Dual multi-component refrigeration cycles for liquefaction of natural gas

A process for liquefying a methane-rich gas stream to produce pressurized liquid product having a temperature above −112° C. (−170° F.) and a pressure sufficient for the liquid product to be at or below its bubble point using two closed, multi-component refrigeration cycles, each refrigerant in said refrigeration cycles comprising constituents of various volatilities, comprising

• (a) liquefying the methane-rich gas stream in a first heat exchanger against a first low-level mixed refrigerant which circulates in a first refrigeration cycle;
• (b) compressing the first low-level mixed refrigerant in a plurality of compression stages and cooling the compressed low-level mixed refrigerant in one or more stages against an external cooling fluid;
• (c) cooling the compressed, cooled first low-level mixed refrigerant against a second low-level mixed refrigerant in a second heat exchanger to at least partially liquefy the compressed first low-level mixed refrigerant before liquefying the methane-rich gas in the first heat exchanger;
• and
• (d) compressing the second multi-component refrigerant in a plurality of compression stages and cooling the compressed second multi-component refrigerant in one or more stages against an external cooling fluid, heat exchanging the compressed, cooled, second multi-component refrigerant in the second heat exchanger to produce a cooled, at least partially liquid second multi-component refrigerant, expanding the cooled, at least partially liquid second multi-component refrigerant to produce a low temperature coolant and passing the low temperature coolant in countercurrent heat exchange with the compressed, cooled, second multi-component refrigerant to at least partially liquefy the first multi-component refrigerant and to at least partially vaporize the second multi-component refrigerant, and recycling the second multi-component refrigerant to the first stage of compression.

A process for liquefaction of a gas rich in methane to produce a pressurized liquid product having a temperature above about −112° C., comprising the steps of:

• (a) cooling and liquefying the gas in a first heat exchanger by heat exchange against a first multi-component refrigerant of a first closed refrigeration cycle;
• (b) cooling said first multi-component refrigerant in a second heat exchanger against a second multi-component refrigerant in a second closed refrigeration cycle;
• (c) said first refrigeration cycle comprising
• pressurizing and cooling the cooled first refrigerant of step (b) in at least one stage of compression and cooling which comprises phase separating the warmed first refrigerant into a vapor phase and a liquid phase, separately pressurizing the vapor phase and the liquid phase, combining the pressurized liquid phase and pressurized vapor phase, and aftercooling the combined phases against an external cooling fluid;
• passing the pressurized first refrigerant through the second heat exchanger to cool the first refrigerant against the second refrigerant;
• passing the pressurized first refrigerant through the first exchanger;
• expanding the pressurized first refrigerant to convert the first refrigerant into a lower temperature mixed refrigerant and passing the expanded first refrigerant through the first heat exchanger in counter-current relationship with itself before expansion and with gas rich in methane, thereby warming the expanded first refrigerant and producing a pressurized liquid having a temperature above about −112° C., and recycling the warmed, expanded first refrigerant to the second heat exchanger;
• and
• (d) said second refrigeration cycle comprising:
• pressurizing and cooling the warmed second refrigerant in at least one stage of compression and cooling which comprises phase separating the warmed second refrigerant into a vapor phase and a liquid phase, separately pressurizing the vapor phase and the liquid phase, combining the pressurized liquid phase and pressurized vapor phase, and aftercooling the combined phases against an external cooling fluid;
• passing the pressurized second refrigerant through the second heat exchanger to cool the first refrigerant against the second refrigerant;
• expanding the pressurized second refrigerant to a lower temperature and passing the expanded second refrigerant through the second heat exchanger in counter-current relationship with itself before expansion and with the first refrigerant, thereby warming the expanded second refrigerant.