Special Topics - Modules in
Pharmaceutical Engineering
ChE 702
Introduction to Mixing
Equipment and Processes
in Pharmaceutical
Operations
Piero M. Armenante
2008 ©
Objectives
 Become familiar with the principles of
single and multiphase mixing in
pharmaceutical processes
 Analyze pharmaceutical processes or
in which mixing is important
 Provide basic tools to conduct
process design analysis and scale-up
of processes or in which mixing is
important
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Relevant Topics
 Classification of Mixing Processes and
Applications
 Mixing Equipment
 Liquid Mixing Fundamentals
 Mixing and Blending in Low Viscosity
Liquids
 High Viscosity Mixing in Stirred Tanks
 Mass Transfer and Mixing
 Solid-Liquid Mixing
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Relevant Topics (continued)
 Liquid-Liquid Mixing
 Gas-Liquid Mixing
 Mixing and Chemical Reactions
 Heat Transfer
 Jet Mixing
 In-Line Mixing
 Mechanical Aspects of Mixing Systems
 Special Topics and Applications
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Classification of Mixing
Processes and Applications
Instructional Objectives of
This Section
By the end of this section you will be
able to:
 Identify basic mixing classes
 Develop an appreciation for the
importance of mixing in industry
 Provide examples of common
pharmaceutical mixing processes
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Definition of Mixing
 Textbook definition:
The term “mixing” refers to all those
operations that tend to reduce nonuniformity in one or more of the
properties of a material in bulk (e.g.,
concentration, temperature)
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Example of Mixing
Tanks/Reactors
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Definition of Fluid Mixing
 “Fluid mixing” refers to mixing
operations in which the continuous
phase is a fluid
 Although a gas can be used as a fluid
(e.g., fluidization) a liquid is
typically the continuous phase in
fluid mixing processes
 In the rest of this course a liquid
phase will always be the
continuous phase
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Single-Phase vs. Multiphase
Mixing
 Single-phase mixing refers to
mixing of miscible fluids. This
operations is typically called
“blending”
 Multiphase mixing refers to mixing
immiscible phases, i.e.:
solid-liquid mixing
liquid-liquid mixing
gas-liquid mixing
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Importance of Mixing in the
Pharmaceutical Industry
 Mixing of a fluid with other media (solids,
liquids) is an extremely common
operation encountered in countless
applications in the pharmaceutical industry
 Many pharmaceutical processes require or
are greatly enhanced by:
 rapid homogenization of miscible components
(in single phase systems)
 intimate contact between two or more distinct
phases (in multiphase systems)
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Examples of Typical Pharmaceutical
Mixing Applications
 Blending
 Precipitation and Crystallization
 Chemical reaction
 Fermentation
 Solid-liquid suspension
 Liquid-liquid emulsification
 Gas sparging
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Economic Impact of MixingRelated Problems
 The impact of poor mixing on
industrial applications has been
estimated to be at 1-10 billion $/year
(1989)
 The additional economic impact
associated with scale-up and start up
problems, waste material and byproducts generation has not been
estimated yet
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Mixing as an Objective or a
Means to an End
 There are operations where mixing itself
is the objective of the process
 These operations are required to produce
homogenization of a system or a product
 Examples:
 Blending of gasoline in large storage tanks
 Dispersion of pigments in paint
 Uniform and stable suspension of API particles
in an oral liquid dosage form
 Formation of stable liquid-liquid emulsions
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Mixing as an Objective or a
Means to an End
 However, in most pharmaceutical
processes involving mixing, mixing
is just a means to achieve a
process objective
 In this case mixing is typically
required to effectively conduct a
primary process (NOT to be
limited by mixing)
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Mixing as an Objective or a
Means to an End
 Examples of processes possibly affected by
mixing:
 Dissolution of an intermediate in a stirred vessel
prior to reaction (mass transfer)
 Precipitation of API or intermediate (crystallization)
 Minimization of impurity formation during
synthesis of a drug product (parallel/consecutive
homogeneous reaction)
 Suspension of a catalyst during heterogeneous
catalysis (mass transfer + heterogeneous reaction)
 Preparation of nano/micro-particles or droplets of
desired particle size distribution (particle size
control)
 Achievement of a uniform temperature in a
crystallizer and temperature control (heat transfer)
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Mixing as an Objective or a
Means to an End
 Mixing operation may involve:
single phase liquids (e.g., blending of
miscible solutions, fast chemical parallel
reactions and impurity formation)
multiphase systems (e.g., solid
dispersion/suspension, emulsification)
 Mixing can improve both single-phase
and mulpiphase processes
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Mixing as a Means to an End
 Example: interfacial mass transfer
  kL A Cinterface  Cbulk   kL  av VL  C
m
A
Cinterface
Cbulk
kL
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Mixing as a Means to an End
 Example: interfacial mass transfer
  kL A Cinterface  Cbulk   kL  av VL  C
m
 Mixing affects:
 state of dispersion or suspension of the
dispersed phase, i.e., degree of macroscopic
homogeneity of the dispersed phase
throughout the continuous phase ( VL, C)
 specific interfacial area (av), and overall
interfacial area (A)
 mass transfer coefficient at the interface (kL)
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Mass Transfer Operations in
Mixing Processes
 All mass transfer processes are
enhanced by:
high mass transfer coefficients
large interfacial area
 Mixing can contribute to achieve
both
 However, most mixing operations are
associated with the generation of
interfacial (contact) area
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Classification of Mixing
Processes
System
Operation
Homogeneous
liquid
Miscible liquids
Pumping, recirculation,
heat transfer
Blending
Solid-liquid
Suspension
Liquid-liquid
(immiscible liquids)
Dispersion
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Mass Transfer Operations in
Mixing Processes
System
Homogeneous
liquid
Miscible liquids
Solid-liquid
Liquid-liquid
(immiscible liquids)
Gas-liquid
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Mass Transfer
Operation
Turbulent and molecular
diffusion
Adsorption, ion exchange
leaching, dissolution
Extraction
Absorption, stripping
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Reactions in Mixing
Processes
System
Reaction
Homogeneous
liquid
Miscible liquids
-
Solid-liquid
Liquid-liquid
(immiscible liquids)
Gas-liquid
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Homogeneous reaction
Heterogeneous reaction,
catalysis, precipitation,
crystallization
Heterogeneous reaction
Gas-liquid reaction
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Single vs. Multiple Mixing
Requirements
 Mixing problems can involve:
a single mixing requirement (e.g.,
suspend solids)
multiple simultaneous mixing
requirements (e.g., suspend solids,
homogenize liquid phase, promote solidliquid mass transfer, transfer heat)
 Even multiple requirements are
typically satisfied by the use of a
single impeller
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Example of Multiple Mixing
Requirements: Crystallizers
In crystallizers a successful process
depends on:
heat transfer (for supersaturation)
bulk blending (for homogenization)
solids suspension (for crystal growth)
effective mass transfer (for crystal
growth)
possible gas removal (boiling systems)
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Critical Mixing Process
 Whenever a process involving a
mixing operation is analyzed one
should ask:
is mixing a critical component of the
process?
if multiple, simultaneous mixing
requirements are present which one is
the most critical?
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Mixing Equipment
Instructional Objective of
This Section
By the end of this section you will be
able to:
 Identify basic types of mixing
equipment
 Describe main components of mixing
equipment
 Describe main features and
characteristics of mixing equipment
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Classification of Mixing
Equipment
Mixing is typically conducted with:
 mechanically stirred tanks
 jet mixed tanks
 in-line dynamic mixers
 in-line static mixers
 high-shear mixing equipment
 mixing equipment for highly viscous
materials (e.g., polymers)
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Mechanically Stirred Tanks
and Reactors
Motor
Gearbox
Shaft
Baffle
Impeller
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Drive (Motor-Gearbox)
Assembly
After Chemineer
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Mechanically Stirred Tanks
and Reactors: Symbols
B
H
Cb
D
T
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Mechanically Stirred Tanks
and Reactors: Symbols
H
S23
S12
Cb
T
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Mechanically Stirred Tanks:
Nomenclature
 Tank shape = cylindrical (occasionally
square cross section)
 T = Internal diameter of tank
 HT = Internal height of tank
 H = Z = Liquid height
 B = Baffle width
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Mechanically Stirred Tanks : Other
Geometric Characteristics
 Shape of tank bottom (flat, dished,
conical, hemispherical)
 Baffle length (full, half)
 Number of baffles
 Baffle position
 Gap between baffles and tank (B)
 Gap between baffles and tank bottom
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Baffles
 Baffles are typically introduced to
prevent vortex formation and
convert tangential (rotational) flow
into axial (vertical) flow
 Baffles are always used in turbulent
flow systems (low viscosity fluids)
 Baffles are not used in laminar flow
(high viscosity fluids)
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Baffles
 Typically four baffles are used
(occasionally three) in fully baffled tanks
 In glass-lined tanks a single baffle placed
midway between the tank wall and the
impeller may be used
 A gap between the baffles and the wall is
introduced to prevent stagnation behind
the baffles and accumulation of material
(e.g., solids)
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Typical Baffle Arrangement
in a Stirred Tank
Baffle
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Typical Baffle Arrangement
in a Glass-Lined Tank
De Dietrich
Vessel
Single
Baffle
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Baffles and Vortexing
Baffled tank:
No vortex
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Unbaffled tank:
Vortex
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The “Standard” Tank
 H/T = 1
 D/T = 1/3
 C/D = 1
 B/T = 1/10 (academic) or 1/12
(industry)
 Number of baffles = 4
 Baffle length = full
 B/T =1/72 or 1/100
 Bottom shape = flat
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Impellers
After Oldshue, 1984
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Impeller Types
Impeller can be classified as follows:
 radial impellers (e.g, Rushton
turbines, paddles, flat-blade turbines,
Smith impellers)
 axial impellers (e.g., marine
propellers, pitched-blade turbines,
fluidfoil impellers such as HE-3s, A310s)
 close-clearance impeller (e.g.,
anchors, helical ribbons, gates)
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Radial Impellers
 Radial impellers pump radially.
 They are used primarily with lowviscosity liquids in baffled tanks.
 Disk turbines can be used for gas
dispersion.
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Radial Impellers
 Common types include:
Rushton turbine (6-blade disk
turbine)
paddle
flat-blade turbines
curve-blade turbine
retreat-blade turbine
Smith impeller
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Examples of Radial Flow
Impellers
After Tatterson, 1991
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Examples of Radial Flow
Impellers
Disk Turbine (Rushton Turbine)
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Examples of Radial Flow
Impellers
Flat-blade turbine (Source: Chemineer)
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Example of Radial Flow Impeller for
High Shear Applications
R500 Sawtooth Impeller (Source: Lightnin)
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Example of Radial Flow
Impeller for Gas Dispersion
Concave-Blade Turbine (Smith Turbine)
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Example of Radial Flow
Impeller for Gas Dispersion
Concave-Blade Turbine (Smith Turbine)
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Flow Generated by Radial
Impellers
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Flow Generated by a Radial
Impeller in a Stirred Tank
After Tatterson, 1991
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Axial Impellers
 Axial impellers pump primarily (but
not exclusively) vertically, either
upwards or downwards.
 They are used mainly with lowviscosity liquids in baffled tanks.
 They are typically used in a
downpumping mode.
 High-solidity impellers are used with
gas.
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Pitch Ratio in Axial
Impellers
 The pitch-to-diameter ratio (or
“pitch ratio”) is the ratio of the
distance the impeller would advance
per rotation to the impeller diameter
 In constant pitch impellers (e.g.,
propellers) the angle of attach
changes along the blade; in variable
pitch impellers (e.g, 45° pitchedblade turbine) the angle is constant
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Constant vs. Variable Pitch
Constant Pitch
(Variable angle
of attack)
Variable Pitch
(Constant angle
of attack)
After Oldshue, 1984
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Axial Impellers
 Common types include:
marine propeller
pitched-blade turbine
fluidfoil impeller (e.g., Chemineer
HE3, Lightning A-310)
high-solidity ratio impellers (e.g.,
Prochem)
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Examples of Axial Flow
Impellers
After Tatterson, 1991
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Examples of Axial Flow
Impellers
Pitched-Blade Turbine
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Example of Axial Flow
(Hydrofoil) Impeller
Chemineer SC-3 Impeller
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Example of Axial Flow
(Hydrofoil) Impeller
Chemineer HE-3 Impeller
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Example of Axial Flow
(Hydrofoil) Impeller
Chemineer HE-3 Impeller
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Example of Axial Flow
(Hydrofoil) Impeller
Maxflow W Impeller
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Example of Glassed
Impellers
De Dietrich
GlasLock System
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Flow Generated by Axial
Impellers
Flow generated by
true axial impellers
(~propeller, A-310, HE-3)
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Flow generated by
mixed-flow impellers
(e.g., 45° pitchedblade turbine)
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Flow Generated by an Axial
Impeller in a Stirred Tank
After Tatterson, 1991
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Close-Clearance Impellers
 Close-clearance impellers are
primarily used with high-viscosity
fluids in unbaffled tanks.
 Close-clearance impellers scrape fluid
off the tank wall and off the impeller.
 They generate a complex flow
pattern and have a pumping action
similar to that of a displacement
pump.
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Close-Clearance Impellers
 Common close-clearance impeller
types include:
anchors
helical ribbons
gates
kneaders
Z- and sigma-blade impellers
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Examples of Close Clearance
Impellers
Anchor Impeller (Source: Chemineer)
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Examples of Close Clearance
Impellers
After Oldshue, 1984
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Examples of Close Clearance
Impellers
After Oldshue, 1984
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Examples of Close Clearance
Impellers
Double Helical Ribbon Impeller (Source: Chemineer)
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Examples of Close Clearance
Impellers
Auger Impeller (Source: Chemineer)
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Examples of Close Clearance
Impellers
After Tatterson, 1991
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Examples of Close Clearance
Agitation System
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Blending Capabilities of
Different Impellers
Impeller
 Open Impellers
- Propellers
- Turbines
- Paddles
 Anchors
 Helical Ribbons
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Viscosity Range
< 100,000 cP
< 200 cP
< 5000 cP
< 100,000 cP
< 50,000
> 30,000
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Characteristics of Common
Radial Impellers
 Rushton turbines (Disk turbine, R100). Strong radial flow, high power
consumption, significant shear, good
for gas dispersion
 Smith impeller. Similar in
performance to Rushton turbine, but
particularly well suited for gas
dispersion
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Characteristics of Common
Radial Impellers
 Paddles. Simple and inexpensive,
medium-to-strong radial flow and
shear, intermediate power
consumption, good for simple
applications at small-to-medium
scales
 Flat-blade turbines. Similar to
paddles but with stronger radial flow
power, consumption, and shear.
Used in transition flow.
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Characteristics of Common
Radial Impellers
 Curve-blade turbine. Similar to flatblade turbines
 Retreat-blade impeller (Pfaudler,
De Dietrich types). Simpler
construction suitable for glass-lined
vessels; reduced power and flow
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Characteristics of Common
Axial Impellers
 Marine propeller (A-100). Oldest
constant-pitched impeller, usually cast
(cannot be easily inserted in a
manhole), expensive, low power
consumption, high pumping rate
 Pitched-blade turbine (A-200).
Very common, simple, usually 45°,
effective for solid suspension; mixed
flow; medium power consumption,
good pumping rate
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Characteristics of Common
Axial Impellers
 Fluidfoil impellers. Many types
exist (Chemineer HE-3, Lightning A310); expensive, near constant pitch
for improved axial flow, low power
consumption, high pumping rate
 High-solidity ratio impellers.
Many types exist (e.g., Maxchem);
low-to-medium power consumption,
high pumping rate, “streamlined”
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Characteristics of Common
Close-Clearance Impellers
 Anchor impellers (A-400). Good
for blending and heat transfer for
liquids with 5000 cP <  < 50,000 cP
 Helical ribbon. Good for blending
high viscosity liquids (up to 25·106
cP)
 Gates. Used in large “squat” tanks.
 Kneaders, Z- and sigma-blade
impellers. Used to mix pastes
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Impellers: Nomenclature
 D = Impeller diameter
 C = Impeller clearance off the tank
bottom measured from the impeller
center
 Cb = Impeller clearance off the tank
bottom measured from the bottom of
the impeller
 Sij = distance between i and j
impellers
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Impellers: Nomenclature
 L = Impeller blade length
 w = Impeller blade width
 wb = Impeller blade width projected
along the vertical axis
 Sij = distance between impellers i and
j
  = Blade angle of attack (if constant)
 Pitch
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Rushton Turbine
L/D=1/4
w/D=1/5
Disk diameter=
3/4·D or 2/3 ·D
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45° Pitched-Blade Turbine
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Typical Ranges for
Geometric Variables
 T = 0.1 m to 10 m (0.3’-33’)
 H/T = 0.3 to 1.2 for single impeller
systems
 D/T = 1/5 to 2/3
 C/D  1
 B/T = 1/10 to 1/12
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Jet Mixers
 Jet mixers rely on the use of a jet,
i.e., a stream of liquid injected at
high velocity in the bulk of another
miscible liquid.
 This is typically achieved with an
external recirculation pump
 Jet mixers are used in:
tanks
tubes and pipes
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Jet Mixer
External recirculation line
Pump
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Jet Mixers in Tanks
 Jet mixers are typically used in large
tanks.
 Jet mixers are used for blending
purposes (e.g., gasoline) or to
suspend solids in unusual processes
(e.g., radioactive material slurry).
 Typically one or more jets are placed
at an angle to provide good
recirculation.
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Axial Jets in Mixing Tanks
Poorly mixed zone
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Angled Jets in Mixing Tanks
Poorly mixed zone
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In-Line Mixers
 In-line mixers are small mixing devices
placed in the same line where the
materials to be mixed are flowing.
 Two types of in-line mixers exist:
dynamic mixers, where the mixing
energy is provided from the outside
static (motionless) mixers where the
fluid itself provides the mixing energy
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In-Line Dynamic Mixers
 In-line dynamic mixers consist of
small high-speed mixers placed
inside a casing fed with a continuous
stream of the materials to be mixed.
 The residence time of in-line mixers
is usually of the order of seconds.
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Example of a Dynamic
In-Line Mixer
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Example of In-Line, High
Shear, Homogenizing Mixer
Greerco (Chemineer)
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Example of a Two-Stage Rotor
Stator for In-Line High Shear Mixer
Greerco (Chemineer)
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Applications of Dynamic
In-Line Mixers
After Oldshue, 1984
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In-Line Static Mixers
 Static mixers consist of mirror image
inserts (elements) placed inside a
pipe, capable of altering the fluid
flow, and rearranging the distribution
of fluid elements across the pipe
cross section.
 Static mixers are only capable of
homogenizing the content of the pipe
across its cross section but not along
its length.
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Static Mixers
Source: Chemineer
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Classification of
Static Mixers
 Static mixers are classified according
to the flow regime under which they
operate.
 Static mixers are available for:
laminar flow
transitional flow
turbulent flow
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Static Mixers for
Laminar Flow
 In laminar flow the only mechanism for
radial mixing is molecular diffusion.
 Each element in a laminar static mixers
typically produces spit and a rotation (90°
or 180°) of the flow, which is then fed to
the next element.
 Such actions result in further sub-divisions
of the flow and the generation of striations
leading to mixing.
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Static Helical Mixer for
Laminar Flow
After Myers et al., Chem. Eng. June 1997
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Static Helical Mixer for
Laminar Flow
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Static Helical Mixer for
Laminar Flow
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Static Mixers for
Turbulent Flow
 In turbulent flow, turbulent eddies
are responsible for radial mixing
 Flow in open pipes produces radial
mixing if enough pipe length is
provided (at least 100 pipe
diameters)
 Static mixers for turbulent flow rely
on vortex generation to produce
mixing
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Static Vortex Mixer for
Turbulent Flow
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Static Vortex Mixer for
Turbulent Flow
Source: Chemineer
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Static Vortex Mixer for
Turbulent Flow
After Myers et al., Chem. Eng. June 1997
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High-Shear Mixing
Equipment
 High-shear mixers are devices used
to generate high velocity gradients
across small distances (resulting in
high shear stress and shear rates) in
order to disperse, break up, or
homogenize a second immiscible
phase.
 Different devices base on different
physical mechanisms are used to
produce high shear.
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High-Shear Equipment
High shear equipment include:
 (high speed) rotor-stator devices
 valve homogeneizers, such as:
valve homogeneizers
ultrasonic homogenizers
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High-Speed, High-Shear
Rotor-Stator Mixer
 High-speed rotor-stator mixers are
devices in which a rotor rotates at
high speed inside a casing provided
with slots. A small gap exists
between the rotor and the stator.
 As the liquid (and its dispersed
phase) move through the rotor-stator
assembly they are subjected to high
shear, resulting in break up effects.
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High-Speed, High-Shear
Rotor-Stator Mixer
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Example of High-Speed, HighShear Rotor-Stator Mixer
Silverson Machines, Inc.
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Example of High-Speed, HighShear Rotor-Stator Mixer
Silverson Machines, Inc.
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Example of High-Speed, HighShear Rotor-Stator Mixer
Silverson Machines, Inc.
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Colloid Mills
 Colloid mills are in-line machines designed
to finely homogenize, disperse solids, and
emulsify immiscible liquids
 Mixing head consist of a rotor and a stator
separated by an extremely small gap
(0.001-0.03 in.)
 Stirring speed are usually extremely high
(2000-14,000 rpm)
 Flow rates are usually small (as a result of
the small rotor-stator gap)
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Colloid Mill
Greerco (Chemineer)
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Colloid Mill
Greerco (Chemineer)
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Colloid Mill
IKA®
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Colloid Mill
Greerco (Chemineer)
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Valve Homogenizers
 Valve homogenizers pump material
at high pressure (30-500 bar)
through small orifices.
 The high velocity in the orifices
produces high shear.
 The equipment operates in line and
can be used to produce emulsions,
dispersion, and suspensions.
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Valve Homogenizer
After Harnby et al., 1985
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Example of Valve
Homogenizer
Five Star Technologies
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Ultrasonic Homogenizers
 Ultrasonic homogenizers pump
material at high pressure (up to 150
bar) through a small orifice placed in
front of a vibrating ultrasonic blade.
 The high velocity in the orifice
produces high shear, and the blade
produces microcavitation that results
in emulsions, dispersion, and
suspensions of the dispersed phase.
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Ultrasonic Homogenizer
After Harnby et al., 1985
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Basic Mechanisms in
Laminar Flow Mixing
 Laminar shear
 Elongation and extensional flow
 Distributive mixing
 Molecular diffusion
 Stresses in laminar flow
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Mixing Equipment for Highly
Viscous Materials
Equipment for highly viscous material
(such as pastes, dough, plastics)
include:
kneaders
single-screw extruders
twin-screw extruders
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Double-Arm Kneader
After Perry and Green, 1984
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Single-Screw Extruder
Feed
Hopper
Die
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Twin-Screw Extruder
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Single-Screw Extruder
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Screw Design to Enhance
Mixing/Compounding Capability in
Single Screw Extruders
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Twin-Screw Extruder with
Clam-Shell Barrel Design
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Gear Mixing Elements in a
Twin-Screw Extruder
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Kneading Paddles in a TwinScrew Extruder
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Final Remarks About
Impellers
 No universal “optimal” impeller
design exists
 Each process needs to be analyzed to
determine what are the controlling
mechanisms
 Impellers can be designed to
optimize the process
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