Page 394 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

There are three key rules to using residue curves to conceptualize distillation processes.
1. Because the temperature increases from top to bottom in a distillation column, the time variable
can be replaced with a height variable in Equation (12.2). This is true only for a packed
column; however, the equivalence of a certain column height and a tray (HETP, height
equivalent to a theoretical plate) makes generalization to tray columns possible.
2. The residue curve is the composition profile in a continuous, packed distillation column at total
reflux.
3. The BVDM (residue curves on triangular diagrams) is useful only for conceptualization, not

numerical calculations, in contrast with the McCabe-Thiele method, which can be used for
numerical calculations.

Therefore, to represent a feasible distillation process qualitatively, the second point combined with the

material balance criterion illustrated in Figure 12.6 (feed, distillate, and bottoms product must lie on
same straight line) means that there must be a straight line connecting the feed, distillate, and bottoms
that intersects the same residue curve, with one end at the distillate and the other end at the bottoms.
It must be remembered that there are an infinite number of residue curves on a triangular plot, even though
only two or three are actually drawn. Figure 12.10 shows examples of feasible and nonfeasible
distillation processes. Figure 12.10(a) is for a system without azeotropes, and Figure 12.10(b) is for a
system with a minimum boiling azeotrope between Components A and B.

Figure 12.10 Feasible and Nonfeasible Distillation Processes