FIGURE 1. DIAGRAM OF APPARATUS
Combustion Rate of Carbon Study of Gas-Film Structure by Microsampling ALMON S. PARKER‘ AND H.C. HOTTEL Massachusetts Institute of Technology, Cambridge, Mass.
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Tu’ PREVIOUS papers of this series (2, 9)
data were presented on the combustion rate of isolated spheres of brush carbon burning under controlled conditions of temperature, ambient gas velocity, and ambient gas composition, and on the combustion rate of carbon disks in the base of a porcelain thimble under conditions permitting the combustion rate to be controlled by varying the effective depth of the thimble. The rate of combustion was found to be inversely proportional to the sum of two resistances in series-one a chemical resistance at the carbon surface, the other a resistance to counterdiffusion of oxygen and combustion products through the gas moving in laminar flow near the carbon surface. The quantitative relation giving diffusional resistance was evaluated from a consideration of those data obtained under conditions in which chemical resistance was negligible either because of 1
Piegent address. Carhide 8: Carbon Chemicals Corporation, Charleston,
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the high temperature (9) or because of the large “diffusional resistance” (8). This resistance to diffusion, expressed in the form of a film resistance, was itself composed of two resistances, one a true diffusional resistance in the gas moving in laminar motion next to the surface and the other an eddy-current resistance; but the data on spheres did not warrant separation of these two. Although the previous work permitted a separation of chemical and diffusional resistance by working alternatively under conditions suppressing the importance of one or the other of these resistances, a technic is desirable which permits a study of chemical resistance a t temperatures a t which diffusional resistance is not negligible and permits a simultaneous study of the latter. Although the work of various inv e s t i g a t o r s on combustion at low absolute pressures permits a partial attainment of the first objective, there is the disturbing factor of possible change in reaction mechanism attending the use of low pressures. For example, llayers ( d ) , in a resume of the status of research on carbon combustion, pointed out that Meyer (6)found in his low-pressure studies of carbon a zero-order oxidation reaction in a certain temperature range, t h a t the rate consequently should be the same a t atmospheric pressure if the mechanism does not change, but t h a t we know from experiment t h a t the rate is much greater. Because of the above-discussed limitations on the interpretation of combustion data in which gas composition is known only in the ambient medium, i t was decided to attempt the analysis of gas a t various points inside t h e gas film, particularly a t the carbon surface itself. The present paper describes the development of the technic of studying combustion mechanism by interfilm gas sampling, and presents the results of such a study made under controlled conditions of temperature and air flow past the burning specimen.
Experimental Method The material chosen for study was brush carbon studied in some detail in the first two papers of this series.2 The specimen was turned to cylindrical form 2.5 cm. in diameter, with hemispherical end, and mounted on a porcelain tube to hang with the hemispherical end downward on the axis of a vertical cylindrical furnace through which metered preheated air was passed. Figure 1 shows the arrangement of apparatus: The flow of air was regulated by valves S, maintained constant by inverted floating bell T,and measured at orifices U. From the orifices the metered air flowed through a supplementary gas-fired preheater external t o the main furnace (added in later experiments to increase capacity), then through an electrical preheater, P, consisting of a chrome1 coil in an annular space in the base of the main furnace, then vertically past the burning carbon specimen, A , and out through the aperture around the tube supporting the specimen. The carbon sample was suspended from spring I terminating in a hook attached to circular plate E, fastened in turn by three hooked wires to the three horizontal rods, C , extending radially from the brass collar into which the porcelain tube supporting the carbon specimen was cemented. Rods C were threaded and provided with balancing nuts. The carbon specimen was cemented and wired to the vertical tube as indicated. The dimensions of the exposed portion of carbon being known, the specific combustion rate could be determined by noting through telescope L the rate a t which point J on the suspension wire moved upward along scale K. Spring I was calibrated during each run by placing a known weight on plate E . 2 Approximately 85 per cent of the present work was carried out with brush carbon from the same lot as that used b y T u (9) and b y Davis ( 2 ) The nea lot used in the last part of the ork shoued no measurable difference in burning characteriatios.
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NOVEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
The temperature of the burning carbon was measured by a thermocouple placed inside the porcelain tube supporting the specimen. The fine leads were coiled to minimize interference with the motion of spring 1 and were secured a t F . When surface gas samples were desired, the carbon specimen could be clamped in position by means not shown in Figure 1. The gas sampling apparatus is shown a t the right of the furnace, and consisted of a 1-mm. quartz sampling tube, B, drawn out a t one end into a 5-inch (12.7-cm.) capillary, 0 (about 0.15 mm. i. d.), bent as shown and clamped rigidly to standard Q. The latter was mounted on carriage D which, in turn, was mounted on track *V in such a way that it could be moved horizontally by a micrometer screw and crank, M . The progress of the ti of capillary 0, which extended through a '/*-inch (3.2-mm3 horizontal silica tube in the wall of the furnace, could be watched through a telescope not shown in Figure 1 but aligned to sight through another l/B-inch silica tube passing through the furnace wall normal to the drawing. The field of view of the telescope is indicated by the small circle at R. Combustion rates could be measured by placing a micrometer eyepiece in this telescope and noting the rate of decrease in the diameter of the carbon specimen; this technic, however, was abandoned in favor of direct weighing with the spring suspension. To obtain gas samples, sampling tube B and 1 or 2 inches (2.5 or 5 cm.) of capillary 0 were filled with mercury from the leveling bottle; carriage D was moved to the left until the tip of the capillary pressed gently against the carbon surface, the stopcock on the line leading to the leveling bottle was opened, and, after the sample had been drawn, the capillary was sealed a t 0 by a flame. Tube B was then removed, and the gas in it was collected by breaking the tip under an inverted capsule in a dish of mercury. The gas samples (0.1 cc. total volume) were analyzed for carbon dioxide, oxygen, and carbon monoxide by means of a microburet built substantially according to the recomniendations of Swearingen, Gerbes, and Ellis (8). One of the runs was made in which gas samples were drawn with the tip of the capillary a t different distances from the carbon surface. These distances were measured by means of a micrometer eyepiece in the observation telescope as previously described. Preliminary measurements showed that the method of measuring carbon temperature described above was adequate if the thermocouple reading remained constant a t the value desired for B run for about 10 minutes just previous to that run. The effect of gas-sampling rate on the surface-gas analyses is shown in Figure 2, in which the content of carbon dioxide, oxygen, or carbon monoxide in t h e gas is plotted against the rate of withdrawing the sample. Since a three hundred fold variation in sampling rate (from 0.003 to 1.00 cc. per minute) produces only a slight change in gas composition, i t is concluded t h a t t h e film structure is not destroyed by the sampling rate (0.05 cc. per minute) used in most of the subsequent experiments. Figure 2 indicates in addition t h a t the reproducibility of sampling is not as good as might be desired, since the carbon dioxide content of the gas at the carbon surface (as determined in runs in which the sampling rate was less than 0.01 cc. per minute) varied from 15.2 to 17.2 per cent, even though the carbon temperature and ambient gas velocity were
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The combustion of brush carbon in air is studied by microanalysis of small gas samples withdrawn from the carbon surface inside the "film" overlying the carbon and by the variation in gas composition through the film. The rate of combustion per unit of superficial carbon surface is also determined under corresponding conditions of ambient gas velocity and temperature. The data are used to study the mechanisms both of combustion at the carbon surface and of diffusion of oxygen and combustion products through the film separating the carbon surface and the ambient air. Equations of combustion are presented which correlate the data.
maintained as nearly constant as possible. Subsequent experiments indicated t h a t this variation corresponds to the expected variation in carbon dioxide content resulting from moving the tip of the sampling tube 0.5 mm. away from the surface of the carbon. Early in the experimental work a set of runs was made in which gas samples were drawn at different distances from the carbon surface. The resulting analyses are plotted against distance in Figure 3, which presents a quantitative picture of the gas film overlying a burning carbon specimen. The short horizontal lines drawn through the data points represent the precision of t h e distance measurements. The surface-gas analyses for various conditions of burning are presented in Table I. The oxygen concentrations given there were plotted against carbon temperature for the various air velocities of the experiments in Figure 4. Below 1000'K. the partial pressure of oxygen appears to approach asymptotically the value 0.21 which i t has in air, and at high temperatures i t appears to approach zero as a limit. At a n y temperature the surface-oxygen concentration increases with the ambient gas velocity. The results of the measurements of combustion rate, generally made in separate experiments from those used for the gas analysis, are presented in Figure 5. There lO3K (where K is grams of carbon consumed per second per sq. cm. of carbon surface) is plotted against carbon temperature for each of the air velocities used in the experiments. The presentation of a t a b l e of values is not warranted by the precision of the results, since combustion rates may be read from the figure within the error of measurement.
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