Microsoft Word - cet-01.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 46, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Peiyu Ren, Yancang Li, Huiping Song Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-37-2; ISSN 2283-9216 Kinetics of the Preparation of Chlorine Dioxide by Sodium Chlorite and Hydrochloric Acid at Low Concentration Zhengbo Mo*a, Songtao Hua, Dedong Hub a Qingdao Technological University,. 11, Fushun Rd, Shibei,Qingdao, Shandong,China; b Qingdao University of Science and Technology, 99 Songling Rd, Laoshan, Qingdao, Shandong, China. 869777852@qq.com The kinetics of the reaction between sodium chlorite and hydrochloric acid is studied at various temperatures and molar concentrations of chlorite and acid at low concentrations. The reaction rate law is established, and macrokinetics formula is obtained. Reaction has been found 1 and 1.39 order with respect to chlorite and hydrochloric acid respectively when preparing chloride dioxide using low concentrations of sodium chlorite and hydrochloric acid. The temperature dependence of the reaction is also investigated and pre-exponential Arrhenius parameter as well as activation energy are determined. 1. Introduction Chloride dioxide (ClO2) is usually used as an oxidant for pulp bleaching and disinfection. For its application in water treatment, chloride dioxide can achieve a good disinfection effect without producing chlorinated organic matters such as trihalomethanes (THMs) and chlorophenol that are carcinogens or mutagens, which was reported by Yu and Zhang (2012). Under normal temperature, chloride dioxide exists as a gas that has an excellent antibacterial and disinfection effect on a variety of microorganisms. Moreover, chloride dioxide does not produce toxic chlorides and possesses a better diffusibility, penetrability and use uniformity compared with liquid chlorine dioxide. These features make chloride dioxide particularly suitable for the prevention and treatment of microbial pollution within a large space. Disinfection with gaseous chlorine dioxide has an antibacterial effect that is 400-2000 times stronger than that of other similar air disinfection and purification techniques. Producing no harmful impact on humans, chlorine dioxide is classified as a disinfectant of A1 safety class by the World Health Organization. Zhang (2012) reported the technology development and application of Chlorine dioxide disinfection. Xi (2012) studied the disinfection effect using chlorine dioxide in public space. Chlorine dioxide is generally prepared using sodium chlorite or sodium chlorate by reduction reaction in acidic conditions. When sodium chlorate is used as the raw material, sulfur dioxide, methanol or hydrogen peroxide is usually chosen as the reductant to produce chlorine dioxide under acidic conditions. But this reaction produces many undesired byproducts, typically chlorine. In recent years, the generating method and kinetics of chlorine dioxide has often been studied. Wang (2008) and Yang (2007) studied the production methods of chlorine dioxide; Chen (2003), Qian (2004), Shi (1999) and Zhu (2011) studied the kinetics of the chlorine dioxide from hydrogen peroxide. Liu (2009) studied the kinetics of chlorine dioxide preparation by R5 method in conventional heating and microwave heating. Jin (2008) studied the kinetic of formation of high- purity chlorine dioxide gas using sodium chlorate as raw materials. Li (2013) studied the kinetics of the reaction for generation of chlorine dioxide from sodium chlorate and hydrochloric acid, and conclude that the rate equation of the reaction system was a formula with mixed- order (combination of first-order and second-order) towards ClO3 -. B.R.Deshwal (2004) studied the reaction between sodium chlorate and sodium chloride in presence of aqueous sulfuric acid and the rate law is established. The temperature dependence of the reaction is also investigated and pre-exponential Arrhenius parameter as well as activation energy are determined. Fang (2013) reported the preparation of chlorine dioxide by sodium chlorite as raw material. His main research is DOI: 10.3303/CET1546009 Please cite this article as: Mo. Z.B., Hu S.T., Hu D.D., 2015, Kinetics of the preparation of chlorine dioxide by sodium chlorite and hydrochloric acid at low concentration, Chemical Engineering Transactions, 46, 49-54 DOI:10.3303/CET1546009 49 the reaction between sodium chlorite and slow release activator (such as sodium persulfate and potassium peroxymonosulfate) to prepare solid controlled-release chlorine dioxide. High-purity chlorine dioxide gas can be prepared by the reaction of sodiumchlorite and hydrochloric acid for ambient air disinfection. The stoichiometric equation is expressed as follows: 2 2 25 4 4 5 2NaClO HCl ClO NaCl H O+ → ↑ + + (1) Since chlorine dioxide gas is explosive and tends to decompose upon heating, it is not suitable for storage and transport, hence it is usually produced immediately before use. For disinfection of ambient air in a large confined space, the concentration of chlorine dioxide needs to be maintained at 0.1-0.28 mg/m3. An excess concentration is harmful to humans, while a low concentration has poor disinfection effect. In order to control the concentration of chlorine dioxide in the ambient air, the reaction needs to be tightly controlled at a low rate. Moreover, the reaction rate needs to be adjusted timely based on the concentration of chlorine dioxide in the air. One effective method to accurately calculate the addition amount of reactants and to reduce the reaction rate is to reduce the concentration of reactants. Research on generating chlorine dioxide with low concentrations of sodium chlorite and hydrochloric acid to generate chlorine dioxide has not been reported. In this study, sodiumchlorite and hydrochloric acid were used to produce chlorine dioxide, and the kinetic characteristics of the reaction at low concentrations were discussed. A macroscopic reaction rate equation was built with the purpose of providing guidance for the design of the reactor and for further applications of the new preparation technique. 2. Experiment 2.1 Experiment reagents Sodium chlorite (A.R), hydrochloric acid (A.R 37.5%), potassium iodide (A.R), sodiumthiosulphate (A.R). 2.2 Equipment Thermostatic water bath, vacuum pump. 2.3 Experimental facilities 1. Thermostatic water bath 2. Reactor 3. Sample injector 4. Feeding funnel and air inlet 5. Absorption bottle for potassium iodide 6. Absorption bottle for potassium iodide 7. Vacuum pump Figure 1: Experimental facilities 2.4 Experimental procedure Thermostatic water bath was adjusted to a certain temperature, and dilute solution of sodiumchlorite was added into the reaction flask. Meanwhile, hydrochloric acid solution was added into the separating funnel. The vacuum pump was started. After pressure adjustment, the separating funnel was opened, and the hydrochloric acid solution was added all in once to initiate the reaction. Time zero started from this moment. For every 2-3 min, the reaction liquid was sampled with a 1 ml syring. The concentration of chlorite in the sample was determined by five-stepiodometry, and the conversion rate of sodium chlorite was calculated at each sampling. The generated chlorine dioxide gas was discharged under negative pressure after passing through potassium iodide solution repeatedly. After the reaction was completed, the thermostatic water bath stopped, the pressure was adjusted to atmospheric pressure, and the vacuum pump was turned off. 2.5 Measurements and analysis The concentration of chlorite was determined by five-step iodometry, and the concentration of hydrogen ions was calculated by subtraction. 50 3. Results and discussion According to the equation of reaction of sodium chlorite and hydrochloric acid (1), if the reaction rate was measured by the amount of sodium chlorite consumed, the reaction kinetic equation can be expressed as follows: 2 2 [ ] [ ] [ ] d NaClO k NaClO HCl dt α β− = (2) Where [NaClO2] and [HCl] are the molar concentrations of sodium chlorite and hydrochloric acid, respectively; α and β are the order of reaction with respect to sodium chlorite and hydrochloric acid, respectively. The reaction conditions, namely, temperature and reactant concentration, were changed, and the reaction liquid was sampled at different time points. By measuring the changes of the concentration of sodium chlorite with time, the order of reaction with respect to the reactant and the reaction rate constants were fitted. On this basis, the reaction kinetic equation was derived. First, the temperature was fixed at 298.15 K and the initial mass concentration of sodium chlorite was at approximately 0.2%. The addition amount of hydrochloric acid was changed, and the variation of the concentration of sodium chlorite was recorded under each addition amount. We got 7 groups of experimental data numbered as group 1 to group 7. Then the initial mass concentration of sodium chlorite was fixed at approximately 0.2% and that of hydrochloric acid at 0.32mol/L. The temperature was changed, and the variation of the concentration of sodium chlorite over time was observed. 7 groups of experimental data were got and numbered as group 8 to group 14. 3.1 Determination of the order of reaction Because sodium chlorite is expensive, excess amount of hydrochloric acid is usually added in real production. This was also the strategy used in the present study. Because the concentration of hydrochloric acid was far excessive, it changed very little during reaction, so the concentration of hydrochloric acid [HCl] can be considered as a constant. Trial calculation and fitting were performed under different preset order of reaction with respect to sodium chlorite. It was found that the ln(C0/Ct) of sodium chlorite (where C0 and Ct were the molar concentrations of sodium chlorite at time 0 and t, respectively) was a linear relationship with time. Thusthe order of reaction α with respect to sodium chlorite was 1, as shown in Fig. 2 and Fig. 3. Figure 2: Relationship between ln(C0/Ct) of sodiumchlorite and time t at 298.15K Figure 3: Relationship between ln(C0/Ct) of sodiumchlorite and time t at different temperature Then the reaction rate equation (2) is changed into 2 2 [ ] [ ] [ ] d NaClO k NaClO HCl dt β− = (3) After sorting, equation (3) becomes 2 2 [ ] [ ] [ ] d N a C lO k H C l d t N a C lO β− = (4) When the temperature is fixed, the reaction rate constant k is a constant.With a great excess of hydrochloric acid,thus [HCl] can be approximated as a constant. Integration is performed for both sides of equation (4)to derive ln(C0/Ct)=k[HCl] β t 51 where C0 and Ct are the molar concentrations of sodium chlorite at time 0 and t, respectively. Therefore, the slopes of each line in Fig. 2 and Fig. 3 are calculated as follows: S=k[HCl]β Logarithm is taken on the two sides: logS=logk+βlog[HCl] When the temperature is fixed on 298.15K, the reaction rate constant k is the same. The slope S of each line fitted using the data group 1 to group 7 and the average concentration of hydrochloric acid before and afterreaction were calculated. The table 1 below was obtained. Table 1: Slopes of lines fitted using each group of data and the corresponding concentrations of hydrochloric acid at 298.15K Group1 Group2 Group3 Group 4 Group 5 Group 6 Group 7 Slope S 0.000503 0.001180 0.003288 0.003518 0.003950 0.004857 0.005798 [HCl](mol/L) 0.1532 0.3113 0.4720 0.6324 0.6194 0.7938 0.9556 log[HCl] -0.8147 -0.5068 -0.3261 -0.1990 -0.2080 -0.1003 -0.0197 logS -3.2984 -2.9282 -2.4830 -2.4537 -2.4034 -2.3137 -2.2367 Straight lines were plotted with log[HCl] on the x axis and logS on the y axis (Table 1). The slope of the line was the reaction order β with respect to hydrochloric acid, which was 1.39 (Fig. 4). Figure 4: Calculation of the reaction order β with respect to hydrochloric acid With the order of reaction obtained, the reaction rate equation changes into 2 1.39 2 [ ] [ ] [ ] d NaClO k NaClO HCl dt − = (5) 3.2 Calculation of activation energy and pre-exponential factor In formula (5), k is rate constant of the reaction expressed as 52 E a R Tk A e − = × where A is pre-exponential factor, and Ea is the activation energy. According to formula (4), [ ] S H C l k β = , thus [ ] E a R T S H C l k A e β − == × . Taking logarithm on the two sides, there is ln ln[ ]ln E a S H C l R T k lnA β= −= − . That is ln ln[ ]S HClβ− = E a R T ln A − . In order to derive the equation of rate constant k, the slope S of each line under different temperatures (Fig. 3) was calculated. The average concentration of hydrochloric acid [HCl] in the reaction liquid under different temperature was also calculated and shown in Table 2. The values of ln ln[ ]S HClβ− and -1/T were further calculated under each temperature. Table 2: Relationship between ln ln[ ]S HClβ− and -1/T under different temperature Group8 Group9 Group10 Group11 Group12 Group13 Group14 T(K) 278.15 283.15 288.15 291. 65 298.15 303.15 308.15 Slope S 0.0003631 0.0004748 0.0005308 0.0007571 0.00118 0.0012 0.00174 [HCl] 0.3158 0.3144 0.3137 0.3133 0.3113 0.3121 0.3106 βln[HCl] -1.6022 -1.6084 -1.6115 -1.6132 -1.6221 -1.6186 -1.6253 lnS-βln[HCl] -6.3187 -6.0443 -5.9297 -5.5728 -5.1201 -5.1069 -4.7286 -1/T -0.003595 -0.003532 -0.003470 -0.003429 -0.003354 -0.003299 -0.003245 Fig. 5 was plotted according to the above data. Figure 5: Gain of activation energy and pre-exponential factor According to Figure 5, the slope S=Ea/R=4559.2 then Ea=8.314×4559.2=37905 J/mol; The intercept lnA=10.036, then the pre-exponential factor A=2.283×104. So the constant of reaction rate 53 3 7 9 0 5 42 . 2 8 3 1 0 R Tk e − = × × . 4. Conclusions To conclude, the macroscopic rate equation for the reaction between sodium chlorite and hydrochloric acid was fitted as 37905 2 1.3 2 4 92.283 10 [ ] [ ] [ ]RT d N aC lO N aC lO C l t e H d − − = × × × × This equation lays the theoretical basis for reactor design and further development and application of the new technique. Acknowledgments This research was supported by Technology Innovation Fund for Small and Medium-sized Enterprises of Jinan Science and Technology Bereau (20140344). References Chen Y., 2003, A Study on Hydrogen Peroxide Based Production of Environmental Benign Oxidant Chlorine Dioxide and Reactive Kinetics [D], South China University of Technology, 06 Deshwal B.R., Lee H. K., 2004, Kinetics and mechanism of chloride based chlorine dioxide generation process from acidic sodium chlorate, Journal of Hazardous Materials B108, 173–182, DOI: 10.1016/j.jhazmat. 2003.12.006 Fang Y., 2013, Study on the preparation of chlorine dioxide with sodium chlorite [M]. Nanjing University of Science and Technology, 03. Jin R.Y., 2008, Study on the kinetic of the formation reaction of high- Purity Chlorine dioxide gas and its sterilization effect to microbe. North University of China, 08. Li X.J., Jiang D.D., Zhang Y.J., 2013, Kinetics of the reaction for generation of chlorine dioxide from sodium chlorate and hydrochloric acid [J], Advanced Materials Research, vol 634-638, 1, 546-550, DOI: 10.4028/www.scientific.net/AMR.634-638.546 Liu S.Q., 2009, Kinetic research of chlorine dioxide preparation by R5 method in conventional heating and microwave heating [M]. Nanjing University of Science and Technology, 05. Qian Y., Chen Y., Jiang Y.B., Ji H.B., 2004, Reaction mechanism and dynamics of H2O2-based chlorine dioxide production [J]. Journal of Chemical Industry and Engineering, 55(10): 1719-1722. Shi L.S., Wang Z.F., Liu M., Liu C.B., Cheng J.L., 1999, Kinetic study of the preparation of chlorine dioxide. Journal of shandong university of technology [J].29(6): 581-585. Wang H.L., 2008,The Manufacture of High Purity Chlorine Dioxide Gas Generator with Chemical Method [M].North University of China, 05. Xi X.Y., 2012, Study on the disinfection effect using chlorine dioxide in public space [M]. North University of China, 06. Yang T.B., 2007, Study of production technology and reaction mechanism of chlorine dioxide gas [M].North University of China, 05. Yu M.X., Zhang W.F., 2012, The research progress of chlorine dioxide disinfectant and sterilization applications [J], Chinese Journal of Disinfection, 29(2):132-135 Zhang S.L., 2012, The technology development and application prospect of Chlorine dioxide disinfection [J]. China science and Technology Review, 28:634 Zhu M.X., Huang H., Dong X., Shen L.N., Xu Y.H., 2011, Kinetics and mechanism study on chlorine dioxide generation with hydrogen peroxide [J]. Journal of Functional Materials, 42: 604-609. 54