Abstract:
Wastewater treatment is becoming increasingly important with the increase of pollution from municipal and industrial sources, driving the development of new processes for the treatment of the wide array of pollutants found in the environment. One such process is photocatalysis, an advanced oxidation process which uses a light activated semi-conductor catalyst to oxidise pollutants. The semi-conductor catalyst is usually applied in a powdered form. However, slow kinetics and mass transfer limitations have so far been hindering industrialisation and it is for this reason that new reactors are being investigated to overcome these limitations. The spinning disc reactor (SDR) is a process intensi cation technology which uses centrifugal force to spread a liquid onto a horizontally spinning disc into a thin lm. The SDR has two tuneable parameters, the ow rate and rotational speed, which allow for a range of ow regimes to form across the surface of the disc and exhibits enhanced mass transfer characteristics. This reactor has not been investigated in detail for its application as a photocatalytic reactor, and the aim of this thesis was to assess the e ectiveness of the SDR as a photocatalytic process intensi cation technology. Highspeed camera images were taken to observe the ow regime in order to select operational parameters representative of di erent operational parameters to be used for the experiments. The e ect of ow rate and rotational speed was investigated on the degradation rate of methylene blue and DHA, and aside from reaction rate maxima found at 15mL.s 1, the surface rate of reaction was constant between the di erent operational parameters. No correlation was found between an increase in the rate of reaction and the ow rate and/or rotational speed, implying that the reaction was not mass transfer controlled. Further investigation into the reaction rate maxima found at 15mL.s 1 and 100 and 200rpm showed that signi cantly enhanced primary molecule and intermediate degradation was caused by a change in the ow found with the highspeed camera images. Further investigation with methylene blue through the adjustment of the nozzle to alter the ow regime found that as the ow became more homogeneous the rate of reaction of the primary molecule increased. This makes the nozzle design an important design consideration in SDRs, which is normally overlooked. The reaction rate maxima of the degradation of methylene blue are likely the result of periodic forcing from the peristaltic pump enhancing the adsorption of the oxygen and/or enhanced electron transfer from the catalyst to the oxygen. Adsorption of oxygen and/or electron transfer to the oxygen are often found to be the rate limiting step in photocatalytic reactions. The enhanced oxygen would improve the charge carrier separation and through that the rate of reaction. The fast rate of degradation of methylene blue achieved with periodic forcing is 3.7 10 7mol.m2.s 1, which is seven times larger than that achieved without. Periodic forcing should be investigated as a possible method of intensifying reaction rate, for the spinning disc reactor and other reactors, as its enhancement e ect is in addition to the increased mass transfer already normally observed in this system. A comparison of the SDR and an annular reactor determined that the SDR is more complex to design, build and maintain. However, the SDR is signi cantly more e cient at utilising the incoming light, as the average photonic e ciency achieved in the SDR is an order of magnitude larger than the maximum photonic e ciency achieved in the annular reactor, 0:19 0:08% versus 0:062 0:009% respectively. 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 3s versus 0:13 0:02 10 4 mol.m 3s, due to the signi cantly smaller volume in the SDR. The maximum surface rate of reaction achieved in the SDR was two times larger than the maximum reaction achieved in the annular reactor, 3:51 0:26 versus 1:66 0:22 107 mol.m 2.s 1 respectively. As the maximum reaction in the annular reactor included a strong degradation due to photolysis, which was not found in the SDR, the photocatalytic rate found in the SDR exceeded both the photocatalytic and photolytic degradation found 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 SDR is performing as a process intensi cation technology.