100 200 300 Time (min)
F0A, and tray 5 is the most sensitive to changes in F0B. Therefore, these locations are selected, as shown in Figure 12.14. The two singular values for this control structure are ctj = 142 and s2 = 17, giving a condition number of 8.4, which indicates that this control structure should perform adequately.
Figures 12.17-12.20 demonstrate that this control structure provides quite acceptable control in the face of all four of the disturbances. Stable base-level regulatory control is attained. The bottoms purity is maintained close to the desired value for all of the disturbances.
Using SVD Analysis. Figure 12.21 shows the U vectors from the SVD analysis. The most sensitive locations are trays 3 and 5, which are those suggested by the steady-state gains. However, there is a third location at tray 12 that is indicated as sensitive. This is unexpected because the gains in this region, as shown in Figure 12.6, are all quite small. However, the DT/ DF0B gain at tray 12 is the largest of the three. Thus, a control structure is evaluated in which the tray 5 temperature is controlled by manipulating F0A and tray 12 temperature is controlled by manipulating F0B.
The results given in Figures 12.22-12.25 demonstrate that this selection of control trays does provide stable base-level regulatory control. However, the purity of the bottoms product is not held as close to its specification as with the original choice of trays 3 and 5. This is particularly true for the changes in feed composition.
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