As has been outlined in the first sections of this chapter, RD processes can efficiently replace reactor-separator flow-sheets by internalization of external recycling loops. This is demonstrated for a simple isomerization reaction in an ideal binary mixture. It is clearly shown that hybrid columns combining non-reactive and reactive sections overcome the restrictions of fully RD columns. The most simple and effective solution for isomerization reactions is a reactive total reboiler with a non-reactive column on top.
The kinetics of a chemical reaction have a significant influence on the products that can be attained from a RD process. The attainable products of counter-current RD columns of infinite height operated at infinite reflux ratio can be obtained as singular points of a reactive reboiler batch process (bottom product) or a reactive condenser batch process (distillate product). The compositions of both products are located on a unique singular point curve. This curve is independent of any special type of reaction kinetics. However, the locations of the top and bottom products on this curve depend on the structure of the rate equation and on the intensity of the reaction (Damkohler number) in the considered reaction system.
In the last sections of this chapter, the determination and analysis of reaction kinetics is discussed. Dimensionless parameter groups are a very efficient tool for evaluating quickly the importance of the physicochemical phenomena occurring in RD columns. Since most of the reactions in RD columns take place in nonideal liquid mixtures at heterogeneous catalysts, the use of activity-based Langmuir sorption isotherms is recommended. This approach results in microkinetic rate expressions in terms of liquid-phase activities. As a most important advantage, these rate equations coincide with the chemical equilibrium in the limiting case of vanishing reaction rate.
The steady-state process behavior of counter-current RD columns operated with heterogeneous catalysts depends strongly on the reaction macrokinetics. This is illustrated using the formation of MTBE in a packed RD column as an example.
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