Fractionating Column | Rectification or Batch Distillation

In this separation method, a part of the condensed liquid is returned back as reflux and a maximum enrichment of the more volatile component in the vapour is obtained by successive partial vaporization and condensation by a multistage contact of the vapour and the liquid. This is achieved in a single unit called a fractionating column or distillation. 

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A fractionating column or fractionating consists of (I) a cylindrical shell divided into sections by a series of perforated trays (ii) a reboiler and (iii) a condenser. A liquid mixture to be separated is introduced in the cylindrical column more or less centrally. The column itself is divided into two sections - rectifying and stripping sections

Rectifying section

The section above the feed plate or tray is called the rectifying section, wherein the vapour is washed to remove the less volatile component with the liquid returned to the column from the top (known as reflux).

Stripping section

The portion below the feed plate including the feed plate is called the stripping section wherein liquid is stripped off a more volatile component by rising vapour. Perforated trays are nothing but gas-liquid contacting devices on which gas/vapour and liquid are brought into intimate contact for mass transfer to occur. 

Vapours are generated in a reboiler and are fed to the bottom of the column. The liquid removed from the fractionator is rich in less volatile components called the bottom product. The vapour issuing from the top of the column is fed to a condenser where the latent heat is removed with the help of a circulated coolant through the condenser. 

A part of the condensed liquid is returned to the column and the remaining part is withdrawn as the top product or distillate which is rich in the more volatile component. As we move down the column, the liquid becomes richer and richer in the less volatile maximum at the bottom and minimum at the top. The part of the condensed liquid returning to the top of the column is called reflux. 

Rectification or Batch Distillation

Batch distillation is used extensively in small-scale production units where the piece of equipment is to be used for many different mixtures and where small quantities of the liquid mixture are to be handled. It is useful when more than one product is to be obtained (different quality products) and when the liquid mixture to be separated is high in solid content tar, etc. as it keeps the solids separated (in reboiler/still) which are removed at the termination of a process.

Batch Distillation Working

In batch distillation, the specific quantity of a liquid mixture is charged to a reboiler/still, heating is applied, vapour generated flows upward through a fractionating column and a part of the liquid from a condenser runs down the column as reflux. The entire fractionating column acts as an enriching section. As the distillate will be rich in more volatile components, the liquid in the reboiler becomes steadily weaker in the more components as the operation proceeds and hence the purity of the product will steadily fail.

Batch distillation operation may be carried out by varying the reflux ratio to get a constant overhead composition. In this case, initially, the column is operated under total reflux and then some value of the reflux ratio is adjusted. But as the distillation proceeds, the top product quality may fail. Thus to keep the top product quality constant, the reflux ratio is increased. The reflux ratio is continually increased till it reaches a maximum value and then it is reduced and the cut is taken into a separate receiver. It may be charged in the next batch. 

Another method of operating a batch distillation unit is to operate a column under a constant reflux ratio. The column is operated under total reflux initially and then reflux is set to a predetermined value. As the distillation proceeds, top product quality will steadily fall but distillate is collected in the same receiver until the average distillate composition is reached at the desired value and then the overhead product is collected in a second receiver till the termination of operation and the same may be charged in the next batch. 

Conclusion

Still another way to operate the unit is to adopt the practice of the cycling procedure wherein the column is operated at total reflux till the desired top temperature is achieved (which is an indication of top product purity) and then all the overhead product is withdrawn at one distillate removal valve is closed and again column is operated under total reflux. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane