Patent Application
20230272971
Production of liquefied natural gas (LNG) through indirect heat exchange against a single mixed refrigerant (SMR) is well-known in the art. A simple, well-known prior art SMR process is described herein and shown in
There have been many attempts to improve the efficiency of SMR processes. For example, U.S. Pat. No. 10,139,157 describes a single mixed refrigerant LNG production cycle in which a single mixed refrigerant stream is cooled and liquefied in a cryogenic exchanger, before passing through a Joule-Thompson valve. Similarly, U.S. Pat. No. 6,334,334 teaches a single mixed refrigerant LNG production cycle in which two mixed refrigerant (vapor and liquid) streams are cooled and liquefied separately in cryogenic exchangers before the resulting liquid is passed through a work producing turbine. The resulting liquefied vapor stream passes through a Joule-Thompson valve. In this process, three compression stages are provided, with liquid produced in the first intercooler being mixed with the discharge of the second stage then cooled in a second intercooler to produce a second liquid and vapor stream, which is subsequently separated. The liquid stream is sent directly to a cryogenic exchanger. In addition, only a portion of the mixed refrigerant is passed through the work producing turbine.
Many of the attempts to improve the efficiency of the SMR process results in processes that are complex to build and/or operate. Accordingly, there is a need for an improved SMR process that better balances increased efficiency and complexity.
Disclosed herein is a simple and efficient SMR process that cools and liquefies a single high pressure ambient two phase mixed refrigerant stream in a cryogenic heat exchanger, expands the liquid refrigerant at the cold end, then vaporizes it in the exchanger to provide refrigeration duty to a natural gas stream being liquefied and a high-pressure mixed refrigerant stream.
An important feature of the exemplary embodiments disclosed herein is a synergistic combination of three stages of compression, two intercoolers (both producing liquid), and the use of a hydraulic turbine to expand the refrigerant before it flows into the main heat exchanger. Providing three stages of compression (with liquid formation as described) enables a high mixed refrigerant discharge pressure to be achieved efficiently. The high mixed refrigerant discharge pressure improved the refrigeration performance of hydraulic turbine and unexpectedly improved the performance of liquefaction system.
Several aspects of the systems and methods are outlined below.
Aspect 1-A method for liquefying a hydrocarbon stream using a mixed refrigerant, the method comprising:
Aspect 2: The method of Aspect 1, wherein the expanded refrigerant stream provides the sole refrigeration duty for step (a).
Aspect 3: The method of any of Aspects 1-2, wherein flow of the refrigerant in steps (a) through (n) defines a closed-loop refrigeration cycle and all of the refrigerant flows through the hydraulic turbine in step (n).
Aspect 4: The method of any of Aspects 1-3, wherein the main heat exchanger comprises a warm end and a cold end and the expanded refrigerant stream is introduced into the main heat exchanger at the cold end.
Aspect 5: The method of any of Aspects 1-4, wherein the vaporized refrigerant stream has a first flow rate in step (b) and the expanded refrigerant stream has a second flow rate in step (n), the first flow rate being equal to the second flow rate.
Aspect 6: The method of any of Aspects 1-5, wherein the cooled two-phase high pressure refrigerant stream has a pressure of at least 1000 PSIA (68.95 bara).
Aspect 7: The method of any of Aspects 1-6, wherein the composition of the refrigerant is the same in the vaporized refrigerant stream, the two phase high pressure refrigerant stream, the condensed refrigerant stream, and the expanded refrigerant stream.
Aspect 8: The method of any of Aspect 1-7, wherein the main heat exchanger comprises a warm bundle and a cold bundle and the method further comprises:
Aspect 9: The method of any of Aspects 1-8, wherein the warm bundle and the cold bundle are each contained within separate shells.
Aspect 10: The method of any of Aspects 1-9, wherein the main heat exchanger further comprises a middle bundle and the method further comprises:
Aspect 11: The method of any of Aspects 1-10, wherein the warm bundle, the cold bundle, and the middle bundle are each contained within separate shells.
Aspect 12: The method of any of Aspects 1-11, wherein the hydrocarbon stream comprises natural gas.
Aspect 13: The method of any of any of Aspects 1-12, wherein step (i) further comprises selectively expanding the condensed refrigerant stream through an expansion valve located on a bypass circuit instead of through the hydraulic turbine.
Aspect 14: A method for liquefying a hydrocarbon stream using a mixed refrigerant, the method comprising:
Aspect 15: The method of Aspect 14, wherein the cooled two-phase high pressure refrigerant stream, the expanded refrigerant stream, and the vaporized refrigerant stream all consist of the mixed refrigerant.
Aspect 16: The method of any of Aspects 14-15, wherein the expanded refrigerant stream provides the sole refrigeration duty for step (a).
Aspect 17: The method of any of Aspects 14-16, wherein flow of the refrigerant in steps (a) through (i) defines a closed-loop refrigeration cycle and all of the refrigerant flows through the hydraulic turbine in step (i).
Aspect 18: The method of any of Aspects 14-17, wherein the vaporized refrigerant stream has a first flow rate in step (a) and the expanded refrigerant stream has a second flow rate in step (i), the first flow rate being equal to the second flow rate.
Aspect 19: The method of any of Aspects 14-18, wherein the composition of the refrigerant is the same in the vaporized refrigerant stream, the cooled two phase high pressure refrigerant stream, the condensed refrigerant stream, and the expanded refrigerant stream.
Aspect 20: A method of designing and fabricating a system for liquefying natural gas using a closed loop single mixed refrigerant process that supplies refrigeration duty to a cryogenic heat exchanger having a plurality of coil wound bundles, each of the plurality of coil wound bundles having an overall tube length, the method comprising:
wherein the sole refrigeration duty for the cryogenic heat exchanger is a stream of the single mixed refrigerant that has been compressed to a pressure of at least 1000 PSIA (68.95 bara) and expanded by a hydraulic turbine.
Aspect 21: The method of Aspect 20, wherein the plurality of coil wound bundles comprises a warm bundle and a cold bundle, the selected refrigeration duty of the warm bundle being less than the selected refrigeration duty of the cold bundle.
The present invention will hereinafter be described in conjunction with the appended drawing figures wherein like numerals denote like elements.
The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention.
Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention.
In the claims, letters are used to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.
In order to aid in describing the invention, directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing and claiming the invention and are not intended to limit the invention in any way. In addition, reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features.
Unless otherwise indicated, the articles “a” and “an” as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.
Unless otherwise stated herein, introducing a stream at a location is intended to mean introducing substantially all of the said stream at the location. All streams discussed in the specification and shown in the drawings (typically represented by a line with an arrow showing the overall direction of fluid flow during normal operation) should be understood to be contained within a corresponding conduit. Each conduit should be understood to have at least one inlet and at least one outlet. Further, each piece of equipment should be understood to have at least one inlet and at least one outlet.
The term “conduit,” as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases.
As used in the specification and claims, the term “flow communication” is intended to mean that two or more elements are connected (either directly or indirectly) in a manner that enables fluids to flow between the elements, including connections that may contain valves, gates, tees, or other devices that may selectively restrict, merge, or separate fluid flow.
The term “natural gas”, as used in the specification and claims, means a hydrocarbon gas mixture consisting primarily of methane.
The terms “hydrocarbon”, “hydrocarbon gas”, or “hydrocarbon fluid”, as used in the specification and claims, means a gas/fluid comprising at least one hydrocarbon and for which hydrocarbons comprise at least 80%, and more preferably at least 90% of the overall composition of the gas/fluid.
The term “mixed refrigerant”, as used in the specification and claims, means a mixture of hydrocarbons, typically comprising hydrocarbon components containing one to five carbon atoms and may contain saturated and/or non-saturated components and/or straight chain and/or branched components, and nitrogen.
The term “ambient cooler”, as used in the specification and claims, means a heat exchange device the cools a fluid against an ambient fluid (typically ambient air).
As used in the specification and claims, the terms “high-high”, “high”, “medium”, “low”, and “low-low” are intended to express relative values for a property of the elements with which these terms are used. For example, a high-high pressure stream is intended to indicate a stream having a higher pressure than the corresponding high pressure stream or medium pressure stream or low pressure stream described or claimed in this application. Similarly, a high pressure stream is intended to indicate a stream having a higher pressure than the corresponding medium pressure stream or low pressure stream described in the specification or claims, but lower than the corresponding high-high pressure stream described or claimed in this application. Similarly, a medium pressure stream is intended to indicate a stream having a higher pressure than the corresponding low pressure stream described in the specification or claims, but lower than the corresponding high pressure stream described or claimed in this application.
Unless otherwise stated herein, any and all percentages identified in the specification, drawings and claims should be understood to be on a mass percentage basis. Unless otherwise stated herein, any and all pressures identified in the specification, drawings and claims should be understood to mean gauge pressure.
As used in the specification and claims, the term “compression system” is defined as one or more compression stages. For example, a compression system may comprise multiple compression stages within a single compressor. In an alternative example, a compression system may comprise multiple compressors.
As used herein, the term “hydraulic turbine” is intended to refer to a work producing liquid expander. In the context of this invention, the primary purpose of the hydraulic turbine is to provide refrigeration to the process by removing enthalpy from the refrigerant stream, and the work produced may be recovered using an electrical generator, used to compress another fluid, or simply dissipated as heat released to the surroundings.
In this example, the feed gas stream 100 is natural gas, which is preferably pre-treated to remove water, acid gases (carbon dioxide and sulfur dioxide) and freezable heavy hydrocarbons. The feed gas stream 100 is preferably near ambient temperature or may have been pre-cooled by another process using known refrigeration techniques (fluid boiling, gas expansion etc.). Typically, the feed gas stream 100 enters a warm end 160 of the cryogenic heat exchanger 130 at a pressure of 40 bara to 80 bara, then exits a cold end 161 of the cryogenic heat exchanger 130 as a product stream 102 in liquid phase, at a temperature of typically between −140 degrees C. and −150 degrees C. The product stream 102 preferably passes through pressure reduction device 138 which may be a Joule-Thompson valve or a work producing hydraulic turbine before being sent to storage (not shown).
In this exemplary process, the cryogenic heat exchanger 130 consists of a single shell 131. Examples of suitable types of heat exchanger types for the cryogenic heat exchanger 130 include a plate and fin heat exchanger or a coil wound heat exchanger. In the case of a coil wound heat exchanger, the expanded refrigerant stream 136 flows through a shell side of the cryogenic heat exchanger 130. If the cryogenic heat exchanger 130 is a plate fin-style exchanger, it may be desirable to employ devices to ensure even distribution of two phase high pressure refrigerant stream 128 among parallel passages and/or exchangers using techniques well known in the art, such as a phase separator liquid pumps and the like.
The feed gas stream 100 could optionally be removed from the cryogenic heat exchanger 130 at an intermediate location (stream 103) and sent to a separation device 150 for heavy hydrocarbon removal, then a predominately methane stream 105 is reintroduced into cryogenic heat exchanger 130.
After providing refrigeration duty, a vapor refrigerant stream 104 is withdrawn from the warm end 160 of the cryogenic heat exchanger 130. The vapor refrigerant stream 104 is preferable at near-ambient temperature and at a pressure of 3-5 bara. The vapor refrigerant stream 104 is then compressed in compressor stage 106 to a pressure typically from 10 to 20 bara, forming a low-pressure refrigerant stream 107. The low-pressure refrigerant stream 107 is then cooled by ambient cooler 108, using cooling water or air. The resulting cooled two-phase refrigerant stream 109 is separated into a liquid stream 113 and a vapor stream 111, using a separator 110. The vapor stream 111 is compressed (via compression stage 114) to a pressure typically between 25 and 30 bara to form a medium pressure refrigerant stream 115. The liquid stream 113 is pumped (using pump 112) to substantially the same pressure as the medium pressure refrigerant stream 115 then combined with the medium pressure refrigerant stream 115. The combined stream 117 is then cooled by ambient cooler 116 using cooling water or air.
The resulting two-phase refrigerant stream 118 is separated into a liquid stream 123 and a vapor stream 121 using a separator 120. The vapor stream 121 is further compressed (via a compression stage 124) to a pressure typically between 40 and 70 bara for form a high pressure refrigerant stream 125. The liquid stream 123 is pumped (via pump 122) to substantially the same pressure as the high pressure refrigerant stream 125, then recombined with the high pressure refrigerant stream 125. The combined stream 127 is then cooled by ambient cooler 126 using cooling water or air to form the two phase high pressure refrigerant stream 128.
The two phase high pressure refrigerant stream 128 is then cooled and condensed in the cryogenic heat exchanger 130, exiting as a condensed refrigerant stream 132 in liquid phase and at a temperature of typically between −140 degrees C. and −150 degrees C. The condensed refrigerant stream 132 is sent to a hydraulic turbine 134 and expanded to form an expanded refrigerant stream 136. Optionally, the hydraulic turbine 134 may have a single-phase liquid outlet followed by a valve or may include a work producing liquid expander with a two-phase outlet.
An optional bypass circuit 139 with an expansion valve 137 (such as a Joule-Thompson valve) could be provided to enable the system to continue to function when the hydraulic turbine 134 is being serviced or in the case of a failure. In addition, an optional expansion valve 162 (such as a Joule-Thompson valve) could be provided downstream from the hydraulic turbine 134 to provide for further expansion of the expanded refrigerant stream 136. The optional bypass circuit 139 and optional expansion valve 162 could also be included in the embodiment shown in
The expanded refrigerant stream 136 is then introduced into the cold end 161 of the cryogenic heat exchanger 130, vaporized (typically at a pressure of 3-5 bara) (providing refrigeration duty for the process), and exits the warm end 160 as the vapor refrigerant stream 104.
In
The process of
If all of the shells 230a, 230b and 230c and bundles of the cryogenic heat exchanger 230 are fabricated at the same time, overall fabrication time is dictated by the bundle that requires the most time to fabricate. If refrigeration duty is split equally among the bundles contained within each of the shells 230a, 230b and 230c, then the bundle contained within the shell 230a would take longer to fabricate than the bundles contained within shells 230b and 230c. The feed gas stream 200 and the two phase high pressure refrigerant stream 228 have a relatively high vapor fraction and relatively low density when flowing through shell 230a, as compared to those same streams when flowing through shell 230b or 230c. This is due to a density increase as the streams 200, 228 are cooled and condensed. This results in the bundle of shell 230a requiring more tubes than the bundles of shell 230b or 230c. If refrigeration duty is shifted from shell 230a to shells 230b and 230c by making the bundle contained within shell 230a shorter and increasing the length of the bundles contained within shells 230b and 230c, the same pressure drop can be achieved with fewer tubes because the length of the tubes is reduced. This reduces the fabrication time of the bundle for shell 230a, and therefore, the overall manufacturing time.
For example, for the exemplary embodiment shown in
In
As shown in columns 1 and 2, there is a 6.0% benefit (i.e., reduction in total power requirement) to adding a hydraulic turbine to the prior art process of
As such, an invention has been disclosed in terms of preferred embodiments and alternate embodiments thereof. Of course, various changes, modifications, and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.
