The flow pattern and oxygen transfer characteristics were studied for laboratory concentric tube air lift reactor operating with air-water mixtures.
Two distributer types (annular dispersion, center dispersion) and three draft tubes (12.4cm, 13.4cm, 14.6cm I.D. each with a thickness of 0.3 cm and a length of 89 cm) were employed throughout this study.
The effects of the air flow rate (0.4 - 5.9 l/s), the distributer type, and the draft tube size on mixing time, the circulation time, the local and overall holdup, and the overall mass transfer coefficient were examined.
The mixing time and the circulation time were measured using capacitor meter, platinized Pt probe, and KCl solution as tracer. Also the local and overall holdup in each section were measured by manometric method.
The oxygen transfer coefficients between air bubble and water were calculated from the amount of the dissolved oxygen (D.O.).
The circulation time was very sensitive to air flow rate at low gas flow rate, while nearly independent at high gas flow rate.
In center dispersion, the mixing time of large draft tube was shorter than that of small draft tube, in annular dispersion, the mixing time profile was crossed at about 2.7 lit/sec.
The overall holdup of large cross-sectional area of riser was higher than that of the small cross-sectional area of riser.
The mass transfer coefficient of annular dispersion was larger than that of center dispersion at low gas flow rate, but the values of each case were nearly the same at high gas flow rate.
The mass transfer coefficients increased linearly with the overall gas holdup.
In this report, some correlations are suggested to represent the effects of gas flow rate on gas holdup and mass transfer coefficient.
$ln(K_La) = -2.3114 + (0.7480)ln\epsilon$
for Annulus Dispersion
$ln(K_La) = -1.2589 + (1.0685)ln\epsilon$
for Center Dispersion