US3175299A - Method of drying sugar crystals with acoustic energy and a gas - Google Patents

Description

March 30, 1965 R. M. G. BOUCHER METHOD OF DRYING SUGAR CRYSTALS WITH ACOUSTIC ENERGY AND A GAS Filed Aug. '2 1961 5 Sheets-Sheet 1 NODE(MOTION) LOOP(PRESSURE SUGAR PRESSURE FIG. 3

2 INVENTOR RAYMOND MARCEL GUT BOUCHER J In ATTORNEYS 3 March 30, 1965 R BOUCHER 3,175,299

METHOD OF DRYING SUGAR CRYSTALS WITH ACOUSTIC ENERGY AND A GAS Filed Aug. 2. 1961 3 Sheets-Sheet 2 [MA/ -i [PRESSURE/MOTION a NODE(MOTION) 3 LOOHPRESSURE) PROCESSING TIME (MINUTES) A IL! B o a z" E Z a g z DJ 0 Z z .2 E 2 *5 m r: g 1:: IL!

o m 05 l 5 I0 50 I00 FREQUENCY (KILOCYCLES PER SECOND) INVENTOR RAYMOND MARCEL GUT BOUCHER ATTO R N EYS March 30, 1965 R. M. G. BOUCHER METHOD OF DRYING SUGAR CRYSTALS WITH ACOUSTIC ENERGY AND A GAS Filed Aug. 2. 1961 3 Sheets-Sheet 3 FIG. 9

INVENTOR RAYMOND MARCEL GUT BOUCHER WM WMBMOW ATTO R N EYS form of surface cavitation.

United States Patent 3,175,299 METHOD OF DRYING SUGAR CRYSTALS WITH ACOUSTIC ENERGY AND A GAS Raymond Marcel Gut Boucher, Metuchen, N..I., assignor to American Sugar Company, Jersey City, NJ, a corporation of New Jersey Filed Aug. 2, 1961, Ser. No. 128,748 1 Claim. (Cl. 34-4) The present invention relates to the drying and conditioning of wet granular crystalline material and of such material in solution, particularly sugar. Although not restricted to sugar, the invention will hereinafter be described as applied to the drying of that substance. The invention provides a method and apparatus whereby the drying of sugar is accelerated and is readily carried to a point at which the sugar so dried will not cake when stored even over prolonged periods of time of the order of months.

In accordance with the invention, sugar is dried by effecting relative movement between the sugar to be dried and a gas, customarily air, which is simultaneously subjected to high intensity acoustic vibrations.

In order to dry moist sugar crystals, and also to produce dry crystalline sugar from a solution of sugar in water, it is desirable for maximum drying rate to match, in the droplets of sugar solution or in the moist partially crystallized particles, the reduction in the concentration of sugar present in solution in the water which results from crystallization against the increase in concentration due to evaporation of the water. In other Words, it is desirable that the concentration of sugar in solution remain substantially constant until the last of the water is removed. When drying is effected solely by passing a stream of gas in contact with the sugar and particularly when the gas is heated or the speed of the gas flow is raised in order to accelerate the evaporation, the result is the formation of a layer of highly supersaturated sugar solution at the surface of the droplets or wet sugar crystals within which layer the rate of crystallization is retarded. The consequence is a reduction rather than an increase in the drying rate. In accordance with the present invention, the formation of such excessively supersaturated solutions is minimized, and the effects of evaporation and crystallization are kept in balance by the application of acoustic energy to the gas stream. This acoustic energy promotes the creation of crystallization nuclei. In addition, it creates a high turbulence in the air, microscopcally considered, in the immediate vicinity of the particles or droplets being dried. This is beneficial in view .of the known fact that the rate of evaporation at the boundary between a liquid and a gas is proportional to the difference between the saturation vapor pressure in the gas at the temperature of and in intimate contact with the liquid and the actual vapor pressure, inversely proportional to the gas pressure, and proportional to a coefficient strongly dependent on and varying directly with the gas turbulence in the immediate vicinity of the gas-liquid boundary. The eflfect of the acoustic energy on the sugar particles or droplets may be likened to a The invention combines this application of acoustic energy with a streaming of the gas relative to the sugar in order to provide the strong macro-turbulence which is necessary to carry away the water expelled by the acoustic energy from the surface of the sugar particles or droplets.

The invention will now be further described with reference to the accompanying drawings in which:

FIG. 1 shows an axial section through a sugar drying apparatus combining acoustic energy with gas streaming according to the invention.

3,175,299 Patented Mar. 30, 1965 FIG. 2 is a cross-section of the apparatus ofFIG. 1 taken on the line 22 of FIG. 1.

FIGS. 3 and 4 are diagrams illustrating preferred relationships between the frequency of the acoustic energy employed and the dimensions of the chamber within which, according to the invention, the sugar and air are subjected to acoustic vibrations.

FIGS. 5 and 6 are graphs useful in describing the invention.

FIG. 7 is a cross-section of an alternative form of apparatus for carrying out the invention.

FIG. 8 is a schematic cross-sectional view of still another form of apparatus for carrying out the invention.

FIG. 9 is a representation of stillanother form of apparatus for carrying out the invention especially adapted for the drying of sugar solutions.

In FIG. 1 a mass of partially dried sugar is shown at 2 in a storage container 4 from which it may be withdrawn in controlled fashion through a valve 6 to the interior of a dryer generally indicated at 8. The dryer includes an inner cylinder 1% supported on rollers as indicated at 12. for rotation about its axis. A motor 14 and gearing 16 may be provided for this purpose. The axis of the cylinder lit may be inclined to the horizontal to promote the flow of sugar from the inlet at the right in FIG. 1 to the outlet end at the left. The interior of the cylinder 1% is provided with buckets or vanes as shown at 18 in FIG. 2, whereby upon rotation of the cylinder, the sugar is continuously lifted and caused to fall across the interior of the cylinder.

An inlet conduit 20 is connected at its right end to a source of air under pressure, not shown. Advantageously this air source heats the air to a controlled temperature which may be of the order of 140 F. and also holds its relative humidity to an acceptably low value dependent upon economic considerations. A relative humidity of 20% for the air on entry into the dryer of FIG. 1 has proven satisfactory. The warmed and dried air supplied at the conduit 20 preferably passes directly into the cylinder 15 in which case the cylinder is imperforate, or it may flow into an annular space 22 between the cylinder 16 and an outer casing 24,. provided in the latter case that the cylinder 10 is foraminous to permit the air to pass into the cylinder through the walls thereof for engagement with the sugar to be dried therein.

.An outlet for the air is shown at 26, from which it may be exhausted if desired, either supplementarilyto or in place of the provision of a supra-atmospheric pressure at the inlet conduit 20.

While FIG. 1 shows the air and sugar as progressing through the cylinder It in the same direction, it is understood that they may move in opposite directions, the air entering at the end from which the dried-sugar emerges.

In one end of the substantially cylindrical space defined by the cylinder 1t and by the end walls 28 and 30 of the dryer, there are mounted one or more sources 32 of acoustic energy. A single source 32 is shown in FIG. 1, but advantageously two or more such sources may beprovided. The source 32 may take the form of a siren, airjet whistle or other acoustic generator. The generator 32 should develop within the chamber a sound intensity level of the order of 0.001 watt/cm. conventionally referred to as decibels above a Zero decibel reference level of 10- watts/cm. Very good results have been achieved with sound intensity levels of decibels (0.01 watt/ cm?) or above.

The frequency of the sound energy, or of a preponderant part of it, and the long dimension of the drying chamher are advantageously chosen together to make that dimension substantially an integral number of half Wave lengths for sound of that frequency. In this manner,

still another form of sugar drying apparatus.

drying of sugar solutions in liquid form. chamber is schematically indicated at 60 into which there standing wave systems may be set up within the drying chamber as schematically illustrated in FIGS. 3 and 4. FIG. 3 shows a drying chamber, the length of which is one-half of a wave length for the sound emitted by the generator 32 of that figure, and FIG. 4 shows a drying chamber having a length of four half wave lengths for the sound developed in the generator 32 of that figure. Under these conditions, the sugar will have some tendency to collect at the nodes of motion in the standing wave pattern, as shown in those figures.

Referring now to FIGS. 5 and 6, FIG. 5 provides data comparing the drying of sugar in accordance with the invention with a drying process employing solely a stream of heated air. The curves A and B in FIG. 5 represent respectively the percentage of moisture removed as a function of drying time for equal masses of sugar treated in the same drying apparatus with and without the employment of sonic energy. The sonic energy employed, in the case of the sample of which the drying is represented by curve A, had a frequency of 9 kilocycles per second and an average intensity level of 140 decibels. Curves A and B are coincident for the first five minutes of processing time. Thereafter, out to a total processing time of ten minutes, the sample dried with acoustic energy reached a residual water content of 0.3 milligram of water per gram of dry sugar. Both samples were subjected to the same flow of air heated to 40 C. and curve B shows that after a processing time of ten minutes, the sample treated without acoustic energy possessed a residual water content of 1.5 milligrams of water per gram of dry sugar.

FIG. 6 is a graph of the relative drying efiiciency, at constant intensity level, of sound energy of different frequencies. FIG. 6 indicates that the optimum or most efiicient range of frequencies is that between about 500 cycles per second and 30,000 cycles per second.

FIG. 7 is a cross-section of another apparatus according to the invention in which plural sound generators 40 are carried by an axial support within a rotating cylinder 42 and are directed to emit their sound substantially radially. The sound from the generators 40 is directed radially outward toward the walls of the cylinder 42 and thus contacts the sugar as it is tumbled by means of vanes 44 in the course of its progress lengthwise of the cylinder.

FIG. 8 illustrates the application of the invention to FIG. 8 shows in cross-section a tunnel dryer having a vibrating screen 50 surmounted by a closed tunnel 52. The sugar is advanced from one end of the tunnel to the other by the vibration of the screen 50 and in the process it is subjected to the action of a heated gas steam and to sonic energy from a plurality of generator 54 arranged in the roof of the tunnel. The gas stream is introduced at one end of the tunnel and is exhausted at a stack 56.

FIG. 9 illustrates the application of the invention to the In FIG. 9, a

extends a pair of coaxial tubes 62 and 64. The sugar solution to be dried is forced, from a source not shown, into the chamber through the inner tube 62, from which it emerges at a series of lateral openings 66 inside the chamber. A source of compressed air, also not shown, connects with a side tube 68 from which it flows through the annular space between tubes 62 and 64. The outer tube 64 terminates short of the openings 66 and the annular stream of air impinges against a hollow resonator 70. The result is atomization of the sugar solution at the location of high intensity sound waves. A baffle 72 cooperates with the resonator 70 in generating the sound waves.

these high levels of supersaturation, the rate of crystallization is retarded rather than accelerated, and the droplets of sugar solution become covered with a layer approaching a disordinate glass structure which is often called the amorphous layer."

In accordance with the invention in the drying of sprayed solutions of crystalline materials, a high rate of evaporation is maintained, without the formation of the undesirable amorphous layer, by irradiation of the droplet interfaces with powerful acoustic waves. These cause the formation of a great number of crystallization nuclei. The apparatus of FIG. 9 effects both atomization of the sugar solution and the generation of a strong acoustic field. The sugar solution, leaving the tube 62 at openings 66 between the bafile 72 and resonator 70, is atomized in a region of powerful acoustic oscillations which help both to disperse the droplets of sugar solution and to nucleate the interfaces thereof.

The invention has been practiced with acoustic power levels of the order of two watts per pound of wet sugar, to achieve drying times of the order of fifteen minutes. The acoustic power level and drying times are of course inversely related.

There will now be given a number of examples of the practice of the invention:

Example I .Twclvc pounds of wet granular sugar were introduced into a tumbler-type dryer of the general character illustrated in FIG. 1. Air at C. and relative humidity of 27% was introduced into the dryer at a rate of thirty-five cubic feet per minute. A sound generator of the stern-jet type was employed to develop within the dryer a sound predominantly of 9,000 cycles per second frequency with an acoustic output between 25 and watts. The sound intensity level inside the drum was held at approximately 139 decibels for five minutes and was increased to 143 decibels during the next succeeding five minutes. After the first five minutes, the moisture content of the sample dropped from an initial level of 0.016 gram of water per gram of dry sugar to 0.0006 gram of water per gram of dry sugar. After ten minutes, the moisture content had fallen below 0.0004 gram of water per gram of dry sugar. The product, after ten minutes of treatment, was free-flowing and did not exhibit any tendency to cake after three months of storage in a closed container. An identical sugar sample treated in the same apparatus under identical conditions, except that the sound generator was not employed, showed at the end of ten minutes of processing a moisture content higher than 0.0015 gram of water per gram of dry sugar and caked after a three months storage period under the same storage conditions.

Example II.Twelve pounds of wet granular sugar were introduced into a dryer similar to that described in Example I from which air introduced at room temperature was exhausted at an average flow rate of forty-seven cubic feet per minute. The air temperature inside the drum was 24 C. and its relative humidity was 58%. An air-operated membrane horn-type sound generator developed sonic energy predominantly of 1,150 cycles per second frequency inside the drum. The intensity level was held at 137 decibels for ten minutes and was increased to 143 decibels during the second ten minutes, the total acoustic output rate being in the neighborhood of 25 watts. After fifteen minutes of processing in this manner, the moisture content of the sugar dropped from an initial value of 0.013 gram of water per gram of dry sugar to 0.0038 gram and, after twenty minutes, to a value below 0.0004 gram of water per gram of dry sugar. The sugar obtained after twenty minutes of processing was freeflowing and did not exhibit any tendency to cake after three months storage in a closed container. An identical sugar sample treated under identical conditions but without sound energy exhibited after processing a moisture content of 0.0016 gram of water per gram of dry sugar am zes and caked after three months of storage under the same conditions.

Example III.Twelve pounds of wet granular sugar were introduced into a dryer similar to that described in Example I from which air introduced at room temperature was exhausted at an average flow rate of forty-four cubic feet per minute. The relative humidity of the air inside the drum was fairly constant at a level in the vicinity of 35%. Two sound generators were employed; one of these was an air-driven membrane horn (Falcon Type U4 with a polystyrene membrane). The sound generated by this generator had a frequency of 1,200 cycles per second and with an air flow therethrough of three cubic feet per minute it produced inside the drum a sound intensity level of 153 decibels; the frequency being related to the dimensions of the drum to provide a resonant standing wave system. The second sound generator was a stem-jet whistle. It generated sound having a frequency of 11,800 cycles per second, and with an air flow through the nozzle of seven cubic feet per minute it produced a sound intensity level of 148 to 150 decibels. Both sound generators were used simultaneously. After twenty minutes of irradiation of the sugar with sound from these sources, the moisture content of the sugar had dropped from an initial value of 0.014 gram of water per gram of dry sugar to 0.00042 gram of water per gram of dry sugar. The final sample was free flowing and did not exhibit any tendency to cake after three months of storage in a closed container. An identical sugar sample treated under the same conditions, but without the dual frequency acoustic field, exhibited after a twenty-minute drying time a moisture content of 0.0007 gram of water per gram of dry sugar.

The figures on moisture content in these examples were measured by the oven technique and the dried sugar content in the second example employing sound energy was checked by the pyridine technique.

The process of the invention can be practiced with apparatus other than that shown herein and described,

including, without limitation, turbo dryers and louvered 4 rotary dryers. Moreover the process of the invention may be carried out even though substances (whether crystalline or not) other than the sugar or the other crystalline sub- 0 stance to be dried are present in the material as subjected to the drying process of the invention.

I claim:

A method of drying moist sugar crystals which comprises introducing a stream of moist sugar crystals into one end of an elongated, substantially horizontally disposed drying zone, generating acoustic energy within said drying zone at the other end thereof, the stream of sugar introduced into said idrying zone being exposed to the influence of said acoustic energy, said acoustic energy being generated at a sound intensity level of at least about decibels and at frequencies in the range 50030,000 cycles per second and the length of said drying zone being such that it is substantially equal to an integral number of half wave lengths of the sound of the generated acoustic energy, contacting said stream of moist sugar crystals for a period of time upwards of about 10 minutes within said drying zone with a stream of air having a relatively low relative humidity and at a temperature in the range from about substantially room temperature to upwards of about F. to eifect substantially complete removal of the moisture from said sugar crystals and withdrawing from the other end of said drying zone a product stream of substantially dry sugar crystals capable of being stored for a prolonged period of time without caking.

References Cited by the Examiner UNITED STATES PATENTS 1,958,702 5/34 Johnston et al. 1,983,434 12/34 Black et al. 2,344,754 3/44 Vang. 2,447,362 8/48 Pessell 23301 X 2,576,297 11/51 Horseley et al 159-48 2,589,310 3/52 Tournier 23-301 2,701,422 2/55 Stewart et al 34-109 2,709,306 5/55 Magn-usson et al. 34-33 2,900,179 8/59 Kaufmann 34-164 X FOREIGN PATENTS 282,480 12/27 Great Britain. 334,318 6/36 Italy.

NORMAN YUDKOFF, Primary Examiner. BENJAMIN BENDETT, Examiner.

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