Stepping Off Trays

As stated above, we consider in this text only feeds in which the vapor and liquid are in equilibrium. Thus the feed point will lie on the equilibrium line. The more general cases are treated in the chemical engineering course on separation processes. In the examples shown here, we have started counting trays at the bottom (or top) and the feed composition conveniently matches one of the intersections with the equilibrium line. In other cases (such as the exercises at the end of this chapter) the feed composition is less conveniently matched to the diagram. In these cases, you might consider starting at the feed composition and stepping off trays upward toward the condenser and downward toward the evaporator. 

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