Internal mass transfer criterion

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

To ensure the system is probing reactions in a kinetically controlled regime, the reaction conditions must be calculated to determine the value of the Wiesz-Prater criterion. This criterion uses measured values of the rate of reaction to determine if internal dififusion has an influence. Internal mass transfer effects can be neglected for values of the dimensionless number lower than 0.1. For example, taking a measured CPOX rate of 5.9 x 10 molcH4 s g results... 

Probably the most widely applied criterion is the one for internal mass transfer limitations in an isothermal catalyst particle, e.g. for pore diffusion. Due to Weisz and Prater (Advances in Catalysis 6 (1954) 143) no pore diffusion limitation occurs, if the Weisz modulus... 

To avoid internal mass transfer resistances in the porous catalytic layer, its thickness, must be limited. To ensure an effectiveness factor of > 0.95 in an isothermal catalyst layer, the following criterion must be fulfilled [14] ... 

In fluid-solid systems, the reaction takes place on the catalyst surface. Prior to this, the reactant molecules have first to reach the catalyst surface and, therefore, the rate of mass transfer is an important operational parameter (Figure 15.3). Two types of mass transfer need to be considered in fluid-solid reactions external and internal mass transfer. In particular, internal mass transfer limitations should be avoided, since they more often limit the performance of the reactor and more strongly influence the product selectivity. The internal mass transfer is characterized by an effectiveness factor, q, defined as the ratio of the observed reaction rate to that at constant concentration throughout the catalyst layer. To ensure an effectiveness factor of q > 0.95 in an isothermal catalyst layer, the following criterion must be fulfilled [16] ... 

Internal Mass Transfer (Pore Diffusion) The simplified criterion for exclusion of an influence of internal mass transfer is given for spherical particles, a first-order reaction, and the assumption Des= 0.1 Di g by Eq. (4.7.19) ... 

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