A form of distillation whereby the process continues without interruption. The process produces at least 2 output fractions; one volatile and the other a bottom/residuum fraction.
When heated the vapour above the liquid varies in composition from the liquid composition. As the mixture is heated the compounds with lower boiling points rise to the top of the column and are cooled and collected in trays.
It follows that the most volatile fluid collect at the top, the least settle at the bottom.
2 main theories govern the design and operation of a distillation tower;
– McCabe Thiele
– The Fenske Equation
The feed is sent to the bottom of the column. The original feedstock is usually pre-heated by means of a pre-heated or furnace. This furnace may be steam heated or natural gas fired or electric.
From the reboiler it goes to the column bottom. From here it is further reheated, within the column. The lighter fractions rise higher and are taken off by trays.
From the trays the distillate goes to a condenser. The condenser produces both reflux and distillate product.
The reflux is fed back to the tower, this helps to cool the rising vapour, and this improves the process efficiency. The more reflux is added the better the efficiency of the process.
The distillates gather at points of different temperatures and pressure. The trays are located at those exact points. They have specific liquid to vapour contact points. The various distillates often have different concentrations, at these points hence they pool at the trays.
Instead of using trays continuous amounts of metal packing can be used.
In the distillation of crude oil however because of its nature a distillate may not be pure. It may be a fluid with multiple boiling points, or have a boiling range. The subsequent distillates may require further chemical treatment before it can be used.
