C

1 bar jy

D2 = 12.54 mol/s xRA = 0.000 xRB = 0.985 xRC = 0.015

Figure 12.62 Two-column configuration for decomposition reaction (HK , IK + LK).

manipulating the reflux ratio. For the distillation column, trays 23 and 8 (T2,23 and T2,8) are chosen as control trays by using SVD analysis. Relay-feedback tests are performed to find controller settings for these three temperature control loops. Table 12.7 gives controlled and manipulated variables and controller parameters.

The performance of this control structure is given in Figure 12.68 for +20% changes in the feed flowrate. Relative fast transients can be achieved, and little deviation in the top and bottoms compositions of column 2 are observed. Specifically, the deviations in product compositions are all less than 0.01%. Because we have two columns in series, the product compositions settle in ^7 h and the temperatures return to setpoints in ~3h. Figure 12.69 shows what happens when composition z0 of fresh feed F0 is changed from pure A (z0A = 1) to a mixture of A and B (z0A = 0.95, z0B = 0.05). Figure 12.70 shows what happens when composition z0 of fresh feed F0 is changed from pure A (z0A = 1) to a mixture of A and C (z0A = 0.95, z0C = 0.05). As shown in Figures 12.69 and 12.70, the process is well controlled using temperature control. For feed composition disturbances, the steady-state offset in product composition is kept below 0.21%, while having fast temperature responses.

12.4.3 Control Structure CS2

We can eliminate composition offsets altogether by implementing two-composition control in the second distillation column. The control scheme is shown in Figure 12.71. The temperature control of the reactive distillation column remains unchanged. However, two-temperature control of column 2 is replaced by two-composition control. Recall that a 4-min analyzer dead time is assumed for composition measurements. Top product purity is maintained at 98.5% B by manipulating the reflux ratio, and the bottoms product composition is controlled at 98% C by adjusting the vapor boilup. Relay-feedback tests are carried out to find ultimate gains and ultimate periods, followed by Tyreus-Luyben tuning with possible detuning. The tuning parameters are given in Table 12.7.

Figure 12.72 shows that much slower responses result for composition control. Product composition responses also show large deviations during transients as well as asymmetric

TABLE 12.6 Steady-State Conditions and Design Parameters for Two-Column Configuration

Column 1 (Reactive Distillation Column)

Fresh feed flowrate of A F0A (mol/s)

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