SCEO_L6 - Selected Chemical Engineering Operations Selected...

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Selected Chemical Engineering Operations Selected Chemical Engineering Operations LECTURE - 6
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Selected Chemical Engineering Operations Properties and Handling of Particulate Solids • Introduction • Particle shape • Particle size – Mixed particle size and size analysis – Screen analysis – Wet and dry screening – Example • Properties of particulate masses • Storage of solids – Pressure in bins and silos – Flow out of bins • Conveying • Mixing of solids
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Selected Chemical Engineering Operations Types of Mixers • In mixing of solid particles, three mechanisms may be involved – Convective mixing, in which groups of particles are moved from one position to another – Diffusion mixing, where the particles are distributed over a freshly developed interface – Shear mixing, where slipping planes are formed • These three mechanisms operate to varying extents in – different kinds of mixers – different kinds of particles • Mixers for dry powders include some machines that are also used for heavy pastes
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Selected Chemical Engineering Operations Types of Mixers • Mixing is by – slow-speed agitation of the mass with an impeller – tumbling – centrifugal smearing and impact • These mixers are of fairly light construction – their power consumption per unit mass of material mixed is moderate • In mixing equipment for pastes, rubber and heavy plastic masses – the material must all be brought to the agitator or the agitator must visit all parts of the mix
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Selected Chemical Engineering Operations Types of Mixers • The action in this machinery is well described as a combination of – Low speed shear – Smearing – Wiping – Folding – Stretching – Compressing • The mechanical energy applied by moving parts directly to the mass of material • The force generated in these mixers are large, the machinery must be ruggedly built, and the power consumption is high
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Selected Chemical Engineering Operations Types of Mixers • The heat evolved per unit mass of the material may be significant – that require cooling to prevent the temperature from reaching a level dangerous to the equipment or the material
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Selected Chemical Engineering Operations Mixers of Cohesive Solids • Some of the most difficult of all mixing problems involve cohesive solids – such as pastes, plastic materials, and rubber • Because of high viscosity of these substances – the mixing equipment must be different and more powerful • With cohesive solids the mixing element cannot generate flow currents – instead they shear, fold, stretch, and compress the material to be mixed
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Selected Chemical Engineering Operations Mixers of Cohesive Solids • The mixers described in this section are – change-can mixers – kneaders, dispersers and masticators – continuous kneaders – mixer-extruders – mixing rolls – mullers and pan mixers – pugmills
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Selected Chemical Engineering Operations Change-Can Mixers • These device blend viscous liquids or light pastes, as in
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SCEO_L6 - Selected Chemical Engineering Operations Selected...

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