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Submitted To:

Engr. Hamayoun Mehmood

Submitted By:

Mehran Rasheed Goraya

10043123-029

Department of Chemical Engineering University of Gujrat, pakistan

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Agitation and MixingFluid Mechanics

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AgitationIt is the induced motion of a material in a specified way, usually in a circulatory pattern in a container

Mixing It is the field of pure chemical Engineering

Introduction

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Mixing

Mixing is the random distribution of material in different phases into another, forming a uniform homogenous phase.

Mixing

Two component

Homogeneous mixture

Reduce viscosity, concentration etc

Promote Heat and Mass transfer

Chemical Reaction involve

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Types of Mixing

Mixing

Gas Liquid Mixing

Single Phase liquid

Mixing

Immiscible Liquids

Solid-Solid Mixing

Liquid Solids Mixing

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•Blending of Petroleum Products

•Mechanical Agitation

Single phase Liquid Mixing

•Production of stable emulsion in food, brewing, Pharmaceutical industry

Immiscible Liquid Mixing

•Aerobic fermentation, waste water treatment, oxidation of hydrocarbon etc require gas liquid contact

Gas Liquid Mixing

• Formation of Composite material (Polymer)

• Carbon Black into Rubber

Liquid solid Mixing

Examples

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Mixing Mechanism

Requirement for Mixing:

There must be bulk flow so that there are no dead zones.

There must be a zone of high shear mixing in which the inhomogenities are broken down.

Mixing Mechanism

Laminar Mixing

Turbulent Mixing

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It is associated with the high viscosity liquid (>10 N s/m2) for either newtonian or non newtonian fluid.

The velocity gradient close to impeller are high. Fluid element in that region deform and stretch.

They repeatedly elongate and become thinner each time the fluid pass through the high shear zone.

Laminar Mixing

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In addition, extensional flow elongational flow also occur simultaneously.

It is the result of convergence of stream line and also increased velocity in direction of flow.

Both these mechanism (shear and elongation) produce stresses which reduce droplet size and increase interfacial area by which desired homogeneity is obtained.

Cont’d

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For low viscosity liquid (< 10 mN s/m2) the bulk flow pattern in vessels is turbulent.

Due to impeller inertia, turbulent eddy diffusion takes place, which is maximum near the impeller.

Due to eddy diffusion turbulent mixing is faster than laminar.

Mixing is rapid in impeller region because of high shear rate due to the presence of vortices.

Large eddies are broken down into the small one and ultimately disappear converting kinetic energy into heat.

Due to eddies homogeneity takes place.

Turbulent Mixing

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The propeller creates an axial flow which may be upward or downward depends upon the direction of rotation.

The radial flow pattern can be achieved by the disc turbine.

Flow Patterns

Propeller mixer

Disc Turbine mixer

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The way of liquid moves depends upon, the type of impeller characteristics of liquid, especially its viscosity, size and proportion of the tank and baffles etc.

The liquid velocity at any point has three component◦ radial, longitudinal, and tangential or rotational

When shaft is in the centre than the vortices are formed due to tangential flow, which is called swirling.

Cont’d

Degree of Agitation is

reduced

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Prevention from Swirling

To improve the mixing and to reduce the vortex formation

Baffles are fitted in the tank To mount the agitator , off center

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Mixing Equipment

Mixing Equipment involve

Mechanical Agitation

Vessels

Impellers

Baffles

Axial flow impeller Radial Flow impeller

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Agitated Vessel

Stirred tanks vary in proportion depending on the purpose for mixing.

D≤4.6 m tank diameter restriction.

Liquid height to vessel diameter H/D ≈ 1

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Impeller

Components of Impeller

Turbine

High Efficiency impeller

propeller

Axial, high speed Impeller

Axial and Tangential motion no vertical motionTo provide more

uniform axial flow and better mixing

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Power Number

Reynold’s number◦ It represent the ratio of the applied to the opposing viscous drag

forces.

Dimensionless Number for mixing

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Froude Number for mixing◦ It represent the ration of the applied to the opposing

gravitational forces.

Weber Number for mixing◦ it represent the ratio of the applied to the opposing surface

tension forces

Cont’d

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Following are the mixing challenges in chemical industries

◦ Producing stable emulsions

◦ Disintegrating and Dissolving solids

◦ Blending liquid of different viscosities

◦ To increase the reaction acceleration

◦ Deagglomeration to avoid lumps

◦ Control rate of heat and mass transfer

Importance of Mixing

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Power consumption

Impeller RPM

Impeller Diameter

Important Design Variable

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Chemical Reactor

Paint Industry

Cement Industry

Fermentation Industry

Food Industry

Pharmaceutical Industry And so on……..

Application in Chemical Industry

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The first step in making paint involve mixing of resin, solvents and additives to form paste.

Sand mill a large cylinder that agitates tiny particle of sand to grind the pigment particle.

Making smaller, to disperse through out mixture.

Dispersion tank contain circular tooth blade.

Paint Industry

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It is vessel designed for chemical reaction.

It contain agitator blade.

Chemical Reactor

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FermenterPharmaceutical

Mixer

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Unit operation of chemical engineering by McCabe and Smith 7th Edition

Chemical Engineering by Coulson and Richardson volume 1st , 6th edition

Mixing in process Industry by

References