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Figure 6.15 MeAc/MeOH/H2O—changes in temperature profile for MeAc and water feed composition changes with product purities fixed.

6.5.4 Invariant Temperature Criterion

Figure 6.15 gives the changes in the temperature profiles for three different feed compositions. The design feed composition is 30/50/20 mol% MeAc/MeOH/H2O. The impurities in the bottoms and in the distillate are kept constant at 0.1 mol% MeAc and 0.1 mol% H2O, respectively. The solid line shows a feed composition in which the MeAc composition is decreased from 30 to 25 mol% and the MeOH composition is increased from 50 to 55 mol%. The dashed line indicates a feed composition in which the MeAc composition is increased from 30 to 33 mol% and the MeOH composition decreased from 50 to 47 mol%. The dotted line indicates a feed composition in which the H2O composition is increased from 20 to 25 mol% and the MeOH composition is decreased from 50 to 45 mol%.

Results show that the temperature on stage 41 does not change for constant product impurities for all of these feed composition disturbances. So there is a conflict between the SVD and the invariant-temperature criteria. One recommends stage 38, while the other recommends stage 41.

6.5.5 Minimum Product Variability Criterion

Figure 6.16 shows how product impurities change for two different temperature control trays as feed composition changes. In the first case, the temperature of stage 38 is fixed at 344.28 K. In the second case, the temperature of stage 41 is fixed at 349.71 K.

The other control degree of freedom that is fixed in this figure is the reflux ratio. Unlike the other systems studied, Table 6.1 shows that the required changes in reflux ratio are smaller than the required changes in the reflux flowrate in this multicomponent, nonideal system. The reflux ratio is fixed at 0.732.

Stage

Figure 6.15 MeAc/MeOH/H2O—changes in temperature profile for MeAc and water feed composition changes with product purities fixed.

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