Slide 1

Properties, Handling and Mixing of Particulate Solids By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore Slide 2 Properties of particulate masses Types of particulate solids Storage of solids Flow out of bins Pressures in bins and silos 2 You will study Slide 3 3 States of Matter GasLiquid o Particles can move past one another o Assumes the shape and volume of container o Compressible o Flows easily o Particles can move past/slide past one another o Assumes the shape of container which it occupies o Not easily compressible o Flows easily Solid o Particles locked into the place o Retains the fixed volume and shape o Not easily compressible o Does not flow easily Slide 4 4 Properties of Particulate Masses / Solids 1. Resist distortion The particles interlock and can not slide past one another that is why they resist distortion when subjected to moderate force. When the force is large enough, failure may occur and one layer of particle slide over other. Slide 5 5 Properties of Particulate Masses / Solids 2. Non-uniform pressure distribution Pressure is not same in all directions. Pressure applied in one direction may create some pressure in other, but it is always smaller than the applied pressure. It is min. in the direction perpendicular to the applied pressure. Slide 6 6 Properties of Particulate Masses / Solids 2. Non-uniform pressure distribution Pressure is not same in all directions. Pressure applied in one direction may create some pressure in other, but it is always smaller than the applied pressure. It is min. in the direction perpendicular to the applied pressure. Slide 7 7 Properties of Particulate Masses / Solids 3. Shear stress in solids Shear stress in solids is transmitted throughout the static mass of the particles unless failure / distortion occurs. Slide 8 8 Properties of Particulate Masses / Solids 4. Solids density Bulk Density: The mass of the particles divided by the volume they occupy. Density of mass vary depending upon the degree of packing of the grains. It is min. when the mass is loose and rises to maximum when particles are packed under pressure. Slide 9 9 Properties of Particulate Masses / Solids 5. Dilation required for flow The tightly packed solid particles must expand or increase in volume to permit interlocking grains to move past one another to start flow. Without dilation, flow is not possible. Slide 10 Types of particulate solids Depending on flow properties, particulate solids are of two classes Cohesive For example; wet clay Non-cohesive For example; dry sand, sugar crystals, grains etc. Slide 11 Storage of solids Bulk storage Bin storage Slide 12 BULK STORAGE Coarse solids like coal and gravels . Outside in large piles Unprotected from the weather Most economical for large amount of material Solid can be removed from pile by dragline or tractor shovel and delivered to conveyer or process May lead to environmental problems Dusting - avoided by providing protective cover Leaching - avoided by providing protective cover or shallow basin Slide 13 13 Slide 14 14 Angle of Repose & Angle of Internal Friction Angle of Repose: The angle measured from the horizontal at which a heap of loose solids will stand without sliding. Angle of Internal Friction: A measure of frictional forces between particulate masses. Slide 15 15 Angle of Repose Angle of repose determines the flow characteristics of particulate masses. Smaller angle Free flowing solid Larger angle Poor flow characteristics Slide 16 BIN STORAGE Valuable or soluble solids cannot exposed in outdoor piles That solids are stored in bins, silos and hoppers. These are cylindrical or rectangular vessels of concrete or metal SILO: tall and relatively small in diameter BIN: not so tall and usually fairly wide HOPPER: small vessel with a sloping bottom for temporary storage before feeding All these vessels are loaded from top and discharged from bottom Slide 17 17 Slide 18 18 Slide 19 FLOW OUT OF BINS Solids tend to flow out of any opening in bottom of storage vessel. Side opening Base opening Its not possible to open entire bottom for discharge. Rather a bottom section is set at the bottom of a bin for discharge; Conical bottom Pyramidal bottom Slide 20 Conical Pyramidal Bottom Section Slide 21 FLOW PATTERNS Mass flow Funnel flow Flow pattern depends on Steepness of the wall in the bottom section Friction between solids and the wall Slide 22 MASS FLOW All the material moves downward uniformly It occurs in cone-bottomed bins with sufficient steep and/or smooth discharge Slide 23 Characteristics of Mass flow Flow is uniform and well controlled Constant density flow Sifting or segregation is minimized No dead zones Described as : FIFO (first in, first out) Free flowing or non cohesive solids follow this pattern Slide 24 FUNNEL FLOW Some material is stationary and rest is moving. A vertical column of solids above the opening moves downward without disturbing material at the sides. It occurs when the bottom section is pyramidal or shallow cone angle Lateral flow Slide 25 Characteristics of Funnel flow Particles segregate and remain segregated Flow is irregular and can cause flooding Density of flow can vary dead zones Described as : LIFO (last in, first out) Cohesive solids follow this pattern Slide 26 26 Slide 27 27 Flow out of a bin The rate of flow of granular solids through opening in the bottom of the bin depends upon the following factors; Diameter of discharge opening Properties of the solids Slide 28 Properties of the solids With cohesive solids, its hard to start flow. They adhere to the vessel walls and support a plug flow over the discharge opening. Vibrators on bin walls are used to start the flow. Flow is easier for non cohesive solids. Discharge opening Not too small - clogging Not too large difficult to control and shut down Small enough to be readily closed during solid discharge Slide 29 Pressures in bins and silos Pressure exerted by solids on the floor of the container is small because of the strong frictional forces between the particles and walls. In granular solids, high pressure does not increase the tendency of the material to flow instead it often packs the grains more tightly and makes flow difficult. In extreme cases, frictional forces cause the particles to arch or bridge, so that they dont fall as the material below them is removed. arch breakers are provided near bottom to avoid particles bridging. Slide 30 30 Arch Breaker