Down-flow bubble column reactors have received great interest in recent years, due to their small size, stability, high gas-liquid interfacial area concentrations and longer residence times. In conventional bubble columns, average bubble sizes of ~5 mm and void fraction or gas hold up of less than ~20% are usually found leading to a typical interfacial area concentration of 200-300 m2/m3. The major limitation of these conventional bubble column reactors is that they have shorter gas phase residence times due to the high rise velocities of bubbles. Down-flow bubble columns provide longer gas phase residence times and gas-liquid contact times compared to conventional bubble columns. Further intensification can be achieved by efficiently generating micro-bubbles at higher void fractions. High gas-liquid interfacial area leads to high mass transfer rates. In the present work, experiments have been performed in a down-flow bubble column reactor with micro-bubbles generated by a novel mechanism to determine the gas-liquid interfacial area concentrations by measuring the void fraction and bubble size distributions using gamma densitometry and a high speed video camera. The results demonstrate that the new bubble generation technique offers high interfacial area concentrations (1,000 to 4,500 m2/m3) in comparison with previous bubble column reactor designs particularly because of micro-bubbles and high void fractions.
