Single drop based modelling of drop residence times in Kühni columns

  • Einzeltropfenbasierte Modellierung von Tropfenverweilzeiten in Kühni-Kolonnen

Buchbender, Florian; Pfennig, Andreas (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2013)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2013

Abstract

Drop-population balance tools such as “ReDrop” have the potential to be beneficial as they allow the compartment geometry along Kühni extraction columns to be quickly and efficiently optimized. In order to reliably use this potential, a quantitative understanding of the effect of compartment geometry and operating conditions such as impeller speed and counter-current flow on mean drop residence time and drop residence-time distribution is needed. Therefore, this work systematically investigates the impact of compartment geometry and operating conditions on the residence time of drops with different diameters, using single drop measurements in lab-scale cells with Kühni compartments. For each operating point, drop movement of up to 100 single drops is recorded with a high-speed camera. Here, drop residence times are not only evaluated for the entire compartment but also for three different compartment zones. These compartment zones are the lower zone 1 between bottom stator and impeller, the impeller zone 2, and the upper zone 3 between impeller and upper stator. Additionally, drop back-mixing between the compartment zones and compartments as well as the number of drops hitting the upper stator or the impeller are evaluated quantitatively. Amongst other things, this detailed evaluation of drop behavior inside the compartment shows that on their way through the compartment, drops spend up to four times longer in zone 3 than in zone 1. It is then shown that it is possible to reduce compartment height by 20% while positioning the impeller asymmetrically inside the compartment without this having any significant effect on mean drop residence time. On the basis of the experimental findings, the Zone-walk model is developed. This model describes drop residence times in Kühni compartments using stochastic methods in order to map drop interaction with internals and the velocity fields inside the compartment. The Zone-walk model is divided into three sub-models, each sub-model describing drop movement in one of the compartment zones. In this model, drops can be back- and forward-mixed between the individual compartment zones as well as between compartments. The Zone-walk model not only accurately describes the influence on drop residence times of the compartment geometries and operating conditions investigated, but can also be successfully extrapolated to further compartment geometries and liquid-liquid systems from the literature. Finally, the Zone-walk model is successfully validated in combination with “ReDrop” by comparing simulation results with data from the literature for two pilot-plant Kühni columns.

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