Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-10-12
2002-02-19
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S613000, C062S619000
Reexamination Certificate
active
06347532
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The liquefaction of natural gas at remote sites, transportation of the liquefied natural gas (LNG) to population centers, and storage and vaporization of LNG for local consumption have been successfully practiced for many years around the world. LNG production sites typically are located on land at remote sites having docking facilities for large LNG tankers which transport the LNG to end users.
Numerous process cycles have been developed for LNG production to provide the large refrigeration requirements for liquefaction. Such cycles typically utilize combinations of single-component refrigeration systems using propane or single chlorofluorocarbon refrigerants operated in combination with one or more mixed refrigerant (MR) systems. Well-known mixed refrigerants typically comprise light hydrocarbons and optionally nitrogen, and utilize compositions tailored to the temperature and pressure levels of specific process steps. Dual mixed refrigerant cycles also have been utilized in which the first mixed refrigerant provides initial cooling at warmer temperatures and the second refrigerant provides further cooling at cooler temperatures.
U.S. Pat. No. 3,763,658 discloses a LNG production system which employs a first propane refrigeration circuit which precools a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a lower temperature than the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed.
An alternative LNG production system, described in U.S. Pat. No. 4,065,278, uses a first propane refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed using a valve and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature below the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This process differs from U.S. Pat. No. 3,763,658 cited above in that the distillation of the feed for heavy component removal occurs at a temperature lower than that provided by the first refrigeration circuit, and a pressure substantially lower than the feed pressure.
U.S. Pat. No. 4,404,008 discloses a LNG production system which employs a first propane refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed using a valve and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature lower than the intermediate temperature of the liquid stream, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This prior art differs from U.S. Pat. No. 3,763,658 in that cooling and partial condensation of the mixed refrigerant of the second refrigeration circuit occurs between compression stages. The resulting liquid is then recombined with the resulting vapor stream at a temperature warmer than the lowest temperature of the first refrigeration circuit, and the combined mixed refrigerant stream is then further cooled by the first refrigeration circuit.
An alternative LNG production system is disclosed in U.S. Pat. No. 4,274,849 which system employs a first mixed component refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature lower than the intermediate temperature of the liquid, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. In
FIG. 7
of this reference, the vapor resulting from the separation of the second refrigerant after precooling is further cooled to a temperature lower than that provided by the first refrigeration circuit and separated into liquid and vapor streams.
U.S. Pat. No. 4,539,028 describes a LNG production system which employs a first mixed component refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a lower temperature than the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This patent differs from that of U.S. Pat. No. 4,274,849 described above by the fact that the second refrigerant is vaporized at two different pressures to provide refrigeration.
The state of the art as defined above describes the vaporization of subcooled mixed refrigerant streams to provide refrigeration for natural gas liquefaction wherein the subcooling is provided by a portion of the refrigeration generated by flashing and vaporizing of the subcooled mixed refrigerant streams. Refrigeration for cooling the mixed refrigerant streams and the natural gas feed is provided by the vaporization of mixed refrigerant streams in a main heat exchange zone. Cooling of the mixed refrigerant vapor during and/or after compression is provided by a separate refrigerant such as propane.
Improved efficiency of gas liquefaction processes is highly desirable and is the prime objective of new cycles being developed in the gas liquefaction art. The objective of the present invention, as described below and defined by the claims which follow, is to improve liquefaction efficiency by providing an additional vaporizing refrigerant stream in the main heat exchange zone. Various embodiments are described for the application of this improved refrigeration step which enhance liquefaction efficiency.
BRIEF SUMMARY OF THE INVENTION
The invention relates to a method for providing refrigeration for liquefying a feed gas which comprises:
(1) providing refrigeration from a first recirculating refrigeration circuit which provides refrigeration in a temperature range between a first temperature and a second temperature which is lower than the first temperature;
(2) providing refrigeration from a second recirculating refrigeration circuit in a temperature range between the second temperature and a third temperature which is lower than the second temperature, wherein the first refrigeration circuit provides refrigeration to the second refrigeration circuit in the temperature range between the first temperature and the second temperature;
(3) withdrawing a mixed refrigerant vapor from a main heat exchange zone in the second recirculating refrigeration circuit and compressing the mixed refrigerant vapor to a final highest pressure to yield a compressed mixed refri
Agrawal Rakesh
Daugherty Tamara Lynn
Roberts Mark Julian
Air Products and Chemicals Inc.
Capossela Ronald
Fernbacher John M.
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