END FLASH SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to GB Patent Application Number 1708514.3, filed May 26, 2017 and entitled "Systems and Methods for Liquefaction of a Gas with the Aid of an End Flash System", the disclosure of which is hereby incorporated herein by reference as if reproduced in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

TECHNICAL FIELD

[0003] The present disclosure relates systems and methods for condensing a gas into a liquid; more particularly, this disclosure relates systems and methods for condensing a gas into a liquid via a common end flash system; still more particularly, the present disclosure relates to systems and methods of using a common end flash system for the liquefaction of natural gas to produce liquid natural gas (LNG).

BACKGROUND

[0004] For reduced volume during transportation and storage, produced natural gas is often converted from a gaseous phase into a liquid phase, referred to as liquid natural gas (LNG), by a liquefaction process. The liquefaction process achieves the phase change through a series of heat exchangers, aided by a refrigerant that transfers heat from the natural gas to a "heat sink," such as ambient air.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] For a detailed description of the disclosed exemplary embodiments, reference will now be made to the accompanying drawings, wherein: [0006] Figure 1 is a schematic view of an embodiment of a liquefaction system in accordance with the principles disclosed herein including multiple refrigerant systems feeding a common end flash system via a liquid expander;

[0007] Figure 2 is a schematic view of an embodiment of a liquefaction system in accordance with the principles disclosed herein including multiple refrigerant systems feeding a common end flash system via a liquid expander;

[0008] Figure 3 is a schematic view of an embodiment of a liquefaction system in accordance with the principles disclosed herein including multiple refrigerant systems feeding a common end flash system via a let-down valve; and

[0009] Figure 4 is a schematic view of an embodiment of a liquefaction system in accordance with the principles disclosed herein including multiple refrigerant systems feeding a common end flash system that includes multiple stages.

SUMMARY

[0010] Herein disclosed is a liquefaction system for removing heat from a process fluid, the system comprising: a plurality of refrigerant systems, wherein each refrigerant system is configured to liquefy at least a portion of the process fluid in a process feed gas line coupled to the plurality of refrigerant systems; and a common end flash system configured to receive process fluid from the plurality of refrigerant systems.

[0011] Also disclosed herein is a method for liquefying a process fluid, the method comprising: (a) operating a plurality of refrigeration systems in parallel, wherein each refrigeration system receives a portion of the process fluid; (b) transferring heat from each portion of the process fluid with the corresponding refrigeration system; (c) supplying the portions of the process fluid from the refrigeration systems to a common end flash system; and (d) lowering the pressure of the portions of the process fluid with the common end flash system to provide a liquefied process fluid and an end flash gas.

[0012] Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein.

DETAILED DESCRIPTION

[0013] The following description is exemplary of certain embodiments of the disclosure. One of ordinary skill in the art will understand that the following description has broad application, and the discussion of any embodiment is meant to be exemplary of that embodiment, and is not intended to suggest in any way that the scope of the disclosure, including the claims, is limited to that embodiment.

[0014] The figures are not necessarily drawn to-scale. Certain features and components disclosed herein may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. In some of the figures, in order to improve clarity and conciseness, one or more components or aspects of a component may be omitted or may not have reference numerals identifying the features or components. In addition, within the specification, including the drawings, like or identical reference numerals may be used to identify common or similar elements.

[0015] As used herein, including in the claims, the terms "including" and "comprising," as well as derivations of these, are used in an open-ended fashion, and thus are to be interpreted to mean "including, but not limited to... ." Also, the term "couple" or "couples" means either an indirect or direct connection. Thus, if a first component couples or is coupled to a second component, the connection between the components may be through a direct engagement of the two components, or through an indirect connection that is accomplished via other intermediate components, devices and/or connections. The recitation "based on" means "based at least in part on." Therefore, if X is based on Y, then X may be based on Y and on any number of other factors. The word "or" is used in an inclusive manner. For example, "A or B" means any of the following: "A" alone, "B" alone, or both "A" and "B." In addition, when used herein including the claims, the word "substantially" means within a range of plus or minus 10%. As may be used herein including the claims, the word "uniform" is equivalent to the phrase "uniform or substantially uniform."

[0016] Numbers and ranges disclosed below may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles "a" or "an", as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents, the definitions that are consistent with this specification should be adopted.

[0017] The use of ordinal numbers (i.e. first, second, third, etc.) to identify one or more components within a possible group of multiple similar components is done for convenience and clarity. Ordinal numbers that may be used outside the claims for members of a particular group of components may not necessarily correspond to the ordinal numbers used in the claims when referring to various members of the same group or a similar group of components.

[0018] As previously described, produced natural gas is commonly condensed from a gas phase into liquid natural gas (LNG) by a liquefaction process. A typical liquefaction process includes a main cryogenic heat exchanger that transfers heat from a feed stream of natural gas to a refrigerant, thereby condensing the feed stream into LNG. The refrigerant is usually passed through a series of compressors and heat exchangers that cool or condense the refrigerant after compression. Refrigerant pressure is dropped, through a valve or expander, prior to passing through the main cryogenic heat exchanger. Many variants of the basic liquefaction process exist, including enhancements such as end flash systems and provisions to separate and extract heavier liquid components.

[0019] The liquefaction process can be augmented by adding an end flash system to cool the LNG further or to complete the liquefaction process after passing through the refrigeration process. The use of a flash system allows the aforementioned primary or secondary heat exchangers to operate at higher temperatures while still achieving a targeted outlet temperature for the LNG after pressure reduction. The flash gas produced may be used as fuel for the process, recycled to feed or exported to other consumers. "Cold" from the end flash gas may be recovered to contribute to cooling the feed gas or refrigerant.

[0020] As will be described in more detail below, embodiments of systems and methods described herein utilize a common end flash system fed by multiple liquefaction units. End flash systems provide an established technique for improving the efficiency of large liquefaction trains, but are not normally cost-effective for smaller train sizes, due to adding complexity to an inherently simple unit. Accordingly, there exists a need for systems and methods of liquefaction that are suitable for liquefaction trains of small, as well as large, sizes, such that the potential to achieve a cost-effective route to the benefits of an end flash system can be realized for cases where liquefaction capacity is built up in multiple small liquefaction trains, as well as for larger trains. In cases where liquefaction capacity is built up in multiple small liquefaction trains, the systems described herein offer the potential to achieve a cost-effective route to the benefits of an end flash system.

[0021] Referring now to Figure 1 , an embodiment of a liquefaction train or system 100 is shown. In general, system 100 is designed to liquefy a process fluid, which in this example is natural gas. Liquefaction system 100 includes a process fluid system 102 and a plurality of refrigerant systems 104. Each refrigeration system 104 includes an inlet 105 in fluid communication with a process gas feed line 1 1 1 via a distribution line 1 13 and an outlet 107 in fluid communication with a line 151 . Systems 104 are operated in parallel between lines 1 13, 151 . Each individual refrigeration system 104 comprises a simple liquefaction process without an end flash system. [0022] Although four refrigerant systems 104 are shown in Figure 1 , in other embodiments, fewer or more refrigerant systems 104 may be provided. In general, the refrigerant systems 104 may operate at the same capacity or at different capacities, depending on the design plan or operational needs. In addition, the refrigerant systems 104 may utilize the same refrigerant or different refrigerants. Still further, the refrigeration systems 104 may utilize the same technology or different technologies.

[0023] Process fluid system 102 includes a shared, common end flash system 150 downstream of the plurality of refrigerant systems 104. End flash system 150 receives LNG from each of the outlets 107 of the plurality of refrigerant systems 104 via line 151 and outputs LNG through a discharge line 1 15.

[0024] In this embodiment, end flash system 150 includes a liquid expander 154, a liquid-gas separator 156, a compressor 160, and a heat exchanger 162, which may be called a compressor after-cooler. In embodiments, liquid expander 154 can comprise a flashing liquid expander. In addition, end flash system 150 includes a cold recovery heat exchanger 158. Heat exchanger 158 may be used to liquefy a side stream of the feed gas supplied by a bypass line 1 13' extending from distribution line 1 13.

[0025] Although shown as comprising an expander and a liquid-gas separator, it is to be understood that, in embodiments, alternative end flash systems are utilized as the common end flash system 150, and such alternative end flash systems and methods are within the scope of this disclosure. By way of non-limiting example, as an alternative to the common end flash systems of Figures 1 -4 employing simple liquid-gas separators, embodiments of this disclosure may utilize distillation via one or more distillation columns to control the purity of the LNG and end flash gas products (e.g., in lines 1 15 and 157, respectively). In such embodiments, a distillation column may be utilized in place of or in addition to a liquid-gas separator 156.

[0026] Referring back to the embodiment of Figure 1 , the inlet of expander 154 is coupled to line 151 and the outlet of expander 154 is coupled to an inlet of liquid-gas separator 156. The liquid discharge port of separator 156 is coupled to discharge line 1 15, and the gas discharge port of separator 156 is coupled to the inlet of cold recovery heat exchanger 158 with line 157. Thus, line 157 supplies relatively cold fluid from separator 156 to heat exchanger 158 to liquefy the side stream of feed gas supplied by bypass line 1 13'.

[0027] A line 159 extends from the outlet of heat exchanger 158 to compressor 160, and delivers the natural gas to cooling heat exchanger 162. In this example, heat exchanger 162 is air-cooled and rejects heat to ambient air. A line 163 delivers gaseous natural gas from heat exchanger 162 to another part of the process for use as fuel, for recycle or for another purpose.

[0028] As previously described, bypass line 1 13' provides fluid communication between distribution line 1 13 and heat exchanger 158, thereby allowing a portion of the feed gas in line 1 1 1 to bypass the plurality of refrigeration systems 104. That portion of the feed gas is liquefied with heat exchanger 158 and passes from heat exchanger 158 into discharge line 1 15. Therefore, heat exchanger 158 condenses additional natural gas from bypass line 1 13' with cooling capacity from the flash gas that exits the separator 156. Figure 1 shows the additional LNG from line 1 13' being combined with the LNG that leaves separator 156 in discharge line 1 15. That combined stream in discharge line 1 15 is routed to storage.

[0029] Conventionally, an end flash system is provided for and integrated with each single liquefaction train, usually with cold recovery integrated with the refrigeration circuit. However, the flash system 150 of system 100 is fed by multiple refrigerant systems 104 and this arrangement may be economically less costly than having separate flash systems for each refrigerant system 104. As another difference, the recovery of cooling capacity in system 100 is applied to directly condense a separate flow of natural gas in line 1 13' rather than to augment the cooling achieved in a primary heat exchanger.

[0030] Referring still to Figure 1 , in some instances, recovering the cooling capacity of the end flash gas within the separate heat exchanger 158 may provide a steady or simple mode of operation for systems that have multiple refrigerant systems 104 feeding a single, common end flash system 150. Thus, coupling multiple refrigerant systems 104 to feed the common end flash system 150 that includes a cold recovery heat exchanger 158 fed by the bypass line 1 13' provides a distinct, beneficial function to system 100. In some embodiments, multiple systems 100 may be used in parallel, fed by a common feed line 1 1 1 .

[0031] Various embodiments of refrigerant systems 104 may be sized to produce a capacity or range of capacities within the range up to approximately 8 million tons per year (Mt/y) of LNG. Adding an end flash system is an economic enhancement for larger liquefaction trains, for example, liquefaction trains sized to produce more than 3 Mt/y. As another example, using a combined end flash system in conjunction with smaller liquefaction trains enables the economic benefit of adding an end flash system for smaller individual liquefaction trains. Although the example system 100 was described as having four refrigerant systems 104, a system 100 may have two, three, ten, or any practical number of refrigerant systems 104, based on economic, engineering, or other considerations.

[0032] In the embodiment of system 100 described above and shown in Figure 1 , the portion of the process gas passing through bypass line 1 13' flows through heat exchanger 158 and into discharge line 1 15 downstream of expander 154 and separator 156. However, in another embodiment shown in Figure 2, the process gas stream passes from bypass line 1 13' through heat exchanger 158 and into line 151 upstream of the LNG expander 154 and separator 156 to increase power recovery from the resulting pressure let-down. In yet another embodiment shown in Figure 3, the expander 154 is replaced with a Joule-Thomson valve 154' and the process gas stream passes from bypass line 1 13' through heat exchanger 158 and into line 151 upstream of the JT valve 154' and separator 156.

[0033] In the embodiment of system 100 described above and shown in Figure 1 , a common end flash system 150 is provided for the plurality of refrigeration systems 104. In addition, the common end flash system 150 comprises a single stage. However, in other embodiments, the common end flash system provided for a plurality of refrigeration systems may include multiple stages. For example, as shown in Figure 4, a common, shared multi-stage end flash system 150' is provided for a plurality of refrigeration systems 104, which offers the potential for further improved performance. In the embodiment shown in Figure 4, the common, shared multi-stage end flash system 150' comprises two Joule-Thomson valves 154', each Joule-Thomson valve 154' being positioned upstream of a corresponding separator 156. Each separator 156 includes a liquid discharge port coupled to discharge line 1 15, and a gas discharge port coupled to the inlet of a corresponding cold recovery heat exchanger 158 via a line 157. As described hereinabove, each cold recovery heat exchanger 158 may be used to liquefy a side stream of the feed gas supplied by bypass line 1 13' extending from distribution line 1 13 via the supply of relatively cold fluid from each separator 156 to the associated downstream heat exchanger 158. A line 159 extends from the outlet of each heat exchanger 158 to a compressor 160, and delivers the natural gas to cooling heat exchanger 162. In embodiments, each heat exchanger 162 is air-cooled and rejects heat to ambient air. A line 163 delivers gaseous natural gas from each heat exchanger 162 to another part of the process for use as fuel, for recycle or for another purpose.

[0034] It is to be understood that, in embodiments, systems comprising a plurality of separators 156 can be operated with one or a plurality (e.g., the same number as the plurality of separators 156) of cold recovery heat exchangers 158, end flash compressors 160, heat exchangers 162, or combinations thereof. For example, in embodiments, lines 157 combine to feed a single cold recovery heat exchanger 158, end flash compressor 160, and/or heat exchanger 162. Also, although Joule-Thomson valves 154' are indicated in the embodiment of Figure 4, it is to be understood that a multi-stage end flash system according to this disclosure can comprise a plurality of LNG expanders, such as expanders 154 depicted in the embodiments of Figures 1 and 2, rather than Joule-Thomson valves 154'.

[0035] Embodiments disclosed herein include:

[0036] A: A liquefaction system for removing heat from a process fluid, the system comprising: a plurality of refrigerant systems, wherein each refrigerant system is configured to liquefy at least a portion of the process fluid in a process feed gas line coupled to the plurality of refrigerant systems; and a common end flash system configured to receive process fluid from the plurality of refrigerant systems.

[0037] B: A method for liquefying a process fluid, the method comprising: (a) operating a plurality of refrigeration systems in parallel, wherein each refrigeration system receives a portion of the process fluid; (b) transferring heat from each portion of the process fluid with the corresponding refrigeration system; (c) supplying the portions of the process fluid from the refrigeration systems to a common end flash system; and (d) lowering the pressure of the portions of the process fluid with the common end flash system to provide a liquefied process fluid and an end flash gas.

[0038] C: A liquefaction system for removing heat from a process fluid, the system comprising: a plurality of refrigerant systems, wherein each refrigerant system is configured to liquefy at least a portion of the process fluid in a process feed gas line coupled to the plurality of refrigerant systems; and a common end flash system configured to receive process fluid from the plurality of refrigerant systems, wherein the end flash system includes an expansion device and a liquid-gas separator.

[0039] D: A method for liquefying a process fluid, the method comprising: (a) operating a plurality of refrigeration systems in parallel, wherein each refrigeration system receives a portion of the process fluid; (b) transferring heat from each portion of the process fluid with the corresponding refrigeration system; (c) supplying the portions of the process fluid from the refrigeration systems to a common end flash system; and (d) lowering the temperature of the portions of the process fluid with the end flash system.

[0040] Each of embodiments A, B, C, and D may have one or more of the following additional elements: Element 1 : wherein the common end flash system includes a cold recovery heat exchanger coupled to the process feed gas line with a bypass line that bypasses the plurality of refrigerant systems, and wherein the cold recovery heat exchanger is configured to produce an additional liquefied process fluid. Element 2: wherein the common end flash system includes an expansion device and a liquid-gas separator, wherein the cold recovery heat exchanger is coupled to the liquid-gas separator and is configured to receive flash gas therefrom, wherein the cold recovery heat exchanger comprises an outlet line for the additional liquefied process fluid, and wherein the outlet line for the additional liquefied process fluid is configured to combine the additional liquefied process fluid with the process fluid from the plurality of refrigerant systems received by the common end flash system, with a liquid discharge line configured for the removal of liquid from the liquid-gas separator, or a combination thereof. Element 3: further comprising an end flash compressor downstream from the cold recovery heat exchanger and configured to compress a gas received therefrom and a compressor after-cooler downstream of the end flash compressor. Element 4: The liquefaction system of claim 4, wherein the compressor after-cooler comprises a cooling heat exchanger that is air-cooled or water-cooled. Element 5: wherein the common end flash system comprises a plurality of stages at different pressures. Element 6: wherein each stage of the common end flash system includes a cold recovery heat exchanger. Element 7: wherein each stage of the common end flash system includes an expansion device and a liquid-gas separator, wherein the common end flash system includes at least one cold recovery heat exchanger coupled to the process feed gas line with a bypass line that bypasses the plurality of refrigerant systems, and wherein the at least one cold recovery heat exchanger is coupled to at least one of the plurality of liquid-gas separators and is configured to receive flash gas therefrom. Element 8: wherein the expansion devices of the plurality of stages are selected from liquid expanders, Joule-Thomson valves, or a combination thereof. Element 9: wherein the end flash system includes an expansion device selected from a flashing liquid expander, a Joule-Thomson valve, or a combination thereof. Element 10: further comprising: bypassing a portion of the process fluid around the plurality of refrigeration systems to the common end flash system; and condensing the portion of the process fluid that bypasses the plurality of refrigeration systems to produce additional liquefied process fluid. Element 1 1 : wherein condensing the portion of the process fluid that bypasses the plurality of refrigeration systems to produce additional liquefied process fluid comprises introducing the portion of the process gas that bypasses the plurality of refrigeration systems into a cold recovery heat exchanger. Element 12: wherein (d) lowering the pressure of the portions of the process fluid with the common end flash system comprises expanding the portions of the process fluid supplied to the common end flash system to provide an expanded fluid and separating the end flash gas from the expanded fluid, and further comprising introducing the separated end flash gas into the cold recovery heat exchanger. Element 13: further comprising subjecting the separated end flash gas to compressing and cooling after passage through the cold recovery heat exchanger. Element 14: wherein cooling is effected via a compressor after-cooler. Element 15: wherein (d) lowering the pressure of the portions of the process fluid with the common end flash system comprises expanding the portions of the process fluid supplied to the common end flash system to provide an expanded fluid, and separating the end flash gas from the expanded fluid. Element 16: wherein expanding comprises introducing into a liquid expander, a Joule- Thomson valve, or a combination thereof. Element 17: wherein the common end flash system is a multi-stage, common end-flash system, and wherein (d) lowering the pressure of the portions of the process fluid with the multi-stage, common end flash system further comprises further expanding the expanded fluid from which the end flash gas has been separated, and again separating gas therefrom. Element 18: wherein the liquefied process fluid comprises LNG. Element 19: wherein common end flash system includes a cold recovery heat exchanger coupled to the process feed gas line with a bypass line that bypasses the plurality of refrigerant systems; wherein the cold recovery heat exchanger is coupled to the liquid-gas separator and is configured to receive flash gas. Element 20: wherein the common end flash system comprises a plurality of stages, wherein each stage of the common end flash system includes an expansion device and a liquid-gas separator. Element 21 : further comprising: bypassing a portion of the process fluid around the plurality of refrigeration systems to the end flash system; condensing the portion of the process fluid that bypasses the plurality of refrigeration systems to produce additional LNG production; routing the additional LNG production through a line to one of the following: storage, a liquid expander in the common end flash system, and a separator in the end flash system.

[0041] While exemplary embodiments have been shown and described, modifications thereof can be made by one of ordinary skill in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations, combinations, and modifications of the systems, apparatuses, and processes described herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. The inclusion of any particular method step or operation within the written description or a figure does not necessarily mean that the particular step or operation is necessary to the method. The steps or operations of a method listed in the specification or the claims may be performed in any feasible order, except for those particular steps or operations, if any, for which a sequence is expressly stated. In some implementations two or more of the method steps or operations may be performed in parallel, rather than serially. The recitation of identifiers such as (a), (b), (c); (1 ), (2), (3); etc. before operations in a method claim are not intended to and do not specify a particular order to the operations, but rather are used to simplify subsequent reference to such operations.