Mass transfer coefficient in stirred tank reactors for xanthan gum solutions

The oxygen transfer rate in Newtonian and non-Newtonian fluids in a stirred tank reactor (STR) of 20 1 of operating volume has been studied. The volumetric mass transfer coefficient, k(L)a, has been measured by a dynamic technique in solutions of xanthan gum for a wide interval of operational conditions. This coefficient has been determined changing many variables, such as the superficial gas velocity (V-s), the stirred speed (N) or the power input by unit of volume (P/V). Also, the properties of the liquid phase, mainly the theologic properties of the liquid, as well as the geometry of the stirrer, have been studied considering the effect of the number and type of stirrers (paddle or turbine), the number of blades of the stirrers and the sparger type (ring and disk). The k(L)n values are dramatically affected by the viscosity of the liquid; to take into account this effect, the rheology of the system has been described using both the Ostwald-de Waele and the Casson models. Dimensional correlations -as a function of the operational conditions- have been obtained, and also dimensionless equations expressing the Sherwood number as a potential function of the Reynolds, Aireation and Weber numbers- have been determined for several stirrer types. (C) 1998 Elsevier Science S.A.