I n d . Eng. Chem. Res. 1990, 29, 778-783
778
Chlorination Kinetics of Silicon Dioxide in the Presence of Carbon Jen-Min Chen and Feg-Wen Chang* Department of Chemical Engineering, National Central I'nzversztt, Chungli, Taiwan 32054, ROC
Ching-Yuan Chang Graduate Institute of E n w o n m e n t a l Engineering, National Taiwan University, Taipei, Taiwan 10764, ROC
T h e chlorination of silicon dioxide/carbon pellet was investigated by a thermal gravimetric analysis (TGA) reaction system over the temperature range 1123-1323 K. T h e effect of gas flow rate, pellet size, pellet forming pressure. C/Si02 molar ratio, and initial grain sizes on the extent of chlorination was determined experimentally. T h e total porosity of the pellets was found to be slightly increased, when t h e reaction proceeded. T h e rate expressions of the chlorination of silicon dioxide/carbon pellet in the chemical-reaction-controlled region were presented. T h e chlorine reaction order was found t o be 0.62, with a n activation energy of 114.7 kJ/mol. A kinetic model was developed, and it gave good agreement with the experimental results. Silicon tetrachloride is the most important halosilane commercially manufactured. Since it is a liquid at room temperature (bp 330.6 K), it may be purified by distillation a t a relatively low cost. For this reason, it finds a wide variety of uses as a raw material for the production of various silicon compounds including high-purity silicon and silicon dioxide. The high-temperature chlorination of silicon dioxide in the presence of carbon is an important step in the conventional process for the production of silicon tetrachloride. Recently, Denki Kagaku Kogyo K. K. (1983a,b, 1984) and Ube Industries, Ltd. (1982a,b, 1983c, 1984) published numerous patents for the production of silicon tetrachloride through a chlorination reaction. In the patents, small amounts of silicon, silicon carbide, or borate were added to accelerate the reaction. Iwai et al. (1983) chlorinated silica in a fixed bed by using activated carbon as a reducing agent. It was found that the reaction rate of chlorination became higher on adding boron trichloride as an accelerant. In addition to the above-mentioned studies, the silicon dioxidelcarbon chlorination reaction system was also applied to high concentration silicon dioxide waste (Basu et al., 1973; Ube Industries, Ltd., 1983a,b) which can result in a fixed ratio of the silicon dioxide/carbon mixture from pyrolysis. In the presence of carbonaceous reducing material, the possible reaction between silicon dioxide and chlorine is thermodynamically feasible, as shown in Figure 1,for the three reactions (Kubaschewski et al.. 1967)
It can be seen that reaction 2 has greater negative free energy values then reaction 1 a t temperatures above 970 K; i.e., it is thermodynamically favorable to produce carbon monoxide in a chlorination reaction at high temperature. However, it consumes more carbon. Carbon monoxide also becomes an ineffective reducing agent when the reaction temperature is increased. The chlorination of silicon dioxide/carbon depends on its physical characteristics and the chlorine source employed. Generally, there are three resistances affecting the
* To whom
correspondence should he addressed
overall reaction rate in gas/solid reactions, namely, the external mass-transfer resistance, pore diffusion resistance, and the chemical reaction. Explicitly, gas flow rate and pellet size affect the external mass transfer, while pellet size and reaction temperature affect pore diffusion. Pellet-forming pressure, reaction temperature, chlorine concentration in the bulk gas, C/Si02 molar ratio, and initial grain sizes of silica and carbon influence the chemical reaction rate. There have been many studies focusing on the manufacture of silicon tetrachloride. However, the kinetics of the chlorination of silicon dioxide/carbon have not received much attention. In the present work, mixtures of silicon dioxide and carbon powder were compacted to spherical pellets and were calcined in a furnace before chlorination. Graphite was selected as the carbonaceous reducing agent due to its excellent compactability in making the pellets. The kinetics and effect of operating variables were extensively studied. A reaction model was also developed to explain the experimental results.
Experimental Section Materials Used and Sample Preparation. The silicon dioxide used in the experiments was supplied by Hayashi Pure Chemical Industries, Ltd., having a minimum purity of 99.2%. Graphite was purchased from Merck & Co. Chlorine was made by Fong-Ming Co. with a purity of 99.9%. High-purity nitrogen was purchased from Chio-Ho Co. with a purity of 99.99%. Silicon dioxide and graphite powder were screened through an ASTM standard sieve to obtain the desired composition. The completely mixed powder was then poured into a cylindrical casting mold with an inner diameter of 1.3 cm and was pressed into a disk by a hydraulic press. Spherical pellets with the desired diameters were cut out of the disk with a sharp knife. The spherical pellets were sealed in a quartz tube and calcined for 6 h a t 1223 E;. Apparatus and Procedure. The experimental chlorination apparatus is shown in Figure 2. A Cahn DT1,-7500 electrical balance was used. The pellet with a known diameter was placed on a quartz disk, which was hung on the arm of the balance and sealed in a quartz reaction tube. High-purity nitrogen was purged into the top of the balance to protect the balance and served as the chlorine diluent gas. The reaction system was flushed with nitrogen at a high flow rate for several minutes, and then the nitrogen was adjusted to the desired flow rate. The reaction tube was placed into the furnace, heating was started, and the sample was brought to the desired tem-
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Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 779 0
Table I. Effect of Reaction Time on the Porosity of the C/Si02 Pelleta reaction reaction time, min porosity time, min porosity 0 0.0908 60 0.1032 15 30
0.0977 0.0999
120
0.1161
- 200
"Calcined a t 1223 K for 6 h; C/Si02 molar ratio, 2.8; pellet diameter, 4 mm; gas flow rate, 625 mL/min; chlorine fraction, 40 vol YO;reaction temperature, 1223 K.
-
:
\
-3 0-
CI 3 - 400
-600 BOO
1000
1200
1400
Figure 1. Standard free energy changes of the chlorination of silicon dioxide in the presence of carbon.
where ps and pc, the densities of silicon dioxide and graphite, are 2.184 X lo3 and 2.368 X lo3 kg/m3, respectively. X-ray diffractometer (Shimadzu, Type XD-5) and scanning electron microscope (Jeol, Type 200cx) have been used to examine the morphology characteristics. Amount of the Reaction. It has been shown thermodynamically that reaction 2 prevails above 970 K. However, when the experiment was carried out a t 1373 K, we found that only carbon dioxide existed in the outlet gas mixture. This could be due to kinetic limitations. Hence, the chlorination of silicon dioxide in the presence of carbon a t temperatures between 1123 and 1323 K may be better represented by reaction 1. From the measured weight loss (due to the removal of silicon tetrachloride as vapor, plus the removal of carbon as carbon dioxide) and the reaction stoichiometry (reaction 11, the amount of silicon dioxide reacted is calculated as follows:
The C/Si02 molar ratio, m, is defined as
The degree of conversion, X s , is given by 1
10
11 12 13 14
15 16
Fdrnace
Box
pump ~ e a c t o r lube NaOH S O I U I O n Ouartz d s r Condenser AIr
Figure 2. Schematic diagram of the apparatus for chlorination.
perature. After flowing through CaS04, a molecular sieve, and a rotameter, chlorine was introduced to the reaction system. The experimental run was begun, and the weight change of sample during the reaction was continuously recorded by a recorder. The outlet gas was absorbed by caustic solution to remove unreacted chlorine. When the experiment was done, the furnace power and chlorine source were turned off. The purge gas of nitrogen in the balance was kept flowing all the time to protect the balance. Measurement of Physical Properties. In the measurement of porosity, the volume and weight of the pressed disk were first measured to calculate the sample's density. By knowing the density and weight of the spherical pellet, one can get the pellet's volume, V,. The sample was placed inside the furnace and burned for 3 h a t 1223 K. The residual was weighed after the carbon in the sample was burned completely. The weights of the carbon and silicon dioxide, W , and Ws, were recorded and the sample porosity (E,) was calculated according to the following equation:
On substitution of eq 5 and eq 6 in eq 7 and rearrangement, the degree of conversion can be formulated as (Ms + mMc)AWp = (8) (Ms + Mc)W,O
xs
Results and Discussion Analysis of Physical Properties. Figure 3 shows the scanning electron micrographs of the structure change with calcination and chlorination. From these photos, the graphite can be seen to be very close to a spherical shape, and its surface is covered with flakes. The shape of the silicon dioxide is piecelike, and the size of the grains are about 2-6 times that of the graphite particles. By comparing the photos taken before and after calcination, it becomes harder to distinguish between silicon dioxide and graphite. This may be attributed to the partial sintering of silicon dioxide and carbon from calcination. It is also observed that after chlorination some pores are formed from part of the reacted silicon dioxide. The X-ray diffraction patterns of neither the calcined silicon dioxide/carbon nor the chlorinated silicon dioxide/carbon show any indication of the presence of crystalline silicon carbide. The diffractograms also show that, with both calcination and chlorination, the morphology of silicon dioxide may not be clearly changed. As listed in Table I, the porosity of silicon dioxide/ carbon pellets increases slightly with time during the chlorination reaction. Such a phenomenon may be due to
780 Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990
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i
A6mm
*'csr""
W8mm
0.06
0.04
0.02
0
40
120
80
160
200
Time ( m i d
Figure 4. Effect of the pellet size on the chlorination of the sample (chlorine fraction, 40 vol 7%; temperature, 1223 K; gas flow rate, 500 mL/min; C/Si02 molar ratio, 2.8; pellet-forming pressure, 4.43 X 105 kPa; initial grain sizes, silicon dioxide and graphite < 45 pm).
A 7.39~10~ kPa 0 4 . 4 3 ~ 1 kPa 0~
0.06
u)
x
0.04
0.02
0 0
40
80
120
160
200
Time (min)
Figure 3. Scanning electron micrographs of the specimen. (a) graphite (2750X), (b) silicon dioxide (%OX), (c) uncalcined silicon dioxide/graphite (275X),(d)calcined silicon dioxide/graphite a t 1223 K for 6 h (275X), (e) chlorinated silicon dioxide/graphite a t 1323 K for 4 h (275X).
the gradual reduction of the grain size of the silicon dioxide and graphite during reaction or due to the consumption of small grains a t the early stage without loss of the large grains. Effect of Gas Flow Rate. When the reaction temperature is 1223 K and the pellet diameter is 4 mm, the external mass-transfer resistance can be neglected if the gas flow rate exceeds 300 mL/min. Effect of Pellet Size. Figure 4 shows the relationship of reaction extent with different pellet diameters a t 1223 K. The reaction rate is affected by the pore diffusion when the pellet diameter is greater than 6 mm. Therefore, when
Figure 5. Effect of the pellet-forming pressure on the chlorination of the sample (chlorine fraction, 40 vol 7%; temperature, 1223 K; gas flow rate, 500 mL/min; C/Si02 molar ratio, 2.8, pellet diameter, 4 mm; initial grain sizes, silicon dioxide and graphite < 45 pm).
the reaction temperature is 1223 K and the pellet diameter is smaller than 6 mm, the overall rate is controlled by the chemical reaction. Effect of Pellet-FormingPressure. Because carbon powder is an effective reducing agent in the metal oxide chlorination system, the distance between carbon and metal oxide will directly affect the reaction rate. When the pellet-forming pressure increases, the distance between carbon and metal oxide will become shorter, and the contact area of these two materials will increase. Consequently, it will accelerate the reaction rate. In this study, the chlorination reaction was carried out a t 1223 K with varying pellet-forming pressures (7.39 X lo4-7.39 X lo5 kPa). From Figure 5, we know that the pellet-forming pressure exhibits a strong effect on the C/Si02 reactivity.
Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990 781 0 08
OC
S102 0 6
I C SI02 1 0
A C SiOp 2 o 0 0 0066
-
oc SI02
2 8
oc s102
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-4
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x"
004
0 02
0 02
0 0
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80
120
160
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60
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Time
Time(min1
Figure 6. Effect of the C / S i 0 2 molar ratio on the chlorination of the sample (chlorine fraction, 40 vol %; temperature, 1223 K; gas flow rate, 500 mL/min; pellet-forming pressure, 4.43 X lo5 kPa; pellet diameter, 4 mm; initial grain sizes, silicon dioxide and graphite < 45 Fm).
When the pellet-forming pressure exceeds 2.22 X lo5 kPa, further increases in pressure have less of an effect. It is thought that, when such a pressure is exceeded, the distance between carbon and silicon dioxide has already reached its minimum and cannot be reduced further. Barin and Schuler (1980) reported that, in the C / T i 0 2 chlorination reaction system, the greater the pellet-forming pressure is, the faster the reaction rate will be. Lin and Lee (1985) have also reported that, in the C/Ti02 system, the reactivity can be enhanced by the pellet-forming pressure, but when the pressure exceeds 5.47 X lo5 kPa, the rate of change will gradually diminish. Effect of C / S i 0 2 Molar Ratio. Owing to the high affinity of silicon to oxygen, it is necessary to add carbon as a reducing agent to accelerate the reaction rate. However, the amount of carbon will affect the contact surface area of silicon dioxide with carbon. Excess amounts of carbon will not favor the reaction and will increase the volume of reactant and waste carbon, while insufficient amounts will affect the reactivity. Therefore, in this study, different C/Si02 molar ratios (0.6-3.5) were chosen to perform the chlorination reaction a t 1223 K. It is observed from Figure 6 that, when the molar ratio is smaller than 2.0, increasing the percentage of carbon increases the rate of chlorination. When the molar ratio is larger than 2.0, there is no significant enhancement in the reaction with additional carbon. The surface of silicon dioxide has been completely occupied by the carbons when the molar ratio is greater than 2.0, and consequently, increasing the carbon amount does not favor the reaction. Denki Kagaku Kogyo K. K. (1983a) and Ube industries, Ltd. (1983~)have also pointed out that the molar ratio of C/Si02 in the range 2-10 is the best. Effect of Grain Size of Silicon Dioxide. Reducing the grain size of silicon dioxide will increase its surface area for the reaction per unit mass. Consequently, the reaction rate is accelerated. This is verified by the experimental results shown in Figure 7. However, when the silicon dioxide grain size is smaller than 53 pm, the effect of silicon dioxide grain size gradually diminishes.
120
160
200
6")
Figure 7. Effect of the grain size of silicon dioxide on the chlorination of the sample (chlorine fraction, 40 vol 70;temperature, 1223 K; gas flow rate, 500 mL/min; pellet-forming pressure, 4.43 X lo5 kPa; pellet diameter, 4 mm; C/Si02 molar ratio, 2.8; initial grain sizes, graphite < 45 I m ) .
0 06
2
0.04
0 02
0 0
40
80
120
160
200
Time ( m i d
Figure 8. Effect of the grain size of carbon on the chlorination of the sample (chlorine fraction, 40 vol 70;temperature, 1223 K; gas flow rate, 500 mL/min; pellet-forming pressure, 4.43 X lo5 kPa; pellet diameter, 4 mm; C/Si02 molar ratio, 2.8; initial grain sizes, silicon dioxide < 45 pm).
Effect of Grain Size of Carbon. The effect of grain size of carbon on the reaction rate is shown in Figure 8. It is observed that there is no significant effect on the reaction when the carbon grains size is small than 63 pm. This could be due to the lower reactivity of C/Si02. Rate Expression for Chemical Reaction Control. When the mass-transfer effect of the gas is absent, the chlorination rate of silicon dioxide can be written as follows: 1 dWs -rs = - _ _ -
wso
=
dt
f ( km, ycl,, P f , dso, dco)
(9)
782 Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990
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IdYel;) Figure 9. Effect of chlorine concentration on the initial chlorination rate of silicon dioxide (temperature, 1323 K; gas flow rate, 500 mL/min; pellet-forming pressure, 4.43 X lo5 kPa; pellet diameter, 4 mm; C/Si02 molar ratio, 2.8; initial grain sizes, silicon dioxide and carbon < 45 pm).
However, the effect is not appreciable when the C/Si02 molar ratio is higher than 2.0, the pellet-forming pressure is over 2.22 x lo5 kPa, and the grain sizes of carbon and silicon dioxide are less than 63 and 53 pm, respectively. Hence, the rate equation can be represented as -rs = f l k YClJ = kYC1t (10) Taking the logarithm of eq 10, we obtain In (-rs) = n In yCl2+ In k (11) The order of chlorine concentration determined from eq 11 and the plot of In (-rso) against in ycl was 0.62. The resulting regression curve is plotted in h g u r e 9. The Arrhenius plots for iZ is given in Figure 10, where the activation energy of the C/Si02 pellet chlorination reaction is 114.7 kJ/mol. The expression for the rate equation is found to be -114.7 X lo3 -rs = 0.9515 exp (12)
Kinetic Model The chlorination of the silicon dioxide/carbon reaction system, involving the reaction of two solid components and a gas, exhibits a higher degree of complexity than that of a single solid component reaction. The mechanism of the reaction is not very clear, and kinetic modeling of this reaction has not been attempted in the literature. A kinetic model which is similar to the grain model (Szekely et al., 1976) is developed here. In this model, the solid pellet may be considered as an agglomerate of grains reacting in the absence of mass transfer. The grains are ordinarily nonporous and assume that the reaction occurs at the surface of each grain according to the shrinking core model. When the structural changes on the reaction are neglected, the tot,al weight of silicon dioxide in the pellet may be formulated as shown below: (13)
-x T
103
(K
’>
Figure 10. Arrhenius plot showing the temperature dependence of the initial chlorination rate of silicon dioxide (chlorine fraction, 40 vol %; gas flow rate, 500 mL/min; pellet-forming pressure, 4.43 X lo5 kPa; pellet diameter, 4 mm; C/Si02 molar ratio, 2.8; initial grain sizes, silicon dioxide and carbon < 45 pm).
In the experiment, the chlorination reaction follows eq 1. Hence, the reaction rate of silicon dioxide on the surface of each grain can be represented by the equation below: 1 m s -rs1 = -4nr2 dt dNc12 = -- 1 8nr2 dt
Since
(16) From eq 14 and eq 16, we can find that the overall reaction rate of silicon dioxide on the grain surface is
From the differential treatment of eq 13 and combination with eq 17, the relationship can be written as (18)
Integrating eq 18 and from the definition of conversion (eq 7), the relationship between conversion and reaction time can be written as 1 - (1 - XS)1/3 = k,t (19) where (20) The experimental conversion-time data a t different tem-
Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990 783 -rsl = surface reaction rate of silicon dioxide on each grain, A
mol mm-' s-1 T = reaction temperature, K t = reaction time, s V,, = total volume of pellet, mm3 Vs = volume of silicon dioxide for each grain, mm3 W , = weight of solid component i in pellet, mg W , = total weight of pellet at time t , mg X s = conversion of silicon dioxide xc = weight fraction of carbon in pellet yCl2= mole fraction of chlorine in bulk gas
1123K 1!73K
A 3
0.04
1223K 1273K 1323K
Superscript
0 = initial condition Subscripts C = carbon
C1, = chlorine p = pellet S = silicon dioxide 0
40
80
120
160
200
Time (min)
Figure 11. Conversion-time data a t different temperatures (chlorine fraction, 40 vol %; gas flow rate, 500 mL/min; pellet-forming pressure, 4.43 X lo5 kPa; pellet diameter, 4 mm; C/Si02 molar ratio, 2.8; initial grain sizes, silicon dioxide and carbon < 45 pm).
perature levels are shown in Figure 11. It can be seen that the plots of 1 - (1 - X S ) l l 3versus time are linear as expected and indicate a kinetically controlled reaction. The foregoing finding is supported by the facts that the reactivity of silicon dioxide/carbon pellet is not high and the structural changes during the reaction are not severe.
Acknowledgment We express our sincere thanks to the National Science Council of the Republic of China for financial support, under Projects NSC75-0402-E008-09 and NSC76-0402E008-13.
Nomenclature dco, dso = initial grain sizes of carbon and silicon dioxide f = functional relationship for k , m, yClz, Pf, dc0, and dso f l = functional relationship for k and yell f2, f 3 = function of yCI2 and x c AGOT = standard Gibbs free energy at temperature T , kJ/mol M O T = heat of formation at temperature T , kJ/mol k = intrinsic rate constant, s-l k' = rate constant in eqs 14, 17, 18, and 20, mm-, s-' k , = rate constant in eqs 19 and 20, Mi = molecular weight of solid component i m = molar ratio of C/Si02 NCIP = moles of chlorine in pellet N s = moles of silicon dioxide in each grain n = reaction order Pf = pellet-forming pressure R = gas constant, 8.314 kJ kmol-' K-' r = grain radius of silicon dioxide, mm -rs = reaction rate of silicon dioxide, mg mg-' s-l
Greek Symbols t = total volume fraction of porosity and carbon in the pellet e,, p,
= porosity of the pellet = density of solid component i in the pellet, kg m-3
change in property Registry No. SiOz, 7631-86-9; C, 7440-44-0.
S =
Literature Cited Barin, I.; Schuler, W. On the Kinetics of the Chlorination of Titanium Dioxide in the Presence of Solid Carbon. Met. Trans. B 1980, l l B , 199. Basu, P. K.; King, C. J.; Lyun, S. Manufacture of Silicon Tetrachloride from Rice Hulls. AIChE J . 1973. 19 (3), 439. Denki Kagaku Kogyo K. K. Manufacture of Silicon Tetrachloride. Japan Kokai Tokkyo Koho J P 83,217,420, 1983a. Denki Kagaku Kogyo K. K. Manufacture of Silicon Tetrachloroide. Japan Kokai Tokkyo Koho JP 83,217,421, 1983b. Denki Kagaku Kogyo K. K. Manufacture of Silicon Tetrachloride. Japan Kokai Tokkyo Koho JP 84,500,17, 1984. Iwai, T.; Mizuno, H.; Miura, M. Silicon Tetrachloride. Eur. Pat. Appl. 77,138, 1983. Kubaschewski, 0.; Evans, E. L.; Alcock, C. B. Tables. In Metallurgical Thermochemistry;Raynor, G. V., Ed.; Pergamon; Oxford, 1967; Vol. 1. Lin, C. I.; Lee, T. J. On the Chlorination of Titanium Dioxide-Carbon Pellet I. Effect of Gas Flowrate, Reaction Temperature, Pellet Size and Pellet Forming- Pressure. J . Chin. Inst. Chem. Eng. 1985, 16 (l), 49. Szekely, J.; Evans, J. W.; Sohn, H. Y. Reactions of Porous Solids. In Gas-Solid Reaction: Academic: New York, 1976. Ube Industries, Ltd. Silicon Tetrachloride. Japan Kokai Tokkyo Koho JP 82 07,813, 1982a. Ube Industries, Ltd. Silicon Tetrachloride. Japan Kokai Tokkyo Koho J P 82 42,524, 1982b. Ube Industries, Ltd. Silicon Tetrachloride. Japan Kokai Tokkyo Koho JP 83 55,330, 1983a. Ube Industries, Ltd. Silicon Tetrachloride. Japan Kokai Tokkyo Koho JP 83 99,116, 1983b. Ube Industries, Ltd. Silicon Tetrachloride. Japan Kokai Tokkyo Koho JP 83 167,419, 1983c. Ube Industries, Ltd. Silicon Tetrachloride. Japan Kokai Tokkyo Koho JP 84 156,908, 1984.
Received for review April 4, 1989 Accepted December 26, 1989
