Abstract:The objective of this study was to investigate the hydrodynamics and the
gas-liquid mass transfer coefficient of an external-loop airlift reactor
(ELAR). The ELAR was operated in three cases: different inlet velocities of
fluids, different alcohols solutions (water, 0.5% methanol, 0.5% ethanol,
0.5% propanol and 0.5% butanol) and different concentration of methanol in
solutions (0%, 0.5%, 1%, 2% and 5%). The influence of superficial gas
velocity and various diluted alcohol solutions on hydrod… Show more
“…g , α gr , α gd , regime transition SAR Koji c et al33,49 0-0.25 0.1, 0.46, 0.5, 1.0, 5.0 wt% AP α g ELAR Miyahara et al31,50 0.015-0.12 0-1.0 wt% AP α g , k l a, k l , a, U Lc , d vs ELAR…”
The Marangoni effect on the hydrodynamics and mass transfer was studied in an internal loop airlift reactor with the addition of ethanol under elevated pressure. The overall gas holdup αg increased by ~10% and the gas–liquid volumetric mass‐transfer coefficient kla increased by ~20% on average for each one bar increase in pressure. When the mole fraction of ethanol xE increased from 0 to 0.012, αg increased by ~110% and kla increased by ~50%. However, when xE increased from 0.012 to 0.11, αg decreased by ~40% and kla decreased by ~25%. This volcano‐shaped relationship between both hydrodynamics and mass transfer behavior and ethanol concentration was determined by the Marangoni effect. Semi‐empirical correlations of αg and kla were proposed by including the Marangoni effect in a wide range of ethanol concentration and operating conditions. These correlations gave good predictions of αg and kla compared with extensive experimental data in the literature.
“…g , α gr , α gd , regime transition SAR Koji c et al33,49 0-0.25 0.1, 0.46, 0.5, 1.0, 5.0 wt% AP α g ELAR Miyahara et al31,50 0.015-0.12 0-1.0 wt% AP α g , k l a, k l , a, U Lc , d vs ELAR…”
The Marangoni effect on the hydrodynamics and mass transfer was studied in an internal loop airlift reactor with the addition of ethanol under elevated pressure. The overall gas holdup αg increased by ~10% and the gas–liquid volumetric mass‐transfer coefficient kla increased by ~20% on average for each one bar increase in pressure. When the mole fraction of ethanol xE increased from 0 to 0.012, αg increased by ~110% and kla increased by ~50%. However, when xE increased from 0.012 to 0.11, αg decreased by ~40% and kla decreased by ~25%. This volcano‐shaped relationship between both hydrodynamics and mass transfer behavior and ethanol concentration was determined by the Marangoni effect. Semi‐empirical correlations of αg and kla were proposed by including the Marangoni effect in a wide range of ethanol concentration and operating conditions. These correlations gave good predictions of αg and kla compared with extensive experimental data in the literature.
The influence of circulating liquid velocity on bubble size distribution (BSD) and turbulence characteristics of an external loop airlift reactor was studied in this paper. The instantaneous and time‐averaged velocity in the riser were studied using particle image velocimetry (PIV), and BSDs were measured by digital image analysis technique based on machine learning. Then turbulence kinetic energy (TKE) and energy dissipation rate (EDR) were calculated through the velocity field. The results indicate that as the circulating liquid velocity increased, the peak value of BSD rapidly decreased from nearly 6 mm to approximately 2 mm. The radial velocity of the liquid gradually decreased and changed direction, eventually increasing again. TKE first decreased and then increased. Compared with bubble flow with a BSD peak of 2–6 mm, bubble flow with a BSD peak of 2 mm had larger TKE. The radial movement of bubbles had great influence on the turbulence characteristics. This study demonstrates that selecting an appropriate circulating liquid velocity can reduce the diameter of bubbles while obtaining greater TKE, thereby improving the mass transfer and reaction efficiency in external loop airlift reactors (EL‐ALR).
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