Dispersion terminal settling velocity

Airborne particulate matter may comprise liquid (aerosols, mists or fogs) or solids (dust, fumes). Refer to Figure 5.2. Some causes of dust and aerosol formation are listed in Table 4.3. In either case dispersion, by spraying or fragmentation, will result in a considerable increase in the surface area of the chemical. This increases the reactivity, e.g. to render some chemicals pyrophoric, explosive or prone to spontaneous combustion it also increases the ease of entry into the body. The behaviour of an airborne particle depends upon its size (e.g. equivalent diameter), shape and density. The effect of particle diameter on terminal settling velocity is shown in Table 4.4. As a result ... 

The terminal settling velocity is given by Equation 8.6 or 8.8. Decanters are normally designed for a droplet size of 150 p,m3,9, but can be designed for droplets down to 100 p,m. Dispersions of droplets smaller than 20 p,m tend to be very stable. The band of droplets that collect at the interface before coalescing should not extend to the bottom of the vessel. A minimum of 10% of the decanter height is normally taken for this3. 

This method can be easily used to show the logic behind the scale-up from original R D batches to production-scale batches. Although scale-of agitation analysis has its limitations, especially in mixing of suspension, non-Newtonian fluids, and gas dispersions, similar analysis could be applied to these systems, provided that pertinent system variables were used. These variables may include superficial gas velocity, dimensionless aeration numbers for gas systems, and terminal settling velocity for suspensions. 

In Fig. 5, the liquid phase axial dispersion coefficient Daxi and the Bodenstein number Bo calculated from this relationship according to Eqs. (9), (10) and (13) are plotted for a range of linear velocities used in fluidized bed adsorption. The physical parameters of the commercial Streamline SP adsorbents (average particle size 247 pm, average particle density 1143 kg/m3, terminal settling velocity 0.0044 m/s, n = 4.7 as described by Chang and Chase [37]) were... 

Kato, et al (5 ) measured solid concentration profiles, solid dispersion coefficients and terminal settling velocities for glass spheres in water, using 6.6, 12.2 and 21.k cm bubble columns. They developed a dimensionless, empirical correlation for the solid dispersion coefficients which agreed with their observed values to within 20%. 

Dilute This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon each other. Specifically, the solids phase is so fully dispersed in the gas that the density of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal settling velocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Gravity settling chambers such as prilling towers and countercurrent-flow spray dryers are two exceptions in which gas velocity is insufficient to entrain the solids completely. 

Sedimentation [22] of the disperse phase usually takes place under the force of gravity. (It may be caused also by an imposed centrifugal force field). A sphere of diameter D and density d under the influence of gravitational force Fg will have a terminal settling velocity Vts in the laminar region governed by Stake s law ... 

This approach is usually not practical for drops less than 100 pm, since for most situations large cross-sectional areas are needed to reduce the bulk velocity below the terminal settling velocities of such drops. An air stream with a 10 ft/s vertical velocity will entrain water drops less than about 700 pm, while a bulk velocity of 1.0 ft/s will entrain drops of about 100 pm or less. Gravity techniques can be used to remove large quantities of die latger-size dispersed-phase materia prior to some other segregation technique. 

The elutriation method is really a reverse sedimentation process in which the particles are dispersed in an upward flowing stream of fluid. All particles with terminal falling velocities less than the upward velocity of the fluid will be carried away. A complete size analysis can be obtained by using successively higher fluid velocities. Figure 1.4 shows the standard elutriator (BS 893)(6i for particles with settling velocities between 7 and 70 mm/s. 

Settling of a particle with radius a in a dilute suspension is hindered by the drag exerted in a dispersion medium. The resistance of the medium is proportional to the settling velocity of the particle. In a very short time, the particle reaches a constant velocity, known as the terminal velocity. The gravitational force on the particle balances the hydrodynamic resistance of the medium as given by ... 

Gravity separations depend essentially on rite density differences of rite gas, solid, or liquids present in the mix. The particle size of the dispersed phase and the properties of die continuous phase are also factors with rite separation motivated by die acceleration of gravity. The simplest representation of this involves rite assumption of a rigid spherical panicle dispersed in a fluid with rite terminal or free-settling velocity represented by... 

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