Control Structures

Because composition analyzers are expensive, require high maintenance, and introduce deadtime into the control loop, it is desirable to use inferential temperature measurements instead of direct composition measurements whenever possible.

In the Eastman control structure, two PI temperature controllers are used to maintain two tray temperatures in the column by manipulating the two fresh feedstreams. Production rate changes are achieved by changing the vapor boilup. The base level is controlled by manipulating the bottoms flowrate.

There are two alternatives for this control structure: a constant reflux flowrate and a constant reflux ratio. The left system in Figure 10.4 shows the control structure (CS7-R) where the reflux flowrate is fixed and the reflux drum level is controlled by manipulating the distillate flowrate. The right system in Figure 10.4 shows the alternative version (CS7-RR) where the reflux drum level is controlled by the reflux flowrate and the distillate flowrate is adjusted to give a constant RR.

Both control structures are single-input, single-output structures with PI controllers for temperature control and P controllers for level control. The relay-feedback method is used to obtain the ultimate gains and ultimate periods. The valves are designed to be half open at steady state. Two 60-s first-order measurement lags are used in both temperature loops. The temperature controllers are tuned using the Tyreus-Luyben tuning method. The column pressure is assumed to be constant, which is achieved by manipulating the condenser heat duty.

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