AU2015388393A1 - Natural gas production system and method - Google Patents

Natural gas production system and method Download PDF

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Publication number
AU2015388393A1
AU2015388393A1 AU2015388393A AU2015388393A AU2015388393A1 AU 2015388393 A1 AU2015388393 A1 AU 2015388393A1 AU 2015388393 A AU2015388393 A AU 2015388393A AU 2015388393 A AU2015388393 A AU 2015388393A AU 2015388393 A1 AU2015388393 A1 AU 2015388393A1
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Prior art keywords
raw material
material gas
refrigerant
heat exchange
production system
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AU2015388393A
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AU2015388393B2 (en
Inventor
Masanori KAMETA
Toshiya MOMOSE
Moritaka Nakamura
Yoshihiro Yamaguchi
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Chiyoda Corp
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Chiyoda Corp
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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
    • F25J1/006—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
    • 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
    • F25J1/0032—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0035—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by gas expansion 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
    • F25J1/003—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
    • 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
    • F25J1/02—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
    • 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
    • F25J2220/64—Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
    • 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
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/08—Cold compressor, i.e. suction of the gas at cryogenic temperature and generally without afterstage-cooler
    • 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
    • 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
    • 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
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/30—Compression of the feed stream
    • 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
    • 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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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Separation By Low-Temperature Treatments (AREA)

Abstract

[Problem] To efficiently liquefy source gas by bringing the source gas cooling curve and the refrigerant temperature rise curve close to each other in source gas cooling using a refrigerant without requiring the addition of external energy. [Solution] A production system 1 is equipped with: a compressor 20 which compresses source gas; a group of refrigerant expanders 54, 63, which includes at least one refrigerant expander for generating power by causing a refrigerant circulating in the production system to expand; and a heat exchanger 25 which cools the source gas compressed by the compressor 20 by heat exchanging the source gas with the refrigerant. The production system 1 is configured so that the compressor 20 compresses the source gas by utilizing the power generated by the refrigerant expander 63 (54).

Description

TITLE OF THE INVENTION NATURAL GAS PRODUCTION SYSTEM AND METHOD TECHNICAL FIELD [0001]
The present invention relates to a natural gas production system and method for producing liquefied natural gas by cooling natural gas.
BACKGROUND ART
[0002]
Natural gas extracted from gas fields is liquefied in a liquefaction plant or other facility so that the gas may be stored and transported in liquid form; that is, as an LNG (liquefied natural gas). Being cooled to around -162 degrees Celsius, liquefied natural gas advantageously has a significantly reduced volume as compared to gaseous natural gas, and is not required to be stored under a high pressure. Generally, a natural gas liquefaction process involves removing impurities such as moisture, acidic gas components and mercury contained in a raw material gas in advance as necessary, and further involves, after removing heavy components (cyclohexane, benzene, toluene, xylene C5+hydrocarbons such as pentane and larger ones or the like) having a relatively high freezing point, liquefying the raw material gas by heat exchange with a refrigerant.
[0003]
Known methods for liquefying raw material gas include various types of liquefaction processes utilizing heat exchange (by a heat exchanger) with a refrigerant, which are widely used. Known refrigerants used for heat exchange include hydrocarbons such as methane, ethane, ethylene, propane and butane, nitrogen and the like as a single-component refrigerant (a refrigerant composed of a single component) or a mixed-component refrigerant. In the liquefaction process of raw material gas, in order to reduce power required for a refrigerant cycle (refrigeration cycle) for cooling a refrigerant (such as power used in a compressor for compressing the refrigerant), it is desirable to reduce a temperature difference between the raw material gas and the refrigerant to be heat-exchanged with each other (that is, to minimize the effective energy loss by bringing the cooling curve of the raw material gas close to the temperature rising curve of the refrigerant).
[0004]
Possible ways to bring the cooling curve of a raw material gas close to the temperature rising curve of a refrigerant include, for example, to perform heat exchange by using different refrigerants each for a corresponding cycle in the process of cooling the raw material (for example, a combination of methane, ethylene, and propane refrigerant cycles) or by using a mixed refrigerant in the entire process. However, the use of different refrigerants or a mixed refrigerant involves a problem of increasing the costs for obtaining, storing and replenishing refrigerants and the facility-related costs for implementing refrigerant cycles. The use of a single refrigerant such as a nitrogen refrigerant in a single phase (gas) can decrease the costs as compared to cases of using different refrigerants or a mixed refrigerant, but involves a problem of a decrease in liquefaction efficiency (refrigeration cycle efficiency etc.) because of the difficulty in bringing the cooling curve of a raw material gas close to the temperature rising curve of the refrigerant to be heat-exchanged.
[0005]
One solution to this problem is shown in US5768912 (Patent Document 1), for example, which discloses a natural gas production system for liquefying a raw material natural gas by cooling the gas with a nitrogen refrigerant, the system including a refrigerant cycle in which a compressed and cooled refrigerant flow is divided into multiple portions and the different portions are fed to respective expansion units and then introduced into respective heat exchangers disposed in series so as to bring the cooling curve of the raw material gas close to the temperature rising curve of the refrigerant, and respective compressors for compressing the refrigerant portions are combined with one another and driven by a common shaft of the expansion units such that energy generated by the expansion of the refrigerant can be recovered. PRIOR ART DOCUMENT (S) PATENT DOCUMENT(S) [0006]
Patent Document 1: US5768912 SUMMARY OF THE INVENTION TASK TO BE ACCOMPLISHED BY THE INVENTION
[0007]
Natural gas production systems of the prior art such as one shown in Patent Document 1 are intended to minimize the effective energy loss by bringing the cooling curve of a raw material gas close to the temperature rising curve of a refrigerant.
[0008]
As a result of intensive research, the inventors of the present invention have found that a liquefaction process of the type disclosed in the above-mentioned Patent Document 1 can be made more efficient without (or with minimized) need for additional energy supplied from outside by pressurizing a raw material gas to be introduced into heat exchangers so as to bring the cooling curve of the raw material gas on the upstream side (high temperature side) close to the temperature rising curve of a refrigerant.
[0009]
The present invention has been made in view of such problems of the prior art, and a primary object of the present invention is to provide a natural gas production system and method for producing liquefied natural gas which can bring the cooling curve of a raw material gas close to the temperature rising curve of a refrigerant, thereby enabling the raw material gas to be liquefied more efficiently without (or with minimized) need for additional energy supplied from outside.
MEANS TO ACCOMPLISH THE TASK
[0010] A first aspect of the present invention provides a natural gas production system (1) for producing liquefied natural gas from a raw material gas including natural gas, the system comprising: a raw material gas compressor (20) for compressing the raw material gas flowing in the production system; a refrigerant expansion unit group (54, 63) including at least one refrigerant expansion unit for generating power by expanding a refrigerant circulating in the production system; and a first heat exchange unit (25) for cooling, by heat exchange with the refrigerant, the raw material gas compressed by the raw material gas compressor, wherein the raw material gas compressor utilizes the power generated by the at least one refrigerant expansion unit to compress the raw material gas.
[0011]
In the natural gas production system according to the first aspect of the present invention, since the raw material gas compressor, which utilizes the power generated by the refrigerant expansion unit group, is located upstream of the first heat exchange unit, the system can reduce the temperature difference between the raw material gas and the refrigerant, thereby enabling the raw material gas to be liquefied more efficiently without (or with minimized) need for additional energy supplied from outside.
[0012]
According to a second aspect of the present invention, in natural gas production system according to the first aspect of the present invention, the system further comprises a second heat exchange unit (26) located downstream of the first heat exchange unit in a flow of the raw material gas for further cooling the raw material gas by heat exchange with the refrigerant.
[0013]
In the natural gas production system according to the second aspect of the present invention, since two or more heat exchangers are used, the system can further reduce the temperature difference between the raw material gas and the refrigerant, thereby enabling the raw material gas to be liquefied more efficiently.
[0014]
According to a third aspect of the present invention, in natural gas production system according to the first or second aspect of the present invention, the refrigerant is comprised of a single component in a single phase.
[0015]
In the natural gas production system according to the third aspect of the present invention, even when the refrigerant is comprised of a single component such as nitrogen in a single phase, which makes it difficult for the system to bring the cooling curve of a raw material gas close to the temperature rising curve of refrigerant to be exchanged, the system ensures that the raw material gas can be liquefied with an improved efficiency.
[0016] A fourth aspect of the present invention provides a natural gas production system (1) for producing liquefied natural gas from a raw material gas including natural gas, the system comprising: a refrigerant expansion unit group (54, 63) including at least one refrigerant expansion unit (63) for generating power by expanding a refrigerant circulating in the production system; a first heat exchange unit (25) for cooling the raw material gas flowing in the system by heat exchange with the refrigerant; a distilling unit (15) located downstream of the first heat exchange unit in a flow of the raw material gas for distilling the raw material gas fed from the first heat exchange unit to minimize or eliminate a heavy component in the raw material gas; a raw material gas compressor (20) located downstream of the distilling unit in the flow of the raw material gas for compressing the raw material gas; and a second heat exchange unit (26) located downstream of the raw material gas compressor in the flow of the raw material gas for further cooling the raw material gas by heat exchange with the refrigerant, wherein the raw material gas compressor utilizes the power generated by the at least one refrigerant expansion unit to compress the raw material gas.
[0017]
In the natural gas liquefaction system according to the fourth aspect of the present invention, since the raw material gas compressor, which utilizes the power generated by the refrigerant expansion unit group, is located downstream of the distilling unit, the system can reduce the temperature difference between the raw material gas and the refrigerant, thereby enabling the raw material gas to be liquefied more efficiently without (or with minimized) need for additional energy supplied from outside. Moreover, in this production system, since the distilling unit located downstream of the heat exchanger, cooling units or other units located upstream of the distilling unit can be omitted. Furthermore, in this production system, since the raw material gas compressor is located between the first and second heat exchangers, the first and second heat exchangers and the raw material gas compressor can be formed into a unitary facility, which enables the entire system to be made compact. In addition, since the distilling unit is located upstream of the raw material gas compressor, the production system can avoid a problem that the raw material gas compressor compresses the raw material gas to a critical state, thereby making it difficult for the distilling unit to treat the raw material gas.
[0018] A fifth aspect of the present invention provides a method for producing liquefied natural gas from a raw material gas including natural gas, the method comprising: a raw material gas compressing step for compressing the raw material gas flowing in a production system; a refrigerant expansion step for generating power by expanding a refrigerant circulating in the production system; and a first heat exchange step for cooling, by heat exchange with the refrigerant, the raw material gas compressed in the raw material gas compressing step, wherein the raw material gas compressing step includes utilizing the power generated by the refrigerant expansion step to compress the raw material gas.
[0019] A sixth aspect of the present invention provides a method for producing liquefied natural gas from a raw material gas including natural gas, the method comprising: a refrigerant expansion step for generating power by expanding a refrigerant circulating in the production system; a first heat exchange step for cooling the raw material gas flowing in the system by heat exchange with the refrigerant; a distilling step for distilling the raw material gas cooled in the first heat exchange step to minimize or eliminate a heavy component in the raw material gas; a raw material gas compressing step for compressing the raw material gas distilled in the distilling step; and a second heat exchange step for further cooling, by heat exchange with the refrigerant, the raw material gas compressed in the raw material gas compressing step, wherein the raw material gas compressing step includes utilizing the power generated by the refrigerant expansion step to compress the raw material gas.
EFFECT OF THE INVENTION
[0020]
As can be appreciated from the foregoing, the present invention allows the natural gas production system to liquefy a raw material gas by cooling the raw material gas with a refrigerant more efficiently by bringing the cooling curve of the raw material gas close to the temperature rising curve of the refrigerant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
Figure 1 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system in accordance with a first embodiment of the present invention;
Figure 2 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system of the prior art as a reference embodiment to be compared with the system of the first embodiment of the present invention;
Figure 3 is a diagram illustrating the cooling curve of a raw material gas and the temperature rising curve of a refrigerant in the natural gas production system shown in Figure 1;
Figure 4 is a diagram illustrating the cooling curve of a raw material gas and the temperature rising curve of a refrigerant in the natural gas production system shown in Figure 2;
Figure 5 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system in accordance with a second embodiment of the present invention; and Figure 6 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system in accordance with a third embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT(S) [0022]
Embodiments of the present invention are described in the following with reference to the appended drawings.
[0023] (First Embodiment)
Figure 1 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system 1 in accordance with a first embodiment of the present invention. The natural gas production system 1 includes units for producing LNG (liquefied natural gas) by cooling a raw material gas including natural gas with a low temperature refrigerant.
[0024]
As shown in Figure 1, after the raw material gas is introduced into the production system 1 via a line LI, the raw material gas is cooled to a temperature that can be liquefied at around atmospheric pressure while being transported through the production system 1. In the present embodiment, the raw material gas supplied to the production system 1 is, for example, a gas containing about 80 to 98 mol% of methane and having a temperature of about 36 degrees Celsius and a pressure of about 5200 kPaA, and which is supplied at a flow rate is about 50,000 kg/hr. However, the present invention is not limited to this embodiment, and the components, temperature, pressure, and flow rate of the raw material gas can be changed as necessary. Examples of such raw material gases include, but not limited to, natural gases obtained in various forms such as shale gas, tight sand gas, coalbed methane, and methane hydrate.
[0025]
As used herein, the term "raw material gas" refers to a processing target material flowing in the production system 1 (including the material in a partially liquefied state in the process), and does not strictly mean the material in a gaseous state. Although detailed descriptions are omitted, the production system 1 may include known natural gas pre-treatment units such as a separation unit for separating natural gas condensate, an acidic gas removal unit for removing acid gas components such as carbon dioxide and hydrogen sulfide, a mercury removal unit for removing mercury, and a moisture removal unit for removing moisture as necessary, and natural gas from which impurities or the like are removed by these units can be used as a raw material gas.
[0026]
The raw material gas introduced in the production system via the line LI is compressed by a compressor 11, cooled by a cooling unit 12, expanded by an expander 13, and then introduced into a distilling unit 15. The compressor 11 is comprised primarily of a centrifugal compressor in which an impeller for compressing the raw material gas is attached to a shaft 16 coaxial with the expander 13. In the cooling unit 12, the raw material gas can be cooled by using part of a refrigerant, which is also used in heat exchangers 25, 26, 27 described later. The expander 13 is comprised primarily of a turbine unit for reducing the pressure of the raw material gas by isentropically expanding the flowing raw material gas and extracting power from the expansion thereof. The power generated by the expansion of the raw material gas in the expander 13 can be transmitted to the compressor 11 via the coaxial shaft 16 and used to compress the raw material gas in the compressor 11.
[0027]
The temperature and pressure of the raw material gas which has been compressed by the compressor 11 are about 39 degrees Celsius and about 5300 kPaA, respectively. The temperature and pressure of the raw material gas which has been cooled by the cooling unit 12 are about -49 degrees Celsius and about 5200 kPaA, respectively, and the temperature and pressure of the raw material gas which has been expanded by the expander 13 are about -54 degrees Celsius and about 4800 kPaA, respectively. The raw material gas is supplied from the expander 13 and introduced to the distilling unit 15 via a line L2.
[0028]
The distilling unit 15 is composed primarily of a rectifying tower including shelves therein, and performs a distilling process for removing heavy components having relatively high freezing points and contained in the raw material gas (i.e. the distilling process reduces each heavy component in concentration to a desired level or less). In the distilling unit 15, a liquid containing relatively high levels of heavy components is discharged from the bottom of the tower of the distilling unit 15. The concentrations of heavy components and the parts of the respective components vary depending on where the natural gas is extracted and other factors, but examples of such heavy components which can be contained in the raw material gas include benzene, toluene, xylene, C5+hydrocarbons such as pentane and larger ones. The distilling unit 15 removes such heavy components from the raw material gas to avoid troubles caused by the coagulation or the like of heavy components in respective units, piping or other parts provided downstream of the distilling unit 15 in the production system 1. Part of the liquid remaining at the bottom of the distilling unit 15 is discharged from the bottom of the tower to a reboiler 18 provided in a line L4 where the liquid is heated by heat medium (steam or oil) supplied from outside in the reboiler 18 and then returned to the distilling unit 15.
[0029]
On the other hand, in the distilling unit 15, the raw material gas containing methane, which has a low boiling point, as a main component (light component) is separated as a tower top distillate, and the separated raw material gas is supplied to a compressor (raw material gas compressor) 20 via a line L5. The temperature and pressure of the raw material gas delivered from the distilling unit 15 to the line L5 are about 27 degrees Celsius and about 4700 kPaA, respectively.
[0030]
The compressor 20 is composed primarily of a centrifugal compressor having an impeller for compressing the raw material gas attached to a shaft 21 coaxial with a refrigerant expander 63 (a second refrigerant expansion unit) as described later, whereby power generated by the expansion of the refrigerant in the expander 63 can be used to compress the raw material gas. In the production system 1, three heat exchangers 25, 26, 27, which function as main heat exchangers, are provided in this order from the upstream side to the downstream side in the flow of the raw material gas in the production system. The raw material gas compressed by the compressor 20 is introduced into the heat exchanger 25 via a line L6. The temperature and pressure of the raw material gas delivered from the compressor 20 to the line L6 are about 98 degrees Celsius and about 10000 kPaA, respectively.
[0031]
Each of the heat exchangers 25, 26, 27 is a plate fin type heat exchanger, and includes a pipe line (hereinafter referred to as "raw material gas pipe line") through which the raw material gas to be cooled flows and a pipe line (hereinafter referred to as "refrigerant pipe line") through which the refrigerant flows. The heat exchangers 25 and 26 are provided with pipe lines for pre-cooling the refrigerant (hereinafter referred to as "pre-cooling pipe line(s)"). The type of the heat exchangers 25, 26, 27 is not limited to this type, and the heat exchangers may be a spool would type heat exchanger.
[0032]
The raw material gas is cooled by flowing through the raw material gas pipe line 31 of the heat exchanger 25 (first heat exchange unit) provided at the most upstream location to thereby have a temperature of about 9 degrees Celsius and a pressure of about 9900 kPaA, and then the cooled raw material gas is introduced into the intermediate heat exchanger 26 (second heat exchange unit) through a line L7. The raw material gas is further cooled by flowing through a raw material gas pipe line 32 of the heat exchanger 26 to thereby have a temperature of about -86 degrees Celsius and a pressure of about 9800 kPaA, and then the further cooled raw material gas is introduced via a line L8 into the heat exchanger 27 (third heat exchange unit) provided at the most downstream location. Thereafter, the raw material gas is further cooled by flowing through a raw material gas pipe line 33 of the heat exchanger 27 to thereby have a temperature of about -155 degrees Celsius and a pressure of about 9600 kPaA and then flows through an expansion valve 35 provided in a line L9 to be introduced into a gas-liquid separating tank 36.
[0033]
When introduced into the gas-liquid separating tank 36, the raw material gas has a temperature of about -162 degrees Celsius and a pressure of about 101 kPaA, respectively, due to throttling expansion in the expansion valve 35, and has become partially in a liquefied state (i.e., LNG). In the gas-liquid separating tank 36, a gas phase component, a part of vaporized natural gas or other gases (for example, nitrogen), is discharged to the outside of the system via a line L10, while a liquid phase component composed primarily of LNG is discharged from a line LI 1. The LNG is sent to a storage unit such as an LNG tank (not shown) by a transporting pump 38 provided on the line LI 1.
[0034]
Any known equipment (or configurations) can be adopted as units used in the production system 1 (including the compressors 11, 20, the cooling unit 12, the expander 13, the distilling unit 15, the heat exchangers 25, 26, 27 and other equipment described later), as long as they achieve necessary functions.
[0035]
Next, a refrigerant cycle in the production system 1 will be described. A refrigerant cycle (refrigeration cycle) using a nitrogen refrigerant is applied to the production system 1, and the nitrogen refrigerant circulates through the system via the heat exchangers 25, 26, 27 for cooling the raw material gas as described above. The nitrogen refrigerant may contain a small amount of gas components other than nitrogen as long as those components do not significantly affect the cooling of the raw material gas. Most preferably, a refrigerant used in the production system 1 is composed primarily of a single component such as nitrogen in a single phase. However any other known refrigerant may also be adopted in the present invention.
[0036]
In the production system 1, the refrigerant which has been used to cool the raw material gas on the upstream side of the raw material gas flow (that is, after being raised in temperature by flowing through a refrigerant pipe line 40 (first refrigerant pipe line) in the heat exchanger 25) is introduced into a compressor (refrigerant compressor) 42 via a line LI2 to be compressed to a prescribed pressure. The temperature and pressure of the refrigerant delivered from the heat exchanger 25 to the line L12 are about 27 degrees Celsius and about 120 kPaA, respectively. The compressor 42 is provided with an intermediate cooling unit 43 for cooling the refrigerant which has been compressed to an intermediate pressure, and a rear cooling unit 44 in a line LI 3 on the downstream side of the compressor 42 for cooling the compressed refrigerant. The temperature and pressure of the refrigerant delivered from the compressor 42 to the line LI3 are about 110 degrees Celsius and about 4300 kPaA, respectively, and the temperature and pressure of the refrigerant after being cooled by the rear cooling unit 44 are about 30 degrees Celsius and about 4200 kPaA, respectively.
[0037]
The refrigerant cooled by the rear cooling unit 44 is introduced into a compressor 45. The compressor 45 includes a centrifugal compressor in which an impeller for compressing the raw material gas is attached to a shaft 46 coaxial with an expander 54 described later. The refrigerant is introduced into the compressor 45 to be compressed to a higher pressure. Two cooling units 47, 48 for cooling the compressed refrigerant are provided in a line L14 on the downstream side of the compressor 45, and the cooled refrigerant is introduced into the heat exchanger 25. The temperature and pressure of the refrigerant delivered from the compressor 45 to the line L14 are about 87 degrees Celsius and about 7000 kPaA, respectively, and the temperature and pressure of the refrigerant after being cooled by the cooling units 47, 48 are about 30 degrees Celsius and about 6900 kPaA, respectively.
[0038]
The refrigerant supplied from the line L14 into the heat exchanger 25 flows through a pre-cooling pipe line 51 (first pre-cooling pipe line), by which the refrigerant is pre-cooled by the refrigerant with a lower temperature flowing in the opposite direction through the refrigerant pipe line 40. The pre-cooled refrigerant is delivered from the heat exchanger 25 to a line L15. The temperature and pressure of the refrigerant delivered to this line LI 5 are about 9 degrees Celsius and about 6900 kPaA, respectively. The downstream end of the line LI 5 branches off into a line LI 6 and a line LI 7 so that one portion of the refrigerant is introduced into an expander 54 via the line LI 6 and the other portion of the refrigerant is introduced into the heat exchanger 26 via the line LI 7. The ratio of the flow rate of the refrigerant portion flowing into the line L16 to that of the refrigerant portion flowing into the line L17 is about 7:3.
[0039]
The expander 54 is a turbine unit for isentropically expanding the flowing refrigerant to reduce the pressure of the refrigerant and taking out power by the expansion thereof. The power generated by the expander 54 can be transmitted to the compressor 45 via the coaxial shaft 46 and used for compressing the refrigerant in the compressor 45. The temperature and pressure of the refrigerant after being expanded by the expander 54 are about -93 degrees Celsius and about 1200 kPaA, respectively. The expanded refrigerant is supplied from the expander 54 to the heat exchanger 26 via a line LI8. More specifically, the downstream end of the line LI 8 is connected to an intermediate portion of a line LI 9 for supplying the refrigerant from the heat exchanger 27 to the heat exchanger 26, and the refrigerant flows through the line LI 8 and joins the refrigerant flowing through the line LI 9 and then introduced into the heat exchanger 26.
[0040]
On the other hand, the refrigerant supplied from the line LI7 into the heat exchanger 26 flows through the pre-cooling pipe line 61 (second pre-cooling pipe line), by which the refrigerant is pre-cooled by the refrigerant with a lower temperature flowing in the opposite direction through a refrigerant pipe line 62 (second refrigerant pipe line). The pre-cooled refrigerant is introduced from the heat exchanger 26 to an expander 63 (second refrigerant expander) via a line L20. The temperature and pressure of the refrigerant delivered from the heat exchanger 26 to the line L20 are about -86 degrees Celsius and about 6900 kPaA, respectively.
[0041]
Like the expander 54, an expander 63 is composed primarily of a turbine unit for reducing the pressure of the refrigerant to and taking out power. As described above, the power generated by the expander 63 is transmitted to the compressor 20 via the coaxial shaft 21 to be used for compressing the raw material gas by the compressor 20. The temperature and pressure of the refrigerant after being expanded by the expander 63 are about -158 degrees Celsius and about 1200 kPaA, respectively. The expanded refrigerant is introduced into the heat exchanger 27 via a line L21.
[0042]
The refrigerant supplied from the line L21 into the heat exchanger 27 flows through a refrigerant pipe line 65, by which the refrigerant is raised in temperature by heat exchange with the raw material gas flowing through the raw material gas pipe line 33 in the opposite direction. Thereafter, the refrigerant is supplied from the heat exchanger 27 to the heat exchanger 26 via the line LI 9. The temperature and pressure of the refrigerant delivered from the heat exchanger 27 to the line L19 are about -88 degrees Celsius and about 1200 kPaA, respectively.
[0043]
The refrigerant supplied from the line LI9 into the heat exchanger 26 flows through the refrigerant pipe line 62, by which the refrigerant is raised in temperature by heat exchange with the raw material gas flowing through the raw material gas pipe line 32 in the opposite direction. Thereafter, the refrigerant is supplied from the heat exchanger 26 to the heat exchanger 25 via a line L22. The temperature and pressure of the refrigerant delivered from the heat exchanger 26 to the line L22 are about -3 degrees Celsius and about 1200 kPaA, respectively. The refrigerant introduced into the heat exchanger 25 flows through the refrigerant pipe line 40 and then is delivered to the line LI 2, by which the circulation of the refrigerant is completed.
[0044] (Reference Embodiment)
Figure 2 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system 100 of the prior art as a reference embodiment to be compared with the above-described system of the first embodiment of the present invention. In the first embodiment and the reference embodiment, like reference numerals refer to like parts, and detailed descriptions as to those parts will be omitted below.
[0045]
The production system 100 of the reference embodiment does not include a compressor corresponding to the compressor 20 located upstream of the heat exchanger 25 in the first embodiment. In the production system 100, a raw material gas with a relatively low-pressure flows through a line L6 and introduced into a heat exchanger 25. The temperature and the pressure of the raw material gas delivered from the line L6 are about 36 degrees Celsius and about 5200 kPaA, respectively. The raw material gas flowing through a raw material gas pipe line 31 of the heat exchanger 25 is cooled to have a temperature of about -45 degrees Celsius and a pressure of about 5000 kPaA and then is introduced into a distilling unit 15 via a line L7a.
[0046]
In the distilling unit 15, a liquid containing a heavy component in relatively high concentration is discharged via a line L3 connected to the bottom of the distilling unit 15. The temperature and pressure of the heavy component liquid discharged from the line L3 are about -48 degrees Celsius and about 4500 kPaA, respectively. On the other hand, in the distilling unit 15, the raw material gas containing methane, which has a low boiling point, as a main component (light component) is separated as a tower top distillate, and the separated raw material gas is introduced into a heat exchanger 26 via a line L7b. The temperature and pressure of the raw material gas delivered from the distilling unit 15 to the line L7b are about -48 degrees Celsius and about 4500 kPaA, respectively.
[0047]
The raw material gas is cooled by flowing through a raw material gas pipe line 32 in the heat exchanger 26 to have a temperature of about -87 degrees Celsius and a pressure of about 4400 kPaA, and then is introduced via a line L8 into a heat exchanger 27. Thereafter, the raw material gas is cooled by flowing through a raw material gas pipe line 33 in a heat exchanger 27 to have a temperature of about -155 degrees Celsius and a pressure of about 4200 kPaA and then flowing through an expansion valve 35 provided in a line L9 to be introduced into a gas-liquid separating tank 36. When introduced into the gas-liquid separating tank 36 from the expansion valve 35, the raw material gas has a temperature of about -162 degrees Celsius and a pressure of about 101 kPaA, respectively, and has become partially in a liquefied state.
[0048]
Next, a refrigerant cycle in the production system 100 will be described. In the production system 100, a nitrogen refrigerant is used as in the first embodiment described above. In the production system 100, the refrigerant after cooling the raw material gas (that is, after being raised in temperature by flowing through a refrigerant pipe line 40 (first refrigerant pipe line) of the heat exchanger 25) is introduced into a compressor 42 via a line L12 to be compressed to a prescribed pressure. The temperature and pressure of the refrigerant delivered from the heat exchanger 25 to the line L12 are about 27 degrees Celsius and about 120 kPaA, respectively. The temperature and pressure of the refrigerant delivered from the compressor 42 to a line LI3 are about 105 degrees Celsius and about 4000 kPaA, respectively, and the temperature and pressure of the refrigerant after being cooled by a rear cooling unit 44 are about 30 degrees Celsius and about 3900 kPaA, respectively.
[0049]
The downstream end of the line LI 3 branches off into a line LI 4a and a line LI 4b so that one portion of the refrigerant is introduced into a compressor 45 via the line LI 4a and the other portion of the refrigerant is introduced into a compressor 145 via the line L14b. The compressor 45 is primarily comprised of a centrifugal compressor in which an impeller for compressing the raw material gas is attached to a shaft 46 coaxial with an expander 54. The refrigerant compressed by the compressor 45 is delivered to the heat exchanger 25 via a line L14c.
[0050]
On the other hand, the compressor 145 is composed primarily of a centrifugal compressor in which an impeller for compressing the refrigerant is attached to a shaft 121 coaxial with an expander 63. Unlike the system of the first embodiment, in the system of this reference embodiment, power generated by the expansion of the raw material gas in the expander 63 is transmitted to the compressor 145 to be used to compress the refrigerant. The refrigerant supplied from the line LI4b into the compressor 14 is compressed by the compressor 145, and then delivered to the heat exchanger 25 via a line L14d.
[0051]
The downstream end of the line LI 4c is connected to an intermediate portion of a line L14d. Thus, after the refrigerant flowing through the line LI4c joins the refrigerant flowing through the line L14d, the refrigerant is cooled in cooling units 47, 48, and then introduced into the heat exchanger 25. When the refrigerant flowing in the line LI4c joins the refrigerant flowing through the line L14d, the temperature and pressure of the joined refrigerant are about 96 degrees Celsius and about 7000 kPaA, respectively, and after being cooled in the cooling units 47, 48, the temperature and pressure of the cooled refrigerant are about 30 degrees Celsius and about 6900 kPaA, respectively.
[0052]
The refrigerant supplied from the line L14d into the heat exchanger 25 flows through a pre-cooling pipe line L51 (first pre-cooling pipe line), by which the refrigerant is pre-cooled by the refrigerant with a lower temperature flowing in the opposite direction through the refrigerant pipe line 40, and then delivered to a line LI 5. The temperature and pressure of the refrigerant delivered to this line LI5 are about -19 degrees Celsius and about 6900 kPaA, respectively. The downstream end of the line LI 5 branches off into a line LI 6 and a line LI 7 so that one portion of the refrigerant is introduced into an expander 54 via the line LI 6. The temperature and pressure of the refrigerant after being expanded by an expander 54 are about -112 degrees Celsius and about 1200 kPaA, respectively. The expanded refrigerant is introduced into the heat exchanger 27 via a line LI8 and a line LI9.
[0053]
On the other hand, the refrigerant flowing through the line LI7 is pre-cooled in the heat exchanger 26 and then supplied from the heat exchanger 26 into the expander 63 via a line L20. The temperature and pressure of the refrigerant after being pre-cooled in the heat exchanger 26 are about -87 degrees Celsius and about 6900 kPaA, respectively. The temperature and pressure of the refrigerant after being expanded in the expander 63 are about -158 degrees Celsius and about 1200 kPaA, respectively. After being expanded in the expander 63, the refrigerant is introduced into the heat exchanger 27 via a line L21.
[0054]
The refrigerant flowing through the line L21 is heated by heat exchange with the raw material gas in the heat exchanger 27 and then supplied from the heat exchanger 27 to the heat exchanger 26 via the line L19. When the refrigerant heated in the heat exchanger 27 joins the refrigerant flowing through the line LI 8, the temperature and pressure of the joined refrigerant are about -104 degrees Celsius and about 1200 kPaA, respectively.
[0055]
The refrigerant is supplied from the line LI9 into the heat exchanger 26, by which the refrigerant is raised in temperature by heat exchange with the raw material gas or the like, and then supplied from the heat exchanger 26 to the heat exchanger 25 via a line L22. The refrigerant introduced into the heat exchanger 25 flows through the refrigerant pipe line 40 and then delivered to the line LI 2, by which the circulation of the refrigerant is completed.
[0056] (Technical effect of the first embodiment)
Figure 3 is a diagram illustrating the cooling curve of a raw material gas and the temperature rising curve of a refrigerant, which is one example of simulation results on the natural gas production system of the first embodiment of the present invention as shown in Figure 1, and Figure 4 is a diagram illustrating the cooling curve of a raw material gas and the temperature rising curve of a refrigerant, which is one example of simulation results on the natural gas production system of the reference embodiment as shown in Figure 2. In Figures 3 and 4, the vertical axis represents the temperatures (degrees Celsius) of the raw material gas and the nitrogen refrigerant, and the horizontal axis represents the heat duties or heat loads (GJ/h) of the raw material gas and the nitrogen refrigerant.
[0057]
As described above, the natural gas production system 1 according to the first embodiment includes the compressor 20 for compressing the raw material gas, the expander 63 for expanding the nitrogen refrigerant to generate the power, the heat exchanger 25 for cooling, by heat exchange with the nitrogen refrigerant, the raw material gas compressed by the compressor 20, wherein the compressor 20 utilizes the power generated by the refrigerant expander 63 to compress the raw material gas. As a result, referring to the cooling curve of the raw material gas and the temperature rising curve of the nitrogen refrigerant of the first embodiment shown in Figure 3, the part of the cooling curve of the raw material gas where the raw material gas is compressed to a higher pressure than a critical pressure in Figure 3 becomes more linear than that of the reference embodiment shown in Figure 4, which means that, in the first embodiment, the system can bring the cooling curve of the raw material gas close to the temperature rising curve of the refrigerant in a middle temperature region (about -30 degrees Celsius to -90 degrees Celsius) and a low temperature region (about -90 degrees Celsius to -158 degrees Celsius).
[0058] (Second Embodiment)
Figure 5 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system in accordance with a second embodiment of the present invention. In the first and second embodiments, like reference numerals refer to like parts and detailed descriptions as to those parts will be omitted below. In the second embodiment, any features which are not discussed below regarding the second embodiment are the same as those in the first embodiment described above.
[0059]
In the second embodiment, a compressor 20 for compressing a raw material gas is connected coaxially with a shaft 46 of an expander 54 for a refrigerant. As a result, the system allows power generated by the expansion of the refrigerant in the expander 54 to be transmitted to the compressor 20 via the coaxial shaft 46 and used to compress the raw material gas in the compressor 20.
[0060]
In this case, the refrigerant cooled by the rear cooling unit 44 is introduced into a compressor 245 via a line L13. The compressor 245 is comprised primarily of a centrifugal compressor in which an impeller for compressing the refrigerant is attached to a shaft 221 coaxial with an expander 63. Two cooling units 47, 48 for cooling the compressed refrigerant are provided in a line L14 downstream of a compressor 42, and the cooled refrigerant is introduced into a heat exchanger 25.
[0061]
In this way, an expander for the refrigerant connected to the compressor 20 for the raw material gas may be changed as appropriate. (The same applies to a third embodiment described later.) In some cases, the system may be configured such that power generated by multiple expanders for the refrigerant is transmitted to and utilized in one or more compressors.
[0062] (Third Embodiment)
Figure 6 is a schematic diagram illustrating a flow of liquefying process in a natural gas production system in accordance with a third embodiment of the present invention. In the first and third embodiments, like reference numerals refer to like parts and detailed descriptions as to those parts will be omitted below. In the third embodiment, any features which are not discussed below regarding the third embodiment are the same as those in the first embodiment described above.
[0063]
In the third embodiment, a distilling unit 15 is located downstream of a heat exchanger 25 in a similar manner to the above-described reference embodiment, and a raw material gas cooled by flowing in a raw material gas pipe line 31 in the heat exchanger 25 is introduced in the distilling unit 15 via a line L7a. In the distilling unit 15, the raw material gas containing methane as a main component (light component) is separated as a tower top distillate, and introduced into a compressor 20 via a line L7b. The compressor 20 is attached to a shaft 21 coaxial with an expander 63 in a similar manner to the first embodiment, which allows power generated in the expander 63 to be used to compress the raw material gas.
[0064]
The raw material gas compressed by the compressor 20 is introduced into a heat exchanger 26 via a line L7c. The raw material gas cooled by flowing through a raw material gas pipe line 32 in the heat exchanger 26 is introduced into a heat exchanger 27 via a line L8, and the subsequent flow of the raw material gas is the same as that in the first embodiment.
[0065]
In this way, the distilling unit 15 may be located between the heat exchanger 25 and the heat exchanger 26. In this case, the tower top distillate of the distilling unit 15 is preferably compressed by the compressor 20 before introducing into the heat exchanger 26.
[0066]
The above-described natural gas production system 1 in accordance with the third embodiment can bring the cooling curve of the raw material gas close to the temperature rising curve of the refrigerant in the same manner as the system of the first embodiment. Moreover, in the production system 1, since the distilling unit 15 is located downstream of the heat exchanger 25, units such as the cooling unit 12 located upstream of the distilling unit 15 as shown in Figure 1 can be omitted. Furthermore, in the production system 1, since the compressor 20 is located between the heat exchangers 25 and 26, the heat exchangers 25, 26, 27 and the compressor 20 can be formed into a unitary facility, which enables the system to be made compact. In addition, since the distilling unit 15 is located upstream of the compressor 20, the production system 1 can avoid a problem that the compressor 20 compresses the raw material gas to a critical state, thereby making it difficult for the distilling unit 15 to treat the raw material gas.
[0067]
Although the present invention has been described based on specific embodiments, these embodiments are merely exemplary and are not intended to limit the scope of the present invention. All the elements of the natural gas production system and method according to the present invention shown in the above embodiments are not necessarily essential and can be appropriately selected as long as they do not deviate from at least the scope of the present invention. All the combinations of elements shown in each of the above embodiments are not necessarily essential and elements may be appropriately selected from different embodiments as long as the combination of the elements do not deviate from at least the scope of the present invention.
GLOSSARY
[0068] 1 production system 15 distilling unit 20 compressor (raw material gas compressor) 25 heat exchanger (first heat exchange unit) 26 heat exchanger (second heat exchange unit) 27 heat exchanger (third heat exchange unit) 31, 32, 33 raw material gas pipe line 40 refrigerant pipe line (first refrigerant pipe line) 42,45 compressor 43 intermediate cooling unit 44 rear cooling unit 47, 48 cooling unit 51 pre-cooling pipe line (first pre-cooling pipe line) 54 expander (refrigerant expansion unit) 61 pre-cooling pipe line (second pre-cooling pipe line) 62 refrigerant pipe line (second refrigerant pipe line) 63 expander (refrigerant expansion unit) 65 refrigerant pipe line

Claims (6)

1. A natural gas production system for producing liquefied natural gas from a raw material gas including natural gas, the system comprising: a raw material gas compressor for compressing the raw material gas flowing in the production system; a refrigerant expansion unit group including at least one refrigerant expansion unit for generating power by expanding a refrigerant circulating in the production system; and a first heat exchange unit for cooling, by heat exchange with the refrigerant, the raw material gas compressed by the raw material gas compressor, wherein the raw material gas compressor utilizes the power generated by the at least one refrigerant expansion unit to compress the raw material gas.
2. The natural gas production system according to claim 1, further comprising a second heat exchange unit located downstream of the first heat exchange unit in a flow of the raw material gas for further cooling the raw material gas by heat exchange with the refrigerant.
3. The natural gas production system according to claim 2, wherein the refrigerant is comprised of a single component in a single phase.
4. A natural gas production system for producing liquefied natural gas from a raw material gas including natural gas, the system comprising: a refrigerant expansion unit group including at least one refrigerant expansion unit for generating power by expanding a refrigerant circulating in the production system; a first heat exchange unit for cooling the raw material gas flowing in the system by heat exchange with the refrigerant; a distilling unit located downstream of the first heat exchange unit in a flow of the raw material gas for distilling the raw material gas fed from the first heat exchange unit to minimize or eliminate a heavy component in the raw material gas; a raw material gas compressor located downstream of the distilling unit in the flow of the raw material gas for compressing the raw material gas; and a second heat exchange unit located downstream of the raw material gas compressor in the flow of the raw material gas for further cooling the raw material gas by heat exchange with the refrigerant, wherein the raw material gas compressor utilizes the power generated by the at least one refrigerant expansion unit to compress the raw material gas.
5. A method for producing liquefied natural gas from a raw material gas including natural gas, the method comprising: a raw material gas compressing step for compressing the raw material gas flowing in a production system; a refrigerant expansion step for generating power by expanding a refrigerant circulating in the production system; and a first heat exchange step for cooling, by heat exchange with the refrigerant, the raw material gas compressed in the raw material gas compressing step, wherein the raw material gas compressing step includes utilizing the power generated by the refrigerant expansion step to compress the raw material gas.
6. A method for producing liquefied natural gas from a raw material gas including natural gas, the method comprising: a refrigerant expansion step for generating power by expanding a refrigerant circulating in the production system; a first heat exchange step for cooling the raw material gas flowing in the system by heat exchange with the refrigerant; a distilling step for distilling the raw material gas cooled in the first heat exchange step to minimize or eliminate a heavy component in the raw material gas; a raw material gas compressing step for compressing the raw material gas distilled in the distilling step; and a second heat exchange step for further cooling, by heat exchange with the refrigerant, the raw material gas compressed in the raw material gas compressing step, wherein the raw material gas compressing step includes utilizing the power generated by the refrigerant expansion step to compress the raw material gas.
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