Characterization of the gas dispersion behavior of multiple impeller stages by flow regime analysis and CFD simulations

Abstract

Multiple impeller reactors are widely used due to their advanced gas utilization and an increased volumetric mass transfer coefficient. However, with the application of Rushton impellers, gas dispersion efficiency varies between the bottom and the upper impeller levels. 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%) and leads to a shift of the cavity and flooding lines in the flow map (Fr-vs Fl-Number) of the upper impellers. A novel analysis of the bubble flow in the dispersed state via a two-phase LES-based CFD model reveals that a more homogenous distribution of air bubbles in the upper compartments leads to high compartment gas hold-up values, but fewer bubbles in the vicinity of the impellers. The measured and simulated data of this study indicate that the upper impellers' efficiency mostly depends on the flow regime of and the pre-dispersion by the bottom impeller rather than on the upper impellers' flow regimes. These results contribute to the understanding of essential mixing processes and scaling of aerated bioreactors.

Analyzing the effect of using axial impellers in large‐scale bioreactors

“…Similar to the experiments, both impeller configurations (A and B) were studied with an impeller speed of 100 min −1 and dosing the salt solution from the top of the reactor. Although the CFD simulation code was verified in the previous works by Witz et al (2016) and Bernauer, Schopf, Eibl, et al (2021), the simulation results were verified in a first step with the experimental data of the mixing time study. For this, the simulated salt concentration at the positions of the two conductivity probes (Figure 1) was compared with the experimental data.…”

“…The calculation of the difference of these two measurements yielded the power draw of the fourth impeller level. This approach was also applied to estimate the top impeller in the Rushton‐only configuration as the power draw of each impeller is different (Bernauer, Schopf, Eibl, et al, 2021). A potential influence of the absent fourth impeller on the power draw of three impeller levels was decided to be negligible as the values were used for a direct comparison of the two impeller types in the same system.…”

“…Earlier work (Bernauer, Schopf, Eibl, et al, 2021) 1. All dimensions of the simulated reactor are identical to the largescale reactor to allow a direct comparison (see Figure 1).…”

“…The calculation of the difference of these two measurements yielded the power draw of the fourth impeller level. This approach was also applied to estimate the top impeller in the Rushton-only configuration as the power draw of each impeller is different (Bernauer, Schopf, Eibl, et al, 2021).…”

“…Froude number (Fr), Flow number (Fl, gas flow rate at the bottom impeller). The green line represents the flooding line and the blue line represents the cavity line as described inBernauer, Schopf, Eibl, et al (2021) 3.1 | Gas hold-upWe characterized the gas dispersion of the two impeller configurations, radial-only (A) and alternating radial/axial (B), by determining the gas hold-up in the 160 m 3 bioreactor. In most cases, the alternating radial/axial impellers exhibit an identical or higher (better) gas hold-up as the Rushton-only configuration for identical operating conditions (impeller speeds and gas flow rates) (Figure4), particularly above a certain stirring speed.…”