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Process
Description for the Manufacture of Oxygen and Nitrogen
from Atmospheric Air
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Five Step Process For The
Manufacture Of Oxygen And Nitrogen
1. AIR COMPRESSION
Air is drawn from atmosphere through Suction Air Filter. (1).Air
is drawn from atmosphere through Suction Air Filter to prevent
dust from getting into the system
The air is then compressed in a three stage Air Compressor
(2) with after cooler to a maximum pressure of 55 Kgs./Cm2
for plant starting conditions and a pressure of 40 - 45 Kgs./Cm2
for best results normal running conditions. Depending on ambient
conditions and good operations the operating pressure of the
Air Separations Unit is brought down to 40 Kg/Cm2 as per past
experience. Air Compressor has inter-coolers between stages
and an After-Cooler after 3rd stage. For further details on
Air Compressor, please refer to Air Compressor Manual supplied
by the Air Compressor manufacturer. The air compressor should
be maintained properly in good condition as it is the main
source of air supply to the plant.
2. AIR -PURIFICATION
The air then enters into cascade an Evaporating Cooler (5)
where it gets cooled to about 20 Deg.C. This unit is optional.
The cooler is a cubical vessel where, there is pipe coil and
is inter connected. The coils are half submerged in water
in the vessel Dry Nitrogen will be bubbled through this water
to become wet gas. As the water vaporizes, it requires latent
heat which is absorbed from water itself. So, water gets cooled.
Thus, air inside the pipe coil will get cooled. Compressed
air, cooled in evaporation cooler will enter into a Moisture
Separator (4 & 8).
Moisture condensed as water will be separated and drained
once in an hour. It is important to drain moisture from the
bottom of the Oil Absorber (9) at regular intervals and also
change the Alumina every 6 to 12 months. After this the air
will pass through an additional cooler called Chilling Unit
(7).
After this the air will pass through Oil Adsorber. (9) Packed
with Alumina balls. Here the Oil Vapor carried over from Air
Compressor will be removed. If this oil vapor is not removed
sufficiently, due to spent carbon or due to high temperature
of process air, the oil vapor will damage the Molecular Sieves.
To obtain a long life of Molecular Sieve ensure the Alumina
is well maintained.
The air then enters one of the Molecular Sieve vessels (11).
The moisture and carbon dioxide in the air will be removed
in this drier. If they are not removed before entry to Cold
Box, they will form Ice and dry Ice which will choke the Heat
Exchanger Tubes and other equipments. There are two driers.
One will be (on line with the process air) in operation for
around 10 hours and the other will be under regeneration.
Regeneration is done by heating and cooling with not-going
Nitrogen. An electric regeneration gas heater (12) is used
for regeneration. For further details, refer separate chapter
on Molecular Sieve Driers.
The dry air is again filtered in a Dust Filter (13) before
entry to Cold Box to avoid any dust entry to Cold Box. In
some plants the air is further cooled through special coils
provided in the Chilling Unit Tank (6), which is called an
equalizing coil as it equalizes the temperature after the
Molecular Sieve drier before Air enters the Cold Box.
3. COOLING OF AIR
The compressed air, cooled to about 15 to 20 Deg.C free of
moisture and carbon dioxide will enter the Cold Box (15).
It initially passes through a Heat Exchanger No.1 (16); the
incoming air will be cooled by the outgoing Oxygen and Nitrogen.
The air will be cooled to around -100 Deg.C. In this Heat
Exchanger. This can be single or divided two parts in series.
The air will then be into two streams. The main air stream
will enter Expansion Engine (14) at 40 - 45 Kgs./Cm2 and will
be expanded to 5 Kgs./Cm2 and -150 to 160 Deg.C the rest of
the air will pass through Heat Exchanger No. 2 (17) to be
cooled to about -160 Deg.C. by the outgoing Oxygen and Nitrogen.
This air will then be expanded by an Expansion Valve V3 to
form liquid air. Both the air streams will now enter bottom
portion of the Lower Column (19). Operating pressure of the
column is around 45 kg/cm2 under normal operating conditions.
As the air enters the Lower Column, after the Expansion Engine
and after Expansion Engine valve V3, a part of this air condenses
into liquid and falls at the bottom of the column. This liquid
is about 40% Oxygen and 60% Nitrogen and is usually called
the "Rich Liquid" and as Nitrogen is more volatile
it rises to top of the lower column where it gets cold from
the condenser and become liquefied. This liquid nearly free
of oxygen collected in the (Pockets in the condenser) trap.
As this liquid poor in oxygen is called poor liquid.
4. AIR SEPARATION
Final separation of the two fractions is achieved in the upper
column. Both the poor liquid are carried into the upper column
by two Expansion Valves and the pressure drops from 4.5/5.0
Kgs. /Cm2 in the lower column to 0.5 Kgs. /Cm2 in the upper
column. The rich liquid enters the middle of the Upper column
and as it flows down, Nitrogen evaporates and Oxygen continues
as liquid. The Liquid Nitrogen (Poor Liquid) enters the top
of the column and as it is flows down the column, it comes
in contact with any evaporating Oxygen and condenses the same
into liquid, while the Nitrogen itself becomes a Gas as it
is more volatile. This process takes place in each Gas as
it is more volatile. This process takes place in each tray.
The entire gaseous Nitrogen is piped out from the top of the
column through Heat Exchangers. Similarly the Liquid Oxygen
at the bottom of the column is carried away to a Liquid Oxygen
Pump from which it is compressed and again passed through
the Heat Exchangers into the Gas Cylinders. As the Liquid
Oxygen travels through the Heat Exchangers, it evaporates
into gaseous oxygen filling the cylinder with gas and giving
up its cold to the incoming air
Generally the purity of Oxygen will be 99.5% and Nitrogen
about 96%, when the plant is operated exclusively for oxygen
production.
The Plant operation should be such that it is not too cold
or too warm. If the cold box is too cold, the Nitrogen will
condense into Liquid Oxygen and the Oxygen Purity will fall.
If the plant is too warm oxygen will evaporate with the Nitrogen
and the quantity of Oxygen produced will go down substantially
and the waste nitrogen will carry more and more oxygen. To
obtain optimum result of the plant, therefore check the purity
of the waste Nitrogen which should not fall below 96%.
When the plant works continuously for a few months, it tends
to accumulate Carbon Dioxide and moisture in its internal
parts. These are to be removed once in about four months.
For details, refer chapter on Defrosting of Plant.
Similarly, the L.O. Pump alone can be defrosted in case of
trouble in pumping (Refer L.O. Pump chapter).
It is advised to give Carbon Tetra Chloride wash to the Cold
Box equipments once in a year to ensure protection against
Hydro Carbon contamination. But when starting during commissioning
CTC wash is a must.
Before starting plant, it is generally defrosted and blown
out. That the cooling/starting is done which will take about
7 to 8 hours. When the plant is stopped for short intervals,
the plant need not be defrosted, but all the cold line valves
are to be closed to prevent outside moisture from entering
the Cold Box.
5. FILLING OF LIQUID /GAS OXYGEN
THE FINAL STEP IS THE FILLING/ COMPRESSION OF LIQUID OXYGEN
PUMP The liquid oxygen is withdrawn from the condenser and
is flows to the liquid oxygen pump where it is compressed
automatically into compressed oxygen. For filling oxygen cylinders
the oxygen will go to the oxygen filling manifold for filling.
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