Ind. Eng. Chem. Res. 1996, 35, 3375-3383
3375
MATERIALS AND INTERFACES The Nitridation Kinetics of Pyrolyzed Rice Husk Tzong-Horng Liou and Feg-Wen Chang* Department of Chemical Engineering, National Central University, Chungli, Taiwan 32054, ROC
Silicon nitride was formed by nitriding acid-leached and pyrolyzed rice husk in a nitrogen atmosphere. The nitridation kinetics of the pyrolyzed husk pellet over a temperature range of 1473-1723 K were studied by a thermal gravimetric analysis (TGA) technique. The effects of gas flow rate, pellet size, pellet-forming pressure, initial grain sizes, and temperature on the extent of nitridation were examined extensively. Experimental results showed that the nitridation rate of the pyrolyzed husk pellet was distinctly faster than the conventional C/SiO2 mixture process. The rate expressions of the nitridation of the pyrolyzed husk pellet were determined. The activation energy was found to be 115 ( 1 kJ/mol. The reaction was mainly controlled by the diffusion of the gaseous reactant through the product layer. A kinetic model of the nitridation process was developed, and it gave good agreement with the experimental results. CO2(g) + C(s) f 2CO(g)
(4)
3SiO(g) + 3C(s) + 2N2(g) f Si3N4(s) + 3CO(g)
(5)
3SiO(g) + 3CO(g) + 2N2(g) f Si3N4(s) + 3CO2(g)
(6)
Introduction Silicon nitride (Si3N4) is an advanced ceramic material. It is suitable for structral applications at high temperature in chemical and material engineering, especially for gas turbines, turbocharger rotors, and diesel engine components. Two methods have been noted for the bulk production of Si3N4 powder. The first method is the direct nitridation of silicon with nitrogen. The second method is the simultaneous reduction and nitridation of silicon dioxide in the presence of carbon. Method 1 is the oldest established method. However, in order to avoid melting or agglomeration of the reactants, the reaction temperature must be kept below the melting point of silicon. This situation makes the reaction proceed slowly. Method 2 is a potentially attractive route because of the low cost of the raw materials, and it is the most economical method for the production of very pure, fine, and uniform particle size silicon nitride powder. Silicon nitride is manufactured by the carbothermal reduction and nitridation of silicon dioxide according to the overall reaction
3SiO2(s) + 6C(s) + 2N2(g) f Si3N4(s) + 6CO(g) (1) The generally accepted reaction mechanism is that an intermediate SiO is formed during the synthesis of silicon nitride and the carbon reduction of the SiO2 to SiO is the rate-determining step (Komeya and Inoue, 1975; Mori et al., 1983; Zhang and Cannon, 1984; Siddiqi and Hendry, 1985; Liou and Chang, 1995). Two elementary reaction steps can be conceived as follows:
SiO2(s) + C(s) f SiO(g) + CO(g)
(2)
SiO2(s) + CO(g) f SiO(g) + CO2(g)
(3)
or
* To whom correspondence should be addressed.
S0888-5885(95)00222-3 CCC: $12.00
and
or
Rice husk is an agroindustrial byproduct. Its main component is organic materials which contain a high ash content. Very high surface area and intimate contact available for the carbon and silicon dioxide mixture can be obtained by heating the rice husk in a non-oxygen atmosphere. Because two solid reactants are involved in reaction 1, pyrolyzed rice husk can provide the source of both carbon and silicon dioxide. Simultaneously, because of the uniform distribution between carbon and silicon dioxide in the rice husk, the reaction can occur more easily than the conventional mechanical mixing technique. This process has the benefit not only to produce valuable silicon nitride powder but also to solve the disposal and pollution problems created during the burning of the rice husk. The production of silicon nitride from rice husk was first reported by a U.S. patent (Cutler, 1974) where the reaction temperature is within the range of 1373-1623 K. There are several reports on the formation of silicon nitride from rice husk (Motoi, 1977; Kasugai, 1979; Hanna et al., 1985; Kaneko et al., 1988; Rahman and Riley, 1989; Patel and Prasanna, 1991). Some reaction parameters have been identified to have an important role in the reaction system. In general, the silicon nitride powder can be prepared from rice husk at temperatures between 1473 and 1723 K under a flow stream of nitrogen. The reaction temperature is relatively lower than that formed from the pure SiO2/C mixture reaction which has been reported by other workers (Hendry and Jack, 1975; Perera, 1987; Cho and Charles, 1991; Durham et al., 1991; Ekelund and Forslund, 1992). It is generally believed that when rice husk is treated with optimum acid solution, it will © 1996 American Chemical Society
3376 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 1. Schematic diagram of the experimental system.
effectively reduce the metallic impurities and obtain silicon nitride powder of high purity. Rahman and Riley (1989) found that the hydrogen addition was benefical in accelerating the rate of silicon nitride formation. However, when the reaction temperature reached beyond 1723 K, byproduct would apparently be formed. Hanna et al. (1985) also found that the amount of silicon nitride increased with iron content to reach an optimum value, and beyond this value, an increase in the amount of silicon carbide formation was noted. The previous studies had focused on the process of manufacturing the silicon nitride from rice husk. However, there is a wide variation in the results reported, and these discrepancies depend considerably on the various parameters selected. The kinetics of the nitridation reaction have not received much attention. In our previous report (Liou and Chang, 1995), we had studied the effect of various factors on the nitridation kinetics of a pure silicon dioxide/carbon mixture. The present work is a continuation of the previous study. 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 nitridation. A thermal gravimetric analysis (TGA) technique was employed in the study for synthesizing a monopellet, spherical precursor to silicon nitride. Experiments were carried out over a wide temperature range to determine the effect of operation variables on the nitridation kinetics. A reaction model was also developed to account for the experimental results. Experimental Section Materials Used and Sample Preparation. An intimately mixed SiO2/C precursor was prepared by pyrolyzing the acid-leached rice husk. Rice husk from a rice mill was chosen as the raw material. High-purity (99.999%) nitrogen gas and argon gas (San-Fu Chem. Co.) were used as the reaction gas and purge gas, respectively. The rice husk was washed well with distilled water to remove adhering soil and dried at 373 K in an air oven for 24 h. In order to leach out as much
metallic contaminations as possible, an acid-leaching technique was used; the method was in accordance with previous studies (Chen and Chang, 1991a). The dried rice husk was refluxed with 3 N hydrochloric acid (Merck & Co., laboratory grade) in a glass, roundbottomed flask at about 373 K for 1 h. It was then filtered and washed repeatedly with warm distilled water until the filtrate was free from acid and then was dried at 373 K for 24 h. The pyrolysis experiments were carried out by placing the treated rice husk in a tubular reactor made of quartz. The reactor was purged with nitrogen at a high flow rate to avoid oxidation of rice husk, and then the nitrogen flow was adjusted to the desired rate. The reactor was inserted into a furnace at the desired temperature for different intervals of time. The pyrolyzed rice husk which contained almost pure SiO2/C mixture was ground and screened through an ASTM standard sieve to obtain the desired grain sizes. Solid composite was made by compacting the powder in a stainless steel die, and the composite was cut into small spherical pellets to minimize the touching distance between sample and crucible. The pellets were then calcined at 973 K for 6 h in a nitrogen-purged tray oven to completely eliminate all of the moisture as well as strengthen the pellet structure. Experimental Apparatus and Procedure. In the present investigation, extensive experiments were carried out on the kinetics of the nitridation reaction using a thermogravimetric analysis technique. A schematic diagram of the experimental apparatus is illustrated in Figure 1. The main components of the system were an electronic microbalance (Cahn, TG 171) which provided a precise means of measuring weight changes in all runs, an alumina flow-through reactor with inlet and outlet, a movable furnace which was rather efficient in both heating and cooling, and a computer to continuously record the weight and temperature for the entire reaction period. A perforated platinum crucible was used as a sample container to avoid the sample crucible effect on the
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3377
reaction (as discussed by Durham et al. (1991)) and to maintain gas flowing around and through the crucible. The crucible was freely suspended by an extension wire from a balance into an alumina tube. A standard type B thermocouple was located near the crucible. All experiments were performed under isothermal conditions at temperatures between 1473 and 1723 K. The off-gas was continuously monitored by the conventional gas chromatography (GC) technique. Several tests were performed to ensure that the sampling technique employed was valid. Experimental accuracy was also confirmed by examining the mixing condition effect on the reduction reaction. Quantitative Method. A significant amount of literature has confirmed that the overall reaction which occurs during carbothermal reduction to silicon nitride is reaction 1. From the analysis of the outlet gas mixture, the CO2 was immeasurably lower than the CO and, consequently, was ignored in the following analysis and calculations. The silicon dioxide conversion in the reacted sample can be calculated as follows: The weight loss of pellet at time t is
∆Wp ) ∆Ws + ∆Wc - ∆Wn
(7)
From the stoichiometry of reaction 1, the amount of carbon reacted and silicon nitride formed can be written as
Mc ∆Wc ) 2 ∆Ws Ms
(8)
1 Mn ∆Ws 3 Ms
(9)
∆Wn )
On substituting eqs 8 and 9 in eq 7, we obtain
∆Ws 3Ms
∆Wp ) (3Ms + 6Mc - Mn)
(10)
The degree of conversion of silicon dioxide, Xs, is defined as
Xs ) ∆Ws/W0s
(11)
With a combination of eqs 10 and 11 and following rearrangement, the conversion of silicon dioxide can be formulated as
Xs )
3Ms∆Wp (3Ms + 6Mc - Mn)W0s
(12)
The conversion can be readily obtained from eq 12 for the known initial silicon dioxide weight (W0s ) and the total weight change read from TGA. The resulting samples were also analyzed chemically by the following procedure. The residual carbon in the nitrided samples was eliminated by burning at 973 K for 1 h. The unreacted silicon dioxide was removed by boiling the residue in sodium hydroxide solution. The crystalline silicon dioxide which might be produced at high temperature was removed by washing with diluted HF solution for a very short duration to avoid decomposition of the silicon nitride. Measurement of Chemical Properties. Chemical analysis of the treated and untreated rice husk was
Table 1. Elemental Composition of Rice Husk composition, wt % water-rinsed acid-leacheda pyrolyzedb
C
H
O
N
S
Cl
ash
42.82 40.90 40.08
5.74 5.23 0.86
40.39 36.81 3.66
0.39 0.35 0.37
ND ND ND
ND ND ND
10.66 16.71 55.03
a Leached by 3 N HCl for 1 h. b Leached by 3 N HCl for 1 h and pyrolyzed at 1173 K for 1 h ND means not detected.
Table 2. Metallic Ingredients in the Rice Husk metallic ingredients as oxides, wt % Mg
Ca
Fe
Mn
Na
K
P
Zn
total
water-rinsed 0.47 1.55 0.07 0.18 0.35 0.90 0.54 0.02 4.08 0.19 0.03 0.05 0.01 0.99 acid-leacheda 0.02 0.62 0.07 0 pyrolyzedb 0.02 0.54 0.03 0 0.14 0.02 0.03 0 0.78 a Leached by 3 N HCl for 1 h. b Leached by 3 N HCl for 1 h and pyrolyzed at 1173 K for 1 h.
performed to determine the amount of carbon, silicon dioxide, and other elements in the samples. The metallic impurities presented in the water-rinsed, acid-leached, and pyrolyzed husk samples were determined using an inductive coupled plasma mass spectrometer (Kontron Plasmakon, Model S-35). The results of elemental analysis were examined with an Heraeus elementary analyzer. Dried rice husk was powdered to 80-mesh size (ASTM), and this powder was employed in the analysis. Measurement of Physical Properties. The BET surface area, pore volume, and average pore diameter of the samples were determined by the N2 adsorption method using standard manufacturer procedures (Micrometric, ASAP 2000). Reactants and products morphology was identified by scanning electron microscope (Hitachi, S-520). The crystalline-phase compositions of the nitrided products were examined by X-ray with Cu KR radiation diffraction (Siemens, D-500). Results and Discussion Analysis of Chemical Properties. The major constituents of rice husk are cellulose, lignin, hemicellulose, and ash. However, the actual composition is variable and changes with variety, climate, and geographic location of growth. The results of the elemental analysis of rice husk treated with different processes are listed in Table 1. The acid-leached rice husk can be regarded as consisting of 16.71 wt % ash, 40.90 wt % carbon, 5.23 wt % hydrogen, 36.81 wt % oxygen, and negligible amounts of sulfur and chlorine. In the pyrolyzed rice husk, it is found that the major component, organic element oxygen, is considerably reduced. The weight percentage of the ash is also found to increase to 55.03 wt %. The amounts of metallic ingredients present in the water-rinsed, acid-leached, and pyrolyzed husk samples are given in Table 2. These results indicate that the impurity content is significantly reduced by the acidleaching process. Therefore, the pyrolyzed husk sample is an important renewable source of reactant for the nitridation reaction. After pyrolyzing the rice husk at 1173 K for 1 h, we found that an almost fixed proportion of C/SiO2 mixture could be obtained, whose molar ratio was around 4:1. The result is in accordance with the report of Chen and Chang (1991a). During the nitridation reaction, excess carbon is often required for increasing the reaction rate (the theoretical molar ratio C/SiO2 ) 2). Hence, the
3378 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 3. Surface Area and Pore Characteristics of the Ground and Unground Samples BET surface area, m2/g total pore vol, mL/g av pore diameter, 4V/A, Å
Aa
Bb
261 0.157 18.2
263 0.146 16.2
a Rice husk was pyrolyzed at 1173K and ground to 325-mesh size (ASTM). b Rice husk was pyrolyzed at 1173K and unground.
pyrolyzed rice husk is suitably used as the starting material in the production of silicon nitride for our current experiment. Analysis of Physical Properties. In order to quantify the effect of starting grain sizes on reaction rate, two batches of pyrolyzed rice husks (ground and unground) were used which had been pyrolyzed under identical conditions. The physical property data are listed in Table 3. The table indicates that the starting grain sizes of the rice husk have no obvious effect on the total specific surface area. In the ground sample, the total cumulative pore volume as well as the average pore diameter are slightly larger than those of the unground sample. It may be inferred from these data that only a small portion of pores have been destroyed when the rice husk is ground. We can conclude that whether the rice husk is ground or not, it will produce no significant change in the total surface area of the samples. The effect of the mechanical treatment is only restricted to the partial pore structure. The result also indicates that the pyrolyzed rice husk is a highly porous material which exhibits a large internal surface area. It is possible that the thermal decomposition of organic matter causes openings in the pore structure of rice husk, which increases the surface area. The morphological features of the pyrolyzed rice husk powder, unreacted pellet, nitrided pellet, and products are shown in Figure 2. Figure 2a shows that a porous surface is obtained when the rice husk is pyrolyzed at high temperature. The result is in accordance with the surface area measurement. From the morphology, we also find that it is difficult to distinguish between silicon dioxide and carbon. This may be due to the fact that silicon dioxide is in intimate contact and bonds to carbon and that these reactants are uniformly dispersed at the molecular level. Remarkable differences arise before and after nitridation. Figure 2b indicates that some small pores are distributed in the unreacted pellet. However, Figure 2c shows an obvious increase in the pore volume when the reaction is completed. By microscopic observation, within the nitrided pellet, Figure 2d shows that clusters of very fine silicon nitride powder have covered the surface of silicon dioxide. This kind of formation dominates most of the silicon nitride formation. Similar morphology was also observed by other workers (Rahman and Riley, 1989; Patel and Prasanna, 1991). The fine white and fibrous shape of silicon nitride whiskers is found to cover the outer surface layer of the pellet as shown in Figure 2e. In addition, a small amount and irregular shape of silicon dioxide remaining as contaminants distributed on the surface of the whiskers is also observed in the figure. Similar whitish deposition was also observed by Krishnarao et al. (1994) for the silicon carbide formation reaction. These phenomena suggest the presence of silicon monoxide gas (SiO) intermediate as a gas-phase reactant. Once silicon monoxide is formed in accordance with reaction 2 or 3, it will continue to react with N2 to form silicon nitride whisker in accordance with reaction
Figure 2. Scanning electron micrographs of the specimen. (a) Powder of pyrolyzed husk (1.2K×); (b) calcined pellet at 973 K for 6 h (500×); (c) nitrided pellet at 1623 K for 2 h (500×); (d) nitrided pellet at 1623 K for 2 h (20.0K×); (e) outer surface layer of the nitrided pellet at 1623 K for 2 h (2.0K×).
5 or 6. Also, it can react with trace oxygen to form silicon dioxide. X-ray diffraction patterns for the products indicate that the phase proportion of the silicon nitride powder produced during the nitridation reaction is predominantly the R form (∼99%) with a small amount of β form. No significant silicon carbide (SiC), silicon oxynitride (Si2N2O), or silicon (Si) peaks were detected in the X-ray observation. In some representative samples, a comparison between the percentage of silicon dioxide conversion calculated from chemical analysis and that from TGA is made. The relative error in total material balance is found to be 5% on average. The error possibly comes either from some volatilization of silicon dioxide as silicon monoxide or from the various analytical methods used. Since good agreement is observed, the
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3379
Figure 3. Effect of pyrolysis temperature on the weight loss of rice husk (leached by 3 N HCl for 1 h).
Figure 4. Effect of gas flow rate on the nitridation of the sample (temperature, 1623 K; pellet-forming pressure, 4.63 × 105 kPa; pellet diameter, 4 mm; initial grain sizes, 38-45 µm).
dominating nitridation reaction in rice husk is confirmed as reaction 1. Effect of Pyrolysis Temperature of Rice Husk. Pyrolysis of rice husk in a flowing nitrogen atmosphere is done to obtain an intimate mixture of carbon and silicon dioxide which is the raw material for the manufacture of silicon nitride. During the pyrolysis process, a great amount of gas volatiles was observed. This indicates that the thermal degradation of rice husk takes place through two main steps. The first step is the volatilization of volatile matter, and the second is the coking of organic matter into carbonaceous material in the residual rice husk char. Figure 3 shows the percentage of weight loss (∆m/ m0) for the acid-leached rice husk as a function of time of heating at different temperatures. The loss in weight (∆m) is mainly attributed to the evolution of volatile matter. The result indicates that the extent of rice husk pyrolysis increased with an increase in temperature. It can be seen that at 573 K, even after 1 h of pyrolysis, the weight loss does not reach a constant value. However, when the temperature is above 973 K, it is found that increasing the pyrolysis period for more than 10 min has no effect on the weight loss. It is presumed that pyrolysis is completed within 10 min at a temperature above 973 K and that more time may not be necessary for this pyrolysis procedure. The result also indicates that weight loss is changed with different pyrolysis temperatures for the same pyrolysis duration (1 h). In the temperature range of 573-773 K, there is a marked change of weight loss for the pyrolysis procedure. However, the effect is not appreciable when the pyrolysis temperature is higher than 973 K. This may be due to the fact that the organic materials in the rice husk are cracked into gaseous volatile matter. For higher pyrolysis temperatures, more gas evolutes are produced, and less tar and char remain during pyrolysis. When the temperature reaches beyond 973 K, the organic materials have been almost completely decomposed to carbon, and a higher pyrolysis temperature will not affect the weight loss. Effect of Gas Flow Rate. For a gas-solid reaction system, interactions between the gas and solid pellets must be taken into consideration. In general, increasing gas flow rates would favor increased nitridation rates.
The effect of gas flow rate on the nitridation of the pellet at 1623 K is shown in Figure 4. From the figure, it can be seen that the conversion of silicon dioxide increased with increasing gas flow rate up to 200 mL/ min. A further increase of gas flow rate has no effect on the conversion. The result indicates that the external mass transfer through gas film resistance of the pellet can be neglected when the gas flow rate exceeds 200 mL/min. The inhibiting effect of CO when even present in a small amount in the nitriding atmosphere had been reported by Cho and Charles (1991) and Ekelund and Forslund (1992). In order to increase the rate of reaction, the partial pressure of CO must be reduced by introducing a higher flow rate of nitrogen. Hence, a higher flow rate of nitrogen would be favorable in maintaining a low CO concentration and would also provide a more efficient reaction of silicon dioxide with nitrogen to silicon nitride. Effect of Pellet Size. When the reactant is a porous solid, diffusion through the pore space is necessary for the reactant gas to react on the solid grain surface. In general, reducing the pellet size would be favorable for the reaction to take place uniformly around the pellet. The experiments were carried out with different pellet diameters at 1623 K, as shown in Figure 5. When the pellet diameters are 7 mm and smaller, the conversion of silicon dioxide is independent of pellet size. This indicates that a homogeous type of reaction is reached, and nitrogen can be allowed to flow easily throughout the pellet. When larger diameters of pellet are used, nitrogen does not easily diffuse into the pellet, and reaction has only occurred on surface layers of the specimen. This situation results in reduction of conversion. These results are consistent with the observation of Ekelund and Forslund (1992). Effect of Pellet-Forming Pressure. Since carbon is an effective reducing agent in the silicon dioxide nitridation system, the physical contact between carbon and silicon dioxide is essential for SiO gas formation (reactions 2 and 3). When the pellet-forming pressure increases, the distance between carbon and silicon dioxide could become shorter, and the contact area of the two components could increase. Consequently, the reaction rate would be accelerated.
3380 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 5. Effect of pellet size on the nitridation of the sample (temperature, 1623 K; gas flow rate, 600 mL/min; pellet-forming pressure, 4.63 × 105 kPa; initial grain sizes, 38-45 µm).
Figure 7. Effect of grain sizes on the nitridation of sample (temperature, 1623 K; gas flow rate, 600 mL/min; pellet-forming pressure, 4.63 × 105 kPa; pellet diameter, 4 mm).
Figure 6. Effect of pellet-forming pressure on the nitridation of the sample (temperature, 1623 K; gas flow rate, 600 mL/min; pellet diameter, 4 mm; initial grain sizes, 38-45 µm).
Figure 8. Effect of reaction temperature on the conversion of silica (gas flow rate, 600 mL/min; pellet-forming pressure, 4.63 × 105 kPa; pellet diameter, 4 mm; initial grain sizes, 38-45 µm).
However, from the results shown in Figure 6, it is found that pellet-forming pressure does not affect the nitridation rate. A similar conclusion is also reported by Chen and Chang (1991b) for the pyrolyzed husk chlorination system. This result is not consistent with our previous report (Liou and Chang, 1995) for the SiO2/C nitridation system and that of Chen et al. (1990) for the SiO2/C chlorination system, where it was noted that the greater the pellet-forming pressure, the faster the reaction rate. The reason for this phenomenon is that the contact between silicon dioxide and carbon in the rice husk is greater than can be achieved by conventional mixing in a synthetically prepared sample. Since the distance between carbon and silicon dioxide has reached its minimal level, excess pellet-forming pressure does not accelerate the nitridation reaction. Effect of Grain Size. The effect of grain size of pyrolyzed rice husk on the nitridation is shown in Figure 7. It is observed that the starting grain sizes appear to have no significant effect on the reaction. The
phenomenon is due to the fact that the carbon and silicon dioxide in the rice husk have been mixed naturally and uniformly dispersed by molecular units. From Table 3, we know that the surface area values of both starting grain sizes of the rice husk are almost the same. Therefore, the pyrolyzed rice husk powders have a large internal surface area, and the mechanical treatment of the rice husk will not improve the reactivity of the samples. This result is consistent with the reported observation of Rahman and Riley (1989). Effect of Reaction Temperature. Nitridation rates varying with temperature between 1473 and 1723 K are shown in Figure 8. It shows that the reaction rates are markedly dependent on the temperature of nitridation, and the increase of temperature causes an increase in the conversion. A nonlinear relation is observed between the conversion and reaction time at each temperature level during carbothermal reduction. The reaction rate proceeds more rapidly during the initial reaction period. The observed gradual decay of the
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3381
reaction rate during the course of the reaction may relate to the formation of silicon nitride product layer. The reacted surface area is decreased as the reactants were gradually covered with the solid products (as shown from SEM observation), and this leads to a decrease in the reaction rate. It is also found that the nitridation rate of the pyrolyzed rice husk is distinctly faster than the rate of pure silicon dioxide/carbon mixture (Liou and Chang, 1995). We can infer from this comparison that the pyrolyzed rice husk has a larger surface area than the pure silicon dioxide/carbon mixture. Also, the degree of contact between silicon dioxide and carbon in the pyrolyzed husk is much more intimate than the pure silicon dioxide/carbon mixture, and the silicon dioxide derived from the original rice husk is dispersed homogeneously with carbonaceous material. Hence, the reaction rate is much faster than with the conventional mechanical mixing method. Rate Expression of the Nitrided Husk Pellet. To quantify the initial reaction rate, kinetic experiments were carried out over a wide temperature range. As mentioned earlier, under the optimum gaseous flow rate and pellet size, the nitridation reaction will occur uniformly throughout the pellet. The nitridation rate of silicon dioxide in the rice husk pellet can be written as follows:
-rs ) -
1 dWs W0s dt
) f(k,Pf,d0g)
(13)
The pellet-forming pressure and grain size of pyrolyzed rice husk do not affect the reaction rate as mentioned earlier. Hence, the rate equation can be expressed as
-rs ) f1(k)
(14)
where k may be a function of PSiO and PN2. A mathematical description of the initial rate equation can then be represented by using an Arrhenius-type equation. Taking the natural logarithm of the above equation yields
ln(-r0s ) ) ln k0 -
(RE)(T1)
(15)
The experiments to determine the activation energy are performed at several different temperatures. A plot of ln(-r0s ) vs 1/T is shown in Figure 9. The observed activation energy obtained by a least-squares fit is 115 ( 1 kJ/mol with a frequency factor of 1.01 s-1. According to the above calculation, the rate expression for the nitridation of rice husk is represented by the equation
(
-rs ) 1.01 exp
)
-115 × 103 RT
mg mg‚s
(16)
The activation energy reported in this work is far lower than the nitridation of pure silicon dioxide/carbon mixture which has an activation energy value of 448 kJ/mol (Liou and Chang, 1995). From the comparison of the activation energy values, it is observed that the nitridation reaction in the pyrolyzed rice husk is easier than in a pure silicon dioxide/carbon mixture. The result is in accordance with previous discussions. Kinetic Model The nitridation of the pyrolyzed rice husk, involving the reaction of two solid components and a gas, exhibits
Figure 9. Arrhenius plot showing temperature dependence of the initial nitridation rate of silica (gas flow rate, 600 mL/min; pelletforming pressure, 4.63 × 105 kPa; pellet diameter, 4 mm; initial grain sizes, 38-45 µm).
a higher degree of complexity than that of a single solid compact reaction. For the pure silicon dioxide/carbon nitridation, a grain model has been proposed in our previous study (Liou and Chang, 1995). The model assumed that the system was considered to be an agglormerate of a number of small grains, and the reaction occurred at the surface of the grain according to a shrinking core model. However, from the micrograph of the nitrided husk pellet (Figure 2d), it can be clearly noted that the reactant is covered intimately with the product layer. The conversion-time curve (as shown in all factors) also clearly indicates that the existence of a nonlinear relationship is due to the product layer gradually formed. In this case, the diffusion of gas reactant through product layer must be taken into consideration. Thus, the grain model may not be represented by the progress of the nitridation of pyrolyzed rice husk pellet. As mentioned above, a mathematical calculation which is similar to Jander’s equation (Szekely et al., 1976) is followed in the gas-solid reaction system. First, the parabolic law is defined here as
dL k1 ) dt L
(17)
L2 ) 2k1t
(18)
or
where L is the film thickness of product at time t and k1 is a proportional constant. The fundamental assumption of the law is that the product layer around the spherical particle is flat. The solid pellet assumed here is visualized to consist of a number of small grains. Within each grain, the reaction front retains its original spherical shape and the product film is uniform in thickness. The assumption is also made that the reaction has uniformly occurred around the pellet and external grain dimension remains constant throughout the reaction. Therefore, L can also be considered as the reacted thickness of the silicon dioxide grain.
3382 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
is considered to be an agglomerate of individual grains would cause the reaction to occur uniformly at the surface of the silicon dioxide grain, and the reaction may be controlled by the diffusion of the gaseous reactant through the product layer. Acknowledgment We express our thanks to the National Science Council of ROC on Taiwan for its financial support under Project NSC 83-0402-E008-001. Nomenclature
Figure 10. Experimental data plotted according to the kinetic model (gas flow rate, 600 mL/min; pellet-forming pressure, 4.63 × 105 kPa; pellet diameter, 4 mm; initial grain sizes, 38-45 µm).
According to the above assumptions, the radius of silicon dioxide grain at initial is
r0s )
(34πρ N)
-1/3
s
(W0s )1/3
(19)
where
4 W0s ) N π(r0s )3ρs 3
(20)
d0g ) initial grain size of pyrolyzed husk E ) activation energy, kJ mol-1 f ) function relationship for k, Pf, and d0g f1 ) function relationship for k k ) nitridation rate constant of pellet, s-1 k0 ) frequency factor of nitrided pellet k1 ) proportional constant, mm2 s-1 k2 ) rate constant in eqs 24 and 25, s-1 L ) film thickness of product or reacted thickness of silicon dioxide at time t, mm Mi ) molecular weight of solid component i m ) total weight of rice husk at time t, mg N ) number of silicon dioxide grain in the pellet Pf ) pellet-forming pressure, kPa R ) gas constant ) 8.314 kJ kmol-1 K-1 rs ) grain radius of silicon dioxide, mm -rs ) reaction rate of silicon dioxide, mg mg-1 s-1 T ) reaction temperature, K t ) reaction time, s Wi ) weight of solid component i in pellet, mg Wp ) total weight of pellet at time t, mg Xs ) conversion of silicon dioxide Superscript
and at time t is
0 ) initial condition
rs )
-1/3 4 πρsN Ws1/3 3
(
)
(21)
where
4 Ws ) N πrs3ρs 3
(22)
(34πρ N)
-1/3
s
[(W0s )1/3 - Ws1/3]
(23)
Combining eqs 18 and 23 and from the definition of conversion (eq 11), the relationship between conversion and reaction time can be formulated as
[1 - (1 - Xs)1/3]2 ) k2t
(24)
2/3 4 k2 ) 2k1 πρsN (W0s )-2/3 3
(25)
where
(
)
c ) carbon n ) silicon nitride p ) pellet s ) silicon dioxide Greek Symbols
By subtracting eq 21 from eq 19, the reacted thickness of the silicon dioxide, L, can be expressed as
L)
Subscripts
On the basis of the conversion of silicon dioxide with time, the data are represented by eq 24. The experimental conversion-time data at different temperature levels are shown in Figure 10. The presence of a straight-line relationship indicates that the nitridation system is followed by the model described above. A reasonable account for the model in which the system
ρs ) density of silicon dioxide in the pellet, kg m-3 ∆ ) change in property
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Received for review April 3, 1995 Revised manuscript received June 3, 1996X IE950222J
X Abstract published in Advance ACS Abstracts, August 15, 1996.
