A system for cooling natural gas with a mixed refrigerant

Abstract

The present disclosure relates to a system for cooling natural gas and specifically concerns but is not limited to a system for cooling natural gas down to its liquefaction, through heat exchange in a multi-stream heat exchanger with a mixed refrigerant. The system comprises a closed thermodynamic refrigeration cycle, wherein the mixed refrigerant is cooled through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization steps, the compression being a two stage compression. 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 corresponding streams of mixed refrigerant from different compression stages, each stream being connected to a respective expansion device and separator, the expansion device being configured to expand and at least partially flash the mixed refrigerant 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 finally 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 is 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 (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).

[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 LNG 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 can be configured as part of a cold box heat exchanger or as coil wound heat exchangers. 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.

[0008] Cold box heat exchangers and coil wound heat exchangers 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 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 compressor are precooled in the multi-stream heat exchanger and then expanded in Joule Thomson 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, 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] The second stage vapor stream is a mixture comprising a fraction of light hydrocarbons and a fraction of mid heavy hydrocarbons (the latter being called inert part) and is sub-cooled to around -162°C before JT cooling. Actually, the inert part is not participating in cold end section sub cooling and should be required in the mid liquefaction section. This increases the refrigerant pre-cooling duty, in particular the refrigerant compressor power and the specific energy consumption (350 kWh / tonLNo).

[0012] However, the use of multi-stream heat exchangers for liquefying a stream of 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.

[0013] 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.

[0014] 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

[0015] 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 two-stage refrigerant compressor, namely a first stage and a second 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 (JT valves) and subsequently used to cool the natural gas and to pre-cool the mixed refrigerant streams from the compressor in a multi-stream heat exchanger. Upstream the JT valves the high pressure liquid fraction and the high pressure vapor fraction of the second stage are pre-cooled in the multi-stream heat exchanger and separated each into an additional liquid fraction and an additional vapor fraction in a respective additional separator. The pre-cooled high-pressure vapor fraction is separated in a high pressure separator and the respective additional high pressure liquid fraction and additional high pressure vapor fraction are further pre-cooled in the multi-stream heat exchanger before being cooled by the JT valves. The precooled high-pressure liquid fraction is cooled in two different stages. First, the precooled high-pressure liquid fraction is expanded to an intermediate-high pressure (and subsequently cooled) by a JT valve and separated into an intermediate-high pressure liquid fraction and an intermediate-high pressure vapor fraction. The intermediate- high pressure vapor fraction is further cooled in the multi-stream heat exchanger before being definitively expanded and cooled by another JT valve and used to cool the natural gas. The intermediate-high pressure liquid fraction is mixed with the precooled intermediate pressure liquid fraction before being used to cool the natural gas. The pre-cooling and separation of the high pressure mixed refrigerant streams 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 additional liquid and vapor streams to the multi-stream heat exchanger, optimizing the heat exchange in the warm section of the multi-stream heat exchanger, and also reducing the mixed refrigerant pre-cooling duty. Moreover, the system forcooling 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.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. l illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a first embodiment;Fig.2 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a second embodiment; andFig.3 illustrates a schematic view of a system for cooling natural gas with a mixed refrigerant according to a third 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 two stage compression, increasing the pressure of the mixed refrigerant from a low pressure to a first stage pressure, or intermediate pressure, and a second stage pressure, or high pressure. In particular, the multi-streamheat 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 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.

[0018] In particular, high pressure vapor from the second stage is pre-cooled in the multi-stream heat exchanger down to around -30°C / -35°C and directed to a pre-cooled separator to separate the lightest hydrocarbon vapor from the mid heavy hydrocarbon liquids. The lightest hydrocarbon vapor fraction from the pre-cooled separator overhead is sub-cooled to around -155°C before JT cooling for the cold end section sub cooling. The mid heavy hydrocarbon liquids from the pre-cooled separator bottoms which is sub cooled to around -115°C before JT cooling for mid liquefaction section cooling duty requirement. The heavy hydrocarbon liquid fraction from the second stage is subcooled to -32°C before JT cooling on additional intermediate separator where hydrocarbon liquid is sent to the separator of the stream from the first stage, while vapor is subcooled to -70°C and sent to a subsequent mid separator for mid liquefaction section cooling duty requirement. As a result, the cold end section refrigerant pre-cooling duty is reduced as the mid heavy hydrocarbons are separated andutilized at the mid liquefaction section. This reduces the refrigerant pre-cooling duty, thereby the refrigerant compressor power and the specific energy consumption (324 / 332 kWh / Ton, down to 7.5-6% reduction).

[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 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.

[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 so called 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, 109, 110 of the mixed refrigerant and a cold passage 105 of the mixed refrigerant. In particular, the hot passage 101 of the natural gas, the hot passages 102, 103, 104, 109, 110of the mixed refrigerant and the cold passage 105 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 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 105 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 hot passages 102, 103, 104, 109, 110 of the mixed refrigerant is connected to a respective expansion device and / or 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 111 or intermediate pressure expansion device 111 and to a first 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 105 at a distance from the warm end of the multistream heat exchanger 100, which is slightly greater than the first distance. A second hot passage 103 or first high pressure mixed refrigerant hot passage 103 has an outlet at a second distance from the warm end of the multi-stream heat exchanger 100, the second distance being greater that the first distance, and is connected to a first high- pressure expansion device 123 and to a second separator 124, with a liquid outlet at the bottom of the separator 124, the liquid outlet being connected to a liquid outlet line 125, and with a vapor outlet at the top of the separator 124, the vapor outlet being connected to a vapor outlet line 126. The vapor outlet line 126 is directed to the multistream heat exchanger 100 to form an intermediate-high-pressure mixed refrigerant hot passage 128, with an inlet at a distance from the warm side of the multi-streamheat exchanger 100, greater than the distance of the outlet of the intermediate-pressure mixed refrigerant hot passage 102 and smaller than the distance of the outlet of the first high-pressure mixed refrigerant hot passage 103. The outlet of the intermediate- high-pressure mixed refrigerant hot passage 128 is arranged at a distance from the warm side of the multi-stream heat exchanger 100 greater than the distance of the outlet of the first high-pressure mixed refrigerant hot passage 103 and smaller than the distance of the outlet of the third high-pressure mixed refrigerant hot passage 111. A second additional expansion device 129 is arranged downstream of the outlet of the intermediate-high-pressure mixed refrigerant hot passage 128 and is configured to expand the mixed refrigerant down to substantially the low pressure. The outlet of the second additional expansion device 129 is connected to an inlet of an additional low pressure separator 130, the additional low pressure separator 130 being configured to separate the low pressure mixed refrigerant into a liquid fraction and a vapor fraction and having a liquid outlet and a vapor outlet, the liquid outlet being connected to a liquid outlet line 131 and the vapor outlet being connected to a vapor outlet line 132, both the liquid outlet line 131 and the vapor outlet line 132 being connected to the mixed refrigerant cold passage 105 of the multi-stream heat exchanger 100 at a distance from the warm end slightly greater than the outlet of the intermediate-high-pres- sure mixed refrigerant hot passage 128.

[0024] A third hot passage 104 or second 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 preferably greater that the first distance and smaller than the second distance, and is connected to a first high pressure mixed refrigerant expansion device 115 and to a high-pressure separator 106 configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction. The high-pressure separator 106 has a liquid outlet and a vapor outlet, the liquid outlet being connected to a liquid outlet line 107 and the vapor outlet being connected to a vapor outlet line 108. The liquid outlet line 107 is connected to a third high-pressure mixed refrigerant hot passage 109. The inlet of the third high-pressure mixed refrigerant hot passage 109 is arranged at a distance from the warm side of the multi-stream heat exchanger 100, slightly greater than the third distance, i.e. the distance of the outlet of the second high pressure mixed refrigerant hot passage 104. The outlet of the third high-pressure mixed refrigerant hot passage 109 is arranged at a distance from thewarm side of the multi-stream heat exchanger 100, greater than the distance of the liquid outlet line 131 and the vapor outlet line 132 of the additional low pressure separator 130. The outlet of the third high-pressure mixed refrigerant hot passage 109 is connected to a second high-pressure expansion device 115 configured to expand the mixed refrigerant to substantially the low pressure. The outlet of the second high-pressure expansion device 115 is connected to a second low pressure separator 116 configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction. The second low pressure separator 116 has a liquid outlet and a vapor outlet, the liquid outlet being connected to a liquid outlet lines 117 and 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 mixed refrigerant cold passage 105 at a slightly greater distance than the distance of the third high-pressure mixed refrigerant hot passage 109.

[0025] The vapor outlet of the high-pressure separator 106 is connected to a vapor outlet line 108 and the vapor outlet line 108 is connected to a fourth high-pressure mixed refrigerant hot passage 110, with an inlet at a distance from the warm side of the multi-stream heat exchanger 100 slightly greater than the distance of the outlet of the second high pressure mixed refrigerant hot passage 104 and an outlet at the cold side of the multi-stream heat exchanger 100. The outlet of the fourth high-pressure mixed refrigerant hot passage 110 is connected to a third high-pressure expansion device 119 configured to expand the mixed refrigerant to substantially the low pressure. The outlet of the third high-pressure expansion device 119 is connected to a third low pressure separator 120 configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction. The third separator 120 has a liquid outlet at the bottom, connected to a liquid outlet line 121, and a vapor outlet at the top, connected to a vapor outlet line 122. Both the liquid outlet line 121 and the vapor outlet line 122 are connected to the cold passage 105 at the cold end of the multi-stream 100.

[0026] Additionally, the system for cooling natural gas through heat exchange with a mixed refrigerant further comprises a two stage 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 105 and an outlet connected to a first stage compressor 202, configured to compress the mixed refrigerant up to an intermediate pressure, the vessel 201 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 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 207. A second stage cooling section 208, namely an air cooler 208 is arranged downstream the second stage compressor 207. A second stage 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 hot 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 hot passage 104 of the multi-stream heat exchanger 100.

[0027] The system operates as follows. The liquefaction of natural gas in natural gas hot passage 101 of the multi-stream heat exchanger 100 is provided by the cold passage of mixed refrigerant, a high pressure vapor stream from the second stage vapor outlet line 211 and two liquid streams, respectively a high pressure liquid stream from the second stage liquid outlet line 210 and an intermediate pressure liquid stream from the first stage liquid outlet line 205. The intermediate pressure liquid stream flows inside an intermediate pressure mixed refrigerant hot passage 102, the high pressure liquid stream flows inside a first high pressure mixed refrigerant hot passage 103 and the high pressure vapor stream flows inside a second high pressure mixed refrigerant hot passage 104. The intermediate pressure subcooled liquid stream inside the intermediate pressure mixed refrigerant hot passage 102 comes out of the warm section of the multi-stream heat exchanger 100 and then is letdown in pressure (and temperature drop because of liquid flash) by the intermediate-pressure expansion device 111 or J- T control valve 111 and is subsequently fed as a two-phase flow at the low pressure to the first low pressure separator 112 operating at slightly higher pressure than the vessel201. In order to get even distribution of vapor and liquid in the cold passage 105, this stream is separated into a liquid fraction and a vapor fraction into the first low pressure separator 112 and is then addressed in a controlled fashion to the cold passage 105.

[0028] The high pressure subcooled liquid stream flowing inside the first high pressure mixed refrigerant hot passage 103 comes out of the middle section of the multistream heat exchanger 100 and then is letdown in pressure (and temperature drop because of liquid flash) by the first high-pressure expansion device 123 or J-T control valve 123. The first high-pressure expansion device 123 is configured to expand the mixed refrigerant to an intermediate-high pressure. A second separator 124 is arranged downstream of the first high-pressure expansion device 123 and is configured to separate the mixed refrigerant into a liquid fraction and a vapor fraction. The heavy liquid fraction from the second separator is further let down in pressure in the first additional separator 127 and then fed to the inlet of the second separator 112 downstream the intermediate-pressure expansion device 111. The high-intermediate light gas from the second separator 124 is fed to the intermediate-high-pressure mixed refrigerant hot passage 128, where it is partially condensed and extracted at mid elevation and letdown in pressure (and temperature drop because of liquid flash) by the second additional expansion device 129 or J-T control valve 129. In order to get even distribution of vapor and liquid in the cold passage 105, this stream is separated into a liquid fraction and a vapor fraction in the additional low pressure separator 130 and is then addressed in a controlled fashion to the cold passage 105.

[0029] The high pressure vapor stream from the second stage separator 209 enters the top warm end of the multi-stream heat exchanger 100 in the second high pressure mixed refrigerant hot passage 104 and it is subcooled and extracted from the core as a two-phase flow, which is directed to the high-pressure separator 106 to be separated into a liquid fraction and a vapor fraction. The heavy liquid from the high-pressure separator 106 is fed to the third high-pressure mixed refrigerant hot passage 109 where it is further subcooled and then extracted at mid-bottom elevation and letdown in pressure (and temperature drop because of liquid flash) by the second high-pressure expansion device 115 or J-T control valve 115. In order to get even distribution of vapor and liquid in the cold passage 105, this stream is separated into a liquid fraction and a vapor fraction into the second low pressure separator 116 and is then addressed in acontrolled fashion to the cold passage 105. Light vapor from the high-pressure separator 106 is fed to the fourth high-pressure mixed refrigerant hot passage 109 where it is further condensed and subcooled and then extracted at the bottom of the multistream heat exchanger 100 and letdown in pressure (and temperature drop because of liquid flash) by the third high-pressure expansion device 119 of J-T control valve 119. In order to get even distribution of vapor and liquid in the cold passage 105, this stream is separated into a liquid fraction and a vapor fraction in the third low pressure separator 120 and is then addressed in a controlled fashion to the cold passage 105.

[0030] The cold mixed refrigerant flows upward in the cold passage 105 from the bottom of the multi-stream heat exchanger 100 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 fourth high-pressure mixed refrigerant hot passage 110. Flowing upwards from the bottom the multi-stream heat exchanger 100, the stream in the cold passage 105 is warmed up to a temperature close to the temperature of the streams from the second low pressure separator 116 and is mixed first with the vapor stream from the second low pressure separator 116 and then with the liquid stream from the second low pressure separator 116. The commingled stream continues its flowing upwards from the bottom of the cold passage 105 to provide refrigeration to the intermediate sections of the streams in the natural gas hot passage 101, the fourth high-pressure mixed refrigerant hot passage 110 and the third high-pressure mixed refrigerant hot passage 109. Flowing upward, the stream in the cold passage 105 is further warmed up to a temperature close to the temperature of the liquid and vapor streams from the second additional expansion device 130 and comingles there with the vapor and liquid streams from the second additional expansion device 130. The comingled mixed refrigerant cold stream continues flowing upwards in the cold passage 105, to provide refrigeration to the top sections of streams in the natural gas hot passage 101, the fourth high- pressure mixed refrigerant hot passage 110, the third high-pressure mixed refrigerant hot passage 109, the intermediate-high-pressure mixed refrigerant hot passage 128 and the first high pressure mixed refrigerant hot passage 103. Flowing upwards, the mixed refrigerant stream in the cold passage 105 is warmed up to a temperature close to the temperature of the liquid and vapor streams from the second separator 112 and comingles there with the vapor and liquid streams from the second separator 112. The comingled mixed refrigerant stream continues its flowing in the cold passage 105 tothe top of the multi-stream heat exchanger 100 and provides refrigeration to the warm end streams in the natural gas hot passage 101, the intermediate pressure mixed refrigerant hot passage 102, the first high pressure mixed refrigerant hot passage 103 and the second high pressure mixed refrigerant hot passage 104. The mixed refrigerant stream comes out of the cold box in the cold passage 105 as superheated vapor at temperature approaching the inlet stream temperatures of the natural gas hot passage101, the intermediate pressure mixed refrigerant hot passage 102, the first high pressure mixed refrigerant hot passage 103 and the second high pressure mixed refrigerant hot passage 104 and then flows to the suction scrubber 201 of the first stage of the mixed refrigerant compressor 202.

[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, 109, 110, 128 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 outlet at the cold end. The mixed refrigerant hot passages102, 103 104, 109, 110, 128 have their respective inlets at the warm end or at different distances from 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, 109, 110, 128, except the second high pressure mixed refrigerant hot passage 104, being subsequently expanded in a respective expansion device 111, 123, 115, 119, 129 before being returned inside the shell 10’, being spread over the section of the coil wound heat exchanger 100’ through distributors, for example spray injectors, to form the mixed refrigerant cold passage 105, 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, 109, 110, 128. According to this embodiment, no separators are arrangedbetween the expansion devices 111, 115, 119, 123, 129 and the respective inlet to the mixed refrigerant cold passage 105.

[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 the additional low pressure separator 130 is not present and the outlet of the second additional expansion device 129 is connected to the inlet of the second low pressure separator 116, together with the outlet of the third high-pressure mixed refrigerant hot passage 109. Accordingly, the outlet of the intermediate-high-pressure mixed refrigerant hot passage 128 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 103 and the outlet of the third high-pressure mixed refrigerant hot passage 109.

[0033] 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, 103, 104) 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 (105) of the mixed refrigerant, configured to exchange heat between the natural gas and the mixed refrigerant, the cold passage (105) 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 (105) 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 stage separator (204) configured to separate the mixed refrigerant at the intermediate pressure into an intermediate-pressure liquid fraction and an intermediate-pressure 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 (207) configured to compress the mixed refrigerant to a second pressure, or high pressure and having an inlet and an outlet, a second stage cooling section (208) with an inlet connected to the outlet of the second stage compressor (207) and an outlet, a second stage separator (209) 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 (209) having an inlet connected to the outlet of the second stage cooling section (208), a liquid outlet connected to a second stage liquid outlet line (210) and a vapor outlet connected to a second stage vapor outlet line (211);- the plurality of mixed refrigerant hot passages (102, 103, 104) of the multi - stream 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 (103), with an inlet connected to the second stage liquid outlet line (210) 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 (104), with an inlet connected to the second stage vapor outlet line (211) and an outlet at a third distance from the warm side of the multi-stream heat exchanger (100, 100’);- a high-pressure separator (106) having an inlet, a liquid outlet and a vapor outlet,- the inlet being connected to the outlet of the second high-pressure mixed refrigerant hot passage (104),- the liquid outlet being connected to a liquid outlet line (107), the liquid outlet line (107) being connected to a third high-pressure mixed refrigerant hot passage (108), 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 third distance, and the outlet being arranged at a fourth distance from the warm side of the multi-stream heat exchanger (100, 100’), thefourth distance being greater than the second distance from the warm end of the multi-stream heat exchanger (100, 100’), i.e. the distance of the outlet of the first high-pressure mixed refrigerant hot passage (103);- the vapor outlet being connected to a vapor outlet line (108), the vapor outlet line (108) being connected to a fourth high-pressure mixed refrigerant hot passage (110), 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 third distance and the outlet being arranged at a fifth distance from the warm side of the multi-stream heat exchanger (100, 100’);- a plurality of expansion devices (111, 115, 119, 123), configured to expand and consequently cool the mixed refrigerant, the plurality of expansion devices (111, 115, 119, 123) comprising an intermediate-pressure expansion device (111), a first high-pressure expansion device (123), a second high-pressure expansion device (115) and a third high-pressure expansion device (119), all the expansion devices (111, 115, 119, 123) being configured to expand the mixed refrigerant 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 (103), of the third high-pressure mixed refrigerant hot passage (109) and of the fourth high- pressure mixed refrigerant hot passage (110) and the outlets being respectively directly or indirectly connected to the mixed refrigerant cold passage (105); wherein the intermediate-pressure expansion device (111), the second high-pressure expansion device (115) and the third high-pressure expansion device (119) are configured to expand and at least partially flash the mixed refrigerant down to substantially the low pressure and the first high-pressure expansion device (123) is configured to expand and at least partially flash the mixed refrigerant down to an intermediate-high pressure, at least 1 bar higher than the low pressure; the outlets of the intermediatepressure expansion device (111), of the second high-pressure expansion device (115) and of the third high-pressure expansion device (119) are connected to the mixed refrigerant cold passage (105), at a slightly greater distance than respectively the first distance, the fourth distance and the fifth distance from the warm end of the multistream heat exchanger (100, 100’); and wherein the system further comprises- an intermediate-high-pressure separator (124) configured to separate a liquidfraction and a vapor fraction and having an inlet, a liquid outlet and a vapor outlet; the inlet being connected to the outlet of the first high-pressure expansion device (123), the liquid outlet being connected to a liquid outlet line (125) and the vapor outlet being connected to a vapor outlet line (126);- a first additional expansion device (127) configured to expand and at least partially flash the mixed refrigerant down to substantially the low pressure and having an inlet connected to the liquid outlet line (125) of the intermediate-high- pressure separator (124) and an outlet connected downstream of the intermediate-pressure expansion device (111);- an intermediate-high-pressure mixed refrigerant hot passage (128), with an inlet and an outlet, the inlet being connected to the vapor outlet line (126) and being arranged at a distance from the warm side of the multi-stream heat exchanger (100, 100’), greater than the first distance, i.e. the distance of the outlet of the intermediate-pressure mixed refrigerant hot passage (102) and smaller than the second distance, i.e. the distance of the outlet of the first high-pressure mixed refrigerant hot passage (103), and the outlet being arranged at a distance from the warm side of the multi-stream heat exchanger (100, 100’), greater than the second distance and smaller than the fourth distance, i.e. the distance of the outlet of the third high-pressure mixed refrigerant hot passage (111); and- a second additional expansion device (129) 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 inter- mediate-high-pressure mixed refrigerant hot passage (128) and the outlet being directly or indirectly connected to the mixed refrigerant cold passage (105).

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, 109, 110) are coil wound tubes (101, 102, 103 104, 109, 110) arranged inside a shell (10’), the outlets of the expansion devices (111, 115, 119, 129) 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 (105) is formed by the mixed refrigerant distributed inside the shell (10’) and flowing over the coil wound tubes (101, 102, 103 104, 109, 110).

3. The system of claim 1, wherein the multi-stream heat exchanger(100) is enclosed in a cold box, the natural gas hot passage (101), the mixed refrigerant hot passages (102, 103 104, 109, 110) and the mixed refrigerant cold passage (105) 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 low pressure separators (112, 116, 120), configured to separate the low pressure mixed refrigerant into a liquid fraction and a vapor fraction, the plurality of low pressure separators (112, 116, 120) comprising a first low pressure separator (112), a second low pressure separator (116) and a third low pressure 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 (111), the second high-pressure expansion device (115) and the third high-pressure expansion device (119), the liquid outlets being connected to respective liquid outlet lines (113, 117, 121) and the vapor outlets being connected to respective vapor outlet lines (114, 118, 122), both the liquid outlet lines (113, 117, 121) and the vapor outlet lines (114, 118, 122) being connected to the mixed refrigerant cold passage (105) at a slightly greater distance than respectively the first distance, the fourth distance and the fifth distance from the warm end of the multi-stream heat exchanger (100).

4. The system of claim 3, wherein the outlet of the second additional expansion device (129) is connected to the inlet of the second low pressure separator (H6).

5. The system of claim 3, wherein the outlet of the second additional expansion device (129) is connected to an inlet of an additional low pressure separator (130), the additional low pressure separator (130) configured to separate the low pressure mixed refrigerant into a liquid fraction and a vapor fraction and having a liquid outlet and a vapor outlet, the liquid outlet being connected to a liquid outlet line (131) and the vapor outlet being connected to a vapor outlet line (132), both the liquid outlet line (131) and the vapor outlet line (132) being connected to the mixed refrigerant cold passage (105) of the multi-stream heat exchanger (100).

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