US3106492A - Apparatus for coating discrete solids - Google Patents

Description

1963 1.. H. MACDONALD ETAL 3,106,492

APPARATUS FOR comma DISCRETE soups Filed Sept. 1, 1961 mm/L United States Patent Office 3,106,492 Patented Oct. 8, 1963 APPARATUS FOR COATING DISCRETE SOLIDS Lee H. Macdonald, Paw Paw Township, Van Buren County, and Charles R. Hine, Portage Township, Kalamazoo County, Mich., assignors to The Upjohn Company, Kalamazoo, Mich., a corporation of Delaware Filed Sept. 1, 1961, Ser. No. 135,660 6 Claims. (Cl. 118-62) This invention relates to an apparatus for coating discrete solids.

The invention is conveniently embodied in an apparatus comprising broadly a vertically disposed lower duct member across the upper portion of which is mounted a rotatable screen for supporting the discrete solids to be coated, a vertically disposed upper duct member mounted above the said screen, and a spray head placed eccentrically above and directed toward the said screen. one duct member and exhausted through the other as a means for accelerating the drying of the coating thus applied. In the operation of this apparatus, the horizontally rotating and tumbling mass of discrete solids is alternately, in the course of each revolution, brought within the spraying zone and the drying zone, a completely enveloping coating being built up through successive stages of coating deposit and at least partial drying.

The coating of discrete solids is of particular interest in the drug, food (especially candy), pesticide, fertilizer, detergent, and the like areas, as well as in coating rocket fuel particles. Conventional pan coating and dip coating techniques have long been used for applying such coatings, but these methods are characterized as timeconsuming and require many units to keep pace with modern machines which produce the discrete solids, such as tablets. In recent years considerable attention has been given to coating methods involving the introduction of the coating material as a spray into a coating chamber containing the solid particles suspended in a moving air stream. These particles are coated as they pass through the spray zone, the particular methods for solids introduction and recovery varying widely. In the application of such techniques to areas of production where product appearance is extremely important, as in the drug industry, it has been found that attrition of the shaped particles during the coating process is often intolera'bly high. This is manifest chiefly in surface imperfections of various types occasioned by the high r velocity impact to which the particles are subjected. Moreover, the fluidized stream containing solids inevitably causes some particles to pass in close proximity to the spray nozzle and hence to receive a localized excess of coating material. While thin coats can be built up by increasing the number of passes in a fluidized stream, an excessive coat is not reversible and becomes exaggerated as the process continues.

The apparatus of this invention represents a novel approach in which the discrete solids are alternately coated and dried on a substantially horizontal rotating screen. each deposit of coating on one revolution of the rotating screen being relatively small in amount and capable of virtually complete drying by an upwardly or downwardly moving gas stream before the coated particles re-enter the spray zone. The tumbling of the solids on the screen results in substantially reduced impact of particle against particle and in addition avoids the damaging blows against stationary surfaces encountered in some of the fluidizing techniques as the fluidized stream recycles through the spray zone. With the present apparatus, the rotating bed of tumbling solids is maintained at a relatively constant distance Gas under pressure is delivered through from the spray nozzle and accordingly does not interfere with the spray pattern. By thus maintaining the desired spray pattern over the randomly oriented solids passing beneath, the uniformity of the coating is more nearly assured.

Referring now to the drawings, it will be seen that FIGURE 1 is a plane view of the screen tray assembly with appurtenant devices.

FIGURE 2 is a cross sectional view of the screen tray assembly taken in the plane II--II of FIGURE 1.

FIGURE 3 is a cross-sectional view of the screen tray assembly taken in the plane IlI--III of FIGURE 1.

FIGURE 4 is a partially exposed cross sectional view of an embodiment of the apparatus described.

FIGURE 5 is a partially exposed isometric view of the apparatus in its preferred embodiment.

FIGURE 6 is a cross sectional view of a conical screen.

FIGURE 1 shows screen tray assembly 1 having disc 2 mounted on circular screen 3 with inner wall 4 and outer wall 5 attached about the circumference-s of disc 2 and screen 3, respectively, to form a bounded annular screen area comprising the screen tray. Shaft 6 is fixedly mounted in disc 2 and serves as central support for screen tray assembly 1. Bafiles 7, joined and supported by cross member 8, are mounted through baflle positioning means 9 on lower duct 10. Leveling arm 11, mounted for vertical movement through leveling arm positioning means 12, is likewise attached to lower duct 10. High pressure gas manifold 13, having a plurality of outlets 14, is mounted below screen tray assembly 1 and secured externally thereof through lower duct 10.

FIGURE 2, representing a cross section taken in plane IIII of FIGURE 1, shows screen tray assembly 1 with disc 2 mounted on screen 3 and having inner wall 4 and outer wall 5 attached about the circumferences of disc 2 and screen 3, respectively, to form a bounded annular screen area. Shaft 6 is fixedly mounted in disc 2 and provides central support for screen tray assembly 1. Visible in this cross sectional view are the two laterally extending bafiies 7 and cross member 8 through which said baffles 7 are mounted for vertical movement on lower duct 10 of FIGURE 1. Shown also in this view is leveling arm positioning means 12 through which leveling arm 11 (not visible) is mounted for vertical movement. High pressure gas manifold 13 is positioned below screen tray assembly 1, and outlets 14 thereof are directed upward toward screen 3.

FIGURE 3 shows the screen tray assembly in a cross section taken in plane II-IIII of FIGURE 1. This view shows particularly leveling arm 11 mounted through leveling arm positioning means 12 on lower duct 10 for vertical movement with respect to screen 3.

In FIGURE 4, screen 3 is mounted within lower duct 10 at its upper end, the duct itself serving as the outer wall of the screen tray assembly described above. The screen is rotated within lower duct 10 through shaft 6 and gear means 15 by motor means 16. Screen 3 receives central support from shaft 6 and peripheral support from screen ring 17 in slidable contact with bearing ring 18 mounted on lower duct 10. The exposed portion of this cross sectional view shows leveling arm 11 attached to leveling arm positioning means 12 for vertical movement above screen 3. Spray nozzle 19 at the end of spray supply line 20 is fixedly mounted in upper duct 21 eccentrically with respect to screen 3. Vertical movement of upper duct 21 is provided by interposed flexible connection 22, whereby said upper duct 21 with spray nozzle 19 and spray supply line 20' attached can be vertically positioned with respect to screen 3. As indicated by the double arrow, the gas stream can move either upwardly or downwardly through screen 3.

FIGURE 5, showing the preferred embodiment of this invention in exposed isometric view, will be described in conjunction with the operation of the apparatus. Discrete solids to be coated, as, for example, tablets, are placed on screen 3 within the screen tray assembly 1. Supporting ring 17 is attached at the base of outer wall of screen tray assembly 1 and rides over bearing ring 18 mounted on rim 2 of upwardly diverging lower duct portion 27. By motor means below, not shown, screen tray assembly 1 is rotated by shaft 6, the solids being tumbled by passage over high pressure gas manifold 13 having a plurality of upwardly directed jets 14, and by collision directly or indirectly with baffies 7. The solids are then leveled by passage beneath vertically adjustable leveling arm 11 and presented for coating at the desired depth on screen 3. Spray nozzle 19 delivers a generally elliptical spray pattern, the principal axis of which coincides approximately with a radius of screen 3. Spray nozzle 19 and spray supply line 20 are adjustable vertically through spray nozzle positioning means 23 in downwardly diverging upper duct portion 24, whereby said spray nozzle 19 can be positioned the desired distance above screen 3. Upper duct 21 with downwardly diverging duct portion 24 is also adjustable vertically through flexible connection 22 in order to serve effectively as a hood. In this embodiment the drying gas stream moves upwardly through lower duct 10 and screen 3 to be exhausted through upper duct 21. In order that the upcoming drying gas stream does not diffuse the spray pattern issuing from spray nozzle 19, bafile plate 25 is secured to the inner surface of rim 26 of upwardly diverging lower duct portion 27 below screen 3 in the path of the spray pattern at screen level. Thus the solids are tumbled and re-oriented, after emerging from the spray pattern, by high pressure gas streams from high pressure gas manifold outlets 14 and subsequently by the combination of baffies 7, and are finally leveled by leveling arm 11 in preparation for re-entering the spray zone.

As used herein, the term discrete solids" means either shaped or irregular surface solids such as beads, pellets, granules, tables, pills, pilules, and the like, which are desired to be coated and which are sufficiently free flowing to permit tumbling on the rotating screen tray assembly.

While a screen has been illustrated in each of the above embodiments as the surface on which the solids are tumbled and rotated, a perforated plate can likewise be utilized and in some cases is advantageous, as Where the solids possess sufiicient weight to render a screen with necessary supporting members unsatisfactory because of interference with elements heretofore described which are positioned below the rotating surface. The term screen as used herein embraces such perforated surfaces. It is apparent that the necessity for and type of supporting structure beneath the screen will depend on the weight placed thereon and on the diameter of the screen. In addition to the preferred combination of supporting ring 17 and bearing ring 18 (FIGURES 4 and 5), a wheel-type structure can be employed in which the hub provides central support and the rim of the wheel, connected with the hub by spokes, is mounted on the lower duct member for peripheral support of the screen or screen tray assembly. Where the screen tray assembly is mounted above the opening of the lower duct member said assembly can be rotated by friction motor means through a friction ring around the outer wall 5 (FIGURE 5) of the assembly. In such a modification the wheel-type supporting structure is particularly advantageous, shaft 6 being journaled in the hub of the wheel for central support with the rim giving peripheral support beneath the outer wall of the screen tray assembly as before. Both the ring combination and wheel structure perform the collateral but important function of preventing the gross escape of up- 'outer wall of the screen tray assembly and the lower duct member or its extended structure.

In the preferred embodiment the screen is mounted horizontally. However, an elevation, less than about 20, contributes to tablet tumbling and can be employed with obvious modifications of tumbling baffles, supporting members and driving means. The term substantially horizontal describes the screen either in the horizontal plane, or as thus elevated.

A variety of screen configurations can be employed as the rotating surface. Preferred is the fiat circular screen illustrated. Alternatively, as shown in FIGURE 6, the screen can comprise a cone having a wide-angle apex (e.g., at least about 135), said conical screen being mounted on its side to give a substantially horizontal segment below the spray nozzle. With the conical screen so positioned, the sloping cone surface imparts a rolling motion to the solids and obviates the necessity for extensive means for solids orientation. Thus the tables are coated as they pass under the spray nozzle directed at the low side of the cone and are tumbled on the high side thereof. Appropriately positioned stationary baffles facilitate distribution of the solids and prevent local accumulation.

While means for orienting the particles are not necessary in some instances, they are generally desirable for most efficient operation. Many types and combinations for accomplishing this purpose will be apparent to those skilled in the art. A satisfactory combination comprises baffles 7 and leveling arm 11 which may be supplemented by the foredescribed high pressure gas manifold 13. Bafiies 7 and leveling arm 11 are stationary during operation and so mounted to produce leveling of the solids before they enter the spray zone. A plurality of leveling arms can likewise be employed, one or more being advantageously positioned at different heights from the screen to promote further movement of the rotating solids. Alternatively or additionally, a spring leg can be attached to the bottom of the rotating screen tray assembly or outer wall 5 thereof for imparting vibration to the screen as the said spring leg passes a stationary block afiixed to the inside of lower duct 10 or rim 26. It is apparent that other means can be employed for vibrating the rotating screen tray assembly and imparting further motion to the rotating solids in order to maximize the new surface presentation on each revolution of the said assembly.

Screen rotation rates can vary over a wide range depending on the bed depth of solids on the screen, particle size of the solids and diameter of the screen. With screens up to about 3 feet in diameter, velocities of from about 10 to about rpm. can be expected to provide satisfactory solids orientation. Higher velocities tend to expel tablets from the rotating surface and lower velocities risk prolonged exposure to the coating spray during each revolution. A bed depth of about 1 inch is advantageously employed with the aforementioned screen rotation velocities, in which case a retaining wall about 2 inches high, identified as outer wall 5, will be sufficient to retain tablets during rotation. Inner wall 4, of approximately the same height as outer wall 5, serves to keep rotating solids from the central portion (disc 2) of screen 3.

The coating spray means represented in spray nozzle 19 can be of conventional type, such as a simple nozzle through which the coating material issues under pressure at least about 5 p.s.i., preferably 5-25 p.s.i., or it can be a fluid atomizing nozzle in which the coating material under at least about 2 p.s.i. is fluidized at the nozzle by a gas stream of about l0-100 p.s.i. The configuration of the spray pattern is not critical but is preferably in the nature of a fiat ellipse which will extend approximately across the screen portion of the screen tray radius. The spray nozzle is eccentrically mounted with respect to the center of the rotating screen assembly in order that the spray will be directed at a limited segment of the screen and afford maximum drying time at each rotating velocity. Vertical positioning of the spray nozzle will depend primarily on the nature, in particular on the viscosity, of the fluid being sprayed, depth of the solids bed, velocity of rotation of the screen tray assembly, particle size of the solids being coated and the efficiency of the means for re-orienting the solids during each revolution. The spray fluid can be a solution, slurry or combination thereof.

The gas flow which serves to partially dry the rotating tablet bed between passages through the spraying zone can pass either upwardly or downwardly. through the screen. Preferably the gas comprises air at a temperature of about 40-l00 C., depending on the nature of the solids and the coating material as well as the efliciency of surface orientation in the drying phase of the cycle. Regardless of the direction of the air stream, the combination of pressure and vacuum must be such as to provide air penetration of the rotating and tumbling bed of solids. When the air stream is moving upward, a somewhat greater pressure but less vacuum is required than when the air direction is downward. In general, with solids of the type represented in pharmaceutical tablets, an upward air flow under pressure of about 5-25 inches of water, coupled with a downstream vacuum of about -20 inches of water, is usually sufficient.

High pressure gas manifold 14 delivers gas, preferably heated air to contribute to drying of the coated particles, at a pressure controlled to give the desired tumbling motion to the said particles without suspending them above the screen.

As seen in FIGURES 4 and 5, the configuration of the terminal portions of the upper and lower ducts is not critical. Thus, in elementary form, as shown in FIG- URE 4, the lower duct 10 in its original diameter can serve as the outer housing for the screen tray assembly and, if desired, outer wall 5 of the screen tray assembly eliminated, as heretofore described. Alternatively, lower duct 10 can diverge upwardly and terminate in an enlarged lower duct member to support the rotating screen tray assembly, with the upper duct 21 of similar and opposing configuration, as in FIGURE 5.

In FIGURE 5 is shown the optional feature of a baffle plate 25 positioned beneath the screen in the path of the spray pattern as determined at screen level. Depending on the character of the spray, an upcoming drying gas stream may diffuse the spray pattern if the spray is not protected from intermixing therewith. This problem is not presented where the drying gas stream is mov ing downwardly through the screen in the same direction as the spray.

In loading the screen tray with particles to be coated, due consideration must be given the necessity for drying gas penetration of the bed of rotating solids. While the ease of penetration is in large measure dependent on the efficiency of the tumbling means, it is obvious that an excessive depth will detract from optimum performance and should be avoided.

In each of the alternatives described above it is to be understood that no critical dimensions or ratios are contemplated, the invention comprising the basic combination of the substantially horizontally rotating screen tray assembly and its obvious equivalents, with spray means directed ata segment of the supporting screen whereby the solids to be coated are passed alternately through the spray zone and a drying zone comprising the remainder of the cycle. It is likewise evident that a variety of means operating in conjunction with said rotating screen tray assembly or its equivalent can be employed for tumbling the solids, said means serving only to facilitate drying and to re-orient the solids for presentation of a new surface for spraying at each revolution. Thus those skilled in the art can readily adapt without difficulty the teachings of this invention to apply the basic concept and apparatus as described to any specific coating problem.

What is claimed is:

1. An apparatus for coating discrete solids comprising: a vertically disposed lower duct member; a substantially horizontal, rotatable screen mounted across the upper portion of said duct member for supporting discrete sol-ids to be coated; means for rotating said screen; a vertically disposed upper duct member mounted above said screen; coating spray means mounted eccentrically above and directed toward said screen; means for delivering gas under pressure to said screen through one duct member; and means for exhausting gas from the opposite side of said screen through the other duct member.

2. An apparatus for coating discrete solids comprising: a vertically disposed lower duct member; a substantially horizontal, circular rotatable screen mounted above said lower duct member, said screen having a solid central portion and walls about the circumferences of both the screen and solid central portion to form an annular screen tray of sufficient depth to prevent escape of discrete solids from the screen surface during rotation of said screen tray; means for'rota-ting said screen tray; a vertically movable upper duct member mounted above said screen tray; vertically movable coating spray means mounted eccentrically above and directed at said screen tray; means for delivering gas under pressure to said screen tray through one duct member; and means for exhausting gas from the opposite side of said screen tray through the other duct member.

3. An apparatus for coating discrete solids comprising: a vertically disposed lower duct member; a substantially horizontal, circular rotatable screen mounted above said lower duct member, said screen having a solid central portion and walls about the circumferences of both the screen and solid central portion to form an annular screen tray of suflicient depth to prevent escape of discrete solids from the screen surface during rotation of said screen tray; means for rotating said screen tray; means for tumbling said discrete solids on said rotating screen tray; a vertically movable upper duct member mounted above said screen tray and 'having an opening diameter approximately that of said screen tray; vertically movable coating spray means mounted eccentrically above and directed at said screen tray; means for delivering heated gas under pressure upward through said lower duct member to said screen tray; baffle means beneath the portion of said screen tray at which said coating spray means is directed to prevent upcoming gas from diffusing the spray pattern from said spray means; and means for exhausting gas through said upper duct memebr.

4. The device of claim 3 in which the means for tumbling discrete solids on the rotating screen tray includes a high pressure gas manifold beneath a portion of said screen tray and having a plurality of outlets directed upwardly, whereby said discrete solids are tumbled on con- Eacg with jets of high pressure gas issuing from said mani- 5. An apparatus for coating discrete solids comprising: a vertically disposed lower duct member; a substantially horizontal, circular rotatable screen mounted above said lower duct member, said screen having a solid central portion and walls about the circumferences of both the screen and solid central portion to form an annular screen tray of sulficient depth to prevent escape of discrete solids from the screen surface during rotation of said screen tray; means for rotating said screen tray; means for tumbling said discrete solids on said rotating screen tray; a vertically movable upper duct member mounted above said screen tray and having an opening diameter approximately that of said screen tray; vertically movable coating spray means mounted eccentrically above and directed at said screen tray; means for delivering heated gas under pressure downward through said upper duct member to said screen tray; and means for exhausting gas through said lower duct member.

6. The apparatus of claim 5 in which the rotatable screen is a conical screen having an apex angle of at least about 135, said conical screen being mounted on its side to present a substantially horizontal segment below the coating spray means.

References Cited in the file of this patent UNITED STATES PATENTS Wurster July 16, 1957 Wilson July 12, 1960 Endicott et a1. Sept. 27, 1960 Mesnard et a1 May 30, 1961

Claims (1)

1. AN APPARATUS FOR COATING DISCRETE SOLIDS COMPRISING: A VERTICALLY DISPOSED LOWER DUCT MEMBER; A SUBSTANTIALLY HORIZONTAL, ROTATABLE SCREEN MOUNTED ACROSS THE UPPER PORTION OF SAID DUCT MEMBER FOR SUPPORTING DISCRETE SOLIDS TO BE COATED; MEANS FOR ROTATING SAID SCREEN; A VERTICALLY DISPOSED UPPER DUCT MEMBER MOUNTED ABOVE SAID SCREEN; COATING SPRAY MEANS MOUNTED ECCENTRICALLY ABOVE AND DIRECTED TOWARD SAID SCREEN; MEANS FOR DELIVERING GAS UNDER PRESSURE TO SAID SCREEN THROUGH ONE DUCT MEMBER; AND MEANS FOR EXHAUSTING GAS FROM THE OPPOSITE SIDE OF SAID SCREEN THROUGH THE OTHER DUCT MEMBER.

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