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

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.