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Figure 12.74 Composition profile for HK , LK + IK with reactive zone marked with NFA and the fraction of total conversion (Rt/Rt) shown in shaded area.

Reboiler Tray Number Condenser

Figure 12.74 Composition profile for HK , LK + IK with reactive zone marked with NFA and the fraction of total conversion (Rt/Rt) shown in shaded area.

Figure 12.75 Temperature profile.

structure where the tray 85 temperature is controlled using the reflux ratio and the vapor boilup is ratioed to the feedflow. Figure 12.77 shows that almost offset free (composition) control can be achieved for feedflow disturbances and fast responses can also be observed. Figure 12.78 demonstrates what happens when the feed composition is changed from pure A (z0A(A) = 1) to a mixture of A and B (z0A(A) = 0.95, z0A(B) = 0.05). Because less A is coming into the system, more B (LK) and C (IK) are going toward the top, with the vapor boilup kept constant. Increased IK C leads to a slight increase in the temperatures in the top of the column, and the temperature controller increases the reflux ratio to maintain the tray 85 temperature. Figure 12.79 shows similar behavior when the feed composition is changed from pure A (z0A(A) = 1) to a mixture of A and C (z0A(A) = 0.95, z0A(C) = 0.05). Because product C is the intermediate boiler, which could appear both in the top and bottom of the column, the reflux ratio will increase to separate products C and B. For all disturbances, the steady-state offsets in product compositions are less than 0.3%.

12.5.3 Control Structure CS2

Using the reflux ratio and vapor boilup as the manipulated variables, the two-temperature control structure is shown in Figure 12.80. Two temperature control trays, tray 85 and tray 5, are selected based on SVD analysis. Using relay-feedback tests, values of ultimate

gains and periods are obtained and provided in Table 12.9. Tyreus-Luyben tuning is used, but some loops are detuned for less oscillatory responses.

Figures 12.81 illustrates the performance of this CS2 control structure under the feedflow disturbances. Tight control can be achieved with a settling time of ^2h. Figure 12.82 shows what happens when the feed composition is changed from pure A (zoa(a> = 1) to a mixture of A and B (z0A(A) = 0.95, z0A(B) = 0.05). Because less A is coming into the system, less vapor boilup and reflux are required. Figure 12.83 shows what happens when the feed composition is changed from pure A (z0A(A) = 1) to a mixture of A and C (z0A(A) = 0.95, z0A(C) = 0.05). Because product C is the intermediate-boiling key, which could appear both in the top and bottom of the column, the reflux ratio will increase to separate product C and product B. For all disturbances, the steady-state error in product composition is less than 0.2%, which is mostly in the top product composition. Compared to CS1, the energy consumption of two-temperature control is less, especially when product B is coming into the system from the feed (Figs. 12.78 and 12.82).

12.5.4 Control Structure CS3

The composition offsets can be eliminated using a temperature/composition cascade as suggested in the case of IK , LK + HK. Will the parallel cascade control also be effective for this single column configuration with heavy reactant, HK , IK + LK?

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