35515811 mixing and agitation
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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