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 volum etric mass transfer coefficient, k(L)a, has been measured by a dynamic tech nique in solutions of xanthan gum for a wide interval of operational condit ions. This coefficient has been determined changing many variables, such as the superficial gas velocity (V-s), the stirred speed (N) or the power inp ut by unit of volume (P/V). Also, the properties of the liquid phase, mainl y the theologic properties of the liquid, as well as the geometry of the st irrer, have been studied considering the effect of the number and type of s tirrers (paddle or turbine), the number of blades of the stirrers and the s parger 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 C asson models. Dimensional correlations -as a function of the operational co nditions- have been obtained, and also dimensionless equations expressing t he Sherwood number as a potential function of the Reynolds, Aireation and W eber numbers- have been determined for several stirrer types. (C) 1998 Else vier Science S.A.