Process and system for liquefaction of hydrocarbon-rich gas stream utilizing three refrigeration cycles

Abstract

A process is disclosed for liquefying a hydrocarbon-rich gas stream such as natural gas by indirect heat exchange with refrigerants in a cascade of three closed loop refrigeration cycles, namely pre-cooling, liquefaction and subcooling. Each of the closed loop refrigeration cycles utilizes a different refrigerant composition. The pre-cooling cycle utilizes a first mixed refrigerant; the liquefaction cycle utilizes a pure refrigerant; and the subcooling cycle utilizes a second next refrigerant. The refrigerants closely match the enthalpy curve of the hydrocarbon-rich gas, thereby improving the energy efficiency of the liquefaction process.

1 . A process for liquefying a hydrocarbon-rich stream by indirect heat exchange with refrigerants in a cascade refrigeration cycle comprising three closed loop refrigeration cycles, characterized in that the refrigerants are subcooled and subjected to expansion, and the resultant cooled gas is heated in indirect heat exchange with the hydrocarbon-rich stream within at least one heat exchanger in each of the three cycles, and each of the resultant vaporized refrigerants is compressed, wherein:
 the first cycle pre-cools the hydrocarbon-rich stream utilizing a pre-cooling mixed refrigerant;   the second cycle liquefies the hydrocarbon-rich stream utilizing a liquefying pure refrigerant; and   the third cycle subcools the hydrocarbon-rich stream utilizing a subcooling mixed refrigerant.   
     
     
         2 . The process of  claim 1  wherein the pre-cooling mixed refrigerant comprises ethane, propane and i-butane; the pure refrigerant comprises ethylene; and the subcooling mixed refrigerant comprises nitrogen, methane and ethane. 
     
     
         3 . The process of  claim 1  wherein the pre-cooling mixed refrigerant consists essentially of between about 10% and about 20% ethane, between about 60% and about 70% propane and between about 15% and about 25% i-butane; the pure refrigerant consists essentially of about 100% ethylene; and the subcooling mixed refrigerant consists essentially of between about 30% and about 40% nitrogen, between about 30% and about 40% methane and between about 30% and about 40% ethane. 
     
     
         4 . The process of  claim 1  wherein the pre-cooling mixed refrigerant consists essentially of about 16% ethane, about 64% propane and about 20% i-butane; the pure refrigerant consists essentially of about 100% ethylene; and the subcooling mixed refrigerant consists essentially of about 33% nitrogen, about 33% methane and about 34% ethane. 
     
     
         5 . The process of  claim 1  wherein the hydrocarbon-rich stream comprises methane. 
     
     
         6 . The process of  claim 1  wherein the hydrocarbon-rich stream comprises natural gas.