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Kinetic

2M2B -> 2M1B

^ 1

Figure 9.10 Six reactions specified.

Figure 9.10 Six reactions specified.

Figure 9.11 (a) Kinetic parameters; (b) specify concentration basis and reactive phase.
Figure 9.11 Continued.

diameter calculated in Tray Sizing is compared with the estimated diameter and the calculations repeated.

The other important issue is the height of liquid on a tray. Hydraulic limitations prevent excessive liquid depth because this would cause large pressure drops. Liquid depths are limited to about 0.1 m. If more liquid holdup is needed, the column diameter can be increased beyond the minimum calculated from sizing calculations. Of course, more reactive trays could also be added to the column. However, reactive distillation columns have the interesting feature that there is an optimum number of reactive stages. Having too few reactive stage results in high energy consumption because the reactant concentrations in the reactive zone must be large, and this requires large vapor rates to keep the reactants from leaving in the bottoms or the distillate. Adding more reactive stages reduces the

Figure 9.12 (a) Specifying reactive trays and holdups; (b) tray holdups.
Figure 9.12 Continued.

vapor boilup requirements because reactant concentrations in the reactive zone decrease. However, beyond some point, adding more reactive stages begins to increase energy consumption because the reactant concentrations in the reactive zone start to increase due to a larger contribution from the reverse reactions.

Figure 9.13 gives a ternary diagram for the isopentane/methanol/TAME system at 4 bar. The phase equilibrium of this system is complex because of the existence of azeotropes. The Unifac physical property package in Aspen Plus is used to model the vapor-liquid equilibrium in all units except the methanol/water column, where the van Laar equations are used because of their ability to accurately match experimental data.

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