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 .

Aeration of a wastewater lagoon environment to accelerate
BNR (Biological Nutrient Removal) is well known and
acknowledged technology. The delivery mechanism
typically consists of submerged, specialized aerators.
Current technology tends to be ineffective and require
significant investments in equipment over time.
The combined mixing and aerating action of a horizontal
directional aeration system may yield improved oxygen
transfer rates over that of vertical aeration technology. A
horizontal aerator combines conventional aspirator
technology with blower assisted aeration which produces
fine bubbles that significantly improve oxygen transfer. It
also induces horizontal flow that keep solids from settling.
CFD simulation of lagoon configurations with vertical and
horizontal aerators was conducted to provide the qualitative
difference between directional and vertical mixing and
quantitative data for Residence Time Distribution (referring
to a perception that directional aerators/mixers may cause
short circuiting between flow in and out).

A horizontal or directional aerator differs from standard
aeration equipment and is particularly suited for use in
lagoons. In horizontal aerators air is injected in front of an
impeller. The spinning impeller provides energy that is
needed for air dispersion and also provides a far reaching
flow of oxygen saturated fluid. Dispersion relies on
turbulence produced by the impeller. Horizontal aerators
also offer benefits in having a compact size, easy placement
at any location in a lagoon, low pressure drop for oxygen
delivery, efficient gas dispersing head, and produce very
high horizontal flow.
Vertical aspirators or high speed floaters are a common
technology used in lagoon configurations. They use
pumping to aerate and the pumping direction may be up or
down. In down pumping air is entrained from a vortex
created by mixer action and goes through an impeller
where it is dispersed into small bubbles. In an up pumping
a stream of water is thrown into the air where it is saturated
with oxygen then mixes with the body of water. All mass
transfer relies on fine spray that develops a large surface
area.
The challenge in selection of equipment for lagoon aeration
is how to select optimum placement of aerators.
Calculation of oxygen transfer requirements is very straight
forward, however the placement requires a great deal of
experience and the support of CFD. A lagoon that is fitted
with vertical aerators only will experience solids deposit at
the bottom. Solids are lifted only in small areas adjacent to
aerators. Get easy pay through payday advance today.

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.