The temperature on the tower plate depends on the air and liquid temperature on it, and the air and liquid temperature depends on its pressure and their oxygen (or nitrogen) concentration. Under certain pressure, high oxygen concentration leads to high temperature; low oxygen concentration (I .e. high nitrogen content) leads to low temperature. As is known to all, no matter in the upper or lower tower, the oxygen concentration of gas and liquid is higher in the lower part, and the higher the oxygen concentration is, the lower the oxygen concentration is. Therefore, their temperature is also higher in the lower part, and the higher the temperature is, the lower the temperature is. In addition, the greater the resistance is, the higher the pressure inside the tower is, the higher the corresponding saturation temperature of gas and liquid is; the lower the pressure is, the lower the corresponding saturation temperature is. Although the liquid at the bottom of the upper tower contains 99.6% oxygen, its saturation temperature is 93.2 K because its pressure is 0.14MPa. However, the gas on the top of the lower tower contains 0.01% oxygen (99.99% nitrogen) and the pressure is 0.6MPa, so its saturation temperature is 96.1 K. At this time, the temperature of nitrogen in the lower tower is higher than that in the upper tower.
It can be seen from the above analysis that when the pressure and purity of the distillation column change, the temperature on the tower plate will also change. For each tower plate, the liquid temperature flowing to the tower plate is lower than the vapor temperature. After gas-liquid contact, the heat transfer process occurs, the liquid partly evaporates, the vapour partly condenses, the oxygen concentration in the liquid increases, the nitrogen concentration in the vapour increases, and finally the temperature reaches nearly equal, then leave the tower plate respectively. Therefore, all the factors that change the pressure and purity, such as the butterfly valve of product oxygen, nitrogen and dirt nitrogen, and the opening change of the throttle valve of liquid air, liquid nitrogen and dirt nitrogen will affect the temperature of the tower plate. When the reflux ratio increases, the amount of liquid increases relatively. After the mixture of gas and liquid, the temperature rise of the liquid is relatively small, and the liquid part is not fully evaporated, which makes the tower temperature lower; the reflux ratio reduces by hours, on the contrary. And the opening degree of gas outlet valve will also affect the pressure of the tower.
