Properties, Handling and Mixing of Particulate Solids By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore

Properties of particulate masses Types of particulate solids Storage of solids Flow out of bins Pressures in bins and silos 2 You will study…

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

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.

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.

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.

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.

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.

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.

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.

Storage of solids  Bulk storage  Bin storage

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

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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.

15 Angle of Repose Angle of repose determines the flow characteristics of particulate masses. Smaller angle  Free flowing solid Larger angle  Poor flow characteristics

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

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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

Conical Pyramidal Bottom Section

FLOW PATTERNS  Mass flow  Funnel flow Flow pattern depends on Steepness of the wall in the bottom section Friction between solids and the wall

MASS FLOW  All the material moves downward uniformly  It occurs in cone-bottomed bins with sufficient steep and/or smooth discharge

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

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

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

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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

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

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 don’t fall as the material below them is removed.  arch breakers are provided near bottom to avoid particles bridging.

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