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Importance of Reaction Kinetics for Catalytic Distillation Processes

K. Sundmacher and Z. Qi 5.1

Introduction

Reactive distillation (RD) processes are often designed from equilibrium assumptions for chemical reactions and interphase phenomena. Equilibrium-based design leads to important implications for the feasibility of RD. However, most of the chemical reactions carried out in RD columns are kinetically controlled and they can be strongly affected by the limitations of inter- or intraphase mass- and energy-transport resistances.

In the first part of this chapter, the general influence of reaction kinetics is discussed using isomerization in an ideal binary mixture as a simple model reaction. For this system, first design principles and important parametric dependencies are discussed.

In the second part, the possible products of kinetically controlled catalytic distillation processes are analyzed using residue curve maps. Ideal, as well as non-ideal, ternary mixtures are considered. Current research activities are presented that are focussed on reaction systems exhibiting liquid-phase splitting phenomena such as the hydration of cyclohexene to cyclohexanol at strongly acidic catalyst particles.

In the third part of this chapter, the experimental determination and the detailed theoretical analysis of reaction kinetics obtained at catalysts used in RD processes are discussed. For reliable column design, activity based microkinetic rate expressions are applied successfully to heterogeneously catalyzed processes. By increasing the particle size of heterogeneous catalysts to be used in RD processes, mass-transport resistances can become relevant and have to be considered for reliable column simulations. This is exemplified by the industrially relevant syntheses of the fuel ethers MTBE and ETBE.

Reactive Ideal Binary Mixtures

First, the role of reaction kinetics is analyzed considering RD processes for the simple reversible reaction Aj o A2 in an ideal binary mixture. The educt Aj is assumed to be the reaction component with the higher boiling point, so the product A2 is obtained in the distillate. The reaction can be carried out in an RD column sequence with an external recycling loop (Fig. 5.1), a non-RD column on top of a reactive reboiler (Fig. 5.2), or a full RD column (Fig. 5.3). More possible configurations are analyzed elsewhere [1].

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