Transport Phenomena

Properties, Handling and Mixing of Particulate SolidsBySidra Jabeen

Department of Chemical Engineering,University of Engineering & Technology Lahore Properties of particulate massesTypes of particulate solidsStorage of solidsFlow out of binsPressures in bins and silos

2You will study3States of Matter

GasLiquid Particles can move past one anotherAssumes the shape and volume of containerCompressible Flows easilyParticles can move past/slide past one anotherAssumes the shape of container which it occupiesNot easily compressible Flows easily

Solid Particles locked into the place Retains the fixed volume and shapeNot easily compressible Does not flow easily4Properties of Particulate Masses / Solids1. Resist distortionThe 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. 5Properties of Particulate Masses / Solids2. Non-uniform pressure distributionPressure 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.6Properties of Particulate Masses / Solids2. Non-uniform pressure distributionPressure 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.7Properties of Particulate Masses / Solids3. Shear stress in solidsShear stress in solids is transmitted throughout the static mass of the particles unless failure / distortion occurs. 8Properties of Particulate Masses / Solids4. Solids densityBulk 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.9Properties of Particulate Masses / Solids5. Dilation required for flowThe 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 clayNon-cohesiveFor example; dry sand, sugar crystals, grains etc.

Storage of solids

Bulk storageBin storage

BULK STORAGE

Coarse solids like coal and gravels . Outside in large pilesUnprotected from the weather Most economical for large amount of materialSolid can be removed from pile by dragline or tractor shovel and delivered to conveyer or processMay lead to environmental problemsDusting - avoided by providing protective coverLeaching - avoided by providing protective cover or shallow basin

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14Angle 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.

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Angle of ReposeAngle of repose determines the flow characteristics of particulate masses.

Smaller angle Free flowing solidLarger angle Poor flow characteristicsBIN STORAGE

Valuable or soluble solidscannot exposed in outdoor pilesThat solids are stored in bins, silos and hoppers.These are cylindrical or rectangular vessels of concrete or metalSILO: tall and relatively small in diameterBIN: not so tall and usually fairly wideHOPPER: small vessel with a sloping bottom for temporary storage before feedingAll 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 openingBase openingIts not possible to open entire bottom for discharge. Rather a bottom section is set at the bottom of a bin for discharge;Conical bottomPyramidal bottom

ConicalPyramidalBottom SectionFLOW PATTERNS

Mass flowFunnel flow

Flow pattern depends on Steepness of the wall in the bottom sectionFriction between solids and the wall

MASS FLOW

All the material moves downward uniformlyIt 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 minimizedNo dead zonesDescribed 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 angleLateral flow

Characteristics of Funnel flow

Particles segregate and remain segregatedFlow is irregular and can cause floodingDensity of flow can varydead zonesDescribed as : LIFO (last in, first out)Cohesive solids follow this pattern26

27Flow 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 openingProperties of the solidsProperties of the solidsWith 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 - cloggingNot too large difficult to control and shut downSmall enough to be readily closed during solid discharge

Pressures in bins and silosPressure 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. 30

Arch Breaker