Abstract:
The present invention provides a fluidized bed for dispensing small quantities of powders. The fluidized bed is made using a porous housing to permit injection and removal of fluid through the surrounding walls to improve the operation of the bed particularly for small particles of the group C size, including significantly reducing powder adhesion to the walls. In one aspect of the invention the fluidized bed housing is rotated about its longitudinal axis, which may be oriented at any suitable angle. A system for changing the volume of the bed is described as is a system for incorporating injection nozzles within the bed for constant agitation and prevention of powder adhering to the walls of the housing.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to devices for the metering of small quantities of powder from fluidized beds through a volumetric measuring device.  
         BACKGROUND OF THE INVENTION  
         [0002]    Accurate metering of a given quantity of powder is often required in various processes including chemical engineering and pharmaceutical processes. When the metered quantity is large, this is relatively easily achieved. However, when the required quantity is very small, this becomes very difficult if high accuracy is required at the same time. In addition, if very fine powder is used, strong interparticle forces cause the powder to agglomerate, thus making the precise metering even more difficult.  
           [0003]    Pulmonary drug delivery represents a new drug administration method that provides many advantages. It provides direct and fast topical treatments to respiratory and lung diseases. It has less first-pass GI (gastrointestinal) metabolism and can provide targeted delivery to heart and brain. Drugs such as peptides can be systemically delivered using the pulmonary channel. Pulmonary drug delivery also allows the use of drugs with low solubility. Antibiotics and even vaccines can be delivered in this manner. Compared to oral in-take, it provides a fast and much more efficient adsorption. Typically, only a few percent of the medication of the oral in-take is required for pulmonary delivery. Compared to intravenous injection, it provides a painless and safe alternative.  
           [0004]    To facilitate pulmonary delivery, drug powders should normally be less than 5 μm so that they become airborne during inhalation. However, powders of such small sizes (typical group C powder in the Geldart classification) have very strong interparticle forces that make them agglomerate and cohesive, and thus very difficult to handle. Since the required dosage for pulmonary delivery is also very small (usually in the order of 1 μg-100 mg), this makes it very difficult to accurately meter such a small quantity and fill them into packages.  
           [0005]    To overcome the interparticle forces, current industrial practice applies two different methods; one involves mixing the ultrafine drug powders with large amounts of coarser powder, and the other the suspension of the powder in liquid. The first method uses a large quantity of excipient (filler) particles that are much larger (normally group A or group A-C powders in the Geldart classification). This makes the small-large powder mixture fluidize and flow easily so that they can be handled more easily. It also significantly increases the volume of each dosage so that the dispensing becomes more accurate when the drug powder is packaged into the Dry Powder Inhaler (DPI). However, only a small fraction of the small drug particles can detach effectively from the large excipient particles during inhalation and the rest stay with the large particles and land in the mouth, limiting the efficiency of final delivery to about 10-15%.  
           [0006]    The second method involves suspending the ultrafine drug powders into liquids such as hydrocarbon propellants and storing them in Metered Dose Inhalers (MDI). When a metered quantity of the propellant is released from the storage canister, the propellant evaporates and expands quickly to disperse the powdered drug into the patients&#39; mouth. The key problem with this method is that the quick expansion of the propellant causes the drug to impact in the back of the throat and other places in the mouth, reducing the amount being inhaled into the lung to less than 10-15%. This method also needs good breath coordination, since it is difficult to predict the amount of drug inhaled if the patients&#39; inhalation does not coincide with the drug releasing.  
           [0007]    Thus both currently practiced methods have significant limitations. It would be ideal if the required small quantity of the fine drug powder could be accurately dispensed alone, without any other chemical or physical constituents. When only the pure drug powder is packaged into the inhaler, the delivery efficiency is expected to increase significantly. However, this tends to be fairly difficult if the quantity to be packaged is extremely small. For example, if each dose contains 0.5 mg or 500 μg of drug powder and the bulk (packed) density of the powder is 0.5 mg/mm 3 (=500 kg/m 3 ), the total volume of the powder withdrawn for each dose is only 1.0 mm 3 .  
           [0008]    Fluidization occurs when particulate materials of sub-micrometers to several millimeters are suspended by up-flowing gas in a vessel or column to form a gas-solid suspension more commonly referred to as a fluidized bed. The fluidized beds formed with the gas-solid suspension are specifically referred to as gas-solid fluidized beds. The term “fluidized bed” applies because the gas-solid suspension formed by the solid particles and the upflowing gas behaves like a fluid. Although primarily gas is used as fluidizing fluid, liquid can also be used. In some cases, both gas and liquid are used together. Those are called liquid-solid fluidized beds and gas-liquid-solid three-phase fluidized beds.  
           [0009]    A gas-solid fluidized bed can operate in several fluidization regimes: particulate, bubbling, slugging and turbulent fluidization regimes (conventional fluidized beds), and fast fluidization and pneumatic transport regimes (high-velocity fluidized beds). In a conventional fluidized bed, there are usually two distinct regions: the upper dilute region (also called the freeboard region) and the bottom dense region which has most of the particles and also contains many more particles per volume than the dilute region. In a high-velocity fluidized bed, almost all particles are carried upwards by the high-velocity upflow gas and almost the entire bed is in a dilute suspension region. There is also a downflow fluidization regime where gas and particles flow co-currently downward in a dilute suspension form.  
           [0010]    A typical design of a fluidized bed includes a gas distributor at the bottom of the fluidized bed column, the main function of which is to uniformly distribute the gas into the fluidized bed. The vessel that contains the fluidized bed can have any suitable shape, but those with cylindrical or rectangular cross-sections and oriented on a substantially vertical axis are commonly used. The vertical walls are usually of solid materials to prevent the gas and solids from escaping from the fluidized beds.  
           [0011]    Sometimes, it is necessary to have solid feeds and withdrawal ports and/or heat transfer tubes or panels mounted on the wall(s) of a fluidized bed. At the top of the bed, there is usually a plate or similar structure that seals the top of the fluidization column. There is usually at least one exit port through the top plate and/or the side wall not far below the top plate that allow gas and entrained solids to leave the fluidized bed and enter into gas-solid separation devices or other vessels or other process units. Those particles that leave the fluidized bed are entrained out by the gas flow, i.e. by solids entrainment.  
           [0012]    Powders may be classified into four groups in gas-solid fluidized systems, according to Geldart&#39;s classifications. Groups B and D powders comprise large particles that typically result in large bubbles when fluidized. Group A powders comprise particles that first experience a significant expansion of the powder bed when fluidized before bubbles begin to appear. Group C powders comprise very small particles for which the interparticle forces significantly affect the fluidization behaviour. As the particle size reduces, interparticle forces increase significantly.  
           [0013]    Those strong interparticle forces cause the fine particles to agglomerate and make them very cohesive. Typical Group C powders comprise particles under 30-45 μm in size, although some very sticky powders larger than these sizes may also belong to Group C powders. Due to strong interparticle forces, Group C powders are either very difficult to fluidize (with channeling and/or very poor fluidization) or mainly fluidize with the large agglomerates as pseudo-particles rather than as individual particles. In either case, fluidization of individual particles cannot be achieved easily so that handling of Group C powders becomes a difficult problem.  
           [0014]    Different measures can be taken to assist the fluidization of Group C powders. Those methods are usually referred to as fluidization aids. Fluidization aids include mechanical stirring, mechanical, acoustic or ultrasonic vibration, addition of much larger particles or other objects to provide extra stirring, addition of finer particles to act as “lubricant”, pulsation of fluidization gas, etc. Some aids are more effective than others for a given Group C powder, but the effectiveness of almost all aids tends to diminish as the powder becomes finer.  
           [0015]    Group C powders also tend to clog up in certain areas of the fluidized bed, such as above the gas distributor, around internals and at exit port(s), and to stick to the internal wall or the ceiling of the bed. Large chunks of powder form in those places and then break from time to time as they grow and become unstable. As those chunks of particles from the ceiling or upper portion of the bed fall back into the fluidized bed, they disturb the flow hydrodynamics inside the bed, causing periodical variation of the bed density and other properties in both the bottom dense phase region and the upper dilute phase region.  
           [0016]    Key characteristics of fluidized beds include easy handling of particles, excellent contact between gas and solids, excellent heat and mass transfer between gas and solids and between gas-solid suspensions and the column wall, good gas and solids mixing, etc. These and other useful characteristics have led to the wide application of fluidized beds in process and other industries. The “easy handling of particles” is due to the uniform solids suspension inside the bed and the relatively free movement of the particles within the gas-solids suspension and of the suspension itself.  
           [0017]    Several well known problems currently exist with the fluidized beds pertaining to the metering of fine particles, especially Group C particles. Solids entrainment can sometimes cause problems to the maintenance of a uniform suspension, since entrained particles may flow out of the fluidized bed with the gas stream from the top exit. In order to maintain a constant suspension, escaped particles must be separated from the gas stream (by cyclone, bag filter and/or other devices) and returned. Because there is limitation on the separation efficiency, some particles may be lost even with several stages of separation, leading to a reduction of the powder inventory. This presents a serious problem in some cases where it is essential not to lose particles, such as in the case where expensive drug powder is handled. In this case, a filter may be installed inside or just at the exit port to stop the entrained particles from flowing out of the bed in the first place. However, such filters are plugged very quickly that periodical purging is essential. In addition, such filters also produce a high pressure drop that a very large filter area has to be created to allow enough gas to flow through.  
           [0018]    The main problem associated with particle loss and with the gradual decrease of solids inventory due to continuous metering out of particles, is the reduction of solids suspension density. Such variations in solids suspension density may reduce the accuracy of powder metering from the fluidized bed. One measure is to continuously add additional particles into the fluidization column. An alternative measure proposed in this invention and discussed hereinafter is to gradually decrease the volume of the fluidized bed by moving one or more side of the column wall inwards.  
           [0019]    Another problem with fluidized beds is that local dead zones or defluidization may occur due to the non-uniform gas distribution at the bottom or due to other reasons such as agglomeration of fine or ultrafine (Group C) particles. This can result in non-uniform and unpredictable suspension density, and other undesirable consequences. For greater certainty, particle agglomeration happens when very fine powder, such as the drug powder for pulmonary drug delivery, is fluidized. Such agglomeration causes non-uniform solids suspension and solids flow, greatly reducing the accuracy of powder metering from the fluidized bed.  
           [0020]    Yet another problem is that some particles tend to stick on the inner wall or the top plate of the fluidization vessel/column. This is especially true when very fine particles are fluidized. This can lead to unwanted solids accumulation on the wall. Accumulation of particles on the wall reduces the solids holdup (=concentration) in the bed, making it difficult to precisely control the fluidized bed density, as desired in some processes. Those particles stuck on the wall may also fall periodically back to the bed (for example, when the accumulation is too thick), changing suddenly the bed density, that is, the solids concentration in the bed. A rotating fluidized bed can be used to overcome this problem. The concept of rotating fluidized bed with porous walls is known, however such beds are rotated to generate centrifugal force to the particles in the bed and are known as centrifugal fluidized beds. In these devices, the cylindrical wall is porous. The porous wall is used as gas distributor for the fluidizing gas to flow inward in all radial directions into the bed and the gas exits through the axial end(s) of the cylinder. The purpose of rotating the cylindrical (horizontal or vertical) vessel is to create a centrifugal force to hold the particles towards the cylindrical wall so that higher fluidization velocity can be used without producing large bubbles in the bed and/or without having significant solids entrainment. This allows the same bed to be operated at higher gas velocity so that the process capacity is increased. Example references that provide the details of such rotating fluidized beds include R Pfeffer, GI Tardos and E Gal, “The use of a rotating fluidized bed as a high efficiency dust filter”, in Fluidization V, eds., K. Ostergaard and A. Sorensen, Eng. Foundation, New York, pages 667-672, 1986; J. Kao, R Pfeffer and G I Tardos, “On partial fluidization in rotating fluidized beds ”, American Institute of Chemical Engineering Journal, Volume 33, pages 858-861, 1987; Qian G-H, I Bagyi, R Pfeffer, H Shaw and J G Stevens, “A parametric study on a horizontal rotating fluidized bed using slotted and sintered metal cylindrical gas distributors”, Powder Technology, Volume 100, pages 190-199, 1998; Qian G-H, I Bagyi, R Pfeffer and H Shaw, “Particle mixing in rotating fluidized beds: inferences about the fluidized state ”, American Institute of Chemical Engineering Journal, Volume 45, pages 1401-1410, 1999; and U.S. Pat. No. 6,197,369.  
           [0021]    However, the key design concepts and the purpose of such prior art centrifugal fluidized beds are significantly different from the rotating and porous fluidized bed dispenser proposed in this invention.  
           [0022]    U.S. Pat. No. 5,826,633 issued to Parks et al. is directed to a powder filling apparatus which uses gravity to assist filling of a metered chamber. The metered chamber is placed below a convergent passageway containing the powder that is being dispensed. While the method and device involves “fluidizing” the powder to overcome inter-particle cohesive forces, they defined fluidizing powder as “the powder is broken down into small agglomerates and/or completely broken down into its constituents or individual particles”. In their definition, upflowing gas is not essential to cause the powder to be fluidized. This is significantly different from the conventional definition of fluidization, as followed in this patent application, that powder is fluidized when it is suspended in an upflowing gas (or liquid). As a result, the device is not per se a fluidized bed since in this device all particles fall unassisted by gravity. In conventional fluidized beds particles are suspended by the fluidizing gas and very few, if any, particles can fall unassisted by gravity. Further, some typical components of a fluidized bed such as an air distributor is missing in this device. In addition, other problems as described above in this invention, such as sticking of particles to the inner surface of the convergent chamber is still problematic with this type of device.  
           [0023]    U.S. Pat. No. 6,183,169 issued to Zhu et al. is directed to a device for precision dispensing of fine powders. This device includes two fluidized bed chambers communicating with each other and operates by first fluidizing a fine powder in one chamber and then using a pressure differential between the chambers to draw the fluidized particles into the second chamber. A solenoid valve attached to the second chamber is opened for a selected period of time to dispense the powder in the form of gas-solid suspension to a collection area. The two-chamber concept utilized by Zhu et al. in U.S. Pat. No. 6,183,169 is different from the one used in the current invention. It uses a two-stage method to dilute and control the gas-solids suspension and a Venturi mechanism to control the powder flow and to transport the powder from one stage to another, while the current invention only has one stage and does not require a Venturi or anything of such kind to control powder flow. In the device disclosed in Zhu et al. powder withdrawal is from the dilute phase in the second chamber.  
           [0024]    Obviously, the key concepts of both U.S. Pat. Nos. 5,826,633 and 6,183,169 are different from the current invention.  
           [0025]    In view of the difficulties and complexities with the prior art, it would be advantageous to provide a single fluidized bed which can dispense quantities of fine powder in an accurate and controlled manner which can be used for either batch or continuous processing of the fine powders. It would also be very advantageous to provide a fluidized bed system that significantly reduces solids accumulation on the walls of the fluidized bed, achieves total solids containment in the bed except for targeted particle withdrawal through selected ports, reduces or eliminates dead zones, and/or allows for the addition and removal of gas at various locations in the fluidized bed.  
         SUMMARY OF THE INVENTION  
         [0026]    This invention utilizes the uniform solids suspension and easy mobility of particles inside the fluidized bed, from which particles are uniformly withdrawn to a fixed-volume cavity so that a definite quantity of particles can be metered out from the fluidized bed. To ensure consistency and accuracy of such powder metering, it is essential to maintain a constant and consistent gas-solids suspension inside the fluidized bed.  
           [0027]    The method disclosed herein involves metering the powder flow from a fluidized bed where the particle suspension has a much lower density than that of packed (bulk) particles so that the withdrawal volume is significantly increased to increase metering accuracy, and where the particles are completely mobile so that a consistent withdrawal can be maintained.  
           [0028]    The present invention also discloses rotating the porous fluidized bed to alternately switch the gas distributor, the bed wall and/or the top gas exit plate, so that particles stuck onto the wall can be continuously back purged off the wall when they are rotated to the bottom of the bed where the gas is introduced in the bed.  
           [0029]    The current invention proposes the following alternatives to provide further agitation to the powder to enhance uniform fluidization: (1) rotating the fluidized bed, with or without adding large beads in the bed; (2) injecting additional gas into the bed at various locations in the bed through gas nozzles; and (3) using gas nozzles with flexible tube that can move randomly inside the bed.  
           [0030]    An object of the present invention is to provide accurate volumetric metering of powder, either by filling a receptacle of given volume or by timing the powder flow at a given volumetric flow-rate, from a fluidized bed. In particular, this invention addresses the problems associated with metering of extremely small quantities (1 μg-100 mg) of ultrafine (&lt;10 μm) powders. To ensure precise metering, the invention provides fluidized bed structures that intend to ensure uniform and relatively constant gas-solids suspension inside the fluidized bed, by minimizing the problems associated with maintaining uniform gas-solids suspension and uniform fluidization. This invention also provides effective means to volumetrically meter and withdraw the required small quantities of powder in a very accurate and controlled manner.  
           [0031]    It is a further object of the present invention to provide a fluidized system that may be used for reduction of solids accumulation on the walls, to provide a system that may be used for reduction or elimination of dead zones, and to provide a system that may be used to add and remove gas at various locations (e.g., along the axial direction).  
           [0032]    Broadly, the present invention relates to metering a small quantity of powder from a fluidized bed using a volumetric method. It can be just any fluidized bed that can provide a steady gas-solid suspension and the withdrawal can be either from the dense phase or the dilute phase of the bed. An element of some suitable shape that has one or more cavities and that can be easily engaged and disengaged to the said fluidized bed with the cavities exposed to the fluidized bed is used for the metering and withdrawal.  
           [0033]    Furthermore, the present invention relates to a fluidized bed structure comprising having a housing defining a fluidized bed chamber, means for introducing primary fluidizing fluid through one or more portion(s) of the surrounding walls into the chamber at one or more side(s) of the chamber and means for permitting the escape of the fluid through one or more portion(s) of the surrounding walls from the chamber at other one or more side(s) of said chamber. At least some of the walls of the chamber have a significant area that is porous, the porous area comprising pores having a size sufficiently small to prevent significant loss of particles from the fluidized bed.  
           [0034]    This invention further relates to a powder metering and withdrawal mechanism that is attached to the fluidized bed. This mechanism includes an element of some suitable shape that has one or more small cavities (pockets, holes) and means (withdrawal port) to engage and disengage such element easily to/from the fluidized bed with the cavities exposed to the gas-solid suspension inside the fluidized bed.  
           [0035]    In one aspect the present invention provides a fluidized bed for dispensing powders, comprising:  
           [0036]    a) a housing defining an enclosure for containing particulate matter, said housing including a fluid injection means for injecting a fluid into said enclosure for fluidizing particulate matter contained within said housing for forming a dilute phase and a dense phase of fluidized powder in said housing; and  
           [0037]    b) volumetric metering means connected to said housing and in flow communication with said enclosure through an outlet passageway for withdrawing pre-selected amounts of said particulate matter from said housing.  
           [0038]    In another aspect of the invention there is provided a fluidized bed for dispensing powders, comprising:  
           [0039]    a) a housing defining an enclosure for containing particulate matter, said housing including at least one porous wall having a suitable porosity to allow flow of fluid through said porous wall while preventing most of the particulate matter contained within said housing from passing through said porous wall, a fluid injection means for injecting a fluid into said enclosure for fluidizing particulate matter contained within said housing for forming either a dilute phase only or a dilute phase and a dense phase of fluidized powder in said housing; and  
           [0040]    b) time controlled powder withdrawal means connected to said housing and in flow communication with said enclosure through an outlet passageway for withdrawing particulate matter from said housing for a pre-selected period of time. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    Further features, object and advantages will be evident from the following detailed description of the present invention taken in conjunction with the accompanying drawings, in which:  
         [0042]    [0042]FIG. 1 is a cross sectional view of a conventional fluidized bed modified in accordance with the present invention to include two powder metering and withdrawal units on the side wall for powder withdrawal, one from the top dilute phase region and the other from the bottom dense phase region;  
         [0043]    [0043]FIG. 2 shows another embodiment of a fluidized bed similar to FIG. 1 in which the powder metering and withdrawal unit includes a porous filter behind the powder collector and an airflow through it (with or without vacuum suction) to enhance the powder withdrawal process;  
         [0044]    [0044]FIG. 3 a  is a cross sectional view of another embodiment of a fluidized bed constructed in accordance with the present invention having porous walls;  
         [0045]    [0045]FIG. 3 b  is a view along arrow A of FIG. 3 a;    
         [0046]    [0046]FIG. 4 a  is a cross sectional view of a porous fluidized bed oriented for rotation about a horizontal axis of the bed housing with the powder metering and withdrawal unit located on the end plate;  
         [0047]    [0047]FIG. 4 b  is a view along arrow A of FIG. 4 a;    
         [0048]    [0048]FIG. 5 is a cross sectional view of an embodiment of an air distribution and/or air withdrawal chamber for the fluidized bed of the present invention;  
         [0049]    [0049]FIG. 6 a  is a cross sectional view of fluidized bed oriented for rotation about the horizontal axis of the housing and having a slide plate for adjusting the volume of the interior of the fluidized bed;  
         [0050]    [0050]FIG. 6 b  is the side view of FIG. 6 a;    
         [0051]    [0051]FIG. 7 a  is a cross sectional view of a fluidized bed oriented for rotation about the horizontal axis of the housing and having a gas purging tube inserted along the horizontal axis of the housing;  
         [0052]    [0052]FIG. 7 b  is the side view along arrow A of FIG. 7 a;    
         [0053]    [0053]FIG. 8 is a cross sectional view of a fluidized bed oriented for rotation about the horizontal axis of the housing and having gas injection nozzles located within the fluidized bed for producing additional agitation and/or jet milling action within the bed;  
         [0054]    [0054]FIG. 9 is a cross sectional view of an embodiment of a fluidized bed having a source of vibration contacting the housing or the particles inside through a mesh for additional agitation of the powder;  
         [0055]    [0055]FIG. 10 is a cross sectional view of an embodiment of a fluidized bed having a source of sonic vibration from the bottom of the bed (within the distributor box) for additional agitation of the powder;  
         [0056]    [0056]FIG. 11 a  is a cross sectional view of fluidized bed oriented for rotation about the horizontal axis of the single cylinder housing and having a slide plate for adjusting the volume of the interior of the fluidized bed, within a sealed casing;  
         [0057]    [0057]FIG. 11 b  is a view along arrow A of FIG. 11 a;    
         [0058]    [0058]FIGS. 12 a  and  12   b  are top and side views respectively of an embodiment of a powder metering and withdrawal unit constructed in accordance with the present invention;  
         [0059]    [0059]FIGS. 13 a  and  13   b  are top and side views respectively of an alternative powder metering and withdrawal unit;  
         [0060]    [0060]FIG. 14 a  is a side view of a fluidized bed with several of the powder metering and withdrawal units of FIG. 12 mounted on a sliding bar;  
         [0061]    [0061]FIG. 14 b  is a partial cross section of the fluidized bed of FIG. 14 a;    
         [0062]    [0062]FIG. 15 a  is a side view of a fluidized bed with a drive mechanism attached to the sliding bar for changing the powder metering and withdrawal units;  
         [0063]    [0063]FIG. 15 b  shows a cross sectional view of the fluidized bed of FIG. 15 a;    
         [0064]    [0064]FIG. 16 a  shows a cross section of an embodiment of a powder dispensing apparatus similar to that shown in FIGS. 11 a  and  11   b  but including a disk-on-bar powder metering and withdrawal device shown in FIG. 14 b  attached at the powder dispensing unit to the top plate;  
         [0065]    [0065]FIG. 16 b  is a view along arrow A of FIG. 16 a;    
         [0066]    [0066]FIG. 17 a  is a cross-sectional view of an embodiment of a non-rotating, porous and single-cylinder fluidized bed with alternating air injection and releasing and with disk-on-bar powder metering and withdrawal device attached to the top plate;  
         [0067]    [0067]FIG. 17 b  is a horizontal cross-sectional view of the device of FIG. 17 a;    
         [0068]    [0068]FIG. 18 a  is a cross-sectional view of an embodiment of a horizontal, rotating, porous and double-cylinder fluidized bed with alternating air injection and releasing and with disk-on-bar powder metering and withdrawal device attached to the end plate;  
         [0069]    [0069]FIG. 18 b  is an end view of the device along arrow A of FIG. 18 a ; and  
         [0070]    [0070]FIG. 19 is a cross section of another embodiment of a single dose metering and withdrawal device attached to a fluidized bed for dose by dose metering and withdrawal. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0071]    The present invention provides improved fluidized bed devices for delivery of metered amounts of ultrafine particles. The fluidized bed devices disclosed herein provide a more uniform suspension of ultrafine particles in the fluidized bed, giving a more constant bed suspension density which allows a steady stream of particles to be withdrawn from the fluidized bed. When timed (controlled), this steady stream of powder flow can provide a controlled quantity of the powder into given containers such as for drug packaging. Alternatively, if the receptacle volume is fixed, the withdrawal quantity can also be controlled by allowing the receptacle to be completely filled.  
         [0072]    [0072]FIG. 1 shows a cross sectional view of a fluidized bed at  20  constructed in accordance with the present invention which includes a housing  22  defining an enclosure  24  containing particles  26  to be dispensed from the fluidized bed. A gas distributor box  28  is attached to the housing  22  along a porous bottom wall (gas distributor)  30  of housing  22  and includes a gas inlet  32 . An air outlet  36  is located in a top wall  34  of housing  22 . It will be understood that gases other than air may be used so long as they are chemically inert in the presence of the powder particles. Two powder metering and withdrawal units  40  and  40 ′ are located in a side-wall  42  of housing  22  for powder withdrawal, unit  40  being for powder withdrawal from the top dilute phase region and unit  40 ′ being for powder withdrawal from the bottom dense phase region of the fluidized powder, the boundary between the top dilute phase and the lower dense phase region being shown by line  46 . The powder withdrawal port may also be located in the bottom plate (not shown) within the distributor box  28 . Side wall  42  includes holes  48  and the powder metering and withdrawal units  40 ,  40 ′ include a powder collection port  50  shown as an annular disc having a central hole  52  in registration with hole  48 . The hole or receptacle  52  has a known volume into which the particles  26  collect during dispensing from the fluidized bed. Port  50  includes a locking mechanism (not shown) to allow the metering units  40 ,  40 ′ to be attached to, or detached from, the side wall  42 .  
         [0073]    [0073]FIG. 2 shows a section of a fluidized bed  60  similar to bed  20  but with a powder metering and withdrawal unit  62  including a porous filter  64  located behind the collection port  50 . The powder dispensing can be assisted or enhanced by airflow from the enclosure  24  through holes  48  and  52  and through filter  64 . In addition, a vacuum port  66  may form part of the metering/withdrawal unit  62  being attached to the outer surface of filter  64  so that a vacuum can be applied from the back of the powder metering/withdrawal unit  62 .  
         [0074]    It is important to provide a steady supply of gas-solid suspension in the fluidized bed  20  or  60 . This can normally be achieved in any fluidized bed, but may be difficult for some fluidized beds especially for those containing cohesive Geldart C powders, since there may be several problems such as solids entrainment, solids attachment on the wall, variable solids inventory due to the above-mentioned two problems, particle agglomeration, channeling, and dead zones. Difficulty also further increases with the decrease of the amount of particles to be withdrawn from the bed. Therefore, certain additional measures preferably should be taken. In particular, the present invention is particularly suited for the dispensing and metering of extremely small quantity (1 μg-100 mg) of fine Geldart group C powders. As discussed above, these powders are very cohesive and tend to form agglomerates which makes it very difficult to fluidize and handle them. The requirement of small quantities of these types of powder in many applications presents another challenge in respect of accuracy of the amounts dispensed. In order to ensure a good and steady gas-solid suspension for Geldart C powder, the above mentioned (six) problems should be properly addressed. Additionally and optionally, the following measures may be taken to help ensure the accurate dispensing and metering of the small quantity of ultrafine cohesive powders. However, the use of the present invention is not limited to addressing the above six issues, but can also provide solutions in other applications such as addition and removal of fluid along a fluidized bed reactor.  
         [0075]    Many of the problems referred to above may be overcome using fluidized beds with porous walls that have a porosity sufficiently small to prevent significant loss of particles through the walls. FIGS. 3 a  and  3   b  show a porous fluidized bed at  70  having a rectangular housing  71  with surrounding walls  72  which are porous and a top plate  74  which may also be porous and/or has an air exit port. The porous bottom plate  76  is used as inlet for primary fluidizing air from the gas distributor box  78  and the top plate  74  and possibly part or all of the porous walls  72  for air outlets. Periodical back air-flow can be used to purge the porous walls  72  and  74  and to blow off the solids accumulation on the walls so that severe solid accumulation is prevented. The use of the porous top plate and surrounding walls as air outlets also reduces the chances of solid entrainment through any open exit such as the exit port  86 , thus giving the benefit of no solids loss with the air exhaust.  
         [0076]    Porosity of the bottom plates  30 ,  76  in FIGS. 1 and 3 a ,  3   b  may be achieved by drilling a number of holes in the distributor plate of distributor box  28  and  78  adjacent to the bottom plate  30  and  76 , aligning two layers of solid materials with holes and meshes sandwiched in between. In the fluidized bed  70  of FIGS. 3 a  and  3   b  the surrounding walls  72  and the top plate  74  may be made using porous materials such as sintered metals, solid materials with micro-holes, or two layers of solid materials with holes and meshes sandwiched in between. In the case of fluidized beds for handling very small particles (e.g. Group C size particles) which is the particle size for which the present invention is particularly suited it is important that the porosity (size of the holes or pores) of the walls be correlated with the particle size so that the pore size is small enough to allow the majority of particles to be retained in the fluidized bed. The porous materials may be hard or rigid materials suitable for structural purposes. If not, solid frames may be used to support the porous structure. One of the typical porous materials used for the present invention is sintered metals, which is very strong and thus can be used to perform structural functions as well.  
         [0077]    In the fluidized bed illustrated in FIGS. 3 a  and  3   b , primary fluidizing fluid such as air is injected as indicated by the arrow  82  through port  80  into the bottom of distribution chamber  78  which then passes through the bottom wall  76  to fluidize solid particles such as a fine powder to form the fluidized bed. Additional fluidizing air different from the primary fluidizing fluid may be injected into the bed  70  through one or more of the walls  72  or top plate  74  or alternatively air may be drawn out or ejected through the porous walls  72  or top plate  74  as indicated by the arrow  84 . Optionally, a primary air outlet  86  can be included in top plate  74  and/or the surrounding walls  72  (the latter embodiment not shown in FIG. 3).  
         [0078]    [0078]FIG. 4 a  shows an embodiment of porous fluidized bed which is constructed for rotation during operation. Fluidized bed  90  includes a cylindrical housing  92  (FIG. 4 a ) having a cylindrical axis  94  with the cylindrical housing oriented horizontally. Housing  92  is adapted to be rotated about the horizontal axis  94  by a motor  96  coupled to a shaft  93  extending along axis  94  from the housing  92 . Cylindrical housing  92  includes a porous cylinder wall  98 , one porous end plate  99  and another porous end plate  100 , and a powder metering/withdrawal unit  102  located in plate  100 . A gas distribution box or manifold  104  is positioned adjacent to the porous cylindrical wall  98 . Manifold  104  remains stationary while cylindrical housing  92  is rotated about axis  94  and since cylindrical housing  92  is porous, the powder on the interior of the housing is constantly being fluidized. A similar design may be used but with the housing oriented vertically and driven by a gear motor about the vertical housing axis.  
         [0079]    There are several significant benefits achieved by having the fluidized bed housing rotated. The first advantage is that the top part (main fluid exit) and the bottom part (primary fluidizing fluid inlet) of the cylindrical wall  98  are switched continuously during rotation. At any given time at least part of the cylindrical wall  98  forms both inlet for primary fluidizing fluid to form the bed as indicated by the arrows at the bottom and the primary outlet for fluidizing fluid at the top. In this case, particles that may be adhering to wall  98  are blown away from the wall by the incoming fluid when that part of the wall is rotated into a position adjacent to manifold  104 .  
         [0080]    Secondly, the relative movement between regions of the fluidized bed and the gas inlet provide a periodic back purge to a greater area of the bed, so that dead zones are effectively reduced or eliminated. Thirdly, the rotation of housing  92  creates additional agitation to the fluidized bed, and thus helps to break up agglomerates and prevent severe channeling of the fluidized bed.  
         [0081]    With the prior art rotating fluidized beds, air is forced to leave the bed axially along the horizontal axis of rotation. The rotating fluidized beds disclosed herein are preferably operated at low rates of rotation, for example 60 revolutions per minute (RPM) has been found to be effective, whereas the centrifugal beds of the prior art rotate much faster since they need to use the rotating action to create large pseudo-gravitational forces.  
         [0082]    If the fluidized bed is designed so that a significant portion or substantially all the fluidizing gas must leave the bed through the porous walls of the housing, there will be little or no particle loss. Even when there is an exit port provided, the fact some gas leaves the housing through the side and top porous walls will reduce the gas velocity through the outlet port and thereby reduce the chances of particle loss.  
         [0083]    Powder withdrawal can be facilitated through one or more withdrawal port at the walls of the fluidized bed, which can be horizontal, vertical or of any other geometry. Fluidized bed  90  in FIGS. 4 a  shows one option of having the withdrawal port  102  on the end plate  100  of the horizontal cylindrical and porous fluidized bed, but it will be understood that it may be placed in other locations on the housing.  
         [0084]    Fluidizing gas may be selectively injected or removed at selected rates through localized areas of the top plate  74  and surrounding side walls  72  as exemplified by the bidirectional arrows shown in FIGS. 3 a  and  3   b  so that air flow into and out of housing  71  is not restricted to the area adjacent to the manifold  78 . Similarly, the bidirectional arrows through the porous cylindrical wall  98  and the porous end walls  99  and  100  of housing  92  in FIG. 4 indicate that bidirectional air flow into and out of the housing  92  is not limited to the portions of cylinder wall  98  adjacent to the gas manifold  104 . Referring to FIG. 5, as an example, a gas inlet manifold  110  positioned against porous wall  112  of the bed housing is formed with three separate compartments  114  into each of which fluid may be selectively injected or removed from each of the compartments through the fluid ports  116  as indicated by the arrows. Such manifolds may extend all around the top and surrounding walls of bed  70  and the full periphery of the cylindrical plus side housing walls for bed  90 , or they may extend only partially around the chamber or bed  70  or  90 . The dimensions of the different compartments of the manifold need not be the same but may be different depending upon the application of the fluidized bed and the fluidizing gas requirements. Furthermore the shape and orientation of the manifolds need not be rectangular nor need they extend axially/horizontally along the full length of the bed housing.  
         [0085]    In another embodiment of the manifold, discrete injector or withdrawal nozzles or pipes may be used (not shown). The nozzles may be mounted to be moveable so that the location of each of the nozzles may be varied if desired.  
         [0086]    Referring now to FIG. 6, a variable-volume porous fluidized bed is shown generally at  130  which includes a movable wall  132  located in cylindrical housing  92  which can be moved back and forth along the cylindrical axis  94  of housing  92  for reducing or increasing the volume of enclosure  24  during the course of operation to provide a relatively consistent gas-solid suspension when the fluidized bed dispenser is in batch operation mode. As particles are withdrawn from the bed, the amount of particles left in the bed will gradually decrease and so will the suspension density. Without volume compensation, particle withdrawal, for example, would require longer withdrawal times to dispense the same amount of material. Such reduction of the bed volume will compensate for the reduction of the particle quantity, so that the gas-solid suspension density can be maintained relatively constant. The volume change is done in a controlled manner such that the decreased volume just compensates for the reduced particle quantity.  
         [0087]    [0087]FIGS. 7 a  and  7   b  illustrate a fluidized bed at  140  modified to have additional gas injection directly into the fluidized bed to provide additional agitation to and to mobilize the fluidized bed, to create preferred gas flow patterns that make the particles/agglomerates to impact among themselves, and to provide purging to the inside of the withdrawal port and dead corners of the bed. In all cases, the opening for the injection gas should be small enough to create a high velocity nozzle flow, to make the gas agitation more effective. In bed  140  shown in FIG. 7 a horizontal tube  142  with holes  144  spaced along the tube  142  is inserted along the rotational axis  94  of housing  92 . The air injection not only provides additional agitation to the bed, but also cleans the edges between the cylindrical housing  92  and the two end plates  99  and  100 . Since the fluidized bed housing  92  itself is rotating, the nozzle gas injection actually “moves” inside the bed relative to the fluidized particles so that it provides agitation periodically to different regions of the bed. This can also effectively reduce or prevent dead zones from arising. It is understood that although there are two holes shown in FIG. 7, there may be more or only one hole as may be required.  
         [0088]    The end of tube  142  is positioned next to the inside of the withdrawal port  102 . Occasionally, particles may accumulate just inside the withdrawal port  102  which can influence the metering speed and accuracy. In this case, this small purge gas can be applied through an end hole facing the powder metering port (not shown in the figure) to blow off the accumulations. Such purging is best done between two consecutive metering/withdrawals or when the new withdrawal units are loaded into the fluidized bed, since purging, if strong enough, may also blow off particles inside the charged cavities and/or create a localized strong gas flow pattern that prevent or reduce the amount of the gas-solid suspension (from) flowing into the withdrawal region. Alternatively, the horizontal tube  142  may be retractable, so that it is retracted away from the withdrawal port but maintains other regions of the fluidized bed aerated.  
         [0089]    [0089]FIG. 8 shows another embodiment of a fluidized bed at  150  having several nozzles  152  distributed in different regions of the bed. Two or more nozzles  152  can be placed facing each other to create a flow pattern for the particles to collide with each other producing a jet milling action. In addition, one or more portions of the tubes extending from the chamber wall to any nozzle  152  may be flexible tubing so that with the proper combination of the tube materials (e.g. rubber) and gas flow rates the nozzle will “dance” randomly due to the air flow from the flexible tube. This produces yet additional agitation to the fluidized bed, and thus reduces or eliminates the problems of particle agglomeration, dead zones and channelling.  
         [0090]    [0090]FIG. 9 illustrates part of a fluidized bed  160  adapted so that mechanical, acoustic or ultrasonic vibration can be applied to the fluidized bed to provide additional agitation. An external source of vibration  162  is coupled to housing  92  in order to generate and pass mechanical, acoustic and/or ultrasonic vibrational energy to the powder inside the housing. Such vibration will be transferred onto the bed wall and/or to some mechanical parts (e.g., preferably a mesh  164  with rigid frame) located in enclosure  24 . The bed wall and/or the frame inside the bed then vibrates at very high frequency, providing additional agitation to break the powder agglomerates. FIG. 10 shows an alternative embodiment in which a vibrational source  163  is positioned in the gas manifold  78 .  
         [0091]    Adding larger beads (for example, 4-20 mm diameter stainless steel balls and/or 5-20 mm diameter marbles, but beads of larger or smaller diameter and of other materials may be used) to the rotating fluidized bed can provide further additional agitation for breakup of particle agglomerates, prevention of dead zones and channelling.  
         [0092]    [0092]FIGS. 11 a  and  11   b  show a fluidized bed  170  including most of the important features discussed above, a variable-volume, rotating and porous fluidized bed dispenser with large beads inside the housing, which is enclosed in a sealed case  172  for practical use.  
         [0093]    It will be understood that while the above description has been exemplified with the gas-solid fluidized bed, the same inventive concepts may also be applied to liquid-solid and gas-liquid-solid three-phase fluidized beds. In addition, more than one powder metering/dispensing unit may be attached to the housing so that powder can be dispensed from multiple locations in the housing.  
         [0094]    [0094]FIGS. 12 a ,  12   b ,  13   a  and  13   b  illustrate different preferred embodiments of the powder metering/dispensing units which are mounted on the fluidized bed housings in flow communication with the interior of the housings. The powder metering/dispensing units comprise a powder collector which includes a front blister plate (or circular disk as shown)  180  having one or more cavities or holes  182  in the disks with the cavities having pre-selected volumes which are used for collecting the powder from the fluidized bed. Each blister disk  180 , also referred to as a blister cell, is mounted on a perforated support plate  184  using a screw  186  with the perforations  185  in plate  184  in registration with holes  182  in the blister plate  180  so that air can flow through the blister plate when it is mounted on the fluidized bed. Filter  64  (FIG. 12 b ) is sandwiched between the blister plate  182  and support plate  184  acts as a porous barrier to prevent particles being drawn through the holes  185  while at the same time allowing air flow by applying a pressure difference on either side of the plate  184 /blister plate  180 , or by a vacuum on the other side of plate  184  on the outside of the chamber. The plate  184  is mounted with the blister disk  180  in registration with aperture  48  in the fluidized bed (FIG. 14 b ) with the cavities  182  facing the interior chamber  24  of the fluidized bed. The blister disks  180  attached to support plate  184  is first mounted on the bar strip  194 , which is then detachably removable from the fluidized bed housing. Particles inside the small cavities  182  are then either kept there for future use (if the purpose of metering is to dispense the powder just into those cavities) or removed (for example by being blown out) from the cavities and transferred to the desired container. FIGS. 13 a  and  13   b  illustrate an alternative disk blister cell  180 ′, with the areas of the disk in the vicinity around the blister holes  182  being partially thinned to reduce the metering volume, while maintaining the mechanical strength by keeping enough thickness for the rest of the front disk  180 ′.  
         [0095]    While the blister disks of FIGS. 12 and 13 are shown mounted on support or back plates  184  ( 184 ′), it will be understood that the support plate  184  could be used itself as the powder collector as long as the cavities  185  located in it are of the desired volume and do not penetrate right through the plate.  
         [0096]    [0096]FIGS. 14 a  and  14   b  show partially a fluidized bed  190  having a powder metering and withdrawal unit  192  which includes a blister cell bar strip  194  for mounting thereon blister disks  180 . FIG. 14 a  shows a bar strip  194  having two circular disks  180  mounted on the strip along its length. Referring to FIG. 14 b , powder withdrawal unit  192  includes a slot  193  at the front that allows the insertion of the bar strip  194 , an inner tube  195  in communication with chamber  24  in the housing, and an outer tube  196  concentric with inner tube  195  and a spring  197  around tube  195  encased in tube  196  pressing a spacer  198 . The slot  193  is facing the opening on one side of the fluidized bed into which strip  194  can be fitted. The purpose of the spring  197  is, through the spacer  198 , to press the bar strip  194  tightly against the side plate of the fluidized bed so that a good seal is maintained. The inner tube  195  is used for outflow air to enhance the powder withdrawal process, which may be enabled by higher pressure inside the fluidized bed or a vacuum at the outside. Strip  194  is also perforated at those positions where the support plates  184  are mounted so that suction through the strips  194 , support plates  184  and blister disks  180  can be obtained for suction assisted filing of the cavities  182 .  
         [0097]    Referring again to FIGS. 14 a  and  14   b  during the metering, one disk blister disk  180  is exposed to the fluidized bed for powder withdrawal. After the holes or cavities  182  of the disk  180  are filled, either by controlling the withdrawal time or by allowing the cavities to be filled up, the strip  194  is moved in the groove a sufficient distance to move the charged disk  180  away from the fluidized bed  190  whereupon disk  180  can be removed from strip  194 . At the same time another disk  180  can either be attached to strip  194  at the same position and reinserted into position adjacent to aperture  48  in the withdrawal position or strip  194  can be slid along to align another disk  180  at another position on the strip  194  with opening  48 .  
         [0098]    Preferably, the blister disks  180  are porous (holes  182  extending right through the disk) so that gas can flow through the disks. This gas flow helps push or suck powder into the cavities  182 . Such gas flow not only accelerates the filling, but also provides increased accuracy since the “driving force” is larger and more steady. Such gas flow can be created by a pressure difference, which can be realized by either applying vacuum suction from the back or increasing the pressure inside the fluidized bed, or both. The gas passage in this case may be a straight hole, produced by making two thin disks (front and back) with the same number of holes in each disk and with their centers lined up to each other. Between the two disks, a thin layer of porous material (filter paper, membrane materials etc.) is used, creating a cavity on each side of the disk, but it is only the front side  182  that is used as the blister cell. Preferably, the holes on the support plate  184  are made larger than those of the blister cell to reduce the resistance to air flow.  
         [0099]    It will be understood that while the above description has been exemplified with the disk shaped metering unit and with cylindrical cavities, other suitable designs with suitable powder receptacles can also be used to meter and collect the powder from the fluidized bed. For example, a bar strip similar to  194  shown in FIG. 14 a  can be made into a powder metering/collecting unit, by having a blister bar with many blister cells coupled with a supporting bar on the back.  
         [0100]    [0100]FIGS. 15 a  and  15   b  show two views of a fluidized bed  200  with associated metering and dispensing unit  202  attached at the aperture  48  and including a drive mechanism  210  attached to the support strip  194  for positioning a series of blister disks  180  at the dispensing unit. Drive mechanism  210  is a manual feed comprising a hand crank  212  driving a screw  214  which drives strip  194 .  
         [0101]    [0101]FIGS. 16 a  and  16   b  show a dispensing apparatus  220  similar to apparatus  170  in FIGS. 11 a  and  11   b  but apparatus  220  includes the powder metering/withdrawal unit  192  of FIG. 14 b.    
         [0102]    [0102]FIGS. 17 a  and  17   b  illustrate an alternative fluidized bed shown generally at  300 . This is a porous, but non-rotating, fluidized bed. In this unit, two cylinders  304  and  306  are used and placed concentrically, with their axes in the vertical direction (but can be horizontal). The inner cylinder  304  may be made of a porous Teflon tube and the outer cylinder  306  may be made of a non-porous Teflon tubing. Between the inner and outer cylinders six sealed chambers  302  are defined. Air can be forced into, and released from, the chambers  302  through six air ports  308  on the outer cylinder  306 .  
         [0103]    Periodic reversal of the airflow direction across the wall of inner cylinder  304  keeps the wall of cylinder  304  clear of significant particle accumulation while still providing enough area for air to leave. Referring to FIG. 17 a , an air distributor box  310  for injection of air into cylinder  304  is located at one end of the concentric cylinders  304  and  306  and includes a porous Teflon plate  312  through which fluidization air is continuously injected. During operation air is being injected into two of the chambers  302  through the associated ports  308  and air is exiting four of the chambers (or alternatively air enters three and exits the other three chambers  302 ) through their associated ports  308 . Solenoid valves (not shown) associated with each of the ports  308  are used to automatically switch the air flow direction into and out of the chambers  302 . The powder withdrawal unit  192  is located at the opposite end of the concentric cylinders  304  and  306  and includes disk collection strip  194 . Since fluidized bed  300  is non-rotating, a seal between the drum and disk collection strip  194  is better maintained so that it minimizes leaking through the sampling port. It also eliminates the air loss before entering the housing  304 . Wear on the unit is also minimized since there is no rotation.  
         [0104]    [0104]FIGS. 18 a  and  18   b  show a unit  320  that combines the features of the unit  220  shown in FIG. 16 (rotating, single cylinder) and the unit  300  shown in FIG. 17 (double-cylinders with sealed air in/out compartments). Essentially, the single cylinder arrangement (housing  92 ) in unit  220  plus the gas distributor box  104  are replaced by the rotating double-cylinder arrangement in unit  300 , in the new unit  320 . In a particular case, a concentric nonporous outer cylinder  306  is used to encase the inner porous cylinder  304  that houses the fluidized bed. The inner cylinder is a porous Teflon tube and the outer cylinder is a non-porous Teflon or stainless steel tubing. A gap is located between the inner and outer cylinders which defines six sealed chambers  302 . Three (or two) chambers  302  located along the bottom may be used for air flow into the inner cylinder while the upper three (or four) chambers along the top may be used for air outflow from the inner cylinder.  
         [0105]    Referring again to FIG. 18 a , located on the left side of the double cylinders  304  and  306  is an end plate  322  which is glued to the end portion of these cylinders. The concentric double cylinders  306 / 304  with end plate  322  attached thereto are mounted in frame  321  for rotation by the rotational drive unit  96  while the end plate  322  which is mounted in frame  321  remains fixed with respect to the cylinders. The powder withdrawal/dispensing unit  192  is mounted on the other end plate  323  at the other end of the double cylinder which is attached to the double cylinder so that it rotates with the double cylinder. The powder withdrawal/dispensing unit  192  is rotatably mounted on the end plate so that as the end plate rotates with the double cylinder, dispensing unit  192  remains stationary and is supported by frame  321 . Fluid injection and escape passageways are located in end plate  322  as indicated by the arrows so that under pressure air is forced into the bottom three (or two) chambers  302  while the air flows out from the top three (or four) chambers  302 . At any moment, two (or three) of the sealed chambers  302  along the bottom of unit  320  are used as the air distributor for the air injection, while the other three (or four) sealed chambers along the top serve as the air exit for air releasing. When rotating the double cylinder together, the air injection and releasing compartments  302  alternate to form periodical airflow reversal across the inner cylinder wall, keeping the wall clear from significant particle accumulation while still providing enough area for air to flow through. The particle withdrawal unit  192  is located at the opposite end of the rotating housing to the air injection, with the powder withdrawal.  
         [0106]    There are several practical benefits to use the double cylinder housing: the inner cylinder can be made from less strong materials such as the more fragile porous Teflon tubing since it does not rotate relative to the outer cylinder. Also because there is no rotating action between the two cylinders, leaking between the housing  304  and distributor chambers  302  are prevented. Erosion of both cylinders is also minimized since there is no relative rotation of the inner cylinder and only small touching area against the outer cylinder at the air distributor box.  
         [0107]    [0107]FIG. 19 shows another embodiment of a powder metering and dispensing unit at  360  where single dosages of powder are dispensed one at a time. The metering/withdrawal port includes a central stationary air inlet tube  362  with a channel  364  down the center of the tube  362 . On the rotating housing  92  a series of calibrated holes  368  in which the powder collects are disposed in a circle around the axis. A vacuum assisted powder loading mechanism  366  includes a tube  370  connected to a vacuum source with the tube  370  being fixed with respect to tube  362 . Tube  370  includes a channel  372  and filter  374  is located in channel  372  having a gauge small enough so that no particles are pulled through the filter  374 . When either (does either mean any of the holes?) hole  368  is aligned in registration with channel  372  powder is pulled into the hole  368 . A guide  376  having a channel  378  is located on the inside of housing  92  and is fixed to tube  362  so it does not rotate. The powder outlet is defined by a tube  380  having a channel  382  which is fixed with respect to tube  362  with channel  382  radially spaced from channel  364  so that as housing  92  rotates holes  368  periodically align with channel  382  whereupon the powder collected when hole  368  was in registration with channel  372  is blown out through channel  382  under pressurized air since that particular hole is in flow communication with pressurized air injected into channel  364 . Therefore, for each rotation of the housing  92  each hole  368  is filed with a pre-selected quantity of powder thereby dispensing powder in a continuous shot-by-shot manner.  
         [0108]    As used herein, the term “comprising” is to be construed as being inclusive and open ended, and not exclusive.  
         [0109]    The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.