Chemical Engineering June 2020 - 37

Table 2. boiling- and freezing-poinT
daTa for gases in air
element
Normal
boiling
point, K
Helium
Hydrogen
Neon
Nitrogen
Oxygen
Argon
Krypton
Xenon
4.2
20.3
27.1
77.4
90.2
87.3
119.8
165
Freezing
point, K
n/a
14.0
24.6
63.2
54.4
83.8
116
161.4
moved. these components can solidify
in the liquid oxygen and block
tubes,
heat
exchanger
passages
and/or piping. this creates other
serious safety concerns, especially
dead-ended boiling in an oxygenrich
environment. dead-ended boiling
has been identified as a cause of
major oxygen-plant energy releases
(incidents) and must be avoided.
one commercial approach to remove
hydrocarbons is to vaporize
and warm the oxygen-rich stream to
roughly 900˚f and send it through a
catalyst bed to react the hydrocarbons,
consuming a small amount of
the oxygen in the stream. However,
water and co2 created by combustion
of the hydrocarbons must be removed
before final separations into
pure product streams.
an alternative to the high-temperature
catalyst method is to replace
the oxygen with an inert gas (typically
ar or n2), which eliminates concentration
of hydrocarbons as a safety
concern. at least one additional distillation
column will be required without
the catalyst system. adsorption
is also used as a purification method
for Kr and xe, but it is most often
used when recovering only one of
the two heavier rare gases. for the
highest-purity products, additional
processing may be recovered to
remove trace compounds that concentrate
with the Kr and xe, such
as cf4 and sf6. they are present in
the air at roughly 44 and 3 parts per
trillion, respectively, but can concentrate
to unacceptable levels when
concentrating air components in the
ppm or ppb range.
Commercial uses
anesthesia using xe/o2 mixtures is
very intriguing and effective. it has
been called a nearly ideal anesthetic
gas. unfortunately, the complexity of
recovering what the patient exhales
and reprocessing the xe-rich gas
makes it difficult to gain widespread
acceptance. plasma televisions and
lasers are two areas where xe is
widely used. in these applications,
xe is mixed with other gases, often
ne and He. xe thrusters are a valuable
technology for deep space
flight and other space applications.
ion thrusters issuing pure xe require
much less mass than conventional
propellants when using xe and can
achieve higher velocities. With the
cost of sending items to space still
in the range of thousands of dollars
per pound, the high price of xe is
not a significant obstacle.
other areas where pure xenon is
used include high-efficiency
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lighting
in automobiles, computer chip
manufacturing, dark matter experiments
in physics research and nuclear
fuel processing.
Krypton has a relatively high mass
for a gas and is an excellent insulator.
it has had many uses over the
years, filling niche markets, including
lighting and window manufacturing.
When the cost is low enough
compared to xe, it can replace xe
in some applications. it can also be
used as a " green " refrigerant.
neon is often the main component
of laser gas mixtures, and is critically
important to the manufacture of
high-capacity computer chips. it has
a distinctive red glow when energized
and is frequently used to make
decorative signs. it is an effective
refrigerant for applications around
30K. the latent heat per gallon of
liquid ne is more than 40 times the
latent heat per gallon of liquid He,
making the design of refrigeration
systems smaller and simpler.
n
Edited by Scott Jenkins
Author
Jim Dray is the founder of the consulting firm JRD
Gastech LLC (Buffalo, NY; Email: jrdgastech@gmail.
com). Prior to founding his own consulting company,
Dray worked at Praxair for 39 years in various roles in
engineering, research and development, and operations.
He retired in 2017 to form JRD Gastech, supporting
the air separation industry.
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