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Select a title to read the abstract.
Experimental and computational investigation of vortex formation in a single-stage Rushton turbine stirred tank reactor under standard and non-standard baffle configurations
An understanding of vortex formation in stirred tank reactors is of great importance, as in some processes, vortices are necessary for a chemical reaction to take place at all or to be accelerated, whilst in other processes, vortex formation is undesirable and can cause high mechanical stresses on the stirrer shaft, sealing and motor drive unit, or may cause undesirable surface aeration or foaming. To gain a better understanding of vortex formation, various baffle systems with different geometries are experimentally investigated concerning power consumption and the resulting vortices on a single-stage Rushton turbine setup. The shapes of the resulting vortices are described mathematically in terms of vortex depth, width and volume, and the stirring systems prone to vortex formation are simulated using a CFD model based on the lattice Boltzmann method. The CFD data obtained are compared, validated and verified against the experimental results in order to fully describe, model and predict vortex formation through simulation. Based on the detailed CFD data, vortex formation can be directly correlated with the swirl number, offering a mechanistic characterization method for the vortex shape in various mixing vessel configurations.
Read the full paper in Processes (open access) (opens in a new tab) · Read the case study
Mixing of miscible liquids: Dimensionless scaling for intermediate-to-large density differences in a stirred tank
Mixing of miscible liquids is an essential process in multiple industrial settings, usually with the intent to homogenize the product. This seemingly simple process is in fact a complex hydrodynamic problem that has a direct impact on the product quality. In this study, numerical simulations of a stirred tank were performed with a 50/50 ratio of liquids and systematically varied the Reynolds and Richardson numbers. A positive correlation between the mixing time and the Richardson number was observed, as reported in the literature. The influence of the Reynolds number was not as pronounced and clear. Based on the Power, Froude and Richardson numbers, we were able to derive an exponential scaling for the dimensionless mixing time that collapsed all our data onto one master curve.
Homogeneous shear distribution improves NK-92 cell cytotoxicity in a clinically relevant 2 L membrane-stirred bioreactor
The immortalized NK-92 cell line is widely used to study natural killer (NK) cell biology and develop immunotherapies. For clinical and commercial applications, large-scale cell expansion requires scalable platforms such as bioreactors. However, conventional bubble-aerated bioreactors generate shear stress and foam, which can impair proliferation during prolonged culture and compromise cell quality. A membrane-based stirring and aeration system was compared to a conventional pitched-blade impeller with microsparger aeration in 2 L stirred-tank bioreactors. Both systems supported comparable growth, viability and metabolic profiles, but cells expanded in the membrane-based system exhibited markedly higher cytotoxicity. Computational fluid dynamics simulations suggest this is attributable to the more homogeneous shear distribution in the membrane-stirred setup.
Read the full paper in Frontiers in Bioengineering and Biotechnology (opens in a new tab)
The impact of lattice Boltzmann method velocity discretization stencils on symmetry and accuracy: applications from pipe flow to stirred tanks
The importance of the velocity discretization stencil in the lattice Boltzmann method (LBM) has been demonstrated several times for turbulent channel and pipe flow simulations. This work presents novel stirred-tank benchmarks and methods to demonstrate anisotropic behavior in application-relevant simulations using the digital bioprocess platform SimVantage. The D3Q19 and D3Q27 LBM velocity discretization stencils are compared in a turbulent pipe and three different stirred tank reactor simulations. The D3Q27 stencil preserves isotropy better in both the pipe and stirred tank simulations; the coherent structure model improves wall shear stress predictions in pipe flow. We recommend that both models be available in every LBM software for engineering applications.
Read the full paper in Chemical Engineering Science (opens in a new tab)
Scale-up of Streptomyces species cultivations based on the morphological response to the energy dissipation rate
Filamentous microorganisms exhibit complex morphologies that influence product formation and are affected by various bioprocess parameters. This study investigates the scale-up of Streptomyces species cultivations from shake flasks to stirred-tank reactors while maintaining comparable morphology. Scale-up based on the average energy dissipation rate led to altered morphology; scale-up based on the maximum energy dissipation rate, quantified by CFD simulations, resulted in improved morphological consistency across scales, suggesting a new scale-up parameter. To overcome oxygen limitations, pure oxygen aeration was implemented, further enhancing morphological comparability.
Analyzing the effect of using axial impellers in large-scale bioreactors
In high-performance industrial fermentation processes, stirring and aeration may account for significant production costs. We apply alternating radial and axial impellers and demonstrate strong gas dispersion and energy-efficient mixing for the first time in a large-scale (160 m³) bioreactor, based on experimental and CFD simulation data. For equal operating conditions this setup yielded similar gas hold-ups and better mixing times (35%) compared to a classical Rushton-only configuration. Applying a radial impeller on an upper level for improving gas dispersion maintains the benefits of axial impellers in terms of reducing energy demand (up to 50%). This effect is significant only at large scale.
Read the full paper in Biotechnology and Bioengineering (opens in a new tab)
Characterization of the gas dispersion behavior of multiple impeller stages by flow regime analysis and CFD simulations
Multiple impeller reactors are widely used due to their advanced gas utilization and an increased volumetric mass transfer coefficient. The present study analyzes the individual flow regime, power input and gas hold-up in each compartment of a reactor equipped with four Rushton impellers. The results indicate that the pre-dispersion of the air introduced by the bottom impeller (up to 80%) plays a key role in a better gas retention efficiency of the upper impellers (>300%). A novel analysis of the bubble flow via a two-phase LES-based CFD model reveals that a more homogeneous distribution of air bubbles in the upper compartments leads to high compartment gas hold-up values.
Read the full paper in Biotechnology and Bioengineering (opens in a new tab)
Chapter Four: LBM for two-phase (bio-)reactors
Studying the hydrodynamics in industrial stirred tanks via Computational Fluid Dynamics has gained a lot of attention in recent years. As a highly parallelizable algorithm, the lattice Boltzmann method has been applied successfully for efficient calculation of single-phase as well as multiphase simulations. A Lagrangian approach for the calculation of the gas phase in industrial bioreactors is presented on top of the standard lattice Boltzmann method. Different fluid phase boundary conditions are compared, turbulence is modeled with Large Eddy Simulation, and local grid refinement as well as porous media models are discussed for production-scale reactors with internal heat exchanger bundles.
Read the chapter in Advances in Chemical Engineering (opens in a new tab)
Local gas holdup simulation and validation of industrial-scale aerated bioreactors
To date, the efficiency of industrial-size bioreactors has mainly been improved based on empirical knowledge. Euler–Lagrange simulations of the two-phase flow in large bioreactors were made possible by the calculation power of graphic cards. Bubble movements were captured via a Lagrangian approach; break-up and coalescence were modeled via stochastic algorithms. A conductivity sensor was used to measure the local gas holdup in a 150 L custom-built acrylic reactor across several flow regimes. Experimental results were in good agreement with the simulation data, especially at low stirring and low aeration rates. To prove the applicability of the code to large-scale problems, a 40 m³ reactor was simulated.
Read the full paper in Chemical Engineering Science (opens in a new tab)
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