Mass and Heat Transfer Effects on Heterogenous Catalytic Reactions

Mass and Heat Transfer Effects on Heterogenous Catalytic Reactions 

If the rates of the chemical steps 3-5 are comparable or higher than the transport processes 1, 2 and 6, 7, significant concentration profiles of and A2 inside the catalyst particle or in the surrounding layer will occur. If the intrinsic rates are very high as compared to the diffusion process in the pores, the reaction will take place only near the external surface, and the observed transformation rate will be controlled by the external mass transfer. The same situation is observed for non-porous pellets or so-called egg-shell catalysts, where the active phase is placed in a layer near the outer pellet surface. If the intrinsic reaction rate is comparable with the diffusion rate within the pores, a pronounced concentration profile of the reactant within the pellet will develop. 

As a consequence of the concentration profiles caused by the transfer phenomena, the observed (effective) reaction rates are modified compared to the rate, which would occur at constant bulk phase concentration. This effect is commonly characterized by an effectiveness factor as defined in Equation 2.132  

Besides the modification of the overall reaction rate, the product selectivity may be changed. This is discussed in detail in the following subsections. 

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Mass and Heat Transfer Effects on Heterogenous Catalytic Reactions 67 we obtain the ratio between heat and mass transfer coefficient ... 

The catalyst particle sizes and shapes (Figure 5.1) vary considerably depending on the reactor applications. In fixed beds, the particle size varies roughly between 1 mm and 1 cm, whereas for liquid-phase processes with suspended catalyst particles (slurry), finely dispersed particles (<100 xm) are used. Heterogeneous catalysis in catalytic reactors implies an interplay of chemical kinetics, thermodynamics, mass and heat transfer, and fluid dynamics. Laboratory experiments can often be carried out under conditions in which mass and heat transfer effects are suppressed. This is not typically the case with industrial catalysis. Thus, a large part of the discussion here is devoted to reaction-diffusion interaction in catalytic reactors. 

The rates at which chemical transformations take place are in some circumstances strongly influenced by mass and heat transfer processes (see Sections 12.3 to 12.5). In the design of heterogeneous catalytic reactors, it is essential to utilize a rate expression that takes into account the influence of physical transport processes on the rate at which reactants are converted to products. Smith (94) has popularized the use of the term global reaction rate to characterize the overall rate of transformation of reactants to products in the presence of heat and mass transfer limitations. We shall find this term convenient for use throughout the remainder of the chapter. Global rate expressions then include both external heat and mass transfer effects on the reaction rate and the efficiency with which the internal... 

While the above criteria are useful for diagnosing the effects of transport limitations on reaction rates of heterogeneous catalytic reactions, they require knowledge of many physical characteristics of the reacting system. Experimental properties like effective diffusivity in catalyst pores, heat and mass transfer coefficients at the fluid-particle interface, and the thermal conductivity of the catalyst are needed to utilize Equations (6.5.1) through (6.5.5). However, it is difficult to obtain accurate values of those critical parameters. For example, the diffusional characteristics of a catalyst may vary throughout a pellet because of the compression procedures used to form the final catalyst pellets. The accuracy of the heat transfer coefficient obtained from known correlations is also questionable because of the low flow rates and small particle sizes typically used in laboratory packed bed reactors. 

We have already considered steady-state one-dimensional diffusion in the introductory sections 1.4.1 and 1.4.2. Chemical reactions were excluded from these discussions. We now want to consider the effect of chemical reactions, firstly the reactions that occur in a catalytic reactor. These are heterogeneous reactions, which we understand to be reactions at the contact area between a reacting medium and the catalyst. It takes place at the surface, and can therefore be formulated as a boundary condition for a mass transfer problem. In contrast homogeneous reactions take place inside the medium. Inside each volume element, depending on the temperature, composition and pressure, new chemical compounds are generated from those already present. Each volume element can therefore be seen to be a source for the production of material, corresponding to a heat source in heat conduction processes. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

Since realistic wood feedstocks are heterogeneous, the uniform entity appropriate for fundamental studies to identify improved conditions for tar formation is the single particle. In it, the intraparticle conditions can be measured and related to process conditions that can be manipulated. In addition, the effect of heat and mass transfer rates on reaction products can be determined. The findings can be rationalized to all reactors in which the studied experimental conditions prevail. The investigation of reacting single particles has proven extremely successful in the development of catalytic reactors (2) and coal pyrolysis (3,4,5). 

Trickle-bed reactors, wherein gas and liquid reactants are contacted in a co-current down flow mode in the presence of heterogeneous catalysts, are used in a large number of industrial chemical processes. Being a multiphase catalytic reactor with complex hydrodynamics and mass transfer characteristics, the development of a generalized model for predicting the performance of such reactors is still a difficult task. However, due to its direct relevance to industrial-scale processes, several important aspects with respect to the influence of external and intraparticle mass transfer effects, partial wetting of catalyst particles and heat effects have been studied previously (Satterfield and Way (1972) Hanika et. al., (1975,1977,1981) Herskowitz and Mosseri (1983)). The previous work has mainly addressed the question of catalyst effectiveness under isothermal conditions and for simple kinetics. It is well known that most of the industrially important reactions represent complex reaction kinetics and very often multistep reactions. Very few attempts have been made on experimental verification of trickle-bed reactor models for multistep catalytic reactions in the previous work. 

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