Flow Field Analysis of Stirred Liquid‐Liquid Systems in Slim Reactors

Abstract

Previous studies have shown the great potential, but also the great challenges, in handling slim reactors often used for polymerization reactions. Experiments and simulations were carried out in reactors with aspect-to-diameter ratios of up to 5, to test and to evaluate the mixing and dispersion efficiency for liquid-liquid systems of single-and multiple-stage impellers. Therefore, power consumption, mixing time and minimum dispersion speed were determined for five different stirrer types under turbulent conditions. It was found that the dimensionless mixing time is highly sensitive to the configuration of the impellers, with almost no dependency on the turbulent power number. Another focus was the analysis of the effect of the baffles. The influence of the baffle length in slim reactors on the mixing time and the macroscopic flow field was determined.

Cross Linking Between the Baffling Effect and Phase Inversion During Liquid–Liquid Monomer Mixing

“…Furthermore, the stirrer speed as well as the concentration of the dispersed phase. All geometric parameters as well as their influence on the fluid dynamics are more extensively described by Maaß et al…”

“…Therefore, the drop size distribution of an agitated system should not be calculated based on a single average value. Based on extensive CFD simulations, PIV (particle image velocimetry) measurements and comparative PBE model variations (see also Maaß et al), an appropriate description of the spatial inhomogeneity was found using a zonal model. One zone with high energy dissipation rates describes the impeller region and one zone with low energy dissipation rates describes the bulk region of the vessel.…”

“…In order to better understand the dynamics of such systems, the classical tank agitated by a stirrer, a system clearly defined by two specific regions with different levels of turbulence and shear stresses, serves as an excellent model . Simplifying such a system into an area surrounding the impellers (a high‐shear stress region, dominated by dispersion) and an area located away from the impellers (a lower‐shear stress region, dominated by coalescence) allows the establishment of a two‐region model .…”

“…Below a certain length of the baffles, the mean drop size was twice as large than above a certain baffle length. Extensive research on the fluid dynamics in a single phase system, experimentally and numerically using CFD, by Maaß et al did not show any reason for this behavior or a reason for the existence of a critical baffle length if the mixing is always under fully turbulent conditions . However, persistent research did show, that phase inversion did occur under certain conditions.…”

Impact of impeller type on methyl methacrylate emulsion polymerization in a batch reactor

“…In fact, it has been reported the multiple impellers have great potential applications for the single‐and two‐phase systems due to their advantages over the single impeller. A flow analysis in slim reactors revealed multiple impellers should be used rather than the single impeller to save power and allow complete dispersion especially for reactors with the ratios of liquid level to reactor diameter >3.…”