Physico-Chemical Aspects of Mass and Heat Transfer in Heterogeneous Catalysis

Heterogeneous catalysis involves, by definition, at least two phases and thus transport of mass and heat between phases become steps which in concert with chemical events, dictate global catalytic behaviour. Indeed transport of heat and mass within the phase of reaction (the catalyst) is of potential importance — in fact of greater significance than is transport between phases. Gradients of concentration and temperature within the usually porous catalyst phase are termed intraphase while those which surround the catalyst are aptly termed interphase. Intra and interphase gradients are local or short range in extent. They exist at localized points within the catalytic reactor which itself may be marked by long-range gradients of concentrations (C) and temperature (T). Thus the conventional fixed bed catalytic reactor hosting an exothermic solid catalyzed reaction will exhibit long range gradients in C and T, i.e. between bed inlet and exit and, at any axial position, between bed center line and wall. At any axial and radial local position, local, short range gradients in C and T can be anticipated.