Patent Grant
7278281
1. Field of the Invention
The present invention relates generally to liquefied natural gas (LNG) terminals, and more particularly to LNG receiving terminals.
2. Background of the Invention
LNG is the liquid state of the same natural gas as used for gas-fired appliances in domestic households and industries, for pipeline sendout, and for electricity generation in gas-fired power plants. While natural gas in its gaseous state is used for domestic and commercial applications, when natural gas is transported from production locations to usage locations over long distances it is usually transported in a liquid state because LNG is about six hundred times smaller in volume than in its gaseous state. This significant reduction in volume makes LNG considerably less expensive than gaseous natural gas to transport over long distances. Hence, many LNG supply networks subject natural gas to liquefying at a production location, transporting between the production location and a usage location, and finally re-gasifying at the usage location prior to distribution to a consumer.
Different natural gas consumers, however, have different requirements for the LNG being re-gasified, such as varying calorific value and/or quality requirements. In order to satisfy different customer requirements, gas companies set strict requirements on the composition of the natural gas sent out of their LNG receiving terminals. These requirements vary from one LNG buyer to another, and often include Ethane (C2), Propane (C3) and heavier components content specifications that are lower than LNG production at existing LNG baseload plants. Exemplary pipeline specifications (Table 1) and LNG baseload plant outputs (Table 2) are provided below.
In many instances, LNG baseload plants cannot be efficiently modified so as to meet the varying specifications. This inflexibility is due, in part, to the configuration and equipment used in typical LNG baseload plants. Specifically, after an initial feed-gas treatment (e.g., acid-gas removal, dehydration, mercury removal, etc.), LNG baseload plants typically remove components from the LNG using a scrub column. As an example, benzene and C5 components may be removed from the LNG to prevent the LNG from freezing in a main cryogenic heat exchanger. Further, C2 components may be removed from the LNG to control the calorific value. Hence, many LNG baseload plants would have to modify the scrub column or alter its operation to satisfy the noted customer requirements.
The scrub columns at many baseload plants, however, cannot be effectively modified to satisfy customer requirements because doing so would reduce the operating pressure of feed gas entering the main cryogenic heat exhanger to unacceptable levels. Specifically, the feed-gas pressure for most baseload LNG plants is greater than 60 bara. If the plant must remove heavier hydrocarbon components to meet a typical North American market calorific value (e.g., about 1,070 btu/cu ft), the scrub column must operate at a pressure of about 40 bara based on the critical pressure of the feed gas. The critical pressure is “critical” because the separation process becomes difficult and very inefficient near the critical pressure while the refrigerant efficiency depends on the operating pressure of feed gas entering the main cryogenic heat exchanger. A lower calorific value, therefore, would require recompression of feed gas from the scrub column to the main cryogenic heat exchanger, which is significantly more expensive. As such, a need exists for a method and apparatus for reducing the amount of various components (e.g., C2 and/or C3+) without raising costs to prohibitive levels.
Other problems with the prior art not described above can also be overcome using the teachings of the present invention, as would be readily apparent to one of ordinary skill in the art after reading this disclosure.
Reference will now be made in detail to exemplary embodiments of the present invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
According to one embodiment of the present invention, imported LNG with excessive heavy components (i.e., C2+ components) is provided as a LNG stream 100 fed into to a LNG receiving terminal. As an example, the LNG stream 100 may contain: 87 mole % C1, 11.4 mole % C2, 1.3 mole % C3, and some additional heavier components. This example LNG composition is used below to illustrate various aspects of the present embodiment. Other compositions are also contemplated.
According to one embodiment of the present invention as shown in
In some embodiments, the fractionation section may process only a portion of the LNG stream 100 as pumped by pump 115. As an example, the fractionation section may process about half of the LNG stream 100. The portion of the LNG stream 100 not processed by the fractionation section may serve as a coolant and/or a mixing component. Additionally, optional bypass 111 may or may not be provided to bypass part of the LNG stream 100. These uses are described in greater detail below.
According to one embodiment of the present invention, the LNG stream 100 can be used as a coolant for coolers 165, 167, 169, and/or 193. Using the LNG stream 100 as a coolant reduces system operating costs as the LNG stream 100 is typically relatively cold as supplied to the system. Further, the LNG stream 100 itself is already being supplied to the system for processing. Thus, additional coolants do not have to be procured and stored. Of course, alternative coolants such as liquid nitrogen could also be used.
According to one embodiment of the present invention, the LNG stream 100 can be used as a mixing component in gas direct-contact condenser 120. Though similarly a gas indirect-contact condenser could also be used. As shown, the LNG stream 100 is split between the C2 extraction column 160 and the gas direct-contact condenser 120. Overhead vapor from the C2 extraction column 160 is partially condensed in condenser/cooler 165 and sent to flash drum 150. Optionally, flash drum 150 lowers a pressure of the overhead vapor to enhance vaporization of dissolved gases in the C1 rich stream 112. The flash gas stream 151 from flash drum 150 is then fed into gas direct-contact condenser 120, where it is mixed with LNG stream 100 to produce a warm condensed LNG. The condensed LNG is then pumped with pump 125 to vaporizer 130 for distribution as lean LNG stream 116. Any overhead gas from the gas direct-contact condenser may be exhausted as fuel 114 for various uses.
As described above, gas direct-contact condenser 120 uses, as a cooling medium 110, the LNG stream 100 to condense the C1 rich stream 112 from the C2 extraction column 160 to produce the lean LNG stream 116. It should be appreciated that other coolants could also be used depending on the type of gas condenser, such as liquid nitrogen. Moreover, other condensing means such as a heat exchanger could also be used with or without using LNG stream 100 as a coolant. Such variations are all considered to be within the spirit and scope of the present invention.
The present embodiment successfully eliminates the need for gas compressors (though they may still be used for various processes) because it uses a gas direct-contact condenser 120 or other condensing means. This reduces system building and operating costs. In addition to lower cost advantages, the present embodiment may achieve a lean LNG stream with not more than about 6 mole % C2. Such an output is a marked improvement over conventional LNG terminals. Other advantages and features of the present invention will also become apparent upon reading this disclosure and practicing various embodiments of the present invention.
According to another embodiment of the present invention, the fractionation section is adapted and configured to provide liquid ethane gas (LEG) 122 and/or liquid propane gas (LPG) 124 for use such as export or fuel. To provide this capability, the fractionation section may include two or more fractionation columns: a C2 extraction column 160 (as previously discussed) and a C2-LPG separator 170 (a second extraction column). This process and the operation of extraction columns 160, 170 is discussed in greater detail below.
The C2 extraction column 160 receives vaporized LNG from the LNG vaporizer 199 and liquid LNG from the LNG feed pump 115. Preferably, the liquid LNG from the LNG feed pump 115 is supplied to one or more column overhead condensers within the C2 extraction column 160. These column overhead condensers may include one of a plate-and-fin type exchanger(s) and a shell-and-tube type exchanger(s) as are well known in the art. Using the column overhead condensers, the C2 extraction column 160 (a first extraction column) produces the C1 rich stream 112 as previously discussed. According to the present embodiment, the C2 extraction column 160 is also configured to produce a first C2 rich stream 144.
In operation, the first C2 rich stream 144 is provided to the C2-LPG separator 170, which produces an LPG stream (a condensed C3 stream) from the bottoms (sent to cooler 167) and a C2 cut (a condensed C2 stream) from the overhead (sent to cooler 193). C2-LPG separator 170 may be of a packed bed or tray column type as are well known in the art. Other configurations are also contemplated.
The C2 cut may be provided to flash drum 190. Flash gas from the flash drum 190 may be sent out as fuel 166 for plant operation or the like. The lean ethane gas, however, is preferably provided to cooler 169 and stored in a tank or distributed as LEG 122 via pump 175. In this manner, the system may be capable of providing lean LEG 122 as well as lean LNG 116.
The LPG stream may be provided to cooler 167 and stored in a tank or distributed as LPG 124 via pump 185. In this manner, the system may be capable of providing lean LPG 124 as well as lean natural gas.
While the present embodiment shows marked improvement over conventional designs, the exact amount of cold feed LNG that the fractionation section processes (roughly 50%) typically depends on the required C2 specification and the extraction column operating requirements. The C2 extraction column 160 usually operates at between about 20 barg to about 50 barg. A lower operating pressure improves separation efficiency, but also increases column size and reduces the fractionation column overhead vapor condensing. The pressure setting must be less than the system critical pressure needed to achieve separation. The C2-LPG separator 170 usually operates at about 20 bara. Other configurations are also contemplated.
According to another embodiment of the present invention as shown in
Within the C3 extraction section, as an example a C3 extraction column 225 may be provided for processing about 8% of the supplied LNG 100 fed to the C3 extraction section. The remaining 11% of the supplied LNG 100 preferably enters the gas direct-contact condenser 295 for use as an absorbent and/or coolant. Operation of the C3 extraction column 225 and gas direct-contact condenser 295 is discussed in greater detail below.
The C3 extraction column 225 may include at least one packed-bed extraction column. Approximately 30% of the LNG that enters C3 extraction column 225 may feed directly to the top as an absorbent. The other 70% first goes to LNG vaporizer 235 which vaporizes the LNG, the vapor then entering the C3 extraction column 225 between the two packed beds as shown and directly enters the column 225. The C3 extraction column 225 separates C3 components from the LNG stream 100 into overhead vapor and a C3 stream.
The overhead vapor may be a lean C1 stream analogous to C1 rich stream 112 in
The C3 stream flows to the C3 flash drum 255, in which light components flash to the top. The C3 stream from the bottom of the flash drum 255 first depressurizes to atmospheric pressure, is cooled with cold LNG, and feeds to the C3 storage tanks. Liquid from the direct-contact condenser 295 is pumped via pump 265 to pipeline required pressure of about 80 barg to about 140 barg, and flows through LNG vaporizer 280 to the export gas pipeline.
The C3 extraction column 225 operating pressure is preferably about 20 barg to about 50 barg. A lower operating pressure improves separation efficiency, but increases column size. According to one aspect of the present invention, four theoretical stages are provided between the liquid and vapor feed and three stages between the vapor feed and bottoms for the C1 and C3 separation. Of course, other numbers of theoretical stages could also be used.
In the extraction column, 90% of the C3 flows to the column bottoms, which contains no more than 10 mole % of C1. Vapor leaving the C3 extraction column 225 is recondensed when mixed with cold LNG in the gas direct-contact condenser. To ensure that the condensed liquid is easily pumped with pump 265, cold LNG flow to the gas direct-contact condenser 295 is at least 20% more than vapor flow.
Preferably, LNG mixes in gas direct-contact condenser 295 with overhead vapor from the C3 extraction column 225. The overhead vapor may be a C1 rich stream analogous to C1 rich stream 112 discussed in reference to
The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. As an example, while the present invention discloses various embodiments as used in a LNG receiving terminal, similar components could also be implemented at a baseload plant. Hence, the embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.
The present invention claims the benefit of and priority to U.S. Provisional Patent Application No. 60/519,267 filed Nov. 13, 2003, the entire contents of which is incorporated by reference herein in its entirety. Additionally, the present invention incorporates by reference the entire contents of “Cost-Effective Design Reduces C2 And C3 At LNG Receiving Terminals” (The Oil & Gas Journal, May 2003).
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