in feed flowrate of 20% are shown in Figure 10.13a. Stable base-level regulatory control is achieved. Note that the bottoms flowrate saturates wide open or completely closed during these transients because of its small flowrate. Product purities return fairly close to their desired levels, but there is a large transient deviation in the bottoms purity for the +20% step increase.
The use of a steam to feed ratio greatly improves this response, as shown in Figure 10.13b. The base-level control output signal resets the reboiler heat input to feed ratio. The steady-state ratio is 0.038689 (in the required Aspen Dynamic units of "GJ/h" per "kmol/h"). The output range of the level controller is changed to 0-0.1, and the TC50 controller is retuned (KC = 3.2 and tj = 29 min). The deviation in bottoms purity is greatly reduced, and saturation of the bottoms valve is also avoided.
Responses to three feed composition disturbances are given in Figure 10.14. The solid lines indicate when the BuOH concentration in the feed is increased from 5 to 7 mol%, with the MeOH reduced accordingly. The dashed lines are when the BuOH concentration in the feed is decreased from 5 to 3 mol%, with the MeOH increased accordingly. The dotted lines indicate when the DME concentration in the feed is decreased from 45 to 40 mol%, with the MeOH increased accordingly. Product purities are held fairly close to their desired values, with the sidestream BuOH composition undergoing the largest changes.
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