Powder bed

The fluid plasticizer (solvent) consists of an energetic compound, eg, nitroglycerin, an inert carrier, and a stabilizer. The system is evacuated to remove volatiles, moisture, and air, and the plasticizer is then pressurized and passed slowly upward through the powder bed while the powder is held stationary by a pressure plate on the powder column. Casting solvent may also be added from the top of the mold. 

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

Vibratory Compaction. Vibratory compaction (38,53) provides a means of forming irregular shape compacts from ungranulated powders. Under agitation, a powder bed tends to rearrange to a configuration of closest packiag. The ampHtude and frequency of vibration required for compaction are dependent on the size and size distribution of the particles present, and the size of the compact, and are determined experimentally. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

The second approach to characterize wetting considers the abihty of the fluid to penetrate a powder bed. It involves the measurement of the extent and rate of fluid rise by capillaiy suction into a column of powder, better known as the Washburn test. Considering the powder to consist of capillaries of radius R, the equilibrium height of rise... 

PLATE 3. Multiple bulking brush discharges on powder bed (time exposure). (From Bartkneeht in Industrial Dust Explosions," ASTM STP 9S8. p. 17S)... 

To avoid ignition via these indirect sparking mechanisms, the resistance to ground of conductive objects that could be encountered by discharges on powder beds should be less than 1 k 2 and preferably less than 100 2, so that the maximum potential attained is less than 1 kV. 

GPl7][R21] Thepressuredropinafilamentous-bedreactor(i.d. 7 pm porosity 0.8),whichwasusedforpropanedehydrogenation,wascomparedwitharandomly packed powder bed (sphere diameter from 100 to 160 pm) of similar hydraulic... 

Another approach to determining the contribution being made by each of the possible com-pression/decompression mechanisms involves monitoring the degree and rate of relaxation in tablets immediately after the point of maximum applied force has been reached. Once a powder bed exceeds a certain yield stress, it behaves as a fluid and exhibits plastic flow [121,122], Certain investigators [122] have studied plastic flow in terms of viscous and elastic elements and have derived the following equation ... 

Fig. 16 Typical force-time trace from an instrumented Zanasi LZ-64 automatic capsule-filling machine. PC, precompression resulting from dipping of dosator into the powder bed C, compression resulting from actual piston tamping R, retention force Ej, ejection D, drag force developing during retraction of piston. (From Ref. 38.)...

Fig. 17 Effect of powder bed height, piston height, and compression force on plug ejection force in an instrumented Zanasi LZ-64 automatic capsule-filling machine (pregelatinized starch lubricated with 0.005% magnesium stearate). Note that the first point of each curve is precompression. Piston height (mm) , 15 -jlf, 14 , 13 , 12. Powder bed height (mm) heavy line, 30 light line, 50. (From Ref. 51.)...

An alternative to the stirred tank system is a column-type device which provides for constant fluid flow through a powder bed. The mass transport process was shown to be primarily determined by the length and cross-sectional area of the cylinder and the fluid flow rate [36],... 

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