Heterogeous Photocatalytic Ozonation Of Phenol In Liquid-Solid Fluidized Bed: Simplified Reactor Performance And Kinetics Modelling

Photocatalysis by titanium dioxide on treatment of environmental contaminants has been studied by many researchers for several decades. Most of the researchers were laboratory scale while not the application in industry. There are two reasons addressed: (1) Requirement of post-process separation of slurry TiO2 from aqueous media is an important obstacle to its practicable applications, although it posesses higher photocatalytic activity; (2) Requirement of study on reaction kinetics and mechanism for the appropriate photoreactors. Thus, it is necessary to promote the photoactivity of immobilized titamium dioxide and to design the photoreactor to give a better performance, and then to set up a suitable model to predict the performance and provide the theoretical basis for application. Langmuir-Hinshelwood model was mostly used to describe the photocatalytic reaction. The presence of ozone could make many reaction steps involved and much more complicated than the slurry TiO2 system. Therefore, it is meaningful to set up a model coupled with all the reaction steps to have a better understanding of TiO2 photocatalysis process.The following three kinds of particles were used to select the appropriate substrate for coating TiO2 and the bed particles base on the hydrodynamics characteristics. The Kureha 0.64 mm spherical activated carbon was chosed according to the observation on fluidization of solid and liquid phase, the prediction of minimum velocity of liquid-solid fluidized bed and gas-liquid-solid fluidized bed and the feasibility of maintenance and operation of the fluidized bed system. The results can be used to select the power equipment and accessories and be the basic knowledge for the reactor design and scale-up.In this study, immobilized TiO2 on spherical activated carbon particles prepared by sol-gel method were selected as the bed particles. The SEM and AES analysis indicated that the TiO2 were coated uniformly on the surface of the activated carbon and the coated mass was about 6.51 wt%. The photocatalytic activity of the prepared catalyst was evaluated by the degradation of phenol in a bench scale test unit before applied to an annular fluidized bed reactor. The results showed that the optimal calcination temperature for TiO2/AC was 500 oC. The coating had a negligible effect on the structure of the original activated carbon, because the adsorption properties did not change obviously after the coating. It was found that the adsoption/desorption of the prepared photocatalysts and original activated carbon can be reasonablely fitted by Freundlich model and the adsoption/desorption was reversible.The performance of the photoreactor is closely related to the light intensity distribution in the reactor. The result of time-series signals of light intensity in the liquid-solid fluidized bed showed that the bed particles moved randomly within a specified zone and the uniform fluidization was realized in the bed. The light intensity distribution model was developed based on the above result and the Lambert-Beer law. It was shown by the model that the light intensity decayed along the radical distance and the decay would accelerate with the increase of solid hold-ups.Effects of both liquid and air flow rates on the phenol degradation rate were examined. The experimental results showed that the increase of liquid and air flow rates may enhance the phenol degradation rate. However, very high liquid flow rate (over 13.8 L/min) and air flow rate (over 3.0 L/min) could lead to the decrease in performance of the three-phase fluidized bed photoreactor. The results on the effect of initial phenol concentration on degradation rate indicated that the photocatalytic reaction in the fluidized bed followed the first order kinetics and could be reasonably fitted by the Langmiur-Hinshelwood kinetics model. Compared to the three-phase fluidized bed in which air is introduced into the bed from the distributor, the liquid-solid fluidized bed in which oxygen is provided by injecting air into the freeboard region of the reactor showed a better phenol destruction performance and is thus preferred for photodegradation of water contaminants.For the photocatalytic oxidation of phenol in the liquid-solid fluidized bed photoreactor, the modeling of the liquid phase phenol concentration distribution along the radical distance was conducted. The integration methodology for the model was used to calculate the average liquid phase phenol concentration. Compared with the experimental data, the relative errors was within 30%, which indicated that the photocatalytic oxidation process in the liquid-solid fluidized bed can be well discribed by developed model.Five oxidation processes, namely O3, UV/O3, UV/O3/AC, TiO2/UV/O2 and TiO2/UV/O3, for phenol degradation in fluidized bed were evaluated and compared, and the photocatalytic ozonation was found to give the highest phenol conversion because of the combined actions of homogenous ozonation in the liquid phase, heterogeneous ozonation on the surface of the catalyst support, i.e. activated carbon, and heterogeneous photocatlytic oxidation on the TiO2 catalyst surface. With the simplified kinetics model, photolytic ozonation was confirmed to predominantly take place on the particle surface comparing the heterogeneous and homogeneous photolytic ozonation. The photocatalytic reaction on the catalyst surface with ozone (TiO2/UV/O3) is much higher than that with oxygen (TiO2/UV/O2), with an enhancement factor ( ) of 3.7, which confirmed that ozone, as a more effective scavenger than oxygen, promoted the reaction process by reducing the recombination of generated holes and electrons. k r ( O3 ) /kr ( O2)...