Control Structure CS1

Figure 12.4 shows a control structure that uses two direct composition measurements. Bottoms product purity (mol% C) is measured and controlled by vapor boilup. The composition of A on tray 5 is measured and controlled by the flowrate of fresh feed F0A. Throughput is set by flow controlling fresh feed F0B. Reflux-drum level is controlled by manipulating the reflux flowrate, and base level is controlled by manipulating the bottoms flowrate.

Tray 5 is selected because this is the location where the composition of A is changing rapidly from tray to tray. The steady-state composition is 38 mol% A. Two 0.5-min firstorder composition measurement lags are inserted in both composition control loops. Composition transmitter spans are 20mol%. All valves are 50% open at steady state.

The controllers are tuned by running relay-feedback tests. Values of ultimate gains and periods are provided in Table 12.2. 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 given in Figures 12.5-12.8. In Figure 12.5 the disturbance is a positive 20% step change in the throughput handle, VS, at 10min. The

12.1 TERNARY SYSTEM WITHOUT INERTS 297 TABLE 12.2 Controller Tuning Parameters for Ternary Without Inerts


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