Ind. Eng. Chem. Res. 1991,30, 2241-2247
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KINETICS AND CATALYSIS The Chlorination Kinetics of Rice Husk Jen-Min Chen and Feg-Wen Chang* Department of Chemical Engineering, National Central University, Chungli, Taiwan 32054, R.0.C
Rice husk was processed by acid-leaching and pyrolysis techniques to remove impurities other than silica and carbon and then further chlorinated to produce silicon tetrachloride. The chlorination of pyrolyzed-husk pellet was investigated by a thermal gravimetric analysis (TGA) reaction system over the temperature range of 973-1373 K. The effects of gas flow rate, pellet size, pellet-forming pressure, initial grain sizes, and temperature on the extent of chlorination were studied extensively. The rate expressions of the chlorination of pyrolyzed-husk pellet in the reaction-controlled region were presented. The reaction order with respect to chlorine concentration, 0.52, and the activation energy, 53.21 kJ/mol, were found. A kinetic model based on the degree of contact between silicon dioxide and carbon was developed, and it gave good agreement with the experimental results. Introduction Silicon tetrachloride is a prominent chemical material having a major impact in the semiconductor industry. It can be used as a silicon source material for the production of organosilicates, silicon esters, organosilicon halides, silicone polymer, etc. Recently, silicon tetrachloride is also used for manufacturing solar-grade Si, highly pure SiOz, Sic, and Si3N4. In the industrial process, silicon tetrachloride is produced as a by-product with other metal chlorides or by the chlorination of Sic, ferrosilicon, or SiOz/C mixture with Clz. Proper utilization of agroindustrial by-products is very important for the national economy; it would not only help in solving the disposal problem, but would also help in reducing the shortages with respect to several materials. A large quantity of husk, which is known to have a fibrous material with a high silica content, is available as waste from rice-milling industries. The major constituents of rice husk are cellulose, lignin, and ash varying with the variety, climate, and geographic location of growth. The ash contains 87-97 wt ?% silica in a hydrated amorphous form and other trace impurities. In previous literature, Basu et al. (1973), Okutani and Nakata (19871, and Ube Industries, Ltd. (1983a,b) have reported in a process for the conversion of ash, obtained by pyrolyzing rice husk, into silicon tetrachloride by chlorination of pyrolyzed rice husk. These reports used rice husk directly, and the impurities contained in the ash can be further chlorinated to inorganic chloride, resulting in only low-purity silicon tetrachloride. A temperature wave theory was also proposed by Basu et al. (1973) to explain the experimental observations for the packed bed reactor. Recently, we have shown that reasonably pure silica can be obtained from rice husk ash by a simple acid-leaching procedure (Chen and Chang, 1991). Considering that rice husk consists of mainly SiOp and an organic element, the rice husk in this paper was treated by an acid-leaching technique to remove the impurities in
* To whom correspondence should be addressed. 0888-5885191f 2630-2241$02.50/0
the ash. After pyrolysis at appropriate temperatures and atmospheres, the almost pure C/SiOz mixture can be obtained, which is much more intimately dispersed than could readily be accomplished by mechanical mixing. Hence, the chlorination of the SiOzin the pyrolyzed rice husk may well proceed more readily than that of a mixture of pure SiOzand C. These products were also chlorinated to produce high-purity SiC14,which can avoid the separation procedure. The previous studies have been focusing on the process of manufacturing the silicon tetrachloride from the rice husk. However, the kinetics of the chlorination reaction have not received much attention. In the present work, the rice husk was processed by an acid-leaching and pyrolysis treatment, and then the solid residue was ground and compacted to spherical pellets before chlorination. The reaction kinetics and effect of operating variables were extensively examined. A kinetic model was also developed to explain the experimental results. Experimental Section Materials Used and Sample Pretreatment. Husk, consisting of the outer shell covering the rice kernel, was obtained from a rice mill. It was washed well with distilled water to remove adhering soil and was dried at 383 K in an air oven. Chlorine and nitrogen was made by FongMing Co. with a purity of 99.9% and 99.99%,respectively. The techniques of acid leaching and pyrolysis reaction were in accordance with our previous studies (Chen and Chang, 1991). In the experiment, the rice husk was refluxed with 3 N HC1 in a glass round-bottomed flask, which was kept at about 373 K within a thermostat for 1 h. After leaching, the husk was thoroughly washed with distilled water and then dried. The pyrolysis reaction was conducted in a tubular reactor at nitrogen atmosphere, with heating at 1173 K for 1 h, and the final husk contained 51.18 wt ?% silicon dioxide. Following this, the husk was ground and screened through ASTM standard sieves to obtain the desired grain sizes. A die compaction method was used to prepare spherical pellets from the powders of pyrolyzed husk. 0 1991 American Chemical Society
2242 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 1
vent
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1. Cas04 2. Molecular siovo 3. Teflon on-off valvo 4. Condonsor 5. Gas p u r i f i o r I.Thormocouplo 7. Microbalanco 1. N O O ~ I O V~IVO 8. F l o w m o t o r 18. Furnaco I I . Air pump 12. Roactor t u b 0 15. NaOH s o l u t i o n 14. Q u a r t z d i s k
Figure 1. Schematic diagram of the apparatus for chlorination.
These powders were pressed into a disk by a hydraulic press, and then the disks were cut into spherical shapes with the desired diameters. Apparatus of Chlorination. In the chlorination experiments, the pellet with a known diameter was placed on a quartz disk, which was suspended on the arm of the microbalance. The weight change of the pellet during the course of an experiment was continuously monitored by a recorder. A schematic diagram of the experimental apparatus is shown in Figure 1. The main components of this system are an electronic microbalance (Cahn Instruments, Inc., Model lOOO), a quartz flow-through reactor with inlet and outlet, and a movable furnace with a PID controller. The furnace had a recommended operation up to the temperature of 1473 K. All experiments were performed under isothermal conditions at temperature between 973 and 1373 K. A series of CaSO, and molecular sieve columns were used to remove any traces of moisture in the feed gases to the reactor; the outlet gas was absorbed by NaOH solution to remove unreacted chlorine. Highpurity nitrogen was purged into the top of the balance to protect the balance and served as the chlorine diluent gas. Auxiliary Techniques. The Hewlett Packard Model 5890A gas chromatograph was used to determine the CO/C02 mole ratio from the outlet gas. Nitrogen, carbon monoxide, and carbon dioxide can be separated by a Carbosieve S-I1 (100/120, 10 f t X 1/8 in.) column. The analytical conditions are temperature 7 min at 308 K, then to 498 K at 305 K/min; flow rate 30 mL/min He; and detector thermal conductivity (10X/8X on range 1 VI. Electron micrographs were obtained by a Hitachi S-520 scanning electron microscope (SEMI. Amount of Reaction. The main reactions of the chlorination of silicon dioxide/carbon pellet can be shown as follow: Si02(11) + C(s)+ 2C12(,) = SiC14(,)+ COz(,) (1) Si02(s)+ 2C(,) + 2C12(,) = Sic&)
+ 2CO,,)
(2)
The thermodynamic consideration has been reported in our previous study (Chen et al., 1990). It was found that
reaction 2 prevails when the temperature is increased. The weight loss of pellet is represented as follows: AW, = AWs + AW, (3) The mole ratio of outlet gas CO/C02, m, is given by m = 2AWs2/AWs1 (4) From the reaction stoichiometry (reactions 1 and 2), and rearranging with eq 4, the amount of silicon dioxide and carbon reacted can be written as (5)
The degree of conversion of silicon dioxide, Xs,which is defined as Xs = AWs/WsO (7) On substituting eqs 5 and 6 in eq 3, and combining with eq 7, the degree of conversion can be formulated as AWP xs = -
1
-)
Mc l + m WSO 1 + 2-(M s 2 + m
(8)
Results and Discussion Gas Analysis. From the outlet gas analysis, the resulting regression curve of the mole ratio of CO/COB against temperature is plotted in Figure 2. It is found that when the temperature is low, reaction 1prevails, and when the temperature is increased the relative importance of reaction 2 is also increased. This is in agreement with thermodynamic analysis. The relationship of the mole ratio of CO/C02 with temperature can be represented by the following equation: m = 0.055T - 53.565 (9) Accordingly, from eq 8, eq 9, and the weight loss from the
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Figure 2. Effect of temperature on the CO/C02 molar ratio (chlorine fraction 40 vol %; gas flow rate 400 mL/min; pellet-forming pressure 4.43 X l@ kPa; pellet size 5 mm; initial grain sizes 38-45 rm).
progress of the reaction, the conversion of silicon dioxide can be calculated quantitatively. Effect of Gas Flow Rate. When the chlorination reaction is studied in the temperature range of 973-1373 K, the gas film resistance can be neglected if the gas flow rate exceeds 200 mL/min. Effect of Pellet Size. The experiments were carried out with different pellet diameters at 973 and 1373 K, as shown in Figures 3 and 4. As seen, when the chlorination was performed at 973 K, the pore diffusion effect did not contribute to the progress of the reaction when the pellet diameters were 6 mm and smaller. Hence, under such conditions, a homogeneous type of reaction is reached and the overall rate is under chemical reaction control. Furthermore, it is observed from Figure 4 that, at 1373 K, the effect of pore diffusion on the chlorination rate is stronger than that at 973 K. The reaction rate is increased by the decrease of pellet diameter, and the diffusion resistance cannot be ignored at higher temperature levels. Effect of Pellet-Forming Pressure. In this chlorination system, the pellet was composed of silicon dioxide and carbon since the degree of contact between the solid constituents may influence the progress of the reaction. When the pellet-forming pressure increases, the distance between silicon and carbon will become shorter and the contact area of these two components will increase. However, from the results as shown in Figure 5, it is found that the pellet-forming pressure does not affect the chlorination rate. This conclusion is not consistent with our previous report (Chen et al., 1990) for the pure Si02/C chlorination system. The reason for this phenomenon is because silicon dioxide and carbon in the husk are much more intimately dispersed at the molecular level than could readily be accomplished by mechanism treatment. Effect of Grain Size. The effect of grain size of pyrolyzed husk on the chlorination is shown in Figure 6. It is observed that there is no significant effect on the reaction. As mentioned above, mechanical treatment will not improve the reactivity of husk.
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Time (min) Figure 3. Effect of pellet sizes on the chlorination of sample (chlorine fraction 40 ~ 0 1 %gas ; flow rate 400 mL/min; pellet-forming pressure 4.43 X 1@ kPa; initial grain sizes 38-45 pm; chlorination temperature 973 K).
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