CFD aided investigation of single droplet coalescence

This article describes the development of a coalescence model using various CFD work packages,and is validated using as toluene water model system.Numerical studies were performed to describe droplet interactions in liquid–liquid test systems.Current models use adjustable parameters to describe these phenomena.The research in the past decades led to different correlations to model coalescence and breakage depending on the chemical system and the apparatus geometry.Especially the complexity of droplet coalescence requires a detailed investigation of local phenomena during the droplet interaction.Computational fluid dynamics(CFD) studies of single droplet interactions were performed and validated with experimental results to improve the understanding of the local hydrodynamics and film drainage during coalescence.The CFD simulations were performed for the interaction of two differently sized droplets at industrial relevant impact velocities.The experimental verification and validation of the numerical results were done with standardized high-speed imaging studies by using a special test cell with a pendant and a free rising droplet.An experimental based algorithm was implemented in the open source code OpenF OAM to account for the contact time and the dimple formation.The standard European Federation of Chemical Engineering(EFCE) test system toluene/water was used for the numerical studies and the experimental investigations as well.The results of the CFD simulations are in good accordance with the observed coalescence behavior in the experimental studies.In addition,a detailed description of local phenomena,like film rupture,velocity gradients,pressures and micro-droplet entrainment could be obtained.

CFD based extraction column design-Chances and challenges

This paper shows that one-dimensional(1-D) [and three-dimensional(3-D) computational fluid dynamics(CFD)] simulations can replace the state-of-the-art usage of pseudo-homogeneous dispersion or back mixing models.This is based on standardized lab-scale cell experiments for the determination of droplet rise,breakage,coalescence and mass transfer parameters in addition to a limited number of additional mini-plant experiments with original fluids.Alternatively,the hydrodynamic parameters can also be derived using more sophisticated 3-D CFD simulations.Computational 1-D modeling served as a basis to replace pilot-plant experiments in any column geometry.The combination of 3-D CFD simulations with droplet population balance models(DPBM)increased the accuracy of the hydrodynamic simulations and gave information about the local droplet size.The high computational costs can be reduced by open source CFD codes when using a flexible mesh generation.First combined simulations using a three way coupled CFD/DPBM/mass-transfer solver pave the way for a safer design of industrial-sized columns,where no correlations are available.