Importance of Reaction Kinetics for Catalytic Distillation Processes

Importance of Reaction Kinetics for Catalytic Distillation Processes 

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 partides. 

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. 

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Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

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