Reaction Kinetics of Oxidative Absorption of Nitric oxide into Sodium Hypochlorite Solution

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1 International Journal of Advanced Research in Science and Technology journal homepage: ISSN (Print) ISSN (Online) Reaction Kinetics of Oxidative Absorption of Nitric oxide into Sodium Hypochlorite Solution Bal Raj Deshwal * and Neha Kundu Department of hemistry, A.I.J.H.M. ollege, Rohtak , Haryana, India *orresponding Author s deshwalbr@yahoo.com A R T I L E I N F O Article history: Received Accepted Available online Keywords: sodium hypochlorite, nitric oxide, order of reaction, activation energy, curved 02 Jan Jan Feb A B S T R A T Absorption of nitric oxide into aqueous sodium hypochlorite solution was studied in a stirred tank reactor. The effect of various operation variables such as nitric oxide () and sodium hypochlorite concentration, initial ph value, and reaction temperature etc. was critically examined. It was observed that the absorption process followed first-order kinetics with respect to both nitric oxide as well as sodium hypochlorite. Nitric oxide absorption reached to maxima when the initial ph value of aqueous sodium hypochlorite solution was taken 5.5 and thereafter absorption decreased slowly. The pre-exponential frequency factor and activation energy of absorption of nitric oxide into aqueous sodium hypochlorite solution were investigated and found to m 3 /(mol s) and kj/mol respectively International Journal of Advanced Research in Science and Technology (IJARST). All rights reserved. Introduction: Nitrogen oxides ( x ) and sulphur oxides (SO x ) are the main air pollutants found in the flue gases emitted from chemical plants and power plants. Technologies for removal of sulphur oxides has attained an advanced stage of development, however, it is not so in case of controlling x emission. More than 90% of x emitted from power plants consist of nitric oxide () which is relatively inert. Absorption of can be carried out either by using strong oxidative absorbent or by complex forming reagents. Wet scrubbers have been the workhorses of the chemical industry for decades and successfully used for removal of several acidic gases. In general, additives are added into scrubbing system to oxidize relatively inert into 2 which can be subsequently removed by alkaline absorbents. Aqueous solutions of numerous oxidative absorbents such as hydrogen peroxide [1], per acid [2], organic tertiary hydro peroxides [3], sodium chlorite [4]-[8], KMnO 4 [9]-[10] and chlorine dioxide [11]-[12] have been investigated to determine their efficiency in the removal of x. Several other liquid absorbents, namely FeSO 4 /H 2 SO 4, Fe(II) EDTA, Na 2 S/NaOH, Na 2 S 2 O 4 /NaOH, Na 2 SO 3, FeSO 4 /Na 2 SO 3, and urea have also been explored in the past to remove x from the exhaust gases. Kustin et al [13] reported the absorption of in aqueous solution of FeSO 4 and found that the reaction follows first order kinetics both with respect to and FeSO 4. Sada et al [14] studied the kinetics of absorption in aqueous alkaline KMnO 4 solutions and reaction was found to be first order with respect to both and KMnO 4. Uchida et al [15] also studied absorption in aqueous alkaline solutions of KMnO 4 in a stirred tank absorber. Absorption of in aqueous mixed solutions of NalO 2 and NaOH has been studied by Sada et al [4]-[5]. The reaction was found to be second order with respect to and first order with respect to NalO 2. Baveja et al [1] have studied the kinetics of liquid phase oxidation of in aqueous H 2 O 2 solutions and reaction was found to be first order with respect to both and H 2 O 2. Takeuchi et al [16] investigated that the absorption of in aqueous solutions of Na 2 SO 3 was accompanied by a fast pseudosecond order reaction with respect to and zero order with respect to Na 2 SO 3. Teramoto et al [17] measured the absorption rates of in aqueous solutions of Fe (II) EDTA. The reaction was found to be first order with respect to both and Fe (II) EDTA. Warshaw [18] studied the absorption of nitrogen oxides into aqueous acidic urea solution. It is pertinent to mention here that aqueous acidic urea solution cannot be used for an exhaust gas which contains mostly (as in the case of flue gas from power plants). Khan and Adewuyi [19] studied the absorption-oxidation process of by aqueous Bal Raj et.al Page 313

2 solution of Na 2 S 2 O 8 in a bubbling reactor. Na 2 S 2 O 8 proved an economical agent for x removal in gasliquid contactors due to its low cost and environmentally benign nature. Guo et al [20] used Fenton reagent for removal. Kinetics of absorption in aqueous iron (II) thiochelate solutions were investigated by Shi et al [21]. Recently, Mondal and helluboyana [22] investigated combined SO 2 and removal from simulated gas stream by NaOl and observed 100% SO 2 and 92% removal efficiencies respectively. hen et al [23] also used sodium hypochlorite as the oxidative absorbent for absorption. Sodium hypochlorite has high oxidative ability in acidic medium and can be used as an effective additive for x control in wet flue gas desulfurization scrubber. Inadequate studies have been made to investigate the oxidative absorption of into sodium hypochlorite solution; therefore, the present manuscript attempts to study the absorption kinetics of into sodium hypochlorite solution and to examine the effect of various operation variables such as concentration of and sodium hypochlorite, initial ph value, and reaction temperature etc. Experimental: The absorption kinetics of this study was carried out in a lab scale stirred tank reaction. A schematic diagram of the experimental system is shown in Fig. 1. The flue gas cleansing unit included a simulated flue gas supply system, bubbling reactor, ph control system, and data acquisition system. Data Acquisition System Output Gas Analysis Sample onditioner Input Gas Analysis Simulated Flue gas Sample onditioner Agitator Vent H 2 SO 4 solution. The inlet and outlet x concentrations were analyzed using the x analyzer (hemiluminiscent type, Model: 42, Thermo Environmental Instruments, USA) after removing the moisture in the sample conditioner. Results and discussion: Reaction between and Sodium hypochlorite: The reaction between and sodium hypochlorite solution in presence of sulphuric acid takes place in various steps as follows: 2NaOl + H 2 SO 4 2HOl + Na 2 SO 4 (1) + HOl 2 + Hl (2) 2 + H 2 O 2H 3 + (3) H 2 O H 3 + H 2 (4) From the above steps, the overall reaction between nitric oxide and aqueous sodium hypochlorite may be written as: 4 + 5NalO + 2H 2 O 3H 3 + H 2 + 5Nal (5) Reaction Kinetics: The absorption rate of into aqueous sodium hypochlorite solution can be expressed by: R = kg(p,b - p) = Ek L( -,b) (6) Here E is the enhancement factor, which is a function of the Hatta number ( H a ). Since concentration of NaOl in the bulk liquid being much greater than the interfacial concentration, thus the kinetics of the reaction becomes pseudo-first order and in such instantaneous reaction, Hatta number is assumed to be equal to enhancement factor E. The Hatta number can be calculated as follows: Gas Mixing MF MF ph ontroller M H 2 SO 4 Solution 1 D (7) H a k( mn) kl NaOl N 2 T Temp. ontroller REATOR Figure: 1. A schematic diagram of experimental apparatus. The simulated flue gas was obtained by controlled mixing of and N 2 using mass flow controllers (MF). The bubbling reactor is made up of acrylic material. The inner diameter and height of the reactor are 15 and 45 cm respectively. ontinuous stirring was provided by mechanical agitator with a speed of 250 rpm. The temperature of the bubbling reactor was controlled within 55 ± 0.1 o, the most common operating temperature for wet scrubbing methods. Initial ph of solution was adjusted by addition of 0.2M Further, kg and k L in Eq. (6) are the gas phase and liquid phase mass transfer coefficients respectively. The magnitude of kg and k L can be obtained by using the method as described by Deshwal and Lee [24]. Partial pressure of at interface i.e. p can be calculated by using Henry s law: p = H (8) Here is the interfacial concentration of in aqueous sodium hypochlorite solution, which is related to the ionic strength of the solution as follows: Bal Raj et.al Page 314

3 R (mol s -1 m -2 ) log, i, iw Where - (K I K I ) (9) K NaOl and NaOl NaOl K H 2 SO 4 H2SO4 H2SO4 are the salting-out parameters for the electrolyte NaOl and H 2 SO 4 respectively. Due to the small concentration of acid, the effects of H + and SO 2 4 ion on can be neglected, so Eq. (9) reduces to: log, i, iw - (K I ) (10) NaOl NaOl to 1000ppm.The effect of gas-liquid interfacial concentration on its absorption rate is shown in Fig. 2, which clearly shows that the absorption rate of increased with the increasing gas-liquid interfacial concentrations. Danckwerts [26] indicated that during gas-liquid reaction, if 1<< M << E i, then gas-liquid reaction can be assumed to be a pseudo m th order reaction, where M is close to the fast reaction enhancement factor and E i is the enhancement factor for instantaneous reaction. 1E-4 The salting-out parameter of an electrolyte is obtained by addition of respective terms for the anions, cations and the gas. In short, it can be given by: K = X a + X c + X g (11) 1E-5 1E-6 [NalO] 0 = 0 [NalO] 0 = 0.04mol/L [NalO] 0 = 0.08mol/L The values of X for various species are available in literatures which are Χ [25], Χ 2- SO [25], Χ [14]. However, Χ lo is not available in the literature, thus the contribution of hypochlorite ion is assumed to be the same as that of chlorite ion i.e. Χ Na lo The rate of reaction between nitric oxide and acidic sodium hypochlorite is assumed to be m th order with respect to and n th order with respect to NaOl. Thus the absorption rate of into aqueous NaOl solution could be expressed by the gas-liquid mass transfer theory as proposed by Danckwerts [26]: R = Where 2 m D k 1 m+ 1 n ( m n) D NaOl (12) is the diffusion coefficient of, which can be calculated from the Wilke-hang equation as shown in Bird et al [27] and its value is found to be cm 2 s -1 at 50 o. After determining the value of m and n, the reaction rate constant k(m n) can be calculated from Eq. (12). The value of rate constants were obtained at various temperatures ranging from 298K to 328K and then activation energy E a and pre-exponential frequency factor A were obtained from Arrhenius Equation. Order of Reaction with respect to : Experiments were performed to investigate the effect of gas-liquid concentration on its absorption rate at ph = 6.0, T = 328K, and of zero, 0.04M and 0.08M. concentration was varied from 200 ppm 1E-7 1E-4 1E-3 (mol m -3 ) Figure 2: Effect of gas-liquid conc. on its absorption rate at ph = 6.0 at 328K. As shown in Fig. 2, the values of the enhancement factor and the instantaneous enhancement factor for this study meet the requirement of 1<< M << E i, as a result, the absorption rate of into aqueous sodium hypochlorite solution can be expressed by Eq. (12). It is obvious that there is a linear relationship between log R and log and the average slope of these lines is close to 1, that is to say, ( m 1) / 2is equal to 1, thus the value of m comes out nearly one i.e. the reaction is first-order with respect to. Order of Reaction with respect to NaOl: Experiments were performed to investigate the effect of NaOl concentration on absorption rate at T = 328K, ph = 6.0 and = mol/l. NaOl concentration was varied from 0.02M to 0.1M. Fig. 3 shows the effect of NaOl concentration on absorption rate. There is a linear relationship between log R and log NaOl,i. The slope of this line is about 0.5, i.e. n /2= 0.5, so the reaction follows 1 st order kinetics with respect to NaOl. Putting the value of order of reaction of and NaOl, the absorption rate under the fast-reaction regime can be expressed by: R = 2 2 D k (13) NaOl Bal Raj et.al Page 315

4 R (x10 5 mols -1 m -2 ) R (x10 5 mols -1 m -2 ) R (mol s -1 m -2 ) 8E-5 6E-5 4E-5 Effect of temperature on absorption rate: (mol m -3 ) Effect of temperature on absorption rate was examined at,0 = 800ppm, initial ph of 6.0, of 0.04M and 0.08M by varying the temperature from 298K to 328K. It is obvious from Fig. 5 that absorption rate increased with increasing temperature. Though higher temperature will certainly decrease the solubility of in the scrubbing solution, but the diffusion coefficient of and moreover the rate constant increased with temperature. The latter effect seems to play significant role on absorption process. Figure 3: Effect of NaOl conc. on absorption rate at 328K, ph = 6.0 and = mol/l respectively. Effect of initial ph value on absorption rate: Experiments were performed to investigate the effect of initial ph value on absorption rate at T = 328K,,0 = 800 ppm and of 0.04M and 0.08M. Fig. 4 shows the effect of initial ph on absorption rate. Initial ph of the solution was varied from 4 to 7. absorption rate increased with the increasing ph value and reached to maximum value at ph 5.5, thereafter absorption rate decreased with increasing ph value. As clear from Eq. 2 to Eq. 4, NaOl firstly oxidizes into 2, which is subsequently absorbed in scrubbing solution. The low ph value is unfavorable to 2 absorption, which eventually hinders absorption. The reduction half-reaction of HOl may be written as: HOl + H + +2e - l - + H 2 O E o = 1.482V (14) According to Nernst equation, the electrode potential - E o ( HOl / l ) decreases with increasing ph value, so the oxidizing ability of scrubbing solution decreases, as a result, absorption rate decreased too = 0.04 mol/l =0.08 mol/l ph 0 Figure 4: Effect of initial ph value on absorption rate at 328K, and,0 = 800 ppm = 0.04mol/L = 0.08mol/L Temperature (K) Figure 5: Effect of temperature on absorption rate at,0 = 800ppm and initial ph of 6.0. Activation Energy and Pre-exponential Frequency factor: It is well known that rate constant is markedly affected by the temperature. The value of rate constants were obtained at various temperatures ranging from 298K to 328K and then graph was plotted between log k vs. 1/T. The activation energy E a and preexponential frequency factor A were obtained from slope and intercept of the Arrhenius plot using logarithm form as follows: E A log k loga - (15) 2.303RT The activation energy E a and frequency factor A were found to m 3 /(mol s) and kj/mol, respectively. onclusion: Reaction kinetics of oxidative absorption of into aqueous NaOl solution was studied in a lab scale bubbling reactor. The operating variables included ppm, mol/L NaOl solution, ph of 4-7, and temperature of o respectively. The reaction was found to be first-order with respect to both and NaOl. absorption reached to the maximum value when the initial ph value of NaOl solution is taken 5.5. The pre-exponential frequency factor and the activation energy were found m 3 /(mol s) and kj/mol respectively. It is pertinent Bal Raj et.al Page 316

5 to mention here that sodium hypochlorite is a potential oxidative absorbent for controlling x emission. Nomenclature: A pre-exponential frequency factor, m 3 /(mol s) Interfacial concentration of at the gas- NalO 3 /NaOH solution interface, mol/m 3 w Interfacial concentration of at the gaswater interface, mol/m 3 D Diffusion coefficient of, m 2 /s E a Activation energy, kj/mol E Enhancement factor E i Instantaneous enhancement factor H Henry s constant of, Pa m 3 /mol I Ionic strength, mol/l K Salting-out parameter k G Gas phase mass transfer coefficient, m/s Liquid phase mass transfer coefficient, m/s k L k mn Rate constant of ( m, n )-order reaction, m 3 /(mol s) M Non-dimensional criterion number of absorption m Reaction order with respect to N absorption rate, mols -1 m -2 ) n Reaction order with respect to NaOl p partial pressure, Pa R Gas constant, J/(mol K) T Absolute temperature (K) X a ontribution to K of the anion, mol/l X c ontribution to K of the cation, mol/l X g ontribution to K of the gas, mol/l μ Dynamic viscosity of the solution, kg/(m s) Subscripts a anion b bulk c cation g gas i interface 0 initial value w water References: 1. K. K. Baveja, D. Subba Rao, M. K. Sarkar, Kinetics of absorption of nitric oxide in hydrogen peroxide solutions, J. hem. Eng. Jpn., 12, , D. Littlejohn, S. hang, Removal of x and SO 2 from flue gas by per acid solution, J. Ind. Eng. hem. Res., 29, , (1990). 3. H. Perlmutter, H. Ao, H. Shaw, Absorption of promoted by strong oxidizing agent: 1. organic tertiary hydro peroxides in n-hexadecane, J. Environ. Sci. Technol., 27, , (1993). 4. E. Sada, H. Kumazawa, I. Kudo, T. Kondo, Absorption of in aqueous mixed solutions of NaIO 2 and NaOH, hem. Eng. Sci., 33, , E. Sada, H. Kumazawa, I. Kudo, T. Kondo, Absorption of lean x in aqueous solutions of NalO 2 and NaOH, Ind. Eng. hem. Proc. Des. Dev., 18, , Brogen, H. T. Karlsson, I. Bjerle, Absorption of in an aqueous solution of NalO 2, J. hem. Eng. Technol., 21, 61-70, H. hu, T. W. hien, B. W. Twu, The absorption kinetics of in NalO 2 /NaOH solutions, J. Hazard. Mater., B84, , H. K. Lee, B. R. Deshwal, K. S. Yoo, Simultaneous removal of SO 2 and by sodium chlorite solution in wetted-wall column, Korean J. hem. Eng., 22-2, , Brogen, H. T. Karlsson, I. Bjerle, Absorption of in an alkaline solution of KMnO 4, J. hem. Eng. Technol., 20, , H. hu, S. Y. Li, T. W. hien, The absorption kinetics of from flue gas in a stirred tank reactor with KMnO 4 /NaOH solutions, J. Environ. Sci. Health, A33, , D. S. Jin, B. R. Deshwal, Y. S. Park, H. K. Lee, Simultaneous removal of SO 2 and by wet scrubbing using aqueous chlorine dioxide solution, J. Hazard. Mater., B135, , B. R. Deshwal, D. S. Jin, S. H. Lee, S. H. Moon, J. H. Jung, H. K. Lee, Removal of from flue has by aqueous chlorine-dioxide scrubbing solution in a labscale bubbling reactor, J. Hazard. Mater., 150, , K. Kustin, I. A. Taub, E. Weinstock, A kinetic study of the formation of the ferrous-nitric oxide complex, Inog. hem., 5, , E. Sada, H. Kumazawa, N. Hayakawa, I. Kudo, T. Kondo, Absorption of in aqueous solutions of KMnO 4, hem. Eng. Sci., 32, , S. Uchida, T. Kobayashi, S. Kageyama, Absorption of nitrogen monoxide in aqueous KMnO 4 /NaOH and Na 2 SO 4 /FeSO 4, solutions, Ind. Eng. hem. Proc. Des. Dev., 22, , H. Takeuchi, M. Ando, N. Kizawa, Absorption of nitrogen oxides in aqueous sodium sulfite and bisulfate solutions, Ind. Eng. hem. Proc. Des. Dev., 16, , M. Teramoto, S. Hiramine, Y. Shimada, Y. Sugimoto, H. Teranishi, Absorption of dilute nitric monoxide in aqueous solutions of Fe(II)EDTA and mixed solutions of Fe(II)EDTA and Na 2 SO 3, J. hem. Eng. Jpn., 11, , A. Warshaw, Removal of nitrogen oxides from a gas stream, US Patent, , N. E. Khan, Y. G. Adewuyi, Absorption and oxidation of nitric oxide () by aqueous solutions of sodium persulfate in a bubble column reactor, Ind. Eng. hem. Res., 49-18, , R. Guo, W. Pan, X. Zhang, J. Ren, Q. Jin, H. Xu, J. Wu, Removal of by using Fenton reagent solution in a lab-scale bubbling reactor, Fuel, 90-11, , Y. Shi, H. Wang, S. hang, Kinetics of absorption in aqueous iron (II) thiochelate solutions, Environ. Prog., 16-4, , M. K. Mondal, V. R. helluboyana, New experimental results of combined SO 2 and removal from simulated gas stream by NalO as low-cost absorbent, hem. Eng. J., 217, 48 53, L. hen,. Hsu,. Yang, Oxidation and absorption of nitric oxide in a packed tower with sodium hypochlorite aqueous solutions, Environ. Prog., 24-3, , B. R. Deshwal, H. K. Lee, Mass transfer in the absorption of SO 2 and x using aqueous euchlorine scrubbing solution, J. Environ. Sci., 21, , Bal Raj et.al Page 317

25. K. Onda, E. Sada, T. Kobayashi, S. Kito, K. Ito, Salting-out parameters of gas solubility in aqueous salt solutions, J. hem. Eng. Jpn., 3-1, 18-24, 1970. 26. P. V.

6 25. K. Onda, E. Sada, T. Kobayashi, S. Kito, K. Ito, Salting-out parameters of gas solubility in aqueous salt solutions, J. hem. Eng. Jpn., 3-1, 18-24, P. V. Danckwerts, Gas-Liquid Reactions, McGraw- Hill, New York, USA, h. I , R. B. Bird, W. E. Stewart, E. N. Lightfoot, Transport phenomena, New York, Wiley, , Author Profile: Dr. Bal Raj Deshwal has competed his Ph.D. in hemistry from M. D. University, Rohtak and presently working as Associate Professor in Department of hemistry, A. I. J. H. M. ollege, Rohtak. He has been awarded Brain Pool Fellowship by Korean Federation of Science and Technology (KOFST), S. Korea and worked as Visiting Scientist in Korea Institute of Energy Research, Daejon, S. Korea for 3 years. He has been awarded Marie urie Fellowship by European ommission too. Bal Raj et.al Page 318

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