Messung und Modellierung des Spaltungs- und Koaleszenzverhaltens von Tropfen bei der Extraktion

• Measurement and modeling of drop breakup and coalescence behaviour during extraction

Klinger, Sigrid Karin Irene; Pfennig, Andreas (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2007, 2008)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2007

Abstract

Conventional models for computation of pulsed liquid extraction columns often neglect the polydispersity of drops. By contrast, new models are based on balances of the drop population in the column. Hence, they promise more detailed results. By means of these models the breakup and coalescence of drops and the resulting drop size alterations are considered. Drop size alterations are significant because they interact with mass transfer and sedimentation process. Prerequisite of the new models are the knowledge of the breakup and coalescence behaviour, e.g. the breakage and coalescence rates. In order to investigate the breakup and coalescence of drops in pulsed packed columns experiments on laboratory scale were conducted. The drop sizes were measured by photography at different heights of the column. Afterwards, the distribution was analyzed. The measurements were conducted with three standard test systems which are characterized by different interface tensions: n butyl acetate (d), toluol (d) and n butanol (d) + water (c). By using different designs of the intake plate the drop size distribution of the fed-in dispersion was adjusted so that either the process of breakage or the process coalescence processes were predominant. The drop size was measured at column heights where equilibrium of breakage and coalescence was not achieved. The influence of fluid dynamics was investigated by varying the pulsation and the flow of dispersed phase. The results show that not only the pulsation intensity af but also the pulsation acceleration af 2 influences the drop size in apparatus. Additionally, the influence of the dispersed phase flow on the drop size could be shown. Based on these results, new models for breakage and coalescence behaviour were derived. An energy balance of a single breaking droplet was considered and the critical drop size diameter was derived which divides the population into breakable and not breakable drops. The breakage probability and the daughter drop size distribution were described similar to packed column models found in literature. The breakage in both column types should be comparable. The coalescence probability of drops was described with an empiric model which considers the drop diameter influence, the holdup and further aspects like the liquid-liquid system. The models were implemented into the simulation program ReDrop (Representative Drops) for extraction columns which was developed by the ‘Department of Chemical Engineering, Thermal Unit Operations, RWTH Aachen University’. The simulation algorithm uses the Monte Carlo method for solving the drop population balance. The probability of events is calculated by using random numbers. The simulation program describes the drop input into the column, the drop movement through the column as well as the breakage and coalescence of drops. The drop size distribution gained through simulation and the measured distribution are displayed for comparison. The implemented models for breakage and coalescence require four parameters. For optimization purposes these parameters were varied in reasonable ranges and adjusted for each liquid-liquid system. Furthermore the modelling of the critical drop size diameter (breakage model) was separately compared with measurement results of packed columns described in literature. Concerning the n-butyl acetate system the simulation and measurement results match very well. Concerning the toluol system the results show the same characteristic in spite of deviations. In addition, simulations for a pilot plant column were conducted with the n-butyl acetate system. They were compared with measurement data. With identical parameters of the laboratory scaled simulations the measured drop sizes in the pilot plant column could also be simulated very well for varying conditions. This means that only one set of parameters for a liquid system can describe the drop sizes under different conditions. In this document, a breakage and coalescence model is presented whose results match well with wide range of experimental data on laboratory and pilot plant scale. The model completes the simulation program ReDrop by adding the calculation of drop population processes in pulsed packed columns. Now, the simulation of this column type is also possible.