Page 178 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GGY Final Pages
Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

Cryogenic Process Engineering 21

the development of long-life, low-cost cryocoolers for ous steps in the analysis. Here, T 0 is the reference temper-
the emerging high-temperature superconductor electronic ature (normally ambient), ˙ m the mass flow rate through
market. the system, and s the change in entropy through the
During this same time period, hydrogen sorption cry- system.
ocoolers have achieved their first successful operation Numerous analyses and comparisons of refrigeration
in space, and closed-cycle, helium, Joule–Thomson cry- and liquefaction cycles are presented in the literature.
ocoolers have continued to make progress in promis- Great care must be exercised in accepting these compar-
ing long-life space applications in the 4 K temperature isons since it is quite difficult to place all processes on a
range. In the commercial area, Gifford–McMahon cry- strictly comparable basis. Many assumptions are gener-
ocoolers with rare earth regenerators have made signifi- ally made in the course of these calculations, and these
cant progress in opening up the 4 K market. can have considerable effect on the conclusions. Assump-
Mixtures of highly polar gases are receiving consider- tions that generally have to be made include heat leak,
able attention as refrigerants for Joule–Thomson (J–T) temperature differences in the exchangers, efficiencies of
cycles since the magnitude of the J–T coefficient in- compressors and expanders, number of stages of com-
creases with nonideality of the gas. New closed-cycle J– pression, fraction of expander work recovered, state of
T or throttle-cycle refrigerators have taken advantage of expander exhaust, purity and condition of inlet gases, and
these mixed refrigerants to achieve low-cost cryocooler pressure drop in the various streams. In view of this fact,
systems in the 65 to 80 K temperature range. Micro- differences in power requirements of 10 to 20% can be
miniature J–T cryocoolers have also been developed over due to differences in assumed variables and can negate the
the past decade using these mixed refrigerants. Fabrica- advantage of one cycle over another. A comparison that
tion of these cryocoolers uses a photolithography pro- demonstrates this point rather well is shown in Table II,
cess in which gas channels for the heat exchangers, which lists some common liquefaction systems described
expansion capillary, and liquid reservoir are etched on earlier using air as the working fluid and based on an inlet
planar glass substrates that are fused together to form gas temperature and pressure of 294.4 K and 0.1 MPa,
the sealed refrigerator. These microminiature refrigera- respectively.
tors have been fabricated in a wide range of sizes and
capacities.
Because of the rapidly increasing availability of cry- IV. SEPARATION AND PURIFICATION
ocoolers, numerous new applications have become pos- OF GASES
sible; many of these involve infrared imaging systems,
spectroscopy, and high-temperature superconductors in
The major industrial application of low-temperature pro-
the medical and communication fields. Many of these ap-
cesses involves the separation and purification of gases.
plications have required additional control of cryocooler-
Much of the commercial oxygen and nitrogen, and all
generated vibration and EMI susceptibility.
the neon, argon, krypton, and xenon, are obtained by the
distillation of liquid air. Commercial helium is separated
F. Comparison of Refrigeration and from helium-bearing natural gas by a well-established
Liquefaction Systems low-temperature process. Cryogenics has also been used
commercially to separate hydrogen from various sources
A thermodynamic measure of the quality of a low-
of impure hydrogen. The low-boiling, valuable compo-
temperature refrigeration and liquefaction system is its
nents of natural gas—namely, ethane, ethylene, propane,
reversibility. The second law, or more precisely the en-
propylene, and others—are recovered and purified by var-
tropy increase, is an effective guide to the degree of ir-
ious low-temperature schemes.
reversibility associated with such a system. However, to
The separation of these gases is dictated by the Gibbs
obtain a clearer picture of what these entropy increases
phase rule. The degree to which they separate is based
mean, it has become convenient to relate such an anal-
on the physical behavior of the liquid and vapor phases.
ysis to the additional work required to overcome these
This behavior is governed, as at ambient temperatures, by
irreversibilities. The fundamental equation for such an
Raoult’s and Dalton’s laws.
analysis is

˙ m s (5)
W = W rev + T 0 A. Air Separation
where the total work W is the sum of the reversible work The simplest air separation device is the Linde
W rev plus a summation of the losses in availability for vari- single-column system, which utilizes the simple Linde