Comparative evaluation of hydrodynamic and gas-liquid mass transfer characteristics in bubble column and airlift slurry reactors

Three pneumatically agitated reactors - a bubble column and two airlift devices - with identical rectangular cross-sections (0.456 m x 0.153 m), working heights (1.64 m) and equivalent gas sparging arrangements were compared in terms of the hydrodynamic and oxygen transfer performance. The two airlift reactors had identical riser-to-downcomer cross-sectional area ratios of 1.0, but differed in being sparged either in the central draft-tube or in the peripheral risers. The reactors produced comparable overall gas holdups for otherwise identical conditions in air-water or air-water-glass bead (0.069 mm particle diameter, 0%-5% (v/v) solids loading) systems. For the airlifts, irrespective of the sparging configuration or the solids loading, the same linear equation could relate the riser and the downcomer gas holdups. The velocity of the induced liquid circulation was not affected by solids loading, but the central draft-tube sparged design produced consistently higher velocities than did sparging in peripheral tubes. The bubble column had the poorest mixing performance. Complete suspension of solids occurred in all reactors in the range of superficial air velocities (0.01-0.08 m s(-1)) tested; however, the distribution of solids was non-uniform in the bubble column. The airlift devices achieved homogeneous distribution. The oxygen transfer capability of the three reactors was comparable, with the bubble column performing slightly better.