CN118076848A - System and method for producing liquefied natural gas - Google Patents

System and method for producing liquefied natural gas Download PDF

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Publication number
CN118076848A
CN118076848A CN202280067712.8A CN202280067712A CN118076848A CN 118076848 A CN118076848 A CN 118076848A CN 202280067712 A CN202280067712 A CN 202280067712A CN 118076848 A CN118076848 A CN 118076848A
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China
Prior art keywords
natural gas
expander
turbine
stream
refrigerant
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CN202280067712.8A
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Chinese (zh)
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H·E·霍华德
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Praxair Technology Inc
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Praxair Technology Inc
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes 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
    • F25J1/0203—Processes 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
    • F25J1/0204—Processes 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 as a single flow SCR cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes 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
    • F25J1/0047—Processes 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
    • F25J1/0052—Processes 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
    • F25J1/0057—Processes 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 after expansion of the liquid refrigerant stream with extraction of work
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes 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
    • F25J1/0047—Processes 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
    • F25J1/005—Processes 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 expansion of a gaseous refrigerant stream with extraction of work
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    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/007—Primary atmospheric gases, mixtures thereof
    • F25J1/0072—Nitrogen
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    • F25J1/02—Processes 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
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    • F25J1/0205—Processes 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 as a dual level SCR refrigeration cascade
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    • F25J1/0203—Processes 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
    • F25J1/0207—Processes 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 as at least a three level SCR refrigeration cascade
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes 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
    • F25J1/0228—Coupling of the liquefaction unit to other units or processes, so-called integrated processes
    • F25J1/0229—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock
    • F25J1/023—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock for the combustion as fuels, i.e. integration with the fuel gas system
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    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes 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
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0262—Details of the cold heat exchange system
    • F25J1/0264—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
    • F25J1/0265—Arrangement 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
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    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes 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
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0281—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc. characterised by the type of prime driver, e.g. hot gas expander
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes 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
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0285—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings
    • F25J1/0288—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes 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
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0294—Multiple compressor casings/strings in parallel, e.g. split arrangement
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2220/00—Processes or apparatus involving steps for the removal of impurities
    • F25J2220/60—Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/62—Separating low boiling components, e.g. He, H2, N2, Air
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    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/20—Integrated compressor and process expander; Gear box arrangement; Multiple compressors on a common shaft
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    • F25J—LIQUEFACTION, 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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    • F25J—LIQUEFACTION, 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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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/42—Processes or apparatus involving steps for increasing the pressure of gaseous process streams the fluid being nitrogen
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    • F25J2240/04—Multiple expansion turbines in parallel
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    • F25J2240/02—Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
    • F25J2240/12—Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream the fluid being nitrogen
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    • F25J2270/00—Refrigeration techniques used
    • F25J2270/14—External refrigeration with work-producing gas expansion loop
    • F25J2270/16—External refrigeration with work-producing gas expansion loop with mutliple gas expansion loops of the same refrigerant

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  • Mechanical Engineering (AREA)
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Abstract

A small to medium lng production system and method are provided. The disclosed lng production system employs a nitrogen-based refrigerant, at least one heat exchanger, three turbines/expanders, and two or more refrigerant compression stages. The expansion ratio of one turbine/expander is significantly lower than the expansion ratio of the other turbine/expander such that the temperature of the exhaust stream from the turbine/expander with the lower expansion ratio is above the critical point temperature of the compressed natural gas containing feed stream but is cooler than-15 ℃.

Description

System and method for producing liquefied natural gas
Technical Field
The present invention relates to the production of Liquefied Natural Gas (LNG), and more particularly to a small or medium liquefied natural gas production system and method using nitrogen-based refrigerants employing at least three turbines/expanders and two or more refrigerant compression stages.
Background
The demand for lng production is rapidly increasing in applications related to energy infrastructure, transportation, heating, power generation. The use of liquefied natural gas as a lower cost alternative fuel also allows for the potential reduction of carbon emissions and other harmful emissions, such as nitrogen oxides (NOx), sulfur oxides (SOx) and particulates that are generally considered to be detrimental to air quality. Because of this demand, there has been a trend to build and operate lower capacity lng production systems in areas where attractive low cost natural gas or methane biogas sources are available and/or where current demand for lng or demand is expected to increase over time.
Small-to medium-scale liquefied natural gas engines may include various energy applications such as well seeding or boil-off gas re-liquefaction, integrated CO 2 extraction and natural gas liquefaction; utility applications such as peak shaving or emergency reserves, lng supply at compressed natural gas fueling stations; and transportation applications including marine applications, off-road transportation applications, and even on-road fleet transportation applications. Other small-scale or medium-scale liquefied natural gas engines may include the production of liquefied natural gas from biological sources such as landfill sites, farms, industrial/municipal waste and wastewater operations.
Most conventional small or medium lng production systems aim to produce lng between 100mtpd and 500mtpd and higher. Many of these liquefaction systems employ mechanical refrigeration or nitrogen-based gas expansion refrigeration cycles to cool the natural gas feed to the temperature required for natural gas liquefaction. The use of nitrogen-based gas expansion refrigeration cycles is a preferred technique for small scale applications due to simplicity, safety, ease of operation, turndown, dynamic responsiveness and maintenance.
The current market for such small natural gas liquefaction systems using nitrogen-based gas expansion refrigeration cycles is dominated by the sales of equipment. While many recent opportunities are driven by environmental considerations, minimizing the installation costs of such natural gas liquefaction systems is also a major factor in the design of the liquefaction process. In designing natural gas liquefaction cycles and liquefaction systems, capital costs and operating efficiencies must be balanced. Such design decisions are highly dependent on site-specific variables, including natural gas feed quality and the intended application and transportation of the liquefied natural gas product.
In conventional high pressure natural gas liquefaction systems employing nitrogen-based gas expansion refrigeration cycles with double expansion, such as shown in fig. 1, there is a need to improve the thermal efficiency of such systems. The use of only two turbines/expanders and the condensing profile of the natural gas lead to significant divergence in the heat exchanger complex. Coupling a turbine/expander to one or more compression stages via an integral gear or "compander" further complicates efforts to improve thermal performance. In particular, the turboexpansion ratio, flow rate, and thermal positioning cannot be simply manipulated independent of the concurrent consideration of the turbomachinery performance, and any such additional turbomachinery complexity may require significant power reduction to offset such additional funds.
Another limiting aspect of the conventional natural gas liquefaction system and process depicted in fig. 1 is with respect to the temperature levels serviced by each turbine/expander. Since the cold turbine/expander provides the subcooling load necessary to prevent any meaningful product loss at depressurization, the outlet state is largely fixed by the Cold End Delta Temperature (CEDT) of the heat exchanger and the saturation conditions (which minimize unit power consumption). The cold turbine/expander inlet conditions are defined by a narrow temperature range within which the coldest part of the compound curve may substantially match. As the inlet temperature of the cold turbine/expander approaches the pseudo dew point inflection point temperature of natural gas, it becomes impossible for the warmed exhaust stream to match the subcooling curve of natural gas. With these factors in mind, and the parallel arrangement, the pressure ratio is largely fixed and/or limited by the cold turbine/expander operation.
The conventional two turbine/expander liquefaction system shown in fig. 1 also exhibits a highly skewed distribution of refrigeration. Since the thermal turbine/expander in such conventional natural gas liquefaction systems is discharged below the natural gas critical point temperature (i.e., -82.6 ℃), its flow absorbs much of the load associated with pre-cooling the refrigerant and natural gas stream and much of the load of NG pseudo-condensation. In the conventional natural gas liquefaction system and process shown in fig. 1, the thermal turbine/expander accounts for about 69% of the recycle refrigerant stream and supplies about 83% of the refrigeration delivered. Thus, the absorption power of pinion #2 coupling the cold turbine/expander to the downstream compression stage is significantly higher than that of pinion #1 coupling the hot turbine/expander to the upstream compression stage. This arrangement complicates the design of the turbomachinery and the ability of the process to fully utilize the capacity of any given "compander" frame.
What is needed, therefore, is a natural gas liquefaction system and process that provides more reasonable power distribution to individual pinions and that exhibits significant capital power benefits with limited additional capital expenditure over conventional two turbine/expander liquefaction systems.
Another natural gas liquefaction system is disclosed in U.S. patent 5,768,912 (Dubar), which discloses a three turbine/expander based natural gas liquefaction cycle. In this prior art publication, three booster-loaded nitrogen expanders are arranged in series and the resulting efficiency of this three turbine/expander liquefaction system is less than ideal, resulting in additional capital costs without a corresponding reduction in power and operating costs.
Accordingly, there is a need for improvements in the overall design and performance of such natural gas liquefaction systems and processes with the objective of minimizing heat exchange liquefaction inefficiency while facilitating turbomachinery design. In this way, power consumption may be minimized. This goal of minimizing heat exchange liquefaction inefficiency is critical to achieving meaningful performance improvements.
Disclosure of Invention
The invention can be characterized by a natural gas liquefaction system having a refrigeration circuit, comprising, inter alia: (i) At least one heat exchanger configured to liquefy and subcool a feed stream comprising compressed natural gas via indirect heat exchange with a refrigerant stream; (ii) Three turbines/expanders configured to expand a portion of the refrigerant stream to produce at least three exhaust streams, the exhaust streams being directed to the at least one heat exchanger to liquefy and subcool the natural gas-containing feed stream via indirect heat exchange and leave the at least one heat exchanger as one or more warmed recycle streams; and (iii) two or more refrigerant compression stages including an upstream refrigerant compression stage and a downstream refrigerant compression stage both configured to compress the warmed recycle stream. The three or more turbo/expanders further comprise a cold turbo/expander configured to expand a cold portion of the refrigerant stream and produce cold exhaust gas that is recycled to the upstream one of the two or more refrigerant compression stages; a first thermal turbine/expander configured to expand a first thermal portion of the refrigerant stream and produce a first hot exhaust gas that is recycled to the upstream one of the two or more refrigerant compression stages; and a second thermal turbine/expander configured to expand a second thermal portion of the refrigerant flow and produce a second hot exhaust gas that is recycled to the downstream refrigerant compression stage of the two or more refrigerant compression stages. The expansion ratio of the second thermal turbine/expander is lower than the expansion ratio of the cold turbine/expander and lower than the expansion ratio of the first thermal turbine/expander.
The invention may be characterized as a method of producing liquefied natural gas comprising the steps of: (a) receiving a feed stream comprising purified compressed natural gas; (b) Liquefying and subcooling a feed stream comprising purified compressed natural gas in at least one heat exchanger via indirect heat exchange with one or more refrigerant streams to produce one or more lower pressure recycle streams and a higher pressure recycle stream; (c1) Compressing the one or more lower pressure recycle streams in an upstream refrigeration compression stage to produce a compressed refrigerant stream; (c2) Compressing the higher pressure recycle stream and the compressed refrigerant stream in a downstream refrigeration compression stage to produce a further compressed refrigerant stream; (d) Cooling the further compressed refrigerant stream in the at least one heat exchanger; (e1) Extracting a cold portion of the further compressed refrigerant stream from the at least one heat exchanger; (e2) Extracting a first hot portion of the further compressed refrigerant stream from an intermediate location of the at least one heat exchanger; (e3) Extracting a second hot portion of the further compressed refrigerant stream from a second intermediate location of the at least one heat exchanger; (f1) Expanding the cold portion of the compressed refrigerant stream in a cold turbine/expander and producing cold exhaust gas having a temperature of less than-145 ℃; (f2) Expanding the first hot portion of the compressed refrigerant stream in a first hot turbine/expander to produce a first hot exhaust gas having a temperature colder than about-90 ℃ and hotter than the cold exhaust gas; (f3) Expanding the second hot portion of the compressed refrigerant stream in a second hot turbine/expander to produce a second hot exhaust gas having a temperature above the critical point temperature of the compressed natural gas containing feed stream and colder than about-15 ℃ and an outlet pressure above the outlet pressures of the cold turbine/expander and the first hot turbine/expander; (g1) Directing the cold exhaust gas and the first hot exhaust gas to the at least one heat exchanger as a refrigeration source and generating one or more lower pressure recycle streams; (g2) Directing the second hot exhaust gas to the at least one heat exchanger as a refrigeration source and generating a higher pressure recycle stream; (h1) Recycling the one or more lower pressure recycle streams to one or more refrigerant compression stages upstream of the plurality of compression stages; and (h 2) recycling the higher pressure recycle stream to a downstream refrigerant compression stage of the plurality of compression stages.
In the present system and method, the first thermal turbine/expander has an expansion ratio between 4.0 and 5.0 and is configured to produce a majority, preferably more than 45%, of the refrigeration, while the cold turbine/expander also has an expansion ratio between 4.0 and 5.0 and is configured to produce less than 25% of the refrigeration. The second thermal turbine/expander preferably has an expansion ratio of between 1.5 and 2.5 and is configured to produce between about 20% and 35% of the refrigeration.
The present system and method is also preferably configured with an integrated gear machine having a drive assembly, a large gear, and a plurality of small gears arranged to drive the refrigerant compression stage and/or for receiving work produced by the turbine/expander. For example, the second thermal turbine/expander and one of the upstream or downstream compression stages are operatively coupled to a first pinion of an integrated gear machine, and the first thermal turbine/expander and at least one of the upstream or downstream compression stages are operatively coupled to a second pinion of the integrated gear machine. The cold turbine/expander may itself be coupled to a third pinion of the integrated gear machine or, preferably, operatively coupled to at least one of the upstream compression stages via the third pinion.
The purified compressed natural gas containing feed stream is preferably at a pressure greater than the critical pressure of natural gas, and more preferably at a pressure between about 50 bar (a) and 80 bar (a). The refrigerant stream is a nitrogen-based refrigerant preferably containing greater than about 80% nitrogen by volume.
Drawings
It is believed that the claimed invention will be better understood when considered in conjunction with the drawings, in which:
FIG. 1 shows a generalized schematic of a process flow diagram of a conventional two turbine and two refrigerant compression stage natural gas liquefaction process known in the art;
FIG. 2 shows a schematic diagram of a process flow diagram of an embodiment of the system and method of the present invention for liquefied natural gas production using three turbines/expanders and two refrigerant compression stages;
FIG. 3 shows a schematic diagram of a process flow diagram of another embodiment of the system and method of the present invention for liquefied natural gas production using three turbines/expanders and three refrigerant compression stages, where two of the three refrigerant compression stages are arranged in parallel; and
Figure 4 shows a schematic diagram of yet another embodiment of the system and method of the present invention for lng production using multiple heat exchangers.
Detailed Description
The design of an efficient liquefaction process employing gas expansion to provide the refrigeration necessary to liquefy and subcool a feed stream containing purified compressed natural gas is a result of both heat transfer and turbomachinery within the system and/or process. Minimization of heat transfer irreversibility is achieved when the divergence of the warming and cooling compound curve (e.g., transferred energy versus temperature) is minimized. The process definition of flow, pressure and temperature controls the resulting compound curve to a large extent. The turbomachinery efficiency is maximized when the process head and flow characteristics agree with empirically based optimal values. These best designs are often characterized by a defined ratio of geometry, flow and head (Ns, ds). Such considerations due to dimensional similarity are well known to the art of gas processing. See, for example, the Kenneth E.Nichols publication entitled "How to Select Turbomachinery for your Application". These optimal turbomachinery conditions are a function of the type of machine under consideration.
In the present system and method, there are particular applications in which multiple centrifugal turbines (particularly three radial inflow turbines) are used. The present systems and methods require, or at least contemplate, that the natural gas feed is a purified compressed natural gas feed stream at a pressure above the critical pressure of natural gas. As used herein, the term purified natural gas feed stream refers to a natural gas feed stream that is substantially free of heavy hydrocarbons, carbon dioxide, water, and other impurities, and may even be a methane-containing biogas. Subsequent and direct liquefaction of the subcritical natural gas feed stream results in a compound curve divergence near the dew point of the mixture. Furthermore, liquefaction of natural gas at pressures below about 40 bar (a) typically results in cooler levels of hot turbine/expander operation, which in turn produces significant losses in terms of unit power consumption. To avoid this loss, the natural gas feed stream is preferably at a pressure above the critical pressure of the natural gas feed stream, and more preferably between about 50 bar (a) and 80 bar (a).
Yet another advantageous feature of the present system and method of producing liquefied natural gas is the use of an integrated gear machine comprising a drive assembly, a large gear, and a plurality of small gears arranged or configured to drive two or more refrigerant compression stages and/or for receiving work produced by three turbines/expanders. The shaft of the large gear may also be connected to the driver assembly via a gear. At least two of the plurality of pinions are net absorbers of power from a drive assembly, which may be an electric motor, a steam turbine, or even a gas turbine. Preferably, the integrated gear machine is configured to distribute power appropriately over the plurality of pinions, and more preferably is arranged or configured such that the power transferred to the two pinions coupled to the refrigerant compression stage differ by no more than 10%. An important aspect or advantage of this integrated gear machine arrangement disclosed herein relates to the specific pairing of turbomachinery on different pinions in a manner that optimizes the performance of the liquefaction system and method of the present invention.
The optimization of turbomachinery begins with consideration of turbine/expander efficiency. Any given process definition (e.g., pressure, temperature, and flow) that results in a viable heat transfer (liquefaction) design also provides the necessary inputs, such as flow and head characteristics, necessary to define the dimensionless characteristics (Ns, ds) required for a given component turbine/expander speed and diameter. It is well known that radial inflow turbines achieve peak efficiency at U/Co (i.e., rotor tip speed/isentropic ejection speed) values approaching 0.70. This ratio is also defined by the following formula: [ U/Co ] = [ NsDs ]/154.
Thus, an efficient process definition will determine the speed and diameter necessary for the turbine/expander to operate at peak efficiency. With respect to gas compression, the process definition determines the compression stage head and the associated turbine/expander on the same pinion determines the rotational speed, which in turn results in a specific speed. The above calculations form part of the overall process optimization. More specifically, the optimization is an iterative process that includes process definition, turbine pairing based on the above calculations, and ultimately consideration of integrated gear pinion power and total input power limitations.
Conventional small and medium lng facilities using nitrogen-based gas expansion as the primary refrigeration source typically employ a centrifugal recycle compression stage for the refrigerant, typically driven by an integral gear wheel housed within a common housing, the integral gear wheel comprising a large diameter large gear wheel having a plurality of meshing pinion gears on the ends of which are mounted respective compression impellers, thereby forming the multiple refrigerant compression stages and expansion impellers of the turbine/expander. The pinion gears may have different diameters to best match the speed requirements of the coupled compression impeller. Each of the plurality of compression impellers and turbine/expanders are typically contained within their respective housings and collectively provide several stages of recycle compression and expansion as needed.
Linde inc. As Linde Group of Companies member also developed a combination of an integral gear machine or single machine that combines a compression stage with an efficient radial inflow expander with up to four pinions in what is known as an integral gear "bridge" machine or BRIM. Linde's "bridge" machines are commonly used in hydrogen/syngas facilities as well as air separation facilities, and typically have different frame sizes, such as between about 90mm and 180 mm. Design studies have examined the use of Linde "bridge" machines to operatively couple multiple radial inflow turbines and centrifugal refrigeration compression stages in a natural gas liquefaction system. The Linde "bridge" machine is fully packaged or integrated with suitable PLC controllers, control valves, safety valves, oil systems, etc., and can be readily equipped with an intercooler and/or an aftercooler. Hardware constraints and limitations of the Linde "bridge" machine are typically a function of the size of the bull gear and driver components. Typically, the Linde "bridge" machine drive associated with the present systems and methods spans a range of about 4MW to 20MW, with the associated maximum pinion speed being in a range of 20,000 to 50,000 rpm. Furthermore, the maximum power delivered to any given pinion or any given turbine-compression stage pairing is preferably limited to less than 50% and in some cases about 35% of the total "bridge" machine drive power.
LNG production with 3 turbo/expander and 2 refrigerant compression stages
Turning to fig. 2, a schematic diagram of a high-level process flow diagram of one embodiment of the system 10 and method of the present invention for lng production using three turbines/expanders and two refrigerant compression stages is shown. The illustrated system 10 further includes a refrigerant circuit having at least one heat exchanger 20 and two aftercoolers 221, 222, an integrated gear wheel 25, a fuel gas circuit 18, and an afterliquefaction conditioning circuit 23 having one or more expansion valves 27 and a phase separator 28 configured to separate nitrogen and other light gases from the subcooled natural gas stream 21.
Purified compressed natural gas feed 12, which is substantially free of heavy hydrocarbons and other impurities and has a feed pressure greater than the natural gas critical pressure (i.e., greater than 46 bar (a)), preferably at a pressure between about 50 bar (a) and 80 bar (a), and more preferably at a pressure between about 60 bar (a) and 75 bar (a), is provided to the natural gas liquefaction system 10 as feed stream 14.
A first major portion of the purified compressed natural gas feed stream 16 is directed to the cooling channels in heat exchanger 20 where it is liquefied and subcooled via indirect heat exchange with the refrigerant stream traversing the warming channels of the heat exchanger. A second minor portion of the purified compressed natural gas feed stream 17 is diverted to a fuel gas loop 18, the fuel gas loop 18 comprising one or more valves 19 configured to expand the second minor portion of the purified compressed natural gas feed stream 17 to a pressure of less than about 6.0 bar (a).
As described above, a first substantial portion of purified compressed natural gas feed stream 16 is liquefied and subcooled within heat exchanger 20 via indirect heat exchange with one or more nitrogen-based refrigerant streams to form subcooled liquefied natural gas stream 21. Subcooled liquefied natural gas stream 21 is then processed in post-liquefaction conditioning circuit 23 wherein the subcooled liquefied natural gas is depressurized via one or more valves 27 or a liquid turbine (not shown) and phase separated using phase separator 28 to separate nitrogen vapor and other light gases. The resulting lng stream 29 constitutes an lng product.
The primary refrigeration source used in the illustrated natural gas liquefaction system 10 is preferably a nitrogen-based gas expansion refrigeration circuit, which preferably includes a refrigerant stream containing greater than about 80% nitrogen by volume. In this illustrated refrigeration circuit, the refrigerant is compressed in two refrigerant compression stages (i.e., an upstream refrigerant compression stage 40 and a downstream refrigerant compression stage 50) arranged in series, with appropriate intermediate and/or post-cooling 221, 222 to counteract the temperature increase caused by the heat of compression. Such post-cooling may be accomplished by indirect contact with air, cooling water, cold water, or other refrigeration medium, or a combination thereof. The compressed refrigerant 55 is then further cooled in the at least one heat exchanger 20 and directed to one or more turbines/expanders 70, 80, 90 configured to expand the compressed refrigerant stream to produce refrigeration.
The embodiment of fig. 2 and 3 depicts a single heat exchanger 20 having multiple warming channels and multiple cooling channels. Alternatively, the at least one heat exchanger may comprise a plurality of heat exchangers or heat exchange cores, with a first heat exchanger 20 or first heat exchange core configured for liquefying the natural gas feed stream 14, and a second heat exchanger 120 or second heat exchange core configured for cooling other streams, such as a portion of a refrigerant stream (as shown in the embodiment of fig. 4), or possibly subcooling the liquefied natural gas stream, or possibly even pre-cooling the natural gas feed stream.
Specifically, the first portion 72 of the compressed refrigerant stream is substantially cooled in the heat exchanger and directed to the cold turbine/expander 70 as a cold portion of the refrigerant stream. The second portion 82 of the compressed refrigerant stream is partially cooled and exits the heat exchanger 20 at an intermediate hotter temperature as a first hot portion, which is then directed to the first thermal turbine/expander 80. The third portion 92 of the compressed refrigerant stream is also partially cooled and exits the heat exchanger 20 as a second hot portion of the compressed refrigerant stream having a temperature that is hotter than the intermediate hotter temperature. The second hot portion 92 of the compressed refrigerant stream is then directed to a second hot turbine/expander 90.
The cold turbine/expander 70 is configured to expand a cold portion 72 of the compressed refrigerant stream to produce a cold turbine exhaust stream 74, the cold turbine exhaust stream 74 being recycled back to the refrigerant compression stages 40, 50 as a warming stream 76 via one or more of the plurality of warming channels in the heat exchanger 20. The partially cooled first hot portion 82 of the compressed refrigerant stream is expanded in the first thermal turbine/expander 80 to produce a first thermal turbine exhaust stream 84 that is also recycled to the one or more refrigerant compression stages 40, 50 as a warmed stream 86 via one or more of the plurality of warmed channels in the heat exchanger 20. The partially cooled second hot portion 92 of the compressed refrigerant stream is expanded in a second thermal turbine/expander 90 to produce a second thermal turbine exhaust stream 94 that is also recycled to the downstream refrigerant compression stage 50 as a warming stream 96 via one or more of the plurality of warming channels in the heat exchanger 20.
The inlet pressures of the three turbines/expanders are approximately equal, but the outlet pressures are different. In particular, the expansion ratio of the cold turbine/expander 70 and the first thermal turbine expander 80 is preferably between about 4.0 and 5.0. With a similar expansion ratio, the cold turbine exhaust 74 and the first hot turbine exhaust 84 may be warmed in the heat exchanger with the same warming pressure. Alternatively, the cold exhaust gas and the first hot exhaust gas may be warmed in separate channels of the heat exchanger and/or may be at different outlet pressures. An important and advantageous feature of the present system and method is that the second thermal turbine/expander 90 has a much smaller expansion ratio than the expansion ratio of the cold turbine/expander 70 and the first thermal turbine/expander 80. Preferably, the second hot turbine/expander has an expansion ratio between 1.5 and 2.5, and since the second hot exhaust gas 94 is at a greater pressure than the cold turbine exhaust gas 74 and the first hot turbine exhaust gas 84, it should be warmed in separate passages of the heat exchanger 20.
Upon exiting heat exchanger 20, warmed cold turbine exhaust stream 76 and warmed first hot turbine exhaust stream 86 are recycled as lower pressure recycle stream 42 to upstream refrigerant compression stage 40, lower pressure recycle stream 42 being compressed in upstream refrigerant compression stage 40 to form stream 44, and then cooled in upstream aftercooler 221 to produce stream 46. The warmed second hot turbine exhaust stream 96 is also recycled as a higher pressure recycle stream and mixed with the compressed refrigerant stream 46 exiting the upstream refrigerant compression stage. This mixed stream 52 is then directed to a downstream refrigerant compression stage 50 where it is further compressed to form a compressed refrigerant stream 54 and then cooled in a downstream aftercooler 222 to form stream 55 and further cooled in heat exchanger 20.
In the depicted embodiment, the cold turbine exhaust stream 74 is at a temperature cooler than-145 ℃, while the first hot turbine exhaust stream 84 is at a temperature cooler than-90 ℃ but hotter than the cold turbine exhaust stream. The second hot turbine exhaust 94 is at a temperature above the critical point temperature of the compressed natural gas feed stream 14 and is hotter than the first hot turbine exhaust stream 84, preferably cooler than about-15 ℃. Moreover, the distribution of the compressed refrigerant flow among cold portion 72, first hot portion 82, and second hot portion 92 is such that first hot turbine/expander 80 is configured to produce more than 45%, and more preferably more than 50%, of the refrigeration for natural gas liquefaction system 10. The cold turbine/expander 70 is configured to produce less than 25%, more preferably less than 20%, refrigeration for the natural gas liquefaction system 10, while the second hot turbine/expander 90 is configured to produce between about 20% and 35% refrigeration for the liquefaction system 10.
The first and second hot and cold turbines/expanders 80, 90, 70 and the upstream and downstream refrigerant compression stages 40, 50 are operatively coupled to the integrated gear machine 25. In particular, the downstream refrigerant compression stage 50 and the first thermal turbine/expander 80 are operatively coupled to the same pinion on the large gear 26 of the integrated gear machine 25, identified as the second pinion 32 of the three-pinion integrated gear machine. Similarly, the upstream refrigerant compression stage 40 and the second thermal turbine/expander 90 are operatively coupled to the same pinion gear of the integrated gear machine 25, illustrated as the first pinion gear 31. The cold turbine/expander 70 is coupled to another pinion, illustrated as the third pinion 33 of the integrated gear machine 25.
LNG production with 3 turbo/expander and 3 refrigerant compression stages
Turning to fig. 3, a schematic diagram of a high-level process flow diagram of another embodiment of the present system 10 and method for lng production using three turbines/expanders and three refrigerant compression stages is shown. Many of the features, components, and streams associated with the natural gas liquefaction system shown in fig. 3 are similar or identical to those described above with reference to fig. 2, and will not be repeated here for the sake of brevity. The key difference of the natural gas liquefaction system shown in fig. 3 compared to the natural gas liquefaction system described above with particular reference to fig. 2 is the addition of a third refrigerant compression stage 40B and the operative coupling of one of the cold turbine/expander 70 and the refrigerant compression stage 40B to the third pinion 33 of the integrated gear machine 25 such that all three pinions are net absorbers of power.
Similar to the embodiment shown in fig. 2, the natural gas liquefaction system 10 shown in fig. 3 also includes a refrigerant circuit having at least one heat exchanger 20 and two aftercoolers 221, 222, an integrated gear train 25, a fuel gas circuit 18, and an afterliquefaction conditioning circuit 23, having one or more expansion valves 27, and a phase separator 28 configured to separate nitrogen and other light gases from the liquefied subcooled natural gas stream 21. The purified compressed natural gas feed 14 is at a feed pressure greater than the critical pressure of natural gas, and preferably at a pressure between about 50 bar (a) and 80 bar (a).
As noted above, the primary refrigeration source is preferably a nitrogen-based gas expansion refrigeration circuit, which preferably includes a refrigerant stream containing greater than about 80% nitrogen by volume. In the embodiment shown in fig. 3, three refrigerant compression stages 40A, 40B, 50 are used to compress the refrigerant flow, with two refrigerant compression stages 40A, 40B of the three refrigerant compression stages being arranged in parallel.
As shown in FIG. 3, warmed cold turbine exhaust 76 and warmed first hot turbine exhaust 86 exiting heat exchanger 20 are recycled as lower pressure recycle streams 42A and 42B to a pair of upstream refrigerant compression stages 40A and 40B, respectively. The pair of upstream refrigerant compression stages includes a first upstream refrigerant compression stage 40A and a second upstream refrigerant compression stage 40B arranged in parallel. The lower pressure recycle stream is divided, with between 60% and 70% of the lower pressure recycle stream 42A being compressed in the first upstream refrigerant compression stage 40A and the remainder of the lower pressure recycle stream 42B being compressed in the second upstream refrigerant compression stage 40B. The parallel flow exiting the first and second upstream refrigerant compression stages 40A, 40B is then cooled in the upstream aftercooler 221.
The warmed second hot turbine exhaust stream is also recycled as higher pressure recycle stream 96 and mixed with the compressed refrigerant stream exiting the first and second upstream refrigerant compression stages, preferably downstream of the upstream aftercooler 221. This mixed stream 52 is then directed to a downstream refrigerant compression stage 50 where it is further compressed to form a compressed refrigerant stream 54 that is then cooled in a downstream aftercooler 222. The cooled compressed refrigerant stream 55 is then further cooled in heat exchanger 20 and directed to one or more turbines/expanders configured to expand the compressed refrigerant stream to produce refrigeration for natural gas liquefaction system 10.
Examples of LNG production
A number of computer simulations were run to characterize the performance of the natural gas liquefaction system and process of the present invention. In one such computer simulation, referred to as example 1, a natural gas liquefaction system designed to produce 175 metric tons of liquefied natural gas per day from a compressed purified natural gas feed stream having a pressure of about 68 bar (a) and a temperature of about 30 ℃ was evaluated using the arrangement disclosed with reference to fig. 2.
Table 1A provides the work distribution in this example of an embodiment using a three-pinion integrated gear machine used in the three turbine/expander and two-refrigerant compression stage system schematically depicted in fig. 2. Similarly, table 1B provides the process flow and refrigerant flow characteristics for this example of the same figure 2 embodiment using a three turbine/expander and two refrigerant compression stage natural gas liquefaction system.
Pinion # of FIG. 2 Service #1 Power (kW) Service #2 Power (kW) Net power (kw)
Pinion #1 N2 Comp#CB1 1916 N2-second heat turbine -399 1517
Pinion #2 N2 Comp#CB2 2511 N2-first heat turbine -994 1517
Pinion #3 - - N2-cold turbine -214 -214
TABLE 1A
FIG. 2 flow description Flow # Temperature (. Degree. C.) Refrigerant flow (in% of total)
CB1 inlet M07 33.48 71.1％
CB2 inlet M12 35.28 100.0％
Cold turbine inlet M03 -113.47 21.5％
Cold turbine exhaust M04 -166.90 21.5％
First WT inlet R02 -34.57 49.6％
First WT exhaust R03 -111.80 49.6％
Second WT inlet S02 22.84 28.9％
Second WT exhaust S03 -27.60 28.9％
Lower pressure of M06 33.48 71.1％
Upstream aftercooler M10 36.00 71.1％
Downstream of M15 36.00 100.0％
TABLE 1B
In the example 1 simulation, the speed of the cold turbine/expander is a variable that constrains the process cycle, and in this example approaches a speed of about 45,000 rpm. It should be noted that the integrated gear or "bridge" machine receives work from the cold turbine/expander on the third pinion, as it is unpaired with any refrigeration compression stage. The other two pinions are net absorbers of power from the drive assembly of the integrated gear machine, and power is distributed to the two pinions in approximately equal or approximately equal proportions. It is noted, however, that the upstream refrigeration compression stage is designed to compress more than 71% of the refrigerant and that this compressed refrigerant is mixed or combined with a higher pressure recycle stream containing the remaining 29% of the refrigerant. The downstream refrigerant compression stage is thus designed to further compress the entire refrigerant flow.
The distribution of the fully compressed refrigerant stream in this example 1 example between the cold turbine/expander, the first thermal turbine/expander, and the second thermal turbine/expander is such that the first thermal turbine/expander is configured to receive nearly 50% of the compressed refrigerant stream and expand the stream from an inlet pressure of 50.2 bar (a) to an outlet pressure of 11.68 bar (a) or an expansion ratio of 4.29. On the other hand, the cold turbine/expander receives more than 21% of the compressed refrigerant stream and expands the stream from an inlet pressure of 49.85 bar (a) to an outlet pressure of 11.78 bar (a) or an expansion ratio of 4.23, while the second hot turbine/expander receives about 29% of the compressed refrigerant stream and expands the stream from an inlet pressure of 50.4 bar (a) to an outlet pressure of 24.08 bar (a) or an expansion ratio of 2.09.
As noted above, the design of small to medium natural gas liquefaction cycles and liquefaction systems involves a number of tradeoffs between capital costs and operating efficiency. The natural gas liquefaction system shown in fig. 2 and operating in a similar manner to the example of example 1 is among the best tradeoffs of thermal performance, capital cost, and adaptation to turbomachinery constraints.
In another computer simulation, referred to as example 2, a natural gas liquefaction system designed to produce 320 metric tons of liquefied natural gas per day from a compressed purified natural gas feed stream having a pressure of about 68 bar (a) and a temperature of about 30 ℃ was evaluated using the three turbine/expander and three refrigerant compression stage arrangement disclosed in fig. 3. Table 2A provides an example work distribution of example 2 of an embodiment of a three pinion integrated gear machine used in a three turbine/expander and three refrigerant compression stage system schematically depicted in fig. 3, while table 2B provides a process flow and refrigerant flow characteristics of the three turbine/expander and three refrigerant compression stage natural gas liquefaction system of fig. 3.
Pinion # of FIG. 3 Service #1 Power (kW) Service #2 Power (kW) Net power (kw)
Pinion #1 N2 Comp#CB1 2408 N2-second heat turbine -748 1660
Pinion #2 N2 Com3#CB2 4321 N2-first heat turbine -1820 2501
Pinion #3 N2 Comp#CB3 1297 N2-cold turbine -360 937
TABLE 2A
FIG. 3 flow description Flow # Temperature (. Degree. C.) Refrigerant flow (in% of total)
CB1 inlet M07 33.48 44.5％
CB2 inlet M12 35.21 100.0％
CB3 inlet M07A 33.48 23.9％
Cold turbine inlet M03 -111.57 19.3％
Cold turbine exhaust M04 -166.90 19.3％
First WT inlet R02 -34.99 49.1％
First WT exhaust R03 -114.00 49.1％
Second WT inlet S02 18.97 31.6％
Second WT exhaust S03 -28.80 31.6％
Higher pressure of S04 33.50 31.6％
Lower pressure of M06 33.48 68.4％
Upstream aftercooler M10 36.00 68.4％
Downstream of M15 36.00 100.0％
TABLE 2B
In the example 2 simulation, which checks for higher capacity, the cold turbine/expander on the third pinion is paired with one of the upstream refrigeration compression stages, while the second hot turbine/expander on the first pinion is paired with the other upstream refrigeration compression stage. The first thermal turbine/expander on the second pinion is paired with a downstream refrigeration compression stage, and all three pinions are net absorbers of power from the drive assembly of the integrated gear machine. In this example 2 example, power is distributed to the three pinions as follows: the second pinion gear coupled to the first thermal turbine/expander and the downstream refrigerant compression stage absorbs 49% of the power, while the first pinion gear coupled to one of the second thermal turbine/expander and the upstream refrigerant compression stage absorbs 32.6% of the power, and the third pinion gear coupled to the other of the cold turbine/expander and the upstream refrigerant compression stage absorbs 18.4% of the power. In this high capacity example of example 2, the integrated gear machine is configured to absorb near maximum total absorbable power for the main body "bridge" machine.
Note, however, that the upstream refrigeration compression stage, which is arranged in parallel, is configured to compress 68% or more of the total refrigerant. Specifically, the first upstream refrigeration compression stage compresses about 65% of the lower pressure recycle stream and the second upstream refrigeration compression stage compresses about 35% of the lower pressure recycle stream exiting the heat exchanger. These compressed streams are combined and directed to an upstream aftercooler, and the resulting cooled stream is mixed or further combined with a higher pressure recycle stream containing the remainder of the refrigerant (near 32%). The downstream refrigerant compression stage is thus designed to further compress the entire refrigerant flow.
The distribution of the fully compressed refrigerant stream between the cold turbine/expander, the first thermal turbine/expander, and the second thermal turbine/expander in this example 2 was such that the first thermal turbine/expander was configured to receive 49.1% of the compressed refrigerant stream and expand the refrigerant stream from an inlet pressure of 52.4 bar (a) to an outlet pressure of 11.68 bar (a) or an expansion ratio of about 4.5. In another aspect, the cold turbine/expander receives about 19.3% of the compressed refrigerant stream and expands the refrigerant stream from an inlet pressure of 52.05 bar (a) to an outlet pressure of 11.78 bar (a) or an expansion ratio of 4.42, while the second hot turbine/expander receives about 31.6% of the compressed refrigerant stream and expands the refrigerant stream from an inlet pressure of 52.6 bar (a) to an outlet pressure of 26.84 bar (a) or an expansion ratio of 1.96.
The natural gas liquefaction process using the three pinion and three turbine/expander arrangement discussed above with reference to fig. 2 and 3 will achieve a minimum power advantage of about 5% relative to a conventional natural gas liquefaction process using the two pinion and two turbine/expander arrangement shown in fig. 1. The actual power advantage achieved by the three pinion and three turbine/expander arrangement, as compared to the conventional two pinion and two turbine/expander arrangement, is very dependent on the operating conditions and constraints imposed on the liquefaction process and the turbine mechanical efficiency and hardware constraints associated with the respective system, but can readily meet or exceed 9.0% of the power advantage, which compensates for not only a small increase in capital cost. The power reduction in the tri-pinion and tri-turbine/expander configurations is also accompanied by an almost 50% increase in heat exchanger UA, which is evidence of a tighter compound curve within the heat exchanger. However, for small to medium natural gas liquefaction systems, adding UA is unlikely to require a second heat exchanger core.
While the natural gas liquefaction system and method of the present invention has been described with reference to several preferred embodiments, it should be understood that various additions, modifications and omissions may be made therein and thereto, without departing from the spirit and scope of the present invention as set forth in the appended claims.

Claims (32)

1. A natural gas liquefaction system, comprising:
a refrigeration circuit, the refrigeration circuit comprising: (i) At least one heat exchanger configured to liquefy and subcool a feed stream comprising compressed natural gas via indirect heat exchange with a refrigerant stream; (ii) Three or more turbines/expanders configured to expand a portion of the refrigerant stream to produce at least three exhaust streams, at least two of the at least three exhaust streams being directed to the at least one heat exchanger to liquefy and subcool the natural gas-containing feed stream via indirect heat exchange and leave the at least one heat exchanger as a warmed recycle stream; and (iii) two or more refrigerant compression stages, the two or more refrigerant compression stages comprising an upstream refrigerant compression stage and a downstream refrigerant compression stage both configured to compress the warmed recycle stream; and
Wherein the three or more turbines/expanders further comprise: (i) A cold turbine/expander configured to expand a cold portion of the refrigerant stream and produce cold exhaust gas that is recycled to the upstream one of the two or more refrigerant compression stages; (ii) A first thermal turbine/expander configured to expand a first thermal portion of the refrigerant stream and produce a first hot exhaust gas that is recycled to the upstream one of the two or more refrigerant compression stages; and (iii) a second thermal turbine/expander configured to expand a second thermal portion of the refrigerant stream and produce a second hot exhaust gas that is recycled to the downstream one of the two or more refrigerant compression stages; and
Wherein the expansion ratio of the second thermal turbine/expander is lower than the expansion ratio of the cold turbine/expander and lower than the expansion ratio of the first thermal turbine/expander.
2. The natural gas liquefaction system of claim 1, wherein the cold turbine exhaust is at a temperature colder than-145 ℃, and the first hot exhaust is at a temperature colder than-90 ℃.
3. The natural gas liquefaction system of claim 2, wherein a temperature of the second hot exhaust gas is greater than a critical point temperature of the compressed natural gas containing feed stream.
4. The natural gas liquefaction system of claim 3, wherein the temperature of the second hot exhaust gas is cooler than about-15 ℃.
5. The natural gas liquefaction system of claim 1, wherein the natural gas containing feed stream is a natural gas feed stream derived from a biological gas source.
6. The natural gas liquefaction system of claim 1, wherein an inlet pressure of the cold turbine/expander and an inlet pressure of the first hot turbine/expander are substantially equal, and an outlet pressure of the cold turbine/expander and an outlet pressure of the first hot turbine/expander are substantially equal.
7. The natural gas liquefaction system of claim 1, wherein the first thermal turbine/expander is configured with an expansion ratio between 4.0 and 5.0, and is further configured to produce greater than 45% refrigeration for the natural gas liquefaction system.
8. The natural gas liquefaction system of claim 1, wherein the cold turbine/expander is configured with an expansion ratio between 4.0 and 5.0, and is further configured to produce less than 25% refrigeration for the natural gas liquefaction system.
9. The natural gas liquefaction system of claim 1, wherein the second thermal turbine/expander is configured with an expansion ratio between 1.5 and 2.5, and is further configured to produce between about 20% and 35% refrigeration for the natural gas liquefaction system.
10. The natural gas liquefaction system of claim 1, further comprising an integrated gear machine having a drive assembly, a large gear, and a plurality of small gears arranged to drive the two or more refrigerant compression stages and/or for receiving work produced by the at least three turbines/expanders.
11. The natural gas liquefaction system of claim 10, wherein the second thermal turbine/expander and the upstream compression stage are operatively coupled to a first pinion gear of the plurality of pinions, and the first thermal turbine/expander and the downstream compression stage are operatively coupled to a second pinion gear of the plurality of pinions.
12. The natural gas liquefaction system of claim 11, wherein the cold turbine/expander is operatively coupled to a third pinion gear of the plurality of pinion gears.
13. The natural gas liquefaction system of claim 10, wherein the two or more refrigerant compression stages further include at least three refrigerant compression stages, and wherein the upstream refrigerant compression stages further include a first upstream refrigerant compression stage and a second upstream refrigerant compression stage arranged in parallel.
14. The natural gas liquefaction system of claim 13, wherein the second thermal turbine/expander and the upstream refrigerant compression stage are operatively coupled to a first pinion gear of the plurality of pinion gears, and the first thermal turbine/expander and the downstream refrigerant compression stage are operatively coupled to a second pinion gear of the plurality of pinion gears.
15. The natural gas liquefaction system of claim 14, wherein the cold turbine/expander and the second upstream refrigerant compression stage are operatively coupled to a third pinion gear of the plurality of pinion gears.
16. The natural gas liquefaction system of claim 15, wherein at least two pinions of the plurality of pinions are net absorbers of power from the drive assembly.
17. The natural gas liquefaction system of claim 15, wherein the drive assembly is an electric motor, a steam turbine, or a gas turbine.
18. The natural gas liquefaction system of claim 15, wherein the power delivered to two of the plurality of pinions differs by no more than 10%.
19. The natural gas liquefaction system of claim 1, wherein the compressed natural gas-containing feed stream is at a pressure greater than a critical pressure of natural gas.
20. The natural gas liquefaction system of claim 1, wherein the feed stream comprising compressed natural gas is at a pressure between about 50 bar (a) and 80 bar (a).
21. The natural gas liquefaction system of claim 1, wherein the refrigerant stream comprises greater than about 80% nitrogen by volume.
22. The natural gas liquefaction system of claim 1, wherein the at least one heat exchanger further comprises a plurality of heat exchangers or heat exchange cores, wherein a first heat exchanger or first heat exchange core is configured to liquefy the natural gas-containing feed stream, and a second heat exchanger or second heat exchange core is configured to cool the natural gas-containing feed stream, cool a hot portion of the refrigerant stream, or subcool the liquefied natural gas stream via indirect heat exchange with one or more of the at least three exhaust streams.
23. A method of producing liquefied natural gas comprising the steps of:
(a) Receiving a feed stream comprising purified compressed natural gas;
(b) Liquefying and subcooling a feed stream comprising purified compressed natural gas in at least one heat exchanger via indirect heat exchange with one or more refrigerant streams to produce one or more lower pressure recycle streams and a higher pressure recycle stream;
(c1) Compressing the one or more lower pressure recycle streams in an upstream refrigeration compression stage to produce a compressed refrigerant stream;
(c2) Compressing the higher pressure recycle stream and the compressed refrigerant stream in a downstream refrigeration compression stage to produce a further compressed refrigerant stream;
(d) Cooling the further compressed refrigerant stream in the at least one heat exchanger;
(e1) Extracting a cold portion of the further compressed refrigerant stream from the at least one heat exchanger;
(e2) Extracting a first hot portion of the further compressed refrigerant stream from an intermediate location of the at least one heat exchanger;
(e3) Extracting a second hot portion of the further compressed refrigerant stream from a second intermediate location of the at least one heat exchanger;
(f1) Expanding the cold portion of the compressed refrigerant stream in a cold turbine/expander and producing cold exhaust gas having a temperature of less than-145 ℃;
(f2) Expanding the first hot portion of the compressed refrigerant stream in a first hot turbine/expander to produce a first hot exhaust gas having a temperature colder than about-90 ℃ and hotter than the cold exhaust gas;
(f3) Expanding the second hot portion of the compressed refrigerant stream in a second hot turbine/expander to produce a second hot exhaust gas having a temperature above the critical point temperature of the compressed natural gas containing feed stream and colder than about-15 ℃ and an outlet pressure above the outlet pressures of the cold turbine/expander and the first hot turbine/expander;
(g1) Directing the cold vent gas and the first hot vent gas to the at least one heat exchanger as a refrigeration source to liquefy and subcool the feed stream comprising purified compressed natural gas and to produce one or more lower pressure recycle streams;
(g2) Directing the second hot vent gas to the at least one heat exchanger as a refrigeration source to pre-cool the feed stream comprising purified compressed natural gas and produce a higher pressure recycle stream; (h1) Recycling the one or more lower pressure recycle streams to one or more refrigerant compression stages upstream of the plurality of compression stages; and
(H2) The higher pressure recycle stream is recycled to a downstream refrigerant compression stage of the plurality of compression stages.
24. The method of claim 23, wherein the first heat turbine/expander is configured with an expansion ratio between 4.0 and 5.0 and is further configured to produce more than 45% refrigeration of the step for liquefying and subcooling the feed stream comprising purified compressed natural gas.
25. The method of claim 23, wherein the cold turbine/expander is configured with an expansion ratio between 4.0 and 5.0 and is further configured to produce less than 25% refrigeration of the step for liquefying and subcooling the feed stream comprising purified compressed natural gas.
26. The method of claim 23, wherein the second heat turbine/expander is configured with an expansion ratio between 1.5 and 2.5 and is further configured to produce between about 20% and 35% refrigeration of the step for liquefying and subcooling the feed stream comprising purified compressed natural gas.
27. The method of claim 23, wherein the cold turbine/expander, the first hot turbine/expander, the second hot turbine/expander, the upstream refrigeration compression stage, and the downstream refrigeration compression stage are operatively coupled via an integral gear machine.
28. The method of claim 27, wherein the cold turbine/expander is operatively coupled to a third pinion of the integrated gear machine.
29. The method of claim 23, wherein the feed stream comprising purified compressed natural gas is a natural gas feed stream derived from a biological gas source.
30. The method of claim 23, wherein the feed stream comprising purified compressed natural gas is at a pressure greater than the critical pressure of natural gas.
31. The process of claim 23, wherein the feed stream comprising purified compressed natural gas is at a pressure between about 50 bar (a) and 80 bar (a).
32. The method of claim 23, wherein the refrigerant stream comprises greater than about 80% nitrogen by volume.
CN202280067712.8A 2021-10-13 2022-04-12 System and method for producing liquefied natural gas Pending CN118076848A (en)

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