emulsion processing - homogenization -
TRANSCRIPT
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Emulsion Processing- Homogenization -
Jochen Weiss
*Food Structure and Functionality LaboratoriesDepartment of Food Science & BiotechnologyUniversity of HohenheimGarbenstrasse 21, 70599 Stuttgart, Germany
Emulsion Workshop
November 13-14th, 2008, Amherst, MA
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Principle of Emulsion Formation
Oil
Water
Primary
Homogenization
Secondary
Homogenization
Premix
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Emulsion Processing:
Homogenizers
Oil
Water Homogenizer
Homogenization is a unit operation using a class of processing
equipment referred to as homogenizers that are geared towards
reducing the size of droplets in liquid-liquid dispersions
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Physiochemical Processes Occurring During Homogenization
Rapid adsorption:Stable droplets
Slow adsorption:Coalescence
DropletDeformation
Disruption
Continuousphase
Dispersedphase
Emulsifier
I. Pre-homogenization
II. Homogenization III. Stabilization
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The order of ingredient addition and homogenizationmay have a large impact on product properties
(A) (B)
General Homogenization Options
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Homogenization:
Process Parameters
� Energy density
- minimum droplet size achievable
� Energy efficiency
- heat losses - manufacturing costs
� Volume Flow Rates
- throughput - production time
� Product rheology
- limitations - materials that can be homogenized
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The Physics of Droplet Disruption
• Maintaining Force. Drop shape maintenance forces (Laplace Pressure)
• Disruptive Force. Drop disruption is due to drop-surface applied tangential stresses
• Weber-Number (We): The ratio between drop disrupting and drop maintaining forces, drop disruption occurs only above a critical Weber number– We < Wecrit or tbr < tbr,crit � droplet
deformation– We > Wecrit or tbr > tbr,crit � droplet disruption
• Deformation Time. Droplets must be exposed to tangential stresses for a sufficient amount of time
1 2
1 1 4cp
r r dγ γ
= + =
4c
dWe
p
τ τ
γ= =
,d
br crit
c
tp
η
τ=
−
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Key Parameter: Energy Density Ev
• The volume specific energy input or the energy density Ev can simply be calculated from the power consumption and the volume flow rate
• The mean droplet diameter may often be empirically related to the energy density, IF, all other parameters are kept constant!
Energy input
homogenized volume v v v
E PE P t
V V= = =
&
( )1,2
b
vx C E=
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How Droplets Are Disrupted: Flow Situations in Homogenizers
Elongational
FlowRotational
Flow
Simple Shear Flow
Flow profiles are complex based on geometry of homogenizer.
Flow may be laminar (rotational, simple shear, elongation) or
turbulent.
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Simple Deformation Scenarios For Liquid-Liquid Dispersions
Rotation of
whole droplet
Circulation of
fluid within droplet
Elongation of
droplet
Disruption of
droplet
“Neck”
Formation
Increased Exertion of Stress Due to Superimposed Flow Profile
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Role of Emulsifiers in Emulsion
Formation and StabilizationStabilizationFormation
Rapidly adsorb
Lower interfacial tensionFacilitate breakup
Generate repulsive forces
Form resistant membranePrevent Coalescence
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Optimum Emulsifier Characteristics for Emulsion Formation
• Objective: to generate small stable droplets
– Rapid adsorption
– Lower interfacial Tension
– Form protective membrane
Disruption Coalescence
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Factors Affecting Droplet Size:
Emulsifier Concentration
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.2 0.4 0.6 0.8
Once the emulsifier
concentration exceeds a
certain level the droplet size depends on the energy input
of the homogenizer.
Corn O/W emulsion - Pandolfe (1995)
Fixed Homogenization Conditions (1000 psi)
Emulsifier/Oil
Mean D
rople
t D
iam
ete
r [µ
m]
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Factors Affecting Droplet Size: Emulsifier Type
0
1
2
3
4
5
0 5 10 15
Fast
Slow
Insufficient
Denaturation
Depends on:
Emulsifier Type
Emulsifier Conc.
Solution Conditions
Mechanical Device
Mean D
rople
t D
iam
ete
r [µ
m]
E [MJm-3]
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Characterizing Emulsifier Efficiency During Homogenization
• Maximum amount of oil that can be homogenized by fixed amount of emulsifier using standardized conditions (homogenization, pH, I, T)– EAI (g oil / g emulsifier) – Emulsifier activity index
• Minimum amount of emulsifier required to achieve a given droplet size using standardized conditions (φ, homogenization, pH, I, T)– cmin (g emulsifier / g oil)
• Minimum droplet size that can be achieved by homogenization– rmin (µm)
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Homogenizers
High Speed Blender
High Pressure Homogenizers
Colloid Mill
High Shear Dispersers
Ultrasonic Disruptor
Membrane Homogenizers
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Homogenizers:
A General Overview (I)
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Homogenizers:A General Overview (II)
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I. High Speed Blenders
• Low volume specific energy input (energy density)
• Energy input highly distributed in the stirred vessel (regions of low and high shear)
• Blender geometry and rotational speed are the prime parameters � turbulent flows preferred.
• Broad particle size distribution, large particles
• Need to avoid air incorporation to avoid foam formation
• Fairly inexpensive usually used for premix production
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II. High Pressure Homogenizer (HPH)
• Most common used homogenizer in the food industry (milk, cream etc.)
• Disruptive energy comes from relaxation of high pressure build up across homogenization valve
• Pressures typically range from 50 to 500 bar (microfluidizer up to 1600 bar)
• Homogenization valve geometry of key importance � influences flow profile
• Homogenization may be single or multiple stage
Shear, Turbulent &Cavitation Forces
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Physical Process Inside the Homogenization Valve
• On entering the homogenization valve, the flow speed greatly increases � pressure drops (Bernoulli) to reach the vapor pressure pD at point A
• Since pD is lower than the external pressure pA � cavitation and two phase fluid flow
• Pressure signal transduction in multi-phase flows is slower than in single phase flow �equilibration with external pressure occurs late (close to exit)
• Sudden pressure jump leads to collapse of cavitational bubbles and the flow reverts to a one-phase flow
• Droplet disruption is therefore due to
– Laminar and turbulent flow at entrance of valve (a)
– Growth of cavitational bubbles in zone (b)
– Collapse of bubbles in zone (c)
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High Pressure Homogenizer: Efficiency
Requirements
• Valve Type
– Flat, toothed or knife edge
• Homogenization Pressure
– Gap Size
• Counter Pressure
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HPH Efficiency Requirements
• Homogenization valves are therefore designed with the following criteria in mind– Ensure sufficient time for growth of cavitational bubbles– “Explosive” increase of pressure towards the exit
– Maximum height of the “Dirac-like” pressure increase
• Base process parameters for the user are homogenization pressure pH and external (exit) pressure pA
• The exit pressure may be increased through– Increased pressure in connected process equipment– Introduction of secondary pressure valve
– Introduction of secondary homogenization valve
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HPH: Influence of Homogenization Pressure (Energy Density)
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HPH: Influence of Counter Pressure
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High Pressure Homogenizer:Influence of Dispersed Phase Viscosity
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High Pressure Homogenizer:Influence of Viscosity Ratio
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Influence of Disperse and Continuous Phase Viscosity
0
2
4
6
8
10
0.001 0.01 0.1 1 10
Viscosity Ratio:Small droplets can only be formed when viscosity ratio of oil to aqueous phase is between 0.05 and 5. It is therefore important to control viscosity ratio.
Laminar Flow Conditions, O/W Emulsions, Fast Emulsifier
(Schubert and Armbruster, 1992)ηd/ηc
Mean D
rople
t D
iam
ete
r [µ
m]
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HPH:
Influence of Valve Geometry
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HDH: Multistep Homogenization
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From HPH to Microfluidizer (MF)
A Small Design Change with a Large Effect
• In addition to pressure changes leading to cavitation, fluid jets also impinge in a mixing chamber
• Extremely turbulent flow situation and very high shear forces cause additional disruption of droplets
• Higher homogenization pressures possible• NO MOVING PARTS! � low maintenance• But: stronger possibility for blockage
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HPH and MF Comparison
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III. Colloid Mills (CM)
• Droplet disruption in colloid mills occurs in a flow channel between a rotor-stator assembly
• The rotor may have a large variety of surface profiles (toothed)
• Typical rotational speeds are n=3000 min-1
� Due to the conical design, the emulsions is transported
without external pressure
application
� The size of the gap alters
residence time in the channel and exerted forces
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IV: High Shear Disperser (HSD) – (Ultra) Turrax
• HSD or Ultra-Turrax Systems are rotor-stator systems similar to colloid mills
• They are composed of coaxial intermeshing rings with radial openings
• The fluid enters in the center of the systems and is accelerated by the rotor
• As it passes through the system, the fluid is accelerated and decelerated multiple times which results in high tangential forces
• Rotational speeds may be as high as n=20000 min-1
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High Shear Disperser (HSD):Principle of Droplet Disruption
• The pre-emulsion is transported through the radial openings of the rotor systems and mixes with the liquid in the gap between the rotor and the stator
• High shear forces and turbulent flow situations arise that lead to a disruption of droplets
Stator
Rotor
Shear-gap
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HSD:
Complexity of Flow Profiles
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HSD:Influence of Viscosity Ratio
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HSD:Influence of Residence Time
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HSD: Influence of Geometry of Rotor-Stator
System
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V. Ultrasonic Homogenizer (UH)
• Piezo-electric or magneto-restrictive transducers generate sound waves with frequencies > 20 kHz
• Associated pressure gradients cause deformation of droplets
• Cavitation caused by drop of local pressure below the vapor pressure of the solvent generates turbulent flow and high shear
• Can be run in batch or continuous using a flow cell
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Typical Design of a Continuous Ultrasonic Homogenizer
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Ultrasonic Homogenizer :Process Parameters
• Frequency– Size of cavitational bubbles decreases with increasing frequency
– Wave intensity
– A minimum pressure drop is required to initiate cavitation � below critical wave intensities, almost no droplet disruption
– Above the critical pressure, direct relationship between applied intensity and achievable particle size
• Sonication time– Disruption and stabilization are kinetic events, thus, a minimum sonication
time is required to achieve droplet disruption
• Hydrostatic pressure– Initially, improved emulsification (up to 5 bar), above 5 bar, decreased
efficiency
• Dissolved gas concentration– Increased number of cavitational events but reduced intensity of cavitational
collapse due to dampening
• Viscosity & Temperature– Influences droplet disruption
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Ultrasonic Homogenizer:Emulsifier Properties Play a Key Role
• The relationship between energy density, volume fraction and achievable mean droplet diameter depends strongly on the emulsifier.
• For slow emulsifiers (bottom), achievable particle sizes increase with increasing volume fraction
• For fast emulsifiers (top), smaller emulsion can be obtained even at high dispersed phase concentrations
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Ultrasonic Homogenizer:Peculiarity in Size Distribution Development
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Ultrasonic Homogenizer:Scale-up of Homogenizers
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VI. Membrane Homogenizers (MH)
• Dispersed phase or emulsion pushed through microporous membrane into continuous phase
• High energy efficiency
• Narrow particle size distribution possible
• Droplet size typically 2-3 × pore size• For high dispersed phase
concentrations, recirculation is needed � time consuming
• Process parameters include:– Membrane pore size and material
– Volume flow rates
– Applied trans-membrane pressure
– Others: temperature, viscosity, etc.
O/W Produced by Membrane
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Direct Comparison of Homogenizers
0.1
1
10
100
0.01 0.1 1 10 100
EV (MJ m-3
)
d32 (µµµµm)
Membrane
MFUS
CM
HSB
HPVH
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Selecting a Homogenizer
� Define desired product characteristics
� Particle size distribution, viscosity
� Define desired production conditions
� Batch/Continuous, throughput, hygiene, temperature
� Identify, Test & Compare Homogenizers
� High speed blender, high pressure valve, colloid mill, ultrasonic, membrane etc.
� Optimize Homogenization Conditions
� Pressure, flow rate, rotation speed, time, temperature, emulsifier type, emulsifier concentration
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Which Homogenizer to Buy?
Fine EmulsionEffective droplet disruption
High specific energy
Coalescence Rate
Narrowdistribution
Viscosity
MembraneHPHUS
HPHCM
CMHSD
lowhigh
highlow
important
Not very important
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Recap:
Factors Affecting Droplet Size
• Homogenizer– Machine Parameters (Pressure, Rotation Speed,
Time)
– Energy Density
• Component Phases– Interfacial Tension
– Viscosity
• Emulsifier– Adsorption Kinetics
– Interfacial Tension Reduction
– Stabilization of Droplets Against Aggregation
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