WO2014088732A1 - Utilisation de fluides frigorigènes de remplacement dans un procédé en cascade optimisé - Google Patents

Utilisation de fluides frigorigènes de remplacement dans un procédé en cascade optimisé Download PDF

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Publication number
WO2014088732A1
WO2014088732A1 PCT/US2013/067814 US2013067814W WO2014088732A1 WO 2014088732 A1 WO2014088732 A1 WO 2014088732A1 US 2013067814 W US2013067814 W US 2013067814W WO 2014088732 A1 WO2014088732 A1 WO 2014088732A1
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Prior art keywords
refrigerant
natural gas
nonflammable
lng
refrigeration cycle
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PCT/US2013/067814
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English (en)
Inventor
Paul Davies
James Lee HARRIS, Jr.
Emery Jay Thomas
Gregg SAPP
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Conocophillips Company
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Publication of WO2014088732A1 publication Critical patent/WO2014088732A1/fr

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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
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
    • C09K5/02—Materials undergoing a change of physical state when used
    • C09K5/04—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa
    • C09K5/041—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa for compression-type refrigeration systems
    • C09K5/042—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa for compression-type refrigeration systems comprising compounds containing carbon and hydrogen only
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
    • C09K5/02—Materials undergoing a change of physical state when used
    • C09K5/04—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa
    • C09K5/041—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa for compression-type refrigeration systems
    • C09K5/044—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa for compression-type refrigeration systems comprising halogenated compounds
    • C09K5/045—Materials undergoing a change of physical state when used the change of state being from liquid to vapour or vice versa for compression-type refrigeration systems comprising halogenated compounds containing only fluorine as halogen
    • 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
    • F25J1/0097—Others, e.g. F-, Cl-, HF-, HClF-, HCl-hydrocarbons etc. or mixtures thereof
    • 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
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0256—Safety aspects of operation
    • 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
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0275—Construction and layout of liquefaction equipments, e.g. valves, machines adapted for special use of the liquefaction unit, e.g. portable or transportable devices
    • F25J1/0277—Offshore use, e.g. during shipping
    • F25J1/0278—Unit being stationary, e.g. on floating barge or fixed platform
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K2205/00—Aspects relating to compounds used in compression type refrigeration systems
    • C09K2205/10—Components
    • C09K2205/12—Hydrocarbons
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K2205/00—Aspects relating to compounds used in compression type refrigeration systems
    • C09K2205/22—All components of a mixture being fluoro compounds

Definitions

  • the present invention relates generally to methods for liquefying natural gas. More particularly, but not by way of limitation, embodiments of the present invention include methods and systems for liquefying natural gas using nonflammable refrigerants.
  • Natural gas is an important resource widely used as energy source or as industrial feedstock used in, for example, manufacture of plastics.
  • Comprising primarily of methane natural gas is a mixture of naturally occurring hydrocarbon gases and is typically found in deep underground natural rock formations or other hydrocarbon reservoirs.
  • Other components of natural gas may include, but are not limited to, ethane, propane, carbon dioxide, nitrogen, and hydrogen sulfide.
  • natural gas is transported from source to consumers through pipelines that physically connect reservoir to market.
  • One alternative method of transporting natural gas involves converting natural gas into a liquefied form via liquefaction process. Because natural gas is gaseous under standard atmospheric conditions, it is typically subjected to thermodynamic processes in order to be liquefied. In its liquefied form, natural gas has a specific volume that is significantly lower than its specific volume in its gaseous form. Thus, the liquefaction process greatly increases the ease of transporting and storing natural gas, particularly in cases where pipelines are not available. For example, ocean liners carrying liquefied natural gas tanks can effectively link a natural gas source to a distant market when separated by an ocean.
  • Converting natural gas to its liquefied form can have other economic benefits as well.
  • storing liquefied natural gas (LNG) can help balance out periodic fluctuations in natural gas supply and demand.
  • LNG can be more easily "stockpiled” for later use when natural gas demand is low and/or supply is high.
  • future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
  • a propane pre-cooled mixed refrigerant is used to cool natural gas.
  • the mixed refrigerant typically includes, but is not limited to, nitrogen, methane, ethane, and propane.
  • natural gas is converted into LNG by utilizing multiple refrigerants in one or more mechanical refrigeration cycles that are used to lower the temperature of a natural gas stream.
  • natural gas is first treated to remove contaminants including, but not limited to, C0 2 , water, and mercury before entering the liquefaction section of an LNG plant.
  • the treated gas is then chilled to approximately -260 °F in successively colder heat exchangers that use propane, ethylene, and methane as refrigerants.
  • the refrigerants are pure or substantially pure substances. In other cases, the refrigerants can be mixtures comprising more than one component.
  • the product leaving the methane exchangers is LNG that is ready for storage. The LNG product is then pumped into insulated storage tanks before being loaded on special ships to be transported to LNG import terminals around the world.
  • Vapor cloud explosion can start the released flammable material forms a vapor cloud within a congested or confined area. Ignition of this cloud produces a flame front that accelerates through the congestion and creates a pressure wave. The severity of the pressure wave depends on several factors including, but not limited to, type of fuel released, size of the cloud within the congested/confined area, and degree of congestion/confinement within the cloud. As processing plants become more congested and confined, risk of explosion can increase.
  • LNG facilities are built in sufficiently open spaces in order to reduce the chances of a vapor cloud explosion in the unlikely case that flammable material is released.
  • Other design considerations can also reduce the risk of explosion.
  • the present invention relates generally to methods for liquefying natural gas. More particularly, but not by way of limitation, embodiments of the present invention include methods and systems for liquefying natural gas using nonflammable refrigerants.
  • One example of a method for liquefaction of natural gas comprises the steps of: a) cooling a natural gas stream via indirect heat exchange with a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoromethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof during a first refrigeration cycle; and b) cooling the natural gas stream via indirect heat exchange with a second refrigerant during a second refrigeration cycle.
  • a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoromethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoro
  • Another example of a method for liquefaction of natural gas comprises the steps of: a) providing at least one nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoroethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof; and b) cooling a natural gas stream in an LNG facility via indirect heat exchange with the nonflammable refrigerant.
  • a nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoroethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof.
  • Yet another example of a method for liquefaction of natural gas comprises the steps of: a) cooling a natural gas stream in a LNG facility via indirect heat exchange with a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoroethane, trifluoromethane, hexafluoroethane, tetrafluoroethane, pentafluorethane, trifluoroethane, pentafluoroethane, any derivative thereof, and any combination thereof during a first refrigeration cycle; b) cooling the natural gas stream in the LNG facility via indirect heat exchange with a second refrigerant during a second refrigeration cycle; and c) cooling the natural gas stream in the LNG facility via indirect heat exchange with a third refrigerant during the third refrigeration cycle.
  • a first nonflammable refrigerant selected from the group consisting of: difluoromethane, pentafluoroethane, trifluoromethane, hex
  • Figure 1 is a plot summarizing effects of nonflammable refrigerants on laminar burning velocity as described in Example 1.
  • Figure 2 is a plot summarizing effects of nonflammable refrigerants on laminar burning velocity as described in Example 1.
  • the present invention relates generally to methods for liquefying natural gas. More particularly, but not by way of limitation, embodiments of the present invention include methods and systems for liquefying natural gas using nonflammable refrigerants.
  • Various parameters can lessen or heighten the risk of a vapor cloud explosion.
  • Some of the parameters affecting the risk of vapor cloud explosions include, but are not limited to, degree of congestion, degree of confinement, gas cloud size gas concentration, gas type (reactivity), ignition location, active mitigation measures, and the like.
  • the risk of vapor cloud explosion may be lowered by addressing any one (e.g., gas type reactivity) or more of the parameters.
  • Conventional refrigerants used during LNG process such as methane have relatively low reactivity while other conventional refrigerants have high reactivity (e.g., ethylene) or medium reactivity (e.g., propane).
  • Fuels are typically considered low reactivity if their laminar burning velocities (LBVs) are lower than about 40 cm/s.
  • Medium reactivity fuels typically have LBVs between about 40 to about 75 cm/s.
  • High reactivity fuels have LBVs of greater than about 75 cm/s.
  • reactivity increases as LBV increases.
  • lowering the reactivity of refrigerants used during LNG processes can lower the risk of vapor cloud explosion as well as the overall safety risk basis arising from LNG facilities and related activities.
  • nonflammable refrigerants particularly in certain LNG processes (e.g., cascade LNG processes, floating LNG facilities, etc.) have been non-existent or limited.
  • Alternative approaches involving nonflammable refrigerants are often limited by technical challenges.
  • the physical properties of many nonflammable refrigerants differ from conventional LNG refrigerants (i.e., propane, ethylene, etc.) such that significant modifications to the design of LNG facilities may be required in order to achieve desirable operating efficiency. For example, use of nonflammable refrigerants may require higher operating pressures on the heavies removal unit.
  • the present invention provides compositions and methods related to lowering safety risk associated with LNG facilities and associated activities.
  • a nonflammable refrigerant that is compatible with LNG processes is provided.
  • the nonflammable refrigerant may be used in place of conventional LNG refrigerants (e.g., methane, propane, ethylene, etc.) or may be used in conjunction with conventional LNG refrigerants to form a refrigerant mixture.
  • Other additives may be added to the refrigerant or refrigerant mixture as desired.
  • advantages of certain embodiments of liquefying natural gas methods and systems described herein include, but are not limited to, one or more of the following:
  • the present invention can be implemented in a process/facility used to cool natural gas to its liquefaction temperature, thereby producing LNG.
  • the LNG process generally employs one or more refrigerants to extract heat from the natural gas and then reject the heat to the environment.
  • Certain LNG processes may comprise multiple refrigerants. For example, a first refrigerant may be used to cool a first refrigeration cycle. A second refrigerant may be used to cool a second refrigeration cycle. A third refrigerant may be used to cool a third refrigeration cycle.
  • first, “second", and “third” refer to the relative position of the cycle with respect to each other.
  • the first refrigeration cycle is positioned just upstream of the second refrigeration cycle while the second refrigeration cycle is positioned upstream of the third refrigeration cycle and so forth.
  • An optimized cascade LNG process typically utilizes propane, ethylene, and methane as the first, second, and third refrigerant respectively.
  • the LNG process in accordance with one or more embodiments of the present invention employs a cascade -type refrigeration process that uses a plurality of multi-stage cooling cycles, each employing a different refrigerant composition, to sequentially cool the natural gas stream to lower and lower temperatures.
  • the LNG process is a mixed refrigerant process that employs a combination of two or more refrigerants to cool the natural gas stream in at least one cooling cycle.
  • Natural gas can be delivered to the LNG process at an elevated pressure in the range of from about 500 to about 3,000 pounds per square in absolute (psia), about 500 to about 1,000 psia, or 600 to 800 psia.
  • the temperature of the natural gas delivered to the LNG process can generally be in the range of from about 0 to about 180 °F (about -18 to about 82 °C), or about 20 to about 150 °F (about -7 to about 66 °C), or 60 to 125 °F (about 16 to about 52 °C).
  • cascade LNG process comprising 3 refrigeration cycles involving 3 refrigerants
  • this is not intended to be limiting. It is recognized that a cascade LNG process involving more or less refrigerants and/or refrigeration cycles may be contemplated. Other variations to the cascade LNG process as well as alternative compatible LNG processes may also be contemplated.
  • the present invention can be implemented in an LNG process that employs cascade -type cooling followed by expansion-type cooling.
  • the cascade-type cooling may be carried out in a mechanical refrigeration cycle at an elevated pressure (e.g., about 650 psia) by sequentially passing the natural gas stream through first, second, and third refrigeration cycles employing respective first, second, and third refrigerants.
  • the first and second refrigeration cycles are closed refrigeration cycles
  • the third refrigeration cycle is an open refrigeration cycle that utilizes a portion of the processed natural gas as a source of the refrigerant.
  • the third refrigeration cycle can include a multi-stage expansion cycle to provide additional cooling of the processed natural gas stream and reduce its pressure to near atmospheric pressure.
  • the refrigerant having the highest boiling point can be utilized first, followed by a refrigerant having an intermediate boiling point, and finally by a refrigerant having the lowest boiling point.
  • the refrigerant can be a hydrocarbon-containing refrigerant.
  • the first refrigerant has a mid-boiling point at standard temperature and pressure (i.e., an STP mid-boiling point) within about 20, about 10, or 5 °F of the STP boiling point of pure propane.
  • the first refrigerant can contain predominately propane, propylene, or mixtures thereof.
  • the first refrigerant can contain at least about 75 mole percent propane, at least 90 mole percent propane, or can consist essentially of propane.
  • the second refrigerant has an STP mid- boiling point within about 20, about 10, or 5 °F of the STP boiling point of pure ethylene.
  • the second refrigerant can contain predominately ethane, ethylene, or mixtures thereof.
  • the second refrigerant can contain at least about 75 mole percent ethylene, at least 90 mole percent ethylene, or can consist essentially of ethylene.
  • the third refrigerant has an STP mid-boiling point within about 20, about 10, or 5 °F of the STP boiling point of pure methane.
  • the third refrigerant can contain at least about 50 mole percent methane, at least about 75 mole percent methane, at least 90 mole percent methane, or can consist essentially of methane. At least about 50, about 75, or 95 mole percent of the third refrigerant can originate from the processed natural gas stream.
  • the first refrigeration cycle can cool the natural gas in a plurality of cooling stages/steps (e.g., two to four cooling stages) by indirect heat exchange with the first refrigerant.
  • Each indirect cooling stage of the refrigeration cycles can be carried out in a separate heat exchanger.
  • core-and-kettle heat exchangers are employed to facilitate indirect heat exchange in the first refrigeration cycle.
  • the temperature of the natural gas can be in the range of from about -45 to about -10 °F (about -43 to about -23 °C), or about -40 to about -15 °F (about -40 to about -26 °C), or about -20 to -30 °F (-29 to about -34 °C).
  • a typical decrease in the natural gas temperature across the first refrigeration cycle may be in the range of from about 50 to about 210 °F (about 10 to about 99 °C), about 75 to about 180 °F (about 24 to about 82 °C), or about 100 to about 140 °F (about 38 to about 60 °C).
  • the second refrigeration cycle can cool the natural gas in a plurality of cooling stages/steps (e.g., two to four cooling stages) by indirect heat exchange with the second refrigerant.
  • the indirect heat exchange cooling stages in the second refrigeration cycle can employ separate, core-and-kettle heat exchangers.
  • the temperature drop across the second refrigeration cycle can be in the range of from about 50 to about 180 °F (about 10 to about 82 °C), about 75 to about 150 °F (about 24 to about 66 °C), or about 100 to about 120 °F (about 38 to about 49 °C).
  • the processed natural gas stream can be condensed (i.e., liquefied) in major portion, preferably in its entirety, thereby producing a pressurized LNG-bearing stream.
  • the process pressure at this location is only slightly lower than the pressure of the natural gas fed to the first stage of the first refrigeration cycle.
  • the temperature of the natural gas may be in the range of from about -205 to about -70 °F (about -132 to about -57 °C), about -175 to about -95 °F (about -115 to about -71 °C), or about -140 to about -125 °F (about -96 to about -87 °C).
  • the third refrigeration cycle can include both an indirect cooling section and an expansion-type cooling section.
  • the third refrigeration cycle can employ at least one brazed-aluminum plate-fin heat exchanger.
  • the total amount of cooling provided by indirect heat exchange in the third refrigeration cycle can be in the range of from about 5 to about 60 °F, about 7 to about 50 °F, or 10 to 40 °F.
  • the expansion-type cooling section of the third refrigeration cycle can further cool the pressurized LNG-bearing stream via sequential pressure reduction to approximately atmospheric pressure.
  • Such expansion-type cooling can be accomplished by flashing the LNG-bearing stream to thereby produce a two-phase vapor-liquid stream.
  • the third refrigeration cycle is an open refrigeration cycle, the expanded two-phase stream can be subjected to vapor- liquid separation and at least a portion of the separated vapor phase (i.e., the flash gas) can be employed as the third refrigerant to help cool the processed natural gas stream.
  • the expansion of the pressurized LNG-bearing stream to near atmospheric pressure can be accomplished by using a plurality of expansion steps (i.e., two to four expansion steps) where each expansion step is carried out using an expander.
  • Suitable expanders include, for example, either Joule-Thomson expansion valves or hydraulic expanders.
  • the third stage refrigeration cycle can employ three sequential expansion cooling steps, wherein each expansion step can be followed by a separation of the gas-liquid product.
  • Each expansion-type cooling step can further cool the LNG-bearing stream in the range of from about 10 to about 60 °F, about 15 to about 50 °F, or 25 to 35 °F.
  • the reduction in pressure across the first expansion step can be in the range of from about 80 to about 300 psia, about 130 to about 250 psia, or 175 to 195 psia.
  • the pressure drop across the second expansion step can be in the range of from about 20 to about 110 psia, about 40 to about 90 psia, or 55 to 70 psia.
  • the third expansion step can further reduce the pressure of the LNG-bearing stream by an amount in the range of from about 5 to about 50 psia, about 10 to about 40 psia, or 15 to 30 psia.
  • the liquid fraction resulting from the final expansion stage is an LNG product.
  • the temperature of the LNG product can be in the range of from about -200 to about -300 °F (-129 to about -184 °C), about -225 to about -275 °F (about -143 to about - 170 °C), or about -240 to about -260 °F (about -151 to about -162 °C).
  • the pressure of the LNG product can be in the range of from about 0 to about 40 psia, about 10 to about 20 psia, or 12.5 to 17.5 psia.
  • the natural gas feed stream to the LNG process will usually contain such quantities of C2+ components so as to result in the formation of a C2+ rich liquid in one or more of the cooling stages of the second refrigeration cycle.
  • the sequential cooling of the natural gas in each cooling stage is controlled so as to remove as much of the C2 and higher molecular weight hydrocarbons as possible from the gas, thereby producing a vapor stream predominating in methane and a liquid stream containing significant amounts of ethane and heavier components.
  • This liquid can be further processed via gas-liquid separators employed at strategic locations downstream of the cooling stages.
  • one objective of the gas/liquid separators is to maximize the rejection of the C5+ material to avoid freezing in downstream processing equipment.
  • the gas/liquid separators may also be utilized to vary the amount of C2 through C4 components that remain in the natural gas product to affect certain characteristics of the finished LNG product.
  • gas-liquid separators may be dependant on a number of parameters, such as the C2+ composition of the natural gas feed stream, the desired BTU content of the LNG product, the value of the C2+ components for other applications, and other factors routinely considered by those skilled in the art of LNG plant and gas plant operation.
  • the C2+ hydrocarbon stream or streams may be demethanized via a single stage flash or a fractionation column.
  • the gaseous methane-rich stream can be directly returned at pressure to the liquefaction process.
  • the resulting heavies-rich liquid stream may then be subjected to fractionation in one or more fractionation zones to produce individual streams rich in specific chemical constituents (e.g., C2, C3, C4, and C5+).
  • the nonflammable refrigerants of the present invention may be used during liquefaction of natural gas.
  • Refrigerants utilized in cascade-type refrigeration processes can have successively lower boiling points in order to maximize heat removal from the natural gas stream being liquefied.
  • cascade-type refrigeration processes can include some level of heat integration.
  • a cascade-type refrigeration process can cool one or more refrigerants having a higher volatility via indirect heat exchange with one or more refrigerants having a lower volatility.
  • cascade and mixed-refrigerant LNG systems can employ one or more expansion cooling stages to simultaneously cool the LNG while reducing its pressure to near atmospheric pressure.
  • the nonflammable refrigerant may be used in a floating LNG (FLNG) process.
  • the nonflammable refrigerant may be used in an optimized cascade LNG process.
  • a refrigerant is a substance used in a heat cycle, which can undergo a reversible phase transition from a liquid to a gas during an LNG process.
  • a nonflammable refrigerant according to one or more embodiments generally comprises simple fluorohydrocarbons that are nonflamamble.
  • the nonflammable refrigerant may include, but not limited to, one or more of the following: difluoromethane (sometimes referred to as "R-32”), pentafluoroethane (sometimes referred to as “R-125" or “FE-25”), trifluoromethane (sometimes referred to as “R-23” or “FE-13”), hexafluoroethane (sometimes referred to as "R-116”), derivatives thereof, and mixtures thereof.
  • fluorohydrocarbons may include, but are not limited to, difluoropropane, trifluoropropane, tetrafluoropropane, pentafluoropropane, and the like.
  • the hydrocarbon portion of the fluorohydrocarbon may comprise one or more carbons.
  • the fluorohydrocarbon may comprise between one carbon to about ten carbons.
  • the fluorohydrocarbon may comprise one or more fluorines.
  • the nonflammable refrigerant may comprise a mixture of difluoromethane and pentafluoroethane.
  • a 1 : 1 mixture (by weight) of difluoromethane and pentafluoroethane is sometimes referred to as "R-410A" and has a boiling point of -55.3 °F (-48.5 °C).
  • the nonflammable refrigerant may comprise a mixture of trifluoromethane and hexafluoroethane.
  • a 46:54 mixture (by weight) of trifluoromethane and hexafluoroethane is sometimes referred to as "R-508B" and has a boiling of -126.94 °F (-88.3 °C).
  • each nonflammable refrigerant may be present in about 0.1% up to about 100% by weight.
  • the specific amount of nonflammable refrigerant present in the mixture may be modified by one of ordinary skill in the art as desired. Such modifications may depend on a number of factors including, but not limited to, desired boiling point, cost, availability, and desired maximum LBV to minimize or eliminate the possibility of ignition and/or reduce the overpressure from an ignited vapor cloud.
  • the nonflammable refrigerant may comprise at least one selected from the group consisting of: R-410A, R-508B, R-23, R-125, and any combination thereof.
  • the nonflammable refrigerant may be an azeotropic mixture. In other embodiments, the nonflammable refrigerant may be a zeo tropic mixture or near-azeotropic mixture.
  • the nonflammable refrigerant is substantially free of hydrocarbons.
  • the nonflammable refrigerant includes a hydrocarbon component in an amount ranging from about 0.1% to about 99.9% by volume or about 0.1 % to about 99.9% by volume.
  • the hydrocarbon may be selected from the group consisting of: ethylene, propane, methane, and any combination thereof.
  • a nonflammable refrigerant comprising a mixture of difluoromethane and pentafluoroethane (e.g., R-410A) may be used as the first refrigerant of an optimized cascade LNG process.
  • R-134a (1,1,1,2-tetrafluoroethane), R-125 (pentafluoroethane), R-404a (a blend of 52 wt% trifluoroethane, 44 wt% R-125, and 4 wt% R-134a), or combinations thereof may be used as the first refrigerant.
  • a nonflammable refrigerant comprising a mixture of trifluoromethane and hexafluoroethane (e.g., R-508B) may be used as the second refrigerant of an optimized cascade LNG process.
  • a nonflammable refrigerant e.g., ethylene, propane
  • the chance of fire and explosion in an LNG facility is reduced.
  • the chance of deflagration to detonation transition may be reduced at least in part due to the reduction in equipment spacing.
  • the use of nonflammable refrigerants also allows greater flexibility in design LNG processes.
  • personnel quarters of a floating LNG facility may be located closer to a nonflammable refrigerant loop which reduces plot spacing and allows the nonflammable refrigerant to be used as a utility for other facility spaces.
  • mixtures of difuloromethane and pentafluoroethane e.g., R-410A
  • condense at a lower temperature than propane which reduces energy and capital requirement for the LNG process.
  • difluoromethane, pentafluoroethane, or both may be added to a flammable refrigerant (e.g., propane) as an additive that can reduce the flame speed of the flammable refrigerant.
  • a flammable refrigerant e.g., propane
  • the flame speed may be reduced below deflagration to detonation transition.
  • the flame speed may be reduce to less than or about the flame speed of methane (see Example 2).
  • trifluoromethane and hexafluoroethane and other nonflammable refrigerants may require modifications to conventional optimized cascade LNG processes. For example, such use may require increased pressure in heavies removal column which causes the column to operate in retrograde condensation region. Furthermore, this increased pressure may require, for example, a booster motor for methane compression system or other rearrangement of gas turbine generator drivers. Methane compressor discharge pressures are typically limited to a value lower than that required for the use of trifluoromethane and hexafluoromethane. Thus, it may be desirable to use a physical solvent and include the heavies removal column upstream.
  • At least one of trifluoromethane and hexafluoroethane may be added to a flammable refrigerant (e.g., ethylene) as an additive which can reduce the flame speed of the flammable refrigerant.
  • a flammable refrigerant e.g., ethylene
  • the flame speed may be reduced below deflagration to detonation transition.
  • the flame speed may be reduce to less than or about the flame speed of methane (-0.4 meters per second).
  • the flame speed of ethylene is about 0.75 meters per second.
  • the measured LBV values are summarized in Table 1 below.
  • the reactivity of a compound increases as LBV increases. This reactivity is typically a function of the strength of hydrogen bonding in the compound and not the heat of combustion or the thermal unit (e.g., BTU) value.
  • FIG. 1 illustrates the results of an experiment in which various nonflammable refrigerants and compounds such as hydro fluoro-olefm (HF01234yf), R- 41 OA, and C0 2 were added to propane. As the nonflammable refrigerant fraction increases, the laminar burning velocity decreases.
  • various nonflammable refrigerants and compounds such as hydro fluoro-olefm (HF01234yf), R- 41 OA, and C0 2 were added to propane.
  • FIG. 2 illustrates the results of an experiment in which various nonflammable refrigerants (R-508B and R-23) were added to ethylene. As the nonflammable refrigerant fraction increases, the laminar burning velocity decreases.
  • FIGS. l and 2 illustrate that nonflammable refrigerants may be added to flammable refrigerants to significantly reduce laminar burning velocity of flammable refrigerants. Such plots may be used to determine mixtures having desired LBV values.
  • R410A was used as a nonflammable refrigerant in place of propane. These characteristics were simulated using REFPROP (version 8) in the Aspen Physical Property System and verified against National Institute of Standards and Technology (NIST) tables. For example, the actual air compressor capacity (ACFM) decreased by 10-30% which allows for a smaller compressor bundle. R-410A also provides a greater vapor density (69.4 kg/m 3 ) as compared to propane (25.6 kg/m 3 ) which permits higher system mass flow and reduces pressure drop losses, allowing smaller diameter piping and smaller equipment to obtain the same refrigerant duty achieved by propane. Table 2 below summarizes simulated properties of R410A using Aspen HYSYS®, a modeling software available through Aspentech Technology, Inc. (Burlington, Massachusetts). These results suggest that R410A may be stable and behaves predictably throughout the given temperature range.
  • confinement and related terms refer to the presence of obstructions that prevent flame propagation in any one of three directions (x, y, or z directions). Objects may be confined in one dimension, two dimensions, or three dimensions.
  • “congestion” and related terms refer to the presence of obstacles that cause a flame front to flow around the obstacles thus generating turbulence and accelerating the flame front. More specifically, the terms “low congestion”, “medium congestion”, and “high congestion” may be a context dependent term. For example, “low congestion” may be defined as having about 15% or less area blockage ratio (ABR) and a pitch of greater than about 8D. In some embodiments, “low congestion” may refer to an area that is easy to walk through relatively unimpeded. The term “medium congestion” may refer to an area having between about 15% to about 30% ABR and a pitch of about 4D to about 8D.
  • ABR area blockage ratio
  • "medium congestion” may refer to an area that can be walked through but requires taking an indirect path.
  • the term “high congestion” may refer to an area having more than about 30% ABR and a pitch of less than about 4D. In some embodiments, "high congestion” may refer to an area that cannot be walked through.
  • area blockage ratio refers to the ratio of the volume of congestion to the total volume available.
  • azeotropic mixture refers to a mixture made up of two or more refrigerants with similar boiling points that acts as a single fluid. The components of an azeotropic mixture typically do not separate under normal operating conditions and can be charged as a vapor or liquid.
  • near-azeotropic mixture refers to a mixture made up of two or more refrigerants with different boiling points that, when in a totally liquid or vapor state, act as one component. However, when changing from vapor to liquid or liquid to vapor, the individual refrigerants evaporate or condense at different temperatures.
  • derivative refers to a compound that is derived from a similar compound.

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Abstract

La présente invention concerne des procédés et des systèmes destinés à liquéfier du gaz naturel à l'aide de fluides frigorigènes ininflammables. Les procédés de liquéfaction consistent à refroidir un flux de gaz naturel par le biais d'un échange de chaleur indirect avec un premier fluide frigorigène ininflammable choisi dans le groupe constitué de : difluorométhane, pentafluorométhane, trifluorométhane, hexafluoroéthane, tétrafluoroéthane, pentafluoréthane, trifluoroéthane, pentafluoroéthane, tout dérivé de ceux-ci, et toute combinaison de ceux-ci pendant un premier cycle de réfrigération ; et à refroidir le flux de gaz naturel par le biais d'un échange de chaleur indirect avec un second fluide frigorigène pendant un second cycle de réfrigération.
PCT/US2013/067814 2012-12-04 2013-10-31 Utilisation de fluides frigorigènes de remplacement dans un procédé en cascade optimisé WO2014088732A1 (fr)

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