Agitation Liquids

8/3/2019 Agitation Liquids

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Agitation Liquids


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Agitation, mixing and blending Whats thedifference?


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MixingObjective: Homogeneity, promoting heat and masstransfer where a system is undergoing a chemicalreaction

Reduce the degree of non-uniformity; for eg:concentration, viscosity, temperatureAchieved by moving the material from one region toanother


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Types of MixingSingle phase liquid mixingMixing of immiscible liquidGas-liquid mixing

Liquid-solid mixingGas-liquid-solid mixingSolid-solid mixing


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Single phase liquid mixing2 or more miscible liquids mixed togetherSimplest type of mixing no heat/mass transfer,no chemical reaction

Example: Blending of petroleum product withdifferent viscosities,


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Mixing of immiscible liquid

2 immiscible liquids stirred together, one phasedispersed as tiny droplets in the second liquid whichforms a continuous phase

Example: Liquid-liquid extraction (solvent extractionand partitioning) a method to separate compounds based on their

relative solubilities in two different immiscible liquids,

usually water and an organic solvent separation of a substance from a mixture bypreferentially dissolving that substance in a suitablesolvent

by this process, a soluble compound is usuallyseparated from an insoluble compound


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Liquid-solid mixing

Mechanical agitation is used to suspend particles in a liquid to promote

mass transfer or chemical reaction

Liquid involved usually of low viscosity and the particles will settle out

when agitation ceases

Example: formation of composite materials fine particles must be

dispersed into a highly viscous Newtonian or non-Newtonian liquid


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Gas-liquid-solid mixing

Eg: catalytic hydrogenation of vegetable oils, slurry

reactors, froth flotation, evaporative crystallization

Efficiency of the process is affected by the extent of

mixing between the three phases


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Solid-solid mixing

Referred as blending

Very complex process and dependent on the character of particles (density,

size distribution, shape and surface properties)

Eg: mixing of sand, cement and aggregate to form concrete, food, drugs and

glass industries


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Mixing Applications

Homogeneity and improve heat transfer

The rotational speed of an impeller in a mixing vessel is selected to achieve

a required rate of heat transfer

Overmixing should be avoided, because it is a wasteful of energy and may

be detrimental to product quality

Eg: in biological operations, excessive high impeller speed/energy input

increase shear rates which damage the micro-organisms present


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In mixing, there are 2 major problems need to beclarified:

1. How to design and select mixing equipment for agiven duty

2. How to assess whether a mixer is suitable for aparticular application

In other to understand this, the following aspects shouldbe understood:

Mechanisms of mixingScale-up similarity criteriaPower consumptionFlow patternsRate of mixing and mixing timeThe range of mixing equipment available and itsselection


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Mixing MechanismsIn liquid mixing device, 2 requirements must be fulfilled:

There must be a bulk/convective flow so that there are nodead/stagnant zonesThere must be a zone of intensive or high shear mixing in whichinhomogeneities are broken down

Both of these processes are energy consuming, mechanical energydissipated as heatThe flow in mixing vessel can be either laminar or turbulent withtransition zone in between the two depending on the properties of theliquid (viscosities)


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Mixing MechanismLaminar mixing

Associated with high viscosity liquids (>10 Ns/m 2),either Newtonian or non-NewtonianThe inertial forces tend to die out quickly, mixerimpeller must cover significant proportion of the crosssection of the vessel to impart sufficient bulk motionVelocity gradient close to the impeller is high, the fluidelements deform and stretch, repeatedly elongate and

become thinner each time the fluid elements passthrough the high shear zone


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Mixing MechanismElongational flow occurs simultaneously as a result ofthe convergence of the streamlines and increasedvelocity in the direction of flowFigure 7.2 shows the process of thinning and flattening offluid elementsThe process of shear and elongation increase stresses inthe liquid and effect the droplet size and interfacial areawhich means the desired homogeneity is achievedRemember that: Ultimate homogenisation of miscibleliquid can only be achieved through molecular diffusion


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A similar mixing process can occur when the liquid is sheared between two rotating

cylinders

During each revolution, the thickness of the fluid element is reduced and molecular

diffusion takes over when the elements are sufficiently thin

This type of mixing is shown in figure 7.3

Mixing can be induced by physically splicing the fluid into smaller units and re-

distributing them in line mixers rely on this mechanism


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Lets watch a video! http://www.youtube.com/watch?v=W3YZ5veN_Bg


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Turbulent mixingFor low viscosity liquids (less than 10mNs/m 2)The bulk flow pattern in mixing vessels with rotating impellersis turbulentThe inertia imparted to the liquid by the rotating impeller issufficient to cause the liquid to circulate throughout the vesseland return to the impeller


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7.3 Scale up of Stirred VesselProblem in designing mixing equipment is todeduce the most satisfactory arrangement for alarge unit from experiments with small units

In order to achieve the same kind of flow pattern intwo units, geometrical, kinematic and dynamicsimilarity and identical boundary conditions mustbe maintained

It is convenient to relate the power used by agitatorto the geometrical and mechanical arrangement of the mixer


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A typical mixer arrangement:


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Froude number

Weber number

G

DN 2

23 N D


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Low Viscosity SystemTypical equipment for low viscosity liquids consists of a vertical cylindrical tank, with a height to diameterratio of 1.5 to 2, fitted with an agitatorFor low viscosity liquids, high-speed propellers of diameter one-third that of the vessel are suitable,running at 10-25 Hz


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Low Viscosity SystemFroude number is usually important when grossvortexing occursIt can be neglected if the value of Reynolds

number is less than 300Thus, in a plot of Power number, N p against Rewith Froude number as parameter, all data fall on asingle line for values of Re < 300, indicating that

in this region Fr has no significant effect on N p


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High Viscosity SystemMixing in highly viscous liquids is slow due to lowdiffusivity and poor bulk flowThe fluid in the immediate vicinity of the impeller

is influenced by the agitator and the flow islaminarFor mixing of highly viscous, non-Newtonianfluids, it is necessary to use specially designed

impellers involving close clearances with thevessel walls


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The prediction of power consumption for agitation of a given non-Newtonian fluid in a particular mixer at adesired impeller speed may be evaluated by thefollowing procedures:

Estimate the average shear rate from equation 7.18

Evaluate the corresponding apparent viscosity, eitherfrom a flow curve or by means of the appropriate flowmodelEstimate the value of the Reynolds number asand then the value of the Power number and hence Pfrom figure 7.8


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Agitation Liquids