US20150013379A1 - LNG Formation - Google Patents

LNG Formation Download PDF

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US20150013379A1
US20150013379A1 US14/378,073 US201314378073A US2015013379A1 US 20150013379 A1 US20150013379 A1 US 20150013379A1 US 201314378073 A US201314378073 A US 201314378073A US 2015013379 A1 US2015013379 A1 US 2015013379A1
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natural gas
refrigerant
refrigeration
cooling
refrigerant mixture
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US14/378,073
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Russell H. Oelfke
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Russell H. Oelfke
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Priority to US201261695592P priority
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Priority to US14/378,073 priority patent/US20150013379A1/en
Priority to PCT/US2013/028906 priority patent/WO2013148075A1/en
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
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    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022Hydrocarbons, e.g. natural gas
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0035Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by gas expansion with extraction of work
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/003Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/004Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by flash gas recovery
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    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/003Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0042Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by liquid expansion with extraction of work
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    • F25J1/0047Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/0052Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
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    • F25J1/0062Light or noble gases, mixtures thereof
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    • F25J1/0087Propane; Propylene
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    • F25J1/0097Others, e.g. F-, Cl-, HF-, HClF-, HCl-hydrocarbons etc. or mixtures thereof
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    • F25J1/0208Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop
    • F25J1/0209Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop as at least a three level refrigeration cascade
    • F25J1/021Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop as at least a three level refrigeration cascade using a deep flash recycle loop
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    • F25J1/0214Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle
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    • F25J1/0211Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle
    • F25J1/0217Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as at least a three level refrigeration cascade with at least one MCR cycle
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    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
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    • F25J1/0211Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle
    • F25J1/0219Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. using a deep flash recycle loop
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J1/0243Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0262Details of the cold heat exchange system
    • F25J1/0264Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
    • F25J1/0265Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer
    • F25J1/0268Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer using a dedicated refrigeration means
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    • F25J2220/00Processes or apparatus involving steps for the removal of impurities
    • F25J2220/60Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/62Separating low boiling components, e.g. He, H2, N2, Air
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    • F25J2220/60Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/64Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general

Abstract

Systems and a method for the formation of a liquefied natural gas (LNG) are disclosed herein. The system includes a refrigeration system configured to chill a natural gas using a refrigerant mixture including a noble gas. The system also includes an autorefrigeration system configured to use the natural g self-refrigerant to form the LNG from the natural gas.

Description

    CROSS-REFERENCE TO RELATED APPLICATION
  • This application claims the benefit of U.S. Provisional Patent Application Nos. 61/695,592 filed Aug. 31, 2012 entitled LNG FORMATION, and 61/618,290 entitled USE OF NOBLE GASES IN LOW TEMPERATURE HYDROCARBON PROCESSING SYSTEMS, APPARATUS, AND METHODS, filed on Mar. 30, 2012, the entirety of each of which is incorporated by reference herein.
  • FIELD OF THE INVENTION
  • The present techniques relate generally to the field of hydrocarbon recovery and treatment processes and, more particularly, to systems and methods that form liquefied natural gas (LNG) via a refrigeration process. Specifically, provided are systems and methods for forming LNG from natural gas using refrigerants that include one or more noble gases.
  • BACKGROUND
  • This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
  • Many low temperature refrigeration systems that are used for natural gas processing and liquefaction rely on the use of refrigerants including hydrocarbon components and nitrogen to provide external refrigeration. Such hydrocarbon components may include methane, ethane, ethylene, propane, and the like. However, the use of refrigerants including hydrocarbon components and nitrogen may not be very efficient, since a large heat transfer area may be required to provide proper refrigeration of the natural gas. In addition, the flammability of the hydrocarbon components within the refrigerants may increase the risks associated with the refrigeration process.
  • Low temperature refrigeration systems that are used for natural gas processing and liquefaction often use synthetic refrigerants, such as R-404A or R-410A, as substitutes for the refrigerants including the hydrocarbon components and the nitrogen. However, such synthetic refrigerants are only suitable for levels of refrigeration that are above around −100° F. In some instances, lower levels of refrigeration may be desirable.
  • International Patent Application Publication WO/2005/072404, by Flynn, et al., describes a cooling system that includes a first refrigerant cycle including a first refrigerant and a second refrigerant cycle including a second refrigerant that is a mixture of cryogenic components. The disclosure is also directed to a cooling system that includes a first refrigerant cycle including a first refrigerant and a second refrigerant cycle including a second refrigerant that is a non-reactive component. The second refrigerant is free of fluorocarbons, chlorofluorocarbons, and hydrocarbons. At least a portion of the second refrigerant is condensed in the second refrigerant cycle. However, the disclosure is not directed to a cooling system that includes any type of autorefrigeration cycle.
  • Related information may be found in U.S. Pat. Nos. 4,533,372, 4,923,493, 5,265,428, 5,062,270, 5,120,338, 6,053,007, and 5,956,971; U.S. Patent Application Publication Nos. 2002/0088249, 2003/0177785, 2007/0193303, 2007/0227185, 2008/0034789, 2008/0087041, 2009/0217701, 2009/0266107, 2010/0018248, 2010/0107684, 2010/0186445, 2012/0031144, 2012/0079852, and 2012/0125043; and International Patent Publication No. WO/2012/015554. Other potentially related information may be found in International Patent Publication No. WO2007/021351; Foglietta, J. H., et al., “Consider Dual Independent Expander Refrigeration for LNG Production New Methodology May Enable Reducing Cost to Produce Stranded Gas,” Hydrocarbon Processing, Gulf Publishing Co., vol. 83, no. 1, pp. 39-44 (January 2004); U.S. Patent Application Publication No. US2003/089125; U.S. Pat. No. 6,412,302; U.S. Pat. No. 3,162,519; U.S. Pat. No. 3,323,315; German Patent No. DE19517116, and J. M. Campbell, “Gas Conditioning and Processing, Vol. 2: The Equipment Modules”, 8th edition, John M. Campbell & Company, 2001.
  • SUMMARY
  • An embodiment provides a system for the formation of a liquefied natural gas (LNG). The system includes a refrigeration system configured to chill a natural gas using a refrigerant mixture including a noble gas. The system also includes an autorefrigeration system configured to use the natural gas as a self-refrigerant to form the LNG from the natural gas.
  • Another embodiment provides a method for the formation of LNG. The method includes chilling a natural gas in a refrigeration system, wherein the refrigeration system uses a refrigerant mixture includes a noble gas. The method also includes liquefying the natural gas to form the LNG in an autorefrigeration system.
  • Another embodiment provides a cascade cooling system for formation of LNG. The cascade cooling system includes a first refrigeration system configured to cool the natural gas using a non-hydrocarbon refrigerant, wherein the first refrigeration system includes a number of first chillers configured to allow for cooling of the natural gas via an indirect exchange of heat between the natural gas and the non-hydrocarbon refrigerant. The cascade cooling system also includes a second refrigeration system configured to chill the natural gas using a refrigerant mixture including a noble gas, wherein the second refrigeration system includes a number of second chillers configured to allow for cooling of the natural gas via an indirect exchange of heat between the natural gas and the refrigerant mixture. The cascade cooling system further includes an autorefrigeration system configured to form the LNG from the natural gas, wherein the autorefrigeration system includes a number of expansion valves or hydraulic expansion turbines, or any combination thereof, and flash drums.
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which:
  • FIG. 1 is a process flow diagram of a single stage refrigeration system;
  • FIG. 2 is a process flow diagram of a two stage refrigeration system including an economizer;
  • FIG. 3 is a process flow diagram of a single stage refrigeration system including a heat exchanger economizer;
  • FIG. 4 is a process flow diagram of a cascade cooling system including a first refrigeration system and a second refrigeration system;
  • FIG. 5 is process flow diagram of an expansion refrigeration system for hydrocarbon dew point control;
  • FIG. 6 is a process flow diagram of an expansion refrigeration system for NGL extraction;
  • FIG. 7 is a process flow diagram of an LNG production system;
  • FIG. 8 is a simplified process flow diagram of a cascade cooling system;
  • FIGS. 9A-B are a more detailed process flow diagram of a cascade cooling system;
  • FIG. 10 is a more detailed process flow diagram of an autorefrigeration system;
  • FIG. 11 is a schematic of a methane pressure-enthalpy (P-H) diagram; and
  • FIG. 12 is a process flow diagram of a method for the formation of LNG.
  • DETAILED DESCRIPTION OF THE DRAWINGS
  • In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the spirit and scope of the appended claims.
  • 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.
  • “Acid gases” are contaminants that are often encountered in natural gas streams. Typically, these gases include carbon dioxide (CO2) and hydrogen sulfide (H2S), although any number of other contaminants may also form acids. Acid gases are commonly removed by contacting the gas stream with an absorbent, such as an amine, which may react with the acid gas. When the absorbent becomes acid-gas “rich,” a desorption step can be used to separate the acid gases from the absorbent. The “lean” absorbent is then typically recycled for further absorption. As used herein a “liquid acid gas stream” is a stream of acid gases that are condensed into the liquid phase, for example, including CO2 dissolved in H2S and vice-versa.
  • As used herein, “autorefrigeration” refers to a process whereby a fluid is cooled via a reduction in pressure. In the case of liquids, autorefrigeration refers to the cooling of the liquid by evaporation, which corresponds to a reduction in pressure. More specifically, a portion of the liquid is flashed into vapor as it undergoes a reduction in pressure while passing through a throttling device. As a result, both the vapor and the residual liquid are cooled to the saturation temperature of the liquid at the reduced pressure. For example, according to embodiments described herein, autorefrigeration of a natural gas may be performed by maintaining the natural gas at its boiling point so that the natural gas is cooled as heat is lost during boil off. This process may also be referred to as a “flash evaporation.”
  • As used herein, a “cascade cycle” refers to a system with two or more refrigerants, where a cold second refrigerant is condensed by a warmer first refrigerant. Thus, low temperatures may be “cascaded” down from one refrigerant to another. Each refrigerant in a cascade may have multiple levels of chilling based on staged evaporating pressures within economizers. Cascade cycles are considered to be beneficial for the production of LNG as compared to single refrigerant systems, since lower temperatures may be achieved within cascade cycles than single refrigerant systems.
  • A “closed-loop refrigeration cycle” refers to a refrigeration cycle wherein substantially no refrigerant enters or exits the cycle during normal operation.
  • A “closed-loop refrigeration system” refers to a refrigeration system comprising compression, heat exchange, and pressure reduction means in which a refrigerant is recirculated without continuous deliberate refrigerant withdrawal. A small amount of refrigerant makeup typically is required because of small leakage losses from the system.
  • A “compressor” or “refrigerant compressor” includes any unit, device, or apparatus able to increase the pressure of a refrigerant stream. This includes refrigerant compressors having a single compression process or step, or refrigerant compressors having multi-stage compressions or steps, more particularly multi-stage refrigerant compressors within a single casing or shell. Evaporated refrigerant streams to be compressed can be provided to a refrigerant compressor at different pressures. Some stages or steps of a hydrocarbon cooling process may involve two or more refrigerant compressors in parallel, series, or both. The present invention is not limited by the type or arrangement or layout of the refrigerant compressor or refrigerant compressors, particularly in any refrigerant circuit.
  • A “Controlled-Freeze-Zone” (CFZ) process is a process that has been proposed to take advantage of the freezing potential of carbon dioxide in cryogenic distillation, rather than avoiding solid carbon dioxide. In the CFZ process, acid gas components are separated by cryogenic distillation through the controlled freezing and melting of carbon dioxide in a single column, without the use of freeze-suppression additives. The CFZ process uses a cryogenic distillation column with a special internal section, e.g., CFZ section, to handle the solidification and melting of carbon dioxide. This CFZ section does not contain packing or trays like conventional distillation columns. Instead, the CFZ section contains one or more spray nozzles and a melting tray. Solid carbon dioxide forms in the vapor space in the distillation column and falls into the liquid on the melting tray. Substantially all of the solids that form are confined to the CFZ section. The portions of the distillation column above and below the CFZ section of the column are similar to conventional cryogenic demethanizer columns. A more detailed description of the CFZ process is disclosed in U.S. Pat. Nos. 4,533,372; 4,923,493; 5,120,338; and 5,265,428.
  • As used herein, “cooling” broadly refers to lowering and/or dropping a temperature and/or internal energy of a substance, such as by any suitable amount. Cooling may include a temperature drop of at least about 1 degree Celsius, at least about 5 degrees Celsius, at least about 10 degrees Celsius, at least about 15 degrees Celsius, at least about 25 degrees Celsius, at least about 50 degrees Celsius, at least about 100 degrees Celsius, and/or the like. The cooling may use any suitable heat sink, such as steam generation, hot water heating, cooling water, air, refrigerant, other process streams (integration), and combinations thereof. One or more sources of cooling may be combined and/or cascaded to reach a desired outlet temperature. The cooling step may use a cooling unit with any suitable device and/or equipment. According to one embodiment, cooling may include indirect heat exchange, such as with one or more heat exchangers. Heat exchangers may include any suitable design, such as shell and tube, plate and frame, counter current, concurrent, extended surface, and/or the like. In the alternative, the cooling may use evaporative (heat of vaporization) cooling and/or direct heat exchange, such as a liquid sprayed directly into a process stream.
  • “Cryogenic temperature” refers to a temperature that is about −50° C. or below.
  • As used herein, the terms “deethanizer” and “demethanizer” refer to distillation columns or towers that may be used to separate components within a natural gas stream. For example, a demethanizer is used to separate methane and other volatile components from ethane and heavier components. The methane fraction is typically recovered as purified gas that contains small amounts of inert gases such as nitrogen, CO2, or the like.
  • 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.
  • A “heat exchanger” broadly means any device capable of transferring heat from one media to another media, including particularly any structure, e.g., device commonly referred to as a heat exchanger. Heat exchangers include “direct heat exchangers” and “indirect heat exchangers.” Thus, a heat exchanger may be a plate-and-frame, shell-and-tube, spiral, hairpin, core, core-and-kettle, double-pipe or any other type of known heat exchanger. “Heat exchanger” may also refer to any column, tower, unit or other arrangement adapted to allow the passage of one or more streams therethrough, and to affect direct or indirect heat exchange between one or more lines of refrigerant, and one or more feed streams.
  • 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 may be present in small amounts. As used herein, hydrocarbons generally refer to components found in natural gas, oil, or chemical processing facilities.
  • “Hydrofluorocarbons” or HFCs are molecules including H, F, and C atoms. Hydrofluorocarbons have H—C and F—C bonds and, depending on the number of carbon atoms in the species, C—C bonds. Some examples of hydrofluorocarbons include fluoroform (CHF3), pentafluoroethane (C2HF5), tetrafluoroethane (C2H2F4), heptafluoropropane (C3HF7), hexafluoropropane (C3H2F6), pentafluoropropane (C3H3F5), and tetrafluoropropane (C3H4F4), among other compounds of similar chemical structure.
  • “Liquefied natural gas” or “LNG” is natural gas generally known to include a high percentage of methane. However, LNG may also include trace amounts of other compounds. The other elements or compounds may include, but are not limited to, ethane, propane, butane, carbon dioxide, nitrogen, helium, hydrogen sulfide, or combinations thereof, that have been processed to remove one or more components (for instance, helium) or impurities (for instance, water and/or heavy hydrocarbons) and then condensed into a liquid at almost atmospheric pressure by cooling.
  • “Mixed refrigerant processes” may include, but are not limited to, a single refrigeration system using a mixed refrigerant, i.e., a refrigerant with more than one chemical component, a hydrocarbon pre-cooled mixed refrigerant system, and a dual mixed refrigerant system. In general, mixed refrigerants can include hydrocarbon and/or non-hydrocarbon components. Examples of suitable hydrocarbon components typically employed in mixed refrigerants can include, but are not limited to, methane, ethane, ethylene, propane, propylene, as well as butane and butylene isomers. Non-hydrocarbon components generally employed in mixed refrigerants can include carbon dioxide and nitrogen. Mixed refrigerant processes employ at least one mixed component refrigerant, but can additionally employ one or more pure-component refrigerants as well.
  • “Natural gas” refers to a multi-component gas obtained from a crude oil well or from a subterranean gas-bearing formation. The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (CH4) as a major component, i.e., greater than 50 mol % of the natural gas stream is methane. The natural gas stream can also contain ethane (C2H6), higher molecular weight hydrocarbons (e.g., C3-C20 hydrocarbons), one or more acid gases (e.g., carbon dioxide or hydrogen sulfide), or any combinations thereof. The natural gas can also contain minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, crude oil, or any combinations thereof. The natural gas stream may be substantially purified prior to use in embodiments, so as to remove compounds that may act as poisons.
  • As used herein, “natural gas liquids” (NGL) refer to mixtures of hydrocarbons whose components are, for example, typically heavier than ethane. Some examples of hydrocarbon components of NGL streams include propane, butane, and pentane isomers, benzene, toluene, and other aromatic compounds.
  • “Noble gas” refers to any of the chemical elements belonging to group 18 of the periodic table. More specifically, the noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The noble gases are characterized by very low chemical reactivity.
  • An “open-loop refrigeration cycle” refers to a refrigeration cycle wherein at least a portion of the refrigerant employed during normal operation originates from the fluid being cooled by the refrigeration cycle.
  • An “open-loop refrigeration system” is a refrigeration system comprising compression, heat exchange, and pressure reduction means in which a refrigerant is recirculated, a portion of the refrigerant is continuously withdrawn from the recirculation loop, and additional refrigerant is continuously introduced into the recirculation loop.
  • A “refrigerant component,” in a refrigeration system, will absorb heat at a lower temperature and pressure through evaporation and will reject heat at a higher temperature and pressure through condensation. Illustrative refrigerant components may include, but are not limited to, alkanes, alkenes, and alkynes having one to five carbon atoms, nitrogen, chlorinated hydrocarbons, fluorinated hydrocarbons, other halogenated hydrocarbons, noble gases, and mixtures or combinations thereof.
  • “Substantial” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may depend, in some cases, on the specific context.
  • Overview
  • Embodiments described herein provide a hydrocarbon processing system and method. Such a hydrocarbon processing system may include or utilize a refrigeration system, such as a cascade cooling system. Further, according to embodiments described herein, the refrigeration system utilizes a refrigerant mixture including a noble gas.
  • Hydrocarbon processing systems include the conventional systems known to those skilled in the art. Hydrocarbon production and treatment processes include, but are not limited to, chilling natural gas for NGL extraction, chilling natural gas for hydrocarbon dew point control, chilling natural gas for CO2 removal, liquefied petroleum gas (LPG) production storage, condensation of reflux in deethanizers/demethanizers, and natural gas liquefaction to produce LNG.
  • Although many refrigeration cycles have been used to process hydrocarbons, one cycle that is used in LNG liquefaction plants is 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. Another cycle that is used in LNG liquefactions plants is the multi-component refrigeration cycle, which uses a multi-component refrigerant in specially designed exchangers. In addition, another cycle that is used in LNG liquefaction plants is the expander cycle, which expands gas from feed gas pressure to a low pressure with a corresponding reduction in temperature. Natural gas liquefaction cycles may also use variations or combinations of these three cycles.
  • LNG is prepared from a feed gas by refrigeration and liquefaction technologies. Optional steps include condensate removal, CO2 removal, dehydration, mercury removal, nitrogen stripping, H2S removal, and the like. After liquefaction, LNG may be stored or fed to a gas pipeline for sale or use. Conventional liquefaction processes can include: APCI Propane pre-cooled mixed refrigerant; C3MR; DUAL MR; Phillips Optimized Cascade; Prico single mixed refrigerant; TEAL dual pressure mixed refrigerant; Linde/Statoil multi fluid cascade; Axens dual mixed refrigerant, DMR; and the Shell processes C3MR and DMR.
  • Carbon dioxide removal, i.e., separation of methane and lighter gases from CO2 and heavier gases, may be achieved with cryogenic processes, such as the Controlled Freeze Zone technology available from ExxonMobil Corporation.
  • While the method and systems described herein are discussed with respect to the formation of LNG from natural gas, the method and systems may also be used for a variety of other purposes. For example, the method and systems described herein may be used to chill natural gas for hydrocarbon dew point control, perform natural gas liquid (NGL) extraction, separate methane and lighter gases from carbon dioxide and heavier gases, prepare hydrocarbons for LPG production, or condense a reflux stream in deethanizers and/or demethanizers, among others.
  • Refrigerants
  • The refrigerants that are utilized according to embodiments described herein may be one or more single component refrigerants, or refrigerant mixtures including multiple components. Refrigerants may include methane, ethane, ethylene, propane, butane, and nitrogen, or combinations thereof. In embodiments described herein, refrigerants in one or more refrigeration stages use non-flammable materials that include noble gases and mixtures of noble gases. Refrigerants may be imported and stored on-site or, alternatively, some of the components of the refrigerant may be prepared on-site, typically by a distillation process integrated with the hydrocarbon processing system. Exemplary mixed refrigerants are disclosed in U.S. Pat. No. 6,530,240.
  • Commercially available refrigerants including fluorocarbons (FCs) or hydrofluorocarbons (HFCs) are used in various applications, as are refrigerants including ammonia, sulfur dioxide, or halogenated hydrocarbons. Exemplary refrigerants are commercially available from DuPont Corporation, including the ISCEON® family of refrigerants, the SUVA® family of refrigerants, the OPTEON® family of refrigerants, and the FREON® family of refrigerants.
  • Multicomponent refrigerants are commercially available. For example, R-401A is a HCFC blend of R-32, R-152a, and R-124. R-404A is a HFC blend of 52 wt. % R-143a, 44 wt. % R-125, and 4 wt. % R-134a. R-406A is a blend of 55 wt. % R-22, 4 wt. % R-600a, and 41 wt. % R-142b. R-407A is a HFC blend of 20 wt. % R-32, 40 wt. % R-125, and 40 wt. % R-134a. R-407C is a hydrofluorocarbon blend of R-32, R-125, and R-134a. R-408A is a HCFC blend of R-22, R-125, and R-143a. R-409A is a HCFC blend of R-22, R-124, and R-142b. R-410A is a blend of R-32 and R-125. R-500 is a blend of 73.8 wt. % R-12 and 26.2 wt. % of R-152a. R-502 is a blend of R-22 and R-115.
  • In embodiments discussed herein, refrigerants in one or more refrigeration stages may also include a noble gas or a noble gas mixture. The six naturally occurring noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). A noble gas can be used alone or in combination with other noble gases, or in combination with other refrigerant components. In some embodiments, the noble gas used as a refrigerant is xenon, krypton, argon, or combinations thereof.
  • Because noble gases are non-flammable, they reduce the risk of handling refrigerants. In addition, because noble gases exist in the atmosphere and are readily collected, any noble gas refrigerant that escapes the refrigeration system can be recycled. Further, if released into the environment, noble gases do not have any ozone depleting potential or greenhouse warming potential.
  • Noble gas refrigerants may provide cooling below about −50° F., or below about −100° F., or below about −120° F., or from about −50° F. to about −162° F., or from about −50° F. to about −244° F., or from about −50° F. to about −303° F. In multistage refrigeration systems, noble gas refrigerants may be utilized in later stages to achieve deeper cooling than provided by hydrocarbon refrigerants, such as below about −50° F., or below about −100° F., or below about −120° F., or from about −50° F. to about −162° F., or from about −90° F. to about −162° F., or from about −100° F. to about −162° F., or from about −120° F. to about −162° F., or from about −50° F. to about −244° F., or from about −90° F. to about −244° F., or from about −100° F. to about −244° F., or from about −120° F. to about −244° F., or from about −50° F. to about −303° F., or from about −90° F. to about −303° F., or from about −100° F. to about −303° F., or from about −120° F. to about −303° F.
  • In various embodiments, any of a number of different types of hydrocarbon processing systems can be used with any of the refrigeration systems described herein. In addition, the refrigeration systems described herein may utilize any of the refrigerants described above.
  • Refrigeration Systems
  • Hydrocarbon systems and methods often include refrigeration systems that utilize mechanical refrigeration, valve expansion, turbine expansion, or the like. Mechanical refrigeration typically includes compression systems and absorption systems, such as ammonia absorption systems. Compression systems are used in the gas processing industry for a variety of processes. For example, compression systems may be used for chilling natural gas for NGL extraction, chilling natural gas for hydrocarbon dew point control, LPG production storage, condensation of reflux in deethanizers or demethanizers, natural gas liquefaction to produce LNG, or the like. Further, other commercial processes that utilize refrigeration may take advantage of the decreased flammability inherent in the noble gases to replace other refrigerants, such as ammonia.
  • FIG. 1 is a process flow diagram of a single stage refrigeration system 100. In various embodiments, the single stage refrigeration system 100 utilizes a refrigerant mixture including a noble gas. The single stage refrigeration system 100 includes an expansion valve 102, a chiller 104, a compressor 106, a condenser 108, and an accumulator 110. A saturated liquid refrigerant 112 may flow from the accumulator 110 to the expansion valve 102, and may expand across the expansion valve 102 isenthalpically. On expansion, some vaporization occurs, creating a chilled refrigerant mixture 114 that includes both vapor and liquid. The refrigerant mixture 114 may enter the chiller 104, also known as the evaporator, at a temperature lower than the temperature to which a process stream 116, such as a natural gas, is to be cooled. The process stream 116 flows through the chiller 104 and exchanges heat with the refrigerant mixture 114. As the process stream 116 exchanges heat with the refrigerant mixture 114, the process stream 116 is cooled, while the refrigerant mixture 114 may at least partially vaporize, creating a saturated vapor refrigerant 118.
  • After leaving the chiller 104, the saturated vapor refrigerant 118, as well as any remaining liquid refrigerant, is compressed within the compressor 106, and is then flowed into the condenser 108. Within the condenser 108, the saturated vapor refrigerant 118 is converted to a saturated, or slightly sub-cooled, liquid refrigerant 120. The liquid refrigerant 120 may then be flowed from the condenser 108 to the accumulator 110. The accumulator 110, which is also known as a surge tank or receiver, may serve as a reservoir for the liquid refrigerant 120. The liquid refrigerant 120 may be stored within the accumulator 110 before being expanded across the expansion valve 102 as the saturated liquid refrigerant 112.
  • It is to be understood that the process flow diagram of FIG. 1 is not intended to indicate that the single stage refrigeration system 100 is to include all the components shown in FIG. 1. Further, the single stage refrigeration system 100 may include any number of additional components not shown in FIG. 1, depending on the details of the specific implementation. For example, in some embodiments, a refrigeration system can include two or more compression stages. In addition, the refrigeration system 100 may include an economizer, as discussed further with respect to FIG. 2.
  • FIG. 2 is a process flow diagram of a two stage refrigeration system 200 including an economizer 202. Like numbered items are as described with respect to FIG. 1. The economizer 202 may be any device or process modification that decreases the compressor power usage for a given chiller duty. Conventional economizers 202 include, for example, flash tanks and heat exchange economizers.
  • As shown in FIG. 2, the saturated liquid refrigerant 112 leaving the accumulator 110 may be expanded across the expansion valve 102 to an intermediate pressure at which vapor and liquid may be separated. The expansion valve 102 may be used to control the downstream temperature and pressure of the saturated liquid refrigerant 112. For example, as the saturated liquid refrigerant 112 flashes across the expansion valve 102, a vapor refrigerant 204 and liquid refrigerant 206 are produced at a lower pressure and temperature than the saturated liquid refrigerant 112. The vapor refrigerant 204 and the liquid refrigerant 206 may then be flowed into the economizer 202. In various embodiments, the economizer 202 is a flash tank that effects the separation of the vapor refrigerant 204 and the liquid refrigerant 206. The vapor refrigerant 204 may be flowed to an intermediate pressure compressor stage, at which the vapor refrigerant 204 may be combined with saturated vapor refrigerant 118 exiting a first compressor 210, creating a mixed saturated vapor refrigerant 208. The mixed saturated vapor refrigerant 208 may then be flowed into a second compressor 212.
  • From the economizer 202, the liquid refrigerant 206 may be isenthalpically expanded across a second expansion valve 214. On expansion, some vaporization may occur, creating a refrigerant mixture 216 that includes both vapor and liquid, lowering the temperature and pressure. The refrigerant mixture 216 may have a higher liquid content than refrigerant mixtures in systems without economizers. The higher liquid content may reduce the refrigerant circulation rate and/or reduce the power usage of the first compressor 210.
  • The refrigerant mixture 216 enters the chiller 104, also known as the evaporator, at a temperature lower than the temperature to which the process stream 116 is to be cooled. The process stream 116 is cooled within the chiller 104, as discussed above with respect to FIG. 1. In addition, the saturated vapor refrigerant 118 is flowed through the compressors 210 and 212 and the condenser 108, and the resulting liquid refrigerant 120 is stored within the accumulator 110, as discussed above with respect to FIG. 1.
  • It is to be understood that the process flow diagram of FIG. 2 is not intended to indicate that the two stage refrigeration system 200 is to include all the components shown in FIG. 2. Further, the two stage refrigeration system 200 may include any number of additional components not shown in FIG. 2, depending on the details of the specific implementation. For example, the two stage refrigeration system 200 may include any number of additional economizers or other types of equipment not shown in FIG. 2. In addition, the economizer 202 may be a heat exchange economizer rather than a flash tank. The heat exchange economizer may also be used to decrease refrigeration circulation rate and reduce compressor power usage.
  • In some embodiments, the two stage refrigeration system 200 includes more than one economizer 202, as well as more than two compressors 210 and 212. For example, the two stage refrigeration system 200 may include two economizers and three compressors. In general, if the refrigeration system 200 includes X number of economizers, the refrigeration system 200 will include X+1 number of compressors. Such a refrigeration system 200 with multiple economizers may form part of a cascade refrigeration system.
  • FIG. 3 is a process flow diagram of a single stage refrigeration system 300 including a heat exchanger economizer 302. Like numbered items are as described with respect to FIG. 1. As shown in FIG. 3, the saturated liquid refrigerant 112 leaving the accumulator 110 may be expanded across the expansion valve 102 to an intermediate pressure at which vapor and liquid may be separated, producing the refrigerant mixture 114. The refrigerant mixture 114 may be flowed into the chiller 104 at a temperature lower than the temperature to which the process stream 116 is to be cooled. The process stream 116 may be cooled within the chiller 104, as discussed above with respect to FIG. 1.
  • From the chiller 104, the saturated vapor refrigerant 118 may be flowed through the heat exchanger economizer 302. The cold, low-pressure saturated vapor refrigerant 118 may be used to subcool the saturated liquid refrigerant 112 within the heat exchanger economizer 302. The superheated vapor refrigerant 304 exiting the heat exchanger economizer 302 may then be flowed through the compressor 106 and the condenser 108, and the resulting liquid refrigerant 120 may be stored within the accumulator 110, as discussed above with respect to FIG. 1.
  • It is to be understood that the process flow diagram of FIG. 3 is not intended to indicate that the single stage refrigeration system 300 is to include all the components shown in FIG. 3. Further, the single stage refrigeration system 300 may include any number of additional components not shown in FIG. 3, depending on the details of the specific implementation.
  • FIG. 4 is a process flow diagram of a cascade cooling system 400 including a first refrigeration system 402 and a second refrigeration system 404. In various embodiments, the first refrigeration system 402 utilizes a refrigerant including a noble gas, such as xenon or krypton, while the second refrigeration system 404 may utilize a different noble gas refrigerant, a fluorocarbon refrigerant, or a hydrocarbon refrigerant. The refrigerants in either refrigeration system 402 or 404 may include mixtures. The cascade cooling system 400 may be used for instances in which a higher degree of cooling than that provided by the refrigeration systems 100, 200, or 300 is desired. The cascade cooling system 400 may provide cooling at very low temperatures, e.g., below −40° C.
  • Within the first refrigeration system 402, a liquid refrigerant stream 406 may be flowed from an accumulator 408 through a first expansion valve 410 and a first heat exchanger 412, which chills a product stream 413. The resulting vapor/liquid stream is separated in a first flash drum 414. A portion of the liquid refrigerant stream 406 may be flowed directly into the first flash drum 414 via a bypass valve 416, which can be used to control the temperature of the liquid in the first flash drum 414, as well as the amount of cooling in the first heat exchanger 412.
  • From the first flash drum 414, a liquid refrigerant stream 418 may be flowed through a second expansion valve 420, and flashed into a second heat exchanger 422, which may be used to further chill the product stream 413. A gas accumulator 424 feeds the resulting vapor refrigerant stream 426 to a first stage compressor 428. The resulting medium pressure vapor refrigerant stream 430 is combined with the vapor refrigerant stream 432 from the first flash drum 414, and the combined stream is fed to a second stage compressor 434. The high pressure vapor stream 436 from the second stage compressor 434 is passed through a condenser 438, which may use cooling from the second refrigeration system 404. Specifically, the condenser 438 may cool the high pressure vapor stream 436 to produce a liquid refrigerant stream 406 using a low temperature refrigerant stream 440 from the second refrigeration system 404. The liquid refrigerant stream 406 from the condenser 438 is then stored in the accumulator 408. A control valve 442 may be used to control the flow of the low temperature refrigerant stream 440 through the condenser 438. From the condenser 438, the resulting vapor refrigerant stream 444 back to the second refrigeration system 404.
  • Within the second refrigeration system 404, a liquid refrigerant stream 448 may be flowed from an accumulator 450 through a heat exchanger 452 that is configured to cool the liquid refrigerant stream 448 via a chilling system 454. The resulting low temperature refrigerant stream 456 may be flowed through a first expansion valve 458 and a first heat exchanger 460, which chills the product stream 413. The resulting vapor/liquid refrigerant stream is separated in a first flash drum 462. A portion of the low temperature refrigerant stream 456 may be flowed directly into the first flash drum 462 via a bypass valve 464, which can be used to control the temperature of the liquid in the first flash drum 462, as well as the amount of cooling in the first heat exchanger 460.
  • From the first flash drum 462, a liquid refrigerant stream 466 may be flowed through a second expansion valve 468, and flashed into a second heat exchanger 470, which may be used to further chill the product stream 413. The resulting vapor/liquid refrigerant stream is separated in a second flash drum 472. A portion of the liquid refrigerant stream 466 may be flowed directly into the second flash drum 472 via a bypass valve 474, which can be used to control the temperature of the liquid in the second flash drum 472, as well as the amount of cooling in the second heat exchanger 470.
  • From the second flash drum 472, a liquid refrigerant stream 476 may be flowed through a third expansion valve 478, and flashed into a third heat exchanger 480, which may be used to further chill the product stream 413. A gas accumulator 482 feeds the resulting vapor refrigerant stream 484 to a first stage compressor 486. The resulting medium pressure vapor refrigerant stream 488 is combined with the vapor refrigerant stream 490 from the second flash drum 472, and the combined stream is fed to a second stage compressor 492. The resulting high pressure vapor refrigerant stream 494 is combined with the vapor refrigerant mixture 496 from the first flash drum 462, and the combined stream is fed to a third stage compressor 497. The resulting high pressure vapor refrigerant stream 498 is flowed through a heat exchanger 499, in which it may be further cooled through indirect heat exchange with cooling water. The resulting liquid refrigerant stream 448 may then be flowed into the accumulator 450.
  • It is to be understood that the process flow diagram of FIG. 4 is not intended to indicate that the cascade cooling system 400 is to include all the components shown in FIG. 4. Further, the cascade cooling system 400 may include any number of additional components not shown in FIG. 4, depending on the details of the specific implementation.
  • FIG. 5 is process flow diagram of an expansion refrigeration system 500 for hydrocarbon dew point control. Condensation of heavy hydrocarbons, e.g., C3-C6, in natural gas within pipes may result in an increase in pressure within the pipes, as well as an increase in the power usage of handling facilities. Therefore, the hydrocarbon dew point may be reduced using the expansion refrigeration system 500 in order to prevent such condensation.
  • As shown in FIG. 5, a dehydrated natural gas feed stream 502 may be flowed into a gas/gas heat exchanger 504. Within the gas/gas heat exchanger 504, the dehydrated natural gas feed stream 502 may be cooled through indirect heat exchange with a low temperature natural gas stream 506. The resulting natural gas stream 508 may be flowed into a first separator 510, which may remove some amount of heavy hydrocarbons 512 from the natural gas stream 508. In various embodiments, removing the heavy hydrocarbons 512 from the natural gas stream 508 decreases the dew point of the natural gas stream 508. The removed heavy hydrocarbons 512 may be flowed out of the expansion refrigeration system 500 through a first outlet valve 514. For example, the heavy hydrocarbons 512 may be flowed from the expansion refrigeration system 500 to a stabilizer (not shown).
  • The natural gas stream 508 may then be flowed into an expander 516. In various embodiments, the expander 516 is a turbo-expander, which is a centrifugal or axial flow turbine. The expansion of the natural gas stream 508 within the expander 516 may provide energy for driving a compressor 518, which is coupled to the expander 516 via a shaft 520.
  • From the expander 516, the resulting low temperature natural gas stream 506 may be flowed into a second separator 522, which may remove any remaining heavy hydrocarbons 512 from the low temperature natural gas stream 506. In various embodiments, removing the heavy hydrocarbons 512 from the low temperature natural gas stream 506 further decreases the dew point of the low temperature natural gas stream 506. The removed heavy hydrocarbons 512 may then be flowed out of the expansion refrigeration system 500 through a second outlet valve 524.
  • The low temperature natural gas stream 506 may be flowed from the second separator 522 to the gas/gas heat exchanger 504, which may increase the temperature of the low temperature natural gas stream 506, producing a high temperature natural gas stream 526. The high temperature natural gas stream 526 may then be flowed through the compressor 518, which may return the pressure of the natural gas stream 526 to acceptable sales gas pressure. The final, decreased dew point natural gas stream 528 may then be flowed out of the expansion refrigeration system 500.
  • In an embodiment, a cooling system, for example, using a noble gas refrigerant may be used to add further cooling to the process. This cooling may be implemented by placing a heat exchanger 530 in the low temperature natural gas stream 506, upstream of the second separator 522. A refrigerant liquid 532 may be flashed across an expansion valve 534, through the chiller 530. The resulting refrigerant vapor 536 can then be returned to the refrigerant system. The chilling may allow for the removal of a much higher amount of condensable hydrocarbons, such as C3s and higher. Further, in some embodiments, the heat exchanger 530 is placed upstream of the expander 516, with a separator located between the heat exchanger 530 and the expander 516 to prevent liquids from flowing into the expander 516.
  • It is to be understood that the process flow diagram of FIG. 5 is not intended to indicate that the expansion refrigeration system 500 is to include all the components shown in FIG. 5. Further, the expansion refrigeration system 500 may include any number of additional components not shown in FIG. 5, depending on the details of the specific implementation.
  • FIG. 6 is a process flow diagram of an expansion refrigeration system 600 for NGL extraction. In various embodiments, NGL extraction may be performed to recover NGLs, which include any number of different heavy hydrocarbons, from a natural gas stream. NGL extraction may be desirable due to the fact that NGLs are often of greater value for purposes other than as a gaseous heating fuel.
  • A dry natural gas feed stream 602 may be flowed into a gas/gas heat exchanger 604 from a dehydration system. Within the gas/gas heat exchanger 604, the dry natural gas feed stream 602 may be cooled through indirect heat exchange with a low temperature natural gas stream 606. The resulting natural gas stream 608 may be flowed into a separator 610, which may remove a portion of NGLs 612 from the natural gas stream 608. The removed NGLs 612 may be flowed from the separator 610 to a deethanizer or demethanizer 614.
  • The natural gas stream 608 may then be flowed into an expander 616. In various embodiments, the expander 616 is a turbo-expander. The expansion of the natural gas stream 608 within the expander 616 may provide energy for driving a compressor 618, which is coupled to the expander 616 via a shaft 620. In addition, the temperature of the natural gas stream 608 may be reduced via adiabatic expansion across a Joule-Thomson valve 622.
  • From the expander 616, the resulting low temperature natural gas stream 606 may be flowed into the deethanizer or demethanizer 614. Within the deethanizer or demethanizer 614, NGLs may be separated from the natural gas stream 606 and may be flowed out of the deethanizer or demethanizer 614 as an NGL product stream 624. The NGL product stream 624 may then be pumped out of the expansion refrigeration system 600 via a pump 626.
  • The deethanizer or demethanizer 614 may be coupled to a heat exchanger 628. In some embodiments, the heat exchanger 628 is a reboiler 628 that may be used to heat a portion of a bottoms stream 630 from the deethanizer or demethanizer 614 via indirect heat exchange within a high temperature fluid 632. The heated bottoms stream 630 may then be reinjected into the deethanizer or demethanizer 614.
  • The separation of the NGL product stream 624 from the natural gas stream 606 within the deethanizer or demethanizer 614 may result in the production of a low temperature natural gas stream that may be flowed out of the deethanizer or demethanizer 614 as an overhead stream 634. The overhead stream 634 may be flowed into a heat exchanger 636, which may decrease the temperature of the overhead stream 634 through indirect heat exchange with a refrigerant mixture 638 including a noble gas. The decrease in temperature can lead to condensation of some of the vapors. The overhead stream 634 may then be separated within a separation vessel 640 to produce the low temperature natural gas stream 606 and a liquid bottoms stream 642. The bottoms stream 642 may be pumped back into the deethanizer or demethanizer 614, via a pump 644, forming a recycle stream.
  • The low temperature natural gas stream 606 may then be flowed through the gas/gas heat exchanger 604. The temperature of the low temperature natural gas stream 506 may be increased within the gas/gas heat exchanger 604, producing a high temperature natural gas stream 646. The high temperature natural gas stream 646 may then be flowed through the compressor 618, which may increase the pressure of the natural gas stream 646. In some embodiments, the high temperature natural gas stream 646 is also flowed through a second compressor 648, which may increase the pressure of the natural gas stream 646 to acceptable sales gas pressure. The natural gas product stream 650 may then be flowed out of the expansion refrigeration system 600.
  • It is to be understood that the process flow diagram of FIG. 6 is not intended to indicate that the expansion refrigeration system 600 is to include all the components shown in FIG. 6. Further, the expansion refrigeration system 600 may include any number of additional components not shown in FIG. 6, depending on the details of the specific implementation.
  • FIG. 7 is a process flow diagram of an LNG production system 700. As shown in FIG. 7, LNG 702 may be produced from a natural gas stream 704 using a number of different refrigeration systems. As shown in FIG. 7, a portion of the natural gas stream 704 may be separated from the natural gas stream 704 prior to entry into the LNG production system 700, and may be used as a fuel gas stream 706. The remaining natural gas stream 704 may be flowed into an initial natural gas processing system 708. Within the natural gas processing system 708, the natural gas stream 704 may be purified and cooled. For example, the natural gas stream 704 may be cooled using noble gas refrigerants, e.g., refrigerant mixtures including one or more noble gases. For example, heavy hydrocarbons 710 may be removed from the natural gas stream 706, and may be used to produce gasoline 712 within a heavy hydrocarbon processing system 714. In addition, any residual natural gas 716 that is separated from the heavy hydrocarbons 710 during the production of the gasoline 712 may be returned to the natural gas stream 704.
  • The natural gas stream 704 may be converted into the LNG 702 within a cryogenic heat exchanger 718. In some embodiments, a mixed refrigerant stream 720 from a mixed refrigeration system 722 is used to cool the natural gas stream 704 within the cryogenic heat exchanger 718. According to embodiments described herein, the mixed refrigerant stream 720 is a refrigerant mixture including one or more noble gases. In other embodiments, a hydrocarbon refrigerant stream (not shown) from a hydrocarbon refrigeration system 724 is used to cool the natural gas stream 704 within the cryogenic heat exchanger 718 to produce the LNG 702.
  • It is to be understood that the process flow diagram of FIG. 7 is not intended to indicate that the LNG production system 700 is to include all the components shown in FIG. 7. Further, the LNG production system 700 may include any number of additional components not shown in FIG. 7, depending on the details of the specific implementation. For example, any number of alternative refrigeration systems may also be used to produce the LNG 702 from the natural gas stream 704. In addition, any number of different refrigeration systems may be used in combination to produce the LNG 702.
  • Cascade Cooling Systems for the Production of Liquefied Natural Gas
  • FIG. 8 is a simplified process flow diagram of a cascade cooling system 800. The cascade cooling system 800 may be used to produce LNG 802 from a raw natural gas 804. The raw natural gas 804 may be flowed into an inlet scrubber 806 within the cascade cooling system 800. The inlet scrubber 806 may remove unwanted particulates from the raw natural gas 804. An inlet meter 808 may monitor the amount and characteristics of the natural gas as it enters the cascade cooling system 800. The natural gas may be passed through an amine treater 810, which can remove hydrogen sulfide, carbon dioxide, and other unwanted gases from the natural gas, and may be chilled within a heat exchanger 812 via indirect heat exchange with propane or any other suitable coolant.
  • The natural gas may be flowed through a first dehydrator 814, which may remove water 816 from the natural gas via a gravity separation process. The removed water 816 may be output from the cascade cooling system 800. The natural gas may then be flowed to a second dehydrator 818, which may remove any remaining water from the natural gas. The second dehydrator 818 may be, for example, a molecular sieve bed or a zeolite bed.
  • A mercury removal system 820, which may include a molecular sieve bed, may remove mercury from the natural gas. In addition, a dry gas filter 822, such as a pleated paper filter, may remove any residual particulates from the natural gas.
  • From the dry gas filter 822, purified natural gas 823 may be sent to a first cold box 824 within a refrigeration system 826. In this example, the first cold box 824 may function as both a heat exchanger and a flash drum. However, in other implementations, a separate flash drum, such as the economizer 202 discussed with respect to FIG. 2, may be used. Thus, the first cold box 824 may cool the natural gas via indirect heat exchange with a first refrigerant mixture 828. The first refrigerant mixture 828 may be a conventional refrigerant, such as a HFC or propane. In addition, the first cold box 824 may act as a vapor-liquid separator, separating the first refrigerant mixture into a vapor refrigerant mixture 830 and a liquid refrigerant mixture. The vapor refrigerant mixture 830 may be generated via flash evaporation of the first refrigerant mixture 828 across an expansion valve 832. The expansion valve 832 may throttle the first refrigerant mixture 828 to decrease the pressure and temperature of the first refrigerant mixture 828, resulting in the flash evaporation of the first refrigerant mixture 828. In some embodiments, the first refrigerant mixture 830 may be entirely vaporized and, thus, no liquid refrigerant mixture may be present within the first cold box 824.
  • The first refrigerant mixture 828 may be continuously recirculated and reused within the refrigeration system 826. For example, after the first refrigerant mixture 828 passes through the first cold box 824, the resulting vapor refrigerant mixture 830 is compressed within a high pressure compressor 834 that can be powered by a first gas turbine 836. The high pressure compressor 834 may be powered by a single gas turbine, for example, by being placed on a common or coupled shaft, or may be powered by electric motors. The vapor refrigerant mixture 830 is then condensed into the liquid refrigerant mixture 828 within a first condenser 838. The liquid refrigerant mixture 828 may then be stored within a surge tank 840, from which it may be flowed back into the first cold box 824 to close the cooling cycle.
  • A second refrigerant mixture 842 can also be used to further cool the purified natural gas 823 within a second cold box 844. In this example, the second cold box 834 further cools the purified natural gas 823 via indirect heat exchange with the second refrigerant mixture 842, which includes at least one noble gas. In addition, the second cold box 844 may act as a vapor-liquid separator, separating the second refrigerant mixture 842 into a vapor refrigerant mixture 846 and a liquid refrigerant mixture. The vapor refrigerant mixture 846 may be generated via flash evaporation of the second refrigerant mixture 842 across an expansion valve 848. The expansion valve 848 may throttle the second refrigerant mixture 842 to decrease the pressure and temperature of the second refrigerant mixture 842, resulting in the flash evaporation of the second refrigerant mixture 842. In some embodiments, the second refrigerant mixture 842 may be entirely vaporized and, thus, no liquid refrigerant mixture may be present within the second cold box 844.
  • The resulting vapor refrigerant mixture 846 exiting the second cold box 844 may be compressed within a low pressure compressor 850 that is powered by a second gas turbine 852, producing a compressed refrigerant mixture 854. The low pressure compressor 850 may be powered by a single gas turbine, for example, by being placed on a common or coupled shaft, or may be powered by electric motors. The compressed refrigerant mixture 854 may then be condensed within a sub-ambient condenser 856, such as an ammonia chiller, to produce the second refrigerant mixture 842. The second refrigerant mixture 842 may be stored within a surge tank 858, from which it may be flowed back into the second cold box 844 to close the cooling cycle.
  • After the natural gas 823 has been cooled within the cold boxes 824 and 844, the natural gas 823 may be further cooled and liquefied within an autorefrigeration system 860, producing the LNG 802. In some embodiments, the autorefrigeration system 860 includes a series of expansion valves (not shown) and flash drums (not shown) that progressively lower the temperature and pressure of the natural gas until it reaches a liquid state at, or near, atmospheric pressure. In addition, prior to being flowed into the autorefrigeration system 860, the natural gas 823 may be flowed through a high pressure nitrogen rejection unit (NRU) (not shown). The NRU may remove some portion of the nitrogen from the natural gas 823 and, thus, may allow for the use of a gas containing a high percentage of nitrogen.
  • The autorefrigeration system 860 may also produce natural gas vapor, which may be used as fuel 862. The fuel 862 may be compressed within a compressor 864 that is powered by a third gas turbine 866 before being flowed out of the cascade cooling system 800. Depending on demand for fuel 862, a large portion of the natural gas vapor may be recombined with the initial purified natural gas 823, and returned to the system for further processing.
  • The produced LNG 802 may be stored within an LNG tank 868 prior to being sent out of the cascade cooling system 800. Gases may be vented out of the LNG tank 868 and pumped back into the autorefrigeration system 860 via a first pump 870. In addition, gas 872 that is separated from the LNG 802 during loading of the LNG 802 at a loading facility, for example, may be pumped back into the autorefrigeration system 860 via a second pump 874.
  • It is to be understood that the process flow diagram of FIG. 8 is not intended to indicate that the cascade cooling system 800 is to include all the components shown in FIG. 8. Further, the cascade cooling system 800 may include any number of additional components not shown in FIG. 8, depending on the details of the specific implementation.
  • FIGS. 9A-C are a more detailed process flow diagram of a cascade cooling system 900. The cascade cooling system 900 may be a cascade, open-loop liquefaction system for the production of LNG. The cascade cooling system 900 may operate at low temperatures, e.g., below about 0° F., or below about −20° F., or below about −40° F. In addition, the cascade cooling system 900 may employ more than one refrigerant and provide refrigeration at multiple temperatures.
  • The cascade cooling system 900 may include a first refrigeration system 902, as shown in FIG. 9A, which may utilize a non-hydrocarbon refrigerant such as a hydrofluorocarbon, e.g., R-404A or R-410a. The cascade cooling system 900 may also include a second refrigeration system 904, as shown in FIG. 9B, which may utilize a refrigerant mixture including at least one noble gas, such as xenon, krypton, argon, or combinations thereof
  • FIG. 10 is a more detailed process flow diagram of an autorefrigeration system 1000. The autorefrigeration system 1000 may be located downstream of the cascade cooling system 900, as discussed further below.
  • A natural gas stream 908 may be flowed into a pipe joint 910 within the cascade cooling system 900. The pipe joint 910 may be configured to split the natural gas stream 908 into two separate natural gas streams. One natural gas stream 914 may be flowed into another pipe joint 912, while the other natural gas stream 916 may be flowed into the autorefrigeration system 1000.
  • Within the pipe joint 912, the natural gas stream 914 may be combined with a natural gas vapor stream 1066 from the autorefrigeration system 1000. The resulting natural gas stream 918 may then be flowed into the first refrigeration system 902 in preparation for cooling of the natural gas stream 918. The natural gas stream 918 may be cooled by being passed through a series of heat exchangers 920, 922, 924, and 926 within the first refrigeration system 902. The heat exchangers 920, 922, 924, and 926 may also be referred to as evaporators, chillers, or cold boxes. The natural gas stream 918 may be cooled within each of the heat exchangers 920, 922, 924, and 926 through indirect heat exchange with a circulating non-hydrocarbon refrigerant. The non-hydrocarbon refrigerant may be a hydrofluorocarbon, such as R-404A or R-410A, or any other suitable type of non-hydrocarbon refrigerant.
  • The non-hydrocarbon refrigerant may be continuously circulated through the first refrigeration system 902, which may continuously prepare the non-hydrocarbon refrigerant for entry into each of the heat exchangers 920, 922, 924, and 926. The non-hydrocarbon refrigerant may exit the first heat exchanger 920 via line 928 as a vapor non-hydrocarbon refrigerant. The vapor non-hydrocarbon refrigerant can be combined with additional vapor non-hydrocarbon refrigerant within a pipe joint 930. The vapor non-hydrocarbon refrigerant is then flowed through a compressor 932 to increase the pressure of the vapor non-hydrocarbon refrigerant, producing a superheated vapor non-hydrocarbon refrigerant. The superheated vapor non-hydrocarbon refrigerant is flowed through a condenser 934, which may cool and condense the superheated vapor non-hydrocarbon refrigerant, producing a liquid non-hydrocarbon refrigerant.
  • The liquid non-hydrocarbon refrigerant may be flowed through an expansion valve 935, which lowers the temperature and pressure of the liquid non-hydrocarbon refrigerant. This may result in the flash evaporation of the liquid non-hydrocarbon refrigerant, producing a mixture of the liquid non-hydrocarbon refrigerant and a vapor non-hydrocarbon refrigerant. The liquid non-hydrocarbon refrigerant and the vapor non-hydrocarbon refrigerant may be flowed into a first flash drum 936 via line 938. Within the first flash drum 936, the liquid non-hydrocarbon refrigerant may be separated from the vapor non-hydrocarbon refrigerant.
  • The vapor non-hydrocarbon refrigerant may be flowed from the first flash drum 936 to the pipe joint 930 via line 940. The liquid non-hydrocarbon refrigerant may be flowed into a pipe joint 942, which may split the liquid non-hydrocarbon refrigerant into two separate liquid non-hydrocarbon refrigerant streams. One liquid non-hydrocarbon refrigerant stream may be flowed through the first heat exchanger 920, partly or completely flashed to vapor, and returned to the pipe joint 930 via line 928. The other liquid non-hydrocarbon refrigerant stream may be flowed to a second flash drum 944 via line 946. The line 946 may also include an expansion valve 948 that throttles the liquid non-hydrocarbon refrigerant stream to control the flow of the liquid non-hydrocarbon refrigerant stream into the second flash drum 944. The throttling of the liquid non-hydrocarbon refrigerant stream within the expansion valve 948 may result in the flash evaporation of the liquid non-hydrocarbon refrigerant stream, producing a mixture of both vapor and liquid non-hydrocarbon refrigerant.
  • The second flash drum 944 may separate the vapor non-hydrocarbon refrigerant from the liquid non-hydrocarbon refrigerant. The vapor non-hydrocarbon refrigerant may be flowed into a pipe joint 950 via line 952. The pipe joint 950 may combine the vapor non-hydrocarbon refrigerant with vapor non-hydrocarbon refrigerant recovered from the second and third heat exchangers 922 and 924. The combined vapor non-hydrocarbon refrigerant may be compressed within a compressor 954 and flowed into the pipe joint 930 via line 956 to be combined with the vapor from flash drum 936 and heat exchanger 920.
  • The liquid non-hydrocarbon refrigerant may be flowed from the second flash drum 944 to a pipe joint 958, which may split the liquid non-hydrocarbon refrigerant into two separate liquid non-hydrocarbon refrigerant streams. One liquid non-hydrocarbon refrigerant stream is flowed through the second heat exchanger 922 and returned to the pipe joint 950 via line 960. The other liquid non-hydrocarbon refrigerant stream is flowed to a third flash drum 962 via line 964. The line 964 also includes an expansion valve 966 that controls the flow of the liquid non-hydrocarbon refrigerant stream into the third flash drum 962. The expansion valve 966 may result in the flash evaporation of the liquid non-hydrocarbon refrigerant stream, producing a mixture of both vapor and liquid non-hydrocarbon refrigerant. Flashing across the valve will reduce the temperature and pressure of the liquid non-hydrocarbon refrigerant stream.
  • The mixture of the vapor and liquid non-hydrocarbon refrigerant may be flashed into the third flash drum 962, further reducing the temperature and pressure. The third flash drum 962 may separate the vapor non-hydrocarbon refrigerant from the liquid non-hydrocarbon refrigerant. The vapor non-hydrocarbon refrigerant may be flowed into a pipe joint 968 via line 970. The pipe joint 968 may combine the vapor non-hydrocarbon refrigerant with vapor non-hydrocarbon refrigerant recovered from the third and fourth heat exchangers 924 and 926. The combined vapor non-hydrocarbon refrigerant may be compressed within a compressor 972 and flowed into the pipe joint 950 via line 974.
  • The liquid non-hydrocarbon refrigerant may be flowed from the third flash drum 962 to a pipe joint 976, which may split the liquid non-hydrocarbon refrigerant into two separate liquid non-hydrocarbon refrigerant streams. One liquid non-hydrocarbon refrigerant stream may be flowed through the third heat exchanger 924 and returned to the pipe joint 968 via line 978. The other liquid non-hydrocarbon refrigerant stream may be flowed through the fourth heat exchanger 926 via line 980. The line 980 may also include an expansion valve 982 that allows the liquid non-hydrocarbon refrigerant to flash, and, thus, lowers the pressure and temperature, of the liquid non-hydrocarbon refrigerant stream as it flows into the fourth heat exchanger 926. From the fourth heat exchanger 926, the liquid non-hydrocarbon refrigerant stream may be compressed within a compressor 984 and sent to the pipe joint 968 via line 986.
  • In one embodiment, a refrigerant mixture including a noble gas is precooled by being flowed through each of the heat exchangers 920, 922, 924, and 926. The refrigerant mixture may be flowed from the second refrigeration system 904 to the heat exchangers 920, 922, 924, and 926 within the first refrigeration system 902 via line 988, as discussed further below.
  • After the natural gas stream has been progressively chilled within each of the heat exchangers 920, 922, 924, and 926, it is flowed into the second refrigeration system 904, shown in FIG. 9B, via line 990. The second refrigeration system 904 may include a fifth heat exchanger 992 and a sixth heat exchanger 994, which may be used to further cool the natural gas stream. The fifth heat exchanger 992 and the sixth heat exchanger 994 may utilize a refrigerant mixture including one or more noble gases, such as xenon or krypton, to cool the natural gas stream.
  • The refrigerant mixture may be continuously circulated through the second refrigeration system 904, which prepares the refrigerant mixture for entry into each of the heat exchangers 992 and 994. The refrigerant mixture may exit the fifth heat exchanger 992 via line 996 as a vapor refrigerant mixture. The vapor refrigerant mixture may be combined with additional vapor refrigerant mixture within a pipe joint 998. The vapor refrigerant mixture may then be flowed through a compressor 1000, which may increase the pressure of the vapor refrigerant mixture, producing a superheated vapor refrigerant mixture. The superheated vapor refrigerant mixture may be flowed through a gas cooler 1002, which may cool the superheated vapor refrigerant mixture, producing a liquid refrigerant mixture. In some cases, if the vapor refrigerant mixture is below ambient temperature, the vapor refrigerant mixture may not be flowed through the gas cooler 1002. The liquid refrigerant mixture may then be flowed through the heat exchangers 920, 922, 924, and 926 within the first refrigeration system 902 via line 988, as discussed above.
  • Once the refrigerant mixture has passed through the heat exchangers 920, 922, 924, and 926, the refrigerant mixture may enter a fourth flash drum 1004 within the second refrigeration system 904 via line 1006. Line 1006 may include an expansion valve 1008 that controls the flow of the refrigerant mixture into the fourth flash drum 1004. The expansion valve 1008 may reduce the temperature and pressure of the refrigerant mixture, resulting in the flash evaporation of the refrigerant mixture into both a vapor refrigerant mixture and a liquid refrigerant mixture.
  • The vapor refrigerant mixture and the liquid refrigerant mixture may be flashed into the fourth flash drum 1004, which may separate the vapor refrigerant mixture from the liquid refrigerant mixture. The vapor refrigerant mixture may be flowed into the pipe joint 998 via line 1010. The liquid refrigerant mixture may be flowed from the fourth flash drum 1004 to a pipe joint 1012, which may split the liquid refrigerant mixture into two separate liquid refrigerant mixture streams. One liquid refrigerant mixture stream may be flowed through the fifth heat exchanger 992 and returned to the pipe joint 998 via line 996. The other liquid refrigerant mixture stream may be flowed through the sixth heat exchanger 994 via line 1014. The line 1014 may also include an expansion valve 1016 that controls the flow of the liquid refrigerant mixture stream into the sixth heat exchanger 994, e.g., by allowing the refrigerant mixture to flash, lowering the temperature and creating a vapor refrigerant mixture and a liquid refrigerant mixture. From the sixth heat exchanger 994, the resulting vapor refrigerant mixture may be compressed within a compressor 1018 and then flowed into the pipe joint 998 to be recirculated.
  • After the natural gas stream has been cooled within the heat exchangers 992 and 994 through indirect heat exchange with the refrigerant mixture including one or more noble gases, the natural gas stream may be flowed into the autorefrigeration system 1000, shown in FIG. 10, via line 1020. The autorefrigeration system 1000 may include various components that are used to liquefy the natural gas, producing LNG.
  • The natural gas stream may be flowed into a pipe joint 1022, which may combine the natural gas stream from line 1020 with a portion of the natural gas stream 916. Initial cooling of the natural gas may be performed within a heat exchanger 1024 prior to flowing the natural gas into the pipe joint 1022 via line 1026.
  • From the pipe joint 1022, the natural gas may be flowed into a reboiler 1028, which may decrease the temperature of the natural gas. The cooled natural gas may be expanded within a hydraulic expansion turbine 1030 and then flowed into a NRU system 1032 via line 1034 to remove excess nitrogen from the natural gas. In various embodiments, the natural gas is flowed into a cryogenic fractionation column 1036, such as a NRU tower, within the NRU system 1032. In addition, heat may be transferred to the cryogenic fractionation column 1036 from the reboiler 1028 via line 1037.
  • The cryogenic fractionation column 1036 may separate nitrogen from the natural gas via a cryogenic distillation process. An overhead stream may be flowed out of the cryogenic fractionation column 1036 via line 1038. The overhead stream may include primarily methane and low boiling point or non-condensable gases, such as nitrogen and helium, which have been separated from the natural gas. The overhead stream may be flowed into an overhead condenser 1040, which may separate any liquid within the overhead stream and return it to the cryogenic fractionation column 1036 as reflux. This may result in the production of one vapor stream, a fuel stream including primarily methane and another vapor stream including primarily low boiling point gases. The fuel stream may be flowed through the heat exchanger 1024 via line 1042. Within the heat exchanger 1024, the temperature of the vapor fuel stream may be increased via indirect heat exchange with the natural gas stream 916, producing a vapor fuel stream. The vapor fuel stream may then be compressed within a compressor 1044 and flowed out of the cascade cooling system 900 as fuel 1046 via line 1048. A liquid stream from the overhead condenser 1040 can be returned to the cryogenic fractionation column 1036 as a reflux stream.
  • The bottoms stream that is produced within the cryogenic fractionation column 1036 includes primarily natural gas with traces of nitrogen. The bottoms stream, as well as the vapor stream from the overhead condenser 1040, may be flowed into a fifth flash drum 1049 via lines 1050 and 1052, respectively. Line 1050 may also include an expansion valve 1054 that controls the flow of the bottoms stream into the fifth flash drum 1049, allowing a portion of the liquid from the bottoms stream to flash, creating a mixed phase stream that is flowed into the fifth flash drum 1049.
  • In addition, some portion of the bottoms stream may be flowed through the overhead condenser 1040 via line 1055. Line 1055 may also include an expansion valve 1056 that controls the flow of the bottoms stream into the overhead condenser 1040. The bottoms stream may be used as refrigerant for the overhead condenser 1040. The resulting vapor exiting the overhead condenser 1040 may be returned to the fifth flash drum 1049 via the line 1052.
  • The fifth flash drum 1049 may separate the mixed phase stream into a vapor stream that includes primarily natural gas and an LNG stream. The vapor stream may be flowed into a pipe joint 1058 via line 1060. The pipe joint 1058 may combine the vapor stream with another vapor stream recovered from a sixth flash drum 1062. The combined vapor streams may be compressed within a compressor 1064 and flowed into the pipe joint 912 within the first refrigeration system 902 via line 1066.
  • The LNG stream may be flowed into the sixth flash drum 1062 via line 1068. The line 1068 may include an expansion valve 1070 that controls the flow of the LNG stream into the sixth flash drum 1062, allowing a portion of the liquid from the LNG stream to flash, creating a mixed phase system that is flowed into the sixth flash drum 1062.
  • The sixth flash drum 1062 may separate the mixed phase stream into LNG and a vapor stream that includes natural gas. The vapor stream may be flowed into a pipe joint 1072 via line 1074. The pipe joint 1072 may combine the vapor stream with another vapor stream recovered from a seventh flash drum 1076. The combined vapor streams may be compressed within a compressor 1078 and flowed into the pipe joint 1058.
  • The LNG stream may then be flowed into the seventh flash drum 1076 via line 1080. The line 1080 may include an expansion valve 1082 that controls the flow of the LNG stream into the seventh flash drum 1076, allowing a portion of the liquid from the LNG to flash. The seventh flash drum 1076 may further reduce the temperature and pressure of the LNG stream such that the LNG stream approaches an equilibrium temperature and pressure, as discussed below with respect to FIG. 11. The produced vapor stream may be flowed into a pipe joint 1084, which may combine the vapor stream with boil-off gas recovered from an LNG tank 1086. The combined vapor streams may be compressed within a compressor 1088 and flowed into the pipe joint 1072.
  • The LNG tank 1086 may store the LNG stream for any period of time. Boil-off gas generated within the LNG tank 1086 may be flowed to the pipe joint 1084 via line 1090. At any point in time, the LNG stream may be transported to an LNG tanker 1092 using a pump 1094, for transport to markets. The additional boil-off gas 1098 generated while loading LNG stream into the LNG tanker 1092, may be recovered in the cascade cooling system 900 by adding it to the pipe joint 1084.
  • It is to be understood that the process flow diagrams of FIGS. 9A, 9B, and 10 are not intended to indicate that the cascade cooling system 900 and the autorefrigeration system 1000 are to include all the components shown in FIGS. 9A, 9B, and 10. Further, the cascade cooling system 900 and/or the autorefrigeration system 1000 may include any number of additional components not shown in FIGS. 9A, 9B, and 10, depending on the details of the specific implementation. For example, in some embodiments, the cascade cooling system 900 includes one or more refrigeration systems that utilize a single mixed refrigerant including at least one noble gas. However, the cascade cooling system 900 and/or the autorefrigeration system 1000 may also include any other types or combinations of refrigeration systems.
  • FIG. 11 is a schematic of a methane pressure-enthalpy (P-H) diagram 1100. The P-H diagram 1100 shows corresponding pressures 1102 and enthalpies 1104 at various temperatures. Like numbered items are as described with respect to FIG. 9. The P-H diagram 1100 includes an equilibrium curve 1106. A left side 1108 of the equilibrium curve 1106 represents a pure liquid, while a right side of the equilibrium curve 1106 represents a pure gas 1110. In addition, if the pressure 1102 and enthalpy 1104 of the methane is within the equilibrium curve 1106, the methane exists as an equilibrium mixture of liquid and gas. If the pressure 1102 and enthalpy 1104 of methane is above the equilibrium curve 1106, the methane is in a critical state.
  • According to the autorefrigeration process described herein, it is desirable to reduce the temperature and pressure 1102 of methane such that the methane exists as a liquid near atmospheric pressure. Each flash evaporation process within the expansion valves 1056, 1070, and 1080 and the flash drums 1049, 1062, and 1076 isenthalpically reduces the temperature and the pressure of the methane. For example, prior to expansion across the hydraulic expansion turbine 1030, the methane may be in a critical state 1112. In many cases, it is difficult to reach such a critical state with typical hydrocarbon refrigerants such as methane. Therefore, xenon may be used for the autorefrigeration process instead of methane in some cases.
  • The hydraulic expansion turbine 1030 may isentropically reduce the temperature and the pressure 1102 of the methane to a first equilibrium state 1114. A NRU may operate at the first equilibrium state 1114 or at a slightly higher pressure. The first equilibrium state 1114 may include a large liquid proportion 1116 and a small gas proportion 1118. The gas may be vented out of the fifth flash drum 1049 such that the methane is in a first pure liquid state 1120. However, the first pure liquid state 1120 may be at a pressure 1102 that is substantially higher than atmospheric pressure. Thus, the methane may be flowed through the expansion valve 1070 and into the sixth flash drum 1062.
  • The expansion valve 1070 may isenthalpically reduce the temperature and the pressure 1102 of the methane to a second equilibrium state 1122. Similarly to the first equilibrium state 1118, the second equilibrium state 1122 may include a large liquid proportion and a small gas proportion. The gas may be vented out of the sixth flash drum 1062 such that the methane is in a second pure liquid state 1124. However, the second pure liquid state 1124 may still be at a pressure 1102 that is substantially higher than atmospheric pressure. Therefore, the methane may be flowed through the expansion valve 1080 and into the seventh flash drum 1076.
  • The expansion valve 1082 may isenthalpically reduce the temperature and the pressure 1102 of the methane to a third equilibrium state 1126. The third equilibrium state 1126 may include a large liquid proportion and a small gas proportion. The gas may be vented out of the seventh flash drum 1076 such that the methane is in a third pure liquid state 1128. In various embodiments, the pressure 1102 of the third pure liquid state 1128 may be near atmospheric pressure. Therefore, the methane may be in the final product form, and may be exported as LNG.
  • Method for LNG Formation
  • FIG. 12 is a process flow diagram of a method 1200 for the formation of LNG. In various embodiments, the method 1200 is implemented within any of the systems 800, 900, or 1000 described above with respect to FIG. 8, 9, or 10, respectively.
  • The method 1200 begins at block 1202, at which the natural gas is chilled in a refrigeration system. The refrigeration system may be a mechanical refrigeration system, valve expansion system, turbine expansion system, or the like. The refrigeration system uses a refrigerant mixture including a noble gas. The noble gas may include xenon, krypton, argon, or any combinations thereof. In addition, the refrigerant mixture may include nitrogen or a hydrocarbon, such as methane, ethane, propane, or butane. According to embodiments described herein, the refrigerant mixture including the noble gas is used in any number of cooling stages to achieve deeper cooling than provided by hydrocarbon refrigerants.
  • In various embodiments, the refrigerant mixture is compressed to provide a compressed refrigerant mixture, and the compressed refrigerant mixture is cooled by indirect heat exchange with a cooling fluid. The compressed refrigerant mixture may be expanded to cool the compressed refrigerant mixture, thereby producing an expanded, cooled refrigerant mixture. The expanded, cooled refrigerant mixture may be passed to a heat exchange area, which may include, for example, a chiller or evaporator. In addition, the natural gas may be compressed and cooled by indirect heat exchange with an external cooling fluid. The natural gas may then be chilled within the heat exchange area using the expanded, cooled refrigerant mixture.
  • The natural gas may be chilled via one or more pre-cooling steps using a first refrigerant mixture. The first refrigerant mixture may include a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof. The natural gas may also be chilled via one or more deep cooling steps using a second refrigerant mixture. The second refrigerant mixture may include a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof.
  • At block 1204, the natural gas is liquefied to form LNG in an autorefrigeration system. In various embodiments, the autorefrigeration system includes a number of expansion valves and flash drums that are used to cool and liquefy the natural gas. The natural gas may be flashed across an expansion valve, lowering the pressure and temperature of the natural gas and producing a vapor fraction and a liquid fraction. The vapor fraction and the liquid fraction may be flashed into a flash drum, which may separate the vapor fraction from the liquid fraction. This process may be repeated within any number of expansion valves and flash drums until a suitable amount of the natural gas has been converted to LNG.
  • It is to be understood that the process flow diagram of FIG. 12 is not intended to indicate that the steps of the method 1200 are to be executed in any particular order, or that all of the steps are to be included in every case. Further, any number of additional steps may be included within the method 1200, depending on the details of the specific implementation. For example, the natural gas may be cooled in a first refrigeration system prior to chilling the natural gas in the refrigeration system. In various embodiments, the first refrigeration system uses a non-hydrocarbon refrigerant.
  • Embodiments
  • Embodiments of the invention may include any combinations of the methods and systems shown in the following numbered paragraphs. This is not to be considered a complete listing of all possible embodiments, as any number of variations can be envisioned from the description above.
    • 1. A system for formation of a liquefied natural gas (LNG), including:
      • a refrigeration system configured to chill a natural gas using a refrigerant mixture including a noble gas; and
      • an autorefrigeration system configured to use the natural gas as a self-refrigerant to form the LNG from the natural gas.
    • 2. The system paragraph 1, including a first refrigeration system configured to cool the natural gas using a non-hydrocarbon refrigerant prior to flowing the natural gas into the refrigeration system.
    • 3. The system of any of paragraphs 1 or 2, including a nitrogen rejection unit upstream of the autorefrigeration system.
    • 4. The system of any of paragraphs 1-3, wherein the system is configured to chill the natural gas for hydrocarbon dew point control.
    • 5. The system of any of paragraphs 1-4, wherein the system is configured to chill the natural gas for natural gas liquid (NGL) extraction.
    • 6. The system of any of paragraphs 1-5, wherein the system is configured to separate methane and lighter gases from carbon dioxide and heavier gases.
    • 7. The system of any of paragraphs 1-6, wherein the system is configured to prepare hydrocarbons for liquefied petroleum gas (LPG) production storage.
    • 8. The system of any of paragraphs 1-7, wherein the system is configured to condense a reflux stream.
    • 9. The system of any of paragraphs 1-8, wherein the refrigerant mixture includes xenon or krypton, or any combination thereof
    • 10. The system of any of paragraphs 1-9, wherein the refrigerant mixture includes xenon, krypton, argon, or nitrogen, or any combinations thereof
    • 11. The system of any of paragraphs 1-10, wherein the refrigeration system includes a mechanical refrigeration system, valve expansion system, or turbine expansion system, or any combinations thereof
    • 12. The system of any of paragraphs 1-11, wherein the refrigerant mixture includes a hydrocarbon, and wherein the hydrocarbon includes methane, ethane, propane, or butane, or any combinations thereof
    • 13. The system of any of paragraphs 1-12, wherein the refrigeration system includes multiple cooling cycles.
    • 14. The system of any of paragraphs 1-13, wherein the refrigeration system includes multiple cooling cycles, including:
      • one or more pre-cooling stages, wherein the refrigerant mixture includes a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof, and
      • one or more deep cooling cycles, wherein the refrigerant mixture includes a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof
    • 15. The system of any of paragraphs 1-14, wherein the refrigerant mixture including the noble gas is utilized in one or more cooling stages to achieve deeper cooling than provided by hydrocarbon refrigerants.
    • 16. The system of any of paragraphs 1-15, including a nitrogen rejection unit, wherein a liquid feed from the bottom of the nitrogen rejection unit is used to provide cooling to a reflux condenser at the top of the nitrogen rejection unit.
    • 17. The system of any of paragraphs 1-16, wherein the refrigerant mixture comprises a pure component refrigerant.
    • 18. A method for formation of a liquefied natural gas (LNG), including:
      • chilling a natural gas in a refrigeration system, wherein the refrigeration system uses a refrigerant mixture including a noble gas; and
      • liquefying the natural gas to form the LNG in an autorefrigeration system.
    • 19. The method of paragraph 18, including cooling the natural gas in a first refrigeration system prior to chilling the natural gas in the refrigeration system, wherein the first refrigeration system uses a non-hydrocarbon refrigerant.
    • 20. The method of any of paragraphs 18 or 19, wherein chilling the natural gas in the refrigeration system includes:
      • compressing the refrigerant mixture to provide a compressed refrigerant mixture;
      • optionally cooling the compressed refrigerant mixture by indirect heat exchange with a cooling fluid;
      • expanding the compressed refrigerant mixture to cool the compressed refrigerant mixture, thereby producing an expanded, cooled refrigerant mixture;
      • passing said expanded, cooled refrigerant mixture to a first heat exchange area;
      • optionally compressing the natural gas;
      • optionally cooling said the natural gas by indirect heat exchange with an external cooling fluid; and
      • heat exchanging the natural gas with the expanded, cooled refrigerant mixture.
    • 21. The method of any of paragraphs 18-20, wherein the noble gas includes xenon or krypton.
    • 22. The method of any of paragraphs 18-21, wherein the refrigerant mixture includes nitrogen or a hydrocarbon, or any combination thereof
    • 23. The method of any of paragraphs 18-22, including liquefying the natural gas to form the LNG via a number of expansion valves or hydraulic expansion turbines and flash drums.
    • 24. The method of any of paragraphs 18-23, including:
      • chilling the natural gas via one or more pre-cooling steps using a first refrigerant mixture, wherein the first refrigerant mixture includes a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof, and
      • chilling the natural gas via one or more deep cooling steps using a second refrigerant mixture, wherein the second refrigerant mixture includes a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof
    • 25. The method of any of paragraphs 18-24, including using the refrigerant mixture including the noble gas in one or more cooling stages to achieve deeper cooling than provided by hydrocarbon refrigerants.
    • 26. A cascade cooling system for formation of a liquefied natural gas (LNG), including:
      • a first refrigeration system configured to cool the natural gas using a non-hydrocarbon refrigerant, wherein the first refrigeration system includes a number of first chillers configured to allow for cooling of the natural gas via an indirect exchange of heat between the natural gas and the non-hydrocarbon refrigerant;
      • a second refrigeration system configured to chill the natural gas using a refrigerant mixture including a noble gas, wherein the second refrigeration system includes a number of second chillers configured to allow for cooling of the natural gas via an indirect exchange of heat between the natural gas and the refrigerant mixture; and
      • an autorefrigeration system configured to form the LNG from the natural gas, wherein the autorefrigeration system includes a number of expansion valves or hydraulic expansion turbines, or any combination thereof, and flash drums.
    • 27. The cascade cooling system of paragraph 26, wherein the first refrigeration system includes a compressor that is configured to compress the non-hydrocarbon refrigerant and a condenser that is configured to cool the non-hydrocarbon refrigerant.
    • 28. The cascade cooling system of any of paragraphs 26 or 27, wherein the second refrigeration system includes a compressor that is configured to compress the refrigerant mixture and a condenser that is configured to cool the refrigerant mixture.
    • 29. The cascade cooling system of any of paragraphs 26-28, wherein the number of first chillers include evaporators configured to cool the natural gas by at least partially vaporizing the non-hydrocarbon refrigerant via a transfer of heat from the natural gas to the non-hydrocarbon refrigerant.
    • 30. The cascade cooling system of any of paragraphs 26-29, wherein the number of second chillers include evaporators configured to chill the natural gas by vaporizing the refrigerant mixture via a transfer of heat from the natural gas to the refrigerant mixture.
    • 31. The cascade cooling system of any of paragraphs 26-30, wherein the LNG includes a liquid fraction and a residual vapor fraction, and wherein the cascade cooling system includes a liquid separation vessel configured to separate the residual vapor fraction from the liquid fraction.
    • 32. The cascade cooling system of any of paragraphs 26-31, including a nitrogen rejection unit upstream of the autorefrigeration system.
    • 33. The cascade cooling system of any of paragraphs 26-32, wherein the refrigerant mixture comprises a pure component refrigerant.
  • While the present techniques may be susceptible to various modifications and alternative forms, the embodiments discussed above have been shown only by way of example. However, it should again be understood that the techniques is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

Claims (33)

What is claimed is:
1. A system for formation of a liquefied natural gas (LNG), comprising:
a refrigeration system configured to chill a natural gas using a refrigerant mixture comprising a noble gas; and
an autorefrigeration system configured to use the natural gas as a self-refrigerant to form the LNG from the natural gas.
2. The system of claim 1, comprising a first refrigeration system configured to cool the natural gas using a non-hydrocarbon refrigerant prior to flowing the natural gas into the refrigeration system.
3. The system of claim 1, comprising a nitrogen recovery unit upstream of the autorefrigeration system.
4. The system of claim 1, wherein the system is configured to chill the natural gas for hydrocarbon dew point control.
5. The system of claim 1, wherein the system is configured to chill the natural gas for natural gas liquid (NGL) extraction.
6. The system of claim 1, wherein the system is configured to separate methane and lighter gases from carbon dioxide and heavier gases.
7. The system of claim 1, wherein the system is configured to prepare hydrocarbons for liquefied petroleum gas (LPG) production storage.
8. The system of claim 1, wherein the system is configured to condense a reflux stream.
9. The system of claim 1, wherein the refrigerant mixture comprises xenon or krypton, or any combination thereof.
10. The system of claim 1, wherein the refrigerant mixture comprises xenon, krypton, argon, or nitrogen, or any combinations thereof.
11. The system of claim 1, wherein the refrigeration system comprises a mechanical refrigeration system, valve expansion system, or turbine expansion system, or any combinations thereof.
12. The system of claim 1, wherein the refrigerant mixture comprises a hydrocarbon, and wherein the hydrocarbon comprises methane, ethane, propane, or butane, or any combinations thereof.
13. The system of claim 1, wherein the refrigeration system comprises multiple cooling cycles.
14. The system of claim 1, wherein the refrigeration system comprises multiple cooling cycles, comprising:
one or more pre-cooling stages, wherein the refrigerant mixture comprises a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof, and
one or more deep cooling cycles, wherein the refrigerant mixture comprises a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof.
15. The system of claim 1, wherein the refrigerant mixture comprising the noble gas is utilized in one or more cooling stages to achieve deeper cooling than provided by hydrocarbon refrigerants.
16. The system of claim 1, comprising a nitrogen rejection unit, wherein a liquid feed from the bottom of the nitrogen rejection unit is used to provide cooling to a reflux condenser at the top of the nitrogen rejection unit.
17. The system of claim 1, wherein the refrigerant mixture comprises a pure component refrigerant.
18. A method for formation of a liquefied natural gas (LNG), comprising:
chilling a natural gas in a refrigeration system, wherein the refrigeration system uses a refrigerant mixture comprising a noble gas; and
liquefying the natural gas to form the LNG in an autorefrigeration system.
19. The method of claim 18, comprising cooling the natural gas in a first refrigeration system prior to chilling the natural gas in the refrigeration system, wherein the first refrigeration system uses a non-hydrocarbon refrigerant.
20. The method of claim 18, wherein chilling the natural gas in the refrigeration system comprises:
compressing the refrigerant mixture to provide a compressed refrigerant mixture;
optionally cooling the compressed refrigerant mixture by indirect heat exchange with a cooling fluid;
expanding the compressed refrigerant mixture to cool the compressed refrigerant mixture, thereby producing an expanded, cooled refrigerant mixture;
passing said expanded, cooled refrigerant mixture to a first heat exchange area;
optionally compressing the natural gas;
optionally cooling said the natural gas by indirect heat exchange with an external cooling fluid; and
heat exchanging the natural gas with the expanded, cooled refrigerant mixture.
21. The method of claim 18, wherein the noble gas comprises xenon or krypton.
22. The method of claim 18, wherein the refrigerant mixture comprises nitrogen or a hydrocarbon, or any combination thereof.
23. The method of claim 18, comprising liquefying the natural gas to form the LNG via a plurality of expansion valves or hydraulic expansion turbines and flash drums.
24. The method of claim 18, comprising:
chilling the natural gas via one or more pre-cooling steps using a first refrigerant mixture, wherein the first refrigerant mixture comprises a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof, and
chilling the natural gas via one or more deep cooling steps using a second refrigerant mixture, wherein the second refrigerant mixture comprises a noble gas, nitrogen, or a hydrocarbon, or any combinations thereof.
25. The method of claim 18, comprising using the refrigerant mixture comprising the noble gas in one or more cooling stages to achieve deeper cooling than provided by hydrocarbon refrigerants.
26. A cascade cooling system for formation of a liquefied natural gas (LNG), comprising:
a first refrigeration system configured to cool the natural gas using a non-hydrocarbon refrigerant, wherein the first refrigeration system comprises a plurality of first chillers configured to allow for cooling of the natural gas via an indirect exchange of heat between the natural gas and the non-hydrocarbon refrigerant;
a second refrigeration system configured to chill the natural gas using a refrigerant mixture comprising a noble gas, wherein the second refrigeration system comprises a plurality of second chillers configured to allow for cooling of the natural gas via an indirect exchange of heat between the natural gas and the refrigerant mixture; and
an autorefrigeration system configured to form the LNG from the natural gas, wherein the autorefrigeration system comprises a plurality of expansion valves or hydraulic expansion turbines, or any combination thereof, and flash drums.
27. The cascade cooling system of claim 26, wherein the first refrigeration system comprises a compressor that is configured to compress the non-hydrocarbon refrigerant and a condenser that is configured to cool the non-hydrocarbon refrigerant.
28. The cascade cooling system of claim 26, wherein the second refrigeration system comprises a compressor that is configured to compress the refrigerant mixture and a condenser that is configured to cool the refrigerant mixture.
29. The cascade cooling system of claim 26, wherein the plurality of first chillers comprise evaporators configured to cool the natural gas by at least partially vaporizing the non-hydrocarbon refrigerant via a transfer of heat from the natural gas to the non-hydrocarbon refrigerant.
30. The cascade cooling system of claim 26, wherein the plurality of second chillers comprise evaporators configured to chill the natural gas by vaporizing the refrigerant mixture via a transfer of heat from the natural gas to the refrigerant mixture.
31. The cascade cooling system of claim 26, wherein the LNG comprises a liquid fraction and a residual vapor fraction, and wherein the cascade cooling system comprises a liquid separation vessel configured to separate the residual vapor fraction from the liquid fraction.
32. The cascade cooling system of claim 26, comprising a nitrogen rejection unit upstream of the autorefrigeration system.
33. The cascade cooling system of claim 26, wherein the refrigerant mixture comprises a pure component refrigerant.
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