APV
PROCESSING OF
EMULSIONS AN
DISPERSIONS
BY HOMOGENIZATION
APV Homogenizer Group
Gaulin Rannie
APV
From its very genesis APV has maintained an impeccable
reputation for excellence worldwide. That standard is perpetuated in the field
of homogenization and high pressure pumps. It's a tradition that dates back to
our invention of the homogenizer in the early 1900's.
That desire to excel in serving our customers is
demonstrated today through innovative design, precision engineering, quality
manufacturing and attention to detail within the APV Homogenizer Group.
This group, created by the unifying of APV Gaulin and APV
Rannie introduced a new era in the industry. With more than 150 years of combined
experience, APV provides our customers with the greatest resource for
homogenization technology ever assembled. While main-taining the distinguishing
characteristics and unique designs of the GAULIN
and RANNIE products, APV is now able to offer a much greater range of
equipment and service from a single source.
The APV Homogenizer Group is ready to meet the challenges
facing us in this exciting, technologically advanced world, and we look forward
to serving you.
World Leader in Homogenization Technology
INTRODUCTION
The APV Homogenizer Group designs and manufactures three
types of emulsion and dispersion processing equipment. They are high pressure Gaulin and Rannie homogenizers (0 to
21,750 psi operating pressure), the low-energy Gaulin HydroshearO mixer/emulsifier (100 to 250 psi operating
pressure) and Gaulin colloid mills. Emulsions
with viscosities up to 5000 cP (for the premix) may be processed, and particle
sizes ranging from 0.1 to 25 micrometers are possible by selecting the correct
unit. The same viscosity and particle size limitations apply in the case of
dispersions. Naturally, there will be some overlapping as to viscosity and
particle size governing the choice of equipment; but, in general, the operating
parameters are sufficiently well defined that an intelligent choice for
evaluations may be made.
Emulsions and dispersions encompass a significant number
of products which would seem to be, on cursory examination, totally unrelated
to each other in form, substance and method of production. However, a more
detailed investigation would reveal that there are many basic elements which
are common to all emulsions and dispersions. To discuss the topics of
homogenization and emulsification in a reasonable space, it is necessary to
relate these processes to the basic elements of emulsions and dispersions.
Therefore, even though this handbook will examine general conditions of
homogenization, it is reasonable to expect that most emulsions and dispersions
will respond in similar ways to these processing techniques.
HOMOGENIZATION
At the World's Fair in Paris in 1900, Auguste Gaulin
exhibited his invention... a process for'lreating" milk. According to the
literature and publications of that time, the word "homogenized" was
first used to describe milk treated by the Gaulin machine. Therefore, the terms
"homogenization" and "homogenize~' historically relate to the
process and equipment developed by Gaulin. The homogenizer basically consists
of a positive-displacement pump to which is attached a homogenizing valve
assembly. The pump forces fluid through the homogenizing valve under pressure.
(This is described in more detail later in this handbook.) The term
"homogenization" refers to the process or action which occurs within
the homogenizing valve assembly. Today, the terms "homogenization"
and "homogenize~' are quite often incorrectly applied to devices which
subject a liquid mixture to conditions different from that in the Gaulin and
Rannie homogenizers. In this handbook "homogenization" and
"homogenize~' refer to the action and equipment related to the classical
homogenizer.
HOMOGENIZERS
To understand how the homogenizer works
and what it does, it is first necessary to trace the path of the liquid mixture
through the homogenizing valve. Figure 1 shows a plug-type homogenizing valve
and standard valve seat. The unhomogenized product enters the valve seat from
the pump cylinder at a relatively low velocity but at a high pressure. For
example, the velocity may be about 10 to 20 feet per second, and the pressure
for this example will be at 3000 psig. This pressure is generated by the
positive-displacement pump and the restriction to flow caused by the valve
being forced against the seat by an actuating force. The positive-displacement
pump provides a relatively constant rate of flow and, therefore, will generate
the required pressures as the flow area between the valve and seat is increased
or decreased.
The liquid flows between the valve and
seat at high velocity, in this case (3000 psig), and the corresponding velocity
would be about 500 feet per second (approximately 340 mph). As the velocity
increases, the pressure decreases producing an instantaneous pressure drop. The
liquid then impinges on the wear ring (impact ring) and is finally discharged as
homog-enized product. The time it takes for the liquid to travel across the
face of the valve seat and to undergo homogenization is less than 50
microsec-onds (5 one hundred thousandths of a second). Therefore, as previously
stated, it can be seen that a large amount of energy is
dissipated in a very short time, imparting a large energy density to the
liquid.
The theories of homogenization that have been presented
over the years have attempted to relate this high energy transition in the
valve to the results observed in the product. It has been cifflicult to prove
or disprove many of these theories, but it appears that the two strongest
contenders today are cavitation and turbulence.
First, something should be said about shear in the, alve,
because many people assume that shear is the main mechanism of homogenization.
When using the term "shear", we are describing the elongation and
subsequent breakup of a dispersed phase droplet because of different fluid
velocities surrounding the droplet. The classical theories of shear effects, as
related to the flow profile in the homogenizing valve, are difficult to
justify. Two arguments against shear are:
(1) the viscosity of many fats and oils is
greater than the maximum value allowed for shearing of the droplets, and
(2) the flow profile in the valve is such that the
bulk of the liquid does not experience large velocity gradients due to boundary
layer effects.
In the cavitation theory the liquid encounters intense
cavitation because of the large pressure drop through the valve. When the
pressure drop is large enough, the vapor pressure of the liquid exceeds the
ambient pressure causing formation of vapour bubbles (cavities in the liquid).
When the cavitation bubbles implode (collapse of the cavities), shock waves are
generated in the liquid. These shock waves break apart the dispersed droplets.
The second homogenization theory, relating to turbulence,
suggests that the energy dissipating in the liquid generates intense turbulent
eddies. These eddies would be of the same size as the average droplet diameter.
The intense energy of the turbulence and localised pressure differences would
tear apart the droplets, reducing their average size. It has been suggested
that some of the effects associated with turbulence and cavitation are similar,
therefore, making it difficult to clearly distinguish between the two.
TYPES OF HOMOGENIZING
VALVES
Different types of APV homogenizing valves are available
for various applications. Figure 2 shows some of the configurations of these
valves. Through experimentation and experience, it has been found that certain
valve geometry is more efficient than others, depending on the product or
process involved.
The standard flat valve (SV) is used in a variety of
food, dairy and chemical applications. This valve is available in different
sizes; and the conditions of operation, such as homogenizing pressure and flow
rate, will determine which size is appropriate. Standard valve material is a
nonferrous alloy; however, tungsten carbide or ceramic would be provided for
highly abrasive applications.
By putting a large inner chamfer on the flat valve, a
knife-edge shape is produced. The knife-edge valve is beneficial when
processing milk or similar emulsions with the lowest homogenizing pressure possible
to achieve normal shelf stability. The knife-edge valve usually requires a
second-stage valve or a back pressure device. The knife-edge valve design is
incorporated into the patented Gaulin Micro-Gap~5 homogenizing valve which is
more fully described in Technical Bulletin #65.
The cell disruption (CD) valve represents a unique
variation of the knife-edge design in a single-stage configuration. This valve
has been found to be the most efficient for processes that require the
disruption of single-cell microorganisms and the subsequent release of proteins
and enzymes. This application is discussed in detail in the booklet, "Cell
Disruption by Homogenization".
The SEO valve is used for high-pressure applications
(over 10,000 psi/690 bar) and for critical emulsions, where small and uniform
particle size is needed, such as liposomes and intravenous emulsions.
Variations in valve configuration, such as piloted versus
plug, do not significantly affect the action or efficiency of the valve in most
cases but are only related to mechanical aspects of the valve such as alignment
and incorporation of hardened surfaces.
HYDROSHEAFY1
MIXERIEMULSIFIER
The Gaulin HydroshearO mixer/emulsifier is a device which
is significantly different from the homogenizer in its fluid flow profile, as
well as in its emulsifying action. It has a unique chamber design which
subjects the processed fluid to a high shear environment. Figure 3 illustrates
the flow profile in the chamber.
The chamber has a double-conical shape. The fluid, under
pressure, enters tangentially at the middle of the chamber wall. This
tangential entrance causes the fluid to flow in a spiral motion, moving from
the outside to the inside of the chamber. Due to the conical shape of the
chamber, the velocity of the fluid layers continually increases as the radius
decreases and the chamber height increases. Because the concentric layers are
moving at different velocities, high shear regions are generated in the fluid
causing emulsification of the dispersed phase. The product is then dis-charged
through a small orifice at the apex of each cone.
The operating pressures and energy level of the
Hydroshear mixer/emulsi-fier are much lower than that of the homogenizer; and,
because of this, the Hydroshear unit is best suited to products which do not
require sub-micron
particle sizes. There are some
applications where low energy batch mixing can be replaced with a
continuous-flow Hydroshear system.
The Hydroshear mixer/emulsifier has been
used for processing many emulsions and dispersions, especially systems that
require dispersing viscosity improvers. A few of these applications are
mentioned later in this handbook.
COLLOID MILL
The Gaulin colloid mill is different from the homogenizer
and Hydroshear system in its action on the processed fluid. The operation of
the colloid mill is similar to the mechanism of a basic mill configuration; the
work on the product takes place between a stationary (stator) and a rotating
plate (rotor). The premix is fed into the area between the rotor and stator
against centrifugal force. With its peripheral speed of 10,000 feet per minute,
the rotor generates high shear fields within the fluid in the working area.
Because of the unique features of the Gaulin colloid mill, the distance between
the rotor and stator can be adjusted to optimise the energy imparted to the
product.
By
decreasing the distance between the rotor and stator, shear on the product is
increased; however, there are limitations in this procedure. Decreasing the gap
will also substantially decrease the flow rate and increase the temperature
rise in the product.
EMULSIONS
An
emulsion may be defined as a heterogeneous system consisting of two immiscible
liquid phases in which one liquid is intimately distributed in the other as
minute droplets whose diameters exceed 0.1 micrometer. In most cases one of the
liquids is water. A third component, known as a surfactant (emulsifying agent
or stabilizer), must be present in amounts adequate to prevent coalescence of
the dispersed phase.
When
oil is dispersed as extremely fine droplets in a continuous phase of water, the
emulsion would be oil-in-water (o/w). Emulsions of this type are
water-dilutable. When the oil is the continuous phase with the water
distributed in the oil as small droplets, the emulsion is water-in-oil. This
type of emulsion is not dilutable with water but will dilute with oil.
The
type of emulsion which forms is greatly dependent upon the nature of the
surfactant used in the formulation. The surfactant or emulsifier selection is a
science in itself and will not be detailed here. However, it can be briefly
stated that the two main functions of the surfactant are (1) lowering the
interfacial tension of the oil and water phases, and (2) preventing
agglomeration, coalescence and breaking of the dispersed droplets after they
are formed.
Obviously,
the amounts of the various ingredients making up an emulsion can vary
significantly, due to the large number of diverse emulsion products. However,
it has been determined that the amount of surfactant required for an emulsion
in many cases is dependent on the energy level (kW/amount of product) used to
generate the emulsion. For example, homogenizing at 4000 psig can generate a
certain type of emulsion with 0.25% emulsifier. This would be equivalent in
average droplet diameter to an emulsion generated by a high shear mixer using
2.25% emulsifier. Therefore, the high energy input of the homogenizer allows a
reduction in amount of emulsifier and, thereby, a substantial savings in emulsifier
costs. This phenomenon is more fully explained in Technical Bulletin #61.
Emulsifiers
The selection of a suitable
emulsifying agent is a complex process. An emulsifier
is categorized in relation to the balance which exists
between the oil-soluble (lipophilic) portion and the water-soluble
(hydrophilic) portion of the molecule. Combinations of emulsifiers are
sometimes used to achieve the desired balance. For references concerning
techniques for selection of emulsifiers, as well as composition, we recommend
contacting companies that sell emulsifiers such as ]Cl Americas, Inc., or BASF Performance Chemicals.
If the emulsifier is 25% oil soluble and 75% water
soluble, then an emulsion prepared with this particular agent will usually be
an oil-in-water type; that is, the oil is dispersed as extremely fine droplets
in a continuous phase of water. If the emulsifier is 75% oil soluble and 25%
water soluble, then the reverse will be true. It will tend to form emulsions of
water-in-oil, where the oil is the continuous phase with the water distributed
in the oil as small droplets. Of course, the amount of solubility in the oil
and water phases, as mentioned here, is only approximate but is used to
illustrate Bancroft's Rule. Simply stated, this rule is that the phase in which
the emulsifying agent is more soluble will be the continuous one.
There are four general types of emulsifiers:
(1) non-ionic, which do not impart a charge to the dispersed phase;
anionic, which provide a negative charge to the dispersed phase;
(2) cationic, which impart a positive charge to the dispersed phase;
lastly,
(3) amphoteric, which will impart either a positive or a negative
charge, depending upon the pH of the system.
At the present time there are several thousand
emulsifying agents commercially available. With this large number of possible
agents, formulating a new emulsion and determining the best emulsifying agent
from cost and efficiency standpoints could require months of work. Fortunately,
all of the agents would not have to be tried, since one would select different
hydrophilic/lipophilic blends and then work with the agents in a particular
chemical grouping, consistent with the blend characteristics.
For a complete listing of emulsifiers and their
manufacturers we suggest "Detergents and Emulsifiers", printed by
McCutcheon's Publications, 175 Rock Road, Glen Rock, New Jersey 07452. New
editions are printed annually. For those interested in the theory of emulsions,
see listings in bibliography.
After having selected the optimum emulsifying agent for a
particular system and adding it to the emulsion formulation, we may ask,
"How does it work?" The emulsifier's oil-soluble end dissolves in the
surface of the oil with the water-soluble portion of the molecule sticking out
into the aqueous phase, so that the oil droplet would resemble a pincushion.
We
now have a system with a large number of small oil droplets bounded by an
emulsifier, and all of these droplets have a charged layer surrounding them
(electrical double layer). Even with nonionic emulsifiers and water-inoil
emulsions, it is believed that some degree of charge potential may exist around
the droplet, possibly generated by a frictional mechanism. With certain
stabilizers such as gums, a protective, uncharged coating forms around the
droplet.
This
emulsifier layer, therefore, either because of its charge effect or purely
physical barrier, causes repulsion between droplets when they approach within a
certain distance of one another. This layer helps stabilize the emulsion,
because it minimizes flocculation and coaiescence of the droplets. Stokes' Law
predicts how the creaming rate is affected by the physical characteristics of
the emulsion. The rate increases as particle size increases (it is proportional
to the square of the radius increases when the two immiscible phases have a
large difference in density and decreases when the viscosity of the outer phase
is large. Therefore, the emulsifier minimizes creaming, because flocculated
droplets represent bigger particles, which separate faster. Also, by preventing
coalescence it minimizes breaking of the emulsion, which results in the
formation of two separate phases (demulsification).
If
the oil concentration is increased to a level where there is so much oil
surface that the particles are physically touching, then a point will be
reached where the viscosity of the emulsion rises very rapidly. A further
increase in oil concentration will result in a breaking or an inversion of the
emulsion to a water-in-oil type. Due to the large surface that can be developed
with the homogenizer, 65% oil is generally the upper limit; although in some
cases where extremely small particle sizes are present, 40% may be the upper
limit. With the colloid mill, where the average particle size is larger and the
surface area produced is not as great as for the homogenizer, it is possible to
make emulsions of 80 to 85% oil phase. This high oil phase level may also be
possible with the Hydroshear mixer/ emulsifier, because of its low energy input.
Why Use Mechanical Energy to Process
Emulsions?
Dispersions
require the application of mechanical energy for preparation. Emulsions,
however, can be prepared spontaneously; that is, if sufficient emulsifying
agent is mixed with oil, the oil can be poured into water and a stable emulsion
prepared. This type of system, however, requires a unique combination of
ingredients and a large amount of emulsifier. Since one can 1rade off'
mechanical energy for the chemical energy of an emulsifier, the mechanical
energy at 3 to 80 per kilowatt hour is infinitely cheaper than an emulsifier at
500 to $1.50 per pound; then, here is the case for high energy equipment.
If
one were to use a standard mixer, the emulsifier level could be reduced to
approximately 5 to 10% as opposed to the 25 to 50% required for a spontaneous
emulsion. If we move to still higher energy levels; for example the colloid
mill or Hydroshear mixer/ emulsifier, the emulsifier level could probably be
reduced to 2 to 5%; with the still higher energy level of the homogenizer, it
can be reduced to as low as 0.2%.
These
figures are approximate, since they depend on the total surface area to be
developed and the efficiency of the particular emulsifier. They do represent,
however, typical reductions that are possible by substituting mechanical energy
for chemical energy.
DISPERSIONS
For
this handbook dispersion is defined as solid primary particles, agglomerates or
aggregates distributed uniformly throughout a continuous medium. These
particles or group of particles can range in size from .001 micrometers on up
to greater than 1 micrometer. Dispersion is colloidal if at least one dimension
is between .001 micrometer and 1 micrometer.' A solid particle dispersed in a
liquid is also called a "suspension".
The
term "aggregate" refers to a group of particles that are more tightly
bound to each other than are particles in agglomerate. The particles in an
aggregate may be joined at their faces or attached to each other by salt
bridges. In agglomerate the particles are less strongly attached at their edges
and corners. An agglomerate or "...floc may be considered as a loose,
irregular, three-dimensional cluster of particles in contact in which the
original particles can still be recognised" .2
When
making these definitions, most authors are referring to dispersions of powders
or pigments. However, because of the diversity of applications involving the
homogenizer and colloid mill, it is sometimes difficult to describe all
homogenized dispersions by a simple definition. Nevertheless, many of the
dispersions encountered can be related to these definitions.
A
few examples of the dispersions processed in the homogenizer or colloid mill
reveal the variety of these applications:
Antacid Carbon Black Dyes
Talc Antiperspirant Catsup
Ink TeflonO Barytes (Barium Suifate)
Cellulose Calcium Stearate Titanium Dioxide
Mica Clay (Bentonite, Kaolin) Fumed Silica
These applications involve solids of one type or another
dispersed into a liquid; but, obviously, some are very different from the
standard pigmenttype dispersion. The basic steps involved in preparing a
dispersion can be given as follows:
ˇ
Wetting (involves the
displacement of air from the particles by the continuous phase);
ˇ
Physical separation
(separation of particles by some mechanical means);
ˇ
Stabilisation
(preventing the tendency for reagglome ration).
The wetting step simply involves adding a dry powder or
pigment to a liquid and mixing in the solid phase, generating a release of air
from the bulk packing of the solid. Sometimes, chemical wetting agents are
needed to facilitate this step.
The second step of the process involves the use of
mechanical equipment. The type of equipment to be used depends on the end
result desired and on the physical nature of the dispersion. The types of
machines used for this step in the process include mills (ball, pebble, sand,
roll, high speed impeller or colloid) and the homogenizer.
The last step involves use of the appropriate chemical
surface-active agents to retard strong re-adhesion of the particles to each
other but not necessarily to avoid all flocculation. In some cases it is
desirable to have some very weak flocculation occur in pigment dispersion, so
that, when the pigment settles, hard packing does not occur but low energy
mixing/ agitation will redisperse the solids. The evaluation and selection of a
chemical agent goes beyond the scope of this handbook but are thoroughly
covered in many reference sources .4
The process of dispersing is sometimes called
"milling" or "grinding", especially in the paint and ink
industries; but, in actuality, particle grinding does not occur.5 The
dispersion process most often involves the reduction of agglomerates to their
primary particle size. 1t is generally accepted that primary particles of
pigments are not significantly reduced during conventional dispersion
processes."' If the desire is to grind particles, then the ball mill would
be used: "...comminution of pigment particles during the dispersion
process is uplikely to be achieved by any machine other than the ball Mill11.7
The type of equipment used would depend on the
characteristics of the product and process. For very high viscosities (over
5000 cIP) the ball, roll or impeller mill would be used.
The sand mill could be used in a continuous, low
viscosity system. The Gaulin colloid mill could be used in a continuous system,
a closed system, an aqueous or a non-aqueous system at Newtonian viscosities below about 5000 cP. The homogenizer could
be used under all these same conditions as the colloid mill; but it is most
efficient at lower viscosities (less than 2000 cP), and it can generate a
smaller primary particle size than the colloid mill, because of its higher
energy input.
The classic Gaulin or Rannie homogenizer, consisting of a
high pressure pump and homogenizing valve assembly, disperses particles through
cavitation and/or turbulence generated in the homogenizing valve. This action
is very different from the effects produced by the previously mentioned
equipment. However, the dispersion process in the homogenizer still involves
the reduction of agglomerates or aggregates to the primary particle size.
"Actual breakdown of small, compact crystals [by most dispersing
equipment) is not likely. One exception is a powder in the form of
needle-shaped crysta1S.118
Considering the fluid action of the homogenizer to be
turbulence or cavitation, the break-up of these agglomerates can be described
by the following statement. "The primary mechanism leading to disruption
of the extended flocs by turbulence is pressure difference on opposite sides of
the floc which cause bulgy deformation and eventual rupture. The pressure
differences are due to the random velocity fluctuations of turbulent
flow."9
The homogenizer can produce a very fine dispersion of
particles, in many cases to the primary particle size. All of the dispersion
material passes through the homogenizing valve, subjecting the total product to
the high energy density in the valve. This means that homogenization would be
more energy efficient than a device such as a ball mill, which requires long
processing time and has an efficiency of less than one percent. Most of the
energy is lost in the friction of the ball charge and is converted into heat.'
The homogenizer also makes unique changes to many
products. It defibrillates the fibers of cellulose to increase its
water-binding tendency. It changes the size of the tomato fibers in catsup to
produce a thicker product. It can cause delamination of clay-like products,
separating the plates in the mineral. It disperses fumed silica, so that a
better gel matrix can occur. Pigments, such as titanium dioxide and carbon
black, are dispersed uniformly by the break-up of agglomerates.
Through experimentation and experience certain conditions
of homogenization which affect dispersions can be described. Some of these
involve equipment and some involve the process.
In many cases, high pressure is beneficial in producing a
good dispersion. However, sometimes increasing pressure beyond a certain value
does not produce a progressively better product. This is because there exists a
threshold energy to disrupt the agglomerates. Once this energy is reached, the
primary particle size is obtained, and no change in particle size is observed,
despite increasing the pressure. Titanium dioxide is an example of this effect.
Multiple passing may be necessary for some dispersions,
if no threshold energy is reached or if unique physical changes are being made
in the product such as occurs with cellulose.
Experimentation suggests that as the Newtonian viscosity
of the continuous phase increases, the efficiency of homogenization decreases.
For a carbon black dispersed in a high viscosity resin, this may mean that high
pressure and more than one pass are required for a good dispersion.
The use of an effective surfactant can make significant
changes in the quality of a dispersion undergoing homogenization. The
surfactant can do this by reducing the energy required to disrupt the
agglomerates or by stabilizing the formed dispersion. For example, a small
amount of sodium hexametaphosphate will produce a finer dispersion of calcium
carbonate than will a large amount of an ineffective emulsifier. An effective
surfactant can also dramatically reduce the viscosity of a dispersion by
preventing interparticle adhesion, either through charge or steric repulsion.'
Results from limited experimentation suggest that the
geometry of the homogenizing valve and seat can affect the efficiency of a
dispersion. A valve configuration that improves the efficiency of
emulsification will also benefit the dispersion process. Therefore, because
Gaulin and Rannie use the optimum designs and materials for their homogenizing
valves, efficient emulsification and dispersion will occur.
Incorporated into the design characteristics of the valve
is the material of construction. Because solids in liquids are significantly
more abrasive than emulsions (liquid-in-liquid), the use of wear-resistant
material on the surfaces of the homogenizing valve and seat and on the pump
valve seats is usually required. The degree of abrasiveness of the dispersion
is very much dependent on the size, shape and hardness of the particles. A
material that is hard (for example, on the Mohs Scale) may not be as abrasive
with a particle size below one micrometer as it would be if the particles were
five micrometers. Also, cubic crystals may be more abrasive than flat plates or
thin needles. Therefore, it can be difficult to predict how abrasive a material
will be without actual test data.
There
are many different methods available for analysis of particle size or quality
of dispersion. Each of these techniques will reveal an aspect of the dispersion
that another may not. Therefore, it may be useful to combine more than one
method to obtain a total picture of quality. These techniques will not be
covered in any detail here, but they are described more fully in other
reference sources or suppliers' literature. These methods include: grind gauge;
microscopy; sedimentation techniques, either gravitational or centrifugal;
Coulter counter; optical turbidity; photon correlation spectroscopy (NICOMP
Analyzer); hydrodynamic chromotography and, of course, simple shelf-life
settling.
ENERGY LEVELS
DEVELOPED
The
following example demonstrates the high intensity energy dissipated through the
homogenizing valve. At a pressure of 3,000 psi, a fluid moves through the valve
at an average velocity of 500 feet per second with a residence time of 15
microseconds. This results in a dissipated energy density of about 1100
kW-per-centimeter-cubed. This demonstrates how the homogenizer concentrates
high energy into a small fluid volume.
The
colloid mill has an energy level equivalent to the homogenizer at 500 to 1000
psi and is capable of preparing an emulsion having an average particle size in
the one to five-micrometer range or a dispersion of comparable quality. With
some products the colloid mill is adequate for the emulsion quality required.
There are always exceptions to these generalities, which is the reason for the
availability of our Customer Service Laboratory.
The
Hydroshear mixer/emulsifier is the ideal device for generating an intermediate
average particle size, two to ten micrometers, from a large particle-size
premix with a low expenditure of energy. Even though the Hydroshear
mixer/emulsifier is a low energy device, it can produce an emulsion or
dispersion with a small particle size under certain conditions, dependent on
the viscosity of oil and the continuous phase, level of oil and emulsifier
package.
Parameters for
Selection of Equipment
In
general, the same parameters apply for both emulsions and dispersions, insofar
as the selection of equipment is concerned. The principle parameter would be
the viscosity of the material as processed, categorized either as Newtonian
(real) or non-Newtonian (apparent). By Newtonian we mean a viscosity which
remains constant with increasing shear rate. A nonNewtonian fluid (thixotropic
and pseudo-plastic) in contrast is, by way of example, a latex paint with a
high apparent viscosity when the system is subjected to a low shear rate; but,
when under a high shear rate, it has a low apparent viscosity. In the case of
non-Newtonian fluids, the actual viscosity under a high rate of shear is the
one that should be considered. A final type of non-Newtonian viscosity that has
to be considered is the category dilatant/rheoplexic, where the material is
fluid at rest but rapidly generates high viscosity when shear is applied.
Normally, dilatant/ rheoplexic systems cannot be handled with high energy
equipment; one exception being crude clay dispersions, which the homogenizer
has been able to prepare.
For further information on the rheology of emulsions, we
would suggest Sherman's "Emulsion Science" (see bibliography).
For guidance, we suggest the following Newtonian
viscosity ranges in centipoise for optimizing efficiency (viscosity of the
internal phase or premix).
Homogenizer 1 to 1000
Colloid Mill 1 to 5000
HydroshearO Mixer/Emulsifier 1 to 2000
The homogenizer shows a drop in efficiency between 200 cP
and 1000 cP; but, normally in this range, either the homogenizer, colloid mill
or Hydroshear mixer/emulsifier may be used. The homogenizer becomes less
efficient when the dispersed-phase viscosity is greater than 500 cP.
With the colloid mill there is no decrease in efficiency
with an increase in viscosity, but the power requirements can become so high as
to make it impractical to use the mill for viscosities above 5000 cP. If the
rotational speed of the mill is reduced to 5000 feet per minute on the
periphery of the rotor, considerably higher viscosities may be handled.
The Hydroshear mixer/emulsifier works by means of shear
between concentric liquid layers and is most efficient below 2000 cP. The
Hydroshear mixer/ emulsifier can be used with many of the emulsion and
dispersion systems that are processed with the colloid mill at gaps larger than
.00Y.
Other parameters that must be considered in determining
the selection of equipment would be factors such as temperature rise and the
physical properties of the ingredients. In systems containing volatile
solvents, where the material must be maintained under pressure, the homogenizer
would be more satisfactory than the colloid mill. If the product must be pumped
through a heat exchanger after processing, the homogenizer would eliminate the
necessity for an additional pump, which is often required with the colloid
mill. Normally, the temperature rise in the homogenizer is considerably less
than in the colloid mill.
The temperature rise (Centigrades)
through the homogenizer can be estimated by the following
equation:
AT = P/41.35 C,d
where P is in atm, C. is specific heat (cal/g/OC) and d
is density (g/mi). With water at 1000 psi (68 atm) homogenizing pressure, a
rise of 30F (1.70C) would occur.
REQUIREMENTS FOR GOOD
HOMOGENIZATION
The flow profile through the homogenizing valve has been
discussed previously, but it is also beneficial to examine what occurs to the
dispersed oil droplet as it undergoes homogenization. The homogenizing pressure
represents the level of energy applied to the liquid as it goes through the
valve. A portion of this energy subdivides the droplet, but most of it is
converted into heat after homogenization is completed. It is estimated that
only 0.01 % of the energy is used for emulsification. This is not to say that
the energy is wasted, because the subdivision of the droplets is a complex
process, and homogenization requires this total energy level to initiate the
action. For example, the initiation step may involve bringing the liquid to a
certain velocity before efficient homogenization can occur.
Interfacial Tension
The work required to break down the droplet (of low
viscosity) is a function of the interfacial tension (related to the surfactant)
and also the diameter of the droplet. As the droplet diameter decreases, the
surface forces become more dominant in relation to bulk forces and resist
disruption to a greater extent. This means, of course, that more energy is
needed to reduce the droplet when its diameter becomes smaller. This is
reflected in the shape of a curve comparing average droplet size to
homogenizing pressure. As the homogenizing pressure increases, the average
droplet size decreases; however, the rate of change in droplet diameter also
decreases. Therefore, the curve is logarithmic and starts to level off at high
homogenizing pressures. Even though high homogenizing pressures are used, there
is a limit to the size reduction of the dispersed phase possible in the
homogenizer. Some sources have suggested that this limiting value is about 0.1
to 0.2 ~1m.
Premix
The condition of the premix to the homogenizer is one of
the most important factors influencing good homogenization. As was stated
above, the energy needed to reduce a large droplet would be less than that
needed to reduce a small droplet. Therefore, to make more efficient use of the
high energy homogenizer, the droplets should be reduced to as small a size as
possible with low energy mixing, before going to the homogenizer. In this way,
the energy of homogenization is not expended on reducing extremely large
droplets to a medium size range, when this can be done with low energy
equipment; but, rather, the homogenizer should be used to reduce medium-size droplets
to a small-size range.
As a general estimate, the premix should contain droplets
which are all under about 20 pim, ideally under 10 pm. The uniformity of the
premix can also be important, because a very broad distribution of droplet
sizes could lead to a homogenized product with a broad size distribution.
Oil Content
Considering the previous description of the energy
transfer in the valve, the effect of increasing oil content can also be
visualized. With the same energy density in the liquid, the increased oil
content will mean that each droplet experiences a smaller portion of the total
energy. The result will be that the emulsion quality may not be quite as good
when the oil percentage is increased. The homogenizer is most efficient when
the oil percentage is less than 50%; and, of course, the efficiency is improved
as the percentage is decreased below this level.
Dispersed-Phase
Viscosity
Experimentation has demonstrated the relationship between
homogenizing efficiency and the viscosity of the dispersed phase or continuous
phase of the emulsion. In an oil-in-water emulsion the Newtonian viscosity of
the oil is very important with respect to the efficiency of homogenization and,
thereby, the average particle size generated.
When measuring average droplet size versus increasing oil
viscosity (for example, between 2 to 200 cP) at constant homogenizing pressure,
it is found that the average droplet diameter steadily increases as viscosity
goes up. Therefore, the homogenizer will produce its best results when the oil
viscosity is low, usually less than 200 cP. Sometimes this can be achieved by
heating the emulsion premix or the oil phase to lower the viscosity of the oil.
In some cases, such as resin emulsions, the oil phase is dissolved in a solvent
which substantially reduces the viscosity of the oil phase. The only reasons
for operating the homogenizer with a high temperature premix would be to lower
the viscosity of the dispersed phase, to keep the dispersed phase in a molten
state or to maintain aseptic processing conditions.
Continuous-Phase Viscosity
It has been found that the continuous-phase viscosity
will also affect homogenizing efficiency. In this case it appears that the
efficiency drops off as the viscosity increases to about 100 cP and then levels
off. This phenomenon may relate to the mechanism of homogenization and is not
totally understood at this time. In summary, then, it can simply be stated that
the homogenizer will operate most efficiently when the dispersed and
continuous-phase viscosities are low.
Particle-Size
Distribution
Many applications require a very uniform droplet-size
distribution in the generated emulsion, either for control of creaming rate or
for some physical action or characteristic required of the emulsion. This can
be accomplished in the homogenizer by passing the product through the valve
more than once. Because the homogenization process is random in nature, the
size distribution follows a log-normal distribution curve. This means that the
curve is asymmetrical and contains a'lail" representing oversized
particles. With one pass through the homogenizer, there is a certain
probability that not all particles are subjected to the same intense energy of
homogenization; therefore, a portion of the particles pass through without
being as reduced in size as others. Another pass through the homogenizing valve
increases the probability of these large particles being reduced. Therefore,
multipassing through the valve narrows the particle-size distribution. The
benefit of multipassing diminishes after nine passes. The effect of
multipassing is shown in Figure 5.
Two-Stage Homogenization
The two-stage homogenizing valve (a Gaulin invention) has
been available for many years. Experimental finds have suggested that the
primary function of the second-stage valve is to influence intensity of the
homogenization effect in the first stage. Therefore, the second-stage valve in
a sense 1ine tunes" the homogenization process. In oil-in-water emulsions
it has been found that the ideal second-stage pressure should be between 10
and 15% of the total homogenizing pressure. It has been
demonstrated with milk that passage through the second-stage valve, itself,
while main-taining the backpressure stated above, does not significantly alter
the condition of the emulsion. Sampling the milk both before and after the
second-stage did not reveal any substantial change in emulsion quality.
The second-stage valve is a convenient means for applying
the correct backpressure to the first-stage homogenizing valve. A homogenizer
processing milk at 2000 psi with a two-stage valve may produce milk emulsion
quality equivalent to a single-stage valve operating at 2200 to 2300 psi. If
the homogenizer has a maximum operating pressure of 2000 psi, then the two-stage
valve configuration extends the achievable homog-enization quality beyond that
produced by a single-stage valve set at this maximum pressure.
There may be some products other than simple emulsions
that have complex formulations that may be affected by the second-stage with
respect to appearance or viscosity. The effect of the second-stage valve is
further described in Technical Bulletin #58.
Homogenizing Efficiency Measurements
To determine the ideal operating pressure of the
homogenizer on a particular product, the first step is to establish a method of
evaluation. The method may be viscosity increase or decrease, average particle
size (as determined microscopically or by instrument), rate of separation,
grind gauge or change in physical appearance, just to mention a few. The next
step is to collect samples from the homogenizer at different pressure settings;
for example, every 1000 psig (or less, if a narrow pressure range is being
evaluated). Finally, using the appropriate method of analysis, the evaluated
parameter is compared or plotted versus homogenizing pressure and the lowest
pressure, which generates the required product, can then be selected. For some
products it may be desirable to determine the effect of multipassing at an
appropriate pressure.
APPLICATIONS FOR GAULIN AND RANNIE EQUIPMENT
Gaulin and Rannie homogenizers are used in the processing
of many varied products. A listing and description of them all would be beyond
the scope of this handbook; however, in order to demonstrate the diversity of
the products benefiting from homogenization, a few will be briefly described.
The homogenizer is extensively used, of course, in the
dairy industry for the processing of milk products and ice cream mixes. In the
food industry the homogenizer is used for processing dispersions such as catsup
and tomato sauce, for emulsions such as orange oil and beverage emulsions and
for frozen whipped toppings. It is also used in the production of cream soups.
In the pharmaceutical field the homogenizer is used for
dispersions such as benzoyl peroxide in cream or lotion bases, for antacid
dispersions and for vitamin suspensions. Homogenization at high pressures is
required for perfluorocarbon emulsions (also known as a blood substitute),
intravenous emulsions and liposomes.
In biotechnology the homogenizer is used for disrupting
microorganisms, such as E. coii or yeast, to release active enzymes or
proteins. The chemical industry uses homogenizers for the preparation of a
myriad of products. Some dispersion applications include titanium dioxide and
other pigments, clay and talc dispersions. Emulsion applications include wax
and silicone oil (usually for low-viscosity oils).
The Hydroshear has successfully processed cosmetic
products such as hair conditioners, skin creams and pharmaceutical creams. Fuel
oil emulsions (w/o) can be generated very economically with the Hydroshear, and
silicone oil emulsions are also easily processed. Food applications include
salad dressings and orange oil premix emulsions.
As
was mentioned above under the description of premix conditions for the
homogenizer, the Hydroshear mixer/emulsifier would be the type of device which
would reduce large droplets to a medium-size range (an average particle size of
two to eight micrometers). This would imply that the Hydroshear unit could be
used as a premix device for the homogenizer. Indeed, experimental work on
flavor emulsions and on pharmaceutical emulsions demonstrated that the
Hydroshear mixer/ emulsifier did generate a good, uniform premix to the
homogenizer, improving the quality of the emulsion produced by one pass through
the homogenizer.
In
general, the Hydroshear mixer/emulsifier can efficiently handle products of
high viscosity such as 2000 cP (Newtonian) and high oil content. Many
applications that are successfully processed with the colloid mill can also be
run on the Hydroshear system.
The
colloid mill is used in a number of applications. Different products should be
evaluated individually on the mill to determine the best balance of flow rate,
temperature rise and emulsion quality. Food applications include the production
of mayonnaise and salad dressings. Formation of silicone oil emulsions (high
viscosity and high oil content) and processing of photographic gelatin are two
chemical applications. Cosmetic products such as hand cleaners and skin creams
have been successfully processed. These are just a few of the mill
applications.
In
general, the mill is best suited to products which are of high viscosity or
high oil content. Because the energy level in the mill is much lower than the
homogenizer, it does not "overwork" those high oil systems which can
be overworked by the homogenizer. This effect is probably due to the type of
particle-size distribution and average particle size generated by the mill. The
total surface area of the emulsions made on the mill is not as large as that
generated by the homogenizer; therefore, the surfactants can better accommodate
this surface area, resulting in a more stable emulsion system.
01996
APV Homogenizer Group
FOOTNOTES
1. G. D.
Parfitt (ed.), Dispersion of Powders in Liquids (New Jersey:
Applied Science Publishers, 1981).
2. D. G. Thomas,
'7urbulent Disruption of Flocs in Small Particle Size
Suspensions", AICHE Journal, 10 (1964), pp. 517-523.
3. V. Buttingnol
and H.L. Gerhart, "Polymer Coating - Pigment Disper-
sions", Industrial and Engineering Chemistry, 60(8)
(1968), pp. 68-79.
4. Parfitt,
loc. cit.
5. Parfitt, op.
cit., p. 334
6. Parfitt, op.
cit., p. 332
7. Parfitt, op.
cit., p. 416
8. E. K.
Fischer, Colloidal Dispersions (New York: Wiley & Sons, Inc., 1950), p. 261
9. Thomas, loc.
cit.
10. Fischer, loc.
cit.
11. M. J. Rosen,
Surfactants and Interfacial Phenomena (New York: John
Wiley and Sons, Inc., 1978).
BIBLIOGRAPHY
Becher, P. Emulsions:
Theory and Practice, New York: Reinhold Publishing Corp., 1965.
"Emulsifier Cost Can Depend on the Type of Mixer
Used", Food Engineering, (September
1981), p. 162.
Encyclopedia of
Emulsion Technology, Vol. 1. Paul
Becher, ed., New York: Marcel Dekker, Inc. (1983)
Loo, C. C. and Carleton, W. M. "Further Studies of
Cavitation in the Homogenization of Milk Products", Journal of Dairy Science, 36, 64 (1953).
Mulder, H. and Walstra, P. "The Milk Fat Globuld', Wageningen, The Netherlands: Centre
for Agricultural Publications and Documents, 1974
Pandolfe, W. D., "Effect of Dispersed and Continuous
Phase Viscosity on
Droplet Size of Emulsions Generated by
Homogenization", Journal of
Dispersion Science and Technology, 2 (4), 459 (1981).
Pandolfe, W. D., "Homogenizers", Encyclopedia of Food Science and Technology,
Y. H. Hui, ed., New York: John Wiley & Sons (1991) p 1413
Pandolfe, W. D., "Effect of Premix Condition,
Sufactant Concentration and Oil Level on the Formation of Oil-In-Water Emulsins
by Homogenization", Journal of
Dispersion Science and Technology, 16 (7), 633
(1995).
Sherman, P., ed., Emulsion
Science, London: Academic Press, 1969
Walstra, P., "Effect of Homogenization on the Fat
Globule Size Distribution in MilW', Netherlands
Milk Dairy Journal, 29, 279 (1975).
APV HOMOGENIZER GROUP PUBLICATIONS
TECHNICAL BULLETINS
No. Title
37 High Capacity
Pump (HCP Application)
40 Double-Packed Cylinder
41 Operation of a Typical Batch-Homogenizing System
45 Operation of Laboratory Homogenizer with High
Temperature Applications
46 Use of a Product Infeed Gauge to Determine Proper Feed
Pressure
58 Effect of the Second-Stage Homogenizing Valve
59 Design and Application of the Hydroshearc
Mixer/Emulsifier
61 An Evaluation of Emulsifier Cost versus Processing
Energy Cost
63 Understanding the EQA
65 Micro-GapO Homogenizing Valve
69 Understanding Product Viscosity
70 Use of the Hydroshear'l' Mixer/Emulsifier as a Premix
Device
71 Multiple-Pass Homogenization by Continuous Recycling
72 Effect of Air on Homogenizing Efficiency and Product
Quality
73 Typical Droplet-Size Distributions for the
Homogenizer, Hydroshear@ Mixer/Emulsifier and Colloid Mill
76 Micro-Gap' Valve Applications - Recommended
Operating Pressures
77 Cylinder
Design for Biotechnology Production Homogenizers
79 Effect of Product Viscosity and
Discharge-Hole Diameter on the HydroshearO Mixer/Emulsifier
80 A Method for Examining Homogenized Milk with
a Phase-Contrast Microscope
81 Operation and Benefits of the Micro-GapO Valve
82 Homogenizer Operation in UHT Plants
83 Simplex versus Triplex Homogenizer
BOOKLETS
3810.00 Cell Disruption by Homogenization
3850.00 Processing of Emulsions and Dispersions by
Homogenization
PROCESS BULLETINS
Title S. 1. C.
Antacid Dispersion 2835.00
Antiperspirants 28445.28
Baby Food - Fruit or Vegetable 20322.21
Baby Food - Meat 20321.21
Carbon Black 28950.00
Cellulose 28230.00
Cellulose Gum Dispersions 28210.50
Cocktail Beverages 20872.15
Egg, Pasteurized Liquid 20179.00
Emulsion Polymerization 28213.00
Fruit Nectars 20334.93
Grease, Lubricating 29920.00
Intravenous Emulsions 28347.00
Latex 30896.81
Mayonnaise 20354.00
Orange Oil Emulsions 20871.00
Peanut Butter 20999.46
Pharmaceutical Products 2834.00
Rosin Emulsions 28612.21
Silica Dispersions, Colloidal 35410.20
Silicone 28695.98
Soluble Soaps 28410.00
Soy Milk 20752.00
Sulfur Dispersions 3295.02
Tomato Products 20336.00
Viscosity Index Improvers 29117.00
Wax Emulsions 29110.71
BROCHURES
1900.00 Gaulin & Rannie Homogenizers and Pumps
2251.50 Hydroshear@ Mixer/Emulsifier
550.02 Colloid Mills
2120.00 Laboratory Equipment/Customer Testing
Laboratory
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