(188a) Phase Separation Kinetics in Liquid-Liquid Extraction and Settler Design

The kinetics of droplet swarm sedimentation and coalescence in liquid/liquid phase separation systems was investigated. Target of this study was to develop an appropriate rate model for settler design. In a first series of batch phase separation experiments the system MIBK/water was investigated. The system properties density, viscosity and surface tension were adjusted by adding PEG 4000 (5 wt% to 40 wt%) and sodium chloride (0.05 mol L^-1 to 1 mol L^-1) to the aqueous carrier phase. Additionally, the system cyclohexanone/water was investigated. Latter system differs from the first series as far as interfacial tension and viscosity increase with increasing amount of PEG 4000, while in the first series the interfacial tension decreases with increasing amount of PEG 4000. Phase separation was monitored by an optical system and by ultrasonic scanning. Since any of the phase separation processes is nonlinear, a modified 1^st order rate model, shown in equation 1, was applied to fit the experimental data.

From single droplet-interface coalescence experiments Cockbaine et al [1] already concluded that the coalescence rate follows 1^st order decay. They assumed that coalescence delay is determined by the rate of film drainage. They proposed separation mechanisms but did not discuss settler design. Detailed investigation of droplet-droplet coalescence was performed by Kamp [2] and Villwok [3]. These studies mainly focused on droplet-droplet coalescence mechanisms, but also did not discuss settler design.

Based on the settler balance shown in equation 2, and the modified 1^st order rate model for nonlinear droplet swarm sedimentation and droplet coalescence, the design of continuous settlers, actually the settler area, can be performed.

The modified 1^st order rate model can address nonlinear droplet swarm sedimentation and droplet-bulk coalescence with two kinetic parameters, the preexponential (b) and the rate constant (k). Basically these parameters are just fit parameters, deduced from batch phase separation experiments. To find a link to the basics of phase separation in liquid-liquid extraction the parameters were compared with the physical properties of the investigated systems. The model parameters compare well with the density, viscosity and interfacial tension. Figure 1 representatively shows the phase separation results for the separation of the system cyclohexanone/water with 15 wt% PEG 4000. The figure shows the experimental data and the modeled separation curves. Figure 2 shows the correlation of the half-life time for coalescence of all experiments with the physical properties.

The results of this study, covering a total number of 21 experiments with a density span of the aqueous phase from 1013 to 1076 kg/m³, a viscosity span from 3 mPas to 30 mPas and a span of the interfacial tension from 3 to 10 mN/m, show a simple correlation of the model parameters and the physical properties. The results also show a correlation of the rate of sedimentation and the rate of coalescence, confirming, that the modified 1^st order rate model offers a simple and easy to apply tool in settler (area) design in liquid-liquid extraction. The results of droplet swarm sedimentation modeling have already been published [4].

Literature:

[1] Cockbain, E.G.; McRoberts, T.S. (1953), The stability of elementary emulsion drops and emulsions, J. Colloid Sci. 8, 440–451

[2] Kamp, J. (2017), Systematic coalescence investigations in liquid/liquid systems – From single drops to technical applications, PhD Thesis, Technische Universität Berlin

[3] Jörn Villwock (2019), Systematische Analyse des Koaleszenzverhaltens von zweiphasigen Flüssigsystemen bei Ionenzugabe, PhD Thesis, Technische Universität Berlin

[4] Pornprapa Bol, Georg Rudelstorfer, Annika Grafschafter, and Matthaeus Siebenhofer, (2021), The Kinetics of Droplet Sedimentation in Liquid-Liquid Extraction, Chem. Ing. Tech. 2021, 93, No. 1–2, 260-272