Natural Gas Liquefaction by a High Pressure Expansion Process

Abstract

A method and system for liquefying a methane-rich high-pressure feed gas stream using a system having first and second heat exchanger zones and a compressed refrigerant stream. The compressed refrigerant stream is cooled and directed to the second heat exchanger zone to additionally cool it below ambient temperature. It is then expanded and passed through the first heat exchanger zone such that it has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone. The feed gas stream is passed through the first heat exchanger zone to cool at least part of it by indirect heat exchange with the refrigerant stream, thereby forming a liquefied gas stream. At least a portion of the first warm refrigerant stream is directed to the second heat exchanger zone to cool the refrigerant stream, which is compressed.

Claims

1. A method for liquefying a feed gas stream rich in methane using a system having first and second heat exchanger zones, where the method comprises: (a) providing the feed gas stream at a pressure less than 1,200 psia; (b) providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; (c) cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; (d) directing the compressed, cooled refrigerant stream to the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream; (e) expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; (f) passing the expanded, cooled refrigerant stream through the first heat exchanger zone to form a first warm refrigerant stream, wherein the first warm refrigerant stream has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone; (g) passing the feed gas stream through the first heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream; (h) directing at least a portion of the first warm refrigerant stream to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream, thereby forming a second warm refrigerant stream; and (i) compressing the second warm refrigerant stream to produce the compressed refrigerant stream.

2. The method of claim 1, wherein the first warm refrigerant stream has a temperature that is cooler, by at least 10 F., than the highest fluid temperature within the first heat exchanger zone.

3. The method of claim 1, wherein a portion of the first warm refrigerant stream remaining within the first heat exchanger zone further exchanges heat within the first heat exchanger zone to produce a third warm refrigerant stream.

4. The method of claim 1, further comprising: combining the second warm refrigerant stream with the third warm refrigerant stream prior to compressing the second warm refrigerant stream.

5. The method of claim 1, further comprising: further cooling the liquefied gas stream within the first heat exchanger zone using a sub-cooling refrigeration cycle, to thereby form a sub-cooled gas stream.

6. The method of claim 1, further comprising: expanding the sub-cooled gas stream in a hydraulic turbine to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream.

7. The method of claim 5, wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant.

8. The method of claim 6, wherein the sub-cooling refrigeration cycle comprises: withdrawing a portion not to exceed 50% of the expanded, sub-cooled gas stream and reducing its pressure in a pressure reduction valve to a range of about 30 to 300 psia to produce one or more reduced pressure gas streams; and passing the one or more reduced pressure gas streams through the first heat exchanger zone as the sub-cooling refrigerant.

9. The method of claim 8, wherein the one or more reduced pressure gas streams comprise two or more reduced pressure gas streams having different pressures from each other.

10. The method of claim 8, further comprising: compressing the sub-cooling refrigerant stream exiting the first heat exchanger zone; and cooling the sub-cooling refrigerant stream by indirect heat exchange with an ambient temperature air or water and then adding the sub-cooling refrigerant to the gas stream.

11. The method of claim 6, wherein at least a portion of the expanded, sub-cooled gas stream is further expanded and then directed to a separation tank from which liquid natural gas is withdrawn and remaining gaseous vapors are withdrawn as a flash gas stream.

12. The method of claim 1, wherein all of the first warm refrigerant stream is directed to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream, thereby forming the second warm refrigerant stream.

13. The method of claim 1, further comprising: prior to directing the feed gas stream to the first heat exchanger zone, compressing the feed gas stream to a pressure no greater 1,600 psia, and then cooling it by indirect heat exchange with an ambient temperature air or water.

14. The method of claim 1, wherein the feed gas stream is cooled to a temperature below an ambient temperature by indirect heat exchange within an external cooling unit prior to directing the feed gas stream to the first heat exchanger zone.

15. The method of claim 1, wherein the compressed, cooled refrigerant stream is cooled to a temperature below the ambient temperature by indirect heat exchange within an external cooling unit prior to directing the compressed, cooled refrigerant stream to the second heat exchanger zone.

16. A system for liquefying a feed gas stream rich in methane, the system having first and second heat exchanger zones and comprising: a feed gas stream at a pressure less than 1,200 psia; a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; a cooler configured to cool the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; at least one heat exchanger within the second heat exchanger zone, the compressed, cooled refrigerant stream being directed to the at least one heat exchanger within the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature and thereby produce a compressed, additionally cooled refrigerant stream; at least one work producing expander arranged to expand the compressed, additionally cooled refrigerant stream, thereby producing an expanded, cooled refrigerant stream; at least one heat exchanger within the first heat exchanger zone, the expanded, cooled refrigerant stream being passed through the at least one heat exchanger in the first heat exchanger zone to form a first warm refrigerant stream, wherein the first warm refrigerant stream has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone; wherein the feed gas stream is passed through the first heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream; wherein at least a portion of the first warm refrigerant stream is directed to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream, thereby forming a second warm refrigerant stream; and a compressor configured to compress the second warm refrigerant stream to produce the compressed refrigerant stream.

17. The system of claim 16, wherein the first warm refrigerant stream has a temperature that is cooler, by at least 10 F., than the highest fluid temperature within the first heat exchanger zone.

18. The system of claim 16, wherein a portion of the first warm refrigerant stream remaining within the first heat exchanger zone further exchanges heat within the first heat exchanger zone to produce a third warm refrigerant stream.

19. The system of claim 16, wherein the second warm refrigerant stream is combined with the third warm refrigerant stream prior to compressing the second warm refrigerant stream.

20. The system of claim 16, further comprising: a sub-cooling refrigeration cycle configured to further cool the liquefied gas stream within the first heat exchanger zone, to thereby form a sub-cooled gas stream.

21. The system of claim 16, further comprising: an additional expander configured to expand the sub-cooled gas stream to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream, wherein the additional expander comprises a hydraulic turbine.

22. The system of claim 20, wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant.

23. The system of claim 21, further comprising: a pressure reduction valve configured to reduce the pressure of a portion, not to exceed 50%, of the expanded, sub-cooled gas stream, to a range of about 30 to 300 psia, thereby producing one or more reduced pressure gas streams; wherein the one or more reduced pressure gas streams is passed through the first heat exchanger zone as the sub-cooling refrigerant.

24. The system of claim 23, wherein the one or more reduced pressure gas streams comprise two or more reduced pressure gas streams having different pressures from each other.

25. The system of claim 20, further comprising: a sub-cooling compressor configured to compress the sub-cooling refrigerant stream exiting the first heat exchanger zone; and an external cooling unit configured to cool the sub-cooling refrigerant stream by indirect heat exchange with an ambient temperature air or water.

26. The system of claim 21, further comprising; an additional expander configured to further expand at least a portion of the expanded, sub-cooled gas stream; and a separation tank to which the expanded, sub-cooled gas stream is directed after passing through the additional expander.

27. The system of claim 16, further comprising: an additional compressor configured to compress, prior to directing the feed gas stream to the first heat exchanger zone, the feed gas stream to a pressure no greater 1,600 psia; and an external cooling unit configured to cool the feed gas stream by indirect heat exchange with an ambient temperature air or water.

28. The system of claim 16, further comprising: a second external cooling unit configured to cool the feed gas stream to a temperature below an ambient temperature by indirect heat exchange within an external cooling unit prior to directing the feed gas stream to the first heat exchanger zone.

29. The system of claim 16, further comprising: a third external cooling unit configured to cool the compressed, cooled refrigerant stream to a temperature below the ambient temperature by indirect heat exchange therein prior to directing the compressed, cooled refrigerant stream to the second heat exchanger zone.

30. The system of claim 26, wherein refrigerant in the primary cooling loop is supplied from one or more of the feed gas stream, the flash gas stream, and boil-off gas of the liquid natural gas.

31. The system of claim 16, wherein at least one heat exchanger within the first heat exchanger zone comprises a brazed aluminum heat exchanger.

32. The system of claim 16, wherein at least one heat exchanger within the second heat exchanger zone comprises a printed circuit heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below.

[0024] FIG. 1 is a schematic diagram of a system for LNG production according to known principles.

[0025] FIG. 2 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0026] FIG. 3 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0027] FIG. 4 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0028] FIG. 5 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0029] FIG. 6 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0030] FIG. 7 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0031] FIG. 8 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0032] FIG. 9 is a schematic diagram of a system for LNG production according to disclosed aspects.

[0033] FIG. 10 is a flowchart of a method according to aspects of the disclosure.

[0034] FIG. 11 is a flowchart of a method according to aspects of the disclosure.

[0035] FIG. 12 is a flowchart of a method according to aspects of the disclosure.

[0036] It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure.

DETAILED DESCRIPTION

[0037] To promote an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. For the sake of clarity, some features not relevant to the present disclosure may not be shown in the drawings.

[0038] At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

[0039] As one of ordinary skill would appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name only. The figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. When referring to the figures described herein, the same reference numerals may be referenced in multiple figures for the sake of simplicity. In the following description and in the claims, the terms including and comprising are used in an open-ended fashion, and thus, should be interpreted to mean including, but not limited to.

[0040] The articles the, a and an are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements.

[0041] As used herein, the terms approximately, about, substantially, and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and are considered to be within the scope of the disclosure. The term near is intended to mean within 2%, or within 5%, or within 10%, of a number or amount.

[0042] As used herein, the term compression unit means any one type or combination of similar or different types of compression equipment, and may include auxiliary equipment, known in the art for compressing a substance or mixture of substances. A compression unit may utilize one or more compression stages. Illustrative compressors may include, but are not limited to, positive displacement types, such as reciprocating and rotary compressors for example, and dynamic types, such as centrifugal and axial flow compressors, for example.

[0043] Exemplary is used exclusively herein to mean serving as an example, instance, or illustration. Any embodiment or aspect described herein as exemplary is not to be construed as preferred or advantageous over other embodiments.

[0044] The term gas is used interchangeably with vapor, and is defined as a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term liquid means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state.

[0045] As used herein, heat exchange area means any one type or combination of similar or different types of equipment known in the art for facilitating heat transfer. Thus, a heat exchange area may be contained within a single piece of equipment, or it may comprise areas contained in a plurality of equipment pieces. Conversely, multiple heat exchange areas may be contained in a single piece of equipment.

[0046] A hydrocarbon is an organic compound that primarily includes the elements hydrogen and carbon, although nitrogen, sulfur, oxygen, metals, or any number of other elements can be present in small amounts. As used herein, hydrocarbons generally refer to components found in natural gas, oil, or chemical processing facilities.

[0047] As used herein, the terms loop and cycle are used interchangeably.

[0048] As used herein, natural gas means a gaseous feedstock suitable for manufacturing LNG, where the feedstock is a methane-rich gas containing methane (C1) as a major component. Natural gas may include gas obtained from a crude oil well (associated gas) or from a gas well (non-associated gas).

[0049] The disclosure describes a process/method and system for liquefying natural gas and other methane-rich gas streams to produce liquefied natural gas (LNG) and/or other liquefied methane-rich gases. In one or more aspects of the disclosure, the primary cooling loop is segmented into two heat exchanger zones. Within the first heat exchanger zone, the primary cooling loop refrigerant is used to liquefy the feed gas. Within the second heat exchanger zone, all or a portion of the primary cooling loop refrigerant is used to cool the high pressure primary cooling loop refrigerant prior to expansion of the refrigerant. The first heat exchanger zone is physically separate from second heat exchanger zone. Additionally, the heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone. One advantage of having two separate heat exchanger zones is that the types of heat exchangers in the two zones can be different from each other. As a non-limiting example, the type of heat exchanger(s) used in the first exchanger zone may include a brazed aluminum heat exchanger, and the type of heat exchanger(s) used in the second heat exchanger zone may be include a printed circuit heat exchanger. It is in the first exchanger zone where more the 90% of the heat transfer needed to liquefy the feed gas occurs. Using the less expensive brazed aluminum heat exchanger here reduces project cost. The significantly more expensive printed circuit heat exchanger may be used in the second heat exchanger zone because it can operate at the required 3,000 psia pressure of the high pressure refrigerant. The use of a printed circuit heat exchanger in the second heat exchanger zone does not significantly impact overall project cost since it is a relatively small heat exchanger. This is because the heat transfer duty within the second heat exchanger zone is significantly smaller than that of the first heat exchanger zone. Both heat exchanger zones may comprise multiple heat exchangers.

[0050] In an aspect, a method for liquefying a gas stream, particularly one rich in methane, includes: (a) providing the gas stream at a pressure less than 1,200 psia; (b) providing a compressed refrigerant with a pressure greater than or equal to 1,500 psia; (c) cooling the compressed refrigerant by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant; (d) directing the compressed, cooled refrigerant to a second heat exchanger zone to additionally cool the compressed, cooled refrigerant below ambient temperature to produce a compressed, additionally cooled refrigerant; (e) expanding the compressed, additionally cooled refrigerant in at least one work producing expander thereby producing an expanded, cooled refrigerant; (f) passing the expanded, cooled refrigerant through a first heat exchanger zone to form a first warm refrigerant, whereby the first warm refrigerant has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone, and whereby the heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone; (g) passing the gas stream through the first heat exchanger zone to cool at least part of the gas stream by indirect heat exchange with the expanded, cooled refrigerant, thereby forming a liquefied gas stream; (h) directing at least a portion of the first warm refrigerant to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant thereby forming a second warm refrigerant; and (i) compressing the second warm refrigerant to produce the compressed refrigerant.

[0051] In another aspect, a method for liquefying a gas stream includes: (a) providing the gas stream at a pressure less than 1,200 psia; (b) compressing the gas stream to a pressure of at least 1,500 psia to form a compressed gas stream; (c) cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream; (d) expanding the compressed, cooled gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream; (e) providing a compressed refrigerant with a pressure greater than or equal to 1,500 psia (f) cooling the compressed refrigerant by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant (g) directing the compressed, cooled refrigerant to a second heat exchanger zone to additionally cool the compressed, cooled refrigerant below ambient temperature to produce a compressed, additionally cooled refrigerant; (h) expanding the compressed, additionally cooled refrigerant in at least one work producing expander thereby producing an expanded, cooled refrigerant; (i) passing the expanded, cooled refrigerant through a first heat exchanger zone to form a first warm refrigerant, whereby the first warm refrigerant has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone, and whereby the heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone; (j) passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the expanded, cooled refrigerant, thereby forming a liquefied gas stream; (k) directing the first warm refrigerant to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant, thereby forming a second warm refrigerant; and (l) compressing the second warm refrigerant to produce the compressed refrigerant.

[0052] In another aspect, a method for liquefying a gas stream includes: (a) providing the gas stream at a pressure less than 1,200 psia; (b) providing a refrigerant stream at near the same pressure of the gas stream; (c) mixing the gas stream with the refrigerant stream to form a second gas stream; (d) compressing the second gas stream to a pressure of at least 1,500 psia to form a compressed second gas stream; (e) cooling the compressed second gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled second gas stream; (f) directing the compressed, cooled second gas stream to a second heat exchanger zone to additionally cool the compressed, cooled second gas stream below ambient temperature to produce a compressed, additionally cooled second gas stream; (g) expanding the compressed, additionally cooled second gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the second gas stream was compressed, to thereby form an expanded, cooled second gas stream; (h) separating the expanded, cooled second gas stream into a first expanded refrigerant and a chilled gas stream; (i) expanding the first expanded refrigerant in at least one work producing expander, thereby producing a second expanded refrigerant; (j) passing the second expanded refrigerant through a first heat exchanger zone to form a first warm refrigerant, whereby the first warm refrigerant has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone, and whereby the heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone; (k) passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the second expanded refrigerant, thereby forming a liquefied gas stream; (l) directing the first warm refrigerant to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled second gas stream, thereby forming a second warm refrigerant; and (m) compressing the second warm refrigerant to produce the refrigerant stream.

[0053] Aspects of the disclosure may include the additional steps of compressing the gas stream to a pressure no greater than 1,600 psia and then cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water prior to directing the gas stream to the first heat exchanger zone. Aspects of the disclosure may also include the additional steps of cooling the gas stream to a temperature below the ambient by indirect heat exchange within an external cooling unit prior to directing the gas stream to the first heat exchanger zone. Aspects of the disclosure may also include the additional steps of cooling the compressed, cooled refrigerant to a temperature below the ambient temperature by indirect heat exchange with an external cooling unit prior to directing the compressed, cooled refrigerant to the second heat exchanger zone. These described additional steps may be employed singularly or in combination with each other.

[0054] Aspects of the disclosure have several advantages over the known liquefaction processes, in which feed compression is required to significantly improve the efficiency of the HPXP. In contrast, the efficiency of the disclosed aspects is more than 16% greater than the efficiency for a comparable configuration according to known liquefaction processes. Aspects of the disclosure may have the additional advantage of allowing significant feed compression (greater than 1,500 psia) without requiring the use of high cost main cryogenic heat exchangers for the first heat exchanger zone. Feed compression by the disclosed method may provide a means of increasing the LNG production of an HPXP train by more than 25% for a fixed amount of power going to the primary cooling and sub-cooling loops. Aspects of the disclosure may also have the advantage of combining the compression service of the feed gas and some of that of the primary cooling loop to reduce equipment count. Such an embodiment provides a highly efficient and compact configuration suitable for small scale LNG applications.

[0055] FIG. 2 is a schematic diagram that illustrates a liquefaction system 200 according to an aspect of the disclosure. The liquefaction system 200 includes a primary cooling loop 202, which may also be called an expander loop. The liquefaction system also includes a sub-cooling loop 204, which is a closed refrigeration loop preferably charged with nitrogen as the sub-cooling refrigerant. Within the primary cooling loop 202, an expanded, cooled refrigerant stream 205 is directed to a first heat exchanger zone 201 where it exchanges heat with a feed gas stream 206 to form a first warm refrigerant stream 208. A portion of the first warm refrigerant 208 is directed to a second heat exchanger zone 210 where, in one or more heat exchangers 210a, it exchanges heat with a compressed, cooled refrigerant stream 212 to additionally cool the compressed, cooled refrigerant stream and form a second warm refrigerant stream 209 and a compressed, additionally cooled refrigerant stream 213. The one or more heat exchangers 210a may be of a printed circuit heat exchanger type, a shell and tube heat exchanger type, or a combination thereof. The heat exchanger types within the second heat exchanger zone may have a design pressure of greater than 1,500 psia, or more preferably, a design pressure of greater than 2,000 psia, or more preferably, a design pressure of greater than 3,000 psia.

[0056] The portion of the first warm refrigerant stream 208 directed to the second heat exchanger zone 210 has a temperature that is cooler by at least 5 F., or more preferably, cooler by at least 10 F., or more preferably, cooler by at least 15 F., than the highest fluid temperature within the first heat exchanger zone 201. The portion of the first warm refrigerant stream 208 that may remain within the first heat exchanger zone (as shown by reference number 208a) further exchanges heat with the feed gas stream to form a third warm refrigerant stream 214. The second warm refrigerant stream 209 from the second heat exchanger zone 210 may be combined with the third warm refrigerant stream 214 from the first heat exchanger zone 201 to produce a fourth warm refrigerant stream 216. The fourth warm refrigerant stream is compressed in one or more compression units 218, 220 to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to form a compressed refrigerant stream 222. The compressed refrigerant stream 222 is then cooled against an ambient cooling medium (air or water) in a cooler 224 to produce the compressed, cooled refrigerant stream 212. Cooler 224 may be similar to cooler 112 as previously described. The compressed, additionally cooled refrigerant stream 213 is near isentropically expanded in an expander 226 to produce the expanded, cooled refrigerant stream 205. Expander 226 may be a work-expansion device, such as a gas expander, which produces work that may be extracted and used for compression.

[0057] The first heat exchanger zone 201 may include a plurality of heat exchanger devices, and in the aspects shown in FIG. 2, the first heat exchanger zone includes first and second main heat exchangers 232, 234, and a sub-cooling heat exchanger 236 exchange heat with the expanded, cooled refrigerant 205. These heat exchangers may be of a brazed aluminum heat exchanger type, a plate fin heat exchanger type, a spiral wound heat exchanger type, or a combination thereof. Within the sub-cooling loop 204, an expanded sub-cooling refrigerant stream 238 (preferably comprising nitrogen) is discharged from an expander 240 and drawn through sub-cooling heat exchanger 236 and second and first main heat exchangers 234, 232. Expanded sub-cooling refrigerant stream 238 is then sent to a compression unit 242 where it is re-compressed to a higher pressure and warmed. After exiting compression unit 242, the re-compressed sub-cooling refrigerant stream 244 is cooled in a cooler 246, which can be of the same type as cooler 224, although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed through first and second main heat exchangers 232, 234 where it is further cooled by indirect heat exchange with part or all of the warm refrigerant stream 208 and expanded sub-cooling refrigerant stream 238. After exiting first heat exchange area 201, the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander 240 to provide the expanded sub-cooled refrigerant stream 238 that is re-cycled through the first heat exchanger zone as described herein. In this manner, the feed gas stream 206 is cooled, liquefied and sub-cooled in the first heat exchanger zone 201 to produce a sub-cooled gas stream 248. Sub-cooled gas stream 248 is then expanded to a lower pressure in expander 250 to form a liquid fraction and a remaining vapor fraction. Expander 250 may be any pressure reducing device, including but not limited to a valve, control valve, Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and the like. The sub-cooled stream 248, which is now at a lower pressure and partially liquefied, is passed to a surge tank 252 where the liquefied fraction 254 is withdrawn from the process as an LNG stream 256, which has a temperature corresponding to the bubble point pressure. The remaining vapor fraction (flash vapor) stream 258 may be used as fuel to power the compressor units.

[0058] FIG. 3 is a schematic diagram that illustrates a liquefaction system 300 according to another aspect of the disclosure. Liquefaction system 300 is similar to liquefaction system 200 and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 300 includes a primary cooling loop 302 and a sub-cooling loop 304. Liquefaction system 300 also includes first and second heat exchanger zones 301, 310. In contrast with liquefaction system 200, all of the first warm refrigerant 308 is directed to the second heat exchanger zone 310 where, in one or more heat exchangers 310a, it exchanges heat with a compressed, cooled refrigerant stream 312 to form a second warm refrigerant 309.

[0059] The first warm refrigerant stream 308 has a temperature that is cooler by at least 5 F., or more preferably, cooler by at least 10 F., or more preferably, cooler by at least 15 F., than the highest fluid temperature within the first heat exchanger zone. The second warm refrigerant stream 309 may be compressed in one or more compressors 318, 320 to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to thereby form a compressed refrigerant stream 322. The compressed refrigerant stream 322 is then cooled against an ambient cooling medium (air or water) to produce the compressed, cooled refrigerant stream 312 that is directed to the second heat exchanger zone 310. The compressed, additionally cooled refrigerant stream 313 is near isentropically expanded in an expander 326 to produce the expanded, cooled refrigerant stream 305.

[0060] The feed gas stream 306 is directed through the first heat exchange area 301 that includes a main heat exchanger 332 and a sub-cooling heat exchanger 336. The number of main heat exchangers in first heat exchanger zone 301 may be reduced since all of the first warm refrigerant 308 is directed to the second heat exchanger zone 310. Within the sub-cooling loop 304, an expanded sub-cooling refrigerant stream 338 (preferably comprising nitrogen) is discharged from an expander 340 and drawn through sub-cooling heat exchanger 336 and main heat exchanger 332. Expanded sub-cooling refrigerant stream 338 is then sent to a compression unit 342 where it is re-compressed to a higher pressure and warmed. After exiting compression unit 342, the re-compressed sub-cooling refrigerant stream 344 is cooled in a cooler 346, which can be of the same type as cooler 324, although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed through main heat exchanger 232 where it is further cooled by indirect heat exchange with part or all of the expanded, cooled refrigerant stream 305 and expanded sub-cooling refrigerant stream 338. After exiting first heat exchange area 301, the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander 340 to provide the expanded sub-cooled refrigerant stream 338 that is re-cycled through the first heat exchange area as described herein. In this manner, the feed gas stream 306 is cooled, liquefied and sub-cooled in the first heat exchanger zone 301 to produce a sub-cooled gas stream 348. Sub-cooled gas stream 348 is then expanded to a lower pressure in expander 350 to form a liquid fraction and a remaining vapor fraction. Expander 350 may be any pressure reducing device, including but not limited to a valve, control valve, Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and the like. The sub-cooled stream 348, which is now at a lower pressure and partially liquefied, is passed to a surge tank 352 where the liquefied fraction 354 is withdrawn from the process as an LNG stream 356, which has a temperature corresponding to the bubble point pressure. The remaining vapor fraction (flash vapor) stream 358 may be used as fuel to power the compressor units.

[0061] FIG. 4 is a schematic diagram that illustrates a liquefaction system 400 according to another aspect of the disclosure. Liquefaction system 400 is similar to liquefaction system 200, and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 400 includes a primary cooling loop 402 and a sub-cooling loop 404. Liquefaction system 400 also includes first and second heat exchanger zones 401, 410. In liquefaction system 400, the sub-cooling loop 404 is an open refrigeration loop where a portion 449 of the expanded, sub-cooled gas stream 448 is recycled and used as the sub-cooling refrigerant stream. Specifically, the portion 449 of the expanded, sub-cooled gas stream is directed through the first heat exchanger zone 401 as previously described before being compressed in a compressor 442, cooled in a cooler 446, and re-inserted into the feed gas stream 406. This sub-cooling refrigerant stream may be one stream, as shown, or may comprise multiple streams at different pressures: for example, a portion of the expanded, sub-cooling gas streamnot to exceed 50% thereofmay be diverted and pass through one or more pressure reduction valves to reduce its pressure to a range of about 30 to 300 psia, to thereby produce one or more reduced pressure gas streams. The reduced pressure gas streams may then be passed through the first heat exchanger zone as the sub-cooling refrigerant. Having multiple streams improves the efficiency of the sub-cooling process. Alternatively, this sub-cooling loop may be configured to be a closed refrigeration loop.

[0062] FIG. 5 is a schematic diagram that illustrates a liquefaction system 500 according to another aspect of the disclosure. Liquefaction system 500 is similar to liquefaction system 200 and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 500 includes a primary cooling loop 502 and a sub-cooling loop 504. Liquefaction system 500 also includes first and second heat exchanger zones 501, 510. Liquefaction system 500 stream includes the additional steps of compressing the feed gas stream 506 in a compressor 560 and then, using a cooler 562, cooling the compressed feed gas 561 with ambient air or water to produce a cooled, compressed feed gas stream 563. Feed gas compression may be used to improve the overall efficiency of the liquefaction process and increase LNG production.

[0063] FIG. 6 is a schematic diagram that illustrates a liquefaction system 600 according to still another aspect of the disclosure. Liquefaction system 600 is similar to liquefaction system 300 and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 600 includes a primary cooling loop 602 and a sub-cooling loop 604. Liquefaction system 600 also includes first and second heat exchanger zones 601, 610. Liquefaction system 600 includes the additional step of chilling, in an external cooling unit 665, the feed gas stream 606 to a temperature below the ambient temperature to produce a chilled gas stream 667. The chilled gas stream 667 is then directed to the first heat exchanger zone 601 as previously described. Chilling the feed gas as shown in FIG. 6 may be used to improve the overall efficiency of the liquefaction process and increase LNG production.

[0064] FIG. 7 is a schematic diagram that illustrates a liquefaction system 700 according to another aspect of the disclosure. Liquefaction system 700 is similar to liquefaction system 200 and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 700 includes a primary cooling loop 702 and a sub-cooling loop 704. Liquefaction system 700 also includes first and second heat exchanger zones 701, 710. Liquefaction system 700 includes the additional step of chilling, using an external cooling unit 770, the compressed, cooled refrigerant 712 in the primary cooling loop 702 to a temperature below the ambient temperature, to thereby produce a compressed, chilled refrigerant 772. The compressed, chilled refrigerant 772 is then directed to the second heat exchanger zone 710 as previously described. Using an external cooling unit to further cool the compressed, cool refrigerant may be used to improve the overall efficiency of the process and increase LNG production.

[0065] FIG. 8 is a schematic diagram that illustrates a liquefaction system 800 according to another aspect of the disclosure. Liquefaction system 800 is similar to liquefaction system 300 and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 800 includes a primary cooling loop 802 and a sub-cooling loop 804. Liquefaction system 800 also includes first and second heat exchanger zones 801, 810. In liquefaction system 800, the feed gas stream 806 is compressed in a compressor 860 to a pressure of at least 1,500 psia to form a compressed gas stream 861. Using an external cooling unit 862, the compressed gas stream 861 is cooled by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream 863. The compressed, cooled gas stream 863 is expanded in at least one work producing expander 874 to a pressure that is less than 2,000 psia but no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream 876. The chilled gas stream 876 is then directed to the first heat exchanger zone 801 where a primary cooling refrigerant and a sub-cooling refrigerant are used to liquefy the chilled gas stream as previously described.

[0066] The sub-cooling loop 804 is a closed refrigeration loop preferably charged with nitrogen as the sub-cooling refrigerant stream. Within the primary cooling loop 802, an expanded, cooled refrigerant stream 805 is directed to the first heat exchanger zone 801 where it exchanges heat with the chilled gas stream 876 to form a first warm refrigerant stream 808. The first warm refrigerant stream 808 is directed to the second heat exchanger zone 810 where it exchanges heat with a compressed, cooled refrigerant stream 825 to additionally cool the compressed, cooled refrigerant stream 825, thereby forming a second warm refrigerant stream 809 and a compressed, additionally cooled refrigerant stream 813. The first warm refrigerant stream 808 has a temperature that is cooler by at least 5 F., or more preferably, cooler by at least 10 F., or more preferably, cooler by at least 15 F., than the highest fluid temperature within the first heat exchanger zone 801. Using one or more compressors 818, 820, the second warm refrigerant stream 809 is compressed to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to form a compressed refrigerant stream 822. The compressed refrigerant stream 822 is then cooled against an ambient cooling medium (air or water) in an external cooling unit 824 to produce the compressed, cooled refrigerant stream 825. After being directed through the second heat exchanger area 810, the compressed, additionally cooled refrigerant stream is near isentropically expanded in an expander 826 to produce the expanded, cooled refrigerant 805. The chilled gas stream 876 is liquefied and sub-cooled in the first heat exchanger zone to produce a sub-cooled gas stream 848, which is further processed as previously disclosed.

[0067] FIG. 9 is a schematic diagram that illustrates a liquefaction system 900 according to yet another aspect of the disclosure. Liquefaction system 900 contains similar structure and components with previously disclosed liquefaction systems and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system 900 includes a primary cooling loop 902 and a sub-cooling loop 904. Liquefaction system 900 also includes first and second heat exchanger zones 901, 910. In liquefaction system 900, the feed gas stream 906 is mixed with a refrigerant stream 907 to produce a second feed gas stream 906a. Using a compressor 960, the second feed gas stream 906a is compressed to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to form a compressed second gas stream 961. Using an external cooling unit 962, the compressed second gas stream 961 is then cooled against an ambient cooling medium (air or water) to produce a compressed, cooled second gas stream 963. The compressed, cooled second gas stream 963 is directed to the second heat exchanger zone 910 where it exchanges heat with a first warm refrigerant stream 908, to produce a compressed, additionally cooled second gas stream 913 and a second warm refrigerant stream 909.

[0068] The compressed, additionally cooled second gas stream 913 is expanded in at least one work producing expander 926 to a pressure that is less than 2,000 psia, but no greater than the pressure to which the second gas stream 906a was compressed, to thereby form an expanded, cooled second gas stream 980. The expanded, cooled second gas stream 980 is separated into a first expanded refrigerant stream 905 and a chilled feed gas stream 906b. The first expanded refrigerant stream 905 may be near isentropically expanded using an expander 982 to form a second expanded refrigerant stream 905a. The chilled feed gas stream 906b is directed to the first heat exchanger zone 901 where a primary cooling refrigerant (i.e., the second expanded refrigerant stream 905a) and a sub-cooling refrigerant (from the sub-cooling loop 904) are used to liquefy the chilled gas stream 906b. The sub-cooling loop 904 may be a closed refrigeration loop, preferably charged with nitrogen as the sub-cooling refrigerant. Within the primary cooling loop 902, the second expanded refrigerant stream 905a is directed to the first heat exchanger zone 901 where it exchanges heat with the chilled feed gas stream 906b to form the first warm refrigerant stream 908. The first warm refrigerant stream 908 may have a temperature that is cooler by at least 5 F., or more preferably, cooler by at least 10 F., or more preferably, cooler by at least 15 F., than the highest fluid temperature within the first heat exchanger zone 901. The second warm refrigerant stream 909 is compressed in one or more compressors 918 and then cooled with an ambient cooling medium in an external cooling device 924 to produce the refrigerant stream 907. The chilled feed gas stream 906b is liquefied and sub-cooled in the first heat exchanger zone 901 to produce a sub-cooled gas stream 948, which is processed as previously described to form LNG.

[0069] Aspects of the disclosure illustrated in FIG. 9 demonstrate that the primary refrigerant stream may comprise part of the feed gas stream, which in a preferred aspect may be primarily or nearly all methane. Indeed, it may be advantageous for the refrigerant in the primary cooling loop of all the disclosed aspects (i.e., FIGS. 2 through 9) be comprised of at least 85% methane, or at least 90% methane, or at least 95% methane, or greater than 95% methane. This is because methane may be readily available in various parts of the disclosed processes, and the use of methane may eliminate the need to transport refrigerants to remote LNG processing locations. As a non-limiting example, the refrigerant in the primary cooling loop 202 in FIG. 2 may be taken through line 206a of the feed gas stream 206 if the feed gas is high enough in methane to meet the compositions as described above. Alternatively, part or all of a boil-off gas stream 259 from an LNG storage tank 257 may be used to supply refrigerant for the primary cooling loop 202. Furthermore, if the feed gas stream is sufficiently low in nitrogen, part or all of the end flash gas stream 258 (which would then be low in nitrogen) may be used to supply refrigerant for the primary cooling loop 202. Lastly, any combination of line 206a, boil-off gas stream 259, and end flash gas stream 258 may be used to provide or even occasionally replenish the refrigerant in the primary cooling loop 202.

[0070] FIG. 10 is a flowchart of a method 1000 for liquefying a feed gas stream rich in methane using a system having first and second heat exchanger zones, where the method comprises the following steps: 1002, providing the feed gas stream at a pressure less than 1,200 psia; 1004, providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; 1006, cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; 1008, directing the compressed, cooled refrigerant stream to the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream; 1010, expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 1012, passing the expanded, cooled refrigerant stream through the first heat exchanger zone to form a first warm refrigerant stream, wherein the first warm refrigerant stream has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone; 1014, passing the feed gas stream through the first heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream; 1016 directing at least a portion of the first warm refrigerant stream to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream, thereby forming a second warm refrigerant stream; and 1018, compressing the second warm refrigerant stream to produce the compressed refrigerant stream.

[0071] FIG. 11 is a flowchart of a method 1100 for liquefying a feed gas stream rich in methane, where the method comprises the following steps: 1102, providing the feed gas stream at a pressure less than 1,200 psia; 1104, compressing the feed gas stream to a pressure of at least 1,500 psia to form a compressed gas stream; 1106, cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water, to form a cooled, compressed gas stream; 1108, expanding the cooled, compressed gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream; 1110, providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; 1112, cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; 1114, directing the compressed, cooled refrigerant stream to a second heat exchanger zone, to additionally cool the compressed, cooled refrigerant stream below ambient temperature, to produce a compressed, additionally cooled refrigerant stream; 1116, expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 1118, passing the expanded, cooled refrigerant stream through a first heat exchanger zone to form a first warm refrigerant stream, whereby the first warm refrigerant stream has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone; 1120, passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the expanded, cooled refrigerant, thereby forming a liquefied gas stream; 1122, directing the first warm refrigerant stream to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream, thereby forming a second warm refrigerant stream;

[0072] and 1124, compressing the second warm refrigerant stream to produce the compressed refrigerant stream.

[0073] FIG. 12 is a method 1200 for liquefying a feed gas stream rich in methane, where the method comprises the following steps: 1202, providing the feed gas stream at a pressure less than 1,200 psia; 1204, providing a refrigerant stream at near the same pressure of the feed gas stream; 1206, mixing the feed gas stream with the refrigerant stream to form a second gas stream; 1208, compressing the second gas stream to a pressure of at least 1,500 psia to form a compressed second gas stream; 1210, cooling the compressed second gas stream by indirect heat exchange with ambient temperature air or water, to form a compressed, cooled second gas stream; 1212, directing the compressed, cooled second gas stream to a second heat exchanger zone, to additionally cool the compressed, cooled second gas stream below ambient temperature, thereby producing a compressed, additionally cooled second gas stream; 1214, expanding the compressed, additionally cooled second gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the second gas stream was compressed, to thereby form an expanded, cooled second gas stream; 1216, separating the expanded, cooled second gas stream into a first expanded refrigerant stream and a chilled gas stream; 1218, expanding the first expanded refrigerant stream in at least one work producing expander, thereby producing a second expanded refrigerant stream; 1220, passing the second expanded refrigerant stream through a first heat exchanger zone to form a first warm refrigerant stream such that the first warm refrigerant stream has a temperature that is cooler, by at least 5 F., than the highest fluid temperature within the first heat exchanger zone; 1222, passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the second expanded refrigerant stream, thereby forming a liquefied gas stream; 1224, directing the first warm refrigerant stream to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled second gas stream, thereby forming a second warm refrigerant stream; and 1226, compressing the second warm refrigerant stream to produce the refrigerant stream.

[0074] The steps depicted in FIGS. 10-12 are provided for illustrative purposes only and a particular step may not be required to perform the disclosed methodology. Moreover, FIGS. 10-12 may not illustrate all the steps that may be performed. The claims, and only the claims, define the disclosed system and methodology.

[0075] The aspects described herein have several advantages over known technologies. For example, the described technology may greatly reduce the size and cost of systems that treat sour natural gas.

[0076] It should be understood that the numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.