Page 182 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Cryogenic Process Engineering 25

flows to the expander. The expander exhaust stream can The design of low-temperature adsorbers requires
contain as much as 20 wt% liquid. This two-phase mixture knowledge of the rate of adsorption and the equilibrium
is sent to the top section of the stabilizer, which separates conditions that exist between the solid and the gas as a
the two phases. The liquid is used as reflux in this unit, function of temperature. The data for the latter are gener-
while the cold gas exchanges heat with fresh feed and is ally available from the suppliers of such adsorbents. The
recompressed by the expander-driven compressor. Many rate of adsorption is generally very rapid, and the adsorp-
variations of this cycle are possible and have been used in tion is essentially complete in a relatively narrow zone
actual plants. of the adsorber. In usual plant operation at least two ad-
sorption purifiers are employed—one in service while the
other is being desorbed of impurities. In some cases there
E. Purification Schemes
is an advantage in using three purifiers—one adsorbing,
The type and amount of impurities to be removed from one desorbing, and one being cooled, with the latter two
a gas stream depend entirely on the type of process in- units being in series. The cooling of the purifier is gen-
volved. For example, in the production of tonnage oxygen, erally performed using some of the purified gas to avoid
various impurities must be removed to avoid plugging of adsorption during this period.
the cold-process lines or to avoid buildup of hazardous Low-temperature adsorption systems continue to find
contaminants. The impurities in air that contribute most an increasing number of applications. For example, sys-
to plugging are water and carbon dioxide. Helium, hy- tems are used to remove the last traces of carbon dioxide
drogen, and neon, on the other hand, accumulate on the and hydrocarbons in many air-separation plants. Adsor-
condensing side of the oxygen reboiler and reduce the rate bents are also used in hydrogen liquefaction to remove
of heat transfer unless removed by intermittent purging. oxygen, nitrogen, methane, and other trace impurities.
The buildup of acetylene, on the other hand, can prove to They are also used in the purification of helium suit-
be dangerous even if its feed concentration in the air does able for liquefaction (grade A) and for ultrapure helium
not exceed 0.04 ppm. (grade AAA, 99.999% purity). Adsorption at 35 K will,
Refrigeration purification is a relatively simple method in fact, yield a helium with less than 2 ppb of neon,
for removing water, carbon dioxide, and certain other con- which is the only detectible impurity in helium after this
taminants from a process stream by condensation or freez- treatment.
ing. (Either regenerators or reversing heat exchangers can Even though most chemical purification methods are
be used for this purpose since a flow reversal is period- not carried out at low temperatures, they are useful in sev-
ically necessary to reevaporate and remove the solid de- eral cryogenic gas separation systems. Ordinarily water
posits.) The effectiveness of this method depends on the vapor is removed by refrigeration and adsorption meth-
vaporpressureoftheimpuritiesrelativetothatofthemajor ods. However, for small-scale purification, the gas can be
components of the process stream at the refrigeration tem- passed over a desiccant, which removes the water vapor
perature.Thus,assumingidealgasbehavior,themaximum as water of crystallization. In the krypton–xenon purifica-
impurity content in a gas stream after refrigeration would tion system, carbon dioxide is removed by passage of the
be inversely proportional to its vapor pressure. However, gas through a caustic, such as sodium hydroxide, to form
due to the departure from ideality at higher pressures, the sodium carbonate.
impurity level generally will be considerably higher than When oxygen is an impurity, it can be removed by re-
that predicted for the ideal situation. Familiarity with these action of the oxygen in the presence of a catalyst with
deviations is necessary if problems are to be avoided with hydrogen to form water. The latter then is removed by
this purification method. refrigeration or adsorption. Palladium and metallic nickel
One of the most common low-temperature methods for have proved to be effective catalysts for the hydrogen–
removing impurities involves the use of selective solid oxygen reaction.
adsorbents. Such materials as silica gel, carbon, and syn-
thetic zeolites (molecular sieves) are widely used as ad-
sorbents because of their extremely large effective surface V. EQUIPMENT FOR CRYOGENIC
areas. Most of the gels and carbons have pores of various PROCESSING
sizes in a given sample, but the synthetic zeolites can be
manufactured with closely controlled pore size openings The achievement and utilization of low temperatures re-
ranging from 0.4 to 1.3 nm. This additional selectivity is quire the use of various specialized pieces of equipment
useful because it permits separation of gases on the basis including compressors, expanders, heat exchangers,
of molecular size. pumps, transfer lines, and storage tanks. As a general rule,