The liquid air and liquid oxygen from the lower column serve as feedstock for the upper column distillation. Therefore, lower column distillation forms the foundation of upper column distillation. The purpose of properly controlling liquid air and liquid oxygen purity is to ensure the purity and yield of oxygen and nitrogen products.
When liquid air purity is high, oxygen purity can be improved. When liquid oxygen purity is high and output is substantial, nitrogen purity can reach ideal levels. However, liquid air and liquid oxygen purity are mutually constrained-when one increases, the other necessarily decreases. Additionally, the purity and output flow rate of each are also interdependent; improving purity inevitably reduces flow rate.
Given these various constraints, there exists an optimal balance point for liquid air and liquid oxygen purity and withdrawal rates, which must be sought through steady, careful, and meticulous adjustment.
Key Operating Point for the Lower Column
The key operating point for the lower column lies in controlling the opening of the liquid oxygen throttle valve. Specifically, while ensuring liquid oxygen purity meets upper column distillation requirements, the withdrawal rate should be maximized as much as possible. This provides more reflux liquid to the upper column's rectifying section. An increased reflux ratio ensures nitrogen purity. Meanwhile, the lower column's reflux ratio will consequently decrease, improving liquid air purity and thereby enhancing oxygen purity.
How to Determine the Proper Valve Opening
The appropriate degree of opening for the liquid oxygen throttle valve can be determined by the difference between liquid oxygen purity and gaseous nitrogen purity. Under normal conditions, sprayed liquid oxygen purity should equal or be slightly lower than the gaseous nitrogen purity leaving the upper column. Liquid oxygen purity is allowed to be 0.51% to 2% lower than gaseous nitrogen purity. The lower the purity, the larger this difference; the higher the purity, the smaller the difference. When gaseous nitrogen purity exceeds 99.9%, liquid oxygen purity should match gaseous nitrogen purity.
Abnormal Condition: High Liquid Oxygen Purity but Lower Gaseous Nitrogen Purity
If an abnormal situation occurs where liquid oxygen purity is very high yet gaseous nitrogen purity is even lower, this indicates insufficient liquid oxygen flow into the upper column. This causes insufficient liquid at the top trays of the upper column, inadequate reflux ratio in the rectifying section, and inability to improve nitrogen purity. Simultaneously, the lower column will experience increased reflux ratio due to the small liquid oxygen throttle valve opening, resulting in decreased liquid air purity and consequently reduced oxygen purity. In this case, the liquid oxygen throttle valve opening must be increased. Although liquid oxygen purity will decrease somewhat, gaseous nitrogen purity will actually improve.
Systems with Both Contaminated and Pure Liquid Oxygen Throttle Valves
In processes equipped with both contaminated liquid oxygen and pure liquid oxygen throttle valves, operation typically involves:
Using the contaminated liquid oxygen throttle valve to control liquid air purity
Using the pure liquid oxygen throttle valve to control liquid oxygen purity
