Drop penetration

Bombs, Earthquake. Large bombs which, when dropped, penetrate and explode beneath the surface of the earth(as deep as 20ft) producing an effect similar to an earthquake(Refs 1 3)... 

The mu. tard-gas drops penetrate with great speed and facility any objects with which they come in contact. They easily penetrate Ipather and rubber boots, uniforms, and other articles of equipment worn by soldiers. The mustard liquid is thus easily carried about by soldiers and spread and evaporated-in other previously uninfected places. Frequently in the late war all the occupants of a dugout w ere contaminaud and made ill by the mustard adhering to the clothing of a. singh soldier whft was not even aware of its pre. ence. 

Beginning with penetration time, Eq. (21-98h) defines key formulation properties controlling capillary rise in powder beds. From considering a distribution of macro- and micropores in the moving powder bed as shown in Fig. 21-102, Hapgood (loc. cit.) determined a total drop penetration time tp of... 

As shown previously, drop wet-in time decreases with increasing pore radius decreasing binder viscosity and increasing adhesion tension. In addition, drop penetration time decreases with decreasing drop size and increasing bed porosity Seff. Effective pore radius R i is related to the surface-volume average particle size ds2, particle shape, and effective porosity of packing Seff by... 

FIG. 21-102 Drop penetration in a moving powder bed. [After Hapgood loc. 

In addition, the fraction of single drops forming individual nuclei (assuming rapid drop penetration) versus the number of agglomerates... 

The spray flux captures the impact of equipment operating variables on nucleation, and as such is very useful for scale-up if nucleation rates and nuclei sizes are to be maintained constant. The overall impact of dimensionless spray flux on nucleation and agglomerate formation is illustrated in Fig. 21-106, with agglomerates increasing with increased spray flux as clearly governed by Eq. (21-106) for the case of rapid drop penetration. 

Regimes of nucleation may be defined (Fig. 21-107) with the help of dimensionless drop penetration time and spray flux / , or... 

A droplet-controlled nucleation regime occurs when there is both low spray flux (relatively few drops overlap) and fast droplet penetration—drops wet into the bed completely before bed mixing allows further drop contact. Nuclei will be formed of the order of drop size. A mechanical dispersion regime occurs at the other extreme of high spray flux, giving large drop overlap and coalescence, and large drop penetration times, promoted by poor wet-in rates and slow circulation... 

Spray distribution generally has a large effect in fluid beds, but in many cases, a small effect in mixers. In fact, fluid-bed granulation is only practical for wettable powders with short drop penetration time, since otherwise defluidization of the bed would be promoted to local pooling of fluid. Mechanical dispersion counteracts this in mixers. There may be a benefit, however, to slowing the spray rate in mixers for formulation with inductive growth behavior, as this will minimize the lag between spray and growai, as discussed previously. 

Is the formulation wettable with fast drop penetration time Is there possible preferential wetting of active vs. excipients Is little growth required, applying only a nucleation stage of granulation ... 

If little growth is desired, granulation may be halted early in growth, or limited to nucleation. If the formulation is wettable by the binding fluid, this is best achieved in the drop-controlled regime of nucleation, which requires fast drop penetration and low spray flux [Eq. (21-107)], Fig. 21-107). Small drop penetration time fp is possible for low binder viscosity, high adhesion tension, coarse feed powders, and... 

Dimensionless spray flux and drop penetration time... 

Equation (21-149) assumes a very fast drop penetration time, and that spray zones do not interact, or multiple passes do not cause drop overlap on previous nuclei. To compensate for this, smaller fluxes may... 

Nucleation in fluidized-bed granulation by necessity occurs within a drop-controlled regime, which requires fast drop penetration and low spray flux [Eq. (21-107), Figure. 21-107]. Spray flux / should be no more than 0.2, and quite possibly much lower. Increasing wettability has been shown to increase nuclei size, presumably due to more stable operation (Fig. 21-99). Figure 21-168 illustrates the impact of increasing spray flux and fluid-bed gas velocity on size distribution. Decreasing dimensional spray flux (which is inverse to... 

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