(480a) From Fundamental Single Drop Analysis to the Description of Liquid/Liquid Dispersions - Part I: Coalescence

There are numerous applications in
industry, where reactions and mass or energy transfer between two immiscible liquids
are the major concerns. When trying to influence or predict the contact area
one has to focus on the underlying principles that lead to the formation of the
drops. The resulting drop size distributions (DSD) are thereby determined by
drop breakage and coalescence. In order to predict the size distributions, both
phenomena have to be understood and described properly. Although comprehensive
scientific research identified a multitude of theoretical models describing
both processes, the prediction of the DSD is only possible with restrictions
when varying for example the power input, material and process parameters.
Consequently, excessive and expensive experimental investigations are still
necessary at different scales of process development. For example the design of
extraction columns still requires pilot plants using high amounts of process
liquids.

To reduce the amount of influencing factors
of the whole process, it is necessary to reduce the problem to the fundamental
behavior of single droplets. Therefore, the impact of influencing parameters
for drop coalescence and breakup needs to be identified and quantified for each
phenomenon separately. The gained knowledge of this microscopic behavior can be
used to prove existing models and develop new ones. These models can either be
semi-empirical or mechanistic. The obtained detailed models can be implemented
into the framework of population balance equations (PBE), which includes
separate kernels for drop breakage and coalescence. The PBE can be applied to
describe the time dependent DSD in technical applications and therefore
connects the microscopic to the macroscopic behavior of droplet swarms. The
microscopically based modeling allows a determination of the model parameters
using single drop experiments with only a small amount of liquid components in
order to adapt the submodels to the present process.

This part is focused on the coalescence of
two droplets and will discuss two characteristics of this phenomenon in detail:
the influence of superimposed mass transfer on the coalescence probability and
the coalescence inhibition due to electrostatic charge at high pH values.

The coalescence probability, which
quantifies if two droplets coalesce or repulse each other after collision, is
quantified systematically using single drop experiments. Therefore, a newly developed
test cell is used where a rising drop collides with a pendant one recorded by a
high speed camera. The drop generation, detachment, collision and the triggered
recording of the process is fully automated. This allows carrying out serial
examinations of droplet collisions under different system conditions (varying
e.g. drop sizes, mass transfer direction and intensity). As the coalescence and
contact time (and consequently the coalescence probability) of drops have a
broad variance, a significant number of experiments has to be conducted to
provide a statistically solid data base. Analyzing the recorded images of the
coalescence process, important quantities like coalescence probability, contact
and coalescence time, relative velocity, momentum and deformation are
evaluated. Conducting these experiments, the EFCE recommended model system
toluene / acetone / water is used.

Analyzing the influence of drop size ratio
on coalescence probability shows the same tendency which commonly used models
predict. If two drops with the same size collide, the coalescence probability
is the lowest and rises with a shift in drop size ratio. Increasing the rising
distance and therefore the relative velocity between the drops decreases the
coalescence probability. This finding is contradictive to the so called energy
models in literature, which assume a higher coalescence probability with higher
kinetic energy (corresponding to a higher relative velocity). As mass transfer
is inherent in extraction and is known to have a significant impact on drop
coalescence, its influence on the coalescence probability is investigated
systematically. Introducing the transferring component acetone into the system
shows that the coalescence behavior is changed significantly over a broad range
of driving concentration differences. A transfer direction from dispersed to
continuous phase (d → c) leads to a decrease in the contact time of two
drops until they coalesce. This results in a significant increase of
coalescence probability to a complete coalescence of all drops independent from
the drop size ratio. Reversing the mass transfer direction (c → d)
generally decreases the coalescence probability, although this effect seems to
be outweighed at distinct drop size ratios. Apparently, the gathered knowledge
of this microscopic behavior offers a basis for model validation and future
model development describing the influence of superimposed mass transfer.

A coalescence inhibition caused by
electrostatic interactions was found in experimental investigations in stirred
toluene/water systems at constant ionic strength of 0.1 mol/L with pH values
higher than 11. Two designated models were used to simulate the transient DSD
in a stirred tank, showing that the influence of droplet charge due to a change
in pH value or ion concentration cannot be predicted satisfactorily by existing
models. This finding motivated a new mechanistic modeling approach implementing
the DLVO theory into the PBE framework. For that reason the coalescence
efficiency is separated in a hydrodynamic and an electrostatic part. Being
independent from each other, the hydrodynamic part of the efficiency can be
described by common models from literature, whereas the developed electrostatic
part is based on the ratio of the repulsing force due to the overlap of the
electrical double layers of two colliding drops to the attractive van der Waals
force between them. This model is able to describe the hindered coalescence at
high pH values and the transient and steady-state Sauter mean diameters of the
DSD fit well with the experimental values. Further investigations will focus on
single drop experiments with a systematic variation of ionic strength, pH and
ionic species and zeta potential measurements. These examinations will build a
basis for further model development.

Fig. 1: Coalescence of two droplets

Partially founded by DFG project
?Coalescence efficiency in binary systems? and DECHEMA
?Max-Buchner-Forschungsstiftung?