Investigation into the effect of flow structure on the photocatalytic degradation of methylene blue and dehydroabietic acid in a spinning disc reactor

The ultraviolet irradiated thin film coated spinning disc reactor is a new technology for the intensification of heterogeneous photocatalytic reactions. This reactor has previously been found to have a reaction rate maxima for the photocatalytic degradation of methylene blue across a spinning disc reactor. The reaction rate maxima occurred at an intermediate flow rate of 15 mL/s and rotational speeds of 100 and 200 rpm, where the reaction kinetics switched from first order to second order with a change in the flow structure. The findings of this work show that the reaction rate maxima is most likely in part caused by periodic forcing from the peristaltic pump increasing the mass transfer of the oxygen. The enhancement in the rate of oxygen transfer to the surface of the disc would increase the charge carrier separation in the catalyst, increasing the reaction rate kinetics. Oxygen being a second limiting reactant would also explain the presence of the second order kinetics. The flow regimes on the surface of the disc change between smooth, spiral and irregular waves depending on the flow rate and rotational speed. The effect of flow rate modulation only occurs when the flow is undisturbed by asymmetric outflow conditions interfering with the flow regime otherwise present on the disc. The initial surface rate of reaction for methylene blue was approximately 0.5 × 10−7 mol/m2/s for most operational conditions, but the fast rate of reaction achieved with periodic forcing was 3.7 × 10−7 mol/m2/s, seven times greater than that achieved without the periodic forcing. Overall, this work shows that periodic forcing should be a key feature in achieving rate enhancements in spinning disc reactors, setting a new precedent in spinning disc reactor operational parameter choice.

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► Thin film photocatalytic spinning disc reactor used for waste degradation.
► Dehydroabietic acid and methylene blue used model waste compounds.
► Flow structure reaction control: it can enhance or reduce the rate of degradation.
► Intermediate periodic forcing gave a seven fold enhancement in rate of reaction.
► Enhancement was due to an increase in mass transfer of oxygen to catalyst surface.