Analysis of a corrugated-plate photocatalytic reactor

Abstract

A new reactor configuration, the corrugated plate reactor, was developed and analyzed through rigorous radiation field simulation, mass transfer measurement and modeling, 4- chlorophenol degradation experiments, as well as degradation kinetics modeling. The novel reactor was then applied to remove 4-chlorophenol, 2,4-dichlorophenol, and 2,4,5- trichlorophenol mixtures from water and to pretreat a contaminated groundwater for subsequent nitrification enhancement. Flat plate as well as slurry reactor systems were also run under otherwise the same operating conditions in order to set up performance benchmarks for the novel reactor.

The radiation fields on Ti02-coated corrugated and flat plates were modeled based on first principles. A special procedure was developed to calculate the effect of multiple reflection on radiative energy absorption. This allowed for the calculation of the local area-specific rate of energy absorption (LASREA) on the surfaces of catalyst films. Based on the results of the radiation model, the LAS REA and the photon absorption efficiency were both found to be quite sensitive to the dimensions of the corrugated plates. Due to the multiple photon reflection between the conjugate wings, corrugated plates possess a superior capability for recapturing longer wavelength photons which would otherwise be reflected out of some classic reactors. This greatly improved the photon absorption efficiency and the uniformity of the radiation distribution on the catalyst films. Compared to the flat plate, corrugations enhanced the radiation absorption efficiency by up to 50% for UV-A fluorescent lamp-powered systems and more than 100% for solar-powered systems.

Mass transfer between the main stream and the surfaces of corrugated as well as flat plates was examined experimentally using the benzoic acid dissolution method. Experimental results were fitted to several relationships which can be used to calculate the average mass transfer coefficients of the tested plates under different flow conditions. A mass transfer model was developed to predict the local mass transfer coefficients on the surfaces of different corrugated plates. Based on the results of the experiments, mass transfer coefficients were identified to be dependent on not only the flow conditions but also the angle of corrugated plates. Within the flowrate range examined, one corrugated plate showed an enhancement of overall mass transfer rates of up to 400% to 600% over that of the flat plate. This enhancement was due primarily to the large illuminated catalyst surface area in the new reactor. In addition, local mass and photon transfer rates on the corrugated plates correlated positively and therefore are complementary to each other. This result is favorable since a higher local photon absorption rate requires a higher mass transfer rate to avoid mass transfer limitation.

Based on the results of 4-chlorophenol degradation experiments, the corrugated plate reactor was up to 150% more efficient than the flat plate reactor. This energy efficiency was only about 15 % lower than that of the slurry reactor. A consistent dependency of the degradation rate on the angle of the corrugated plate was observed. The optimal angle was identified to be near 7°. Under the conditions examined, the reactions in corrugated plate reactors were found to be affected by the transfer of both 4-chlorophenol and oxygen to the catalyst surfaces.

The novel reactor efficiently mineralized mixtures of 4-chlorophenol, 2,4- dichlorophenol, and 2,4,5-trichlorophenol. Photocatalytic mineralization of the groundwater contaminants was slow even after carbonate and bicarbonate were eliminated by lowering the pH. However, nitrification was enhanced significantly after only a short period of pretreatment in the photoreactor. This indicated the potential for using photocatalysis to remove inhibition from biological nitrification systems.

With only three experimentally determined parameters, the new kinetic model incorporates reaction kinetics, mass transfer as well as photon transfer. The performance of the corrugated plate reactors predicted with the model was found to agree with the experimental data reasonably well. Corrugated plates with small angles were predicted to substantially alleviate the mass transfer limitation in flat plate reactors. However, this limitation can never be essentially eliminated unless sufficiently weak radiation sources are adopted. This is probably the bottleneck of aqueous phase photocatalysis.