Although the potential and beneficial characteristics of photocatalysis in the degradation of a good number of emerging pollutants have been widely studied and demonstrated, process design and scale-up are restrained by the lack of comprehensive models that correctly describe the performance of photocatalytic reactors. Together with the kinetics of degradation reactions, the distribution of the radiation field in heterogeneous photocatalytic systems is essential to the optimum design of the technology. Both the Local Volumetric Rate of Photon Absorption (LVRPA) and the Overall Volumetric Rate of Photon Absorption (OVRPA) help to understand this purpose. This work develops a Six-Flux radiation absorption–scattering model coupled to the Henyey–Greenstein scattering phase function to evaluate the LVRPA profile in a LED-assisted photocatalytic reactor. Moreover, the OVRPA has been calculated and integrated into the kinetic equation, accounting for the influence of the radiation distribution on the reaction rate. The model has been validated with experimental data for the degradation of S-Metolachlor (MTLC), and the set of operating variables that maximize the reactor performance, 0.5 g/L of TiO2 P25 and pH 3, has been determined.