US2576297A - Sonic spray drying - Google Patents

Description

Nov. 21, 1951 C. B. HQRSLEY Em 2,516,291

` soNIc SPRAY nvmc Filed July 29, 1947 2 SHEETS- 51mm 2 INVENToRs -Patented Nov. 27, 1951 UNITED STATES PATENT OFFICE Caperton B. Horsley,

Stamford, Conn., and Harold W. Danser, J r.,Belmont, Mass., asslgnors to Ultrasonic Corporation, Cambridge, Mass., a corporation of Massachusetts Application July 29, 1947, Serial No. 764,384

gas in cyclone separators, while the smallest ones .pass off into the atmosphere. Inasmuch as the drying action of the stream of hot gas is much more efficient when the treated material is in finely divided form, spray drying has become an' important and widely adopted technique.

Our invention is based upon the discovery that sound waves can advantageously be used to improve the efficiency and economy of spray drying, and, in some cases, to improve the quality of the nal product. l

The rate at which a wet particle loses moisture depends to an important degree upon its diameter, its speed relative to the surrounding air, and the temperature and relative humidity of the air. The most important object of our invention is to improve spray drying by employing sound waves to increase the relative speed between the hot gas and the wet particles.

Although very fine particles dry more readily than larger ones. it often happens that the end product is disadvantageouslyv affected by the presence ofl fines. For example, soap powders containing very small soap particles are undesirable because the fines cause sneezing. In many instances the optimum product is one in which the particle sizes vary only through4 a small range. Another object of our invention is to improve spray drying by producing dried particles of substantially uniform size, through the agglomerating action of sound waves on the wet particles.

Furthermore, in conventional spray driers significant amounts of the substance treated are often carried off as fines too small for separation by cyclones or comparable apparatus. This loss constitutes not only a waste but a public nuisance in areas surrounding the drier. Another object of our invention is to provide a simple, efficient method for eliminating these and other losses in spray driers.

One problem ordinarily encountered in spray drying is the fouling of the drier due to material depositingy on the walls of the tanks. The extent of deposition depends somewhat on the nature of the material being dried. In' many cases, however, it has been found necessary periodically vto close down the drier in order to remove these deposits from the walls. .Furthermore, these deposits frequently constitute a i'lre hazard. Another object of our invention is to improve the efficiency and safety of spray drier operation by preventing fouling.

The most important feature of the invention resides in applying sound waves at frequencies vand intensities suihcient to develop greatly increased velocities between the droplets or wet particles and the hot gas, thus, in effect, flowing a much greater volume of drying gas over each particle.

Obviously, the method here discussed is applicable to the evaporation of various liquids into gases of many different chemical compositions. For convenience we shall refer generally to water evaporating into hot dry air. Those skilled in the art will readily appreciate the factors involved in adapting the method for use with other "materials In carrying out .the method of our invention we spray the material to be dried into the top of a cylindrical tower or tank and simultaneously pass through the tank a stream of hot dry air. The result is that an aerosol is formed in the tower, including wet particles or droplets of various sizes depending upon the nature of the substance to be dried and upon the design of the spray nozzles. Through the medium of a powerful sound Wave generator, there is maintained inside the tower a mean sound wave intensity of preferably 150 decibels or db above zero reference level, or higher, where zero reference level is understood by` convention to be 0.0002 dynes/cm.2 root mean square pressure. We have found that a modified form of the variable frequency sound wave generator disclosed in the copending application of C. B. Horsley et al.. Ser. No. 722,240 led January 15, 1947, and now 'Patent No. 2,535,680, provides a satisfactory source of sound at frequencies between 1 and 100 kc., which range appears to be the most suitable for carrying out the method of our invention.

The generator develops an intense sound field within the drying tank, and the sound waves act to develop large velocities between the drying gas and the wet particles. Consequently the evaporation rate is significantly increased. It follows that in spray driers where the humidity of the gas leaving the drier is significantly lower than but where the final product has been satisfactorily same flow of exit water vapor. Conversely if the the drying gas under normal conditions. This time factor largely determines the height of the drying tower. The evaporation of the moisture is so speeded up in our method that the time factor is greatly reduced. Therefore, we can employ a much smaller drying tower than has been formerly used to dry comparable quantities of material. That is to say, we are able to increase the dry product output per cubic foot of drying tank.

Furthermore advantage may be taken of the agglomerating effect of sound waves acting in an aerosol. For example where the size of the dried particles is ordinarily undesirably small, we may select a frequency such that small wet particles agglomerate before dryingresulting in cohesive dried particles of satisfactorily large diameter. The nal product will contain particles whose range of size is therefore reduced. Where the problem relates more to separation of the dried particles from the gas, we may select a frequency at which the dried particles will at least temporarilyl agglomerate to sizes large enough to` facilitate separation in conventional cyclone separators or the like.

Finally, one action ofthe sound waves, as will be more fully discussed, is to strip deposited material from the walls of the drier, thus enabling continuous operation of the equipment and preventing clogging or fouling. Here the properties of the treated material may affect the degree to which sonic stripping becomes effective.

The selection of the amount of acoustic energy to be developed in the drier by the generator is a straightforward matter, since both evaporation and agglomeration effects vary with the intensity above an effective threshold which varies somewhat with di'erent conditions but lies in the neighborhood of 150 db mean sound pressure above zero reference level.

The selection of an'operating frequency is much more complicated matter. It must first be decided whether agglomeration of either the wet or dry particles is desired. If no agglomeration is required, the frequency will be selected from the high end of the range. At relatively high frequencies the gas will move in response to sound wave forces but the particles will not be significantly aected. The result is that the gas will flow by the particles and the relative velocity between the particles and the gas is at a maximum. As the frequency is reduced there is a. range' in which the gas and the finer, and drier, particles are moved in response to sound wave forces but the larger particles are relatively stationary. At still lower frequencies the differential velocities between particles of differing mass-to-drag ratios become maximum. Consequently the agglomerating effects of the sound waves depend primarily on the distribution of particle sizes in the aerosol in the area in question. This distribution varies as the drying proceeds, with the result that a frequency may be selected at which maximum agglomeration of undried particles will take place or by choosing a different frequency the bulk of the agglomeration will be among dried particles.

In many cases, if wet particles are agglomerated -the solid nuclei will be more cohesive when dried than if the agglomeration operates on particles already dried, although wet agglomeration may slow down the drying process. These factors serve to determine the choice of agglomerating conditions mentioned above.

The method of determining the optimum frequency range for each installation will be further evident from a reading of our copending application Ser. No. 696,628, led September 12. 1946, and now Patent No. 2,535,679, for Process of Agglomerating Aerosols. It will appear that the proper frequency for agglomerating wet particles will be lower than that for the agglomeration of dried particles, since in general the wet particles will be heavier than the dry particles. It must be pointed out, however, that there can be no sharp demarcationbetween the agglomeration of wet or dry particles. That is to say, a frequency selected to agglomerate wet particles will inevitably bring about some agglomeration among dried particles and vice versa.

As an example, if the particle size range of wet particles extends from 1 to 30 microns, it will i be found that at 3.5 kc. considerable agglomeration will take place. It is the differential velocity between the air and the particles which controls evaporation vbut the differential velocity between particles of dierent sizes which probably controls agglomeration. Consequently it is possible simultaneously to agglomerate the wet particles and also to evaporate the moisture from them. In that case the overall size of the particles is progressively diminished dueto evaporation but the solid nuclei are continuously increased because of agglomeration. Obviously, the

production of relatively dry agglomerates facilitates the subsequent separation of the solids from the gas. Indeed, our method may be practiced to render possible the recovery of materials often lost in the form of particles so fine as to be unresponsive to the action of cyclone separators or the like.

The practice of our method is not confined to the drying of materials hitherto spray dried. but

offers considerable scope for improving the qualities of products not previously believed well adapted for spray drying. It. should be emphasized that the practice of our novel method is by no .means limited to operations involving equipment of thetype shown in the drawing. Those skilled in the art will readily appreciate how presently existing systems may be adapted to include means for developing the required sound field.

In order to illustrate the practice of our meth- Aod, we have selected a preferred installation plan 5 ing to the'demands of the material and the vrelation of the drying operation to other process operations and equipment. As shown, the drying and agglomerating steps are carried out in a large cylindrical steel tank I provided with a cover member 20 carrying an elongated housing 30 extending from the center of the cover 20 radially oil.' to one side and provided with a depression or well I6 in the` bottom of which is seta nozzle I2 connected to a conduit I4 of reduced diameter, through which the wet material is forced under pressure to the nozzle. Inasmuch as the type of nozzle used will vary considerably with the nature of the substance to be dried, we have shown it only diagrammatically; for example, it may be arranged to jet the material radially to form in effect a circular horizontal spray indicated by the broken lines I3 and radiating from the centrally disposed nozzle `outwardly toward the wall of the tank I0. Surrounding the well I6 is a frusto-conical baille I8 inclined bodily inwardly and terminating at its upper end in an outwardly directed smoothly curved ange I9 to form with the wall of thewell I6 a restricted annular nozzle affording access from the housing 30 to the interior of the tank II).` Supported within the housing 30 by a suitable base 32 is a heat exchanger 34, shown schematically in broken lines.

Connected to the housing 30 is a blower 36 arranged to draw air through a lter 38 and deliver it under pressure to the heat exchanger 34. The blower and heat exchanger may be so operated that, for example, dry air at 300 F. will flow at the rate of 20,000 cubic feet perminute between the baille I8 and the well I6 to enter the tank I0 as an annular blast surrounding the upper chamber a turbine wheel 43 and in the lower chamber a hollowvrotor 40 with a series of slots at itsperiphery and justaposed to stator ports 39 formed in the casing 31 and communieating with an annular exponential horn 4I. A conduit 46 supplies air under pressure to both of the chambers in the casing 31. The jets used to drive the turbine wheel 43 have been omitted for sake of clarity in the drawing but are of conventional character. The sound generator assembly is contained within a housing 44 suitably secured to the Walls of the tank I0 and has frustoconical form at its upper end as well as a conical hood 42 secured adjacent the top of the housing 44 and spaced therefrom to provide an exhaust passage for the air used to drive the turbine wheel 43; an exhaust pipe 45 leads from the upper chamber of the casing 31 into the top of the housing 44. As shown in Fig. 1 the conduit 46 is connected to a suitable compressor 48 disposed outside the tank I0. The sound generator may conveniently comprise a modiiled version of the generator disclosed in the copending application of C. B. Horsley et al., Ser. No. '722.240, filed January 15, 1947 for Method and Apparatus for Generating Sound Waves.

The bottomof the tank or chamber I0 comprises a wall 52 inclined to merge with the upper end of -a wallot a second cylindrical tank 54 smaller in diameter than the tank I0, and in turn merging into a hopper 56 which may be exhausted by a valve 58 disposedat its apex. A large steel casing or housing 50, substantially pentagonal in vertical cross section, is disposed partly within the tank I0 and"partly within the second tank 54,l its .dimensions being such that a restricted annular passage I is formed between'the housing 50 and the top of the tank 54. The bottom of the casing 50 is'open; the top of it is secured to the bottom of the generator 40. Entering through the side of the tank 54 is a large duct 62 which terminates in a vertical section 60 disposed within the casing 50. At its other end the duct 62 is connected to a cyclone separator 54 which in turn discharges through a duct 66 into a-second cyclone separator B8. A stack leads from the second cyclone 58 to the atmosphere and, as is usual, solids are withdrawn from the bottom of each cyclone separator. When it is desired to dry material, the blower 36 is operated to force air through the entire system thus far described, at any desired rate. The air is heated in the heat exchanger 34. The materia to be dried is sprayed from the nozzle I2 at the top of the tank I0, as previously described,y and the sound generator is operated to develop an intense sound iield within the tank I0. I

In the zone immediately surrounding the nozzle I2 the eiect of the sound waves is in any case to increase the normal rate of evaporation of the moisture from the wet particles in the spray and also to cause agglomeration of the smaller wet particles, if desired, if the correct frequency is employed. In the area lying interlmediate the nozzle and the bottom 52 of the tank. I0, the sound waves continue to promote evaporation and optionally to cause agglomeration of dried particles. There is, of course, no sharply dened border separating the zones of wet and dry agglomeration. It is obvious that the particles reaching the lower portion of the 5 tank I0 will be drier, generally speaking, than those momentarily at the top of the tank which have entered it relatively recently. The agglomerating eiect obtained within the tank will be suilicient to build up larger particles which 50 fall in to the funnel-shaped portion 56 and can be withdrawn through the valve 58 as completely dried, solid material. Other particles will be carried into the housing and through the conduit to the cyclones 64 and 68. There the solid particles are separated from the air by the cyclone separators operating in conventional fashion.

It should be noted that the shape of the air inlet passage formed by the baille I8 and the well I6 provides for smooth flow of air into the tank I0 but minimizes `the dimensions of the passages through which sound can escape. It will be seen that the orifice formed for the vintroduction of the hot air is a nozzle in which the velocity of the entering air increases to maximum at the narrowest point with consequent drop in pressure. As the nozzle walls diverge the pressure rises and the velocity falls. Hence the air is introduced into the tank through a A70 minimum opening but in a smooth, non-turbulent manner so that a relatively small over-all pres.

sure drop is required, while access of the sound waves to the exterior is diminished as Iar as possible. The same comments apply to the channel found between the members 50 and 52..

7, Y' In operating conventional driers some of the Viirielydivided material will deposit on the walls until large masses are thus collected, clogging and fouling the drier. We have observed that in many suitably intense sound waves act to prevent excessive accumulation of such deposits. It would seem fthat the sound waves develop pressure diiferentials in the interstices of the deposit and, 'in eect, blast the particles from the wall. In the past the accumulation of such deposits 'has proved a great handicap; the wet solids pack on the walls in masses until the drier is completely choked and useless. Then follows the task of removing the deposits before drying can be resumed.

The apparatus shown may often be operated indefinitely without having to be shut down for cleaning. Thedrying of the material is accomplished more enlciently than has heretofore been possible. Finally, the operation may be not only more economical from the standpoint of power expended, but also because the flnes hitherto carried -off into the atmosphere in the stream of at least 150 db above 0.0002 dynes/cm.2 root mean square pressure and at a frequency of about 3.5

kc., whereby the sound waves not only promote evaporation of the moisture but also form dry REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,980,171 Amy Nov. 13, 1934 2,297,726 Stephanoinl Oct. 6,1942 2,361,940 Hall Nov. 7, 1944 2,392,866 Stephanoi Jan. 15, 1946 2,407,462 Whitely Sept. 10, 1946 2,413,420 Stephanoii.' Dec. 31, 1946 2,448,372 Horsley Aug. 31, 1948 2,456,706 Horsley Dec. 21, 1948 2,512,743 Hansell June 27, 1950 FOREIGN PATENTS Number Country Date 110,343 Sweden Apr. 1l, 1944 Certificate of Correction Patent No. 2,576,297 November 27, 1951 CAPERTUN B. HORSLEY ET AL.

It is hereby certied that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 4, line 44, after relatively insert large;

[SEAL] THOMAS F. MURPHY,

Assistant ommssz'oner of Patents.

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