Optimal catalyst thickness in titanium dioxide fixed film reactors: Mathematical modelling and experimental validation

• Experimental results show an optimal catalyst thickness for sprayed TiO2 of 5 μm.
• Liquid-side and catalyst-side illumination produce different behaviour.
• Model for diffusion, advection and rate of reaction in porous media is proposed.
• First principle model shows very good agreement with experimental results.
• Results can be used for optimisation of large-scale reactors.

The use of immobilised TiO2 for the purification of polluted water streams introduces the necessity to evaluate the effect of mechanisms such as the transport of pollutants from the bulk of the liquid to the catalyst surface and the transport phenomena inside the porous film. Experimental results of the effects of film thickness on the observed reaction rate for both liquid-side and support-side illumination are here compared with the predictions of a one-dimensional mathematical model of the porous photocatalytic slab. Good agreement was observed between the experimentally obtained photodegradation of phenol and its by-products, and the corresponding model predictions. The results have confirmed that an optimal catalyst thickness exists and, for the films employed here, is 5 μm. Furthermore, the modelling results have highlighted the fact that porosity, together with the intrinsic reaction kinetics are the parameters controlling the photocatalytic activity of the film. The former by influencing transport phenomena and light absorption characteristics, the latter by naturally dictating the rate of reaction.