Basics Of Reactive Distillation

Reactive distillation is attractive in those systems where certain chemical and phase equilibrium conditions exist. We will discuss some of its limitations in Section 1.4. Because there are many types of reactions, there are many types of reactive distillation columns. In this section we describe the ideal classical situation, which will serve to outline the basics of reactive distillation.

Consider the system in which the chemical reaction involves two reactants (A and B) producing two products (C and D). The reaction takes place in the liquid phase and is reversible.

For reactive distillation to work, we should be able to remove the products from the reactants by distillation. This implies that the products should be lighter and/or heavier than the reactants. In terms of the relative volatilities of the four components, an ideal case is when one product is the lightest and the other product is the heaviest, with the reactants being the intermediate boiling components.

Figure 1.1 presents the flowsheet of this ideal reactive distillation column. In this situation the lighter reactant A is fed into the lower section of the column but not at the very bottom. The heavier reactant B is fed into the upper section of the column but not at the very top. The middle of the column is the reactive section and contains Nrx trays. Figure 1.2 shows a single reactive tray on which the net reaction rate of the reversible reaction depends on the forward and backward specific reaction rates (kF and kB) and the liquid holdup (or amount of catalyst) on the tray (Mn). The vapor flowrates through the reaction section change from tray to tray because of the heat of the reaction.

As component A flows up the column, it reacts with descending B. Very light product C is quickly removed in the vapor phase from the reaction zone and flows up the column. Likewise, very heavy product D is quickly removed in the liquid phase and flows down the column.

The section of the column above where the fresh feed of B is introduced (the rectifying section with NR trays) separates light product C from all of the heavier components, so a distillate is produced that is fairly pure product C. The section of the column below where the fresh feed of A is introduced (the stripping section with NS trays) separates nn Trays

ns Trays

Figure 1.1 Ideal reactive distillation column.

ns Trays

Figure 1.1 Ideal reactive distillation column.

Rn - Mn (kF[T„)xnkxnB ~ kB(T„)xrGxno)

Figure 1.2 Reactive tray.

heavy product D from all of the lighter components, so a bottom is produced that is fairly pure product D. The reflux flowrate and the reboiler heat input can be manipulated to maintain these product purities. Figure 1.3 gives typical composition profiles for this ideal case. The specific numerical case has 30 total trays, consisting of 10 stripping trays, 10 reactive trays, and 10 rectifying trays. Trays are numbered from the bottom. Note that the concentrations of the reactants peak at their respective feed trays (tray 11 for A, tray 20 for B). The purities of the two products are both 95 mol%, with B the major impurity in the bottoms and A the major impurity in the distillate.

One of the most important design parameters for reactive distillation is column pressure. Pressure effects are much more pronounced in reactive distillation than in conventional distillation. In normal distillation, the column pressure is selected so that the separation is made easier (higher relative volatilities). In most systems this corresponds to low pressure. However, low pressure implies a low reflux-drum temperature and low-temperature coolant. The typical column pressure is set to give a reflux-drum temperature high enough (49 °C, 120 °F) to be able to use inexpensive cooling water in the condenser and not require the use of much more expensive refrigeration.

In reactive distillation, the temperatures in the column affect both the phase equilibrium and chemical kinetics (Fig. 1.4). A low temperature that gives high relative volatilities may

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