The liquid in the lower tower mainly comes from the liquid flowing from the first tower plate at the bottom. The liquid flowing down from the top Tower plate contacts with the air entering the tower on the first tower plate, part of the liquid evaporates, and the oxygen concentration in the liquid increases before flowing into the liquid kettle. However, its oxygen concentration is limited by the oxygen concentration (20.9% O2) of the air entering the tower, which is always lower than its equilibrium concentration. For example, when the pressure of the tower is 0.55 MPA, the oxygen concentration in the liquid which is balanced with the vapor containing 20.9% oxygen is 40.8%, while the actual oxygen concentration in the air should be lower than this value.
Equilibrium concentration is related to pressure. With the increase of pressure, the equilibrium concentration decreases, and the oxygen concentration in liquid air also decreases. For example, when the pressure is 0.6MPa, the equilibrium concentration in the liquid is 40.1% O2.
In addition, according to the energy balance of the tower, the air entering the tower should be wet. For the oxygen generator in the medium pressure process, some liquid should be contained after the mixture of expansion air and throttling air; for the oxygen generator in the low pressure process, the cold end air of the switching heat exchanger is slightly overheated, then a part of air must be liquefied in the liqufier to make it wet in the comprehensive state. The greater the cooling loss of the unit is, or the production of liquid products requires the higher moisture content of the air entering the tower. However, the oxygen concentration in this part of liquid entering the tower is relatively low, therefore, the size of moisture content will affect the oxygen concentration in the liquid space. The higher the cold loss is, the lower the oxygen concentration in liquid. For example, for the equipment which produces gas and oxygen, the oxygen concentration in liquid is 38% O2 ~ 40% O2; for the equipment producing liquid oxygen, the oxygen concentration in the liquid space is 35% O2 ~ 37% O2.
In the actual operation, the oxygen purity of the liquid is also restricted by the material balance of the lower tower. If the liquid nitrogen regulating valve is turned off too small and the amount of liquid space increases, the purity of liquid oxygen will certainly decrease while the purity of liquid nitrogen will increase. Otherwise, the purity of oxygen in liquid will increase.
The purity of liquid-air oxygen has an influence on the rectification. Its influence degree can not be generally speaking, but should be analyzed specifically. For example, under the condition that the purity of liquid nitrogen remains unchanged and the return flow is certain, the higher the purity of liquid space is, the higher the purity of oxygen will be; the lower the purity of liquid space is, the more difficult it is to improve the purity of oxygen. Of course, it depends on whether the tower has potential or not, and the operator had better operate according to the specified index.
