Microkinetic analysis of heterogeneous catalytic systems

Department of Chemical Engineering, Bogazi University, Istanbul, Turkey 

This chapter deals with the microkinetics of gas-solid catalytic reaction systems. An applied approach is adopted in the discussion, which starts with the formulation of intrinsic rate equations that account for chemical processes of adsorption and surfece reaction on solid catalysts and then proceeds with the construction of global rate expressions that include the individual and simultaneous effects of physical external and internal mass and heat transport phenomena occurring at the particle scale. 

Accordingly, in order to arrive at the rate equation(s) appropriate for macrokinetic analysis at the reactor scale, microkinetic analysis has to take into account several chemical and physical rate processes at the particle scale  

1 Transfer of reactant(s) from the bulk gas stream to the exterior catalyst surface 

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Hydrogenation is an important industrial reaction that often requires the presence of a heterogeneous catalyst to achieve commercial yields. Ethylene, C2H4, is the smallest olefin that can be used to investigate the addition of hydrogen atoms to a carbon-carbon double bond. Even though many experiments and theoretical studies have been carried out on this simple system, the reaction is still not completely understood. Microkinetic analysis provides insights into the relevant elementary steps in the catalytic cycle. 

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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