Scale-up of Aerated Industrial Multistage Rushton Impeller Bioreactors with Complex Rheology

The power input and gas-liquid mass transfer rank among the most important industrial fermentation process parameters. The present study analyzes the power input and gas hold-up as a function of the flow regime, impeller diameter, and rheological properties in a pilot scale reactor (160 L) equipped with four Rushton impellers. This leads to four dimensionless numbers for predicting measurements in pilot and industrial bioreactors (110 and 170 m3) with a standard deviation of 7 % to 29 %. This is unparalleled for the underlying aerated and non-Newtonian fermentation broths. Several existing correlation equations are discussed to be dissatisfying (up to 130 % deviation), and might be sufficiently valid only within scale or for small scaling factors. The introduced approach predicts adequately accurate over three orders of magnitude. Based on these encouraging results, we identified the Galilei number and the power concept as the central elements in combination with the consequent dimensional analysis for an efficient scaling betweeen pilot and industrial scale.

Application of CFD to Analyze the Hydrodynamic Behaviour of a Bioreactor with a Double Impeller

Stirred bioreactors are commonly used unit operations in the pharmaceutical industry. In this study, computational fluid dynamics (CFD) was used in order to analyze the influence of the impeller configuration (Segment–Segment and Segment–Rushton impeller configurations) and the impeller rotational speed (an operational parameter) on the hydrodynamic behaviour and mixing performance of a bioreactor equipped with a double impeller. A relatively close agreement between the power values obtained from the CFD model and those measured experimentally was observed. Various parameters such as velocity profiles, stress generated by impellers due to the turbulence and velocity gradient, flow number, and mixing time were used to compare the CFD simulations. It was observed that the impeller’s RPM could change the intensity of the interaction between the impellers when a Segment–Rushton impeller was used. In general, increasing the RPM led to an increase in total power and the stress acting on the cells and to a shorter mixing time. At a constant RPM, the Segment–Rushton impeller configuration had higher total power and stress acting on cells compared to the Segment–Segment impeller configuration. At lower RPM values (i.e., 50 and 100), the Segment–Segment impeller provided a shorter mixing time. Conversely, at the highest RPM (i.e., 150) the Segment–Rushton impeller had a shorter mixing time compared to the Segment–Segment impeller; this was attributed to the high level of turbulence generated with the former impeller configuration at high RPM.

A Study of Dispersed Phenomena on Rushton and Rushton V-CUT

The flow behavior of liquid-solid particles in mixing tanks using a modified Rushton impeller, called a Rushton V-cut impeller, was studied. Both the Rushton and Rushton V-cut impellers were compared at a 300 rpm stirring speed and a 10 % wt solid concentration. Hydrodynamic behaviors, such as solid volume fraction, velocity, pressure, and shear stress, in both the Rushton and Rushton V-cut impellers were investigated. Computational fluid dynamics (CFD) software able to understanding of hydrodynamics of stirring liquids which contain solid particles. CFD programme was to predict the mixing flow of the highly viscous system. Therefore, the present work was carried out using CFD software with the Eulerian-Eulerian approach with a turbulent k-ε model. The simulation of mixing tanks was consisted of moving and stationary zones by using moving references frame method or MFR. The results were observed that Rushton V-cut can dramatically reduce pressure up to 20% and the shear stress up to 64.38% while keeping the liquid-solid mixing at a considerable degree. Therefore, this design can reduce the power consumption.

The Influence of Impeller Combination on the Gas-Liquid Dispersion Performance of a Coaxial Mixer in Viscous Fluids

As a kind of mixer with wide adaptability, coaxial mixer has a wide-application in the process industry. With the help of experiment and numerical simulation, the gas-liquid dispersion performance of four impeller combinations in viscous system was studied in terms of the global gas holdup, local gas holdup, local bubble size and liquid phase flow field. Wall-scraping anchor is the common outer impeller of the four impeller combinations, and the four inner impellers are Rushton turbine, six-straight-blade turbine (SBT), 45° six-pitched-blade turbine pumping downwards (PBTD) and 45° six-pitched-blade turbine pumping upwards (PBTU). The experimental results indicate that the global gas holdup of the Rushton impeller combination is the highest among the four impeller combinations and that of PBTU is the lowest. The local gas holdup of Rushton and PBTU combinations are all high, but the gas distribution with the Rushton combinations is more uniform. Besides, the local bubble size of the four impeller combinations has the same tendency with the local gas holdup. Under the same conditions, the flow field and distribution of gas holdup of the four impeller combinations in stirred tank were simulated numerically. Compared with the experimental results, the simulation results show a good agreement on the value and distribution of local gas holdup. Among the four coaxial mixers, the Rushton impeller combination is more suitable for gas-liquid dispersion.

Study of the Turbulent Flow Structure around a Standard Rushton Impeller

The velocity field around the standard Rushton turbine was investigated by the Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) measurements. The mean ensembleaveraged velocity profiles and root mean square values of fluctuations were evaluated at two different regions. The first one was in the discharge stream in the radial direction from the impeller where the radial flow is dominant and it is commonly modelled as a swirling turbulent jet. The validity range of the turbulent jet model was studied. The second evaluated region is under the impeller where flow seems to be at first sight rather rigorous but obtained results show nonnegligible values of fluctuation velocity.

CFD Simulation of the Discharge Flow from Standard Rushton Impeller

The radial discharge jet from the standard Rushton turbine was investigated by the CFD calculations and compared with results from the Laser Doppler Anemometry (LDA) measurements. The Large Eddy Simulation (LES) approach was employed with Sliding Mesh (SM) model of the impeller motion. The obtained velocity profiles of the mean ensemble-averaged velocity and r.m.s. values of the fluctuating velocity were compared in several distances from the impeller blades. The calculated values of mean ensemble-averaged velocities are rather in good agreement with the measured ones as well as the derived power number from calculations. However, the values of fluctuating velocities are obviously lower from LES calculations than from LDA measurements.