A distillation column is a critical piece of equipment in the refining of crude oil. It acts very much like a still, separating product into its different chemical components based on differences in volatility. Oil refineries use two main types of distillation columns. They are:
Packed Type Columns – A packed distillation column is a vertical tower packed in sections with ceramic raschig rings, ceramic saddles or steel pall rings. The packing material is used to increase the surface area for mass transfer between gas and liquid phases during the distillation process.
The process stream is heated before entering the column, which causes partial vaporization. As it moves up the tower, the vapor cools. Light components will continue to rise and heavier components will condense and fall to the bottom as liquid.
A reboiler at the bottom of the column adds heat to create vapor, which strips light components out of the liquid. Packed towers are typically onlyused to produce a top and bottom product.
Tray Type Column – Tray type distillation columns operate on the same principle as packed columns; however, instead of using packed material they use trays situated at various heights within the tower. The trays have a weir to maintain a layer of liquid across the surface.Hot vapor bubbles through the liquid to strip out light components. As the liquid flows over the weir, it is directed to the tray below.
There are three main types of trays in use: sieve, valve and bubble cap trays. Bubble cap trays generally achieve the most efficient separation of product components. The bubble cap design distributes the vapor more extensively, forcing it to bubble through the liquid flowing across the tray.
In general, tray type columns are more efficient than packed type columns, but are also more expensive. They do, however, facilitate multi-stage separation of products via side streams in the tower, and are therefore much preferred for crude oil distillation.