Five Step Pocess 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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