Fixed bed photocatalytic reactor for formaldehyde degradation: Experimental and modeling study

• Treatment of formaldehyde-containing solution with TiO2 coated Raschig rings.
• Residence time distribution of a fixed bed photocatalytic reactor.
• Modeling based on hydraulics, chemical reactions and mass transfer mechanisms.
• Kinetic constants of photocatalytic degradation of formaldehyde.

Formaldehyde is toxic to humans and is classed as a category 1 carcinogen. Methods have been developed to degrade this compound, but for industrial application, relevant mathematical models are required. This study models a fixed bed photocatalytic reactor designed to degrade formaldehyde constituted of TiO2-coated Raschig rings illuminated by UV-A lamps. Initially, the reactor’s hydraulic behavior was described based on an experimental residence time distribution. This model takes into account hydraulics, light distribution, chemical kinetics and mass transfer in the reactor. The dispersion model satisfactorily represented the reactor’s hydraulic behavior. This model, combined with a Langmuir–Hinshelwood kinetic model, was then used to calculate variations in concentration at the reactor output. By adding the transfer flux between the bulk and the surface to the material balance equation, it is possible to distinguish between mass transfer and chemical reaction limitation and determine the chemical kinetics. Experimental data from different initial concentrations were used to calculate the Langmuir–Hinshelwood kinetic constants. The dispersion model with chemical reaction was validated under various irradiation and flow rate conditions. The results show that the fixed bed photocatalytic reactor efficiently degrades formaldehyde in an aqueous solution. The chemical constants of photocatalytic degradation obtained for formaldehyde are necessary parameters if this technology is to become the basis of industrial applications. This study provides a new tool, integrating mass transfer limitations and light distribution, to design photo-reactors.

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