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Photocatalytic oxidation of toluene in the gas phase: modeling of annular photocatalytic reactor

The photocatalytic degradation of gas phase toluene (used as a model VOC) over illuminated titanium dioxide (TiO2) was investigated at room temperature using continuous flow annular reactor. The dependence of the overall process efficiency upon operating variables such as initial concentration of toluene and water in air stream and total flow rate of gas mixture was analyzed. Results of kinetic analysis showed that photocatalytic oxidation of toluene obey Langmuir-Hinshelwood kinetics. Deactivation of TiO2 catalyst was observed and attributed to adsorption of the partial oxidation products on the active sites of the catalyst. The presence of the benzaldehyde and benzoic acid on the surface of the deactivated photocatalysts was confirmed by using FT-IR spectroscopy. Different matematical model of the photocatalytic reactor were developed and experimentally verified. Introduction Volatile organic compounds (VOCs) are an important class of pollutants usually found in the atmosphere of all urban and industrial areas. Photochemical oxidation, PCO (also referred to as advanced oxidation processes, AOP) have become increasingly popular as alternative for environmental VOC decontamination purposes [1, 2]. Photocatalysis on TiO2 is advantageous compared to other oxidation methods because it takes place at room temperature, so no thermal activation is necessary and only low-intensity UV lights are needed. Moreover, TiO2 is relatively stable and an inexpensive photocatalyst. While most relevant studies have been concerned with photodegradation in the liquid phase, the degradation of gaseous organic compounds has gained importance recently, predominantly for air purification [3]. Recently, important research area becomes investigation of optimal reactor configurations. There are numerous laboratory reactor designs, such as batch reactor [4], fixed bed annular reactor [5], a microchannel reactor [6], etc. The objective of this work is investigation of the photocatalytic oxidation of toluene in the gas phase with particular emphasis on presentation of the methodology for photoreactor analysis and design based on chemical reaction engineering and mass transfer fundamentals. Experimental The commercial catalyst used in this work was supplied by Degussa&reg ; (P-25). The TiO2 layer was coated onto the internal glass surface of reactor using TiO2 isopropanol slurry. The TiO2-coated tube was heated at 110oC for 2 hours. A TiO2 loading density of 22.3 &micro ; g cm-2 was obtained by weighting the tube before and after the coating. The physico-chemical properties of the TiO2 catalyst were studied by different techniques, such as SEM, XRD, FT-IR spectroscopy, TG/DSC and nitrogen adsorption/desorption measurements. An annular photoreactor was constructed using a Pyrex glass tube with a 53.8 mm internal diameter of outer tube and 23, 8 of inner tube. Illumination was provided by a 8 W fluorescent lamp. The high purity synthetic air was used as a gas carrier. The concentration of toluene and moisture in the contaminated atmosphere was obtained by vaporization of organic compound and water using predetermined values of flow rate applying mass flow controllers (Cole Parmer&reg ; ). Prior to switching on the UV or visible light illumination, the catalyst was first exposed to the polluted air stream until dark adsorption equilibrium was reached. Toluene concentrations were measured on-line by a gas chromatograph equipped by Carbowax 20M packed column (Restek&reg ; ) and FID detector. Results and discussion The photocatalytic oxidation of toluene was carried out over TiO2 in an annular reactor that had been developed to study the treatment of organic pollutants in gas phase. The experiments were performed at different initial concentration of toluene (0.8-8.9 g m-3) and water content in the air (10-60 %) as well as at different total flow rate of gas mixture. It was observed that conversion of toluene decreased with the increase of initial concentration of toluene. As expected, toluene conversion increased with increase of the space time in the reactor. On the other hand, water content had only small influence on the observed conversion. After several hours of using in the reaction, the TiO2 catalyst was deactivated. Additional FT-IR analysis of surface species was confirmed the presence of small quantities of the benzaldehyde and benzoic acid. However, assumption of the constant catalyst activity was taken into consideration during reactor analysis and modeling, due to the fact that kinetic experiments were performed in the relativelly small time intervals. Generally, approach to the modeling of an annular photocatalytic reactor is not different of modelling other similar configuration, for example of a monolithic catalyst structure. In this study reactor models were derived based on the physical picture, especially regarding on hydrodynamics of gas phase inside the reactor and mode of transfer of toluene from gas to the catalyst surface. The main assumptions made during deriving of the models were: a) reactor was considered as steady-state tubular reactor, i.e. inlet and outlet characteristic variables were independent of time ; b) isothermal conditions ; c) catalyst was deposited on the inner wall of outer tube of annular reactor in very thin layer and therefore intraphase diffusion was neglected in the overall reaction path. Following such approach, several mathematical models were used, ranging from the simple 1D heterogeneous model, based on assumption of the ideal flow conditions to the complex 2D heterogeneous model taking into account the radial concentration gradients due the mass transfer by diffusion and laminar flow of gas mixture. The proposed models were verified by comparing the experimental data with theoretical predictions. The most realistic situation was achieved using the last one, which is based on the fact that imaginary boundary layer between gas and solid phase is broadened to the whole space between two concentric tubes of the annular reactor. The mass transfer in this region is due to diffusion. This model is very complex but it's very important advantage in comparison with other models is in the fact that the diffusion coefficient of toluene in gas mixture, mainly consisted of air, can be easily calculated. Accuracy of the empirical calculation of mass transfer coefficient is much lower in the case of the 1D heterogeneous model. Therefore, much better approximation and estimation of kinetic constants can be made using 2D heterogeneous model. Obviously, 2D model with laminar flow is the best approximation of physical nature of the photocatalytic annular reactor. Thus, it is useful to optimize by this model the operating conditions and design parameters of the photocatalytic reactor. References [1] H. d. Lasa, B. Serrano, M. Salaices, Photocatalytic Reaction Engineering, Springer, London, 2005. [2] G. Ertl, H. Knözinger, J. Weitkamp, Handbook of Heterogeneous Catalysis, Wiley-VCH, Weinheim, 1997. [3] J. Zhao, X. Yang, Building and Environment, 38, 645-654 (2003). [4] S. B. Kim, H. T. Hwang, S. C. Hong, Chemosphere 48, 437-444 (2002). [5] R. M. Alberici, W. F. Jardim, Applied Catalysis B: Environmental, 14, 55-68 (1997). [6] H. Ge, G. Chen, Q. Yuan, H. Li, Catalysis Today, 110, 171– 178 (2005).

volatile organic compounds; photocatalytic oxidation; annular photocatalytic reactor

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