Patent Application
20190101327
The disclosure relates generally to liquefied natural gas (LNG) production. More specifically, the disclosure relates to LNG production at high pressures.
This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as an admission of prior art.
Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, which are great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (LNG) for transport to market.
In the design of an LNG plant, one of the most important considerations is the process for converting the natural gas feed stream into LNG. Currently, the most common liquefaction processes use some form of refrigeration system. Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) the “cascade cycle,” which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature; (2) the “multi-component refrigeration cycle,” which uses a multi-component refrigerant in specially designed exchangers; and (3) the “expander cycle,” which expands gas from feed gas pressure to a low pressure with a corresponding reduction in temperature. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
The refrigerants used in liquefaction processes may comprise a mixture of components such as methane, ethane, propane, butane, and nitrogen in multi-component refrigeration cycles. The refrigerants may also be pure substances such as propane, ethylene, or nitrogen in “cascade cycles.” Substantial volumes of these refrigerants with close control of composition are required. Further, such refrigerants may have to be imported and stored, which impose logistics requirements, especially for LNG production in remote locations. Alternatively, some of the components of the refrigerant may be prepared, typically by a distillation process integrated with the liquefaction process.
The use of gas expanders to provide the feed gas cooling, thereby eliminating or reducing the logistical problems of refrigerant handling, is seen in some instances as having advantages over refrigerant-based cooling. The expander system operates on the principle that the refrigerant gas can be allowed to expand through an expansion turbine, thereby performing work and reducing the temperature of the gas. The low temperature gas is then heat exchanged with the feed gas to provide the refrigeration needed. The power obtained from cooling expansions in gas expanders can be used to supply part of the main compression power used in the refrigeration cycle. The typical expander cycle for making LNG operates at the feed gas pressure, typically under about 6,895 kPa (1,000 psia). Supplemental cooling is typically needed to fully liquefy the feed gas and this may be provided by additional refrigerant systems, such as secondary cooling and/or sub-cooling loops. For example, U.S. Pat. Nos. 6,412,302 and 5,916,260 present expander cycles which describe the use of nitrogen as refrigerant in the sub-cooling loop.
Previously proposed expander cycles have all been less efficient thermodynamically, however, than the current natural gas liquefaction cycles based on refrigerant systems. Expander cycles have therefore not offered any installed cost advantage to date, and liquefaction cycles involving refrigerants are still the preferred option for natural gas liquefaction.
Because expander cycles result in a high recycle gas stream flow rate and high inefficiency for the primary cooling (warm) stage, gas expanders have typically been used to further cool feed gas after it has been pre-cooled to temperatures well below −20° C. using an external refrigerant in a closed cycle, for example. Thus, a common factor in most proposed expander cycles is the requirement for a second, external refrigeration cycle to pre-cool the gas before the gas enters the expander. Such a combined external refrigeration cycle and expander cycle is sometimes referred to as a “hybrid cycle.” While such refrigerant-based pre-cooling eliminates a major source of inefficiency in the use of expanders, it significantly reduces the benefits of the expander cycle, namely the elimination of external refrigerants.
U. S. Patent Application US2009/0217701 introduced the concept of using high pressure within the primary cooling loop to eliminate the need for external refrigerant and improve efficiency, at least comparable to that of refrigerant-based cycles currently in use. The high pressure expander process (HPXP), disclosed in U. S. Patent Application US2009/0217701, is an expander cycle which uses high pressure expanders in a manner distinguishing from other expander cycles. A portion of the feed gas stream may be extracted and used as the refrigerant in either an open loop or closed loop refrigeration cycle to cool the feed gas stream below its critical temperature. Alternatively, a portion of LNG boil-off gas may be extracted and used as the refrigerant in a closed loop refrigeration cycle to cool the feed gas stream below its critical temperature. This refrigeration cycle is referred to as the primary cooling loop. The primary cooling loop is followed by a sub-cooling loop which acts to further cool the feed gas. Within the primary cooling loop, the refrigerant is compressed to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia. The refrigerant is then cooled against an ambient cooling medium (air or water) prior to being near isentropically expanded to provide the cold refrigerant needed to liquefy the feed gas.
In the expander loop 102, a compression unit 108 compresses a refrigerant stream 109 (which may be a treated gas stream) to a pressure greater than or equal to about 1,500 psia, thus providing a compressed refrigerant stream 110. Alternatively, the refrigerant stream 109 may be compressed to a pressure greater than or equal to about 1,600 psia, or greater than or equal to about 1,700 psia, or greater than or equal to about 1,800 psia, or greater than or equal to about 1,900 psia, or greater than or equal to about 2,000 psia, or greater than or equal to about 2,500 psia, or greater than or equal to about 3,000 psia, thus providing compressed refrigerant stream 110. After exiting compression unit 108, compressed refrigerant stream 110 is passed to a cooler 112 where it is cooled by indirect heat exchange with a suitable cooling fluid to provide a compressed, cooled refrigerant stream 114. Cooler 112 may be of the type that provides water or air as the cooling fluid, although any type of cooler can be used. The temperature of the compressed, cooled refrigerant stream 114 depends on the ambient conditions and the cooling medium used, and is typically from about 35° F. to about 105° F. Compressed, cooled refrigerant stream 114 is then passed to an expander 116 where it is expanded and consequently cooled to form an expanded refrigerant stream 118. Expander 116 is a work-expansion device, such as a gas expander, which produces work that may be extracted and used for compression. Expanded refrigerant stream 118 is passed to a first heat exchanger 120, and provides at least part of the refrigeration duty for first heat exchanger 120. Upon exiting first heat exchanger 120, expanded refrigerant stream 118 is fed to a compression unit 122 for pressurization to form refrigerant stream 109.
Feed gas stream 106 flows through first heat exchanger 120 where it is cooled, at least in part, by indirect heat exchange with expanded refrigerant stream 118. After exiting first heat exchanger 120, the feed gas stream 106 is passed to a second heat exchanger 124. The principal function of second heat exchanger 124 is to sub-cool the feed gas stream. Thus, in second heat exchanger 124 the feed gas stream 106 is sub-cooled by sub-cooling loop 104 (described below) to produce sub-cooled stream 126. Sub-cooled stream 126 is then expanded to a lower pressure in expander 128 to form a liquid fraction and a remaining vapor fraction. Expander 128 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 126, which is now at a lower pressure and partially liquefied, is passed to a surge tank 130 where the liquefied fraction 132 is withdrawn from the process as an LNG stream 134, which has a temperature corresponding to the bubble point pressure. The remaining vapor fraction (flash vapor) stream 136 may be used as fuel to power the compressor units.
In sub-cooling loop 104, an expanded sub-cooling refrigerant stream 138 (preferably comprising nitrogen) is discharged from an expander 140 and drawn through second and first heat exchangers 124, 120. Expanded sub-cooling refrigerant stream 138 is then sent to a compression unit 142 where it is re-compressed to a higher pressure and warmed. After exiting compression unit 142, the re-compressed sub-cooling refrigerant stream 144 is cooled in a cooler 146, which can be of the same type as cooler 112, although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed to first heat exchanger 120 where it is further cooled by indirect heat exchange with expanded refrigerant stream 118 and expanded sub-cooling refrigerant stream 138. After exiting first heat exchanger 120, the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander 140 to provide a cooled stream which is then passed through second heat exchanger 124 to sub-cool the portion of the feed gas stream to be finally expanded to produce LNG.
U. S. Patent Application US2010/0107684 disclosed an improvement to the performance of the HPXP through the discovery that adding external cooling to further cool the compressed refrigerant to temperatures below ambient conditions provides significant advantages which in certain situations justifies the added equipment associated with external cooling. The HPXP embodiments described in the aforementioned patent applications perform comparably to alternative mixed external refrigerant LNG production processes such as single mixed refrigerant processes. However, there remains a need to further improve the efficiency of the HPXP as well as overall train capacity. There remains a particular need to improve the efficiency of the HPXP in cases where the feed gas pressure is less than 1,200 psia.
U. S. Patent Application 2010/0186445 disclosed the incorporation of feed compression up to 4,500 psia to the HPXP. Compressing the feed gas prior to liquefying the gas in the HPXP's primary cooling loop has the advantage of increasing the overall process efficiency. For a given production rate, this also has the advantage of significantly reducing the required flow rate of the refrigerant within the primary cooling loop which enables the use of compact equipment, which is particularly attractive for floating LNG applications. Furthermore, feed compression provides a means of increasing the LNG production of an HPXP train by more than 30% for a fixed amount of power going to the primary cooling and sub-cooling loops. This flexibility in production rate is again particularly attractive for floating LNG applications where there are more restrictions than land based applications in matching the choice of refrigerant loop drivers with desired production rates. Although liquefying the feed gas at high pressures has advantages, it was found that for liquefaction pressures greater than 1,500 psia the choice of suitable cryogenic heat exchangers for the primary cooling and sub-cooling loops were limited to options significantly high in cost, weight and with reduced fluid processing capabilities. For example, the use of printed circuit heat exchangers, which are capable of operating at pressures greater than 4,500 psia, was shown to significantly increase project cost compared to the more widely sourced brazed aluminum heat exchanger type where proven operating pressures are less than 1,500 psia. This significant increase in cost may limit the practical application of feed compression to up to 1,500 psia. Thus, there remains a need to further improve the HPXP without requiring feed compression or feed compression greater the 1,500 psia. Additionally, there remains an additional need to allow the use of significant feed compression with HPXP without requiring the use of high-cost main cryogenic heat exchangers such as printed circuit heat exchangers.
The present disclosure provides 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 the following steps: providing the feed gas stream at a pressure less than 1,200 psia; providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; 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; expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 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; 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; 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 compressing the second warm refrigerant stream to produce the compressed refrigerant stream.
The present disclosure also provides a system for liquefying a feed gas stream rich in methane, the system having first and second heat exchanger zones. A feed gas stream at a pressure less than 1,200 psia is provided. A compressed refrigerant stream is provided with a pressure greater than or equal to 1,500 psia. A cooler is 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 is provided 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 is arranged to expand the compressed, additionally cooled refrigerant stream, thereby producing an expanded, cooled refrigerant stream. At least one heat exchanger is provided within the first heat exchanger zone. The expanded, cooled refrigerant stream passes 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. The feed gas stream passes 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. 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. A compressor is configured to compress the second warm refrigerant stream to produce the compressed refrigerant stream.
The present disclosure also provides a method for liquefying a feed gas stream rich in methane, where the method comprises the following steps: providing the feed gas stream at a pressure less than 1,200 psia; compressing the feed gas stream to a pressure of at least 1,500 psia to form a compressed gas stream; cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water, to form a cooled, compressed gas stream; 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; providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; 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; expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 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; 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; 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; and compressing the second warm refrigerant stream to produce the compressed refrigerant stream.
The disclosure also provides a system for liquefying a feed gas stream rich in methane and having a pressure less than 1,200 psia. The system comprises: a compressor for compressing the feed gas stream to a pressure of at least 1,500 psia, to form a compressed gas stream; a cooler for cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water, to form a cooled, compressed gas stream; at least one work producing expander for expanding the cooled, compressed gas stream 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; a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; a refrigerant cooler for cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; a heat exchanger zone through which the compressed, cooled refrigerant stream is directed to be additionally cooled below ambient temperature, to produce a compressed, additionally cooled refrigerant stream; an additional work producing expander for expanding the compressed, additionally cooled refrigerant stream, thereby producing an expanded, cooled refrigerant stream; an additional heat exchanger zone through which the expanded, cooled refrigerant stream is passed, to thereby form a first warm refrigerant stream, whereby the 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 chilled gas stream is passed through the additional 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; wherein the first warm refrigerant stream is directed to the heat exchanger zone to cool by indirect heat exchange the compressed, cooled refrigerant stream, thereby forming a second warm refrigerant stream; and an additional compressor for compressing the second warm refrigerant stream to produce the compressed refrigerant stream.
The foregoing has broadly outlined the features of the present disclosure so that the detailed description that follows may be better understood. Additional features will also be described herein.
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.
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.
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.
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.
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.”
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.
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.
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.
“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.
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.
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.
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.
As used herein, the terms “loop” and “cycle” are used interchangeably.
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).
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.
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.
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.
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.
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.
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.
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.
The first heat exchanger zone 201 may include a plurality of heat exchanger devices, and in the aspects shown in
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.
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.
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.
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.
Aspects of the disclosure illustrated in
and 1124, compressing the second warm refrigerant stream to produce the compressed refrigerant stream.
The steps depicted in
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.
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.
This application claims the priority benefit of U.S. Provisional Application No. 62/565,725 filed Sep. 29, 2017 entitled “Natural Gas Liquefaction by a High Pressure Expansion Process,” the entirety of which is incorporated by reference herein. This application is related to U.S. Provisional Application No. 62/565,733, titled “Natural Gas Liquefaction by a High Pressure Expansion Process,” being commonly owned and filed on an even date herewith, the disclosure of which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62565725 | Sep 2017 | US |
