RADIATION-FIELD MODELING IN A PHOTOCATALYTIC MONOLITH REACTOR

An incidence model is formulated from the basic principles of radiatio n heat transfer to predict the light intensity profile inside a photoc atalytic monolith reactor, where a light-absorbing and light-reflectin g catalytically active thin film is coated on the inner walls of monol ith channels of either circular or square cross section. A diffuse ext ernal light source and diffusely reflecting wall coatings were assumed . The mathematical model representation of the light Bur distribution to the monolith wall and through the ross section of the monolith chan nel take the form of integral equations. In dimensionless form, these equations reveal that, for a given channel type, light intensity profi les are controlled by channel aspect ratio and film reflectivity. The equations were solved numerically using Gauss-Legendre quadrature to g ive quantitative estimates of the radiation intensity profile down the length of the monolith channel for a specified incident light intensi ty distribution at the entrance of the channel and an assumed thin fil m reflectivity. Experimental cross-sectional light intensity profiles for square channeled, uncoated ceramic monoliths with two different ce ll densities confirmed the prediction of the model that dimensionless profiles are not dependent on absolute channel dimensions, but rather are uniquely determined by the channel aspect ratio. Experimental inte nsity data for titania-coated monoliths were well described by model p redicted profiles assuming an average reflectivity of 40%. For identic al aspect ratio channels, model predictions reveal that the light inte nsity profiles for square and circular channels are quite similar. Mod el predictions indicate that radiation field gradients are large, with relatively little light penetrating beyond a length equivalent to thr ee channel widths. This prediction implies that, for monoliths with ty pical commercial aspect ratios, a large fraction of the coated channel wall is not effectively irradiated. (C) 1998 Elsevier Science Ltd. Al l rights reserved.