POWER CONSUMPTION IN UNBAFFLED TANKS: SUB AND SUPER-CRITICAL REGIMES

Abstract

Unbaffled stirred tanks are increasingly recognized as a viable alternative to common baffled tanks for a range of processes (e.g. crystallization, food and pharmaceutical processes, etc) where the presence of baffles is undesirable for some reason. Also, in the case of bioreactors for animal cell cultivation, where cell damage is mainly related to bubbles bursts at the air –liquid interface, unbaffled tanks have been shown to be able to provide sufficient mass transfer through the free surface vortex. As a consequence bubble formation and subsequent bursting is conveniently avoided (Scargiali et al., 2012). The same feature clearly makes unbaffled vessels potentially advantageous for any foaming gas-liquid system, provided that process rates, and relevant gas consumption needs, are compatible with the relatively small gas transfer rates achievable. Notwithstanding the increasing industrial interest towards unbaffled tanks, available experimental information on unbaffled tanks behavior is still scant, even for basic quantities such as mechanical power drawn. In this work the influence of Reynolds and Froude numbers on power consumption characteristics of unbaffled stirred tanks operating both in sub-critical conditions (the free surface vortex has not yet reached the impeller) and in super-critical conditions (the free surface vortex has reached the impeller and a gas phase is ingested and dispersed inside the reactor) is presented. Experimental results show that in the former case Power Number dependence on Reynolds and Froude numbers is consistent with previous results by Rushton et al. (1950). At rotational speeds higher than Ncrit (super-critical regime) air entrapment and dispersion inside the reactor occurs while a steep reduction on power number is observed. A novel correlation for power number prediction as a function of Reynolds and Froude number is finally proposed.