Oxygen mass transfer characteristics in an internal‐loop airlift reactor with preset trumpet‐shaped riser

A size‐adjustable trumpet‐shaped riser was installed into a conventional airlift reactor to form a modified internal‐loop airlift reactor (ILAR). The volumetric oxygen mass transfer coefficient ( k L a ) and gas holdup of the ILAR were heavily influenced by the height ratio of riser to static fluid level above the spargers ( h / H ), the cross‐sectional area ratio of riser to downcomer ( A r / A d ) and the superficial gas velocity in the riser ( U g ) when deoxidized distilled water was used as the oxygen absorbent. An oxygen transfer rate of (2.17 ± 0.11) × 10 −5  kg m −3  s −1 , k L a of (27.88 ± 1.12) × 10 −3  s −1 and gas holdup of 0.031 were obtained with the ILAR of A r / A d  = 1.09 and h / H  = 0.73 at U g  = 18 mm s −1 . As the well‐constructed preset trumpet‐shaped riser provided ideal conditions for complete circulation and blending of the fluid, enhanced oxygen transfer efficiency was achieved. The gas holdup and k L a were increased by 16.2% and 10.2% at 19 °C, as compared with those of the conventional airlift reactor without this preset riser. The fluid rising velocity in the riser ( V R ) and falling velocity in the downcomer ( V D ) may be expressed by power‐law functions of U g and A r / A d , respectively. These metrics indicate that the ILAR has better performance, as compared with a conventional airlift reaction, for oxygen transfer and microorganism culture for chemical engineering, biological fermentation and environmental protection. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.