Overall mass transfer coefficients and controlling regimes in catalytic monoliths

It is well known that the performance of a catalytic monolith is bounded by two limits: the kinetic regime at low temperatures (or before ignition for the case of exothermic reactions) and the external mass transfer controlled regime at sufficiently high temperatures (or after ignition). The washcoat diffusional resistance can also be significant over an intermediate range of temperatures. The transition temperatures at which the controlling regime changes from kinetic to washcoat diffusion to external mass transfer depend on the various geometric properties of the monolith, flow properties, the catalyst loading and washcoat properties. We present analytical criteria for determining these transition temperatures. These are derived using the recently developed low-dimensional model and the concepts of internal and external mass transfer coefficients. The criteria are more general than those in the literature and are useful in analyzing the experimental data. Further, we present an explicit expression for the experimentally measurable dimensionless apparent mass transfer coefficient (Sh(app)) in terms of individual transfer coefficients in each phase. It is shown that Sh(app) can be lowered by orders of magnitude compared to the theoretical upper bound obtained in the limit of external mass transfer control. Low values of Sh(app) are obtained due to a small value of effective diffusivity in the washcoat, low catalyst loading or a reaction with low activation energy. The analytical criteria may be used for the design of monolith properties and experimental conditions so that the performance of the monolith approaches the upper limit defined by the external mass transfer controlled limit. (C) 2009 Elsevier Ltd. All rights reserved.