Transverse External Mass Transfer

The existence of transverse gradients is associated with external mass transfer limitation. The lack of gradients in a CFD simulation is associated with kinetically limited processes. A more quantitative measure of transport or kinetic limitations can be done using an estimate (order of magnitude) of the second Damkholer 

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External mass transport. Involves diffusion of species from gas phase to the surface of the coated catalyst. This may be estimated as the transverse diffusion or external mass transfer time %) defined as follows (Equation (4.1)) ... 

Additionally, two parameters are used for estimating the effect of mass transfer phenomena. First, the transverse Peclet number (P), which is defined as the ratio between characteristic times for transverse (to convective flow) gas-phase diffusion and convection process. The magnitude of P (Equation (4.6)) allows establishing the upper bound on conversion that can be attained in a monolith in the external mass transfer controlled regime ... 

Effect of the face gas velocity (mass transfer limitations). It is well known how these monolithic catalysts work in laminar flow because of the small size of their channels. There are transversal gradients of concentration in them, thus. This effect has been quantify in this research by two ways 1st) Some tests were made with the same gas hourly space-velocity [10,000 h (450 C)] varying the face gas velocity (equivalent to Ae superficial gas velocity in fixed beds) from 28 to 83 cm/s, The monoliths were then cut to different lengths (from 10 to 30 cm). Results on oxidation of EC (1,000 ppm at inlet) are shown in Figure 3 for two BASF catalysts, and indicate some external diffusion control at low face gas velocities. To keep it to a minimum extent, most of further tests were made at the maximum (in this facility) face velocity 83 cm/s. 2nd) Mass transfer limitations in these monoliths have also been studied by the well established procedures in chemical engineering, following the detailed... 

This noninvasive method could allow the differentiation between the various packing materials used in chromatography, a correlation between the chromatographic properties of these materials that are controlled by the mass transfer kinetics e.g., the coliunn efficiency) and the internal tortuosity and pore coimectivity of their particles. It could also provide an original, accurate, and independent method of determination of the mass transfer resistances, especially at high mobile phase velocities, and of the dependence of these properties on the internal and external porosities, on the average pore size and on the parameters of the pore size distributions. It could be possible to determine local fluctuations of the coliunn external porosity, of its external tortuosity, of the mobile phase velocity, of the axial and transverse dispersion coefficients, and of the parameters of the mass transfer kinetics discussed in the present work. Further studies along these lines are certainly warranted. 

Heat and mass transfer coefficients can be used to interrogate the importance of external transport phenomena and how to choose reactor size. The latter controls (i) pressure drop, (ii) residence time and thus reactant conversion or flow rate and thus power generated, (iri) the effective reaction rate and thus the process efficiency, (iv) the temperature and (v) whether a system is kinetically controlled and thus ideal for extraction of catalytic kinetics. Another application of Nu and Sh is that a 2D or 3D problem can be reduced to a computationally tractable problem by approximating the transverse transport phenomena using overall transport correlations. Such pseudo-2 D models (also called heterogeneous ID models for catalytic systems) have been used to explore the stability and performance of microbumers with a significantly lower computational effort than CFD models (e.g. [23-25]). 

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