Reflux in Distillation

If the distillation tower works perfectly, it would produce a pure product but unfortunately the lighter products usually contain some heavier fraction while the heavier products contain some lighter fraction, this chase is sometimes referred to as overlap.


Distillation system uses several methods to maximize the purity of the products and one of these methods is called refluxing. The vapor is coming of the top of the tower are condensed in a condenser and then collected in the receiver. Part of the liquid from the receiver is sent to storage or other unit in the plant as tower overhead product. The rest of the liquid is pumped back in the top of the tower. The liquid that is re-introduced into the tower is called external reflux. Because it consists of liquid that was cooled in the condenser, the external reflux is cooler than the liquid in the top of the tower. As the external reflux cools the top of the tower, vapors made of heavier fraction condense and liquid made of heavier faction flows down the tower and it’s referred to as internal reflux. Meanwhile the top of the tower is still hot enough to keep the lighter fraction in vapor form. The vapor is drawn off the top of the tower into the condenser.

Refluxing increases the purity of the overhead products because condensing the vapor made heavier fraction keep them out off the stream of vapor that leaves the top of the tower.

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