Charge dissipation

In the case of charged powder entering a silo, the Maurer discharge mechanism continuously dissipates charge as shown in Fig. 10. The details of this mechanism—thresholds, inlet flow requirements, etc.—are not as well-documented or understood as they should be. 

For powders and granular solids, there are two types of antistatic agents surface- and volume-active additives. Surface-active agents, which increase the surface conductivity of individual particles, are effective because triboelectric charge is always situated on the surfaces of individual particles. Most if not all surface-active agents are hygroscopic and thus attract a thin film of water to the surface it is this moisture that is responsible forthe increased surface conduction (van Drumpt, 1991). The effective bulk resistivity of the particles % -may be estimated by assuming that the particles are spherical and of radius R (Jones, 1995). 

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Features common to most high-voltage dc powder supplies include rev ersible polaritv, short-circuit and current-limiting protection, and automatic residual-charge dissipation to ground. 

The key to safe operation is to provide an adequate means of charge dissipation from charged materi s to ground. This requires mobility of charges in or on the charged materialp/ii.s electrical continuity from the material to ground. 

Flammable liquids are considered particularly static-prone if their elec trical conductivity is within the range of 0.1 to 10 pS/m. If no particulates or immiscible liquid are present, these prodlic ts are considered safe when their conductivity has been raised to 50 pS/m or higher. Blending operations or other two-phase mixing may cause such a high rate of charging that a conductivity of at least 1000 pS/m is needed for safe charge dissipation (British Standard 5958, part 1, Control of Undesirable Static Electricity, para. 8, 1991). 

It is essential that persons be grounded in hazardous (classified) locations. For most (memical operations, the resistance to ground from the body should not exceed 100 megohms. A lower allowable resistance may be specified for locations where the presence of pri-maiy explosives, hydi ogen-air mixtures, oxygen-enriched mixtures, or certain solid-state devices requires faster charge dissipation. 

Knowing and p (or k), the relaxation time (r) can be estimated. The determination of relaxation time allows the rate of charge dissipation to be calculated for any process obeying Ohm s law. 

The hyperbolic relaxation equation (A-5-2.4.1 a) contains charge carrier mobility as a variable, which should be sensitive to oil viscosity. This is found to be the case for some viscous nonconductive liquids. These have much slower rates of charge dissipation equivalent to an Ohmic liquid whose conductivity is 0.02 pS/m (5-2.5.4). 

Table (b) shows charge dissipation times to reduce the charge density to 5% of the initial values for a series of initial charge densities (Qg) and for... 

TABLE (b). Charge Dissipation Times for Hyperbolic Relaxation (e, = 2-4)... 

Antistatic Material that minimizes static charge generation, provides controlled static charge dissipation, or both. 

Wurr, E., Electrostatic charge dissipation of FIBCs, Powder Hand. Proc., 4 77-79(1992)... 

Bocian, Lindsey and co-workers studied sandwich complex nanocapacitors comprised of porphyrin and phthalocyanine ligands separated by lanthanide metals [133]. A triple-decker sandwich of phthalocyanine-Eu-phthalocyanine-Eu-porphy-rin, with two phenylethynyl linker wires from the porphyrin, potentially has up to nine accessible oxidation states (—4 to +4). SAMs of monomers, dimers, trimers, and oligomers of this sandwich, anchored at one or both ends by thioacetyl groups, gave charge densities up to 10 10 mol cm-2, electron-transfer rates up to 105 electrons s-1, and charge-dissipation half-lives in the 10-50 s range. 

An alternate test for charge-dissipating performance is the charge decay test, in which the time of charge decay is measured after a potential of 5 kV has been applied to the specimen (Federal Test Method Standard 101C, Method 4046.1). For many purposes, a... 

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