IMPELLER-AGITATED AEROBIC REACTOR - THE INFLUENCE OF TINY BUBBLES ON GAS HOLD-UP AND MASS-TRANSFER IN HIGHLY VISCOUS-LIQUIDS

Gas hold-up structure in aerated viscous media (mu > 0.1 Pa s), which is distinctively characterised by a nearly biomodal bubble size distri bution, has been investigated in an impeller agitated reactor having a standard geometric configuration. Experiments were performed with aqu eous solutions of CMC, castor oil and rapeseed oil in a glass vessel o f 0.3 m internal diameter agitated by a standard six-bladed disc turbi ne. Large bubbles, some as large as the impeller, were formed, while t iny bubbles (d(t) = 0.1-3 mm) were found to accumulate for a while dur ing aeration. As a result, the gas hold-up was found to vary with time . The dynamic variation of tiny bubble hold-up could be described by t he equation epsilon(t) = epsilon(tf)(1 - e(-t/tau)), epsilon(tf) being the steady-state hold-up. The characteristic time constant, tau, was evaluated from this equation and its value was found to depend on the rheological and interfacial properties of liquids, impeller speed and gas velocity. Tiny bubbles were found to constitute as high as 70-80% of the total gas hold-up, and their contribution was also found to be a function of the aforementioned parameters. The effect of the formati on of tiny bubbles on mass transfer rates, in particular oxygen transf er rates in an aerobic bioreactor, has been discussed. Earlier researc hers who have recognised the formation of tiny bubbles in such highly viscous media have tended to assume that these bubbles have sufficient ly long residence times which enable them to attain an oxygen partial pressure which is in equilibrium with the dissolved oxygen level in th e liquid. It now appears that this may not be the case. Tiny bubbles d o actively contribute to oxygen transfer, and their contribution is mo re significant in impeller-agitated reactors than in bubble columns. A theoretical framework to establish when the contribution of tiny bubb les to oxygen transfer can be significant is presented. This has been done by defining a new dimensionless group N(D) = (k(L)a)t(tau)RT/epsi lon(tf)H where (k(L)a)t is the mass transfer coefficient due to tiny bubbles, H is the Henry's constant at the temperature T, and R is the universal gas constant. High values of N(D) would imply that the tiny bubbles have attained equilibrium, while low values indicate that the y are actively transferring solute. An expression for the ratio rate o f oxygen transfer from tiny bubbles to that from large bubbles is dedu ced in terms of N(D).