Product Composition (Mole Fraction)
A 0.8725 0.0025
B 0.0928 0.0175
C 0.0347 0.9800
5.1.4 Effect of Pressure
Now that we have a design procedure and a base case, we are ready to see how various parameters affect the ternary reactive column without inerts. Figure 5.3 shows the effect of changing the column operating pressure. The top left graph in Figure 5.3a shows that the optimum pressure is 7 bar in this ternary system. Remember that the optimum pressure is 8 bar in the quaternary system. The lower pressure is a result of not having the very light product that tends to reduce temperatures in the quaternary system. The reaction zone temperatures in the ternary system at 7 bar are similar to the reaction zone temperatures in the quaternary system at 8 bar.
The lower graphs in Figure 5.3a give the compositions of reflux xDj and bottoms product xBj. They change very little with pressure. Temperature profiles at different pressures are given in Figure 5.3b.
In the quaternary system, increasing the reactive tray holdup decreases energy consumption. The same is true in the ternary system, as demonstrated in Figure 5.4. Thus, adding more reactive tray holdup improves the steady-state designs of both systems. There are no counterintuitive effects of reactive tray holdup.
In the quaternary system in Chapter 2, we demonstrated that there is an optimum number of reactive trays at which vapor boilup is minimized. Having too few or too many reactive trays increases the steady-state energy consumption. This effect is counterintuitive. Does the same occur in the ternary system?
Figure 5.5 shows that it does not. Adding more reactive trays improves the steady-state design of the ternary system because vapor boilup decreases as reactive trays are added. Figure 5.5 also shows that the compositions in the top and bottom of the column do not change much. Figure 5.6 illustrates that the same is true for the temperature profile. Figure 5.7 gives the composition profiles throughout the column for three values of Nrx.
Note the sharp liquid composition changes that occur between the top of the column on tray NT and the reflux drum (stage NT + 1). This is caused by the total reflux operation. The vapor composition on tray NT is higher for lighter component A than the liquid composition. The reverse is true for heavier component B. Therefore, the liquid leaving the total condenser is richer in A than the top tray and leaner in B. Thus, the liquid composition profiles rise for A and drop for B at the top of the column.
These results demonstrate a fundamental difference between a two-product reactive column (quaternary system) and a one-product reactive column (ternary system). This is a good example of the complexity and challenges associated with reactive distillation.
In the quaternary system in Chapter 2, we demonstrated that increasing the number of fractionating trays in the stripping and rectifying sections tends to decrease vapor boilup. In the ternary system, the column has only stripping trays. The impact of changing the number from the base case of NS = 5 is shown in Figure 5.8.
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