Particle interception with surface

The contribution of atmospheric dust to surface dust depends on the dust falling to the earth. This occurs either as dry dust fall or wet washout with rain, snow or hail (1-6,8-10). Dry dust fall occurs by s imentation, impaction, interception or diffusion. Sedimentation, the fall under gravity, may be estimated using Stoke s law which relates the density and diameter of particles to their falling velocity. A particle of density 1.0 g cm"3 and diameter around 0.1 pm would fall with a velocity of around 9 x 10" cm s" ... 

It has been found that a plot of pg / Vg [1-pgl vs pg is hnear for the pressure range of 0.05 to 0.4, with aslope of (C - 1) / (Vmono C ) and intercept of 1/ (Vmono C ). Let us now do a simple calculation using BEH data obtained. Suppose we have a 20 gm. sample having a density of 2.0. We measure the surface area as 6 m. From the area of a sphere. A = r d2, and the volume of a sphere, V = 4/37tD3., we find the total volume of n spheres to be 10 cc, i.e.- n 4/3 r D = 10. The surface area of n 7rD2 spheres is 6 m3. The total number of spheres present, n, is the same in both formulas. Therefore, by substitution, we find D= 10 p. If we obtain a particle diameter by some other method and find that it is mueh smaller than that of the BET method, we infer that the peatieles are porous. We thus speak of the porosity and need to correct for the pore surface area if we are to meike a reasonable estimate of the true diameter by the BET method. 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

Determine the medium resistance Rm. Rm is the (true) y intercept in Fig. 14.3, approximately 0.24 x 10n m 1. (However, use this result with caution, because it is hard to establish operating conditions at t = 0.) It frequently happens that the intercept of the p/Qxq) line is negative. Such a result generally implies (1) migration of fine particles, with subsequent blinding of medium or cake, or (2) sedimentation on a horizontal filter surface facing up. 

For the ACs the data are representative of the samples after heat-treatment at all three temperatures since during their fabrication these materials have already been treated at temperatures in excess of 850°C. However, for the alumina and clay samples the surface areas and pore volumes are shown after treatment at each temperature as these materials undergo various phase transitions that lead to sintering of the samples and shifts in their relative pore size distributions with heat-treatment. The particle size was determined from the corresponding MIP curve for the powder raw material. The Sbet in the case of microporous ACs should be considered as an apparent surface area due to the micropore filling mechanism associated with these materials [15]. The external area and micropore volumes were calculated from the slope and intercept of the t-plots of the corresponding isotherms. The total pore volume was taken as the amount of gas adsorbed at a relative pressure of 0.96 on the desorption isotherm, equivalent to a pore diameter of 50 nm. The mesopore volume was calculated from the difference in the total pore volume and the micropore volume. 

This mechanism is similar to that of a deep-bed filtration process with some differences (12). In the filtration process the particle-size to pore-size ratio is small, and the particles are mostly captured on the media surface. Thus interceptive capture dominates, and this capture does not alter the flow distribution in the porous medium. Permeability reduction is not significant and is ignored. On the other hand, the emulsion droplet size is generally of the same order of the pore size, and the droplets are captured both by straining and interception. This capture blocks pores and results in flow redistribution and a reduced permeability. 

The particle transfer coefficient k has dimensions of velocity and is often called the deposition velocity. At a given location on the collector surface the dimensionless group kL/D, known as the Sherwood number, is a function of the Reynolds. Peclet, and interception numbers. Rates of particle deposition measured in one fluid over a range of values of Pe, Re, and R can be u.sed to predict deposition rate.s from another fluid at the same values of the dimensionless groups. In some cases, it is convenient to work with the Schmidt number Sc = u/D = Pe/Re in place of Pe as one of the three groups, because Sc depends only on the nature of the fluid and the suspended particles. 

An aerosol with particles in the micron size range flows around a smooth solid sphere a few millimeters in diameter. At sufficiently high Reynolds numbers, a laminar boundary layer develop.s over the sphere from the stagnation point up to an angle of about 110 at which. separation takes place. The removal of particles by direct interception can be calculated from the velocity distribution over the forward. surface of the sphere, up to 90 from the forward stagnation point (Fig. 4.P4). 

The coefficients a(p, c) and tj(p, c) describe chemical and physical effects on the kinetics of deposition. The transport of particles from the bulk of the flowing fluid to the surface of a collector or media grain by physical processes such as Brownian diffusion, fluid flow (direct interception), and gravity are incorporated into theoretical formulations for fj(p, c), together with corrections to account for hydrodynamic retardation or the lubrication effect as the two solids come into close proximity. Chemical effects are usually considered in evaluating a(p, c). These include interparticle forces arising from electrostatic interactions and steric effects originating from interactions between adsorbed layers of polymers and polyelectrolytes on the solid surfaces. 

---