(530a) Influence of Ions and Fluid Dynamics on Coalescence in Liquid/Liquid Dispersions

Influence
of ions and fluid dynamics on coalescence in liquid/liquid dispersions

Jörn
Villwock¹; Johannes Kamp¹;
Felix Gebauer²; Hans-Jörg Bart²; Matthias Kraume¹

¹ Chair of
Chemical & Process Engineering, Technische Universität Berlin, Germany

² Chair of
Separation Science and Technology, University of Kaiserslautern, Germany

Dispersions of two
immiscible liquids are an integral part of several unit operations. The drop
size distribution, which is the result of the competing phenomena drop breakage
and coalescence, influences a decisive part of the overall process efficiency.
Existing models for the coalescence rate include the physical properties, but
also fluid dynamic characteristics like energy dissipation rate and relative
velocity. However, these influencing parameters are implemented in existent
models with different proportionalities and in some cases even contradictorily.
To resolve this issue the system is reduced to the fundamental behaviour of
single droplet coalescence and the impact of the influencing parameters is
quantified separately.

With a fully automated test
cell - based on the principle to observe the collision of a pendant drop and a
second rising drop using high speed imaging (see Fig. 1) - serial examinations
of binary droplet collisions under variable system conditions are possible. All
experimental results were achieved using the standard test system for
liquid/liquid extraction toluene/water.  In this work the systematic analysis
of influencing parameters on coalescence in liquid/liquid dispersions is presented.
It focuses on the binary droplet coalescence and the following quantities will
be discussed in detail: coalescence probability and coalescence time depending
on the addition of different ion types in varying concentrations and on drop
size and relative velocity. The experimental results are used to validate and
refine existing models or to develop new models, which can be implemented in
population balance equations to describe the drop size distribution in
liquid/liquid dispersions.

Investigations with various
ions (Cl^-, ClO₄^-, SO₄^2-,
SCN^- and OH^-) in a concentration range of 10^-5
- 1 mol/L showed a strong decrease of the
coalescence probability with increasing concentration. However, general
explanations failed as no clear influence on interfacial tension and zeta
potential could be observed. The coalescence time increases up to an apparent
critical ion concentration and subsequently decreases. Coalescence times also
became longer with increasing drop size ratio as well as increasing collision
velocity. With respect to coalescence probability a critical collision velocity
was found. While below this critical value the coalescence probability mainly
depends on the drop size, nearly no coalescence occurred above it. To test the transferability,
the experiments are currently repeated with the system heptane/water having a
higher interfacial tension.

Fig.
1: Sample image sequence for coalescing droplets

Financially supported by DFG project KR 1639/19-2
?Coalescence efficiency in binary systems?.