Reaction Kinetics of Ozonation of Trichloroethylene and Benzene in Gas and Liquid Phase

Chinese J. Ch. E. (Vol 8, No s)coefficient is independent of the gas velocities in each 4.3 Model estimationsriser and the liquid circulation speed. The regressivEstimation of the liquid circulation rate and gasvalue of the total frictional loss coefficient was 9.7 for hold-up on the basis of the gas fow rate and geome-the MT-ALR of D2 =9.0cm. a decrease in the di- try is of practical importance for operation and scaleameter of downcomer leads to an increase in the total up of ALR. here the liquid circulation rates obtainedfrictional loss coefficient, as shown in Table 1. More- at different gas aow rates and different diameters ofover, a minimum Et is obtained at A2/A1 =0.366, downcomer were calculated with Eq(16)using the avwhich may be due to the suitable distribution of gas erage frictional loss coefficients and the semi-empiricalmomentum in the liquidcorrelation[eq.(1))forgashold-up[8l.comparisonofthe calculating results and experimental data of liquidTable 1 Total frictional loss coeficients for differentdiameters of downcirculation velocity is shown in Fig. 6, in which a fairlygood agreement is obtained and the average error isD3,cm9.6% with the maximum of 30%. This demonstrates0.0123222±30the reliability of this model for estimation of the liquid0095213.3±4.0circulation speed in MT-ALR.6.8425970士230Figure 6 Comparison of calculating values of thediameters of downcomerFigure 4 Liquid circulation rate in downcomer (T2)as a function of gas flow rate of riser 1(T1)forNOMENCLATUREdifferent diameters of downcomer (T2)(v3=0.012m-1)A s cross-sectional area of tube i, m2model estimations;◇o△ experimental dataD2,cm:◇3.0;05.0;x7.0△90Ex energy input by gas, J-sEr energy dissipation,J-8-IEX mean kinetic energy, Jg-1coeleration due to gravity, m 8-2Ho liquid height without gaB,meQ volume flow rate,u velocity, ms-10.010.020.030.040.05中国煤化工CNMHG6 Total frictional loss coefFcient as a functionof gas fow rate of TI for different gas fow rates ofnumber of tube, il-4(1 and 3 are risers, while 2T3(D2=9.0cm)and 4 are downcomerst-.3,m-1:◇0;口0.012;△0.024X0.047Liquid Circulation in a Multi-tube Air-lift Loop Reactors gas liquid separatorSection )(in Chinese), 12, 73(1996)superficial6 Orazem, M.E., Fan, L. T, Erickson, L E, "Bubble flow inthe downflow section of an airlift tower", Biotechnol. Bio-z channeleng,21,1579(1979)7 Kawas, Y, Moo-Young, M ,"Mathematical model for tubulence", Chem. Eng J., 43, B19(1987).REFERENCES8 Liu,Y.M, Sun, G. D Yuan, N. J,"Experimental studyof gas holdup in a multi-tube airlift loop reactor", Chemical1 Chisti, M. P, Moo- Young, M, "Airlift reactor: charactreis-Engineering(in Chinese), 26(2), 23(1998)s and design considerations", Chem. Eng9 Joshi, J. B, Sharma. M. M., A circulation model for bu(1987)2 Bakker, w. A M, van Can, H J L, Tramper, J de gooible columns with Newtonian and non-NewtoniarTrans. Inst. Chem. Eng, 57, 244(1979)jer, C D, " Hydrodynamics and mixing in a multiple air-lift 10 Lehrer, I. H "Gas agitation of liquids", Ind. Eng. ChemProc.De.Dev,7,226(1986)3 Gasner, L. L,Development and application of the thin 11 Wallis, G.B., One Dimensional Two Phase Flow,McGraw.channel rectangular air lift mass transfer reactor to fermtation and waste-water treatment systems", Biotechonok. 12 Verlaan, P, Tramper J, Van't Riet K,"A hydroynamicBioeng,16,1】79(1974)model for an airlift loop bioreactor with externall loap4 Margaritis, A. Sheppard, J. D, "Mixing time and oxygenChem. Eng J, 38, B43(1986).transfer characteristics of a double draft tube fermentor", 13 Bello, R. A, Robinson, C. w, Moo-Young, M,"LiquidBiotechnol. Bioeng, 23, 2117(1981)circulation and Mixing characteristics of airlift contactor5 Liu, Y.M., Hu, H, Ding, F.x., Yuan, N. J,ExperimenCan. J. Chem. Eng, 62, 573(1984)14 Garcia Calvo, E,"A fluid dynamic model for airlift looploop reactor", Acta Petrolei Sinica(Petroleum Processingreactors",Chem. Eng. Sci, 44, 321(1989)中国煤化工CNMHGchin··J,Ch,E.8(8)267(2000)Chinese J of Chem. Eng, 8(3)272-275(2000)RESEARCHReaction Kinetics of Ozonation of Trichloroethylene andBenzene in Gas and Liquid PhasesZHONG Li(钟理)College of Chemical Engineering, South China University of Technology, G510641, ChinaKuo Chiang-HaiDepartment of Chemical Engineering, Mississippi State Univeraity, MS 39762, USAAbstract The kinetics of ozonation reactions of trichloroethylene (TCe)and benzene in gas and liquid phases at101.3 kPa and 298 K was investigated in this paper. The ozonation of TCE is first order with respect to the ozone con-nd the TCE ozonation in aqueous medium is first order with reapect to both ozone and trichloroethylene with theaverage rate constant 6. 30(mol-L-)-l-s-I. The ozonation of benzene in the gas phase is first order in ozone butindependent of the benzene concentration with the average reaction rate constant 0.0011s-I. The overall kineticaof reaction between ozone and benzene in aqueous solution is found to be frat order with one-half order in bothbezene, with the averay2.67=-. It is found that the ozonation rate of pollutantsKeywor cker than that of self-decomposition of ozone in both gas and aqueous phase.1 INTRODUCTIONa syringe into the reactor to commence a kinetic exVolatile organic compounds such as trichloroethylene periment. Changes in the concentration(absorbance)(TCE)and benzene can escape readily into the am. of the limiting reactant, ozone, during the reactionbient air from waste treatment or industrial opera- were measured using an ozone monitor(PCI Modeltions because of relatively high vapor pressures. These HC-lP, high concentration ozone monitor with tem-ompounds can react with many oxidants in air to perature and pressure compensation). Details of thethe control of these volatile pollutants is important an early paper/ay dure and analyses are described inform secondary and tertiary pollutants[1, 2. Therefore, experimental procein both air and water pollution abatement. ThoughThe vapor phase ozonation of TcE or benzenethese pollutants can be removed from a gas or a water is predominated by direct oxidation of the organictream by physical and biological methods, further reactant and ozone molecules, accompanied by seltreatment of the waste gas and wastewater may be decomposition reaction of ozone 4). The overall reac-needed for conversion of these pollutants to harmless tions can be expressed asor less harmful productsO3→+15022 KINETIC EXPERIMENTS IN GASaO3+B(pollutant)- productsPHASEAccording to the above reactions, the rate of depletionThe kinetic experiments were conducted in a batch of ozone is the sum of the rates of self-decompositionglass reactor(6. 4 liters in volume)equipped with ex- and ozonation reactionternal jacket for water circulation to maintain a con-stant temperature of 298K=+ K. Ozone gas was pro-dca/dt=k1c+kac咱duced from an ozone generator(Ozoteq Model L-50)using extra dry, pure oxygen. Following careful prepa-The rate of pollutant(B)destruction by ozonation re-ration, the reactor was filled with the ozone-oxygenaction isgas mixture till the pressure in the reactor systemd cB/dt= k2caapproached atmospheric pressure, and an appropri- Prel中国煤化工 rried out toate amount of liquid TCE or benzene(spectropho- mCNMHGtometric grade of over 99% purity)was injected by tieate that oneReceived 1999-10-21, accepted 2000-03-23Supported by Guangdong Province Natural Science Foundation(No. 970457)To whom correpondence should be addressed.Reaction Kinetics of Osonation of Trichloroethylene and Benzene in Gas and Liquid Phasesmole of ozone is required for initial conversion of where the apparent rate constant, k', can be obtainedeach mole of Tce or benzene to other intermediates from the regression analysis. According to Eq (0),aor final products. Thus, the stoichiometric ratio, a, straight line can be drawn to yield the slope as n bycan be taken as an unity for determination of the plotting k'-k1 against the initial concentration of or-kinetic parameters. Analyses of the ozone concen- ganic like TCE or benzene, cB, on the logarithmictration data indicate that the self-decomposition and scale for a series of experiments carried out under theozonation reactions are first order (p=m= 1)with same conditions, as illustrated in Figs. 3 and 4.Therespect to the concentration of ozone, as shown in regression analysis gives the following expressions ofFigs. 1 and 2. For the self-decomposition of ozone reaction rate for TCE and benzene ozonation, respecwithout any contaminant, the average rate constant tively.k1 is 0.00012s- at 101 3kPa and 298K, as deter-mined from regression analysis of the kinetic data for -dca/ dt=57.30cacB.(for TCE ozonation)(8)different experimental runs-dca/dt=0.001lca(for benzene ozonation)( 9)0This shows that the overall kinetics for ozonation ofTCE is first order with respect to the concentration ofozone and one and one- half order with respect to theconcentration of TCE with the average rate constantk2=57.30(mol- L-)-1 s-I, and that the overall re-action for benzene ozonation is first order in ozone,but independent of benzene concentration with the8000average rate constant k2=0.0011s-1Figure 1 Osone self-decomposition in gas phase at298K and 101.13kPa0,moL-1:◆00081:■0.00049:▲0.00054hn(c非Figure s Dependence of rate constant of osonation0100200300400500on TCE in gas phase at 298K and 101.13 kPaFIgure 2 Osonation of TCE or bensen in gas phaseO ozonation of TCE, ca.0=0.00019 mol L-I0.002 molLe ozonation of benzene, c,o=0.00043mol-L-1如2氵中国煤化工dc/dt=(1+k2cB)c=kc.CNMHGk=(k1+k2)ure 4 Dependence of rate constant of osonationIn(k'-k1)=mk2 +nIncBbenzene in gas phme at 298K and 101.13 kPaJchL·()2Q200)274Ch. E.(Vol 8, No. 3)3 KINETIC EXPERIMENTS IN LIQUIDPHASEA stopped-Hlow spectrophotometer system(High-TechScientific, Model SF-51)was applied to investigate thekinetics of the reaction in aqueous solutions(pH=5. 4)at 298 K. Details of the experimental procedure arereferred to Ref [ 6. The analyses are similar to thosein gas phase. A series of experiments were carriedout in this work. For the self-decomposition of ozoneand ozonation of TCE, the dimensionless concentra-tion of ozone can be plotted against reaction timethe semi-logarithmic scale to obtain a straight line foreach run, as demonstrated in Fig. 5. This implies thatFigure 5 Correlation of kinetic data for ozonethe self-decomposition can be considered first orderelf-decomposition and TCE ozonation in aqueousInwith the rate constant 0.00078s-I and that(pH=.4)at208Kthe ozonation of tce is first order with respect tozone self-decomposition, C,, D =0.0002 mol-L△。 zonation of tce,c,0=0.0002molL-1,ozone concentration. It is observed from Fig. 5 thatcB.0(TCE)=0.001 mol-L-Ithe reaction of ozonation of TCE is much faster thanthat of ozone self-decomposition, hence little deviationgas with small quantities of water to accelerate themay be made if ozone self-decomposition is ignored removal rate if the off-gas is not saturated with waterduring the analysis of kinetic data for the ozonation initially. The reaction orders are different for the twoprocess of TCE. The overall kinetics of ozonation of phases, which implies the reaction mechanisms mayTCe is found to be second order with first order for be different. It is technically feasible to remove TCEeach reactant and the reaction rate constant is 6.30 or benzene from either a water or a gas stream bymolL-)- s-l in distilled water(pH=5. 4)at 298K. ozonation processfor the ozonation of benzene under the same con-ditions, the measurements in this work confirm thatthe overall reaction is first order with one-half order NOMENCLATUREtric ratio of ozonation for TCE orin ozone concentration and one-half order in benzeneconcentration, respectively, and the average ozonationrate constant is 2.67s-l. The results of regressiveozone concentration, mol-L-lanalysis for ozonation of TCE or benzene are listed cB pollutant(TCE or beuzene)concentrationin Table 1since the rate of ozonation of TCE or benzene in cB. o initial pollutant(TCE or benzene)concentrationthe liquid phase is much faster than that in the gasmol-L-lphase, it may be advantageous to spray a contami-k1 rate constant of onone self-decomposition, a -1rate constant of ozonation for pollutantsTable1 Rate constants of osonation of TCe or benzene in aqueous solutions(pH=5. 4)at 298K6.3000020.02510.000.05050.0002中国煤化工CNMHG0.215DReaction Kinetics of Ozonation of Trichloroethylene and Benzene in Gas and Liquid Phase275apparent rate constant of ozonation for pollutants, 3 Zhong, L, Kuo, C.H., Zappi, M. E,Kinetics of gas4 Kuo, C. H, Zhong, L, Zappi, M. E,"Vapor and liquidphase ozonation of benzene", Ozone Sci. Eng.,19, 109(1997)REFERENCES5 Zhong, L, Kuo, C H, Zappi, M. E, "TreaI Glaze, w. H, Kang, J. wcontaminated water and off-gas by the ozonationest of a kinetic model forProc. of the Int. Conf on Envrion. Eng. and Chpounds with ozone and hyide in a semibatchuangzhou, China, 79(10(1989)6 Zhong, L, Kuo, C. H, "Investigation of degra2 Altshuller, A. P, Bufalini, J.air pollution: a review", Environ. Sci. Tech,, 5(4),39aqueous solutions", Guangdong Huagong,(3), 24(1998)(1971)中国煤化工CNMHG.ch.起晶仂20