Light-off criterion and transient analysis of catalytic monoliths

A one-dimensional two-phase model is used to derive an analytical light-off criterion for a straight channeled catalytic monolith with washcoat, in which the flow is laminar. For the case of uniform catalyst loading and a first order reaction, the light-off criterion is given by Here, Tf,in is the inlet fluid temperature, ΔTad is the adiabatic temperature rise, is one-half the channel hydraulic radius (, , cross-section area, perimeter), L is the channel length, ū is the fluid velocity, De is the reactant effective diffusivity in the washcoat, δc is the effective washcoat thickness, kf is the fluid thermal conductivity and kv(Tf,in) is the first order rate constant per unit washcoat volume at the inlet fluid temperature. NuH,∞ is the asymptotic Nusselt number in the channel. The function f accounts for diffusional limitations in the washcoat and is given by f(ϕ)=1 for ϕ 0.5. The factor g(Peh) depends on the solid conductivity, or more precisely, the heat Peclet number, , where δw(kw) is the effective wall thickness (thermal conductivity). The function g(Peh) decreases monotonically from 2.718 for Peh=0 to unity for Peh=∞. We also show that if the second term is negligible and the first exceeds unity, then ignition occurs at the back-end. If the second term exceeds unity then ignition occurs at the front-end. If the sum exceeds unity with the second term less than unity and not negligible compared to the first term then ignition occurs in the middle of the channel. This analytical ignition criterion is verified by numerical simulations using an accurate transient model that uses position dependent heat and mass transfer coefficients. We show that the plot of exit concentration versus time consists of two regions: kinetically controlled transient region and the mass transfer controlled steady-state asymptote. For the case of high solid conductivity, we present an analytical expression for the transient time at which the monolith shifts from the kinetically controlled to the mass transfer controlled regime. We also determine the influence of various parameters such as the washcoat thickness, channel dimensions, catalyst loading and initial solid temperature on this transient time and the cumulative emissions. Examination of the influence of solid conduction and channel geometry on cumulative emissions showed that designs that are optimum for steady-state operation lead to higher transient emissions and vice versa. Finally, we discuss the transient and steady-state behavior of the monolith for the special case of Lef<1 (hydrogen oxidation).

[1]  Donald W. Schwendeman,et al.  Light-off Behavior of Catalytic Converters , 1994, SIAM J. Appl. Math..

[2]  J. Cavendish,et al.  Carbon Monoxide Oxidation in an Integral Reactor: Transient Response to Concentration Pulses in the Regime of Isothermal Multiplicities , 1978 .

[3]  L. Hegedus.  Temperature excursions in catalytic monoliths , 1975 .

[4]  Hsueh-Chia Chang,et al.  A theory for fast‐igniting catalytic converters , 1995 .

[5]  Bruce A. Finlayson,et al.  Mathematical models of the monolith catalytic converter: Part I. Development of model and application of orthogonal collocation , 1976 .

[6]  V. Balakotaiah.  Simple runaway criteria for cooled reactors , 1989 .

[7]  Jacob A. Moulijn,et al.  Monoliths in Heterogeneous Catalysis , 1994 .

[8]  James Wei.  The Catalytic Muffler , 1975 .

[9]  R. E. Hayes,et al.  Introduction to Catalytic Combustion , 1998 .

[10]  S. Oh,et al.  Multiple steady states in an isothermal, integral reactor: the catalytic oxidation of carbon monoxide over platinum--alumina , 1977 .

[11]  Pio Forzatti,et al.  Adequacy of lumped parameter models for SCR reactors with monolith structure , 1992 .

[12]  Nikunj Gupta,et al.  Heat and mass transfer coefficients in catalytic monoliths , 2001 .

[13]  Dan Luss,et al.  Explicit runaway criterion for catalytic reactors with transport limitations , 1991 .

[14]  James Wei,et al.  Mathematical modeling of monolithic catalysts , 1976 .

[15]  Hsueh-Chia Chang,et al.  Designing a fast-igniting catalytic converter system , 2001 .

[16]  Pio Forzatti,et al.  A comparison of lumped and distributed models of monolith catalytic combustors , 1995 .

[17]  Pio Forzatti,et al.  Mathematical Models of Catalytic Combustors , 1999 .

[18]  G. Comini,et al.  Laminar forced convection in ducts , 1979 .

[19]  G. Eigenberger.  On the dynamic behavior of the catalytic fixed-bed reactor in the region of multiple steady states—I. The influence of heat conduction in two phase models , 1972 .

[20]  Runaway limits for adiabatic packed‐bed catalytic reactors , 1998 .

[21]  James C. Cavendish,et al.  Transients of monolithic catalytic converters. Response to step changes in feedstream temperature as related to controlling automobile emissions , 1982 .

[22]  V. Balakotaiah,et al.  Shape normalization and analysis of the mass transfer controlled regime in catalytic monoliths , 2002 .

[23]  Carmo J. Pereira,et al.  Computer-Aided Design of Catalysts , 1993 .