Reactor with internal transport limitation

A unique benefit of biofilm reactors for research purposes is the fact that due to the distinct separation between the L and the S phase (difference in density — Pl) high relative velocities can be realized. As a result, external transport limitation can be excluded or easily studied simultaneously with internal transport limitations in the case of uniform and controlled biofilm thickness. In this respect, biofilm reactors are superior to the conventional... 

Heterogeneously catalyzed reactions. Macroscopic fluid models are combined with microscopic transport models in the catalyst particles to describe how concentration changes with time and position in a catalytic reactor. Special considerations must be given to the selection of experimental temperature and catalyst particle size to minimize (and hopefully eliminate) internal transport limitations on the catalytic reaction rate. The next requirement is that the flow pattern in the reactor Is accurately represented by the well-mixed or plug-flow assumption. The subsequent discussion applies to gas-phase reactants. 

The internal recycle reactor as described elsewhere ( ) used for the experiments was charged with about 60 g of catalyst which was thermally pretreated and reduced with hydrogen before the synthesis reaction. During the synthesis recycle ratios (recycled volume per time and weight of catalyst divided by space velocity under operating conditions) of more than 20 were used to establish ideal mixing as well as isothermal operation and to avoid transport limitation due to film resistance. 

For interphase limitations (boundary layer effects) the situation seems, at first glance, as simple as that for internal gradients, since most correlations for heat-and mass-transfer eoeffieients show a proportionality to the flow velocity of the surrounding fluid, u", where normally 0.6 < n < 1. At the lower velocities associated in particular with laboratory reactor operation, however, n tends to be closer to 0.6 than to 1, and the transport coefficients become insensitive to flow velocity and changing flow velocity is not an effective diagnostic. 

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... 

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