Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
2003-03-18
2004-06-01
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S114000, C062S335000
Reexamination Certificate
active
06742357
ABSTRACT:
BACKGROUND OF THE INVENTION
Multiple-loop refrigeration systems are widely used for the liquefaction of gases at low temperatures. In the liquefaction of natural gas, for example, two or three closed-loop refrigeration systems may be integrated to provide refrigeration in successively lower temperature ranges to cool and liquefy the feed gas. Typically, at least one of these closed-loop refrigeration systems uses a multi-component or mixed refrigerant which provides refrigeration in a selected temperature range as the liquid mixed refrigerant vaporizes and cools the feed gas by indirect heat transfer. Systems using two mixed refrigerant systems are well-known, in some applications, a third refrigerant system using a pure component refrigerant such as propane provides initial cooling of the feed gas. This third refrigerant system also may be used to provide a portion of the cooling to condense one or both of the mixed refrigerants after compression. Refrigeration in the lowest temperature range may be provided by a gas expander loop that is integrated with a mixed refrigerant loop operating in a higher temperature range.
In a typical multi-loop mixed refrigerant process for liquefying natural gas, the low level or coldest refrigeration loop provides refrigeration by vaporization in a temperature range of −30 to −165° C. to provide final liquefaction and optional subcooling of cooled feed gas. The refrigerant is completely vaporized in the coldest temperature range and may be returned directly to the refrigerant compressor, for example, as described in representative U.S. Pat. Nos. 6,119,479 and 6,253,574 B1. Alternatively, the completely vaporized refrigerant may warmed before compression to provide precooling of the feed gas as described in U.S. Pat. Nos. 4,274,849 and 4,755,200 or for cooling of refrigerant streams as described in Australian Patent AU-A-43943/85. A common characteristic feature of these typical liquefaction processes is that the refrigerant in the low level or coldest refrigeration loop is completely vaporized while providing refrigeration in the lowest temperature range. Any additional refrigeration provided by the refrigerant prior to compression thus is effected by the transfer of sensible heat from the vaporized refrigerant to other process streams.
In known liquefaction processes that use three integrated closed-loop refrigeration systems, the size of the process equipment in the third or lowest temperature refrigeration system may be smaller relative to the two warmer refrigeration systems. As the process liquefaction capacity is increased, the sizes of the compression and heat exchange equipment in the two warmer systems will reach the maximum sizes available from equipment vendors, while the sizes of the corresponding equipment in the lowest temperature refrigeration system will be smaller than the maximum sizes. In order to further increase the production capacity of this liquefaction process, parallel trains would be needed because of compression and/or heat exchanger size limitations in the two warmer refrigeration systems.
It would be desirable to increase the maximum production capacity of this liquefaction process at the limits of available compressor and heat exchanger sizes, thereby allowing the use of larger single-train liquefaction processes. This need is addressed by the present invention, which provides an integrated refrigeration system having increased production capacity without requiring duplicate parallel equipment for the warmer refrigeration systems.
BRIEF SUMMARY OF THE INVENTION
In one embodiment, the invention relates to a method for liquefying a gas which comprises cooling a feed gas stream successively through first and second temperature ranges to provide a liquefied product, wherein refrigeration for cooling the feed gas stream in the first temperature range is provided by a first vaporizing refrigerant and refrigeration for cooling the stream in the second temperature range is provided by a second vaporizing refrigerant, and further wherein the second vaporizing refrigerant provides additional refrigeration by vaporization at temperatures above a lowest temperature in the first temperature range.
In another embodiment, the invention relates to a method for liquefying a gas which comprises cooling a feed gas stream (
1
) successively through first, second, and third temperature ranges to provide a liquefied product (
13
), wherein refrigeration for cooling the feed gas stream in the first temperature range is provided by a first vaporizing refrigerant (
117
), refrigeration for cooling the stream in the second temperature range is provided by a second vaporizing refrigerant (
213
), and refrigeration for cooling the stream in the third temperature range is provided by a third vaporizing refrigerant (
315
), and further wherein the third vaporizing refrigerant provides additional refrigeration by vaporization (
317
) at temperatures above a lowest temperature in the second temperature range.
The first temperature range may be between 35 and −70° C., the second temperature range may be between 0 and −140° C., and the third temperature range may be between −90 and −165° C. The feed gas stream (
1
) may be natural gas.
The cooling of the feed gas stream (
1
) may be effected by:
(a) cooling the feed gas stream (
1
) in the first temperature range by indirect heat exchange with the first refrigerant (
117
) vaporizing in a first heat exchange zone (
310
) to provide a first partially cooled feed stream (
5
) and a first refrigerant vapor (
101
);
(b) further cooling the partially cooled feed stream (
5
) in the second temperature range by indirect heat exchange with the second refrigerant (
213
) vaporizing in a second heat exchange zone (
311
) to provide a second partially cooled feed stream (
9
) and a second refrigerant vapor (
201
); and
(c) further cooling the second partially cooled feed stream (
9
) in the third temperature range by indirect heat exchange with the third refrigerant (
315
) vaporizing in a third heat exchange zone (
312
) to provide the liquefied product (
13
) and a two-phase refrigerant stream (
316
).
Another embodiment of the invention relates to a method for liquefying a gas which comprises cooling a feed gas stream (
1
) successively through first, second, and third temperature ranges to provide a liquefied product (
13
), wherein refrigeration for cooling the feed gas stream is provided by a first refrigerant (
117
) vaporizing in the first temperature range, a second refrigerant (
213
) vaporizing in the second temperature range, and a third refrigerant (
315
) vaporizing in the third temperature range and further vaporizing at temperatures above a lowest temperature in the second temperature range, wherein the cooling of the feed gas stream (
1
) is effected by:
(a) cooling the feed gas stream (
1
) in the first temperature range by indirect heat exchange with the first refrigerant (
117
) vaporizing in a first heat exchange zone (
310
) to provide a first partially cooled feed stream (
5
) and a first refrigerant vapor (
101
);
(b) further cooling the partially cooled feed stream (
5
) in the second temperature range by indirect heat exchange with the second refrigerant (
213
) vaporizing in a second heat exchange zone (
311
) to provide a second partially cooled feed stream (
9
) and a second refrigerant vapor (
201
); and
(c) further cooling the second partially cooled feed stream (
9
) in the third temperature range by indirect heat exchange with the third refrigerant (
315
) vaporizing in a third heat exchange zone (
312
) to provide the liquefied product (
13
) and a two-phase refrigerant stream (
316
);
wherein the third refrigerant (
315
) may be a multi-component mixture comprising two or more components selected from the group consisting of nitrogen and hydrocarbons having from one to five carbon atoms and the third refrigerant may comprise (in mole %) 5-15% nitrogen, 30-60% methane, 10-30% ethane, 0-10% propane, and 5-15% i-pentane. The third refrige
Air Products and Chemicals Inc.
Doerrler William C.
Fernbacher John M.
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