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.
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.