Separation Process Principles- 2n - Seader & Henley - Solutions Manual

Assuming startup conditions of total reflux use eq 13

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Unformatted text preview: , V. N. Kiva, G. A. Meski, and M. Morari, Chem. Eng. Sci., 49, 3033-3051 (1994). They find that for a ternary mixture, reflux and boilup ratios can be found that the composition of the middle vessel can tend to one of the three components. For a multicomponent mixture, both light and heavy impurities can be removed simultaneously. Exercise 13.20 Subject: Effect of holdup on batch rectification. Given: Appreciable column holdup. Find: (a) After total reflux conditions are established, why is the composition of the charge to the still lower in the light component compared to the charge? (b) Why is the separation more difficult than with zero holdup? Analysis: (a) When the holdups in the column (trays or packing) and condenser-reflux drum are appreciable compared to the original charge, the composition in the boiler will become higher in the heavier components because of the fractionation that occurs during the establishment of the total reflux condition. The average composition of the holdups above the boiler will be higher in the lighter components, and, thus, the average composition of the boiler holdup will be lower in the light components, compared to the original charge. (b) When distillation with distillate accumulation begins, the mixture in the boiler is now less rich in the lighter component than it would be if the column and condenser-reflux drum holdups were zero. Thus, the separation into a nearly pure light component is more difficult. Exercise 13.21 Subject: Effect of holdup on batch rectification. Given: Appreciable column holdup. Find: Why tray compositions change less rapidly when column holdup is appreciable compared to zero holdup. Why is the degree of separation improved with appreciable column holdup? Analysis: Column (plate or packed) holdup causes a flywheel-like inertia effect, which decreases the rate of change of stage compositions. This can be observed in Eq. (13-36), where the rate of change in stage composition is inversely proportional to the stage holdup. Thus, the degree of separation is improved because the composition does not as rapidly as it would with zero holdup. Exercise 13.22 Subject: Effect of holdup on batch rectification. Given: Statements in Exercises 13.20 and 13.21. Find: Reasons why it is difficult to predict the effect of column holdup. Analysis: When the holdups in the column (trays or packing) and condenser-reflux drum are not negligible compared to the original charge, the composition in the boiler will become higher in the heavier components because of the fractionation that occurs during the establishment of the total reflux condition. The average composition of the holdups above the boiler will be higher in the lighter components, and, thus, the average composition of the boiler holdup will be lower in the light components, compared to the original charge. When distillation with distillate accumulation begins, the mixture in the boiler is now less rich in the lighter component than it would be if the column and condenser-re...
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