US Patent 9863697 Integrated methane refrigeration system for liquefying natural gas

Described herein is a method and system for liquefying a natural gas feed stream to produce an LNG product. The natural gas feed stream is liquefied, by indirect heat exchange with a gaseous methane or natural gas refrigerant circulating in a gaseous expander cycle, to produce a first LNG stream. The first LNG stream is expanded, and the resulting vapor and liquid phases are separated to produce a first flash gas stream and a second LNG stream. The second LNG stream is then expanded, with the resulting vapor and liquid phases being separated to produce the second flash gas stream and a third LNG stream, all or a portion of which forms the LNG product. Refrigeration is recovered from the second flash gas by using said stream to sub-cool the second LNG stream or a supplementary LNG stream.