Effect of design configurations on hydrodynamic behavior and gas-liquid mass transfer in the internal-loop airlift contactor

Abstract

The effect of design configurations on hydrodynamic behavior and gas-liquid mass transfer in three types of internal-loop airlift contactors were investigated. The model systems were (i) conventional concentric tube airlift contactor so-called ALC, (ii) ALC with baffles installed in the riser so called ALC-B, and (iii) ALC with a vertically split draft tube so called ALC-S. Each experiment was performed with a specific power input (PG/VL) ranging from 55-357 W/cubic m. Baffles were found to obstruct a flow of liquid and gas bubbles, and to induce more bubble coalescence resulting in a larger bubble diameter. This negatively influenced gas-liquid interfacial area (a) in the contactor. However, the baffles encouraged more turbulence in the system and led to a higher value of mass transfer coefficient (kL). The effect on "kL" found to be more outstanding than the effect on "a" and thus the overall gas-liquid mass transfer coefficient (kLa) in the ALC-B was higher than in the ALC. This is true only for the system at the specific power input less than 200 W/cubic m. At higher values of PG/VL the effect of baffles disappeared and the performance of ALC-B was close to that of the conventional ALC due to the existence of dead zones below each baffle plate. In the ALC-S was expected that there was a resistant layer in the space between the upper and lower draft tubes of the contactor which obstructed the liquid flow. This resulted in a lower liquid circulation in the ALC-S when compared to the liquid circulation in the ALC. However, it was observed that this configuration facilitated the occurrence of small bubbles resulting in an increase in the specific gas-liquid interfacial area (a). Due to this larger "a", a more favorable value of mass transfer was obtained in the ALC-S than in the ALC. However, at high PG/VL (>200 W/cubic m.) the effect of vertically split draft tube on the mass transfer tended to significantly decrease. The reason for this phenomenon can not be realized from the existing experimental setup, and more investigation is needed.