A method and system for controlling the nitrogen concentration in an LNG product and fuel from flash gas within preferred ranges. A cooled LNG stream is separated into a nitrogen-enriched vapor stream, a fuel stream, and an LNG product stream using a plurality of phase separating devices, such as flash drum or rectifying column. A portion of the vapor stream is recycled to the rectifying column as reflux. A portion of a stream having a higher concentration of nitrogen is combined with the fuel stream to maintain the fuel stream within a desired nitrogen concentration range.

A liquid natural gas (LNG) reforming system of the present invention may include a reformer provided to receive LNG from an LNG tank; a C02 PSA unit connected to the reformer and configured to extract carbon dioxide from off-gas generated from the reformer; a cooler connected to the C02 PSA unit and configured to cool and liquefy the carbon dioxide extracted by the C0.sub.2 PSA unit using the LNG supplied from the LNG tank to the reformer; a storage tank connected to the cooler and provided to store liquid carbon dioxide of the cooler therein; and a circulation pump provided to pump the liquid carbon dioxide from the cooler into the storage tank and circulate a part of the liquid carbon oxide into the cooler.

A pressurized CO.sub.2 rich gas is cooled down to condense at least part of the stream in a heat exchanger. A bulk of the CO.sub.2 is separated by partial condensation and distillation in order to obtain at least one non-condensable gas from a separation vessel. The non-condensable gas is optionally heated up to a temperature lower than −20° C. (membranes performances is greatly enhanced by low temperature operation). The non-condensable gas is introduced into a membrane permeation unit, producing a residue stream and a permeate stream (the permeate stream is enriched in CO.sub.2). The permeate stream is recycled to the process, optionally after compression. The method is auto-refrigerated, i. e. no external refrigerant is used to provide cooling below 0° C.

One or more specific embodiments disclosed herein includes a method for separating hydrogen from an olefin hydrocarbon rich compressed effluent vapor stream, employing a integrated heat exchanger, multiple gas-liquid separators, external refrigeration systems, and a rectifier attached to a liquid product drum.

A compressing system includes a compression section that compresses a target gas to an intermediate pressure, which is equal to or higher than a critical pressure and lower than a target pressure to generate an intermediate supercritical fluid, a cooling section that cools the intermediate supercritical fluid generated in the compression section to near a critical temperature to generate an intermediate supercritical pressure liquid, and a pumping section that compresses the intermediate supercritical pressure liquid generated in the cooling section to a pressure that is equal to or higher than the target pressure. At least one of the intermediate supercritical pressure liquid compressed in the pumping section, a low-temperature liquid generated by extracting the intermediate supercritical pressure liquid on the upstream side of the pumping section to reduce pressure to near the critical pressure, and an external cooling medium is used as a cooling medium in the cooling section.

A system and method for liquefaction of natural gas using two distinct refrigeration circuits having compositionally different working fluids and operating at different temperature levels is provided. The turbomachinery associated with the liquefaction system are driven by a single three-pinion or four-pinion integral gear machine with customized pairing arrangements. The system and method of natural gas liquefaction further includes the conditioning of a lower pressure natural gas containing feed stream to produce a purified, compressed natural gas containing stream at a pressure equal to or above the critical pressure of natural gas and substantially free of heavy hydrocarbons to be liquefied.

A system and method for liquefaction of natural gas using two distinct refrigeration circuits having compositionally different working fluids and operating at different temperature levels is provided. The turbomachinery associated with the liquefaction system are driven by a single three-pinion, three-turbine integral gear machine with customized pairing arrangements. The system and method of natural gas liquefaction further includes the conditioning of a lower pressure natural gas feed stream to produce a purified, compressed natural gas stream at a pressure equal to or above the critical pressure of natural gas and substantially free of heavy hydrocarbons to be liquefied.

A system and method for liquefaction of natural gas using two distinct refrigeration circuits having compositionally different working fluids and operating at different temperature levels is provided. The turbomachinery associated with the liquefaction system are driven by a single three-pinion, three-turbine integral gear machine with customized pairing arrangements. The system and method of natural gas liquefaction further includes the conditioning of a lower pressure natural gas containing feed stream to produce a purified, compressed natural gas stream at a pressure equal to or above the critical pressure of natural gas and substantially free of heavy hydrocarbons to be liquefied.

A system and method for liquefaction of natural gas using two distinct refrigeration circuits having compositionally different working fluids and operating at different temperature levels is provided. The turbomachinery associated with the liquefaction system are driven by a single three-pinion or four-pinion integral gear machine with customized pairing arrangements. The system and method of natural gas liquefaction further includes the conditioning of a lower pressure natural gas containing feed stream to produce a purified, compressed natural gas stream at a pressure equal to or above the critical pressure of natural gas and substantially free of heavy hydrocarbons to be liquefied.

A system and method for liquefaction of natural gas using two distinct refrigeration circuits having compositionally different working fluids and operating at different temperature levels is provided. The turbomachinery associated with the liquefaction system are driven by a single three-pinion or four-pinion integral gear machine with customized pairing arrangements. The system and method of natural gas liquefaction further includes the conditioning of a lower pressure natural gas containing feed stream to produce a purified, compressed natural gas stream at a pressure equal to or above the critical pressure of natural gas and substantially free of heavy hydrocarbons to be liquefied.