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Tj = minimum batch time for the separation using lowest plate holdup (2%) T2 = minimum batch time for the separation using optimal plate holdup Ts = % time saved = (T, - T2) x 100/7,

Tj = minimum batch time for the separation using lowest plate holdup (2%) T2 = minimum batch time for the separation using optimal plate holdup Ts = % time saved = (T, - T2) x 100/7,

3.5.3.1. Effect of Plate Holdup on the Column Performance

Column configurations (NT) and separation requirements (x*D) for several cases are presented in Table 3.3. The condenser holdup was fixed to 2% of the total initial charge and the column hold up is varied as a percentage of the total initial charge to the column. The initial charge to the column (B0) is 5 kmol with a light component mole fraction (xB0) of 0.6. Also the amount of distillate product required (D*) is set to 3 kmol. The column operates under constant condenser vapour load strategy (section 3.2.2) with a vapour load of 3 kmol/hr for all cases.

fitmnt holdup

Figure 3.18a. Minimum Batch Time vs Column Holdup at different q'

fitmnt holdup

Figure 3.18a. Minimum Batch Time vs Column Holdup at different q'

A series of minimum time problems (Chapter 5) were solved at different values of q with increasing holdup for each case. Figures 3.18a and 3.18b show the minimum time solution vs. percent total holdup in the column for different mixtures at different q and Figures 3.19a and 3.19b show the corresponding optimum reflux ratio (required to get the separation in minimum time) vs. percent total holdup of the column. The results are summarized in Table 3.3 which shows, for each given separation, the optimum value of holdup to achieve the best performance out of the given column. The corresponding best minimum batch time and the optimum reflux ratio to achieve that are also presented in the table for each case.

The last column of Table 3.3 shows the percent reduction in batch time achieved using the optimum plate holdup compared to the batch time with the lowest plate holdup (2%). It clearly shows that the column performance in terms of minimum batch time is improved significantly with increasing plate holdup for easy separation {q < 0.60). For one case 23% batch time saving is observed (case 10).

c Reprinted from Chem. Eng. Set, 53, Mujtaba, I.M. and Macchietto, S., Holdup issues in batch distillation-binary mixture, 2519-2530 , Copyright (1998), with permission from Elsevier Science .

However, for difficult separation (q > 0.60) this is reversed. Figures 3.18a and 3.18b clearly show that for q > 0.60 the column performance, in terms of minimum batch time, is improved significantly with decreasing plate holdup and suggests that for difficult separations the column holdup should be kept as minimum as possible. This is also clear from the results presented in Table 3.3 which show that for difficult separations optimum column holdup is very close to the minimum (Mujtaba and Macchietto, 1998 used 2% as the minimum column holdup). The minimum batch times for both cases (using minimum and optimum holdup) are almost alike and no time saving could be realized when compared to one another (last column of Table 3.3). The results discussed so far clearly show that holdup may have a dramatic effect on the operation.

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