Influence of Viscosity on Mass Transfer Performance of Unbaffled Stirred Vessels

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled vessels. However they may be expected to provide significant advantages in a wide range of applications (e.g. crystallization, food and pharmaceutical processes, etc) where the presence of baffles is often undesirable. Moreover, in plants or animal cell cultivation bioreactors, where cell damage is often caused by bursting bubbles at the air -medium interface (Barret et al., 2010), they can provide sufficient mass transfer through the free surface vortex, so bubble formation and subsequent bursting inside the reactor can be conveniently avoided (Scargiali et al., 2012). In this work the influence of viscosity on oxygen transfer performance of an unbaffled stirred vessel is investigated in view of its use as a biochemical reactor for animal cell growth. Liquid viscosity was increased by adding weighted amounts of polyvinylpyrrolidone (PVP) to distilled water. Experimental results show that at rotational speeds lower than the critical one (Ncrit, at which the free surface vortex reaches the impeller), despite the absence of gas dispersion inside the reactor and relevant cell damage due to bubble bursting, gas-liquid mass transfer is not adversely affected by viscosity and the systems remains able to provide sufficient oxygen for typical animal cell cultures. At rotational speeds higher than Ncrit air entrapment and dispersion occurs inside the reactor and an increase of mass transfer performance is observed while increasing viscosity, probably due to smaller bubble coalescence rates due in turn to the viscosity increase itself as well as to gas-liquid interface modifications by PVP.