A system for cooling natural gas with a mixed refrigerant

Abstract

The present disclosure relates to a system for cooling natural gas, in particular down to its liquefaction, through heat exchange in a multi-stream heat exchanger with a mixed refrigerant that, after having absorbed heat from the natural gas 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 three stage compression. In particular, the multi-stream heat exchanger is provided with: - a hot passage of the natural gas; - a cold passage of the mixed refrigerant; and - a plurality of hot passages for pre-cooling of corresponding streams of mixed refrigerant from different compression stages, and a respective expansion device and separator, the expansion device being configured to expand and at least partially flash 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 routed to the mixed refrigerant cold passage.

Description

A system for cooling natural gas with a mixed refrigerant DescriptionTECHNICAL FIELD

[0001] The present disclosure relates to a system for cooling natural gas. Embodiments disclosed herein specifically concern but are not limited to a system for cooling a natural gas down to its condensation temperature, to obtain liquefied natural gas production.BACKGROUND ART

[0002] 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.

[0003] 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.

[0004] 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 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).

[0005] 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.

[0006] 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 save costs. To this aim, the components of the liquid natural gas production unit are carefully designed.

[0007] An important solution to increase the efficiency of heat exchange is the use of multi-stream heat exchangers, which are often part of so called cold boxes. Generally, a cold box 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.

[0008] Cold boxes allow very compact layout and offer a highly efficient thermal insulation, 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 simple due to an optimized design, making construction a very quick and simple step and reducing pre-commissioning.

[0009] 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.

[0010] 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 hydrocarbons of themixture. 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 compressor are precooled in the multi-stream heat exchanger and then expanded in Joule Thomson valves where flashing occurs, generating the required cooling effect to condense the natural gas and to precool the mixed refrigerant streams as well, followed by vaporizing and exchanging heat with the natural gas. The vapor stream from the second stage separator is used for mid liquefaction and sub-cooling sections by Joule- Thomson cooling followed by vaporizing and exchanging heat with the natural gas.

[0011] However, the use of multi-stream heat exchangers for liquefying a stream of a natural gas can still be improved in order to further increase the efficiency of the system, which is according to the prior art, generally non-optimal.

[0012] 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.

[0013] Accordingly, an improved multi-stream heat exchanger to address the issues of conversion rate of the systems of the current art would be beneficial and would be welcomed in the technology.SUMMARY

[0014] In one aspect, the subject matter disclosed herein is directed to a system for cooling natural gas, the system comprising a multi-stream heat exchanger configured to cool the natural gas by exchanging heat with a mixed refrigerant, wherein the system comprises a closed thermodynamic refrigeration cycle of the mixed refrigerant operated with a three-stage refrigerant compressor, namely a first stage, a second stage and an intermediate stage, wherein each stage of the compressor is configured to compress a mixed refrigerant at a respective pressure, wherein a cooling section and a separator are arranged downstream each stage of the compressor to condensate and separate the mixed refrigerant from the stage of the compressor into a liquid fraction and a vapor fraction and wherein the liquid fractions of each stage and the vapor fraction of the second stage are cooled by Joule-Thomson valves and subsequently used to cool the natural gas. The closed thermodynamic refrigeration cycle of the mixed refrigerantoperated with a three-stage mixed refrigerant compressor allows for compressing colder gas and reducing the compressor absorbed power, reducing the duty of the mixed refrigerant cooling section downstream each compression stage, providing one additional liquid stream to the multi-stream heat exchanger, optimizing the heat exchange in the warm section of the multi-stream heat exchanger, and also slightly reducing the mixed refrigerant pre-cooling duty. Moreover, the system for cooling natural gas according to the present disclosure allows for reducing the temperature difference between the mixed refrigerant and the natural gas in the different sections of the multi-stream heat exchanger, in particular in the cold end section of the multi-stream heat exchanger. The system also provides for the following advantages over the prior art: reduction in specific energy; reduction in carbon foot print;- reduction of the overall weight / space of the natural gas liquefaction system and / or increase of the production of liquefied natural gas with the same overall weight / space; and improvement of the mixed refrigerant flexibility during turndown operating mode.

[0015] In another aspect, the subject matter disclosed herein concerns a system for cooling natural gas wherein the vapor from the separator downstream the second stage compressor is withdrawn from the multi-stream heat exchanger and routed to an additional separator to be split into a liquid stream and a vapour stream, in order to reduce the precooling duty of the hot mixed refrigerant, thus reducing the specific power needed by the system. Moreover, the reduced pressure ratio of each compression stage, combined with dedicated anti-surge valves leads to an increased efficiency at partial load due a larger compressor operating envelope area as resulting in more margin on surge control line compared to traditional two stages SMR cycle.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A more complete appreciation of the disclosed embodiments of the invention and many of the attended 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 view of a system for cooling natural gas with a mixed refrigerant according to a first embodiment;Fig. la illustrates a line graph of the temperature of the cold stream and the hot streams inside the multi-stream heat exchanger as a function of the exchanged heat flow in a natural gas liquefaction system according to the prior art;Fig. lb illustrates a line graph of the temperature of the cold stream and the hot streams inside the multi-stream heat exchanger as a function of the exchanged heat flow in a natural gas liquefaction system according to the system of Fig.1;Fig.2 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a second embodiment;Fig.3 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a third embodiment;Fig.4 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a fourth embodiment;Fig.5 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a fifth embodiment;Fig.6 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a sixth embodiment; andFig.7 illustrates a section view of a system for cooling natural gas with a mixed refrigerant according to a seventh embodiment.DETAILED DESCRIPTION OF EMBODIMENTS

[0017] According to one aspect, the present subject matter is directed to systems and methods for cooling natural gas down to its liquefaction, through heat exchange in a multi-stream 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 three stage compression, increasing the pressure of the mixed refrigerant from a low pressure to a first stage pressure, or intermediate pressure, an intermediate stage pressure, or intermediate-high pressure and a second stage pressure, or high pressure. In particular, the multi-stream heat exchanger has a warm end and a cold end, the temperature inside the multi-stream heat exchanger decreasing with the distance from the warm end, and being provided with:a hot passage of the natural gas, with an inlet on the warm end and an outlet on the cold end 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 end of the multi-stream heat exchanger and a plurality of inlets, at different intermediate elevations between the warm end and the cold end of the multi -stream heat exchanger; and- a plurality of hot passages for corresponding streams of mixed refrigerant from different compression stages, with respective inlets on the warm end of the multistream heat exchanger and respective outlets at different intermediate distances from the warm end 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 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 routed to the mixed refrigerant cold passage.

[0018] According to one aspect, the first stage compressor, the second stage compressor and the intermediate stage compressor are allocated in one single casing. This configuration allows for greater compactness and reduced footprint. For the same reason, the intermediate compressor and the second stage compressor are preferably arranged in-line and in back to back configuration with the first stage compressor. Alternatively, the first stage compressor and the intermediate compressor can be arranged in-line and the second stage compressor in back to back configuration.

[0019] 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 placesthroughout 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.

[0020] 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.

[0021] The terms low pressure, intermediate pressure, intermediate-high 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.

[0022] 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, in particular a brazed aluminum heat exchanger enclosed in a cold box heat exchanger, with a casing 10 or box 10, a hot passage 101 of the natural gas, a plurality of hot passages 102, 103, 104, 105 of the mixed refrigerant and a cold passage 106 of the mixed refrigerant. In particular, the hot passage 101 of the natural gas, the hot passages 102, 103, 104, 105 of the mixed refrigerant and the cold passage 106 of the mixed refrigerant are comprised of plates and fins arranged side by side and surrounded by an insulating material inside a box 10. The hot passage 101 of the natural gas has an inlet on an end, or warm end of the multi-stream heat exchanger 100, and an outlet on the opposite end, or cold end of the multi-stream heat exchanger 100. The temperature inside the multi-stream heat exchanger 100 is higher at the warm end and decreases with the distance from the warm end of the multi-stream heat exchanger 100. The hot passages 102, 103, 104, 105 of the mixed refrigerant have respective inlets on the warm end of the multi-stream heat exchanger 100 and respective outlets at different intermediate distances from the warm end of the multi-stream heat exchanger 100, while the cold passage 106 of the mixed refrigerant has an outlet on the warm end of the multi-stream heat exchanger 100 and a plurality of inlets, at different intermediate distances from the warm end of the multistream heat exchanger 100.

[0023] The outlet of each hot passage 102, 103, 104, 105 of the mixed refrigerant is connected to a respective expansion device and separator. In particular, a first hot passage 102, or intermediate pressure mixed refrigerant hot passage 102, has an outlet at a first distance from the warm end of the multi-stream heat exchanger 100 and is connected to a first expansion device 107 or intermediate pressure expansion device 107 and to a first separator 108, with a liquid outlet at the bottom of the separator 108, the liquid outlet being connected to a liquid outlet line 109, and with a vapor outlet at the top of the separator 108, the vapor outlet being connected to a vapor outlet line 110, both the liquid outlet line 109 and the vapor outlet line 110 being connected to the mixed refrigerant cold passage 106 at a distance from the warm end of the multi-stream 100, which is slightly greater than the first distance. A second hot passage 103 or intermediate-high pressure mixed refrigerant hot passage 103 has an outlet at an intermediate distance from the warm end of the multi-stream heat exchanger 100, the intermediate distance being greater that the first distance, and is connected to an inter- mediate-high-pressure expansion device 111 and to a second separator 112, with a liquid outlet at the bottom of the separator 112, the liquid outlet being connected to a liquid outlet line 113, and with a vapor outlet at the top of the separator 112, the vapor outlet being connected to a vapor outlet line 114, both the liquid outlet line 113 and the vapor outlet line 114 being connected to the mixed refrigerant cold passage 106 at a distance from the warm end of the multi-stream 100, which is slightly greater than the second distance. A third hot passage 104 or first high pressure mixed refrigerant hot passage 104 has an outlet at a third distance from the warm end of the multi-stream heat exchanger 100, the third distance being greater that the second distance, and is connected to a first high pressure mixed refrigerant expansion device 115 and to a third separator 116, with a liquid outlet at the bottom of the third separator 116, the liquid outlet being connected to a liquid outlet line 117, and with a vapor outlet at the top of the third separator 116, the vapor outlet being connected to a vapor outlet line 118, both the liquid outlet line 117 and the vapor outlet line 118 being connected to the cold passage 106 at a distance from the warm end of the multi-stream 100, which is slightly greater than the third distance. A fourth hot passage 105 or second high pressure mixed refrigerant hot passage 105 has an outlet at the cold end of the multi-stream heat exchanger 100, and is connected to a second high pressure expansion device 119 and to a fourth separator 120, with a liquid outlet at the bottom of the fourth separator 120, the liquid outlet being connected to a liquid outlet line 121, and with a vaporoutlet at the top of the fourth separator 120, the vapor outlet being connected to a vapor outlet line 122, both the liquid outlet line 121 and the vapor outlet line 122 being connected to the mixed refrigerant cold passage 106 at substantially the cold end of the multi-stream 100.

[0024] Additionally, the system for cooling natural gas through heat exchange with a mixed refrigerant further comprises a multistage compression section 200, which comprises a vessel 201 at a first pressure, or low pressure, having an inlet connected to an outlet of the cold passage 106 and an outlet connected to a first stage compressor202, configured to compress the mixed refrigerant up to an intermediate pressure, the vessel 101 being configured as a suction scrubber of the first stage compressor 202. The outlet of the first stage compressor 202 is connected to a first stage cooling section203, namely one or more air coolers 203. A first stage separator 204 is arranged downstream the air coolers 203, the first stage separator 204 being configured to separate the mixed refrigerant at intermediate pressure into an intermediate pressure liquid fraction and an intermediate 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 intermediate pressure mixed refrigerant hot passage 102 of the multi-stream 100, and a vapor outlet connected to a first stage vapor outlet line 206. The first stage vapor outlet line 206 is connected to an intermediate stage compressor 207 configured to compress the mixed refrigerant up to an intermediate-high pressure. An intermediate stage cooling section 208, namely an air cooler 208 is arranged downstream the intermediate stage compressor 207. An intermediate stage separator 209 is arranged downstream the intermediate stage cooling section 208 and is configured to separate the mixed refrigerant at intermediate-high pressure into an intermediate-high-pressure liquid fraction and an intermediate-high-pressure vapor fraction, the intermediate stage separator 209 having a liquid outlet connected to an intermediate stage liquid outlet line 210, which is connected to the inlet of the intermediate-high-pressure hot passage 103 of the multi-stream 100, and a vapor outlet connected to an intermediate stage vapor outlet line 211. Finally, the intermediate stage vapor outlet line 211 is connected to a second stage compressor 212. A second stage cooling section 213, namely an air cooler 213 is arranged downstream the second stage compressor 212. A second stage separator 214 is arranged downstream the second stage cooling section 213 and is configured to separate the mixed refrigerant at high pressure into a high-pressure liquidfraction and a high-pressure vapor fraction, the third stage separator 214 having a liquid outlet connected to a third stage liquid outlet line 215, which is connected to the inlet of the first high-pressure mixed refrigerant hot passage 104 of the multi-stream 100, and a vapor outlet connected to a second stage vapor outlet line 216, which is connected to the inlet of the second high-pressure mixed refrigerant hot passage 105 of the multi-stream 100.

[0025] According to one aspect, the first stage compressor 202, the second stage compressor 212 and the intermediate stage compressor 207 are allocated in one single casing. Alternatively, the first stage compressor 202, the second stage compressor 212 and the intermediate stage compressor 207 can be arranged in line, or the first stage compressor 202 and the intermediate compressor 207 are arranged in-line and the second stage compressor 212 is arranged in back to back configuration or the first stage compressor 202 is arranged in-line and the intermediate compressor 207 and the second stage compressor 212 are arranged in back to back configuration.

[0026] The system operates as follows. The mixed refrigerant stream from the cold passage 106, a superheated vapor at the low pressure and at a temperature approaching the temperature of the mixed refrigerant flowing through the hot passages 101, 102, 103, 104, 105, flows from the multi-stream 100 to the suction scrubber 201 of the first stage mixed refrigerant compressor 202. In case of upset conditions, any liquid carryover from the multi-stream 100 is separated in the vessel 201 before the vapor stream is fed into the first stage compressor 202, which is the first of three compressor stages202, 207, 212 in series. Each stage discharge of the three stage mixed refrigerant compressor is cooled by a respective cooling section, namely air coolers 203, 208, 213. The mixed refrigerant stream from the discharge of the first stage compressor 202, at a first stage pressure or intermediate pressure, flows to the first stage cooling section203, namely an air cooled mixed refrigerant cooler 203. At this point, the mixed refrigerant is partially condensed, namely at least part of the components of the mixed refrigerant having a higher boiling point are condensed. The outlet temperature of the first stage cooling section 203 is controlled at a temperature higher than the mixed refrigerant dewpoint at the corresponding pressures to ensure no liquid dropout. The partially condensed mixed refrigerant enters the first stage separator 204 to separate the vapor and liquid. The vapor exiting from the first stage separator 204 is routed to the intermediate stage compressor 207 and the separated first stage liquid is routed, ata first stage pressure or intermediate pressure, to the intermediate-pressure mixed refrigerant hot passage 102 of the multi-stream heat exchanger 100. The mixed refrigerant stream from the discharge of the intermediate stage compressor 207, at a intermediate stage pressure or intermediate-high pressure, flows to the intermediate stage cooling section 208, namely an air cooled mixed refrigerant condenser 208, where the mixed refrigerant is partially condensed, the remaining components of the mixed refrigerant having a higher boiling point being condensed. The partially condensed mixed refrigerant enters an intermediate stage separator 209 to separate the vapor and liquid. The intermediate stage vapor exiting the intermediate stage separator 209 is routed to a second stage compressor 212 and the separated intermediate stage liquid is routed, at a second stage pressure or intermediate-high pressure, to the intermediate- high pressure mixed refrigerant hot passage 103 of the multi-stream heat exchanger 100. The mixed refrigerant stream from the discharge of the second stage compressor212, at a second stage pressure or high pressure, flows to a second stage cooling section213, namely an air cooled mixed refrigerant condenser 213, where the mixed refrigerant at high pressure is partially condensed, the remaining components of the mixed refrigerant having a higher boiling point being condensed. The partially condensed high pressure mixed refrigerant enters a second stage separator 214 to separate a high- pressure vapor and a high-pressure liquid. The high-pressure liquid exiting the second stage separator 214 is routed to a first high-pressure mixed refrigerant hot passage 104 of the multi-stream heat exchanger 100 and the separated high pressure vapor is routed to a second high-pressure mixed refrigerant hot passage 105 of the multi-stream heat exchanger 100. The liquefaction of natural gas in the multi-stream heat exchanger 10, namely in pass 101 is provided by cold passage 106 of mixed refrigerant, a high-pressure vapor stream flowing in the second high pressure mixed refrigerant hot pass 105 and three liquid streams (the high pressure liquid stream of the first high-pressure mixed refrigerant hot pass 104, the intermediate-high pressure liquid stream of the intermediate-high pressure mixed refrigerant hot pass 103 and the intermediate pressure liquid stream of the intermediate pressure mixed refrigerant hot pass 102) that flow from the mixed refrigerant separators 204, 209, 214 separately and go to the second high pressure mixed refrigerant hot pass 105 (wherein a high pressure vapor from the separator 214 flows), the first stage high pressure mixed refrigerant hot pass 104, the intermediate-high pressure mixed refrigerant hot pass 103 (wherein an intermedi-ate-high pressure liquid from the intermediate separator 209 flows) and the intermediate pressure mixed refrigerant hot pass 102 (wherein an intermediate pressure liquid from the first stage separator 204 flows) of the cold-box heat ex changer 100.

[0027] The intermediate pressure subcooled liquid stream from the intermediate pressure mixed refrigerant hot pass 102 comes out of the warm section of the multistream heat exchanger 100 and then is letdown in pressure (and temperature drop because of liquid flash) by the or intermediate pressure expansion device 107 (or J-T control valve 107) and fed as two-phase flow to the first separator 108 (or warm separator 108) operating at slightly higher pressure than the suction scrubber 201. In order to get even distribution of vapor and liquid in the cold pass 106, this stream is separated in a liquid fraction and a vapor fraction into the warm separator 108 and is then directed to the cold pass 106. The intermediate-high pressure subcooled liquid of the intermediate-high pressure mixed refrigerant hot pass 103 comes out of the middle section of the multi-stream heat exchanger 100 and then is letdown in pressure (and temperature drop because of liquid flash) by the expansion device 111 (or J-T control valve 111) and fed as two-phase flow to the second separator 112 (mid warm separator 112) operating at slightly higher pressure than the first separator 108. In order to get even distribution of vapor and liquid in the cold pass 106, this stream is separated into a liquid fraction and a vapor fraction in the mid warm separator 112 and is then directed in a controlled fashion to the cold pass 106. The high pressure subcooled liquid stream of the first high-pressure mixed refrigerant hot pass 104 comes out of the mid-bottom section of the multi-stream heat exchanger 100 and then is letdown in pressure (and temperature drop because of liquid flash) by the expansion device 115 or J-T control valve 115 and fed as two-phase flow to the third separator 116 (or mid cold separator 116) operating at slightly higher pressure than the mid warm separator 112. In order to get even distribution of vapor and liquid in the cold pass 105, this stream is separated into a liquid fraction and a vapor fraction in the mid cold separator 116 and is then is directed in a controlled fashion to the cold pass 105. The high-pressure vapor stream from the second stage separator 214 enters the warm end of the multi-stream heat exchanger 100 in the second high-pressure mixed refrigerant hot pass 105 and is condensed and fully subcooled and extracted from the cold end of the multi-stream heat exchanger 100, then letdown in pressure (and temperature drop because of liquid flash) by the expansion device 119 (or J-T control valve 119) and fed as two-phase flow tothe fourth separator 120 (or cold separator 120) operating at slightly higher pressure than the mid cold separator 116. In order to get even distribution of vapor and liquid in the cold pass 106, this stream is separated into a liquid fraction and a vapor fraction in the cold separator 120 and is then directed in a controlled fashion to the cold passage 106.

[0028] The cold mixed refrigerant travels up from the bottom of the cold pass 106 to provide refrigeration to the cold ends of streams in the natural gas hot pass 101 and in the second high-pressure mixed refrigerant hot pass 105. Traveling up from the cold end of the multi-stream heat exchanger 100, the mixed refrigerant stream in the cold pass 106 is warmed up to a temperature close to the streams from the mid cold separator 116 and mixes there first with the vapor stream from the mid cold separator 116 and then with the liquid stream from the mid cold separator 116. The mixed refrigerant stream continues its flowing inside the cold pass 106 to provide refrigeration to the intermediate sections of streams in the natural gas hot pass 101, the second high-pressure mixed refrigerant hot pass 105 and the first high-pressure mixed refrigerant hot pass 104. Traveling upwards the mixed refrigerant stream in the cold pass 106 is warmed up to a temperature close to the streams from the mid warm separator 112 and mixes there with the vapor and liquid streams from the mid warm separator 112. The mixed refrigerant stream continues its travelling in the cold pass 106 to provide refrigeration to the top sections of the mixed refrigerant streams in the natural gas hot pass 101, the second high-pressure mixed refrigerant hot pass 105, the first high-pressure mixed refrigerant hot pass 104 and the intermediate-high-pressure mixed refrigerant hot pass 103. Traveling upwards, the mixed refrigerant stream in the cold pass 106 is warmed up to a temperature close to the temperature of the streams from the warm separator 108 and mixes there with the vapor and liquid streams from the warm separator 108.

[0029] The mixed refrigerant stream continues its travelling in the cold pass 106 to the top of the multi-stream heat exchanger 100 and provides refrigeration to the warm end streams in the natural gas hot pass 101, the second high-pressure mixed refrigerant hot pass 105, the first high-pressure mixed refrigerant hot pass 104, the intermediate- high-pressure mixed refrigerant hot pass 103 and the intermediate-pressure mixed refrigerant hot pass 102. The mixed refrigerant stream of the cold pass 106 comes out ofthe multi-stream heat exchanger 100 as superheated vapor at a temperature approaching the inlet stream temperatures of the natural gas stream in the natural gas hot pass 101 and of the mixed refrigerant streams in the second high-pressure mixed refrigerant hot pass 105, the first high-pressure mixed refrigerant hot pass 104, the intermediate- high-pressure mixed refrigerant hot pass 103 and the intermediate-pressure mixed refrigerant hot pass 102 and then flows to the suction scrubber 201 of the mixed refrigerant first stage compressor 202.

[0030] The system disclosed with reference to Fig.1 allows for a better efficiency in the cooling of the natural gas. Fig. la illustrates a line graph of the temperature of the cold stream and the hot streams inside the multi-stream heat exchanger as a function of the exchanged heat flow in a natural gas liquefaction system according to the prior art. Fig. lb illustrates a line graph of the temperature of the cold stream and the hot streams inside the multi-stream heat exchanger as a function of the exchanged heat flow in a natural gas liquefaction system according to the system of Fig.1. It is evident the reduction of the temperature differential between the cold stream and the hot stream according to the system of the present disclosure.

[0031] With continuing reference to Fig. 1, a further embodiment of a system for cooling natural gas with a mixed refrigerant is shown in Fig. 2. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig. l and described above, and which will not be described again. The main difference between the system shown in Fig.2 and the system previously described with reference to Fig.1 is the kind of multi-stream heat exchanger, a coil wound heat exchanger 100’ being shown in Fig.2. according to this embodiment, the natural gas hot passage 101 and the mixed refrigerant hot passages 102, 103 104, 105 are coil wound tubes helically arranged inside a shell 10’, with a warm end at the bottom and a cold end at the top of the coil wound heat exchanger 100’ . The natural gas hot passage 101 has an inlet into the shell 10’ of the coil wound heat exchanger 100’ at the warm end and an out let at the cold end. The mixed refrigerant hot passages 102, 103 104, 105 have their respective inlets at the warm end and their respective outlets at different distances from the warm end, the mixed refrigerant of each mixed refrigerant hot passage 102, 103 104, 105 being subsequently expanded in a respective expansion device 107, 111, 115, 119 before being returned inside the shell 10’, being spread over the section of the coil wound heat exchanger 100’ through distributors, for example sprayinjectors, to form the mixed refrigerant cold passage 106, in particular in the form of droplets of liquid mixed refrigerant, flowing over the external surface of the coil wound tubes 101, 102, 103 104, 105. According to this embodiment, no separators are arranged between the expansion devices 107, 111, 115, 119 and the respective inlet to the mixed refrigerant cold passage 106.

[0032] With continuing reference to Fig. 1, a further embodiment of a system for cooling natural gas with a mixed refrigerant is shown in Fig. 3. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig. l and described above, and which will not be described again. The system shown in Fig.3 differs from the system of Fig.1 in that a high-pressure separator 123 is arranged along the second high-pressure mixed refrigerant hot passage 105, which is consequently divided into a first portion 105’ and a second portion 105”. The high-pressure separator 123 is configured to separate the mixed refrigerant from the second high-pressure mixed refrigerant hot passage 105 into a liquid fraction and a vapor fraction, the high-pressure separator 123 comprising an inlet, a liquid outlet and a vapor outlet, the inlet being connected to an outlet of the first portion 105’ of the second high-pressure mixed refrigerant hot passage 105. Preferably the distance of the outlet of the intermediate-high-pressure mixed refrigerant hot passage 103 from the warm end of the multi-stream heat exchanger 100 is adjusted to be the same or slightly smaller than the distance of the outlet of the first portion 105’ in order to optimize the efficiency of the multi-stream heat exchanger 100.

[0033] The liquid outlet of the high-pressure separator 123 is connected to a liquid outlet line 124 and the vapor outlet of the high-pressure separator 123 is connected to a vapor outlet line 125, with an inlet into the multi-stream 100 at a distance from the warm end slightly greater than the outlet of the first portion 105’ of the second high- pressure mixed refrigerant hot passage 105. The liquid outlet line 124 is connected to a third high-pressure mixed refrigerant hot passage 105’”, with an outlet arranged at a distance from the warm side of the multi-stream heat exchanger 100, which is greater than the distance of the outlet of the first high-pressure mixed refrigerant hot passage 104, and lower than the distance of the outlet of the second high-pressure mixed refrigerant hot passage 105. The vapor outlet of the high-pressure separator 123 is connected to a vapor outlet line 125, which is connected to the second portion 105” of the second high-pressure mixed refrigerant hot passage 105, with an outlet at the cold sideof the multi-stream heat exchanger 100, the outlet being connected to the inlet of the third expansion device 119.

[0034] According to this embodiment, a first additional expansion device 126 is additionally present and configured to expand the mixed refrigerant down to substantially the low pressure, with an inlet connected to the outlet of the third high-pressure mixed refrigerant hot passage 105”’ and an outlet connected to the inlet of a first additional separator 127. The first additional separator 127 is configured to separate the mixed refrigerant expanded by the first additional expansion device 126 into a liquid fraction and a vapor fraction and comprises a liquid outlet and a vapor outlet, the liquid outlet being connected to a liquid outlet line 128 and the vapor outlet being connected to a vapor outlet line 129, both the liquid outlet line 128 and the vapor outlet line 129 being connected to the mixed refrigerant cold passage 106 at a slightly greater distance from the warm side of the multi-stream heat exchanger 100 than the outlet of the third high- pressure mixed refrigerant hot passage 105’”.

[0035] This embodiment operates as already described with reference to the embodiment of Fig.1 and additionally as follows. The high-pressure vapor stream of the second high pressure mixed refrigerant hot passage 105 has an inlet at the top warm end of the multi-stream heat exchanger 100, is precooled and then extracted from the core as a two-phase flow to be separated into a liquid fraction and a vapor fraction in the high-pressure separator 123 (or pre-cool separator 123). Heavy liquid from the precool separator 123 is fed to the third high-pressure mixed refrigerant hot passage 105’” where it is further subcooled and then extracted at mid-bottom elevation of the multistream heat exchanger and letdown in pressure (and temperature drop because of liquid flash) by the expansion device 126 (a J-T control valve 126) and fed as two-phase flow to the first additional separator 127 operating at slightly higher pressure than the third separator 116 (or mid cold separator 116). In order to get even distribution of vapor and liquid in the cold pass 106, this stream is separated into a liquid fraction and a vapor fraction in the first additional separator 127 and is then addressed in a controlled fashion to the cold pass 106.

[0036] Light vapor from the high-pressure separator 123 is fed to the second portion 105” of the second high-pressure mixed refrigerant hot passage 105 where it is further condensed and subcooled and then extracted at the bottom and letdown in pressure(and temperature drop because of liquid flash) by the second high pressure expansion device 119 or J-T control valve 119 and fed as two-phase flow to the fourth separator 120 (or cold separator 120) operating at slightly higher pressure than the first additional separator 127.

[0037] The cold mixed refrigerant flows upward from the bottom of the cold pass 106 to provide refrigeration to the cold ends of the natural gas stream in the natural gas hot passage 101 and of the mixed refrigerant in the second portion 105” of the second high-pressure mixed refrigerant hot passage 105. Flowing upward from the cold end of the multi-stream heat exchanger 100, the mixed refrigerant stream in the cold pass 106 is warmed up to a temperature close to the temperature of the mixed refrigerant streams from the first additional separator 127 and mixes there first with the vapor stream from the first additional separator 127 and then with the liquid stream from the first additional separator 127. The mixed refrigerant stream continues its flowing upwards in the cold pass 106 to provide refrigeration to the other sections of the hot passages 101, 102, 103, 104 and 105 as already explained previously with reference to Fig.l.

[0038] With continuing reference to Fig. l, Fig.4 illustrates a further embodiment of a system for cooling natural gas with a mixed refrigerant. The same reference numbers used in Fig.l are used in Fig.4 to designate the same or corresponding parts, components or elements, which will not be described again. The embodiment of Fig.4 differs from the embodiment of Fig.l mainly in that the liquid outlet and the vapor outlet of the third separator 116 are not directed to the cold passage 106. In fact, according to this embodiment, the first high-pressure expansion device 115 is configured to expand the mixed refrigerant down to a pressure, at least 1 bar higher than the intermediate- high pressure and a second additional expansion device 130 is arranged along the liquid outlet line 117 of the third separator 116 to expand the mixed refrigerant down to substantially the intermediate-high pressure, the liquid outlet line 117 downstream the second additional expansion device 130 being connected to the inlet of the second separator 112. The vapor outlet line 118 of the third separator 116 is connected to an additional mixed refrigerant hot passage 131 with an inlet at a distance from the warm end slightly greater than the third distance, i.e. the distance of the outlet of the first high pressure mixed refrigerant hot passage 104 and an outlet at an intermediate distance between the third distance and the fourth distance, i.e. the distance of the outletof the second high pressure mixed refrigerant hot passage 105. A third additional expansion device 132 is arranged downstream of the outlet of the additional mixed refrigerant hot passage 131 and is configured to expand the mixed refrigerant down to substantially the low pressure. The outlet of the third additional expansion device 132 is connected to an inlet of a second additional separator 133, the second additional separator 133, configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction and having a liquid outlet and a vapor outlet, respectively connected to a liquid outlet line 134 and to a vapor outlet line 135, both the liquid outlet line 134 and the vapor outlet line 135 being connected to the mixed refrigerant cold passage 106 of the multi-stream heat exchanger 100 at a distance slightly greater than the distance of the outlet of the additional mixed refrigerant hot passage 131.

[0039] With continuing reference to Figs.1 and 4, a further embodiment of a system for cooling natural gas with a mixed refrigerant is shown in Fig.5. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Figs.l and 4 and described above, and which will not be described again. The system shown in Fig.5 differs from the system of Fig.4 in that the outlet of the third additional expansion device 132 is connected to the inlet of the fourth separator 120. In this embodiment, the outlet of the additional mixed refrigerant hot passage 131 is consequently moved to the cold end of the multi-stream heat exchanger 100.

[0040] Various combinations of the embodiment disclosed with reference to Figs. l- 5 are possible. For example, with continuing reference to Figs. l, 3 and 4, the embodiment of a system for cooling natural gas with a mixed refrigerant shown in Fig.6 is a combination of the embodiments previously described with reference to Figs. 3 and 4. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Figs.l, 3 and 4 and described above, and which will not be described again. In particular, the system shown in Fig.6 differs from the system of Fig.1 in that it comprises both the high-pressure separator 123 and related equipment characterizing the embodiment of Fig.3 and an additional mixed refrigerant hot passage 131 of the vapor outlet of the third separator 116 and related equipment, already disclosed with reference to Fig.4.

[0041] Finally, with continuing reference to Figs.1, 3 and 5, a further embodiment of a system for cooling natural gas with a mixed refrigerant is shown in Fig.7. The samereference numbers designate the same or corresponding parts, elements or components already illustrated in Figs. l, 3 and 5 and described above, and which will not be described again. The system shown in Fig.7 differs from the system of Fig.1 in that it comprises both the high-pressure separator 123 and related equipment disclosed with reference to Fig.3 and an additional mixed refrigerant hot passage 131 of the vapor outlet of the third separator 116 and related equipment as described with reference to Fig.5. The equipment related to the additional mixed refrigerant hot passage 131 of the vapor outlet of the third separator 116 differs from those described with reference to Fig.5 in that the outlet of the third additional expansion device 132 is connected to the inlet of the first additional separator 127, together with the outlet of the third high- pressure mixed refrigerant hot passage 105”’. Accordingly, in this embodiment, the outlet of the additional mixed refrigerant hot passage 131 is arranged at a distance from the warm end of the multi-stream heat exchanger 100, which is comprised between the outlet of the first high pressure mixed refrigerant hot passage 104 and the outlet of the third high-pressure mixed refrigerant hot passage 105’”.

[0042] 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 form the spirt 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.

Claims

A system for cooling natural gas with a mixed refrigerantCLAIMS1. A system for cooling natural gas, the system comprising:- a multi-stream heat exchanger (100, 100’) configured to cool the natural gas by exchanging heat with a refrigerant fluid composed of two or more components, i.e. a mixed refrigerant, the multi-stream heat exchanger (100, 100’) including:- a hot passage (101) of the natural gas, with an inlet on an end, or warm end of the multi-stream heat exchanger (100, 100’), and an outlet on the opposite end, or cold end of the multi-stream heat exchanger (100, 100’), the temperature inside the multi-stream heat exchanger (100, 100’) being higher at the warm end and decreasing with the distance from the warm end;- a plurality of hot passages (102, 104, 105) of the mixed refrigerant, with respective inlets on the warm end of the multi-stream heat exchanger (100, 100’) and respective outlets at different distances from the warm end of the multi-stream heat exchanger (100, 100’); and- a cold passage (106) of the mixed refrigerant, configured to exchange heat between the natural gas and the mixed refrigerant, the cold passage (106) having an outlet on the warm end of the multi-stream heat exchanger (100, 100’) and a plurality of inlets, at different distances from the warm end of the multistream heat exchanger (100, 100’);- a closed thermodynamic cycle system (200), configured to compress the mixed refrigerant through cyclic thermodynamic transformations, the closed thermodynamic cycle system (200) including- a vessel (201) at a first pressure, or low pressure, having an inlet connected to an outlet of the cold passage (106) and an outlet, a first stage compressor (202) configured to compress the mixed refrigerant to a second pressure, or intermediate pressure and having an inlet connected to the outlet of the vessel (201) and an outlet, a first stage cooling section (203) with an inlet connected to the outlet of the first stage compressor (202) and an outlet, a first stageseparator (204) configured to separate the mixed refrigerant at the intermediate pressure into an intermediate-pressure liquid fraction and an intermediatepressure vapor fraction, the first stage separator (204) having an inlet connected to the outlet of the first stage cooling section (203), a liquid outlet connected to a first stage liquid outlet line (205) and a vapor outlet connected to a first stage vapor outlet line (206); a second stage compressor (212) configured to compress the mixed refrigerant to a third pressure, or high pressure and having an inlet and an outlet, a second stage cooling section (213) with an inlet connected to the outlet of the second stage compressor (212) and an outlet, a second stage separator (214) configured to separate the mixed refrigerant at the high pressure into a high-pressure liquid fraction and a high-pressure vapor fraction, the second stage separator (214) having an inlet connected to the outlet of the second stage cooling section (213), a liquid outlet connected to a second stage liquid outlet line (215) and a vapor outlet connected to a second stage vapor outlet line (216);- the plurality of mixed refrigerant hot passages (102, 104, 105) of the multistream heat exchanger (100, 100’) including- an intermediate-pressure mixed refrigerant hot passage (102), with an inlet connected to the first stage liquid outlet line (205) and an outlet at a first distance from the warm end of the multi-stream heat exchanger (100, 100’),- a first high-pressure mixed refrigerant hot passage (104), with an inlet connected to the second stage liquid outlet line (215) and an outlet at a second distance from the warm end of the multi-stream heat exchanger (100, 100’), the second distance being greater than the first distance, and- a second high-pressure mixed refrigerant hot passage (105), with an inlet connected to the second stage vapor outlet line (216) and an outlet at a third distance from the warm side of the multi-stream heat exchanger (100, 100’), the third distance being greater than the second distance;- a plurality of expansion devices (107, 115, 119), configured to expand and consequently cool the mixed refrigerant, the plurality of expansion devices (107, 115, 119) comprising an intermediate-pressure expansion device (107), a first high-pressure expansion device (115) and a second high-pressure expansion device (119), all the expansion devices (107, 115, 119) being configured to expand and at least partially flash the mixed refrigerant down to substantially the lowpressure and having a respective inlet and a respective outlet, the inlets being respectively connected to the outlet of the intermediate-pressure mixed refrigerant hot passage (102), of the first high-pressure mixed refrigerant hot passage (104) and of the second high-pressure mixed refrigerant hot passage (105) and the outlets being respectively directly or indirectly connected to the mixed refrigerant cold passage (106), at a slightly greater distance than respectively the first distance, the second distance and the third distance from the warm end of the multi-stream heat exchanger (100, 100’); wherein the system further comprises- an intermediate stage compressor (207) configured to compress the mixed refrigerant to a fourth pressure, or intermediate-high pressure and having an inlet connected to the first stage vapor outlet line (206) and an outlet, an intermediate stage cooling section (208) with an inlet connected to the outlet of the intermediate stage compressor (207) and an outlet, an intermediate stage separator (209) configured to separate the mixed refrigerant at the intermediate-high pressure into an intermediate-high-pressure liquid fraction and an intermediate-high-pres- sure vapor fraction, the intermediate stage separator (209) having an inlet connected to the outlet of the intermediate stage cooling section (208), a liquid outlet connected to an intermediate stage liquid outlet line (210) and a vapor outlet connected to an inlet of an intermediate stage vapor outlet line (211), the outlet of the intermediate stage vapor outlet line (211) being connected to the inlet of the second stage compressor (212);- an intermediate-high-pressure mixed refrigerant hot passage (103), with an inlet connected to the intermediate stage liquid outlet line (210) and an outlet at an intermediate distance from the warm side of the multi-stream heat exchanger (100, 100’), the intermediate distance being greater than the first distance and smaller than the second distance;- an intermediate-high-pressure expansion device (111) configured to expand and at least partially flash the mixed refrigerant down to substantially the low pressure and having an inlet and an outlet, the inlet being connected to the interme- diate-high-pressure mixed refrigerant hot passage (103) and the outlet being directly or indirectly connected to the mixed refrigerant cold passage (106), at a slightly greater distance than the intermediate distance from the warm end of the multi-stream heat exchanger (100, 100’).

2. The system of claim 1, wherein the multi-stream heat exchanger is a coil wound heat exchanger (100’), the natural gas hot passage (101) and the mixed refrigerant hot passages (102, 103 104, 105) are coil wound tubes (101, 102, 103 104, 105) arranged inside a shell (10’), the outlets of the expansion devices (107, 111, 115, 119) are connected to the inside of the shell (10’) through respective distributors at respective distances from the warm end of the coil wound heat exchanger (100’) and the cold passage (106) is formed by the mixed refrigerant distributed inside the shell (10’) and flowing over the coil wound tubes (101, 102, 103 104, 105).

3. The system of claim 2, wherein the second high-pressure mixed refrigerant hot passage (105) is divided into a first portion (105’) and a second portion (105”) and the system further comprises:- a high-pressure separator (123) having an inlet, a liquid outlet and a vapor outlet,- the inlet being connected to an outlet of the first portion (105’) of the second high-pressure mixed refrigerant hot passage (105), at a fifth distance from the warm side of the multi -stream heat exchanger,- the liquid outlet being connected to a liquid outlet line (124), the liquid outlet line (124) being connected to a third high-pressure mixed refrigerant hot passage (105’”), with an inlet and an outlet, the inlet being arranged at a distance from the warm side of the coil wound heat exchanger (100’), slightly greater than the fifth distance, and the outlet being arranged at a sixth distance from the warm side of the coil wound heat exchanger (100’), the sixth distance being greater than the second distance from the warm end of the coil wound heat exchanger (100’), i.e. the distance of the outlet of the first high-pressure mixed refrigerant hot passage (104), and lower than the third distance from the warm end of the coil wound heat exchanger (100’), i.e. the distance of the outlet of the second high-pressure mixed refrigerant hot passage (105);- the vapor outlet being connected to a vapor outlet line (125), the vapor outlet line (125) being connected to the second portion (105”) of the second high- pressure mixed refrigerant hot passage (105), with an inlet and an outlet, the inlet being arranged at a distance from the warm side of the coil wound heat exchanger (100, 100’), slightly greater than the fifth distance and the outlet being arranged at the third distance from the warm side of the coil wound heatexchanger (100, 100’), the outlet being connected to the inlet of the third expansion device (119);- a first additional expansion device (126) configured to expand the mixed refrigerant down to substantially the low pressure and having an inlet and an outlet, the inlet being connected to the outlet of the third high-pressure mixed refrigerant hot passage (105’”); and the outlet being directly connected to the mixed refrigerant cold passage (106), at a slightly greater distance than the sixth distance from the warm side of the coil wound heat exchanger (100’).

4. The system of claim 3, wherein the fifth distance is substantially the same as or slightly greater than the intermediate distance from the warm end of the coil wound heat exchanger (100’), i.e. the distance of the outlet of the intermediate- high-pressure mixed refrigerant hot passage (103).

5. The system of any of the previous claims 1 to 4, wherein the first high-pressure expansion device (115) is configured to expand and at least partially flash the mixed refrigerant down to a fifth pressure, at least 1 bar higher than the intermediate-high pressure and wherein the system further comprises:- a second additional expansion device (130) configured to expand the mixed refrigerant down to substantially the intermediate-high pressure and having an inlet connected to the liquid outlet line (117) of the third separator (116) and an outlet connected to the inlet of the second separator (112);- an additional mixed refrigerant hot passage (131) with an inlet and an outlet, the inlet being connected to the vapor outlet line (118) of the third separator (116);- a third additional expansion device (132) configured to expand the mixed refrigerant down to substantially the low pressure and having an inlet and an outlet, the inlet being connected to the additional mixed refrigerant hot passage (131) and the outlet being directly connected to the mixed refrigerant cold passage (106), at a distance from the warm side of the coil wound heat exchanger (100’) greater than the distance of the outlet of the first high-pressure mixed refrigerant hot passage (104) and smaller that the distance of the outlet of the second high- pressure mixed refrigerant hot passage (105).

6. The system of claim 1, wherein the multi-stream heat exchanger is enclosed in a cold box heat exchanger (100), the natural gas hot passage (101), themixed refrigerant hot passages (102, 103 104, 105) and the mixed refrigerant cold passage (106) are heat exchange plates and fins arranged inside a box (10) filled with a heat insulating material and the system further comprises:- a plurality of separators (108, 112, 116, 120), configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction, the plurality of separators (108, 112, 116, 120) comprising a first separator (108), a second separator (112), a third separator (116) and a fourth separator (120) having a respective inlet, a respective liquid outlet and a respective vapor outlet, the inlets being respectively connected to the intermediate-pressure expansion device (107), the inter- mediate-high-pressure expansion device (111), the first high-pressure expansion device (115) and the second high-pressure expansion device (119), the liquid outlets being connected to respective liquid outlet lines (109, 113, 117, 121) and the vapor outlets being connected to respective vapor outlet lines (110, 114, 118, 122), both the liquid outlet lines (109, 113, 117, 121) and the vapor outlet lines (110, 114, 118, 122) being connected to the mixed refrigerant cold passage (106) at a slightly greater distance than respectively the first distance, the intermediate distance, the second distance and the third distance from the warm end of the cold box heat exchanger (100).

7. The system of claim 1 or 6, wherein the second high-pressure mixed refrigerant hot passage (105) is divided into a first portion (105’) and a second portion (105”) and the system further comprises:- a high-pressure separator (123) having an inlet, a liquid outlet and a vapor outlet,- the inlet being connected to an outlet of the first portion (105’) of the second high-pressure mixed refrigerant hot passage (105), at a fifth distance from the warm side of the multi -stream heat exchanger,- the liquid outlet being connected to a liquid outlet line (124), the liquid outlet line (124) being connected to a third high-pressure mixed refrigerant hot passage (105’”), with an inlet and an outlet, the inlet being arranged at a distance from the warm side of the multi-stream heat exchanger (100, 100’), slightly greater than the fifth distance, and the outlet being arranged at a sixth distance from the warm side of the multi-stream heat exchanger (100, 100’), the sixth distance being greater than the second distance from the warm end of the multi-streamheat exchanger (100, 100’), i.e. the distance of the outlet of thefirst high-pressure mixed refrigerant hot passage (104), and lower than the third distance from the warm end of the multi -streamheat exchanger (100, 100’), i.e. the distance of the outlet of the second high-pressure mixed refrigerant hot passage (105);- the vapor outlet being connected to a vapor outlet line (125), the vapor outlet line (125) being connected to the second portion (105”) of the second high- pressure mixed refrigerant hot passage (105), with an inlet and an outlet, the inlet being arranged at a distance from the warm side of the multi-streamheat exchanger (100, 100’), slightly greater than the fifth distance and the outlet being arranged at the third distance from the warm side of the multi-stream heat exchanger (100, 100’), the outlet being connected to the inlet of the third expansion device (119);- a first additional expansion device (126) configured to expand and at least partially flash the mixed refrigerant down to substantially the low pressure and having an inlet and an outlet, the inlet being connected to the outlet of the third high- pressure mixed refrigerant hot passage (105’”); and- a first additional separator (127) having an inlet, a liquid outlet and a vapor outlet, the inlet being connected to the outlet of the first additional expansion device (126), the liquid outlet being connected to a liquid outlet line (128) and the vapor outlet being connected to a vapor outlet line (129), both the liquid outlet line (128) and the vapor outlet line (129) being connected to the mixed refrigerant cold passage (106) at a slightly greater distance from the warm side of the multistream heat exchanger (100) than the sixth distance.

8. The system of claim 7, wherein the fifth distance is substantially the same as or slightly greater than the intermediate distance from the warm end of the multi-stream heat exchanger (100, 100’), i.e. the distance of the outlet of the interme- diate-high-pressure mixed refrigerant hot passage (103).

9. The system of any of the previous claims 1 and 6 to 8, wherein the first high-pressure expansion device (115) is configured to expand the mixed refrigerant down to a fifth pressure, at least 1 bar higher than the intermediate-high pressure and wherein the system further comprises:- a second additional expansion device (130) configured to expand and at least partially flash the mixed refrigerant down to substantially the intermediate-highpressure and having an inlet connected to the liquid outlet line (117) of the third separator (116) and an outlet connected to the inlet of the second separator (112);- an additional mixed refrigerant hot passage (131) with an inlet and an outlet, the inlet being connected to the vapor outlet line (118) of the third separator (116);- a third additional expansion device (132) configured to expand the mixed refrigerant down to substantially the low pressure and having an inlet and an outlet, the inlet being connected to the additional mixed refrigerant hot passage (131) and the outlet being connected alternatively to i. the inlet of the fourth separator (120); or ii. an inlet of a second additional separator (133), the second additional separator (133) also having a liquid outlet and a vapor outlet, the liquid outlet being connected to a liquid outlet line (134) and the vapor outlet being connected to a vapor outlet line (135), both the liquid outlet line (134) and the vapor outlet line (135) being connected to the mixed refrigerant cold passage (106) of the multi-stream heat exchanger (100, 100’).

10. The system of any of the preceding claims, wherein the first stage compressor (202), the second stage compressor (212) and the intermediate stage compressor (207) are allocated in one single casing.

11. The system of claim 10, wherein the intermediate compressor (207) and the second stage compressor (212) are arranged in-line and in back to back configuration with the first stage compressor (202).

12. The system of claim 10, wherein the first stage compressor (202) and the intermediate compressor (207) are arranged in-line and the second stage compressor (212) is arranged in back to back configuration.

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