An experimental and theoretical study of a catalytic monolith to control automobile exhaust emissions

Abstract

Experimental investigations of automobile exhaust emissions were examined by combusting a mixture of propane and air within a multi-channel monolith. Chemical kinetics, mass transfer and heat transfer effects were studied using appropriate temperature and flow conditions to separate the effects. The results were used to construct both a one- and two-dimensional mathematical model. Simulations of monolith behaviour were then compared with observed performance. First-order chemical kinetics were observed for the low hydrocarbon concentrations examined in the temperature range 557–648 K, while mass transfer limitation was apparent at temperatures between 736 K and 769 K. Perturbations to inlet concentration and temperature were effected while studying monolith performance, and the responses recorded. Computer simulations using the two mathematical models predicted correct trends, but did not agree quantitatively with the experimental results. The one-dimensional model predicts both concentration and temperature responses to a change in inlet conditions better than the more comprehensive two-dimensional model, even when heat losses are taken into account. This is because experimentally determined heat and mass transfer coefficients are used for computations relating to the one-dimensional model, whereas these parameters were calculated theoretically in the two-dimensional model. Further computer simulations revealed discontinuities in the values of Nusselt numbers, values depending on elapsed time following a step change in inlet conditions and axial position along the monolith channel. This unusual feature is accounted for by a reversal in heat transfer between wall and bulk fluid as the reaction develops along the monolith channel.

Radiation heat transfer in catalytic monoliths

“…Most have included conduction heat transfer in the catalytic surface (monolith walls or flat plate), with the exception of Heck et al (19761, Mori et al (1977), T i e n (1981a,b), Schefer (1980), Brown et al (19831, Bruno et al (1983), Fakheri andBuckius (19841, Markatou et al (1991, 19931, andLeighton andChang (1995). Some have included radiation heat transfer, as well (Lee and h i s , 1977;Sinkule and Hlavhcek, 1978;Ryan et al, 1991;Bennet et al, 1992;Hayes et al, 1992Hayes et al, , 1996Boehman et al, 1992;Hayes and Kolaczkowski, 1994;Groppi et al, 1995;Kolaczkowski and Worth, 1995;Frye and Boehman, 1996;Boehman et al, 1997a,b). The level of detail in heterogeneous chemistry models has varied from generic global rate laws to global rate laws modified by effectiveness factors that account for pore diffusion in the catalytic layer (Young and Finlayson, 1976a,b: Hayes and Kolaczkowski, 19941, to solution of the governing equations for the surface diffusion of species in the catalytic layer (Zygourakis and Ark, 1983).…”

“…It is convenient to characterize these efforts in terms of their description of the gas phase within the catalytic passage, that is, whether the channel is one-dimensional (relying on heat-and mass-transfer coefficient correlations), two-dimensional (assuming axisymmetry, parallel flat plates, or external boundary-layer flow), or three-dimensional (no spatial simplifications). These studies have included: one-dimensional considerations of single passageways (Votruba et al, 1975;Heck et al, 1976;Young and Finlayson, 1976a,b;Sinkule and HlavLcek, 1978;Finlayson and Young, 1979;Marteney and Kesten, 1981;T'ien, 1981a,b;Oh and Cavendish, 1982;Prasad et al, 1983;Ahn et al, 1986;Zygourakis, 1989;Bennet et al, 1992;Tien and T'ien, 1992;Montreuil et al, 1992;Groppi et al, 1993;O h and Bissett, 1993;Leighton and Chang, 1995), multiple passageways (Flytzani-Stephanopoulos et al, 1986;Chen et al, 1988;Kolaczkowski et al, 1988;Kolaczkowski and Worth, 19951, two-dimensional solutions for single passageways (Heck et al, 1976;Young and Finlayson, 1976a,b;Finlayson and Young, 1979;Bennet et al, 1992;Lee and Aris, 1977;Harrison and Ernst, 1978;Zygourakis and Ark, 1983;Bruno et al, 1983;Ryan et al, 1991;Hayes et al, 1992Hayes et al, , 1996Boehman et al, 1992;Hayes and Kolaczkowsi, 1994;Groppi et al, 1995;Frye and Boehman, 1996;Boehman et al, 1997a,b), exter...…”

“…They present their method of solution (Hayes et al, 19921, comparison with experimental reactor measurements (Bennet et al, 1992), and explanation of discrepancies between experiment and prediction (Hayes and Kolaczkowski, 1994). In their formulation for radiative heat transfer, Hayes et al make use of finite area shape factors to describe the exchange between sections of the monolith wall in the finite-element model.…”

“…The influence of radiation heat transfer on catalytic converter behavior has been included in simulations by Sinkule and Hlavicek (1978) using a 1-D model, and Lee and Aris (1977), Hayes and coworkers (Bennet et al, 1992;Hayes and Kolaczkowski, 19941, Ryan et al (19911, and Boehman et al (1992) Axial Distance, cm Table 1. Sinkule and HlavAcek (1978) performed steady-state simulations that considered only black surfaces and a passage with L/D = 20.…”

“…Hayes and co-workers developed a finite element method for simulating both catalytic converters and catalytic combustors, which they applied to propane (C,H,) and CO oxidation (Bennet et al, 1992;Hayes et al, 1992;Hayes and Kolaczkowski, 1994). They present their method of solution (Hayes et al, 19921, comparison with experimental reactor measurements (Bennet et al, 1992), and explanation of discrepancies between experiment and prediction (Hayes and Kolaczkowski, 1994).…”