Chemical Engineering June 2020 - 35

directly to the low-pressure column.
these features can reduce neon recovery
by 20% or more. other processes
remove nitrogen gas from the
high-pressure column, removing ne,
He and H2 at the same time. When
significant quantities of nitrogen gas
are removed from the top of the highpressure
column, the liquid nitrogen
reflux to the low-pressure column is
removed a few stages below the top
of the high-pressure column, significantly
changing the concentration of
non-condensables at the top of the
high-pressure column. if the reboiler
is designed for non-condensable
recovery, the ne will concentrate to
roughly 1% in the non-condensable
vapor stream, along with most of
the He and H2. the remainder of the
stream is nitrogen.
When the
low-pressure column
reboiler design makes concentrating
the non-condensables to high levels
impractical, a stripping column can
be used to collect ne. the liquid
nitrogen that flows from the highpressure
column to the low-pressure
column as reflux is passed through
a stripping column on the way to
the low-pressure column. the nerich
vapor steam is withdrawn from
the top of the column. the nitrogen
liquid headed to the low-pressure
column as reflux is withdrawn from
the bottom of the stripping column
(figure 1). the stripping column
can improve reboiler performance
because the non-condensables do
not concentrate to such high levels,
minimizing the apparent temperature
difference across the reboiler,
and increasing ne recovery. this reduces
pressure in the high-pressure
column and air compressor power.
in either case, the ne-containing
stream is at the pressure close to
that of the high-pressure column
(nominally 60 psi). it is fed to a column/condenser
system, where it is
condensed against low-pressure (1
psi) liquid nitrogen at 78K. this will
concentrate the ne up to around
50%. a potential improvement to
this operation is to use a vacuum
pump to reduce the liquid n2 boiling
temperature to just above nitrogen's
freezing point (63.2K). reducing the
condensing temperature reduces the
n2 content in the crude ne stream.
crude neon is typically shipped by
tube trailer to a central processing
facility, so transportation, equipment
and operating cost tradeoffs
are used to evaluate whether or not
a vacuum pump system is desirable.
a catalyst bed operating above
ambient temperature is typically
used to react hydrogen from the
stream, with oxygen added as required
to feed the reaction. adsorption
is typically used to remove the
water and most of the remaining nitrogen
and oxygen, yielding a crude
neon stream that contains roughly
75% ne and 25% He. the compressed,
crude ne stream is then
cooled to around the freezing point
of n2, where it is passed through a
trap (adsorber bed) to remove trace
concentrations of oxygen and nitrogen
that would freeze when the
ne-rich stream was cooled close
to its dew point. the cooled ne/He
mixture is fed to a small distillation
column, where the pure ne collects
in the bottom as liquid and the He,
trace H2 and some ne leave from
the top of the column. the ne product
can be recovered as liquid or gas
from the column.
if it is recovered as liquid, then the
refrigeration source for the top condenser
has to be sized to provide
the refrigeration needed. mechanical
refrigeration and liquid H2 are two
potential refrigeration sources. the
column pressure is an important variable
for designing the separation. increasing
column pressure can minimize
the ne lost out the vent with the
He. it also increases the temperature
difference between the boiling and
freezing points of ne in the process.
at atmospheric pressure, the difference
between the boiling and freezing
points of ne is only 2.6K. increasing
the column pressure increases
the boiling point of ne and makes the
column easier to operate and restart
after a shutdown.
Krypton and xenon production
the boiling points of Kr and xe are
above that of liquid oxygen, so Kr
and xe will concentrate with the
oxygen product at the bottom of the
low-pressure column. the process
and main condenser design have a
huge impact on potential Kr and xe
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