Experimental and modeling studies for real time simulations of catalytic monolithic reactors.

摘要

We present accurate low-dimensional models for real time simulation, control and optimization of monolithic catalytic converters used in automobile exhaust treatment. These are derived directly by averaging the governing equations and using the concepts of internal and external mass transfer coefficients. They are expressed in terms of three concentration and two temperature modes and include washcoat diffusional effects without using the concept of the effectiveness factor. The models reduce to the classical two-phase models in the limit of vanishingly thin washcoat. The models are validated by simulating the transient behavior of a three-way converter for various cases and comparing the predictions with detailed solutions. It is shown that these new models are robust and accurate with practically acceptable error, speed up the computations by orders of magnitude, and can be used with confidence for the real time simulation and control of monolithic and other catalytic reactors.;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. Further, the criteria are used to evaluate the controlling regimes during the oxidation of H2, C3H6 and CH4 on a Pt/Al2O3 monolithic catalyst. The analysis reveals that methane oxidation is kinetically limited over a wide range of temperatures whereas the propylene oxidation has a more classical transition between a kinetic and external transport limited regime. Finally, we demonstrated the application of the analysis for optimum design of catalytic monoliths.