The coalescence phenomenon of drops in liquid/liquid systems is reviewed with particular focus on its technical relevance and application. Due to the complexity of coalescence, a comprehensive survey of the coalescence process and the numerous influencing factors is given. Subsequently, available experimental techniques with different levels of detail are summarized and compared. These techniques can be divided in simple settling tests for qualitative coalescence behavior investigations and gravity settler design, single-drop coalescence studies at flat interfaces as well as between droplets, and detailed film drainage analysis. To model the coalescence rate in liquid/liquid systems on a technical scale, the generic population balance framework is introduced. Additionally, different coalescence modeling approaches are reviewed with ascending level of detail from empirical correlations to comprehensive film drainage models and detailed computational fluid and particle dynamics.

This study aims at a better understanding of the effects of flocculants and adsorbents on membrane bioreactor (MBR) mixed liquor. A total of 12 different additives (metal salts, chitosans, starches, synthetic polymers and PAC) were tested with regards to their impact on particle size distribution in activated sludge. Of further interest was the shear stability and dewaterability of sheared sludge. This was tested in a range of shear rates dominating in MBR. For most additives, a significant effect on the capillary suction time (CST) was observed. Most additives formed aggregates that were stable in the tested shear range (0–4000 s-1). Nevertheless, only the tested chitosans and polymers were able to significantly increase the volume based particle size (up to 127%). In order to examine the long term effect of shearing on particle size three of the tested additives were surveyed in pilot plant experiments. Here the increase in particle size was only 17–18% for the tested polymers. In lab scale tests these ...

A low-impact acoustic levitation system has been developed to study immobilised single drops in liquid–liquid systems. The ability to observe liquid drops several millimetres in diameter for days enables fundamental research into a wide range of mechanisms. Non-invasive optical measurements with excellent optical accessibility are possible. This experimental work provides the basis for mass transfer studies, emphasizing the precise volume determination, signal noise, reproducibility, and the impact of the acoustic field on the drop and its surrounding environment. The setup can be effectively controlled and proves beneficial for research objectives provided that all liquid phases are entirely degassed, and there are no compressible voids present within the liquids. In addition to the precise, uniform, and reliable measurement conditions, we observed no acoustic streaming in the proximity of the drop and there was no significant vibration of the drop. Qualitative observations using rainbow schlieren deflectometry indicate that the nodal or anti-nodal planes of the standing waves can act as barriers to the dispersion of inhomogeneous dissolved substances in the continuous phase.

The dynamic behavior of a toluene/NaOH solution system was studied in a batch stirred tank. Characterization by step response experiments showed that the drop size response to stirrer speed steps behaves as a first‐order system, whereas the stirrer speed responds as a second‐order system. The respective time constants differed by at least one order of magnitude, suggesting effective control of the drop size is feasible by adjusting the stirrer speed. Based on experimental data, simple, linear system models were derived for the mean drop diameter and the interfacial area using the stirrer speed as input variable. Validation experiments showed good model qualities of about 70 % and the design of test trajectories proved the model's usability in an open‐loop control.