Surface dust size distribution

Particles in the atmosphere come from different sources, e.g., combustion, windblown dust, and gas-to-particle conversion processes (see Chapter 6). Figure 2-2 illustrates the wide range of particle diameters potentially present in the ambient atmosphere. A typical size distribution of ambient particles is shown in Fig. 2-3. The distribution of number, surface, and mass can occur over different diameters for the same aerosol. Variation in chemical composition as a function of particle diameter has also been observed, as shown in Table 4-3. 

One of the characteristic features of Reg soils is the vesicular nature of the uppermost soil horizon. The size distribution of the vesicles is up to a few mm in diameter. Similar vesicular structures were also observed in lithosols and takyr-like alluvial soils and were always associated with the presence of stones or thin, hard crusts that sealed the soil surface. It forms mostly through accumulation of aeolian dust (McFadden et al., 1998). 

Attrition of particulate materials occurs wherever solids are handled and processed. In contrast to the term comminution, which describes the intentional particle degradation, the term attrition condenses all phenomena of unwanted particle degradation which may lead to a lot of different problems. The present chapter focuses on two particular process types where attrition is of special relevance, namely fluidized beds and pneumatic conveying lines. The problems caused by attrition can be divided into two broad categories. On the one hand, there is the generation of fines. In the case of fluidized bed catalytic reactors, this will lead to a loss of valuable catalyst material. Moreover, attrition may cause dust problems like explosion hazards or additional burden on the filtration systems. On the other hand, attrition causes changes in physical properties of the material such as particle size distribution or surface area. This can result in a reduction of product quality or in difficulties with operation of the plant. 

The size distribution of a dust as measured by a microscope is as follows. Convert these data to obtain the distribution on a mass basis, and calculate the specific surface, assuming spherical particles of density 2650 kg/m3. 

The relative inflammability of a few common explosive dusts, according to particle-size, is shown in Figure 71. These data are from studies by Price and Brown (1922) and show the decrease in relative inflammability with increasing particle-size. It is necessary to point out that the samples tested are not clean-cut size-distributions. They include unknown fractions of particles less than stated size, so that relationships to surface cannot be determined. However they are indicative of the effect of increasing size on relative inflammability. 

The data shown in Fig. 20.4 are for tests in which dusts with narrow size distributions were used. Hertzberg and Cashdollar (1987) pointed out that with broad size distributions, strong size dependencies tend to be blurred, hut the trends should remain the same. With small mean particle sizes, all flammable material is volatilized from the particles, and the combustion process is homogeneous. As the particle size increases, only the surface material is volatilized so that more dust is required overall to sustain combustion. Finally, above some upper limit of particle size, the volatilization rate is such that a minimum concentration of combustible volatile material is not achieved. 

The surface areas of dust samples as determined by optical and electron microscope have also been compared [167]. Pore size distributions of thin films of AI2O3, as measured by TEM, have also been compared with those determined by gas adsorption/desorption [168]. It has also been suggested that electron microscope gives a truer estimate of surface area than gas adsorption techniques [169]. Further information can be obtained in a recent review of specimen preparation for TEM [170]. 

The National Physical Laboratory has surface area and pore size distribution standards. AC fine test dust is available from General Motors for calibrating classifiers and ASME also produce standards for this purpose. 

Dust must have particle size distribution capable of propagating flame The sensitivity of a dust cloud to ignition and the resulting explosion violence (severity) increase with a decrease in particle size. This is because the combustion process involves chemical reaction at the solid-oxidant interface. Therefore, as the dust particles get finer the total surface area that is available for oxidation will increase. It should be noted that, in practice, very often dust clouds are made up of particles with sizes ranging from fine to coarse. As the fines become airborne more readily, they play a more prominent role in the initial ignition and explosion propagation. 

Particles having high bulk density, low hygroscopicity, high sphericity, dust-free, narrow particle size distribution, and smoother surface can be produced. 

---