-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
1/13
Invited review paper
Aerodynamic dispersion of cohesive powders: A review of understanding
and technology
G. Calvert, M. Ghadiri *, R. Tweedie
Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e i n f o
Article history:
Received 19 August 2008Accepted 10 September 2008
Keywords:
Dispersion
Aerodynamic
Cohesive powders
Particle characterisation
Dry powder inhalers
a b s t r a c t
Dispersion is the desired disintegration of particle clusters down to their primary constituents through
the application of external forces, which overcome the interparticle attraction forces. This method is ben-
eficial for many processes but especially for the characterisation of particulate systems and therapeutic
drug delivery via the lungs from dry powder inhalers (DPIs). Dry powder dispersion is becoming increas-
ingly popular as a method of sample preparation for a range of instruments such as a laser diffraction
measurement device. There are many advantages for dry dispersion compared to wet methods. However,
complete dispersion of fine cohesive powders is difficult due to the relatively large interparticle attraction
forces compared to separating forces arising from fluid energy. This review identifies the current state of
theoretical and experimental understanding of powder dispersing in a gaseous medium. The approaches
to relate bulk powder properties to dispersion, the stresses produced on a particulate structure due to
aerodynamic forces and possible approaches for linking the two are discussed. Furthermore, the available
dispersion technology is reviewed with a discussion of individual dispersers and commercial devices
used for dispersing bulk powder. Also, the review highlights the research needed in this field to gain a
better understanding of how bulk powders interact with a dispersing fluid.
2008, The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder
Technology Japan. All rights reserved.
1. Introduction
The efficient dispersion of particulate solids is of great impor-
tance in a number of industries such as pharmaceutical, bulk
chemical and food. The majority of the products in these industries
are primarily in powder form; therefore, particle size analysis is
essential when producing powders as even small differences in size
and/or shape as well as surface properties affect process ability or
performance and end product attributes. Consequently, powder
dispersion is widely used to break up loose aggregate clusters as
a means to determine particle size distributions; this can be
achieved using laser diffraction [1], time-of-flight techniques
[2,3] and microscopy methods [4]. Dispersion of powders can be
done in a gas or a liquid phase and a variety of theoretical and
experimental researches are available which investigate liquid sys-
tems[514]. In contrast, the understanding of the interactions be-
tween bulk powder and gaseous medium and the transformation
from an aggregate state to a dispersed state are lacking. Despite
this, aerodynamic dispersion remains a popular method of sample
preparation [15,16]. There are several reasons, including most
notably, ease of use, speed of measurement, absence of liquids
and a high optical property contrast between the particle and the
gas phase.
A further area where aerodynamic dispersion is highly impor-
tant is in therapeutic drug delivery via the lungs using dry pow-
der inhalers (DPIs). There are many devices on the market
including Rotahaler, Aerohaler, and Diskhaler. Replacement
of the chlorofluorocarbon (CFC)-driven pressurised metered dose
inhalers (pMDIs) was necessitated by the concerns of CFC damage
to the earths ozone layer. This resulted in the reformulation of
pMDIs with the chlorine-free propellant hydrofluoroalkane gas
(HFA-134a and HFA-227). However, HFAs are also greenhouse
gases, and although their potency is less than CFCs, their effect
is still 2000 times greater than that of carbon dioxide. Therefore,
it is inevitable that these substances are going to be subjected to
future controls [17]. A major alternative to the pMDI is the
breath-actuated DPI which has obvious environmental benefits.
In order to effectively deliver a drug to the desired region of
the lung, the turbulent air stream created by any DPI must pro-
vide adequate power to disperse the powder that produces a
cloud of respirable fine particles. This involves a balance between
DPI design, drug formulation and the patient-generated inspired
flow rate [18,19].
The ability to control dispersion of a wide range of powders,
from friable to robust and free-flowing to extremely cohesive
material, is an area of great interest and importance. However, it
0921-8831/$ - see front matter 2008, The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved.doi:10.1016/j.apt.2008.09.001
* Corresponding author.
E-mail address: [email protected](M. Ghadiri).
Advanced Powder Technology 20 (2009) 416
Contents lists available at ScienceDirect
Advanced Powder Technology
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p t
mailto:[email protected]://www.sciencedirect.com/science/journal/09218831http://www.elsevier.com/locate/apthttp://www.elsevier.com/locate/apthttp://www.sciencedirect.com/science/journal/09218831mailto:[email protected] -
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
2/13
is recognised that the complete dispersion of fine cohesive
powders, especially in the size range below 20 lm[20], is difficult
due to the relatively large interparticle attraction forces, namely
van der Waals, electrostatic and liquid bridge, compared to sepa-
rating forces[21]. In general, the relative strength of these forces
increases with decreasing particle size. The van der Waals force,
the main interparticle attraction force as particle size decreases,
is approximately 100 times greater than gravity for 10lm particles
[22]. As a consequence, a great amount of energy is required to de-
form and disintegrate these clusters completely to their primary
constituents. Unfortunately, the application of such large disper-sive energies may lead to particle breakage, an obvious disadvan-
tage when measuring particle size. Additionally, particle
morphological features such as shape and surface asperities also
affect particle interactions and hence dispersion performance
[23]. As reported by Kousaka et al. [24], irregular-shaped particles
disperse more easily compared to spherical particles.
A wide range of disperser types are available for academic and/
or commercial applications. Typical dispersion units include educ-
tor, nozzle, capillary tube, Venturi, mixer type, and fluidised bed.
These different configurations are discussed in the technology
section.
In this paper, the current understanding and the technology
available for dry powder dispersion are critically reviewed and
summarised. Furthermore, the available research and commercial
devices used for dispersing bulk dry powders are reviewed. The re-
search needed in this field to gain a more comprehensive under-
standing of how bulk powders interact with a dispersing fluid is
also highlighted.
2. Interparticle forces affecting bulk flow properties
Insufficient powder dispersion in gas flow arises due to inter-
particle forces. Simons [25] classified these forces according to
their relative magnitudes, from strongest to weakest: (a) solid
bridge forces; (b) liquid bridge forces; (c) van der Waals; (d) elec-
trostatic; and (e) magnetic. Extensive reviews of theories andexperimental studies are available in the literature [21,22,2631].
Aerodynamic dispersion of bulk powders is the successful appli-
cation of separating forces to overcome bulk powder attraction
forces producing a cloud of primary particles. Consequently it is
of interest to investigate the factors which influence these forces
that give rise to aggregate strength. Obvious factors include the
primary particle size and shape, density, packing structure, poros-
ity and contact area, and the interparticle bond strength.
When considering powder dispersibility, there are two intrinsic
bulk powder properties of interest: the bulk tensile strength and
the shear strength. In the literature, a number of models are avail-
able, which relate the microscopic interparticle interactions to the
tensile strength of a particle cluster. Rumpf[32]introduced a mod-
el which, described the mechanical stability of aggregates; he con-sidered the pull-off force necessary to separate the structure along
Nomenclature
Latin lettersA Hamaker constant (Number of atoms/m3)CF drag coefficient ()c ratio of diameter of the circular contact area to the par-
ticle diameter ()
Dp particle diameter (m)DpW particle diameter corresponding to the inertia parame-
ter, Wc(m)Dp50 median particle diameter (m)Dp50s mass median primary particle diameter by sedimenta-
tion method (m)DS dispersibility parameter ()Dv volume equivalent diameter (m)Dp frequency distribution curve intersection between f0
andfd (m)d capillary tube diameter (m)dpag particle cluster diameter (m)d0 orifice disperser diameter (mm)ds impact obstacle diameter (m)E elastic modulus (Pa)
Fd separating force (N)Fad interparticle attraction force (N)Fp characteristic value of re-entrainment (Pa)F0p characteristic value of interparticle cohesion force (Pa)fd/0 particle size frequency distribution curves (%/lm)H interparticle separation distance (m)k proportionality constant ()kn particle coordination number (-)mA/B particle mass (kg)n impeller rotational speed (radians/s)DP pressure drop (Pa)Q0 air flow rate (l/min)Re Reynolds number ()R particle radius (m)RfA/B fluid resistance forces acting on a particle (N)r surface asperity radius (m)
t time (s)Dt duration of impact (s)u air velocity (m/s)ur particlefluid relative velocity (m/s)up particle velocity (m/s)
u average air velocity (m)vi impact velocity (m/s)We Weber number ()W powder flow rate (kg/s)Z particle diameter ratio ()
Greek symbolsC interface energy (J/m2)D agglomerate breakage dimensionless group ()/ packing fraction ()Wc inertia parameter ()b dispersion efficiency ()bi correction factor to account for second particles pres-
ence ()c velocity gradient acting across the sphere (1/s)cA/B surface energy (J/m2)e porosity ()ed specific dissipated power (J/m
3s)gT target efficiency ()j dynamic shape factor ()l fluid viscosity (Pa s)qa air density (kg/m
3)qf density of the fluid (kg/m
3)qp particle density (kg/m
3)r tensile strength/stress (Pa)rdisp dispersion strength (Pa)rc bending moment (Pa)s shear stress (Pa)sc critical shear stress for re-entrainment (Pa)
G. Calvert et al. / Advanced Powder Technology 20 (2009) 416 5
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
3/13
its cross section into two halves and calculated tensile strength, r,
of a particle cluster as given by [33]:
r /knFad
pD2p1
where / is the packing fraction, Fad is the attraction force between
particles of diameter Dpand knis the particle coordination number.
If the particles are dry and assuming that van der Waals interactionsare the only attraction forces, Fad is given by
Fad A
6H2R1R2
R1 R2
2
where A is the Hamaker constant, His the interparticle separation
distance and R1 and R2 are the radii of the contacting spheres,
respectively. However, this approach is limited when evaluating
real situations as structures present heterogeneous properties. As
a result, particle clusters break along the line of least mechanical
resistance.
Several attempts have been made to improve this model by
adding various terms. For example, it is possible to include surface
features such as particle roughness[34]:
Fad A
6H2rR
r R
R
1 rH
2" #
3
whereris the radius of the asperity on the surface. A common ap-
proach for improving the flow properties of cohesive powders is the
use of spacer particles, which are discussed later. It has been shown
that the above model was able to correlate the tensile strength
when spacer particles, several nanometres in size, were added to
cohesive powders[35]. In this situation, the surface asperities with
radius rwere substituted by the spacer aggregate size. However, the
tensile strength is reduced only as a result of size and not by consid-
ering the different chemical compositions of the spacer aggregates.
Dispersion performance is affected by particle interactions, andmethods are available for measuring bulk powder tensile strength
[3641]or shear strength[42]. Iinoya and Masuda[43]carried out
an experimental study on the performance of three different types
of disperser: a mixer type; a fluidised bed and an eductor. They
determined the cohesion force per unit area by using a shear tester.
It is seen that the finer more cohesive powder, metallic silicon, dis-
perses less well compared with calcium carbonate, which has a lar-
ger mass median diameter, and is less cohesive.
In these standard approaches, the forces acting on the bulk pow-
der are mechanical forces. It is of great interest to elucidate if such
methods also apply to the highly dynamic forces which powders
experience during dispersion. Masuda et al. [44]developed a parti-
cle re-entrainment test, which can measure the rupture phenome-
non of aggregated particles caused by the flow of a gas over a
powder bed. The apparatus used in the particle re-entrainment test
is shown inFig. 1. The test cell is packed with powder and dry air is
fed into the test cell. The air flow rate is controlled so that the cross
sectional average velocity increases by 1 m/s every minute. Once
the cross-sectional average velocityreaches a critical value, the flow
begins to entrain aggregated particles. The process is observed by a
microscope with a video system, and is detected based on the con-
tact electrification of particles.
At the onset of entrainment the critical shear stress, sc, is calcu-
lated using the following equations:
sc CFqau
2
2
CF 16Re1 Re< 2300
CF 0:0791Re14 ReP 2300
4
where qa is the air density, CFis the drag coefficient, u is the air
velocity and Reynolds number (Re) is based on the equivalent diam-
eter of the rectangular channel. Previously, Masuda et al. [44]found
that for fly ash no. 10 (JIS Z8901), the critical shear stress defined in
Eq.(4) was proportional to the tensile strength, r, of the powdersc kr k 0:38 for the fly-ash no: 10 5
Eq.(1) introduced by Rumpf[33]has been used by Masuda and Got-
oh [45] for evaluating the capability of using the re-entrainment
test to estimate the dispersion performance of a given powder. They
applied this approach to a mixer type and an eductor disperser (see
Table 1). In order to estimate the coordination number necessary to
calculate the tensile strength in Eq. (1), the RidgwayTarbuck
empirical equation was used[46]
kn 13:8 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
175 232/p
6
Substituting Eq.(1) into Eq.(5) results in the following:
kFad
pD2p sc
/kn F
p 7
The right-hand side of Eq.(7), defined as the characteristic value of
re-entrainment,Fp, can be determined by the re-entrainment test. It
is seen that for most powders, Fpincreases with the packing fraction
due to the increase in the critical shear stress and tend to a maxi-
mum value asymptotically. This maximum value is defined as the
characteristic value of interparticle cohesive force, F0p. Furthermore,
Masuda and Gotoh[45]introduced the following approach for eval-
uating the dispersion efficiency, b:
b
Z Dp0
fd dDp
Z 1Dp
f0 dDp 8
wheref0 is the particle size distribution of fully dispersed primaryparticles measured by a centrifugal sedimentation method, fd is
the particle size distribution of the aerodynamically dispersed pow-
der and Dp represents the intersection of the two frequency distri-
butions. The above method relates the size distribution of the
dispersed powder to the size distribution if the powder was fully
dispersed. The re-entrainment test has been shown to correlate well
with the dispersion efficiency defined in Eq. (8)[45]. However, this
approach is based upon the powder critical shear strength being lin-
early proportional to the tensile strength. This may not be the case
for all powders.
Recently Weiler et al. [47] introduced a theoretical model for
describing the complete disintegration of dry powder clusters. A
number of assumptions were introduced to simplify powder dis-
persion, including the clusters disintegrated entirely and instantly,so that the forces affected the complete surface area of all particles;
MonitorVideo
TV Camera
Microscope
Buffer volume withdiffusion layer
Test section
PumpValve
Flow meter
Test cell
Testedpowder
Detector
Fig. 1. Apparatus for particle re-entrainment test[45].
6 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
4/13
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
5/13
the clusters and primary particles were spherical; and the only
cohesive forces were in the direct surroundings of the particles.
This model considers the breakage of every contact inside the
agglomerate to estimate dispersion strength, rdisp
rdispFad1 ekn
2pDpag
D3p9
where dpagis the particle cluster diameter and e is porosity. Eq.(9) islimited to agglomerates, where DpagDp. In the case where the
particle size ratio Dpag/Dp< 50, Eq.(9) is modified to the following:
rdispFad1ekn
2pD2pag
D3pag
D3p 4f
b2
where
b Dp
DpagDp f 1 arccosb
180
10
Unfortunately, at present there is no empirical evidence to validate
this approach. However, it is expected that it will underestimate the
difficulty of powder dispersion. It is unrealistic for all contacts to be
broken at the same time due to forces from the dispersing fluid act-
ing only upon the aggregate surface exposed to the fluid forces. It is
reasonable to assume the fluid within the porous structure remains
essentially quiescent[48]. Therefore, the fluid force acting on a par-
ticle cluster will affect particles exposed to the fluid flow, which
may disperse the exposed particles or the force might propagate
to particles within the cluster. This will result in the deformation
and eventual break up of loose aggregates [12].
In the literature, attempts have been made to analyse the effect
of bond strength on agglomerate breakage using the distinct ele-
ment method (DEM)[4954]. Kafui and Thornton[49] investigated
the effect of surface energy on the strength of regularly packed
agglomerates and related the breakage of interparticle contacts
to the Weber number,We, as defined by
We qpDpu
2p
C 11
whereqpis the particle density,upis the particle velocity and C is
the interface energy, which is defined by the Dupr equation[28]as
C cA cB cAB 12
where cA and cB are the surface energies of two particles made of
different materials, A and B , in contact with each other and cAB is
the interaction energy between them. For materials which are the
same, cAB is zero and therefore C = 2c. However, in later work
Thornton et al. [50] modified Eq. (11) by including a lower limit
of impact velocity, below which no contact is broken. It was shown
that this approach was better for relating the breakage of interpar-
ticle contacts. Subero et al. [52]used DEM to analyse the effect of
interface energy in the range 0.55.0 J/m2 in the randomly packed
agglomerates. They found that the results were in good agreementwith Thornton et al.[50]. However, if the interface energy was in-
creased well beyond one order of magnitude, as investigated above,
then the standard and modified Weber numbers no longer provided
an adequate description of the trend[53].
MorenoAtanasio and Ghadiri [54] introduced a mechanistic
model, relating the number of broken contacts in a particle assem-
bly due to impact velocity, interparticle adhesion energy and the
properties of the particles which form the cluster. They assumed
that the energy used to break contacts during impact was propor-
tional to the incident kinetic energy of the particle assembly. The
number of broken contacts was shown to depend on the following
dimensionless group, D:
D qpD
5=3p E
2=3u2p
C5=3 13
where Eis the elastic modulus. Their simulation results showed that
the effect of surface energy on agglomerate breakage was better de-
scribed by this mechanistic model than by the Weber number
alone, as previously used to characterise the impact strength of
agglomerates.
3. Aerodynamic dispersion mechanisms
The interaction between loose aggregates and dispersing fluid
has been investigated both from a theoretical and experimental
point of view. Loosely aggregated particles suspended in fluid
flows experience several kinds of forces caused by rapid accelera-
tion, deceleration, turbulent eddies, etc. When considering powder
fluid interactions, the following mechanisms are used to describe
the dominating dispersion process[55]:
(1) dispersion by rapid acceleration or deceleration and/or shear
flow;
(2) dispersion of particle clusters by impact onto a stationary or
moving target;
(3) dispersion by other mechanical forces (e.g. fluidisation, mix-
ing, vibration and scraping).
The dispersion of dry powders using one or a combination of the
above mechanisms is most likely divided into the subsequent three
zones[56]:
(1) Delivery the powder is introduced to the dispersion unit,
for example, by a vibratory tray.
(2) Dispersion and/or entrainment the powder is dispersed by
forces acting upon the loose aggregates without breaking
primary particles.
(3) Transport and presentation the powder is subsequently
conveyed from the dispersing device and presented to a
region for a specific purpose downstream.
3.1. Powder delivery
The powder delivery stage of dispersion suffers fromall theprob-
lems associated with powder flow, and will impact upon dispersion
performance. If the dominant dispersion mechanism is powder
acceleration, an important parameter is the relative velocity differ-
ence between the powder and dispersing fluid [24]. Hence, it is
desirable that the particles have comparatively negligible velocity
when enteringthe dispersionregion. The nature of interparticle col-
lisions andcollisions with devicewalls may aiddispersion butcould
result in tribocharging which might hinder the process. Further-
more, environmental conditions such as relative humidity will im-
pact upon dispersion performance and reproducibility.
3.2. Powder dispersion
To effectively disperse bulk powders, the efficient application of
a separating force is paramount. In addition, it is well practiced in
industry to reduce the interparticle attraction force, hence reduc-
ing the separating force necessary for efficient dispersion, a tech-
nique commonly adopted in the DPI industry. A reduction in
interparticle attraction can be achieved through the use of spacer
particles, by coating particle surfaces or using carrier particles.
These approaches are discussed later in this section.
3.2.1. Dispersion by acceleration in a uniform flow field
The greatest stress acting upon loose aggregates occurs at max-imum acceleration; this is as soon as the cluster enters the fluid
8 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
6/13
flow. The stresses on a sphere in simple flow fields when the
sphere is initially at rest have been analysed by Bagster and Tomi
[48]. It has been shown that the maximum stresses, tensile, rmax,
and shear,smax, in a uniform flow field may be approximated by
rmax andsmax3lur
Dp14
where l is the fluid viscosity and uris the particlefluid relativevelocity. This particular approach adopts a major simplifying
assumption, whereby the aggregates are homogeneous spheres
undergoing bulk motion as single entities and possess characteristic
structural properties. Furthermore, the fluid within the porous
structure remains stationary. However, this is a preliminary at-
tempt in relating the variables, which govern the break up of aggre-
gates due to aerodynamic forces. When analysing dispersion
mechanisms, it is helpful to treat a loose aggregate as a cluster con-
sisting of a number of primary particles, which are bonded by inter-
particle attraction. The separation of two particles forming a
doublet[24,58]and three particles forming a triplet [59]has been
considered in a uniform flow field. Kousaka et al. [24] analysed
the separating force between the two particles of a doublet using
Newtons second law of motion (Fig. 2)
mAdupdt
RfA Fd
mBdupdt
RfB Fd15
whereRfAand RfB are the fluid resistance forces acting on each par-
ticle, Fd is the force acting on each particle, mA and mB are the
masses of the respective particles, which can be found if the parti-
cles diameters and densities are known.
Kousaka et al. [24] assumed that the densities of the particles
are equal and operating under Stokes law with the following rela-
tions for the respective resistance forces, RfA and RfB:
RfA 3plurDpARfB 3plurDpB
16
where DpA and DpB are the diameters of particles A and B, respec-tively. Substituting Eq. (16) into Eq. (15) the product can reduce
to give the force acting on the contacting area between the two
particles
Fd 3plurDpADpBDpB DpA
D2pA DpADpB D2pB
17
Thus, if the attraction force is known, Eq. (17)will estimate whether
the doublet particles will theoretically detach from one another.
Furthermore, if the doublet consists of equal-sized spheres then
based on the above approach dispersion will not occur, as the sep-
arating force is zero.
A similar approach to that outlined above was adopted by Yuu
and Oda [57], when investigating the dispersion process due to
acceleration through an orifice-type disperser. In addition, thismodel allows for the dispersion of clusters consisting of more than
two particles. For example, this is achieved by approximating a
cluster to a doublet that disperses into two smaller doublets which
eventually disperse into primary particles. Therefore the clusters of
more than two particles are assumed to be doublets which break
up to form doublets. This research introduced a correction factor,
bi, similar to Reed and Morrison [60] to account for the effect of
the presence of a second sphere
mA dupdt 3pbAurDpA Fd
mBdupdt
3pbBurDpB Fd18
Assuming that van der Waals interaction is the only attractive force
acting between particles, Yuu and Oda [57] derived a population
balance equation using Eq.(18)for predicting the change of the par-
ticle size distribution due to acceleration (or deceleration). In a
number of studies, the importance of the diameter ratio between
two particles in contact, Z, has been highlighted[24,5759].
ZDpADpB
19
Using the approach of Yuu and Oda, it is possible to calculate the
possible dispersion range as a function ofZ, i.e. for what values of
Zunder given flow conditions that will result in particle dispersion.As expected, with an increase in inertia the dispersion range in-
creases. Unfortunately, this model has unrealistic assumptions: a
particle cluster consisting of more than two particles is dispersed
to two clusters, each consisting of two particles, which are then dis-
persed to two single particles. This is over simplifying the aerody-
namic dispersion phenomenon, which will involve complex loose
aggregates, dispersion through particle erosion and collisions, etc.
Furthermore, the correction factor, bi, is not yet fully characterised
and the model only considers the effect of van der Waals forces.
Kousaka et al. [58]extended the separating force well beyond
the Stokes regime, Reynolds numbers up to 104, whereby the dis-
persion force acting on a doublet can be calculated using
Fd 0:119qa
u2r
D2
pBZ2jA ZjB 2:07lq
au3
rD6
pBZ3
1=2
jA Z3=2jB 9:05lurDpBZjA Z
2jBZ3 1
1 20
where j is the dynamic shape factor (=1 for a spherical particle).
When jA= jB= 1 andZ= 0.5, then the separating force is maximum
for the dispersing conditions. As previously mentioned, a doublet of
two equal sized spheres cannot be dispersed because the separating
force acting at the contact is zero. This has been experimentally ob-
served by Endo et al.[59], when the percentage of doublets seemed
not to vary significantly with pressure. If particles A and/or B are
themselves loose aggregates, the dispersion force is also calculated
from Eq.(20), taking into account the volume equivalent diameter,
Dv, instead ofDpand the dynamic shape factor for each particle.
Particle B
Particle Az
0
Fig. 3. Illustration of a model doublet in a simple shear flow field.
a b
Fig. 2. Illustration of a model doublet in a uniform flow field.
G. Calvert et al. / Advanced Powder Technology 20 (2009) 416 9
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
7/13
3.2.2. Dispersion in a shear flow field
In addition to dispersion in a simple uniform flow field, there
have been attempts to understand the behaviour of particles,
which are placed in a simple shear flow field[24,48,61,62], as illus-
trated inFig. 3.
Bagster and Tomi[48]analysed the stress induced on a spheri-
cal particle in a shear flow field and reported that the shear stress,
s, acting on a sphere is at its maximum in the central plane, the va-lue of which is expressed as follows:
s 8:5lc 21
where c is the velocity gradient acting across the sphere. This ap-
proach is obviously limited to simple systems, and a more extensive
analysis is necessary for dense phase powder dispersion.
With respect to two particles forming a doublet, Kousaka et al.
[24]discussed theoretically the dispersion in a shear flow field. It is
found that when assuming particleBis sufficiently larger than par-
ticleA, a bending moment is induced, rc, which is approximated
using the following equation:
rc93lcpc3
22
where cis the ratio of the diameter of the circular contact area tothe diameter of the particle, and is dependent on the contact config-
uration. In general, it is seen thatchas a value much less than unity
and as a result the bending moment induced is greater than the
shear stress, suggesting that the bending moment is dominant in
a shear flow field. Furthermore, it is suggested that the stress on
small particles in a shear flow field is less important than that of
acceleration in a uniform flow.
3.2.3. Dispersion by impaction
The breakage of agglomerates due to impaction has been exten-
sively investigated due to agglomerate strength being important in
a vast number of manufactured products in the chemical, pharma-
ceutical and food industries [4954,6365]. However, little atten-
tion has been made to aerodynamic dispersion involvingimpaction.
Particles in a fluid flow will collide with each other and walls,
depending upon their inertia. As expected, if the stress induced
by an impact is greater than the strength of the loose aggregates
they will deform, break or disintegrate aiding dispersion. Kousaka
et al. [24] investigated the stress induced and the probability of
impaction in a fluid flow field. Assuming the particle is spherical,
the compressive stress,ri, at the plane through the particle centre
is expressed as follows:
ri2
3qpDpag
vi
Dt
23
where viis the particle impact velocity and Dtis the duration of the
impact. If the stress given by Eq. (23)is assumed to be much largerthan the strength of the aggregates because of a sufficiently large
value of vi/Dt, the breakage of the particle cluster depends upon
the probability of impact with the obstacle, which is given as, gT
gT fWc where WcqpurD
2pag
18lds24
where Wcis the inertia parameter anddsis the diameter of the im-
pact obstacle. Suppose a certain value ofWcwhen the target colli-
sion efficiency is 50%, i.e. half of the clusters impacting upon the
obstacle; the particle diameter, DpW, is given as follows:
DpW
18ldsWcqpur
!12
25
If the particles, half of which impact during dispersion, are observed
experimentally, the diameter of the dispersed particles is repre-
sented by Eq.(25). It is shown that the impaction force, the com-
pression stress, and the inertia parameter increase with the
diameter of the loosely aggregated particles. Therefore, indepen-
dent of the structure of the cluster, dispersion by impaction is effec-
tive for larger particle clusters.
3.2.4. Reduction of interparticle attraction force
The reduction of interparticle attraction is an approach which
can be beneficial for the dispersion of dry powders. Techniques in-
clude powder coating[45,66,67], spacer or glidant particles[35,68]
or the addition of secondary material [23,6971].
Particle coatings can be used to reduce the Hamaker constant
and consequently reduce van der Waals attraction between parti-
cles. Spacer or glidant particles are also used within industry to im-
prove the flow properties of powders. Tedeschi et al. [68]reported
improved dispersion performance of 1545 nm thin aluminium
flakes when mixed with fumed silica. The addition of the fumed sil-
ica increased the interparticle spacing, thus reducing the van der
Waals forces between the flakes. The silica was shown to have
maximum effectiveness at 4 wt%.DPI formulations usually contain an exipient with a common
powder being fine lactose. It has been shown that an increase in
dispersion is possible when lactose is added to a DPI formulation,
due to lactose having a lower interparticle attraction[69]. The abil-
ity to improve powder dispersibility without additional material
but by increasing surface roughness has also been investigated
[23]. The asperities reduce particle contact area therefore reducing
cohesion/adhesion. The advantage of these methods is that the
choice of inhaler and required entrainment flowrate are less criti-
cal. Chew et al. [70] quantified the degree of surface corrugation
with powder dispersion by the surface fractal dimension (Ds) ob-
tained by light scattering and showed that increasing Ds slightly
from 2.06 to 2.18 enhanced the FPF from 27% to 41%. This is ex-
plained through an increase in particle separation and a decrease
in contact area. However, a further increase in Ds did not improve
the FPF.
The addition of ternary components within dry powder aerosols
has been analysed [71]. The fine particle fraction (FPF) from ternary
mixtures was dependent on carrier type, ternary concentration,
and ternary component type. Ternary mixtures produced higher
FPF than binary mixtures, except those containing superfine pow-
der. They suggested that the carrier particles have sites of varying
degrees of adhesion.
Masuda and Gotoh[45]evaluated the effect of treating powders
with ethanol vapour and used the previously discussed re-entrain-
ment test. By treating powders with ethanol vapour for 12 h, a sig-
nificant decrease in cohesion was observed, accompanied with a
small increase in dispersion performance. However, treating pow-
ders with ethanol vapour is time dependent with powders revers-ing back to their original state when exposed to air.
Further powder engineering approaches are available in the lit-
erature, which are designed to lower powder attraction, including
needle crystals[72], AIRTM particles[73], aerogel powders[74]and
spray-freeze dried-particles[75].
4. Technology overview
Dry powder dispersion is widely used for many technological
applications, and in this section the available devices are intro-
duced and discussed. The focus is upon devices used within re-
search and commercial applications for particle characterisation.
A comprehensive review of the DPI market is already available[17,76]. Ashurst et al. [76] concluded that the DPI market is full
10 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
8/13
of creative designs yet no design seems to fulfil all the require-
ments of an ideal DPI. Recently, Chan[77]discussed the critical is-
sues associated with the development of a dry powder inhaler
delivery system. It is suggested that a greater understanding of sur-
face phenomena with regards to powder production methods, for-
mulation properties, and powder dispersion is needed to improve
DPI performance.
4.1. Typical dispersion devices
Many different types of dispersers are available [78] and can
be classified according to the main dispersion mechanisms which
have been previously discussed. Typical devices are listed in
Table 1. The dominant dispersion mechanisms of each are also
given.
4.1.1. Eductor disperser
A typical eductor disperser, shown inTable 1(a), has two feed
inlets, one provides pressurised air and the second contains the
powder to be dispersed. The powder may enter under the influence
of various forces such as gravity or an existing pressure difference
between the two inlets forcing the powder towards the dispersionregion. Dispersion is intended to occur at the throat of the device,
where the pressurised air and the powder feed meet with the dom-
inant dispersion force being rapid acceleration and/or shear flow.
Additionally, particleparticle and particlewall collisions are
likely to aid the dispersion process as shown in Fig. 4. In contrast
to the eductor depicted inTable 1,the pressurised air and powder
feed may be reversed.
In the literature, a number of studies have investigated the
eductor disperser and its ability to disperse fine powders. Several
of these investigations [45,58,59,79] have used the eductor de-
signed by Nisshin Flour Milling Co. [80]. Kousaka et al. [58]used
this particular device to disperse aggregate polystyrene latex par-
ticles with diameters of 2 and 5.2 lm. The experimental results
suggested that particles consisting of 5 lm primary particles were
almost entirely dispersed by acceleration. Furthermore, Masuda
and Gotoh[45]have demonstrated that the Nisshin Flour Milling
Co. eductor[80]performs approximately the same as a mixer-type
disperser; seeTable 1(f) and Section4.1.8, so long as the mass flow
ratio is kept constant.
Endo et al. [59] investigated the dispersion of fine powders
(around 1 lm) using an eductor disperser [80] and attempted to
relate dispersion performance to powder cohesion. The aggregated
particles were sucked into the eductor through a stainless steel
tube. At the eductor throat, the powder is dispersed by using high
pressure (0.310 MPa) nitrogen gas. The dispersed particles were
collected by gravitational settling and in each experiment 500
1000 particles were examined using optical or electron micros-
copy. A dimensionless dispersibility parameter (DS) defined as
the ratio of the generated dispersion force (Eq. (20)) to the inter-particle attraction force (Eq. (2)) was used to predict powder
dispersion
DS Fd2Fad
26
Using Eqs. (2) and (20), numerical simulations of aggregates of poly-
disperse spheres have been calculated, and are compared with
experimental observations[59]. For some powders including mono-
disperse and polydisperse latex particles, SiC and TiO2, the experi-
mental results were in good agreement with the simulated data.
This suggests that the theoretical dispersion and relative cohesionforce are adequate tools for characterising the dispersion state of
these powders. On the contrary, for Al2O3and CaCO3particles there
was little correlation. Endo et al. [59] suggest that the cohesion
force is greater for Al2O3 and CaCO3than the value calculated on
the assumption of spherical particle shape. This is supported with
scanning electron micrographs showing an increase in contact area
between adjacent particles[59].
Recently, Tang et al. [81] investigated a simple, cost effective
eductor as a device for dispersing powders. The eductor was con-
nected to the Malvern Instruments Mastersizer 2000 laser diffrac-
tion system for particle size measurement. A comparison was
made between the performance of this device and the small-scale
powder disperser (SSPD) model 3433 (TSI, Shoreview, USA) and the
Scirocco dry powder disperser (Malvern Instruments, Malvern,UK). The eductor introduced by Tang et al. [81] worked as effi-
ciently as the two commercial devices, and furthermore introduced
the possibility of using swirl flow as an approach for aiding powder
dispersion.
4.1.2. Venturi disperser
A standard Venturi disperser uses converging and expanding
flows, and the powder may already be entrained within the air flow
or enter at the secondary inlet (Table 1(b)). Kousaka et al.[24]had
previously shown that the dominant dispersion mechanism in this
type of disperser is acceleration or deceleration, which can be fairly
effective as long as the relative velocity between particle and fluid
is large. However, Kousaka et al.[24]report that with a Venturi de-
vice it is difficult to obtain a large relative velocity, which may ac-count for the lack of further investigation.
4.1.3. Nozzle disperser
A typical nozzle disperser contains a particle laden pipe with
pressurised air as shown in Table 1(c). As a result, the particles
are already at a relatively high velocity before the pipe diameter
is reduced to accelerate the air stream and to disperse aggregates
even further. This device is referred to as a typical disperser device
[55]; however, there appears to be limited information confirming
or debating its dispersion performance ability. As illustrated in
Table 1(g), it is possible for nozzle dispersers to be used in conjunc-
tion with a secondary dispersion step such as an impaction plate.
4.1.4. Capillary tube disperser
A capillary tube disperser, illustrated inTable 1(d), is generally
regarded as using a velocity gradient acting across the cross section
of the device to shear powders promoting dispersion. Initially, one
Fig. 4. Particleparticle and particlewall collisions.
G. Calvert et al. / Advanced Powder Technology 20 (2009) 416 11
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
9/13
might think that the capillary tube disperser would be dominated
by shear forces. However, it is suggested that dispersion occurs due
to impaction as the powder enters the capillary tube. This was
investigated by changing the length of the capillary tube disperser
from 0.06 to 0.5 m, which had no effect on particle size[24]. This
implies that the surviving particle clusters do not disperse further
along the tube length by the prevailing shear. A possible explana-
tion could be that the particles tend towards the centre of the pipewhere shear is minimal.
Yamamoto et al. [83,84] investigated capillary tube dispersers of
various diameters. In the range of 3080% relative humidity they
showed that the environmental conditions can have a large impact
upon certain, but not all, materials. This is an area of dispersion
which needs a greater understanding. In agreement with that ob-
served by Kousaka et al.[24], the dispersed particles reach an equi-
librium size distribution over a short distance, approximately 5 mm
in a capillarytube which has a total length of 500 mm. Additionally,
an empirical relationship was found between the median particle
diameter and the specific dissipated power of the air stream at
thevena contracta,ed, which is related to the pressure drop, DP
Dp50 15e0:2d
ed DP
u2:5d
27
whereDp50is the median particle diameter, d is the diameter of the
capillary tube, and u is the average air velocity. Eq.(27)can be re-
lated to the dispersion performance of different materials in a cap-
illary disperser, i.e. the required power necessary for effective
dispersion.
A new dispersing approach from those shown in Table 1 has
been developed by Kousaka et al.[85,86], which was termed boil-
ing method. After suspending a powder consisting of submicron
particles in pure water or Fluorinert, the suspension is continu-
ously heated and boiled in a capillary tube to generate aerosol par-
ticles dispersed into primary particles. The vapourised liquid is
separated from the particles by a cooling condenser thereby pro-
ducing dry particles dispersed in air.
4.1.5. Orifice disperser
An orifice disperser is a device which adopts a sudden reduction
followed by a sudden increase in cross sectional diameter produc-
ing rapid acceleration and deceleration; furthermore, impaction
may occur upon the constricting walls of the orifice, see Table
1(e). Kousaka et al. [24] categorised the dominating dispersion
mechanism as impaction in this device.
Yamamoto and Suganuma [87] applied the dissipated power
approach developed for the capillary tube disperser to an orifice
disperser. For all materials investigated, a relationship between
the degree of dispersion and the intensity of the air stream through
the orifice was found
Dp50
Dp50s 31:3e0:2d
ed 0:4DPu
d0
28
where Dp50s is the mass median diameter of primary particles and
d0 is the diameter of the orifice. The dissipated power is related to
the air flow rate, Q0, using the following:
ed 2:7 106Q
30
d70
29
Eqs.(28) and (29)are used to determine the dissipated power nec-
essary to fully disperse the powder (i.e. Dp50/Dp50s= 1), based on
which the required air flow rate can be calculated. However, this
may not be the case for all types of powder due to differences in
their individual material properties. Hence, such an approach needsto include the influence of cohesion, shape, etc.
4.1.6. Mixer-type disperser
As shown inTable 1(f), a mixer-type disperser can be used to
generate different dispersion mechanisms. Kousaka et al. [24]ob-
served that the dominant dispersion mechanism is dependent
upon how the powder is introduced to the system. If the powder
is already reasonably well dispersed as a particle cloud, then accel-
eration is deemed as the dominating mechanism; however, if the
powder enters as aggregated clusters then inertial impaction onthe rotating blades is likely to be dominant. Usually, the dispersed
powder exits via an outlet tube attached tangentially at the top of
the mixer vessel. It has been shown that a great advantage of the
mixer disperser is concentration control of the exiting particles
[43]. As a result of this, the mixer-type disperser has been studied
in detail[8992].
Masuda and Gotoh [93] evaluated different mixer-type dis-
perser sizes to understand the effects of disperser scale on the dis-
charge air flow rate and the stable operation range, which is related
to the deposition of particles. The discharged air flow rate is di-
rectly proportional to the impeller rotational speed and the dis-
charge coefficient is nearly proportional to the 5/3 power of the
impeller length. A semi-empirical equation was derived for deter-
mining the mass median diameter from a mixer-type disperser
Dp50 7:0 102 1:65 107W2:5n2 Dp50s 30
wheren is the impeller rotational speed andWis the powder flow
rate. Eq.(30)provides a useful tool for defining the optimal rota-
tional speed, where the minimum mass median diameter is ob-
tained at a constant concentration and a good correlation
between experimental data and Eq.(30)has been observed[93].
4.1.7. Impact disperser
Table 1(g) shows a typical impact disperser having particles
which are entrained within an air flow and pass through a nozzle
subsequently colliding with an obstacle that is perpendicular to
the flow. As the particles are accelerated through a nozzle, some
dispersion may occur but the main dispersion mechanism is
impaction upon the plate. It has been suggested that this particulardispersion arrangement is the most aggressive, even being de-
scribed as a crusher [55].Additionally, a typical impact disperser
may provide some dispersion in the wake region due to particle
particle collisions as observed by Gomes and Vincent [82].
Kousaka et al.[24]investigated a novel approach for impact dis-
persion using eight wire screens, and concluded that dispersion
dominated by impaction was the most efficient mechanism. This
device has been tested in the relative humidity range 40100%.
There is very little difference in the dispersed particle size over this
range of environmental conditions. It is demonstrated that the size
reduction by impact can be estimated from the existing theory of
the target efficiency of a particle in an air stream (Eq. (25)) [24].
Pressurised
air inlet
Powder inlet
Dispersed powder
Fig. 5. Malvern Scirocco dry powder dispersion device[95].
12 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
10/13
On the other hand, impaction may be regarded as an aggressive
dispersion approach that could result in primary particle breakage
if the material is fragile.
4.1.8. Fluidised bed disperser
Fluidised beds are used in many applications such as drying of a
powder bed or gas cleaning [88], and for dispersing particles. Astandard fluidised bed disperser, as shown inTable 1(h), generates
forces through impaction and, less desirably, attrition by neigh-
bouring fluidising particles. Cohesive powders can be fluidised by
using large fluidising particles, dispersion aids, which mix the
powder and collide with cohered material freeing the primary par-
ticles to be subsequently entrained in the fluid flow.
Iinoya and Masuda [43] have shown that effective dispersion
with a fluidised bed is achievable through the use of dispersion
aids. An important parameter is the quantity of dispersion aids
needed to optimise performance. It is seen that the mass median
particle diameter is typically smaller when the dispersion aid bed
height is between 0.01 and 0.02 m. When the bed height is optimal,
the performance of the fluidised bed disperser is comparable to a
mixer-type disperser investigated in this study [43]. However,
the upper limit of gas velocity is the point when the dispersion aids
are entrained. If this is the case, denser dispersion aids may be
used, but the effect of various dispersion aids has not been
documented.
4.2. Commercial devices
A wide variety of commercial dispersion devices are available
for the process of particle characterisation. Most systems that areemployed for dispersion consist of a constant rate feeder and an
entrainment apparatus. A typical example would be the Wright
dust feeder[94]. In this device, the powder is placed within a rotat-
ing dust cylinder which allows for the powder bed to be scraped
and entrained into a carrier gas with any large particle clusters
being dispersed by impaction. Usually, the details of commercial
devices are not always fully disclosed, and there is little work in
the literature comparing the performance of different designs.
However, it does seem that commercial devices incorporate differ-
ent dispersion mechanisms in order to disperse a wide range of
powders more efficiently.
Dispersion in the dry feeder, known as the Scirocco, coupled
with the Malvern Instruments Mastersizer 2000 ( Malvern, Worcs,
UK) laser diffraction instrument is achieved by accelerating pow-
ders in a vertical eductor arrangement (Fig. 5). Dispersion occurs
due to great particle acceleration along with shear forces and par-
Powder feeding
region
Powder dispersion
region
Dispersedpowder
Fig. 6. Sympatec Rodos dry powder dispersion device.
Dispersed
powder
Powder bed
Pressurisedair
Shear
region
Fig. 7. The Aero-Disperser used in the AerosizerTM [100].
G. Calvert et al. / Advanced Powder Technology 20 (2009) 416 13
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
11/13
ticleparticle and particlewall collisions, especially at the right
angle region downstream.
The air inlet pressure can be varied from 0.1 to 4.0 bar to deter-
mine the optimum pressure that can disperse the powder without
damaging individual particles. It is possible to calibrate the ideal
dispersing pressure through a comparison with wet dispersion
particle size analysis.
Sympatec GmbH (Zellerfeld, Germany) produces a dry powderdisperser called Rodos[96](Fig. 6). This device is specified as being
capable of dispersing particles down to 0.1 lm[97]. Depending on
the degree of powder cohesion, it is possible to change the dosing
method to the device. For relatively free-flowing materials, a vibra-
tory feeder and an inlet funnel connected to the dosing line are
used; however, for extremely fine or cohesive materials a rotating
table having a grove with a compacting roller is recommended
(Fig. 6). The powder is dispersed by using pressurised air in order
to accelerate the particles as soon as they arrive in the dispersing
line [1]. The pressure of the air feed can be varied from 0.1 to
6.0 bar to achieve a fully dispersed state. Furthermore, there are
different dispersing lines available (4, 6, and 10 mm) to enhance
the dispersion performance of more cohesive materials.
The AerosizerTM LD (Amherst Process Instruments, Hadley, MA) is
a time-of-flight measurement device capable of measuring the size
of particles in the range of 0.2700 lm. The Aero-DisperserTM used
in conjunction with the AerosizerTM analyser uses pulsed jet tech-
nology to lift the sample from the sample holder and then applies
high shear flow to complete dispersion down to the primary parti-
cles(Fig.7) [3]. There are four controllable variablesavailable to im-
prove dispersion performance of various materials, including shear
force, feed rate, transport velocity from the fluidised bed to the dis-
perser pin, and pin vibration[98]. This device has been shown to
disperse cohesive material without evidence of attrition [99].
Beckman Coulter GmbH (Krefeld, Germany) produces a dry
powder disperser called the TornadoTM [101], which breaks up a
powder by using a swirling air flow, sucked through a ring gap
above the sample via rotation of the sample container support,
leading to strong impaction with controllable intensities [102].Again pressure optimisation (0.14.0 bar) is recommended to find
the ideal pressure (Fig. 8).
The dispersing unit manufactured by Fritsch GmbH (Oberstein,
Germany) uses mechanical and pneumatic forces to disperse pow-
ders [103]. An amplitude-controlled vibratory feeder is used for
material dosing and the dispersion occurs in a two-phase annular
gap nozzle, where powders are accelerated at a high flow rate. The
default pressure is set at 3 bar with a maximum of 4 bar; however,
it can be reduced to 1 bar for fragile materials to avoid attrition.
The dispersing device manufactured by Horiba Instruments(Japan), PowderJet II, is a coaxial vertical eductor [104]designed
Pressurised
air inlet
Powder inlet
riAriA
Laser light
Vacuum suction
Fig. 9. The Horiba Instruments PowderJet II device[105].
Non-pressu
rised ambientair enters the space
between the suction probeand the sample holder and
forms a downward flow
High shear force generated
by a change in air velocity
direction
Dry powder bed
Dispersed dry powder
delivered to a particle
analyser via vacuum
Vortices created by tapered
lower end of the suction probecreating low energy particle to
wall and particle to particle
collisions
Fig. 8. The basic principle of the Tornado dry powder system [101].
Nominal isokinetic
sample removal
Powder sampleRotating table
Venturi throat
Exhaust to filter
Capillary tube
Dispersed powder output
Fig. 10. The TSI small-scale powder disperser.
14 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
12/13
to provide a straight particle flow path to minimise physical impact
of the particles (Fig. 9). The disperser has the option of three differ-
ent nozzles, which can be used for dispersing various materials: a
larger nozzle for less cohesive larger materials; an intermediate
nozzle; and a small nozzle to be used with fine cohesive materials.
It is suggested that the smaller nozzle can disperse fine titanium
dioxide down to 0.1 lm[105].
The small-scale powder disperser (SSPD) manufactured by TSI(Shoreview, USA) is depicted inFig. 10.The powder to be dispersed
is placed over the surface of one of the three annular rings of abra-
sive paper, which in turn is glued to the top of a rotating table.
Powder is removed from the rotating table by suction through a
Venturi aspirator and capillary delivery tube. A region of low pres-
sure is created by the increased velocity of air through the Venturi
throat, which draws the fluid up through the capillary tube. The
device has been found to work well under the following operating
conditions: flowrate through the Venturi throat 16.5 L/min;
velocity through the Venturi throat 310 m/s; flowrate through
the capillary delivery tube 2.0 L/min; and velocity through the
capillary delivery tube 32.5 m/s[106]. An evaluation of the SSPD
can be found in the literature[107].
5. Conclusions and recommendations
The ability to completely disperse a wide range of powders from
cohered clustersto their primaryconstituentsis of great importance
for many reasons, be it to fully characterise a powder or delivery
therapeutic drugs to the lung. There is still a need to further under-
stand how bulk powders interact with a dispersing fluid. There are
modelsfor therelatively simple systems, butlittle is known regard-
ingcomplex fluid flows with bulk solid.In addition,there appears to
be little understanding of the limit of interparticle cohesion, for
which acceleration and/or shear flow is no longer effective.
A number of methods are available for characterising bulk pow-
der cohesion such as tensile test, shear test, or the re-entrainment
test. However, there is still little evidence confirming, whetherthese approaches are reliable tools for determining dispersion per-
formance. In addition, there is little understanding that links bulk
powder properties and aerodynamic dispersion performance.
A large number of dispersion devices are available and a num-
ber of investigations have been conducted evaluating their perfor-
mance. It is seen that dispersion by impaction is the most efficient
mechanism; however, this may not be the case with all powders,
particularly if weak or friable. Furthermore, a great deal of atten-
tion has been given to the impeller mixer-type disperser due to
ability to control the particle concentration leaving the disperser.
The performance of an eductor disperser is similar to the mixer
type when the mass flow ratio is the same.
A relatively untouched area is the use of computational ap-
proaches to further understand dispersion behaviour. New devel-opments in coupling the distinct element method (DEM) with
computational fluid dynamics (CFD) enables a more fundamental
study to be carried out.
Acknowledgement
The authors gratefully acknowledge the financial support from
the Engineering and Physical Sciences Research Council (EPSRC)
and Malvern Instruments.
References
[1] K. Leschonski, S. Rothele, U. Menzel, A special feeder for diffraction patternanalysis of dry powders, Part. Charact. 1 (1984) 161166.
[2] J.P. Mitchell, M.W. Nagel, Time-of-flight aerodynamic particle size analysers:their use and limitations for the evaluation of medical aerosols, J. Aerosol.Med. 12 (4) (1999) 217240.
[3] T.B. Fields, Aerodynamic time-of-flight particle size measurement, J. Dispers.Sci.Technol. 23 (5) (2002) 729735.
[4] T. Allen, Particle Size Measurement, fifth ed., Chapman & Hall, London, 1997.[5] G.D. Parftitt, The dispersion of powders in liquids an introduction, Powder
Technol. 17 (1977) 157162.[6] G.K. Batchelor, J.T. Green, The hydrodynamic interaction of two small
freely-moving spheres in a linear flow field, J. Fluid Mech. 56 (1972) 375
400.[7] R.C. Sonntag, W.B. Russel, Structure and breakup of flocs subjected to fluid
stresses I. Shear experiments, J. Colloid Interface Sci. 113 (1986) 399413.[8] R.C. Sonntag, W.B. Russel, Structure and breakup of flocs subjected to fluid
stresses III. Converging flow, J. Colloid Interface Sci. 115 (1987) 390395.[9] T.G.M. van der Ven, in: Colloidal Hydrodynamics, Academic Press, London,
1989.[10] K. Higashitani, N. Inada, T. Ochi, Floc breakup along centreline of contractile
flow to orifice, Colloids Surf., A 56 (1991) 1323.[11] K. Higashitani, K. Yoshida, N. Tanise, H. Murata, Dispersion of coagulated by
ultrasonication, Colloids Surf., A 81 (1993) 167175.[12] K. Higashitani, K. Iimura, H. Sanda, Simulation deformation and breakup of
large aggregates in flows of viscous fluids, Chem. Eng. Sci. 56 (2001) 29272938.
[13] M. Fanelli, D.L. Feke, I. Manas-Zloczower, Prediction of the dispersion ofparticle clusters in the nanoscale Part I: steady shearing responses, Chem.Eng. Sci. 61 (2006) 473488.
[14] M. Fanelli, D.L. Feke, I. Manas-Zloczower, Prediction of the dispersion ofparticle clusters in the nanoscale Part II: unsteady shearing responses,Chem. Eng. Sci. 61 (2006) 49444956.
[15] A.P. Tinke, K. Vanhoutte, F. Vanhoutte, M. De Smet, H. De Winter, Laserdiffraction and image analysis as a supportive analytical tool in thepharmaceutical development of immediate release direct compressionformulations, Int. J. Pharm. 297 (2005) 8088.
[16] J. Dodds, G. Rasteiro, B. Scarlett, R. Weichert, R. Williams, From particulatesize analysis (PSA 1970) to particulate systems analysis (PSA 2003), Chem.Eng. Res. Des. 82 (A12) (2004) 15331540.
[17] I.J. Smith, M. Parry-Billings, The inhalers of the future? A review of drypowder devices on the market today, Pulm. Pharmacol. Ther. 16 (2) (2003)7995.
[18] H. Steckel, B.W. Muller, In vitro evaluation of dry powder inhalers I: drugdeposition of commonly used devices, Int. J. Pharm. 154 (1997) 1929.
[19] T. Srichana, G.P. Martin, C. Marriott, Dry powder inhalers: the influence ofdevice resistance and powder formulation on drug and lactose depositionin vitro, Eur. J. Pharm. Sci. 7 (1998) 7380.
[20] D. Geldart, Types of gas fluidization, Powder Technol. 7 (5) (1973) 285292.[21] J. Visser, Van der Waals and other cohesive forces affecting powder
fluidization, Powder Technol. 58 (1) (1989) 110.[22] M. Corn, in: C.N. Davis (Ed.), Aerosol Science, Academic Press, London, 1966,
pp. 359390.[23] N.Y.K. Chew, H.K. Chan, Use of solid corrugated particles to enhance powder
aerosol performance, Pharm. Res. 18 (11) (2001) 15701577.[24] Y. Kousaka, K. Okuyama, A. Shimizu, T. Yoshida, Dispersion mechanism of
aggregate particles in air, J. Chem. Eng. Jpn. 12 (2) (1979) 152159.[25] S.J.R. Simons, Modeling of agglomerating systems: from spheres to fractals,
Powder Technol. 87 (1) (1996) 2941.[26] H.C. Hamaker, The London-van der Waals attraction between spherical
particles, Physica (The Hague) 4 (1937) 10581072.[27] K.L. Johnson, K. Kendall, A.D. Roberts, Surface energy and the contact of elastic
solid, Proc. R. Soc. Lond. A 324 (1971) 301313.[28] J.N. Israelachvili, Intermolecular and Surface Forces: With Applications to
Colloidal and Biological Systems, 1985.[29] C.U. Yurteri, M.K. Mazumder, N. Grable, G. Ahuja, S. Trigwell, A.S. Biris, R.
Sharma, R.A. Sims, Electrostatic effects on dispersion, transport, anddeposition of fine pharmaceutical powders: development of anexperimental method for quantitative analysis, Part. Sci. Technol. 20 (1)
(2002) 5979.[30] M.K. Mazumder, C.U. Yurteri, G. Ahuja, N. Grable, H.K. Chan, N. Chew, Bipolar
electrostatic charging of fine pharmaceutical powders by dispersion andtransport processes and its implications to lung deposition of inhaled DPIaerosols, Respiratory Drug Delivery VIII, Tucson, Arizona, vol. II, 2002, pp.667670.
[31] S. Matsusaka, H. Masuda, Electrostatics of particles, Adv. Powder Technol. 14(2) (2003) 143166.
[32] H. Rumpf, Grundlagen und Methoden des Granulierens, Chem. Ing. Tech. 30(3) (1958) 144158.
[33] H. Rumpf, Zur Theorie der Zugfestigkeit von Agglomeraten beiKraftbertragung an Kontaktpunkten, Chem. Ing. Tech. 42 (8) (1970) 538540.
[34] H. Rumpf, K. Sommer, K. Steier, Mechanismen der Haftkraftverstrkung beider Partikelhaftung durch plastisches Verformen, Sintern undviskoelastisches Flieen, Chem. Ing. Tech. 48 (4) (1976) 300307.
[35] K. Meyer, I. Zimmermann, Effect of glidants in binary powder mixtures,Powder Technol. 139 (2004) 4054.
[36] M.D. Ashton, R. Farley, F.H.H. Valentin, An improved apparatus for measuringthe tensile strength of powders, J. Sci. Instrum. 41 (1964) 763765.
G. Calvert et al. / Advanced Powder Technology 20 (2009) 416 15
-
7/26/2019 X Aerodynamic dispersion of cohesive powders.pdf
13/13
[37] M.D. Ashton, D.C. H. Cheng, R. Farley, F.H.H. Valentin, Some investigationsinto the strength and flow properties of powders, Rheolog. Acta 4 (1965) 206218.
[38] T. Eaves, T.M. Jones, Cohesion and tensile strength of bulk solids, Rheolog.Acta 10 (1971) 127134.
[39] A. Schweiger, I. Zimmermann, A new approach for the measurement of thetensile strength of powders, Powder Technol. 101 (1999) 715.
[40] J.M. Valverde, A. Castellanos, A. Ramos, A. Prez, M.A. Morgan, P.K. Watson,Rev. Sci. Instrum. 71 (7) (2000) 27912795.
[41] A. Castellanos, J.M. Valverde, M.A.S. Quintanilla, The sevilla powder tester: a
tool for characterizing the physical properties of fine cohesive powders atvery small consolidations, KONA 22 (2004) 6681.
[42] J. Schwedes, Review on testers for measuring flow properties of bulk solids,Granul. Matter 5 (2003) 143.
[43] K Iinoya, H. Masuda, in: K. Willeke (Ed.), Generation of Aerosols, Ann ArborScience, Ann Arbor, 1980, pp. 189202.
[44] H. Masuda, S. Matsusaka, S. Ikumi, Particle re-entrainment from powder bedof fine particles in a rectangular air-flow channel, Kagaku Kogaku Ronbun. 11(1985) 4854.
[45] H. Masuda, K. Gotoh, Performance evaluation of dry dispersers, Adv. PowderTechnol. 6 (4) (1995) 305315.
[46] K. Ridgway, K.J. Tarbuck, The random packing of spheres, Br. Chem. Eng. 12(1967) 384388.
[47] C. Weiler, M. Wolkenhauer, M. Trunk, P. Langguth, Dry powderdeagglomeration of spherical, submicron particles, in: PARTEC 2007Conference, 2007.
[48] D.F. Bagster, D. Tomi, Stresses within a sphere in simple flow fields, Chem.Eng. Sci. 29 (8) (1974) 17731783.
[49] K.D. Kafui, C. Thornton, Computer simulated impact of agglomerate, in: C.Thornton (Ed.), Powders & Grains 93, The Proceedings of the SecondInternational Conference on Micromechanics of Granular Media. A.A.Balkema, Rotterdam, 1993, pp. 401406.
[50] C. Thornton, K.K. Yin, M.J. Adams, Numerical simulation of the impact fractureand fragmentation of agglomerates, J. Phys. D: Appl. Phys. 29 (1996) 424435.
[51] C. Thornton, M.T. Ciomocos, M.J. Adams, Numerical simulations ofagglomerate impact breakage, Powder Technol. 105 (1999) 7482.
[52] J. Subero, Z. Ning, M. Ghadiri, C. Thornton, Effect of interface energy on theimpact strength of agglomerates, Powder Technol. 105 (1999) 6673.
[53] R. Moreno, Computer simulation of impact of spherical agglomerates usingdistinct element method, Ph.D. Thesis, University of Surrey, Guildford, UK,2003.
[54] R. Moreno-Atanasio, M. Ghadiri, Mechanistic analysis and computersimulation of impact breakage of aglomerates: effect of surface energy,Chem. Eng. Sci. 61 (2006) 24762481.
[55] K. Gotoh, H. Masuda, K. Higashitani, in: H. Masuda, K. Higashitani, H. Yoshida(Eds.), Powder Technology Handbook, CRC/Tayler and Francis, 2006.
[56] M.B. Ranade, S.T. Tedeschi, K. Powers, H. El Shall, Gas dispersion of ultrafine
powders, in: AIChE Spring National Meeting, Conference Proceedings,Orlando, FL, United States, April 2327 2006.
[57] S. Yuu, T. Oda, Disruption mechanism of aggregate aerosol particles throughan orifice, AIChE J. 29 (2) (1983) 191198.
[58] Y. Kousaka, Y. Endo, T. Horiuchi, T. Niida, Dispersion of aggregate particles byacceleration in air stream, Kagaku Kogaku Ronbun. 18 (2) (1992) 233239.
[59] Y. Endo, S. Hasebe, Y. Kousaka, Dispersion of aggregates of fine powder byacceleration in an air stream and its application to the evaluation of adhesionbetween particles, Powder Technol. 91 (1) (1997) 2530.
[60] L.D. Reed, F.A. Morrison, The slow motion of two touching fluid spheres alongtheir line of centers, Int. J. Multiphase Flow 1 (1974) 573584.
[61] S.L. Goren, The hydrodynamic forces on touching spheres along the line ofcenters exerted by a shear field, J. Colloid Interface Sci. 36 (1) (1971) 9496.
[62] Z. Tadmor, Forces in dispersive mixing, Ind. Eng. Chem. Fundam. 15 (4) (1976)346348.
[63] H. Rumpf, The strength of granules and agglomerates, in: W.A. Knepper (Ed.),Agglomeration-Proceedings of the First International Symposium onAgglomeration, Philadelphia, 1962, pp. 379418.
[64] K.J. Kendall, Agglomerate strength, Powder Metall. 31 (1988) 2831.
[65] R. Moreno, M. Ghadiri, S.J. Antony, Effect of impact angle on the breakage ofagglomerates, Powder Technol. 130 (2003) 132137.
[66] V.A. Philip, R.C. Mehta, P.P. Deluca, M.K. Mazumder, E-SPART analysis of poly(D,L-lactide-co-glycolide) microspheres formulated for dry powder aerosols,Part. Sci. Technol. 15 (3) (1997) 303316.
[67] K. Bechtold-Peters, H. Nguyen, G. Rowley, Surface modification of powderswith fatty acid or fatty alcohol derivatives or poloxamer for inhalation,US2003007932 A1, 2002.
[68] S.T. Tedeschi, K. Powers, M.B. Ranade, H. El Shall, Dispersion of high aspectratio particles in air, in: AIChE Spring National Meeting, ConferenceProceedings, Orlando, FL, United States, April 2327 2006.
[69] X.M. Zeng, G.P. Martin, S.K. Tee, C. Marriott, The role of fine particle lactose onthe dispersion and deaggregation of salbutamol sulfate in an air streamin vitro, Int. J. Pharm. 176 (1) (1998) 99110.
[70] N.Y.K. Chew, P. Tang, H.K. Chan, J. Raper, How much particle surfacecorrugation is sufficient to improve aerosol performance of powders?,Pharm Res. 22 (1) (2005) 148152.
[71] M.D. Louey, P.J. Stewart, Particle interactions involved in aerosol dispersion ofternary interactive mixtures, Pharm. Res. 19 (10) (2002) 15241531.
[72] K. Ikegami, Y. Kawashima, H. Takeuchi, H. Yamamoto, N. Isshiki, D.I. Momose,K. Ouchi, Improved inhalation behavior of steroid KSR-592 in vitro with
Jethaler by polymorphic transformation to needle-like crystals (b-form),Pharm. Res. 19 (10) (2002) 14391445.
[73] R. Vanbever, J.D. Mintzes, J. Wang, J. Nice, D. Chen, R. Batycky, R. Langer, D.A.Edwards, Formulation and physical characterization of large porous particlesfor inhalation, Pharm. Res. 16 (11) (1999) 17351742.
[74] K. Lee, G. Gould, Aerogel powders for inhalation therapy, Int. Appl. 2002
(2002) 0704.[75] Y.F. Maa, P.A. Nguyen, T. Sweeney, S.J. Shire, C.C. Hsu, Protein inhalation
powders: spray drying vs. spray freeze drying, Pharm. Res. 16 (2) (1999) 249254.
[76] I. Ashurst, A. Malton, D. Prime, B. Sumby, Latest advances in the developmentof dry powder inhalers, Pharm. Sci. Technol. Today 3 (7) (2000) 246256.
[77] H. K. Chan, Dry powder aerosol dru delivery opportunities for colloid andsurface scientists, Colloids Surf., A 284-285 (2006) 5055.
[78] R. Dennis, in: Handbook on Aerosols, Technical Information Center, EnergyResearch and Development Administration, Washington, DC, 1976.
[79] Y. Endo, Y. Kousaka, Dispersion of aggregate particles by acceleration in ahigh-pressure air stream, Kagaku Kogaku Ronbunshu 18 (5) (1992) 760763.
[80] Y. Yamada, S. Doi, M. Yasuguchi, Powder dispersers, GB 2142978A (1985).[81] P. Tang, D.F. Fletcher, H.K. Chan, J.A. Raper, Simple and cost-effective powder
disperser for aerosol particle size measurement, Powder Technol. 187 (1)(2008) 2637.
[82] M.S.P. Gomes, J.H. Vincent, The effect of inertia on the dispersion of particlesin the flow around a two-dimensional flat plate, Chem. Eng. Sci. 57 (8) (2002)13191329.
[83] H. Yamamoto, A. Suganuma, D. Kunii, Dispersion of agglomerated finepowder by a high speed air stream, Kagaku Kogaku Ronbunshu 3 (1) (1977)1218.
[84] H. Yamamoto, A. Suganuma, D. Kunii, Dispersion of airborne aggregates byhigh speed air stream, Kagaku Kogaku Ronbunshu 6 (1) (1980) 103105.
[85] Y. Kousaka, T. Horiuchi, Y. Endo, Generation of aerosol particles by boiling ofsuspensions, Aerosol Sci. Technol. 8 (21) (1994) 236240.
[86] Y. Kousaka, Y. Endo, S. Nakai, A new sizing technique for fine powdersdispersed in air, Powder Technol. 100 (1998) 1519.
[87] H. Yamamoto, A. Suganuma, Dispersion of dust aggregates into air by orifice,Kagaku Kogaku Ronbunshu 9 (2) (1983) 183188.
[88] R. Clift, M. Ghadiri, M.J. Cooke, Gas cleaning, US 4475931, 1984.[89] H. Masuda, S. Kawaguchi, K. Gotoh, Effect of powder feed rate and rotational
speed of impeller on the performance of a mixer-type disperser, J. Soc.Powder Technol., Jpn. 27 (1990) 515519.
[90] K. Gotoh, M. Takahashi, H. Masuda, The dispersion mechanism of a mixer-type disperser, J. Soc. Powder Technol. Jpn. 29 (1992) 1117.
[91] K. Gotoh, H. Asaoka, H. Masuda, Effect of impeller shape on the performance
of the mixer-type disperser, Adv. Powder Technol. 5 (4) (1994) 353364.[92] K. Gotoh, T. Yoshida, H. Masuda, Improvement of the mixer-type powder
disperser, Adv. Powder Technol. 5 (4) (1994) 323337.[93] H. Masuda, K. Gotoh, Dry dispersion of fine particles, Colloids Surf., A 109
(1996) 2937.[94] B.M. Wright, A new dust-feeder mechanism, J. Sci. Instrum. 27 (1) (1950) 12
15.[95] P. Rajniak, K. Dhanasekharan, C. Sinka, N. MacPhail, R. Chern, Modeling and
measurement of granule attrition during pneumatic conveying in alaboratory system, Powder Technol. 185 (3) (2008) 202210.
[96] K. Leschonski, S. Rothele, Apparatus for producing a gas solid two phase flowjet having a constant mass or volume flow rate and predetermined velocity,US 4 573 801, 1986.
[97] M. Puckhaber, Increasing the quality of metal powder with qualitativeparticle size analysis, Powder Metall. April (2000) 1617.
[98] J.F. Bohan, Dry powder dispersion system for particle size analysis usingaerodynamic time-of-flight, Powder Handl., Proc. 8 (1) (1996) 5161.
[99] M. Hindle, P.R. Byron, Size distribution control of raw materials for drypowder inhalers using the Aerosizer with the Aero-Disperser, Pharm.
Technol. June (1995) 6478.[100] T.A. Poole, Powder disperser for aerodynamic particle sizing system, US 4 895
034, 1990.[101] T. Breen, B. Schmitz, Non-pressurised dry powder dispensing apparatus, US
6454141 B1, 2002.[102] U. Koenig, A new quality for dispersing powders in particle size analysis, GIT
Lab. Fachz. 47 (3) (2003) 224225.[103] G. Bumcke, Particle size analysis in the laboratory technology and equipment,
Fritsch GmbH: Oberstein, Germany, 1990.[104] T. Yamaguchi, Dry particle size distribution measuring apparatus and
method, EP1319937A1, 2003.[105] Horiba Instruments, Optimization of dry powder particle size analysis,
Pharmaceutical Online, 2008.[106] D.B. Blackford, K.L. Rubow, A small-scale powder disperser, World Congress
Particle Technology 110 (1986) 645655.[107] B.T. Chen, H.C. Yeh, B.J. Fan, Evaluation of the TSI small-scale powder
disperser, J. Aerosol Sci. 8 (1995) 13031313.
16 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416