Continuous distillation with reflux (Rectification)

It is the process of separation of components of comparable volatility. Production of vapor by boiling liquid mixture and condensing the vapor and allowing part of condensed liquid to still is the working principle of rectification.

In every distillation process, vapor formed in kettle is richer in more volatile component than in remaining liquid. However, the vapor contains certain amount of both components and distillate is rarely a pure substance. In order to increase concentration of volatile component in vapor and to separate the less volatile one, vapor is brought into contact with the stream of boiling liquid that runs down the column.

A stream of liquid with high concentration of volatile component is introduced at the top of the column. The most volatile components are concentrated in vapor phase and least volatile in liquid phase. This way vapor becomes richer in volatile component as it goes up the column, while liquid phase becomes richer in heavier component as it descends the column and separation is achieved.

Separation of mixtures depends upon relative volatilities of components, number of contact stages and reflux ratio. Rectification unit consist of

  1. Steam or reboiler in which vapor is generated
  2. Rectifying or fractionating column, which is divided into large number of perforated trays that permit upward flow of vapor.
  3. Condenser, condense all vapor, leaving the top of column, sending a part of condensed liquid (reflux) back to column to descend counter to raising vapor and delivery the part of condensed liquid as product.

Typical diagram for rectifying column is shown as below.

In the above figure, feed is introduced into the column ‘A’ at the point that is nearly its center. The plate on which feed enters is called feed plate. All plates above the feed constitute rectifying section. The plates below the feed, including the plate itself constitute stripping section.

The feed flows down the stripping section to the bottom of the column in which definite level of liquid is maintained. Liquid flows by gravity to reboiler ‘B’. Reboiler generates steam (vapor) and returns it to the bottom of the column, which then passes up to entire column.

Fig: Schematic diagram of reboiler

At one end of reboiler is a weir (dam) and downstream side of this weir collects bottom product which is withdrawn and passed through cooler ‘G’. This cooler also preheats the feed by heat exchange with the hot bottoms.  The vapors rising through rectifying section are completely condensed in condenser ‘C’ and the condensate is collected in accumulator ‘D’.

Reflux pump ‘F’ takes liquid from accumulator and delivers it to the top plate of tower. This liquid stream is called ‘external reflux’.

Condensate not picked up by reflux pump is cooled in heat exchanger ‘E’ called product cooler and withdrawn as overhead product.



About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.