Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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1. INTRODUCTION
Granular materials are substances made up of many distinct solids (“grains”)
that have been present in human activity since very early history in forms such as
cereal grains or construction materials. Granular materials are important constituents
in numerous industrial processes. Such industries as: chemicals, cosmetics,
pharmaceuticals, biotechnology, ceramics, food, energy, paper/wood, metallurgy,
cement, glass, minerals, consumer products, plastics strongly depend on granular
materials. A single shift in conditions can drastically change performance of a process
in those industries. Following growing industrial use of granular materials a number
of branches of engineering evolved devoted to understanding how to deal with these
materials, among them powder technology, soil mechanics, geotechnology, foundation
engineering, earthquake engineering, erosion control and mining engineering.
The most important technologies of process engineering involving granular materials
as listed at ‘Powder, bulk solids’ portal are: pneumatic conveying, transport, size
reduction, spheroidization, screening, coating, mixing (blending), segregation,
product consistency, weighing, metering, packaging and bagging, storage, stratification,
dust collection, instrumentation and control, feeding, quality control. Each
of the above applies specific equipment. For example a group of particle enlargers
and formers constitute of: briquetters, coaters, compactors, conditioners, dedusters,
densifiers, disc pelletizers, drum flakers, drum pelletizers, encapsulators, extruders,
flakers, fluid bed agglomerators, granulators, instantizers, kneaders, laboratory mixing
agglomerators, pelletizers, pinmixers, powder coaters, powder presses, rewetting
agglomerators, roller presses, rotary agglomerators, rotating pans, screens,
spheroidizers, spray agglomerators, spray congealers, tablet coaters, tablet presses,
vibratory agglomerators.
As compared to liquid granular material reveal three distinct differences
in mechanical behavior:
Granular materials are characterized by higher than zero angle of internal
friction that in the case of liquids is zero. As a result of that static pressure in
liquids is not dependent on direction, while in granular material pressure may
vary with direction of measurement. Static granular material may carry shear
stress, while liquid cannot. Therefore the surface of static liquid is flat, while
the free surface of static granular material has conical shape.
In granular material tangent stress under condition of shear load does not
depend on velocity of deformation, but depends on the mean stress. In liquid
shear stress depends on velocity of deformation (as an effect of viscosity),
but does not depend on pressure.