The Impact of Contaminants on the Hydrodynamics and Mass Transfer Characteristics of Pilot-Scale Microbubble Aerated Columns

Abstract

Gas-liquid contacting plays a key role in many industrial operations, where achieving high volumetric mass transfer coefficient values (kLa) is crucial for enhanced performance. However, contaminants present in almost all natural and industrial streams significantly reduce the liquid-side mass transfer coefficient, kL, a challenge that can be addressed by employing fine and ultra-fine bubbles. These bubbles create a large specific interfacial area of contact between phases, a factor which is critical for enhancing mass transfer performance. This thesis focuses on real-world industrial applicability and comprises four original research papers addressing the aforementioned issues and how to mitigate them. To ensure relevance to industrial practice, the overall research findings highlighted the importance of conducting systematic experiments, using contaminated G/L systems, and collecting accurate and reproducible data generated in pilot-scale contactors. The first paper in this series addresses the critical task of minimizing errors and biases in estimating kLa, a key element in evaluating the gas utilization efficiency encountered under highly intensified conditions. A novel method for enhancing the accuracy and reliability of the kLa estimates is developed, which significantly reduces the errors and biases introduced by conventional approaches. Subsequent papers further investigate the importance of generating and maintaining substantial interfacial contact areas between the phases and maintaining desirable flow patterns in contaminated systems. Experiments were conducted using a 170 L, 5.7 m tall pilot-scale microbubble-aerated setup equipped with a novel adjustable dual-phase sparger. An innovative DGD technique was developed and used to characterize the G/L contacting patterns achieved. The ensuing information database (comprising 313 highly accurate and reproducible runs) provides insights into the hydrodynamic characteristics in different sections of the column under varying gas flow rates, initial bubble sizes, and contaminant concentrations. Gas holdups of 36% and a specific interfacial area of 5,470 m2/m3 were achieved at superficial gas velocity as low as 50 mm/s. Lastly, the study introduces a novel criterion, 'Coefficient of Variation', for identifying flow regimes encountered under highly intensifying contacting conditions in the bubble column and their transitions.