Lecture Slides by Prof. Imitaz Ali at Ghulam Ishaq Khan Institute for Engineering Sciences and Technology (GIKI).

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Oxygen and Nitrogen
Uses
• Oxygen
– It is used to produce oxyacetylene flame to cut and weld the metals
– Used in oxygen steelmaking process
– Used as artificial respiration for patients
– Used by mountain climbers, sea divers and high attitudeairplane
pilots
• Nitrogen
– Used in manufacture of synthetic nitrogen oxide, ammonia, nitric acid
– Used in manufacture of organic nitrates e.g; propellants and
explosives
– Syntheticallyproduced nitrates are key ingredients of industrial
fertilizers
– Applied to create inert atmosphere

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Introduction
Oxygen
• Oxygen (O2) composed of two atoms of the element at (O)
bind to form dioxygen, a very pale blue, odorless, tasteless
diatomic gas.
Introduction
Oxygen
• It constitutes 20.95% of the volume of air
• It is necessary to sustain life
• It is the highly reactive nonmetallic element that
readily forms compounds or oxides with almost all
other elements
• It is a strong oxidizing agent and has the second-
highest electro negativity after fluorine than of all the
elements
• By mass, after hydrogen and helium, oxygen is the
third-most abundant element in the universe

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Introduction
Oxygen
• Oxygen was independently discovered by Carl Wilhelm Scheele and
Joseph Priestley in 1773 and 1774 respectively, but work was first
published by Priestley.
• Antoine Lavoisier named as oxygen in 1777, whose experiments
with oxygen helped to discredit the then-popular phlogiston theory
of combustion and corrosion.
• Oxygen is produced industrially by
– Cryogenic Separation : fractional distillation of liquefied air
– AbsorptiveSeparation : use of zeolites/ silica gel with pressure-cycling
to concentrateoxygen from air
– MembraneSeparation:
– Electrolysis of water : used only to an insignificantextent
Introduction
Nitrogen
• Nitrogen (N2) is a colorless, odorless, tasteless,
and mostly inert diatomic gas at standard
conditions, constituting 78.09% by volume of
Earth's atmosphere.

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Introduction
Nitrogen
• Nitrogen occurs in all living organisms, primarily
in amino acids, proteins and in the nucleic acids
(DNA and RNA).
• The human body contains about 3% by weight of
nitrogen, the fourth most abundant element
after oxygen, carbon, and hydrogen.
• Nitrogen was discovered by Daniel Rutherford in
1772, who called it noxious air or fixed air.
• He also explains that nitrogen does not support
combustion.
Introduction
Nitrogen
• At the same time by Carl Wilhelm Scheele, Henry
Cavendish, and Joseph Priestley, referred it as burnt air or
phlogisticated air.
• Antoine Lavoisier referred nitrogen as inert gas and as
"mephitic air" or azote, in which animals died and flames
were extinguished.
• English word nitrogen entered the language in 1794.
• The extremely strong bond in elemental nitrogen causing
difficulty for both organisms and industry in breaking the
bond to convert the nitrogen into useful compounds, but
large amounts of useful energy released when the
compounds burn, explode, or decay back into nitrogen gas.

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Introduction
Analysis of Air
• Air mainly consist of two gases oxygen and nitrogen,
which are practically considered to constitute 1/5 and
4/5 of air by volume respectively. The list of various
gases present in air by weight percent is as under
By volume By weight
Oxygen 20.95 23.2
Nitrogen 78.09 75.47
Argon 0.933 1.28
Carbon Dioxide 0.03 0.046
Neon 0.0018 0.0012
Helium 0.0005 0.00007
Krypton 0.0001 0.0003
Gas
Ratio compared to Dry Air(%)
Introduction
Analysis of Air
• Except CO2 the concentration of all the gases
present in air are constant.
• Water vapors and traces of ozone and iodine are
present in air in variable amounts.
• Composition of air depends on altitude and
distance to sea, in neighborhood of industry,
built up urban areas, nearby volcanic
phenomena.
• Other gases such as CO, H2S and NO2 are also
present in air.

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Introduction
Kinetics and theory of gases
• At lower pressure the average distance
between molecules is large, and at high
pressure the molecules are brought near to
one another.
• Similarly, with fall of temperature average
distance between molecules diminishes and,
the molecules come closer together.
Introduction
Critical temperature
• When by decreasing the temperature, the molecules of a
gas are brought close together, the gas assumes the liquid
form provided the repulsive tendency has been diminished
beyond a certain point known as critical temperature
which is different for different gases.
• Above the critical temperature any gas cannot be liquefied by
compression. The critical temperature is the temperature
below which a gas can be liquefied by mere application of
pressure.
• Below the critical temperature, the gas is termed as vapor
(gas co-exists in equilibrium with its corresponding liquid) and
above the critical temperature it is called a gas.

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Introduction
Critical pressure
• The minimum pressure under which gas
liquefies at the critical temperature is called as
critical pressure.
• Above critical temperature the gas will never
liquefy under any pressure.
• Therefore air should be cooled at very high
pressure and low temperature for cooling
purpose.
Introduction
The critical temperature and critical pressure of
some gases are as follows
Gases Critical temperature(oC) Critical pressure(atm.)
Ethylene 9.5 50.65
Methane -82.85 45.6
Nitrogen -147.13 33.49
Hydrogen -239.9 12.8
Oxygen -118.75 49.7
Acetylene 35.5 61.55
Ammonia 132.5 112.3
Carbon monoxide -138.7 34.6
Carbon dioxide 31.3 72.9

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Liquefaction
• The substances which are gaseous at ordinary
temperatures can be converted into liquid
state if sufficiently cooled and simultaneously
subjected to a high pressure.
• So, the liquefaction of gases is linked with the
production of low temperatures.
• There are various methods of liquefaction of
gases.
Liquefaction of air by Joule - Thomson
effect
• If a gas is allowed to expand through a fine
nozzle or a porous plug, so that it issues from
a region at a higher pressure to a region at a
lower pressure there will be a fall in
temperature of the gas provided the initial
temperature of the gas should be sufficiently
low.

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Liquefaction of air by Joule - Thomson
effect
Liquefaction of air by Joule - Thomson
effect

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Liquefaction of air by Joule - Thomson
effect
• CO2 free air is compressed to 200atm and is
cooled by water.
• The condensed water is removed by passing
through activated alumina.
• Then air is passed through inner coil of heat
exchangers.
• The valve with nozzle is provided at the end of
the inner coil.
• Then gas is allowed to suddenly expand by
opening the valve, which result in decrease of
temperature of air.
Liquefaction of air by Joule - Thomson
effect
• After expanding the cold air goes out through the outer
coil, is then recompressed to 200atm pressure, cooled
by water and then again allowed to transverse the
inner coil.
• The temperature of the incoming air further falls due
to the presence of cold air in the outer coil.
• Now as the cooled air suddenly expands through the
nozzle, the air suffers cooling, the temperature
becomes lower than in the first operation.
• The colder air now passes through the outer coil
producing an atmosphere of lower temperature.

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Liquefaction of air by Joule - Thomson
effect
• The cooled compressed air passes repeatedly through the
inner coil and subsequently undergoes Joule-Thomson
effect, the temperature of the air further drops.
• In this way progressive cooling takes place until the
temperature of air falls below the critical temperature of
oxygen and nitrogen.
• When this happens air undergoes liquefaction in the inner
coil, so on opening the valve liquid air falls in the container.
• A part of liquid air evaporates, through the outer coil,
maintaining the low temperature below the critical
temperature.
Manufacture
• Linde’s Process
– The first rectification of N2 and O2 using Joule Thomson
effect was carried out by Linde in 1906.
– After six year Claude rectified them by combined effect of
external work and internal work in cooling the air to
liquefaction point.
• Raw materials
– Basis: 1000kg Oxygen (95%)
– Air = 3600Nm3
– Steam = 1750kg
– Cooling water = 5000kg
– Electricity = 450-480kWH

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Manufacture - Linde’s Process
Manufacture - Linde’s Process
• The distillation tower is specially designed bubble cap tray double
columns arranged one above another.
• The two distillation columns are having intermediate distillation dome for
effectiveseparation of liquid enriched with O2.
• The column feed is liquefied air at 200atm pressure introduced at the
bottom of the column. Since the boiling point of O2 (-183oC) and N2 (-
195oC) are very low, column does not require any external heating.
• Distillationtake place only due to release of vacuum.
• Thus a number of recycling from lower column to upper column and lower
columnto dome is required. The construction of dome includes number of
internal pipes so that distillate of the lower column collides to the roof
and is returned back to the column as reflux.
• The compressed air which arrives from the first section of the plant which
acts as the heating fluid in the heater at the base of the enrichment
column.

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Manufacture - Linde’s Process
• The same air, always containedwithin a tube, passes out from the Iower column of
the tower only to reenter it higher up after the pressure to which it is subjected is
reduced by means of a valve, resulting in the lowering of its temperature.
• Nitrogenwith a small oxygen impurity collects at the top of the enrichment
column,and after expansion to atmospheric pressure; this nitrogen is sent to back
as the refluxin the rectificationcolumn situatedabove.
• The liquid which collectsin the heater at the base of the enrichment column is fed,
after expansion to atmospheric pressure onto a suitableplate of the rectification
column.
• Only after number of recycling,liquid with 82% concentrationof O2 is taken in
outer part of dome.
• This liquid goes to further rectificationin upper column where it is refluxed with N2
rich liquid coming from lower column.
• The final separation in the upper column takes place which has less number of
trays. Gaseous N2 is the top product of the column and the bottom product is
liquid O2.
Linde’s Process
• Disadvantage of Linde’s air liquefaction
method
– It results in a lower yield of around 10%.
– The working pressure is very high resulting in high
power consumption and requiring robust
equipment.

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Manufacture - Claude Process
• In Claude process, progressive cooling of compressed air is done by
external work (isentropic and isenthalpic expansion) and Joule -
Thomson effect. 70% of air is cooled by external work and 30% by
Joule - Thomson effect.
• Two variants of Claude process
– Molecularsieve variant
Cooling of air is brought about primarily by expansion with the
performanceof work. Therefore, there is no need to equip the plant
with cycles that use refrigerants or make use of very high pressures
which are employed when free expansion is used, in order to produce
cooling.
– Evaporationto diffusion
Cooling of the air can be adopted such as causing liquids which can
evaporate to diffuse air. It is then safe to reabsorb them when
necessary.
Linde’s Process

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Claude’s Process
Manufacture
a) Water wash cooler; b) Reversing heat exchanger; c) Expansion turbine; d) Double
column rectifier; e) Condenser; f) Subcooler; g) Adsorber; h) Compressor; i) Filter

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Manufacture - Claude Process
– Kinetics and thermodynamics
Liquefied air is subjected to rectification to separate the oxygen
and nitrogen components present in it. In liquid state both are
miscible in all ratios and they do not form azeotropic mixtures;
so they could not separate by boiling the solution.
• Rectification is carried out in a 'plate column', inside which
repeated condensations and evaporations take place on
plates, which lead to a continuous change in the
composition of the binary system throughout the length of
the column.
• This is continued until one of its pure components exists at
top of the column and the other at the bottom of the
column.
Manufacture - Claude Process
Operation of a rectification column
• Pn is the reference plate having
temperatureTn and liquid
compositionL, vapour
compositionV.
• As pressure is released the more
volatilecomponent i. e. N2 is
evaporated out partly and goes to
the upper plate Pn+1.
• The composition of liquid L2 is
having less concentrationof N2 at
temperatureTn-1.
• Similarlyliquid below the
reference plate is Pn-1 has higher
concentrationof O2 and vapour
V1 having higher composition of
O2 at temperature Tn+1.
The liquids which fall down from the plates
toward the heater in the base of the column
(L2, L, L1) become progressivelyricher in the
less volatile component (oxygen)
The vapours which rise toward the top of the
column (V1, V, V2) gradually become enriched
in the more volatile component (nitrogen)

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Manufacture - Claude Process
• For, perfect operation of a rectification column
always requires that:
– The liquid should always be introduced onto a
plate which supports a liquid of the same
composition as that of the feedstock liquid
– Part of the distillate from the top of the column is
recycled in the form of a 'reflux' with the aim of
repeated washing on all of the plates which refine
the vapors moving towards the top of the column
Manufacture - Claude Process
• Two-section fractionating tower
– Economically acceptable solution
– The upper column (rectification column) is about twice the
height of the lower column (enrichment), and both of
them are fitted with plates spaced at specific intervals
– Lower column has a boiler. no condenser at the top
– Lower column operate at 6 atm
– Upper column operates only slightly above atmospheric
pressure
– The heat exchanger between the two columns acts as a
condenser with respect to the lower column and a boiler
with respect to the upper column precisely as a result of
the two different pressures in the two compartments.

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Adsorptive Separation
• Nitrogen production by pressure-swing
adsorption (PSA) employs carbon molecular
sieves.
• The feed air is compressed to 5 – 12 bar, passed
through a water-cooled heat exchanger and a
condensate separator, and then fed to the
adsorbers , which alternate between adsorption
and regeneration.
• Water, carbon dioxide and oxygen are removed
by adsorption on carbon molecular sieves.
Adsorptive Separation
• The unadsorbed nitrogen leaves the adsorber during
the adsorption phase, goes to a buffer vessel at 4 – 11
bar (i.e., ca. 1 bar below the selected inlet pressure)
and is delivered from the vessel.
• At the same time, the second adsorber is being
regenerated by pressure reduction.
• In principle, the pressure level during the adsorption
and regeneration phase can also be lowered; the
compressor is replaced by a blower, and a vacuum
pump is used to reduce the pressure for regeneration.

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Adsorptive Separation
Schematic of a PSA plant for the production of
nitrogen from air on carbon molecular sieve
a) Compressor; b) Heat exchanger; c) Condensate
separator; d1), d2) Adsorbers; e) Buffer vessel
Adsorptive Separation
• Compressed air enters the
adsorption bed at high pressure.
• Typical adsorbents are usually based
on zeolites.
• Nitrogen is adsorbed to an
appreciably greater extent and
oxygen-enriched air is obtained at
the outlet.
• The bed is then blown down to a
lower pressure to desorb nitrogen.
• A purge step may sometimes be
used to sweep nitrogen out of the
bed before a new cycle begins

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Membrane separation
• Nitrogen containing less
than 10 ppm by volume of
oxygen can be produced
economically by using a
membrane/Deoxo hybrid
system; oxygen is removed
by reducing it to water
over a catalyst with
hydrogen.
• The gas leaving the catalyst
bed is cooled, dried, and
delivered as product.
bulk liquid / PSA
membrane + deoxo /
liquid assist cryogenic