Author
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Nelson, Stuart |
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Submitted to: American Institute of Chemical Engineers Annual Meeting
Publication Type: Proceedings Publication Acceptance Date: 10/8/2004 Publication Date: 11/7/2004 Citation: Nelson, S.O. 2004. Useful relationships between dielectric properties and bulk densities of granular and powdered materials. In: Proceedings of the American Institute of Chemical Engineers Annual Meeting. Fourth World Congress on Microwave and RF Applications. New York. p. 136-140. CDROM. Interpretive Summary: The dielectric properties of cereal grains and oilseeds are used in the determination of moisture content by electrical and electronic moisture meters. The dielectric properties of materials are those electrical characteristics that determine the interaction of the material and electric fields that can be used to sense moisture content instantaneously in properly designed moisture meters. However, the packing, or bulk density, of such granular materials interferes with accurate moisture measurement if differences in packing are not taken into account. This paper provides a discussion of the influence of bulk density on the dielectric properties of granular and powdered materials. It also summarizes results of research to define the relationships between the dielectric properties and the bulk density of such materials as wheat, wheat flour, and pulverized coal, limestone, and some plastics. A recommended equation for correcting the dielectric properties for changes in the bulk density of such materials is presented. This information will be useful to those designing moisture meters for grain and seed and other granular or pulverized materials to provide more reliable moisture information for the management of grain and seed harvesting, storage, marketing and processing operations. Consequently, maintenance of better quality in agricultural grain and oilseed crops and products from those crops will benefit both producers and consumers. Technical Abstract: When knowledge of the dielectric properties, or permittivity, of granular or powdered materials is important, as in dielectric heating applications or in the sensing of moisture content by radio-frequency or microwave instruments, some reliable relationship between the dielectric properties and the bulk density of the material is required. Linear relationships between functions of the permittivity and bulk density have been observed, and dielectric mixture equations have been explored to obtain useful relationships for determining the dielectric properties of granular and powdered materials. These findings are reviewed, useful permittivity-density relationships are identified, and an equation is provided that permits reliable calculation of the permittivity of a particulate material at any given bulk density, if the permittivity is known or measured at another known bulk density. The relationships are illustrated for pulverized coal, wheat, ground wheat, limestone, and a coal-limestone mixture. The useful mixture equation is valid for granular and powdered materials with permittivities in the same range as those for grain, coal and limestone. |
