Figure 5.18 Effect of reactive tray holdup on composition profiles.
A is lost because less A is fed (upper right graph). and the concentration of A in the distillate is lower (lower left graph).
Figure 5.18 gives the composition profiles for three different reactive tray holdups. Increasing holdup reduces the concentrations of both reactants in the reactive zone, so less A is lost out the top.
In the base case, the flowrates of both the fresh feed of B (Fob) and reflux (R) are specified. The production rate is basically set by F0B. In the base case the reflux is 70mol/s. Increasing the reflux flowrate for a fixed flowrate of F0B should require more vapor boilup, but it should improve the separation. If the improvement in separation results in a significant reduction in the loss of reactants, the optimum design might not correspond to the point of minimum vapor boilup.
The results given in Figure 5.19 show that vapor boilup is minimized at a reflux flowrate of 66 mol/s. However, at this reflux flowrate there is a very large loss of reactant A in the distillate. Both F0A and xDA increase rapidly for reflux flowrates lower than 69 mol/s. At higher reflux flowrates, F0A and xDA again increase. Note that using somewhat more reflux than that giving minimum vapor boilup involves a small penalty in energy and yield. However, using a reflux that is only a very small amount less than that giving minimum vapor boilup produces a very severe penalty in both energy and yield. Therefore, a reasonable value for the reflux flowrate is probably 70 mol/s because this provides a little cushion between the operating point and "going over the cliff' into a region of very high yield losses.
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