(479a) Effect of Ions on Coalescence in Liquid Two-Phase Systems

Liquid/liquid processes, e.g., extraction processes or polymerization, play an important role in industrial applications. The efficiency of these processes is mainly determined by the drop size distribution (DSD), which is the result of the competing phenomena drop breakage and coalescence. These phenomena were investigated extensively in the last decades. However, due to its complexity, especially coalescence is by no means completely understood. To get a comprehensive understanding of the coalescence process considering influencing parameters such as fluid dynamics and ion addition and to evaluate/optimize model descriptions of the DSD, a systematic experimental analysis of the influence of ion addition and fluid dynamics on coalescence was conducted.

A bottom-up approach was used to systematically quantify the impact of the described parameters in the system toluene/water separately: first the fundamental behavior of single drop coalescence was investigated in a fully automated test cell using high speed imaging [1]. With this setup, serial and detailed examinations (e.g. coalescence efficiency and time) of binary droplet collisions under variable system conditions are possible. In the second step of the approach, lab-scale experiments in a stirred tank and a Kühni-column (both DN150) were conducted, measuring the drop size distribution and the height of the dense-packed zone, respectively, for the addition of different ions in varying concentrations.

In the single drop experiments, a significant coalescence inhibition was observed for increasing ion concentrations (see Figure 1). Additionally, drop size and ion species had a major impact on the coalescence efficiency and coalescence time (hence the large error bars in Figure 1). The dependence of the coalescence behavior on the ion species could also be observed in lab-scale stirred tank experiments, as can be seen in Figure 2 for different ions. At concentrations above 10^-3 mol/L all ions decrease the Sauter mean diameter to a specific degree, especially at a prompt stirrer speed decrease the strong impact of ion species on coalescence behavior becomes apparent. A first approach to model the coalescence behavior with addition of OH^-ions (=increasing pH) and the implementation into the population balance equation [2] showed promising results and is now tested for application for the investigated ions in our system.

The comprehensive experimental and numerical results of the systematic analysis will be presented and discussed.

 

Figure 1. Coalescence efficiency for single drops with varying drop diameter as a function of ionic strength.

Figure 2. Transient behavior of the Sauter mean diameter in a stirred tank for the addition of different ions.

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

[1] J. Villwock, F. Gebauer, J. Kamp, H.-J. Bart, M. Kraume, Chem. Eng. Technol. 37(7), 1103-1111 (2014)

[2] J. Kamp, M. Kraume, Chem. Eng. Sci. 126, 132-142 (2015)