June 1951
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
local mass velocity of suspended water, weight per cent area per unit time = mass of suspended matter transported radially per unit area = coordinate perpendicular t o axis of duct, length = first root of zero-order Bessel function of first kind = local velocity of air stream, length per unit time = average air velocity, length er unit time = coordinate parallel t o axis ofduct, with origin at nozzle = distance from duct wall, length, g = D / 2 - r = eddy diffusivity, length squared per unit time = time = zero-order Bessel function of first kind = first-order Bessel function of first kind
=
M
r
R1 u
z g
e Jo J1
BIBLIOGRAPHY
Amstead, B. H., “Techniques for Determining Liquid Droplet Concentration in High Velocity Air Streams,” M.S. thesis, Univ. of Texas, Austin, Tex., 1949. (2) Coldren, C. L., M.S. thesis in chemical engineering, University of Illinois, Urbana, 1950.
(1)
1331
(3) Gilliland, E. R., and Sherwood, T. K., IND.ENG.CHEM.,26,
516 (1934).
(4) Grimmett, H. L., “Entrainment in ilir Jets,” Ph.D. thesis,
University of Illinois, 1949. (5) Kalinske, A. A., “Investigations of Fluid Turbulence and Suspended Material Transportation” in “Fluid Mechanics and Statistical Methods in Engineering,” Philadelphia, University of Pennsylvania Press, 1941. ( 6 ) Nukiyama, S., and Tanasawa, Y., Trans. SOC. Mech. Engrs. ( J a p a n ) ,4, No. 15,86 (1938). (7) Schubauer, C. B., Natl. Advisory Comm. Aeronaut., Tech. Rept. 524 (1935). (8) Sherwood, T. K., “Mass Transfer and Friction in Turbulent Flow” in “Fluid Mechanics and Statistical Methods in Engineering,” Philadelphia, University of Pennsylvania Press, 1941. (9) Sherwood, T. K., and Woertz, B. B., Trans. Am. Inst. Chem. Engrs., 35,517 (1939). (10) Towle, W. L., and Sherwood, T. K., IND.ENG.CHEM.,31,457 (1939). RECEIVED January 3, 1951.
Preparation of Solid- and Liquid-in-Air Suspensions For Use in Air Pollution Studies R. D.CADLE AND P.L. MAGILL S T A N F O R D R E S E A R C H I N S T I T U T E . S T A N F O R D . CALIF.
A
*
. I
N OVER-ALL investigation of air Artificial smogs have been found useful in verifying the accuracy of current pollution calls for a number of distinct knowledge of the composition of natural smogs. The preparation of atmosstudies, including meteorology, methods pheres containing gaseous additives is usually relatively simple, b u t the prepof collection, and physical and chemical aration of atmospheres containing dispersed liquids and solids is much more analysis, These studies reveal much difficult. This paper describes techniques for preparing such atmospheres about both the mechanism of occurrence on a continuous basis. and the nature of smog, but they do not Aerosol generators were developed, which disperse liquids by an aspirating directly explain its disagreeable effects. action and can also be used for dispersing certain solids. I n a device for conSynthetic smogs are, therefore, useful in tinuously dispersing powders a t a uniform and easily controllable rate, t h e verifying the accuracy of current knowlpowder is spread on a long brass trough which is drawn beneath an airedge of the true composition and physoperated glass aspirator. Chambers i n which the atmospheres are blended ical make-up of natural smog. Experiand tested are described. ments with such smogs are valuable beThe methods and equipment developed have been very useful in air pollucause the mechanisms by which natural tion studies, and should be useful in many more studies of dispersions i n air. smog exerts its effects are not well understood and can best be studied empirically. Synthetic smogs may be considered as atmospheres prepared prepared in this laboratory had a median particle size less than under carefully controlled conditions and containing one or 50 microns, and many less than 1 micron. more of the constituents of natural smog. These constituents DISPERSION OF POWDERS can be in gaseous or aerosol form, or both. The preparation of atmospheres containing gaseous additives of known type and The dispersion of powders into air has been discussed by a composition is usually relatively simple; the problems involved number of authors. Dautrebande ( I , 2 ) and Sinclair (4, 9 )have have been studied in detail (8). However, the preparation of described methods which depend on pneumatic dispersions. T h e atmospheres containing dispersed liquids and solids of definite physiological and optical properties of aerosols are often studied concentration and physical form is a much more difficult task. more conveniently on continuously renewed systems than on It is with the latter problem that this paper is primarily conrelatively “static” systems that change by settling and coagulacerned. The main contribution is a description of techniques for tion. The powders may be continuously dispersed a t a uniform uniformly dispersing particles into a moving air stream, as conand easily controlled rate. A relatively simple device was found trasted with the preparation of relatively static aerosols. t o be especially effective for this purpose (Figure 1). The extent to which the atmospheric dispersions can be conThe powder to be dispersed was spread on a brass trough about sidered aerosols is a matter of definition. The upper particle 1.5 meters long and 6 mm. deep which was drawn mechanically size limit for aerosols has been defined as 50 microns (9) and 1 at a rate of about 4 em. per minute beneath an air-operated glass micron ( 1 ) . Both limits are higher than those usually accepted aspirator. The sides of the trough made a 90’ angle where they for colloidal particles in general. Practically all of the dispersions came together. The aspirator sucked the powder from t h e
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INDUSTRIAL AND ENGINEERING CHEMISTRY
trough, and the blast of air tended t o break u p coagulates. The portion of the aspirator which dipped into the trough narrowed to a slit about 1mm. in width, which during a run was kept about 2 mm. above the apex of the trough. The air jet in the aspirator was about 1 mm. in diameter. Air was forced through this jet at a rate of about 20 liters per minute. The powder to be dispersed was usually spread to a depth of 1 t o 4 mm. in the trough, depending upon the desired concentration in the final
Vol. 43, No. 6
ticles were allowed t o settle on the slides for 24 hours. Experience has shown that almost all particles over 0.2 micron settle out under these conditions. Particles on the screens were examined with the electron microscope. The results demonstrated the removal of large particles by the separators, but not necessarily the removal of coagulates, since coagulation could occur in the settling box. Figure 3, A and B, shows that the larger particles of powdered graphite are removed by the. separators. Figure 3, C, D, E , and F , similarly demonstrates the removal or breaking up of the larger particles from screened dust collected in the Los Angeles area with an electrostatic precipitat,or.
B
A
Figure 1.
Dust-Dispersing Apparatus
aerosol. The resulting dispersion was drawn through a settling chamber to remove the largest particles and coagulates. The settling chamber was either the premixing box of the smog chamber (described below) or the glass settling column shown on the right in Figure 2. This column was 76 em. high and 15 em. in diameter. Further removal of the larger particles could be effected by passing the aerosol from the glass settling column through a series of cyclone separators, also shown in Figure 2. These were about 76 cm. high and decreased progressively in diameter from 8 t o 2.5 cm. They were similar in design to those described by Dautrebande (1). Figure 3 shows electron micrographs of particulate material collected from dispersions before and after passing through the settling chamber and cyclone separators. The concentration of particulate material in the air before passing into the settling chamber was about 2000 p.p.m. by weight. The dispersions from which particulate material was t o be collected were passed into a wooden settling box. Parlodian-covered screens ( 1 1 ) mounted on microscope slides were placed in the box and par-
C
Figure 3. Gold-Shadowed Electron Micrographs of Particulate Material Collected from Aerosols Prepared by Dust-Dispersing Apparatus
Figure 2. Settling Column and Cyclone Dust Separators M a s k used during studies of eye-Irritating properties of high concentrations of particulate material
Particulate material wasCollected as it camedirectly from t h e aspirator and after it had passed through t h e t r a i n of cyclone separators (X500) A. Powdered graphite from aspirator 2Op B. Powdered graphite: from train 1/16 in. X 25,0OO/*/in. = 1500/500 x = 3 p C D. Dust f r o m Los Angeles, f r o m aspirator, 12 and 6s f . Dust f r o m Los Angeles, from train, 2p G. L a m e black. from ascllrator H. L a m p black; from t r a l n
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June 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
A
P
1333
B
Figure 5. Sodium Chloride Crystals Settled from Aerosols Produced by Liquid Dispersers ( M O O ) A. E.
A
E
Figure 4.
Glass Liquid bispersers
Figure 3, G and H , electron micrographs of lamp black, shows the coagulation which may have occurred after leaving the cyclone separators. This method of dispersion produced aerosols of easily reproducible concentration, demonstrated by means of dispersions prepared in the smog chamber. Theoretical concentrations of material in the chamber were calculated from the weight of material dispersed, and the rate of air flow through the chamber (about 3 cubic meters per minute). The air in the chamber was analyzed by collecting the material on metal plates by means of a n electrostatic precipitator and measuring the gain in weight of the plates (6). When carbon black was dispersed a t a theoretical concentration of 4 p.p.m. by weight, the concentration found by repeated analysis was 1.0 * 0.2 p.p.m. by weight. The loss of carbon occurred mainly in the premixing chamber and the blower.
Aerosol f r o m aspirating-type disperser Aerosol f r o m splash-type disperser
Figure 6. Sodium Chloride Crystals Settled Aerosols Scrubbed with Water (X500) A. E.
from
Aerosol f r o m aspirating-type dlsperser Aerosol f r o m splash-type disperser
lower of the two sets of baffles. Removal of the larger particles was accomplished as before by the combination of impingement on the original liquid and the reversal of the direction of flow. This design avoided the clogging of capillaries which a t times occurred with the aspirating disperser. The simplicity of design of these dispersers makes them easy to construct. The size of the particles in the aerosols produced by these devices was determined with dispersions of 12 % aqueous solutions of sodium chloride. The aerosols were first passed through the glass settling column (described above), where the water evaporated from the droplets. The aerosol was then passed into a box, where it was allowed t o settle for 24 hours on Parlodian-covered screens. Electron micrographs (gold shadowed) of the salt crystals collected on the screens are shown in Figure 5. The
DISPERSION O F LIQUIDS
Several methods were useN for dispersing liquids. Figure 4, A , shows a glass disperser resembling somewhat the metal ones described by Dautrebande (3). The disperser was about 20 cm. high and 2.5 cm. in diameter. Liquids to be dis ersed were placed in the bottom of the apparatus. Compressef; air a t about 5 pounds per square inch pressure was admitted through the side arm. Liquid was drawn upward through the ca illary tubes by an aspirating action, and the dispersion f o r m e g a t the jet impinged upon the original solution where the direction of flow was reversed. The impingement and the centrifugal force on the particles resulting from the flow reversal removed the larger droplets. The dispersion then passed through two sets of baffles and out of the top of the apparatus. This disperser differed from the Dautrebande design in causing the aerosol t o impinge on the original solution and in the resultant reversal of direction of flow.
A similar but simpler design is shown in Figure 4, B. T h e liquid, splashed onto the central tube by the jet of air, flowed down the tube until i t was dispersed at the jet. The collection of splashed drops by the central tube was aided by the
Figure 7.
Diagrammatic Sketch of Smog Chamber and Premixing Box
INDUSTRIAL AND ENGINEERING CHEMISTRY
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filtering through coarse filter paper, such as Whatman No. 4. The filtrate, which contained suspended lamp black, nas nebulized by one of the dispersers, and the resulting aerosol waa bubbled through water to remove most of the alcohol which had evaporated As alcohol also evaporated from the suspension in the disperser, the concentration of lamp black in this suspension increased unless alcohol was added. This alcohol was introduced through a side arm, attached to a dropping funnel, sealed t o the lower part of the generator. This method of dispersion was somewhat more cumbersome than that using the moving brass trough and it was difficult t o scrub all of the alcohol from the aerosol. Therefore, this method was not used in the preparation of synthetic smog. How,ever, it had the advantage of simplicity of equipment and uses for this technique may be found. TESTING CHAMBERS
Figure 8.
Premixing Box and Smog Chamber
apparent frame of the micrographs was produced by the wire of the screen. The largest particles obtained were about 4 microns in diameter and the median size was about 0.7 micron. In an attempt to decrease fui ther the maximum particle size, the aero901s were bubbled through about 1 em. of water in a flask before passing into the settling column and the wooden settling box ( 3 ) . Electron niicrographP of the settled crystals are shown in Figure G. Apparently, particles over about 1 micron in size had been removed by the water. Because the salt crystals were obtained by evaporation, the diameter of the original droplets must have been considerably greater. This equipment could be used to disperse solids, provided a satisfactory suspension of the solid in a volatile liquid could be prepared. .4 suspension of lamp black in 95% ethyl alcohol could be obtained by stirring lamp black into eth? I alcohol and
Thorough blending of the aerosols and gaseous constituents of a synthetic smog before it is to be tested is as important as the proper metering of the gases and the preparation of the aerosols. This was accon~plishedin a premixing chamber. The premixing box, shown in Figures 7 and 8, was a galvanized metal box of 1 cubic meter capacity, containing two baffles, through which part or all of the air entering the smog chamber could be drawn. A control panel and equipment for metering gaseous constituents into the pi eniiving chamber were mounted on top of this chamber. The dust-dispersing unit. described above, is shown a t the loiter right in Figure 8. Aerosol from this unit entered the premixing chamber directly. Above the chamber on the right were mounted two mechanically driven syringes for dripping solutions of volatile contaminants a t a constant rate on hot surfaces. The resulting vapors mere swept into the premixing rhaniber with a stream of nitrogen. The glass dispersers weie placed within the chamber. -4high voltage ozone generator v,as mounted behind the control panel. The blended ingredients ere then blown from the premising chamber into a 10 cubic meter galvanized iron box (generally referred to as the smog chamber), where the effect of the synthetic smogs on human subjects could be studied. The smog chamber has been described by Magill ( 8 ) . The gasrs from the siiiog c h a m b e r w e r e e i t h e r exhausted into the open atniosphere or were passed by means of a duct into a transmissometer tube, where the amount of light scatteiing produced by the p a r t r l r s in the synthetic smog could be measured (Figures 9 and IO). The transmissometer consisted essentially of a light source, a 40-foot tube at each end of which neere mirrors, a photoelectric cell, and electronic equipment for amplifying the current from the cell. It has been desc'iibed in detail (10). ANALYSIS O F SYNTHETIC SMOGS
Figure 9.
Transmissometer Used t o Measure Effect of Various Substances in Decreasing Visibility
The composition of synthetic smogs prepared in this l a b o r a t o r y vias based on analyses of the Los Angeles atmosphere. There-
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
lune 1951
EXnAUr t
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L.. Highman, B., Alford. X. C., and Weaver., T. L., Ihid.,76, 247 (1948). (4) La Mer, V. K., Umberger, J. Q., Sinclair, D., and Buchwalter, F. E., “Optical Characterization of Any -4erosol in the Laboratory or Field. Production of Aerosols from Powdered Solid Materials,” Office of Scientific Research and Development, OSRD 4904,Div. 10-601.2-MI (Oct. 31, 1944). (5) Magill, P. L., Am. I n d . Hug. Assoc. Qiaart., 11, 5 5 (RIarch 1950). Figure 10. Schematic Arrangement of Transmissometer (6) Magill, P. L., 1x0. ENG. CHEM., 41, 2476 (1949). (7) h‘lann, H. B., and Xhitney D. R., Ann. Math. Statistics, 18, 50 (1947). fore, these synthetic smogs were analyzed by the same methods (8) Silver., S. D., J . Lab. Clin. Med., 31, 1153 (1946). used on the Los Angeles atmosphere, in order t o make certain (9) Sinclair, David, “Handbook on Aerosols,” Washington, D. C., that the desired concentrations had been obtained. Analytical Atomic Energy Commission, Chap. 5, 6, 1950. methods used in this laboratory have been described by Magill ( 5 ) . (10) Stanford Research Institute, “The Smog Problem in Los Angeles eounty,” Second Interim Report, 1949. RESULTS O B T A I N E D W I T H S Y N T H E T I C SMOGS (11) Wyckoff, R. W. G., “Electron Microscopy. Technique and Applications,” New York, Interscience Publishers, 1945. An atmosphere containing the following contaminants was RECEIVED January 3, 1951. investigated in this laboratory and can be used as an example of experiments with synthetic smogs: AIR INTAKE
(3) Dautrebande,
P.p.m. by Weight
Oil (40% Diesel, 60% used crankcase oil) Wac1
so2 SO.
OZODE!
Formaldehyde Formic acid Acrolein Carbon bjack Nitric acid
t
.
These substances were known to be present in intense Los Angeles smog in about the concentrations listed. A test panel of twenty-five persons, picked a t random from the Stanford Research Institute staff, was used t o compare the eye, nose, and throat irritation which the panel indicated was produced by this atmosphere with t h a t which the panel indicated was produced by fresh air. Persons were introduced into the chamber in pairs and they recorded their reactions each minute for 10 minutes by a system of check marks on a record sheet. The results were treated statistically, using the rank test of Mann and Whitney (7‘). A thorough discussion of the results obtained is out of place in this paper. However, it may be mentioned that significant eye and nose irritation was produced by this atmosphere, but insignificant throat irritation. The irritative properties of this formula were not eliminated by the removal of any single contaminant. The results were an important part of the study of Los Angeles smog, since probably the most disagreeable property of this smog is the eye irritation it produces. ACKNOWLEDGMENT
Acknowledgment is due Joan Gates for the preparation of the electron micrographs. It is also due the Smoke and Fumes committee of the Western Oil and Gas Association, whose financial support and encouragement for the study of smog in Los Angeles made this work possible. LlTERATURE CITED
(1) Dautrebande, L., Alford, W. C., and Highman, B.,
Tosicol., 30, 108 (1948).
J. Ind. Hug.
(2) Dautrebande, L., and Capps, Roquell, Arch. intern. pharmacodvnamie, 82, 505 (1950).
~~
COURTESY
U S DEPARTMENT
OF AGRICULTURE
Centrally Controlled Device Keeps Aircraft Free from Hitch- Hiking I nsects A flexible manifold system consists essentially of a n aerosol supply t a n k connected by copper tubin w i t h electrically controlled valves contalning aerosol nozzles. #he nozzles are operated by a slngle swltch w i t h automatic t i m e control, and t h e insecticidal aerosol completely fllls t h e plane. A counting devlce registers each t i m e t h e aerosol is released.
