Petroleum refinery wastewater (PRW) is becoming a big problem for both people and the environment, so researchers have been trying harder to find better ways to treat it. One option that looks really promising is using advanced oxidation processes (AOPs) to break down all the pollutants inside this wastewater. In this study, new-designed photoreactor was used to conduct a photocatalysis approach that aimed to remove the chemical oxygen demand (COD) from PRW collected from Al-Diwaniya refinery in the south of Iraq. The geometrically optimized photoreactor enhances the formation of the free radicals required for efficient degradation by maximizing mass transfer and light dispersion. Bare TiO₂ and SiO₂/TiO₂ were fabricated as photocatalysts with the sol-gel technique and then examined by the XRD, FTIR, FESEM, AFM, BET, UV-DRS, and PL analysis. Results showed that, the optimal SiO₂/TiO₂ ratio was 5/95% (S-5) achieving 90% COD reduction efficiency with a photocatalyst dosage of 2 g/L, a reaction time of about 4 h, and using 8 UV-C lamps (8 W each). Under these conditions the process consumed 0.512 kWh/L electricity energy (EE/O). The results demonstrated that COD reduction exhibits behavior consistent with pseudo first order kinetics. In addition, the scavenger experiments revealed that the COD was primarily degraded by the ^⦁O_₂^- radical. After five cycles, the photocatalyst S-5 maintained its stability and continued to remove over 82% of the COD. Refinery effluent was significantly better treated after using this innovative reactor design in conjunction with the SiO₂/TiO₂ photocatalyst. It provides an ecological, more efficient, and less expensive way to lower COD in PRW.