The previous section showed no surprises regarding the effect of tray holdup. In this section we look at changing the number of reactive trays. Intuition would lead us to think that the more trays the better. This is certainly the case in conventional distillation. However, as we will see, this is not the case with a steady-state reactive distillation column for this type of reaction (two reactants, two products).
Figure 2.5 Effect of number of reactive trays.
Figure 2.5 demonstrates the effect of changing the number of reactive trays NRX with all other parameters held constant at base case values. The important graph is the upper left one that shows how vapor boilup (or energy) changes as the number of reactive trays is varied. It is quite unexpected that there is a minimum in this curve, which says there is an optimum value for the number of reactive trays in terms of energy consumption. This is certainly different than in conventional distillation in which adding more trays always reduces energy consumption. Figure 2.6 gives temperature profiles with several numbers of reactive trays.
The explanation of this counterintuitive result can be deduced from the composition profiles shown in Figure 2.7. With few reactive trays, a lot of vapor boilup is required to keep the reactants from leaving the column. The top left graph shows that the concentration of A is quite high in the rectifying section of the column with an NRX value of 7. Likewise, the top right graph shows that the concentration of B is quite high in the stripping section of the column for this case. This occurs because the consumption of the reactants is not as large as they move up or down through the reactive zone because there are few trays. Therefore, a lot of vapor boilup and reflux is needed in order to keep reactant B from dropping out of the bottom and to keep reactant A from going out of the top.
The situation is just the reverse with a large number of reactive trays. The concentrations of the reactants at the ends of the column where they are fed become large because much of the other reactant has been consumed on the many trays in the reactive zone before it reaches the opposite end. There is little B arriving near the bottom of the reactive zone, so the concentration of A is large near where it is fed. Likewise, there is little A arriving near the top of the reactive zone, so the concentration of B is large near where it is fed.
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