Photographic Study of Boiling J. W. WESTWATER AND J. G. SANTANGELO' University of Illinois, Urbana, I l l .
Boiling is a very complex phenomenon. Interest in some of its aspects has been greatly magnified with the advent of nuclear reactors, jet engines, and other equipment where large amounts of heat must be removed from a small zone. The author published last year in the Scienfific American [190, No. 6, 64 (1 954)] an article of the popular type outlining his explanations for the mechanisms involved in boiling. Here he presents a discussion of photographic equipment and techniques used in his studies of boiling, and an evaluation of the results obtained.
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HE use of photography has a strong appeal to scientists. Seeing a phenomenon stopped or slowed down can be a great aid to understanding it. Boiling is a form of heat transfer which lends itself to photographic techniques. As early as 1936 investigators (9, 14) were using photography as a part of their research on boiling, but the resulting pictures were often unsuited for detailed analyses. Even recent investigators (IO) have reported "disappointing results" from photographic observations. The need for short exposure times was evident from the start. Sauer (bl), Castles [reported by McAdams ( 1 6 ) ] , and more recently Corty and Foust (4)obtained still photographs at 10-5second exposure. These pictures showed that boiling is a fastaction phenomenon and that slowspeed photographs can be misleading. Because boiling is a dynamic occurrence, motion pictures are preferable to still ones. Normal-speed movies (16 or 24 frames per second) are interesting and helpful ( 5 , 8),but such films show blurred action in each single frame. Drew and Mueller (8) took some motion pictures a t 64 frames per second, and Jakob (13, 14) used even higher speeds of about 500 frames per second. These fihns are an important part of the literature on boiling, but even film a t 500 frames per second shows blurring in individual frames, indicating that speeds higher than 500 frames per second are desirable. Dew (6) obtained motion pictures of the phenomenon of surface boiling-localized boiling of a cold liquid-at 2000 frames per second. His film is discussed by several authors (17, $0). Castles (3) photographed nucleate and film boiling of water a t a hot wire using a camera speed of 3000 frames per second. Gunther ( l a ) , using a highly specialized camera, succeeded in taking movies a t 20,000 frames per second. The field of view of the boiling was limited to a small fraction of a square inch, and the length of a sequence on 35-mm. film was limited to about 5 feet. The work reported here was undertaken to obtain high-quality motion pictures of all three regions of boiling: nucleate, transition, and film boiling. The speed of the film was to be great enough to permit quantitative studies of the events such as bubble growth 1
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or film oscillation. Lastly, to provide good illustrations for the conclusions based on observations of the movies, still photographs were to be taken a t 10-6-second exposure. CHOICE OF BOILING APPARATUS
A sketch of the apparatus is shown in Figure 1 A steamheated, horizontal, copper tube was used as the hot element. An electric heating element is definitely not suited for work in the transition region of boiling, as shown by the results of Nukiyama ( 1 8 )and subsequent workers. The difficulty in transition boiling arises because an increase in the temperature driving force causes a decrease in the coefficient of boiling heat transfer. If a stearnheated tube is used, this decrease results in a decrease in steam flow, and the process is self-regulating But with an electric element, the decrease in the heat transfer coefficient does not cause a decrease in the electric current. The result is an accumulation of heat in the heater element and a corresponding undesirable sudden rise in temperature. This occurrence is discussed in detail by McAdams and others ( 1 6 ) . The heating tube was of bayonet construction, so that it could be inserted easily into the boiler through an opening in a brass cover plate a t one end of the boiler. The outer tube was of copper a/& inch in outside diameter, and the inner tube was of ropper 0.177 inch in outside diameter. Steam entered the small tube, and condensate was removed from the larger one. The immersed length of the outer tube was about 6 inches. The bayonet construction was desirable from the photographic standpoint; a single, straight tube was viewed by the camera. Methanol was chosen as the boiling liquid. It exhibits all three types of boiling within a range of temperature driving forces which can be achieved with laboratory steam available a t 100 pounds per square inch. Because methanol boils a t 148' F., the over-all driving force could be varied from 64" to 190" F. For a small number of tests, the apparatus was moved to the university power plant, where high-pressure steam was available, so that some data TT ere obtained with temperature differences of nearly 300" F. T o facilitate photography, a flat-sided, rectangular, glass boiler was employed. Approximatery 1 gallon of liquid was contained in the boiler, covering the steam tube to a depth of 3 or 4 inches. The methanol vapor formed during boiling was condensed in an overhead condenser and was either recycled to the boiler or diverted for metering. The apparatus was kept a t atmospheric pressure. Safety was realized by having a manometer, containing methanol, connected to the boiler. If an unsafe pressure developed, the manometer discharged t o a collecting vessel; the boiler contents followed close behind. The glass boiler measured 10 inches long, 10 inches high, and 5.5 inches front to back along the camera axis. The glass thickness was about 1/4 inch. The boilers as purchased were of onepiece construction, open at one end, and were being sold for use as a q u m a . However, none broke while in use as boilers. BOILING PROCEDURE
A large number of variables affect boiling, and great care is necessary if reliable data are desired. Riany papers in this field betray a failure to control experiments adequately. The commonest faults are failure to control the surface texture of the hot solid or the dissolved gas content of the boiling liquid. For the present work, the heating tube was cleaned immediately before each run. This is a necessary nuisance. Emery cloth (No. 3/0) was rubbed against the tube until the tube was bright and shiny. The apparatus was then closed, and methanol, which had been degassed previously by boiling, was put in the
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boiler. The tube was then aged in a standard manner: Steam a t 90 pounds per square inch gage was fed to the heating tube and film boiling was allowed to occur outside the tube. At the end of 20 minutes the steam was adjusted to the desired pressure and a run was made. A run required 10 to 30 minutes, during which time heat-transfer data were recorded and photographs were made. A t the end of each run the apparatus was disassembled.
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f t . ' F., it was possible to calculate estimates for the individual boiling coefficients and driving forces. Future experiments will measure metal temperature directly, for these measurements are superior to calculated values. The boiling heat-transfer data are shown in Figure 2. The estimated individual, boiling-side coefficients are based on an assumed steam-side coefficient of 4000. As the actual steam coefficient was probably less than this, especially for the steam-side area blanked off by condensate, the boiling-side coefficients in Figure 2 may be considered as the minimum possible values.
tVENT STILL PHOTOGRAPHS
1
OPENTUBE MONOMETER
1
The still photographs used as illustrations for this paper were taken a t lO-E-second exposure using a Crown Graphic camera having a double extension bellows. The film was 4 by 5 inch, Kodak Super XX. An aperture of f-9 was used, and the lens-tosubject distance was usually 8.5 inches. The short exposure time was achieved by using a General Electric flash-tube photolight. The camera shutter was opened while the gas-filled tube discharged in second. The light was 48 inches from the object; the light axis was at a 45-degree angle with the camera axis.
GLASS B O I L E R
METHANOL CONDENSATE
ii
REDUCING VALVE STEAM
a
LIGHTS \
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Figures 3 to 8 were taken in this manner. Figure 1 shows the general setup for photography.
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? 56001
I
I
I
1
I
I180
CAMERA
1
SUBCOOLER
WATER STEAM CONDENSATE COLLECTION
Figure 1.
Arrangement of boiling and photographic equipment
Scattered heat-transfer data resulted whenever the boiling liquid contained dissolved gas. This has been noted by other workers (19). Nonreproducible data will result if the metal surface texture is not carefully controlled ( 4 , 7 ) . The purpose of the cleaning and aging was to provide the same metal surface texture for all runs. With the, standardized technique, reproducible data were obtained easily a t every point along the boiling curve, including all of the transition region. The heat-transfer data consisted of heat flow rates and the temperature driving forces. Heat input was measured by metering the steam condensate. Heat output was determined by metering the methanol condensate and measuring its temperature (during a short batchwise operation) and by measuring the heat pickup by the condenser water (during continuous operation). At the higher heat loads, these values agreed to within a few per cent. A t the lowest heat flow rates the agreement was poorer. The single worst discrepancy, 50%, occurred a t the minimum heat output of 280 B.t.u. per hour. Rut in all cases, the heat input exceeded the output, so it is felt that the discrepancy might be due t o heat lost to the surroundings, even though an attempt was made to account for these losses by making dry runs with no methanol. The final heat-transfer coefficients were based on the methanol condensed in all cases. The steam temperature was calculated from the measured steam pressure. The heat transfer coefficients and the temperature driving forces are most accurately calculated on the over-all basis. As no dirt or scale existed on the outside of the heating tube, and the inside was in contact with clean steam which should have a condensing coefficient of about 3000 or 4000 B.t.u./hr. sq.
AT^, O V E R A L L TEMPERATURE D I F F E R E N C E , ~ F AT^, ESTIMATED TEMPERATURE D R O P ACROSS METHANOL FILM,?
Figure 2.
Boiling curve for methanol
HIGH-SPEED MOTION PICTURES
The motion pictures were taken a t 4000 frames per second using a Western Electric, Fastax camera. A 100-foot reel of 16-mm. film was run through the camera in about 1 second. The lens aperture was f-4,and the lens-to-object distance was 15 inches. The lighting was critical. As conventional relationships between exposure times and light intensities do not hold for very short exposure times, it was necessary to use a trial and error technique to discover the proper lighting. The best lighting was found to consist of four lamps, bunched together, located 18 inches from the object and forming an angle of 45 degrees with the
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Figure 3. Point A on boiling curve.
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Nucleate boiling of methanol
Over-all A T = 67'F.
camera axis. The lights totaled roughly 400,000 candle power and were all focused on the heating tube. The lamps were three R-2-750 spot lights plus an RSP2 photoflood. They were operated a t 125 volts, instread of the rated 120 volts, so as to achieve an illumination in excess of the rated values. An idea of the necessary lighting for 4000 frames per second was obtained by taking preliminary movies a t 16, 64, and 1000 frames per second. Five or six reels of film were consumed a t 4000 frames per second before photographs of good quality were obtained. An argon timer, inside the camera housing, made possible a determination of the exact speed a t which any portion of the film passed the camera lens. The argon tube produced 120 flashes per second, which were evident later as exposed spots along the edge of the film. The timer marks showed that about half of a reel of film was run with acceleration and that the last half-reel ran a t constant speed. The constant-speed portions were used for quantitative measurements.
Figure 4. Point B on boiling curve. Over-all A T = 99.5'F.
Heat transfer, 76,900 B.t.u./hr. sq. f t .
OBSERVATIONS FROM PHOTOGRAPHS
Qualitative conclusions were obtained by repeated observations of the motion pictures projected at 16 frames per second. This speed gives a slowdown in the action by a factor of 250 to 1. The phenomenon of boiling has a breath-taking appearance at this speed, as confusion vanishes and order appears. Bubbles form slowly and move in a stately manner. Bubble sizes and spacings were determined quantitatively from direct measurements of the images on a projector screen. For nucleate and film boiling the desired measurements were obtained while the film was running. For the fastest action (bubble growth during transition boiling) film was projected one frame at a time, using a stop-action projector. The enlargement on the screen was such that a bubble whose image was 1 inch in diameter had a true diameter of 0.115 inch. A 1-inch scale, included in the field of view of the camera, served to establish the true magnification.
Critical boiling Heat transfer is a t m a x i m u m , 172,000 B.t.u./hr. sq. f t .
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Figure 5. Point C on boiling curve.
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Transition boiling
Over-all A T = 112'F.
Numerous measurements of the times for events to occur were made by timing the slowed events on the projector screen-for example, 20 vapor bursts occurred on a particular 1-inch length of heater tube during 30 seconds of slow motion. Six check counts all were within the range of 18 to 22 and this reliability was deemed satisfactory. Thirty seconds of slow motion correspond to 0.1 2 second of real time. For the most rapid events, time determinations were obtained by counting the actual number of motion-picture frames required for the events-for example, the life of one vapor burst during transition boiling was found to be 12 frames. This amounts to 0.75 second of slow motion or 0.003 second in reality. Since the lens aperture for the motion pictures was large (f-4), the individual frames are photographically inferior to the still shots (taken a t f-9). The illustrations in this paper are stills. The high-speed motion picture ( 2 3 ) is available to interested persons, upon request. A description of what is visible in the movie is given below. Nucleate Boiling. During nucleate boiling, bubbles form at point sources. When the over-all AT is 70' F., the over-all coefficient, U , is 1350 B.t.u./hr. sq. ft. ' F., and the time for one bubble to grow to its maximum size on the tube is 0.059 second, on the average. As soon as one bubble breaks loose, a new one starts to grow in its place a t the identical spot on the tube. Thus the frequency of bubble emission is about 17 per second at each active point. The rapid emission of bubbles results in good agitation and this is part of the reason why nucleate boiling is such an efficient method for transferring heat. Some variation occurs between different active points, SO that the range of values IS from 15.5 to 20.1 bubbles per second. The size of the bubbles a t the instant of leaving the tube varies somewhat also. The smallest is 0.10 inch in diameter, the largest is 0.20 inch, and the average is 0.17 inch. The active nuclei are not equally spaced. For the test described above, the spacing of adjacent nuclei varied from 0.058 to 0.46 inch, with an average value of 0.109 inch. The number of active nuclei is a function of the over-all temperature difference ( 4 ) . The space between active centers is bare and free of bubbles. This is evident in Figure 3. As the AT increases, so does the number of active centers, and vice versa. I t is still uncertain as
Heat transfer, 69,000 B.t.u./hr. a q . f t .
to what an active center is. It may be a tiny sharp point on the solid, a trace of impurity, or a pit containing some adsorbed inert gas. The last view seems most promising a t present. As the temperature difference increases, a critical condition arises a t which the solid becomes well peppered with active nuclei and the bubbles are in contact with their neighbors. This corresponds to the condition of maximum heat transfer. Figure 4 illustrates this condition, especially for the left portion of the tube. For methanol boiling in contact with copper, the critical over-all temperature difference is very close to 100' F. The value depends on the liquid and the solid as well as on the pressure. An examination of Figure 4 gives the impression that nucleate boiling is occurring along the left portion of the tube and that the transition state of boiling (described below) is beginning along the right portion. Kucleate boiling is the ordinary type of boiling that occurs in a tea kettle. As long as the temperature difference is less than the critical value, the amount of vapor generated can be increased by increasing the metal temperature. The characteristic feature is that small bubbles form rapidly and repeatedly a t randomly located specific nuclei. Some of the hot surface may exhibit no nuclei, and even though liquid-solid contact occurs in those areas, no bubbles will be produced there. Transition Boiling. If the temperature driving force is increased beyond the critical value by a few degrees, the transition type of boiling begins. Most prior workers have failed to realize that this boiling is entirely different from both nucleate boiling and film boiling. No active nuclei exist. I n fact, no liquid-solid contact exists either. The tube is completely blanketed by a film of vapor, but the film is not smooth nor stable. The film is irregular and is in violent motion. Vapor is formed by sudden bursts a t random locations along the film. Liquid rushes in toward the hot tube, but before the two can touch, a miniature explosion of vapor occurs and the liquid is thrust back violently. The newly formed slug of vapor finally ruptures, and the surrounding liquid again surges toward the tube. The process is repeated indefinitely. Figure 5 , taken from (a$),illustrates transition boiling. I n this photograph a downward-surging slug of vapor is seen a t the far right, lower corner.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 6.
Transition boiling
Point D on boiling curve. Over-all A T = 124'F.
One observer ( 1 1 ) of these high-speed motion pictures has expressed a n opinion that occasional liquid-solid contact does occur during transition boiling. If so, these contacts are rare and of exceedingly short duration. The present writers do not believe there is a real contact. The frequency of the vapor bursts is surprisingly high. For an over-all AT of 133' F. (and a U of 164 B.t.u./hr. sq. ft. O F.) each inch of the photographed side of the tube exhibited 84 bursts per second. The bursts occur so suddenly and unexpectedly that even in slow motion they resemble explosions. A typical burst has a life of 0.003 second. Bursts seen in profile show that the surrounding liquid is shoved back about 0.16 inch in this time. At the end of this time the vapor film has a distinct bulge, which begins to disintegrate into a number of bubbles of assorted sizes which rise to the surface. The decrease in the rate of heat transfer, which occurs as soon as the critical temperature difference is exceeded, is caused by the insulating effect of the vapor on the hot solid. If the temperature difference is increased still further, the blanket becomes thicker and more stable. Figure 6, taken at an over-all A T which was 12" F. higher than the A T for Figure 5 , shows less violent action than Figure 5. The increase in the vapor thickness results in a better damping of the explosive bursts until finally a thick,
Figure 7. Point E o n boiling curve.
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Heat transfer, 27,200 B.t.u./hr. sq. ft.
rather stable film results and no more sudden bursts occur. This is the beginning of film boiling. For methanol against copper, the transition region extends over the range of over-all temperature differences of 100' t o 140' F. The rate of heat transfer a t the higher AT is only 3% of the heat flow a t the lower A T . Film Boiling. The occurrence of the stable vapor blanket corresponds to the beginning of film boiling. No active nuclei exist; no explosive bursts occur. The slow-motion action is the slowest, most orderly, and best defined of the three types of boiling. It is significant that film boiling has been the first type of boiling to yield to theoretical considerations. The equations of Bromley and others (1, 9 ) give reasonable predictions for boiling heat transfer in the film region. During film boiling the hot tube is completely encased in a slowly moving film of vapor. Heat flows across this film by conduction and radiation, and new vapor is generated continuously a t the vapor-liquid interface. No vapor is generated a t the solid itself. For this reason, the smoothness of the solid and its chemical composition have little effect on the heat transfer. Newly formed vapor flows upward around the side of the tube. A distinct series of ripples flows up the tube. For an over-all AT of 184"F., the ripples move up the tube at a rate of 74 each second on each side of the tube. Ripples are visible in Figures 7 and 8 a s
Film boiling
Over-all A T = 148'F.
Heat transfer, 12,970 B.t.u./hr. sq. ft.
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Figure 8. Point F on boiling curve.
Film boiling
Over-all AT = 181’F.
well as in the photographs of Sauer ( $ 1 ) . Ripples are not visible in the film by Castles (S), presumably because he used a fine wire instead of a tube. As vapor gathers a t the top of the tube, a rodlike mass forms along the top of the entire length of tube. At first the rod is smooth and rather uniform in thickness, as seen in Figure 8, although faint movements occur constantly. As the gas rod increases in volume, peaks and valleys form until the upper boundary becomes sinusoidal in shape. Then the entire rod ruptures, between all the nodes, and a horizontal row of bubbles rise side by side. Figure 7 shows a gas rod in the process of rupture. After the rupture, the whole process is repeated. One cycle requires 0.06 second when the over-all temperature difference is 184’ F. A second gas rod ruptures a t points directly under the centers of the last released individual bubbles. This means that the even-numbered rows of bubbles are displaced sideways by a half space from the odd-numbered ones. Viewed from the side, the bubbles are similar to marchers arranged in triangular spacing rather than the conventional square spacing. This fact is evident in Figure 7 and also in the motion pictures ( 3 , 2 3 ) . Bubbles released during film boiling are much larger than those formed in nucleate boiling. At the instant of release the average diameter is about 0.3 inch, with a range from 0.20 to 0.36 inch The horizontal spacing of the bubbles is usually about 0.5 inch, center to center, although occasionally a spacing as great as 0.75 inch occurs. I n 1 second, about 22 bubbles are released per inch of tube length. Bubbles are released from the top of the tube only, in sharp contrast to the other two forms of boiling. The history of a bubble after its release is very different from that depicted in the drawings of rising bubbles in numerous books and journals. A bubble which is about to break free from a hot surface has a vapor filament connecting it to the surface. When the filament breaks, it contracts elastically and rams into the main body of the bubble. The effect is like that of a fist punched into a rubber balloon. At this instant the bubble becomes umbrella shaped. The deformed surface then snaps back and proceeds to vibrate as the bubble rises. The true shape of rising bubbles is clear in Figure 8 as well as in the photographs of Jakob and others ( I S , 14).
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Heat transfer, 5,470 B.t.u./hr. sq. ft.
ford, R. S. Leiser, E. L. Dougherty, A. B. Pizzato, F, S. Pramuk, W. T. Godsey, V. E. Putnam, and L. V. Peterson. Additional fruitful suggestions were made by R. L.Pigford. Support for the research was given by the National Science Foundation. LITERATURE CITED
(1) Bromley, L. A , Chem. Eng. Progr., 46, 221-7 (1950). (2) Bromley, L. A., LeRoy, N. R., and Robbers, J. A., IND.ENG. CHEM.,45, 2639-46 (1953). (3) Castles, J . T., McAdams, W. H., and Addoms, J. N., Mass. Inst. Technology, Cambridge, Mass., Motion Picture, “Visual Studies in Boiling,” 1947. (4) Corty, Claude, and Foust, A. S., Annual Meeting, 4 m . Inst. Chem. Engrs., St. Louis, Mo., Preprint 1 (1953). (5) Corty, Claude, and Foust, A. S., Univ. Mich., Ann Arbor, Mich., Motion Picture, “Surface Variables in Nucleate Boiling,” 1953. (6) Dew, J. E., Mass. Inst. Technology, M.S. thesis in chemical engineering, 1948. (7) Dougherty, E. L., Jr., Univ. Ill,, M.S.thesis in chemical engineering (1951). (8) Drew, T. B., and Mueller, A. C., E. I. du Pont de Nemours & Co., Wilmington, Del., Motion Picture, “Boiling,” 1937. (9) Drew, T. B., and Mueller, A. C., Trans. Am. Inst. Chem. Engrs., 33. 449-71 (1937). (10) Farbkr, E. A.,’and Scorah, R. L., Trans. Am. SOC.Mech. Engrs., 70, 369-84 (1948). (11) Foust, A. S., Lehigh Univ., Bethlehem, Pa., Drivate communication, October 1954. (12) Gunther, F. C., Trans. Am. SOC.Mech. Engrs., 73, 115 (1951). (13) Jakob, Max, “Heat Transfer,” p. 629, Wiley, New York, 1949. (14) Jakob, Max, Mech. Eng., 58, 643-60 (1936). (15) McAdams, W. H., Chem. Eng. Progr., 46, 121-30 (1950). (16) -McAdams, W. H., Addoms, J. N., Rinaldo, P. >I., and Day, R. S., Ibid.,44, 63946 (1948). (17) McAdams, W. H., Kennel, W. E., Minden, C. S., Carl, R.. Picornell, P. M., and Dew, J. E., IND. ENC.CHEM.,41,1945-53 (1949). (18) Nukiyama, Shiro, J. SOC.Mech. Engrs. Japan, 37, 367 (1934). (19) Pike, F. R., Miller, P. D., Jr., and Beatty, K. O., Jr., Annual Meeting, Am. Inst. Chem. Engrs., St. Louis, Mo., Preprint 2 (1953). (20) Rohsenhw, TV. >I., and Clark, J. A., Trans. Am. SOC.Mech. Engrs., 73, 609-20 (1951). (21) Sauer, E. T., Mass. Inst. Technology, M.S. thesis in chemical engineering, 1937. (22) Westwater, J. W., Sci. American, 190, No. 6, 64-8 (1954). (23) Westwater, J. W., and Santangelo, J. G., Univ. Ill., Urbana, Ill., Motion Picture, “Photographic Study of Boiling,” 1954.
ACKNOWLEDGMENT
Valuable contributors to the development of the boiling apparatus and the photographic techniques were W. M. Ruther-
RECEIVEDfor review J u l y 26, 1954. ACCEPTED January 24, 1955. Presented before the Division of Industrial and Engineering Chemistry a t the 127th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Cincinnati. Ohio.
