Gas liquefaction system with multiple refrigerant cycles

Abstract

The liquefaction system comprises a high-temperature refrigerant circuit and a low-temperature refrigerant circuit. The system further comprises a first high-temperature heat exchanger, wherein the feed gas is in heat exchange with a first stream of vaporizing first refrigerant and is cooled thereby, and a second high-temperature heat exchanger, wherein compressed first refrigerant of the first refrigerant circuit and compressed second refrigerant of the second refrigerant circuit are in heat exchange with a second stream of vaporizing first refrigerant and are cooled thereby. A low-temperature heat exchanger is further provided, wherein cooled feed gas from the first high-temperature heat exchanger and cooled second refrigerant from the second high-temperature heat exchanger are further cooled and liquefied in heat exchange with a flow of vaporizing second refrigerant.

1 . A system for liquefying a pressurized feed gas, the system comprising:
 a high-temperature refrigerant circuit, comprising: a first compression arrangement, a first heat rejection device, and a first pressure reduction device;   a low-temperature refrigerant circuit, comprising: a second compression arrangement, a second heat rejection device, and a second pressure reduction device;   a first high-temperature heat exchanger, wherein the feed gas is in heat exchange with a first stream of vaporizing first refrigerant and is cooled thereby;   a second high-temperature heat exchanger wherein compressed first refrigerant of the first refrigerant circuit and compressed second refrigerant of the second refrigerant circuit are in heat exchange with a second stream of vaporizing first refrigerant and are cooled thereby; and,   a low-temperature heat exchanger, wherein cooled feed gas from the first high-temperature heat exchanger and cooled second refrigerant from the second high-temperature heat exchanger are further cooled and liquefied in heat exchange with a flow of vaporizing second refrigerant;   wherein the high-temperature refrigerant circuit comprises:   a main refrigerant line, which extends through a hot side of the second high-temperature heat exchanger to the first pressure reduction device;   a first secondary refrigerant line, which extends from the main refrigerant line, downstream of the second high-temperature heat exchanger, through the first pressure reduction device and to the first high-temperature heat exchanger; and,   a second secondary refrigerant line, which extends from the main refrigerant line, downstream of the second high-temperature heat exchanger, through the first pressure reduction device and to the second high-temperature heat exchanger.   
     
     
         2 . The system of  claim 1 , wherein a vaporized first refrigerant collection duct from the first high-temperature heat exchanger and a vaporized first refrigerant collection duct from the second high-temperature heat exchanger are fluidly coupled to a single compressor train of the first compression arrangement. 
     
     
         3 . The system of  claim 1 , wherein the first pressure reduction device comprises: a first pressure reduction unit between the first heat rejection device and the first high-temperature heat exchanger; and a second pressure reduction unit between the first heat rejection device and the second high-temperature heat exchanger. 
     
     
         4 . The system of  claim 1 , wherein:
 the first compression arrangement comprises:
 a first compressor and a second compressor arranged in sequence; 
 an intercooler is arranged between a discharge side of the first compressor and a suction side of the second compressor; 
 a liquid/gas separator is arranged between the intercooler and the second compressor; 
 the liquid/gas separator being adapted to separate liquid from a flow of partially compressed first refrigerant delivered by the first compressor; 
 a liquid outlet of the liquid/gas separator is fluidly coupled to the second high-temperature heat exchanger; and, 
 a third pressure reduction unit being arranged between the liquid outlet of the liquid/gas separator and the second high-temperature heat exchanger. 
   
     
     
         5 . The system of  claim 1 , comprising a further liquid/gas separator between the second compression arrangement and the low-temperature heat exchanger; wherein the further liquid/gas separator comprises:
 a gas outlet fluidly coupled to the low-temperature heat exchanger; and,   a liquid outlet fluidly coupled to the low-temperature heat exchanger.   
     
     
         6 . The system of  claim 5 , wherein the second pressure reduction device comprises:
 a fourth pressure reduction unit between the gas outlet of the further liquid/gas separator and the low-temperature heat exchanger; and,   a fifth pressure reduction unit between the liquid outlet of the further liquid/gas separator and the low-temperature heat exchanger.   
     
     
         7 . The system of  claim 1 , further comprising a heavy hydrocarbon removal system. 
     
     
         8 . The system of  claim 1 , further comprising a feed gas pre-treatment unit. 
     
     
         9 . The system of  claim 8 , wherein the feed gas pre-treatment unit comprises a feed gas pre-cooler; wherein the feed gas pre-cooler comprises a pre-cooling heat exchanger with a hot side adapted to circulate feed gas and a cold side adapted to circulate first refrigerant from the first refrigerant circuit. 
     
     
         10 . The system of  claim 1 , wherein the feed gas is natural gas. 
     
     
         11 . The system of  claim 1 , wherein the first refrigerant is a mixed refrigerant and the second refrigerant is a mixed refrigerant. 
     
     
         12 . A method for liquefying a pressurized feed gas, the method comprising the following steps:
 circulating a first refrigerant in a high-temperature refrigerant circuit comprising: a first compression arrangement, a first heat rejection device, a first pressure reduction device, a first high-temperature heat exchanger, and a second high-temperature heat exchanger;   circulating a second refrigerant in a low-temperature refrigerant circuit comprising: a second compression arrangement, a second heat rejection device, a second pressure reduction device, and a low-temperature heat exchanger;   flowing a stream of first refrigerant from the first heat rejection device through a hot side of the second high-temperature heat exchanger;   downstream of the second high-temperature heat exchanger, dividing the stream of first refrigerant into a first stream of expanded and vaporizing first refrigerant and a second stream of expanded and vaporizing first refrigerant;   flowing the first stream of expanded and vaporizing first refrigerant in the first high-temperature heat exchanger in heat exchange with feed gas and cooling the feed gas therewith;   flowing the second stream of expanded and vaporizing first refrigerant in the second high-temperature heat exchanger in heat exchange with compressed and cooled second refrigerant circulating in the low-temperature refrigerant circuit, and cooling the second refrigerant therewith; and,   further cooling and condensing the feed gas in the low-temperature heat exchanger in heat exchange with a stream of vaporizing second refrigerant circulating in the low-temperature refrigerant circuit.   
     
     
         13 . The method of  claim 12 , wherein the first stream of vaporizing first refrigerant is expanded in a first pressure reduction unit and the second stream of vaporizing first refrigerant is expanded in a second pressure reduction unit. 
     
     
         14 . The method of  claim 12 , further comprising the following steps:
 partially compressing the first refrigerant in a first compressor;   cooling the partially compressed first refrigerant in an intercooler;   separating a liquid phase and a gaseous phase of the partially compressed and cooled first refrigerant;   expanding the liquid phase of the partially compressed and cooled first refrigerant and feed the expanded first refrigerant obtained from expansion of the liquid phase through the second high-temperature heat exchanger;   further compressing the gaseous phase of the first refrigerant; and,   cooling the further compressed refrigerant in the second high-temperature heat exchanger and splitting and expanding the cooled further compressed first refrigerant downstream of the second high-temperature heat exchanger into said first stream and second stream.   
     
     
         15 . The method of  claim 12 , further comprising the following steps:
 collecting the cooled second refrigerant from the second high-temperature heat exchanger in a liquid/gas separator;   separating a liquid phase from a gaseous phase of the cooled second refrigerant in the liquid/gas separator; and,   separately expanding the cooled liquid phase and the cooled gaseous phase of the second refrigerant to generate said flow of vaporizing second refrigerant.   
     
     
         16 . The method of  claim 12 , further comprising the step of collecting vaporized first refrigerant from the first high-temperature heat exchanger and vaporized first refrigerant from the second high-temperature heat exchanger in a single compression train of the high-temperature refrigerant circuit. 
     
     
         17 . The method of  claim 12 , further comprising the step of diverting a flow of liquefied first refrigerant from the high-temperature refrigerant circuit and pre-cooling the feed gas by heat exchange with the diverted flow of liquefied first refrigerant. 
     
     
         18 . The method of  claim 12 , wherein the feed gas is natural gas. 
     
     
         19 . The method of  claim 12 , wherein the first refrigerant is a mixed refrigerant and the second refrigerant is a mixed refrigerant.