The steady-state simulation of a two-column heat-integrated distillation system was developed in Chapter 5. The two columns run "neat," so all the energy released in the condenser of the high-pressure column is used in the reboiler of the low-pressure column. In the steady-state design, this balance was achieved by using a Flowsheeting Options Design Spec that split the total feed between the two columns such that the two heat duties balanced.
Now we want to study the dynamics of this complex flowsheet. The problem of balancing the heat duties becomes one of the major aspects of designing an effective control structure. The other issue is what to manipulate to control a tray temperature in each column. In the high-pressure column, reboiler heat input can be manipulated. But in the low-pressure column, reboiler heat input is not adjustable since it is fixed by the condenser heat duty in the high-pressure column. Table 8.3 summarizes the major design features of the two columns.
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