Chemical Engineering Science 59 (2004) 4653 – 4655
www.elsevier.com/locate/ces
Shorter Communication
Effect of Ca
(OH)
2
/fly ash weight ratio on the kinetics ofthe reaction of
Ca
(OH)
2
/fly ash sorbents with SO
2
at low temperatures
Chiung-Fang Liu, Shin-Min Shih
∗, Ren-Bin Lin
Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
Received 18 February 2003; received in revised form 4 December 2003; accepted 17 June 2004 Available online 24 August 2004
Abstract
Ca(OH)2/fly ash sorbents were prepared with different weight ratios. Their reactions with SO2have been studied under the conditions
similar to those in the bag filters ofthe spray-drying flue gas desulfurization system. The sorbent reactivity increased in general with decreasing Ca(OH)2/fly ash weight ratio. The reaction kinetics was well described by the surface coverage model which assumes that
the sorbent was made up ofplate grains and the rate was controlled by the chemical reaction on the grain surface and takes into account the surface coverage by products. For sorbents with Ca(OH)2/fly ash weight ratios than 10/90, the effect of weight ratio on the reaction was entirely represented by the effects ofthe intial specific surface area and the Ca content ofthe sorbent.
䉷2004 Elsevier Ltd. All rights reserved.
Keywords: Flue gas desulfurization; Ca(OH)2; Kinetics; Mathematical modeling; Multiphase reaction; pollution
1. Introduction
Many researchers have shown that sorbents prepared from hydrated lime[Ca(OH)2] and fly ash have higher SO2
captures and Ca utilizations than hydrated lime alone has (Jozewicz and Rochelle, 1986; Ho, 1987; Ho and Shih, 1992, 1993; Garea et al., 1997a, b; Liu et al., 2002; Lin et al., 2003). Hence, the Ca(OH)2/fly ash sorbents have a good potential for use in the dry and semidry flue gas desul-furization processes. The literature on the kinetic model for the reaction ofCa(OH)2/fly ash sorbents with SO2,
how-ever, is scarce. Garea et al. (1997b) proposed a nonideal surface adsorption model for sorbents with a Ca(OH)2/fly ash weight ratio of 13. Recently we reported a surface cov-erage model for sorbents with a weight ratio of 70/30 (Liu et al., 2002). In this work, the effect of Ca(OH)2/fly ash
weight ratio on the kinetics ofthe reaction ofCa(OH)2/fly
ash sorbents with SO2 was investigated, with the aim to
obtain a kinetic model for sorbents prepared with different Ca(OH)2/fly ash weight ratios.
∗Corresponding author. Tel.: 886-2-23633974, fax: 886-2-23623040.
E-mail address:smshih@ccms.ntu.edu.tw(S.-M. Shih).
0009-2509/$ - see front matter䉷2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.06.031
2. Experimental section
The Ca(OH)2 reagent, fly ash, sorbent preparation pro-cedure, and experimental setup and procedure for sulfation experiments used in this work were the same as those de-scribed inLiu et al. (2002)andLin et al. (2003). Sorbents with different Ca(OH)2/fly ash weight ratios, as those listed
in Table 1, were prepared by first slurrying the solids at a water/solid weight ratio of10/1 and 65◦C for 16 h and subsequently vacuum-drying the slurries at 105–110◦C f or 8–10 h. The dry cake was crushed into powder. The mean particle diameters,dp, ofthe sorbents were in the range of
4–35m, as shown inTable 1. The sorbents were made to react with SO2mainly at 60◦C, 70% relative humidity (RH)
and 1000 ppm SO2 using a differential fixed-bed reactor.
For each run, about 30 mg ofsample was dispersed into the quartz wool contained in the quartz sample pan, which had dimensions of10 mm o.d. and 15 mm height.
The conversion, X, ofa reacted sample was defined as its SO23−/Ca2+molar ratio. The SO23−content in a sample was determined by iodometric titration, and the Ca2+ content by EDTA titration. The experimental error in X was about
4654 C.-F. Liu et al. / Chemical Engineering Science 59 (2004) 4653 – 4655
Table 1
Ca(OH)2/fly ash weight ratios, sorbent weights per mole ofCa, mean diameters, BET specific surface areas, and values ofk1andk−12 in Eq. (1) for Ca(OH)2/fly ash sorbents
Ca(OH)2/ Fly ash M dp (m) Sg0(m2/g) k1 (min−1) k−12
(wt. ratio) (g sorbent/mol Ca)
Raw Ca(OH)2 75 7 9.6 0.032 0.195 100/0 76 5 11.8 0.049 0.241 90/10 86 4 19.8 0.115 0.413 70/30 110 9 38.0 0.165 0.620 50/50 163 24 38.7 0.329 0.752 30/70 252 35 27.5 0.354 0.694 10/90 677 13 42.3 0.581 1.163
Slurrying conditions: 65◦C,L/S = 10/1, and 16 h. Reaction conditions: 60◦C, 70% RH, and 1000 ppm SO2.
0 10 20 30 40 50 60 70 0.0 0.2 0.4 0.6 0.8 1.0 1.2 E q.(1) Ca(OH)2 100/0 90/10 70/30 30/70 50/50 10/90 X t, min
Fig. 1. Effect of Ca(OH)2/fly ash weight ratio on the reaction ofCa(OH)2/fly ash sorbents with SO2. Slurrying conditions: 65◦C,
L/S=10/1, and 16 h. Reaction conditions: 60◦C, 70% RH, and 1000 ppm SO2.
The Ca2+content ofa sorbent was determined by EDTA titration, and its reciprocal,M, the initial sorbent weight per mole ofCa, is listed inTable 1. TheM value increases with decreasing Ca(OH)2/fly ash weight ratio. The BET specific
surface area,Sg0, ofeach sorbent is also listed inTable 1. The
Sg0ofa Ca(OH)2/fly ash sorbent is much greater than that
ofCa(OH)2alone due to the formation of foil-like calcium
silicate hydrates in the sorbent (Liu et al., 2002; Lin et al., 2003).
3. Results and discussion
The experimental results for the reaction of the sorbents with SO2are shown inFig. 1. In general, the sorbent
reactiv-ity increases with decreasing Ca(OH)2/fly ash weight ratio. The reaction ofeach sorbent is seen to stop at an ultimate conversion after about a 20 min reaction time. The ultimate conversion ofthe sorbent with a ratio of10/90 exceeds 1.0; this is because the contribution ofthe reactive compounds of Na, K, and Mg other than Ca present in the fly ash particles was also included when calculating the conversion.
0 1x104 2x104 3x104 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Ca(OH)
2/fly ash sorbent raw Ca(OH) 2 Ca(OH) 2, Ho et al., 2002 100/ 0 90/10 70/30 50/50 30/70 10/90 k1 , min -1 Sg0M,m2 /mol Ca
Fig. 2. Plot ofk1versusSg0M. Slurrying conditions: 65◦C,L/S = 10/1, 16 h. Reaction conditions: 60◦C, 70% RH, and 1000 ppm SO2.
The curves inFig. 1are the least-squares fitting curves of the data using the following equation given by the surface coverage model (Shih et al., 1999):
X = [1 − exp(−k1k2t)]/k2. (1)
Like Ca(OH)2alone (Ho et al., 2002) and the sorbent with a ratio of70/30 (Liu et al., 2002) the reaction behavior of the sorbents with other ratios are seen to be well described by the surface coverage model. The model assumes that the sorbent is made up ofplate grains and the rate is controlled by the chemical reaction on the grain surface, and takes into account the surface coverage by products.
The values of k1 and k2−1 in Eq. (1) obtained for each
sorbent are listed inTable 1. Bothk1 andk−12 values vary
significantly with the Ca(OH)2/fly ash ratio. However, as
seen inFig. 2,k1andSg0M for each sorbent follow a linear
relationship, except the data for the sorbent with a 10/90 ratio. The linear least-squares fitting line inFig. 2gave
C.-F. Liu et al. / Chemical Engineering Science 59 (2004) 4653 – 4655 4655 0 5 10 15 20 25 30 35 40 45 0.0 0.2 0.4 0.6 0.8 1.0 1.2 100/0 90/10 30/70 70/30 50/50 10/90 Ca(OH)
2/fly ash sorebnt raw Ca(OH) 2 Ca(OH) 2, Ho et al.,2002 Sg0, m2 /g 1/ k2
Fig. 3. Plot ofk−12 versusSg0. Slurrying conditions: 65◦C,L/S = 10/1, 16 h. Reaction conditions: 60◦C, 70% RH, and 1000 ppm SO2.
with a correlation coefficient of 0.986. Furthermore, as shown inFig. 3, there is a linear relationship between k−12 andSg0, except the data for the sorbent with a 10/90 ratio. The linear least-squares fitting line inFig. 3gave
k−12 = (1.96 ± 0.12) × 10−2× Sg0 (3) with a correlation coefficient of 0.945. According to Eq. (1),
k−12 equals the ultimate conversion. Therefore, Eq. (3) im-plies that the ultimate conversion ofa sorbent is proportional to itsSg0and independent ofitsM.
Eqs. (2) and (3) were obtained under the reaction condi-tions of60◦C, 70% RH, and 1000 ppm SO2. According to
Ho et al. (2002) andLiu et al. (2002),k1 and k2 are also
functions of RH, temperature, and SO2 concentration, but
only RH affects k1 and k2 significantly. In order to know
whether the relationship betweenk1 or k2 andSg0 andM
is affected by the RH, samples of Ca(OH)2(M = 74 g/mol
Ca,Sg0= 10.3 m2/g) and the sorbent with a ratio of30/70
(M = 248 g/mol Ca, Sg0= 25.6 m2/g) were reacted with
1000 ppm SO2 at 60◦C and different RHs (30–80%). The k1/(Sg0M) and k2Sg0values obtained were found to be well
correlated with the RH using the correlations that have been given for the Ca(OH)2/fly ash sorbent with a 70/30 ratio by
Liu et al. (2002). Thus, the linear relationship between k1
andSg0M or between k2−1andSg0is valid at different RHs (30–80% RH). Furthermore, the equations representing the effects of RH, temperature(T ), and SO2concentration(y)
onk1andk2given in the article byLiu et al. (2002), Eqs.
(13) and (14), are valid for sorbents with ratios > 10/90. Those two equations can be rewritten by combining with Eqs. (2) and(3), respectively, as
k1= 1.26 × 10−4Sg0Me0.0234RHy0.10e−9096/RT, (4)
k2= 1930Sg0−1RH−0.864. (5)
It should be noted that the numerical values ofthe parameters in Eq. (13) in the previous article were miswritten and should be corrected as
k1= 0.0112Sg0e0.0234RHy0.10e−9096/RT. (6)
Conversions calculated by Eq. (1) together with Eqs. (4) and (5) were compared with the experimental results (Fig. 1) except those for the sorbent with a 10/90 ratio, both were in good agreement with a standard deviation of0.07.
4. Conclusion
From the above analyses, we can conclude that for a Ca(OH)2/fly ash sorbent prepared with a weight ratio larger
than 10/90, its reaction with SO2 at low temperatures can
well be described by the surface coverage model, Eq. (1) together with Eqs. (4) and (5). The effect of Ca(OH)2/fly ash weight ratio on the reaction kinetics was entirely repre-sented by the effects of the initial specific surface area and the Ca content ofa sorbent on the kinetic parameters.
Acknowledgements
This research was supported by the National Science Council ofRepublic ofChina.
References
Garea, A., Fernandez, J., Viguri, J.R., Ortiz, M.I., Renedo, M.J., Irabien, A., 1997a. Fly ash/calcium hydroxide mixtures for SO2removal: structural properties and maximum yield. Chemical Engineering Journal 66, 171–179.
Garea, A., Viguri, J.R., Irabien, A., 1997b. Kinetics ofthe flue gas desulfurization at low temperature: fly ash/calcium (3/1) sorbent behavior. Chemical Engineering Science 52, 715–732.
Ho, C.S., 1987. Reaction ofCa(OH)2and fly ash/Ca(OH)2slurry with SO2. MS. Engineering Thesis, Department ofChemical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C.
Ho, C.S., Shih, S.M., 1992. Ca(OH)2/fly ash sorbents for SO2removal. Industrial & Engineering Chemistry Research 31, 1130–1135. Ho, C.S., Shih, S.M., 1993. Characteristic and SO2 capture capacities
ofsorbents prepared from products ofspray-drying flue gas desulfurization. The Canadian Journal of Chemical Engineering 71, 934–939.
Ho, C.S., Shih, S.M., Liu, C.F., Chu, H.M., Lee, C.D., 2002. Kinetics of the sulfation of Ca(OH)2at low temperatures. Industrial & Engineering Chemistry Research 41, 3357–3364.
Jozewicz, W., Rochelle, G.T., 1986. Fly ash recycle in dry scrubbing. Environmental Progress 5, 219.
Lin, R.B., Shih, S.M., Liu, C.F., 2003. Structural properties and reactivities ofCa(OH)2/fly ash sorbents for flue gas desulfurization. Industrial & Engineering Chemistry Research 42, 1350–1356.
Liu, C.F., Shih, S.M., Lin, R.B., 2002. Kinetics ofthe reaction of Ca(OH)2/fly ash sorbent with SO2 at low temperatures. Chemical Engineering Science 57, 93–104.
Shih, S.M., Ho, C.S., Song, Y.S., Lin, J.P., 1999. Kinetics ofthe reaction ofCa(OH)2 with CO2 at low temperature. Industrial & Engineering Chemistry Research 38, 1316–1322.
