The whole low pressure oxygen generator has certain rectification potential because of the distillation section of the upper tower, so the air after expansion can directly enter the upper tower to participate in the rectification within a certain limit. The state of air after expansion and the amount of introduction will have a direct impact on the rectification working condition. Considering from the benefit of rectification, since the air and liquid on the tower plate are in the saturated state, it is better to reach the saturated state when the inflated air is blown into the upper tower. Considering the cold damage of the compensation Tower, it is better to use gas-liquid mixture containing a small amount of liquid in the state of inflated air entering the tower.
However, due to the limitation of the structure of the expansion machine, the air cannot reach saturation at the outlet of the expansion machine, let alone produce liquid. Otherwise, the expansion machine will have a liquid shock accident, causing damage to the expansion machine. Due to this reason, the air temperature after expansion must be higher than the saturation temperature under its pressure, that is, there is a certain degree of superheat.
When the inlet and outlet pressure of the expansion machine is fixed, the air temperature after expansion is determined by the temperature before expansion. The air temperature before expansion is high, the enthalpy value is high, the function during expansion is strong, the unit refrigerating capacity is large, and the temperature drop is also large, this is the so-called "high temperature and high enthalpy drop" of expansion working medium ".
According to experience, when the inlet air temperature of the expansion machine is about-140℃, the expansion temperature of the expansion machine drops only about 30℃; when the inlet temperature is-100℃, after expansion, the temperature drop can reach above 50℃. Under the condition of obtaining the same refrigerating capacity, the unit refrigerating capacity of the expansion working medium is large, and the required expansion capacity is small. At present, when designing the oxygen generator, the inlet temperature of the expansion machine is generally-90 ~ -100℃, the temperature after expansion is-140 ~ -150℃. The pressure after expansion is generally 0.13 ~ 0.135MPa. Its corresponding saturation temperature is-188 ~ -189℃, The superheat is about 40℃. Although the expansion volume is small, the superheat is increased. The amount and superheat of the expanded air in the upper tower both have great influence on the rectification. The liquid on the tower plate at the entrance of the air expansion will evaporate in large quantities, the temperature of the tower plate will rise, the reflux ratio of the distillation section will decrease, and the rectification capacity will decrease. If the blowing volume is too large, overheating and overheating will even destroy the rectification working condition, the purity of the product will decrease and the extraction rate will decrease.
In recent years, in order to reduce the influence of air expansion blowing into the tower, when designing the distillation tower to upper the tower, the number of tower plates from the liquid-air inlet to the section of Air expansion blowing into the tower is increased. In the past, this section was only 2 ~ 3 boards, and now it has increased to 5 ~ 6 Tower plates.
In addition, in the design, increase the temperature before the expansion machine to achieve the purpose of reducing expansion amount. At the same time, in the process design, the heat exchanger after expansion is adopted, and the cooling of sewage nitrogen is used to cool the air after expansion, so as to reduce the superheat of the air after expansion and reduce the superheat of the air after expansion to 10 ~ Blow into the tower after 20℃. In the new air separation process, the adoption of pressurized expansion can also reduce the expansion amount and superheat of the air.
