The following are not listed as design variables for the distillation unit operations in Table 5.4:

(a) Condenser heat duty

(b) Stage temperature

(c) Intermediate-stage vapor rate

(d) Reboiler heat load

Under what conditions might these become design variables? If so, which variables listed in Table 5.4 would youeliminate?

Typical Variable Specifications for Design Cases Table 5.4 Variable Specification Case I, Component Recoveries Case II, Number of Equilibrium Stages Specified Specified No Unit Operation 1. Recovery of 1. Number of (a) Absorption (two inlet streams) 2N + 2C +5 one key component stages 1. Condensate at saturation temperature 2. Recovery of light key component 3. Recovery of heavy key component 2N +C+9 1. Condensate at saturation (b) Distillation (one inlet stream, total condenser, partial reboiler) Total condenser Divider temperature 2. Number of stages above feed stage 3. Number of 2 stages below feed stage Partial 4. Reflux ratio 4. Reflux ratio reboiler 5. Distillate flow (> minimum) 5. Optimal feed stage 1. Recovery of light key rate (2N +C + 6) 1. Number of stages above feed stage (c) Distillation (one inlet Partial condenser component 2. Recovery of heavy key stream, partial condenser, partial reboiler, 2. Number of stages below feed stage 3. Reflux ratio component 3. Reflux ratio (> minimum) 4. Optimal feed stage 4. Distillate flow vapor distillate rate only) Partial reboiler