ABSTRACT
Introduction ........................................................................................................105 Gas Hold-Up Structure in Viscous Liquids ..................................................106 Effect of Tiny Bubbles on Mass Transfer .......................................................109 Experimental Observations on Gas Hold-Up in Impeller Agitated Systems ............................................................................................... 111 Conclusions ........................................................................................................ 116 References............................................................................................................. 118
Viscous media are frequently encountered in aerobic biotechnological processes, and gas bubbling into such media is an essential feature. An efficient contact between gas and liquid phases is necessary to maintain high reaction rates. The production capacity of aerobic bioreactors is often limited by the rate at which oxygen is made available to microorganisms. High viscosity of certain biological media (e.g., xanthan gum and polysaccharide fermentation) makes it more difficult to transfer oxygen. It is therefore extremely important to understand and decipher the mechanism of oxygen transfer, especially in media that are highly viscous and non-Newtonian. The key variable indicating the efficiency of gas-liquid contacting, and in turn influencing oxygen transfer, is gas hold-up, i.e., fraction of the dispersion constituting the gas phase. This chapter examines the gas hold-up structure in impeller agitated viscous liquids, reviews the link between operating variables and gas hold-up, and presents models that can be used to analyze gas-liquid mass transfer in such systems.
