An experimental and modelling investigation of the effect of the flow regime on the photocatalytic degradation of methylene blue on a thin film coated ultraviolet irradiated spinning disc reactor

In this work, the impact of wave regime and operational parameters on the photocatalytic degradation of methylene blue was investigated on a thin film coated ultra-violet spinning disc. In the employed experimental setup, the wave regimes of spiral, unstructured and crisscross waves as well as smooth film could be observed at disc rotational speeds of 50-200 rpm and flow rates of 5-20 mL/s with a calculated average thickness of 160-450 μm. The glass discs were coated with anatase TiO2 by a sol-gel procedure followed by heat treatment at 500 ° C for 1 h. The reactor was irradiated by a low pressure mercury UV lamp producing an irradiance of 12-23 W/m2 on the disc surface. The reactant was saturated with oxygen and the effect of spinning speed, flow rate and the resulting wave regime on the degradation rate and kinetics of methylene blue and its reaction intermediates determined. Reactions followed pseudo-second-order kinetics, suggesting dimerisation and/or mass transfer limitations given that the two reactions with the highest conversion observed at 15 mL/s and 100 and 200 rpm, were pseudo-first-order. The spinning disc reactor was, however, not photon transfer limited. The wave regimes showed no impact on the reaction rate, since the flow was mainly laminar with no interfacial mass transfer of oxidant required.