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2 different number of stages N for process configuration in

Fig. 5.11. Mole fraction of product A2 in distillate versus stage Damkohler number at

2 different number of stages N for process configuration in

Fig. 5.11. Mole fraction of product A2 in distillate versus stage Damkohler number at

are obtained for other reaction orders. At n = 3 multiple attainable compositions are possible.

Fig. 5.11 illustrates how the attainable distillate composition xDmlx, predicted from (5.23), compares with the distillate concentration xD obtained from column simulations. At a finite number of trays N, above a certain Das/R ratio the chemical equilibrium xce is overcome and a maximum distillate mole fraction is obtained. Any further increase of the Damkohler number leads to a continuous decrease of the distillate concentration down to the chemical equilibrium level. Depending on the number of column stages N, the simulated maximum value of xD can be significantly lower than the maximum attainable distillate composition xDmax. As an example, for a 20-stage column with Dacon/R = 0.1, the compositions are xD = 0.82 from simulation and xDmax = 0.89 from (5.23). The McCabe-Thiele diagram in Fig. 5.12 reveals that the corresponding operating line of the RD section and the VLE line are in a tangent pinch situation at the given parameters.

Comparing the performance of the three different RD processes considered, one can conclude that a non-RD section on top of reactive total reboiler seems to be the best configuration for both productivity and reliability of design. The minimum reflux ratio Rmin of this configuration can be estimated from (5.17) and (5.15). This Rmin value can be significantly reduced by installation of a pre-reactor [1]. A fully RD column often suffers from splitting of the product in the upper column section owing to backward reaction.

Fig. 5.12. Real and attainable distillate product composition illustrated in McCabe-Thiele diagram for configuration in Fig. 5.3 (OL = operating line)

Fig. 5.12. Real and attainable distillate product composition illustrated in McCabe-Thiele diagram for configuration in Fig. 5.3 (OL = operating line)

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