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.

Pulp fibre suspensions display non-Newtonian rheology,
including a yield stress. Under certain mixer operating
conditions this creates caverns (regions of active mixing)
around the impellers with the cavern size affecting the
extent and quality of mixing attained. Due to the opacity
of pulp suspensions it is not possible to measure cavern
size with direct optical techniques, like photography.
Consequently two non-invasive techniques suitable for use
in opaque media were evaluated for determining the
cavern dimensions: electrical resistance tomography
(ERT) and ultrasonic Doppler velocimetry (UDV). The
agitation of several pulp suspensions in a 38 cm diameter
cylindrical vessel was studied using these methods over a
range of operating conditions. ERT is a non-invasive
technique that images differences in conductivity between
regions in the mixer using voltage measurements made at
the vessel periphery. Cavern measurement by ERT is very
rapid (data are collected within a few seconds) but it
suffers from poor spatial resolution (approximately 5 to
10% of the vessel diameter – from 1.9 to 3.8 cm in our
case). Two methods were evaluated for creating the
conductive environment imaged by ERT – the injection of
saline solution or the addition of small metallic tracer
particles to the region surrounding the impeller. UDV was
used to determine the cavern boundary by measuring the
locations at which suspension velocity fell to zero for
multiple linear paths through the vessel. While UDV
provided better spatial resolution of the cavern than ERT
(about 2 mm), multiple measurements (and consequently
significant time) were needed to build up the profile of the
cavern boundary.
Cavern size as a function of impeller rotation speed is
reported for a range of pulp suspension mixing conditions
(hardwood and softwood pulps, suspension mass
concentrations from 2 to 4%, two impeller offsets from the
wall, and two suspension height-to-chest diameter ratios)
in the 38-cm diameter cylindrical chest. A scaled version
of a commercially available axial flow impeller designed
for use in pulp suspension agitation (the Maxflo,
Chemineer Inc.) was used in the standard side-entering
configuration used for pulp stock chests. Measured cavern
diameters were compared against the axial force model
developed by Ammaullah et al. (1998) for predicting
cavern diameters in non-Newtonian fluids. The
discrepancy between the experimental data and model
predictions were fairly large, although they decreased with
increasing yield stress Reynolds number. The discrepancy
was attributed to the proximity of the impeller to the
vessel walls in the side-entering configuration studied. An
alternative correlation is presented for predicting the
cavern volume in pulp suspensions in this mixing
configuration based on the suspension yield stress

increasing yield stress Reynolds number. The discrepancy
was attributed to the proximity of the impeller to the
vessel walls in the side-entering configuration studied. An
alternative correlation is presented for predicting the
cavern volume in pulp suspensions in this mixing
configuration based on the suspension yield stress

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).