Mixing, or Segregation, can be defined using three dimensions. The instantaneous state of segregation has two dimensions:
the scale of segregation, and the intensity of segregation. The intensity of segregation is reported as the CoV in a blending
application, while the scale of segregation is reported as the striation thickness distribution, the drop size distribution, as
examples. Previous work has shown that the two dimensions contain different and independent information. The CoV tells
us nothing about the scale of segregation, and the scale of segregation contains no information about the range of
concentrations observed.
The exposure dimension conbines the intensity and scale of segregation with a third characteristic of the system to give a rate
of reduction in segregation. Many examples of exposure equations are given in the literature. The most familiar is the mass
transfer rate, where the scale of segregation can be related to the interfacial area, the intensity of segregation to the local
concentration gradients, and the tendency of the system to reduce segregation to the mass transfer coefficient, or the
molecular diffusivity. In this case, the exposure dimension is an integral combination of the local area and intensity of
segregation, so while it is correlated to both the scale and the intensity of segregation, it is not a linear combination of the
average measures.
In this talk, the exposure dimension will be reviewed in the context of existing literature and models. The goal is to
determine the underlying mixing variables which consistently drive a reduction in segregation, and the role that these
variables play in achieving a range of process objectives

PVC is in terms of revenue one of the most important
products of the chemical industry. Globally over 50% of
PVC manufactured is used in construction. Worldwide
80 % Percent of PVC is produced by suspension
polymerisation. In such processes mechanical agitation is
used to mix the monomer droplets into an aqueous liquid
phase. Growing markets and growing economies lead to
higher PVC production rates. Limits and demands in
space and transportation are changing the outfit of the
used mixing reactors. The height (H) is increasing with
constant diameter (D). Did most of the apparatuses start
with a ratio of height vs. diameter of one, ratios of two to
three are normal today and ratios of over four are expected
for the nearer future. Such unique geometries need to
fulfil the still growing exigencies in economy and
ecology. Therefore the analysis and optimisation of such
liquid/liquid systems is of major interest for the chemical
industry.
The step of scaling up a reactor from pilot plant to
industrial scale is an issue where much empiricism is still
used and where expensive and time-consuming
experimental programs are usually required [VivaldoLima
et al. 1997] and only accurate prediction of system
behaviour will change that situation. To develop such
prediction methods cooperation was set up between the
Vinnolit GmbH & Co. KG and the TU-Berlin.
From the different tasks for scale up and for the
production process of PVC the dispersion of the two
immiscible liquids is of major interest for this work. So
the drop size distributions of two model systems,
chlorobutanol/water and toluene/water, were analysed.
Here parameter variation for reactor height vs. diameter
(1.0 to 4.5), stirrer type (Rushton turbine, Retreat Curve
Impeller, single and multi-stage stirrer systems),
dissipation rate, dispersed phase fraction (5 to 50 Percent)
and influence of colloids were carried out for the named
systems. For the mathematical description of such drop
size distributions (DSD) a quantitative understanding of
drop breakage and coalescence mechanisms is essential to
develop predictive models. The mathematical model used
here is the Population Balance Equation (PBE).
After adaptation and enhancements of classical models
from the literature (Coulaloglou & Tavlarides 1977;
Kumar & Ramkrishna 1996, Alopaeus et al. 2002)
simulations for the presented system were carried out. The
use of colloids is inevitable for the suspension
polymerisation and resulting in a strong inhibition of
coalescence. So a major focus on breakage submodels of
the PBE was set. Therefore single drop breakage events

were carried out to analyze crucial influence parameters of
the breakage rate like breakage time and energy
dissipation rate. These results were used to validate and
enhance the breakage submodels of the PBE. Then the
simulation results from different models were compared
with the experimental data and each other.
METHODS
A special in-situ endoscope technique has been developed
[Ritter & Kraume 2000; Maaß et al. 2007b]. With this
technique, drop size distributions for all phase fractions
even under transient conditions can be determined with
high time resolution . The Population Balance
Equation is applied with the intention to calculate these
transient drop size distributions in the stirred system. In
order to solve the transient space averaged PBE, the
commercial numerical solver PARSIVAL® (Particle Size
Evaluation) [Wulkow et al. 2001] is applied. For the
parameter estimation the experimental data of the stirred
vessel are used. The fitted parameters had to be
significant, i.e. the confidence interval was required to be
small compared to the value of the parameter, and they
had to be independent from each other.
The single drop experiments are carried out in an in house
developed breakage cell (Maaß et al. 2007a).

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.