Industrial Sonic Agglomeration and Collection,Systems HAROLD W. DANSER AND ERNEST P. NEUMANN Ultrasonic Corporation, Cambridge, Mass. Physical variables involved in the design of industrial sonic agglomeration and collection systems for the recovery of fine particles in gases are discussed. Included are intensity and frequency of the sound field and exposure time of the aerosol therein. Examples are presented of installations used industrially t o collect sulfuric acid fog, as well as soda ash from the recovery boiler of a paper mill. Finally, evaluation is made of the characteristics of those particular types of industrial fine particle collection problems which have been found to lend themselves to sonic agglomeration and collection treatment, and illustration is given of the many process industries having collection problems for which sonic collectors may be used.
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Figure 1. Model U4 Sonic Generator for Laboratory, Pilot Plant, or Small Commercial Installations
T
HE technical literature on the subject of the sonic agglomera-
tion of aerosols dates back to the 1860’s, and an abundant supply of information on the subject has followed since. Bergmann ( I ) , for example, is replete with references that treat theory in some detail and also offer data on small scale experiments. T o summarize superficially this published information (,%-+f), the following explanation of the subject may be presented. Sound waves cause the small particles in an aerosol to vibrate, thereby increasing the normal collision rate of the particles. The particles will on collision adhere to each other. This adhesion may be visualized by considering the small size of the particles involved-less than 10 microns. For these small particles the surface forces tending to hold them together are large compared to those that would disunite them. By a series of collisions and subsequent adhesions, small particles can be agglomerated. The published literature describes, in general, laboratory scale investigations processing aerosols in small flow rates (10 to 100 cubic feet per minute) by utilizing electronic and air jet sound generators. These generators are characterized by an acoustic power output of the order of 10 to 100 watts. If laboratory data are extrapolated to industrial scale collection problems of 10,000 to 100,000 cubic feet per minute, a sound generator with an output of the order of 10 to 50 kilowatts is needed. No means has yet been devised that will permit economical congtruction of large scale electronic type sound generators and the jet type suf-
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fers from its low efficiency. However, the recent development of an efficient siren-type sound generator (6) makes possible an output of this order of magnitude. One such generator appears in Figure 1, and the method of construction is shown ip Figure 2. These generators consist of a rotor, 1 (Figure 2), facing a stator, 2, with precision matched ports, 3, around the periphery of each. A compressed gas is passed through the ports of the rotor and then of the stator. As the rotor turns, alternately opening and closing the ports of the stator, the gas flows out intermittently through the stator ports. An intense sound wave is thereby created at the interface, 4, and is directed by an acoustic horn, 5, from the generator. Intensity of the sound wave is governed by the pressure of the compressed gas and frequency by the speed a t which the rotor turns. T o govern the rotor speed, a variable speed motor may be used in conjunction with an autotransformer. About 225 cubic feet per minute of gas, compressed to 8 pounds per square inch gage, are required to operate this gener-
Figure 2.
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Cross Section of Model U4 Sonic Generator
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 11,
minute): the intensity of the sound field in which he aerosol is treated; the exposure time in this field; and the frequency of the sound.
INTENSITY OF SOUND FIELD The intensity or sound pressure level in an acoustic field is usually measured in decibels (db.) above watt per square em. The threshold value causing noticeable agglomeration in a reasonable time is approximately 140 db. However, for effective industrial practice, upwards of 150 db. appear to be required. High intensities are achieved by utilizing a siren-type sound generator convei ting the energy of a compressed gas into sound. Effective conversion of this energy to sound depends on details of the generator design. The application of the sound depend? on the geometry of the treating chamber and ducts attached thereto. I n practice, the design of the treating chamber reflects largely a compromise between acoustic, aerodynamic, structural. and economic requirements. In a properly designed installation, from 40 to 60% of the energy in the compressed gas may presently be converted into sound.
n
Figure 3.
Sonic Collection System Installation
ator; this gas flow may be obtained from a compressor requiring about a 10-hp. drive motor. Up to 3000 cubic feet per minute of dust-laden gas may be processed by one such generator when installed in the manner shown in Figure 3. One installation such as this may serve advantageously as a pilot plant, or several may be used in parallel to handle full commercial scale gas cleaning problems ranging up to about 15,000 cubic feet per minute. Larger models of sound generators, of the type shown in Figure 4, have also been developed; each has a gas treating capacity of up to 50,000 cubio feet per minute. For somewhat larger gas flows, several of these generators may be used in parallel. It is not intended to discuss here the factors underlying the available theories relative to aerosol coagulation, nor are new theories or data t o be presented. Rather, the object of this paper is to discuss briefly the present status of the development of industrial scale sonic collection systems. Accordingly, the principal variables entering into the application of high intensity sound waves to the coagulation of aerosols are reviewed and a general evaluation is made of the characteristics of those particular large scale aerosol collection problems which may be solved by currently available sonic equipment. Research and development experience indicates that there aye three principal variables to be considered in the agglomeration of aerosols in large gas volumes (5,000 to 50,000 cubic feet per
EXPOSURE TIME Since the process of agglomeration depends on collision between the particles involved, it follows that as exposure time is increased a larger number of collisions will ensue. However, there appears to be an upper limit beyond which an increase in exposure time will not significantly improve the degree of agglomeration. FOP a given aerosol and field intensity, this limit will obtain when agglomeration has proceeded to the point 1% here the particles become few in number compared to the volume they occupy. I n general, given a sufficiently intense acoustic field of proper frequency, exposure time may be of the order of only a few seconds. FREQUENCY OF THE SOUND For work on an aerosol having a given particle size, there will be a range of favorable frequency of operation. Consider the mechanics of vibration of a particle in an aerosol. The forces that tend to overcome the inertia of the particle arise from two principal sources-namely, shear or drag between the particle and the gas or pressure forces. Shear exists when there is a relative velocity between the particle and the gas. Pressure forces exist when the particle is in a pressure gradient and there will be different pressures on its opposite faces causing a resultant force. Given a sufficiently intense acoustic field, if the frequency of the sound is high and the inertia of the particle is large-that is, its mass relatively large-the shear and pressure forces will not be large enough to set the particle in motion during the short times in which they act. However, if the frequency of the sound is low or the mass small, the particle is readily set in motion and will follow very nearly the motion of the gas. Consider next an aerosol containing particles of a given size distribution. There will be a frequency at which great relative motion is caused between the different size particles resulting in a maximum number of collisions. Experience indicates this best frequency range may extend down to the order of 1.0 kilocycle for particles of 10 microns in size, and somewhat higher for particles as small as 0.01 micron. T o arrive at the best frequency for given conditions, one must also keep in mind that the generator efficiency as well as the treating chamber effectiveness are also functions of frequency; consequently, a compromise is made and selection is obtained through development experience.
EXAMPLES OF KNDUSTRIAL INSTALLATION
Figure 4.
Large Commercial Sonic Generator
Any particular instaIIation must reflect a balance of the dictates of the three variables. An example of such balance may be talien from an experimental installation which Ultrasonic Corporation has made in conjunction with Chemical Construction Corporation
November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Here again a sound pressure field of high intensity is produced, to which the fine particles are exposed for less than 4 seconds. I n such a paper mill installation, the intrinsic value of the agglomerated material collected by the cyclones may return the cost of the investment in the sonic collector in from 12 to 20 months. Significantly, engineering studies reveal that in many cases the installed cost of a sonic collection system is appreciably less than the cost of electrostatic collectors. The installed cost of industrial scale sonic collection systems ranges downward to below $1.00 per cubic foot of installed capacity. Over-all operating cost, including power plus operating and maintenance labor, is likewise low, ranging downward to below 0.7 mill per thousand cubic feet of gas treated.
Figure 5 .
Sulfuric Acid Fog Sonic Collector
of New York, N. Y. This installation has been made for the collection of sulfuric acid fog from exit stack gases of a contact type sulfuric acid manufacturing plant. This installation appears in Figure 5; Figure 6 is a flow plan of the installation. The exit stack gases leaving the sulfuric acid absorption tower enter a humidification chamber, where sulfur trioxide, passing through water sprays, is converted to sulfuric acid mist; the particles of the mist are believed to range from 0.5 to 5 microns in diameter. The gas stream carrying this mist measures 24,000 cubic feet per minute at 125' F. It is introduced tangentially into the bottom of a cylindrical agglomeration tower 8 feet in diameter and 35 feet tall where it is treated and subsequently passed through cyclones to the atmosphere. When the sound generator is not operating, the acid fog particles pass vertically through the tower, entirely through the cyclones, and into the atmosphere. With the sound generator operating, however, a sound pressure field of over 150 db. is produced throughout the tower. Recent initial operations of this installation showed that the gas needs to be exposed to the acoustic field for less than 4 seconds in order for the fine particles to be agglomerated to such size that over 90% by weight of the acid may be removed from the gas stream, about half of the collection occurring on the inner walls of the tower, the remainder within the cyclones. The exit gases leaving the sonic collector then contained between 2 and 3 mg. of 100% sulfuric acid per cubic foot. A second example of an industrial sonic collection system, this time for recovery of fine solid particles, may be taken from an installation which Ultrasonic Corporation has made in conjunction with Combustion Engineering-Superheater, Inc., of New York, N. Y. This installation is for the sonic collection of soda ash fume from the stack gases leaving a recovery boiler in a paper mill. The gases enter an agglomeration tower similar in size to that shown in Figure 5; subsequently the gases pass through cyclone separators for recovery of the agglomerates, the cleaned gas finally passing to the atmosphere. Gas flow rate measuresabout 50,000 cubic feet per minute a t 200" F. The fume particles leaving the furnace average less than 1 micron in diameter.
FIELDS FOR USE OF SONIC COLLECTORS I n addition to the agglomeration and recovery of sulfuric acid fog and soda ash fume, as mentioned in the above examples, it appears that sonic collection techniques may be applied to recover from a gas numerous other types of fine liquid and solid particles. An analysis may now be made of the characteristics of particular industrial fine particle collection problems which have been found to lend themselves to sonic agglomeration and collection treatment, either on a wet or on a completely dry collection basis. I n evaluating the proper fields of use for sonic collectors, it is apparent first that this new tool is applicable to the recovery from a gas of particles smaller in size than those readily collected by conventional cyclone separators (less than 10 microns). Such small particles are so light that they tend to remain airborne and to pass directly through a cyclone. It is on such smaller particles, particularly below 1 micron, that sonic treatment may effectively be used to so agglomerate them and to permit their cyclonic separation. I n fact, there appears to be no lower limit of particle size in a visible fume or dust, below which sonic agglomeration becomes ineffective. Secondly, temperature, within reasonable limits, does not impose restrictions on the use of sonic agglomeration. Thus gas
DISCHARB STACK
SIIEKER
CYCLONE IEASSEPARATORS LINED
(Ola IN
FSAR OF STACK
NOT SI{OVN IN DIAGRAM)
1
Figure 6.
Flow Diagram of Acid Fog Collector
INDUSTRIAL AND ENGINEERING CHEMISTRY
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temperatures may range from below 0’ F. to substantially above 1000 F. without impairing the efficiency of sonic agglomeration. At extreme temperatures, of course, special consideration must be given to the materials of which the collection system is constructed. Thirdly, sonic agglomeration of fine particles appears to be independent of the electrical characteristics of the particles. Thus, inert particles and those characterized as insulators may effectively be collected, whereas such properties may exclude their collection by electrostatic means. A fourth consideration lies in the absence within a sonic collection system of any fire hazard. Thus flammable liquid or solid materials as well as combustible gases may be treated without the fire or explosion hazard existing in some other collection systems in which electrical discharges, arcs, or short circuits may occur. The gas passed through the sound generator may be air, steam, any other inert gas, or the process gas itself which is being treated; the latter may be tapped off downstream from t,he sonic collector and at that point is compressed for operating the sound generator. A further consideration is that there must be a sufficient number of particles in each cubic foot of gas so that, as the particles are vibrated in the acoustic field, there may result the proper number of collisions between the particles. If the particles are too widely separated in the gas, an adequate degree of agglomeration may not be achieved. The required particle weight per cubic foot will vary with the average particle size. As a rough approximation 1 grain per cubic foot is sufficient when the partiO
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cle sizes range from 1 to 10 microns, but the grain loading figure may decrease somewhat for aerosols in which the average particle is smaller than 1 micron, as a larger number of collision targets is available. When the normal grain loading is too small, various techniques may be employed to increase it-for example, water 01 other liquid may be sprayed (or condensed) into the aerosol or a second aerosol may be mingled with the first. In some cases the grain loading may be effectively increased by cooling the gas t o diminish its volume. I n conclusion, there is a broad range of recovery problems where sonic collection may advantageously be used. This area appears to include the recovery of many valuable materials throughout the smelting, petroleum, chemical, steel, carbon black, cement, lime and rock products, sulfur, paper, and other process industries. Moreover, many industries are faced with serious nuisance abatement problems arising from their random discharge of obnoxious fumes, dusts, and smog. Sonic collection techniquw offer a n wonomical solution to many such problems. LITERATURE CITED (1) Bergmann, L., “Ultrasonics,” New York, John Wiley & Sons, 1944. (2) Brandt, O.,Freund, H., and Hiedemann, E., KoZloid Z., 77, 103
(1936); Trans. Faraday Soe., 32, 1101 (1936). (3) Hiedemann, E.,KoZZoid-Z., 34, 494 (1933). (4) St. Clair, H.W., Spendlove, M. C., and Potter, E. V., U.8.Bur. Mines, Rept. Invest. 4218 (March 1948). (5) Ultrasonic Corporation, patents applied for (1945--49). RECEIVEDMarch 7, 1949.
Design of Exhaust Ventilation for Soli Materials Handling FUNDAMENTAL CONSIDERATIONS R. T. PRING, J. F. KNUDSEN,
AND
RICHARD DENNIS
Kennecott Copper Corporation, Garfield, Utah Solid materials handling includes dumping, storing, discharging, feeding, conveying, elevating, screening, mixing, loading, and filling of solids; in these operations no change of state of the material occurs. Dust is the atmospheric contaminant and is dispersed into the workroom primarily by air currents set up by the movement of the solid particles. Exhaust capacity requirements to eliminate dust dispersion have heretofore been determined experimentally or by the use of arbitrary standards having little relationship to actual induced air volumes. Laboratory tests under controlled conditions have shown that the air volume, Qa, entrained by falling droplets of water may be expressed by QA
=
K
U‘x (V, - VI) N
where A, = the cross-sectional area of the pattern, V, = the maximum velocity attained by the falling droplets, V I = the velocity intercept a t Q A = 0, N = the number of particles in a unit system of single successive drops falling in line, A, = the projected area per particle, L = the length, the width of the pattern cross section. Under and W the conditions of the tests, K = 3.21 X 10-2 and V I = 642
feet per minute. Practical application of this expression requires redefinition of A, and N and the evaluation of factors for particle shape and surface roughness, relative enclosure, and crowding of particles, all under field conditions. Details of proper hooding and enclosure of material transfer points, to reduce the amount of solids carried into the ventilating system, are illustrated.
MOSPIIERIC contamination within industrial plants is the result of the dispersion of solid or liquid particulate matter (dusts, fumes, mists), gases, or vapors from an infinite variety of operations or machines. Such dispersion is ordinarily effected by air currents set up by convection, cross drafts, the motion of machines, or the movement of materials. The control of these air currents thus becomes the important factor in the prevention of air contamination and is readily accomplished by properly designed local exhaust ventilation. I n this paper, discussion is confined to exhaust ventilation of dusty processes or, more specifically, of solid materials-handling operations. No special problems in duct design or equipment selection are involved, whatever the source of dust; however, certain fundamental principles, peculiar to materials-handling
