VOLUMETRIC MASS-TRANSFER COEFFICIENT IN CONCENTRIC-TUBE AIRLIFT BIOREACTORS

Higbie's penetration theory is combined with a model of gas hold-up in order to set up an expression for the prediction of the volumetric ox ygen mass transfer coefficient, K(L)a, in airlift bioreactors. The res ults indicate that the K(L)a is a strong function of the superficial g as velocity, gas hold-up, liquid circulation velocity, bubble diameter and bubble rise velocity. A 2501 pilot-scale concentric cylinder airl ift bioreactor having a shell diameter of 0.317 m and a draught tube d iameter of 0.211 m is used to obtain local measurements of the K(L)a a s a function of the superficial gas velocity in the riser during the f ermentation of Saccharomyces cerevisiae. The results show that in the downcomer, K(L)a, increases with increasing height of the liquid from the sparger while in the riser the K(L)a is relatively uniform and has a value similar to that observed in the lower section of the downcome r. The K(L)a measurements are described and discussed using the propos ed model.