| Microchannel reactors are a fundamental building block of chemical reaction engineering. One may find them in a permeable catalytic material as an idealized geometry for a single pore, in a fabricated microreactor, or as a cell in a monolith honeycomb. At different scales, all these structures materialize the concept of process intensification through mass/heat transfer enhancement and/or by miniaturization. The behavior of these systems is determined by the interplay of convection, diffusion and reaction in the open channel and surrounding catalyst domains. In this study, we propose an analysis of these interactions, using scaling and approximate analytical methods. First, we considered the determination of the conversion of reactant in the problem of mass transfer in channel flow with finite linear wall kinetics, for different degrees of the concentration profile development. Then, the analysis was extended to the case where a nonlinear reaction occurs, in the limits of kinetic and mass transfer control. These results were compared with numerical simulations and found to be in reasonable agreement.;A uniformly valid description concerning the degree of development of the concentration (or temperature) profile was also pursued. For this purpose, we developed the application of an asymptotic technique to the series which is the solution of the classical problem. The result can be written as a combination of the predictions from limiting theories and the intermediate region appears well characterized. The extension of this transition zone is bounded between the inlet regime length and the distance at which the profile can be considered fully developed, both given explicitly in terms of the parameters and of an appropriate criterion which can be set as desired.;Concerning the competition between mass transfer towards the catalyst and reaction at the wall, scaling analysis suggests that the correct scales for external and internal transport should be included in the criterion for diffusional limitation. This gives origin to the concept of a rescaled Damkohler number Da* . These order-of-magnitude predictions are confirmed in more detail by correlations for the degree of mass transfer control theta . It is proposed that boundaries for kinetic and mass transfer control should be plotted for specified values of theta in a diagram, with the Damkohler and Graetz's numbers as axes.;At the scale of the catalyst coating, the internal reaction-diffusion processes were considered and boundaries between limits derived explicitly in terms of the operating temperature. The relationship with regimes defined by external phenomena was examined and the dimensionless group which establishes the overall picture in diffusional limitations at both channel and catalytic coating was identified. An improved calculation method for the effectiveness factor was proposed, based on a typical geometrical characteristic of thin coatings. This has found application in the description of nonlinear kinetics, nonuniform geometries and egg-shell catalyst particles.;The interaction between the same mechanisms appears in the analysis of perfusive catalyst particles and walls, where intraparticular convection is possible due to the existence of 'large pores'. We have derived an expression for the effectiveness factor in a monolith with a permeable wall and shown that the conditions under which the performance enhancement is maximum correspond to strong convective transport, but only if this is 'matched' by a fast reaction. In a slab-shaped catalyst with a zeroth-order exothermic reaction, we estimated the effectiveness factor and maximum temperature in a number of regimes, represented in an operating diagram with axes defined by the intraparticular Peclet number, Thiele modulus and Lewis number. |
