Page 528 - Mechanical Engineers' Handbook (Volume 4)

7 Examples of Cryogenic Processing 517

split into two streams. The larger is cooled in part of the main exchanger, expanded in a
turboexpander, and rewarmed to supply much of the warm end refrigeration, after which it
is sent to the distribution system. The smaller fraction is cooled both by methane and by N 2
refrigeration until it is largely liquid, whereupon it goes to storage. Heavier liquids are
removed by phase separation along the cooling path. Low-temperature refrigeration is sup-
plied by a two-stage Claude cycle using N as working fluid.
2
The LNG is stored in very large, insulated storage tanks. Typically such a tank might
be 300 ft in diameter and 300 ft high. The height is made possible by the low density of
LNG compared to other hydrocarbon liquids. LNG tanks have been built in ground as well
as aboveground and of concrete as well as steel. However, the vast majority are aboveground
steel tanks.
In designing and building LNG tanks the structural and thermal requirements added to
the large size lead to many special design features. A strong foundation is necessary, and so
the tank is often set on a concrete pad placed on piles. At the same time the earth underneath
must be kept from freezing and later thawing and heaving. Thus electric cables or steam
pipes are buried in the concrete to keep the soil above freezing. Over this pad a structurally
sound layer of insulation, such as foam glass, is put to reduce heat leak to the LNG. The
vertical tank walls are erected onto the concrete pad. The inner one is of stainless steel, the
outer one is usually of carbon steel, and the interwall distance would be about 4 ft. The
walls are field erected with welders carried in a tram attached to the top of the wall and
lifted as the wall proceeds. The wall thickness is, of course, greater at the bottom than it is
higher up.
The floor of the tank is steel laid over the foam glass and attached to the inner wall
with a flexible joint. This is necessary because the tank walls will shrink upon cooling and
expand when reheated. The dish roof is usually built within the walls over the floor. When
the walls are completed, a flexible insulating blanket is put on the inside wall and the rest
of the interwall space is filled with perlite. The blanket is necessary to counter the wall
movement and prevent settling and crushing the perlite. At the end of construction the roof
is lifted into position with slight air pressure. Usually this roof has hanging from it an
insulated subroof that also rises and protects the LNG from heat leak to the roof. When this
structure is in place, it is welded in and cover plates are put over the insulated wall spaces.
For safety considerations these tanks are usually surrounded by a berm designed to
confine any LNG that escapes. LNG fire studies have shown such a fire to be less dangerous
than a fire in an equivalent volume of gasoline. Still, the mass of LNG is so large that
opportunities for disaster are seen as equally large. The fire danger will be reduced if the
spill is more closely confined, and hence these berms tend to be high rather than large in
diameter. In fact, a concrete tank berm built by the Philadelphia Gas Works is integral with
the outside tank wall. That berm is of prestressed concrete thick enough to withstand the
impact of a major commercial airliner crash.
Revaporization of LNG is done in large heat exchangers using air or water as heat sink.
Shell and tube exchangers, radiators with fan-driven air for warming, and cascading liquid
exchangers have all been used successfully, although the air-blown radiators tend to be noisy
and subject to icing.

Base-Load LNG Plant
Table 13 lists the base-load LNG plants in operation in 1994. Products from these plants
produce much of the natural gas used in Europe and in Japan, but United States use has
been low, primarily because of the availability of large domestic gas fields.
In contrast to peak-shaving plants, liquefiers for these projects are large, primarily lim-
ited by the size of compressors and heat exchangers available in international trade. Also,