Kinetics and Reaction Mechanism of Catalytic Oxidation of Ammonium Sulfite

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1 Asian Jounal of hemisty; Vol., No. 1 (1), Kinetics and Reaction Mechanism of atalytic Oxidation of Ammonium Sulfite SHAOPENG GUO 1, JIAHUA WANG, XIN HEN, JIA ZHANG 1, RUI ZHANG 1 and JUN LU 1,* 1 Key Laboatoy of oal Gasification, Ministy of Education, East hina Univesity of Science and Technology, Shanghai 7, P.R. hina MWH Envionmental Engineeing (Shanghai) o., Ltd., Shanghai 11, P.R. hina SINOPE Ningbo Engineeing o., Ltd., Ningbo 151, P.R. hina *oesponding autho: lujun@ecust.edu.cn Received: 1 Febuay 1; Accepted: May 1; Published online: Decembe 1; AJ-17 The effects of five catalysts on the oxidation ate of ammonium sulfite wee compaed by intinsic expeiments using self-designed bath eacto. Fe + was tuned out to be a good catalyst fo oxidation of ammonium sulfite. Using stied bubbling eacto, the kinetics of catalytic oxidation of sulfite in the ammonium-based wet flue gas desulfuization pocess wee investigated by vaying concentations of (NH ), (NH ) and Fe +, ph, ai flow ate and tempeatue. The eaction was found to be fist ode with espect to ammonium sulfite,. ode with espect to Fe + and the oxidation could each the maximum ate when ph value was close to. The mechanism of the oxidation was discussed and concluded that the oxidation is contolled by diffusion of oxygen. Keywods: Kinetics, Mechanism, atalyst, Ammonium sulfite, Ammonia-based flue gas desulphuization. INTRODUTION Ammonia-based flue gas desulfuization pocess is a wellknown technique with high efficient emoval of and the ammonium sulfite is obtained as a bypoduct consequently. If ammonium sulfite is used diectly as a poduct, it may decompose into again which will poduce new envionmental pollution 1,. ompaed with ammonium sulfite, ammonium sulfate-the oxidation poduct of ammonium sulfite, is much moe valuable and stable when it is used as a fetilize in agicultue. Howeve, due to the low oxidation ate, most of the injected ai is wasted duing the foced oxidation pocess of flue gas desulfuization, esulting in excess investment and enegy. Theefoe, it is cucial to tun ammonium sulfite into ammonium sulfate efficiently and economically to ealize lage-scale application of ammonia-based flue gas desulfuization. Many scholas have studied the poblem of the oxidation of aqueous ammonium sulfite. Gukan et al. made a eseach on the kinetics of the heteogeneous oxidation of ammonium sulfite with oxygen tansfeing fom the gas phase and poposed that the eaction mechanism and consequently the ode of the intinsic ate of eaction wee dependent on the oxygen to sulfite atio. Zhou et al. 5 conducted expeiments of the heteogeneous oxidation of concentated ammonium sulfite and found the eaction to be. ode with espect to sulfite above a citical sulfite concentation and fist ode with espect to oxygen. Long et al. studied a homogeneous catalyst system fo the oxidation of concentated ammonium sulfite. The sulfite oxidation ate was found to be independent of the initial sulfite concentation, the eaction ode with espect to oxygen was1.. Jia et al. 7 investigated the kinetics of oxidation of total sulfite using bubbling appaatus, the eaction was found to be -.5 ode with espect to sulfite in the tempeatue ange of K. Except fo ammonium sulfite, the catalytic oxidation eactions of othe kinds of bypoducts in wet flue gas desulfuization have eceived much attention duing the last yeas. Zhao et al. 9 studied the eaction ate of sodium sulfite oxidation with cobalt ion catalyst, the eaction was found to be fist ode with espect to sulfite when the eaction ate was contolled by eaction kinetics and zeo ode with espect to sulfite when the eaction ate was contolled by the gas-liquid ate of mass tansfe chaacteistics. Lancia et al. 1 conducted a study of catalytic oxidation of calcium bisulfite with manganese. The eaction was found to be fist ode with espect to bisulfite and.5 ode with espect to patial pessue of oxygen and manganese ion concentation. Kaatza et al. 11 found the eaction of calcium bisulfite catalytic oxidation to be ode with espect to bisulfite and fist ode with espect to feous ion concentation. Kaatza et al. 11 continued to finish a eseach on eaction ate of sulfite oxidation catalyzed by cupous ions, the eaction was found to be ode with espect to bisulfite, zeo ode with espect to dissolved oxygen and. ode with

2 7 Guo et al. Asian J. hem. espect to feous ion concentation. They also daw a conclusion that coope is less effective than ion as a catalyst 1. Though the oxidation of ammonium sulfite has been studied by seveal eseaches and has esulted in diffeent conclusions, it should be noted that most of these pevious investigations dealt with oxygen absoption into ammonium sulfite solutions without catalyst o just with cobalt ions. They did not taken othe tansition metal ions into consideation which was quite diffeent fom the catalytic oxidation of calcium sulfite. In addition, these pevious investigations wee conducted unde conditions significantly diffeent fom those encounteed in ammonia-based flue gas desulfuization plants. Theefoe, it can be concluded that moe data and a close study on the mechanism is necessay. The pesent wok was undetaken to make a systematic study of the catalytic oxidation of ammonium sulfite, unde conditions elevant to ammonia based wet flue gas desulfuization plants. The intinsic eaction of sulfite oxidation was caied in an intemittent eacto (Fig. ). Fistly, a known quantity of high puity wate was added into the intemittent eacto which was installed in a magnetic stiing constant-tempeatue bath. The stie stated at a pedetemined speed and a dissolved oxygen pobe was fixed in the solution. A cetain quantity of ammonium sulfite solution was then added into the eacto to a total volume of 1 L and the eaction stated at the same time. The time was noted and the concentation of dissolved oxygen was ecoded duing the eaction pocess. The concentation of sulfate could be calculated using the following fomula: - +.5O - () EXPERIMENTAL All the chemicals used wee of analytical gade in ode to educe the influence of impuities. The softwae Oigin.5 was used fo egessive analysis of the expeimental data. The oxidation of ammonium sulfite was caied out in a stied bubbling eacto which was installed in a magnetic stiing constant-tempeatue bath (Fig. 1). The stied bubbling eacto was a glass cylindical vessel, 11. cm in intenal diamete and 15.5 cm in height. Afte adding 1 ml solution consist of cetain amount of ammonium sulfite and ammonium sulfate pepaed by high puity wate without oxygen and a set amount of catalyst into the eacto, the stie stated at a pedetemined speed of 1 pm.the time was noted and the ai was injected into the solution in a steady manne. Hl and NaOH wee used to adjust the ph. The eaction ceased at the scheduled time and the concentation of sulfate was then measued by means of ion chomatogaphy (IS-5, Dionex, USA)which was detemined accoding to the standad cuve of sulfate. The obseved oxidation ate was calculated using the following fomula: = d / dt (1) Fig.. Expeimental appaatusof intinsic eaction (1- quantitative liquid filling injecto; - dissolved oxygen mete; - magnetic stiing constant-tempeatue wate bath; - intemittent eacto; 5- ubbe stoppe; - magnetic stie; 7- themocouple) RESULTS AND DISUSSION In this section, effects of five catalysts, including o +, Mn +, Zn +, Fe + and Fe + ae compaed by intinsic expeiment sat fixed concentation of ammonium sulfite and tempeatue (Fig. ). It is obvious that Fe + has much bette catalytic effectthan othe catalyst when the catalysts ae in the same concentations. The simila conclusions wee dawn by Kaatza and Lancia, when they studied the catalytic oxidation of calcium bisulfite 1,1. Theefoe, Fe + is an effective catalyst which is suitable fo applying in wet flue gas desulfuization system (mol L 1 S 1 ) 1 Fe + Fe + Zn + Mn + o + Fig. 1. Expeimental appaatus of ammonium sulfite oxidation (1- ai compesso; - ai flow mete; - ph mete; - magnetic stiing constant-tempeatue wate bath; 5- stied bubbling eacto; - magnetic stie; 7- aeation heade; - themocouple) cal 1 9 (mol L 1 ) Fig.. Effect of diffeent catalysts on the oxidation ate [(NH ) = mmol/l; T = 9.15 K]

3 Vol., No. 1 (1) Kinetics and Reaction Mechanism of atalytic Oxidation of Ammonium Sulfite 71 Results of oxidation of ammonium sulfite Effect of - concentation: The elation of sulfite oxidation ate and ammonium sulfate concentation in this wok is shown in Fig.. It can be seen that the oxidation ate deceases as the ammonium sulfate concentation inceases. While the ammonium sulfate concentation was aised to mol/l, the oxidation ate of sulfite was educed by about 1 % compaed to the zeo magnesium sulfate concentation. The esult cetificated that too high concentation of sulfate will inhibit the oxidation of sulfite.the simila tendency has been epoted by Zhou et al. 5. One eason that sulfate ion hampes the oxidation may be due to the decease of the solubility of oxygen in solution and the diffusion coefficient changes 1, (mol L 1 min 1 ) (NH ) (mol L 1 ) Fig. 5. Effect of sulfite concentation on the oxidation ate [(NH ) = mmol/l; (Fe + ) = mol/l; ph = 5.5; Q = 1 L/h; T = 9.15 K] 1 (mol L 1 min 1 ) (NH ) (mol L 1 ) 1 (mol L 1 min 1 ) Fig.. Effect of sulfate concentation on the oxidation ate [(NH ) = mmol/l; (Fe + ) = mol/l; ph = 5.5; Q = 1 L/h; T = 9.15 K] Effect of concentation of - : Fig. 5 shows that the oxidation ate of sulfite inceases as the ammonium sulfite concentation inceases. The oxidation ate of sulfite is found to be fist ode with espect to ammonium sulfite. As epoted by Zhao et al. 9, the eaction with o + was fist ode with espect to the sodium sulfite when contolled by the intinsic eaction, which is simila to ou esults. Howeve, Wang et al. 19 found that the un-catalyzed oxidation of satuated magnesium sulfite is zeo ode with espect to sulfite. Jia et al. 1 found the eaction without catalyst to be -.5 ode with espect to ammonium sulfite 7. Diffeent kinds of sulfite have diffeent chaactes and eaction conditions, such as ionic stength, catalysts o inhibitos, ange of sulfite concentation, ph and solubility, which may stongly affect the expeimental esults. Effect of Fe + concentation: The effect of Fe + concentation on the oxidation date was investigated using Fe + concentation fom to mol/l. The esult is shown in Fig.. As the Fe + catalyst, the oxidation ate can be significantly inceased by solely inceasing the Fe + concentation, but the ate of incease was not constant. A simila tendency was descibed by Zhao et al. 9,17 when they studied the foced oxidation of calcium sulfite with catalysts of o + and Mn +. Howeve, the most significant diffeence with pevious liteatues is that the oxidation ate kept inceasing with the inceasing concentation of Fe +, the phase eaction ate keeps steady had not been found. The eason may be that Fe + (mol L 1 ) Fig.. Effect of Fe + concentation on the oxidation ate [(NH ) = mmol/l; (NH ) ) = mmol/l; ph = 5.5; Q = 1 L/h; T = 9.15 K] the sulfite oxidation ate was always detemined by the eaction kinetics with lowe ph contolling. The oxidation ate was found to be. ode with espect to the concentation of Fe +. Effect of ph: The expeimental esults indicating the dependence of ph on the oxidation ate of ammonium sulfite ae shown in Fig. 7. The oxidation ate of sulfite inceased slowly with ph fom.5 to 5.5 and inceased quickly with ph fom 5.5 to., while the oxidation ate began to decease apidly with ph above.5. This tendency is simila to that epoted by Zhang and Milleoon 1 fo pesence of sulfite in seawate. Howeve, Jia et al. 7 eached an discepant conclusion with a theoy that the sulfite oxidation mechanism depends on the bisulfite concentation and the atio of bisulfite concentation to sulfite concentation deceases with the ph inceases. It's difficult to explain the phenomenon on a specific theoy. Linek and Vacek found that the oxidation effect of oxygen significantly educed unde low ph. Theefoe, the changes of the eaction ate with diffeent ph can be consideed due to the inteaction of diffeent impact factos. Unde lowe ph, the bisulfite concentation maintained at a high level while the effect of oxygen was geatly educed, esulting in a lowe

4 7 Guo et al. Asian J. hem (mol L 1 min 1 ).5. 1 (mol L 1 s 1 ) Fig.7. Effect of ph on the oxidation ate [ (NH ) = mmol/l; (NH ) = mmol/l; Fe + = mol/l; Q = 1 L/h; T = 9.15 K] oxidation ate. When ph value aised, the effect of oxygen incease while the bisulfite concentation declined apidly, causing the coesponding eduction of the oxidation ate. Effect of ai flow ate: Fig. showed that the oxidation ate of sulfite inceased quickly with the ai flow up to15 L/h, above which it inceased slowly. This tendency is simila to that epoted by Shen et al. 19. The eason may be that when the ai flow was below 15 L/h, the bubbled ai dispesed unifomly, esulting in a quiet bubbling aea and had little stiing effect on the eaction solution, the gas-liquid contact aea inceased as the ai flow inceased. When the ai flow inceased up to 15 L/h, some ai foth collided and agglomeated, esulting in a tubulent bubbling aea and a little change of gas-liquid contact aea thus mass tansfe ate of oxygen inceased slowly. 1 (mol L 1 min 1 ) Fig Effect of ai flow ate on the oxidation ate [ (NH ) = mmol/l; (NH ) = mmol/l; Fe + = mol/l; ph = 5.5; T = 9.15 K] Effect of tempeatue: Fig. 9 showed that the oxidation ate of sulfite inceases as the tempeatue inceases. Accoding to the Ahenius fomula: ln k ph Q (L h 1 ) = E 1 ln k a R T () Fig Temp. (K) Effect of ai flow ate on the oxidation ate [ (NH ) = mmol/l; (NH ) = mmol/l; Fe + = mol/l; ph = 5.5; Q = 1 L/h] It can be seen that a plot of ln k vesus 1/T is a staight line, the activation enegy E a can be calculated accoding to the slope of the line. The eaction ate constant k which quantifies the speed of a chemical eaction inceases with a ise in tempeatue. This is why the oxidation ate inceases as the tempeatue incease. The appaent activation enegy fo the oxidation was calculated to be 7.7 kj/mol,which is much lowe than the peviously epoted activation enegy fo the oxidation of ammonium sulfite 5-7. Mechanism of oxidation of ammonium sulfite: The ammonium sulfite oxidation may include two steps: an intinsic eaction of sulfite in the solution and diffusion of oxygen fom ai bubbles into the solution on the basis of the oxidation pocesses of calcium sulfite,. Intinsic eaction: The intinsic expeiments of ammonium sulfite oxidation wee conducted by vaying concentation of (NH ), Fe + and tempeatue. The esults of intinsic eaction wee shown in Figs Fig. 1 shows that the eaction ate was acceleated by inceasing concentation of Fe + and it's found to be.5 ode with espect to Fe + concentation. Fig. 11 illustated that the eaction ate inceases as the ammonium sulfite concentation inceases. The eaction is found to be second ode with espect to ammonium sulfite. Futhemoe, the dissolved oxygen concentation deceased linealy with eaction time unde excess of sodium sulfite, indicating that the eaction was zeo ode with espect to dissolved oxygen concentation. Based on the expeimental esults above, the intinsic eaction ate can be expessed as:.5 i k(nh ) + Fe = () The effect of tempeatue on the intinsic eaction ate is shown in Fig. 1. The appaent activation enegy was calculated to be 7.9 kj/mol with a fequency facto of L/(mol s). If intinsic eaction wee the contolling step, the eaction ate calculated by eqn. 1 should be mol/(l min) when the concentation of dissolved sulfite is mmol/l at 9.15 K. Fig. shows that the oxidation ate is only

5 Vol., No. 1 (1) Kinetics and Reaction Mechanism of atalytic Oxidation of Ammonium Sulfite 7 DO (mol L 1 ) Fig. 1. Effect of Fe + concentation on the intinsic oxidation ate [ (NH ) = mmol/l; T = 9.15 K] DO (mg L 1 ) Fig. 11. Effect of sulfite concentation on the intinsic oxidation ate [ Fe + = mmol/l; T = 9.15 K] DO (mg L 1 ) mmol/l 1 mmol/l 5 1 mmol/l mmol/l 1 1 mmol/l 1 5 t (s) 1. mmol/l mmol/l. mmol/l.5 mmol/l. mmol/l t (s) 9.15 K 9.15 K.15 K.15 K 1.15 K 1 5 t (s) Fig. 1. Effect of tempeatue on the intinsic oxidation ate [ (NH ) = mmol/l; Fe + = mmol/l] 1 - mol/(l min), which is much lowe than the calculated intinsic eaction ate. Theefoe, it is confimed that the intinsic eaction is not the contolling step. Diffusion of oxygen: In the quiet bubbling aea (Q 15 L/h), it was assumed: (1) the ai foth bubbles wee equal in size and showed little agglomeation duing thee ise pocess; () the oxygen concentation in the single ai foth emained constant; () the pessue of the ai foth emained constant duing the ise pocess, since the height of eacto was elatively low, the pessue of the ai foth was equal to the atmospheic pessue. The assumption mentioned above lead to: Q = Mπ g (5) = M M = π The mass tansfe ate of oxygen is contolled by the liquid film mass tansfe pocess and the diffusion ate of oxygen is obtained accoding to the double film theoy: Q g M () = * M K L'( O O ) (7) Futhe, as the chemical eaction was apid: * m 1 n K L ' = D k( O O ) ( ) m 1 () + The concentation of dissolved oxygen in the solution was close to zeo and the intinsic eaction ode of oxygen, m = zeo and that of sulfite, n =. Theefoe, the diffusion ate of oxygen can be witten as: M D m + 1 O k( * O ) m n * DO k O 1 (9) 1 Q * = DO K O π (1) g The catalytic effect of Fe + was egaded as magnification of the eaction coefficient k and was simplified as follows: Q * = DO KO Fe + πg 1 (11) Accoding to eqn. 11, it was concluded that the diffusion ate of oxygen was popotional to the ai flow ate, the oxidation ate was fist ode with espect to sulfite and.5 ode with espect to Fe + concentation. The eaction in this wok was found to be fist ode with espect to ammonium sulfite and. ode with espect to Fe +. The expeimental esults wee geneally consistent with eqn. 11. Based on the discussions above, it can be consideed that the diffusion of oxygen is the contolling step of the oxidation of ammonium sulfite. onclusion Though the ammonia-based wet flue gas desulfuization pocess has been inceasingly used on the industial boile in ecent yeas, the catalytic oxidation of ammonium sulfite only gained vey little attention. In this pape, the catalytic oxidation of ammonium sulfite with feous ion elevant to ammoniabased wet flue gas desulfuization plants, including the kinetics of sulfite oxidationunde elated opeating paametes and the mechanism of the oxidation, has been systematically studied.

6 7 Guo et al. Asian J. hem. Effects of five catalysts, including o +, Mn +, Zn +, Fe + and Fe + ae compaed by intinsic expeiments at fixed concentation of ammonium sulfite and tempeatue. The esults showed that Fe + is the most suitable one fo flue gas desulfuization systems. Using stied bubbling eacto, the kinetics of ammonium sulfite oxidation was studied by vaying (NH ) concentation, (NH ) concentation, Fe + concentation, ph, ai flow ate and tempeatue. The eaction was found to be fist ode with espect to sulfite,. ode with espect to Fe + and the appaent activation enegy fo the oxidation was calculated to be 7.7 kj/mol. The oxidation ate can be speeded up by inceasing the ammonium sulfite concentation and Fe + concentation, aising the ai flow and maintaining the ph value at, while the sulfate concentation and the tempeatue can also affect the oxidation ate. The mechanism of the oxidation of ammonium sulfite was discussed, it was concluded that the oxidation was contolled by diffusion of oxygen. This finding will be beneficial fo designing o optimizing the oxidation pocess in the ammonium-based flue gas desulfuization pocess. Notation Fe + = oncentation of feous ion (mol/l) (NH ) = oncentation of ammonium sulfite (mol/l) (NH ) = oncentation of ammonium sulfate (mol/l) O = oncentation of oxygen in the solution (mol/l) * O = Equilibium concentation of oxygen at the inteface (mol/l) = oncentation of sulfite (mol/l) D O = Diffusion coefficient of oxygen in the solution (m /s) E a = Appaent activation enegy (kj/mol) k = The eaction ate constant k = Fequency facto K L = oncentation of feous ion, m/s M = Amount of foth in unit time m = Reaction ode of oxygen n = Reaction ode of sulfite Q = Ai flow (L/h) R = Gas contant,.15 J/(mol K) = The eaction ate, mol/(l min) g = Radius of balls (m) i = Intinsic eaction ate, mol/(l s) M = Diffusion ate of oxygen in single foth, mol/(l s) T = Tempeatue (K) t = Reaction time (s) AKNOWLEDGEMENTS The pesent wok was suppoted by the poject coopeating with SINOPE Ningbo Reseach Institute of Technology and Shanghai Leading Academic Discipline Poject (No. B5). REFERENES 1. Z.M. Zhou and Z. hen, Guangdong hem. Ind. (hina),, 5(7).. H.W. Kang, Y.B. Zhou and J.P. Li, Guangdong hem. Ind. (hina), 7, 1 (9).. L.D. Wang and Y. Zhao, hem. Eng. J., 1,1 ().. T. Gükan, A. Nufal and I. Eoglu, hem. Eng. Sci., 7, 1 (199). 5. J.H. Zhou, W. Li and W.D. Xiao, hem. Eng. Sci., 55, 57 ().. X.L. Long, W. Li, W.D. Xiao and W.K. Yuan, J. Hazad. Mate., 19, (). 7. Y. Jia, Q. Zhong, X.Y. Fan and X.R. Wang, hem. Eng. J., 1, 1 ().. V. Linek and V. Vacek, hem. Eng. Sci.,, 177(191). 9. B. Zhao, Y. Li, H.L. Tong, Y.Q. Zhuo, L. Zhang, J. Shi and.h. hen, hem. Eng. Sci.,, (5). 1. A. Lancia, D. Musmaa, M. Pisciandao and M. Tammao, hem. Eng. Sci., 5, 19 (1999). 11. D. Kaatza, M. Pisciandao, A. Lancia and D. Musmaa, hem. Eng. Sci.,, 197 (5). 1. D. Kaatza, M. Pisciandao, A. Lancia and D. Musmaa, hem. Eng. J., 15, 5 (). 1. A. Lancia and D. Musmaa, Envion. Sci. Technol,., 191 (1999). 1. E. Naita, F. Lawson and K.N. Han, Hydometallugy, 1, 1 (19). 15. Y. Iwai, H. Eya, Y. Itoh, Y. Aal and K. Takeuchi, Fluid Phase Equilib.,, 71 (199). 1. L.D. Wang, Y.L. Ma, G. Yuan, W. Zhang, Y. Zhang and J.M. Hao, Sci. Sinica him. (hina),, 117 (1). 17. B. Zhao, Y. Li, Y. Zhou and.h. hen, Koean J. hem. Eng.,, 71 (). 1. J.Z. Zhang and F.J. Milleo, Geochim. osmochim. Acta, 55, 77 (1991). 19. Z. Shen, S. Guo, W. Kang, K. Zeng, M. Yin, J. Tian and J. Lu, Ind. Eng. hem. Res., 51, 19 (1).