Ozone Mass Transfer with Combined Effects ofSelf-decomposition of Ozone and Ozonationof Pollutants in a Stirred Vessel

• Main Authors: ChunYuChiu, 邱浚祐
• Other Authors: ChingYuanChang
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/21188137402847339461

博士 === 國立臺灣大學 === 環境工程學研究所 === 88 === The mass transfer aspect in isothermal ozonation systems is studied. A theoretical analysis is performed employing the film model to described the mass transfer of ozone absorption, decomposition, and reaction with pollutants in aqueous solution with the decomposition and reaction rate expressions of general reaction orders (not necessarily integers). The effects of ozone self-decomposition and reaction with pollutants on the mass transfer of ozone absorption are investigated. An enhancement factor Er , defined as the ratio of mass absorbed per unit area in time t with chemical reaction (or mass flux of chemical adsorption, NAr) to that without chemical reaction or of purely physical absorption (or mass flux of physical adsorption, NA ), is introduced as an index to evaluate the effect of chemical reactions. The model is also extended to consider the the ozone absorption with ultraviolet radiation. The effects of system parameters on the ozone mass transfer rate are examined. These parameters include the ozone self-decomposition reaction rate parameters (MAm , MBn , MIu ), the pollutants reaction rate parameters (MDB , MRBr’ , MIB), the reaction orders (m, n, u, a’, b’, r’, f’, u’), the pH value of solution, and the physical liquid phase mass transfer coefficient (kL0). The results indicate that, for smaller values of MAm0.5 and MBn0.5, the effect of chemical reaction on the absorption rate is not significant. However, when MAm0.5>1 and MBn0.5>1 the dissolved ozone gas is greatly consumed by the self-decomposition reaction. Also, the change of Er is significnt and important. For the given values of MAm0.5 and MBn0.5, the effect of reaction order on Er is of minor important. The combined enhancing effect of self-decomposition reaction (r) and UV radiation (u) on the ozone absorption rate in terms of NAru/NA is most significant for the situation with larger values of radiation intensity [I], as well as with small values of kL0. Also, for any particular finite value of [I] the enhancing effect due to UV radiation alone in terms of NAru/NAr reaches a local maximum at some pH value. The utilization of UV in addition to self-decomposition reaction is thus most efficient at this specific pH value. The mathematical model proposed by Anselmi et al. (1984 and 1985) for a semibatch stirred gas-liquid contactor is refined to described the mass transfer of ozone absorption, decomposition and reaction with pollutants in the aqueous solution with the decomposition and reaction rate expression of general reaction orders (not necessarily integers). Four system equations are employed to describe the ozone concentrations in the bulk liquid (CALb), the hold-up gas (CAGi), and the outlet gas in the free volume above the liquid surface (CAGe), and the pollutant (B) concentrations in the bulk liquid (CBTLb), respectively. The combined effects of ozone decomposition and reaction with pollutant on the mass transfer, in terms of enhancement factor (Er) are also considered in the refined model. Furthermore, the model also takes into account the variation of Er with CALb and CBTLb, which varies with time during the course of gas-liquid contacting. This analysis thus extends the applicabilities of model of Anselmi et al. (1984). Comparison of predicted values of proposed model with the experimental data of Sotelo et al. (1989a) (the ozone decomposition system), Anselmi et al. (1985) (the reaction with the benzenesulfonic acid system), Beltran et al. (1990) (the reaction with the o-cresol system) also indicates good agreement, and thus verifies the validity of present model. It is of special importance for the ozone mass transfer in the cases of basic solutions, of high reaction rate constants, of low mass transfer coefficients, in which the combined effects of decomposition and reaction with pollutants on absorption is significant, and in the system with varied liquid phase ozone concentration. The time to reach a specific conversion of pollutant decreases, but the enhancement factor increase as the MDB increases in the semibatch reactor. The application of this model can be extended to describe the CSTR (continuous stirred tank reactor) operation. Comparison of predicted values of concentration of proposed model with the experimental data of Sheffer and Esterson (1982) indicates good agreement. For larger value of MDB, the enhancement factor and the pollutant concentration of effluent and the emitted exit ozone gas decrease as the number of countercurrent flow reactors (constant total volumn) increase. It is important that the utilized fraction of ozone gas ((in — out) / in ) reach 75 % at the MDB 5 and Nt=5. The model can also be used to analyze the experimental data to obtain the mass transfer coefficient (kL0 or kL0ae) of purely physical absorption of ozone. The value of kL0 or kL0ae does not depend on the self-decomposition of ozone. For the ozonation systems with the same factors affecting the physical absorption, such as geometry, shape, agitation, blade, diffuser and absorbent liquid, their values of kL0 or kL0ae are the same. This syudy distinguishes the effects of chemical reaction, in terms of Er, from those of purely physical absorption (kL0 or kL0ae) on the mass transfer of ozone, so as to better and clearly describe the mass transfer in ozonation systems. The present model is of general reactor orders and applicable to various ozonation systems. The results are of very importance for the application of ozonation on pollution control.