Retrograde distillation

The feed point should give the most equal slopes on both sides of the feed stage in a key ratio plot (Fig. 2,20). Too high a feed causes excessive retrograde distillation below the feed, and too low a feed causes excessive retrograde distillation above it. In order to reach the optimum feed point, the feed stage should be moved from the sharp maximum (or minimum) toward the flat one (Fig. 2.20). 

In multicomponent separations, the sensitivity of temperature to the key components is important. Figure 18.2a and b shows composition and temperature profiles for a depropanizer separating propane (C3) and lighter components from butane (C4) and heavier components. The temperature is sensitive to the composition of the keys between tray 3 and tray 13. Below tray 3 and above tray 13 the temperature is more sensitive to the concentration of nonkeys than to the concentration of the keys. Trays 8 to 10 show some tendency toward retrograde distillation (recognized by the maximum in the C4 concentration curve) and are best avoided. Moczek et al. (287) provide a detailed demonstration of the anomaly in temperature response in the retrograde distillation region. This leaves trays 3 to 7 and trays 11 to 13 as those suitable for temperatime control. 

Use UV-Vis Spectrophotometer to determine the retrogradation characteristics of starch samples take the supernatant of starch paste which stand a period of time, make the reference by the distilled water, and determine the transmittance under the 650nm wave. 

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