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Ind. Eng. Chem. Fundam., Vol. 18, No. 4, 1979
EXPERIMENTAL TECHNIQUES
A Pressurized Thermobalance Apparatus for Use at Extreme Conditions John M. Forgac and John C. Angus' Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44 106
A cold wall pressurized thermobahnce was developed for obtaining thermogravimetric rate data at extreme conditions. A solid state, electronic balance was completely enclosed in a pressurized water-cooled vessel. Samples suspended from the balance were heated with an induction coil within the vessel. Design calculations indicate temperatures of 2000 K at 136 atm (2000 psi) hydrogen pressure are obtainable. Experiments were performed at sample temperatures up to 1633 K and pressures to 70 atm. Hydrogen, helium, methane, and argon were used. The measured rates were accurate to within 5 YO. Signal fluctuations were correlated using the Grashof number, which reflects effects of natural convection
Introduction The pressurized thermobalance is a device for measuring mass at different temperatures in a pressurized environment. Dobner et al. (1976) have reviewed the published reports. Mechanical and electronic balances, mass transducers, and quartz springs have been used. Successful results have been obtained at pressures as high as 500 atm. Weighing devices have been incorporated into systems capable of 1573 K. However, the combination of high temperature and pressure in the range used here or the use of induction heating in a pressurized thermobalance have not been reported. The device discussed here was developed for studying the rate of carbon formation from methane pyrolysis at elevated temperatures and pressures. However, a wide range of other uses is possible, e.g., S02-dolomite reactions, catalyst coking, coal or char hydrogasification, and combustion of solids. Apparatus Overall Design. The vessel was designed for use with hydrogen-containing gas mixtures at pressures up to 100 atm. Desired sample temperatures were as high as 2000 K. These extreme conditions dictated a cold-wall pressure vessel. Structural strength was obtained with an outer body of 316 stainless steel, protected from hydrogen embrittlement with a shrink-fitted liner of type-A286 austenitic stainless steel. The walls were kept cool with water flowing through grooves in the liner and by thermal isolation of the sample by a series of four concentric cylindrical radiation shields. Convective and radiative heat transfer calculations indicate sample temperatures as high as 2000 K could be obtained with the internal wall temperature not rising above 350 K. The cold walls give rise to the principal limitation of the device, viz., its inability to accommodate condensable gases. Essential to the success of the system was the ability to place the microbalance within the pressurized chamber. A Perkin-Elmer Model AM-1 Autobalance was used. In 0019-7874/79/1018-0416$01.00/0
this servo-controlled device the current required to null the position of the balance beam is measured. The position sensor and weighing mechanism were removed from their normal housing and placed within the vessel on a water-cooled plate. The balance beam was shortened from 10.2 cm to 5.1 cm. This reduced the sensitivity by a factor of 2; however, this was unimportant in light of fluctuations introduced from other sources. The ultimate sensitivity and response were not determined by the balance but rather by convection and electromagnetic coupling of the sample with the induction coil. Details of Apparatus. The vessel, shown in Figure 1, was constructed by Autoclave Engineers, Inc. The internal diameter and depth of the inner liner are 5 and 14 in., respectively, giving a volume of 4.5 L. The wall thickness of the 316 outer shell and the A286 liner are 1 and 1/2 in., respectively. The lid is attached to vertical guide rails and can be raised and lowered with a hand winch. Closure is made with ten 7/8-14 bolts through the lid and an O-ring between the lid and flange. The vessel is designed for a nominal pressure of 5000 psi and was hydrostatically tested to 2375 psi. A water-cooled plate with a 1-in. center hole is suspended from the lid. The electronic balance rests on this plate and the sample is suspended through the hole. A quartz sight window is centered in the lid, providing a view of the heated sample and permitting temperature measurement by infrared pyrometry. To reduce convection currents, a horizontal baffle, not shown in Figure 1, is attached to the gas inlet tube. Other accessories attached to the lid include a Statham pressure transducer, a Conax electrical feedthrough for the electronic balance, a thermowell for measuring the temperature in the upper portion of the vessel, inlet and outlet gas lines, inlet and outlet water lines for the cooling plate, and a valve attached to a vacuum line. The two water-cooled copper feed-throughs for the induction coil enter the bottom of the vessel. These copper 0 1979 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 18, No. 4, 1979 417
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Figure 1. High pressure thermobalance: 1,lid; 2, viewing port; 3, 316 stainless steel outer shell; 4,A286 inner steel liner; 5, cooling water grooves; 6, electronic balance; 7,water cooled plate; 8, gas inlet tube (top portion through lid not shown); 9, induction heating coil; 10, water cooling inlet; 11, Be-Cu insert; 12, Teflon liner.
tubes are brazed to shaped Be-Cu inserts which in turn fit into Teflon liners which insulate the coil from the vessel. The assembly is compressed with external nuts, aided by the internal pressure. The sample is suspended in the heated zone with a 15 cm long hangdown wire made from nichrome, platinum, and iridium. The sample holders were cylindrical crucibles made from 0.025-mm platinum foil. A 5OO-kHz, 5-kW, induction heater acting on coiled graphitic foil susceptors (Grafoil, Union Carbide) was used to heat the sample. Alumina and sillimanite pieces (Coors) support the susceptor and provide electrical and thermal insulation. Operation. The thermobalance was operated as a static system. The large volume of the reactor gives essentially constant composition throughout a run. Although we have not done so, it should be possible to run with a continuous gas flow. In order to perform an experiment, a sample and holder are attached to the hang-down wire with the lid raised. The lid is lowered and bolted onto the reactor. The vessel is then pressurized, the induction heater is turned on, and the sample is heated to the desired temperature. Samples can be heated to operating temperatures in 3 to 5 min. If desired the temperature can be increased during a run. Mass, temperature, and pressure are continuously recorded on strip chart recorders. The outer walls of the vessel remained very cool during operation. Even a t the highest temperatures used (1633 K), a maximum increase in outer wall temperature of only 10 K over ambient conditions was observed. The temperature inside the chamber, above the cooling plate in the vicinity of the electronic balance, never went above 355 K, which is well below the maximum allowable temperature of 373 K for the microbalance. The thermobalance has proved durable. Over one hundred experiments have been performed. The balance has been routinely operated a t 50 atm and 1433 K. The device has been operated with sample temperatures as high as 1633 K annd pressures to 70 atm. Example Results Carbon obtained from pyrolysis of methane was deposited on 0-l-pm diamond powder with specific surface
Figure 2. Carbon deposition rate vs. reciprocal temperature: Q26.6 atm, 0.19 CHI, 0.81 H,; 0,11.0 atm, 0.30 CHI, 0.70 Hz. The symbols rn and 0 indicate the two predicted equilibrium temperatures below which substrate etching occurs.
area of approximately 10 m2/g. Experiments were performed at temperatures from 1073 to 1623 K and pressures from 1to 50 atm. Hydrogen, helium, methane, and argon gas mixtures were used. In these experiments the rate of carbon deposition was constant with time and, consequently, average rates were obtained by fitting a straight line to the mass-vs.-time trace. (Linear regression was used to fit a series of mass readings taken at discrete time intervals.) Some selected average rate data are given in Figure 2. Conditions near reaction equilibrium were used, the equilibrium conditions being identified computationally (Gordon and McBride, 1971). Below the equilibrium temperature, negative deposition rates were observed. These are not shown in Figure 2. As the temperature was increased above the equilibrium temperature, the deposition rate increased rapidly. A nucleation rate process is believed to be governing. Measured average rates covered four orders of magnitude (lo4 to g/g min) for deposition on 20 to 140-mg diamond powder samples. The scatter in the data is a strong function of the conditions being studied. In the present case the percentage error of the measurements ranged from 2 to 15% with an average of 5%. Here percentage error is defined as 100 standard deviations divided by the mean rate.
Sources of Error The rates measured with the thermobalance were compared with integral mass changes obtained from before-and-after mass measurements made on an analytical balance. The integral rates were in agreement with the thermobalance rates but had significantly more variance. The difference is attributed to small amounts of sample loss occurring during the heat-up and cooling-off period. Since instantaneous rates are measured with the thermobalance, such sample losses are of little consequence. At severe operating conditions, it was possible to regig produce the carbon deposition rates greater than min to within 33%. The rather low reproducibility is a characteristic of the system studied and arises from the extreme sensitivity of the measured rate to temperature; it does not represent any inherent defect in the device. For example, the slope of the less steeply sloped line in Figure 2 is -8.1 X lo4 K. Based on this value, an error in the temperature of only 6 K, typical for the present situation, will cause a 33% reproducibility error at 1250 K. A rate
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process with a less sensitive temperature response would display better reproducibility without any change in the apparatus. Bouyancy and convection are important at these extreme conditions. During pressurization, the thermobalance would display an apparent mass increase. This could be accounted for by a volume difference between the sample side of the beam and the counterweight side of -0.023 cm2. Moreover, the apparent mass increases suddenly upon heat-up. The signal recovers but not to the initial value. The net average shift in apparent mass is 2.4 mg with a standard deviation of 1.7 mg. The mass increase is too great to be accounted for by the diminished bouyancy of the heated gas in the vicinity of the sample. Electromagnetic forces from incomplete shielding of the sample from the induction coil is the suspected cause of this additional effect. The effect of newly deposited mass on bouyancy is minimal. The maximum possible apparent mass reduction at extreme conditions is only 0.4% of the deposited carbon. This amount is well below the accuracy of the measurements. Spurious mass changes are observed in the course of operating the apparatus. These occur outside the run period and do not affect the rate measurements. They are attributed to small displacements in the position of the balance, or adsorption or outgassing from the balance parts. The time when these mass changes occur and their approximate magnitudes are: when the lid is raised or lowered, 0.16 mg; when evacuating air from the vessel, -0.25 mg; when cooling the vessel, -0.10 mg; over long times (16 h) at vacuum, 0.25 mg. Fluctuation in the mass signal is a major source of uncertainty in the rate measurement. The likely cause is the natural convection of the gases rising from the heated zone. The standard error of estimate of the mass in milligrams, obtained from the linear regression of the mass-vs.-time data, was used as a measure of fluctuation. A correlation was developed between the mass signal fluctuation and the Grashof number, a dimensionless quantity commonly used to characterize natural convection. The Grashof number is p2/3gATD3 Gr = F
where p is density, /3 is expansivity of the gas, g is the acceleration of gravity, AT is the temperature difference between the fluid and ambient atmosphere, p is the viscosity, and D is a characteristic dimension. The natural convection was modeled as if occurring from a finite body in an infinite medium. The characteristic length of the heated body was chosen to be the internal diameter of the heated zone (this parameter was not changed in any experiment). A plot of fluctuation vs. Grashof number is presented in Figure 3. The data were collected from results with pure and mixed gases of hydrogen, helium, methane, and argon. The total mass of the platinum holder and sample in each experiment was approximately 570 mg. Although the correlation is specific to this system, it does reveal in broad outline the nature of changing signal accuracy with severity of conditions. For example, it is clear that for Gr < IO' other sources of error are influencing the measurements in addition to convective effects. For Gr >> lo7 convection effects appear to dominate. Severe noise is introduced into the measurement at Gr > lo9; above this level, use of this thermogravimetric
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technique would not be practical without changes in design. Whether or not mass transfer effects influence the observed rate will, of course, depend on the relative values of the reaction rate and mass transfer coefficients. Since gas motion past the sample was induced solely by natural convection, mass transfer rates at the sample surface could be independently varied only by control of the sample size and geometry. Nevertheless, this apparatus could be adapted with a limited gas flow in the reaction zone to regulate mass transfer rates external to the sample. An analysis which utilized the methods of Satterfield (1970) indicated that there could be interparticle mass transfer effects under some conditions investigated. Evidence for interparticle diffusion limitations was obtained only at higher rates; smaller, 20-mg, samples displayed higher specific rates than the larger 140-mg samples at similar conditions. Most rate measurements were outside the mass-transfer-limited regime. Eddies within the vessel introduced noise in the temperature measurement; the greatest effect took place at the highest temperature and pressure. The percent error (100 standard deviations divided by the average temperature) ranged from 0.3% to 1.2% with an average of 0.6%. Conclusions The pressurized thermobalance is a very useful tool for obtaining gas-solid reaction rates at extreme conditions. Pressures up to 70 atm, temperatures up to 1633 K, and hydrogen mole fractions of 99 mol% were the most severe conditions employed; sample temperatures of 2000 K and pressure of 136 atm (2000 psia) are believed to be achievable. Instantaneous rates can be obtained and experimental conditions can be changed during the course of a run. Induction heating permits rapid heat-up rates. Fluctuations in the apparent mass are partially quantifiable through a Grashof number correlation. It should be possible to obtain improvements in accuracy, primarily through the reduction of convection effects. The device can find application in carbon deposition studies, coal or char hydrogasification, S02/limestone reactions, shale oil pyrolysis, and solid combustion reactions. The cold walls do not permit the use of condensable gases. Literature Cited Dobner. S., Kan. G., Graff, R . A,, Squires, A. M., Thermochim. Acta, 16, 251 (1976).
Gordon, S., McBrlde, 8. J., NASA SP-273 (1971). Satterfield, C. N., "Mass Transfer in Heterogeneous Catalysis", MIT Press, Cambridge, Mass., 1970.
Received for review June 28, 1978 Accepted J u n e 22, 1979
