Heat and Mass Transfer in Monoliths in the Presence of an Exothermic Superficial Reaction

Heat and mass transfer phenomena are investigated in a catalytic monolith with a fast exothermic superficial reaction at fully developing laminar flow for different values of the kinetic parameters. The behavior of a channel of the monolith reactor is simulated with a three-dimensional model with cylindrical symmetry, whose unsteady Navier-Stokes equations, discretized by adopting the control volume approach, are solved by means of the CFD-ACE+ package. The perturbation generated by heat production associated with the reaction of the flow field, temperature and concentration profiles determines at the ignition of the reaction a new entrance effect with consequent enhancement of mass and heat transfer. Hence Nu and Sh curves exhibit a not monotonic trend along the dimensionless axial coordinate, with a spike in correspondence of the ignition. Such enhancement is strongly dependent on the kinetic parameters of the surface reaction, and is not predictable with available correlations. Mass and heat transfer coefficients collapse only if a different definition of Nu and Sh, expressed as functions of the actual driving force, embedding the adiabatic temperature rise and the kinetic parameters, is used.