A falling film photocatalytic solar reactor (FFR) and a tubular reactor with compound parabolic collectors (CPCR) were modeled. An empirical reaction mechanism was proposed for the heterogeneous photocatalytic degradation of organic contaminants using TiO2, and H2O2. An analytical kinetic rate was deducted from the mechanism in order to describe the change of H2O2 concentration and the rate of destruction of total organic carbon (TOC). The profiles of the absorption rate of photons in the heterogeneous medium were estimated by using the absorption-scattering radiation six-flux model (SFM). A dimensionless number that we named Damkohler of a photocatalytic reactor was proposed (Da(ph)). Finally, the modeling was validated against experimental data for the TOC destruction obtained from the mineralization of dicloxacillin (DCX) and 4-chlorophenol (4-CP), which we considered as model pollutants. The modeling fit the experimental data with a standard deviation of 1.30 ppm. The kinetic constants were independent of the geometry, this is because the mass transport, radiant energy, and an intrinsic kinetic expression were taken into account. Therefore, the constants can be used in the design and simulation of new geometries. Under the best-tested conditions, 58.7 mol of organic carbon were converted to CO2 per each 100 mol of photons absorbed (quantum yield near to 0.5). Also, Da(ph) is expressed mathematically by the contributions of the catalyst, the radiant field, the substrate and the mean residence time within the reactor. The latter is key to the large-scale sizing of these photocatalytic solar reactors for industrial applications. Then, Da(ph) indicates how the designs of new reactors can be improved.