Steady-state axial profiles of dissolved oxygen in tall bubble column bioreactors

A model is developed for prediction and interpretation of the observed stea dy-state axial dissolved oxygen concentration profiles in tall bubble colum ns. The observed concentration profiles are non-linear, unlike what would b e expected if the hydrostatic pressure alone influenced the profiles. The n on-linear profiles result from the axial mixing of liquid in the column. Se veral other factors influence the profiles, including the overall gas holdu p, the volumetric overall gas-liquid mass transfer coefficient, and the sta tic height of liquid in the column. The effect of mixing can be adequately accounted for using an axial dispersion coefficient. Because the axial disp ersion coefficient is sensitive to the diameter of the column and to gas fl ow rate, the overall behavior of the profile is affected by the aspect rati o of the column and the superficial gas velocity in it. The mass transfer c oefficient and the axial dispersion coefficient have mutually opposing effe cts on the shape of the profile. Because both those variables increase with increasing gas flow rate, the shape of the profile is affected less than w ould be the case if only mixing influenced the profile. The non-linearity o f concentration profiles increases with increasing overall height of the co lumn especially when the height exceeds about 2 m in a 0.24 m diameter colu mn. The model-predicted axial concentration profiles agree closely - within +/- 3% - with the measured data. Using the measured profile, the model all ows for calculation of the liquid-phase axial dispersion coefficients. This method does not require the use of tracers. Being a steady-state method, t he operation of the bioreactor does not need to the interrupted in any way for the determination of the axial dispersion coefficient or the state of m ixing. Consequently, the proposed method is particularly suited to characte rizing the axial dispersion coefficient in an operating bioreactor without disturbing the operation. If the axial dispersion coefficient is known, the model allows for quantifying the spatial inhomogeneities in oxygen concent ration in a bioreactor vessel. (C) 1999 Elsevier Science Ltd. All rights re served.