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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',

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A spinning disc reactor was investigated as a process intensification technology for photocatalysis and compared with a conventional annular reactor. It was found that the average photonic efficiency achieved in the SDR was three times larger than the maximum photonic efficiency achieved in the annular reactor, 0.19±0.08% versus 0.062 ± 0.009%, indicating that the SDR is significantly more efficient at utilising the incoming light. Similarly, the average volumetric rate of reaction for the SDR was an order of magnitude larger than that of the annular reactor, 3.6 ± 1.5 × 10 −4 mol.m −3 .s −1 versus 0.13 ± 0.02 × 10 −4 mol.m −3 .s −1 , due to the significantly smaller volume in the SDR. However, the average surface rate of reaction is more useful for comparison in an immobilised catalyst system. In the SDR, the initial surface rate of reaction was approximately the same (within the margin of error) as the photocatalytic reaction in the annular reactor. This suggests that both reactors exhibit the same rate limiting step. Given the significantly higher mass transfer rate in the SDR over the annular reactor, it is likely that the rate limiting step is either the adsorption of oxygen onto the catalyst or the electron transfer from the catalyst to the oxygen, often found to be the rate limiting step in photocatalytic reactions. However, the maximum surface rate of reaction achieved in the SDR (at a flow rate of 15mL.s −1) was two times larger than the maximum reaction achieved in the annular reactor-this suggests that at this condition the rate limiting step is being overcome, and that when operated at this condition the photocatalytic SDR is performing as a process intensification technology.

The ultraviolet irradiated thin film coated spinning disc reactor is a new technol ogy 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 15mL/s and rotational speeds of 100 and 200rpm, 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 separa tion 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 mod ulation 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/m 2 /s for most operational conditions, but the fast rate of reaction achieved with periodic forcing was 3.7×10 −7 mol/m 2 /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.