Multicomponent systems present a much more difficult problem than binaries. Discussions of the techniques used can be found in references 39 43, 45.

In addition there are two approximations that can be used for multicomponents. One (the Smith-Brinkley) is a rating method used if the number of stages and internal flow rates are known (42). The other technique (Fenske-Underwood-Gilliland) is actually a design method. In the case of the latter the first step is to rank the components in terms of their volatilities relative to the least volatile (Table 5-6).

The components closest to the point of separation are B and C; most of the B is in the overhead stream and a little in bottoms and vice versa for C. This means that B is the light key (lk) and C is the heavy key (hk).

Minimum stages for the system of Table 5-6 can be determined from the Fenske equation (45).

(5-22)

where X represents the desired mole fractions of the heavy or light keys in the distillate ( D) and bottom ( W) products. The ?' is the volatility of the light key relative to the heavy key (in the case cited above ?' equals 3.6/1.7). Just as we assume for binary mixtures, a minimum number...