Answer: only when the quantity and state of materials entering and exiting the distillation tower and each stock of materials completely accord with the material balance, component balance and energy balance of the whole distillation tower can the distillation working condition maintain stable operation. Generally, the gas, nitrogen and oxygen products derived from the distillation column are in the state of dry saturated vapour, so the state of air processing in the distillation column should also be the state of dry saturated vapour under its pressure. Generally, the lower tower pressure is 0.5 ~ 0.6MPa, its corresponding saturation temperature is about 100 ~ 101 K, that is, the state of processing air entering the tower is 100 ~ 101 k dry saturated steam.
However, due to the cold loss of the distillation column, and the air entering the upper tower after expansion is overheated air, in order. compensating the cooling capacity, which leads to the processing air entering the state of the lower tower not only reaching saturation, but also containing a small amount of liquid, that is, the state of the processing air entering the lower tower should be gas-liquid mixture. For towers that only produce gas products, the processed air in the tower contains 3% ~ 5% liquid; for the distillation column producing gas and liquid products at the same time, due to the increase of the cold loss of the tower and the addition of some liquid products with low enthalpy value leaving the tower, in order to maintain the cooling balance of the tower, the proportion of liquid contained in the processed air entering the tower is bound to increase.
In the small medium pressure oxygen generator, the liquid air in the processed air is provided by partial air liquefaction after high pressure air throttling; in the whole low pressure switching process, the liquid part in the processed air of the lower tower is produced in the liquid purifier; in the purification process of the whole low-pressure molecular sieve, a small amount of liquid space in the processed air of the lower tower, after the positive flow air in the cold section of the main heat exchanger is cooled, part of it is generated by liquefaction.
If the air entering the tower is not a gas-liquid mixture, but saturated or overheated, the cooling volume inside the tower will lose balance, and the liquid in the tower plate in the distillation tower will evaporate excessively, the temperature of the tower plate will rise, and the liquid will decrease, even if the liquid disappears, the rectification can not be carried out normally, and the air can not be separated.
