Diffusion charging

Fig. 12. Comparison of actual and predicted charging rates for 0.3-pm particles in a corona field of 2.65 kV/cm (141). The finite approximation theory (173) which gives the closest approach to experimental data takes into account both field charging and diffusion charging mechanisms. The curve labeled White (141) predicts charging rate based only on field charging and that marked Arendt and Kallmann (174) shows charging rate based only on diffusion.

The potential difference across the mobile part of the diffuse-charge layer is frequently called the zeta potential, = E(0) — E(oo). Its value depends on the composition of the electrolytic solution as well as on the nature of the particle-hquid interface. 

For many particles, the diffuse-charge layer can be characterized adequately by the value of the zeta potential. For a spherical particle of radius / o which is large compared with the thickness of the diffuse-charge layer, an electric field uniform at a distance from the particle will produce a tangential electric field which varies with position on the particle. Laplace s equation [Eq. (22-22)] governs the distribution... 

Other Considerations In general, dry ESPs operate most efficiently with dust resistivities between 5x10 and 2 x lO ohm-cm. In general, the most diffieult particles to collect are those with aerodynamie diameters between 0.1 and 1.0 / m. Particles between 0.2 and 0.4 m usually show the most penetration. This is most likely a result of the transition region between field and diffusion charging. 

The traditional unipolar diffusion charging model is based on the kinetic theory of gases i.e., ions are assumed to behave as an ideal gas, the properties of which can described by the kinetic gas theory. According to this theory, the particle-charging rate is a function of the square of the particle size dp, particle charge numbers and mean thermal velocity of tons c,. The relationship between particle charge and time according White s... 

The most straightforward approach is to assume that the field charging and diffusion charging are independent processes i.e., particle charge can be presented as a sum of charges due to field (s ) and diffusion (sj) charging. Another simple approach to estimating the combined effect is... 

According to this approximation, the drift velocity is proportional to the square of the electric field. This is a clear indication of the importance of the electric field inside an electrostatic precipitator. Equation (13.60) is a valid approximation for large particles [dp > 0.5 m), provided that particle charge is close to the saturation level. In the case of small particles, the effect of diffusion charging must be taken into account. 

Pui, D. Y. H. Experimental Study of Diffusion Charging of Aerosols. Thesis, Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota (1976). 

The overall charge (Qt) consumed to oxidize the film by a potential step from Ec to E has two components the charge consumed to relax the compact structure, which will be called the relaxation charge (Qr), and the charge consumed under diffusion control to complete the oxidation, called the later diffusion charge (Qd)- The following equation is obeyed ... 

The integration of Eq. (36) yields the diffusion charge consumed until a given time in those regions where the structure was opened ... 

The diffuse charge distribution in the semiconductor s surface layer leads to a drastically lower cell capacitance at the semiconductor-electrolyte interface. Typical... 

The chemical and structural features of the membrane and cell wall are extensively discussed elsewhere in this volume (see Chapters 2, 6, 7 and 10). They usually contain numerous charged groups, which, as far as they are not internally compensated by counterions, give rise to the formation of an electric double layer at the interphase. The net charge of membrane surface plus cell wall is counterbalanced by a diffuse charge with opposite sign. This so-called diffuse... 

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