Ternary Azeotropic System

Up to this point we have looked at systems with fairly ideal vapor-liquid equilibrium behavior. The last separation system examined is a highly nonideal ternary system of methyl acetate, methanol, and water. Methyl acetate and methanol form a homogeneous minimum-boiling azeotrope at 1.1 atm with a composition of 66.4 mol% methyl acetate and a temperature of 329 K. This means that the overhead product from the distillation column cannot have a composition greater than this azeotropic composition.

The design objectives are to produce a distillate product with 0.1 mol% water and a bottoms product with 0.1 mol% methyl acetate. The feed flowrate is 1kmol/s, and the design feed composition is 30 mol% methyl acetate (MeAc), 50 mol% methanol (MeOH), and 20 mol% water. Column pressure is set at 1.1 atm. The column has 42 stages and is fed on stage 36 (the stage that minimizes reboiler heat input at design feed composition). The reflux ratio required to achieve the specified purities is 0.7323.

6.5.1 Slope Criterion

The upper graph in Figure 6.13 gives the temperature profile at design conditions. The lower graph shows the differences between the temperatures on adjacent trays. The maximum change occurs on stage 37. There is also a large change in temperature at the very bottom of the column that is due to buildup of the heaviest component water.

6.5.2 Sensitivity Criterion

The upper graph in Figure 6.14 gives the openloop gains between tray temperatures and the two manipulated variables. These curves show that stage 38 is sensitive to changes in heat input and stage 27 is sensitive to changes in reflux.

6.5.3 SVD Criterion

The lower graph in Figure 6.14 gives the U\ and U2 values from the SVD analysis. The results suggest that stage 38 can be controlled by either reflux or reboiler heat input. There is a second smaller peak in U2 at about stage 28 that could be controlled by heat input. The singular values of the steady-state gain matrix are oj = 0.5965 and o2 = 0.0855, which gives a condition number CN = oj/o2 = 6.98.

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