Optical and hydrodynamic performance of photocatalytic monoliths of different shapes in a solar photoreactor with compound parabolic collector

Radiative transfer and hydrodynamic analyses are carried out to study the performance of solid macroscopic pieces (monoliths) of TiO2 photocatalyst in a CPC solar reactor. The absorbed photonic flux distribution was solved through Monte Carlo simulations for different monolith geometries: asterisk and flower shapes. In the case of the asterisk geometry, different numbers of arms (lobes) are considered. The performance of the monoliths in a tubular Pyrex photoreactor is numerically studied under both diffuse and collimated UV solar irradiation on the CPC. Also, the case of a homogeneous diffuse illumination on the glass tube, without CPC reflection, is considered. Hydrodynamic simulations are also carried out to determine the overall friction factors in the presence of the different pieces. Significant differences are observed for the results between different monoliths. Absorbed power varies up to 36%. On the other hand, the peak flux is the same for all pieces, but the standard deviation of its distribution may be up to 65%. The effect of partial specularity in the reflection by the TiO2 surface is also studied, resulting in a maximum deviation of just 4.2% in the flux values between 25% and 75% specularity. Two different positions for a monolith inside the glass tube were considered, with some significant differences in the flux distribution, but with changes below 4% in total absorbed power. With regards to the hydrodynamics, laminar flow of water along the pieces inside the tube was considered. Differences higher than one order of magnitude are observed in the friction factor for different pieces, being the void fraction inside the tube the main parameter affecting this change. The results of this work are relevant to understanding the potential effect of using catalyst pieces with different geometry on a photocatalytic reactors performance; inadequate geometrical design of this component may lead to diminished light collection or substantially increased pressure drops for scaled up systems.