A general schematic diagram of a multistage counter-current binary distillation operation is shown in Figure 4.10. The operation consists of a column containing the equivalent N number of theoretical stages arranged in a two-section cascade; a condenser in which the overhead vapor leaving the top stage is condensed to give a liquid distillate product and liquid reflux that is returned to the top stage; a reboiler in which liquid from the bottom stage is vaporized to give a liquid bottom products and the vapor boil off returned to the bottom stage; accumulator is a horizontal (usually) pressure vessel whereby the condensed vapor is collected; Heat exchanger where the hot bottoms stream is used to heat up the feed stream before it enters the distillation column.
The feed enters the column at feed stage contains more volatile components (called light key, LK) and less volatile components (called heavy key, HK). At the feed stage feed may be liquid, vapor or mixture of liquid and vapor. The section above the feed where vapor is washed with the reflux to remove or absorb the heavy key is called enriching or rectifying section. The section below the feed stage where liquid is stripped of the light key by the rising vapor is called stripping section.