Simulation of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions

The replacement of conventional-packed beds of pellets with "high conductivity" honeycomb catalysts in industrial externally cooled multitubular fixed-bed reactors is investigated by modeling and simulation for the oxidation of methanol to formaldehyde and for the epoxidation of ethylene to ethylene oxide, which involve a consecutive and a parallel reaction scheme, respectively. Results suggest that near-isothermal operation of the fixed-bed reactors can be achieved using monolithic catalyst supports based on relatively large volume fractions of highly conductive materials. Pressure drops are reduced to less than 1%. The selectivity is favored by the excellent control of the intraporous diffusional resistances resulting from the thin catalytic washcoats. Reactor designs based on larger tubes are feasible at the expense of greater volume fractions of catalyst support. A critical aspect is represented by the restrictions on the specific load of catalyst per reactor volume resulting from the poor adhesion of very thick catalyst layers onto metallic surfaces. Such a difficulty can be circumvented by maximizing the geometric surface area of the monolith (e.g. minimizing the honeycomb pitch), enhancing the catalytic activity (e.g. increasing the load of active components), and increasing the coolant temperature (if the selectivity is not adversely affected). (C) 2001 Elsevier Science B.V. All rights reserved.