The usual sizing calculations are performed, the flowsheet is pressure-checked, and the file is exported to Aspen Dynamics. Flow controllers are installed on the feed, distillate, and sidestream. Base level is controlled by manipulating bottoms flowrate. Reflux drum level is controlled by manipulating reflux flowrate.
The major control structure issues of this sidestream column are how to manipulate the sidestream flowrate and how to manipulate the distillate flowrate. They cannot be fixed or just ratioed to the feed flowrate because changes in feed composition required that the flowrate of the distillate and the sidestream must change to achieve the desired purities. Figure 10.2 shows a control structure that provides effective control of this complex column. Figure 10.3 gives the controller faceplates. Note that two of the flow controllers are on "cascade" (remote setpoints).
The control scheme shown in Figure 10.2 controls the temperature on stage 3 by manipulating the distillate flowrate and controls the temperature on stage 51 by manipulating the reboiler heat input. These stages are selected by studying the temperature and composition profiles shown in Figure 10.4. The two temperature controllers (TC3 and TC51) have one-minute deadtimes and 100 K temperature transmitter spans. They are tuned by running a relay-feedback test and using the Tyreus-Luyben settings.
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