A distillation column is a crucial component used in separating liquid mixtures into their individual parts or fractions based on volatility differences. Fractionating columns are employed in both small-scale laboratory distillations and large-scale industrial distillations.
Types of Distillation Columns
There are various types of distillation columns, each designed for specific separation purposes and differing in complexity.
In batch operation, the column is charged with a ‘batch’ of feed, and then the distillation process is conducted. Once the desired objective is achieved, a new batch of feed is introduced.
In contrast, continuous columns process a continuous feed stream. There are no interruptions unless there is an issue with the column or surrounding process units. Continuous columns are capable of handling high throughputs and are more commonly used. We will focus only on this type of column.
Main Components of a Distillation Column
A distillation column consists of several components, each serving to transfer thermal energy or enhance material transfer. A typical distillation column includes:
- Vertical housing for the separation of liquid components
- Column internals, such as trays or packing, for enhanced component separation
- A reboiler that provides the necessary vaporization for the distillation process
- A condenser for cooling and condensing the vapor exiting the top of the column
- A reflux tank to hold condensed vapor from the top of the column and circulate the liquid (reflux) back to the column
Basic Operation and Terminology
The liquid mixture to be processed is known as the feed, which is introduced near the middle of the column on a tray called the feed tray. The feed tray divides the column into an enriching (or rectification) section at the top and a stripping section at the bottom. The feed flows down the column and collects at the bottom in the reboiler.
Heat is supplied to the reboiler to generate vapor. The heat source can be any suitable fluid, although steam is commonly used in most chemical plants. In refineries, the heating source may come from the output streams of other columns. The vapor generated in the reboiler is reintroduced at the bottom of the column. The liquid extracted from the reboiler is known as the bottoms product or simply bottoms.
The vapor moves up the column and, upon exiting the top, it is cooled by a condenser. The condensed liquid is stored in a holding vessel called the reflux drum. Some of this liquid is recycled back to the top of the column, and it is referred to as reflux. The condensed liquid removed from the system is known as the distillate or top product.
As a result, there are internal flows of vapor and liquid within the column, as well as external flows of feeds and product streams into and out of the column.