Question

Reactor Systems and Separation Systems

1. Please explain how to break a flood on a distillation column.
2. Explain how the temperature is controlled on the
   - top
   - middle
   - bottom part of the tower,
   in a distillation column

Answer 1

1] The distillation process is one of the most common means of separating two or more components from a fluid. or we can say distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction.

Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

2] The temperature decreases as the material move higher in the tower. The gradual decrease in temperature from the bottom of the distillation tower to the top is called the temperature gradient.

For maintaining the proper temperature gradient, temperature at the critical point in the system must be controlled. Temperatures in the distillation system are typically controlled in 3 ways or there are 3 types.

Pre-heater
One way is to control the temperature of the mixture by using a pre-heater, this regulate the temperature at the feed point.

Pump around
Some distillation system contains equipment known as pump around. The purpose of pump around is to remove hot liquid from the tower and pumps it to a cooler.

Pressure
Since pressure effect the boiling temperature of the liquid, it is an important factor in the distillation process. If pressures in the distillation are not correct, product purity could decrease.