STUDIES ON MASS-TRANSFER CHARACTERISTICS OF A MODIFIED AIRLIFT FERMENTER

An approach to modify external loop airlift bioreactor is presented th at examines its performance with respect to mass transfer. There are v arious designs of airlift fermenter [1]. In the proposed system [2] th e riser has been replaced by a tube of irregular geometry, in the form of converging-diverging sections (CDT-ALF), so that better mass trans fer may be obtained due to better liquid mixing caused by the bubble f low, pulsation effect and early transition to turbulence. Mass transfe r characteristics of the modified airlift fermenter CDT-ALF were studi ed and compared with those of a conventional one, UT-ALF. Overall volu metric mass transfer coefficient, K-L alpha, was determined by sulfite oxidation method. K-L alpha was determined with respect to U-G for di fferent h(i). Higher K-L alpha was always observed in CDT-ALF compared to that in UT-ALF under any operating condition of h(i) and U-G. If t he K-L alpha values are compared in both the systems under their optim um conditions of h(i) and U-G, CDT-ALF showed 122.5% higher values of K-L alpha compared to UT-ALF. However, when both the systems were oper ated at the lowest experimental conditions of U-G, the k(L) alpha in C DT-ALF was found to be 170% higher. In UT-ALF while with the decrease of U-G, k(L) alpha decreased, in CDT-ALF the reverse was observed i.e. at low U-G, K-L alpha was higher. However with the increase of h(i), K-L alpha decreased in both the systems. To predict volumetric mass tr ansfer coefficient K-L alpha, empirical correlations were developed by dimensional analysis for both the reactors. The correlations were exp erimentally verified to determine their reliability to predict mass tr ansfer coefficient and the deviation was found within reasonable limit .