The effectiveness of this control structure is demonstrated in Figures 10.5 and 10.6. The responses of the system to 20% step increases and decreases in feed flowrate are shown in Figure 10.5. The compositions of all three products are returned to their specified values within ~ 1.5 h. The largest transient deviations occur in the bottoms composition. This could be improved by using a heat input to feed ratio.

Note that the increase in feed flowrate causes the distillate valve to go completely shut for about 20 min while the decrease in feed flowrate saturates the valve wide open. Remember that the distillate flowrate is very small. Note also in Figure 10.3 that the steady-state controller output signal to the distillate valve (V12) is only 13% instead of the normal 50%. The size of the control valve and the pump work exported from Aspen Plus were both increased to get more rangeability in the distillate flowrate. The valve size is changed by clicking the icon, then right-clicking and selecting Forms and All Variables. Then the value of COmax is increased (see Fig. 10.6).

To change the pump power, click the pump P12 icon, then right-click and select Forms and Configure (see Fig. 10.7). Use the dropdown arrow to change UseCurves from True to False, and increase the EpowerR.

Figure 10.8 gives responses for three feed composition disturbances. The solid lines show what happens when the DME in the feed is increased from 5 to 7 mol% DME (with MeOH reduced by a corresponding amount). The dashed lines show what

Q V12.AllVariables Table

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