Chemical Engineering June 2020 - 33

HighNeon
Air
pressure
column
O2
pressure
column
Catalyst
bed
Cooler
H2O
Dryer
H2O
N2
umn
is usually the economic choice. taking
gaseous nitrogen from the top of the
high-pressure column would reduce the
liquid available for reflux to the low-pressure
column, reducing oxygen recovery.
Before the air is fed to the distillation
columns, it must be compressed, purified
and cooled. this prevents impurities
from freezing in the heat exchangers,
piping and columns, and thus avoids
plugging the equipment, which would
lead to poor performance and possibly
unsafe conditions.
once the purified air begins its journey
through the cold box, the incoming air is
cooled further via heat exchangers that
transfer the warm air's heat to the outgoing
cold products. the incoming air
gets colder and the outgoing products
that have been processed are warmed
to near ambient temperature. this heat
exchange activity minimizes the total energy
required, but there are still losses
and heat leaks into the system.
the two main methods of providing
refrigeration to overcome losses are liquid
addition (liquid oxygen or liquid nitrogen
from a tank or separate liquefier)
and gas expansion. Gas expansion can
be done as part of the asu system, in
the same cold box. it is done with a turbine,
which spins at high-speed as the
gas passes through it and expands. air
and nitrogen are the two fluids that are
typically used in turbines.
argon can be recovered as a valuable
byproduct from an asu that produces
oxygen by adding one or more additional
columns to the system.
the boiling points of each of the rare
gases determines where in the air separation
process it is recovered. the normal
boiling point of ne is 27.1K, warmer
only than He and H, which are also
present in air. these three components
N2
adsorber
Compressor
75% Ne
25% He
Ne/He distillation
N2
~50% Ne, ~30% N2, 15% He, 5% H2
(H2 removal)
He + Ne
Figure 1. The diagram
shows the production of
neon coupled to an airseparation
unit
Pure neon
concentrate with the nitrogen streams
at the top of the columns, where nitrogen
is the coldest stream of the three
major components. the three lowestboiling-point
gases are commonly referred
to as noncondensables, because
they will concentrate if the process is not
designed to remove them before they
build up to unacceptable levels.
Economics of production
ne, Kr and xe exist in air at low levels
(18.2, 1.14 and 0.087 ppm, respectively)
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Chemical Engineering June 2020

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