Use of alternate refrigerants in optimized cascade process

Abstract

Methods and systems for liquefying natural gas using nonflammable refrigerants are provided. Methods of liquefaction include cooling a natural gas stream via indirect heat exchange with a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoromethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof during a first refrigeration cycle; and cooling the natural gas stream via indirect heat exchange with a second refrigerant during a second refrigeration cycle.

1 . A method for liquefaction of natural gas comprising:
 a) cooling a natural gas stream via indirect heat exchange with a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoromethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof during a first refrigeration cycle; and   b) cooling the natural gas stream via indirect heat exchange with a second refrigerant during a second refrigeration cycle.   
     
     
         2 . The method of  claim 1  wherein the first refrigerant or the second refrigerant includes a hydrocarbon component in an amount ranging from about 0.1% to about 99% by volume. 
     
     
         3 . The method of  claim 2  wherein the hydrocarbon is selected from the group consisting of: ethylene, propane, and any combination thereof. 
     
     
         4 . The method of  claim 1  wherein the first refrigerant or the second refrigerant is an azeotropic mixture or a near-azeotropic mixture. 
     
     
         5 . The method of  claim 1  wherein the first refrigerant is selected from a group consisting of: a 1:1 mixture of difluoromethane and pentafluoromethane; pentafluoroethane; a mixture of trifluoroethane, pentafluoroethane and tetrafluoroethane; and any derivative thereof. 
     
     
         6 . The method of  claim 1  wherein the second refrigerant is a nonflammable refrigerant comprising a mixture of trifluoromethane and hexafluoroethane. 
     
     
         7 . The method of  claim 1 , further comprising:
 c) producing liquefied natural gas.   
     
     
         8 . The method of  claim 7 , further comprising:
 d) vaporizing the liquefied natural gas.   
     
     
         9 . A method for liquefaction of natural gas comprising:
 a) providing at least one nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoroethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof; and   b) cooling a natural gas stream in an LNG facility via indirect heat exchange with the nonflammable refrigerant.   
     
     
         10 . The method of  claim 9  wherein the LNG facility is located offshore. 
     
     
         11 . The method of  claim 9  wherein the refrigerant includes a hydrocarbon component in an amount ranging from about 0.1% to about 99% by volume. 
     
     
         12 . The method of  claim 11  wherein the hydrocarbon is selected from the group consisting of: ethylene, propane, and any combination thereof. 
     
     
         13 . The method of  claim 9  wherein at least three nonflammable refrigerants are used to cool the natural gas stream and produce liquefied natural gas. 
     
     
         14 . The method of  claim 13  wherein the liquefied natural gas is vaporized. 
     
     
         15 . The method of  claim 9  wherein a first nonflammable refrigerant is used to cool the natural gas stream in a closed-loop refrigeration cycle and second nonflammable refrigerant is used to cool the natural gas stream in an open-loop refrigeration cycle. 
     
     
         16 . The method of  claim 9  wherein the refrigerant is an azeotropic or near-azeotropic mixture. 
     
     
         17 . A method for liquefaction of natural gas comprising:
 a) cooling a natural gas stream in a LNG facility via indirect heat exchange with a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoroethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof during a first refrigeration cycle;   b) cooling the natural gas stream in the LNG facility via indirect heat exchange with a second refrigerant during a second refrigeration cycle; and   c) cooling the natural gas stream in the LNG facility via indirect heat exchange with a third refrigerant during the third refrigeration cycle.   
     
     
         18 . The method of  claim 17  wherein the first refrigerant is selected from a group consisting of: a 1:1 mixture of difluoromethane and pentafluoromethane; pentafluoroethane; a mixture of trifluoroethane, pentafluoroethane and tetrafluoroethane; and any derivative thereof. 
     
     
         19 . The method of  claim 17  wherein the second refrigerant is a mixture of trifluoromethane and hexafluoroethane. 
     
     
         20 . The method of  claim 17  wherein the third refrigerant is methane. 
     
     
         21 . The method of  claim 17  wherein the first refrigerant or the second refrigerant includes a hydrocarbon component in an amount ranging from about 0.1% to about 99% by volume. 
     
     
         22 . The method of  claim 21  wherein the hydrocarbon is selected from the group consisting of: ethylene, propane, and any combination thereof. 
     
     
         23 . The method of  claim 17  wherein the LNG facility is located offshore. 
     
     
         24 . The method of  claim 17 , further comprising:
 d) producing liquefied natural gas.   
     
     
         25 . The method of  claim 24 , further comprising:
 e) vaporizing the liquefied natural gas.