Jörn Villwock

Flooding remains a major challenge in gravity separators for coalescence-hindered liquid–liquid systems, where extended dense-packed zones (DPZs) can form below the coalescence front. Reliable design and scale-up require a quantitative description of how dispersed-phase load and inlet droplet-size distribution (DSD) affect the steady-state DPZ height and local dispersed-phase hold-up. However, experimental data obtained under conditions where the dispersed-phase load and inlet DSD are varied independently remain scarce. These effects were investigated for two organic–aqueous liquid–liquid systems using a vertical separator that allowed independent variation of the dispersed-phase load and the inlet DSD. For all investigated conditions, the steady-state DPZ height increased with dispersed-phase load. DSDs with smaller characteristic droplet sizes generally resulted in higher DPZs, particularly at higher loads. A power-law relationship adequately described the entire investigated load range only for the largest DSDs, whereas extrapolation of the low-load correlations increasingly underestimated the DPZ height at higher loads for smaller DSDs. Local hold-up measurements showed that the DPZ was not a homogeneous packed layer but consisted of a compacted upper zone near the coalescence front and a less compact lower zone toward the sedimentation zone.

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.

Acoustic levitation can provide significant benefits for many fundamental research questions. However, it is important to consider that the acoustic field influences the measurement environment. This work focuses on the dissolution of immobilised drops using acoustic levitation in liquid–liquid systems. Previous work demonstrated that the acoustic field of standing waves impacts mass transfer by affecting the spread of dissolved substances in the continuous phase in two distinct ways: (I) solutes may either pass through nodal planes of the standing waves or (II) not pass. The binary systems examined for case (I) are 1-hexanol–water and 1-butanol–water, and for case (II), n-butyl acetate–water and toluene–water. This work quantifies the intensification effect of acoustic levitation on dissolution for the two types of behaviour, by comparing them with reference measurements of mechanically attached dissolving drops. The system was designed to ensure minimal intensification. The minimum intensification of mass transfer for levitating drops in the used setup of case (I) was 25%, and for case (II), it was 65%, both increasing with decreasing surface-equivalent diameter. With this understanding, acoustic levitation can be used more accurately in the field of mass transfer studies.

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 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.