Photocatalysis of organic effluents
Heterogeneous photocatalytic oxidation is a relatively new technology for the destruction of toxic organic substances in water and so far has concentrated on reaction mechanisms and kinetics, but the engineering side was hardly touched. In this study a detailed investigation for suitable reactor design has been carried out using three different reactor types. An annular photoreactor was used with two different lamps with different intensities; low pressure and medium pressure mercury lamps. Two types of Ti02 powder catalysts Degussa P-25 and Tioxide PCi were used as slurry solutions and Degussa P-25 was found more photocatalytically active than the other. Two main organic reactants salicylic acid and 4-chlorophenol were studied to assess the light intensity effect. Although better reaction rates were achieved with the higher intensity medium pressure lamp, quantum yield results showed that low pressure lamp photons were utiised more effectively. The falling film reactor (FFR) was used to increase the light intensity utilisation with the stronger medium pressure lamp and using the same reactants in slurries catalysts Degussa P-25 gave superior results to those in annular reactor. Because of the thin film characteristics, FFR provided good oxygen transfer at the film/air interface. A commercial supported catalyst system as a fibre glass gauze was also used inside the reactor and was found as effective as slurry systems. In order to improve the thin film characteristics and combine these with an immobiised catalyst, a spinning disc reactor (SDR) with catalyst coated borosilicate disc was used to degrade aqueous solutions of 4- chlorophenol and salicylic acid. It was found that the efficiency of the photocatalyst process was dependent upon the type of UV source used. Lamps supplying shorter wavelength UV radiation were found to be more efficient than those whose emissions lay mainly in the near UV region. The method used to coat the disc of the SDR does not appear to meet its operational requirements.