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University of Groningen
Fundamentals of the high-shear pelletisation processRamaker, Johanna
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Publication date:2001
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Citation for published version (APA):Ramaker, J. (2001). Fundamentals of the high-shear pelletisation process. Groningen: s.n.
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In pharmaceutical application, an agglomeration process that results in agglomerates of a
rather wide size distribution within the range of 0.1 – 2.0 mm, with a high intra-agglomerate
porosity (about 20 – 50 %) is named a granulation process, and the agglomerates are called
granules.
If the final agglomerates are spherical, free flowing, and of a narrow size distribution in the
size range of 0.5 – 2.0 mm, and a low intra-agglomerate porosity (about 10 %), the process is
often referred to as pelletisation process, and the agglomerates are called pellets.
Particle size enlargement by pelletisation is often desirable for several reasons:1,2
1. Prevention of segregation of co-agglomerated components, resulting in an improvement
of the content uniformity.
2. Prevention of dust formation resulting in an improvement of the process safety, as fine
powders can cause dust explosions and the respiration of fines can cause health problems.
3. Increasing bulk density and decreasing bulk volume.
4. The defined shape and weight may improve the appearance of the product.
5. Improvement of the handling properties due to the free-flowing properties of pellets.
6. Controlled release application of pellets due to the ideal low surface area-to-volume ratio
that provides an ideal shape for the application of film coatings.
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Typical disadvantages of pellets and pellet production are:
1. Often pellets can not be pressed into tablets because they are too rigid. In that case, pellets
have to be encapsulated into capsules.
2. The production of pellets is often an expensive process and/or requires highly specialised
equipment.
3. The control of the production process is difficult e.g. the amount of water to be added is
very critical and overwetting occurs easily.
Pelletisation is used in various industries, like the pharmaceutical industry (controlled release
preparations), agricultural industry (fertilisers and herbicides), mineral processing (iron ore
pelletisation), food and detergents industry.
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Granulation in general can be divided into low-shear granulation, for example in a fluid bed,
where the movement of the particles is induced by an air steam, and medium/high-shear
granulation, where the particle movement is induced by means of forceful mechanical mixing
(for example using an impeller or a rotating plate). Medium-shear granulation occurs for
example in a rotating drum or planetary mixer. High-shear granulation is performed in a
rotary processor (marumerizer) and a high-shear mixer (figure 1.1).
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Figure 1.1. Schematic presentation of different pelletisation devices: a. rotating drum; b. high-shear
mixer Gral; c. rotary processor (marumerizer); d. extruder.
Pelletisation can be performed in medium-shear mixers as well as in high-shear mixers. The
following apparatuses are described in literature as pelletisation equipment (figure 1.1):
- pan and drum mixers, which have been described by many authors3-7;
- high-shear mixers, as described systematically by Schæfer and Kristensen8-10;
- rotary processors, as extensively described by Holm et al.11-13 and Vertommen14;
- extrusion (and spheronisation) equipment, as introduced by Reynolds15, and reviewed by
Vervaet et al.16
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This thesis focuses on the pelletisation process in high-shear mixers (typically a Collette Gral
or a coffee grinder). The results will often be compared with other high-shear granulation
processes, and with the rotating drum pelletisation process (which is a medium-shear
process).
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The pelletisation process in a high-shear mixer can be divided into several stages:
1. premixing of the solids;
2. liquid addition stage;
3. wet massing stage;
4. drying stage.
In the equipment used in this thesis, the first three stages take place inside the high-shear
mixer. The drying stage occurs in an oven (tray-drying).
The formation of pellets occurs during the liquid addition stage and the wet massing stage.
Growth (see 1.3.3) of pellets starts with the nucleation stage, occurring during the liquid
addition stage. The nucleation stage will be discussed in chapter 2. Further growth of pellets
occurs by fast growth (linear as well as exponential growth). Breakage of pellets also takes
place. Depending on the ratio of the amount of growth and breakage, an equilibrium stage can
be obtained in pellet growth.
The Collette Gral (figure 1.1b) high-shear mixer is equipped with:
- an impeller, rotating at the bottom of the bowl;
- a chopper, rotating near the wall of the bowl;
- a nozzle to supply the binder liquid.
The impeller rotational speed can easily be changed during processing. Therefore it is
possible have different rotational speeds, for example during the different stages of the
pelletisation process.
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Granulation processes in a high-shear equipment have often been considered as ‘black-boxes’
in which the starting material is converted to granules17. Changes of the process variables are
mainly based on trial and error. A real understanding of what’s going on is still missing.
Therefore, one has to take a careful look at the process and literally take the cover of the bowl
to look inside the ‘black-box’ (figure 1.2).
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Figure 1.2. Black-box approach.
By taking a careful look at the high-shear pelletisation process, three remarkable facts can be
seen. First, properties of the produced pellets depend strongly on the starting materials, the
apparatus, and the process conditions. This certainly can be a problem during upscaling,
because apparatus variables may change during upscaling resulting in changed pellet
properties.
Second, the pellet size distribution and pellet shape change during pelletisation, and depend
on the binder content (and the kind of binder used), processing time and impeller rotational
speed. More binder liquid or an increased processing time generally results in larger and more
spherical pellets, until the breakage of pellets becomes more important than the growth of
pellets, resulting in a decrease of the mean pellet size and the sphericity. A higher impeller
rotational speed causes faster growth and breakage of pellets. Depending on the ratio of
growth rate and breakage rate, this results in larger and more spherical pellets (more growth
than breakage), or smaller and less spherical pellets (more breakage than growth). Finally,
especially for the high-shear pelletisation process, a characteristic flow profile (e.g. torus) is
observed as soon as the pellets have been formed (see chapter 4).
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In order to know what happens on the scale of a particle, you can try to imagine yourself
being a particle inside a high-shear mixer. Try to understand what happens to you as a
particle. You will be swept through the bowl by the impeller. You will meet the wall and
collide with many other particles. At a specific moment, a splash of water will be dropped on
you, which wets you within a short period of time, leaving a small layer of liquid on you. As
soon as you meet some other particles again, they will stick on to you, and a granule has been
formed. A few moments later, you are being cut into several pieces by the impeller or the
chopper. That’s granulation!
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The method of ‘trial and error’ is still widely used for changing the process variables of the
granulation process to improve pellet properties. This method highly depends on the
experiences and creativity of the pharmaceutical scientist and the technician. Applying
experimental design is a more sophisticated approach18-21. With this procedure, the influence
of different processing variables as well as their mutual interactions can be investigated. And
the sensitivity of a number of processing variables can be investigated at the same time.
Although the effect of the processing variables can be described with this method, these
effects are often not (mechanistically) understood and extrapolation of the results is therefore
not allowed. Therefore, it is better to try to understand the influences of the processing
variables on the granule properties from a mechanistic point of view.
To understand the high-shear pelletisation process, one can try to develop mechanistic models
for this process. In literature, some fruitful modelling has been performed by several authors,
like Hounslow et al.22 who used the population balance modelling to model the particle size
distribution during the granulation process, Iveson et al.23 who gave an overview of the whole
granulation process by drawing a granulation regime map, and Wellm24 who modelled the
torque development during the granulation process.
These approaches make it possible to look inside the black-box and try to understand what
really goes on. Such a method also gives the possibility to predict the influence of a change of
the apparatus, the process, or the formulation (i.e. ingredients).
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Changes of the design and functionality of the impeller may increase the efficacy of the
energy input for the process and therefore diminish the unused dissipation of this energy.
Holm25 and Schæfer et al.26 changed the impeller design in order to create more densification
of the granules, which finally resulted in a narrower particle size distribution and more
spherical granulesCovering the wall of the bowl with PTFE-tape or coating was used by
several authors25,27,28. Due to this tape, less adhesion of granules on the wall was found,
resulting in a more homogeneous liquid distribution, resulting in a more controllable
granulation process.
Another way of changing the apparatus was recently published. A new sampling method was
described by Thies and Kleinebudde28, a cylinder of 2 cm was placed in the mixer wall.
Through this cylinder, several representative samples can be taken from one batch during
granulation without interrupting the process.
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Changes of the process conditions can be performed in order to reduce the amounts of fines
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and oversize granules which otherwise have to be either disposed of or recycled into the
process. So, process changes can be introduced in order to reduce the costs and the
environmental contamination. These changes include an optimisation of the impeller
rotational speed, chopper rotational speed, processing time, load of the bowl, the method or
rate of liquid addition, etc. There are many sound articles concerning the high-shear
pelletisation process24,29,30, the comparable melt pelletisation process5,8, and the production of
pellets in the rotary processor11,14.
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Microcrystalline cellulose (MCC) is world-wide the most used pelletisation aid. It would be
interesting to look for other excipients that also can be used as pelletisation aids. Because not
much literature about this subject is available, and the high-shear pelletisation process has
some aspects in common with the extrusion and spheronisation process, some literature about
this last field will be discussed. Kleinebudde et al.31 used a mixture of microcrystalline
cellulose (50-70 %, MCC), low-substituted hydroxypropylcellulose (0-20 %, L-HPC), and
acetaminophen (30%) for the preparation of pellets with the extrusion and spheronisation
method, and found a decreased water-sensitivity of the process and good dissolution
properties of acetaminophen from the pellets. But still, more than half of the formulation
existed of MCC. In comparison, the minimal amount of MCC needed to form a continuous
network, the so-called percolation threshold, is about 14 %32. At any volume concentration
higher than 14 %, MCC has formed a continuous network. So, in order to find other
excipients than MCC (for example a mixture of different materials) suitable for pelletisation,
the amount of MCC in this mixture has to be below 14 %. In 1994, Lindner and Kleinebudde
reported a study using powdered cellulose as pelletisation aid33. The pellets obtained with
powdered celluloses showed higher porosities and faster releasing properties compared to
those made with MCC. Since this study, a few more studies were reported concerning the
search towards more products that could be used as pelletisation aids. Chatlapalli et al.34
prepared pellets containing hydroxypropylmethylcellulose (HPMC) and hydroxyethyl-
cellulose (HEC) and used isopropyl alcohol as granulation liquid. Both HMPC and HPC were
found to be suitable pelletisation aids. Also a mixture of MCC (11 %) and β-cyclodextrine (89
%) was reported as a suitable extrusion / spheronisation agent giving satisfactory products35.
With this formulation, the amount of water needed to obtain good quality pellets was highly
decreased.
As a conclusion it can be stated that questions like “why is microcrystalline cellulose such a
good pelletisation agent”, and “is it possible to find other excipients except from
microcrystalline cellulose that can be used for the preparation of pellets” still are mostly
unanswered. Using melt pelletisation as an alternative for the high-shear pelletisation
technique, i.e. using a meltable binder, a series of pelletisation agents can be used, such as
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polyethylene glycol (PEG)5,8,36, Gelucire37, glycerol monostearate38 or stearic acid39. Because
MCC is so frequently applied it has also been used in the work described in this thesis.
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Granulation became a subject for scientific study in the second half of the 1950s; the forces
between the primary particles were among the first subjects investigated. Rumpf40 identified
in his fundamental work the categories of forces holding the granules together (figure 1.3):
- Attractive forces, like gravitational, magnetic, electrostatic and van der Waals forces. All
those forces are important during the handling of fine dry powders. In case of the
pelletisation process, those forces are more than one order of magnitude smaller than the
capillary forces, and thus not significant.
- Solid bridges, due to inorganic bonding, chemical reaction, crystallisation, melting at
points of contact due to frictional pressure, and sintering. Solid bridges can also be formed
after hardening of the binder liquid, which is of great importance during melt pelletisation.
- Particle interlocking. These bonds can be very important in the pelletisation process
because of the different shapes of the starting materials - crystals, rounded, or elongated -
and the high-shear forces acting on the pellets during processing.
- Liquid bridges, which can act in several ways to hold the granule together. The strength of
the liquid bridge is caused by the capillary pressure due to the curved surfaces, and the
interfacial tension. Liquid can also be adsorbed on the particle surface forming
multilayers. These multilayers can act as a lubricant to reduce the inter-particle friction.
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Figure 1.3. The most important inter-particle forces for pelletisation: a. solid bridges; b. particle
interlocking; c. liquid bridges.
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Newitt and Conway-Jones3 described in their pioneering research on the granulation process,
the different stages of liquid bonding in a granule as the pendular, funicular, capillary and the
droplet stage (see figure 1.4).
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Figure 1.4. Different stages of saturation for liquid bonds in granules: a. pendular stage; b. funicular
stage; c. capillary stage; d. droplet stage.
Before any liquid is present, and no liquid bridges can be formed, small aggregates of powder
can exist, held together by attractive forces like van der Waals forces. In the pendular stage,
only minor amounts of liquid are present, and the particles are held together by liquid bridges
present only at the contact points of individual primary particles. In the funicular stage, the
amount of liquid is increased, so that some of the liquid bridges can coalesce and form links
between more than two primary particles. The voids between the liquid bridges are partly
saturated with liquid. In the capillary stage, the voids are fully saturated with liquid, which
provides the strongest granule. At the surface, the liquid is drawn back into the pores under
capillary action, and inside the granules, the particles are completely surrounded with liquid.
If more liquid is added, the strength of the granule decreases rapidly, and the granule will be
converted into a system with particles suspended into the liquid or a paste. This is the so-
called droplet stage.
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A number of granule growth and breakage mechanisms, described by several authors 3,41-43
are illustrated in figure 1.5.
Figure 1.5. Mechanisms of granule growth and breakage, after Sastry et al.42
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Granule growth starts with nucleation, where primary particles stick together after being
wetted by a liquid drop. Also the engulfment of powder particles in a droplet is a part of the
nucleation mechanism.
Coalescence is the mechanism in which two granules collide with each other and form one
larger granule. After some surface deformation, a new spherical granule can be formed. The
maximal granule size above which no more coalescence takes place, and the chance of
sticking or non-sticking (=rebound) of a granule, have been topics of several investigations
(for example Ouchiyama and Tanaka44, and Ennis et al.45).
Layering, also called snowballing, is the mechanism in which many primary particles (e.g. the
non-granulated starting material) stick on the surface of a larger granule, due to the formation
of capillary bridges.
There is no distinct difference between the mechanisms coalescence and layering. In fact,
only the size of the initial particles differs. Coalescence assigns all successful collisions
between two granules, while layering is the mechanism in which primary particles stick on to
a larger granule.
Breakage of granules has been divided in literature into several mechanisms43. First of all
crushing, in which smaller granules are crushed and subsequently distributed over the surface
of the remaining granule by layering. Crushing can occur by shattering, fragmentation, or
abrasion. The other breakage mechanism referred to in literature is abrasion transfer. In this
mechanism material is transferred between two colliding granules, leaving both intact. This
mechanism has been identified experimentally41, but is thought to have a negligible effect on
the final granule size distribution.
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The strength of a liquid bridge formed by a collision between two particles determines
whether rebound or coalescence occurs. This force, which is among others a function of the
amount of liquid between the (spherical) particles relative to their volume, has been calculated
by Rumpf40. The maximum value of this force can be described as:
plcap dF αγ= (1.1)
where 1.9 < α < π depending on the moisture content (volume liquid to volume particle ratio),
γl relates to the surface tension of the binder liquid, and dp to the granule diameter.
For the breakage of granules, another equation based on fundamental research of Rumpf40 has
to be used. The tensile strength of granules (σt) in the funicular or capillary stage (see figure
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1.4) is defined as:
( )θ⋅γ
⋅ε
ε−⋅⋅=σ cos1
pp
lt d
SC (1.2)
in which C is the coordination number, depending on the particle shape, S is the liquid
saturation, ε is the intra granular porosity, dpp is the diameter of the primary particle, and θ is
the contact angle. The liquid saturation of a granule is defined as:
l
sHSρρ
⋅ε
ε−⋅= 1(1.3)
where H is the moisture content, which can be calculated as the ratio between the liquid mass
and the (dry) solid mass. The powder- and liquid densities are assigned by ρs and ρl,
respectively. The characteristic relationship between the liquid saturation and the tensile
strength of granules, as investigated by Schubert46, is schematically given in figure 1.6.
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Figure 1.6. Relationship between tensile strength of wet granules and the saturation, after Schubert46. Sp
denotes the end of the pendular state, and Sc the end of the capillary state.
Besides the properties of the starting material, the tensile strength of a granule highly depends
on the porosity and the liquid saturation. A high saturation and a low porosity provide strong
granules. Equation 1.2 is based on the static strength of the liquid bridges between the
particles. Under dynamic conditions, this equation does not exactly give the granule tensile
strength, which will be discussed in greater depth later.
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Ouchiyama and Tanaka44,47 derived a model for the estimation of the critical diameter above
which no coalescence occurs. This critical diameter (dc) can be calculated with:a
tc Kcd
σ⋅⋅= 23
(1.4)
Here, a and c are constants, K is a deformability constant, σt is the tensile strength of the
granule. The diameter dc is the specific diameter at which the chance on coalescence between
two granules theoretically is zero. The deformability constant K is defined as the ratio
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between the contact area (A) and the compaction force (F):
F
AK = (1.5)
This equation is only valid in case of total plastic behaviour of the granule.
Kristensen et al.48,49 measured the mechanical properties of moist granules, and used an
extended model of Ouchiyama and Tanaka to calculate the tensile strength of these moist
granules. The calculated tensile strength values are based on the principle that a limiting
strength of the moist granules must be gained before growth by coalescence becomes
possible. The granules gain strength by densification facilitated by the addition of binder
solution and agitation. Densification is a decrease in intra-granular porosity, which results in
an increase of the pellet saturation (eq. 1.3) leading to an increased pellet deformability, and
an increase of the tensile strength of the pellet (eq. 1.2) as long as S < Sc.
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Ennis et al.45,50 recognised the importance of the effect of viscosity on the strength of a liquid
bridge and developed an agglomeration model based on the dynamic liquid bridge.
Depending on the kinetic energy during the approach and the energy dissipated during the
collision, either rebound (non-successful collision) or coalescence (successful collision) of the
pellet occurs (figure 1.7). Ennis et al.45 defined the dimensionless viscous Stokes’ number for
such a system:
ηρ
=9
8 0rvSt p
v (1.6)
where ρp is the pellet (or granule) density, v0 is the relative velocity between the two spheres, r
is the pellet (or granule) radius, and η is the viscosity of the binder liquid.
K
KD
Y�
U
Figure 1.7. Ennis approach of the collision of two pellets.
This viscous Stokes’ number is a measure of the ratio of the collision energy to the viscous
dissipation brought about by the binder liquid. In a high-shear mixer, the impact velocity is
directly related to the tip velocity of the impeller (vtip = πND, with N is the impeller rotational
speed, and D the diameter of the bowl).
A collision between two pellets results either in rebound or coalescence. Since the collision
gives rise to loss of kinetic energy, the velocity before impact (v0) will be larger than the
velocity after impact (v). Coalescence occurs if Stv is lower than a critical value of the viscous
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Stokes’ number (Stv*). This critical viscous Stokes’ number is given by:
+=
av h
h
eSt ln
11* (1.7)
where e is the coefficient of restitution based on linear velocity differences of the pellets after
and before impact (as will be discussed in greater depth in chapter 4). The thickness of the
liquid layer on the surface of the pellet is given by h, and ha represents the characteristic
length of the surface asperities (the radius of the starting primary particles can be taken as a
measure herefore, see figure 1.7).
Three granulation regimes have been defined in terms of the magnitude of the Stokes’ number
(Stv) in comparison to the critical Stokes’ number (Stv*):
Stv « Stv* non-inertial regime, all collisions are successful;
Stv = Stv* inertial regime, some collisions are successful;
Stv » Stv* coating regime, no collisions are successful.
Granule growth by coalescence is promoted at low values of Stv and high values of Stv*. From
equations 1.6 and 1.7, it can be seen that the probability for a successful collision, and as a
consequence the granules growth rate, is increased by a lower particle density, a lower
impeller speed, a smaller granule size, a higher binder viscosity, a lower coefficient of
restitution, an increased surface liquid layer, and a smoother surface (or smaller primary
particles). Because some of these variables also depend on the time-effects and the moisture
content, it will be difficult to estimate the exact values of h, ha, and e experimentally.
Therefore, the use of this theory is limited to retrospective argumentation.
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Material properties of the materials involved are the most important variables during the
pelletisation process. For a material to be pelletised, the combination between the plastic and
the elastic properties of this material must have an optimal value. An improvement of the
model of Ouchiyama and Tanaka44 and Kristensen et al.49 has been made by Iveson et al.7,23
The latter compared the deformation behaviour, process intensity and liquid saturation with
the granule growth behaviour of granules made from varying materials. The rate of granule
consolidation was described by Iveson et al.23 with the deformation number, e.g. the ratio of
the impact pressure of the impeller (ρpvi2) and the yield pressure of the wet granules or pellets
(Yp). The relationship between the deformation number and the granule saturation resulted in
the design of a regime map for granule growth (figure 1.8). The granule growth regime map is
based on an extensive literature research for different materials and granulation processes and
some additional experiments.
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Pore Saturation =
Nucleation onlyInduction
Decreasig Induction Time
Increasing Growth Rate
Steady Growth
RapidGrowth
Slurry/Over-Wet
Mass
“Crumb”“Dry”Free-
FlowingPowderIncreasing
DeformationNumber,
100 %0 %
Crumb
Crushing&
Layering
Coalescence
Coalescence
H⋅ρρ⋅
εε−
l
s
min
min1
p
ip
Y
vDe
2ρ=
Figure 1.8. Granulation regime map, after Iveson et al.23
As can be seen in figure 1.8, a low deformation number leads either to nucleation or to
induction time growth. Higher liquid saturation generally causes faster granule growth due to
the increased surface plasticity and surface free-liquid present to form capillary bonds.
Furthermore, a high process intensity or a low wet granules deformation propensity increase
the granule growth rate. Considering the high-shear pelletisation process, a high mixing
intensity is obtained (high vi). In combination with the plastic deformation properties of the
wet pellets, a steady growth should be expected. More liquid saturation should finally lead to
rapid growth. If more liquid would be added, overwetting occurs and a slurry would be
formed.
This regime map is the first of its kind. It includes a number of parameters of interest, and has
a great potential to predict granulation behaviour from product and process characteristics. It
should make the control of granulation processes much easier.
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Randolph and Larson51 developed the first principles of the population balance describing the
number balance of the formation of crystals. As the granulation process can be described
similarly, the approach of Randolph and Larson has been adopted into this field. The basis
equation of the population balance is given by:
( ) DBGnt
n −=⋅∇+∂∂
(1.8)
where n is the number density of granules of a specific size at a specific time; G is the growth
rate of granules; B is the birth rate density of granules; and D is the death rate density of
granules.
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This equation can not be solved analytically, so solutions have to be obtained numerically by
discretisation. Hounslow et al.22 have developed the population balance equation further by
using different geometrically scaled size intervals, in which the size of the particles in the next
size class is twice the size (volume) of the particles in the current one.
The population balance of Hounslow et al.22 is given by:
∑ ∑∑−
=
∞
=
−−−−
−
=−−
+− β−β−β+β=1
1,,
211,1
2
11,1
1 22
12
i
j ijijjiijji
ijiii
i
jijji
iji NNNNNNNdt
dN(1.9)
where Nj represents the number of particles of size class j, dNi/dt is the change of the number
of particles of size class i as a function of time, and β is the coalescence kernel (note that this
kernel is a rate-constant, not a nucleus as described in chapter 2).
Assumptions of this population balance are:
- only binary collisions take place;
- conservation of volume during coalescence, which excludes any porosity changes during
the process;
- the conditions are uniform throughout the granulator, segregation of granules is not
included in the balance, and the impact forces are supposed to be constant in the whole
granulator;
- the coalescence kernel (β) is known;
- coalescence is the only granule growth mechanism, excluding nucleation;
- no granule breakage occurs. But during granulation in high-shear mixers breakage by the
impact of the impeller and the chopper does certainly occur.
There is no procedure for choosing the coalescence kernel form. Even if a good fit is found,
there is no guarantee that this is unique, that it is the best, or that it has any physical basis52.
There is a large number of proposed kernels in the literature42,43,45,53, but there is no a priori
justification which kernels are appropriate for a given granulation system, and a physical
interpretation of the coalescence kernel is still missing.
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Down-scaling experiments can be performed in such a way that these experiments are
representative for the large production scale. This is crucial. Other requirements for small-
scale apparatus and small-scaled processes are:
- rapid and reproducible experiments;
- cheap experiments;
- similar formulation;
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- geometric similarity with the large scale is not necessary, provided that dynamic similarity
of the particles exists (equal forces on the particles as a function of time). In fact this
means that the experiments are representative for the full scale.
Being more specific and remembering the part ‘imagine yourself being a particle’ (section
1.2.1.1), the following aspects are important:
- who you meet: smaller particles (for example primary particles), colleagues (of almost the
same size) and large particles (lumps), the impeller, the wall, or water droplets;
- under what conditions: water content and velocity differences;
- how often: number of meetings, in total or as a function of time (frequency).
If these aspects are identical at different scales, the development of the properties of the
particles (e.g. diameter, porosity) as a function of time, P(t), will be the same. This is the
fundamental basis of scale down, a concept that will show-up from time to time in this thesis.
Some remarks regarding these requirements can be made:
1. Not all aspects are always important.
2. A typical scale-down experiment is often limited to one or two aspects (e.g. the influence
of tip speed on mixing, or on growth and breakage of pellets). However, this is often also
true for classical (scale-up) experiments of complicated processes (due to the impossibility
to keep all relevant dimensionless numbers at a constant value during scale-up).
3. The advantage of scale-down is two fold. First it gives more experimental room for
manoeuvre (no geometric similarity needed). Second it provides rapid insight in the
relative importance of the different mechanisms that are potentially involved.
4. It is not always necessary to realise exactly the same value of the aspects at different scales
of operation. The same order of magnitude is often sufficient (e.g. to find the most
important mechanisms involved).
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There are several dimensionless numbers, which can be used for scale-up purposes:
- Power number: relationship between the power input and the power transferred to kinetic
energy (∆P/N3D5ρ).
- Reynolds number: describing the relationship between frictional forces and the
consistency of the wet mass (ρND2/η).
- Relative swept volume: fractional volume of the total batch size which is displaced by the
impeller in a specific time range (~N)
- Tip velocity: velocity of the impeller at the tip of the impeller arm (πND). The tip velocity
of the impeller arm can be used for scale-up in order to maintain a constant maximal shear
rate.
- Froude number: relationship between the centrifugal force and the gravity force (N2D/g).
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In literature, different methods of scale-up have been reported. The relative swept volume has
been considered to relate to the work input on the material which is assumed to provide
densification of the wet mass54,55.
Horsthuis et al.56 used the Froude number in order to scale-up in the Gral, whereas the relative
swept volume and a constant tip speed did not result in a comparable process.
A power consumption curve has been used by Leuenberger et al.57 for scale-up purposes. The
relationship between the power consumption and the amount of moisture added looks very
similar to the saturation curve derived by Schubert46 (figure 1.6). The process is stopped as
soon as the power consumption curve is at the desired value and the wet mass reaches the
desired consistency.
Cliff and Parker58 showed that the ratio between the power number and a pseudo Reynolds
number was independent of the scale of operations used (a pseudo Reynolds number was used
because the wet mass consistency ‘η’ was measured with a mixer torque rheometer in the
dimensions Nm instead of Pas). This method could therefore be a useful tool during scale-up.
Landin et al.59 and Faure et al.60 incorporated also the Froude number into this ratio in order
to find a scalable function.
Dry granules have another tensile strength than moist pellets, and tend to undergo brittle
fracture43. Therefore it is desirable to know the material properties of the wet pellets instead
of the dry pellet properties. The granule strength only gives important information about the
granulation process if it is measured as a wet-granule strength. The mixer torque rheometer
can give important information about the rheology of the granulate. The mixer torque
rheometer61-64 can easily be used as a down-scaled high-shear mixer. But, using this device,
one has to bear in mind the differences in apparatus-design, and impeller speed. The impact
velocity of the mixer torque rheometer and of the high-shear mixer should be of the same
order of magnitude in order to be able to use the rheometer as a down-scaled high-shear
mixer. Otherwise there is no dynamic similarity, which could result in the development and
measurement of non-relevant material properties in the mixer torque rheometer.
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Microcrystalline cellulose (MCC) is described as a purified, partially depolymerised cellulose
prepared by treating α-cellulose, obtained as a pulp from fibrous plant material with mineral
acids65. The cellulose fibres in the starting material are composed of millions of microfibres.
In the microfibres, two different regions can be distinguished: a paracrystalline region, which
is an amorphous and flexible mass of cellulose chains, and a crystalline region, which is
composed of tight bundles of cellulose chains in a rigid linear arrangement65. As an effect of
controlled hydrolysis, the amorphous fraction has largely been removed, yielding aggregates
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of the more crystalline portions of cellulose fibres. After purification by filtration and spray
drying, dry porous agglomerated microcrystals are obtained.
The requirements for the formation of pellets from a wet mass are as follows:66
1. The wet mass must possess sufficient mechanical strength if wet, yet it must be brittle
enough for lumps to be broken down to pellets, but not be so friable that the pellets
disintegrate completely.
2. The wet mass must be sufficiently plastic to enable the formation and spheronisation of
pellets in the high-shear mixer, thus decreasing the surface roughness.
The function of MCC herein is to control the distributions of water through the wet powder
mass during pelletisation, and to modify the rheological properties in the mixture, conferring
a degree of plasticity which allows for rapid pelletisation.
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The rheological properties of wet mass just before and after the formation of the pellets highly
depend on the liquid content of the wet mass. Only in a narrow range of liquid content it is
possible to produce round pellets of the desired size. Below this specific moisture content, the
plasticity of the wet mass is insufficient: in this case pelletisation parameters such as torque,
power consumption and temperature increase, and no pellets can be formed. Exceeding the
specific moisture content, results in lower values for the pelletisation parameters: the pellets
are soft and easily deformable. As a consequence, uncontrolled growth due to coalescence of
pellets occurs, or crumb is formed.
These observations can be described with strength of static liquid bonds in moist granules,
and lead to the model of different liquid states in moist granules3,40.
The liquid saturation model describes the relationship between the amount of powder and
liquid and the granule tensile strength, based on the different liquid saturation stages:
pendular, funicular, capillary, and droplet stage (see section 1.3.2). The same model is used to
describe the process during pelletisation. When water is added to a powder, the liquid will
occupy the spaces between the powder particles. The saturation of the powder mass can be
defined as the fraction of the pores between the powder particles, which is occupied by water
(eq. 1.3). The strength of the pellets depends on the saturation, porosity and size of the
starting material:
( )θ⋅γ
⋅ε
ε−⋅⋅=σ cos1
pp
lt d
SC (1.2)
Some restrictions of the liquid saturation model are:67
1. the equation of the tensile strength (eq. 1.2) of granules was developed for a liquid
saturation between about 25 % and 90 %;
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2. the tensile strength of wet pellets in the liquid saturation model is based on static liquid
bridges only;
3. particles are assumed to be spherical;
4. (partly) dissolving of the solid by the liquid is not included in this model;
5. porosity is assumed to be constant during process, and swelling of the solid in the liquid is
not included in the liquid saturation model.
During the extrusion and the pelletisation process with MCC, it has been observed that the
calculated saturation of pellets can be about 100 %68, the strength of the liquid bridge depends
also on the viscosity of the binder liquid50, the size and shape of primary MCC-particles are
not spherical and rigid69, and pellets containing MCC shrink during drying70. For these
reasons, the saturation model is not valid to explain the behaviour of MCC during extrusion
and/or pelletisation.
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Due to the passive water binding properties of microcrystalline cellulose (MCC), an amount
of about 16 – 26 % of water can be bound to MCC71. If more water is present (for example in
a MCC-water mixture during granulation), it has to be bounded in another way. Therefore,
MCC was addressed as a ‘molecular sponge’72 (figure 1.9a).
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Figure 1.9. Schematic illustration of water in the cellulose samples. a. sponge model73; b. crystallite-gel
model.
Each particle of MCC would behave as a porous sponge and each particle would be able to
absorb a large quantity of water. Part of the water in MCC is absorbed in the pores inside the
cellulose fibres and amorphous regions, and part is located between the fibres with
obstruction and hydration interactions with the fibres73. All pores are supposed to be
completely filled with water. Under pressure the water would be partly squeezed out and
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lubricate a particle rearrangement. Water can also be taken-up again after releasing the
pressure while the volume increases. MCC particles remain intact during the process of
pelletisation, extrusion and spheronisation and should be of the same size, shape and volume
in the finished product compared to the original MCC powder74.
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Kleinebudde proposed the crystallite-gel model in which a gel is formed during extrusion /
spheronisation with MCC67. The concept of the crystallite-gel model could also be valid for
the pelletisation process.
It has been shown75 that powder particles of MCC are broken down into smaller sub-units due
to the presence of water and shear (for example during granulation and extrusion). Single
crystallites with a size of a few micron can be obtained. These single particles are able to form
a crystallite-gel and immobilise the water (see figure 1.9b). The crystallites or their
agglomerates can form a network by cross-linking with hydrogen bonds at the amorphous
ends. The viscosity of the gel depends on the water content and the degree of cross-linking
(e.g. the size of the resulting structural components). At increasing liquid content, the fraction
of gelling agent in the gel decreases and the deformability increases. The gel is not sticky,
because the gelling agent is not soluble in water.
The formation of hydrogen bonds in the amorphous ends of the crystallites during drying can
be described as an autohesion effect resulting in a stable matrix. (Autohesion is defined as the
mutual inter-diffusion of free polymer chain ends across the particle-particle interface of high
molecular weight polymers resulting in a stable link76.) This provides an explanation for the
disintegrating77 and dissolution properties19,78 of pellets.
The particle size of the MCC powder does not have any influence on the amount of binder
liquid needed for pelletisation (while it is reported that the amount of binder liquid needed for
pelletisation is influenced by the source of MCC67), which can be explained using the
crystallite-gel concept. After all, after formation of the crystallite gel, the size of the starting
material is not important anymore, whereas the amount of contaminations of the starting
material is.
The surface structure of MCC pellets is completely different from the structure of MCC
powder. The structure of the original powder particles disappeared completely and turned into
a coherent network. This suggests the formation of a network during pelletisation. And,
shrinking of the pellets during drying is supposed not to occur in the sponge-like approach,
which is another argument using the crystallite-gel model.
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The aim of this thesis is to obtain insight in the most important mechanisms involved during
the high-shear pelletisation process. The mechanisms of pellet growth and breakage are
investigated, as well as the flow profile of pellets inside the mixer, using a coffee-grinder as a
down-scaled high-shear mixer. By knowing the locations of pellets inside the mixer, and the
influence of the impeller, chopper, wall and other pellets on the pellet deformability and
strength, it should be possible to predict the pellet growth behaviour for a specific
combination of powder mixture, apparatus and process parameters.
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a constant
A contact area (m2)
B birth rate density function (s-1)
C coordination number
c constant
d diameter (m)
D diameter of the bowl (m)
D death rate density function (s-1)
e coefficient of restitution based on linear velocity differences
F force (N)
g gravitational acceleration constant (m·s-2)
G growth rate (s-1)
h thickness of liquid layer on granule surface (m)
ha characteristic length of the surface asperities (m)
H moisture content
K deformability constant (Pa-1)
N impeller rotational speed (s-1)
N number of particles
n number density of granules
P power input (W)
r granule radius (m)
S liquid saturation
Stv viscous Stokes number
Stv* critical viscous Stokes’ number
t time (s)
v relative velocity (m.s-1)
Y yield pressure (Pa)
Greek symbols
α constant
β coalescence kernel
ε (intra-granular) porosity
γ surface tension (N.m-1)
ρ density (kg.m-3)
η viscosity (Pa.s)
θ contact angle (°)σt tensile strength (Pa)
Subscripts
c critical
i,j size classes
l liquid
p granule or pellet
pp primary particle (starting material)
s solid
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,1752'8&7,21
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