Review Study and Comparison of Cryogenic
Distillation, Pressure Swing Adsorption and
Membrane Technology for the Gas Separation
Operations
Vishal R. Thakkar1, Prof. H.N. Pandya2
Chemical Engineering Department, L.D. College of Engineering, Ahmedabad-380013
vish.thkkr2@gmail.com, hiral_chem30@yahoo.com
Abstract:
1.Introduction:[6][8]
Our surrounding air mostly consists of nitrogen,
oxygen, carbon dioxide and some inert gases.
Also the exhaust gases liberating from various
industries contain these gases in various
amounts. Nowadays, different technologies
have been developed to separate out these
gases from the gas mixtures or from the air.
These gases have their potential uses in various
day to day applications. These gases found their
use in most industrial applications. In this paper
the usage of various gases including nitrogen,
oxygen and hydrogen have been discussed in
detail. Also the air separation technologies like
cryogenic distillation, pressure swing adsorption
and membrane technology are included with
detailed technical description. The paper
contains the comparison between aforesaid
technologies considering various factors ranging
from product purity to the overall costs and
shutdowns. This paper helps to distinguish
between main three air separation technologies
and provides means for setting criteria for the
selection of these techniques.
Air separation is the most common process
used to extract one or all of the main
constituents of atmospheric air. The three
main components are Nitrogen (78.1%),
Oxygen (20.9%) and Argon (0.9%). The
remaining gases in the air are in trace
amounts and normally not recovered. In
very large air separation units (ASU) Neon,
Xenon and Krypton are recovered in small
amounts.
Keywords: Air separation technologies,
Cryogenic distillation, Pressure Swing
Adsorption, Membrane technology,
Comparison.
The most common method for air
separation
is
cryogenic
distillation.
Cryogenic air separation units (ASUs) are
built to provide nitrogen or oxygen and
often co-produce argon. Other methods
such as Membrane, pressure swing
adsorption (PSA) and Vacuum Pressure
Swing Adsorption (VPSA) are commercially
used to separate a single component from
ordinary air.
High purity gases like nitrogen, oxygen
and argon used for semiconductor device
fabrication require cryogenic distillation.
Similarly, the only viable sources of the rare
gases like neon, krypton, and xenon is the
distillation of air using
two distillation columns.
at
least
2.Applications/Usage of Air Separation:
[1][5][6][8]
Air separation as described above is used to
separate main three components of air i.e.
nitrogen, oxygen and argon.
Nitrogen gas
Nitrogen gas has a variety of applications,
including serving as an inert replacement
for air where oxidation is undesirable.








As a modified atmosphere, pure or
mixed with carbon dioxide, to preserve
the freshness of packaged or bulk foods
(by delaying rancidity and other forms
of oxidative damage).
In incandescent light bulbs as an
inexpensive alternative to argon.
In photolithography in deep ultraviolet,
to avoid a strong oxygen absorption.
Dried and pressurized, as a dielectric
gas for high-voltage equipment.
In the manufacturing of stainless steel.
Used in some aircraft fuel systems to
reduce fire hazard.
On top of liquid explosives as a safety
measure.
Filling automotive and aircraft tires due
to its inertness and lack of moisture
or oxidative qualities, as opposed to air.
Nitrogen is commonly used during sample
preparation procedures for chemical
analysis. It is used to concentrate and
reduce the volume of liquid samples.
Nitrogen tanks are also replacing carbon
dioxide as the main power source
for paintball guns.
Oxygen
This gas is used in various industrial
chemical applications. It is used to make
acids, sulfuric acid, nitric acid and other
compounds. Its most reactive variant is
ozone O3. It is applied in assorted chemical
reactions. The goal is to boost reaction rate
and oxidation of unwanted compounds. Hot
oxygen air is required to make steel and
iron in blast furnaces. Some mining
companies use it to destroy rocks.
Industries use the gas for cutting, welding
and melting metals. The gas is capable of
generating temperatures of 3000 0C and
2800 0C. This is required for oxy-hydrogen
and oxy-acetylene blow torches.
Argon
Argon is effective in the field of thermal
insulation and can be used as filler for
windows with double glazing. Because of its
low conductivity, this gas improves
insulation substantially and hence increases
windows thermal efficiency. It helps to cut
down on energy loss, which in turn reduces
electricity (or any other resources)
consumption for heating purposes.
 An inert gas, Argon is used as
a protective atmosphere for certain
foodstuffs,
notably fruits
and
vegetables.
 In metallurgy and welding, Argon is
also used to create an inert
protective atmosphere between the
liquid metal and surrounding air.
The gas offers protection against
oxidation risks and reduces smoke
emissions.
 Argon is also used in lighting notably
for the filling of incandescent and
fluorescent light bulbs. Since it
doesn’t react with the filament,
even at high temperatures, it
protects it.
3.Technical Description Of Technologies:
Cryogenic Distillation:[3][4][7][9]
The technology of air separation with the
help of cryogenic temperatures is by far not
new. The principle of cryogenic plants
operation is based on the air liquefaction
and its subsequent separation with the
recovery of nitrogen, oxygen, and argon.
This method of gas production is called
deep air freezing. The feed air is initially
compressed, and, after passing though the
heat exchangers, is expanded in the turbine
expander or on the expansion valve with
the resulting temperature decrease down
to the 93 °K point, where it is turned into
liquid.
Subsequent separation of liquefied air,
which mainly consists of liquid nitrogen and
liquid oxygen, is based on the difference
in the boiling points of its components:
oxygen — 90.18 °K, nitrogen — 77.36 °K.
By gradual evaporation of liquid air,
nitrogen is being preferentially evaporated,
while the residual liquid is becoming
increasingly saturated with oxygen. The
process is then multiply repeated at the air
separation rectifying tray with the
production of liquid oxygen, nitrogen, and
argon of the required purity.
Pressure Swing Adsorption: [1][2]
The adsorption technology is based on the
sorption of certain substances by molecular
sieves with the resulting air mixture
separation. The adsorption technology
allows efficiently producing such gases
as nitrogen and oxygen from atmospheric
air.
Plants operation is based on the pressure
swing adsorption principle with the feed air
pressure exceeding the atmospheric level
at the
adsorption
stage,
and
depressurization
to the
atmospheric
pressure at the desorption stage.
When air passes through one of the two
alternating adsorbers filled with adsorbent the carbon-molecular sieve - oxygen
is preferentially adsorbed by the sieve,
while the gas media is becoming saturated
with nitrogen. After the molecular sieve
is saturated with oxygen, the air is diverted
to the other adsorber. The used adsorber
is then depressurized and purged with part
of the product nitrogen, while the sieve
is cleared from the adsorbed oxygen and
is regenerated. The adsorption based
process of air separation is performed
within the temperature range of 10 0C to
40 0С.
Membrane Technology:[1][5][11]
The pivot of the membrane technology
is the membrane responsible for the gas
separation process. The modern gas
separation membrane no longer represents
a flat plate or film, but is shaped as hollow
fibers.
Membrane
separation
technologies
currently use a hollow-fiber membrane
consisting of porous polymer fibers coated
with a separation layer. A porous fiber has
a complex asymmetric structure, with the
polymer density increasing towards the
fiber external surface. The application
first
cooling it
until it
liquefies,
then
selectively
distilling
the compo-nents at
their
various
boiling
tempera - tures
technology
used to
separate
some gas
species
from a
mixture of
gases
under
pressure
according
to the
species'
molecular
characteris
tics and
affinity for
an adsor-bent
material
separated
by synthetic
membranes
made from
polymers
such as
polyamide
or cellulose
acetate, or
from
ceramic
materials.
Separation
Mean
Boiling
Point
Tempe - rature
Difference
Adsorption
Affinity
Differential
velocity
with which
various gas
mixture
components
permeate
membrane
substance.
Temperature
It requires
temperatur
e below
00C since
boiling
point of air
is below 1500C.
Ambient
Air Tempe-rature.
Depends
upon the
type of the
membrane.
Pressure
Before cold
box, it is >
20 bar.
Operating
Pressure is
in the
range of 2
to 6 kg/cm2
in case of
air
separation.
Typical
range of
pressure is
10 to 40
bars.
>10 bar
Product Purity
90 – 98%
>99%
90 to 98%
Feed
Composition
30 – 75%
75- 90%
30 – 90%
of porous support layers with asymmetric
structure allows separating gases under
high pressures (up to 6.5 MPa). The
thickness of the fiber gas separation layer
does not exceed 0.1 µ, ensuring a high
relative permeability of gases across the
polymer membrane.
The existing level of the technological
development
makes
possible
the
production of polymers with a high
selectivity for various gases, and,
consequently, capable of delivering highpurity gaseous
products.
A modern
membrane module used for the membrane
gas separation technology comprises
a removable membrane cartridge and
a body. The density of fibers packaging
in the cartridge is estimated at some 500–
700 square meters per the cartridge cubic
meter, which helps to minimize the
dimensions of gas separation plants.
The membrane technology based gas
mixture separation utilizes the difference
in partial pressures on the external and
internal
surfaces
of a hollowfiber membrane. Highly permeating gases
(e.g. H2, CO2, O2, water vapors, higher
hydrocarbons) penetrate the fibers and exit
the membrane cartridge through one of the
pipes. Less permeating gases (e.g. CO, N2,
CH4) exit the membrane modules through
the other outlet pipe.
4.Comparison between Three Main Air
Separation Techniques:[1][3][4][6][8]
Factors
Concept
Cryogenic
Distillation
Pure gases
can be
separated
from air by
Pressure
Swing
Adsorption
Membrane
Technology
Pressure
swing
adsorption
(PSA) is a
Gas
mixtures
can be
effectively
Feed Phase
Feed Criteria
Energy
Consumption
Shutdown
Gas
Liquid
(after
wards)
Gas
Pretreatment
for
removal of
water
particles
Pretreatment
for main
taining
feed
quality.
Pretreatment
for removal
of
components
that are
harmful to
membranes
used.
Highest in
compa - rison with
the rest of
the two
Moderate
to high
with
compared
to the
membrane
technology
.
Moderate
number of
shutdowns
than
membrane
separation.
Lowest
Frequent
Shutdowns
Gas
Lesser
Shutdowns
and mostly
requires for
membranes
replacements or regenerations.
Utility
requirements
Air Compressor
and
Coolers,
Chillers,
Water
separators.
Air Compressor.
Air
treatment.
Air
Compressor
Air
treatment.
Cost
High maintenance
cost and
Capital
cost.
Low main
tenance
cost. High
capital
cost.
Requires
large sites.
Lower cost
compared
to PSA and
Cryogenics.
5. Conclusion:
For same capacity and provided utilities,
cryogenic distillation consumes more
energy and thus economically more
expensive with compared to pressure swing
adsorption.
Though both the technologies have their
pros and cons, for single gas separation,
pressure swing adsorption is the more
preferable option according to the cost
analysis.
The membrane technology is the latest one
of all but it is expensive mainly because of
complicated structured membranes. If cost
of the project affects the most to the
decision then pressure swing adsorption is
again more preferable option over the
membrane technology.
6. References:
1).Perry, R.H. and Green D. , Perry’s Chemical
Engineer’s Handbook, McGraw-Hill book
company, ed. 8th, pg no. 16-61 to 16-64, 20-57
to 20-62.
2).John J. Mcketta and William Aaron
Cunningham, Encyclopedia of Chemical
Processing And Design, Volume-2, Adsorption
design.
3).Robert E. Treybal, Mass-Transfer Operations,
McGraw-Hill International Editions, Chemical
Engineering series, pg no. 585.
4).S.B. Thakore, B.I.Bhatt, Introduction to
Process Engineering and Design, Tata McGraw
Hill Education Pvt. Ltd.
5).Kaushik Nath, Membrane Separation
Processes, PHI Learning Pvt Ltd., pg no 167.
6).David R. Vinson, Air separation control
technology < computers and chemical
engineering, Science Direct>
7).Barron, Randall F., Cryogenic Systems, 2nd
ed., Oxford University Press, New York,
1985, pp.199-211.
8).J Klosek, A.R smith, A review of air separation
technologies and their integration with energy
conversion processes. <Fuel processing
technologies, volume70, issue 2>
9).Prof. Jennifer Wilcox, Cryogenic Distillation
and Air Separation, <http://link.springer.com/>
10).Gas separation by Pressure Swing
Adsorption (PSA), Japan envirochemicals
11).G.K. Fleming and W.J. Koros
Membrane-based gas separation, < Journal of
membrane science, volume 83, issue 1>