DESCRIPTION

TECHNICAL FIELD

[0001] The present disclosure relates to feed gas liquefaction systems and methods. Embodiments disclosed herein specifically refer to liquefaction of natural gas, such as methane or a mixture of light-weight hydrocarbons.

BACKGROUND ART

[0002] In several industrial applications, a feed gas needs to be chilled and liquefied, for instance for transportation purposes.

[0003] Specifically, natural gas requires to be liquefied and obtain liquefied natural gas (LNG) to reduce the volume thereof and ensure safe transportation thereof, in so- called LNG vessels. Other gaseous products, such as oxygen or nitrogen, may also require liquefaction for the purpose of ease of transportation,

[0004] Several liquefaction cycles have been developed in the art, in an attempt to improve the liquefaction process and make it more efficient also from the point of view of energy consumption.

[0005] Liquefaction of natural gas proved to be particularly challenging. Natural gas is extracted from gas fields, treated in scrubber units to remove impurities, such as water, mercury (Hg), heavier hydrocarbons and the like, and subsequently compressed, chilled and liquefied. Natural gas consists mainly of methane (CH4), but may include percentages of heavier hydrocarbons. The chemical composition of the natural gas may fluctuate over time in an unpredictable manner. The pressure at which natural gas is delivered from the gas field may also fluctuate over time in a way which cannot be predicted.

[0006] The liquefaction system shall quickly react to pressure or compositional fluctuations of the natural gas, to prevent negative effects on the operation of the liquefaction system, such as fouling of the heat exchangers due to precipitation of solidified heavier hydrocarbons contained in the incoming flow of natural gas. [0007] The liquefaction systems of the current art are not satisfactory from this point of view. The large thermal inertia of the components of these systems make adaptation of the operating conditions to changing pressure and/or composition of the feedgas particularly slow.

[0008] A gas liquefaction system and a liquefaction method overcoming or alleviating the drawbacks of the current systems and methods mentioned above would be welcomed in the art.

SUMMARY

[0009] According to one aspect, disclosed herein is a system for liquefying a pressurized feed gas, the system including a high-temperature refrigerant circuit and a low- temperature refrigerant circuit. The first high-temperature refrigerant circuit includes a first compression arrangement, a first heat rejection device and a first pressure reduction device. The low-temperature refrigerant circuit includes a second compression arrangement, a second heat rejection device and a second pressure reduction device.

[0010] The high-temperature refrigerant circuit further includes a first high-temper- ature heat exchanger, adapted to circulate the feed gas in heat exchange with a first stream of vaporizing first refrigerant and cool the feed gas by heat exchange with the first stream of vaporizing first refrigerant. Furthermore, the high-temperature refrigerant circuit includes a second high-temperature heat exchanger, adapted to circulate compressed first refrigerant of the first refrigerant circuit and compressed second refrigerant of the second refrigerant circuit in heat exchange with a second stream of vaporizing first refrigerant and cool the compressed first refrigerant and second refrigerant by heat exchange with the second stream of vaporizing first refrigerant.

[0011] The high-temperature refrigerant circuit further comprises a main refrigerant line, which extends through a hot side of the second high-temperature heat exchanger to the first pressure reduction device. The high-temperature refrigerant circuit further comprises a first secondary refrigerant line and a second secondary refrigerant line. The first secondary refrigerant line 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. The second secondary refrigerant line 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.

[0012] The cooled first refrigerant, which exits the first heat rejection device, is therefore further cooled in the second high-temperature heat exchanger and split into two streams after having passed through the hot side of the second high-temperature heat exchanger. At this stage the first refrigerant is still in a liquid state or almost in a liquid state and is split into first and second secondary streams which are then delivered to the first high-temperature heat exchanger and to the second high-temperature heat exchanger. In some embodiments, the full stream of compressed and chilled first refrigerant is split into separate first and second secondary streams before entering the first pressure reduction device.

[0013] In embodiments, the first pressure reduction device comprises a first pressure reduction unit, arranged in the first secondary refrigerant line, and a second pressure reduction unit, arranged in the second secondary refrigerant line. The cooled first refrigerant stream exiting the hot side of the second high-temperature heat exchanger is thus expanded after splitting into two secondary streams, which are then directed through the cold side of the first high-temperature heat exchanger and second high- temperature heat exchanger.

[0014] The low-temperature refrigerant circuit further comprises 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.

[0015] The first high-temperature heat exchanger, wherethrough the feed gas flows, is small compared to the heat exchangers of the prior art and has therefore a smaller thermal inertia. A faster reaction of the system to pressure and composition fluctuations of the feed gas is thus achieved.

[0016] According to a further aspect, disclosed herein is a method for liquefying a pressurized feed gas. The method includes the following steps: circulating a first refrigerant in a high-temperature refrigerant circuit comprising: a first compression arrangement, a first heat rejection device, and a first pressure reduction device, a first high-temperature heat exchanger, and a second high-temper- ature 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.

[0017] Further features and embodiments of the method and of the system of the present disclosure are outlined below and set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Reference is now made briefly to the accompanying drawings, in which:

Fig.l illustrates a simplified schematic of a system according to the present disclosure;

Fig.2 illustrates an embodiment of a system according to the present disclosure;

Fig.3 illustrates a further embodiment of a system according to the present disclosure; and

Fig.4 illustrates a yet further embodiment of a system according to the present disclosure. DETAILED DESCRIPTION

[0019] Fig. l illustrates a simplified diagram of a feed gas liquefaction system 1 according to the present disclosure in one embodiment. The liquefaction system 1 comprises a high-temperature refrigerant circuit 3 and a low-temperature refrigerant circuit 5.

[0020] In the following description, reference will be made specifically to natural gas as feed gas, but some of the advantages of the system and method according to the present disclosure can be achieved also with feed gas of different nature.

[0021] The refrigerants of the high-temperature refrigerant circuit 3 and low-temperature refrigerant circuit 5 can each include a pure gas, or a gas mixture. In preferred embodiments both refrigerants are mixed refrigerants, usually with a different gas composition. Therefore, in the following description reference will usually be made to “mixed refrigerants”.

[0022] The high-temperature mixed refrigerant circuit 3 and the low-temperature mixed refrigerant circuit 5 are interlaced, as will be described in more detail below, in that the refrigerants flowing in both circuits is chilled in a high-temperature heat exchanger common to both low-temperature mixed refrigerant circuit 3 and high-tem- perature mixed refrigerant circuit 5.

[0023] The high-temperature mixed refrigerant circuit 3 includes a first compression arrangement 7, a first heat rejection device 9, and a first pressure reduction device 11. Similarly, the low-temperature mixed refrigerant circuit 5 includes a second compression arrangement 13, a second heat rejection device 15 and a second pressure reduction device 17. Reference number 7M indicates a driver, such as an electric motor, a gas turbine, a steam turbine, or the like, to drive the first compression arrangement 7. Reference number 13M indicates a driver, such as an electric motor, a gas turbine, a steam turbine or the like, to drive the second compression arrangement 13.

[0024] Each heat rej ection devices 9 and 15 can include one or more heat exchangers, wherein the respective first mixed refrigerant and second mixed refrigerant is cooled in heat exchange with a coolant fluid, such as air or water.

[0025] The pressure reduction devices may include one or more throttling or lamination valves, one or more expanders, or combinations thereof. As will be explained in detail below, the first pressure reduction device 11 includes two separate pressure reduction units 11 A, 1 IB, through which separate streams of the first mixed refrigerant are expanded. Each pressure reduction unit can include one or more pressure reduction members, such as valves and/or expanders.

[0026] The natural gas liquefaction system 1 further includes a feed gas (natural gas) duct 19, which extends through a pre-treatment section 21, a high-temperature heat exchanger arrangement 23 and a low-temperature heat exchanger 25 and leads to a liquefied natural gas collection reservoir or the like, schematically shown at 27. The liquefied natural gas (LNG) stored in the reservoir 27 can be loaded in an LNG vessel or delivered through a pipeline. The high-temperature heat exchanger arrangement 23 forms part of the high-temperature refrigerant circuit 3 and the low-temperature heat exchanger 25 forms part of the low-temperature refrigerant circuit 5.

[0027] In short, compressed feed gas, in the exemplary embodiment natural gas, is pre-treated in the pre-treatment section 21 to remove impurities, such as moisture, mercury and other contaminants, and is subsequently cooled and liquefied by removing heat therefrom in the high-temperature heat exchanger arrangement 23 and low- temperature heat exchanger 25.

[0028] The high-temperature mixed refrigerant circuit 3 and the low-temperature mixed refrigerant circuit 5 are interlaced, in that the high-temperature heat exchanger arrangement 23 is used to chill the feed gas, as well as mixed refrigerant circulating both in the high-temperature mixed refrigerant circuit 3 and in the low-temperature mixed refrigerant circuit 5.

[0029] More in detail, the high-temperature heat exchanger arrangement 23 comprises a first high-temperature heat exchanger 23 A and a second high-temperature heat exchanger 23B. The first mixed refrigerant circuit comprises a primary refrigerant line 30, which delivers compressed and cooled first mixed refrigerant from the first heat rejection device 9 through the high-temperature heat exchanger arrangement 23. The primary refrigerant line 30 extends through a hot side of the second high-temperature heat exchanger 23B, in which the compressed and cooled first mixed refrigerant flowing from the first heat rej ection device 9 is further cooled by heat exchange with a flow of vaporizing first mixed refrigerant counterflowing in the cold side of the second high-temperature heat exchanger 23B.

[0030] Downstream of the second high-temperature heat exchanger 23B the primary refrigerant line 30 is bifurcated at 33 into a first secondary refrigerant line 35A and a second secondary refrigerant line 35B. The first pressure reduction unit 11A of the pressure reduction device 11 is located in the first secondary refrigerant line 35 A and the second pressure reduction unit 1 IB of the pressure reduction device 11 is located in the second secondary refrigerant line 35B. In the schematic diagram of Fig.1 the first pressure reduction unit 11 A and the second pressure reduction unit 1 IB are represented as expansion or throttling valves, for instance Joule-Thomson valves.

[0031] The compressed and cooled stream of first mixed refrigerant from the first heat rejection device 9 is thus split into a first stream which is depressurized in the first pressure reduction unit 11A and flows through the first high-temperature heat exchanger 23 A, and a second stream which is depressurized in the second pressure reduction unit 1 IB and flows through the second high-temperature heat exchanger 23B.

[0032] The feed gas duct 19 extends through the hot side of the first high-temperature heat exchanger 23 A such that the feed gas is chilled by heat exchange against the vaporizing first stream of first mixed refrigerant.

[0033] The vaporizing second stream of first mixed refrigerant, which expands in the second pressure reduction unit 1 IB, flows through the second secondary refrigerant line 35B and through the cold side of the second high-temperature heat exchanger 23B and removes heat from the stream of first mixed refrigerant flowing along the primary refrigerant line 30 through the hot side of the second high-temperature heat exchanger 23B. The first mixed refrigerant exiting the first high-temperature heat exchanger 23 A and the second high-temperature heat exchanger 23B is collected at the suction side of the first compression arrangement 7 for compression and delivery to the first heat rejection device 9.

[0034] The low-temperature mixed refrigerant circuit 5 includes a line 41 which extends from the second heat rejection device 15 through a hot side of the second high- temperature heat exchanger 23B. Therefore, the vaporizing second stream of first mixed refrigerant flowing through the second secondary refrigerant line 35B and through the cold side of the second high-temperature heat exchanger 23B removes heat also from the pressurized second mixed refrigerant delivered by the second heat rejection device 15 and circulating in the low-temperature mixed refrigerant circuit 5.

[0035] The low-temperature mixed refrigerant circuit further extends through a hot side of the low-temperature heat exchanger 25. The pressurized and chilled second mixed refrigerant exiting the hot side of the low-temperature heat exchanger 25 is expanded in the pressure reduction device 17 and the expanded and vaporizing second mixed refrigerant flows through the cold side of the low-temperature heat exchanger 25 in heat exchange with the feed gas flowing through line 19 and in further heat exchange with the pressurized second mixed refrigerant along line 41.

[0036] The second mixed refrigerant exiting the cold side of the low-temperature heat exchanger 25 is delivered to the suction side of the second compression arrangement 13 for compression and delivery to the second heat rejection device 15.

[0037] Thus, chilling capacity of the vaporizing expanded first mixed refrigerant stream flowing through the first high-temperature heat exchanger 23 A is entirely used to cool the incoming compressed feed gas (natural gas), while the chilling capacity of the vaporizing expanded first mixed refrigerant stream flowing through the second high-temperature heat exchanger 23B is used to chill the compressed first mixed refrigerant circulating in the high-temperature mixed refrigerant circuit 3, and the compressed second mixed refrigerant circulating in the low-temperature mixed refrigerant circuit 5.

[0038] The dimension, and therefore the thermal inertia, of the first high-temperature heat exchanger 23A can therefore be small, to provide a fast adaptation of the liquefaction system 1 to variable conditions of the feed gas entering line 19, for instance in response to fluctuations of the pressure and/or chemical composition of the feed gas.

[0039] Fig.2 illustrates a more detailed diagram of a natural gas liquefaction system according to the invention in an embodiment. The same reference numbers used in Fig. l are used in Fig.2 to designate the same or corresponding components, elements or parts.

[0040] The liquefaction system 1 of Fig.2 comprises a high-temperature mixed refrigerant circuit 3 and a low-temperature mixed refrigerant circuit 5.

[0041] The high-temperature mixed refrigerant circuit 3 includes a first compression arrangement 7, a first heat rejection device 9, and a first pressure reduction device 11. In the embodiment of Fig.2, the first compression arrangement 7 includes a compressor train driven into rotation by a driver 7M, such as an electric motor, a gas turbine or a steam turbine. The compressor train of the compression arrangement 7 includes a first compressor 7A and a second compressor 7B. The delivery side of the first compressor 7A is fluidly coupled to a suction side of the second compressor 7B through an intercooler 9A, which forms part of the first heat rejection device 9. The first heat rejection device 9 further includes a heat exchanger 9B fluidly coupled to the delivery side of the compression train, i.e. to the delivery side of the second compressor 7B.

[0042] Between the intercooler 9A and the suction side of the second compressor 7B a liquid/gas separator 8 is provided, which separates liquefied first mixed refrigerant, collecting at the bottom of the liquid/gas separator 8, from gaseous first mixed refrigerant, collecting at the top of the liquid/gas separator 8. A further liquid/gas separator 10 can be arranged upstream of the suction side of the first compressor 7 A, to remove liquid first mixed refrigerant, if any. Liquid collecting at the bottom of the liquid/gas separator 10 can be recovered or disposed of in conventional manner.

[0043] Similarly, the low-temperature mixed refrigerant circuit 5 includes a second compression arrangement 13, a second heat rejection device 15 and a second pressure reduction device 17. Reference number 13M indicates a driver, such as an electric motor, a gas turbine, a steam turbine or the like, to drive the second compression arrangement 13. A liquid/gas separator 16 can be arranged upstream of the suction side of the compression arrangement 13. Liquid collecting at the bottom of the liquid/gas separator 16 can be recovered or disposed of in conventional manner.

[0044] The low-temperature mixed refrigerant circuit 5 can further include a liquid/gas separator 18 adapted to receive compressed and cooled second mixed refrigerant from the second heat rejection device 15 and separate liquid second mixed refrigerant at the bottom and gaseous second mixed refrigerant at the top thereof.

[0045] The pressure reduction devices may include one or more throttling or lamination valves, one or more expanders or combinations thereof. As in the embodiment of Fig.1, the first pressure reduction device 11 includes two pressure reduction units 11 A, 1 IB, such as two Joule-Thomson or other expansion valves, through which separate streams of first mixed refrigerant are expanded.

[0046] The natural gas liquefaction system 1 further includes a feed gas duct 19, which extends through a pre-treatment section 21, a high-temperature heat exchanger arrangement 23 and a low-temperature heat exchanger 25 and leads to a liquefied natural gas collection reservoir or the like, schematically shown at 27. The liquefied natural gas (LNG) stored in the reservoir 27 can be loaded in an LNG vessel or delivered through a pipeline.

[0047] As shown in the schematic diagram of Fig.1, similarly to Fig.2, in the embodiment of Fig. 2 the high-temperature heat exchanger arrangement 23 comprises a first high-temperature heat exchanger 23 A and a second high-temperature heat exchanger 23B. The first mixed refrigerant circuit comprises a primary refrigerant line 30, which delivers compressed and cooled first mixed refrigerant from the heat exchanger 9B of the first heat rejection device 9 through the high-temperature heat exchanger arrangement 23. More specifically, the primary refrigerant line 30 extends through a hot side of the second high-temperature heat exchanger 23B, in which the compressed and cooled first mixed refrigerant flowing from the first heat rejection device 9 is further cooled by heat exchange with a flow of vaporizing first mixed refrigerant counterflowing in the cold side of the second high-temperature heat exchanger 23B.

[0048] Downstream of the second high-temperature heat exchanger 23B the primary refrigerant line 30 is bifurcated at 33 into a first secondary refrigerant line 35A and a second secondary refrigerant line 35B. The first pressure reduction unit 11A of the pressure reduction device 11 is located in the first secondary refrigerant line 35 A and the second pressure reduction unit 1 IB of the pressure reduction device 11 is located in the second secondary refrigerant line 35B.

[0049] The compressed and cooled stream of first mixed refrigerant from the heat exchanger 9B is therefore split into a first stream which is depressurized in the first pressure reduction unit 11A and flows through the first high-temperature heat exchanger 23 A, and a second stream which is depressurized in the second pressure reduction unit 1 IB and flows through the second high-temperature heat exchanger 23B. [0050] The feed gas duct 19 extends through the hot side of the first high-temperature heat exchanger 23 A such that the feed gas is chilled by heat exchange against the vaporizing first stream of first mixed refrigerant flowing through the first secondary refrigerant line 35 A.

[0051] The vaporizing second stream of first mixed refrigerant, which expands in the second pressure reduction unit 1 IB, flows through the second secondary refrigerant line 35B and through the cold side of the second high-temperature heat exchanger 23B and removes heat from the stream of first mixed refrigerant flowing along the primary refrigerant line 30 through the hot side of the second high-temperature heat exchanger 23B. The first mixed refrigerant exiting the first high-temperature heat exchanger 23 A and the second high-temperature heat exchanger 23B is processed through the liq- uid/gas separator 10, if present, and collected at the suction side of compressor 7 A for compression and delivery to the intercooler 9A of the first heat rejection device 9.

[0052] The low-temperature mixed refrigerant circuit 5 includes a line 41, which extends from the second heat rejection device 15 through a hot side of the second high- temperature heat exchanger 23B, such that the vaporizing second stream of first mixed refrigerant flowing through the second secondary refrigerant line 35B and through the cold side of the second heat high-temperature exchanger 23B removes heat from the pressurized second mixed refrigerant delivered by the second heat rejection device 15 and circulating in the low-temperature mixed refrigerant circuit 5.

[0053] The low-temperature mixed refrigerant circuit 5 further extends through a hot side of the low-temperature heat exchanger 25. The pressurized and chilled second mixed refrigerant exiting the hot side of the low-temperature heat exchanger 25 is expanded in the pressure reduction device 17 and the expanded and vaporizing second mixed refrigerant flows through the cold side of the low-temperature heat exchanger 25 in heat exchange with the feed gas flowing through line 19 and in further heat exchange with the pressurized second mixed refrigerant along line 41 to remove heat therefrom.

[0054] The second mixed refrigerant exiting the cold side of the low-temperature heat exchanger 25 is delivered to the suction side of the second compression arrangement 13 for compression and delivery to the second heat rejection device 15. [0055] As mentioned with regard to Fig.1 , also in the liquefaction system 1 of Fig.2 the chilling capacity of the vaporizing first mixed refrigerant stream flowing through the first high-temperature heat exchanger 23 A is entirely used to cool the incoming compressed feed gas (natural gas), while the chilling capacity of the vaporizing first mixed refrigerant stream flowing through the second high-temperature heat exchanger 23B is used to chill the compressed first mixed refrigerant circulating in the high-tem- perature mixed refrigerant circuit 3, and the compressed second mixed refrigerant circulating in the low-temperature mixed refrigerant circuit 5.

[0056] In the embodiment of Fig.2, liquefied first mixed refrigerant collecting at the bottom of the liquid/gas separator 8 is delivered to the second high-temperature heat exchanger 23B and expands in an additional pressure reduction unit 11C and the vaporizing first mixed refrigerant resulting from the expansion flows though the cold side of the second high-temperature heat exchanger 23B.

[0057] Similarly, liquefied second mixed refrigerant collecting at the bottom of the liquid/gas separator 18 is expanded in a pressure reduction unit 17A and the vaporizing second mixed refrigerant resulting from the expansion flows in through the cold side of the low-temperature heat exchanger 25.

[0058] The liquefaction system 1 of Fig.2 further includes a heavy hydrocarbon removal system 50, known to those skilled in the art and not disclosed herein in detail. Liquefied petroleum gas from the heavy hydrocarbon removal system 50 can be collected in a reservoir 52.

[0059] A further embodiment of a liquefaction system according to the present disclosure is illustrated in Fig.3. The same reference numbers are used to designate the same components, parts or elements already shown in Fig.2 and described above. These parts will not be described again.

[0060] The embodiment of Fig.3 differs from the embodiment of Fig.2 mainly in that the feed gas pre-treatment section 21 includes a gas cooling circuit using liquefied first mixed refrigerant from the high-temperature mixed refrigerant circuit to pre-cool the pressurized feed gas and remove moisture therefrom. Incoming feed gas is fed along duct 19. [0061] The pre-treatment section 21 may have any suitable configuration known to those skilled in the art. The pre-treatment section comprises a pre-cooling heat exchanger 61 having a hot side through which partially pre-treated compressed feed gas flows, downstream of a gas sweetener 63, for instance. The pre-cooled feed gas is then processed through a drier 65 for moisture removal. Liquefied first mixed refrigerant from the liquid/gas separator 8 flows through the cold side of the pre-cooling heat exchanger 61 to remove heat from the feed gas. The exhausted first mixed refrigerant from the pre-cooling heat exchanger 61 is returned to the liquid/gas separator 10.

[0062] The heat exchangers 23 A, 23B and 25 can be configured as wound coil heat exchangers for example. In other embodiments, the heat exchangers 23 A, 23B and 25 can be configured as brazed aluminum heat exchangers. Fig.4 illustrates an embodiment using brazed aluminum heat exchangers 23 A, 23B, 25. Parts having the same function as those described in connection with Fig. 2 are labeled with the same reference numbers and are not described again. In the embodiment of Fig.4 a pre-treatment section using liquefied first mixed refrigerant to pre-cool the feed gas can be foreseen, as shown in Fig.3.

[0063] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.