DISTILLATION COLUMNS (or TOWERS)

Distillation columns or towers are constructed to behave in the same way as a series of separate stills as discussed earlier. Each 'still' section consists of a number of 'TRAYS' or contacting devices arranged vertically above one another in the column. These trays or contactors bring liquid and vapour into intimate contact in order to obtain the required separation of the mixture. The height of the tower and the number of trays or contacting devices it contains, depends upon the purity of the 'Fractions' required.

Columns for the distillation process can be of the following types:

1. The 'PACKED' Tower
2. The 'TRAY' Tower

1. THE PACKED TOWER

As its name implies, the packed tower is a vertical, steel column which contains 'Beds' of packing material which are used to bring the rising vapours into intimate contact with falling liquid within the tower. The heat added to the mixture before entering the tower partially vaporises the mixture and the vapours rise up the tower and begin to cool.

The liquid falls towards the bottom of the tower. At the tower bottom, in general, more heat is added to the liquid by a 'Reboiler' which may be steam heated or a fuel fired furnace type.

The addition of heat here causes more vapours to rise up the column. As the two phases of the mixture - falling liquid and rising vapour - come together, light components are stripped out of the liquid and enter the gas phase while heavy components in the vapour are condensed into the liquid phase.

In this way, as the vapour rises and gradually cools, it becomes lighter and, as the liquid falls, it becomes hotter and heavier.

With this type of distillation column there is generally only a top and bottom product. The quality of the products depends upon the height of the tower, the number of contacting devices, the tower temperature and pressure and their control, and the velocity of the rising vapours.

The type of packing materials used, also plays a part in the separation process. The packing can be of such types as:

Ceramic Raschig Rings, Stainless Steel Pall Rings or Ceramic Saddles .. etc. See Figure: 10.

Figure: 10