Diffusion limitations in stagnant photocatalytic reactors

•Analyzes of uneven absorption of light in photoreactors vs. counteracting diffusion.•Describes the extremes of optically thick reactors and optically thin reactors.•Provides a criterion for maximum allowable reaction rate for kinetic experiments.

This paper provides a simple criterion to determine when the performance of an unmixed photocatalytic slurry reactor becomes limited by diffusion. We use a 1D description of the reactor and the two-flux intensity model to describe the concentration profile in unmixed photoreactors. We show that the effect of diffusion limitation in optically thick photoreactors is negligible when the Dahmköhler number based on reactor length is smaller than 0.1τγ, where ττ is the optical thickness and γγ is the exponent that describes how the reaction rate varies with light intensity. For optically thin reactors, in contrast, we find that the maximum Dahmköhler number scales with the inverse of τγτγ.