Batch distillation is often preferable to continuous distillation in cases where relatively small quantities of material are to be handled at irregularly scheduled periods.The simplest case of batch distillation is one in which the material to be separated is charged to a heated kettle,fitted with a total condenser and product receiver.The material is distilled without reflux until a definite quantity of one of the components of the mixture has been recovered or until a definite change in composition of the still contents has been effected.
In a batch rectification set up,vapors generated in the column pass up the column currently to liquid that is passing down the column.The column packing provides a good contact between the liquid and vapor.As liquid passes up the column,it comes in contact with thin film of liquid formed around the packing surface.This results in a transfer of mass and heat.Most of the heat transferred between the phases,and effective for vaporization,is that due to the latent heat of the components.If the molal latent heats of vaporization of the components are nearly the same,as is often the case,then the moles of vapor condensed are approximately equal to the moles of the liquid vaporized. As a result,themolal up-flow throughout the column remains constant.Also the liquid down-flow from top to bottom remains constant.
In the column,mass is transferred in such a way that the more volatile component pass from the liquid to the vapor and the less volatile component from the vapor to the liquid. Thus there is an increase in the concentration of the more volatile component in the vapor as they pass up the column. There is, also, an increase in concentration of the less volatile component in the liquid as it passes down the column.In all-types of batch distillation,a quantity of feed is charged to a still pot, or kettle, and heat is applied to it.The vapor, which is usually passed through a fractionating column,is then condensed giving the overhead product while a less volatile residue remains in the kettle at the end of distillation.Continuous distillation is a steady state process because once equilibrium has attained, conditions at any given point remains constant, whereas batch distillation is an unsteady state process,the concentration of the more volatile component decreasing continually so that the temperature and composition of the mixture at a point in the system must alter as the distillation proceeds.
The method of operation of a batch fractionation still is similar to that of a continuous still, reflux being returned to the top of the column to enable fractionation to take place. At the start of distillation, the column is either run at total reflux until equilibrium has attained; or the reflux ratio is fixed at the required value from the beginning, in which case the distillate is recycled to the still until conditions are stable.The operation of batch still can be either of the two methods:
1.Constant reflux ratio variable product composition:
By this method, it is possible to split a given feed into a number of fraction in the one operation by running the column under constant reflux and taking several cuts for the distillate.
2.Variable reflux ratio constant product composition:
When it is desired to produce a distillate of specific constant composition, the reflux ratio must be continuously increased and the rate of output consequently decreases.
Animation showing a single tray of a distillation column