The first example is the ternary mixture of methyl acetate, methanol, and water. In Chapter 5 we found that the economic optimum column has 27 stages, is fed on stage 21, has a reflux ratio of 1.002, and operates at 1.1 atm. The feed is 0.1 kmol/s of a 30/50/ 20 mol% mixture of MeAc/MeOH/H2O.
The diameter is 1.38 m. The diameters of the reflux drum and column base that give 10-min holdups are 1.34 and 1.27 m, respectively. The temperature gradient is the largest at stage 23, so this will be controlled at 342 K by manipulating reboiler heat input. The constant reflux ratio policy was found in Chapter 5 to be better than the constant reflux to feed policy, so this control structure will be implemented.
The file is exported to Aspen Dynamics, and the additional controllers are installed in the normal way. Figure 8.1 gives the control structure and the controller faceplates.
Note that the distillate mass flowrate is measured and fed to the multiplier "RR" as input1. Input2 is the constant 1.002. The output of the multiplier is the mass flowrate of the reflux stream.
A 1-min deadtime is used in the temperature loop. Relay-feedback testing gives controller tuning constants KC = 2.0 and tj = 7.9 min. The temperature transmitter span is 300-400 K.
Figure 8.2 shows the response of the system to positive and negative 20% step changes in feed flowrate. The control structure provides stable regulatory level control. Note that the reflux flow changes directly with the distillate.
Distillation Design and Control Using Aspen™ Simulation, By William L. Luyben Copyright © 2006 John Wiley & Sons, Inc.
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