HYDRODYNAMIC BEHAVIOR OF A LIQUID-LIFT, EXTERNAL-LOOP BIOREACTOR USING A SPINNING SPARGER

The liquid-lift bioreactor is conceptually similar to the familiar air -lift bioreactor. A liquid is sparged into the base of a column contai ning a second, immiscible liquid of higher density. The two phases ris e cocurrently to the top of the column, where they are separated. The dense phase is then recycled to the base of the riser, whereas the lig ht phase is removed from the bioreactor. The hydrodynamic characterist ics of a 12 L, liquid-lift, external-loop bioreactor have been investi gated using water and oleic acid as the continuous and dispersed phase s, respectively. The experimental unit had a working height of 1.7 m a nd a downcomer to riser area ratio of 0.43. A spinning sparger consist ing of six, 1 mm diameter orifices spread evenly on a 4.4 cm diameter circle was incorporated near the base of the riser to allow for enhanc ed control of the produced droplets. Experimental studies were underta ken at superficial dispersed-phase velocities up to 4 cm/min and sparg er spinning speeds up to 350 rpm (maximum orifice tangential velocity of 0.8 m/s). Uniform droplets were produced at diameters ranging from 1 to 5 mm, while liquid holdups and circulation velocities reached up to 2% and 3 cm/s, respectively. The droplet size data were best fit to an empirical model, and the well-known drift-flux theory of Zuber and Findlay was used to predict the dispersed-phase holdup. The circulati on velocity of the continuous phase was predicted using an energy bala nce around the loop. The model was found to provide reasonable predict ions of droplet diameter, dispersed phase holdup, and circulation velo city as functions of both the dispersed-phase superficial velocity and the spinning speed of the sparger.