SIMPLE (DIFFERENTIAL) DISTILLATION
We will consider a binary mixture of A (more volatile) and B (less volatile). The set-up is as shown in the Figure below. The system consist of a batch of liquid (fixed quantity) inside a kettle (or still) fitted with heating element or steam jacket, and a condenser to condense the vapour produced. The condensed vapour is known as the distillate. The distillate is collected in a condensate receiver. The liquid remaining in the still is known as the residual.
A simple material balance for the process is shown in the Figure below.
In this process, subsequent collection of distillates (see Figure) will yield a lower concentration of the MVC.
The concentration changes can be analysed using the phase diagram, and detailed mathematical calculations carried out using the Rayleigh Equation.
The separation can be extended to several more stages by re-heating the distillate collected, and collecting the new distillate, as shown in the Figure.
The distillate product from the second stage will be purer in A compared to the first stage, but the quantity of distillate collected in the second stage is lower than that of the first stage. As more stages are added, purer distillate product can be obtained, but the quantity collected will be lower than the previous stage.