Single refrigerant mixed LNG production process

Abstract

A simple and efficient single mixed refrigerant process for cooling and liquefying hydrocarbon feed streams such as natural gas. The process employs a closed loop single mixed refrigerant process for cooling duty. Refrigerant compressed to high pressure using at least three stages of compression and two intercoolers, both producing liquid. A hydraulic turbine is used to expand the high pressure refrigerant before it flows into the main heat exchanger.

Description

[Background technology]

[0001] The production of liquefied natural gas (LNG) by indirect heat exchange against a single mixed refrigerant (SMR) is well known in the art. A simple, well-known prior art SMR process is described herein and illustrated in FIG.

[0002] There have been many attempts to improve the efficiency of the SMR process. For example, U.S. Patent No. 10,139,157 describes a single mixed refrigerant LNG production cycle in which a single mixed refrigerant stream is cooled and liquefied in a cryogenic exchanger before passing through a Joule-Thomson valve. Similarly, U.S. Patent No. 6,334,334 teaches a single mixed refrigerant LNG production cycle in which two mixed refrigerant (vapor and liquid) streams are cooled and liquefied separately in a cryogenic exchanger before the resulting liquid passes through a work-producing turbine. The resulting liquefied vapor stream passes through a Joule-Thomson valve. In this process, three compression stages are provided, and the liquid produced in the first intercooler is mixed with the discharge of the second stage and then cooled in a second intercooler to produce a second liquid and vapor stream, which are then separated. The liquid stream is sent directly to the cryogenic exchanger. In addition, only a portion of the mixed refrigerant passes through the work-producing turbine.

[0003] Many of the attempts to improve the efficiency of SMR processes result in processes that are complex to build and / or operate. Thus, there is a need for improved SMR processes that better balance increased efficiency and complexity. Summary of the Invention

[0004] Disclosed herein is a simple and efficient SMR process that cools and liquefies a single high pressure ambient two-phase mixed refrigerant stream in a cryogenic heat exchanger, expands the liquid refrigerant at the cold end, and then vaporizes it in the exchanger to provide refrigeration duty to the natural gas stream and the high pressure mixed refrigerant stream being liquefied.

[0005] A key feature of the exemplary embodiment disclosed herein is the synergistic combination of three stages of compression, two intercoolers (both producing liquid), and the use of a hydro turbine to expand the refrigerant before it flows into the main heat exchanger. Providing three stages of compression (with liquid formation as described) allows for a high mixed refrigerant discharge pressure to be efficiently achieved. The high mixed refrigerant discharge pressure improved the cooling performance of the hydro turbine and unexpectedly improved the performance of the liquefaction system.

[0006] Several aspects of the systems and methods are outlined below.

[0007] Aspect 1. A method for liquefying a hydrocarbon stream using a mixed refrigerant, comprising: (a) cooling and condensing the hydrocarbon stream and the cooled two-phase high pressure refrigerant stream in a main heat exchanger against the expanded refrigerant stream to form a liquefied hydrocarbon stream, a condensed refrigerant stream, and a vaporized refrigerant stream; (b) compressing the vaporized refrigerant stream in the first compression stage to a first pressure to form a low pressure compressed refrigerant stream; (c) cooling the low pressure compressed refrigerant stream in a first ambient cooler to form a cooled two-phase refrigerant stream; (d) separating the cooled two-phase refrigerant stream into a first cooled vapor stream and a first cooled liquid stream; (e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form an intermediate pressure compressed stream; (f) cooling the intermediate-pressure compressed stream in a second ambient cooler to form a cooled intermediate-pressure compressed stream; (g) separating the cooled intermediate pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream; (h) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a two-phase high pressure compressed stream; (i) cooling a first two-phase high pressure stream including the two-phase high pressure compressed stream in a third ambient cooler to form a cooled two-phase high pressure compressed stream; (j) expanding the condensed refrigerant stream to form an expanded refrigerant stream, where at least a portion of the expansion is performed using a hydro turbine; and (k) combining the first cooled liquid stream with a fluid stream downstream of the cooled two-phase refrigerant stream and upstream of the cooled two-phase high pressure compressed stream; (l) combining a second cooled liquid stream with a fluid stream downstream of the cooled two-phase refrigerant stream and upstream of the cooled two-phase high pressure compressed stream.

[0008] Aspect 2. 2. The method of embodiment 1, further comprising: (m) modifying a pressure of the first cooled liquid stream prior to performing step (k).

[0009] Aspect 3. The method of aspect 2, wherein step (m) comprises increasing the pressure of the first cooled liquid stream to the second pressure or to a third pressure before performing step (k).

[0010] Aspect 4. The method of embodiment 1, further comprising: (n) modifying the pressure of the second cooled liquid stream prior to performing step (l).

[0011] Aspect 5. The method of aspect 4, wherein step (n) comprises reducing the pressure of the second cooled liquid stream to the first pressure before performing step (l).

[0012] Aspect 6. The method of aspect 4, wherein step (n) comprises increasing the pressure of the second cooled liquid stream to a third pressure before carrying out step (l).

[0013] Embodiment 7. The method of embodiment 1, wherein the expanded refrigerant stream provides the sole cooling duty for step (a).

[0014] Aspect 8. The method of aspect 1, wherein the flow of refrigerant in steps (a)-(l) defines a closed-loop refrigeration cycle, and wherein all of the refrigerant flows through the hydro-turbine in step (n).

[0015] Aspect 9. The method of aspect 8, wherein the main heat exchanger comprises a warm end and a cold end, and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.

[0016] Aspect 10. The method of aspect 1, wherein the vaporized refrigerant stream has a first flow rate in step (b) and the expanded refrigerant stream has a second flow rate in step (n), the first flow rate equal to the second flow rate.

[0017] Aspect 11. The method of aspect 1, wherein the cooled two-phase high pressure refrigerant stream has a pressure of at least 1000 PSIA (68.95 bara).

[0018] Aspect 12. The method of aspect 1, wherein the composition of the refrigerant is the same in the vaporized refrigerant stream, the two-phase high pressure refrigerant stream, the condensed refrigerant stream, and the expanded refrigerant stream.

[0019] Aspect 13. A method for liquefying a hydrocarbon stream using a mixed refrigerant, comprising: (a) cooling and condensing the hydrocarbon stream and the cooled two-phase high pressure refrigerant stream in a main heat exchanger against the expanded refrigerant stream to form a liquefied hydrocarbon stream, a condensed refrigerant stream, and a vaporized refrigerant stream; (b) compressing the vaporized refrigerant stream in the first compression stage to a first pressure to form a low pressure compressed refrigerant stream; (c) cooling the low pressure compressed refrigerant in a first ambient cooler to form a cooled two-phase refrigerant stream; (d) separating the cooled two-phase refrigerant stream into a first cooled vapor stream and a first cooled liquid stream; (e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form an intermediate pressure compressed stream; (f) pumping the first cooled liquid stream to a second pressure to form a pumped first cooled liquid stream; (g) combining the pumped first cooled liquid stream with the medium-pressure refrigerant stream to form a combined medium-pressure refrigerant stream; (h) cooling the combined medium-pressure refrigerant stream in a second ambient cooler to form a cooled combined medium-pressure refrigerant stream; (i) separating the cooled combined medium-pressure refrigerant stream into a second cooled vapor stream and a second cooled liquid stream; (j) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a high pressure compressed stream; (k) pumping the second cooled liquid stream to a third pressure to form a pumped second cooled liquid stream; (l) combining the pumped second cooled liquid stream with the high pressure compressed stream to form a two-phase high pressure refrigerant stream; (m) cooling the two-phase high pressure refrigerant stream in a third ambient cooler to form a cooled two-phase high pressure refrigerant stream; (n) expanding the condensed refrigerant stream through a hydraulic turbine to form an expanded refrigerant stream.

[0020] Aspect 14. The method of aspect 13, wherein the expanded refrigerant stream provides the sole cooling duty for step (a).

[0021] Aspect 15. The method of aspect 13, wherein the flow of refrigerant in steps (a)-(n) defines a closed-loop refrigeration cycle, and wherein all of the refrigerant flows through the hydro-turbine in step (n).

[0022] Aspect 16. The method of aspect 15, wherein the main heat exchanger comprises a warm end and a cold end, and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.

[0023] Aspect 17. The method of aspect 13, wherein the vaporized refrigerant stream has a first flow rate in step (b) and the expanded refrigerant stream has a second flow rate in step (n), the first flow rate being equal to the second flow rate.

[0024] Aspect 18. The method of aspect 13, wherein the cooled two-phase high pressure refrigerant stream has a pressure of at least 1000 PSIA (68.95 bara).

[0025] Aspect 19. The method of aspect 13, wherein the composition of the refrigerant is the same in the vaporized refrigerant stream, the two-phase high pressure refrigerant stream, the condensed refrigerant stream, and the expanded refrigerant stream.

[0026] Aspect 20. A method according to claim 1, wherein a main heat exchanger comprises a hot bundle and a cold bundle, the method comprising: (o) when performing step (a), providing a first cooling duty within the warm bundle; 14. The method of claim 13, further comprising: (p) providing a second cooling duty in the cold bundle when performing step (a), the second cooling duty being less than the first cooling duty.

[0027] Aspect 21. The method of aspect 20, wherein the warm bundle and the cold bundle are each contained within a separate shell.

[0028] Aspect 22. The method according to claim 1, wherein the main heat exchanger further comprises an intermediate bundle, the method further comprising: 21. The method of embodiment 20, further comprising: (q) providing a third cooling duty in the intermediate bundle when performing step (a), the third cooling duty being less than the first cooling duty.

[0029] Aspect 23. The method of aspect 22, wherein the warm bundle, the cold bundle, and the intermediate bundle are each contained within a separate shell.

[0030] Aspect 24. The method of aspect 13, wherein the hydrocarbon stream comprises natural gas.

[0031] Aspect 25. The method of aspect 13, wherein step (i) further comprises selectively expanding the condensed refrigerant stream through an expansion valve located on the bypass circuit instead of through the hydro turbine.

[0032] 26. A method for liquefying a hydrocarbon stream using a mixed refrigerant, comprising: (a) cooling and condensing the hydrocarbon stream and the cooled two-phase high pressure refrigerant stream in a main heat exchanger against the expanded refrigerant stream to form a liquefied hydrocarbon stream, a condensed refrigerant stream, and a vaporized refrigerant stream; (b) expanding the condensed refrigerant stream to form an expanded refrigerant stream, where at least a portion of the expansion is performed using a hydro turbine; and (c) compressing the vaporized refrigerant stream in the first compression stage to a first pressure to form a low pressure compressed refrigerant stream; (d) cooling the low pressure compressed refrigerant stream in a first ambient cooler to form a cooled two-phase refrigerant stream; (e) separating the combined cooled two-phase refrigerant stream into a first cooled vapor stream and a first cooled liquid stream; (f) compressing the first cooled vapor stream in a second compression stage to a second pressure to form an intermediate pressure compressed stream; (g) pumping the first cooled liquid stream to a third pressure to form a pumped first cooled liquid stream; (h) cooling the intermediate-pressure compressed stream in a second ambient cooler to form a cooled intermediate-pressure compressed stream; (i) separating the cooled intermediate pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream; (j) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a two-phase high pressure compressed stream; (k) combining the pumped first cooled liquid stream with the two-phase high pressure compressed stream to form a combined two-phase high pressure compressed stream; (l) cooling the combined two-phase high pressure compressed stream in a third ambient cooler to form a cooled two-phase high pressure compressed stream; (m) expanding the second cooled liquid stream through an expansion valve to form an expanded cooled stream; (n) combining the expanded cooled stream with the cooled two-phase refrigerant stream to form a combined cooled two-phase refrigerant stream.

[0033] Aspect 27. The method of aspect 26, wherein the expansion in step (b) is provided by a hydro turbine, followed by an expansion valve.

[0034] Embodiment 28. The method of embodiment 26, wherein the second compression stage operates at a temperature of about 96.8°F.

[0035] Aspect 29. The method of aspect 26, wherein the expanded refrigerant stream provides the sole cooling duty for step (a).

[0036] Aspect 30. The method of aspect 26, wherein the flow of refrigerant in steps (a)-(n) defines a closed-loop refrigeration cycle, and wherein all of the refrigerant flows through the hydro-turbine in step (l).

[0037] Aspect 31. The method of aspect 26, wherein the main heat exchanger comprises a warm bundle and a cold bundle contained within separate shells.

[0038]

[0036] Aspect 32. The method of aspect 26, wherein the main heat exchanger further comprises an intermediate bundle located between the warm bundle and the cold bundle.

[0039] Embodiment 33. The method of embodiment 26, wherein the hydrocarbon stream comprises natural gas.

[0040] Aspect 34. The method of aspect 26, wherein the main heat exchanger has a warm end and a cold end, and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.

[0041] Aspect 35. A method of designing and fabricating a system for liquefying natural gas using a closed loop single mixed refrigerant process providing cooling duty to a cryogenic heat exchanger having a plurality of coil wound bundles, each of the plurality of coil wound bundles having an overall tube length, the method comprising: (a) selecting a cooling duty for each of the plurality of coil wound bundles that minimizes a difference in overall tube length for each of the plurality of coil wound bundles; (b) fabricating a system to provide the cooling duty selected in step (a);

[0042] A method in which the sole cooling duty of the cryogenic heat exchanger is a single mixed refrigerant stream that has been compressed to a pressure of at least 1000 PSIA (68.95 bara) and expanded by a hydro turbine.

[0043] Aspect 36. The method of aspect 35, wherein the plurality of coil wound bundles comprises a warm bundle and a cold bundle, and the selected cooling duty of the warm bundle is less than the selected cooling duty of the cold bundle. [Brief description of the drawings]

[0044] The present invention is hereinafter described in conjunction with the accompanying drawings, in which like numerals refer to like elements and in which:

[0045] [Figure 1] FIG. 1 is a schematic flow diagram depicting an improved SMR process. [Diagram 2] FIG. 2 is a schematic flow diagram depicting the improved SMR process of FIG. 1 modified to include a coil-wound heat exchanger having multiple shells. [Diagram 3] FIG. 1 is a schematic flow diagram depicting an alternative improved SMR process modified to include an expansion valve to reduce the temperature of the fluids in the process and to reduce power requirements. [Figure 4] FIG. 1 is a schematic flow diagram depicting the prior art PRICO® LNG process. [Diagram 5] 4 is a table comparing key parameters from the prior art process shown in FIG. 4 with several variations of the exemplary embodiment of the present invention shown in FIGS. 1-3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] The following detailed description provides only preferred exemplary embodiments and is not intended to limit the scope, applicability, or configuration of the present invention. Rather, the following detailed description of preferred exemplary embodiments will provide those skilled in the art with an effective description for implementing the preferred exemplary embodiments of the present invention. It should be understood that various changes can be made in the function and arrangement of elements without departing from the spirit and scope of the present invention.

[0047] In the claims, letters are used to identify the claimed steps (e.g., (a), (b), and (c)). These letters are used to aid in referencing the method steps and are not intended to indicate the order in which the claimed steps are performed, unless such order is specifically recited in the claims.

[0048] To aid in describing the present invention, directional terms may be used in the present specification and claims to describe portions of the present invention (e.g., top, bottom, left, right, etc.). These directional terms are intended merely to aid in describing and claiming the present invention and are not intended to limit the present invention in any manner. In addition, reference numerals introduced herein in connection with a drawing may be repeated in one or more subsequent drawings without additional description herein to provide context for other features.

[0049] Unless otherwise indicated, as used herein, the articles "a" and "an" mean one or more when applied to any feature in the embodiments of the invention described herein and in the claims. The use of "a" and "an" does not limit the meaning to a single feature unless such a limitation is specifically stated. The article "the" preceding a singular or plural noun or noun phrase refers to the particular named feature or to certain named features and may have a singular or plural connotation depending on the context in which it is used.

[0050] Unless otherwise specified herein, introducing a stream to a location is intended to mean introducing substantially all of said stream to that location. All streams discussed herein and shown in the drawings (typically represented by lines with arrows indicating the general direction of fluid flow during normal operation) should be understood to be contained within corresponding conduits. Each conduit should be understood to have at least one inlet and at least one outlet. Additionally, each piece of equipment should be understood to have at least one inlet and at least one outlet.

[0051] As used herein and in the claims, the term "conduit" refers to one or more structures through which a fluid may be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and / or gases.

[0052] As used in this specification and claims, the term "flow communication" is intended to mean that two or more elements are connected (directly or indirectly) in a manner that allows fluid to flow between the elements, including connections that may include valves, gates, tees, or other devices that may selectively restrict, combine, or separate fluid flow.

[0053] As used in this specification and claims, the term "natural gas" means a hydrocarbon gas mixture consisting primarily of methane.

[0054] As used herein and in the claims, the terms "hydrocarbon", "hydrocarbon gas" or "hydrocarbon fluid" refer to a gas / fluid that contains at least one hydrocarbon and that comprises at least 80%, more preferably at least 90%, of the total composition of the gas / fluid.

[0055] As used herein and in the claims, the term "mixed refrigerant" means a mixture of hydrocarbons, typically comprising hydrocarbon components containing from 1 to 5 carbon atoms, and which may include saturated and / or unsaturated components and / or straight chain and / or branched components, and nitrogen.

[0056] As used herein and in the claims, the term "ambient cooler" means a heat exchange device that cools a fluid against an ambient fluid, typically ambient air.

[0057] As used herein and in the claims, the terms "high-high", "high", "medium", "low" and "low-low" are intended to represent relative values ​​of properties of the elements for which they are used. For example, a high-high pressure stream is intended to refer to a stream having a higher pressure than the corresponding high pressure stream or medium pressure stream or low pressure stream described or claimed in this application. Similarly, a high pressure stream is intended to refer to a stream having a higher pressure than the corresponding medium pressure stream or low pressure stream described or claimed in this application, but a lower pressure than the corresponding high-high pressure stream described or claimed in this application. Similarly, a medium pressure stream is intended to refer to a stream having a higher pressure than the corresponding low pressure stream described or claimed in this application, but a lower pressure than the corresponding high pressure stream described or claimed in this application.

[0058] Unless otherwise specified herein, any and all percentages specified in the specification, drawings, and claims should be understood to be on a mass percentage basis. Unless otherwise specified herein, any and all pressures specified in the specification, drawings, and claims should be understood to mean gauge pressure.

[0059] As used herein and in the claims, the term "compression system" is defined as one or more compression stages. For example, a compression system may include multiple compression stages within a single compressor. In an alternative example, a compression system may include multiple compressors.

[0060] As used herein, the term "hydraulic turbine" is intended to refer to a work-producing liquid expander. In the context of the present invention, the primary purpose of a hydraulic turbine is to provide cooling to a process by removing enthalpy from a refrigerant stream, and the work produced may be recovered using an electrical generator, used to compress another fluid, or simply dissipated as heat released to the surroundings.

[0061] Figure 1 illustrates a single mixed refrigerant (SMR) natural gas liquefaction process. In this exemplary process, a feed gas stream 100 and a two-phase high pressure refrigerant stream 128 are cooled against an expanded refrigerant stream 136. For the processes of Figures 1 and 2, the term "refrigerant" should be understood to mean a mixed refrigerant.

[0062] In this example, the feed gas stream 100 is natural gas, and is preferably pretreated to remove water, acid gases (carbon dioxide and sulfur dioxide), and heavy hydrocarbons that can be frozen. The feed gas stream 100 is preferably near ambient temperature, or may be pre-cooled by another process using known cooling techniques (fluid boiling, gas expansion, etc.). Typically, the feed gas stream 100 enters the warm end 160 of the cryogenic heat exchanger 130 at a pressure of 40 bara to 80 bara, and then exits the cold end 161 of the cryogenic heat exchanger 130 as a product stream 102 in the liquid phase, typically at a temperature of -140°C to -150°C. The product stream 102 preferably passes through a pressure reduction device 138, which may be a Joule-Thompson valve or a work-producing hydraulic turbine, before being sent to storage (not shown).

[0063] In this exemplary process, the cryogenic heat exchanger 130 consists of a single shell 131. Examples of heat exchanger types suitable for the cryogenic heat exchanger 130 include a plate and fin heat exchanger or a coil wound heat exchanger. In the case of a coil wound heat exchanger, the expanded refrigerant stream 136 flows through the shell side of the cryogenic heat exchanger 130. In the case of a plate fin style exchanger, it may be desirable to employ devices to ensure uniform distribution of the two-phase high pressure refrigerant stream 128 between the parallel passages and / or exchangers using techniques well known in the art, such as a phase separator liquid pump.

[0064] The feed gas stream 100 may optionally be removed from the cryogenic heat exchanger 130 at an intermediate location (stream 103) and sent to a separation device 150 for heavy hydrocarbon removal, and then the primarily methane stream 105 is reintroduced into the cryogenic heat exchanger 130.

[0065] After providing the cooling duty, a vapor refrigerant stream 104 is withdrawn from the warm end 160 of the cryogenic heat exchanger 130. The vapor refrigerant stream 104 is preferably at a temperature close to ambient temperature and at a pressure of 3-5 bara. The vapor refrigerant stream 104 is then compressed in a compressor stage 106 to a pressure of typically 10-20 bara to form a low pressure refrigerant stream 107. The low pressure refrigerant stream 107 is then cooled by an ambient cooler 108 using cooling water or cooling air. The resulting cooled two-phase refrigerant stream 109 is separated using a separator 110 into a liquid stream 113 and a vapor stream 111. The vapor stream 111 is compressed (via a compression stage 114) to a pressure of typically 25-30 bara to form an intermediate pressure refrigerant stream 115. Liquid stream 113 is pumped (using pump 112) to substantially the same pressure as medium pressure refrigerant stream 115 and then combined with medium pressure refrigerant stream 115. The combined stream 117 is then cooled by ambient cooler 116 using cooling water or cooling air.

[0066] The resulting two-phase refrigerant stream 118 is separated into a liquid stream 123 and a vapor stream 121 using separator 120. Vapor stream 121 is further compressed (via compression stage 124) to a pressure of typically 40-70 bara to form high pressure refrigerant stream 125. Liquid stream 123 is pumped (via pump 122) to substantially the same pressure as high pressure refrigerant stream 125 and then recombined with high pressure refrigerant stream 125. Combined stream 127 is then cooled by ambient cooler 126 using cooling water or cooling air to form two-phase high pressure refrigerant stream 128.

[0067] The two-phase high pressure refrigerant stream 128 is then cooled and condensed in a cryogenic heat exchanger 130 and exits as a condensed refrigerant stream 132 in the liquid phase and at a temperature typically between −140° C. and −150° C. The condensed refrigerant stream 132 is sent to a hydro turbine 134 and expanded to form an expanded refrigerant stream 136. Optionally, the hydro turbine 134 may have a valve following the single-phase liquid outlet or may include a work-producing liquid expander with a two-phase outlet.

[0068] An optional bypass circuit 139 with an expansion valve 137 (such as a Joule-Thomson valve) may be provided to allow the system to continue to function when the hydro turbine 134 is being serviced or in the event of a failure. Additionally, an optional expansion valve 162 (such as a Joule-Thomson valve) may be provided downstream of the hydro turbine 134 to provide further expansion of the expanded refrigerant stream 136. The optional bypass circuit 139 and optional expansion valve 162 may be included in the embodiment shown in FIG.

[0069] The expanded refrigerant stream 136 is then introduced into the cold end 161 of the cryogenic heat exchanger 130, where it is evaporated (typically at a pressure of 3-5 bara) (providing the cooling duty for the process) and exits the warm end 160 as vapor refrigerant stream 104.

[0070] In Figure 2, all items are labeled with reference numbers in the format 2XX. Unless otherwise noted herein, elements in Figure 2 having reference numbers with the same last two digits as elements in Figure 1 should be understood to be substantially the same as the corresponding elements in Figure 1. For example, separator 210 in Figure 2 is substantially the same as separator 110 in Figure 1.

[0071] The process of FIG. 2 is very similar to that of FIG. 1, except that the cryogenic heat exchanger 130 of FIG. 1 is replaced by a cryogenic heat exchanger 230 having three shells 230a, 230b, and 230c, each arranged in series and each containing a coil wound tube bundle (not shown). The duty can be divided between two or three coil wound exchangers in separate shells with interconnecting piping (253-257) as shown. Alternatively, the cryogenic heat exchanger 230 can be built with two or three coil wound bundles in a common shell. Dividing the service can help keep the individual bundles within manufacturing limitations, e.g., limitations on maximum length to stay within the capabilities of current manufacturing facilities. The construction of the cryogenic heat exchanger 230 allows the cooling duty to be divided unevenly between the bundles and selected in a manner that shortens manufacturing time.

[0072] If all of the shells 230a, 230b, and 230c and bundles of the cryogenic heat exchanger 230 are fabricated simultaneously, the overall fabrication time is determined by the bundle that requires the most time to fabricate. If the cooling duty is divided evenly between the bundles contained in each of the shells 230a, 230b, and 230c, the bundles contained in the shell 230a will take longer to fabricate than the bundles contained in the shells 230b and 230c. The feed gas stream 200 and the two-phase high pressure refrigerant stream 228 have a relatively high vapor fraction and a relatively low density when flowing through the shell 230a, compared to those same streams when flowing through the shells 230b or 230c. This is due to the increased density of the streams 200, 228 as they cool and condense. This causes the bundles in the shell 230a to require more tubes than the bundles in the shells 230b or 230c. Shifting the cooling duty from shell 230a to shells 230b and 230c by shortening the bundles in shell 230a and increasing the length of the bundles in shells 230b and 230c reduces the length of the tubes and therefore allows the same pressure drop to be achieved with fewer tubes, which reduces the fabrication time of the bundles for shell 230a and therefore reduces the overall fabrication time.

[0073] For example, in the exemplary embodiment shown in Figure 2, the duty may be divided such that shell 230a has 27% of the total duty, shell 230b has 35% of the total duty, and shell 230c has 38% of the total duty. In general, for an SMR system with three heat exchanger shells, the division of duties preferably results in the warmest shell (shell 203a in Figure 2) having less than 30% of the total cooling duty of the heat exchanger. Similarly, for an SMR system with only two heat exchanger shells, the division of duties preferably results in the warmest shell having less than 45% of the total cooling duty of the heat exchanger.

[0074] In Figure 3, all items are labeled with reference numbers in the format 3XX. Unless otherwise stated herein, elements in Figure 3 having reference numbers with the same last two digits as elements in Figure 1 or Figure 2 should be understood to be substantially the same as the corresponding elements in Figure 1 or Figure 2. For example, phase separator 310 in Figure 3 is substantially the same as phase separator 110 in Figure 1 and phase separator 210 in Figure 2.

[0075] The process of FIG. 3 is very similar to that of FIG. 2. After providing cooling duty, the vapor refrigerant stream 304 is withdrawn from the warm end of the cryogenic heat exchanger 330 and then compressed in the compressor stage 306 to form a low pressure compressed refrigerant stream 319. Note that in this example, the warm end of the heat exchanger 330 is at the top end of the heat exchanger 330. In some other applications, such as a brazed aluminum heat exchanger ("BAHX"), the warm end would be at the bottom end. The low pressure compressed refrigerant stream 319 is then cooled by an ambient cooler 308. The resulting cooled two-phase refrigerant stream 309 is combined with an expanded cooling stream 354 (discussed herein) and then separated into a first cooled liquid stream 313 and a first cooled vapor stream 311 using a phase separator 310. Both phases in the phase separator 310 are below ambient temperature. Vapor stream 311 is compressed via compression stage 314 to form intermediate-pressure compressed stream 315. The first cooled liquid stream 313 is pumped using pump 312 to substantially the same pressure as high-pressure compressed stream 331 (discussed herein) to form a high-pressure cooled liquid stream high-pressure compressed stream. The intermediate-pressure compressed stream 315 is then cooled in ambient cooler 316 to form a cooled intermediate-pressure compressed stream 318, which is then separated into liquid stream 323 and vapor stream 325 using phase separator 320. Vapor stream 325 is compressed via compression stage 324 to form a two-phase high-pressure compressed stream 331.

[0076] The liquid stream 323 is cooled and expanded through a suitable expansion valve 352, e.g., a Joule-Thomson valve, to form an expanded cooled stream 354, at least a portion of which is a vapor. The expanded two-phase stream 354 is mixed with the two-phase refrigerant stream 309 upstream of the phase separator 310. This mixing reduces the temperature of the two-phase refrigerant stream 309 entering the phase separator 310, which further reduces the power requirements of the second compressor stage 314 and the total power needed to operate the system. This arrangement also requires one less pump than the illustrative examples of Figures 1 and 2.

[0077] In this illustrative example, high pressure cooling liquid stream 322 is combined with high pressure refrigerant stream 331 downstream of compressor stage 324, and the combined stream is then cooled by ambient cooler 326 to form a two-phase high pressure refrigerant stream 328, which is then cooled and condensed in cryogenic heat exchanger 330.

[0078] In Figure 4, all items are labeled with reference numbers in the format 4XX. Unless otherwise stated herein, elements in Figure 4 having reference numbers with the same last two digits as elements in Figure 1 or Figure 2 should be understood to be substantially the same as the corresponding elements in Figure 1 or Figure 2. For example, phase separator 410 in Figure 4 is substantially the same as phase separator 110 in Figure 1 or phase separator 210 in Figure 2.

[0079] 4 shows the PRICO® LNG process, a prior art single mixed refrigerant process commonly used in small and medium-sized LNG plants. In this process, two compression stages 406, 414 and a single intercooler 408 and drum 410 are used. Liquid 413 from drum 410 is pumped (using pump 412) and then mixed with vapor stream 411 from the final compression stage 414 before being cooled in aftercooler 416. A Joule-Thomson valve 434 is used to expand the liquefied mixed refrigerant stream 432 before being sent to exchanger 430 where it is vaporized to provide cooling. This process is provided to form a basis for comparing performance with the processes disclosed herein in the examples provided below. EXAMPLES

[0080] Figure 5 is a table comparing the prior art process shown in Figure 4 with several variations of the exemplary embodiment shown in Figures 1 and 3 for a constant production rate. The data in Figure 5 was generated in a process simulator with operating parameters of pressure, temperature, and mixed refrigerant composition selected using a numerical optimization program. Design assumptions such as LNG production rate, ambient temperature, pressure drop, minimum approach temperature of the exchangers, compressor efficiency, etc. were the same for all five examples. The relative power values ​​for each configuration are the ratio of the total power required to operate the system in each configuration to the power required to operate the system using the embodiment shown in Figure 4.

[0081] As shown in columns 1 and 2, there is a 6.0% benefit (i.e., reduction in total power requirements) from adding a water turbine to the prior art process of FIG. 4. A comparison of columns 1 and 3 shows a 5.2% benefit from adding a third compression stage, a second intercooler, and a second pump to the prior art process of FIG. 4. Based on these results, the total expected benefit of the process of FIG. 1 over the process of FIG. 4 is expected to be 11.2%. The actual improvement was 13.2%, significantly exceeding expectations. This unexpected result is believed to be due to the synergistic effect of providing a higher mixed refrigerant discharge pressure (due to the additional compression stage 124) to the water turbine 134. A comparison of the prior art process of FIG. 4 (column 1) with the process of FIG. 3 (column 5) shows a 15.6% benefit from adding the water turbine 334, the third compression stage 324, the second intercooler 316, the second pump 312, and the expansion valve 352 to the prior art process of FIG. 4. The suction temperature of the second compression stage 314 is reduced due to the Joule-Thomson effect of the liquid from the phase separator 320 therein, reducing the total power required to operate the system. These modeled calculations did not consider any recovery of the work performed by the water turbine 134. It is therefore possible that additional benefits may be realized from embodiments including a water turbine.

[0082] The present invention has thus been disclosed in terms of the preferred and alternative embodiments thereof. Naturally, various changes, modifications, and variations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention be limited only by the terms of the appended claims.

Claims

1. 1. A process for liquefying a hydrocarbon stream using a mixed refrigerant, comprising: (a) cooling and condensing the hydrocarbon stream and the cooled two-phase high pressure refrigerant stream in a main heat exchanger against an expanded refrigerant stream to form a liquefied hydrocarbon stream, a condensed refrigerant stream, and a vaporized refrigerant stream; (b) compressing the vaporized refrigerant stream in a first compression stage to a first pressure to form a low pressure compressed refrigerant stream; (c) cooling the low pressure compressed refrigerant stream in a first ambient cooler to form a cooled two-phase refrigerant stream; (d) separating the cooled two-phase refrigerant stream into a first cooled vapor stream and a first cooled liquid stream; (e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form an intermediate pressure compressed stream; (f) cooling the intermediate-pressure compressed stream in a second ambient cooler to form a cooled intermediate-pressure compressed stream; (g) separating the cooled intermediate pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream; (h) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a two-phase high pressure compressed stream; (i) cooling a first two-phase high pressure stream comprising the two-phase high pressure compressed stream in a third ambient cooler to form a cooled two-phase high pressure compressed stream; (j) expanding the condensed refrigerant stream to form the expanded refrigerant stream, wherein at least a portion of the expansion is performed using a hydro turbine; and (k) combining the first cooled liquid stream with a fluid stream downstream of the cooled two-phase refrigerant stream and upstream of the cooled two-phase high pressure compressed stream; (l) combining the second cooled liquid stream with a fluid stream downstream of the cooled two-phase refrigerant stream and upstream of the cooled two-phase high pressure compressed stream.

2. 10. The method of claim 1, further comprising: (m) modifying a pressure of the first cooled liquid stream prior to performing step (k).

3. 3. The method of claim 2, wherein step (m) comprises increasing the pressure of the first cooled liquid stream to the second pressure or the third pressure before performing step (k).

4. 10. The method of claim 1, further comprising: (n) modifying the pressure of the second cooled liquid stream prior to performing step (l).

5. 5. The method of claim 4, wherein step (n) comprises reducing the pressure of the second cooled liquid stream to the first pressure before performing step (l).

6. 5. The method of claim 4, wherein step (n) comprises increasing the pressure of the second cooled liquid stream to the third pressure before performing step (l).

7. The method of claim 1 , wherein the expanded refrigerant stream provides the sole cooling duty for step (a).

8. 2. The method of claim 1, wherein the flow of the refrigerant in steps (a) through (l) defines a closed-loop refrigeration cycle, and all of the refrigerant flows through the hydro turbine in step (n).

9. 9. The method of claim 8, wherein the main heat exchanger comprises a hot end and a cold end, and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.

10. 2. The method of claim 1, wherein the vaporized refrigerant stream has a first flow rate in step (b) and the expanded refrigerant stream has a second flow rate in step (n), the first flow rate being equal to the second flow rate.

11. 2. The method of claim 1, wherein the cooled two-phase high pressure refrigerant stream has a pressure of at least 1000 PSIA (68.95 bara).

12. 2. The method of claim 1, wherein the composition of the refrigerant is the same in the vaporized refrigerant stream, the two-phase high pressure refrigerant stream, the condensed refrigerant stream, and the expanded refrigerant stream.

13. 1. A process for liquefying a hydrocarbon stream using a mixed refrigerant, comprising: (a) cooling and condensing the hydrocarbon stream and the cooled two-phase high pressure refrigerant stream in a main heat exchanger against an expanded refrigerant stream to form a liquefied hydrocarbon stream, a condensed refrigerant stream, and a vaporized refrigerant stream; (b) compressing the vaporized refrigerant stream in a first compression stage to a first pressure to form a low pressure compressed refrigerant stream; (c) cooling the low pressure compressed refrigerant in a first ambient cooler to form a cooled two-phase refrigerant stream; (d) separating the cooled two-phase refrigerant stream into a first cooled vapor stream and a first cooled liquid stream; (e) compressing the first cooled vapor stream in a second compression stage to a second pressure to form an intermediate pressure compressed stream; (f) pumping the first cooled liquid stream to the second pressure to form a pumped first cooled liquid stream; (g) combining the pumped first cooled liquid stream with the medium-pressure refrigerant stream to form a combined medium-pressure refrigerant stream; (h) cooling the combined medium-pressure refrigerant stream in a second ambient cooler to form a cooled combined medium-pressure refrigerant stream; (i) separating the cooled combined medium pressure refrigerant stream into a second cooled vapor stream and a second cooled liquid stream; (j) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a high pressure compressed stream; (k) pumping the second cooled liquid stream to the third pressure to form a pumped second cooled liquid stream; (l) combining the pumped second cooled liquid stream with the high pressure compressed stream to form a two-phase high pressure refrigerant stream; (m) cooling the two-phase high pressure refrigerant stream in a third ambient cooler to form the cooled two-phase high pressure refrigerant stream; (n) expanding the condensed refrigerant stream through a hydraulic turbine to form the expanded refrigerant stream.

14. 14. The method of claim 13, wherein the expanded refrigerant stream provides the sole cooling duty for step (a).

15. 14. The method of claim 13, wherein the flow of the refrigerant in steps (a) through (n) defines a closed-loop refrigeration cycle, and all of the refrigerant flows through the hydro turbine in step (n).

16. 16. The method of claim 15, wherein the main heat exchanger comprises a hot end and a cold end, and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.

17. 14. The method of claim 13, wherein the vaporized refrigerant stream has a first flow rate in step (b) and the expanded refrigerant stream has a second flow rate in step (n), the first flow rate equal to the second flow rate.

18. 14. The method of claim 13, wherein the cooled two-phase high pressure refrigerant stream has a pressure of at least 1000 PSIA (68.95 bara).

19. 14. The method of claim 13, wherein the composition of the refrigerant is the same in the vaporized refrigerant stream, the two-phase high pressure refrigerant stream, the condensed refrigerant stream, and the expanded refrigerant stream.

20. the main heat exchanger comprises a hot bundle and a cold bundle, and the method comprises: (o) when performing step (a), providing a first cooling duty within the warm bundle; 14. The method of claim 13, further comprising: (p) when performing step (a), providing a second cooling duty in the cold bundle, the second cooling duty being less than the first cooling duty.

21. 21. The method of claim 20, wherein the warm bundle and the cold bundle are each contained within a separate shell.

22. the main heat exchanger further comprising an intermediate bundle, the method comprising:

21. The method of claim 20, further comprising: (q) providing a third cooling duty in the intermediate bundle when performing step (a), the third cooling duty being less than the first cooling duty.

23. 23. The method of claim 22, wherein the warm bundle, the cold bundle, and the intermediate bundle are each contained within a separate shell.

24. The method of claim 13 , wherein the hydrocarbon stream comprises natural gas.

25. 14. The method of claim 13, wherein step (i) further comprises selectively expanding the condensed refrigerant stream through an expansion valve located on a bypass circuit instead of through the water turbine.

26. 1. A process for liquefying a hydrocarbon stream using a mixed refrigerant, comprising: (a) cooling and condensing the hydrocarbon stream and the cooled two-phase high pressure refrigerant stream in a main heat exchanger against an expanded refrigerant stream to form a liquefied hydrocarbon stream, a condensed refrigerant stream, and a vaporized refrigerant stream; (b) expanding the condensed refrigerant stream to form the expanded refrigerant stream, wherein at least a portion of the expansion is performed using a hydro turbine; and (c) compressing the vaporized refrigerant stream in a first compression stage to a first pressure to form a low pressure compressed refrigerant stream; (d) cooling the low pressure compressed refrigerant stream in a first ambient cooler to form a cooled two-phase refrigerant stream; (e) separating the combined cooled two-phase refrigerant stream into a first cooled vapor stream and a first cooled liquid stream; (f) compressing the first cooled vapor stream in a second compression stage to a second pressure to form an intermediate pressure compressed stream; (g) pumping the first cooled liquid stream to a third pressure to form a pumped first cooled liquid stream; (h) cooling the intermediate-pressure compressed stream in a second ambient cooler to form a cooled intermediate-pressure compressed stream; (i) separating the cooled intermediate pressure compressed stream into a second cooled vapor stream and a second cooled liquid stream; (j) compressing the second cooled vapor stream in a third compression stage to a third pressure to form a two-phase high pressure compressed stream; (k) combining the pumped first cooled liquid stream with the two-phase high pressure compressed stream to form a combined two-phase high pressure compressed stream; (l) cooling the combined two-phase high pressure compressed stream in a third ambient cooler to form the cooled two-phase high pressure compressed stream; (m) expanding the second cooled liquid stream through an expansion valve to form an expanded cooled stream; (n) combining the expanded cooled stream with the cooled two-phase refrigerant stream to form the combined cooled two-phase refrigerant stream.

27. 27. The method of claim 26, wherein the expansion in step (b) is provided by a water turbine followed by an expansion valve.

28. 27. The method of claim 26, wherein the second compression stage operates at a temperature of about 96.8°F.

29. 27. The method of claim 26, wherein the expanded refrigerant stream provides the sole cooling duty for step (a).

30. 27. The method of claim 26, wherein the flow of the refrigerant in steps (a) through (n) defines a closed-loop refrigeration cycle, and all of the refrigerant flows through the hydro turbine in step (l).

31. 27. The method of claim 26, wherein the main heat exchanger comprises a warm bundle and a cold bundle contained within separate shells.

32. 27. The method of claim 26, wherein the main heat exchanger further comprises an intermediate bundle located between the warm bundle and the cold bundle.

33. 27. The method of claim 26, wherein the hydrocarbon stream comprises natural gas.

34. 27. The method of claim 26, wherein the main heat exchanger comprises a warm end and a cold end, and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.

35. 1. A method of designing and fabricating a system for liquefying natural gas using a closed loop single mixed refrigerant process providing cooling duty to a cryogenic heat exchanger having a plurality of coil wound bundles, each of the plurality of coil wound bundles having an overall tube length, the method comprising: (a) selecting a cooling duty for each of a plurality of coil wound bundles that minimizes a difference in overall tube length for each of the plurality of coil wound bundles; (b) fabricating the system to provide the cooling duty selected in step (a); A method wherein the sole cooling duty of said cryogenic heat exchanger is a single mixed refrigerant stream compressed to a pressure of at least 1000 PSIA (68.95 bara) and expanded by a hydro turbine.

36. 36. The method of claim 35, wherein the plurality of coil wound bundles comprises a hot bundle and a cold bundle, the selected cooling duty of the hot bundle being less than the selected cooling duty of the cold bundle.

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