(292b) Influence of Ions on Coalescence in Liquid/Liquid Dispersions - a Systematic Experimental Analysis

In several industrial processes liquid/liquid dispersions
play an important role. The efficiency of these processes is mainly determined
by the drop size distribution, which is the result of the competing phenomena drop
breakage and coalescence. Although existing models for the coalescence rate include
physical properties and fluid dynamic characteristics like energy dissipation
rate and relative velocity, other parameters such as the influence of ions are
either not or merely insufficiently implemented. In a first step, the system is
reduced to the fundamental behaviour of single droplet coalescence to quantify
the impact of these parameters separately and in particular the influence of
ion species and concentration. 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, a) - serial examinations of binary
droplet collisions under variable system conditions are possible.

With this setup the influence of different ion
species in various concentrations on the coalescence probability for the system
toluene/water was investigated systematically. It was observed that coalescence
is inhibited with increasing ion concentration and that coalescence times are significantly
influenced by the ion species. The experiments were repeated with n-heptane/water
to test the reproducibility of results for different test systems.

To validate the single drop results, experiments
in a lab-scale stirred tank (DN 150) were conducted (see Fig. 1, b). The drop
size distributions were measured with an endoscope technique for selected ion
species and concentrations. Additionally, further important parameters like
dispersed phase fraction and specific energy dissipation rate were varied and
analysed. The comprehensive systematic parameter analysis is presented and single
drop and stirred tank results for the investigated test systems are compared
and discussed.

Fig.
1: Systematic coalescence analysis: from single drops (a) to droplet swarms (b)

Financial support within the DFG project “Coalescence
efficiency in binary systems” is gratefully acknowledged.