METHOD OF AND APPARATUS FOR FLUIDIZING PARTICULATE SUBSTANCE
United States Patent 3731393
A fluidized bed of solid particles, for instance of spray dried milk, is formed along a funnel wall by conveying the particles supplied to the upper funnel edge from above along the wall or by a body of gas which is introduced through a plurality of slits in the wall preferably in a direction substantially tangential to the circular cross section thereof to form a gas vortex. The gas vortex is preferably so distributed that its velocity is in direct proportion to the diameter of the conical wall.
US Patent References:
METHOD AND APPARATUS FOR CARRYING OUT PHYSICAL AND CHEMICAL REACTIONS
Geiger et al. - February 1970 - 3494047
Plastic material dryer
Maus - August 1967 - 3337969
Concentrator
Stebbins - September 1924 - 1509915
Process of and apparatus for drying liquids
Douthitt - October 1931 - 1829477
Apparatus for the physical and/or chemical treatment of granular solids or fine dusts
Schoppe - July 1964 - 3140862
METHOD AND APPARATUS FOR CARRYING OUT PHYSICAL AND CHEMICAL REACTIONS
Geiger et al. - February 1970 - 3494047
Plastic material dryer
Maus - August 1967 - 3337969
Concentrator
Stebbins - September 1924 - 1509915
Process of and apparatus for drying liquids
Douthitt - October 1931 - 1829477
Apparatus for the physical and/or chemical treatment of granular solids or fine dusts
Schoppe - July 1964 - 3140862
Inventors:
Okada, Katsuto (Tokyo, JA)
Kato, Fumio (Kawasaki, JA)
Kato, Fumio (Kawasaki, JA)
Application Number:
05/151363
Publication Date:
05/08/1973
Filing Date:
06/09/1971
View Patent Images:
Export Citation:
Assignee:
Morinaga Nyugyo Kabushiki Kaisha (Tokyo, JA)
Primary Class:
International Classes:
A23C1/00; A23C1/04; B01J2/16; B01J8/44; F26B3/092; F26B17/10; B01J8/24; F26B3/02; F26B17/00; F26B3/08
Field of Search:
34/10,57R,57E,174
US Patent References:
| 3518777 | HEAT EXCHANGE APPARATUS FOR FLUIDIZING PARTICULATE MATERIAL | July 1970 | Kono |
Primary Examiner:
Sprague, Kenneth W.
Claims:
We claim
1. A method of forming a fluidized bed of solid particles on a downwardly tapering funnel wall having a circular cross section of downwardly decreasing diameter, comprising the steps of
2. supplying the solid particles to an upper edge of the funnel wall from above,
3. introducing a body of gas through a plurality of slits in the wall in a direction substantially tangential to the circular cross section thereof to form a whirling stream or vortex of the body of gas along said wall, and
4. distributing the vortex of gas so that the velocity thereof is in direct proportion to the diameter of the wall, the gas vortex fluidizing the solid particles and conveying them downwardly along the wall in a thin annular bed.
5. The method of claim 1, wherein the solid particles are supplied continuously, the particles are conveyed along the wall in circles by the vortex of gas, and are continuously discharged through a coaxial port at the lower end of the funnel.
6. The method of claim 1, wherein the body of gas is cold, dehumidified air whereby the particles are cooled in the fluidized bed.
7. The method with claim 1, wherein the body of gas is introduced through the slits from a sealed chamber surrounding the conical wall, the gas being supplied to said chamber at a velocity of about 5 - 30 m/sec.
8. An apparatus for forming a fluidized bed of solid particles, said apparatus comprising in combination:
9. The apparatus of claim 5, wherein the slits have substantially tangentially extending baffle members for directing the body of gas therethrough and inwardly.
10. The apparatus of claim 5, wherein the casing is conical, extending substantially concentrically about the funnel and forming an annular fluid-tight chamber therewith.
11. The apparatus of claim 5, wherein the gas escape and discharge ports are coaxial with the conical wall, the inlet port being arranged near the periphery of the cover member.
1. A method of forming a fluidized bed of solid particles on a downwardly tapering funnel wall having a circular cross section of downwardly decreasing diameter, comprising the steps of
2. supplying the solid particles to an upper edge of the funnel wall from above,
3. introducing a body of gas through a plurality of slits in the wall in a direction substantially tangential to the circular cross section thereof to form a whirling stream or vortex of the body of gas along said wall, and
4. distributing the vortex of gas so that the velocity thereof is in direct proportion to the diameter of the wall, the gas vortex fluidizing the solid particles and conveying them downwardly along the wall in a thin annular bed.
5. The method of claim 1, wherein the solid particles are supplied continuously, the particles are conveyed along the wall in circles by the vortex of gas, and are continuously discharged through a coaxial port at the lower end of the funnel.
6. The method of claim 1, wherein the body of gas is cold, dehumidified air whereby the particles are cooled in the fluidized bed.
7. The method with claim 1, wherein the body of gas is introduced through the slits from a sealed chamber surrounding the conical wall, the gas being supplied to said chamber at a velocity of about 5 - 30 m/sec.
8. An apparatus for forming a fluidized bed of solid particles, said apparatus comprising in combination:
9. The apparatus of claim 5, wherein the slits have substantially tangentially extending baffle members for directing the body of gas therethrough and inwardly.
10. The apparatus of claim 5, wherein the casing is conical, extending substantially concentrically about the funnel and forming an annular fluid-tight chamber therewith.
11. The apparatus of claim 5, wherein the gas escape and discharge ports are coaxial with the conical wall, the inlet port being arranged near the periphery of the cover member.
Description:
The present invention relates to improvements in a method and apparatus for forming and conveying a fluidized bed of solid particles, wherein a body of gas, for instance dehumdified, cold air, is mixed with a mass of solid particles to impart fluidity to the mass so that it flows like a body of fluid.
Fluidized beds of solid particles have been variously used, for instance to transport such particles, to desiccate particles containing moisture, to impart a desired temperature thereto, i.e., to heat or cool them, to subject them to chemical processing, and the like.
It is a primary object of this invention to provide an improved method and apparatus for fluidizing solid particles, including powders.
It is another object of the invention to provide a continuous processing system wherein solid particles may be supplied at one end, fluidized for uniform processing, and continuously discharged at the other end.
It is still a further object to provide a fluidizing apparatus operating without any mechanical agitating means but capable of forming and conveying a uniform fluidized bed of solid particles for producing a product of uniform quality.
It is yet another object to provide an efficient cooling and drying system for solid particles in a fluidized bed containing only relatively small amounts of resident particles.
The above and other objects and advantages are accomplished in accordance with the present invention by forming a fluidized bed of solid particles on a downwardly tapering conical funnel wall having a circular cross section of downwardly decreasing diameter. The solid particles are supplied to the funnel from above near or at its upper edge, and a body of gas is introduced through a plurality of slits in the funnel wall in such a manner that a vortex of gas circles or whirls along the wall to mix with the solid particles and thereby to produce a fluidized bed thereof and move the particles downwardly along the wall in a thin swirling bed. The gas may be introduced into the funnel at a uniform velocity but, for most effective fluidizing of the particles, the body of gas is introduced through the slits in a direction substantially tangential to the circular cross section of the funnel wall, and the vortex of gas is distributed so that the velocity thereof is in direct proportion to the diameter of the conical wall, thus forming a uniform fluidized bed of substantially constant thickness along the funnel wall. In this case, the body of gas is introduced through the slits from a sealed chamber or wind box surrounding the funnel, the gas being blown into the chamber to form a whirling stream or vortex of gas in the chamber whence it passes thruogh the slits into admixture with the solid particles. The resultant fluidized bed does not bubble in the manner of a boiling liquid as it moves along the funnel wall, as has been the case in some conventional fluidizing methods. The fluidized bed moves in the form of a thin layer of particles along the wall.
The apparatus of this invention comprises a funnel with a downwardly tapering conical wall having a circular cross section of downwardly decreasing diameter. The funnel has an open top, a discharge port at the lower end for discharging the solid particles, and a plurality of slits, preferably arranged in adjacent circular rows along the wall. A cover member covers the open funnel top and has an inlet port for supplying the solid particles to the conical wall at or near its upper edge, and a gas escape port. A casing surrounds the funnel and forms a chamber therewith, and a duct is connected to the casing for supplying a body of gas to the chamber, the body of gas being passed inwardly from the chamber through the slits into the funnel.
The above and other features of the invention will become more apparent from the following detailed description of a now preferred embodiment thereof, taken in conjunction with the accompanying drawing wherein:
FIG. 1 is a longitudinal sectional view of an apparatus for fluidizing solid particles according to the invention;
FIG. 2 is a cross sectional view along line 2--2 of FIG. 1;
FIG. 3a is a developed view of the slitted conical wall of the apparatus;
FIG. 3b is an enlarged perspective view of a preferred embodiment of the slits in the conical wall;
FIGS. 3c and 3d are respective sectional views taken along lines c--c and d--d of FIG. 3b; and
FIG. 4 is a view similar to that of FIG. 3a, showing a slitted blank welded together from a plurality of pre-cut segmental portions for forming the funnel wall.
If desired and advantageously, the illustrated fluidizing apparatus may form the bottom portion of a drying tower for spray drying milk so that the solid milk particles are received in the funnel of the apparatus from the spray drying tower, for instance in a tower of the type described and claimed in our copending application Ser. No. 820,902, filed May 1, 1969.
Referring now to the drawing and first to FIG. 1, there is shown a funnel with a downwardly tapering conical wall 1 having a circular cross section of downwardly decreasing diameter. The funnel has an open top which is covered by cover member 3 having an inlet port 10 adjacent the periphery thereof for supplying solid particles to the funnel close to the upper edge thereof, and a gas escape or exhaust port 9 coaxial with the vertical axis of the funnel. The lower end of the funnel has a coaxial discharge port 11 for discharging the solid particles. A casing 2 surrounds the funnel and forms a sealed wind box chamber therewith, the upper end of the casing having a flange mating with annular flanges of the cover member 3 and the funnel wall 1, which flanges are fastened together by bolts 6 cooperating with nuts 7, gaskets 4 and 5 being interposed between the flanges to provide a fluid-tight seal. A flexible conduit 14 is clamped to the lower end of the casing 2 and the discharge port 11 by clamping rings 15 and 16 to provide another fluid-tight seal.
While it would be possible to connect inlet gas duct 8 to the casing 2 at any point of the casing, for instance in a radial direction, the preferred embodiment shown in FIG. 2 illustrates the duct connected to the casing for supplying a body of gas to the sealed wind box chamber in a direction tangential to the circular cross section of the funnel wall 1 at or near the upper, large-diameter end thereof. Gas, such as dehumidified cold air, is blown into the duct 8 from a blower (not shown).
The funnel wall has a plurality of slits 13 over its entire area, the illustrated embodiment showing a plurality of adjacent rows of slits extending circularly around the conical wall 1 in circles of diminishing diameters, the slits being substantially parallel to each other and extending in the direction of the axis of the funnel in their longer dimensions.
As shown in FIG. 3a, the wall 1 may be formed from a semi-circular slitted plate, in which case the angle of the conus is 60°.
As will appear from FIGS. 3b, 3c and 3d, each slit 13 has a substantially tanGentially extending, depressed portion 13b forming a baffle member for directing a body of gas therethrough and inwardly, this depressed baffle member ending at the scoring line 13a. Slits of this shape will prevent powder particles moving in the fluidized bed along the conical wall 1 from falling outwardly through the slits and will efficiently direct the gas inwardly.
As shown in FIG. 4, the slitted wall may also be formed from a plurality of segmental plates 1a which may be fastened together, preferably by welding, to form the conical wall 1'.
The method of the present invention and the operation of the above-described apparatus will now be described with particular reference to FIGS. 1 and 2, wherein the solid arrows indicate the flow of the solid particles and the arrows in broken line indicate the flow of the whirling gas stream or vortex.
For instance, if spray dried milk particles coming from a drying tower according to our above-mentioned copending application are to be cooled while conveyed in a fluidized bed, the apparatus of FIG. 1 forms the bottom of such a tower to receive the spray dried milk particle through inlet port 10, cold and dehumidified air being blown in through inlet duct 8. As shown, the air swirls in the annular wind box chamber 12 in a vortex and is passed inwardly from this sealed chamber through the multiplicity of slits 13 tangentially along the funnel wall. The air thus admitted moves in a whirling movement from the wall towards its axis and is subsequently exhausted through gas escapt port 9 in cover 3 upwardly away from the apparatus.
As the solid powder particles fall by gravity through the inlet port 10 onto the upper edge of the conical wall 1, they will be gradually moved downwardly along the wall in circles in a whirling motion as they are mixed with the air vortex. A relatively thin fluidized bed of milk particles is thus formed along the wall and constantly moves towards discharge port 11 while the particles are cooled by the whirling air. During this movement of the particles, the force of gravity exerted thereupon is substantially balanced by an upwardly directed counterforce due to the centrifugal force caused by the whirling motion and an ascending component of the air current coming from the lower slits in the conical wall 1. This balance of forces eliminates variations in the residence time of the particles on the wall 1.
Due to the centrifugal force exerted thereupon, the gas vortex in the wind box is under higher pressure at the top, where the funnel diameter is largest and the distance of the gas vortex from the funnel axis is, therefore, largest, and decreases gradually in direct proportion to the diminishing diameter towards the lower end. In this manner, the amount of air introduced into the funnel decreases from the upper to the lower end thereof in direct proportion to the decreasing pressure of the gas. This causes the particles in the fluidized bed to swirl about at a correspondingly decreasing speed as they are conveyed in the bed in a thin layer along the funnel wall while being cooled. Since the diameter of the annular fluidized bed of particles decreases in the same manner, the thickness of the fuidized bed remains constant along the entire length of the funnel wall.
If, on the other hand, the duct is so connected to the casing 2 as to deliver the gas thereinto in a radial direction, rather than tangentially, the air will not whirl but pass into the funnel through the slits at the same pressure along the entire funnel wall. In this case, the fluidized bed will be non-uniform, i.e., thicker at the bottom than on top.
It may be noted that the same effect will be acheived if the casing 2 were cylindrical but such a structure would increase the bulk of the apparatus unnecessarily. Furthermore, since some particles may fall out of the funnel through slits 13, the funnel shape of the casing will enable it to gather such stray particles and discharge them at the lower end.
In the manufacture of powdered milk in the apparatus described and claimed in our above-mentioned copending application, the spray dried powder tends to agglomerate at the bottom of the drying tower if the dried particles have not been cooled down below the solidifying temperature of the fat contained therein. Therefore, a cooling zone must be provided in the lower part of the drying tower to cool the milk powder down to room temperature. By using the present apparatus as the cooling zone, the height of this portion of the tower may be considerably reduced and the drying tower efficiency correspondingly improved.
For instance, the height and diameter of the cylindrical or main part of the drying tower may be 15 m and 5 m, respectively, while the total height of the funnel-shaped bottom part thereof, built in accordance with the present invention, may be 4.3 m, with the height of the funnel 1 being 2 m. With such an apparatus, we have sprayed milk of 50 percent solids concentration from a single spray nozzle under a pressure of 190 kg/sq. cm. and at a rate of 5000 k/h. The sprayed particles of milk were entrained by hot air introduced into the drying tower at a temperature of 150° C. and at a velocity of 1 - 2 m/sec. to be dried while descending within the cylindrical part of the tower, whereupon they were admitted into the cooling zone through inlet port 10. They were cooled in the funnel while being conveyed therethrough in an annular fluidized bed along the wall of the funnel which had a total of about 5,000 slits each having a width of about 1 mm and a length of about 20 mm. Dehumidified air of a temperature below 20°C. was blown into inlet duct 8 at a velocity of about 5 - 30 m/sec. so that the milk particles swirled along the funnel wall in a fluidized bed having a substantially uniform thickness of about 2 to 3 mm.
The described fluidizing apparatus is highly efficient and may be readily cleaned because it has not auxiliary mechanical devices for scraping the milk particles off the funnel wall. Furthermore, since the fluidized bed is thin, all particles will be uniformly cooled and there will be no rejected portion of the dried particles, due to scorching during the drying operation. The particles in the fluidized bed will freely move along the funnel wall while being cooled, without the need for agitators, vibrators or the like.
While the invention has been particularly described and illustrated in conjunction with a now preferred embodiment, it will be apparent that variations and modifications may occur to those skilled in the art without departing from the spirit and scope thereof, as defined in the appended claims.
Fluidized beds of solid particles have been variously used, for instance to transport such particles, to desiccate particles containing moisture, to impart a desired temperature thereto, i.e., to heat or cool them, to subject them to chemical processing, and the like.
It is a primary object of this invention to provide an improved method and apparatus for fluidizing solid particles, including powders.
It is another object of the invention to provide a continuous processing system wherein solid particles may be supplied at one end, fluidized for uniform processing, and continuously discharged at the other end.
It is still a further object to provide a fluidizing apparatus operating without any mechanical agitating means but capable of forming and conveying a uniform fluidized bed of solid particles for producing a product of uniform quality.
It is yet another object to provide an efficient cooling and drying system for solid particles in a fluidized bed containing only relatively small amounts of resident particles.
The above and other objects and advantages are accomplished in accordance with the present invention by forming a fluidized bed of solid particles on a downwardly tapering conical funnel wall having a circular cross section of downwardly decreasing diameter. The solid particles are supplied to the funnel from above near or at its upper edge, and a body of gas is introduced through a plurality of slits in the funnel wall in such a manner that a vortex of gas circles or whirls along the wall to mix with the solid particles and thereby to produce a fluidized bed thereof and move the particles downwardly along the wall in a thin swirling bed. The gas may be introduced into the funnel at a uniform velocity but, for most effective fluidizing of the particles, the body of gas is introduced through the slits in a direction substantially tangential to the circular cross section of the funnel wall, and the vortex of gas is distributed so that the velocity thereof is in direct proportion to the diameter of the conical wall, thus forming a uniform fluidized bed of substantially constant thickness along the funnel wall. In this case, the body of gas is introduced through the slits from a sealed chamber or wind box surrounding the funnel, the gas being blown into the chamber to form a whirling stream or vortex of gas in the chamber whence it passes thruogh the slits into admixture with the solid particles. The resultant fluidized bed does not bubble in the manner of a boiling liquid as it moves along the funnel wall, as has been the case in some conventional fluidizing methods. The fluidized bed moves in the form of a thin layer of particles along the wall.
The apparatus of this invention comprises a funnel with a downwardly tapering conical wall having a circular cross section of downwardly decreasing diameter. The funnel has an open top, a discharge port at the lower end for discharging the solid particles, and a plurality of slits, preferably arranged in adjacent circular rows along the wall. A cover member covers the open funnel top and has an inlet port for supplying the solid particles to the conical wall at or near its upper edge, and a gas escape port. A casing surrounds the funnel and forms a chamber therewith, and a duct is connected to the casing for supplying a body of gas to the chamber, the body of gas being passed inwardly from the chamber through the slits into the funnel.
The above and other features of the invention will become more apparent from the following detailed description of a now preferred embodiment thereof, taken in conjunction with the accompanying drawing wherein:
FIG. 1 is a longitudinal sectional view of an apparatus for fluidizing solid particles according to the invention;
FIG. 2 is a cross sectional view along line 2--2 of FIG. 1;
FIG. 3a is a developed view of the slitted conical wall of the apparatus;
FIG. 3b is an enlarged perspective view of a preferred embodiment of the slits in the conical wall;
FIGS. 3c and 3d are respective sectional views taken along lines c--c and d--d of FIG. 3b; and
FIG. 4 is a view similar to that of FIG. 3a, showing a slitted blank welded together from a plurality of pre-cut segmental portions for forming the funnel wall.
If desired and advantageously, the illustrated fluidizing apparatus may form the bottom portion of a drying tower for spray drying milk so that the solid milk particles are received in the funnel of the apparatus from the spray drying tower, for instance in a tower of the type described and claimed in our copending application Ser. No. 820,902, filed May 1, 1969.
Referring now to the drawing and first to FIG. 1, there is shown a funnel with a downwardly tapering conical wall 1 having a circular cross section of downwardly decreasing diameter. The funnel has an open top which is covered by cover member 3 having an inlet port 10 adjacent the periphery thereof for supplying solid particles to the funnel close to the upper edge thereof, and a gas escape or exhaust port 9 coaxial with the vertical axis of the funnel. The lower end of the funnel has a coaxial discharge port 11 for discharging the solid particles. A casing 2 surrounds the funnel and forms a sealed wind box chamber therewith, the upper end of the casing having a flange mating with annular flanges of the cover member 3 and the funnel wall 1, which flanges are fastened together by bolts 6 cooperating with nuts 7, gaskets 4 and 5 being interposed between the flanges to provide a fluid-tight seal. A flexible conduit 14 is clamped to the lower end of the casing 2 and the discharge port 11 by clamping rings 15 and 16 to provide another fluid-tight seal.
While it would be possible to connect inlet gas duct 8 to the casing 2 at any point of the casing, for instance in a radial direction, the preferred embodiment shown in FIG. 2 illustrates the duct connected to the casing for supplying a body of gas to the sealed wind box chamber in a direction tangential to the circular cross section of the funnel wall 1 at or near the upper, large-diameter end thereof. Gas, such as dehumidified cold air, is blown into the duct 8 from a blower (not shown).
The funnel wall has a plurality of slits 13 over its entire area, the illustrated embodiment showing a plurality of adjacent rows of slits extending circularly around the conical wall 1 in circles of diminishing diameters, the slits being substantially parallel to each other and extending in the direction of the axis of the funnel in their longer dimensions.
As shown in FIG. 3a, the wall 1 may be formed from a semi-circular slitted plate, in which case the angle of the conus is 60°.
As will appear from FIGS. 3b, 3c and 3d, each slit 13 has a substantially tanGentially extending, depressed portion 13b forming a baffle member for directing a body of gas therethrough and inwardly, this depressed baffle member ending at the scoring line 13a. Slits of this shape will prevent powder particles moving in the fluidized bed along the conical wall 1 from falling outwardly through the slits and will efficiently direct the gas inwardly.
As shown in FIG. 4, the slitted wall may also be formed from a plurality of segmental plates 1a which may be fastened together, preferably by welding, to form the conical wall 1'.
The method of the present invention and the operation of the above-described apparatus will now be described with particular reference to FIGS. 1 and 2, wherein the solid arrows indicate the flow of the solid particles and the arrows in broken line indicate the flow of the whirling gas stream or vortex.
For instance, if spray dried milk particles coming from a drying tower according to our above-mentioned copending application are to be cooled while conveyed in a fluidized bed, the apparatus of FIG. 1 forms the bottom of such a tower to receive the spray dried milk particle through inlet port 10, cold and dehumidified air being blown in through inlet duct 8. As shown, the air swirls in the annular wind box chamber 12 in a vortex and is passed inwardly from this sealed chamber through the multiplicity of slits 13 tangentially along the funnel wall. The air thus admitted moves in a whirling movement from the wall towards its axis and is subsequently exhausted through gas escapt port 9 in cover 3 upwardly away from the apparatus.
As the solid powder particles fall by gravity through the inlet port 10 onto the upper edge of the conical wall 1, they will be gradually moved downwardly along the wall in circles in a whirling motion as they are mixed with the air vortex. A relatively thin fluidized bed of milk particles is thus formed along the wall and constantly moves towards discharge port 11 while the particles are cooled by the whirling air. During this movement of the particles, the force of gravity exerted thereupon is substantially balanced by an upwardly directed counterforce due to the centrifugal force caused by the whirling motion and an ascending component of the air current coming from the lower slits in the conical wall 1. This balance of forces eliminates variations in the residence time of the particles on the wall 1.
Due to the centrifugal force exerted thereupon, the gas vortex in the wind box is under higher pressure at the top, where the funnel diameter is largest and the distance of the gas vortex from the funnel axis is, therefore, largest, and decreases gradually in direct proportion to the diminishing diameter towards the lower end. In this manner, the amount of air introduced into the funnel decreases from the upper to the lower end thereof in direct proportion to the decreasing pressure of the gas. This causes the particles in the fluidized bed to swirl about at a correspondingly decreasing speed as they are conveyed in the bed in a thin layer along the funnel wall while being cooled. Since the diameter of the annular fluidized bed of particles decreases in the same manner, the thickness of the fuidized bed remains constant along the entire length of the funnel wall.
If, on the other hand, the duct is so connected to the casing 2 as to deliver the gas thereinto in a radial direction, rather than tangentially, the air will not whirl but pass into the funnel through the slits at the same pressure along the entire funnel wall. In this case, the fluidized bed will be non-uniform, i.e., thicker at the bottom than on top.
It may be noted that the same effect will be acheived if the casing 2 were cylindrical but such a structure would increase the bulk of the apparatus unnecessarily. Furthermore, since some particles may fall out of the funnel through slits 13, the funnel shape of the casing will enable it to gather such stray particles and discharge them at the lower end.
In the manufacture of powdered milk in the apparatus described and claimed in our above-mentioned copending application, the spray dried powder tends to agglomerate at the bottom of the drying tower if the dried particles have not been cooled down below the solidifying temperature of the fat contained therein. Therefore, a cooling zone must be provided in the lower part of the drying tower to cool the milk powder down to room temperature. By using the present apparatus as the cooling zone, the height of this portion of the tower may be considerably reduced and the drying tower efficiency correspondingly improved.
For instance, the height and diameter of the cylindrical or main part of the drying tower may be 15 m and 5 m, respectively, while the total height of the funnel-shaped bottom part thereof, built in accordance with the present invention, may be 4.3 m, with the height of the funnel 1 being 2 m. With such an apparatus, we have sprayed milk of 50 percent solids concentration from a single spray nozzle under a pressure of 190 kg/sq. cm. and at a rate of 5000 k/h. The sprayed particles of milk were entrained by hot air introduced into the drying tower at a temperature of 150° C. and at a velocity of 1 - 2 m/sec. to be dried while descending within the cylindrical part of the tower, whereupon they were admitted into the cooling zone through inlet port 10. They were cooled in the funnel while being conveyed therethrough in an annular fluidized bed along the wall of the funnel which had a total of about 5,000 slits each having a width of about 1 mm and a length of about 20 mm. Dehumidified air of a temperature below 20°C. was blown into inlet duct 8 at a velocity of about 5 - 30 m/sec. so that the milk particles swirled along the funnel wall in a fluidized bed having a substantially uniform thickness of about 2 to 3 mm.
The described fluidizing apparatus is highly efficient and may be readily cleaned because it has not auxiliary mechanical devices for scraping the milk particles off the funnel wall. Furthermore, since the fluidized bed is thin, all particles will be uniformly cooled and there will be no rejected portion of the dried particles, due to scorching during the drying operation. The particles in the fluidized bed will freely move along the funnel wall while being cooled, without the need for agitators, vibrators or the like.
While the invention has been particularly described and illustrated in conjunction with a now preferred embodiment, it will be apparent that variations and modifications may occur to those skilled in the art without departing from the spirit and scope thereof, as defined in the appended claims.