Ind. Eng. Chem. Res. 1987,26, 2007-2011 Re = reactor Reynolds number S = saturation ratio s1 = monomer surface area, cm2 t = residence time, s T = temperature, K u = fluid velocity, cm/s u1 = monomer volume, cm3 V‘ = aerosol volume density, cm3/cm3 x ’ = axial coordinate, cm W = polydispersity index Y = aerosol yield y’ = Cartesian coordinate perpendicular to x axis, cm Greek Symbols
0 = dimensionless average residence time = aerosol surface tension, dyn/cm Z = dimensionless surface tension group T
= time constant for infinitely fast and lamellar mixing
theories, s Superscripts -’ = dimensional = average
Subscripts s = seed particle A = ammonia
N = nitric acid AN = ammonium nitrate 0 = inlet I = infinitely fast mixing theory L = lamellar mixing theory 1 = limiting reactant Registry No. NH3, 7664-41-7; HN03, 7697-37-2; NH4NO3, 6484-52-2.
Literature Cited Angst, W.; Bourne, J. R.; Sharma, R. N. Chem. Eng. Sci. 1982,37, 585. Belevi, H.; Bourne, J. R.; Rys, P. Chem. Eng. Sci. 1981, 36, 1649. Bowen, H. K. Mater. Sci. Eng. 1980, 44, 1. Brandner, J. D.; Junk, N. M.; Lawrence, J. W.; Robins, J. J. Chem. Eng. Data 1962, 7, 227. Canteloup, J.; Tueta, R.; Braguier, M. Conf. Proc. Int. Symp. Plasma Chem., 4th 1979,2, 438. Countess, R. J.; Heicklen, J. J. Phys. Chem. 1973, 77, 444. Dannenberg, E. M. J . Inst. Rubber Znd. 1971,5, 190. Friedlander, S. K. Smoke, Dust and Haze; Wiley Interscience: New York, 1977. Friedlander, S. K. Ann. N . Y. Acad. Sci. 1983, 404, 354. Heberlein, J. V. R. Mater. Res. SOC.Symp. Proc. 1984, 30, 101.
2007
Henry, J. F.; Gonzales, A.; Peters, L. K. Aerosol Sci. Technol. 1983, 2, 321. Herrmann, E. Characterizationof Powder Surfaces; Parfitt, G . D., Sing, K. S. W., Eds.; Academic: New York, 1976. IMSL IMSL Contents Document, 8th ed.; International Mathematical and Statistical Libraries: Houston, 1980. Iribarne, A.; Frantisak, F.; Hummell, R. L.; Smith, J. W. AZChE J. 1972, 18, 689. Jenson, V. G. Chem. Eng. Sci. 1983, 38, 1151. Kato, A.; Hojo, J.; Okabe, Y. Mem. Fac. Eng. Kyushu Uniu. 1981, 41, 319. Katz, J. L.; Donohue, M. D. J. Colloid Interface Sci. 1982,85, 267. Kingery, W. D. Introduction to Ceramics;Wiley: New York, 1960. Kodas, T. T.; Pratsinis, S. E.; Friedlander, S. K. J . Colloid Interface Sci. 1986, 111 , 102. Kodas, T. T. “Aerosol Dynamics In Tubular Flow Reactors”, Ph.D. Dissertation, University of California, Los Angeles, 1986. Lay, J. R.; Iya, S. K. ZEEE Photovoltaic Spec. Conf., 15th 1981,565. Liu, B. Y. H.; Levi, J. In Generation of Aerosols and Facilities for Exposure Experiments; Willeke, K., Ed.; Ann Arbor Science: Ann Arbor, MI, 1980. Mao, K. W.; Toor, H. L. AIChE J . 1970, 16,49. McKelvey, K. N.; Yieh, H.; Zakanycz, S.; Brodkey, R. S. AIChE J. 1975, 21, 1165. Medallia, A. I.; Rivin, D. In Characterization of Powder Surfaces; Parfitt, G. D., Sing, K. S. W., Eds.; Academic: New York, 1976. Miller, T. J.; Potkay, E.; Yuen, M. J. “Review of Chemistry and Mechanism of Deposition for Optical Waveguide Fabrication by Vapor Deposition from a Combustion Flame”, AIChE Meeting, Anaheim, CA, 1984. Naumann, E. B.; Buffham, B. A. Mixing in Continuous Flow Systems; Wiley Interscience: New York, 1983. Okuyama, K.; Kousaka, Y.; Motouchi, T. Aerosol Sci. Technol. 1984, 3, 353. Olszyna, K. J.; DePena, R. G.; Heicklen, J. J . Aerosol Sci. 1974,5, 421. Ottino, J. M. AIChE J . 1981, 27, 184. Pratsinis, S. E.; Kodas, T. T.; Dudukovic, M. P.; Friedlander, S. K. Ind. Eng. Chem. Process Des. Deu. 1986a, 25, 634. Pratsinis, S. E.; Kodas, T. T.; Dudukovic, M. P.; Friedlander, S. K. Chem. Eng. Sci. 1986b,41, 693. Prochazka, S.; Greskovich, C. Cer. Bull. 1978, 57, 579. Rutner, E.; Goldfinger, P.; Hirth, J. P. Condensation and Euaporation of Solids; Gordon and Breach: New Yorli, 1964. Stelson, A. W.; Friedlander, S. K.; Seinfeld, J. H. Atmos. Enuiron. 1979, 13, 369. Ulrich, G. D.; Subramanian, N. S. Combust. Sci. Technol. 1977, 17, 119. Vance, J. L.; Peters, L. K. Ind. Eng. Chem. Fundam. 1976,15,202. Warren, D.; Seinfeld, J. H. J. Colloid Interface Sci. 1984, 3, 135.
Received for review May 6, 1986 Revised manuscript received April 9, 1987 Accepted June 10, 1987
An Internally Heated Weighed Reactor Thermobalance for Gas-Solid Reaction Studies Michael H. Treptau and Dennis J. Miller* Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824-1226
A novel thennobalance apparatus is described which alleviates some problems traditionally associated with thermogravimetric reaction studies. The new design provides direct measurement of sample temperature, elimination of external mass-transfer resistances via gas flow directly through the sample bed, and added safety using a cold-wall vessel design. The sensitivity of the balance is approximately f10 mg for a 2-g carbon sample. The apparatus consists of a pressure vessel, a counterweighted balance arm, and a top-loading electronic balance. The pressure vessel is internally heated and insulated; the solid sample is a fixed or fluidized bed through which reactant gas flows directly. Results from gasification with C02and O2 show that the balance adequately measures sample weight during reaction. Thermogravimetric analysis has long been recognized as a useful and straightforward method for ‘measuring 0888-5885/87/2626-2007$01.50/0
gas-solid reaction rates and adsorption phenomena. Thermobalance apparatus of many different configurations 0 1987 American Chemical Society
2008 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
have been developed; Dobner et al. (1976) and Gardner et al. (1980) review thermobalance technology up to around 1979. Thermogravimetric analysis at atmospheric and subatmospheric pressures can be carried out in several commercially available instruments. For studies at elevated pressures, apparatus have been constructed as modifications of the commercial apparatus or based on concepts different from commercial instruments. The earliest and most commonly used of these is the hanging basket thermobalance, first developed by Feldkirchner and Johnson (1968) and modified by Gardner et al. (1974). In this balance, the sample is held in a small basket located in a thick-wall tube surrounded by a furnace. The basket is suspended by a wire from the balance assembly; the balance assembly is shielded from the heated zone by a combination of distance and a cooled purge gas stream. This balance design, although workable, has the combined problem of cumbersome size and high temperature of the pressure vessel walls. Other hanging basket balances have been developed to resolve these problems. Li and Rogan (1978) describe a modification of a Du Pont thermogravimetric analyzer that eliminates the size problem by using cooling coils and a cooled purge stream. Forgac and Angus (1979) constructed a cold-wallthermobalance in which a heating coil is placed inside the pressure vessel around the sample basket. The pressure vessel is cooled by water flowing through grooves in the vessel wall. Sears et al. (1982) improved on this design by placing insulation between the heating element and vessel wall. Overall, the cold-wall design affords greater safety and a thinner vessel wall, reducing cost and weight. Hanging basket balances are capable of very accurate solid weight measurements. However, difficulties inherent to the design make measurement of intrinsic reaction kinetics unreliable under many conditions. First, it is difficult to measure the sample temperature during reaction, as placing a thermocouple directly in the solid disrupts weight measurement. Second, reactant gas flows around the sample basket and thus does not directly contact the sample. External mass transfer from gas to sample and mass transfer within the solid bed are potentially major transport resistances that disguise reaction kinetics (Wigmans et al., 1983). Also, the thermobalance cannot be run in either a differential or integral mode with respect to the gas phase, as most of the gas passes through the reactor without contacting the sample. The composition of the gas actually in contact with the sample is also unknown if mass-transfer resistances are present (Holstein, 1983). Third, basket materials can influence the reaction. Catalytically active stainless steel and platinum, or quartz, which is strongly attacked by alkali metals, have been used as basket materials. To avoid these difficulties, a new concept in thermobalance design, in which the entire reactor and its contents are weighed during reaction, has been developed by Gardner et al. (1980). Their apparatus consists of a massive (136 kg), externally heated pressure vessel suspended from a counterweighted balance arm. The reacting solid is in the form of a fixed bed through which reactant gas flows and in which thermocouples are located, thus alleviating interparticle mass transfer and providing direct temperature measurement. Kapteijn et al., (1982) constructed a balance of similar concept in which the reaction vessel was placed directly on a balance platform but gave few details of its operation.
New problems are generated as a result of the entire reactor and contents being weighed: gas lines and thermocouples exert forces which affect reactor weight, temperature and gas pressure fluctuations change reactor weight, and natural convection between furnace and vessel causes noise in the weight output. All of these factors lower the sensitivity of the measurements.
Description of Thermobalance The thermogravimetric instruments described above for elevated pressure studies each have certain desirable features which improve performance. The apparatus developed in our laboratory and described here combines the desirable features of several existing instruments; the resulting design represents a significant improvement in thermobalance technology. Specifically, the balance incorporates the following concepts of other apparatus: (1) The entire reactor and contents are weighed during reaction. The solid sample is therefore in the form of a bed within the pressure vessel through which gas flows and into which thermocouples are inserted. (2) The sample heater is located inside the pressure vessel; the vessel wall is maintained at low temperature by using insulation between the wall and heater. (3) Small sample size is employed to minimize gradients within the solid reactant. (4)The sample is held in a ceramic tube to eliminate extraneous catalytic effects. The pressure vessel is designed for conducting carbon gasification experiments a t conditions up to 970 OC and 600 psi simultaneously. Because the vessel weight (10 kg) exceeds the capacity of any balance with sufficient sensitivity for kinetic measurements, the vessel rests on one end of a counterweighted balance arm. The counterweight, which is suspended from the other end of the arm, rests on the pan of a Mettler PE 360 top-loading electronic balance with a capacity of 60 f 0.001 g. In this configuration, loss of sample weight during reaction is recorded as a weight gain on the balance. Detailed descriptions of the pressure vessel and overall apparatus are given in the following sections. Pressure Vessel. A sketch of the pressure vessel and fittings is shown in Figure 1. The shell is constructed of 316 stainless steel and consists of a 0.6-cm-thick by 10cm-0.d. by 16.5-cm-tall cylindrical tube t o which a 1.27cm-thick bottom plate and flange ring are welded. The flange cover is 2.5-cm thick and is sealed to the flange ring by eight 5/,:in. bolts and a Viton O-ring. The flange cover includes fittings for heater and thermocouple connections, Swagelok fittings for gas lines, and the internal nut and O-ring assembly to hold the alumina sample tube in place. Inside the pressure vessel, a 2.5-cm-thick by 15-cm-tall annular ring of K-20 firebrick is fit snugly to the stainless steel shell and sealed with a high-temperature, nonporous ceramic impregnant. The alumina sample tube (1.25-cm id.) is fixed to the cover plate by a cemented metal ring, nut, and Kalrez O-ring assembly to prevent short-circuiting of the reaction gas. Inside the sample tube, the reacting solid (typically 2 g) is placed between thin layers (3-7 mm) of 28-40-mesh alumina powder supported on a quartz wool plug. Two chromel-alumel thermocouples (1-mm 0.d.) extend into the sample bed to different depths to measure sample temperature. The sample tube is surrounded by a 750-W band heater; the lower bed thermocouple is used as the control thermocouple because it remains in contact with the shrinking reactant bed throughout reaction. The heater is centered on the ceramic tube about the sample. Reactant gas flows downward through the annulus between the firebrick and sample tube, where it is preheated by the heater, and then flows upward through the sample
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2009
c
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Figure 1. Internally heated pressure vessel.
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Figure 2. Schematic of apparatus.
tube and out of the pressure vessel. The internal volume of the pressure vessel is about 100 cm3. Balance Assembly. A schematic of the balance assembly is given in Figure 2. The balance arm and support fulcrums are from a 30-lb-capacity double-pan balance. The pressure vessel rests on a metal plate affixed to one end of the balance arm; the counterweights are suspended on 3-mm cables from the other end of the arm. The balance arm assembly is located on a platform of 1.25-cm plywood and 0.6-cm steel plate bolted by angle iron to the wall of the room. The top-loading balance, upon which the counterweights are placed, rests on a similar platform located 45 cm below the balance arm platform. To provide projectile shielding and minimize air currents, the apparatus is located in the corner of the room and surrounded by 0.6-cm-thick Lexan shields. Gas flow lines from the pressure vessel consist of 1.5mm-0.d. annealed copper tubing wound in a loose spiral
and directed from the balance assembly along the axis of the balance fulcrum. The gas lines are mounted on brackets connected to the room wall. Gas flow control is provided by a mass flow controller or a metering valve; a three-way ball valve directs either purge or reactant gas to the pressure vessel. Power leads for the heater and thermocouple wires are routed directly from the pressure vessel to a bracket 2 f t from the balance assembly. Gas lines and electrical connections in this configuration have a minimum effect on vessel weight as recorded by the balance. Operating Characteristics. The operating procedure is designed specificallyto minimize extraneous changes in pressure vessel weight during reaction. Two types of extraneous weight change are of concern: relatively largescale drift in sample weight as a result of perturbing the apparatus (Le., changing gas flow rate and pressure or heating the sample), and small fluctuations in weight from building vibrations and air currents. Large-scale drift is generally on the order of 0.1-0.5 g/h, while small fluctuations are usually on the order of *0.005-0.05 g. Overall sensitivity of the apparatus, based on measurements with calibration weights, is about zkO.01 g. The apparatus is prepared for an experiment by loading the sample into the tube, attaching the tube to the vessel cover (which also imbeds the thermocouples into the sample), and lowering the cover onto the pressure vessel. The vessel is sealed and placed on the balance arm, and gas and electrical connections are secured. The balance assembly is then allowed to stabilize for 1-2 h until the balance readout fluctuates less than k0.005 g. Because the moment arms of the balance are of different lengths, the response of the Mettler balance to change in weight of the pressure vessel is determined by using calibration weights. The experiment is begun by applying power to the heater. The desired sample temperature is reached within 5 min, but the pressure vessel takes 2-21/2 h to reach steady-state temperature. The vessel walls reach temperatures of 150-190 "C at steady state for a reactant bed temperature of 800 "C; expansion of the vessel walls and fittings exerts a force on the balance resulting in an apparent weight change of approximately 0.5 g during heatup. Once the weight is stabilized, purge and reactant gas pressures are matched and flow is switched from purge to reactant gas. This switch is accompanied by a rapid change in reactor weight caused by differences in molecular weight of the purge and reactant gases. This change is complete within 5 min; weight change recorded subsequently represents solid reaction rate. Weight change during reaction is generally free of noise; however, the balance is sometimes prone to sudden weight shifts on the order of 0.05 g and to slow drift of weight on the order of 0.1 g/h.
Gasification Studies To characterize the operating features of the balance, uncatalyzed and K2C03-catalyzed gasifications of 50200-mesh activated coconut charcoal (S = 750 m2/g, 3% ash) have been conducted in the presence of O2 and C02. Experiments were carried out in 15 psig reactant gas at 650-800 "C for C02and 350-500 "C for 02.The heating period was approximately 2 h; reaction times ranged from 90 to 120 min. Nitrogen was used as the purge gas in all experiments; the shift in weight when changing from nitrogen to reactant gas was on the order of 0.1 g for O2 and 0.4 g for C 0 2 . The carbon sample size in all experiments was 2 g. Results of the experiments are intended to il-
2010 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
-
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Figure 3. Raw data for COPgasification a t 700 "C. 231
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lustrate the feasibility and reproducibility of rate measurements in the balance. The raw data obtained from experiments are given as the balance output; the actual carbon weight vs. time is determined by dividing the balance output by a conversion factor (approximately0.7) equal to the ratio of the moment arms of the balance and subtracting this curve from the initial carbon weight. A sample of the raw data, shown as individual data points recorded in 15-s intervals, is given in Figure 3 for C02 gasification at 700 "C. The absolute value of the ordinate is arbitrary and is different for each experiment. The data for carbon weight vs. time for C02gasification at several temperatures are given in Figure 4. The points represent sample weight for 16.6% K2C03-loadedcarbons at temperatures between 650 and 800 "C and for uncatalyzed carbon at 800 "C. In separate experiments, the weight loss during heatup before gasification was found to be 0.35 for catalyzed samples and 0.09 g for the uncatalyzed sample. This weight, which is regained upon exposure to air, is attributed to water in both carbon and catalyst and is thus substract4 from the initial 2-g sample weight in Figure 4. Steady-state temperature as measured by the two thermocouples located at opposite ends of the sample bed differed at most by 15 "C during reaction. Gas flow rate was set at 300 cm3 (STP)/min, below the fluidization velocity of the particles in the bed. In all experiments the rate of gasification was nearly constant at low conversion and decreased at higher conversion. The weight loss curves are smoothest at the beginning of gasification and become less smooth as solid is consumed and bed configuration changes. Because the bed is an integral reactor under these conditions, the rate expression reported by Kapteijn and Moulijn (1983) is used to calculate activation energy:
where k = rate constant [mol/(g of initial C-s)], Ke = equilibrium constant for C02 + C = 2C0, and K3 = inhibition constant. The rate expression was integrated over CO, conversion in the bed to yield a value of k for each run. The Arrhenius
0
20
40
60
80
100
TIME (min)
Figure 6. Carbon weight and temperature vs. time for uncatalyzed O2 gasification; (--) 425 "C; (-) 375 O C ; ( - - - ) T I ;(- -) T,.
plot is shown in Figure 5; the apparent activation energy using average bed temperature in each run is 36.3 kcal/ gmol. The calculated value of k at 1050 K is 4.4 X mol/ (g of initial Cas). In comparison, Kapteijn and Moulijn (1983) report a value of k = 3 X mol/(g of initial C-s) for a similar carbon containing 10 wt % K2C03. If their rate is normalized linearly with catalyst loading, the corresponding value for 16.6 wt % K2C03is 5 X mol/(g of initial Cas), in excellent agreement with the value obtained in our experiments. Carbon weight vs. time curves for uncatalyzed 0, gasification at 375 and 425 "C are given in Figure 6. The sample ignited and burned at all temperatures in the range 350-500 "C; the oxygen flow rate (180-200 cm3/min (STP)) limited the gasification rate in all reactions. The temperature in the bed varies widely during reaction, ranging to 300 "C above and 100 "C below the set point temperature. The dotted lines in Figure 6 are the temperatures recorded by the thermocouples for the run at 425 "C. They indicate that combustion occurred in a narrow zone at the bottom of the sample bed near the heater control thermocouple (TI);the high temperature of TIresulted in the heater shutting down, thus allowing the rest of the bed to cool (T2).The results of O2 gasification do not represent any meaningful rate measurements but do illustrate the capability of the balance for measuring weight and sample conditions during reaction.
Discussion The behavior of the thermobalance as illustrated in the experiments performed points out both advantages and problems associated with the apparatus. It is clear from experiments that the balance does fulfill the requirement of recording sample weight and temperature simultaneously during reaction. The major problem associated with the balance is the occasional drift in weight recorded during steady-state operation from factors other than sample weight change. The magnitude of this drift is a maximum of 0.1 g/h but is significant enough to result in differences in actual and recorded sample weight during experiment. Because this drift occasionally occurs, operation of the balance at present is limited to gas-solid reactions where significant weight changes of the sample occur over a relatively short period of time. The weight drift could result from external
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2011 disturbances (vibration or temperature) or from internal changes within the apparatus (temperature or pressure) and could be reduced by better isolation of the balance apparatus. Other problems associated with the balance are the relatively long time (2 h) required to reach a steady state and entrainment of very fine particles in the gas flow through the bed. The change in apparent weight during heatup results from forces exerted by the gas lines as the pressure vessel expands. Several gas line configurations were used in an attempt to eliminate this problem, and while the magnitude of the weight change has been reduced, it has not been eliminated. Because of this long heatup time, it is not possible to study solids which thermally decompose before reaching reaction temperature. Entrainment may make it necessary to agglomerate or pelletize materials composed of fine particles before reaction; however, significant entrainment was not observed in the experiments reported in this paper. The design concept of the apparatus offers a combination of desirable features not found in earlier apparatus. One immediately apparent advantage is safety; the coldwall design and small pressurized volume make the risk from vessel rupture minimal. Also, simultaneous measurement of temperature and sample weight provides information about sample conditions in the bed; with the sample weight alone, false conclusions could be drawn about what is actually occurring during reaction (Boero, 1982). Even though only two thermocouples were used in our experiments, it is possible to place several more in the bed without affecting balance performance. Gas flow through the bed of solids is another major improvement over hanging basket apparatus. For the flow conditions (300 cm3/min, Re = 0.8) in the bed during C02 gasification at 800 OC, the mass-transfer coefficient calculated from the j-factor correlation is k, = 40 cm/s; there results less than 0.1% change in C02 concentration between bulk gas and carbon particle surface. External mass-transfer resistances are therefore eliminated. Finally, the potential exists to regulate gas flow such that the solid reactant bed is fluidized during reaction. Fluidized beds operate at near isothermal conditions and with the solids well-mixed; thermogravimetric studies of gas-solid reactions with fluidized solid would be very at-
tractive both for fundamental kinetic studies and for simulating reaction under actual conditions (i.e., in a fluidized bed gasifier). However, to operate in the fluidization regime, a narrow range of particle size would be desirable to best predict minimum fluidization velocity; also, entrainment of partially consumed particles would have to be accounted for in the weight analysis.
Conclusions A novel thermobalance apparatus for carbon gasification studies has been constructed, and operating characteristics have been illustrated. Attractive aspects of the apparatus include measurement of sample weight and temperature simultaneously, elimination of external mass-transfer resistances through operation as a fixed bed, and safe operation as a result of a cold-wall design. Present shortcomings of the balance include an extended heatup period and weight drift of up to 0.1 g/h during reaction. These problems presently limit balance operation to relatively fast reactions in which large weight changes occur. Acknowledgment This work is supported in part by the National Science Foundation under Grant CPE-83-07963.
Literature Cited Boero, J. F. R. Carbon 1982,20, 535. Dobner, S.; Kan, G.; Graff, R. A.; Squires, A. M. Thermochem. Acta 1976, 16, 251. Feldkirchner, H. L.; Johnson, J. L. Rev. Sci. Znstrum. 1968,39,1227. Forgac, J. M.; Angus, J. C. Znd. Eng. Chem. Fundam. 1979,18,416. Gardner, N. C.; Samuels, E.; Wilkes, K. ACS Adv. Chem. Ser. 1974, 131, 209. Gardner, N. C.; Leto, J. J.; Lee, S.; Angus, J. C. NBS Spec. Publ. 1980, 580, 235. Holstein, W. L. Fuel 1983, 62, 259. Kapteijn, F.; Coevert, P.; Moulijn, J. A. J. Phys. E 1982, 15, 1064. Kapteijn, F.; Moulijn, J. A. Fuel 1983, 62, 221. Li, K.; Rogan, F. H. Thermochem. Acta 1978,26, 185. Sears, J. T.; Maxfield, E. A,; Tamhankar, S. S. Znd. Eng. Chem. Fundam. 1982,21, 474. Wigmans, T.; van Cranenburgh, H.; Elfring, R.; Moulijn, J. A. Carbon 1983,21, 23.
Received for review June 26, 1986 Accepted May 18, 1987
