Reduced order models for describing dispersion and reaction in monoliths

We consider a detailed model describing the transient diffusion, convection and reaction in a monolith channel with a porous washcoat layer and average it over the small transverse scales to obtain a reduced order model expressed in terms of physically meaningful or measurable concentration modes. We provide a physical interpretation of the various effective transport coefficients and also obtain the appropriate inlet and initial conditions to be used on the reduced order model. We examine various limiting cases, present the solutions of the reduced order model and compare our results with the exact solution and those in the literature. For the case of no reaction, the low-dimensional model may be expressed as a hyperbolic equation in terms of the cup-mixing concentration. For the steady-state case, the model may be expressed in multi-mode form with intra and interphase mass transfer coefficients. However, in the general transient case, we show that the traditional external mass transfer coefficient concept is not applicable as the interfacial flux depends on three concentration modes. Finally, we compare the predictions of the reduced order model with those of the traditional two-phase model (for time varying inlet conditions) and show that the latter may lead to significant errors due to neglect of the dispersion in the fluid phase.

The dispersion curve for non-reacting case has longer tail; and contribution of diffusion into washcoat on dispersion of a solute in a monolith may be large even for small transverse Peclet numbers, if the diffusivity of solute in the washcoat is much less as compared to that in the fluidFigure optionsDownload high-quality image (310 K)Download as PowerPoint slideHighlights
► A four-mode reduced order model is derived for the transient operation of a catalytic monolith.
► Accuracy of the model is validated by comparing with the exact solution for linear kinetics.
► Interfacial flux depends on interfacial as well as two external concentration modes.
► The dispersion curve has longer tail if the diffusivity of solute is small.