The effect of backflow on column pressure.
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I heard today that as the backflow increases, the tower pressure first drops and then rises. What’s going on?
Backflow is equivalent to cooling capacity; as backflow increases, the temperature at the top of the tower decreases, and since temperature and pressure are related to each other, the tower pressure also decreases. The reflux is the light components at the top of the tower; after vaporization, it occupies a relatively large volume. That’s all I can say. Not professional
The material exiting the top of the tower contains a high proportion of light components; it is liquefied through a condenser, which reduces the volume of the gas. At the same time, by increasing the amount of reflux (cold material) that returns to the bottom of the tower, the temperature of the material in the bottom of the tower decreases (under unchanged heating conditions), the amount of evaporation decreases, and as a result the pressure inside the tower drops. However, by continuing to heat, the evaporation rate increases sharply (as the light components brought in by reflux raise the content of low-boiling substances at the bottom of the tower), resulting in an increase in pressure inside the tower. Therefore, after the backflow increases, the pressure inside the tower first decreases and then increases.
While maintaining a basically constant temperature at the top of the distillation column, increasing the reflux flow rate at the top appropriately will cause the pressure at the top to drop. However, by continuing to apply heat, the amount of vaporization increases sharply due to the light components brought in by the reflux, which raises the content of low-boiling substances in the bottom of the column, thereby causing the pressure inside the column to rise. Therefore, after the backflow increases, the pressure inside the tower first decreases and then increases