Method and system for cooling a hydrocarbon stream using a gas phase refrigerant

A method for liquefying a natural gas feed stream to produce an LNG product, the method comprising:

• passing a natural gas feed stream through and cooling the natural gas feed stream in the warm side of some or all of a plurality of heat exchanger sections so as to liquefy and subcool the natural gas feed stream, the plurality of heat exchanger sections comprising a first heat exchanger section in which a natural gas stream is liquefied, a second heat exchanger section in which the liquefied natural gas stream from the first heat exchanger section is subcooled, and a third heat exchanger section in which a natural gas stream is precooled prior to being liquefied in the first heat exchanger section, the liquefied and subcooled natural gas stream being withdrawn from the second heat exchanger section to provide an LNG product;
• and
• circulating a refrigerant, comprising 20-70 mole % nitrogen and 30-80 mole % methane, in a refrigeration circuit comprising the plurality of heat exchanger sections, a compressor train comprising a plurality of compressors and/or compression stages and one or more intercoolers and/or aftercoolers, a first turbo-expander, a second turbo-expander and a first J-T valve, wherein the circulating refrigerant provides refrigeration to each of the plurality of heat exchanger sections and thus cooling duty for liquefying and subcooling the natural gas feed stream, and wherein circulating the refrigerant in the refrigerant circuit comprises the steps of:
• (i) splitting a compressed and cooled gaseous stream of the refrigerant to form a first stream of cooled gaseous refrigerant and a second stream of cooled gaseous refrigerant;
• (ii) expanding the first stream of cooled gaseous refrigerant down to a first pressure in the first turbo-expander to form a first stream of expanded cold refrigerant at a first temperature and said first pressure, the first stream of expanded cold refrigerant being a gaseous or predominantly gaseous stream containing no or substantially no liquid as it exits the first turbo-expander;
• (iii) passing the second stream of cooled gaseous refrigerant through and cooling the second stream of cooled gaseous refrigerant in the warm side of at least one of the plurality of heat exchanger sections, splitting the resulting further cooled second stream of cooled gaseous refrigerant to form a third stream of cooled gaseous refrigerant and fourth stream of cooled gaseous refrigerant, and passing the fourth stream of cooled gaseous refrigerant through and further cooling and at least partially liquefying the fourth stream of cooled gaseous refrigerant in the warm side of at least another one of the plurality of heat exchanger sections comprising at least the second heat exchanger section to form a liquid or two-phase stream of refrigerant;
• (iv) expanding the liquid or two-phase stream of refrigerant down to a second pressure by throttling said stream through the first J-T valve to form a second stream of expanded cold refrigerant at a second temperature and said second pressure, the second stream of expanded cold refrigerant being a two-phase stream as it exits the J-T valve, the second pressure being lower than the first pressure and the second temperature being lower than the first temperature;
• (v) expanding the third stream of cooled gaseous refrigerant down to a third pressure in the second turbo-expander to form a third stream of expanded cold refrigerant at a third temperature and said third pressure, the third stream of expanded cold refrigerant being a gaseous or predominantly gaseous stream containing no or substantially no liquid as it exits the second turbo-expander, the third temperature being lower than the first temperature but higher than the second temperature;
• (vi) passing the first stream of expanded cold refrigerant through and warming the first stream of expanded cold refrigerant in the cold side of at least one of the plurality of heat exchanger sections, comprising at least the third heat exchanger section and/or a heat exchanger section in which all or part of the second stream of cooled gaseous refrigerant is cooled, passing the third stream of expanded cold refrigerant through and warming the third stream of expanded cold refrigerant in the cold side of at least one of the plurality of heat exchanger sections, comprising at least the first heat exchanger section and/or a heat exchanger section in which all or a part of the fourth stream of cooled gaseous refrigerant is further cooled, and passing the second stream of expanded cold refrigerant through and warming the second stream of expanded cold refrigerant in the cold side at least one of the plurality of heat exchanger sections, comprising at least the second heat exchanger section, wherein the first and second streams of expanded cold refrigerant are kept separate and not mixed in the cold sides of any of the plurality of heat exchanger sections, the first stream of expanded cold refrigerant being warmed to form all or part of a first stream of warmed gaseous refrigerant and the second stream of expanded cold refrigerant being warmed and vaporized to form all or part of a second stream of warmed gaseous refrigerant;
• and
• (vii) introducing the first stream of warmed gaseous refrigerant and the second stream of warmed gaseous refrigerant into the compressor train, whereby the second stream of warmed gaseous refrigerant is introduced into compressor train at a different, lower pressure location of the compressor train than the first stream of warmed gaseous refrigerant, and compressing, cooling and combining the first stream of warmed gaseous refrigerant and second stream of warmed gaseous refrigerant to form the compressed and cooled gaseous stream of the refrigerant that is then split in step (i);
• and wherein either:
• (a) the third pressure is the substantially the same as the second pressure, the third stream of expanded cold refrigerant passing through and being warmed in the cold side of at least the first heat exchanger section and the second stream of expanded cold refrigerant passing through and being warmed in the cold side of at least the second heat exchanger section and then passing through and being further warmed in the cold side of at least the first heat exchanger section where it mixes with the third stream of expanded cold refrigerant;
• the third heat exchanger section is a coil wound heat exchanger section comprising a tube bundle having tube-side and a shell side;
• and the plurality of heat exchanger sections further comprise a fourth heat exchanger section in which a natural gas stream is precooled and/or in which all or a part of the second stream of cooled gaseous refrigerant is cooled, the first stream of expanded cold refrigerant passing through and being warmed in the cold side of one of the third and fourth heat exchanger sections to form the first stream of warmed gaseous refrigerant, and a mixed stream of the second and third streams of expanded cold refrigerant from the first heat exchanger section passing through and being further warmed in the cold side of the other of the third and fourth heat exchanger sections to form the second stream of warmed gaseous refrigerant;
• or
• (b) the third pressure is the substantially the same as the first pressure, the third stream of expanded cold refrigerant and first stream of expanded cold refrigerant being mixed and warmed in the cold side of at least one of the plurality of heat exchanger sections, the third and first streams of expanded cold refrigerant being mixed and warmed to form the first stream of warmed gaseous refrigerant;
• the plurality of heat exchanger sections further comprise a fourth heat exchanger section in which a natural gas stream is precooled and/or in which all or a part of the second stream of cooled gaseous refrigerant is cooled, and a fifth heat exchanger section in which a natural gas stream is liquefied and/or in which all or a part of the fourth stream or a fifth stream of cooled gaseous refrigerant is further cooled, wherein said fifth stream of cooled gaseous refrigerant, where present, is formed from another portion of the further cooled second stream of cooled gaseous refrigerant;
• and the second stream of expanded cold refrigerant, after passing through and being warmed in the cold side of the second heat exchanger section, is passed through and is further warmed in the cold side of at least the fifth heat exchanger section and then the fourth heat exchanger section.