The simulation and full predictive design of large-scale photocatalytic reactors can be carried out by solving the conservation equations of momentum, energy and mass on the system, similarly to conventional chemical reactors. The momentum and mass balance equations take the same expressions as those corresponding to thermal reactors, as they depend on the fluid dynamic behavior of the reactor (well-mixed, plug-flow, axial dispersion, etc.), the relevant mass transfer mechanism (diffusion and/or convection) and the expression corresponding to the reaction rate taking into consideration the stoichiometry of the chemical reactions. In contrast, the classic enthalpy balance that describes the heat flows and temperature profiles in thermal chemical reactors is substituted in photochemical reactors by the radiative energy balance. The reason for this is that reactions are photochemically and not thermally activated. Consequently, the temperature profiles and thermal effects on the chemical reaction step can usually be neglected, although they can be relevant in some of the physical processes coupled to the reaction, such as mass transfer and adsorption. On the other hand, resolution of the radiation transfer equation (RTE) becomes critical for the simulation of photocatalytic reactors.