temperature on stage 12 by manipulating the vapor sidestream flowrate. This is a direct-acting controller; an increase in stage 12 temperature increases the vapor sidestream flow-rate to the rectifier. The TC12 temperature controller is tuned by running a relay-feedback test with the other two controllers on manual and using the Tyreus-Luyben settings: KC = 0.22 and tj = 12 min.
The modified control structure provides stable regulatory control for both feed flowrate and feed composition. However, as shown in Figure 10.25, the 20% increase in feed flow-rate produces a large transient disturbance in the purity of the distillate from the main column with a peak xD1(MeOH) of mol% MeOH. This can be improved by using feedforward control. Installing a sidestream to heat input ratio as shown in Figure 10.26, with the ratio reset by the TC12 temperature controller, improves the load response of the system. The temperature controller is retuned (KC = 0.10 and tj = 12 min). A maximum controller output of 40 is used since the normal Si/Qri ratio is 21.
Figure 10.27 compares the responses to a 20% increase in feed flowrate for the two cases, with and without the use of the ratio. The improvement the purity of the DME product is striking. Figure 10.28a gives results for changes in the DME feed concentration from 35 mol% to either 40 or 30 mol% DME (with corresponding changes in MeOH). Figure 10.28b gives results for changes in the water feed concentration from 30 mol% to either 25 or 35 mol% H2O (with corresponding changes in MeOH). The control structure handles these many disturbances quite effectively.
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