Fluent material handling – with receiver or receiver coacting mea – Filling or refilling of dispensers
Reexamination Certificate
2001-12-17
2004-02-03
Huson, Gregory L. (Department: 3751)
Fluent material handling, with receiver or receiver coacting mea
Filling or refilling of dispensers
C141S045000, C141S054000, C141S067000, C141S236000, C141S286000, C141S302000, C141S307000, C222S195000, C222S630000
Reexamination Certificate
active
06684917
ABSTRACT:
FIELD OF THE INVENTION
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
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.
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.
To facilitate pulmonary delivery, drug powders should normally be less than 5 &mgr;m 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 &mgr;g-100 mg), this makes it very difficult to accurately meter such a small quantity and fill them into packages.
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%.
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' 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' inhalation does not coincide with the drug releasing.
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 &mgr;g 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
.
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.
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.
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.
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.
Powders may be classified into four groups in gas-solid fluidized systems, according to Geldart'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. Those strong interparticle forces cause the fine particles to agglomerate and make them very cohesive. Typical Group C powders comprise particles under 30-45 &mgr;m 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 particle
Ma Ying Liang
Wen Jianzhang
Zhang Hui
Zhu Jesse
Hill & Schumacher
Huson Gregory L.
Schumacher Lynn C.
The University of Western Ontario
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