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There is only one reactant in the ternary decomposition reaction. Unlike the quaternary two-feed, two-product systems and ternary synthesis reactions with two feeds and one product, the stoichiometric balance of the reactants is no longer a problem. The control strategy will focus on holding product purities at setpoints. Thus, the control problem is similar to that of conventional distillation column.

Reboiler Tray Number Condenser

Figure 12.48 Composition profile for A , B + C with reactive zone marked with NFA and the fraction of total conversion (Ri/Rt) shown in shaded area.

Reboiler Tray Number Condenser

Figure 12.48 Composition profile for A , B + C with reactive zone marked with NFA and the fraction of total conversion (Ri/Rt) shown in shaded area.

Figure 12.49 Temperature profile.

12.3.2 Control Structure CS1

A two-temperature control structure is considered first. Selecting the vapor boilup and reflux ratio as manipulated variables, the control problem then is to find the best locations for the two temperature control trays. Figure 12.50 shows the steady-state changes in tray temperatures throughout the column for several small changes in vapor boilup and reflux ratio.

We used SVD to select the control trays. Figure 12.51 shows the U vectors from the SVD analysis. The most sensitive locations are trays 26 and 5. Figure 12.52 shows the control structure. The reflux-drum level is controlled by manipulating the distillate flowrate, and the base level is controlled by manipulating the bottoms flowrate. A third-order measurement lag with a time constant of 10 s is assumed in each temperature controller. The controllers are tuned by running relay-feedback tests. Ultimate gains and periods values are given in Table 12.5. Tyreus-Luyben tuning is used in most cases, but some loops are detuned to give larger closed-loop damping coefficients.

Results for the CS1 control structure are presented in Figures 12.53-12.55. In Figure 12.53 the disturbances are positive and negative 20% step changes in the feed flow-rate at a time of 100 min. The temperature dynamics are very fast and steady state can be reached in a few minutes. The composition dynamics are a little slower and settle in 50 min. However, 0.1 mol% steady-state offsets in product purities are observed. Figure 12.54 shows what happens when composition z0 of fresh feed F0 is changed from pure reactant A (z0A = 1) to a mixture of A and B (z0A = 0.95, z0B = 0.05). When light

product B is fed into the column, the reflux ratio decreases and the boilup rate decreases to maintain the column temperatures. This will lead to lower top product purity and purer heavy product in the bottoms. Figure 12.55 demonstrates what happens when composition z0 of the fresh feed is changed from pure A (z0A = 1) to a mixture of A and C (z0A = 0.95,

TABLE 12.5 Controller Tuning Parameters for Ternary Decomposition Reaction: IK LK 1 HK

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