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UNIT – 4
GAS SEPARATION AND GAS PURIFICATION
SYSTEMS
Introduction:
cryogenic industry is huge owing to the various
applications of the cryogens, both in liquid and gaseous
states. For example, the use of inert gases like argon in
chemical and welding industries has increased in the
recent past. Liquid Nitrogen is used as precoolant in most
of the cryogenic systems. Also, cryogens like LOX, LH2
are used in rocket propulsion. In the recent past, LH2 is
being considered as a fuel for an automobile. Production
of Ammonia in Rashtriya Chemicals & Fertilisers Ltd.
(RCF) industry requires separation of purge gases like
Nitrogen, Argon and other inert gases at cryogenic
temperatures.
For most practical purposes, Air is considered as a
mixture of 78% N2 + 21% O2 + 1% Ar. The other
ingredients are Helium, Neon, Krypton etc. which occur
in negligible quantities.
Air is the raw material for the production of most of the
gases and the process of separation of any gas mixture
into its individual components is called as Gas Separation.
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In other words “Gas Separation” deals with separation of
various gas mixtures and their purification.
Different techniques of gas separation commonly
used are
• Synthetic membranes
• Adsorption
• Absorption
• Cryogenic distillation
Synthetic membranes:
Synthetic membranes are the porous media which allow
only a certain gas molecules to pass through.
Semi – permeable membrane is a film which allows only
one kind of gas to pass through but not the other.
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The membrane in the figure allows only Gas A to pass
and hence the separation occurs.
For example, a thin sheet of palladium allows to pass
through.
Adsorption:
Adsorption is the physical processes in which only a
certain kind of gas molecules are adhered to the adsorbing
surface.
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The adsorbate in the figure adheres only Gas A to the
surface and hence the separation occurs.
For example, finely divided Nickel adsorbs hydrogen on
to its surface.
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Absorption:
Absorption is a chemical process in which a substance in
one physical state is taken into other substance at a
different physical state.
• For example, liquids being absorbed by a solid or gases
being absorbed by a liquid.
• When an incoming stream containing CO2 is passed
through a solution of Sodium hydroxide, the later absorbs
the gas and hence decreases the CO2 content in the
outgoing stream.
• Hence, this chemical process helps in the separation of
the mixture.
Cryogenic distillation: Distillation is a process of separation based on the
differences in the volatilities (boiling points).
• If the process of distillation occurs at cryogenic
temperatures, it is called as Cryogenic Distillation.
• The commercial production of gases like O2, N2,
Argon, Neon, Krypton & Xenon is obtained by cryogenic
distillation of Liquid Air.
The separation of a mixture can be done at both room
temperature and cryogenic temperature.
• For example in the case of Air, the following processes
are possible.
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• Some of the advantages of Cryogenic separation over
Room Temperature separation are
• The separation at lower temperatures is most
Economical.
• There is an increased difference in the boiling points of
the ingredients.
• A large quantities of the gas can be separated.
• A high purity of the gas can be obtained.
Is Gas Mixing Reversible?
Consider a closed chamber filled with Gas A and Gas B
as shown in the figure.
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• Initially, the gases are separated by an impervious wall.
• If the wall is removed, the gases would mix.
• However, the replacement of wall would not result in
the separation of gases.
• It is clear that the mixing of two different gases is an
irreversible process because unmixing or separation of the
mixture requires work input.
• The system in which all the processes are reversible is
called as an Ideal System.
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• Although in reality such a system does not exist
Some of the system are used to separate and purify the
cryogenic liquids. Separation of mixtures at cryogenic
temperature is the most economical separation method.
Commercially produced oxygen, nitrogen, argon, neon, krypton
and xenon are obtained through rectification of liquid air. Other
separation methods physical adsorption and refrigeration
purification.
Thermodynamically Ideal separation system:
Thermodynamically ideal system would be one in which all
processes are reversible, semipermeable membranes practically
employed to separate certain gases. Ex: a thin sheet of palladium
will allow molecules of hydrogen to pass through it, but will not
allow other gases to pass through. This process imagined to be
reversible. Mixing of gases is an irreversible process as it
requires some work to separate the mixture into its original
constituents. However reversible mixing is possible with semi –
permeable membranes which allow passage of one gas only
from a mixture and work obtained while mixing is used to
unmix the gases.
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Let us consider the separation of mixture of two gases (gas A
and Gas B) in double piston arrangement (closed chamber).
The left hand piston is permeable to gas A only and the right
hand piston permeable to gas B only and not A. The separation
takes place at constant temperature, the work required to
separate gases as the process is reversible and in compression.
The molecule of gas A exerts no pressure on left hand piston but
exerts pressure on B. similarly the molecules of B exerts no
pressure on right hand side but exerts on left hand side. If both
pistons moves inward until individual gases are separated. Initial
temperature of the gases is (ambient) and pressure ( ). The
work requirement may be determined from temperature entropy
diagram for each gas.
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Isothemal reversible work requirement for a gas =
=
( ) ( ) ---------- 1
Total work to separate unit mass of mixture =
= (
) (
)
+ (
) (
) ------- 2
1 – before separation
2 – after separation
= isothermal work
m = unit mass
= ambient temperature
= ambient pressure
Substituting for ideal work for each component from eqn 1
results in
= {
( ) (
) ( )} - {
(
) (
) ( )} ---- 3
From equation 3 we can observe that ideal work is decreased as
the separation process is carried out at decreased temperature.
Entropy of ideal gas PV = mRT, S = ln T – R ln p +
Enthalpy of ideal gas is h = T+
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Where, are the entropy and enthalpy values at some
reference state.
When gas come in contact with solid. If the gas molecules
penetrate into the solid called – absorption, if the gas molecules
stuck on the solid surface one or two layers called adsorption, if
there is strong chemical bond between molecules and solid –
chemical adsorption – weak contact – physical adsorption.
Ideal work of separation for an ideal-gas mixture is
= {(
) (
) (
) (
)}
The process is carried out at constant temperature and constant
volume .
Individual pressure ratios =
=
=
In terms of moles =
=
=
(since, mR = nR = mR/M)
Where, =
= mole fraction of gas A.
Work requirement per unit mole of mixture is given by
=
= { (
) (
)}
Separation of mixture of several gases, =
=
∑ (
)
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= mole fraction of the jth component of the mixture.
FOM =
=
Principles of Gas separation:
Simple condensation and evaporation: Separation of mixtures
can be obtained by partial condensation for the elements which
have widely different boiling points. ex: mixture of - ,
- etc can be separated effectively by a single partial
condensation.
It is not possible to separate air into practically pure components
by a single condensation.
Ex: The mixture of and has composition 80% and 20% on
molar basis at 20 atm. During condensation at 111.2 K,
attained 99.35%. but further lower the temperature to 80K the
composition of the liquid is 99.43% . The composition of the
vapor above the liquid is 6.4% . During this process we have
condensed 79% of the original gas.
When pure substances are required this process is not adequate.
– 77.36K 3.379 atm & 77 – 23% at 1 bar
- 4.214K 2.26 at 81 – 0.161y = 0.836
– 90.18 50.10 atm 78.8 – 1.00y = 0.94
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Liquid – 93% – 7%
– 80% - 20% He 20 bar molar basis
110K – 0.179%y = 0.242 the vapor above liquid
80K – 0.791% = 0.936
94% and 6% oxygen.
Principle of Rectification:
Rectification is the cascading of several evaporations and
condensations carried out in counterflow in order to obtain as far
as possible pure gases or liquids.
The equipment which carries out these processes is called as a
Rectification Column.
The figure below shows the schematic of a Rectification
column.
It is a vertical column which is closed by spherical domes, both
at the top and at the bottom.
•These are spherical in shape in order to minimize surface area
(less heat in -leak) and accommodate high pressures (1 –5 atm).
•The column is well insulated because, it is usually operated at
cryogenic temperatures.
The top dome houses a Condenserand the bottom dome houses
a Boiler.The two phase mixture is first expanded isenthalpically.
It can be liquidor liquid + vaporor vapor.
This expanded product is introduced into the column as Feed.
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Schematic of a rectification column
The rectification process shown schematically on temperature-
composition diagram.
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If a saturated vapor is introduced at point 1 in the column in
ideal steady state operation, the liquid layer above feed inlet has
the same composition as the feed, even though the liquid at point
2 is at lower temperature than feed. But the vapor flows upward
through the liquid, the liquid flows downwards as shown in fig.
The vapor transfer the heat to the liquid and forms bubble.
Higher boiling point component moves downwards and lower
boiling point component moves upwards. The process continues
as long as feed continuous. The upper portion becomes richer
and richer with nitrogen and lower portion becomes richer and
richer with oxygen. At the top portion of the column some heat
is removed to provide liquid in the upper portion and some heat
is provided to form vapors in the boiler or kettle.
The process continues till to attain required purity. By using
large number of layers, quite high purity of both components
can be achieved.
To achieve the high perfection more number of plates are
required theoretically, since the vapor does not leave the plate at
the same temperature as the liquid on the average.
Measure of the perfection of the plate or layer of the liquid is
given by Murphree efficiency of the plate.
Murphree efficiency: The ratio of actual change in mole
fraction of the lower boiling point component in the vapor
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during its travel through the liquid layer to the maximum
possible change in the mole fraction.
=
Where,
= mole fraction of more volatile component in the vapor
phase leaving the jth plate.
=mole fraction of more volatile component in the vapor
phase rising to the jth plate.
= composition of vapor in equilibrium with liquid leaving the
jth plate.
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Air – separation systems:
Linde Single-Column Air separation System:
Linde single-column gas-separation system.
The Linde single-column system introduced in 1902 is the
simplest airseparation system used. In this scheme, shown in.
Fig. above, water vapor and carbon dioxide are removed from
the air after the latter has been compressed isothermally. The air
then passes through a precooling heat exchanger. This heat
exchanger is a three-channel unit if gaseous oxygen is produced
and a two-channel unit if liquid oxygen is produced. For a
gaseous oxygen product, the cold returning gas is used to cool
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the incoming air whereas a liquid oxygen product does not
provide any cooling capabilities for the system since it is
removed from the bottom of the column. The precooled air
passes through a coil in the bottom of the column, where it is
further cooled and liquefied. The coil serves as the reboiler in a
typical distillation column. The liquefied air is then expanded
through a Joule-Thomson throttling valve to the column
pressure, where some of the air is vaporized. The part liquid,
part vapor feed is directed to the top of the column with the
liquid serving as the reflux for the separation process. If gaseous
oxygen is the final product, the entering air must be compressed
to apressure of 3-6 MPa; if the final product is to be liquid
oxygen, apressure near 20 MPa is necessary.
The Linde single-column separation system is obtained by
replacing the liquid reservoir of the simple Linde liquefaction
cycle with a stripping column.
However, any of the other liquefaction processes could be used
just as weIl to furnish liquid for the column.
The major problem of the Linde single-column system is that,
although the oxygen purity is high, too much of the oxygen in
the feed is lost in the nitrogen emuent system. In many cases, the
equilibrium vapor concentration in the nitrogen waste stream is
6% oxygen for an initial liquid mixture of 21 % oxygen and
79% nitrogen. This contamination prevents any use of the
nitrogen product in applications requiring high-purity gas.
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Linde double column air separation:
Linde double-column gas-separation system
The Linde double-column system was introduced in 1910 to
solve the problem of oxygen losses in the nitrogen stream of the
Linde single-column system. As noted earlier, the maximum
purity of the top product in the single column is approximately
94 mol % nitrogen. If this purity had been attained in Example
6.12, nearly 25% of the oxygen in the feed would have been
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removed in the top product.
In the double-column scheme shown in Fig. above, two columns
are placed one on top of the other; the lower column is generally
operated at a pressure of 0.5 or 0.6 MPa while the upper column
is operated at about 0.1 MPa. This difference in column pressure
provides the needed temperature difference to operate the
condenser-reboiler located between the two columns. In this
arrangement, the nitrogen vapor in the lower column condenses
at approximately 95 K while the oxygen liquid in the upper
column vaporizes near 90 K. The condensed nitrogen from the
lower column provides reflux for both the upper and lower
columns. Note that if the desired product is liquid oxygen, the
heat exchanger becomes a two-channel heat exchanger and the
gaseous oxygen product stream is deleted.
The double-column system works like the single-column system
except for the addition of the rectification section. In the double-
column system, entering air is introduced in the middle of the
lower column instead of at the top. Part of the liquid nitrogen
product stream from the lower column is throttled to the
operating pressure of the upper column and sent to the top of the
upper column as reflux. The enriched liquid air from the lower
reboiler is also throttled and introduced as feed into the middle
of the upper column.
Depending on the number of plates used, any practical purity
level of either or both components may be obtained. When
extremely high-purity products are desired, the argon present in
the air must be considered as a third component of the mixture
and removed in a draw-off stream from the upper column.4 The
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operation of such a column can best be shown with the aid of an
example.
The actual efficiency of the air-separation process is
considerably below
the theoretical value. There are three major sources of
inefficiency: (1) the
non ideality of the refrigeration process, (2) the imperfection of
the heat
exchangers, and (3) losses of refrigeration through non ideal
insulation. 5
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ARGON AND NEON SEPARATION SYSTEMS:
Argon-recovery subsystem.
Since air contains 0.934 % argon and the boiling point of argon
is between those of nitrogen and oxygen, it can remain as an
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impurity in either the nitrogen or oxygen product or both.
Because of the relative quantities of nitrogen, oxygen, and argon
in air, the oxygen product will contain about 5% argon if the
latter is all removed in the oxygen product. If the argon is
collected with the nitrogen, the nitrogen product will have an
impurity of approximately 1.3 % argon. Thus, when extremely
pure oxygen and nitrogen are required, the argon must be
removed. 9
In the early days of air separation, argon was considered to be an
undesirable impurity, but now there is an extensive commercial
demand for argon for use in inert-gas welding and in
incandescent lamps. Accordingly, it makes economic sense to
consider a system that can also produce argon when separating
air.
An argon-separation system usually consists of two basic parts:
(1) a recovery subsystem as shown in Fig.6.31 and (2) a
purification subsystem.
The argon concentration is highest in the lower portion of the
upper column of a double-column air separation system where
there is also a high oxygen concentration. At this point, part of
the oxygen-argon-nitrogen mixture is removed and sent to an
argon column where the crude argon is recovered.
The reboiler liquid from this column is almost pure oxygen and
is returned at an appropriate feed point in the upper part of the
double column while the crude argon is sent to the argon-
purification system.
There are two basic types of argon-purification subsystems in
industrial
use: (1) the catalytic-combustion system as shown in Fig. 6.32
and (2) the adsorption system. The first subsystem adds
hydrogen to the argon stream removed from the crude argon
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separation column. This mixture is compressed to 0.5 MPa and
the oxygen is removed by combustion or combination with the
hydrogen in a catalytic-combustion furnace. This results in an
argon
Argon-purification subsystem utilizing a combined catalytic combustion
and rectification scheme.
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product containing only 1 ppm by volume of oxygen, 1 %
hydrogen, and 1 % nitrogen. A water-cooled condenser followed
by an alumina drier removes any water vapor formed during
combustion. The remaining hydrogen and nitrogen are removed
by condensing the argon stream with the incoming cold crude
argon. The final argon product contains less than 20 ppm
impurities by volume, where the major impurities are nitrogen (1
to 10 ppm), oxygen (0 to 5 ppm), and carbon dioxide (0 to 5
ppm).
The second subsystem removes oxygen by adsorption instead of
catalytic combustion. A rectification column is used to remove
the nitrogen from the argon-oxygen mixture. The latter mixture
collects in the reboiler of the column where it is then passed
through a heat exchanger and two adsorbent traps in parallel.
These traps preferentially adsorb the oxygen. The argon gas is
filtered to remove any adsorbent particles and condensed.
Separation of Neon and Helium:
After a double-column air-separation system has been in
continuous operation over long periods of time, the effectiveness
of the heat transfer in the condenser- reboiler is observed to
decrease. This is attributed to the gradual accumulation of neon
and helium in the nitrogen, which lowers the partial pressure of
the nitrogen so that it will not condense at the condenser design
temperature. This problem can be solved by periodically venting
some of the accumulated gas from the dome of the condenser
either to the atmosphere or to a neon-separation system.
A neon-separation system consists of two subsystems: (1) the
recovery subsystem, and (2) the purification subsystem.
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Neon-recovery subsystem. erude neon and helium are recovered
from the dome of the lower-column condenser in a double-column
air separation system.
The recovery subsystem shown in Fig. above consists of a small
condenser-rectifier where the vent gas is sent to remove the neon
and helium from the nitrogen. Once these have been separated,
the crude neon-helium gas mixture from the dome of the
condenser- rectifier passes through a liquid-nitrogen-refrigerated
charcoal trap, to remove most of the remaining nitrogen. The gas
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leaving the charcoal trap contains about 95% neon and helium
and about 5% nitrogen and hydrogen.
The neon-helium mixture is compressed and enters the
purification subsystem, as shown in Fig. 6.34. The remaining
nitrogen is removed in another liquid-cooled charcoal trap. The
mixture passes through a copper-oxide
furnace to remove the hydrogen by oxidation to water vapor.
The latter is frozen out in a mo ist ure trap. The remaining neon-
helium mixture is directed through another charcoal trap where
the neon is adsorbed. The helium passes through a gaseous-
discharge tube, which indicates when the charcoal becomes
saturated with neon. This signals the need to divert the flow
from the moisture
trap through a parallel set of charcoal traps to continue the neon
adsorption process and permit the saturated traps to be desorbed
of neon upon warming.
The desorbed neon gas is pumped through the discharge tube to
indicate its approximate purity. The actual neon purity is
checked using analytical techniques such as gas
chromatography. Once the purity level is sufficiently high, the
neon is sent to a final charcoal trap where any final traces of
helium are removed. After all of the neon is adsorbed, the trap is
warmed up and the pure neon is transferred to a gas storage
system.
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Neon-purification subsystem utilizing charcoal adsorption.
Adsorption process:
Physical adsorption:
When a gas is brought in contact with a solid, generally a part of
the gas is taken up by the solid. If the gas molecules penetrate
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into the solid, the process is usually called absorption. If the gas
molecules stick to the surface of the solid in one or more layers,
the process is called adsorption. If there is a very strong reaction
between the solid and the gas, the adsorption process is called
chemical adsorption.
Materials such as silica gel, alumina gel, charcoal, and synthetic
zeolite (molecular sieves) are widely used as adsorbents because
their porous physical structures create large effective surface
areas. Most of the gel and carbon adsorbents have pores of
varying sizes in a given sample, but the synthetic zeolites are
manufactured with closely controlled pore size openings ranging
from 4 to about 13Å. This makes them even more selective than
other adsorbents since it permits separation of gases on the basis
of molecular size.
The equilibrium between the solid and gas and the rate of
adsorption. Equilibrium data for the common systems generally
are available from the suppliers of such material. The rate of
adsorption is usually very rapid and the adsorption is essentially
complete in a relatively narrow zone of the adsorber.
If the concentration of the adsorbed gas is considerably more
than a trace, then heat of adsorption mayaiso be a factor of
importance in the design. The heat of adsorption is usually of the
same order or larger than the normal heat
of condensation of the gas being adsorbed. Under such
situations, it is generally advisable to design the purification in
two steps, i.e., first removing a significant portion of the
impurity either by condensation or chemical
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reaction and then completing the purification with a low
temperature adsorption system.
A number of expressions have been developed to predict the
volume of gas that may be adsorbed by an adsorbent. One of the
more widely used relationships is that of Brunauer, Emmett, and
Teller which assumes that a molecule is retained on the
adsorbent surface if its energy is lower than the interaction
energy between the molecules and the adsorbent atoms. The
resulting relationship for the volume of a gas adsorbed on a
surface per unit mass of adsorbent based on this assumption is
(
)
(
)[ ( )(
)]
1
where V is the volume of gas adsorbed at 0.1013 MPa and
273.15 K, the mass of the adsorbent, the volume of gas
needed to form a monomolecular layer of gas over the total
adsorbent surface per unit mass of adsorbent, p the partial
pressure of the gas being adsorbed, and the saturation
pressure of the gas being adsorbed at the temperature of the
adsorbent. The parameter z is defined as
z = exp[( )/RT] 2
where is the interaction energy between the surface and the
gas for the first adsorbed layer, and is the condensation
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energy for the next layers. The latter differs from since these
layers are deposited on a layer of adsorbed gas rather than on the
solid surface. Again, R is the specific gas constant and T the
temperature of the gas being adsorbed. Some numerical values
for and are presented in Table 6.9 for several
adsorbent-gas combinations.
It is evident from Eqs. (1) and (2) that increased adsorption is
achieved when the difference between and is large and the
adsorbent temperature is reduced.
The parameter is a function of the size of the adsorbed
molecules and the ratio of the adsorbent surface area to the
adsorbent mass. This can be represented mathematically by
=
3
Where N is the number of adsorbed molecules required to cover
the total adsorbent surface A, M the molecular weight of the
adsorbed gas, R the specific gas constant, Avogadro's
number, and the mass of adsorbent.
and are the standard temperature (273.15 K) and pressure
(0.1013 MPa), respectively. When the adsorbed molecules have
the same diameter D and are close packed on the surface, N can
be obtained from
N =1.155A/
If only n layers of a gas can be adsorbed on a surface, Eq. (1)
must be modified to reflect this restriction and results in the
following expression:
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= [ (
) ]
[ (
)]
[ ( )(
)
(
)
]
[ ( )(
) (
)
]
4
Equation (1) and Eq. (4) yield practically the same values for the
volume of gas adsorbed for n > 4 when the gas partial pressure
is less than 0.4 . As n becomes very large, results from the
two relations become identical.
PSA systems:
Pressure swing adsorption (PSA) is a technology used to
separate some gas species from a mixture of gases under
pressure according to the species' molecular characteristics and
affinity for an adsorbent material. It operates at near-ambient
temperatures and differs significantly from cryogenic distillation
techniques of gas separation. Specific adsorptive materials (e.g.,
zeolites, activated carbon, molecular sieves, etc.) are used as a
trap, preferentially adsorbing the target gas species at high
pressure. The process then swings to low pressure to desorb the
adsorbed material.
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Schematic drawing of PSA process ("aria" = air input)
Process
Pressure swing adsorption processes rely on the fact that under
high pressure, gases tend to be attracted to solid surfaces, or
"adsorbed". The higher the pressure, the more gas is adsorbed;
when the pressure is reduced, the gas is released, or desorbed.
PSA processes can be used to separate gases in a mixture
because different gases tend to be attracted to different solid
surfaces more or less strongly. If a gas mixture such as air, for
example, is passed under pressure through a vessel containing an
adsorbent bed of zeolite that attracts nitrogen more strongly than
it does oxygen, part or all of the nitrogen will stay in the bed,
and the gas coming out of the vessel will be enriched in oxygen.
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When the bed reaches the end of its capacity to adsorb nitrogen,
it can be regenerated by reducing the pressure, thereby releasing
the adsorbed nitrogen. It is then ready for another cycle of
producing oxygen-enriched air.
This is the process used in medical oxygen concentrators used
by emphysema patients and others who require oxygen-enriched
air to breathe.
Using two adsorbent vessels allows near-continuous production
of the target gas. It also permits so-called pressure equalisation,
where the gas leaving the vessel being depressurised is used to
partially pressurise the second vessel. This results in significant
energy savings, and is common industrial practice.
Adsorbents
Aside from their ability to discriminate between different gases,
adsorbents for PSA systems are usually very porous materials
chosen because of their large specific surface areas. Typical
adsorbents are activated carbon, silica gel, alumina and zeolite.
Though the gas adsorbed on these surfaces may consist of a
layer only one or at most a few molecules thick, surface areas of
several hundred square meters per gram enable the adsorption of
a significant portion of the adsorbent's weight in gas. In addition
to their selectivity for different gases, zeolites and some types of
activated carbon called carbon molecular sieves may utilize their
molecular sieve characteristics to exclude some gas molecules
from their structure based on the size of the molecules, thereby
restricting the ability of the larger molecules to be adsorbed.
Applications
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Aside from its use to supply medical oxygen, or as a substitute
for bulk cryogenic or compressed-cylinder storage, which is the
primary oxygen source for any hospital, PSA has numerous
other uses. One of the primary applications of PSA is in the
removal of carbon dioxide (CO2) as the final step in the large-
scale commercial synthesis of hydrogen (H2) for use in oil
refineries and in the production of ammonia (NH3). Refineries
often use PSA technology in the removal of hydrogen sulfide
(H2S) from hydrogen feed and recycle streams of hydrotreating
and hydrocracking units. Another application of PSA is the
separation of carbon dioxide from biogas to increase the
methane (CH4) ratio. Through PSA the biogas can be upgraded
to a quality similar to natural gas.
PSA is also used in
Hypoxic air fire prevention systems to produce air with a
low oxygen content.
On purpose propylene plants via propane dehydrogenation.
They consist of a selective medium for the preferred
adsorption of methane and ethane over hydrogen.
Industrial Nitrogen generator units which employ the PSA
technique produce high purity nitrogen gas (up to
99.9995%) from a supply of compressed air. But such PSA
are more fitted to supply intermediate ranges of purity and
flows. Capacities of such units are given in Nm³/h, normal
cubic meters per hour, one Nm³/h being equivalent to 1000
liters per hour under any of several standard conditions of
temperature, pressure, and humidity.
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o for nitrogen : from 100 Nm³/h at 99,9 % purity, to
9000 Nm³/h at 97% purity ;
o for oxygen : up to 1500 Nm³/h with a purity between
88% and 93%.
Research is currently underway for PSA to capture CO2 in large
quantities from coal-fired power plants prior to
geosequestration, in order to reduce greenhouse gas production
from these plants.
PSA has also been discussed as a future alternative to the non-
regenerable sorbent technology used in space suit Primary Life
Support Systems, in order to save weight and extend the
operating time of the suit.
Variations of PSA technology
Double Stage PSA
(DS-PSA, sometimes referred to as Dual Step PSA). With this
variation of PSA developed for use in Laboratory Nitrogen
Generators generation of nitrogen gas is divided into two steps:
in the first step, the compressed air is forced to pass through a
carbon molecular sieve to produce nitrogen at a purity of
approximately 98%; in the second step this nitrogen is forced to
pass into a second carbon molecular sieve and the nitrogen gas
reaches a final purity up to 99.999%. The purge gas from the
second step is recycled and partially used as feed gas in the first
step.
In addition, the purge process is supported by active evacuation
for better performance in the next cycle. The goals of both of
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these changes is to improve efficiency over a conventional PSA
process.
The DS-PSA is also applied to up levels oxygen concentration in
this case a zeolite aluminum silica based adsorb Nitrogen in the
first stage focusing Oxygen 95%, and in the second stage the
molecular sieve carbon-based adsorbs the residual nitrogen in a
reverse cycle, concentrating to 99% oxygen.
Rapid PSA
Rapid pressure swing adsorption or RPSA is frequently used in
portable oxygen concentrators. It allows a significant reduction
in the size of the adsorbent bed when high purity is not essential
and feed gas can be discarded. It works by quickly cycling the
pressure while alternately venting opposite ends of the column
at the same rate. This means that unadsorbed gases progress
along the column much faster and are vented at the distal end,
while adsorbed gases do not get the chance to progress and are
vented at the proximal end.