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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.