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-, SO42-, 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”.