Investigation Into Photocatalytic Degradation Of VOCS &the Nano-TiO₂ Agglomerate Fluidization In An Annular Fluidized Bed Reactor

Volatile organic compounds (VOCs) belongs to the organic chemicals with boiling point at 50-260℃and saturated vapor pressure at 133.32 Pa, which released or diffused from building materials, indoor furnishings, smoking, automobile exhaust and fuels combustion. Due to people stayed indoors for long time, some of VOCs may have short- and long-term adverse health effects on humsn beings. Photocatalytic oxidation (PCO) technology using semiconductors has some advantages over other VOCs controlling processes. The multiphase flow of photocatalyst particles coupling with PCO technology was put forward in this paper, and the photocatalytic degradation of gaseous benzene and cyclohexane in an annular fluidized bed reactor designed was investigated in details.The fluidization characterization of nano-titania (P25) agglomerates in the dedigned annular fluidized bed reactor was investigated by experimental data, numerical simulation, and theoretical analyses. The model of minimum fluidization velocity related to fluidization materials and reactor structure was developed based on the modified traditional models. For initial fluidization stage, the bed voidage and friction coefficient between agglomerates and reactor walls were determined using linear-fitting method, respectively. In addition, the average dimension values of elutriation agglomerates were evaluated by Navier-Stokes equation. Finally, the bed voidage distribution model considering bed diameter was developed.The model of nano-titania agglomerates in the annular fluidized bed was also presented considering the friction effect between agglomerates and reactor walls. According to the model, the following conclusions were given. (1) The friction between agglomerates and reactor walls should be considered unless the distance between the inner and outer wall is larger than 0.075 m; (2) If an agglomerate collides with other whose diameter is 0.298 times less than or equal to the former, the two agglomerates may coalesce. The dimensionless diameter coefficient exceeds 0.298, the two agglomerates may separate. Using the above-mentioned model, the relationship of diameter of agglomerate and gas velocity is explored.The adsorption experiments of benzene and cyclohexane in the annular fluidized bed indicated that (1) their adsorption efficiencies were a degressive function of concentration at the fixed gas velocity and RH; that (2) adsorption active sites were a function of gas velocity based on the analyses of kinetic data, and the maximum value of adsorption active sites for cyclohexane was 2.71×10-4 mmol g^-1 at fluidization number of 2.11 and the minimum value of adsorption active sites for benzene was 0.81×10-4 mmol g^-1 at fluidization number of 1.37, respectively; and that (3) the adsorption efficiencies of benzene and cyclohexane were decreaed with increasing RH, which was explained by the variation of adsorbate structure and adsorption active sites.Photocatalytic degradation of benzene and cyclohexane gave the following conclusions. Firstly, the concentration of target has obvious influences on photocatalytic degradation efficiency, and the degradation efficiency decreased with increasing concentration. Secondly, the photocatalytic degradation efficiency was approximated to the maximum values at the optimal fluidization numbers 1.62 and 1.8 for cyclohexane and benzene respectively. Thirdly, the influences of RH on photocatalytic degradation of cyclohexane and benzene were similar, and there was an inflexion point corresponding to the maximum degradation efficiency at variation of concentrations. Fourthly, the relationship of RH and target concentration was explored asC_H2O=(1+K_AC_M)/2K_Hwithout consideration of adsorption of the products and intermediates yielded in photocatalysis.Using FTIR, GC-MS, and UV-Spectrum, the intermediates yielded in photocatalytic degradation of cyclohexane were cyclohexanol, cyclohexanone and 2-cyclohexen^-1-one, while the intermediates for benzene were phenol and carboxylic acid. We can infer the photocatalytic degradation mechanisms of benzene and cyclohexane and the causes of photocatalyst deactivation. Based on the adsorption active sites occupied by intermediates, the regeneration and recycling experiments were performed, suggesting that the photocatalyst can be regenerated and recycled over three times.Photocatalysis kinetic models based on the elementary reactions explores the roles of water molecule in this process.