What is the effect of the pressure in the pre-distillation tower on the distillation process?

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The overhead product of the pre-distillation tower is usually a light gasoline fraction or a reforming feedstock (Tower II in the two-tower process). When water is used as the cooling medium and the product is cooled to around 40°C, with the reflux tank operating at a pressure of 0.11–0.3 MPa, almost all of the oil is condensed. Therefore, distillation is generally carried out under pressure slightly higher than atmospheric pressure, which is how the name \"atmospheric pre-distillation column\" came about. When the content of non-condensable gases in the feedstock is high, increasing the pressure can reduce the amount of light gasoline lost during the emission of inert gases. Appropriately increasing the operating pressure of the tower can enhance its processing capacity; when the operating pressure is raised from 0.11 MPa to 0.3 MPa, the production capacity of the tower can increase by 70%. An increase in the tower’s pressure also raises the operating temperature of the entire tower, which facilitates heat exchange between the side products and the feedstock. The disadvantage is that as the pressure increases, the relative volatility decreases, making separation difficult. To achieve the same separation precision, it is necessary to increase the reflux ratio at the top of the tower, which in turn increases the load on the condenser at the tower top. Furthermore, since the feed temperature cannot be increased arbitrarily, the vaporization rate of the feed will decrease as pressure rises.