FLUIDIZED BED COATING APPARATUS

A fluidized bed coating system for coating substrate particles with liquid and powder. The system includes a vessel having a particle separating partition that divides an internal chamber of the vessel into up-bed and down-bed regions. An upstanding spray nozzle within the particle separating partition for directing pressurized coating liquid upwardly into the up-bed region such that particles are liquid coated as they circulate through the up-bed and down-bed regions. A coating powder directing system is provided that includes a controller managed powder delivery manifold for directing coating powder and pressurized gas under the particle separating partition and below the discharge orifices of the spray nozzle for coating the liquid coated particles with a uniform thickness of coating powder as they are recirculated through the up-bed and down-bed regions.

FIELD OF THE INVENTION

The present invention relates generally to fluidized bed coating systems, and more particularly, to fluidized bed coating systems in which particles are covered with a liquid and a powder.

BACKGROUND OF THE INVENTION

Known fluidized bed coating systems circulate a pressurizing gas through a chamber, usually under a slight vacuum, creating an up-bed region where particles rise and a down-bed region where particles fall. A nozzle sprays the particles with an atomized liquid as they move through the chamber. The liquid dries to create a solid coating on the particles.

It is often desirable to further coat particles in powder. Uniformity of the powder coating can be critical in, for example, the medical, pharmaceutical, and other chemical processing fields. Existing fluidized bed coating systems have been unable to apply powder coatings to particles uniformly.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a fluidized bed coating system adapted to coat particles more effectively with liquid and powder with the liquid and powder drying to form a uniform coating on the particles.

Further advantages of the invention will be apparent upon reviewing the illustrative examples set out in the detailed description with reference to the drawings.

The invention is susceptible to modifications and alternative constructions. Illustrative examples thereof are shown in the drawings and described below in detail. The invention is not limited to the specific examples disclosed. To the contrary, the invention covers all modifications, alternative constructions, and equivalents falling within the spirit and scope of the claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now more particularly toFIG.1, there is shown are illustrated Wurster-type fluidized bed coating system100in accordance with the invention. The coating system100includes a bowl or vessel110defining a coating chamber112in which substrate particles are circulated. A hollow particle separating partition120in this case in the shape of a truncated cone divides the chamber112into an inner up-bed region114and an outer down-bed region116. As understood by a person skilled in the art, the substrate particles20to be coated may be introduced into the coating chamber112from an appropriate port in the fluidized bed bowl110.

Fluidizing gas10such as vacuum air enters the chamber112and flows through a perforated gas plate130. The perforations are unevenly distributed across the surface of the perforated gas plate130such that the perforated gas plate130defines a greater density of perforations below the inner region114of the chamber112while defining a lesser density of perforations below the outer region116. The uneven distribution of the perforations in the perforated gas plate130causes the fluidizing gas10to define an inner higher-velocity region10aand an outer lower-velocity region10b.

The higher-velocity fluidizing gas10alifts the substrate particles20within the up-bed region114. Eventually, the substrate particles20exit the up-bed region114through the top of the particle separating partition120(not shown) to fall within the down-bed region116toward the perforated gas plate130. The substrate particles20collect above the perforated gas plate130in a pile132that is lightly fluidized by the lower-velocity fluidizing gas10bbefore migrating under the particle separating partition120and repeating the cycle.

In accordance with an important aspect of the present embodiment, the coating system100applies a coating22of powder30and liquid40to the substrate particles20as they circulate in the chamber112by rising from the lightly fluidized pile132into the up-bed region114and then falling in the down-bed region116. The process can be terminated, for example, once a certain mass of powder has been injected into the coating system100. As will be shown below, this can be estimated by measuring the weight of powder30remaining in a powder distribution system with load cells212(FIG.2)

In the illustrated embodiment a nozzle140configured to spray a mixture of liquid40and pressurizing gas42is disposed within the up-bed region114. The nozzle140includes porting142connected to a first source40a(e.g., a tank or a pump) of liquid40and a second source42a(e.g., a tank, a compressor, or a fan) of pressurizing gas42. The nozzle porting142leads to a discharge orifice144of the nozzle140including one or more discharge orifices144from which a mixture of the liquid40and the pressurizing gas42is sprayed onto the substrate particles20rising in the up-bed region114. The nozzle140in this case is an upstanding generally cylindrical structure having one or more discharge orifices144in a terminal end.

The liquid40can be a solution including one or more coating agents dissolved in a liquid base, such as water. The pressurizing gas42, like the fluidizing gas10, can be compressed air. The pressurizing gas42atomizes the liquid40sprayed from the discharge orifice144, providing energy to break the stream of liquid40into droplets. The liquid40mixes with the powder30to develop the liquid coating22on the outer surface of the substrate particles20. As further discussed below, the substrate particles20can receive a coating of liquid40before powder30is injected into the coating system100.

In carrying out this embodiment, a powder distribution assembly150is provided that includes one or more powder tubes160configured to spray a mixture of powder30and pressurizing gas32into the up-bed region114. Each powder directing tube160includes an inlet162connected to a powder distribution manifold280and an outlet164configured to direct a mixture of powder30and pressurizing gas32into the up-bed region114(FIG.3). The powder tube outlet164includes one or more outlet ports radially extending with respect to the major axis of the powder tube160. The ports are configured to spray the mixture of powder30and pressurizing gas32into the up-bed region114at a location below the liquid discharge orifices144of the spray nozzle140. In other examples, the outlet ports of the powder tube outlet164are axial with respect to the major axis of the powder tube160or horizontal.

Each powder tube160in this case fits through a sealed port112in the vessel110. The illustrated powder tubes160are slanted downward, placing the tube inlet162above the tube outlet164. The powder tube outlets164are located underneath the particle separating partition120along an outer perimeter of the higher-velocity fluidizing gas flow10a.

The powder tube outlets164are disposed upstream of the discharge orifices144such that the substrate particles20, during a given cycle, break the plane of the powder tube outlets164before breaking the plane of the liquid discharge orifices144. Once the process reaches a state in which both powder30and liquid40are injected into the coating system100, the substrate particles20receive powder30and atomized liquid40effectively at the same time.

The substrate particles20preferably are circulated through the fluidized bed coating system100while the powder distribution system150is offline to receive an initial coating of the liquid40from the nozzle140. Once the coating22has attained certain properties (e.g., a measured dew point temperature of the fluidizing gas10has reached a predetermined value or a predetermined amount of time has elapsed), the powder distribution system150is activated while the nozzle140continues to spray liquid40.

Throughout the process, the mass flow rate of the fluidizing gas10,10a,10bcan be increased to overcome the added weight of coating22on the substrate particles20. For example, the mass flow rate of fluidizing gas10,10a,10bcan be increased once the powder distribution system150comes online.

Examples of substrate particles20include seeds, beads, pellets, sugar spheres, pebbles, and minerals. Examples of powder30include polymers and pharmaceutical active ingredients. The powder30can be designed to adhere to the substrate particles20. For example, the powder30can include a coating agent configured to, upon mixing with the liquid40, create a chemical bond between the other ingredients of the powder30and the substrate particles20. According to some examples, the properties of the liquid40are changed when the powder distribution system150is brought online (e.g., a new liquid source42ais added, an existing liquid source42ais disconnected, or existing liquid sources42aare supplied at an updated volume or mass ratio).

FIG.2shows a powder distribution system200for supplying a mixture of powder30and pressurizing gas32to the coating system100through a powder manifold280. Powder30leaves powder manifold280in a dry state. The pressurizing gas32(e.g., compressed air) propels powder30into the chamber112of the coating machine100.

A hopper210funnels dry powder30into an auger conveyer220having a cylindrical outer housing222in which a helical screw blade224mounted on a driveshaft226is turned by a motor228. The helical screw blade224advances powder30toward an intermediate manifold230. Although shown as a separate element inFIG.2, manifold230can be an outlet of auger conveyer220.

One or more load cells212measure the weight of powder30in the hopper210and/or the auger conveyer220. For example, a controller231including one or more processors can be configured to subtract a previously measured weight of the auger conveyer220and hopper210before powder30is loaded from a current weight to determine (e.g., estimate) a current weight of the powder30remaining within the hopper210and/or the auger conveyer220.

Based on an amount of time between different weight measurements of the remaining powder30, the controller231determines (e.g., estimates) a mass flow rate of powder30into the coating system100. Based on this information, the controller231adjusts a speed of auger motor228to maintain the current mass flow rate of powder30into the coating system110(also called a feed rate) at a desired value (e.g., a single value or within an acceptable range of values).

The intermediate manifold230is connected to a pressure system240including a filter242and a pressure sink246. The pressure system240is configured to control the gas pressure within auger conveyer220to prevent the eductor from creating a vacuum within the auger housing222that would draw extra powder30from the hopper210. For example, if the air pressure within the auger housing222is below ambient pressure, then ambient air will flow from the pressure sink246(e.g., ambient air), through the filter242, and into the auger housing222.

According to some examples, no valves244are provided in the pressure relief system240such that the interior of the auger housing222is in continuous fluid communication with the pressure sink246via the filter242. According to other examples, the pressure relief system240includes one or more valves244configured to selectively open and close the fluid path between the filter242and the pressure sink246.

The filter242is configured to trap powder30, preventing it from reaching the pressure sink246. The filter242is also configured to trap particles within fluid flowing from the pressure sink246(e.g., particles in ambient air), preventing those particles from mixing with the powder230. The filter242is disposed at the top of the manifold/auger outlet230and can be, for example, a screen between ambient environment and the auger outlet230.

Downstream of manifold/conveyer outlet230, powder30enters an ejector260through a first inlet port262. The ejector260includes a second inlet port264connected to a source32aof dry pressurizing gas32, such as a fan, a compressor, or a pressurized gas tank. Eductor260can be a jet pump that relies on the venturi effect. The velocity of powder30at the outlet266of eductor260is greater than the velocity of powder at the first inlet262of eductor260. Pressurizing gas32pushes powder30from eductor outlet266, through manifold280, into the powder distribution assembly150.

FIG.3shows a second powder distribution assembly300for use in the fluidized bed coating system100. The second powder distribution assembly300can replace the first powder distribution assembly150shown inFIG.1. Alternatively, the first and second powder distribution assemblies150,300can be used in parallel.

The second powder distribution assembly300includes one or more powder tubes310and a powder distribution ring320. Each powder tube310fits through a sealed port112in the vessel110.

The illustrated powder tubes310are slanted downwards such that each tube inlet312is disposed above the tube outlet314. The inlets312connect to the powder manifold280to receive the mixture of powder30and pressurizing gas32from the powder distribution system200. The outlets314spray or otherwise deposit the mixture of powder30and pressurizing gas32into the powder distribution ring320. Each tube outlet314can include one or more outlet ports, which can be, for example, axially or radially oriented with respect to the major axis of the powder tube310.

The powder distribution ring320is welded to the particle separating partition120and defines an annular chamber322. The annular chamber322serves as a manifold in which the mixture of powder30and pressurizing gas32circulate about a circumference of the particle separating partition120. The powder distribution ring320and its annular chamber322surround the circumference of the particle separating partition120.

One or more through holes122across the particle separating partition120join the annular chamber322of the powder distribution ring320with the up-bed region114. The mixture of powder30and pressurizing gas32circulating within the annular chamber322flows from the through holes122into the up-bed region114. The through holes122eject or spray the mixture of powder30and pressurizing gas32into the up-bed region114upstream of the discharge orifice144.

Referring to the cross-sectional plan view inFIG.4taken the perspective of section4-4inFIG.3, the second powder distribution assembly300defines a ring about the particle separating partition120. The annular chamber322is a circumferentially extending void.

FIG.5is a cross-sectional plan view taken from section5-5inFIG.3. A view of a powder tube310has been added. Views of the down-bed region116and the vessel110have been omitted. The through holes122are spaced about the circumference of particle separating partition120. Although not shown, the through holes122can be at different vertical heights.

The powder distribution ring320enables powder to be sprayed into up-bed region114at a number of locations greater than the number of powder tubes160, enhancing coating uniformity. For example, inFIG.5, a single powder tube160feeds a mixture of powder and pressurizing gas32into the annular chamber322of the powder distribution ring320. Four through holes122then feed (e.g., spray) powder30and the pressurizing gas32into the up-bed region114. In other examples, there are two, four, or eight times more through holes122than powder distribution tubes160. The mass flow rate of powder30in each individual powder tube160be significantly (e.g., at least 50% or 100%) greater than the mass flow rate of powder30through each individual hole22.

InFIG.3, the major axis of each through hole122is shown as being downwardly inclined with respect to the horizontal. In other examples, the major axis of each through hole122is horizontal or upwardly inclined with respect to the horizontal such that the mixture of powder30and pressurizing gas32is sprayed at the discharge orifices144of the nozzle140.

FIG.6illustrates stages of a substrate particle20and its coating22as the substrate particle20circulates within fluidized bed coating system100. The substrate particle begins in a first stage602upstream of the discharge orifice144and the powder tube outlet164. Here, the substrate particle20is a clean seed such as a pre-coated particle or a raw material. In a second stage604, atomized liquid40sprayed from discharge orifice144has formed a wet coating22on the outer surface of substrate particle20. After the second stage604, the powder distribution assembly150,300is brought online to begin injecting powder30into the coating system100. In a third stage606, the coating22includes a wet mixture of the powder30and the liquid40, including any solid coating agents dissolved in the liquid40.

After the third stage606, the coating22dries while the substrate particle20rises in the up-bed region114, falls in the down-bed region116, and rests in the lightly fluidized pile132. In a fourth stage608, the substrate particle20has a dry coating22including solid residue from the powder30adhered to the substrate particle20. The dry coating22can also include solid residue from the liquid40.

After the fourth stage608, the substrate particle20repeats the cycle to build another layer of coating22. In the fifth stage610, the substrate particle20has a drier inner coating22aincluding residue from both the powder30and the liquid40, and a wetter outer coating22bincluding a mixture of the powder30and the liquid40. The cycle can be terminated once a certain mass of powder30has been dispensed as measured by the load cells212of the powder distribution system200.

Multiple fluidized bed coating systems100can be combined into a single assembly. Each of the coating systems100in the assembly can define a unique up-bed region114. The down-bed regions116of adjacent coating systems110can overlap. Nozzle140and coating system100can be configured to operate as described in U.S. Publication No. 2008/0000419 to Bender et al. (“Bender”), which is hereby incorporated by reference in its entirety.

While examples have been provided in the drawings and foregoing detailed description, such disclosure is illustrative and exemplary, not restrictive. Changes and modifications may be made to the claims without departing from their spirit and intended scope.