Dual mixed refrigerant cycle for gas liquefaction

Details Dual mixed refrigerant cycle for gas liquefaction Dual mixed refrigerant cycle for gas liquefaction

2000-08-07

2001-08-07

Doerrler, William (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Liquefaction

C062S335000

Reexamination Certificate

active

06269655

ABSTRACT:

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 are usually 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.
The objectives in the design and operation of current LNG process cycles and equipment have been to minimize energy consumption and maximize LNG production while operating at changing product demand rates and varying ambient temperature conditions. Since LNG production facilities are typically land-based in remote locations, the land area required for plant battery limits has not been a critical factor in plant design and layout.
Numerous mixed refrigerant (MR) LNG cycles have been disclosed in the art. These cycles generally use a first refrigerant which vaporizes at a higher temperature (i.e., the warm or high level MR) in a first heat exchanger (i.e., the warm or high level exchanger) and a second refrigerant which vaporizes at a lower temperature (i.e., the cold or low level MR) in a second heat exchanger (i.e., the cold or low level exchanger). U.S. Pat. No. 4,274,849 describes a dual mixed refrigerant process in which feed gas is first cooled in a separate exchanger using the refrigerant fluid exiting the cold or low level MR heat exchanger. The precooled feed is then further cooled and liquefied in the cold MR exchanger. The vaporized low level refrigerant after compression is cooled against the warm or high level refrigerant in the warm or high level MR exchanger. A disadvantage of this process is that an extra heat exchanger is required for feed precooling.
U.S. Pat. No. 4,112,700 discloses a dual MR process in which the high level MR is boiled at three different pressure levels with interstage compression. This requires the use of multiple heat exchangers or multiple heat exchange zones, which requires multiple return streams to the compressor. Such multiple heat exchange/compression stages have a disadvantage from a thermodynamic perspective, since non-equilibrium streams of differing compositions are mixed interstage in the warm mixed refrigerant compression train. The mixing of streams causes a thermodynamic irreversibility which will result in reduced cycle efficiency.
A dual mixed refrigerant process is described in U.S. Pat. No. 4,525,185 wherein the high level MR is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones, and leads to multiple vessels, valves, and piping associated with the interstage feeds to the high level MR compressor, and increases the area required for the plant. In this process, the feed is first cooled using low level MR exiting the low level MR heat exchanger. The disadvantage of this approach is that an extra heat exchanger is required as in U.S. Pat. No. 4,274,849 cited above. In this process cycle, non-equilibrium streams are mixed interstage in the high level mixed refrigerant compression train, which causes thermodynamic irreversibility and reduces cycle efficiency.
U.S. Pat. No. 4,545,795 discloses a dual MR process wherein the high level MR is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones in the high level MR heat exchanger. In this process, the feed is first cooled using the fluid exiting the low level MR exchanger, and this requires an additional heat exchanger as in U.S. Pat. No. 4,274,849 cited above. This flowsheet also has a disadvantage from a thermodynamic perspective, since non-equilibrium streams are mixed interstage in the high level MR compression train which causes thermodynamic irreversibility as earlier discussed.
A dual mixed refrigerant process is U.S. Pat. No. 4,539,028 in which the high level MR is boiled at three different pressure levels, which requires the use of multiple heat exchangers or heat exchange zones. The low level mixed MR is boiled at two different pressure levels, which also requires the use of multiple heat exchangers or heat exchange zones. In this process, the feed is first cooled using the low level MR, which requires an extra heat exchanger, a disadvantage shared by several of the processes cited above. This cycle also has a disadvantage from a thermodynamic perspective, since non-equilibrium streams are mixed interstage in the mixed refrigerant compression train. This mixing causes a thermodynamic irreversibility which will result in reduced cycle efficiency.
A paper entitled “Liquefaction of Associated Gases” by H. Paradowski et al presented at the 7
th
International Conference on LNG, May 15-19, 1983 describes a dual MR process in which the high level mixed refrigerant is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones. In addition, the feed is first cooled using the low level MR exiting the low level MR exchanger, and this requires an extra heat exchanger. This process also has a disadvantage from a thermodynamic perspective, since high level MR streams are generally not in thermal equilibrium with the interstage stream before the high level and interstage MR streams are mixed in the refrigerant compression train. This mixing of streams into the main flow of the compressor causes a thermodynamic irreversibility which will result in reduced cycle efficiency.
U.S. Pat. No. 4,911,741 discloses a dual MR process in which the high level MR is boiled at three different pressure levels. This requires the use of multiple heat exchangers or heat exchange zones and also has a disadvantage from a thermodynamic perspective as earlier discussed, since streams which are potentially at different temperatures are mixed interstage in the high level mixed refrigerant compression train. This mixing of streams causes thermodynamic irreversibility which will result in reduced cycle efficiency.
A dual MR process is described in U.S. Pat. No. 4,339.253 in which the high level MR is boiled at two different pressure levels. In addition, an interstage liquid stream from the high level MR is boiled at a third pressure. This requires the use of multiple heat exchangers or heat exchange zones. In this process, the feed is initially cooled before heavier hydrocarbon removal by heat exchange with the low level MR vapor exiting warm end of the low level MR exchanger. The disadvantage of this approach is that an extra heat exchanger is required. This heat exchange also increases the pressure drop of the low level MR stream before compression. As in several of the processes described above, this process has a thermodynamic disadvantage since non-equilibrium streams are mixed interstage in the high level MR compression train. The mixing of streams into the main flow causes thermodynamic irreversibility which will result in reduced cycle efficiency.
U.S. Pat. No. 4,094,655 describes a dual MR process where the low level MR is boiled at two different pressure levels, which requires the use of multiple heat exchangers or heat exchange zones. In this process, the high level MR is first cooled using the fluid from the low level MR exchangers, rather than being cooled by the high level mixed refrigerant loop itself. The disadvantage of this approach is that an extra heat exchanger is required. As in several of the processes described above, this process has a thermodynamic disadvantage s

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