Spatially resolved gas fraction measurements in a bubble column through three-angled X-ray tomography

Abstract

Bubble columns are a popular choice for large-scale bioprocesses, yet model-based performance predictions provide major uncertainties in scale-up. High-resolution experimental data on local gas fraction and bubble size distribution are essential for reducing these uncertainties. Unfortunately, experimental characterization of gas-phase dynamics is often challenging. This study investigates a 3-angled X-ray tomography setup for measurement of spatially resolved gas fraction profiles in a 20 cm internal diameter bubble column with 100 cm static liquid height. X-ray scatter and beam hardening are identified as main sources of measurement error, and corrections were proposed and tested. It was found that software-based post-processing image corrections could correct for the effects of both beam hardening and cross-scatter resulting in gas fraction profiles that closely matched literature data for superficial gas velocity (u_g,s) of 1.7 cm s^-1, 4.6 cm s^-1, 5.8 cm s^-1 and 8.6 cm s^-1. Comparison between X-ray tomography and radial profiles from optical fiber probe and pressure probe measurements showed a discrepancy, with the fiber probe and pressure probes reporting roughly 20 % lower gas fraction values. An extra correction term might be needed to bring X-ray tomography into quantitative agreement with the complementary measurement methods. Overall, our work shows that three-angled X-ray tomography allows for rapid and reliable acquisition of gas fraction profiles in scale-invariant bubble columns. The method is a factor twelve faster than optical fiber probes, with increased radial resolution, but lacking the spatial and temporal resolution to identify single bubbles. The combination of X-ray tomography for gas fraction and fiber probes for bubble size distribution is an attractive setup for systematic studies gas phase dynamics in bubble columns.