Drawdown of floating solids in stirred tanks: scale-up study using CFD modeling

Floating particles mixing characteristics in an eccentric stirred tank coupled with dislocated fractal impellers

The mixing characteristics of floating particle dispersion process in an eccentric stirred tank with dislocated fractal impellers were investigated using computational fluid dynamics (CFD) and experimental analyses. Solid concentration distribution, axial solid concentration profile, cloud height, solid integrated velocity, power consumption and just drawdown speed were investigated. Results showed that dislocated fractal impeller can enhance solid integrated velocity and fluid turbulent fluctuation intensity compared with dislocated pitched blade impeller, and eccentric agitation coupled with dislocated fractal impeller could destroy the typical circulation loops and symmetric flow field and improve the axial circulation efficiency of floating particles on the basis of dislocated fractal impeller. Eccentric agitation coupled with dislocated fractal impeller could further enhance the floating particle dispersion homogeneity (MI) and decrease the just drawdown speed (N jd) on the basis of dislocated fractal impeller and dislocated pitched blade impeller at the specific power consumption. Meanwhile, eccentric ratio of 0.3 or 0.4 was optimal for the floating particle mixing process in this work.

Validation of Novel Lattice Boltzmann Large Eddy Simulations (LB LES) for Equipment Characterization in Biopharma

Detailed process and equipment knowledge is crucial for the successful production of biopharmaceuticals. An essential part is the characterization of equipment for which Computational Fluid Dynamics (CFD) is an important tool. While the steady, Reynolds-averaged Navier–Stokes (RANS) k − ε approach has been extensively reviewed in the literature and may be used for fast equipment characterization in terms of power number determination, transient schemes have to be further investigated and validated to gain more detailed insights into flow patterns because they are the method of choice for mixing time simulations. Due to the availability of commercial solvers, such as M-Star CFD, Lattice Boltzmann simulations have recently become popular in the industry, as they are easy to set up and require relatively low computing power. However, extensive validation studies for transient Lattice Boltzmann Large Eddy Simulations (LB LES) are still missing. In this study, transient LB LES were applied to simulate a 3 L bioreactor system. The results were compared to novel 4D particle tracking (4D PTV) experiments, which resolve the motion of thousands of passive tracer particles on their journey through the bioreactor. Steady simulations for the determination of the power number followed a structured workflow, including grid studies and rotating reference frame volume studies, resulting in high prediction accuracy with less than 11% deviation, compared to experimental data. Likewise, deviations for the transient simulations were less than 10% after computational demand was reduced as a result of prior grid studies. The time averaged flow fields from LB LES were in good accordance with the novel 4D PTV data. Moreover, 4D PTV data enabled the validation of transient flow structures by analyzing Lagrangian particle trajectories. This enables a more detailed determination of mixing times and mass transfer as well as local exposure times of local velocity and shear stress peaks. For the purpose of standardization of common industry CFD models, steady RANS simulations for the 3 L vessel were included in this study as well.

Critical review of different aspects of liquid-solid mixing operations

Mechanically stirred slurry tanks are utilized in several industries to perform various unit operations such as crystallization, adsorption, ion-exchange, suspensions polymerization, dispersion of solid particles, leaching and dissolution, and activated sludge processes. The major goal of this review paper is to critically and thoroughly analyse the different aspects of previous research works reported in the literature in the field of liquid-solid mixing. This paper sheds light on the advantages and limitations of various particle concentration measurement methods employed to assess the suspension quality and the extent of solid suspensions in slurry reactors. Attempts are being made to identify and compare various mathematical models and methods to quantify particle dispersion and distribution in slurry reactors. It has been shown that various factors such as geometric configurations, agitation conditions, and physical characteristics of liquid and solid have pronounced influence on local suspension quality and power consumption. Computational fluid dynamics (CFD) modeling can be extremely useful in assessing the suspension of solid particles in slurry tanks. A critical review of different scale-up procedures employed for solid suspension and distribution in liquid-solid systems is presented as well. The findings of this review paper can be useful for future research works in liquid-solid mixing.