A class of air contaminants that has received much attention is volatile organic compounds (VOCs), which are produced and used in numerous industrial fields. Current VOC abatement technologies in the industrial sector are based on techniques that allow for either the separation and isolation of unwanted compounds (adsorption, absorption, condensation processes) or the elimination of pollutants through their oxidation (biological, thermal, catalytic processes). Currently, the most used processes in the industrial sector to remove VOCs are based on catalytic combustion, which however require high temperatures and therefore considerable operating costs. Photocatalytic oxidation is an interesting alternative solution, as it allows for high abatement efficiencies at room temperature and produces harmless final products (CO2 and H2O). To investigate the efficient applicability of photocatalysis to the treatment of gaseous effluents, where energy costs are significantly lower than with conventional technologies, in the previous year a plant for the abatement of organic pollutants using a photocatalytic process was set up at the RSE laboratories. The plant is designed for one step operation, so that it is possible to evaluate its efficiency in treating high volumes of gaseous streams. The reactor consists of a steel cylinder with a volume of approximately 23 l, inside which there are 5 15 W Wood’s lamps (or black light lamps) (emission at 360 nm). Nanometric titanium dioxide (TiO2) was used as a catalyst, anchored on a support made up of hollow Pyrex glass cylinders (6×6 mm), whose preparation procedure was developed in the previous year. In order to analyze the composition of the gaseous stream entering and exiting the photocatalytic reactor, the plant is equipped with a micro gas chromatograph that monitors the concentration of VOCs and a continuous analyzer for the quantitative measurement of the CO2 produced by the photocatalytic mineralization reaction of the VOCs themselves.
In the reference period, the system was tested using synthetic mixtures with variable VOC concentrations and different contact times, in order to evaluate the removal kinetics and detect the possible formation of reaction by-products. In particular, during one step operation, mixtures of benzene (20 and 10 ppmv) in nitrogen were used to which 3% (v/v) oxygen was added. The feed flow rate to the reactor was set at 300 Nl/h or 150 Nl/h, corresponding to a GHSV (Gas Hourly Space Velocity) of 15 h-1 and 7.5 h-1 and contact times on the catalyst of 240s and 480s, respectively. By operating with a flow rate of 300 Nl/h, abatement efficiencies of 35.5% and 22.0% were obtained, with benzene concentrations of 10 and 20 ppm respectively, while by operating with a flow rate of 150 Nl/h the abatement efficiency increases to 62.5% and 36.5%, again for benzene concentrations of 10 and 20 ppm, respectively.
Due to a sudden failure of the micro gas chromatograph, during the tests it was not possible to monitor the concentration of benzene (C6H6) exiting the photocatalytic reactor. Therefore, the abatement efficiency was determined on the basis of the analysis of the CO2 developed by the complete mineralization reaction of the C6H6 and may have been underestimated as it could not take into account the benzene adsorbed on the photocatalyst.