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Figure 11.28 Reactive and recovery columns with 20% excess of reactant B.

flowrate of fresh feed F0B. The setpoint of this total flowrate controller is ratioed to flowrate F0A.

3. Pressures in both columns are controlled by condenser duties (not shown in Fig. 11.29).

4. Base levels are controlled by bottoms flowrates.

5. Reflux-drum levels are controlled by reflux flowrates.

6. Reflux ratios are maintained by measuring distillate flowrates and adjusting reflux flowrates.

7. The temperature on tray 17 in the reactive column is controlled by manipulating vapor boilup VS1 in the reactive column.

8. The temperature on tray 18 in the recovery column is controlled by manipulating vapor boilup VS2 in the recovery column.

The ratio is set to give a 20% excess of B. For the numerical case considered in this example, the ratio is 1.2, that is, FB,tot = 1.2 F0A.

The same disturbances used in the previous studies of the single reactive column are imposed on the system. The throughput change is now a step in F0A. Figure 11.30 gives the response for a 20% increase in F0A. The first thing to note is that the time scale has been increased from that used for the one-column case. The system with recycle has significantly larger time constants than the single column. It takes 2-3 h for the transients to die

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