35515811 mixing and agitation

MIXING AND AGITATION

Be able to: –Differentiate between Mixing and agitation or stirring–Describe the purpose of mixing operations.–Describe various mixing mechanisms.–Describe label various parts of agitated or stirred tank.– Know various impeller types.–Know various flow patterns.–Ways to prevent swirling

MIXING AND AGITATION

• Agitation is an induced circularly motion of the material in a container in a desired way.

• Material is agitated by a stirrer or mixer that is made up of shaft that connects motor and the impeller.

• Stirring is synonymous with agitation• Mixing is a random distribution of materials in

different phases into another forming a homogenous phase.

GENERAL PURPOSES FOR AGITATION

Homework give an example of each of the following • Suspending solids in liquids. e.g.• Homogenization (blending miscible liquids). e.g.• Sparging (dispersing a gas through the liquid in

the form of bubbles. e.g.• Emulsifying (dispersing two immiscible liquids

within each other to form emulsion). e.g.• Promoting heat transfer between liquids and a

heat transfer surface e.g. – McCabe et al 2001:229

Typical Blending Applications

• Chemical Reactions• Polymerization• Simple blending of miscible fluids• Make-Up Tanks.• Storage, Feed, or Holding Tanks

Suspensions

degree of are suspension has a significant effect on the mixer power.

Five degrees of suspension :

• Solids Just Suspended

• Off Bottom Suspension

• Moderate Uniformity

• Nearly Uniform Suspension.

• Uniform Suspension

Heat Transfer Mixing Applications

• Chemical Reactions

• Fermentations

• Polymerizations

• Esterifications

• Hydrogenations

Mixing Mechanisms

ConvectionImpeller action, moves fluids through the

different parts of vessel,

Macro-mixing

Turbulent flow causes a wide range of vortices. Bulk of fluid separates into smaller elements.

Laminar shear

Fluid elements are dispersed by laminar shearing. Elements are stretched, distorted and folded.

Micro-mixing

Diffusion of reactants by concentration gradient.

Mixing/Stirred/Agitated tanks/vessels/reactors

Mixing/Stirred/Agitated tanks/vessels/reactors are common chemical engineering equipments.

Most stirred Tanks a cylindrical with rounded bottom

Vertical impeller spindle or shaft

Major components of Stirred Tanks are :• Vessel/tank.• Mixer or impeller.• Impeller or Mixer motor.• Baffles There a various types of each of these.

• 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

Mixing Tanks or Stirred tanks

Mixing Tanks or Agitated Vessels

NomenclatureWhere:

D = Impeller Diameter

C = Impeller off Bottom Clearance

N = Impeller speed

Z = liquid Depth

T = Vessel Diameter

Task 1 Label the symbols in the diagram

M N

S

I

Impeller TypeMixing impeller converts the rotational energy of

the mixer shaft into the correct combination of flow, shear and turbulence to achieve the desired mixing .

• No one impeller is one size fits all. • There is range of impellers to select from for any

given mixing requirements.• The following slide shows various impellers

along predominant flow patterns over a certain range of viscosities.

• Hence impellers are classified into two classes according to flow patterns e.g. Axial and radial flow patterns.

Impeller Types

Zlokarnik 2001:7

Axial and radial flow patterns.

• Axial Flow pattern– Parallel with axis of

impeller shaft

Zlokarnik 2001:7

Radial Flow– Tangential flow

Impeller Types

• Other Impeller types:

• Propeller Impeller type.– Axial, high speed impeller

• Turbine impeller type.– Axial and tangential motion no vertical motion

at the impeller.

Some visual examples

3-blade marine Impeller

McCabe 2001:241

Simple straight blade Turbine

Six disk Turbine

Concave Six disk Turbine Pitched blade Turbine

• Impeller at the centre and swirling

Swirling

Tangential flow

Swirling and Impeller positions

Mc Cabe 2001:244

Swirling and Impeller positions

Impellers can be positioned to avoid swirling

(see diagrams below) Mc Cabe 2001:245