Coalescence, liquid-liquid phase separation, and settlers

Gravity settlers are used for the separation of liquid-liquid dispersions because of their reliability, operating flexibility and high capacity. The design methods, which are available today, allow equipment design for translucent systems using data from standardized lab-scale settling experiments. These experiments on laboratory scale require roughly one liter of the technical two-phase system. Such experiments are essential, because trace components strongly affect the coalescence behavior. The batch-settling experiment is recorded on video and evaluated with a model that explicitly accounts for the sedimentation as well as the interactions between the droplets, namely their coalescence. After evaluating the parameters for the original material system, the simulation tool allows to design horizontal and vertical settlers depending on the specified operating parameters. In addition, Ruckes expanded the models to design systems with solid impurities, which may lead to crud formation. Recently Kopriwa unified the coalescence model and showed that the coalescence in settlers, where the drops are in direct contact in a close-packed layer, as well as in an extraction column, where drops only eventually meet, can be described with an identical approach. The model approach applied characterizes the coalescence probability as product of collision frequency, which is a function of fluid dynamic behavior, and coalescence efficiency. The latter in turn characterizes the time the drops stay in contact, which is again a function only of fluid dynamics, and the time required for coalescence. Only the latter influence is solely dependent on the coalescence properties of the material system.

Real technical systems encountered in industry often have a higher viscosity and are opaque. Due to the higher viscosity wide drop-size distribution occur, for which the models have to predict the fraction of drop phase remaining in the continuous phase due to the very small drops, which do not have sufficient time to reach the interface. Thus, our current work approaches exactly these challenges.

Current projects are :

• ERICAA: Saving Energy and Resources by Innovative CFD-based Design of Liquid-Liquid Gravity Settlers
• Accounting for wide drop-size distributions in settler design under gravity and in the centrifugal field 
• Characterizing crud formation and its influence on settling behavior 
• Cascaded Option Trees for biobased processes 

Publications

• N. Kopriwa, A. Pfennig: Characterization of Coalescence in Extraction Equipment Based on Lab-Scale Experiments. Solvent Extr. Ion Exc. 34(7) (2016) 622-642. 
• S. Ruckes, A. Pfennig: Untersuchungen zum Einfluss von Mulm auf das Abscheideverhalten organisch- wässriger Stoffsysteme. Final Project Report, 2010. 
• L. Schlieper, M. Chatterjee, M. Henschke, A. Pfennig: Liquid-Liquid Phase Separation in Gravity Settler with Inclined Plates. AIChE J. 50(4) (2004) 802-811. 
• M. Henschke, L.H. Schlieper, A. Pfennig: Determination of a coalescence parameter from batch-settling experiments. Chem. Eng. J. 85 (2002) 369–378. 
• H. Speth, A. Pfennig, M. Chatterjee, H. Franken: Coalescence of secondary dispersions in fiber beds. Sep. Purif. Technol. 29(2) (2002) 113-119.

Contact(s) : PFENNIG Andreas