Natural gas liquefaction by a high pressure expansion process

A method for liquefying a feed gas stream rich in methane, comprising:

• providing the feed gas stream at a pressure less than 1,200 psia;
• compressing the feed gas stream to a pressure of at least 1,500 psia to form a compressed gas stream;
• cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water to form a cooled, compressed gas stream;
• expanding the cooled, compressed gas stream in at least one first work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the feed gas stream was compressed to form a chilled gas stream;
• passing the chilled gas stream to a first heat exchanger zone downstream from the at least one first work producing expander, the first heat exchanger zone comprising a first main heat exchanger, a second main heat exchanger, and a sub-cooling heat exchanger sequentially downstream from the at least one first work producing expander;
• providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia;
• cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream;
• directing the compressed, cooled refrigerant stream to a second heat exchanger zone comprising at least one heat exchanger to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream;
• expanding the compressed, additionally cooled refrigerant stream in at least one second work producing expander to produce an expanded, cooled refrigerant stream;
• passing the expanded, cooled refrigerant stream through the second main heat exchanger of the first heat exchanger zone but not the sub-cooling heat exchanger to form a first warm refrigerant stream having a temperature that is cooler, by at least 5° F., than a highest fluid temperature within the first heat exchanger zone;
• cooling at least part of the chilled gas stream in the first heat exchanger zone by indirect heat exchange with the expanded, cooled refrigerant stream to form a liquefied gas stream;
• wherein the chilled gas stream passes sequentially through the first main heat exchanger, the second main heat exchanger, and the sub-cooling heat exchanger to form the liquefied gas stream, and
• neither the chilled gas stream nor the liquefied gas stream are passed through the second heat exchanger zone;
• directing a first portion of the first warm refrigerant stream to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream to form a second warm refrigerant stream;
• directing a second portion of the first warm refrigerant stream to the first main heat exchanger, such that the second portion of the first warm refrigerant stream bypasses the second heat exchanger zone, to form a third warm refrigerant stream exiting the first main heat exchanger;
• combining the second warm refrigerant stream and the third warm refrigerant stream to produce a fourth warm refrigerant stream;
• and
• compressing the fourth warm refrigerant stream to produce the compressed refrigerant stream.