WO2026013308A1

WO2026013308A1 - A single mixed refrigerant natural gas liquefaction method and system with integrated chilling for gas turbine air supply

Info

Publication number: WO2026013308A1
Application number: PCT/EP2025/070021
Authority: WO
Inventors: Marco TOGNOTTI; Tiziano PELLEGRINI; Ramesh Mamillapalli
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2024-07-12
Filing date: 2025-07-11
Publication date: 2026-01-15
Anticipated expiration: 2027-01-12

Abstract

The present disclosure relates to a method and a system of cooling a stream of natural gas by exchanging heat with a mixed refrigerant, i.e. a refrigerant fluid composed of two or more components, the mixed refrigerant being cooled in a closed thermodynamic cycle, wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization, the method comprising the following cyclic steps: - compressing a main stream of mixed refrigerant to different pressure values by a multi-stage compressor (300) driven by a gas turbine (400), a mixed refrigerant liquid fraction being obtained from each stage of the multi-stage compressor (300) and a mixed refrigerant vapor fraction being obtained from the last stage of the multi-stage compressor (300); - cooling the mixed refrigerant liquid fractions and the mixed refrigerant vapor fraction by exchanging heat with the main stream of sub-cooled mixed refrigerant, to obtain a plurality of streams of sub-cooled mixed refrigerant at different pressure, - flashing the streams of sub-cooled mixed refrigerant at different pressure to one common pressure, for further cooling the streams of sub-cooled mixed refrigerant and obtaining a plurality of streams of sub-cooled mixed refrigerant at a same pressure, - re-combining the streams of subcooled mixed refrigerant to obtain the main stream of sub-cooled mixed refrigerant, - exchanging heat between the main stream of sub-cooled mixed refrigerant and the stream of natural gas, and - returning the main stream to the compressing step, wherein the method further comprises the following steps: - splitting a portion of the mixed refrigerant liquid fraction from one of the stages of the multi-stage compressor (300), to obtain an auxiliary stream of mixed refrigerant, - cooling the auxiliary stream of mixed refrigerant by exchanging heat with the main stream of sub-cooled mixed refrigerant in a dedicated heat exchanger pass, to obtain an auxiliary stream of slightly sub-cooled mixed refrigerant, - flashing the auxiliary stream of slightly sub-cooled mixed refrigerant, obtaining an auxiliary stream of sub-cooled mixed refrigerant, - exchanging heat between the auxiliary stream of sub-cooled mixed refrigerant and the inlet air directed to the gas turbine (400) or alternatively between the auxiliary stream of sub-cooled mixed refrigerant and a gas turbine inlet air chilling fluid, - re-combining the auxiliary stream of sub-cooled mixed refrigerant with the main stream of mixed refrigerant, downstream of the step of exchanging heat between the main stream of sub-cooled mixed refrigerant and the stream of natural gas, and - returning the main stream of mixed refrigerant to the compressing step.

Description

A single mixed refrigerant natural gas liquefaction method and system with integrated chilling for gas turbine air supply

Description

TECHNICAL FIELD

[0001] The present disclosure concerns a method and a system for cooling a natural gas through a single mixed refrigerant with an integrated gas turbine air chilling system. Embodiments disclosed herein specifically concern but is not limited to a system for cooling a natural gas down to its condensation temperature, to obtain liquefied natural gas production.

[0002] In particular, the present disclosure concerns an integrated gas turbine air chilling system and a related method to maximize the gas turbine full power, even during adversely high ambient temperature conditions, thus increasing the overall efficiency of the liquefaction plant, by directly or indirectly cooling the air inlet of the gas turbine through heat exchange with a portion of the mixed refrigerant.

BACKGROUND ART

[0003] Natural gas is a naturally occurring hydrocarbon gas mixture comprising primarily of methane, but commonly including little amounts of other hydrocarbons, mainly light alkanes like propane and butane.

[0004] For practical and commercially viable transport of natural gas, its volume has to be greatly reduced. To do this, the gas is liquefied by means of refrigeration to less than -161°C (the boiling point of methane at atmospheric pressure). Each liquid natural gas production plant consists of one or more liquefaction and purification facilities to convert natural gas into liquefied natural gas.

[0005] The liquefaction process involves a preliminary removal of certain components, such as dust, acid gases, water, mercury and heavy hydrocarbons, which could cause problems in the liquefaction or even in downstream equipment. The natural gas is then condensed into a liquid (LNG) with a vapor pressure close to atmospheric pressure by cooling it to approximately -162°C; maximum transport pressure is set at around 25 kPa (4 psi).

[0006] In order to reduce the temperature of natural gas, the heat of the natural gas is transferred to a refrigerant fluid in controlled conditions through the use of heat exchangers. After having absorbed heat from the natural gas, in order to be reused, the refrigerant fluid is conveniently cooled in a closed thermodynamic refrigeration cycle, wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization.

[0007] In order to obtain the liquefaction of natural gas through heat exchange with a refrigerant fluid, efficiency of heat exchange is a key issue in order to achieve better performances and save costs. To this aim, the components of the LNG plant are carefully designed.

[0008] An important solution to increase the efficiency of heat exchange is the use of cryogenic multi-stream heat exchangers. Cryogenic multi-stream heat exchangers can be configured as cold box heat exchanger or as coil wound heat exchangers. In the following, the terms cryogenic multi-stream heat exchangers and the term multistream heat exchangers will be used to indicate the same device. Generally, a cold box heat exchanger is a complete package of brazed aluminum heat exchangers contained in a casing with structural support, thermal insulation containment, and protection for the internal equipment. The thermal insulation of heat exchangers and piping can be obtained in a single casing, making use of a common insulation, for example by using insulating materials inside the casing and by pressurizing and purging through dry nitrogen gas. Differently, a coil wound heat exchanger is a multi-stream heat exchanger, wherein the streams to be cooled flow inside coil wound tubes helically arranged inside a shell and the refrigerant is a liquid spread at different elevations within the shell through distributors to flow over the external surface of the coil wound tubes.

[0009] Cold box heat exchangers allow very compact layout and offer a highly efficient thermal heat exchange thanks to the use of low temperature approach, without maintenance needed, to the heat exchange between natural gas and refrigerant fluid. Additionally, on-site installation work is very limited and access to connection piping is relatively simple due to an optimized design, making on site activities a very quick and simple step and reducing pre-commissioning.

[0010] Mixed refrigerants are the most widely used technology for liquefying natural gas. In particular, a mixed refrigerant is a refrigerant fluid composed of two or more refrigerant fluids that are blended together to achieve specific properties for a particular refrigeration or liquefaction application. These mixtures are used in place of single refrigerants because they allow achieving higher thermodynamic efficiency, which can lead to reduced energy consumption and operating costs. This is because the different refrigerant components in a mixed refrigerant have different boiling points and heat capacities, which allows for a more efficient heat transfer process. Additionally, mixed refrigerants can operate over a wider range of temperatures than single refrigerants.

[0011] In a conventional natural gas liquefaction system, after having absorbed heat from the natural gas, the mixed refrigerant is cooled in a closed thermodynamic refrigeration cycle operated with a two-stage refrigerant compressor. Downstream the first stage compressor, the mixed refrigerant is cooled down to a temperature lower than the boiling point of part of its components, namely the heavier hydrocarbon parts of the mixture. The condensate fraction is separated while the remaining non condensed fraction is routed to the second stage compressor and subsequently to cooling and separation of a second liquid condensate. The liquid streams from the first and second stage compressors are precooled and the vapor stream from the second stage separator is condensed and subcooled (i.e. cooled at a temperature lower than its boiling temperature) in the multi-stream heat exchanger in separated passes and then expanded in respective Joule Thompson valves (herein after also called JT valves) where flashing occurs, generating the required cooling effect to condense the natural gas and to precool the mixed refrigerant streams as well.

[0012] The second stage vapor stream is a mixture comprising a fraction of light hydrocarbons sub-cooled to around -160°C before JT cooling.

[0013] It is known that natural gas liquefaction plants using single mixed refrigerant technology can employ gas turbines as driver for mixed refrigerant compressors. It is also known that conventional gas turbine air chilling systems use external refrigeration cycles to provide cooling duty to supply chilled air to a gas turbine. In fact, gas turbines are subject to performance loss, i.e. decrease of power output, in case of high ambient temperature, especially aero-derivative machines. Using a chiller system, the gas turbine is kept at full power, so that the liquefaction plant is not affected by shortage of gas turbine power, even during high ambient temperature conditions. For example, with a typical aero-derivative machine, chilling the inlet air from 30 to 10°C can improve the gas turbine power output by more than 25%. External chilling systems use an external refrigeration loop and run a refrigerant fluid (HFO or HCFC) in closed loop to provide the required chilling duty using intermediate glycol water circuit to the gas turbine inlet air. These systems typically use screw compressors, evaporators, condensers, expansion valves and require an additional plot plant aside to the liquefaction module and also external power supply.

[0014] The above described technology negatively affects the operation costs of the system and requires large footprint and therefore it can still be improved in order to further increase the efficiency of the system.

[0015] Therefore, there is a need for improved designs of a multi-stream heat exchanger that can address these challenges and provide efficient and reliable performance in various industrial applications.

[0016] Accordingly, an improved method and system for cooling natural gas to address the issues of cost and size of the systems of the current art would be beneficial and would be welcomed in the technology.

SUMMARY

[0017] In one aspect, the subject matter disclosed herein is directed to a method and a system for cooling natural gas, comprising a multi-stream heat exchanger configured to cool the natural gas by exchanging heat with a mixed refrigerant in a closed thermodynamic cycle system, employing one or more gas turbines as driver for mixed refrigerant compressors, wherein a small portion of the mixed refrigerant is used to provide cooling duty to supply chilled air to the gas turbines. The method and system include a multi-stage compressor with multiple compression stages driven by a gas turbine, cooling sections, and separators for separating the mixed refrigerant into liquid and vapor fractions. The multi-stream heat exchanger has passages for the natural gas and mixed refrigerant, with expansion devices and phase separators downstream to provide the refrigeration duty. Additionally, the method and system feature an auxiliary mixed refrigerant heat exchange cycle with an auxiliary warm passage for a portion of the mixed refrigerant liquid fraction from one of the separators, an auxiliary expansion device for flashing, i.e. expanding and separating into a liquid fraction and a vapor fraction and consequently additionally cooling the mixed refrigerant from the auxiliary warm passage, and a heat exchanger for shifting the cooling duty from the mixed refrigerant to the inlet air of the gas turbine, optionally via an intermediate cooling medium. The mixed refrigerant of the auxiliary mixed refrigerant heat exchange cycle is split from the liquid fraction of one of the separators downstream of the compression stages, preferably of the last compression stage, and is kept separate from the vapor fraction of the separator. The system may also include a gas turbine inlet air chilling circuit with an air chilling coil to cool the inlet air for the gas turbine. The mixed refrigerant circulating through the system, provides cooling for the natural gas and the gas turbine inlet air. In particular, the precooling required for the auxiliary mixed refrigerant in the auxiliary warm passage is minimized, most of the cooling being achieved by the auxiliary expansion device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig.1 illustrates a schematic of a single mixed refrigerant natural gas liquefaction system with integrated chilling system for gas turbine air supply, according to an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0019] According to one aspect, the present subject matter is directed to a method of cooling a stream of natural gas by exchanging heat with a mixed refrigerant, i.e. a refrigerant fluid composed of two or more components (hydrocarbons and not-hydro- carbons), the mixed refrigerant being cooled in a closed thermodynamic cycle, wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization, the method comprising the following cyclic steps:

- compressing a main stream of mixed refrigerant to different pressure values by a multi-stage compressor driven by a gas turbine, a mixed refrigerant liquid fraction being obtained from each stage of the multi-stage compressor and a mixed refrigerant vapor fraction being obtained from the last stage of the multi-stage compressor;

- cooling the mixed refrigerant liquid fractions and the mixed refrigerant vapor fraction by exchanging heat with the main stream of sub-cooled mixed refrigerant, to obtain a plurality of streams of sub-cooled mixed refrigerant at different pressure,

- flashing the streams of sub-cooled mixed refrigerant at different pressure to one common pressure, for further cooling the streams of sub-cooled mixed refrigerant and obtaining a plurality of streams of sub-cooled mixed refrigerant at a same pressure,

- re-combining the streams of sub-cooled mixed refrigerant to obtain the main stream of sub-cooled mixed refrigerant,

- exchanging heat between the main stream of sub-cooled mixed refrigerant and the stream of natural gas, and

- returning the main stream to the compressing step, wherein the method further comprises the following steps:

- splitting a portion of the mixed refrigerant liquid fraction from one of the stages of the multi-stage compressor, to obtain an auxiliary stream of mixed refrigerant,

- cooling the auxiliary stream of mixed refrigerant by exchanging heat with the main stream of sub-cooled mixed refrigerant in a dedicated heat exchanger pass, to obtain an auxiliary stream of slightly sub-cooled mixed refrigerant,

- flashing the auxiliary stream of slightly sub-cooled mixed refrigerant, obtaining an auxiliary stream of sub-cooled mixed refrigerant,

- exchanging heat between the auxiliary stream of subcooled mixed refrigerant and the inlet air directed to the gas turbine or alternatively between the auxiliary stream of subcooled mixed refrigerant and a gas turbine inlet air chilling fluid,

- re-combining the auxiliary stream of subcooled mixed refrigerant with the main stream of mixed refrigerant, downstream of the step of exchanging heat between the main stream of sub-cooled mixed refrigerant and the stream of natural gas, and - returning the main stream of mixed refrigerant to the compressing step.

[0020] In particular, the step of cooling the auxiliary stream of mixed refrigerant by exchanging heat with the main stream of sub-cooled mixed refrigerant, aims at slightly sub-cooling the saturated liquid mixed refrigerant split from the mixed refrigerant liquid fraction from one of the stages of the multi-stage compressor. In fact, an excessive sub-cooling could undermine the effect of the subsequent flashing step.

[0021] The method comprises the following optional features: splitting a portion of the mixed refrigerant liquid fraction is obtained by splitting a portion of the mixed refrigerant liquid fraction from the last stage of the multi-stage compressor; the temperature of the auxiliary stream of cooled mixed refrigerant from the step of cooling the auxiliary stream of mixed refrigerant by exchanging heat with the main stream of sub-cooled mixed refrigerant, is comprised between approximately 0 and 25°C; the temperature of the auxiliary stream of sub-cooled mixed refrigerant from the step of expanding the auxiliary stream of cooled mixed refrigerant, is comprised between approximately -30 and -5°C; the pressure of the auxiliary stream of sub-cooled mixed refrigerant from the step of expanding the auxiliary stream of cooled mixed refrigerant, is comprised between approximately 4 and 7 bara; the amount of the auxiliary stream of mixed refrigerant from the step of splitting a portion of the mixed refrigerant liquid fraction from one of the stages of the multi-stage compressor, is up to approximately 50% of the mixed refrigerant liquid fraction from the stage of the multi-stage compressor.

[0022] According to one aspect of the present disclosure the method comprises a step of compressing a main stream of mixed refrigerant to different pressure values by a multi-stage compressor.

[0023] According to another aspect, the present subject matter is directed to systems for cooling natural gas down to its liquefaction, through heat exchange in a multistream heat exchanger with a mixed refrigerant that, after having absorbed heat from the natural gas, in order to be reused, is cooled in a closed thermodynamic refrigeration cycle, wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization steps, the compression being a two stage compression, increasing the pressure of the mixed refrigerant from a low pressure to a first stage pressure, or medium pressure, and a second stage pressure, or high pressure, wherein a gas turbine is used as driver for the compression and wherein a portion of the mixed refrigerant is used in an auxiliary mixed refrigerant heat exchange cycle to cool the inlet air of the gas turbine, the auxiliary mixed refrigerant heat exchange cycle comprising auxiliary cooling steps, including an auxiliary expansion step and a heat exchanging step for shifting the cooling duty from the mixed refrigerant to the inlet air of the gas turbine. In particular, the multi-stream heat exchanger has a warm side and a cold side, the temperature inside the multi-stream heat exchanger decreasing with the distance from the warm side, and being provided with: a passage of the natural gas, with an inlet on the warm side and an outlet on the cold side of the multi-stream heat exchanger; a cold passage of the mixed refrigerant, at the low pressure, the cold passage being configured to exchange heat between the natural gas and the mixed refrigerant and having an outlet on the warm side of the multi-stream heat exchanger and a plurality of inlets, at different intermediate distances between the warm side and the cold side of the multi -stream heat exchanger;

- a plurality of warm passages for corresponding streams of mixed refrigerant from different compression stages, with respective inlets on the warm side of the multi-stream heat exchanger and respective outlets at different intermediate distances from the warm side of the multi-stream heat exchanger, each outlet being connected to a respective expansion device and separator, the expansion device being configured to expand the mixed refrigerant down to substantially the low pressure or an intermediate pressure and subsequently cool the mixed refrigerant, the separator being configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction, which are further pre-cooled and finally routed to the mixed refrigerant cold passage, and

- an auxiliary warm passage of the portion of the saturated liquid mixed refrigerant from one of the compression stages, to be used to cool the inlet air of the gas turbine, being kept separate from the saturated vapor from the compression stages.

[0024] Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0025] When introducing elements of various embodiments the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0026] The terms low pressure, medium pressure and high pressure are used to refer to relative values, in order to distinguish the different pressure values inside the system, and do not have any limiting value.

[0027] Moreover, the terms cold box, cold box heat exchanger and multi-stream heat exchanger are all used to indicate a same device.

[0028] Referring now to the drawings, Fig.1 shows a schematic of an exemplary system for cooling natural gas through heat exchange with a mixed refrigerant, the system comprising a multi-stream heat exchanger 100, or cold box 100, comprising a brazed aluminum heat exchanger 10 with a passage 101 of the natural gas, a plurality of warm passages 102, 103, 104 of the mixed refrigerant and a cold passage 105 of the mixed refrigerant. In particular, the passage 101 of the natural gas, the warm passages 102, 103, 104 of the mixed refrigerant and the cold passage 105 of the mixed refrigerant are comprised of different layers of aluminum plates and fins arranged side by side, all these metal parts being submerged in an insulating material inside the cold box 100.

[0029] Alternatively, the multi-stream heat exchanger can be a coil wound heat exchanger, wherein the natural gas passage, the warm passages, and the auxiliary passage are configured as heat exchange tubes or coils passing through a shell and wherein the shell is configured as the cold passage of the coil wound heat exchanger.

[0030] The passage 101 of the natural gas has an inlet at an end, or warm side of the multi-stream heat exchanger 100, and an outlet at the opposite end, or cold side of the multi-stream heat exchanger 100. The temperature inside the multi-stream heat exchanger 100 is higher at the warm side and decreases with the distance from the warm side of the multi-stream heat exchanger 100. The warm passages 102, 103, 104 of the mixed refrigerant have respective inlets on the warm side of the multi-stream heat exchanger 100 and respective outlets at different intermediate distances from the warm side of the multi-stream heat exchanger 100, while the cold passage 105 of the mixed refrigerant has an outlet on the warm side of the multi-stream heat exchanger 100 and a plurality of inlets, at different intermediate distances from the warm side of the multistream heat exchanger 100.

[0031] The outlet of warm passages 102, 103, 104 of the mixed refrigerant is connected to a respective expansion device and/or separator. In particular, a first warm passage 102, or medium pressure mixed refrigerant warm passage 102, has an outlet at a first distance from the warm side of the multi-stream heat exchanger 100 and is connected to a first expansion device 112 and to a first separator 122, with a liquid outlet at the bottom of the separator 122, the liquid outlet being connected to a liquid outlet line, and with a vapor outlet at the top of the separator 122, the vapor outlet being connected to a vapor outlet line, both the liquid outlet line and the vapor outlet line being connected to the mixed refrigerant cold passage 105 at a distance from the warm side of the multi-stream heat exchanger 100, which is slightly greater than the first distance, i.e. the distance of the outlet of the warm passage 102 from the warm side of the multi-stream heat exchanger 100. A second warm passage 103 or first high pressure mixed refrigerant warm passage 103 has an outlet at a second distance from the warm side of the multi-stream heat exchanger 100, the second distance being greater that the first distance, and is connected to a second expansion device 113 and to a second separator 123, with a liquid outlet at the bottom of the separator 123, the liquid outlet being connected to a liquid outlet line, and with a vapor outlet at the top of the separator 123, the vapor outlet being connected to a vapor outlet line, both the liquid outlet line and the vapor outlet line being connected to the mixed refrigerant cold passage 105 at a distance from the warm side of the multi-stream heat exchanger 100, which is slightly greater than the second distance, i.e. the distance of the outlet of the warm passage 103 from the warm side of the multi-stream heat exchanger 100. A third warm passage 104 or second high pressure mixed refrigerant warm passage 104 has an outlet at the cold side of the multi-stream heat exchanger 100 and is connected to a third expansion device 114 and to a third separator 124, with a liquid outlet at the bottom of the separator 124, the liquid outlet being connected to a liquid outlet line, and with a vapor outlet at the top of the separator 124, the vapor outlet being connected to a vapor outlet line, both the liquid outlet line and the vapor outlet line being connected to the mixed refrigerant cold passage 105 at the cold side of the multi-stream heat exchanger 100.

[0032] The system for cooling natural gas through heat exchange with a mixed refrigerant further comprises a closed thermodynamic cycle system 200, wherein the mixed refrigerant from the outlet of the cold passage 105 of the mixed refrigerant at the warm side of the multi-stream heat exchanger 100 is subjected to a cooling effect produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization. In particular, the closed thermodynamic cycle system 200 includes a two-stage compressor 300, downstream of a vessel 201 at a first pressure, or low pressure, the vessel 201 having an inlet connected to an outlet of the cold passage 105. The outlet of the low-pressure vessel 201 is connected to a first stage compressor 301, configured to compress the mixed refrigerant up to the medium pressure, the vessel 201 being configured as a suction scrubber of the first stage compressor 301. The outlet of the first stage compressor 301 is connected to a first stage cooling section 203, namely one or more coolers 203. A first stage separator 204 or medium pressure separator 204 is arranged downstream the coolers 203, the first stage separator 204 being configured to separate the mixed refrigerant at medium pressure into a medium pressure liquid fraction and a medium pressure vapor fraction, the first stage separator 204 having a liquid outlet connected to a first stage liquid outlet line 205, which is connected to the inlet of the medium pressure mixed refrigerant warm passage 102 of the multi-stream heat exchanger 100, and a vapor outlet connected to a first stage vapor outlet line 206. The first stage vapor outlet line 206 is connected to a second stage compressor 302. A second stage cooling section 208, namely a cooler 208 is arranged downstream the second stage compressor 302. A second stage separator 209 or high pressure separator 209 is arranged downstream the second stage cooling section 208 and is configured to separate the mixed refrigerant at high pressure into a high pressure liquid fraction and a high pressure vapor fraction, the second stage separator 209 having a liquid outlet connected to a second stage liquid outlet line 210, which is connected to the inlet of the first high pressure mixed refrigerant warm passage 103 of the multi-stream heat exchanger 100, and a vapor outlet connected to a second stage vapor outlet line 211, which is connected to the inlet of the second high pressure mixed refrigerant warm passage 104 of the multi-stream heat exchanger 100.

[0033] The two-stage compressor 300 is driven by a gas turbine 400 through a common shaft or any other mechanical coupling allowing transmission of kinetic energy of the gas turbine 400 to the compressor 300. The gas turbine 400 comprises an air chilling system to chill the air and keep the gas turbine at full power, even during high ambient temperature cases. The air chilling system comprises a gas turbine inlet air chilling circuit 500, circulating a gas turbine inlet air chilling fluid, an air chilling heat exchanger 501 configured to cool the inlet air directed to the gas turbine 400 by heat exchange with the gas turbine inlet air chilling fluid. The cooling duty of the gas turbine inlet air chilling fluid is provided in a heat exchanger 126 allowing heat exchange with a part of the mixed refrigerant from the multi-stream heat exchanger 100. In particular, an auxiliary warm liquid passage 106, or auxiliary passage 106, of saturated liquid mixed refrigerant, is split from the liquid outlet line 210, the auxiliary passage 106 having an inlet on the warm side of the multi-stream heat exchanger 100 and an outlet configured to withdraw the mixed refrigerant from the multi-stream heat exchanger 100 at a temperature set as defined herein below. The auxiliary passage 106 is a small (few layers) and short passage to slightly sub-cool the saturated liquid mixed refrigerant split from the liquid outlet line 210 of the separator 209. In an exemplary embodiment, this stream is extracted from the multi-stream heat exchanger 100 at approximately 0-25°C. The mixed refrigerant from the auxiliary passage 106 is directed to an auxiliary expansion device 116, typically a JT control valve or a hydraulic expander located outside the multi-stream heat exchanger 100. Due to the liquid flash of the light components of the mixed refrigerant occurring in the auxiliary expansion device 116, a considerable cooling effect is obtained. In an exemplary embodiment, the pressure of the mixed refrigerant is reduced in the auxiliary expansion device 116 down to 4-7 bara and the temperature down to approximately -30/-5°C. The mixed refrigerant is then directed to heat exchanger 126 to cool the gas turbine inlet air chilling fluid, typically a mixture of glycol and water running in a closed loop from 10-20°C to 3-10°C, depending on the selected air/water temperature profiles. The refrigerated gas turbine inlet air chilling fluid is then returned to the air chilling heat exchanger 501 to cool the ambient air directed to the gas turbine 400. A return line 136 of the mixed refrigerant, downstream of the heat exchanger 126 allows the mixed refrigerant from the heat exchanger 126 to be routed to the vessel 201, or suction scrubber of the first stage compressor 301, where it mixes with the warm mixed refrigerant from the cold passage 105. Alternatively, the return line 136 of the mixed refrigerant can be connected to the return line 115 of the cold passage 105. Optionally, the return line 136 of the mixed refrigerant can be routed to the first stage separator 204 or medium pressure separator 204, this option being only available when the auxiliary warm liquid passage 106 of saturated liquid mixed refrigerant is split from the liquid outlet line 210 of the second stage separator 209 or high-pressure separator 209.

[0034] According to an auxiliary embodiment, recirculation of water condensate from the gas turbine air chilling heat exchanger 501 to one of the mixed refrigerant cooling sections of the multistage compressor 300, in particular, according to the embodiment of Fig.1 to the second stage cooling section 208. In fact, according to an embodiment, the second stage cooling section 208 is configured as an aircooler with an air suction side with fans. When using the GT air duct chilling heat exchanger 501 to cool the air entering the combustion chamber of the gas turbine 400, a considerable amount of air humidity is condensed, especially in summer conditions. For example, chilling around 190kg/s of air from 31°C and 50%HR to 10°C, approx. 4.6 ton/h of water at 10°C are condensed. This water can be accumulated in a small tank and then, via a pump, can be addressed to several nozzles located around the mixed refrigerant aircooler fans air suction side of the second stage cooling section 208, to form a fine water spray. This spray, coming in contact with the warm air, evaporates and cools down the air flow to the mixed refrigerant aircooler of the second stage cooling section 208, while increasing the air humidity (air humidification). In this case, the decrease in temperature is around 1.5-2°C, with a 20% humidity increase. These two factors improve the heat exchange in the aircooler of the second stage cooling section 208 (10-12%), this is particularly advantageous in summer conditions, when the aircooler can have problems in keeping the right set point. At the end, the liquid water condensed in the air chilling heat exchanger 501 is returning back to atmosphere as vapor humidity, with no need of disposal.

[0035] According to alternative embodiments, the auxiliary passage 106 of saturated liquid mixed refrigerant can be split from the liquid outlet line 205 of the first stage separator 204 or medium pressure separator 204 or from the liquid outlet line 210 of the second stage separator 209 or high-pressure separator 209. When the auxiliary warm liquid passage 106 of saturated liquid mixed refrigerant is split from the liquid outlet line 205 of the first stage separator 204 or medium pressure separator 204, the return line 136 of the mixed refrigerant can only be routed to the vessel 201 at the low pressure. However, the spillage of mixed refrigerant from the liquid outlet line 210 of the high-pressure separator 209 is the preferred solution due to a higher pressure than that of the liquid outlet line 205 of the medium pressure separator 204 and a greater concentration of lighter components, thus increasing the liquid flash downstream the expansion device 116 and the generated cooling. Importantly, the warm liquid passage 106 is always kept separate from the first stage vapor outlet line 206 of the first stage separator 204 and from the second stage vapor outlet line 211 of the second stage separator 209. In an exemplary embodiment, with respect to the total flow of the mixed refrigerant, the mixed refrigerant split from the liquid outlet line 210 can be up to approximately 50%, preferably is around 3-10%, depending on the duty of the heat exchanger 126.

[0036] The impact on the production of liquified natural gas due to the spillage of mixed refrigerant for chilling purposes is limited to the refrigeration power spent on chiller duty plus the pre-cooling duty of the chiller pass. Anyway, most of the refrigeration effect is generated outside the multi-stream heat exchanger 100, by throttling the pressure in the expansion device 116 and flashing the liquid mixed refrigerant. The liquid flow of mixed refrigerant for chilling purpose can be modulated depending on ambient condition and gas turbine load. In winter conditions, when gas turbine inlet air chilling is not required, the multi-stream heat exchanger 100 can even be run with no flow on the auxiliary mixed refrigerant heat exchange cycle and in particular on the auxiliary passage. [0037] According with the purpose of the present disclosure, the solution of the present disclosure has the benefit that the capital cost and the required plot plan of integrated chilling system versus the standard external chiller are consistently reduced, due to less equipment involved and to the use of same fluid for liquefying the natural gas and to chill the GT air.

[0038] While the invention has been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing from the spirit and scope of the claims. In addition, unless specified otherwise herein, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.

[0039] For example, the cooling duty of the portion of mixed refrigerant, which is split from the liquid fraction of one of the separators, and which is kept separate from the vapor fraction of the separators, can be used for other chilling services of the plant when needed.

Claims

A single mixed refrigerant natural gas liquefaction method and system with integrated chilling for gas turbine air supply CLAIMS

1. A method of cooling a stream of natural gas by exchanging heat with a mixed refrigerant, i.e. a refrigerant fluid composed of two or more components, the mixed refrigerant being cooled in a closed thermodynamic cycle, wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization, the method comprising the following cyclic steps:

- compressing a main stream of mixed refrigerant to different pressure values by a multi-stage compressor (300) driven by a gas turbine (400), a mixed refrigerant liquid fraction being obtained from each stage of the multi-stage compressor (300) and a mixed refrigerant vapor fraction being obtained from the last stage of the multi-stage compressor (300);

- re-combining the streams of subcooled mixed refrigerant to obtain the main stream of sub-cooled mixed refrigerant,

- splitting a portion of the mixed refrigerant liquid fraction from one of the stages of the multi-stage compressor (300), to obtain an auxiliary stream of mixed refrigerant, - cooling the auxiliary stream of mixed refrigerant by exchanging heat with the main stream of sub-cooled mixed refrigerant in a dedicated heat exchanger pass, to obtain an auxiliary stream of slightly sub-cooled mixed refrigerant,

- exchanging heat between the auxiliary stream of sub-cooled mixed refrigerant and the inlet air directed to the gas turbine (400) or alternatively between the auxiliary stream of subcooled mixed refrigerant and a gas turbine inlet air chilling fluid,

- re-combining the auxiliary stream of sub-cooled mixed refrigerant with the main stream of mixed refrigerant, downstream of the step of exchanging heat between the main stream of sub-cooled mixed refrigerant and the stream of natural gas, and

- returning the main stream of mixed refrigerant to the compressing step.

2. The method of claim 1, wherein the step of splitting a portion of the mixed refrigerant liquid fraction is obtained by splitting a portion of the mixed refrigerant liquid fraction from the last stage of the multi-stage compressor (300).

3. The method of claim 1 or 2, wherein the temperature of the auxiliary stream of cooled mixed refrigerant from the step of cooling the auxiliary stream of mixed refrigerant by exchanging heat with the main stream of sub-cooled mixed refrigerant, is comprised between approximately 0 and 25°C.

4. The method of one or more of the previous claims, wherein the temperature of the auxiliary stream of sub-cooled mixed refrigerant from the step of expanding the auxiliary stream of cooled mixed refrigerant, is comprised between approximately -30 and -5°C.

5. The method of one or more of the previous claims, wherein the pressure of the auxiliary stream of sub-cooled mixed refrigerant from the step of expanding the auxiliary stream of cooled mixed refrigerant, is comprised between approximately 4 and 7 bara.

6. The method of one or more of the previous claims, wherein the amount of the auxiliary stream of mixed refrigerant from the step of splitting a portion of the mixed refrigerant liquid fraction from one of the stages of the multi-stage compressor (300), is up to approximately 50% of the mixed refrigerant liquid fraction from the stage of the multi-stage compressor (300).

7. A system for cooling natural gas, the system comprising a multistream heat exchanger (100) configured to cool the natural gas by exchanging heat with a mixed refrigerant, i.e. a refrigerant fluid composed of two or more components, the mixed refrigerant being cooled in a closed thermodynamic cycle system (200), wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization; the closed thermodynamic cycle system (200) comprising

- a multi-stage compressor (300) comprising a plurality of compression stages (301, 302) in a sequence, comprising at least one first compression stage (301) and one last compression stage (302), the plurality of compression stages (301, 302) being driven by a gas turbine (400) and being configured to compress the mixed refrigerant to different pressure values, a respective cooling section (203, 208), comprising at least one first cooling section (203) and one last cooling section (208), being connected downstream of each compressor stage (301, 302), a respective separator (204, 209), comprising at least one first separator (204) and one last separator (209), being connected downstream of each cooling section (203, 208) and being configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction, the vapor fraction of each separator (204) except the last separator (209) being directed to a subsequent compression stage, the liquid fraction of each separator (204, 209) and the vapor fraction of the last separator (209) being directed to a respective passage (102, 103, 104) of a plurality of passages (102, 103, 104) or warm passages (102, 103, 104), of the multi-stream heat exchanger (100) through a respective liquid outlet line (205, 210) or vapor outlet line (211); the multi-stream heat exchanger (100) comprising

- a passage (101) of the natural gas and the plurality of warm passages (102, 103, 104) of the mixed refrigerant, the passage (101) and the warm passages (102, 103, 104) having an inlet on the same side or warm side of the multi-stream heat exchanger (100), the warm passages (102, 103, 104) comprising warm liquid passages (102, 103) of saturated liquid mixed refrigerant from respective liquid outlet lines (205, 210) and a warm vapor passage (104) of saturated vapor mixed refrigerant from the last separator (209); each warm passage (102, 103, 104) having a respective outlet from the multi-stream heat exchanger (100) at a respective distance from the warm side of the multi-stream heat exchanger (100);

- a cooling section of each warm passage (102, 103, 104) of mixed refrigerant, downstream of the outlet from the multi-stream heat exchanger (100), each cooling section comprising an expansion device (112, 113, 114), configured to expand and consequently cool the mixed refrigerant and a respective phase separator (122, 123, 124) connected downstream the expansion device (112, 113, 114) and configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction, the liquid fraction and the vapor fraction of each phase separator (122, 123, 124) being connected to a further passage (105) or cold passage (105) of the mixed refrigerant in the multi-stream heat exchanger (100), the mixed refrigerant of the cold passage (105) flowing in a countercurrent direction with respect to the flow direction of the natural gas of the passage (101) and of the mixed refrigerant of the warm passages (102, 103, 104), the cold passage (105) being connected to the first compression stage (301) of the multi-stage compressor (300) through a return line (115), downstream of the cold passage (105); wherein the system further comprises an auxiliary mixed refrigerant heat exchange cycle, comprising

- an auxiliary warm liquid passage (106) or auxiliary passage (106) of saturated liquid mixed refrigerant, the mixed refrigerant being split from one of the liquid outlet lines (205, 210) and being kept separate from the vapor outlet line (211), the auxiliary passage (106) having an inlet on the warm side of the multi-stream heat exchanger (100) and an outlet configured to withdraw the sub-cooled mixed refrigerant liquid from the multi-stream heat exchanger (100) at a first temperature;

- an auxiliary expansion device (116), downstream of the auxiliary passage (106) and configured to expand and consequently cool the mixed refrigerant from the first temperature down to a second temperature;

- heat exchanging means (126, 501), downstream of the auxiliary expansion device (116), configured to exchange cold duty between the mixed refrigerant and the inlet air directed to the gas turbine (400) or alternatively through an intermediate medium or gas turbine inlet air chilling fluid; and - a return line (136) of the mixed refrigerant, downstream of the heat exchanging means (126, 501) and connected upstream of one of the plurality of compression stages (301, 302).

8. The system of claim 7, further comprising a gas turbine inlet air chilling circuit (500), circulating the gas turbine inlet air chilling fluid and comprising:

- a heat exchanger (126), wherein the gas turbine inlet air chilling fluid is cooled by exchanging cold duty with the portion of mixed refrigerant from the expansion device (116);

- an air chilling heat exchanger (501), downstream of the heat exchanger (126) and configured to cool the inlet air directed to the gas turbine (400) by heat exchange with the gas turbine inlet air chilling fluid, the outlet of the gas turbine inlet air chilling fluid from the air chilling heat exchanger (501) being connected to the inlet of the gas turbine inlet air chilling fluid of the heat exchanger (126).

9. The system of claim 8, wherein the heat exchanger (126) is arranged outside the multi stream heat exchanger (100).

10. The system of one or more of claims 7-9, wherein the auxiliary passage (106) is split from the liquid outlet line (210) downstream the last compressor (302) of the multi-stage compressor (300).

11. The system of one or more of claims 7-10, wherein the position of the outlet of the auxiliary warm passage (106) from the multi-stream heat exchanger (100) is set to allow the auxiliary expansion device (116) to lower the temperature of the liquid mixed refrigerant down to the second temperature.

12. The system of one or more of claims 7-11, wherein the return line (136) of the mixed refrigerant is connected to a vessel (201), or suction scrubber of the first compression stage (301).

13. The system of one or more of claims 7-11, wherein the return line (136) of the mixed refrigerant is connected to the return line (115) of the cold passage (105).

14. The system of one or more of claims 7-13, wherein the multi-stage compressor (300) is a two-stage compressor (300).

15. The system of one or more of claims 7-13, wherein the multi-stage compressor is a three-stage compressor.

16. The system of one or more of claims 7-15, wherein the multi-stream heat exchanger (100) is a brazed aluminum heat exchanger or a coil wound heat exchanger.

17. The system of one or more of claims 7-16, wherein the first temperature, i.e. the temperature of the mixed refrigerant at the outlet of the auxiliary passage (106) from the multi-stream heat exchanger (100), is comprised between approximately 0 and 25°C.

18. The system of one or more of claims 7-17, wherein the second temperature, i.e. the temperature of the mixed refrigerant downstream of the liquid split expansion device (116), is comprised between approximately -30 and -5°C.

19. The system of one or more of claims 7-18, wherein the pressure of the mixed refrigerant downstream of the auxiliary expansion device (116), is comprised between approximately 4 and 7 bara.

20. The system of one or more of claims 7-19, wherein the gas turbine chilling fluid is a mixture of water and glycol.

21. The system of one or more of claims 7-20, wherein a flow valve is arranged along the return line (136) of the mixed refrigerant.

22. The system of one or more of claims 7-21, wherein the temperature of the gas turbine inlet air chilling fluid inside the gas turbine air chilling circuit (500) in comprised between a minimum temperature downstream of the heat exchanger (126), comprised between approximately 3 and 10°C and a maximum temperature downstream of the air chilling heat exchanger (501), comprised between approximately 10 and 20°C.

23. The system of one or more of claims 7-22, wherein the amount of mixed refrigerant, split from one of the liquid outlet lines (205, 210) and directed to the auxiliary passage (106) is up to approximately 50% of the mixed refrigerant flowing inside the liquid outlet line (205, 210).