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Tray

Inerts; SVD; F0A/F0B

Inerts; SVD; F0A/F0B

Tray

Figure 12.43 Inerts SVD analysis.

Tray

Figure 12.43 Inerts SVD analysis.

be negative (direct acting: an increase in temperature increases F0A). This loop was tuned and worked well when the other temperature loop was on manual.

The second temperature on tray 5 was controlled by manipulating F0B. The process gain is positive, so the controller gain should be positive (reverse acting: an increase in temperature decreases F0A). This loop did not work, as illustrated in Figure 12.44. The tray 3 temperature controller is on manual, that is, F0A is fixed. No disturbance is made. After about 10min, the temperature on tray 5 begins to drop. The tray 5 temperature controller increases F0B up to its limit (twice the steady-state value), but the temperature does not recover. We were unable to make this control structure work using trays 3 and 5.

There is a slight hump in the U curve at tray 13 shown in Figure 12.43. We tested a control structure that used F0A to control the tray 5 temperature and F0B to control the tray 13 temperature. These two loops were successfully tuned, but the tray 13 controller had a very large ultimate period (55 min). Table 12.3 gives the controller tuning parameters.

The performance of this control structure is given in Figure 12.45 for a +20% change in VS. The system is stable, but the bottoms purity and distillate purity are not well controlled. Figure 12.46 shows that the response to a 20% decrease in VS is even worse in terms of product purities. The bottoms composition drops to 76 mol% C and the distillate composition drops slightly.

These results demonstrate that a two-temperature control scheme does not provide effective control of the ternary system with inerts. The two-temperature control structure works for the quaternary system and for the ternary without inerts, but not for the ternary with

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