Page 187 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

30 Cryogenic Process Engineering

of the reversing heat exchanger, are critical to the proper In the process of cooldown, the warm feed stream de-
functioning of these types of exchangers. posits impurities on the cold surface of the packing. When
the streams are switched, the impurities are reevaporated
in the cold stream while simultaneously cooling the pack-
4. Regenerators
ing. Thus, the purifying action of the regenerator is based
Anothermethodforthesimultaneouscoolingandpurifica- on the same principles as for the reversing exchanger, and
tion of gases in low-temperature processes is based on the the same limiting critical temperature differences must be
use of regenerators, first suggested by Fr¨ankl in the 1920s. observed if complete reevaporation of the impurities is to
Whereas in the reversing exchanger the flows of the two take place.
fluids are continuous and countercurrent during any one Regenerators quite frequently are chosen for appli-
period, the regenerator operates periodically, storing heat cations where the heat-transfer effectiveness, defined as
in a high heat-capacity packing in one-half of the cycle Q actual /Q ideal , must approach values of 0.98 to 0.99. It is
and then giving up the stored heat to the fluid in the other clear that a high regenerator effectiveness requires a high
half of the cycle. heat capacity per unit volume and a large surface area per
Such an exchanger normally consists of two identical unit volume.
columns packed with a material of high heat capacity and The low cost of the heat-transfer surface along with
high heat-transfer area through which the gases that are to the low pressure drop are the principal advantages of
be cooled or warmed flow. Such regenerator materials and the regenerator. However, the intercontamination of fluid
geometries generally fall into three groups, based on the streams by mixing due to periodic flow reversals and the
temperature range over which they are to be used. The first difficulty of regenerator design to handle three or more
group includes woven screen materials of stainless steel, fluids have restricted its use and favored the adoption of
bronze, or copper used over the temperature range from brazed aluminum exchangers.
30 to 300 K. In the range between 10 and 30 K, lead and
antimony spheres are used becauase their heat capacity is
higher than any of the screen materials. However, below VI. STORAGE AND TRANSFER SYSTEMS
10 K, lead loses 89% of its room-temperature specific heat,
and its volumetric heat capacity is less than that of helium Once a cryogen has been produced, it must be stored,
at a pressure of 1 MPa. In the late 1980s, a third category transferred, or transported to its end use. The effective-
of essentially heavy rere-earth intermetallic compounds ness of the cryogenic storage transfer or transport system
was developed with the potential for enhancing the heat depends on how well it reduces the loss of the cryogen due
capacity at temperatures below 10 K. The increase in spe- to unavoidable heat leak into the system and how well it
cific heat of two of these rare-earth compounds is shown maintains the purity of the cryogen. Good design, with a
in Fig. 13. knowledge of the heat-transfer mechanisms and the prop-
erties of available insulations, is essential in minimizing
the boil-off losses due to heat leak. Proper operating pro-
cedures, on the other hand, are necessary if product purity
is to be maintained.

A. Insulation Concepts
Since heat leak is a major concern in storage and transfer
systems of cryogenic liquids, selecting the proper insu-
lation to use in such systems is vitally important. The
normal design strategy is to minimize radiative and con-
vective heat transfer while introducing a minimum of solid
conductance media. The choice of insulation, however, is
generally governed by an attempt to balance the cost of in-
stalled insulation with the savings anticipated by lowered
boil-off losses.
The various types of insulation used in the storage and
transfer of cryogenic liquids can be divided into five cate-
FIGURE 13 Volumetric specific heat of two rare-earth intermetal- gories: (1) vacuum, (2) multilayer, (3) powder and fibrous,
lic compounds and lead. (4) foam, and (5) special. The boundaries between these