unit 4 gas separation and gas purification systems1

Cryogenics [email protected]

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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

https://en.wikipedia.org/wiki/Activated_carbon


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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.

https://en.wikipedia.org/wiki/Hydrotreating


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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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38 | P a g e

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.

unit 4 gas separation and gas purification systems1

Engineering