Primary particles estimating particle size

Figure 14-120 presents an early calculated estimate of mesh efficiency as a fraction of mist-particle size. Experiments by Calvert (R-12) confirm the accuracy of the equation of Bradie and Dickson (Joint Symp. Pioc. Inst. Mech. Eng./Yoikshiie Bi Inst. Chem. Eng., 1969, pp. 24-25) for primary efficiency in mesh separators ... 

Precise data on the primary particle size of the silica gels could not be obtained from the manufacturer. Fortunately, particle size can be estimated, based on theoretical considerations. 

It was pointed out in chapter 1 that Sheinfain s globular theory5 allows an estimation of the average primary particle size if the specific surface area is known," according to... 

By electron microscopy the size of the primary particles in the aggregates is estimated to be about 10 nm. Particle size measurements using a nanosizer show the size of aggregates dispersed in a well wetting solvent to be in the range of 100 nm. Laser diffraction of fumed silica dispersed in air provides sizes of agglomerates larger than 5 pm. 

Applying basic concepts of solid-state diffusion to transport across the boundaries of coalescing submicron panicles is difficult. Information is lacking on the crystalline state and the nature of the struclural imperfections in the colliding particles. However, values of the solid-.state diffusion coefficient can provide qualitative guidance in estimating the effects of material properties on primary particle size as discussed in a later section. 

ESTIMATION OF AVERAGE PRIMARY PARTICLE SIZE METHOD OF CHARACTERISTIC TIMES... 

This expression is the starting point for estimating the average primary particle size. Initially, for a hot gas containing many small particles that coalesce almost instantaneously, Tf 0 and Am A ph. As the gas cools, Tf starts to increase (hat is. coalescence is not quite instantaneous. This is seen from (12.6) and the strong (Arrhenius) dependence of D on T. Next we assume that ii = and we take the minimum surface area, in which... 

Estimation of Average Primary Particle Size Method of Characteristic Times 346 Primary Particle Size Effects of Aerosol Material Properties 350 Particle Neck Formation 353 Particle Crystal Structure 355 Basic Concepts 355 Experimental Obsetyafions 355 Problems 356 References 357... 

Results of XAFS measurements are compiled in Table 1 with the results of the estimates of the TEM measurements (for details on TEM results see ref. [ 13 ]). The catalysts are named according to the stabilizing polymer and particle size determined by TEM. Except for the PVP stabilized catalyst reduced with methanol, an excellent agreement exists between the size of the metal particles estimated from TEM and EXAFS. The methanol reduced sample shows the smallest particle size when estimated from EXAFS, while it appears to be much larger in TEM. At present we would like to speculate that this is due to the formation of agglomerates (diameter 3.5 nm) formed from very small particles (diameter 1.2 nm). Consequendy, the eatalyst is named Rh/PVP/3.5-a to indicate the difference with the sol containing primary particles of 3.5 nm in diameter. 

Ciliary action removes deposited particles from both the bronchi and bronchioles. Though it is generally thought that mucocilliary action rapidly transports most particles deposited here toward the pharynx, a fraction of these particles are cleared more slowly. Evidence for this is found in human studies. For humans, retention of particles deposited in the lungs (BB and bb) is apparently biphasic. The slow action of the cilia may remove as many as half of the bronchi- and bronchiole-deposited particles. In human bronchi and bronchiole regions, mucus moves more slowly the closer to the alveoli it is. For the faster compartment, it has been estimated that it takes about 2 days for particles to travel from the bronchioles to the bronchi and 10 days from the bronchi to the pharynx. The second (slower) compartment is assumed to have approximately equal fractions deposited between BB2 and bb2 and both with clearance half-times estimated at 20 days. Particle size is a primary determinant of the fraction deposited in this slow thoracic compartment. A small fraction of particles deposited in the BB and bb regions is retained in the airway wall for even longer periods (BBseq and bbseq). 

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