Chemical Engineering June 2020 - 32

Cover Story
rare noble Gases:
production and
use of ne, Kr and xe
Jim Dray
Jrd Gastech LLc
In BrIef
Basics of air
separation
economics of
production
neon production
Krypton and xenon
production
t
he term " rare gases " typically refers
to the elements neon (ne), krypton
(Kr) and xenon (xe). they are noble
gases that are collected and separated
by cryogenic separation of air, the primary
method of producing two of the world's
largest commercial chemicals (oxygen and
nitrogen). the concentration in air of ne, Kr
and xe total less than 20 parts per million
(ppm). argon (ar) is also a noble gas recovered
from air, but its concentration in air is
comparatively much higher (0.93%) and is
not considered rare. Helium (He) is present
at about 5 ppm in air, but is much more
easily recovered from certain natural gas
deposits, and is not usually commercially recovered
from air. all noble gases lighter than
radon (He, ne, ar, Kr and xe) are valued for
their inertness and other unique properties.
this article provides information about the
separation of ne, Kr and xe from air and their
respective uses.
Basics of air separation
clean, dry air is made up of approximately
78% nitrogen and 21% oxygen, 0.93%
argon and trace amounts of other components.
the concentrations of other impurities
(water, carbon dioxide, hydrogen, nitrous
oxide, methane, ethane and miscellaneous
hydrocarbons) are a function of ambient
conditions and local circumstances.
production of oxygen and nitrogen from air
with a cryogenic air separation unit (asu) is
typically done in two distillation columns operated
around -300˚f (-185˚c)±20 degrees.
the low temperatures are required to convert
air, oxygen and nitrogen from gas to liquid.
Liquid contacting vapor is what makes the
distillation columns work. the two columns
32
Part 2
Rare noble gases neon, krypton and xenon have unique properties and can be obtained from
air, but the economics of isolating them at air-separation units depends on demand and pricing.
This article provides information on their production and use
are typically built with a low-pressure column
that sits above a high-pressure column.
the stacked configuration allows for efficient
heat exchange between liquid at the bottom
of the low-pressure column and vapor at the
top of the high-pressure column.
the distillation columns, heat exchangers
and cold piping are all enclosed in a " cold
box " that is filled with perlite insulation to minimize
heat leaking into the system. the lowpressure
column typically operates at around
5 psi at the top of the column. the high-pressure
column operates at around 60 psi. the
higher pressure is required to raise the boiling
point temperature of nitrogen enough that it
condenses at a higher temperature than the
liquid oxygen, boiling at the bottom of the
low-pressure column. this occurs in a heat
exchanger (known as the main condenser)
located in the bottom of the low-pressure column,
where the liquid oxygen collects.
the high-pressure column separates the
nitrogen from the oxygen, producing highpurity
nitrogen (typically with single-digitparts-per-million
levels of o2) at the top of
the column and a low-purity oxygen stream
at the bottom (35 to 40% o2). roughly 45%
of the air feed to the high-pressure column is
removed from the top of the high-pressure
column and sent to the top of the low-pressure
column as reflux liquid, or as nitrogen
product. the rest is returned to the highpressure
column as reflux to achieve the desired
ppm-o2 level in the nitrogen.
oxygen is produced from the bottom of
the low-pressure column. High-purity nitrogen
can be produced from the top of either
or both columns. for large volumes of nitrogen
(more than 10% of the feed air or half the
oxygen flow), the top of the low-pressure colChemiCal
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