Influence of internal heat removal upon effectiveness, stability of an exothermic catalytic film: Implications for process intensification

Abstract

Microreactors employing thin catalytic films present a unique situation in catalyst design owing to the removal of symmetric internal boundary condition (i.e. zero heat flux) employed in catalyst design analysis to-date. The allowance for substantial heat removal and/or addition rates at the internal boundary of the catalytic film via process intensification (e.g. heat-exchanger reactors, recuperative combustors) can have significant implications on catalyst effectiveness and ignition/extinction behavior. This manuscript presents a simplified one-dimensional model of two catalytic thin films exchanging heat through a common dividing wall to demonstrate the importance of catalyst design for the cases of (i) internal cooling or heating of a catalytic film, and (ii) coupling of exothermic and endothermic reactions via separate catalytic films. The former case is analogous to microreactor designs for heat removal from an exothermic process, while the latter addresses catalyst design for heat-exchanger reactors coupling endothermic reactions (e.g. steam reforming) with exothermic reaction (e.g. partial oxidation). For a single film catalyzing an exothermic reaction, internal cooling was found to expand the region of steady-state multiplicity and simultaneously decrease catalyst effectiveness, whereas internal heating resulted in stabilization. For the case of two films separately catalyzing exothermic and endothermic reactions and exchanging heat via a common supporting wall, a combination of high wall thermal conductance and intermediate Thiele modulus for the endothermic reaction was capable of preventing ignition/extinction behavior while maintaining individual film effectivenesses in excess of unity. These findings not only give practical insight into the intrinsic behavior of catalysts employed in recuperative and heat-exchanger reactors, but also illustrate the need for future development of rigorous stability criteria for such systems.