Ind. Eng. Chem. Res. 1991,30,2340-2344
2340
where cd is a desorption constant and E' the activation energy of the desorption step. The difference in the activation energies defines a positive heat of adsorption q. E'-E=q (4) Physical adsorption is in general a nonactivated process, i.e., E = 0. The adsorption equilibrium is defined by dn, dnd -=(5) dt dt and it follows at equilibrium p/o = p + c where
2
c = c, erp(
R LT )
(7)
The fractional coverage 8 can be expressed as the ratio of the adsorbed moles m a t pressure p and the maximum adsorption m, at saturation coverage 8 = m/m, (8) This yields the linear relationship -P = - +P m ma
ma
C
p / m = kg
+ ko
or (8b)
used in the evaluation of the adsorption data. Registry No. CHI, 74-82-8; CBHB,115-07-1; C3Hs, 74-98-6; CIHlo, 106-97-8; CBHI4,110-54-3; CHaOH, 67-56-1; C2H60H, 6417-5; H20, 7732-18-5; CO, 430-08-0; C02, 124-38-9; N2,7727-37-9; 02,7782-44-7; neopentane, 463-82-1.
Literature Cited Aharoni, C.; Tompkins, F. C. Kinetics of Adsorption and Desorption and the Elovich Equation. In Advances in Catalysis and Related
Subjects; Eley, D. D., Pines, H., Weisz, P., Eds.; Academic Press: New York, 1970; pp 1-49. Barrer, R. M. Zeolite Structures. In Zeolites: Science and Technology; Martinus Nijhoff: Dordrecht, 1984. Brunauer, S.; Emmett, P. H.; Teller, J. Adsorption of Gases in 1938, 60, 309-319. Multimolecular Layers. J. Am. Chem. SOC. Dessau, R. M. Selective Sorption Properties of Zeolites. In Adsorption and Ion Exchange with Synthetic Zeolites-Principles and Practice; Flank, W. H., Ed.; ACS Symposium Series 135; American Chemical Society: Washington, DC, 1980, pp 123-135. Flanigen, E. M.; Grose, R. W. Crystalline Silica Adsorbent. US Patent 4,061,724, 1977. Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Silicalite, a New Hydrophobic Crystalline Silica Molecular Sieve. Nature 1978,271, 512. Hayward, D. 0.;Trapnell, B. M. W. Chemisorption; Butterworths: London, 1964. Holborow, K. A.; Loughlin, K. F. Multicomponent Sorption Equilibria of Hydrocarbon Ga~esin 5A Zeolite, Paper 32. In Molecular Sieves-II; Katzer, J. R., Ed.; ACS Symposium Series 40; American Chemical Society: Washington, DC, 1977; pp 379-392. Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. J. Structure of Synthetic Zeolite ZSM-5. Nature 1978,272,437-438. Low, M. J. D. Kinetics of Chemisorption of Gases on Solids. Chem. Rev. 1960,60, 267-312. Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J. Crystal Structure and Structure-Related Properties of ZSM-5. J. Phys. Chem. 1981,85, 2238-2248. Otto, K. Adsorption of Methane on Active Carbons and Zeolites. In Hydrocarbon Technology Environment-Alternative Energy Sources IV; Veziroglu, T. N., Ed.; Ann Arbor Science Publishers, Butterworths: Stoneham, MA, 1982; Vol. 6, pp 241-260. Rabo, J. A. Zeolite Chemistry and Catalysis; American Chemical Society: Washington DC, 1976. Reich, R.; Ziegler, W. T.; Rogers, K. A. Adsorption of Methane, Ethane, and Ethylene Gases and Their Binary and Ternary Mixtures and Carbon Oxide on Activated Carbon at 212-301 K and Pressures to 35 Atmospheres. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 336-344. Szostak, R. Molecular Sieves, Principles of Synthesis and Identification; Van Nostrand Reinhold New York, 1989. Received for review August 14, 1990 Revised manuscript received May 29, 1991 Accepted June 10,1991
Adsorption Behavior of Chlorofluorocarbons in Zeolitic Pores. 1. Adsorption Isotherm Satoru Kobayashi,* Koichi Mizuno, Satoshi Kushiyama, Reiji Aizawa, Yutaka Koinuma, and Hideo Ohuchi National Research Institute for Pollution and Resources, 16-3 Onogawa, Tsukuba, Ibaraki, 305 J a p a n
The adsorption of CFC-12 on Nay, KY, and CsY zeolites was carried out by use of a conventional static adsorption apparatus, and the data were discussed in terms of the best fit adsorption isotherms. An inflection point was observed on each isotherm a t the adsorption amount of ca. 10 molecules per unit cell. The heat of adsorption calculated by Clausius-Clapeyron equation between 0 and 15 "C on NaY zeolite increased with increasing adsorption amount of CFC-12. These data were explained in terms of the nonlocalized equation proposed by Hill.
Introduction Chlorofluorocarbons (CFCs) are widely used in many fields as solvents, refrigerants, foam rubber blowing agents, and propellants. However, the emission of CFCs is considered to result in the destruction of the stratospheric ozone layer (Molina and Rowland, 1974). The removal from gas emission by adsorption is one of the desirable methods for resolving this problem, and a variety of adsorption plants for the recycling have been already commercialized. In contrast to the progress in the practical application, fundamental studies concerning the adsorption of CFCs
on microporous adsorbents have been very scarce. Urano et al. (Urano and Yamamoto,1985) reported the adsorption and desorption of CFC-11 in waste gas from a polyurethane form factory using activated carbons. However, the study placed an emphasis upon the engineering and economic aspect of the recovery rather than the kinetics and mechanism of the adsorption. Therefore, in this series of studies, we aimed at elucidating the adsorption behavior of CFCs in zeolitic pores through adsorption experiments with a variety of CFCs and zeolites. By the way, many investigators have studied adsorption isotherms for the gas phase, and various isotherm equa-
0888-5885/91/2630-2340$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2341 Table I. Properties of Zeolites zeolite type NaY KY CSY molecular formulan (Naaa)(*) (K48Nas)(*) (Cs,Na,,)(*) molecular weight, g/cell 12765 13508 15 681 surface area, cm2/g 768 660 446 water content, wt % 24.1 12.6 18.0 a
*, AlMSilMOBM.
tions have been proposed by Langmuir (Langmuir, 1918), Dubinin (Dubinin and Astakhov, 1971), and others. Recently, a simplified statistical model (Ruthven, 1982) and a vacancy solution model (Suwanayuen and Danner, 1980) have been widely considered (Takeuchi and Chihara, 1984) in connection with the adsorption of multicomponent mixtures. However, it seems that little investigation has been reported concerning isotherm equations applicable over a wide range of concentration. Especially, detailed investigation of adsorption isotherms in the low concentration region is very scarce. The activated carbons generally used have disadvantages in that they are flammable and are difficult to regenerate. On the other hand, zeolites do not have such disadvantages and we have found recently that zeolites exhibit adsorption capacity equal to or better than activated carbons in the low concentration region (Kobayashi, unpublished results). In this paper, we describe the result of adsorption of dichlorodifluoromethane (CFC-12)on various zeolites and consider the adsorption behavior of CFC-12 in zeolitic pores based on the applicability of various isotherm equations, mainly, in the low concentration region.
1.0
0.1
0.01
100
10
Equilibrium Pressure p [torr] Figure 1. Adsorption isotherm for the adsorption of CFC-12 on Nay, KY, and CsY zeolites at 0 O C . I
Experimental Section CFC-12 used was purchased from ASAHI FLON Company. The main properties of CFC-12 at 25 "C are as follows: vapor pressure 4800 Torr; heat of evaporation 4.83 kcal/ mol; molecular size (calculated by van der Waals radius) 6.2 A; liquid density 1.29 g/cm3; gas density 0.036 g/cm3. Adsorbents used were Nay, KY, and CsY zeolites. NaY was a faujasite-type zeolite provided by TOSOH Corp. (HSZ-320NAA). It was used as received. KY and CsY were prepared by ion exchange of NaY with KCl or CsCl aqueous solution at 70 "C overnight. The content of Na, K, and A1 were analyzed by inductively coupled plasma spectrophotometry after the samples were dissolved by acid. Cs was determined by ion chromatography. The compositions and properties are shown in Table I. The adsorption was carried out with a conventional static adsorption apparatus which is made up of a He and CFC-12 gas reservoir, a pressure gauge, vacuum joints and valves, and an adsorption tube in a water bath. All joints and valves were grease-free. Experimental procedures are as follows: (1)sampling about 0.2 g of the adsorbent in adsorption tube; (2) drying under vacuum at 250 "C for 2 h; (3) determining the dead volume of the adsorption tube by He; (4) introducing the appropriate amount of CFC-12 and measuring the pressure decrease in the adsorption tube caused by adsorption; (5) repeating procedure 4 until the final pressure of ca. 100 Torr was reached. Adsorption amounts were calculated by the ideal gas equation. They are hereafter represented by the number of CFC-12 molecules per unit structure of zeolite, that is, the unit cell whose molecular formula is given by (Na,) (A166Si1360384) for NaY (represented as molecules/ cell), except for the Dubinin-Astakhov plot. In the case of the Dubinin-Astakhov plot, adsorption amounts were represented by the liquid volume of adsorbed CFC-12 per
0
05
1/p
1.0
1.5
Wtorrl
Figure 2. Langmuir plot for the adsorption of CFC-12 on NaY zeolite.
unit weight of zeolite (mL/g).
Results and Discussion Figure 1shows semilogarithmic plots that represent the relationship between adsorption amount and equilibrium pressure for the adsorption of CFC-12 on Nay, KY, and CsY zeolites at 0 "C. On NaY zeolite, the adsorption amount is the smallest in the low-pressure region, i.e., in low gas concentration, and the saturated adsorption amount is the largest among the three zeolites. On the other hand, on CsY, the adsorption amount is the largest in the low-pressure region, but the saturated adsorption amount is the smallest. The adsorption amount for KY lies between that of NaY and CsY. First, the Langmuir equation and the Dubinin-Astakhov equation often used for adsorption on zeolite are examined. Figure 2 shows the relation between the reciprocal of the adsorption amount and the reciprocal of equilibrium pressure for the adsorption of CFC-12 on NaY zeolite based on the following Langmuir equation: -Q= KP (1) Qo 1 + K P This equation is based on the mechanism that molecules adsorb in a monolayer on the adsorbent surface and the interaction between molecules does not exist. The figure reveals that the data cannot be represented by one straight line, indicating that the behavior of CFC-12 in zeolitic pore is not completely explained in terms of this mechanism.
2342 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 -1 0
-2-
-
0
OT
0
15'C
0
30'C
-
30
-15-
e
L
2
0
-20-
-25
I
P,b,
I
0
Equilibrium Pressure P [torr] Figure 5. Adsorption isotherm of CFC-12 on NaY zeolite at 0 "C based on detailed data. Table 11. Values of Parameters for Hill Adsorption Isotherm ~ _ _ _ _ _ _ ~~~
o',
I
'C
zeolite type NaY
temp, OC 0
15 30 KY
/
0
30 CSY
0
30 /
0
VO,.
molecules/cell 51 (52) 51 (51) 51 50 (44) 47 (44) 36 (38) 35 (38)
K1,
1/Torr 0.217 0.101 0.0462 1.516 0.326 5.802 1.28
9,
kcal/mol 8.0 8.4 8.3
K2 3.78 3.38 2.95 3.72 2.76 3.84 2.9
Experimental values are in parentheses. 1
2
Equilibrium Pressure
4
3
5
P [torr]
Figure 4. Adsorption isotherm of CFC-12 on NaY zeolite at 0,15, and 30 "C in low coverage.
Figure 3 is the isotherm represented by the following Dubinin-Astakhov equation: W = Woexp[-(A/E)"] (2) where n is a positive integer less than 6 and Figure 3 is represented by taking n = 3. A is a decrease in Gibbs free energy and is given by RT In ( p s / p ) . As can be seen, the isotherm splits into two straight lines. Even when n is 6, a little deviation from a straight line was seen in the lowpressure region. This indicates that the adsorption is not based on a Dubinin-type mechanism. Figure 4 is the experimental data in the low-pressure region for the adsorption of CFC-12 on NaY at 0,15, and 30 O C . Careful inspection of the data leads us to find the fact that there exists an inflection point on each isotherm at about 10 molecules/cell. This can be clarified by Figure 5, which shows detailed data at 0 OC in a extended scale. Yoshida et al. reported (Yoshida et al., 1989)that a similar inflection point was observed on the adsorption isotherm of xenon in zeolitic pore, and that the isotherm was in good agreement with the equations proposed by Hill (Hill, 1946) and Kieselev (Kieselev, 1958). Thus we attempt first to apply the following Hill's nonlocalized equation to the data: (3)
where 0 is coverage and is equal to Q/Q@ Solid lines in Figure 4 and 5 are the fitting curves calculated from the (3) by using a nonlinear least-squares method (Marquardt method). It is recognized that a good fit is obtained; es-
pecially the inflection point can be successfully reproduced by the equation. In order to confirm more accurately that (3) is the best fit for the data, the correlation coefficients (r)between all the experimental data and the estimates obtained by fitting each equation were calculated by using a least-squares method for the adsorption on NaY at 0 "C. The results are follows: r(Langmuir) = 0.714 r(Dubinin-Astakhov) = 0.693 r(Hil1) = 0.959 Since the correlation coefficient for Hill's equation is the nearest to 1, it is certain that Hill's equation is suitable for adjusting adsorption isotherm of CFC-12 on zeolites. Solid lines already shown in Figure 1 are the fitting curves by (3). It can be seen that good agreement is also given between the observed and the calculated values for KY and CsY zeolites. Table I1 lists the parameters obtained by the above calculation for Nay, KY, and CsY zeolites. Qo is the saturated adsorption amount forming a monolayer. Almost the same values of Qo are obtained at different adsorption temperatures for each adsorbent. The data in parentheses for Qo were experimentally obtained by extrapolation to saturated vapor pressure, and these values are nearly equal to the calculated ones. K , is the equilibrium constant between adsorbent surface and adsorption molecules, and is equal to the Langmuir constant. These values are positive and decrease with increasing temperature. K 2 is the interaction coefficient between the adsorbed molecules and is represented by the following equation (Hill, 1946): K 2= 2a/bkT (4) Since a and b correspond to the van der Waals constant, K 2 must be positive and does not depend on the property
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2343 ?
Langmuir, Dubinin, or Kieselev equation, but Hill's equation could rationally explain the experimental data. Among others, the unusual phenomena of the increase of heat of adsorption with an increase in adsorption amount could be explained by the equation. It should be noticed, however, that Hill's equation is the one derived on the basis of a mobile first layer obeying a two-dimensional van der Waals equation. This means the three-dimensional interaction between adsorbates and between adsorbate and adsorbent is not considered. However, the interactions are likely to have influenced the data obtained here, because the pore of the zeolite is very small. Therefore, it is surprising that the data can be successfully described by the equation. The space in the micropore may exhibit the same function as the suPface of flat plate. This will be discussed in detail in a successive paper.
a.l
o e
$ 1 I :
,
,
,
5
10
15
,I
0 0
Adsorption Amount
20
Q [molec./celll
Figure 6. Heat of adsorption of CFC-18 on NaY zeolite calculated between 0 and 15 OC.
of adsorbents. Furthermore, according to (4), K 2 must decrease with raising the temperature. It can be seen from Table I1 that the experimental values that are obtained on different adsorbents at the same temperatures are positive and are equal and, moreover, these values decrease with raising the temperature. Figure 6 is the heat of adsorption calculated by the Clausius-Clapeyron equation between 0 and 15 "C on Nay zeolite. It is recognized that the heat of adsorption increases with increasing adsorption amount, Le., coverage. This phenomenon is rather unusual since the heat of adsorption is generally known to decrease with increasing surface coverage. However, it was found that the phenomenon can also be interpreted by applying the Hill's equation as the following. Heat of adsorption defined by Clausius-Clapeyron is represented as follows. q = RP(r3 In P/BT), (5) If this equation is rearranged by substituting (3), the following equation is obtained. q
%)e
= -RP(
- RP8(
$) e
(6)
The first term corresponds to the heat of adsorption derived from the Langmuir equation and is independent of 8. On the other hand, the second term is dependent on 8. Since aK2/aT is negative as has been mentioned, (6) shows that q increases with the increase in coverage 8. This explains well the rising tendency of the heat of adsorption in Figure 6. Furthermore, the values of q at 8 = 0 calculated from the first term of (6) are shown in Table 11. The value for NaY between 0 and 15 OC is 8.0 kcal/mol and is equal to the one estimated at 8 = 0 from Figure 6. The applicability of the localized adsorption equation of Kieselev was also examined (eq 7 ) .
However, the parameter (K;)for the adsorption equilibrium constant between adsorbates which was obtained by curve fitting was negative and the parameter for saturated adsorption amounts were largely deviated from the results. Moreover, the equation obtained by substituting this equation into (5) showed that q decreased with the increase in 8; namely, it was impossible to explain the rising tendency of the heat of adsorption. Conclusion The experimental data for the adsorption of CFC-12 on Nay, KY, and CsY zeolites could not be explained by the
Acknowledgment We gratefully acknowledge Mr. Susumu Takahashi, who made the adsorption apparatus. Data collected by Ms. E. Goto were also utilized in this study, and her contribution is gratefully acknowledged.
Nomenclature A = Gibbs free energy, cal/mol a = two-dimensional van der Waals constant, cm4 Torr/
(molecule)2 9
b = two-dimensionalvan der Waals constant, cm2/molecule E = characteristic adsorption energy, cal/mol K = Langmuir constant, Torr-' K1= Langmuir constant on Hill's equation, Torr-' K,' = Langmuir constant on Kieselev's equation, Torr-' K 2 = adsorbate-adsorbate interaction constant on Hill's
equation
K,' = adsorbate-adsorbate interaction constant on Kieselev's equation k = Boltzmann's constant, cal/deg n = constant in (2) p = adsorption equilibrium pressure, Torr pa = saturated pressure, Torr Q = adsorption amount, molecules/cell Qo = saturated adsorption amount, molecules/cell q = heat of adsorption, kcal/mol R = gas constant, cal/(deg mol) T = absolute temperature, K W = adsorption amount, mL/g W, = saturated adsorption amount, mL/g Greek Letter 0 = coverage Registry No. CC12F2,75-71-8.
Literature Cited Dubinin, M. M.; Astakhov, V. A. Description of Adsorption Equilibria of Vapors on Zeolites over Wide Ranges of Temperature and Pressure. Molecular Sieve Zeolites II; Academic Press: New York, 1971; p 69. Hill, L. T. Statistical Mechanics of Multimolecular Adsorption XI. Localized and Mobile Adsorption and Absorption. J.Chem. Phys. 1946, 14, 441.
Kieselev, A. V. Appearance of Adsorbate-Adsorbate Interaction in Gas Adsorption on Graphitize Carbon Black. 1. Gas Adsorption Isotherm Equation Considering Adsorbate-Adsorbate Interaction. Kolloidn. Zh. 1958, 20, 338. Langmuir, L. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. SOC.1918,40, 1361. Molina, M. J.; Rowland, F. S. Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom-Catalyzed Destruction of Ozone. Nature 1974, 249, 811. Ruthven, D.M.A Simple Theoretical Isotherm for Zeolites: Further Comments. Zeolites 1982, 2, 242.
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Znd. Eng. Chem. Res. 1991,30, 2344-2349
Suwanayuen, S.; Danner, R. P. A Gas Adsorption Isotherm Equation Based on Vacancy Solution Theory. AIChE J. 1980,26, 68. Takeuchi, Y.; Chihara, K. Kyuchaku (Adsorption). Kagaku Kogaku 1984, 48, 262.
Urano, K.; Yamamoto, E. Removal and Recovery of Freon 11 Vapor from a Polyurethane Form Factory by Activated Carbon. Int. Frog. Urethanes 1985, 4, 35.
Yoshida, T.; Koizumi, J.; Ashida, M.; Akai, Y. Adsorptive Behavior of Xenon in Zeolitic Pore of Faujasite. Nippon Kagaku Kaishi 1989,458.
Receioed for review November 1, 1990 Revised manuscript received May 6 , 1991 Accepted June 10, 1991
Characteristics of the Hydrogen-Chlorine Flame and the Effect of Different Parameters for the Synthesis of Tungsten Powders Vithal Revankar,* Guo Ying Zhao, and Vladimir Hlavacek Laboratory for Ceramic and Reaction Engineering, Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
As technology advances, needs for fine homogeneous powders have increased. Among many new powder synthesis technologies, a diffusion flame offers many advantages. T h e present paper deals with the reaction engineering strategy for the design and scale-up of a hydrogen-chlorine flame reactor. The details of the temperature distribution pattern within the flame and the reactor with total flow rate and hydrogen to chlorine ratio are presented. The different regions of the flow during the operation are described. The paper discusses the effect of residence time, temperature, and supersaturation on the particle size of tungsten powder produced by the reduction of tungsten hexachloride. The particle sizes of the generated tungsten powder, which range from 30 to 600 nm, can be controlled by manipulating the flame characteristics. The generated powders are very active and must be protected from an oxygen environment.
Introduction There is considerable evidence in the literature that metal or ceramic compacts made by sintering ultrafine metallic or ceramic powders exhibit improved mechanical physical and chemical properties. The pronounced effect of grain size on the mechanical properties of iron and steel (Petch, 1953; Cracknell and Petch, 1955; Smith et al., 1954; Edwards et al., 1939; Jaffe et al., 1953),copper (Carreker and Hibbard, 1953),zinc (Greenwood and Quarrel, 1954), and other metals (Hopkin, 1956) are well documented. In general, a reduction in grain size is accompanied by an increase in hardness, yield strength, and fracture strength and by a decrease in the ductile-brittle transition temperature. Ultrafine grain structure is also said to be the key to improving the properties of a dispersion-strengthened metal and alloy system (Grant and Preston, 1957; Lenel et al., 1957). Recently, it was found that the sintered parts of tungsten (or tungsten alloys) using submicron powders possess superplasticity. Submicron powders are also desirable for the fabrication of materials using a lower sintering temperature (hopefully without sintering aids) and resulting in a finer microstructure. The lower sintering temperature of the ultrafbe powders may be explained by high surface free energy. A special need in modem powder metallurgy is for composite powders of the metal-ceramics type. Flame synthesis has a great but largely unexplored potential. The composite powders that might result could have wide application in the formation of dispersionstrengthened metals containing several volume percent of ceramic dispersion. The direct synthesis of the composite has distinct advantages over the mixing of conventional powders. The ceramic and metallic particles can be prepared in a submicron size without extensive and contaminating milling. Also, because of the agglomerates typically present in ultrafine powders, flame processing from the
* To whom correspondence should be addressed. 0888-5885/91/2630-2344$02.50/0
gas phase can yield a more uniform mixture than those obtained by milling. The processing technique of using cocurrent diffusion flame offers many advantages. It is a clean process that permits cold nonreactive walls. The reaction volume is well-defined. The ability to maintain steep temperature gradients in the effective thermal environment, and thus well-defined reaction zone, appears to allow precise control of the nucleation rates, growth rates, and exposure times, thus permitting the nucleation and growth of the very fine particles. In the 1970s a group at the Oak Ridge Gaseous Diffusion Plant (ORGDP) of the Union Carbide Nuclear Division performed interesting and valuable research on the preparation of tungsten and tungsten alloy powders by reduction of the corresponding fluorides in a hydrogenfluorine flame. The powders prepared via the flame route were not pyrophoric and possessed high surface area (White and Duffy, 1959). Unfortunately the research results were never commercialized, apparently because of corrosive power of the H2/HF mixture and the unfavorable economics of the hydrogen-fluorine flame process. More information about this process can be found in papers published by the Oak Ridge Group (Smiley et al., 1965, 1966; Merriman et al., 1967; Smiley and Pashley, 1967). The synthesis of hydrogen chloride by burning hydrogen in chlorine produces a concentrated and highly pure product (Maude, 1941; Kirk and Othmer, 1982). Our laboratory used this exothermic flame process to generate different ultrafine microcrystalline and high surface area ceramic powders (Zhao et al., 1990a,b). The objective of this paper is to provide a reaction engineering strategy for the design and scale-up of a H2-ClZflame aerosol generator by using effectively the heat released from the combustion reactions. The tungsten powder produced in the WCg + Hzsystem is investigated in detail. Numerous experiments were performed on the effects of several process variables on 0 1991 American Chemical Society
