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Figure 10.16 Rangeability with C and D in F0A.

Figure 10.16 Rangeability with C and D in F0A.

Both designs can handle big changes in the feed composition of F0A without shutting down. However, the product purities drop faster for the disturbance in the feed composition of F0A for the 5/13/5 design. This makes the acceptable change in F0A composition smaller for the 5/13/5 design than for the 5/10/5 design.

We have no explanation for this counterintuitive result. It may be caused by differences in the shapes of the composition profiles among the various numbers of reactive trays. Figure 10.15 gives composition profiles for the three cases.

Two changes occur as more reactive trays are added:

1. The peaks in reactant A and B concentrations increase as the number of reactive trays is increased. This is counterintuitive because we would expect the presence of more catalyst to promote more reaction and lower the reactant concentrations. However, this can be explained by observing that the concentration of one reactant increases while the concentration of the other reactant decreases.

2. The impurities in the two products change from being essentially the adjacent volatility component (mostly B impurity in the bottoms with product D and mostly A impurity in the distillate with product C) to being a mixture of both reactants at both ends.

We cannot explain these unexpected results, but they do help to explain why the optimum number of reactive trays is 7. Recall that vapor boilup increases as we increase Nrx from 7 to 10 to 13 (see Table 10.1). This occurs because the larger reactant concentrations at the ends of the reactive zone require more energy to keep these impurities out of the product streams. We have no physical explanation of why these changes make the column more sensitive to disturbances in the composition of F0A.

Two other types of impurities in the fresh feeds are also possible. There may be some of the product components (C or D) in the fresh feedstreams. The rangeability of the system for the 5/10/5 case is shown in Figures 10.16 and 10.17. In Figure 10.16 either C or D are impurities in fresh feed F0A. The system can handle higher concentrations of D because the F0A stream is fed near the bottom of the column from which D is removed. The impurity levels can be fairly large.

However, Figure 10.17 shows that the impurities levels are much lower for F0B, particularly for D impurities because this heavy product has to work itself all the way down through the column from the high feed location of F0B.

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