Page 391 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

For a simple ternary distillation process, a curve can be drawn by connecting the compositions on each
stage. This is equivalent to the operating line in the McCabe-Thiele method. This is illustrated in Figure
12.6. It is observed that the curves for the rectifying and stripping section intersect. This intersection
implies a feasible distillation process, just as intersection of the operating lines for rectifying and
stripping implies a feasible binary distillation on the McCabe-Thiele diagram. If the curves do not
intersect, then the distillation operation is not feasible.

Each point shown on the curves on the triangular diagram is analogous to the highlighted points on the
McCabe-Thiele diagram, where the stepping process intersects the operating lines. In the McCabe-Thiele
method, the optimum feed location is determined by the intersection of the operating lines. It is possible to
place the feed in a different location; however, the optimum location minimizes the number of stages
required. For simple ternary distillation, the feed tray is fixed at the point of intersection of the rectifying
and stripping curves on the triangular diagram. To trace the tray-to-tray compositions, one follows a
curve for one section and then switches to the curve for the other section at the intersection of the curves.
The closely spaced points not in the range of operation are those analogous to the points on a McCabe-
Thiele diagram obtained by not switching operating lines in the stepping process and pinching as the
equilibrium curve is approached.

It is also observed that the feed point lies on a straight line connecting the distillate and bottoms product
concentrations. This is a consequence of the lever rule and is similar to the representation of mixing and