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

14 Cryogenic Process Engineering

Superfluid Designation for helium II existing below cations. For example, the phenomenon of superconduc-
2.17 K and exhibiting negligible viscosity. tivity occurring at low temperatures has been successfully
exploited in the development of high-field magnets for
various uses. Space simulation is another application us-
CRYOGENICS is a term commonly associated with low
ing a low-temperature concept. In this case, cryopumping,
temperatures. However, the point on the temperature scale
or the freezing of residual gases in a chamber on a cold
at which refrigeration in the ordinary sense of the term
surface, is used to provide the ultrahigh vacuum represen-
ends and cryogenic engineering begins is not well de-
tative of outer space. This concept has been encompassed
fined. Most scientists and engineers working in cryogenic
in several commercial vacuum pumps.
engineering restrict this term to a temperature range be-
Freezing as a means of preserving food dates back
low 125 K. This is a reasonable dividing line since the
to 1840. However, today the food industry uses large
normal boiling points of the more permanent gases, such
amounts of liquid nitrogen for this purpose and as a refrig-
as helium, hydrogen, neon, nitrogen, oxygen, and air, lie
erant in frozen-food transport systems. The use of cryo-
below 125 K, while the more common refrigerants have
genics in biology and medicine has generated such interest
boiling points above this temperature. Thus, cryogenic
that work in these low-temperature areas is now identified
process engineering is concerned with the industrial devel-
as cryobiology and cryomedicine, respectively. For exam-
opment, utilization, and improvement of low-temperature
ple, liquid nitrogen-cooled containers are routinely used
techniques, processes, and equipment.
to preserve whole blood, bone marrow, and animal semen
for extended periods of time. Liquid helium is used to cool
I. GENERAL APPLICATIONS the magnets in the MRI units employed by most modern
hospitals. Cryogenic surgery is an acceptable procedure
The industrial production and utilization of temperatures for curing such involuntary disorders as Parkinson’s dis-
below 125 K are commonly referred to as cryogenic en- ease. Finally, one must recognize the role of cryogenics
gineering or cryogenic process engineering. This field of in the chemical processing industry with the treatment of
endeavor has grown significantly since World War II. It natural gas streams to recover valuable heavy components
is now a major business in the United States with a na- or upgrade the heat content of fuel gas, the recovery of use-
tional value in excess of $2.5 billion annually, based on ful components from air, and the purification of various
the previously defined temperature range. If the defini- process streams.
tion is broadened slightly to include the production of
somepetrochemicalsthatutilizelow-temperatureprocess- II. LOW-TEMPERATURE PROPERTIES
ingintheirmanufacture,suchasethylene,theannualvalue
rapidly escalates to over $12 billion.
Familiarity with the properties and behavior of materials
An examination of cryogenic engineering shows it to be
used in any system operating at low temperatures is essen-
a very diverse supporting technology, a means to an end
tial for proper design considerations. Since there are sev-
and not an end in itself. For example, oxygen, one of the
eral significant effects among materials that become evi-
most important industrial gases, is obtained by the low-
dent only at low temperatures, it is risky to obtain needed
temperature separation of air. Fifty percent of the oxygen
properties by an extrapolation of the variation in prop-
produced in this manner is used by the steel industry to
erties observed at ambient conditions. For example, the
reduce the cost of high-grade steel, while another 20% is
vanishing of specific heats, the phenomenon of supercon-
used in the chemical process industry to produce a variety
ductivity, and the onset of ductile–brittle transitions in car-
of oxygenated compounds. Liquid hydrogen production
bon steel cannot be inferred from property measurements
since the mid-1950s has risen from laboratory quantities
obtained at ambient temperatures. Accordingly, there is
to a level of more than 250 tons/day (227,000 kg/day).
no substitute for test data on a truly representative sample
Similarly,theneedforliquidheliumhasincreasedbymore
specimen when designing for the limit of effectiveness of
than a factor of 15, requiring the construction of large
a cryogenic material or structure.
plants to separate helium from natural gas by cryogenic
means. Demands for energy have likewise accelerated the
A. Fluid Properties
construction of tonnage base load liquefied natural gas
(LNG) plants around the world and have been responsible Numeroustabulationsofthermodynamicpropertydataare
for the associated domestic LNG industry of today with available in the literature. For example, a very recent tab-
its use of peak-shaving plants. ulation of thermodynamic data by Jacobsen, et al (1997)
An introduction of cryogenics would be incomplete covers all of the cryogenic fluids of interest. Sufficient
without brief mention of some of the many current appli- detail on the models used for each fluid is available so