Fully Reactive Distillation Column

In Fig. 5.3, a fully RD column is depicted in which the chemical reaction Aj A2 takes place. The educt Aj is fed at the point of its highest concentration within the column, that is the reboiler. In general, the Damkohler numbers of the reboiler, DUfeb. of the condenser, nd of the reactive stages, Da, can be chosen independently. Their corresponding mass balances are given by 

For design purposes, it is desirable to know the maximum attainable distillate concentration of the reaction product Aj. This composition can be found from the intersection of the reactive condenser operating line with the VLE line. At the intersection point, the following equation is valid 

In a similar manner, the minimum attainable bottom concentration is determined 

For R S 1, (5.23) and (5.24) correspond to the feasibility relations proposed first by Chadda et al. [3], which were derived from a cascadic flash calculation. From Fig. 5.10, one can see that the attainable top concentration for n = 1 is continuously decreasing with increasing Damkohler number Da aa due to the increased intensity of the backward reaction of A2 to Aj in the upper column section. For Da on = approaches the chemical equilibrium composition Similar trends 

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 of this configuration can be estimated from (5.17) and (5.15). This R,ni 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. 

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For example 2, Figs. 4.4(a) and (b) show the bifurcations of all singular points with respect to the Damkohler numbers of the reactive condenser and the reactive reboiler, respectively. As can be seen from the feasibility diagram in Fig. 4.4(c), at Damkohler numbers Dac > 0.830, two possible condenser products - that is, the top products of a fully reactive distillation column, are predicted. The kinetic azeotrope in the reactive reboiler is always the possible bottom product of a column. 

Figure 4.9(a) and (b) illustrate the system behavior at a total pressure of 15 atm and 8 atm, respectively. As can be seen from the location of the PSPS, this system has similar features as the ideal system example 1 which has an elhpse-shaped PSPS (see Fig. 4.2(a)), as discussed above. Due to the boiling sequence of the reaction components, the PSPS is fully located outside the physically relevant composition space and, as a consequence, no reactive azeotrope can appear. It is worth noting that inside the phase-splitting region, the PSPS of the real heterogeneous system and the PSPS of the pseudohomogeneous system are different However, this does not affect the feasible top and bottom products of a fully reactive distillation column. 

Knowledge of the equilibrium is a fundamental prerequisite for the design of non-reactive as well as reactive distillation processes. However, the equilibrium in reactive distillation systems is more complex since the chemical equilibrium is superimposed on the vapor-liquid equilibrium. Surprisingly, the combination of reaction and distillation might lead to the formation of reactive azeotropes. This phenomenon has been described theoretically [2] and experimentally [3] and adds new considerations to feasibility analysis in RD [4]. Such reactive azeotropes cause the same difficulties and limitations in reactive distillation as azeotropes do in conventional distillation. On the basis of thermodynamic methods it is well known that feasibility should be assessed at the limit of established physical and chemical equilibrium. Unfortunately, we mostly deal with systems in the kinetic regime caused by finite reaction rates, mass transfer limitations and/or slow side-reactions. This might lead to different column structures depending on the severity of the kinetic limitations [5], However, feasibility studies should identify new column sequences, for example fully reactive columns, non-reactive columns, and/or hybrid columns, that deserve more detailed evaluation. 

The integration of the recycling reactor directly into the distillation column leads to the process configuration shown in Fig. 5.2, in which the reaction takes place within the column total reboiler. On top of the reboiler a fully non-RD section is installed. This process can be seen as a simple hybrid RD column with only one reactive tray. Comparing the curves in Fig. 5.4 and Fig. 5.6, the operational characteristics of the two processes, recycling system and distillation column with reactive reboiler, are identical at 93 = 00 and R = co. 

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