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.