Control Structure CS3

In Chapter 10 we investigated a two-temperature control structure for the quaternary, two-product system. We demonstrated that an internal composition measurement is not required in that system to provide the extremely precise balancing of the stoichiometry of the reaction, that is, feeding exactly the right amount of reactants so that no excess builds up in the column. Will a similar two-temperature control structure be effective in the ternary system without inerts? This structure is shown in Figure 12.14. The two fresh feeds are manipulated to control the temperatures on two trays.

The first issue is to find what trays to control. In Chapter 10 we found the steady-state gains and used SVD to help in the selection of control trays. The same approach is applied in the ternary system.

Using Steady-State Gains. Figure 12.15 shows the steady-state changes in tray temperatures throughout the column for several small changes in vapor boilup. These are obtained by making small changes in vapor boilup from its steady-state value and converging to a new steady state. The level controllers are on automatic during this procedure. The D VS must be quite small to obtain the true linear gains (gain values do not change as the size of the DVS is decreased). These results show that tray 4 is the most sensitive for vapor boilup changes.

The same procedure is used for the other two inputs into the system: fresh feed flowrates F0A and F0B. Results are provided in Figure 12.16. Tray 3 is the most sensitive to changes in

>B

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