Theoretical prediction of the minimum stirrer speed in mechanically agitated suspensions

The objective of this article is to derive theoretical equations for the minimum stirrer speed in vessels containing suspensions. There are a variety of equations for the prediction of this stirrer speed (Zwietering, Chem. Eng. Sci. 8 (1958) 244–253; Kneule and Weinspach, Verfahrenstechnik 1 (12) (1967) 531–540; Nienow, Chem. Eng. Sci. 23 (1968) 1453–1459; Einenkel, VDI-Forschungsheft Nr. 595, Düsseldorf, 1979; Einenkel and Mersmann, vt-Verfahrenstechnik 11 (2) (1977) 90–94; Niesmak, Thesis TU Braunschweig, 1982; Molerus and Latzel, Chem. Eng. Sci. 42 (6) (1987) 1423–1437; Zehner, Chem. Ing. Technol. 58 (10) (1986) 830–831). However, nearly all of them are derived from experimental results obtained in a limited range of vessel size and suspension properties [1, 2, 3]. Some relationships based on experiments carried out in small vessels are contradictory and not useful for scale-up. Reliable models are necessary not only for the design of stirred vessels but also for the reduction of power consumption. It is important to distinguish between two decisive processes: the ‘avoidance of settling’ and ‘off-bottom lifting’.