WO2017013475A1 - Liquefying natural gas - Google Patents

Liquefying natural gas Download PDF

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Publication number
WO2017013475A1
WO2017013475A1 PCT/IB2015/055596 IB2015055596W WO2017013475A1 WO 2017013475 A1 WO2017013475 A1 WO 2017013475A1 IB 2015055596 W IB2015055596 W IB 2015055596W WO 2017013475 A1 WO2017013475 A1 WO 2017013475A1
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Prior art keywords
energy
compression
unit
refrigeration cycle
natural gas
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PCT/IB2015/055596
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French (fr)
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Mehdi MEHRPOOYA
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Mehrpooya Mehdi
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Priority to PCT/IB2015/055596 priority Critical patent/WO2017013475A1/en
Publication of WO2017013475A1 publication Critical patent/WO2017013475A1/en

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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
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0005—Light or noble gases
    • 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
    • F25J1/0052—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
    • 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
    • F25J1/0211—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle
    • F25J1/0214—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR 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
    • F25J1/0225—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 other external refrigeration means not provided before, e.g. heat driven absorption chillers
    • F25J1/0227—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 other external refrigeration means not provided before, e.g. heat driven absorption chillers within a 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
    • 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.
    • F25J1/0281—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
    • F25J1/0284—Electrical motor as the prime mechanical driver
    • 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
    • F25J2240/00—Processes or apparatus involving steps for expanding of process streams
    • F25J2240/70—Steam turbine, e.g. used in a Rankine 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
    • F25J2260/00—Coupling of processes or apparatus to other units; Integrated schemes
    • F25J2260/30—Integration in an installation using renewable energy
    • 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
    • F25J2270/90—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
    • F25J2270/906—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration by heat driven absorption chillers

Definitions

  • the invention relates to a system and a method for liquefying natural gas.
  • Liquefied natural gas is natural gas, predominantly methane, that has been converted to liquid form, for example, for ease of storage. Natural gas may be liquefied at cryogenic temperature of approximately ⁇ 160°C. Refrigeration of the natural gas to the cryogenic processes is generally an energy consuming and costly process. Conventional refrigeration methods are only based on vapor compression-refrigeration cycles, generally with high energy consumption and low efficiency.
  • EP 2564139 A2 describes a processes and apparatus for the liquefaction of natural gas wherein an expansion of a refrigerant through a turbo-expander as part of a refrigeration cycle is used to drive a compressor which increases the pressure of the natural gas feed to the liquefaction process.
  • the process for liquefaction of natural gas comprising the steps of: (a) providing a refrigeration cycle comprising the steps of: (i) compressing a fluid refrigerant; (ii) cooling the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed refrigerant; (iii) work-expanding at least a first portion of the cooled compressed refrigerant from step (ii) in a first turbo-expander to provide an expanded cooled refrigerant; (iv) reheating the expanded cooled refrigerant from step (iii) in a liquefaction heat exchange system to provide a reheated refrigerant; and (v) returning the reheated refrigerant from step (iv) to step (i); (b) providing a natural gas feed stream; (c) compressing the natural gas feed stream; (d) passing the compressed natural gas feed stream directly or indirectly to heat exchange with the expanded cooled refrigerant from step (iii
  • a first aspect of the invention provides a system for liquefying natural gas, the system comprising: - a gas passage for providing a feed natural gas stream; - an absorption refrigeration unit arranged and configured for receiving the feed natural gas stream and for producing a pre-cooled gas stream by cooling the feed natural gas stream in an absorption refrigeration cycle; - a first compression-refrigeration unit arranged and configured for receiving the pre-cooled gas stream and for producing a liquefied gas by cooling the pre-cooled gas stream in a first compression-refrigeration cycle; - a second compression-refrigeration unit integrated with the first compression-refrigeration unit and arranged and configured for receiving the liquefied gas and producing a sub-cooled liquefied gas by cooling the liquefied gas in a second compression-refrigeration cycle; - a hybrid energy generating unit for generating energy, the hybrid energy generating unit comprising a fuel cell and a steam turbine for generating the energy; wherein the energy
  • the above measures involve a gas passage for providing a feed natural gas stream.
  • the gas passage may be, for example, a pipe.
  • the passage may be directly or indirectly connected to a natural gas source.
  • the above measures further involve an absorption refrigeration unit arranged and configured for receiving the feed natural gas stream and for producing a pre-cooled gas stream by cooling the feed natural gas stream in an absorption refrigeration cycle.
  • Absorption refrigeration unties and cycles are generally known in the art per se.
  • An absorption refrigeration cycle may, for example, comprise an evaporation step wherein a liquid refrigerant, an absorption step and a condensation step through a heat exchanger to replenish the supply of liquid refrigerant.
  • the above measures further involve a first compression-refrigeration unit arranged and configured for receiving the pre-cooled gas stream and for producing a liquefied gas by cooling the pre-cooled gas stream in a first compression-refrigeration cycle.
  • Compression-refrigeration units and cycles are generally known in the art per se.
  • a compression-refrigeration unit may, for example, use a circulating liquid refrigerant as the medium which absorbs and removes heat from a medium to be cooled and subsequently rejects that heat elsewhere.
  • the above measures further involve a second compression-refrigeration unit integrated with the first compression-refrigeration unit and arranged and configured for receiving the liquefied gas and producing a sub-cooled liquefied gas by cooling the liquefied gas in a second compression-refrigeration cycle.
  • two integrated cycles may comprise, for example, only one condenser or other components serving both the cycles. Integration of compression-refrigeration units and cycles are generally known in the art per se.
  • the above measures further involve a hybrid energy generating unit for generating energy, the hybrid energy generating unit comprising a fuel cell and a steam turbine for generating the energy, wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle, the second compression-refrigeration cycle.
  • Fuel cells and steam turbines for generating energy are generally known in the art.
  • a fuel cell is a device that generates electricity by a chemical reaction.
  • the fuel cell may be any fuel cell type, e.g. a solid oxide fuel cell, phosphoric acid fuel cell, proton exchange membrane fuel cell, magnesium-Air fuel cell, etc., suitable for generating the energy.
  • the fuel cell and the steam turbine may be configured to generate heat energy and electricity.
  • the electricity and the heat generated by the fuel cell and the steam turbine may be at least partially directly or indirectly to drive the first compression-refrigeration cycle, the second compression-refrigeration cycle and the absorption refrigeration cycle.
  • waste heat generated by the fuel cell and/or the steam turbine may be used, for example to provide a portion or all of a heat due required, for example, in the absorption refrigeration unit.
  • electricity generated by the fuel cell and/or the steam turbine may be used, for example, to drive an electrically powered motor in the first compression-refrigeration unit, the second compression-refrigeration unit and/ or the absorption refrigeration unit so as to drive the first compression-refrigeration cycle, the second compression-refrigeration cycle and/ or the absorption refrigeration cycle.
  • the invention is based on the insight that liquefaction efficiency is improved when the system according to the invention is used for liquefying natural gas wherein an absorption refrigeration unit is coupled to two integrated compression-refrigeration units. Furthermore, the energy consumption of the system for liquefying natural gas is reduced, in particular, by using the hybrid energy generating unit comprising a fuel cell and a steam turbine, according to the invention.
  • the hybrid energy generating unit may generate electricity and waste heat.
  • the waste heat in the hybrid energy generating unit may be, for example, used to at least partially provide a required heat duty in the absorption refrigeration unit.
  • the fuel cell generates a first portion of the energy and the hybrid energy generating unit is arranged and configured such that the first portion of the energy is partially used to drive the steam turbine. This is advantageous in reducing energy consumption of the system.
  • the steam turbine is arranged and configured to receive a portion of the feed natural gas stream and generate a second portion of the energy. This is advantageous in reducing energy consumption of the system.
  • the energy generating unit further comprises a solar unit arranged and configured for generating a third portion of the energy and for providing the third portion of the energy at least partially to the absorption refrigeration unit to drive the absorption refrigeration cycle.
  • a solar unit arranged and configured for generating a third portion of the energy and for providing the third portion of the energy at least partially to the absorption refrigeration unit to drive the absorption refrigeration cycle.
  • the third portion of the energy is partially used to heat ambient air, and the heated ambient air is provided to an air inlet of the fuel cell for generating the first portion of the energy. This is advantageous in further enhancing efficiency of the system and reducing energy consumption.
  • the solar unit further comprises an energy storage sub-unit for storing solar energy.
  • an energy stored in the energy storage sub-unit is used as the third portion of the energy. This is advantageous in reducing energy consumption of the system.
  • the first compression-refrigeration unit and the second compression-refrigeration unit are mixed refrigerant vapor compression units.
  • a first mixed refrigerant used in the first compression-refrigeration unit comprises methane, ethane and ethylene and wherein a second mixed refrigerant used in the second compression-refrigeration unit comprises nitrogen, methane, ethane and ethylene.
  • the absorption refrigeration unit is an ammonia-water mixture absorption unit.
  • the system further comprises a treatment unit arranged and configured for receiving the feed natural gas from the gas passage, purifying the feed natural gas stream and providing the feed natural gas stream to the absorption refrigeration unit.
  • Treatment unites for purifying natural gas are generally known in the art. Natural gas may be purified by, for example, removing common contaminants such as water, carbon dioxide and hydrogen sulfide.
  • the fuel cell comprises a steam reformer arranged and configured for receiving a portion of the feed natural gas from the gas passage, receiving steam from a steam source and generating hydrogen gas.
  • Steam reforming is a generally known method in the art for producing hydrogen, carbon monoxide, or other useful products from hydrocarbon fuels such as natural gas.
  • a reformer may react steam at high temperature with a fuel.
  • the generated hydrogen may be used in different applications e.g. it may be used as a primary feedstock for chemical industry or as commodity for oil refineries.
  • the steam source is the steam turbine. This is advantageous in reducing energy consumption of the system.
  • a method for liquefying natural gas comprising
  • Fig. 1 shows a system 100 for liquefying natural gas.
  • the system 100 comprises a gas passage 105 for providing a feed natural gas stream 05.
  • the system 100 further comprises an absorption refrigeration unit 110 arranged and configured for receiving the feed natural gas stream 05 and for producing a pre-cooled gas stream 010 by cooling the feed natural gas stream 05 in an absorption refrigeration cycle.
  • An absorption refrigeration cycle may comprise an evaporation step wherein liquid refrigerant evaporates by an evaporator in a low partial pressure environment, thus extracting heat from its surroundings e.g. the refrigerator's compartment.
  • the absorption refrigeration cycle may further comprise an absorption step wherein a gaseous refrigerant is absorbed by another liquid e.g. a salt solution, reducing its partial pressure in the evaporator and allowing more refrigerant to evaporate.
  • the absorption refrigeration cycle may further comprise a refrigeration step wherein the refrigerant-saturated liquid is heated, causing the refrigerant to evaporate out, causing an increase in its partial pressure, without a change in total pressure. The refrigerant may be then condensed through a heat exchanger to replenish the supply of liquid refrigerant in the evaporator.
  • a compression-refrigeration unit may use a circulating liquid refrigerant as the medium which absorbs and removes heat from a medium to be cooled and subsequently rejects that heat elsewhere.
  • a compression-refrigeration unit may comprise four components: a compressor, a condenser, a thermal expansion valve also known as a throttle valve or metering device and an evaporator.
  • a circulating refrigerant enters the compressor in a thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well.
  • the hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air.
  • That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the unit and the rejected heat is carried away by either the water or the air.
  • a plurality of each of the component may be used, for example to enhance efficiency of a cycle.
  • the system 100 further comprises a second compression-refrigeration 130 integrated with the first compression-refrigeration unit 120 and arranged and configured for receiving the liquefied gas 020 and producing a sub-cooled liquefied gas 030 by cooling the liquefied gas in a second compression-refrigeration cycle.
  • a second compression-refrigeration 130 integrated with the first compression-refrigeration unit 120 and arranged and configured for receiving the liquefied gas 020 and producing a sub-cooled liquefied gas 030 by cooling the liquefied gas in a second compression-refrigeration cycle.
  • fuel cells and steam turbines for generating energy are generally known in the art.
  • the fuel cell and the steam turbine may be configured to generate heat energy and electricity.
  • the electricity and the heat generated by the fuel cell and the steam turbine may be at least partially directly or indirectly to drive the first compression-refrigeration cycle, the second compression-refrigeration cycle and the absorption refrigeration cycle.
  • Fig. 2 shows a system 200 for liquefying natural gas comprising a gas passage 205 for providing a feed natural gas stream 020, a treatment unit 207 for purifying the feed natural gas stream 020 and producing a purified natural gas stream 027.
  • Treatment unites for purifying natural gas are generally known in the art.
  • Natural gas may be purified by, for example, removing common contaminants such as water, carbon dioxide and hydrogen sulfide.
  • the system 200 further comprises an absorption refrigeration unit 210 arranged and configured for receiving the purified feed natural gas stream 027 and for producing a pre-cooled gas stream 021 by cooling the purified feed natural gas stream 027 in an absorption refrigeration cycle.
  • the absorption refrigeration unit may be an ammonia-water mixture absorption unit.
  • the system 200 further comprises a first compression-refrigeration unit 220 arranged and configured for receiving the pre-cooled gas stream 021 and for producing a liquefied gas 022 by cooling the pre-cooled gas stream 021 in a first compression-refrigeration cycle.
  • the system 200 further comprises a second compression-refrigeration 230 integrated with the first compression-refrigeration unit 220 and arranged and configured for receiving the liquefied gas 022 and producing a sub-cooled liquefied gas 023 by cooling the liquefied gas in a second compression-refrigeration cycle.
  • the first compression-refrigeration unit 220 and the second compression-refrigeration unit 230 may be mixed refrigerant vapor compression units.
  • a first mixed refrigerant used in the first compression-refrigeration unit 220 may comprise methane, ethane and ethylene and a second mixed refrigerant used in the second compression-refrigeration unit 230 may comprise nitrogen, methane, ethane and ethylene.
  • the system 200 further comprises a hybrid energy generating unit 240 for generating energy 024, the hybrid energy generating unit 240 comprising a fuel cell 250, a steam turbine 260 and a solar unit270 for generating the energy 024, wherein the system 200 is arranged and configured such that the energy 024 is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle, the second compression-refrigeration cycle.
  • the fuel cell 250 may generate a first portion of the energy 024 and the hybrid energy generating unit 240 is arranged and configured such that the first portion of the energy is partially used to drive the steam turbine 260.
  • the steam turbine is 260 may receive a portion of the feed natural gas stream 020 and generate a second portion of the energy.
  • the hybrid energy generating unit 240 may further comprises a solar unit 270 arranged and configured for generating a third portion of the energy and for providing the third portion of the energy at least partially to the absorption refrigeration unit 210 to drive the absorption refrigeration cycle.
  • the third portion of the energy may be partially used to heat ambient air, and the heated ambient air may be provided to an air inlet of the fuel cell 250 for generating the first portion of the energy.
  • the solar unit 270 may further comprises an energy storage sub-unit (not shown) for storing solar energy. An energy stored in the energy storage sub-unit may be used as the third portion of the energy.
  • the hybrid energy unit 240 may generate a further output 028 which may, for example comprise, water or hydrogen produced in the fuel cell 250 or further energy which may be used elsewhere in other systems which may be coupled to the system 200, etc.
  • the fuel cell 250 may comprises a steam reformer (not shown) arranged and configured for receiving a portion of the feed natural gas 020 from the gas passage 205, receiving steam from a steam source and generating hydrogen gas 028.
  • the steam source may be the steam turbine 260.
  • Fig. 3 shows an example of a process diagram of a system 300 for liquefying natural gas, comprising absorption refrigeration cycle 302, a first compression-refrigeration cycle 304, a second compression-refrigeration cycle 306, a hybrid energy generation unit 308 and a solar unit 310.
  • Ammonia water mixture may be used as an absorption refrigeration cycle solution in the absorption refrigeration cycle 302.
  • a required heat duty in a generator T-1 tower in the absorption refrigeration cycle 302 may be provided by both the solar unit 310 and a fuel cell stack exhaust gas of the hybrid energy generation unit 308.
  • a required heat duty of a steam turbine power plant may be supplied by a solid oxide fuel cell unit in a heat exchanger 40.
  • Inlet water may be pressurized to about 70bar in P-3 and P-4 pumps.
  • the water may be vaporized by a combustion chamber outlet in a heat exchanger H-2 and then it may follow to steam turbines T-1 and T-2 and produce a portion of a required power in the process.
  • a portion of the outlet stream from T-2 turbine, may follow to the fuel cell unit and provide a required steam for reforming of a fuel gas.
  • Natural gas feed may enter a compressor C-4 and outlet stream may follow to a mixer M-2.
  • the stream may enter a pre-reformer reactor. Reforming reactions up to 25% conversion may be done in the reactor.
  • Pre-reformer outlet may enter a heat exchanger H-3 which may be heated up to 800 oC.
  • the stream may enter the solid oxide fuel cell stack.
  • the solid oxide fuel cell stack outlet may enter an inverter and an outlet of the inverter may be provided as apportion of required power which may be about 55 %.
  • Fuel utilization factor may be 85%. solid oxide fuel cell stack outlet may follow to a combustion chamber.
  • a temperature of combustion chamber outlet may be about 1000 oC and stream may follow to heat exchangers H-3, H-4, H-21, H-2 and H-1 and provide a required heat duty for preheating an inlet stream to the solid oxide fuel cell stack.
  • a portion about 25% of fuel gas reforming may be done in the pre-reforming reactor.
  • Combustion chamber outlet at 1100 oC may follow to heat exchangers H-3 and H-4 and provide a required heat duty in fuel cell unit and also it may follow to heat exchanger H-2 and provide a portion of a required heat duty for heating an inlet air to the fuel cell unit. Then the stream may enter heat exchanger H-2 in a steam cycle.
  • Specifications of the solid oxide fuel cell stack are, as example, presented in Table 1.
  • a solar parabolic trough unit streams may provide a portion of the required heat duty in the fuel cell unit and the absorption refrigeration cycle.
  • Outlet stream from solar field 43 at 390 oC may follow to a heat exchanger HS-1.
  • outlet stream may follow to a generator of the refrigeration cycle R-1 and may provide a portion of its heat duty.
  • a thermal energy storage unit may be used in order to storage a hot working fluid in a condition that a produced heat duty is higher than a required value.
  • Two tanks as hot salt tank and cold salt tank may be used.
  • results of a simulation show that achievable overall efficiency according to the system 300 described above is 60%. It is noted that required power is about 30% lower than conventional mixed fluid cascade process (MFC) which generally use three mixed refrigerant cycles are for liquefaction of the natural gas. Furthermore, a required heat transfer area in multi stream heat exchangers which is of important may decrease up to 35%. As such, operating and capital costs of the liquefaction process compared to an MFC process may decrease significantly.
  • MFC mixed fluid cascade process
  • Fig. 4 shows a method 400 for liquefying natural gas.
  • the method 400 comprises providing 410 a feed natural gas stream.
  • the method 400 further comprises cooling 420 the feed natural gas stream in an absorption refrigeration cycle so as to produce a pre-cooled gas stream.
  • the method 400 further comprises cooling 430 the pre-cooled gas stream in a first compression-refrigeration cycle so as to produce a liquefied gas.
  • the method 400 further comprises cooling 440 the liquefied gas in a second compression-refrigeration cycle integrated with the first compression-refrigeration cycle so as to produce a sub-cooled liquefied gas.
  • the method 400 further comprises energy generating 450 comprising generating a first portion of the energy by a fuel cell and a second portion of the energy by a steam turbine, wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle and the second compression-refrigeration cycle.

Abstract

A system and a method are provided for liquefying natural gas. The system comprises a gas passage for providing a feed natural gas stream and an absorption refrigeration unit arranged and configured for producing a pre-cooled gas stream by cooling the feed natural gas stream in an absorption refrigeration cycle. The system further comprise a first compression-refrigeration unit arranged and configured for producing a liquefied gas by cooling the pre-cooled gas stream in a first compression-refrigeration cycle. The system further comprise a second compression-refrigeration integrated with the first compression-refrigeration unit and arranged and configured for producing a sub-cooled liquefied gas by cooling the liquefied gas in a second compression-refrigeration cycle. The system further comprise a hybrid energy generating unit for generating energy, the hybrid energy generating unit comprising a fuel cell and a steam turbine for generating the energy, wherein the energy is at least partially used to drive the absorption refrigeration cycle, first compression-refrigeration cycle and the second compression-refrigeration cycle.

Description

LIQUEFYING NATURAL GAS
The invention relates to a system and a method for liquefying natural gas.
Liquefied natural gas (LNG) is natural gas, predominantly methane, that has been converted to liquid form, for example, for ease of storage. Natural gas may be liquefied at cryogenic temperature of approximately −160°C. Refrigeration of the natural gas to the cryogenic processes is generally an energy consuming and costly process. Conventional refrigeration methods are only based on vapor compression-refrigeration cycles, generally with high energy consumption and low efficiency.
EP 2564139 A2 describes a processes and apparatus for the liquefaction of natural gas wherein an expansion of a refrigerant through a turbo-expander as part of a refrigeration cycle is used to drive a compressor which increases the pressure of the natural gas feed to the liquefaction process. The process for liquefaction of natural gas comprising the steps of: (a) providing a refrigeration cycle comprising the steps of: (i) compressing a fluid refrigerant; (ii) cooling the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed refrigerant; (iii) work-expanding at least a first portion of the cooled compressed refrigerant from step (ii) in a first turbo-expander to provide an expanded cooled refrigerant; (iv) reheating the expanded cooled refrigerant from step (iii) in a liquefaction heat exchange system to provide a reheated refrigerant; and (v) returning the reheated refrigerant from step (iv) to step (i); (b) providing a natural gas feed stream; (c) compressing the natural gas feed stream; (d) passing the compressed natural gas feed stream directly or indirectly to heat exchange with the expanded cooled refrigerant from step (iii) in the liquefaction heat exchange system; and (e) withdrawing a cooled and at least partly condensed natural gas product from the liquefaction heat exchange system; wherein the first turbo-expander is used to drive a compressor to compress the natural gas feed stream in step (c).
It would be advantageous to have a system or method for a more efficient and less costly liquefaction of natural gas.
To better address this concern, a first aspect of the invention provides a system for liquefying natural gas, the system comprising:
- a gas passage for providing a feed natural gas stream;
- an absorption refrigeration unit arranged and configured for receiving the feed natural gas stream and for producing a pre-cooled gas stream by cooling the feed natural gas stream in an absorption refrigeration cycle;
- a first compression-refrigeration unit arranged and configured for receiving the pre-cooled gas stream and for producing a liquefied gas by cooling the pre-cooled gas stream in a first compression-refrigeration cycle;
- a second compression-refrigeration unit integrated with the first compression-refrigeration unit and arranged and configured for receiving the liquefied gas and producing a sub-cooled liquefied gas by cooling the liquefied gas in a second compression-refrigeration cycle;
- a hybrid energy generating unit for generating energy, the hybrid energy generating unit comprising a fuel cell and a steam turbine for generating the energy;
wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle, the second compression-refrigeration cycle.
The above measures involve a gas passage for providing a feed natural gas stream. The gas passage may be, for example, a pipe. The passage may be directly or indirectly connected to a natural gas source.
The above measures further involve an absorption refrigeration unit arranged and configured for receiving the feed natural gas stream and for producing a pre-cooled gas stream by cooling the feed natural gas stream in an absorption refrigeration cycle. Absorption refrigeration unties and cycles are generally known in the art per se. An absorption refrigeration cycle may, for example, comprise an evaporation step wherein a liquid refrigerant, an absorption step and a condensation step through a heat exchanger to replenish the supply of liquid refrigerant.
The above measures further involve a first compression-refrigeration unit arranged and configured for receiving the pre-cooled gas stream and for producing a liquefied gas by cooling the pre-cooled gas stream in a first compression-refrigeration cycle. Compression-refrigeration units and cycles are generally known in the art per se. A compression-refrigeration unit may, for example, use a circulating liquid refrigerant as the medium which absorbs and removes heat from a medium to be cooled and subsequently rejects that heat elsewhere.
The above measures further involve a second compression-refrigeration unit integrated with the first compression-refrigeration unit and arranged and configured for receiving the liquefied gas and producing a sub-cooled liquefied gas by cooling the liquefied gas in a second compression-refrigeration cycle. It is noted that two integrated cycles may comprise, for example, only one condenser or other components serving both the cycles. Integration of compression-refrigeration units and cycles are generally known in the art per se.
The above measures further involve a hybrid energy generating unit for generating energy, the hybrid energy generating unit comprising a fuel cell and a steam turbine for generating the energy, wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle, the second compression-refrigeration cycle.
Fuel cells and steam turbines for generating energy are generally known in the art. A fuel cell is a device that generates electricity by a chemical reaction. The fuel cell may be any fuel cell type, e.g. a solid oxide fuel cell, phosphoric acid fuel cell, proton exchange membrane fuel cell, magnesium-Air fuel cell, etc., suitable for generating the energy.
The fuel cell and the steam turbine may be configured to generate heat energy and electricity. The electricity and the heat generated by the fuel cell and the steam turbine may be at least partially directly or indirectly to drive the first compression-refrigeration cycle, the second compression-refrigeration cycle and the absorption refrigeration cycle. As such, waste heat generated by the fuel cell and/or the steam turbine may be used, for example to provide a portion or all of a heat due required, for example, in the absorption refrigeration unit. In addition, electricity generated by the fuel cell and/or the steam turbine may be used, for example, to drive an electrically powered motor in the first compression-refrigeration unit, the second compression-refrigeration unit and/ or the absorption refrigeration unit so as to drive the first compression-refrigeration cycle, the second compression-refrigeration cycle and/ or the absorption refrigeration cycle.
The invention is based on the insight that liquefaction efficiency is improved when the system according to the invention is used for liquefying natural gas wherein an absorption refrigeration unit is coupled to two integrated compression-refrigeration units. Furthermore, the energy consumption of the system for liquefying natural gas is reduced, in particular, by using the hybrid energy generating unit comprising a fuel cell and a steam turbine, according to the invention. For example, the hybrid energy generating unit may generate electricity and waste heat. The waste heat in the hybrid energy generating unit may be, for example, used to at least partially provide a required heat duty in the absorption refrigeration unit.
Optionally, the fuel cell generates a first portion of the energy and the hybrid energy generating unit is arranged and configured such that the first portion of the energy is partially used to drive the steam turbine. This is advantageous in reducing energy consumption of the system.
Optionally, the steam turbine is arranged and configured to receive a portion of the feed natural gas stream and generate a second portion of the energy. This is advantageous in reducing energy consumption of the system.
Optionally, the energy generating unit further comprises a solar unit arranged and configured for generating a third portion of the energy and for providing the third portion of the energy at least partially to the absorption refrigeration unit to drive the absorption refrigeration cycle. This is advantageous in further enhancing efficiency of the system and reducing energy consumption.
Optionally, the third portion of the energy is partially used to heat ambient air, and the heated ambient air is provided to an air inlet of the fuel cell for generating the first portion of the energy. This is advantageous in further enhancing efficiency of the system and reducing energy consumption.
Optionally, the solar unit further comprises an energy storage sub-unit for storing solar energy.
Optionally, an energy stored in the energy storage sub-unit is used as the third portion of the energy. This is advantageous in reducing energy consumption of the system.
Optionally, the first compression-refrigeration unit and the second compression-refrigeration unit are mixed refrigerant vapor compression units.
Optionally, a first mixed refrigerant used in the first compression-refrigeration unit comprises methane, ethane and ethylene and wherein a second mixed refrigerant used in the second compression-refrigeration unit comprises nitrogen, methane, ethane and ethylene.
Optionally, the absorption refrigeration unit is an ammonia-water mixture absorption unit.
Optionally, the system further comprises a treatment unit arranged and configured for receiving the feed natural gas from the gas passage, purifying the feed natural gas stream and providing the feed natural gas stream to the absorption refrigeration unit. Treatment unites for purifying natural gas are generally known in the art. Natural gas may be purified by, for example, removing common contaminants such as water, carbon dioxide and hydrogen sulfide.
Optionally, the fuel cell comprises a steam reformer arranged and configured for receiving a portion of the feed natural gas from the gas passage, receiving steam from a steam source and generating hydrogen gas. Steam reforming is a generally known method in the art for producing hydrogen, carbon monoxide, or other useful products from hydrocarbon fuels such as natural gas. A reformer may react steam at high temperature with a fuel. The generated hydrogen may be used in different applications e.g. it may be used as a primary feedstock for chemical industry or as commodity for oil refineries.
Optionally, the steam source is the steam turbine. This is advantageous in reducing energy consumption of the system.
In a further aspect of the invention, a method is provided for liquefying natural gas, the method comprising
- providing a feed natural gas stream;
- cooling the feed natural gas stream in an absorption refrigeration cycle so as to produce a pre-cooled gas stream;
- cooling the pre-cooled gas stream in a first compression-refrigeration cycle so as to produce a liquefied gas;
- cooling the liquefied gas in a second compression-refrigeration cycle integrated with the first compression-refrigeration cycle so as to produce a sub-cooled liquefied gas;
- energy generating comprising generating a first portion of the energy by a fuel cell and a second portion of the energy by a steam turbine;
wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle and the second compression-refrigeration cycle.
It will be appreciated by those skilled in the art that two or more of the above-mentioned embodiments, implementations, methods, and/or aspects of the invention may be combined in any way deemed useful.
Modifications and variations of the system, which correspond to the described modifications and variations of the system, can be carried out by a person skilled in the art on the basis of the present description.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings,
Fig.1
shows a system for liquefying natural gas.
Fig.2
shows a system for liquefying natural gas.
Fig.3
shows a process diagram of a system for liquefying natural gas.
Fig.4
shows a method for liquefying natural gas.
Fig. 1 shows a system 100 for liquefying natural gas. The system 100 comprises a gas passage 105 for providing a feed natural gas stream 05. The system 100 further comprises an absorption refrigeration unit 110 arranged and configured for receiving the feed natural gas stream 05 and for producing a pre-cooled gas stream 010 by cooling the feed natural gas stream 05 in an absorption refrigeration cycle.
It is noted that absorption refrigeration unties and cycles are generally known in the art per se. An absorption refrigeration cycle may comprise an evaporation step wherein liquid refrigerant evaporates by an evaporator in a low partial pressure environment, thus extracting heat from its surroundings e.g. the refrigerator's compartment. The absorption refrigeration cycle may further comprise an absorption step wherein a gaseous refrigerant is absorbed by another liquid e.g. a salt solution, reducing its partial pressure in the evaporator and allowing more refrigerant to evaporate. The absorption refrigeration cycle may further comprise a refrigeration step wherein the refrigerant-saturated liquid is heated, causing the refrigerant to evaporate out, causing an increase in its partial pressure, without a change in total pressure. The refrigerant may be then condensed through a heat exchanger to replenish the supply of liquid refrigerant in the evaporator.
The system 100 further comprises a first compression-refrigeration unit 120 arranged and configured for receiving the pre-cooled gas stream 010 and for producing a liquefied gas 020 by cooling the pre-cooled gas stream 010 in a first compression-refrigeration cycle.
It is noted that compression-refrigeration units and cycles are generally known in the art per se. For example, a compression-refrigeration unit may use a circulating liquid refrigerant as the medium which absorbs and removes heat from a medium to be cooled and subsequently rejects that heat elsewhere. Such a compression-refrigeration unit may comprise four components: a compressor, a condenser, a thermal expansion valve also known as a throttle valve or metering device and an evaporator. For example, a circulating refrigerant enters the compressor in a thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the unit and the rejected heat is carried away by either the water or the air. It is noted that optionally, a plurality of each of the component may be used, for example to enhance efficiency of a cycle.
The system 100 further comprises a second compression-refrigeration 130 integrated with the first compression-refrigeration unit 120 and arranged and configured for receiving the liquefied gas 020 and producing a sub-cooled liquefied gas 030 by cooling the liquefied gas in a second compression-refrigeration cycle.
The system 100 further comprises a hybrid energy generating unit 140 for generating energy 040, the hybrid energy generating unit 140 comprising a fuel cell 150 and a steam turbine 160 for generating the energy 040, wherein the system 100 is arranged and configured such that the energy 040 is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle, the second compression-refrigeration cycle.
It is noted that fuel cells and steam turbines for generating energy are generally known in the art. The fuel cell and the steam turbine may be configured to generate heat energy and electricity. The electricity and the heat generated by the fuel cell and the steam turbine may be at least partially directly or indirectly to drive the first compression-refrigeration cycle, the second compression-refrigeration cycle and the absorption refrigeration cycle.
Fig. 2 shows a system 200 for liquefying natural gas comprising a gas passage 205 for providing a feed natural gas stream 020, a treatment unit 207 for purifying the feed natural gas stream 020 and producing a purified natural gas stream 027. Treatment unites for purifying natural gas are generally known in the art. Natural gas may be purified by, for example, removing common contaminants such as water, carbon dioxide and hydrogen sulfide.
The system 200 further comprises an absorption refrigeration unit 210 arranged and configured for receiving the purified feed natural gas stream 027 and for producing a pre-cooled gas stream 021 by cooling the purified feed natural gas stream 027 in an absorption refrigeration cycle. The absorption refrigeration unit may be an ammonia-water mixture absorption unit.
The system 200 further comprises a first compression-refrigeration unit 220 arranged and configured for receiving the pre-cooled gas stream 021 and for producing a liquefied gas 022 by cooling the pre-cooled gas stream 021 in a first compression-refrigeration cycle. The system 200 further comprises a second compression-refrigeration 230 integrated with the first compression-refrigeration unit 220 and arranged and configured for receiving the liquefied gas 022 and producing a sub-cooled liquefied gas 023 by cooling the liquefied gas in a second compression-refrigeration cycle. The first compression-refrigeration unit 220 and the second compression-refrigeration unit 230 may be mixed refrigerant vapor compression units. A first mixed refrigerant used in the first compression-refrigeration unit 220 may comprise methane, ethane and ethylene and a second mixed refrigerant used in the second compression-refrigeration unit 230 may comprise nitrogen, methane, ethane and ethylene.
The system 200 further comprises a hybrid energy generating unit 240 for generating energy 024, the hybrid energy generating unit 240 comprising a fuel cell 250, a steam turbine 260 and a solar unit270 for generating the energy 024, wherein the system 200 is arranged and configured such that the energy 024 is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle, the second compression-refrigeration cycle. The fuel cell 250 may generate a first portion of the energy 024 and the hybrid energy generating unit 240 is arranged and configured such that the first portion of the energy is partially used to drive the steam turbine 260. The steam turbine is 260 may receive a portion of the feed natural gas stream 020 and generate a second portion of the energy. The hybrid energy generating unit 240 may further comprises a solar unit 270 arranged and configured for generating a third portion of the energy and for providing the third portion of the energy at least partially to the absorption refrigeration unit 210 to drive the absorption refrigeration cycle. The third portion of the energy may be partially used to heat ambient air, and the heated ambient air may be provided to an air inlet of the fuel cell 250 for generating the first portion of the energy. The solar unit 270 may further comprises an energy storage sub-unit (not shown) for storing solar energy. An energy stored in the energy storage sub-unit may be used as the third portion of the energy. The hybrid energy unit 240 may generate a further output 028 which may, for example comprise, water or hydrogen produced in the fuel cell 250 or further energy which may be used elsewhere in other systems which may be coupled to the system 200, etc.
It is noted that the fuel cell 250 may comprises a steam reformer (not shown) arranged and configured for receiving a portion of the feed natural gas 020 from the gas passage 205, receiving steam from a steam source and generating hydrogen gas 028. The steam source may be the steam turbine 260.
Fig. 3 shows an example of a process diagram of a system 300 for liquefying natural gas, comprising absorption refrigeration cycle 302, a first compression-refrigeration cycle 304, a second compression-refrigeration cycle 306, a hybrid energy generation unit 308 and a solar unit 310. Ammonia water mixture may be used as an absorption refrigeration cycle solution in the absorption refrigeration cycle 302. A required heat duty in a generator T-1 tower in the absorption refrigeration cycle 302 may be provided by both the solar unit 310 and a fuel cell stack exhaust gas of the hybrid energy generation unit 308. A required heat duty of a steam turbine power plant may be supplied by a solid oxide fuel cell unit in a heat exchanger 40. Inlet water may be pressurized to about 70bar in P-3 and P-4 pumps. The water may be vaporized by a combustion chamber outlet in a heat exchanger H-2 and then it may follow to steam turbines T-1 and T-2 and produce a portion of a required power in the process. A portion of the outlet stream from T-2 turbine, may follow to the fuel cell unit and provide a required steam for reforming of a fuel gas.
Natural gas feed may enter a compressor C-4 and outlet stream may follow to a mixer M-2. The stream may enter a pre-reformer reactor. Reforming reactions up to 25% conversion may be done in the reactor. Pre-reformer outlet may enter a heat exchanger H-3 which may be heated up to 800 ºC. The stream may enter the solid oxide fuel cell stack. The solid oxide fuel cell stack outlet may enter an inverter and an outlet of the inverter may be provided as apportion of required power which may be about 55 %. Fuel utilization factor may be 85%. solid oxide fuel cell stack outlet may follow to a combustion chamber. A temperature of combustion chamber outlet may be about 1000 ºC and stream may follow to heat exchangers H-3, H-4, H-21, H-2 and H-1 and provide a required heat duty for preheating an inlet stream to the solid oxide fuel cell stack. A portion about 25% of fuel gas reforming may be done in the pre-reforming reactor. Combustion chamber outlet at 1100 ºC may follow to heat exchangers H-3 and H-4 and provide a required heat duty in fuel cell unit and also it may follow to heat exchanger H-2 and provide a portion of a required heat duty for heating an inlet air to the fuel cell unit. Then the stream may enter heat exchanger H-2 in a steam cycle. Specifications of the solid oxide fuel cell stack are, as example, presented in Table 1.
A solar parabolic trough unit streams may provide a portion of the required heat duty in the fuel cell unit and the absorption refrigeration cycle. Outlet stream from solar field 43 at 390 ºC may follow to a heat exchanger HS-1. Next, outlet stream may follow to a generator of the refrigeration cycle R-1 and may provide a portion of its heat duty. Also a thermal energy storage unit may be used in order to storage a hot working fluid in a condition that a produced heat duty is higher than a required value. Two tanks as hot salt tank and cold salt tank may be used. When the solar unit heat duty is not enough the required heat duty may be provided by changing a pressure drop in the steam cycle turbines.
Results of a simulation show that achievable overall efficiency according to the system 300 described above is 60%. It is noted that required power is about 30% lower than conventional mixed fluid cascade process (MFC) which generally use three mixed refrigerant cycles are for liquefaction of the natural gas. Furthermore, a required heat transfer area in multi stream heat exchangers which is of important may decrease up to 35%. As such, operating and capital costs of the liquefaction process compared to an MFC process may decrease significantly.
Fig. 4 shows a method 400 for liquefying natural gas. The method 400 comprises providing 410 a feed natural gas stream. The method 400 further comprises cooling 420 the feed natural gas stream in an absorption refrigeration cycle so as to produce a pre-cooled gas stream. The method 400 further comprises cooling 430 the pre-cooled gas stream in a first compression-refrigeration cycle so as to produce a liquefied gas. The method 400 further comprises cooling 440 the liquefied gas in a second compression-refrigeration cycle integrated with the first compression-refrigeration cycle so as to produce a sub-cooled liquefied gas. The method 400 further comprises energy generating 450 comprising generating a first portion of the energy by a fuel cell and a second portion of the energy by a steam turbine, wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle and the second compression-refrigeration cycle.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or stages other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Figure PCTXMLIB-appb-I000001

Claims (14)

  1. A system (100) for liquefying natural gas, the system (100) comprising:
    - a gas passage (105) for providing a feed natural gas stream (05);
    - an absorption refrigeration unit (110) arranged and configured for receiving the feed natural gas stream (05) and for producing a pre-cooled gas stream (010) by cooling the feed natural gas stream (05) in an absorption refrigeration cycle;
    - a first compression-refrigeration unit (120) arranged and configured for receiving the pre-cooled gas stream (010) and for producing a liquefied gas (020) by cooling the pre-cooled gas stream (010) in a first compression-refrigeration cycle;
    - a second compression-refrigeration unit (130) integrated with the first compression-refrigeration unit (120) and arranged and configured for receiving the liquefied gas (020) and producing a sub-cooled liquefied gas (030) by cooling the liquefied gas (020) in a second compression-refrigeration cycle;
    - a hybrid energy generating unit (140) for generating energy (040), the hybrid energy generating unit comprising a fuel cell (150) and a steam turbine (160) for generating the energy (040);
    wherein the energy (040) is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle and the second compression-refrigeration cycle.
  2. The system according to claim 1, wherein the fuel cell generates a first portion of the energy and the hybrid energy generating unit is arranged and configured such that the first portion of the energy is partially used to drive the steam turbine.
  3. The system according to any of claim 1 or 2, wherein the steam turbine is arranged and configured to receive a portion of the feed natural gas stream and generate a second portion of the energy.
  4. The system according to any of claims 1 to 3, wherein the energy generating unit further comprises a solar unit arranged and configured for generating a third portion of the energy and for providing the third portion of the energy at least partially to the absorption refrigeration unit to drive the absorption refrigeration cycle.
  5. The system according to claim 4, wherein the third portion of the energy is partially used to heat ambient air, and the heated ambient air is provided to an air inlet of the fuel cell for generating the first portion of the energy.
  6. The system according to any of claims 4 and 5, wherein the solar unit further comprises an energy storage sub-unit for storing solar energy.
  7. The system according to claim 6, wherein an energy stored in the energy storage sub-unit is used as the third portion of the energy.
  8. The system according to any of the preceding claims, wherein the first compression-refrigeration unit and the second compression-refrigeration unit are mixed refrigerant vapor compression units.
  9. The system according to 8, wherein a first mixed refrigerant used in the first compression-refrigeration unit comprises methane, ethane and ethylene and wherein a second mixed refrigerant used in the second compression-refrigeration unit comprises nitrogen, methane, ethane and ethylene.
  10. The system according to any of the preceding claims, wherein the absorption refrigeration unit is an ammonia-water mixture absorption unit.
  11. The system according to any of the preceding claims, wherein the system further comprises a treatment unit arranged and configured for:
    - receiving the feed natural gas from the gas passage;
    - purifying the feed natural gas stream; and
    - providing the feed natural gas stream to the absorption refrigeration unit.
  12. The system according to any of the preceding claims, wherein the fuel cell comprises a steam reformer arranged and configured for:
    - receiving a portion of the feed natural gas from the gas passage;
    - receiving steam from a steam source; and
    - generating hydrogen gas.
  13. The system according to any of the preceding claims, wherein the steam source is the steam turbine.
  14. A method (400) for liquefying natural gas, the method (400) comprising:
    - providing (410) a feed natural gas stream;
    - cooling (420) the feed natural gas stream in an absorption refrigeration cycle so as to produce a pre-cooled gas stream;
    - cooling (430) the pre-cooled gas stream in a first compression-refrigeration cycle so as to produce a liquefied gas;
    - cooling (440) the liquefied gas in a second compression-refrigeration cycle integrated with the first compression-refrigeration cycle so as to produce a sub-cooled liquefied gas;
    - energy generating (450) comprising generating a first portion of the energy by a fuel cell and a second portion of the energy by a steam turbine;
    wherein the energy is at least partially used to drive the absorption refrigeration cycle, the first compression-refrigeration cycle and the second compression-refrigeration cycle.
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