Chemical Engineering August 2019 - 48

Brian Warrick and AES
Gas entering
the main heat
Air enters the main
air compressor,
beginning the
process.
Molecular sieve
beds remove CO2
and hydrocarbons
from the air
exchangers is cooled
by the outgoing cold
products
Nitrogen
(-195oC)
Argon
(-185oC)
Oxygen
(-183oC)
Methane
(-161oC)
Gas moves into the
" cold box, " which
contains the distillation
columns
Finally, purified gas
continues on to the
factory for storage
and use
Hot
Cold
FIGURE 4. Air-separation units, like the one shown here, separate gases according to differing boiling points,
using cryogenic distillation
be considered, as well as whether
the purifier will fit into the end-user's
space constraints. Optimization of
the final product is possibly the most
critical step in purification. Beyond a
laboratory-scale purifier, consistency
must be considered to ensure the
purifier aligns with existing and potential
future equipment design.
The final packaged purifier then
needs to be shipped to the end
user. Many zeolites and oxides can
be sent via standard shipping methods,
but many metal catalysts and
alloys may require hazardous shipping.
The International Air Transport
Association (IATA) defines regulations
for shipment of all hazardous
materials sent via aircraft. Depending
upon the hazardous material
class and package group, limits on
shipping quantities may apply. In
these cases, a special permit may
be required. Determination of hazardous
material and special provision
requirements are determined
by IATA regulations.
Impurity concentrations
Not all 99.999%-pure gases have
the same impurity loading and not
all 99.999%-pure gases contain exactly
10 parts per million (ppm) of
impurities. When selecting a purifier,
a common question is " what is the
impurity concentration? " This is the
most important aspect of purifier
selection and has significant impact
on cost and lifetime of the purification
system. Nitrogen generated in
cryogenic form provides a good ex46
ample.
As nitrogen passes through
the distillation process, trace
amounts of carbon dioxide, moisture,
and all hydrocarbons, as well
as components with boiling points
higher than that of nitrogen, liquify
into a waste stream. Nitrogen exits
the column containing hydrogen
and carbon monoxide, which are
not liquified, because of their higher
boiling points. Further, if the nitrogen
is liquified for cryogenic storage, hydrogen
typically flashes from the liquid
due to its boiling point. Therefore,
nitrogen from a cryogenic source is
typically closer to a 99.9999%-pure
product. While some specialized
distillation processes exist, nitrogen
generated from a distillation column
is typically the same across grades.
What may vary is how the nitrogen
is processed following distillation.
For higher purity grades, enhanced
transport and analytical testing is
used to ensure the delivered product
is of the highest purity. The gas
may have a certificate of conformity.
Other gases follow similar behavior,
specifically that the impurity content
within the gas may be predicted by
how the product was generated.
A common element of the separation
methods referenced above is
the use of an adsorbent or catalyst
to remove or separate components.
Purification is simply an extension
of these gas-generation technologies.
With the exception of liquid
hydrogen, the purity of each technology
is 99.999% or greater. For
many applications within the electronics
industry, purity on the order
of 99.9999999% is required. Therefore,
enhanced technologies are required
to further drive purity to these
levels. A plot in the online version of
this article demonstrates oxygen,
moisture and carbon dioxide impurities
removed to less than 50 ppt,
When discussing the purity of a gas,
a common terminology is to refer to
the number of nines used to express
the purity level. For example, a gas
of 99.999% purity is referred to as a
" 5 nines " (5N) gas and may contain
up to 10 ppm of total contaminants.
For industrial gas purification, purity
of 7 nines (7N; less than 100 ppb of
impurities) to 9 nines (9N; less than 1
ppb of impurities) is common.
7N or higher purification technology
exists for most industrial gases,
but not all impurities can be removed
from each gas. An example is oxygen,
which contains high-ppm levels
of inert gases. Standard purification
technologies do not offer a means of
removing inert gases from oxygen.
Therefore, purified oxygen may be
only a 5N gas, even though methane,
moisture, hydrogen, carbon
monoxide and carbon dioxide are
removed to low-ppb levels.
n
Edited by Scott Jenkins
Authors
Brian Warrick is director of purification
technology for Applied Energy
Systems Inc., (180 Quaker
Lane, Malvern, PA 19355; Email:
bwarrick@appliedenergysystems.
com; Phone: 610-647-8744) ARM
Purification division, a position he
has held since 2013. Warrick has
over 20 years of professional experience
within the industrial gases
and equipment industries, and has demonstrated expertise
in research and development, product line development,
sales, manufacturing, proposals and contracts.
Prior to joining AES/ARM, Warrick worked in various capacities
for Praxair Inc. He is author or coauthor of six U.S.
patents. Warrick holds a master's degree in physics from
the University of Colorado and is a Six Sigma Green Belt.
Dan Spohn is the director of purification
business development Applied
Energy Systems (same address
as above; Email: dspohn@
appliedenergysystems.com;
Phone: 610-647-8744). Prior
to
joining AES/ARM, Spohn held technical
sales and marketing positions
for several companies and was an
engineering manager and mechanical
engineer. Spohn is a veteran of the U.S. Navy and was
educated at the U.S. Naval Nuclear Power School.
Editor's note: To view additional graphics associated
with this article, visit the online version of this article at
www.chemengonline.com
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AUGUST 2019

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Chemical Engineering August 2019

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