Chemical Engineering June 2020 - 36

recovery. many older designs used oxygen
compressors that were fed with gas
from from the low-pressure column. reboilers
of that era were set in a pool of
liquid oxygen that was recirculated and
maintained at a high level to ensure that
liquid and gas both flow out the top of
the reboiler. this minimizes the chance
of boiling to dryness and concentrating
hydrocarbons or other impurities (co2
and n2o, for example) to unacceptable
levels in oxygen. the primary hydrocarbons
in the main condenser of an asu
are methane and ethane, but propane
and other trace contaminants are also
present. these hydrocarbons have boiling
points above oxygen, close to those
of Kr and xe, and present major safety
concerns when recovering Kr and xe.
the older asu designs have the potential
for Kr and xe recoveries greater
than 90% because almost all the liquid
flowing down the low-pressure column
is vaporized and returned to the column
or removed as oxygen product. Kr and
xe could concentrate to roughly 60 ppm
each, with zero liquid oxygen withdrawal.
newer asu processes, commonly
known as liquid pumped cycles, typically
remove liquid oxygen (instead of
gas) from the low-pressure column. the
liquid oxygen withdrawn from the lowpressure
column is typically pumped
to the desired pressure and vaporized
against high-pressure air in the main
heat exchanger. this approach eliminates
the oxygen compressor in most
applications, but makes Kr and xe recovery
more difficult, because the Kr
and xe in the sump liquid is roughly 5.5
ppm Kr and 0.4 ppm xe.
in new designs without Kr and xe recovery,
the liquid oxygen is withdrawn
from the sump of the high-pressure
column, where the main condenser is
located. this results in lower concentrations
of hydrocarbons and other impurities
in the sump liquid, but also removes
all the Kr and xe from the process. it is
element
Oxygen
Argon
Neon
Krypton
Xenon
36
Tons per day
100
5.5
0.0055
0.0014
0.0002
possible to recover most of the Kr and
xe with other methods, but they increase
process complexity and costs.
one approach to achieve nominally
70% xe recovery and lower Kr recovery
is to take the liquid oxygen product
from the low-pressure column before it
reaches the main condenser. the liquidto-vapor
ratio at the bottom of the column
varies with the chosen cycle, but
is close to 1.4 to 1 in most cases. the
liquid oxygen removed as product never
reaches the reboiler, so the portion of
the Kr and xe in the air feed is lost. it is
much easier to recover xe than Kr with
this type of process. figure 2 (from u.s.
patent 5,069,698) illustrates the impact
of the liquid o2-withdrawal rate on the
recovery of Kr and xe from the reboiler
sump. High xe recovery can be achieved
with a relatively small liquid o2 feed to a
Kr/xe concentrating column, requiring
smaller equipment and less energy than
a high-Kr recovery system.
there are other, more complex processes
that can be used to recover Kr
and xe from liquid pumped cycles. they
tend to have more significant process
impact on, and increase the cost of, the
asu process to which they are attached.
concentration of Kr and xe is usually
accomplished in a series of unit operations.
the large difference in boiling
points among o2, Kr and xe makes it
easy to separate the molecules by distillation.
the first step is a short column
that raises Kr and xe concentrations to
between 0.1 and 0.5%. the main complication
is that the boiling points of hydrocarbons
common in the air are close
to those of Kr and xe. the hydrocarbons
must be removed or diluted relative to
the oxygen before the rare gases can be
concentrated further. another key operation
is to remove trace quantities of co2
and n2o from the oxygen stream. this is
most often done by adsorption. co2 and
n2o will reach unacceptable levels in the
concentration process if they are not reTable
1. UniT Comparison of ConTained flows of oxygen, argon and rare gases in a
100-Tons-per-day oxygen planT
ft3/hr (at normal
temp., pressure)
100,600
4,464
8.74
0.55
0.042
Liquid gallons per
year
7.66 × 106
3.48 × 105
396
52
4.9
gas liters per
year
2.50 × 1010
1.10 × 109
2,159,048
135,237
10,321
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