Heat transfer in heterogeneous catalysis

Physico-chemical aspects of mass and heat transfer in heterogeneous catalysis. 

J.J. Carberry, Physico-Chemical Aspects of Mass and Heat Transfer in Heterogeneous Catalysis in Catalysis, Science and Technology, Vol. 8, Springer, Berlin, 1987, p. 131. 

Hetereogeneous catalysis in industrial applications have a variety of different geometrical shapes and porosity. Therefore, understanding of mass and heat transfer in heterogeneous catalysis is essential for the description of the processes involved in getting the reactants to the catalyst surface and removal or addition of energy from the catalyst particles. 

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

Comprehensive treatments of the theory and application of diffusion and chemical reaction have been given in the following classical works [D.A. Frank-Kamenetskii, Diffusion and Heat Transfer in Chemical Kinetics, 2nd ed., Plenum Press, NY, (1969) C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, MIT Press, Cambridge, MA, (1970) R. Aris, The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts, Clarendon Press, Oxford, England, (1975)]. 

Kondelik, P. J., J. Horak and J. Tesarova. Heat and Mass Transfer in Heterogeneous Catalysis. Variations of Local Void Fraction in Randomly Packed Beds of Equilateral Cylinders. Inc. Eng. Chem. Proc. Des. Devel. 7 (1968) 250-252. 

The simulation of reacting flows in packed tubes by CFD is still in its earliest stages. So far, only isothermal surface reactions for simplified geometries and elementary reactions have been attempted. Heterogeneous catalysis with diffusion, reaction, and heat transfer in solid particles coupled to the flow, species, and temperature fields external to the particles remains a challenge for the future. 

By definition, the turnover frequency is expressed per number of active sites. So, catalytic samples that differ only in the amount active sites must exhibit the same values of turnover frequency. If not, heat and mass transfer phenomena are present. Specifically, the correct measurement of intrinsic kinetic data in heterogeneous catalysis is difficult due to the effect of heat and mass transfer, especially inside the pores of high specific-area materials. The turnover frequency reveals these phenomena. In other words, in the case of supported... 

The intrinsic catalytic properties of enzymes are modified either during immobilization or after they were immobilized [25-27], In heterogeneous catalysis such as is carried out by immobilized enzymes, the rate of reaction is determined not simply by pH, temperature and substrate solution, but by the rates of proton, heat and substrate transport, through the support matrix to the immobilized enzyme. In order to estimate this last phenomenon, we have studied the internal mass transfer limitation both in hexane and in SC C02, with different enzymatic support sizes. 

Detailed treatments of mass and heat transfer effects in heterogeneous catalysis can be found in standard texts of reaction engineering and catalysis [11-15], Here, a brief overview and analysis must suffice. 

Physical transport processes can play an especially important role in heterogeneous catalysis. Besides film diffusion on the gas/liquid boundary there can also be diffison of the reactants (products) through a boundary layer to (from) the external surface of the solid material and additionally diffusion of them through the porous interior to from the active catalyst sites. Heat and mass transfer processes influence the observed catalytic rates. For instance, as discussed previously the intrinsic rates of catalytic processes follow the Arrhenius... 

First elementary reaction steps at an isolated reaction center have been considered and then the increasing complexity of the catalytic stem when several reaction centers operate in parallel and communicate. This situation is common in heterogeneous catalysis. On the isolated reaction center, the key step is the self repair of the weakened or disrupted bonds of the catalyst once the catalytic cycle has been concluded. Catalytic systems which are comprised of autocatalytic elementary reaction steps and communication paths between different reaction centers, mediated through either mass or heat transfer, may show self-organizing features that result in oscillatory kinetics and spatial organization. Theory as well as experiment show that such self-organizing phenomena depend sensitively on the size of the catalytic system. When the system is too small, collective behavior is shut down. 

Chapter 4 concerns differential processes, which take place with respect to both time and position and which are normally formulated as partial differential equations. Applications include heterogeneous catalysis, tubular chemical reactors, differential mass transfer, heat exchangers and chromatography. It is shown that such problems can be solved with relative ease, by utilising a finite-differencing solution technique in the simulation approach. 

The most likely opportunities for exploitation will come from mass transfer-limited reactions and the combination of unit operations in one device. Examples of reactions mentioned earlier include polymerization, condensation reactions, crystallization, and heterogeneous catalysis. Combined unit operations are illustrated by reactive distillation, polymer devolatilization with pelletization, and the use of heat exchangers (reboilers and condensers) with distillation. 

In addition to the chemical steps, which are the only steps involved in stoichiometric or in homogeneous catalysis reactions, heterogeneous catalysis reactions involve also physical steps, i.e. transport (transfer) of organic molecules (and heat) from the reaction mixture to the active sites of the solid catalyst and vice versa.113-151 Another difference deals with the chemical steps, which do not occur in the fluid phase, but for part of them involve both fluid and solid phases (chemisorption and desorption), the other part occurring at the surface of the catalyst.113-151... 

Dittmeyer R, Emig G. Simultaneous heat and mass transfer and chemical reaction. In Ertl G, Knozinger H, Weitkamp J, editors. Handbook of heterogeneous catalysis. 2nd ed. Weinheim Wiley-VCH Verlag GmbH Co. KGaA 2008. p. 1727-84. 

Floyd, S. Choi, K.Y. Taylor, T.W. Ray, W.H. Polymerization of olefins through heterogeneous catalysis. IV. Modeling of heat and mass transfer resistance in the polymer particle boundary layer. J. Appl. Polym. Sci. 1986, 31, 2231-2265. 

G.W. Roberts, "The influence of mass and heat transfer on the performance of heterogeneous catalysts in gas/liquid/solid systems in P.N. Rylander and H. Greenfield (eds.). Catalysis in Organic Synthesis, Academic Press, New York (1976) 1. 

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