Patent ID: 7552598

Claim:
A process for sub-cooling an LNG stream obtained by cooling using a first refrigeration cycle, the process comprising the following steps: (a) introducing the LNG stream at a temperature of less than −90° C. into a first heat exchanger; (b) sub-cooling the LNG stream in the first heat exchanger by heat exchange with a refrigerating fluid comprising a mixture of nitrogen and methane; (c) subjecting the refrigerating fluid to a closed second refrigeration cycle which is independent of said first cycle, the closed second refrigeration cycle comprising the following successive phases: (i) heating the refrigerating fluid issuing from the first heat exchanger in a second heat exchanger and keeping the refrigerating fluid at a low pressure; (ii) compressing the refrigerating fluid issuing from the second heat exchanger in a compression apparatus to a high pressure greater than a critical pressure of the refrigerating fluid; (iii) cooling in the second heat exchanger the refrigerating fluid originating from the compression apparatus; (iv) dynamically expanding in a cold turbine at least a portion of the refrigerating fluid issuing from the second heat exchanger to a low pressure; (v) introducing the refrigerating fluid issuing from the cold turbine into the first heat exchanger; and following step (iii), (iii1) after the refrigerating fluid passes through the second heat exchanger, separating the refrigerating fluid issuing from the compression apparatus into a sub-cooling stream and a secondary cooling stream; (iii2) expanding the secondary cooling stream in a secondary turbine; (iii3) mixing the secondary cooling stream issuing from the secondary turbine with the refrigerating fluid stream issuing from the first heat exchanger so as to form a stream of refrigerating mixture; (iii4) placing the sub-cooling stream issuing from step (iii1) in a heat exchange relationship with the stream of refrigerating mixture in a third heat exchanger; and (iii5) introducing the sub-cooling stream issuing from the third heat exchanger into the cold turbine, wherein the sub-cooling stream issuing from step (iii1) is placed in the heat exchange relationship with the stream of refrigerating mixture in the third heat exchanger, without placing the stream of refrigerating mixture issuing from step (iii1) in a heat exchange relationship with the LNG stream.