At the minimum re
ux condition, an innite number of stages for a given separation
is required, and at least one zone of constant composition (pinch zone) of all the
components exists in the column (King, 1980). In general, there will be one pinch zone
in the rectifying column section and one pinch zone in the stripping column section
(Smith, 2005). For a binary distillation at minimum reflux, the two pinch zones are
usually adjacent to the feed stage, as shown in Figure 2.1a. For a multi-component
distillation at minimum reflux, the pinch zones are located just above and below the
feed stage if all components are distributing (exist in both top and bottom products),
which is also shown in Figure 2.1a; if light non-key and heavy non-key components are
not distributing (only exist in either top or bottom product), the pinch zones in the
rectifying and stripping sections are away from the feed stage, which is shown in Figure
2.1b (Smith, 2005, p.167).