Dispersing of gases or immiscible liquids within another continuous liquid fluid phase
is a standard operation for the processing of multiphase systems in the industrial
areas like food, cosmetics pharmaceuticals and fine chemicals. The commonly used
dispersing tools are rotor-stator devices with tooth-/pin geometries arranged in
circular, concentric or axial, stacked disc configurations. Depending on the gap width
between the rotor-stator pins or tooth elements and the viscosity function of the twophase
system, laminar, turbulent or transitional flow conditions act in the dispersive
mixing procedure. – A new generation of dispersing devices are the static or dynamic
membrane devices in which the disperse drops or bubbles are formed once and
detached from the membrane surface by cross- or co-flowing fluid streams. This
procedure means significantly reduced mechanical treatment of the multiphase fluid
system and allows to tailor narrow disperse size distributions.
Rotor-Stator Dispersing Processes
Our recent work concerning Rotor-Stator (R/S) dispersing process developments
has mainly focused on laminar to transition domain flow conditions. We have
investigated bubble and drop break-up in some detail under steady and transient
shear or elongation as well as mixed flow conditions. Different types of R/S (model)
flow apparatus were applied comprising concentric  /eccentric  cylinder-, four
roller- , single-  and multi-toothed  geometries. From respective experiments
expanded maps of critical bubble-/drop break-up characteristics (critical Weber
(WeC) or critical Capillary (CaC) numbers as a function of viscosity ratio
λ, deformation rate G, total deformation D and flow type α have been derived.
Figure 1 exemplarily demonstrates CaC(α) for drop break-up under pure shear (α =
0), equibiaxial (α = -1), or planar (α = 1) elongation as well as for mixtures of these
flow types. Steady and transient drop break-up were investigated experimentally (a),
by numerical flow simulation using CFD (b) and by a non- equilibrium
thermodynamics modeling approach (c) /1-3/. Consistent results from these three
approaches (a-c) were received for surfactant free as well as for surfactant covered
drop interfaces.

Fig.1: Critical Ca- number for different flows

Fig.2: Surfactant distribution at drop
interface in shear flow

For surfactant covered drop interfaces a criterion to distinguish between diffusion
and convection driven interfacial coverage with surfactant molecules was defined as
the ratio of Peclet number (Pe) / Capillary number (Ca), denoted as α and
implemented into a convection diffusion equation which forms the bases for
respective CFD calculations. As a result of these calculations surfactant
concentration distributions along the interfacial contour of drops deformed in shear,
elongation and mixed flows were received and satisfying comparability with
experimental drop deformation data was found. Figure 2 shows such calculated
concentration distributions of surfactant at the deformed drop interface for different
Capillary numbers and a viscosity ratio λ of 4.
The impact of transient shear and elongation flows has been investigated within an
eccentric cylinder gap and transferred to a complex multi toothed rotor-stator
dispersing geometry. CFD based simulations applying a particle tracking procedure
along distinct particle flow tracks allowed us to quantify the transient drop
deformation history of selected drops along their paths through the dispersing
apparatus. Comparisons with respective experimental results demonstrated again
good agreement as demonstrated in Figure 3.

Fig. 3: left: Transient deformation and Ca-number; right: multi-toothed R/S geometry
Membrane / Micro-channel Dispersing Processes
In addition to rotor-stator flow devices we considered also channel / nozzle / pore
flows with respect to their dispersive mixing performance. New microfluidics devices
have been developed in our Laboratory at ETH Zürich in close collaboration with the
University of Queensland in Brisbane (Australia); Prof. J. Cooper-White. Within the
lasts two years we investigated drop formation in co-flow and cross-flow micro- and
macro channels. By means of micro particle imaging the velocimetry (Micro-PIV) we
accessed velocity fields around respective drops and used this information for
optimizing the dispersing channel flow geometries and to derive scale up criteria
(e.g. We = f (Re) characteristics) over several orders of magnitude like demonstrated
in figure 4 .
As a scaleable solution with application relevance, derived from micro channel cross
flow results, a Rotating Membrane Device (ROME) with Controlled Pore Distance
(CPD) was developed. The cross flow is generated by the rotational motion of a
membrane cylinder within a surrounding concentric housing through which the
continuous fluid phase is axially pumped. The disperse fluid or gas-phase enters

Fig 4: Micro-/macro channel co-flow dispersing; experimental data / M. Duxenneuner
through a hollow shaft into the rotating cylinder membrane body and forms disperse
liquid droplets or bubbles at the membrane surface, from which the cross flowing
continuous fluid phase flow detaches them as soon as a critical shear stress is
exceeded .

Mixing in agitated tanks represents a fundamental part of
the chemical, petrochemical, pharmaceutical, food and
paper industries. This unit operation is commonly carried
out under steady flow conditions with the impeller
centered with respect to the vessel and rotating at constant
speed in one direction only. Mixing has been a popular
subject for the last years. Most studies have been focused
on power consumption, measurements of mixing times
and visualization of flow patterns. A number of techniques
have been developed for visualizing flow patterns in
stirred vessels. Mavros (2001) reviewed the use of very
sophisticated techniques for observing the flow behaviour
in stirred vessels including particles image velocimetry
(PIV), planar laser induced fluorescence (pLIF) and
thermal pulses, among others.
Mixing operations are usually carried out in some
applications at low to moderate Reynolds number, in
which the turbulent regime cannot be achieved. The
presence of important segregated regions and dead zone
generated with open impellers at low Reynolds number
has been extensively reported in the literature (Solomon et
al., 1981; Lamberto et al., 1996). Such pathologies persist
even if baffles and mixed flow impellers are used resulting
in very long mixing times, high energy consumption and
undesirable by-products due to poor reaction selectivity
can be generated. The most common way to vanish and
eliminate these regions consists of increasing the
rotational speed, however, this leads to an energy
consuming process, long mixing times, and in some cases
it may be detrimental to the final product especially when
mixing shear sensitive media.
It has been demonstrated that the flow structures generated
under steady flow conditions can be destroyed if the flow
is continuously perturbed if the impeller is lightly
displaced from tank centreline in the radial direction as
well as by using time-dependent revolutions. The works
reporting the use of spatial and temporal conditions
promoting chaotic flows in stirred vessels are reviewed in
this paper.
STEADY-STATE FLOW
Mixing operations are usually carried out at low to
moderate Reynolds number. The presence of important
segregated regions and dead zone generated with open
impellers at low Reynolds number has been extensively
reported in the literature (Solomon et al., 1981; Lamberto
et al., 1996). Those pathologies persist even if baffles and
mixed flow impellers are used resulting in very long
mixing times, high energy consumption and undesirable
by-products due to poor reaction selectivity can be
generated. The most common way to enhance mixing by
minimizing the effect of such non-homogeneities consists
of using wide impeller or increasing the rotational speed.
However, the latter option leads to high-energy levels and
in some cases it could be detrimental for materials
sensitive to shear.
Solomon et al (1981) reported for the first time the
observations of well-mixed regions around the impellers
surrounded by stagnant fluid, which are known as caverns
in the case of yield stress fluids. Considering a cavern as a
sphere with center on a Rushton turbine they proposed
empirical expressions for determining the geometrical
characteristics of such flow structures. The observations of
these flow structures were later confirmed by Alvarez et al
(2002) and Ascanio et al (2002).
UNSTEADY-STATE FLOW
The most common way to enhance mixing by minimizing
the effect of flow structures such caverns (well-mixed
regions) and isolated regions (static or quasi-static zones)
consists of using wide impellers or by increasing the
rotational speed. However, the latter option leads to highenergy
levels and in some cases it could be detrimental for
materials sensitive to shear. Therefore, other alternatives
based on the theory of chaos principles have been
developed and reported in the literature. They consist
basically on the use of spatial and temporal scenarios as
described below.

Eccentric impellers
Different alternatives based on pioneering studies on
chaotic laminar mixing have been reported in the
literature, in which it was demonstrated that
homogenization could be quickly achieved by using
eccentric cylinders rotating alternatively in both directions
during short times (Ottino et al 1988; Swanson and Ottino
1990; Muzzio et al 1991; Muzzio et al 1992). One of the
main reasons of using eccentric impellers is that they
provide the best mixing action where a vortex is not
required or not desirable.
Alvarez et al (2002) proposed the use of spatial conditions
based on the radial position of the impeller with respect to
the tank centreline. They showed that a minimal radial
displacement of the impeller can enhance the mixing
performance in terms of the mixing times. Similar results
were found by Ascanio et al (2002) by using different one
or two off-centered impellers in the stirred vessel.
Dynamic perturbations
Lamberto et al 1996 proposed the use of time-dependent
stirring speed to destroy the caverns generated under
steady mixing conditions. They demonstrated that the
mixing of Newtonian fluids could be clearly enhanced if
the fluid dynamics was continuously perturbed. This
method was shown to prevent the formation of segregated
regions close to the impeller, confirming the theoretical
results of Aref (1984). Using time-dependent rotation
directions changes conditions, Ascanio et al (2002)
demonstrated that segregated regions can be gradually
vanished and finally destroyed if the flow is continuously
perturbed.
It is clear from the above contributions based on the
fundamental concepts of chaos theory that nonconventional
mixing strategies appear to be promising to
enhance mixing in stirred tanks in laminar regime. During
the symposium a general overview of the works reporting
scenarios based spatial and temporal conditions will be
given for preventing the formation of coherent segregated
regions in the vicinity of the impeller resulting in energyefficient
processes.

Microencapsulation is a widespread technology that has many applications, like the protection and controlledrelease
of active ingredients in the medical and cosmetics industries, or the fabrication of fragranced fabrics in the
textile industry.
This work focuses on the emulsification step of an interfacial polymerization microencapsulation process. Firstly,
an emulsion is prepared that comprises a population of droplets. This dispersed phase contains a monomer. In a
second step, another monomer, which is soluble in the continuous phase, is added to the system to begin the
reaction at the interface of droplets.
In industry, microencapsulation by interfacial polymerization is usually performed in stirred-tank reactors, where
both the emulsification and encapsulation steps are carried out. But this process is very costly in energy due to
the power input necessary for the generation of a fine dispersion, as well as the time needed to get the right drop
size distribution. Moreover, the characteristics of the final product, such as the particle size distribution with
respect to the target size, and the membrane thickness and structure, are not necessarily well controlled. These
characteristics are strongly dependant on the hydrodynamic conditions of the different steps. In particular, it is
crucial to control the drop size of the emulsion in order to control the microcapsule size distribution resulting from
this process.
In this study, the emulsification process is carried out using Sulzer SMX mixers. Such mixers are usually
employed for the dispersion of viscous liquids in the laminar flow regime. However, it is demonstrated in this study
that they are also well adapted for liquid-liquid dispersion in turbulent flow.
The influence of the dispersed phase concentration, the flow velocity and the number of mixing elements on the
drop size distribution under various turbulent flow conditions is investigated. The drop size distribution is
characterized in terms of the mean surface-volume drop diameter and standard deviation, which are measured
with a laser diffraction device. The emulsions are cyclohexane-in-water stabilised with Tween 80, which are the
same fluids involved in the system chosen for the encapsulation process.
A correlation of the Sauter mean diameter with the Weber number and the Reynolds number is proposed for the
flow rate range studied and compared with the correlation given by Streiff (1977) for SMV Sulzer mixers at low
energy input.
The dispersion process in turbulent flow is governed by the ratio of the stress forces outside the drop to the
surface forces at the interface of the drop. The external stress forces are the turbulent drag forces on the drop
surface created by local velocity differences, which are promoted by turbulent eddies. In this case, the smallest
drop size corresponds to the microscale of turbulence and the size can be correlated with the specific energy
dissipation in the mixer. The specific energy can be determined from flowrate and pressure drop through the static
mixer.
Since the correlations available to calculate the pressure drop in SMX mixers are valid for single phase Newtonian
fluid flow, the pressure drop of the liquid-liquid flow is measured in this study and used to calculate the specific
energy dissipation. A correlation of the maximum drop diameter with specific energy dissipation is proposed and
compared with that given by Hinze (1955) for isotropic turbulent flow.
Finally, the minimum number of mixer elements required to obtain a stable drop diameter is given for different
hydrodynamic conditions and dispersed phase concentrations.
The work carried out has enabled the emulsification conditions in the static mixer to be optimized, which should
allow the encapsulation process to be performed in the best conditions. Moreover, the SMX static mixers show
good performance for emulsification in turbulent flow in terms of droplet size and energy consumption compared
with the conventional stirred-tank reactor.