Info

* Fixed

* Fixed

Reflux Ratio Levels 0.712 0.819 0.841 0.942 0.660 0.781

Case 1

Switching Times, hr t = 0 2.10 3.56 4.78 6.21 6.99 7.67

Reflux Ratio Levels 0.721 0.814 0.790 0.912 0.628 0.767

Case 2

SwitchingTimes.hr t = 0 2.14 3.55 4.33 6.08 6.66 7.43

Reflux Ratio Levels 0.708 0.784 0.817 0.919 0.663 0.821

Case 3

Switching Times, hr t = 0 1.82 3.01 4.07 5.91 6.86 7.77

Reflux Ratio Levels 0.719 0.812 0.721 0.905 0.589 0.719

Case 4

Switching Times, hr t = 0 2.11 3.53 4.20 5.93 6.22 6.90 Figure 6.5. Reflux Ratio Profiles for Ternary Distillation (Simple Model)11

In case 3, the sales value of the second main-cut is much higher compared to that of the first main-cut. Therefore, a larger recovery of component 2 is now obtained to maximise the profit. Relative to the base case, less of main-cut 1 is produced in a shorter time but with very little loss of valuable component 2 and with low reflux ratios (Figure 6.5). In off-cut 1, a higher composition of xJu = 0.70 ensures a small loss of component 2. In case 4, although the sales value of main-cut 1 is increased relative to the base case, the recovery of component 1 in main-cut 1 was similar to that of case 2. Any higher recovery would require longer processing

Reflux Ratio Levels 0.712 0.819 0.841 0.942 0.660 0.781

Switching Times, hr t = 0 2.10 3.56 4.78 6.21 6.99 7.67

Reflux Ratio Levels 0.721 0.814 0.790 0.912 0.628 0.767

SwitchingTimes.hr t = 0 2.14 3.55 4.33 6.08 6.66 7.43

Reflux Ratio Levels 0.708 0.784 0.817 0.919 0.663 0.821

Switching Times, hr t = 0 1.82 3.01 4.07 5.91 6.86 7.77

Reflux Ratio Levels 0.719 0.812 0.721 0.905 0.589 0.719

Switching Times, hr t = 0 2.11 3.53 4.20 5.93 6.22 6.90 Figure 6.5. Reflux Ratio Profiles for Ternary Distillation (Simple Model)11

time for task 1 and therefore would produce lower profit. The operation time for task 3 is much shorter than that in the base case, with lower recovery at lower reflux ratios. Also, a lower off-cut composition in this case significantly reduced the time required for task 2. In most cases high reflux ratios are required in task 2 (off-cut production) to ensure that the very high purity requirements for component 2 in the subsequent cut (task 3 operation) can be achieved.

Mujtaba and Macchietto (1993) reported that the outer loop solution typically required 4-6 function and 3-4 gradient evaluations with 7-10 function and 5-6 gradient evaluations in each inner loop problem. One complete function and gradient evaluation of the outer loop required about 8-10 minutes CPU and 15-18 minutes, respectively, on a SPARC-1 Workstation.

For the sake of comparison the case 2 problem of Table 6.4 was rerun with the same product specifications on the main-cuts but without any off-cut production (the STN for this operation is shown in Figure 6.7).

The results are summarised in Table 6.5. The accumulated and instant distillate composition profiles are shown in Figure 6.8. This operation requires a longer batch time and higher reflux ratios and gives more than 20% less profit compared to the results for case 2 (Table 6.4). This clearly shows the benefit of producing an intermediate off-cut for this specific separation problem.

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