- Email: [email protected]
A study on the conversion of trona to sodium bicarbonate
Available online at www.sciencedirect.com
Powder Technology 184 (2008) 58 – 63 www.elsevier.com/locate/powtec
A study on the conversion of trona to sodium bicarbonate Kyungmin Jacob Cho a,⁎, Timothy C. Keener a , Soon-Jai Khang b a b
Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, United States Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221, United States Received 7 March 2007; received in revised form 9 July 2007; accepted 8 August 2007 Available online 15 August 2007
Abstract The deactivation model was used to explain kinetics underlying the conversion reaction of trona to NaHCO3 (sodium bicarbonate). The model showed good agreement with the experimental data obtained from the conversion reaction of trona to NaHCO3. It gave the value of 0.94 as an average correlation coefficient with the experimental data. However, at lower temperature, the model was in poor agreement with the data. This would be related to the structural variation of trona particles at the lower temperature. A trona particle is initially nonporous and then it begins to crack. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, the fissures on the surface of the particles are not fully developed during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term articulating the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. The deactivation rate constant, kd is strongly temperature dependent and the change of the slope suggests the reaction mechanism changes as the reaction temperature increases. © 2007 Elsevier B.V. All rights reserved. Keywords: Deactivation model; Trona; SO2; Desulfurization
1. Introduction Numerous models published in the literature have been explained solid transformation reactions controlled by the kinetics of gas–solid reactions. Kinetic models have been gradually complicated to more closely elucidate underlying phenomena involved in gas–solid reactions, such as chemical reaction, diffusion through gas film or ash layer, external mass transfer, heat transfer, and etc.[1] Those kinetic models can be categorized into based on the assumptions used in models. Szekely et al. proposed the grain model, regarding a porous pellet as consisting of initially non porous small grains with same size which react independently according to the shrinking core model [2]. Due to its correspondence with the structure of porous solids, the grain model has been extensively used in kinetic models [1]. However, numerous extensions to the original model have been developed due to its simplified approximations such as pseudo steady state, isothermal
⁎ Corresponding author. E-mail address: [email protected] (K.J. Cho). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.08.005
condition, equimolar reaction, and first order with respect to a reactant gas. The pseudo steady state approximation used by the majority of models is generally valid for gas–solid reactions, which ignores the accumulation in gas phase [3,4]. However, Wen and Wang stated that the term expressing heat accumulation can not be neglected in the case of nonisothermal models [5]. The equimolar counter diffusion assumption indicates that reactive and product gas flux are equal but opposite. It is only valid when the stoichiometric coefficients for gases in reactant and product are equal under the pseudo steady state approximation. However, this assumption is most frequently used in kinetic models due to the simplifying of the diffusional transportation in a pellet [1]. Several researchers investigated the effect of total flux under non equimolar condition to complement the assumption [6–8]. The isothermal assumption is also widely used in the model for the reaction of gas with solid. However, when reaction is exothermic or endothermic, this assumption is not valid for the initial condition to develop kinetic models [1]. Cannon et al., Shen et al. and Ishida et al. discussed a specific characteristic of exothermic gas–solid reaction, using the shrinking core model with the steady state assumption [9–11]. Beveridge and Goldie investigated the
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
59
Fig. 1. SEM observation of parent trona.
importance of the term expressing heat transfer in the heat balance, showing that the theoretically possible steady state is never attained in practical environment [12]. Also, Sampath et al. proposed a nonisothermal transient grain model taking into account of external mass transfer and heat transfer [13,14]. 1.1. Conversion of trona Previous studies have shown Na-based sorbents have higher reactivity toward SO2 compared to Ca-based sorbents in dry injection systems[15]. Samuel et. al and Kaplan et. al reported that trona(Na2CO3·NaHCO3·2H2O) and nahcolite consisting of 90% of NaHCO3 are very efficient for SO2 reduction in dry injection systems[16,17]. Among those Na-based sorbents, sodium bicarbonate (NaHCO3) is more efficient in removing SO2 than sodium carbonate (Na2CO3) [15,18]. This is due to tremendous reactive surface area that is created as sodium bicarbonate decomposes to sodium carbonate prior to the reaction with SO2 [19–22]. Also, Na2CO3 prepared by decomposing of NaHCO3 can be completely utilized to remove SO2 while calcined limestone can be converted only up to about 40% and dense limestone can be converted up to
about 75% [23]. Another advantage of Na2CO3, the decomposition product of NaHCO3, is that the reaction with SO2 takes place rapidly at the temperature range of 373 K–474 K while limestone requires about 1173 K to commence the reaction with SO2 [20]. However, sodium bicarbonate is not as cost efficient as sodium carbonate or trona [24]. Trona is composed of approximately 46% sodium carbonate (Na2CO3) and 36% sodium bicarbonate (NaHCO3), which is relatively abundant in nature. Therefore, the key achievement would be in developing a cost-effective method of converting sodium carbonate fraction of parent trona to sodium bicarbonate to achieve more efficient removal of SO2 in dry injection systems. Trona is known to be stable until 57 °C under dry conditions, and create intermediates such as wegschiderite (Na2CO3·3NaHCO3) and sodium monohydrate (Na2CO3·H2O) between 57 °C and 160 °C [25–30]. Above 160 °C, trona decomposes to sodium carbonate [27]. Since NaHCO3 will be the final product, its chemical properties should be contemplated for the conversion. NaHCO3 decomposes to Na2CO3 in the temperature range of 100 °C–180 °C [25,31,32]. In theory, therefore, the feasible temperature for converting trona to
Fig. 2. SEM observation of modified trona.
60
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
Fig. 3. Schematic diagram of the fixed bed reactor for the conversion.
sodium bicarbonate is in the range of 57 °C to 100 °C. The overall conversion reaction of Na2CO3 to NaHCO3 is shown in the following reaction [27,30,32,33].
Fig. 5. Comparison of experimental data obtained at lower temperature with the deactivation model.
Na2 CO3 ðsÞ þ CO2 ðgÞ þ H2 OðgÞ↔2NaHCO3 ðsÞ
Fernandez Bertos et al. reported that minerals must be inorganic in nature for effective carbonation and some free
water is necessary for the reaction [37]. However, excessive water can plug pores so that the carbonation reaction is impeded as the diffusion rate of CO2 decreases. Johannesson et al. described that minerals with lower surface area need less water to have optimum carbonation [38]. Diffusivity of CO2 is affected by physical properties of a solid, such as particle size, surface area, porosity, and permeability to CO2 [37]. Hills et al. reported that finer powders show greater carbonation at higher water contents due to more surface areas to react with CO2 [39]. Also, CO2 diffusivity can be obstructed by low porosity in the infrastructure of a solid [40,41].
Fig. 4. Conversion–time relationship at different molar ratio of water vapor in gas phase.
Fig. 6. Comparison of experimental data obtained at higher temperature with the deactivation model.
The forward of this reaction is highly exothermic [31,34] and water vapor and CO2 are essential factors for the conversion reaction. The carbonation rate increases with the amount of CO2 in gas phase and the relative humidity level of 50–70%. However, excessive and lower humidity level obstruct the carbonation rate [35,36]. 1.2. Reactivity and diffusivity of CO2
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
61
Table 1 Correlation coefficient of the model with the experimental data at lower temperature
limestone[45]. Integration of Eqs. (1) and (2) gives following expression for the activity and the conversion.
Temperature (°C)
Correlation coefficient
22 32
0.83 0.94
a ¼ ekd t X ¼
1.3. Model development
k ð1 ekd t Þ kd
ð4Þ
The experimental data obtained from the carbonation reaction of trona were compared with the deactivation model.
The most general type of heterogeneous reaction for gas– solid reaction can be represented by AðgÞ þ bBðsÞ→dDðsÞ in which A is a gaseous reactant, B is a solid reactant, and D is a solid product. The most common models used in gas–solid reactions are the unreacted shrinking core model with chemical reaction control or diffusion control, and the grain model. With referring Figs. 1 and 2, we can observe the structural change of trona particles as the carbonation reaction proceeds. It indicates structural variations are quite significant in the conversion of trona to NaHCO3. The reactivity of solid reactant might also change as reaction proceeds. For gas–solid reactions with significant structural change, the unreacted shrinking core model does not well fit to experimental data [15]. However, the deactivation model can be applied to explain such complex reactions. A number of non-catalytic gas–solid reactions were simulated using the deactivation model showing satisfactory predictions [15,42–46]. For the deactivation model, the changes of active sites and the reactivity of active sites can be expressed with an activity term “a” in the rate expression [44,46]. dX ¼ka dt
2. Experimental procedure The conversion experiments were carried out using a fixed bed reactor in order to convert trona to NaHCO3. A schematic diagram of the fixed bed reactor is shown in Fig. 3. In the fixed bed experiments, 20 g of parent trona were placed in a sample basket located on the base of the reactor. Then, air inside the reactor was evacuated by a vacuum pump, and 100% CO2 was injected into the reactor until the pressure was 19 psia. The pressure of CO2 in the fixed bed reactor was maintained during the experiments. 3. Results and discussion Experimental conversion data were obtained as weight changes measured by wet titration (Standard Methods 2320) during the conversion of trona to NaHCO3. In the experiment, the pressure of CO2 was kept at 19 psia and the mole fraction of water vapor in the gas phase (yH2O) was controlled by increasing the temperature of water to 22 °C, 32 °C, 42 °C, 50 °C, and 70 °C. Reaction time was varied from 10 min to 120 min. When the temperatures of water were 22 °C, 32 °C, 42 °C, 50 °C, and 70 °C, the mole fraction of water vapor in the
ð1Þ
where k is the apparent rate constant including the dependence of the reaction rate on the partial pressure of gaseous reactant. The variation of the activity of the solid reactant is expressed to be proportional to the activity itself and the deactivation approach is similar to catalyst deactivation.
ð3Þ
da ¼ kd a dt
ð2Þ
Here, kd is the deactivation rate constant and Doğu introduced this approach to the reaction of SO 2 with
Table 2 Correlation coefficient of the model with the experimental data at higher temperature Temperature (°C)
Correlation coefficient
42 50 70
0.96 1.00 1.00
Fig. 7. Temperature dependence of the deactivation rate constant.
62
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
Table 3 Parameters of the deactivation model for the carbonation of trona to NaHCO3 Temperature (°C)
kd (s− 1)
k/kd
22 32 42 50 70
0.0003 0.0170 0.0587 0.154 0.232
21.274 1.172 0.9536 0.9012 0.8750
gas phase (yH2O) were 0.02, 0.04, 0.06, 0.10, and 0.24, respectively. Fig. 4 illustrates the conversion–time relationship under different molar ratio of water vapor in the gas phase at 22 °C, 32 °C, 42 °C, 50 °C, and 70 °C. Comparison of the predictions from the deactivation model with the experimental data obtained from different experimental conditions is illustrated in Figs. 5 and 6. Correlation coefficients of the model with the experimental data are reported in Table 1 and Table 2, which were calculated by Statistica 6.0. As shown in Fig. 5 and Fig. 6, while the deactivation model shows poor agreement with the experimental data at the lower temperature like 22 °C and 32 °C, it shows satisfactory results for the carbonation reaction of trona at higher temperature like 42 °C, 50 °C, and 70 °C. This would be related to the structural variation of trona particles at the lower temperature. As shown in Fig. 1 and Fig. 2, a trona particle is initially nonporous and then it begins to crack so that fissures appear on the surface. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, a trona particle takes longer time to change its structure. In other words, the fissures on the surface of the particles are not fully developed at the lower temperature during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term expressing the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. Furthermore, as mentioned earlier, Wen and Wang reported that the term expressing heat accumulation can not be neglected in the case of nonisothermal models [5]. Therefore, heat transfer or structural variation would explain the difference between the prediction and the experimental data. The best advantage of the deactivation model, however, is simple and can easily predict the conversion– time relationship for the trona conversion to NaHCO3. Deactivation rate constant was evaluated at different temperatures by regression analysis of the conversion–time data as depicted in Fig. 7. Also, the values of kd and k/kd are given in Table 3. The deactivation rate constant is quite strongly temperature dependent. However, it indicates a change in reaction mechanism as temperature increases. Line 1 in Fig. 7 was fitted using the experimental data from lower reaction temperature in the range of 22 °C to 50 °C and line 2 was evaluated by the data from higher temperature conditions such as 50 °C, and 70 °C. Line 3 was estimated using the entire experimental data, giving the R-squared value of 0.78. The activation energy for the deactivation rate constant, kd, was
found as 26,190 cal/mol for line 1, 4492 cal/mol for line 2, and 41,148 cal/mol for line 3 from each slope in Fig. 7. 4. Conclusions The experimental data were obtained from the conversion reaction of trona to NaHCO3. Increased water vapor significantly affects the conversion of trona to NaHCO3 and a particle structure changes during the conversion reaction. The deactivation model shows good agreement with the experimental data and gives the value of 0.94 as an average correlation coefficient. However, the deactivation model gives poor prediction at the lower temperature. This would be related to the structural variation of trona particles at the lower temperature. A trona particle is initially nonporous and then it begins to crack. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, the fissures on the surface of the particles are not fully developed during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term articulating the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. The deactivation rate constant, kd is strongly temperature dependent and the change of the slope suggests the reaction mechanism changes as the reaction temperature increases. Nomenclature a Activity of solid reactant b Stoichiometric coefficient of solid reactant k Apparent reaction rate constant, s− 1 kd Deactivation rate constant, s− 1 X Conversion of trona to NaHCO3
References [1] F. Patisson, M. Galant Francois, D. Ablitzer, A non-isothermal, nonequimolar transient kinetic model for gas–solid reactions, Chemical Engineering Science 53 (4) (1998) 697. [2] J. Szekely, J.W. Evans, H.Y. Sohn, Gas–Solid Reactions, Academic Press, New York, 1976. [3] K.B. Bischoff, Accuracy of the pseudo steady state approximation for moving boundary diffusion problems, Chemical Engineering Science 18 (11) (1963) 711. [4] S.K. Bhatia, On the pseudo steady state hypothesis for fluid–solid reactions, Chemical Engineering Science 40 (1985) 868–872. [5] C.Y. Wen, S.C. Wang, Thermal and diffusional effects in solid gas reactions, Industrial and Engineering Chemistry 62 (1970) 30–51. [6] H.Y. Sohn, H.J. Sohn, Effect of bulk flow due to volume change in the gas phase on gas–solid reactions: initially nonporous solids, Industrial and Engineering Chemistry, Process Design and Development 19 (2) (1980) 237. [7] H.Y. Sohn, O.A. Bascur, Effect of bulk flow due to volume change in the gas phase on gas–solid reactions— initially porous solids, Industrial & Engineering Chemistry Process Design and Development 21 (4) (1982) 658.
K.J. Cho et al. / Powder Technology 184 (2008) 58–63 [8] D.L. Ulrichson, D.J. Mahoney, Pore closure models for gas–solid reactions — the effect of bulk flow and reversibility, Chemical Engineering Science 35 (3) (1980) 567. [9] K.J. Cannon, K.G. Denbigh, Studies on gas–solid reactions-II: causes of thermal instability, Chemical Engineering Science 6 (4–5) (1957) 155. [10] J. Shen, J.M. Smith, Diffusional effects in gas–solid reaction, Industrial and Engineering Chemistry Fundamentals 4 (3) (1965) 293–301. [11] M. Ishida, C.Y. Wen, Comparison of kinetic and diffusional models for solid–gas reactions, AIChE Journal 14 (2) (1968) 311–317. [12] G.S.G. Beveridge, P.J. Goldie, Effectiveness factors and instability in noncatalytic gas–solid reactions: the effect of solid heat capacity, Chemical Engineering Science 23 (8) (1968) 913. [13] B.S. Sampath, P.A. Ramachandran, R. Hughes, Modelling of non-catalytic gas–solid reactions-I. Transient analysis of the particle-pellet model, Chemical Engineering Science 30 (1) (1975) 125. [14] B.S. Sampath, P.A. Ramachandran, R. Hughes, Modelling of non-catalytic gas–solid reactions-II. Transient simulation of a packed bed reactor, Chemical Engineering Science 30 (1) (1975) 135. [15] C. Guldur, G. Dogu, T. Dogu, Kinetics of trona sulfur dioxide reaction, Chemical Engineering and Processing 40 (1) (2001) 13. [16] E.A. Samuel DAF, R.L. Ostop, SO2 removal using dry sodium compounds, Am. Inst. Eng. Semp. 77 (211) (1981). [17] S. Kaplan, B.J. Garrick, P. Bieniarz, On the use of Bayes' Theorem in assessing the frequency of anticipated transients, Nuclear Engineering and Design 65 (1) (1981) 23. [18] L.J. Musio, J.K. Arand, Bench-scale study of the dry removal of SO2 with nahcolite and trona, EPRI, California, 1981. [19] T. Dogu, Effect of port structure on the mechanism of SO2 sorption on activated soda, Wiley, New York, 1984. [20] W. Hu, J.M. Smith, T. Dogu, G. Dogu, Kinetics of sodium bicarbonate decomposition, AIChE Journal 32 (9) (1986) 1483–1490. [21] S. Kimura, J.M. Smith, Kinetics of the sodium carbonate-sulfur dioxide reaction, AIChE Journal 33 (9) (1987) 1522–1532. [22] T.C. Keener, W.T. Davis, Study of the reaction of SO2 with NaHCO3 and Na2CO3, Journal of the Air Pollution Control Association 34 (6) (Jun 1984) 651–654. [23] M. Hartman, J. Hejna, Z. Beran, Application of the reaction kinetics and dispersion model to gas–solid reactors for removal of sulfur dioxide from flue gas, Chemical Engineering Science 34 (4) (1979) 475. [24] J. Maziuk, J. Kumm, Comparison of dry injection acid–gas control technologies, A&WMA 95th annual conference and exhibition, 2002, (Baltimore, Maryland; 2002). [25] A. Ekmekyapar, H. Ersahan, S. Yapici, Nonisothermal Decomposition Kinetics of Trona, Industrial and Engineering Chemistry Research 35 (1) (January 08, 1996) 258–262. [26] R.S. Gartner, G.J. Witkamp, Wet calcining of trona and bicarbonate in a mixed solvent, Journal of Crystal Growth 237–239 (2002) 2199–2204. [27] R.S. Gartner, G.J. Witkamp, Wet calcining of trona (sodium sesquicarbonate) and bicarbonate in a mixed solvent, Journal of Crystal Growth 237–239 (1–4 III) (2002) 2199–2204. [28] K.H. Buchel, H.-H. Moretto, P. Woditch, Metals and their Compounds, 2 ed.WILEY-VCH, Weinheim, Germany, 2000.
63
[29] F. Ray Thompson CBE, Sodium Carbonate, The Royal Society of Chemistry, UK, 1995. [30] G. Braune, H. Scheider. Sodium carbonate 1993. [31] P.K. Heda, D. Dollimore, K.S. Alexander, D. Chen, E. Law, P. Bicknell, A method of assessing solid state reactivity illustrated by thermal decomposition experiments on sodium bicarbonate, Thermochimica Acta 255 (1995) 255–272. [32] W.-Y. Kuu, R. Chilamkurti, C. Chen, Effect of relative humidity and temperature on misture sorption and stability of sodium bicarbonate powder, International Journal of Pharmaceutics 166 (1998) 167–175. [33] T.C. Keener, S.-J. Khang, Kinetics of the sodium bicarbonate-sulphur dioxide reaction, Chemical Engineering Science 48 (16) (Aug 1993) 2859–2865. [34] M. Dogan, C. Guldur, G. Dogu, T. Dogu, Soda ash production from Trona in a spray dryer, Journal of Chemical Technology and Biotechnology 68 (2) (1997/2) 157–162. [35] J.C. Walton, S. Bin-Shafique, R.W. Smith, N. Gutierrez, A. Tarquin, Role of carbonation in transient leaching of cementitious wasteforms, Environmental Science and Technology 31 (8) (1997) 2345. [36] N.I. Fattuhi, Concrete carbonation as influenced by curing regime, Cement and Concrete Research 18 (3) (1988) 426. [37] M. Fernandez Bertos, S.J.R. Simons, C.D. Hills, P.J. Carey, A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2, Journal of Hazardous Materials 112 (3) (2004) 193. [38] B. Johannesson, P. Utgenannt, Microstructural changes caused by carbonation of cement mortar, Cement and Concrete Research 31 (6) (2001) 925. [39] D. Johnson, R.E. Booth, A.S. Whiteley, M.J. Bailey, D.J. Read, J.P. Grime, et al., Plant community composition affects the biomass, activity and diversity of microorganisms in limestone grassland soil, European Journal of Soil Science 54 (4) (2003) 671. [40] Johnson DC. Accelerated Carbonation Of Waste Calcium Silicate Materials. SCI Lecture Papers Series. 2000. [41] C.D. Hills, S.J.T. Pollard, The influence of interference effects on the mechanical, microstructural and fixation characteristics of cement-solidified hazardous waste forms, Journal of Hazardous Materials 52 (2–3) (1997) 171. [42] B. Gullett, W. Jozewicz, L.A. Stefanski, Reaction kinetics of Ca-based sorbents with HCl, Industrial and Engineering Chemistry Research 31 (11) (1992) 2437. [43] W. Duo, N.F. Kirkby, J.P.K. Seville, R. Clift, Alteration with reaction progress of the rate limiting step for solid–gas reactions of Ca-compounds with HCl, Chemical Engineering Science 50 (13) (1995) 2017. [44] N. Yasyerli, T. Dogu, G. Dogu, i. Ar, Deactivation model for textural effects of kinetics of gas–solid noncatalytic reactions “char gasification with CO2”, Chemical Engineering Science 51 (11) (1996) 2523. [45] T. Dogu, The importance of pore structure and diffusion in the kinetics of gas–solid non-catalytic reactions: reaction of calcined limestone with SO2, Chemical Engineering Journal 21 (3) (1981) 213. [46] S. Balci, G. Dogu, T. Dogu, Structural variations and a deactivation model for gasfication of coal, Industrial and Engineering Chemistry Research 26 (7) (1987) 1454.
Powder Technology 184 (2008) 58 – 63 www.elsevier.com/locate/powtec
A study on the conversion of trona to sodium bicarbonate Kyungmin Jacob Cho a,⁎, Timothy C. Keener a , Soon-Jai Khang b a b
Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, United States Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221, United States Received 7 March 2007; received in revised form 9 July 2007; accepted 8 August 2007 Available online 15 August 2007
Abstract The deactivation model was used to explain kinetics underlying the conversion reaction of trona to NaHCO3 (sodium bicarbonate). The model showed good agreement with the experimental data obtained from the conversion reaction of trona to NaHCO3. It gave the value of 0.94 as an average correlation coefficient with the experimental data. However, at lower temperature, the model was in poor agreement with the data. This would be related to the structural variation of trona particles at the lower temperature. A trona particle is initially nonporous and then it begins to crack. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, the fissures on the surface of the particles are not fully developed during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term articulating the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. The deactivation rate constant, kd is strongly temperature dependent and the change of the slope suggests the reaction mechanism changes as the reaction temperature increases. © 2007 Elsevier B.V. All rights reserved. Keywords: Deactivation model; Trona; SO2; Desulfurization
1. Introduction Numerous models published in the literature have been explained solid transformation reactions controlled by the kinetics of gas–solid reactions. Kinetic models have been gradually complicated to more closely elucidate underlying phenomena involved in gas–solid reactions, such as chemical reaction, diffusion through gas film or ash layer, external mass transfer, heat transfer, and etc.[1] Those kinetic models can be categorized into based on the assumptions used in models. Szekely et al. proposed the grain model, regarding a porous pellet as consisting of initially non porous small grains with same size which react independently according to the shrinking core model [2]. Due to its correspondence with the structure of porous solids, the grain model has been extensively used in kinetic models [1]. However, numerous extensions to the original model have been developed due to its simplified approximations such as pseudo steady state, isothermal
⁎ Corresponding author. E-mail address: [email protected] (K.J. Cho). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.08.005
condition, equimolar reaction, and first order with respect to a reactant gas. The pseudo steady state approximation used by the majority of models is generally valid for gas–solid reactions, which ignores the accumulation in gas phase [3,4]. However, Wen and Wang stated that the term expressing heat accumulation can not be neglected in the case of nonisothermal models [5]. The equimolar counter diffusion assumption indicates that reactive and product gas flux are equal but opposite. It is only valid when the stoichiometric coefficients for gases in reactant and product are equal under the pseudo steady state approximation. However, this assumption is most frequently used in kinetic models due to the simplifying of the diffusional transportation in a pellet [1]. Several researchers investigated the effect of total flux under non equimolar condition to complement the assumption [6–8]. The isothermal assumption is also widely used in the model for the reaction of gas with solid. However, when reaction is exothermic or endothermic, this assumption is not valid for the initial condition to develop kinetic models [1]. Cannon et al., Shen et al. and Ishida et al. discussed a specific characteristic of exothermic gas–solid reaction, using the shrinking core model with the steady state assumption [9–11]. Beveridge and Goldie investigated the
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
59
Fig. 1. SEM observation of parent trona.
importance of the term expressing heat transfer in the heat balance, showing that the theoretically possible steady state is never attained in practical environment [12]. Also, Sampath et al. proposed a nonisothermal transient grain model taking into account of external mass transfer and heat transfer [13,14]. 1.1. Conversion of trona Previous studies have shown Na-based sorbents have higher reactivity toward SO2 compared to Ca-based sorbents in dry injection systems[15]. Samuel et. al and Kaplan et. al reported that trona(Na2CO3·NaHCO3·2H2O) and nahcolite consisting of 90% of NaHCO3 are very efficient for SO2 reduction in dry injection systems[16,17]. Among those Na-based sorbents, sodium bicarbonate (NaHCO3) is more efficient in removing SO2 than sodium carbonate (Na2CO3) [15,18]. This is due to tremendous reactive surface area that is created as sodium bicarbonate decomposes to sodium carbonate prior to the reaction with SO2 [19–22]. Also, Na2CO3 prepared by decomposing of NaHCO3 can be completely utilized to remove SO2 while calcined limestone can be converted only up to about 40% and dense limestone can be converted up to
about 75% [23]. Another advantage of Na2CO3, the decomposition product of NaHCO3, is that the reaction with SO2 takes place rapidly at the temperature range of 373 K–474 K while limestone requires about 1173 K to commence the reaction with SO2 [20]. However, sodium bicarbonate is not as cost efficient as sodium carbonate or trona [24]. Trona is composed of approximately 46% sodium carbonate (Na2CO3) and 36% sodium bicarbonate (NaHCO3), which is relatively abundant in nature. Therefore, the key achievement would be in developing a cost-effective method of converting sodium carbonate fraction of parent trona to sodium bicarbonate to achieve more efficient removal of SO2 in dry injection systems. Trona is known to be stable until 57 °C under dry conditions, and create intermediates such as wegschiderite (Na2CO3·3NaHCO3) and sodium monohydrate (Na2CO3·H2O) between 57 °C and 160 °C [25–30]. Above 160 °C, trona decomposes to sodium carbonate [27]. Since NaHCO3 will be the final product, its chemical properties should be contemplated for the conversion. NaHCO3 decomposes to Na2CO3 in the temperature range of 100 °C–180 °C [25,31,32]. In theory, therefore, the feasible temperature for converting trona to
Fig. 2. SEM observation of modified trona.
60
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
Fig. 3. Schematic diagram of the fixed bed reactor for the conversion.
sodium bicarbonate is in the range of 57 °C to 100 °C. The overall conversion reaction of Na2CO3 to NaHCO3 is shown in the following reaction [27,30,32,33].
Fig. 5. Comparison of experimental data obtained at lower temperature with the deactivation model.
Na2 CO3 ðsÞ þ CO2 ðgÞ þ H2 OðgÞ↔2NaHCO3 ðsÞ
Fernandez Bertos et al. reported that minerals must be inorganic in nature for effective carbonation and some free
water is necessary for the reaction [37]. However, excessive water can plug pores so that the carbonation reaction is impeded as the diffusion rate of CO2 decreases. Johannesson et al. described that minerals with lower surface area need less water to have optimum carbonation [38]. Diffusivity of CO2 is affected by physical properties of a solid, such as particle size, surface area, porosity, and permeability to CO2 [37]. Hills et al. reported that finer powders show greater carbonation at higher water contents due to more surface areas to react with CO2 [39]. Also, CO2 diffusivity can be obstructed by low porosity in the infrastructure of a solid [40,41].
Fig. 4. Conversion–time relationship at different molar ratio of water vapor in gas phase.
Fig. 6. Comparison of experimental data obtained at higher temperature with the deactivation model.
The forward of this reaction is highly exothermic [31,34] and water vapor and CO2 are essential factors for the conversion reaction. The carbonation rate increases with the amount of CO2 in gas phase and the relative humidity level of 50–70%. However, excessive and lower humidity level obstruct the carbonation rate [35,36]. 1.2. Reactivity and diffusivity of CO2
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
61
Table 1 Correlation coefficient of the model with the experimental data at lower temperature
limestone[45]. Integration of Eqs. (1) and (2) gives following expression for the activity and the conversion.
Temperature (°C)
Correlation coefficient
22 32
0.83 0.94
a ¼ ekd t X ¼
1.3. Model development
k ð1 ekd t Þ kd
ð4Þ
The experimental data obtained from the carbonation reaction of trona were compared with the deactivation model.
The most general type of heterogeneous reaction for gas– solid reaction can be represented by AðgÞ þ bBðsÞ→dDðsÞ in which A is a gaseous reactant, B is a solid reactant, and D is a solid product. The most common models used in gas–solid reactions are the unreacted shrinking core model with chemical reaction control or diffusion control, and the grain model. With referring Figs. 1 and 2, we can observe the structural change of trona particles as the carbonation reaction proceeds. It indicates structural variations are quite significant in the conversion of trona to NaHCO3. The reactivity of solid reactant might also change as reaction proceeds. For gas–solid reactions with significant structural change, the unreacted shrinking core model does not well fit to experimental data [15]. However, the deactivation model can be applied to explain such complex reactions. A number of non-catalytic gas–solid reactions were simulated using the deactivation model showing satisfactory predictions [15,42–46]. For the deactivation model, the changes of active sites and the reactivity of active sites can be expressed with an activity term “a” in the rate expression [44,46]. dX ¼ka dt
2. Experimental procedure The conversion experiments were carried out using a fixed bed reactor in order to convert trona to NaHCO3. A schematic diagram of the fixed bed reactor is shown in Fig. 3. In the fixed bed experiments, 20 g of parent trona were placed in a sample basket located on the base of the reactor. Then, air inside the reactor was evacuated by a vacuum pump, and 100% CO2 was injected into the reactor until the pressure was 19 psia. The pressure of CO2 in the fixed bed reactor was maintained during the experiments. 3. Results and discussion Experimental conversion data were obtained as weight changes measured by wet titration (Standard Methods 2320) during the conversion of trona to NaHCO3. In the experiment, the pressure of CO2 was kept at 19 psia and the mole fraction of water vapor in the gas phase (yH2O) was controlled by increasing the temperature of water to 22 °C, 32 °C, 42 °C, 50 °C, and 70 °C. Reaction time was varied from 10 min to 120 min. When the temperatures of water were 22 °C, 32 °C, 42 °C, 50 °C, and 70 °C, the mole fraction of water vapor in the
ð1Þ
where k is the apparent rate constant including the dependence of the reaction rate on the partial pressure of gaseous reactant. The variation of the activity of the solid reactant is expressed to be proportional to the activity itself and the deactivation approach is similar to catalyst deactivation.
ð3Þ
da ¼ kd a dt
ð2Þ
Here, kd is the deactivation rate constant and Doğu introduced this approach to the reaction of SO 2 with
Table 2 Correlation coefficient of the model with the experimental data at higher temperature Temperature (°C)
Correlation coefficient
42 50 70
0.96 1.00 1.00
Fig. 7. Temperature dependence of the deactivation rate constant.
62
K.J. Cho et al. / Powder Technology 184 (2008) 58–63
Table 3 Parameters of the deactivation model for the carbonation of trona to NaHCO3 Temperature (°C)
kd (s− 1)
k/kd
22 32 42 50 70
0.0003 0.0170 0.0587 0.154 0.232
21.274 1.172 0.9536 0.9012 0.8750
gas phase (yH2O) were 0.02, 0.04, 0.06, 0.10, and 0.24, respectively. Fig. 4 illustrates the conversion–time relationship under different molar ratio of water vapor in the gas phase at 22 °C, 32 °C, 42 °C, 50 °C, and 70 °C. Comparison of the predictions from the deactivation model with the experimental data obtained from different experimental conditions is illustrated in Figs. 5 and 6. Correlation coefficients of the model with the experimental data are reported in Table 1 and Table 2, which were calculated by Statistica 6.0. As shown in Fig. 5 and Fig. 6, while the deactivation model shows poor agreement with the experimental data at the lower temperature like 22 °C and 32 °C, it shows satisfactory results for the carbonation reaction of trona at higher temperature like 42 °C, 50 °C, and 70 °C. This would be related to the structural variation of trona particles at the lower temperature. As shown in Fig. 1 and Fig. 2, a trona particle is initially nonporous and then it begins to crack so that fissures appear on the surface. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, a trona particle takes longer time to change its structure. In other words, the fissures on the surface of the particles are not fully developed at the lower temperature during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term expressing the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. Furthermore, as mentioned earlier, Wen and Wang reported that the term expressing heat accumulation can not be neglected in the case of nonisothermal models [5]. Therefore, heat transfer or structural variation would explain the difference between the prediction and the experimental data. The best advantage of the deactivation model, however, is simple and can easily predict the conversion– time relationship for the trona conversion to NaHCO3. Deactivation rate constant was evaluated at different temperatures by regression analysis of the conversion–time data as depicted in Fig. 7. Also, the values of kd and k/kd are given in Table 3. The deactivation rate constant is quite strongly temperature dependent. However, it indicates a change in reaction mechanism as temperature increases. Line 1 in Fig. 7 was fitted using the experimental data from lower reaction temperature in the range of 22 °C to 50 °C and line 2 was evaluated by the data from higher temperature conditions such as 50 °C, and 70 °C. Line 3 was estimated using the entire experimental data, giving the R-squared value of 0.78. The activation energy for the deactivation rate constant, kd, was
found as 26,190 cal/mol for line 1, 4492 cal/mol for line 2, and 41,148 cal/mol for line 3 from each slope in Fig. 7. 4. Conclusions The experimental data were obtained from the conversion reaction of trona to NaHCO3. Increased water vapor significantly affects the conversion of trona to NaHCO3 and a particle structure changes during the conversion reaction. The deactivation model shows good agreement with the experimental data and gives the value of 0.94 as an average correlation coefficient. However, the deactivation model gives poor prediction at the lower temperature. This would be related to the structural variation of trona particles at the lower temperature. A trona particle is initially nonporous and then it begins to crack. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, the fissures on the surface of the particles are not fully developed during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term articulating the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. The deactivation rate constant, kd is strongly temperature dependent and the change of the slope suggests the reaction mechanism changes as the reaction temperature increases. Nomenclature a Activity of solid reactant b Stoichiometric coefficient of solid reactant k Apparent reaction rate constant, s− 1 kd Deactivation rate constant, s− 1 X Conversion of trona to NaHCO3
References [1] F. Patisson, M. Galant Francois, D. Ablitzer, A non-isothermal, nonequimolar transient kinetic model for gas–solid reactions, Chemical Engineering Science 53 (4) (1998) 697. [2] J. Szekely, J.W. Evans, H.Y. Sohn, Gas–Solid Reactions, Academic Press, New York, 1976. [3] K.B. Bischoff, Accuracy of the pseudo steady state approximation for moving boundary diffusion problems, Chemical Engineering Science 18 (11) (1963) 711. [4] S.K. Bhatia, On the pseudo steady state hypothesis for fluid–solid reactions, Chemical Engineering Science 40 (1985) 868–872. [5] C.Y. Wen, S.C. Wang, Thermal and diffusional effects in solid gas reactions, Industrial and Engineering Chemistry 62 (1970) 30–51. [6] H.Y. Sohn, H.J. Sohn, Effect of bulk flow due to volume change in the gas phase on gas–solid reactions: initially nonporous solids, Industrial and Engineering Chemistry, Process Design and Development 19 (2) (1980) 237. [7] H.Y. Sohn, O.A. Bascur, Effect of bulk flow due to volume change in the gas phase on gas–solid reactions— initially porous solids, Industrial & Engineering Chemistry Process Design and Development 21 (4) (1982) 658.
K.J. Cho et al. / Powder Technology 184 (2008) 58–63 [8] D.L. Ulrichson, D.J. Mahoney, Pore closure models for gas–solid reactions — the effect of bulk flow and reversibility, Chemical Engineering Science 35 (3) (1980) 567. [9] K.J. Cannon, K.G. Denbigh, Studies on gas–solid reactions-II: causes of thermal instability, Chemical Engineering Science 6 (4–5) (1957) 155. [10] J. Shen, J.M. Smith, Diffusional effects in gas–solid reaction, Industrial and Engineering Chemistry Fundamentals 4 (3) (1965) 293–301. [11] M. Ishida, C.Y. Wen, Comparison of kinetic and diffusional models for solid–gas reactions, AIChE Journal 14 (2) (1968) 311–317. [12] G.S.G. Beveridge, P.J. Goldie, Effectiveness factors and instability in noncatalytic gas–solid reactions: the effect of solid heat capacity, Chemical Engineering Science 23 (8) (1968) 913. [13] B.S. Sampath, P.A. Ramachandran, R. Hughes, Modelling of non-catalytic gas–solid reactions-I. Transient analysis of the particle-pellet model, Chemical Engineering Science 30 (1) (1975) 125. [14] B.S. Sampath, P.A. Ramachandran, R. Hughes, Modelling of non-catalytic gas–solid reactions-II. Transient simulation of a packed bed reactor, Chemical Engineering Science 30 (1) (1975) 135. [15] C. Guldur, G. Dogu, T. Dogu, Kinetics of trona sulfur dioxide reaction, Chemical Engineering and Processing 40 (1) (2001) 13. [16] E.A. Samuel DAF, R.L. Ostop, SO2 removal using dry sodium compounds, Am. Inst. Eng. Semp. 77 (211) (1981). [17] S. Kaplan, B.J. Garrick, P. Bieniarz, On the use of Bayes' Theorem in assessing the frequency of anticipated transients, Nuclear Engineering and Design 65 (1) (1981) 23. [18] L.J. Musio, J.K. Arand, Bench-scale study of the dry removal of SO2 with nahcolite and trona, EPRI, California, 1981. [19] T. Dogu, Effect of port structure on the mechanism of SO2 sorption on activated soda, Wiley, New York, 1984. [20] W. Hu, J.M. Smith, T. Dogu, G. Dogu, Kinetics of sodium bicarbonate decomposition, AIChE Journal 32 (9) (1986) 1483–1490. [21] S. Kimura, J.M. Smith, Kinetics of the sodium carbonate-sulfur dioxide reaction, AIChE Journal 33 (9) (1987) 1522–1532. [22] T.C. Keener, W.T. Davis, Study of the reaction of SO2 with NaHCO3 and Na2CO3, Journal of the Air Pollution Control Association 34 (6) (Jun 1984) 651–654. [23] M. Hartman, J. Hejna, Z. Beran, Application of the reaction kinetics and dispersion model to gas–solid reactors for removal of sulfur dioxide from flue gas, Chemical Engineering Science 34 (4) (1979) 475. [24] J. Maziuk, J. Kumm, Comparison of dry injection acid–gas control technologies, A&WMA 95th annual conference and exhibition, 2002, (Baltimore, Maryland; 2002). [25] A. Ekmekyapar, H. Ersahan, S. Yapici, Nonisothermal Decomposition Kinetics of Trona, Industrial and Engineering Chemistry Research 35 (1) (January 08, 1996) 258–262. [26] R.S. Gartner, G.J. Witkamp, Wet calcining of trona and bicarbonate in a mixed solvent, Journal of Crystal Growth 237–239 (2002) 2199–2204. [27] R.S. Gartner, G.J. Witkamp, Wet calcining of trona (sodium sesquicarbonate) and bicarbonate in a mixed solvent, Journal of Crystal Growth 237–239 (1–4 III) (2002) 2199–2204. [28] K.H. Buchel, H.-H. Moretto, P. Woditch, Metals and their Compounds, 2 ed.WILEY-VCH, Weinheim, Germany, 2000.
63
[29] F. Ray Thompson CBE, Sodium Carbonate, The Royal Society of Chemistry, UK, 1995. [30] G. Braune, H. Scheider. Sodium carbonate 1993. [31] P.K. Heda, D. Dollimore, K.S. Alexander, D. Chen, E. Law, P. Bicknell, A method of assessing solid state reactivity illustrated by thermal decomposition experiments on sodium bicarbonate, Thermochimica Acta 255 (1995) 255–272. [32] W.-Y. Kuu, R. Chilamkurti, C. Chen, Effect of relative humidity and temperature on misture sorption and stability of sodium bicarbonate powder, International Journal of Pharmaceutics 166 (1998) 167–175. [33] T.C. Keener, S.-J. Khang, Kinetics of the sodium bicarbonate-sulphur dioxide reaction, Chemical Engineering Science 48 (16) (Aug 1993) 2859–2865. [34] M. Dogan, C. Guldur, G. Dogu, T. Dogu, Soda ash production from Trona in a spray dryer, Journal of Chemical Technology and Biotechnology 68 (2) (1997/2) 157–162. [35] J.C. Walton, S. Bin-Shafique, R.W. Smith, N. Gutierrez, A. Tarquin, Role of carbonation in transient leaching of cementitious wasteforms, Environmental Science and Technology 31 (8) (1997) 2345. [36] N.I. Fattuhi, Concrete carbonation as influenced by curing regime, Cement and Concrete Research 18 (3) (1988) 426. [37] M. Fernandez Bertos, S.J.R. Simons, C.D. Hills, P.J. Carey, A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2, Journal of Hazardous Materials 112 (3) (2004) 193. [38] B. Johannesson, P. Utgenannt, Microstructural changes caused by carbonation of cement mortar, Cement and Concrete Research 31 (6) (2001) 925. [39] D. Johnson, R.E. Booth, A.S. Whiteley, M.J. Bailey, D.J. Read, J.P. Grime, et al., Plant community composition affects the biomass, activity and diversity of microorganisms in limestone grassland soil, European Journal of Soil Science 54 (4) (2003) 671. [40] Johnson DC. Accelerated Carbonation Of Waste Calcium Silicate Materials. SCI Lecture Papers Series. 2000. [41] C.D. Hills, S.J.T. Pollard, The influence of interference effects on the mechanical, microstructural and fixation characteristics of cement-solidified hazardous waste forms, Journal of Hazardous Materials 52 (2–3) (1997) 171. [42] B. Gullett, W. Jozewicz, L.A. Stefanski, Reaction kinetics of Ca-based sorbents with HCl, Industrial and Engineering Chemistry Research 31 (11) (1992) 2437. [43] W. Duo, N.F. Kirkby, J.P.K. Seville, R. Clift, Alteration with reaction progress of the rate limiting step for solid–gas reactions of Ca-compounds with HCl, Chemical Engineering Science 50 (13) (1995) 2017. [44] N. Yasyerli, T. Dogu, G. Dogu, i. Ar, Deactivation model for textural effects of kinetics of gas–solid noncatalytic reactions “char gasification with CO2”, Chemical Engineering Science 51 (11) (1996) 2523. [45] T. Dogu, The importance of pore structure and diffusion in the kinetics of gas–solid non-catalytic reactions: reaction of calcined limestone with SO2, Chemical Engineering Journal 21 (3) (1981) 213. [46] S. Balci, G. Dogu, T. Dogu, Structural variations and a deactivation model for gasfication of coal, Industrial and Engineering Chemistry Research 26 (7) (1987) 1454.