External interphase transport processes

For the heterogeneous catalytic process to be effective, the reactants present in the surrounding fluid phase must be transported to the surfece of the solid catalyst, and after the reaction, the products formed must be carried back from the surface to the bulk fluid. The path of the physical rate processes at the particle scale is divided into two parts, as depicted in the 7-step sequence of the continuous reaction model used in microkinetic analysis  

1 Transfer of reactants from the bulk fluid to the exterior surface of the catalyst particle (external or interphase transport) 

2 Diffusion of reactants from the exterior surface to the active interior surface of the porous particle (internal or intraparticle transport) 

The last two steps, steps 6 and 7, of the sequence involve the analogous transport of products back to the bulk fluid. Steps 1 and 7 are external physical rate processes which are in series with chemical steps 3-4-5 of adsorption-reaction-desorption in other words, they are separated from the chemical steps. Steps 2 and 6, on the other hand, are internal physical rate processes which are concurrent with chemical steps and require simultaneous treatment of chemical and physical rate processes. Heat transfer between bulk fluid and outer catalyst surfece and within the porous particle is also treated by the same reasoning. 

In this section, the catalyst particle or the catalytic surface is assumed to be nonporous, and the 7-step sequence is reduced to a 5-step sequence with steps 1, 3-4-5, and 7. In this case, the only mass transfer resistance involved is between the fluid and the outer surfece of the particle. The rates of external mass transfer depend on (i) the temperature, pressure, and physical properties of the fluid phase under these conditions, (ii) the gas velocity relative to the solid surface, and (iii) the intrinsic rate of the surface reaction. In other words, the rate at which mass is transported from the fluid to the surface is determined by the relative magnitudes of  

---

In the most general case, i.e. when intraparticlc and interphase transport processes have to be included in the analysis, the effectiveness factor depends on five dimensionless numbers, namely the Thiele modulus the Biot numbers for heat and mass transport Bih and Bim, the Prater number / , and the Arrhenius number y. Once external transport effects can be neglected, the number of parameters reduces to three, because the Biot numbers then approach infinity and can thus be discarded. 

Gas-solid (catalytic) reactions. Mass transfer is likely to be more important within the pellet than in the external film, and heat transfer more important in the film than within the pellet. In other words, intraphase mass transfer and interphase heat transfer would normally be the dominant transport processes. Thus the pellet can reasonably be assumed to be isothermal. 

Physical transport processes can play an especially important role in heterogeneous catalysis. Besides film diffusion on the gas/hquid boundary, there can also be diffusion of the reactants (products) through a boundary layer to (from) the external surface of the sohd material, as well as through the porous interior to (for reactants) or from (for products) the active catalyst sites. Fleat and mass transfer processes influence the observed catalytic rates. For instance, as discussed previously, the intrinsic rates of catalytic processes follow the Arrhenius law, while mass transfer hinders such pronounced dependence, decreasing the apparent activation energy. The intraparticle and interphase mass transfer coefficients display a lower temperature dependence as visualized in Fig. 10.2 and discussed later. 

---