 . The system for the liquefaction of natural gas according to  claim 18 , further comprising a fifth cooling unit for cooling the material gas compressed by the first compressor at a point upstream of the second heat exchanger by using a water, air or propane refrigerant. 
     
     
         20 . The system for the liquefaction of natural gas according to  claim 18 , further comprising a third heat exchanger for exchanging heat between the material gas compressed by the first compressor and the top fraction from the distillation unit. 
     
     
         21 . A system for the liquefaction of natural gas that cools the natural gas to produce liquefied natural gas, comprising:
 a first expander for generating power by expanding natural gas under pressure as material gas;   a distillation unit for reducing or eliminating a heavy component in the material gas by distilling the material gas depressurized by expansion in the first expander;   a first compressor for compressing the material gas from which the heavy component was reduced or eliminated by the distillation unit by using power generated in the first expander; and   a liquefaction unit for liquefying the material gas compressed by the first compressor by exchanging heat with a refrigerant.   
     
     
         22 . A method for the liquefaction of natural gas by cooling the natural gas to produce liquefied natural gas, comprising:
 a first expansion step for generating power by expanding natural gas under pressure as material gas;   a first cooling step for cooling the material gas depressurized by expansion in the first expansion step;   a distillation step for reducing or eliminating a heavy component in the material gas by distilling the material gas cooled in the first cooling step;   a first compression step for compressing the material gas from which the heavy component was reduced or eliminated in the distillation step by using the power generated in the first expansion step; and   a liquefaction step for liquefying the material gas compressed in the first compression step by exchanging heat with a refrigerant.