Fractional Distillation Tower

Distillation tower schematic

One of the key elements of many process plants is a distillation system. It’s important for operator to understand how distillation system works because its parts of their job to make sure the system is run properly.

The purpose of any distillation system is to separate a liquid mixture into two or more components. The separation takes place in distillation columns or tower. The liquid mixture is called feed or charge and the component into which separated are called cuts or fractions.

In the schematic above the feed is stored in a tank, during operation a pump is used to move the feed from tank through a pre-heater. In the pre-heater the mixture is heated under pressure to just below its boiling point. The pressure in the tower is lower than the pressure in the pre-heater so when the feed enters the tower it starts to boil. The vapors from boiling liquid which consist primarily lighter components in the feed rises in the tower. The remaining liquid which consists primarily of the heavier components in the feed moves down the tower and collected at the bottom.

Some of this liquid is drawn-off as the bottom products and some of it is routed to a device called re-boiler which is connected to the bottom of tower.

The re-boiler is usually a heat exchanger designed to vaporize the lighter component that remain in the liquid at the bottom of the tower. Vapors from re-boiler or in some chases a mixture of vapor and liquid re-enter the tower, the vapors then rise up in the tower. These vapors and the heat they contain are often referred to as boil-up. The hot boil-up provides heat that’s needed for the distillation process to take place in the tower.

The vapors that rise up on the tower are routed to a condenser. The purpose of the condenser is to cool and condenses the vapor into liquid. From the condenser the liquid flows-in to a receiver or accumulator. The receiver provides the reservoir for the liquid. Part of the liquid from the receiver is pumped back in to the top of the tower and part of it is drawn-off as tower overhead product.

At the heart of distillation system is the distillation tower. Basically a distillation tower is a series of distillation processes or stills stack-on top of each other. The process of distillation is some time referred to as fractional distillation or fractionation and distillation towers are sometimes called fractionators. Fractionation is a distillation that occurs at different level of the tower.

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