1、PDF外文：http:/ 1 英文文献 Modeling of organic pollutant destruction in a stirred2tank reactor by ozonation CHENGJiang1 , YANG Zhuo2ru1 , CHEN Huan2qin1 , KUO C. H. 2 , ZAPPI E.M. 2 Abstract :Destruction of organic contaminants in water by ozonation is a gas2liquid process which involve
2、s ozone mass transfer and fast irreversible chemical reactions. Ozonation reactor design and process optimizing require the modeling of the gas2liquid interactions within the reactor. In this paper a theoretical model combining the fluid dynamic and reaction kinetic parameters is proposed for predic
3、ting the destruction rates of organic pollutants in a semi2batch stirred-tank reactor by ozonation. A simple expression for the enhancement factor as our previous work has been applied to evaluate the chemical mass transfer coefficient in ozone absorption. 2 ,42dichlorophenol (2 ,42DCP) and 2
4、,62DCP or their mixture are chosen as the model compounds for simulating , and the predicted DCP concentrations are compared with some measured data. Keywords : dichlorophenol destruction ； ozonation ； stirred2tank reactor ； enhancement factor Introduction Because of the high oxidation
5、 potential of ozone (O3 ) , ozonation has been regarded as a promising method for drinking and waste water treatment. A wide range of organic pollutants in water can be degraded by O3 , O3 combined with H2O2 or UV light , which are known as Advanced Chemical Oxidation Processes (AOPs) . Compared to
6、the traditional treatment technologies , such as activated carbon adsorption or biodegradation , chemical oxidation with ozone offers the advantages of greater rate and extent of contaminant destruction. Although there are numerous reports (Hoigne , 1983 ； David , 1991) on the ozonation kinetics res
7、earch regarding reaction rate constant , stoichiometric ratio and the identification of intermediates , application of these reaction kinetics to yield essential information for successful reactor and process design has not been received sufficient attention ( Yue , 1992) . This may be partly due to
8、 the lack of the chemical mass transfer coefficient of ozone in a specific reactor. It is well known that the mass transfer rate of a gaseous solute in absorption is enhanced by chemical reactions. The extent of this influence is expressed in terms of the enhancement factor , E , which is defined as
9、 the ratio of the mass transfer coefficient of the chemical absorption to that of physical absorption. In general , it is hard to determine the chemical mass transfer coefficient by experiment especially in absorption processes accompanied by complex reactions while the physical mass transfer coeffi
10、cient may easily be obtained experimentally or from semi-empirical approaches. Based on the film theory Kuo (Kuo ,1982) proposed an iteration method for predicting the enhancement factor of mass transfer by ozone self2decomposition and ozonation reactions. Because the derived enhancement factor is a
11、n implicit expression , it is inconvenient in application to simulating the degradation rates of organic pollutants in an ozonation reactor. In this paper a simple explicit expression of the enhancement factor (Cheng , 2000) relating to the Danckwerts surface renewal model in ozone abs
12、orption with a first order ozone self2decomposition and a second order ozonation or a series of parallel ozonation reactions ( ajA + Bj pj Pj , j = 1 ,2 n) has been applied to predict the DCP destruction rate by ozonation in a semi2batch stirred tank reactor. 1 Mathematical model 1. 1 De
13、struction of one single organic pollutant in aqueous solution by ozonation When ozone is bubbled into a semi-batch stirred tank containing one organic pollutant solution , both gas and liquid phases can be assumed well mixed. The mass balance for ozone in the gas phase can be expressed
14、as (Qiu , 1999) : where cA , G , cA , G,0 , cA , i and cA ,L represent the concentration of ozone in the gas bulk , in the influent stream of the reactor ,at the gas2liquid interface and in the liquid bulk respectively. H , uG , G are the liquid height in the reactor , the velocity of gas phas
15、e and the gas holdup fraction , respectively , and t is the absorption time. kL as is the overall volumetric ozone physical mass transfer coefficient ( kL is the physical mass transfer coefficient , as is the specific interfacial area ) , E is the enhancement factor , and their product , kL as E , d
16、enotes the chemical mass transfer coefficient as discussed above. Because no dissolved ozone was detected in most experiments ( Kuo , 1982 ； Qiu , 1999) , i. e. cA ,L 0 , all the fast ozonation reactions can be assumed to complete within the liquid film. Then the depletion rate o
17、f the organic component in the liquid phase can be written as (Sotelo , 1990) : where cB ,L refers to the concentration of organic compound B in the liquid bulk , and a is the stoichiometric ratio of the ozonation reaction. In the above two equations the interfacial concentration of ozone cA
18、, i can be expressed as cA , i = cA , G Sr. The solubility ratio of ozone Sr is 0. 21 0. 28 in the pH range of 5 9 (Qiu , 1999) and an average value 0. 24 is adopted here. A simple explicit expression of the enhancement factor in ozone absorption with ozone self2decomposition and a sec
19、ond order ozonation reaction was derived in our previous work (Cheng , 2000) based on the surface renewal model as where DA and DB are the diffusivity of ozone and organic B respectively. k is the second order reaction constant , kd is the first order ozone self2decomposition reaction constant
20、. It should be noted that Eq. (2) is valid only if the following condition holds M = Md + M1 1 - ( E - 1) /Q > 4. 1. 2 Destruction of the mixture of organic pollutants in aqueous solutio
21、n by ozonation When two or more contaminants are present initially in the liquid , the absorption of ozone is accompanied by parallel ozonation reactions. If the competition between these reactions is considered to be dependent only on the reaction constants ,the depletion rate of orga
22、nic component Bj in the liquid phase can be derived as The mass balance for ozone A in the gas phase is the same as Eq. (1) . For the enhancement factor in ozone absorption with parallel ozonation reactions , an approximate expression can also be deduced relating to the surface renewal model a
23、s (Cheng , 2000) where Ej represents the supposed enhancement factor in ozone absorption accompanied by a first order ozone self2 decomposition reaction and a second order ozonation reaction of a single reactant Bj . 2 Simulating and experimental results 2 ,42DCP and 2 ,62D
24、CP isomers were chosen as the model compounds for simulatingwhich are the least and most reactive species with ozone in the DCP isomers respectively. Dichlorophenols have been widely used in the production of pesticides , dyes and other industrial chemicals. They are listed among the 65 priority pollutants by the EPA in the Clean Water Act of 1977. The water quality criteria recommended by the
