Page 526 - Mechanical Engineers' Handbook (Volume 4)

7 Examples of Cryogenic Processing 515

gas. Then the stored liquid is vaporized and added to the network. These ‘‘peak-shaving’’
plants consist of a small liquefier, an immense storage capacity, and a large capacity vapor-
izer. They can be found in most large metropolitan areas where winters are cold, especially
in the northern United States, Canada, and Europe.
The second situation is that of the oil/gas field itself. These fields are likely to be at
long distances from the market. Oil can be readily transported, since it is in a relatively
concentrated form. Gas is not. This concentration is done by liquefaction prior to shipment,
thus reducing the volume about 600-fold. Subsequently, revaporization occurs at the port
near the market. These ‘‘base-load’’ LNG systems consist of a large liquefaction plant, rel-
atively modest storage facilities near the source field, a train of ships moving the liquid from
the field to the port near the market, another storage facility near the market, and a large
capacity vaporizer. Such a system is a very large project. Because of the large required
investment, world political and economic instability, and safety and environmental concerns
in some developed nations, especially the United States, only a few such systems are now
in operation or actively in progress. See Table 13 for data on world LNG trade.

Peak-Shaving Plants
The liquefaction process in a peak-shaving installation is relatively small capacity, since it
will be operating over the bulk of the year to produce the gas required in excess of normal
capacity for two to six weeks of the year. It usually operates in a region of high energy cost
but also of readily available mechanical service and spare parts, and it liquefies relatively
pure methane. Finally, operating reliability is not usually critical because the plant has ca-
pacity to liquefy the required gas in less time than in the maximum available.
For these reasons efficiency is more important than system reliability and simplicity.
Cascade and various expander cycles are generally used, although a wide variety of processes
have been used including the Stirling cycle.
Figure 39 shows a process in which an N expander cycle is used for low-temperature
2
refrigeration, whereas the methane itself is expanded to supply intermediate refrigeration.
This is done because of the higher efficiency of N expanders at low temperature and the
2
reduced need for methane purification. The feed natural gas is purified and filtered and then

Table 13 Data on World LNG Trade
World’s LNG Plants, 1994 World’s LNG Imports, 1994 World’s LNG Trade

Capacity, Quantity, Amount,
Million Metric Parallel Million Metric Million Metric
Location Tons/yr Liq. Trains Country Tons/yr Year Tons/yr
Kenai, Alaska 2.9 2 Japan 38.9 1980 22
Skikda, Algeria 6.2 8 S. Korea 4.4 1990 65
Arzew, Algeria 16.4 12 Taiwan 1.7 2000 90–95 (est)
Camel, Algeria 1.3 1 France 6.6 2010 130–160 (est)
Mersa, Libya 3.2 4 Other Europe 7.8
Das Is., Abu Dhabi 4.3 2 U.S.A. 1.7
Arun, Indonesia 9.0 5
Bontang, Indonesia 13.2 7
Lamut, Brunei 5.3 5
Bintulu, Malaysia 7.5 3
Barrup, Australia 6.0 3