DETERMINATION OF MASS-TRANSFER PARAMETERS IN SURFACE AERATED BIOREACTORS WITH BUBBLE ENTRAINMENT

The volumetric oxygen mass transfer coefficient k(L)a is a critical de sign parameter for most bioreactors and has been extensively studied. However, few reliable correlations are available for its estimation. I n this context, the k(L)n and specific interfacial area a of 3.5 1 and 16 1 surface-baffled and aerated double-helical-ribbon impeller (HRI) bioreactors were evaluated. This bioreactor configuration is characte rized by significant surface bubble generation and entrainment into th e liquid phase and has been shown to be highly efficient in the cultur e of fragile plant, mammalian and insect cells. The k(L)a and a parame ters were determined independently using the dynamic gassing-in method and a modified sulphite method respectively. A second order model wit h proper validity criteria was developed to account for the effect (up to 90% underestimation) of headspace gas mixing dynamics on the k(L)a determination of surface-aerated bioreactors. For a 3.5 1 bioreactor stirred at 250 rev min(-1) a surface k(L)a value of 52 h(-1) was obser ved while the value for the 16 1 bioreactor mixed at 150 rev min(-1) w as 44 h(-1). The liquid mass transfer coefficients k(L) obtained by di viding the measured k(L)a by the corresponding measured specific inter facial area are within the ranges reported in the literature for surfa ce aerated (2.0-3.2x10(-5) m s(-1)) and sparged (9-37x10-5 m s(-1)) co nventional systems.