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The very small bottoms flowrate implies that the base level should be controlled by reboiler heat input and not by bottoms flowrate. The temperature on stage 50 is controlled by manipulating the bottoms flowrate. It is important to note that the base-level controller must be set on automatic for the temperature controller to work because changing the bottoms flowrate has no direct effect on stage 50 temperature. The level loop is "nested" inside the temperature loop.

The key control structure issue is how to set the vapor sidestream flowrate. It certainly must change as disturbances enter the column. One effective way to achieve this is to ratio the sidestream flowrate to the reboiler heat input. This control structure is shown in Figure 10.11. The two temperature controllers (TC3 and TC50) have 1-min deadtimes and 100 K temperature transmitter spans. They are tuned by running a relay-feedback test and using the Tyreus-Luyben settings. The TC3 controller manipulating reflux flowrate is tuned first with the TC50 controller on manual. Controller tuning constants are KC = 0.46 and tj = 9.2 min. Then the TC50 controller manipulating bottoms flowrate is tuned with the TC3 controller on automatic (sequential tuning). The tuning constants are KC = 3.8 and tj = 30 min. Note that this second loop is slower than the first. Controller faceplates are shown in Figure 10.12. The FCD flow controller is "on cascade" since its setpoint comes from the TC3 temperature controller. The FCS flow controller is also "on cascade" since its setpoint comes from the S/QR ratio. Note that the controller output signal for the bottoms flow controller FCB is only 14%. The control valve size and the bottoms pump power were increased to provide more rangeability of the very small bottoms flowrate.

The effectiveness of this control structure is demonstrated in Figures 10.13 and 10.14 for feed rate and feed composition disturbances. Responses to step increases and decreases dead3

Figure 10.11 Control structure for vapor sidestream column.

Figure 10.11 Control structure for vapor sidestream column.

Figure 10.12 Controller faceplates.

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