Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
  • F25J1/0022—Hydrocarbons, e.g. natural gas
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
  • F25J1/0032—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
  • F25J1/004—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by flash gas recovery
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
  • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
  • F25J1/005—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by expansion of a gaseous refrigerant stream with extraction of work
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/003—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
  • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
  • F25J1/007—Primary atmospheric gases, mixtures thereof
  • F25J1/0072—Nitrogen
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
  • F25J1/0203—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
  • F25J1/0203—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
  • F25J1/0204—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle as a single flow SCR cycle
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
  • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
  • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
  • F25J1/0285—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings
  • F25J1/0288—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J2270/00—Refrigeration techniques used
  • F25J2270/14—External refrigeration with work-producing gas expansion loop
  • F25J2270/16—External refrigeration with work-producing gas expansion loop with mutliple gas expansion loops of the same refrigerant

Definitions

• the present invention relates generally to the technology of natural gas liquefaction, and more particularly to a system and method for natural gas liquefaction employing a plurality of turboexpanders configured into a series arrangement.
• LNG liquefied natural gas
• Many LNG liquefaction plants utilize a refrigeration cycle with mixed refrigerants where cooling is generally achieved by heat exchange using refrigerant with one or more compositions including propane, propylene, ethane, ethylene, methane and nitrogen or mixtures thereof, in a closed loop or open loop configuration.
• Mixed refrigerant cycles are efficient as they can closely approach the cooling curve of the natural gas and multi-component refrigerants at different stages of the liquefaction process utilizing the latent heat of refrigerant vaporization.
• Nitrogen Expander cycles utilizing dual/three turboexpanders in a closed-loop nitrogen refrigeration cycle.
• the nitrogen stream is split into two/three streams before expanded by two/three expanders in parallel to reach different cooling temperature for the liquefaction of natural gas.
• the flow rate adjusting of the split nitrogen streams makes the cooling curves a close fit, which thus improves the process efficiency.
• FIG 12 there is provided a natural gas liquefaction system with a dual expanders in a parallel configuration in the prior art.
• the feed gas 101, gas treatment module 102, main cryogenic heat exchanger 103, natural gas heat exchange means 131 natural gas pressure reduction means 104, flash drum 105, LNG 106, flash gas heat exchange means 132, flash gas compressor 107, flash gas aftercooler 108, fuel gas 109, first refrigerant compressor 111, first refrigerant aftercooler 112, second refrigerant compressor 113, second refrigerant aftercooler 114, first refrigerant recompressor 115 , second refrigerant recompressor 117, fourth refrigerant aftercooler 118, first refrigerant heat exchange means 133 and second refrigerant heat exchange means 134 are similar to the corresponding components shown in FIG 1 (described in detail hereinbelow).
• the first turboexpander 119 and second turboexpander 120 are configured into a parallel arrangement so that the nitrogen stream from the first refrigerant heat exchange means 133 is split into two, one feeding into the first turboexpander 119 and the other into the second turboexpander 120, the downstreams from both expanders are directly fed back into the second refrigerant heat exchange means 134.
• the refrigerant mass flow rate in these processes is high.
• One objective of this invention is to provide an LNG production system with improved refrigeration efficiency.
• the LNG production system comprises a main cryogenic heat exchanger, a natural gas liquefaction subsystem, and a refrigeration subsystem comprising a plurality of refrigerant compressors configured into a series arrangement to perform multi-stage compressions of a refrigerant, a plurality of aftercoolers each of which being coupled to each of the plurality of refrigerant compressors to cool the compressed refrigerant, a plurality of turboexpanders coupled to the last aftercooler and configured into a series configuration to perform multi-stage expansions of the compressed refrigerant, and a plurality of refrigerant heat exchange means coupled to both the first of the plurality of refrigerant compressors and the last of the plurality of turboexpanders, so that all components form a close refrigeration cycle; wherein the main cryogenic heat exchanger facilitates heat exchange between a pressurized natural gas passing through the natural gas liquefaction subsystem
• the main cryogenic heat exchanger is a multi-stream heat exchanger.
• the natural gas liquefaction subsystem comprises a gas treatment module for treating the pressurized natural gas so as to make it suitable for being liquefied, a natural gas heat exchange means fluidly/gaseously coupled with the gas treatment module and disposed within the main cryogenic heat exchanger for enabling the passing-through pressurized natural gas to exchange heat with countercurrent refrigerant flows, and a natural gas pressure reduction means fluidly/liquidusly coupled with the natural gas heat exchange means for controlling the reduction of the pressures of the pressurized liquefied natural gas from the natural gas heat exchange means so as to further reduce the temperature of the pressurized liquefied natural gas, yielding LNG and flash gas.
• the natural gas pressure reduction means is Joule-Thomson (J-T) valve, two-phase expander or liquid expander.
• the refrigeration subsystem comprises a first refrigerant compressor, a first refrigerant aftercooler coupled to the first refrigerant compressor, a second refrigerant compressor coupled to the first refrigerant aftercooler, a second refrigerant aftercooler coupled to the second refrigerant compressor, a first refrigerant recompressor coupled to the second refrigerant aftercooler, a third refrigerant aftercooler coupled to the first refrigerant recompressor, a second refrigerant recompressor coupled to the third refrigerant aftercooler, a fourth refrigerant aftercooler coupled to the second refrigerant recompressor, a first refrigerant heat exchange means disposed within the main cryogenic heat exchanger and coupled to the fourth refrigerant aftercooler to intermediate cool the compressed refrigerant, a first turboexpander coupled to first refrigerant heat exchange means to first expand the compressed refrigerant, a second turboexpander coupled to first refrigerant heat
• the refrigeration subsystem further comprises a third refrigerant heat exchange means disposed within the main cryogenic heat exchanger, wherein the upstream inlet of the third refrigerant heat exchange means is coupled to one dowiistream outlet of the first turboexpander while the downstream outlet of the third refrigerant heat exchange means is coupled to one upstream inlet of the second refrigerant compressor; and wherein during operation, the refrigerant after the expansion by the first turboexpander is split into two streams with a ratio of 30/70 to 60/40, one stream (30-60% of full stream) being introduced into the third refrigerant heat exchange means to serve as a cold stream in the main cryogenic heat exchanger, and the other stream (40-70% of full stream) being further expanded by the second turboexpander and then being introduced into the second refrigerant heat exchange means to serve as the coldest stream for the sub-cooling of the liquefied natural gas.
• a third refrigerant heat exchange means disposed within the main cryogenic heat exchanger, wherein the upstream
• the refrigeration subsystem further comprises an inter-cooler disposed within the main cryogenic heat exchanger between the first and second turboexpanders. In another embodiment of the LNG production system, the refrigeration subsystem further comprises an inter-cooler disposed between one stream from the first turboexpander and the second turboexpander. - In a further embodiment of the LNG production system, the refrigerant subsystem further comprises a third expansion device and a second intercooler, both being disposed between the second turboexpander and second refrigerant heat exchange means.
• the first turboexpander has the option to provide two split refrigerant streams, one feeding back to a fourth refrigerant heat exchange means disposed within the main cryogenic heat exchanger and the other to the second expansion device via the first intercooler.
• the second turboexpander has the option to provide two split refrigerant streams, one feeding back to a fifth refrigerant heat exchange means disposed within the main cryogenic heat exchanger and the other to the third expansion device via the second intercooler.
• the method comprises providing a main cryogenic heat exchanger in which heat exchange occurs, providing a pressurized natural gas stream that flows through the main cryogenic heat exchanger to get liquefied, and providing cold energy to the main cryogenic heat exchanger by a refrigeration device; wherein the refrigeration device comprises a plurality of refrigerant compressors configured into a series arrangement to perform multi-stage compressions of a refrigerant, a plurality of aftercoolers each of which being coupled to each of the plurality of refrigerant compressors to cool the compressed refrigerant, a plurality of turboexpanders coupled to the last aftercooler and configured into a series configuration to perform multi-stage expansions of the compressed refrigerant, and a plurality of refrigerant heat exchange means coupled to both the first of the plurality of refrigerant compressors and the last of the plurality of turboexpanders, so that all components form a close refrigeration cycle.
• the refrigeration device comprises a plurality of refrigerant compressors configured into a series arrangement to perform multi
• FIG 1 is a schematic drawing showing an LNG production system according to one embodiment of the present invention.
• FIG 2 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 3 shows the Heat Flow-Temperature curves of the LNG production system shown in FIG 2.
• FIG 4 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 5 shows the Heat Flow-Temperature curves of the LNG production system shown in FIG 4. "
• FIG 6 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 7 shows the Heat Flow-Temperature curves of the LNG production system shown in FIG 6.
• FIG 8 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 9 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 10 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 1 1 is a schematic drawing showing an LNG production system according to another embodiment of the present invention.
• FIG 12 is a schematic drawing showing an exemplary LNG production system in the prior art.
• the present invention provides a system and method to liquefy the natural gas using expander based refrigeration cycle in a simple and efficient way.
• the system and method uses turboexpanders in series arrangement, resulting in the advantages of simplicity and flexibility, low refrigerant flow rate requirement, low expansion ratio requirement for each expander, and competitive efficiency and power consumption, when compared with the existing prior art processes.
• FIG 1 there is provided an LNG production system in accordance with one embodiment of the present invention.
• the LNG production system 100 comprises a main cryogenic heat exchanger 3, a natural gas liquefaction subsystem, and a refrigeration subsystem, where the main cryogenic heat exchanger 3 facilitates the heat exchange between the natural gas passing through the natural gas liquefaction subsystem and the refrigerant passing through the refrigeration subsystem so that the natural gas in the natural gas liquefaction subsystem is liquefied by the refrigerant in the refrigeration subsystem.
• the main cryogenic heat exchanger 3 is a multi-stream heat exchanger, which integrates the heat transfer of cold and warm streams and optimizes the integrated cooling curve.
• the natural gas liquefaction subsystem comprises a gas treatment module 2 for ensuring that the feed gas 1 is suitable for the natural gas liquefaction process, a natural gas heat exchange means 31 disposed within the main cryogenic heat exchanger 3 for enabling the passing-through natural gas to exchange heat with countercurrent refrigerant or other gaseous flows (e.g., flash gas flow as discussed below), a natural gas pressure reduction means (e.g., Joule-Thomson (J-T) valve, two- phase expander or liquid expander) 4 for controlling the reduction of the pressures of the cooled natural gas from the natural gas heat exchange means 31 so as to further reduce the temperature of the natural gas, yielding two-phase (vapor and liquid) streams, and a flash drum 5 for separating the two-phase streams into LNG 6 and flash gas.
• a natural gas pressure reduction means e.g., Joule-Thomson (J-T) valve, two- phase expander or liquid expander
• All components are fluidly coupled by conventional pipes/conduits; a 2/31 conduit coupling the downstream outlet of the gas treatment module 2 with the upstream inlet of the natural gas heat exchange means 31; a 31/4 conduit coupling the downstream outlet of the natural gas heat exchange means 31 with the upstream inlet of the J-T valve 4, and a 4/5 conduit coupling the downstream outlet of the J-T valve 4 and the upstream inlet of the flash drum 5.
• the LNG 6 is stored by conventional means.
• the natural gas liquefaction subsystem further comprises a flash gas heat exchange means 32 disposed within the main cryogenic heat exchanger 3 for recovering the cold energy by the natural gas flowing through the natural gas heat exchange means 32, a flash gas compressor 7 for compressing the cold energy recovered flash gas, and a flash gas aftercooler 8 for cooling the compressed flash gas to yield fuel gas 9.
• the feed gas 1 from an external source is usually with a certain pressure
• the high pressure natural gas after gas treatment goes through the natural gas heat exchange means 31 disposed within the main cryogenic heat exchanger 3 where it is been liquefied.
• the high pressure liquid which exits from main cryogenic heat exchanger 3 passes to the J-T valve 4 to reduce the pressure to -1.2 bara.
• the pressure reduction of the high pressure stream results in the temperature drop to around -161 °C and formation of a two-phase stream, which is further separated the vapor and liquid in the flash drum 5.
• the liquid is the LNG product, and transferred to a LNG storage tank.
• the flash gas is recovered the cold energy in the main cryogenic heat exchanger 3.
• the cold flash gas serves as part of the refrigerant to recover the cold energy in the main cryogenic heat exchanger 3 before further compressed and used for fuel gas.
• the refrigeration subsystem comprises a plurality of refrigerant compressors performing multi-stage compressions, a plurality of aftercoolers, a plurality of turboexpanders performing multi-stage expansions, and a plurality of refrigerant heat exchange means, where all components are coupled in a series configuration to form a close refrigeration cycle.
• the "plurality" means two or more in the present application.
• the preferable refrigerant is nitrogen (N 2 ).
• the first and second refrigerant compressors 11, 13 can be driven by either electrical motor, gas engine or gas turbine, while the first and second recompressors 15, 17 driven by the second and first turboexpanders 20, 19, respectively.
• the aftercoolers are typically aircooler or watercooler, and cool the compressed refrigerant to a temperature, for example ⁇ 40 °C, depending on the ambient condition.
• the refrigerant discharged from the downstream outlet of the second refrigerant heat exchange means 34 has low pressure (typically 6 bara); the low pressure refrigerant is first compressed to -50 bara by the first and second refrigerant compressors 11, 13, and then further compressed to 90-100 bara by the first and second recompressors 15, 17.
• Each of the downstreams of the refrigerant compressors and recompressors is cooled by one of the four aftercoolers 12, 14, 16, 18, respectively to a temperature depending on the ambient condition.
• the high pressure refrigerant stream from the downstream outlet of the fourth aftercooler 18 enters into the first refrigerant heat exchange means 33 disposed in the main cryogenic heat exchanger 3 to be cooled down to an intermediate temperature, typically ⁇ -27 °C, then into the first turboexpander 19 to expand to a pressure of -24 bara, and then into the second turboexpander 20 to reduce the pressure to ⁇ 7 bara and reach a temperature of— 153 °C.
• the refrigerant stream from the downstream outlet of the second turboexpander 20 at cryogenic temperature passes into the second refrigerant heat exchange means 34 disposed within the main cryogenic heat exchanger 3 and provides the main cold energy to liquefy the natural gas. After the cold is recovered in the main cryogenic heat exchanger 3, the refrigerant stream flows into the first refrigerant compressor 11 again, and re-circulates in the close refrigerant loop.
• the refrigeration subsystem further comprises a third refrigerant heat exchange means 35 disposed within the main cryogenic heat exchanger 3.
• the upstream inlet of the third refrigerant heat exchange means 35 is coupled to one downstream outlet of the first turboexpander 19 via a 19/35 conduit while the downstream outlet of the third refrigerant heat exchange means 35 is coupled to one upstream inlet of the second refrigerant compressor 13 via a 35/13 conduit.
• the refrigerant after the expansion by the first turboexpander 19 is split into two streams with a ratio of 30/70 to 60/40, one stream (30-60% of full stream) being introduced into the third refrigerant heat exchange means 35 to serve as a cold stream in the main cryogenic heat exchanger 3, and the other stream (40-70% of full stream) being further expanded by the second turboexpander 20 and then being introduced into the second refrigerant heat exchange means 34 to serve as the coldest stream for the sub-cooling of the pressurized liquefied natural gas.
• the stream split after the first turboexpander 19 can better distribute the cold energy according to heat demand for the natural gas liquefaction, since more cold energy is required for the condensing service at intermediate temperature than the sub- cooling for the natural gas at low temperature.
• This process in FIG 2 is flexible to the change of natural gas feed pressure, as the temperature difference between the cold and warm streams is evenly approached along the heat flow as shown in FIG 3. With the downstream split after the first turboexpander, the efficiency/specific power requirement improves 15-20% over the process described in FIG 1.
• the refrigeration subsystem further comprises an inter- cooler 36 disposed between the first and second turboexpanders 19, 20.
• the upstream inlet of the inter-cooler 36 is coupled to the downstream outlet of the first turboexpander 19 via a 19/36 conduit, and the downstream outlet of the inter-cooler 36 is coupled to the upstream inlet of the second turboexpander 20.
• the high pressure refrigerant stream from the downstream outlet of the first turboexpander 19 is expanded to a pressure of -24 bara before it enters the inter-cooler 36 to further cool down to— 136 °C, and then enter the second turboexpander 20 to reduce the pressure to -15 bara and reach a temperature of— 153 °C.
• This downstream of the second turboexpander 20 at cryogenic temperature passes into the main cryogenic heat exchanger 3 and provides the main cold energy to liquefy the natural gas. After the cold is recovered in the main cryogenic heat exchanger 3, the refrigerant stream flows into the first refrigerant compressor 11 again, and re-circulates in the close refrigerant loop.
• FIG 5 shows the cooling curve for the above described system shown in FIG 4. With the inter-cooler between the first and second expanders, the efficiency/specific power requirement improves 8-10% over the system and process described in FIG 1.
• FIG 6 there is provided a LNG production system in accordance with another embodiment of the present invention.
• the refrigerant from the first turboexpander 19 is split into two streams.
• the inter-cooler 36 disposed between one stream from the first turboexpander 19 and the second turboexpander 20 via a 37/20 conduit.
• the high pressure refrigerant stream is cooled down to an intermediate temperature, e.g.—32 °C, before entering the first turboexpander 19, and expanded to a pressure of ⁇ 30 bara before it is split into two streams.
• an intermediate temperature e.g.—32 °C
• One stream enters the intercooler 37 disposed within the main cryogenic heat exchanger 3 again to further cool down to— 111 °C, and then enter the second turboexpander 20 to reduce the pressure to -10 bara and reach a temperature of—153 °C.
• This downstream of expander 20 passes into the main cryogenic heat exchanger 3 and provides the main cold energy at low temperature to liquefy the natural gas.
• Another stream split from the downstream of expander 19 directly passes through main cryogenic heat exchanger 3 to provide the cold energy at warm temperature to liquefy the natural gas.
• the both refrigerant streams flow into the first and second refrigerant compressors 11, 13 respectively, and re-circulate in the close refrigerant loop.
• the refrigerant split after the first turboexpander helps better distribute the heat flow of refrigerant between warm and cold temperatures, thus better match the heat requirement of the natural gas during the de-superheating, condensing and sub-cooling phases from the warm to cold temperatures.
• the process in FIG 6 has further 5-7% efficiency improvement and 2-5% refrigerant flow rate reduction from the process in FIG 4.
• the cooling curves for the above processes in FIG 6 are shown in FIG 7.
• FIG 8 there is provided a LNG production system in accordance with another embodiment of the present invention.
• the description of the similar features shown in FIGS 1, 2, 4 and 6 will be omitted if not necessary.
• the refrigerant from the second turboexpander 20 is split into two streams.
• the refrigeration subsystem further comprises a third refrigerant compressor 22 and a fifth refrigerant aftercooler 23 disposed between the second refrigerant compressor 13 and the first refrigerant recompressor 15, a fifth refrigerant heat exchange means 38 for receiving one stream from the second turboexpander 20, a sixth refrigerant heat exchange means 39 for receiving another stream from the second turboexpander 20, and a second J-T valve disposed between the sixth and second refrigerant heat exchange means 39, 34 for low pressure stage expansion.
• the refrigeration subsystem further comprises a third recompressor 26 with an aftercooler 25, and a third turboexpander 24 to substitute the second J-T valve.
• FIGS 8, 9, 10, and 11 show the embodiments for the process using three- stage expansion, to provide three cold streams to supply the cold energy for the natural gas liquefaction.
• the refrigeration subsystem comprises an inter-cooler disposed between one stream from the first turboexpander and the second turboexpander, and a second inter- cooler disposed between one stream from the second turboexpander and the third expander.
• the downstream of the second expanders 20 has the option to split into two streams, with one stream serves as cold stream to supply cold for the natural gas, while the other stream is further cool down to a lower temperature, before it is further expanded by a 3 rd expansion device 21 or 24, as shown in FIGS 8, 9, and 1 1.
• the downstream of the first expanders 19 also has the option to split into two streams, with one stream serves as cold stream to supply cold for the natural gas, while the other stream is further cool down to a lower temperature, before it is further expanded by the second expander 20, as shown in FIGS 8 and 9.
• the overall pressure expansion ratio for the three stages expansion is around 12-15, higher than the two stages expansion in processes described in FIGS 4 and 6. With the increased expansion ratio, the process efficiency can be further improved 4-8%, with refrigerant flow reduction of 30-40% from the process in FIG 6.
• the configuration of the two expanders in series minimized the expansion ratio of each expander compared with a single expander process.
• the expansion ratio for each expander process is ⁇ 4 in dual N 2 expander cycle in series arrangement, while it requires an expander ratio of 9.4 in the single expander process.
• the dual expanders in series configuration possesses the flexibility to accommodate the scenario where no flash gas is produced in the LNG production while still maintaining the temperature approach in the main cryogenic heat exchanger not to exceed 30 °C which is required by widely used ALPEMA standard.
• the flow rate reduction of 19/35 can increase the inlet stream flow of second expander 20, thus can increase the expansion ratio to reach a lower outlet temperature, which further liquefy the natural gas to a lower temperature and reduced the flash gas amount after J-T expansion.
• FIG 1 Dual N 2 Expander in Series - no intercool 0.56 100%
• FIG 2 0.50 118% intercool 14.8
• FIG 4 Dual N 2 Expander in Series - with intercool 0.51 152%
• FIG 6 0.49 149%

Landscapes

• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Mechanical Engineering (AREA)
• Thermal Sciences (AREA)
• General Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Separation By Low-Temperature Treatments (AREA)

Priority Applications (7)

Applications Claiming Priority (1)

Publications (1)

Family

ID=47116264

Family Applications (1)

Country Status (7)

Cited By (3)

Families Citing this family (19)

Citations (2)

Family Cites Families (14)

• 2012
  • 2012-09-07 BR BR112015002174A patent/BR112015002174A2/pt not_active Application Discontinuation
  • 2012-09-07 EP EP12780894.7A patent/EP2893275A1/en not_active Withdrawn
  • 2012-09-07 US US14/417,789 patent/US20150204603A1/en not_active Abandoned
  • 2012-09-07 SG SG11201503594SA patent/SG11201503594SA/en unknown
  • 2012-09-07 CN CN201280075097.1A patent/CN104520660B/zh not_active Expired - Fee Related
  • 2012-09-07 MX MX2014014750A patent/MX2014014750A/es unknown
  • 2012-09-07 WO PCT/SG2012/000327 patent/WO2014039008A1/en active Application Filing

Patent Citations (2)

Non-Patent Citations (3)

Also Published As

Similar Documents

Legal Events