I n d . Eng. Chem. Res. 1988,27, 2203-2210 Schultz, W. J.; Etter, M. C.; Pocius, A. V.; Smith, S. “A New Family of Cation Binding Compounds: threo-a,w-Poly(cyc1ooxalkane)diyl”. J . Am. Chem. SOC.1980, 102,7981-7982. Schultz, W. J.; Katritzky, A. R. ”Polymers Containing 2,5Oxolanylene Segments”. US Patent 4 309 516, 1982. See, T. M. S.; Loh, P. C.; Newell, R. “A New Approach to Tyre Compounding with Particular Reference to Motor Cycle Tyres”. Proc. Int. Rubb. Conf., Kuala Lumpur, Malaysia, 1985; p 352. Stellman, Z. M.; Woodward, A. E. “Chain Folding in Poly(trans1,4-butadiene) Crystals”. J . Polym. Sci., Part B, Polym. Lett. 1969, 7, 755-759. Stellman, J. M.; Woodward, A. E.; “Chain Folding in Poly-trans1,Cbutadiene Crystals Grown from Various Solvents”. J. Polym. Sci. Part A-2, Polym. Phys. 1971, 9, 59-66. Swern, D. Organic Peroxides; Wiley Interscience: New York, 1971; VOl. 2. Swern, D.; Billen, G. N.; Scanlan, J. T. “Hydroxylation and Epoxidation of Some 1-Olefins with Peracids”. J . Am. Chem. SOC. 1946,68, 1504-1507. Swern, D.; Findley, T. W.; Billen, G. N.; Scanlan, J. T. “Determination of Oxirane Oxygen”. Anal. Chem. 1947, 19, 414-415. Terry, R. W.; Jacobs, A. F. “Modified Diene Polymers”. Brit. Patent Appl. 2 008 125, 1979. Tutorskii, I. A,; Khodzhaeva, I. D.; Novikov, S. V.; Dogadkin, B. A. “Conformational Changes of the Macromolecular Coil during the Epoxidation of Polyisoprene”. Vyskomol. Soedin, Ser. A. 1973, 15, 2282.
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Tutorskii, I. A.; Khodzhaeva, I. D.; Dogadkin, B. A. “Reactions of Polyisoprene with Peracids”. Vysokomol Soedin, Ser. A 1974,16, 157. Tutorskii, I. A.; Khodzhaeva, I. D.; Dogadkin, B. A. “Reactions of Diene Polymers with Peracids”. Khim. Khim. Tekhnol. Tr. Yubileinoi Konf. Posvyashch. 70-Letiyu Znst. (Mosk. Znst. Tonkoi Khim. Tekhnol.) 1970, 187; Chem. Abstr. 1974, 81, 106968j. Udipi, K. “Polymer Epoxidation Process”. US Patent 4 131 725, 1978. Udipi, K. “Epoxidation of Styrene-Butadiene Block Polymers I”. J. Appl. Polym. Sci. 1979a,23, 3301-3309. Udipi, K. “Epoxidation of Styrene-Butadiene Block Polymers 11”. J. Appl. Polym. Sci. 1979b,23, 3311-3321. Ube Industries Ltd. “Hot Melt Adhesives and Coatings”. Japan Kokai Tokyo Koho 8 098 202, 1979. U. S. Fed. Reg. 1963,27, 2445. Wichacheewa, P.; Woodward, A. E. “Kinetics of Epoxidation of Poly(trans-l,4-butadiene)Crystals in Suspension”. J . Polym. Sci., Polym. Phys. Ed. 1978, 16, 1849-1859. Witnauer, L. P.; Swern, D. ”X-ray Diffraction and Melting Point Composition Studies on the 9,lO Epoxy and Dihydroxystearic Acids and 9,lO-Epoxyoctadecanols”.J. Am. Chem. SOC.1950, 72, 3364-3368. Zuchowska, D. “Polybutadiene Modified by 1,2 Epoxidation”. Polymer 1980, 21, 514; 1981, 22, 1073. Received for review March 29, 1988 Accepted August 30, 1988
KINETICS AND CATALYSIS Analysis and Modeling of the Direct Sulfation of CaC03 Mohammad R. Hajaligol, John P. Longwell,* and Adel F. Sarofim Department of Chemical Engineering and Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Isothermal sulfation studies of single-crystal CaC03 and porous CaO confirm that the higher rates, a t high conversion, for direct sulfation are due to formation of a more porous calcium sulfate layer. A shrinking core model showed that reaction rate and diffusion control are both important. For direct sulfation, calculated diffusivities above 50% conversion were about 2 orders of magnitude greater than for diffusion through the sulfate layer formed during CaO sulfation. Thus, diffusivities depend on porosity. Porosity increases with the rate of COPgeneration, and the resulting temperature effect can be correlated by an activation energy of 35 kcal/mol. Natural limestones, with a porosity of 2-3 % , doubled conversion at 3 h for 38-45-pm particles. By choice of direct sulfation conditions, high calcium conversions can be achieved in the solid residence times typical of dense-phase fluidized bed combustion. The conventional method for removing sulfur dioxide from coal combustion products involves decomposing limestone to lime, which then reacts with sulfur dioxide. Stone utilization with this method is generally low unless another process is also used, such as repeated rehydration of the lime coupled with the use of a chemical such as methanol (Moran et al., 1987). An alternative approach is direct sulfation of limestone, where stone reacts directly with sulfur dioxide in an oxidizing atmosphere to produce sulfate. It has recently been demonstrated that, in an environment that prevents CaCO, decomposition, com-
* Person t o whom correspondence should be addressed. 0888-5885/88/2627-2203$01.50/0
plete utilization of stone is feasible (Snow et al., 1987). The chemistry involved, when SO2is reacted with CaC03 and excess oxygen, is usually CaC03(s) + SO&)
+ (1/2)02(g) = CaS04(s) + COP(g) (1)
However, in the presence of some catalysts, SO,(g) can be converted to S03(g), and the following reaction is also plausible: CaC03(s) + S03(g) = CaS04(s) + C02(g)
(2)
Studies in this area have been done by Van Houte et al. (1978, 1981), Glasson and O’Neill (198Qa,b),Fee et al. 0 1988 American Chemical Society
2204 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 Table I. Chemical Analyses of Limestones (Mole Fractions) comDd Iceland mar" Fredoniab Ripsbr" CaC03 0.9915 0.9863 0.964 Mg'333 0.0051 0.018 SiOz 0.00855 0.0029 0.000 NazO 0.0001 0.0031 Ah03 0.0047 0.015 0.0014 0.0004 FeA S 0.0003
"Data taken from Snow (1985). bData taken from Harvey (1971). Table 11. BET Surface Areas of Unreacted Limestones limestone surface area. m2/a Iceland spar" (45-53 pm) 0.207 Rigsby' (45-53 pm) 0.576 0.75 Fredoniab (60 pm)
Q
z
0 c W Q
Lz LL
0
60 120 180 REACTION T I M E , Minutes
240
Figure 1. Effects of temperature on the sulfation of cSc03 for particles with 2-3-pm diameter.
"Data taken from Snow (1985). bData taken from Borgwardt (1987).
(1982), and very recently a t MIT by Snow et al. (1987). A thorough review of this literature has been given by Snow (1985) and Snow et al. (1987) and will not be repeated here. Snow et al. (1987) found diffusional resistance at high conversion to be lower during direct sulfation, which they related to the formation of a porous structure, in contrast to the nonporous product layer characteristic of CsO sulfation. They hypothesized that the evolution and escape of CO, during direct sulfation of CaC03 with SO2 formed this porous structure. The objectives of the present study were to extend the information available on the effects of reaction time, temperature, particle diameter, particle porosity, and water vapor pressure for the direct conversion of carbonate to sulfate; to enlarge our understanding of the structural changes in limestone during the process; and to develop a detailed model that can be used to improve sulfur-capture processes.
Experimental Section Pure calcite crystal (Iceland spar) was used in most of the work reported here. The effects of stone porosity were investigated using two other natural limestones, Rigsby and Fredonia. A summary of the chemical and physical properties of limestones are presented in Tables I and 11. More detailed information regarding these properties has been presented elsewhere (Snow, 1985; Harvey, 1971; Borgwardt, 1987). Particles with average diameters of 2-3, 10-12, 19-21, and 38-45 pm were used in this study. Sample fractionation for diameters smaller than 38 pm was carried out in a centrifugal separator, an ACUCUT machine Model A12 made by Donaldson, Inc., Minneapolis, MN. A thermogravimetric analyzer (TGA) (Cahn Instruments, Inc., Cerritos, CA, System 113) was used in these experiments. The sulfation procedure consisted of heating a sample at 50-60 "C/min to the desired temperature, establishing isothermal conditions with the gas stream of 95% C02 and 5% 02,and then adding SOz in a concentration of 3000 ppm, at a total pressure of 1 atm. Isothermal reaction temperature was varied from 500 to 940 "C. Sulfation of limestone in the TGA can be sample size dependent because of the external mass-transfer resistance. Samples of different sizes were carefully tested, and samples smaller than 5 mg with 2-3-~mparticle diameter, when spread carefully on the platinum pan in the TGA, were found to be independent of external resistances. A
0.0
v0
f
60
I
I
120
180
24 C
REACTION TIME Mlnutes ~
Figure 2. Effects of temperature on the sulfation of CaCO, for particles with 5-7-pm diameter.
D, =10-12)m
0
60 120 100 REACTION TIME M i n u t e s
240
Figure 3. Effects of temperature on the sulfation of CaC03 for particles with 10-12-pm diameter.
detailed description of this technique is given by Snow (1985). In order to provide enough sample for porosity measurements, some experiments were conducted in a bench-scale fluidized bed at 750 "C for 38-45-pm particles and a fixed bed at 800 and 900 "C for particles of 5-7-pm and 10-12-km diameters, respectively. Exactly the same procedure was followed as when the TGA was used. Porosity of the samples was measured with a mercury porosimeter, Model, Autopore 9200 of Micromeritics Instrument Corp., Norcross, GA. In those experiments in which CaO was used for sulfation, CaO was prepared from Iceland spar limestone at the same temperatures a t which direct sulfation was carried out. The procedure used consisted of heating CaC03 in a pure COPgas stream, and once the desired temperature was reached, the gas stream was switched from COZ to He.
Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 2205
-I
Q Z
0
0.4
t 600'C
500'C
0.0
60 120 1 a0 REACTION TIME Mlnutes
0
LL 0.0 0
I
240
60
120
180
240
REACTION TIME Minutes
Figure 4. Effects of temperature on the sulfation of CaCOS for particles with 19-21-~mdiameter.
Figure 6. Effects of particle diameter on the sulfation of CaCO, at 900 OC temperature.
T=90O0C
Dpz38-45Ym
0 8 - ICELAND SPAR
0.01
600
I
700 REACTION
I
I
800
900
I 1000
TEMPERATURE;^ Figure 5. Effects of temperature and reaction time on the sulfation of CaCOa for 10-12-rm particles.
Immediately upon completion of stone calcination, sulfation was begun in a stream of 5% 02,3000 ppm SO2, and the rest helium.
Results Figures 1-4 show the effects of temperature for different particle diameters, 2-3,5-7,10-12, and 19-21 pm; and as a function of reaction time. At a fixed reaction time, conversion increased as temperature increased for any particle diameter. Although temperature was most effective for smaller particle diameters, i.e., 2-3 pm, for any particle diameter and above 800 "C the changes in conversion with temperature were striking. Around the temperature where CaC03 decomposition occurred at atmospheric pressure and 95% C02 (-900 "C), conversion leveled off. Above this temperature, conversion relative to 900 "C increased for a shorter reaction time but decreased for longer reaction times. This behavior for 10-12-pm particles is clearly shown in Figure 5. Up to the decomposition temperature, the reaction involved for sulfur capture is given in eq 1. Above this temperature, the calcium carbonate decomposes to calcium oxide and sulfation occurs as CaO(s) + SOz(g) + (1/2)02(g) = CaS04(s) Since CaO is more reactive and has a larger surface area than CaC03, the sulfation rate of CaO is faster for lower reaction times. But as time goes on, diffusion through the impervious CaSO, product layer surrounding the CaO becomes the rate-controlling fador. With direct conversion of CaC03 to CaSO,, the escape of COz seems to develop porosity in the CaS04 product layer and allows easier access of SO2 and O2 to the CaCO, surface. The effects of particle diameter at 900 "C temperature are presented in Figure 6. As expected, for a given re-
0
I
I
60
120
I
180
240
REACTION TIME , M i n u t e s
Figure 7. Effect of porosity on the sulfation of CaC08 for 38-45-rm particles a t 900 OC temperature.
z
0 VI a w > z
8 -1
1 P+ V
4
a LL
FREDONIA 0
-------
60 120 180 REACTION T I M E , M i n u t u s
240
Figure 8. Effects of porosity on the sulfation of CaCO, at 900 OC temperature.
action time, conversion of CaC03 to CaS04 decreases as particle diameter increases. The effects of the limestone's original porosity on sulfation at 900 OC are shown in Figures 7 and 8. Measurements were made for two natural limestones, Fredonia and Rigsby. In contrast to the single-crystal Iceland spar, these stones are a conglomerate of small calcite crystals with approximately 2-3 % overall porosity (Figure 9). For larger particle diameter, the rate of sulfation for porous limestone was significantly higher than for the single-crystal Iceland spar (Figure 8). For smaller particles, however, the difference is not so large. This finding can be attributed to the size of a single crystal in natural limestone. As the SEM photograph of Fredonia and Rigsby limestones shows (Figure 9), the single-crystal sizes for these limestones are roughly in the ranges 2-5 and 1-3-pm, respectively. The 2-5-wm crystal size of Fredonia
2206 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 1000
k
I
I
I
1
Dp s10-12ym
0
0 I
I
I
0 0 U ??
1
1
1
o
'""1
o 0" 6 0.2
0
0.4
0.6
P
ATM
co2
0.8
1.0
Figure 11. Effect of COBpartial pressure on the CaC03decomposition temperature. a
--
Dp=10-12ym
c
z
0
U
f 2 z 0 v) a W
>
900%
6
:
U
2 2 0
16'
0
20
60
40
80
100
REACTION TIME, Minutes
Figure 12. Comparison of CaO sulfation with CaCOS sulfation. Figure 9. SEM photographs from the particle surface of Fredonia (a) and Rigsby (b) limestones.
-
Dp = 38 4 5 y m
u
z 0
fl 0.8 [1:
w
>
5
0.6 j
V
5 0.4 0 U
0-
01
I
0.0 ~~
0
~~
60 120 REACTION TIME ,Minutas
180
Figure 10. Effects of water vapor on the sulfation of CaCOSat 900 "C temperature and for 10-12-pm particles.
limestone is very close to the smaller diameter fraction (4-8 pm) used in this study. Water vapor is a constituent of the flue gas stream in coal combustors. In this study, the gas stream was saturated with water vapor a t 40 and 50 "C and then passed over the sample a t 900 "C. Figure 10 shows the results for a particle with 10-12-pm diameter and water partial pressure of 0.0, 0.06, and 0.12 atm, respectively. While water vapor probably does not affect the reaction a t the CaC03 interface, it has a positive effect on the total conversion. Increasing water vapor concentration from 6 to 12 mol 70 had a smaller effect than the first 6 mol %. C02is always present in coal combustors and flue gases, and its partial pressure plays a significant role in deter-
0.1 FRACTIONAL CONVERSION
I 0.2
Figure 13. Porosity variation of sulfated Iceland spar with conversion.
mining the maximum temperature that can be reached without stone decomposition. Figure 11 shows decomposition temperature versus COPpartial pressure a t a total pressure of 1 atm. Up to 850 OC, a C02 concentration in the gas stream of about 10% by volume was sufficient to prevent stone decomposition. For the 95% C02 mixture generally used, decomposition temperature was about 920 "C in our TGA. Figure 12 compares the direct conversion process with sulfation of CaO, using Iceland spar. The ratio of CaO to CaCOnconversion was higher than unity for lower reaction times, particularly a t lower temperatures, but was reversed a t higher temperatures (900 " C ) and above 15 min. Measurements were made on the porosity of the CaSO,, product layer that covers the unreacted CaC03 as a function of the extent of the reaction. There was a de-
Ind. Eng. Chem. Res., Vol. 27, No. 12,1988 2207
Figure 1.5. SEM photographs of sectioned particles of partially (50%) sulfated Iceland spar limestone (to support shrinking; core model).
'
