Agitation and Mixing

Agitation:

Agitation refers to the induced motion of a material in a specified way, usually in a circulatory pattern inside some sort of container. Purpose is to make homogeneous phase.

Mixing:

Mixing is the random distribution, into and through one another, of two or more initially separate phases.

A single homogeneous material, such as tankful of cold water, can be agitated, but it can’t be mixed until some other material (such as a quantity of hot water or some powdered solid) is added to it.

Purposes of Agitation

Liquids are agitated for number of purposes which are given as follows

Suspending solid particles

Blending miscible liquids, e.g. Methyl alcohol and water Dispersing a gas through the liquid in the form of small bubbles.

Dispersing a second liquid, immiscible with the first, to form an emulsion or suspension of fine drops.

Promoting heat transfer between the liquid and a coil or jacket.

Agitated Vessel and Its Accessories

Flow Pattern in Agitated Vessels The Flow Pattern in agitated vessels depends on the following factors;

Type of Impeller Characteristics of the fluid Size and proportions of the tank Baffles Agitator

The velocity of the fluid at any point in the tank has three components and overall flow pattern in the tank depends on the variations in these three components.

Radial Component (It acts in a direction perpendicular to the shaft of the impeller)

Longitudinal Component (It acts in a direction parallel with the shaft)

Tangential / Rotational Component (It acts in a direction tangent to a circular path around the shaft)

Vertically Mounted Shaft Radial and Tangential components are in horizontal plane

Longitudinal component is in vertical plane

Radial and longitudinal are useful for mixing action

Tangential component is generally disadvantageous when shaft is vertically mounted.

Tangential component follows a circular path around the shaft and creates a vortex in the liquid.

If the solid particles are present in the liquid, circulatory currents tends to throw the particles to the outside by centrifugal force and they move downward and to the Centre of the tank at the bottom.

In an un baffled vessel circulatory flow is induced by all types of impellers i.e. axial or radial

For strong swirling, flow pattern is same regardless of design of Impeller and at high speed the vortex may be so deep to reach at the impeller surface.

• Prevention of swirling Swirling or circulatory flow can be prevented by any of three ways In small tanks the impeller can be mounted off center (shaft is moved away

from center then tilted in a plane perpendicular to the direction of move) In Large tanks, the agitator may be mounted in the side of the tank with shaft

in horizontal plane but at an angle with radius. In large tanks with vertical agitators, swirling can be prevented by installing

baffles. Four baffles are sufficient to prevent swirling and vortex formation (Even two

have a strong effect on swirling effect). For turbines width of baffle need be no more than one-tweflth of vessel

diameter and for propellers no more than one eighteenth of tank diameter. No baffles are required for side entering, inclined or off center propellers

Impellers Impellers are divided into two major classes

Axial Flow Impellers (These generates current parallel with the axis of impeller shaft)

Radial Flow Impellers (These generate currents in tangential or radial directions)

Following are the three main types of impellers Propellers Paddles Turbines

There are also various other subtypes of impellers but the above mentioned three types solves perhaps 95% of all liquid agitated problems

Propellers It is an axial flow, high speed impeller for liquids of low viscosity.

Smaller propellers runs at either 1150 or 1750 rev/min and larger ones can run at 400 to 800 rev/min.

The flow currents leaving the impeller continue through the liquid in a given direction until deflected by floor or wall of the vessel. The propeller blades vigorously cut or shear the liquid.

A revolving propeller traces out a Helix in the fluid.

One full revolution of propeller (provided no slip between liquid and Propeller) would move the liquid longitudinally a fixed distance depending on the Angle of inclination of propeller blades. The ratio of this distance to propeller diameter is known as Pitch of blade. A Propeller blade with pitch 1 is called square Pitch.

Propellers rarely exceed 18 in. in diameter.

Sometimes two propellers work in opposite directions or in “push-pull” to create highly turbulent zone.

In deep tank, two or more may be mounted on the same shaft

Propellers are used when strong vertical currents are desired e.g. when heavy solid particles are to be kept in suspension.

They are not ordinarily used when viscosity of liquid is greater than 50P.

Agitator consists of flat paddle turning on vertical shaft

They pushed the liquid radially and tangentially with almost no vertical motion at impeller unless blades are Pitched.

The currents they generates travel outward to the vessel wall and then either upward or downward.

In deep tanks several paddles are mounted one above the other on the same shaft.

Anchors are useful for preventing deposits on a heat transfer surfaces (as in a jacketed processes) but hey are poor mixers.

Industrial paddle agitators turn at speeds between 20 and 150 rev/min

The total length of paddle impeller is typically 50 to 80percent of inside diameter of vessel.

Width of blade is one-sixth to one-tenth its length.

Slow speed paddles gives mild agitation and can work in an unbaffled tanks but at higher speeds baffles become necessary

Paddles

Turbines

They resembles multi bladed paddle agitators with short blades, turning at high speeds on a shaft mounted centrally in the vessel.

Blades may be straight or curved, pitched or vertical.

Diameter of impeller turbine is smaller than with paddles, ranging from 30 to 50% of vessel diameter.

The principle currents generated by turbines are radial and tangential ( the tangential component induces vortexing and swirling which must be stopped by baffles

Draft tube Draft tube is a cylindrical duct slightly

larger than the impeller diameter and is positioned around the impeller

Used with axial impellers to direct the suction and discharge flows.

The impeller draft tube system acts as a low efficiency axial flow pump

The top to bottom circulation flow is of significance for flow controlled processes, suspension of solids and dispersion of gases.

They are particularly useful for tall vessels having large ratio of height to diameter.

Standard Turbine Design

Power Consumption

The important consideration in the design of an agitated vessel is the power required to drive the impeller.

• When flow is turbulent in the tank the flow can be obtained by “q” and “Ek” per unit volume of fluid

“q” can be obtained from flow number

“Ek” per unit volume of fluid is equal to following

Where V2 is slightly smaller than the tip speed u2. ratio of And we know u2 = απnDa and the power requirement is

P = q x Ek

In Dimensionless form

The left hand side is the power number Np.

Power CorrelationPower required to rotate a given impeller, the empirical correlations of power with other variables of the system are needed. These can be represented in dimensionless form and are listed out as follows;

Measurement of tank and Impeller Distance of the impeller from the tank floor The liquid depth Dimensions of the baffles Number and arrangement of the baffles Number of blades in the impeller The viscosity and density of the fluid Speed of agitator Dimensionless constant gc (because newton’s law is applicable) Absence / Presence of swirling and vortex Acceleration due to gravity when the liquid is lifted up (due to agitation) from certain

average heightVarious linear measurements can all be converted to dimensionless ratios, called “shape factors”, by dividing each of them by one of their number which is arbitrarily chosen as a basis. Diameter of tank , Dt, or the diameter of impeller, Da, are the suitable choices for the base measurement.

Let the shape factors, so defined, be denoted by S1, S2, S3, ……………Sn. (the diameter of the impeller, Da, is taken as base measurement.

Two mixers of the same geometrical proportions throughout but of different sizes will have identical shape factors but will differ in magnitude of Da. Devices meeting these requirements are called geometrically similar.