The photon path length inside the radiant field of a photocatalytic solar reactor (with a compound parabolic collector (CPC)) crossing a slurry fluid with suspended particles of TiO2 was studied. The modeling included three sub-models: a ray-tracing algorithm to determine the boundary conditions in the absorber wall, a boundary layer to calculate the path length of the photon before its extinction and finally, the six flux scattering-absorption model (SFM) to estimate the absorbed energy profiles. A simple expression for the average of the local volumetric rate of photon absorption (LVRPA) in terms of a reference state was proposed. The recent concept of a radiant boundary layer was extended to CPC reactors, and a new calculation of the photon path length is proposed. We found that a catalyst load higher than the optimum value leads to a shorter photon path length causing a dark zone within the absorber where the catalyst activation does not occur. Optimal conditions for six different commercial TiO2 catalysts were determined. The best catalysts are the Fluka and Fischer catalysts, with 20% of radiant energy absorption over the total irradiated energy. The efficiency is independent of the diameter if the optimal catalyst load is guaranteed. The load diameter optimal pairs share an absorber optical thickness (t(a,opt)= 13.18 for the Degussa catalyst), which can be used to scale-up the reactor. The path length at maximum photon absorption was 3/4 of the absorber diameter. Reactor diameters of 1-5 cm had optimum catalyst loads of 230-46 mg/I. The turbulent regime favors (LVRPA). The angle between the normal axis to the reactor surface and the sun position may vary up to 10 with minor losses of up to 0.5%. (C) 2017 Elsevier B.V. All rights reserved.