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Figure 6.16 (a) MeAc/MeOH/H2O—changes in product impurities for MeAc feed composition changes with fixed reflux ratio and stage temperature; (b) MeAc/MeOH/H2O—changes in product impurities for water feed composition changes with fixed reflux ratio and stage temperature.

Figure 6.16a gives results for changes in the MeAc composition of the feed. The control tray recommended by SVD (stage 38) does a better job in holding distillate close to its specified value of 0.1 mol%. The changes in the bottoms composition are about the same for both stages 38 and 41, but in opposite directions.

Figure 6.16b give results for changes in the water composition of the feed. Controlling the temperature on stage 38 keeps both products closer to their specification than when stage 41 is controlled.

6.5.6 Closedloop Multiplicity

One interesting feature of nonlinear systems is the possible appearance of multiple steady states. Most researchers have explored openloop multiplicity. We found that closedloop multiplicity occurs in the methyl acetate/methanol/water system.

The two product impurity levels are fixed at their specified values, and feed composition is varied over a range of values. The required reflux flowrate and reflux ratio are calculated for each case. Figure 6.17 shows what happens when we start with a feed composition of 30/50/20 mol% MeAc/MeOH/H2O. The reflux ratio is 0.732, and the reflux flowrate is 0.364 kmol/s with this feed composition. As the feed composition is increased, the required reflux and reflux ratio increase. At a feed composition of 33/47/20 mol% MeAc/MeOH/H2O, the reflux ratio is 0.862 and the reflux flowrate is 0.444 kmol/s. This is labeled "Profile 1" in Figure 6.17.

However, if the feed composition is changed to 34/46/20 mol% MeAc/MeOH/H2O, there is a huge increase in the required reflux ratio to 1.94 and the reflux flowrate to 0.991 kmol/s. Further increases in MeAc in the feed continue to increase RR and R.

Now if the feed composition is reduced back to 33/47/20 mol% MeAc/MeOH/H2O (labeled "Profile 2" in Fig. 6.17), the reflux ratio does not return to 0.862 but changes only to 1.87. Likewise, the reflux flowrate does not return to 0.444 kmol/s but changes

Feed comp (mf MeAc) Feed comp (mf MeAc)

Figure 6.17 MeAc/MeOH/H2O—closedloop multiplicity.

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