So the reflux flowrate is fixed at 1.0797 kmol/s in Figure 6.4, and the temperature on one stage is held constant (stage 2, 8, 12, 20, 29, or 37). The abscissas in the plots show the mole fraction of propane in the feed; the ordinates, the purity of the distillate xD and the impurity of the bottoms xB.

The results shown in Figure 6.4 display some counterintuitive results. Controlling the temperature on stage 8 near the top of the column does a better job in maintaining bottoms purity than does controlling stage 29 near the bottom. The bottoms impurity is held quite close to or under the desired 2 mol% propane. On the other hand, controlling the temperature on stage 29 near the bottom of the column does a better job of maintaining the purity of the distillate at or above the desired 98 mol% propane. Conventional wisdom dictates that a tray located nearer the product stream will hold its purity more constant.

These results indicate that either stage 8 or stage 29 does a fairly good job in maintaining product purities in this binary system when single-end temperature control is used. If dual-temperature control were used and the temperatures at the two ends of the column were controlled, product compositions would be held exactly at their desired values under steady-state conditions if pressure changes do not occur.

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