The literature on micromixing and local-specific energy dissipation rate,ε T (on which it depends), for two-phase systems is limited and conflicting. Here, the competitive iodide/iodate reaction scheme has been used to study the effect of particles and gas flow rate on micromixing in a Rushton turbine agitated vessel. Gassing rates up to 1.5 vvm did not show any effect on product distribution compared to the ungassed at constant mean-specific energy dissipation rate (ε T ) g =ε T for a feeding near the impeller. Near the upper liquid surface, micromixing improved with increasing flow rate because it increased fluid turbulence. These results confirm the limited literature. With 500 m glass beads at concentrations up to 2.5 wt.%, micromixing was unaffected near the impeller and near the surface. The related literature is very unclear and it is difficult to draw any precise conclusions. At ∼12 wt.% when cloud formation was observed, micromixing was significantly worse, especially, it is shown for the first time, in the clear layer above the cloud. The latter finding is significant for processes such as precipitation where micromixing determines the particle characteristics.

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Detailed measurements and computational fluid dynamics (CFD) investigation of the hydrodynamics in a bubble column containing internal features causing flow disturbances are presented for both air and helium gases. An optical needle probe has been used to measure profiles of bubble size, bubble velocity, and gas holdup at different locations across the cross section of the column. An approach combining CFD with population balances is able to represent observed multiphase flow phenomena such as the effect of the pipes to remix and redistribute the gas as well as the tendency of the gas to channel through a slit in the pipes rather than go around the pipes. The comparison of CFD simulation to experimental measurements reveal that the overall decrease in gas holdup observed when switching from air to helium gas can be explained by swarm effects, whereas the steeper decrease in the gas holdup profile across the column is due to coalescence effects. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3764–3774, 2018

Bioprocesses for the production of renewable energies and materials lack efficient separation processes for the utilized microorganisms such as algae and yeasts. Dissolved air flotation (DAF) and microflotation are promising approaches to overcome this problem. The efficiency of these processes depends on the ability of microorganisms to aggregate with microbubbles in the flotation tank. In this study, different new or adapted aggregation models for microbubbles and microorganisms are compared and investigated for their range of suitability to predict the separation efficiency of microorganisms from fermentation broths. The complexity of the heteroaggregation models range from an algebraic model to a 2D population balance model (PBM) including the formation of clusters containing several bubbles and microorganisms. The effect of bubble and cell size distributions on the flotation efficiency is considered by applying PBMs, as well. To determine the sensitivity of the results on the model assumptions, the modeling approaches are compared, and suggestions for their range of applicability are given. Evaluating the computational fluid dynamics (CFD) of a dissolved air flotation (DAF) system shows the heterogeneity of the fluid dynamics in the flotation tank. Since analysis of the streamlines of the tank show negligible back mixing, the proposed aggregation models are coupled to the CFD data by applying a Lagrangian approach.

Bioprocesses for the production of renewable energies and materials lack efficient separation processes for the utilized microorganisms such as algae and yeasts. Dissolved air flotation (DAF) and microflotation are promising approaches to overcome this problem. The efficiency of these processes depends on the ability of microorganisms to aggregate with microbubbles in the flotation tank. In this study, different new or adapted aggregation models for microbubbles and microorganisms are compared and investigated for their range of suitability to predict the separation efficiency of microorganisms from fermentation broths. The complexity of the heteroaggregation models range from an algebraic model to a 2D population balance model (PBM) including the formation of clusters containing several bubbles and microorganisms. The effect of bubble and cell size distributions on the flotation efficiency is considered by applying PBMs, as well. To determine the impact of the model assumptions, the modeling approaches are compared and classified for their range of applicability. Evaluating computational fluid dynamics (CFD) of a DAF system shows the heterogeneity of the fluid dynamics in the flotation tank. Since analysis of the streamlines of the tank show negligible backmixing, the proposed aggregation models are coupled to the CFD data by applying a Lagrangian approach.