Particle electrification

Limits on Particle Charging. The electrical charge carried by a particle resides on the surface. Thus, a fundamental upper limit for particle electrification may be computed by imposing the constraint that the electric field at the surface can not exceed the dielectric strength of dry air, Eb 30 kV/cm. According to this hypothesis, the upper limit upon surface charge density becomes... 

Guyton concludes that with nonconductors the charging process is due to particle electrification either upon impact or during passage through the... 

The phenomena of the electrification of solids are complex. In gas-solid flows, surface contact by collisions, ion collection, and thermionic emission are known to be the major modes of particle electrification. Details of these three charging modes are introduced in the discussion that follows. 

Wet scrubbing uses liquid droplets to remove fine dust in a gas stream. In all types of wet scrubbing, the basic cleaning mechanism involves the attachment of particles to the droplets. The function of the droplets in scrubbers is similar to that of spherical fibers in filtration. Likewise, the primary collection mechanisms in scrubbing are similar to those in filtration, i.e., inertial impaction, interception, and diffusion [e.g., Fan, 1989]. Secondary collection mechanisms include thermophoresis due to temperature gradients, coagulation of particles due to particle electrification, and particle growth due to liquid condensation. 

Particles carrying charges of opposite polarity due to contact electrification will be attracted to opposite electrodes when passing through an electric field and thus can be separated from each other. 

Contact Electrification This form of charging involves the contact and separation of solid-sohd, sohd-hquid, or hquid-liqmd surfaces. Pure gases do not cause charging unless they cany droplets or dust particles. 

Charging by contact electrification is an active mechanism whenever dissimilar particles make and break contact with each other, or whenever they slide over a chute or an electrode. This charging mechanism is most frequently used to charge selectively and obtain an electrostatic separation of two species of dielectric materials as realized in a free fall electrostatic separator. 

Triboelectric Series. Prediction of triboelectric behavior in granular solids is hampered by difficult-to-control factors such as particle shape, prior mechanical contacts, material purity, and particle moisture content (which is usually related to airborne humidity). In the absence of any reliable predictive model for powder electrification, the practical requirements of industry necessitate an empirical approach (Taylor and Seeker,... 

Electrostatic Charging The primary cause of electrostatic charging is contact electrification, which takes place when two different materials are brought into contact and separated. Other causes include induction charging, the formation of sprays, and impingement of charged mist or particles on an ungrounded conductor. 

Corn (C7) has given a recent review of adhesive forces between particles in which he concludes that inadequate data on static electrification are available to arrive at any definite conclusion as to the relative role of electrostatics in particle adhesion. A detailed evaluation of the analysis presented by Russell (R9), however, sheds considerable light on the subject. 

Kll. Kunkel, W. B., The static electrification of dust particles on dispersion into a cloud, J. Appl. Phys. 21, 820 (1950). 

The argument is so general that its particularization for the metal/electrolyte interface was only for convenience. One could have carried out the discussion with equal validity for the gas/electrolyte or the glass (container)/electrolyte boundary of the electrolyte. Of course, one would have had to note the difference between the particles that constitute gases and glass and those that compose a metal. In all these systems, the conclusion would be reached that forces are direction dependent at the phase boundary and therefore new and compromise arrangements are assumed by the particles (of the two phases) in the phase boundary. If the particles are charged or are dipoles, not only is there a redistribution of particles but also an electrification of the interface and the development of a potential difference across it... 

Static Electrification of Dust Particles. Kunkel (l Y) has made an extensive study of the charge and size distribution of particles ranging from 0.5- to 30-micron radius in dust clouds in air and has investigated both calm and turbulent conditions. 

The material properties of solids are affected by a number of complex factors. In a gas-solid flow, the particles are subjected to adsorption, electrification, various types of deformation (elastic, plastic, elastoplastic, or fracture), thermal conduction and radiation, and stresses induced by gas-solid interactions and solid-solid collisions. In addition, the particles may also be subjected to various field forces such as magnetic, electrostatic, and gravitational forces, as well as short-range forces such as van der Waals forces, which may affect the motion of particles. 

When solid particles are exposed to a high-temperature environment, typically for T > 1,000 K, charging by thermal electrification becomes important. The electrons inside the solid can acquire the energy from the high-temperature field and be freed by overcoming the energy barrier or the work function. By losing electrons in such a thermionic emission process, the particles are thermally electrified. 

It is noted that the rate of electrification is not constant. Once the tendency is established for an electron to escape from the solid particle by thermionic emission, the charge buildup occurs on the particle, which then attempts to recapture the to-be-freed electron by the attracting Coulomb force. Therefore, the equilibrium of thermal electrification of solid particles in a finite space is possible. Details on the equilibrium and the rate of electrification concerning the thermionic emission are available in Soo (1990). 

An aerosol is a dispersion of discrete particles in a stream of gas. Starch and cellulose aerosols are potential fire hazards in granaries where friction between the moving, micronized particles causes electrification, whereupon separate accumulations of positive and negative charges may discharge as an electric spark and ignite the combustible solute (contact electrification synonymous with triboelectrification Ross and Morrison, 1988). 

Charge Taken by Particles—Since 1909 several investigations have been made on the nature of dust electrification. Knoblauch (1901) determined the charge on particles of many different materials placed on plates of platinum, sulfur, glass, and paraffin. This dust was then dropped on a grounded copper plate and the charge noted with an electroscope connected to the different plates. Knoblauch s experiments were carefully made. He found that platinum and paraffin... 

C 20 line cohesive interparticle forces e.g., flour channel rather than fluidize pressure drop < particle weight fluidization improved by stirring vibration addition of submicrons anti-electrification ... 

A second particle-charging mechanism is static electrification. This mechanism arises from one or a combination of several other mechanisms, making theoretical interpretation in terms of a single mechanism very difficult, if not impossible (and most experimenters have attempted to interpret their results in terms of a single mechanism). Five basic mechanisms can result in static electrification. These are examined for their importance in aerosol physics. 

Flame ionization. A final static electrification method is the ionization of particles in a flame. This effect was first observed as early as 1600, and it has recently become the subject of much interest because of potential application in such diverse areas as direct generation of electricity, control of combustion processes by applied electric fields, and the like (Lawton... 

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