Coarse dispersion flow

For the remainder of this paper, we shall illustrate the general capacity of the above fluid dynamic scheme to adequately describe coarse dispersion flow. With this purpose in mind, we shall briefly consider the results of applying this scheme... 

The basic problem that developers have been trying to solve is the need for a dry powder to flow, for dosing and emptying purposes, yet disperse adequately to make an inhalable aerosol. In terms of conventional powder technology these two requirements are usually mutually exclusive. Coarse powders flow well but contain very little respirable material. Fine powders possess a high respirable content but generally exhibit very poor flow properties. The focus then has been on trying to find ways to make these contradictory requirements compatible. 

Thus, in order to render the stability theory completely determinate, we need to specify in an unequivocal form both the conservation equations governing macroscopic suspension flow and all the rheological equations of state. This is easily seen to be possible for coarse dispersions of small particles. For such dispersions, normal stresses in the dispersed phase may be approximately described in terms of the particulate pressure as explained in Section 4, and this pressure can be evaluated for uniform dispersion states with the help of Sections 7 and 8. As a result, particulate pressure appears to be a single-valued function of mean variables characterizing the uniform dispersion state under study and of the physical properties of its phases. This single-valued function involves neither unknown quantities nor arbitrary parameters. On the other hand, if the particle Reynolds number is small, all interphase interaction force constituents also can be expressed in an explicit consummate form with help from the theory in reference [24]. This expression for the fluid-particle interaction force recently has been employed as well in stability studies for flows of collisionless finely dispersed suspensions [15,60]. 

The severe implications of these facts have been partially uncovered in reference [5] as a result of formulating a kinetic theory for granular flow without interaction with the ambient medium. These implications, as well as additional difficulties due to the necessity to calculate the energy supply to the particle fluctuations, make somewhat problematic, at the present state of the art, the formulation of a reliable and sufficiently simple hydrodynamic model even for coarse dispersions. We have succeeded in this respect only at the expense of making certain supplementary assumptions. These assumptions cne ... 

A typical application of iron oxidation for As(III) was the SONO filter. Inside the filter, there is a top layer composed of coarse river sands (CRS), which is an inactive material as a coarse particulate filter, disperser, flow stabilizer and providing mechanical stability in the filter (Hussam et al., 2007), but generates high concentrations of soluble iron and precipitate as Fe(OH)3 to oxide the groundwater. 

The effective viscosity of a coarse dispersion of two liquids of viscosities r], and 770 can be described by the equation for two-phase flow ... 

Commonly, the most important feature of a nozzle is the size of droplet it produces. Since the heat or mass transfer that a given dispersion can produce is often proportional to (1/D ) , fine drops are usually favored. On the other extreme, drops that are too fine will not settle, and a concern is the amount of liquid that will be entrained from a given spray operation. For example, if sprays are used to contact atmospheric air flowing at 1.5 m/s, drops smaller than 350 [Lm [terminal velocity = 1.5 m/s (4.92 ft/s)] will be entrained. Even for the relative coarse spray of the hoUow-cone nozzle shown in Fig. 14-88, 7.5 percent of the total hquid mass will be entrained. 

Blood agents have also been adsorbed onto a solid carrier, such as pumice, to facilitate dispersal of the agent and increase their persistency after their release. These compositions may appear as free-flowing powders or as a coarse grit. Odors may vary from the unmodified agent. 

Where there is a wide range of particle sizes in the powder, fluidisation will be more even in a bed that is tapered so as to provide the minimum cross-section at the bottom. If the pressure gradient is low and the gas does not therefore expand significantly, the velocity will decrease in the direction of flow. Coarse particles which will then tend to become fluidised near the bottom of the bed assist in the dispersion of the fluidising gas. At the same time, the carry-over of fines from the top will be reduced because of the lower velocity at the exit. 

Such small particles usually are generated by air-jet micronization and less frequently by controlled precipitation or spray drying. As bulk powder, they usually tend to be very cohesive and exhibit poor flow and insufficient dispersion because of large interparticle forces such as van der Waals and electrostatic forces (Zeng et al. 2001 Podczeck 1998 Hickey et al. 1994). The control of sufficient powder flow and deaggregation (dispersion) is thus of utmost importance to ensure efficient therapy with a dry-powder aerosol. Two different formulation approaches are used currently in marketed DPI preparations to fulfill the requirements. Most often, coarse particles of a pharmacologically inactive excipient, usually a-lactose monohydrate, are added that act as a carrier and provide sufficient powder flow to the mixture. Other carbohydrates, amino acids, and phospholipids have been suggested frequently (Crowder et al. 2001). 

Furcelleran occurs as a brown or tan to white, coarse to fine powder. It is soluble in water at a temperature of about 80°, forming a viscous, clear or slightly opalescent solution that flows readily. It disperses in water more readily if first moistened with alcohol, glycerin, or a saturated solution of sucrose in water. 

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