Interplay between channel and catalyst operating regimes in wall-coated microreactors

When mass transfer to the wall of a microchannel interacts with the nonlinear reaction–diffusion processes occurring inside the catalyst coating, several operating regimes are possible in the overall system behavior. The conditions for which these regimes occur and the transition criteria are derived here using theoretical analyses for mass transfer in a catalytic layer and in channel flow with a nonlinear wall reaction. The results are expressed in terms of the variables of operation (in temperature diagrams), inlet reference conditions and criteria which can be set as strict as desired (the effectiveness factor or the degree of mass transfer control). Two main behaviors are identified regarding the major source for transport limitation, for different values of internal and external “diffusion velocities”. The transition between them is identified and the implications on the strategies to achieve overall kinetic or mass transfer control are discussed. Our analysis allow us to map all regimes analytically (inclusively for nonlinear kinetics), whereas in previous literature, numerical evaluation is required even for the simpler case of first-order reactions. The more complex effects introduced by kinetic nonlinearities are highlighted, namely the influence of the reactant local distribution.

► Analytical description of boundaries for overall and localized controlling regimes.
► Regime mapping in temperature-Graetz and temperature-Thiele diagrams.
► Transition between internal and external mass transfer limiting behaviors.
► Effects of nonlinear kinetics on regime boundaries.
► Limitations in common experimental tests for mass transfer resistance evaluation.