The Effect of Rheological Complexities on Power Consumption in an Aerated, Agitated Vessel

Abstract

Power for agitation has been measured under aerated and unaerated conditions in a 0.29 m vessel of Rushton dimensions at specific powers up to 18 W/kg. The lluids studied were water. Newtonian solutions up to 19 m Pas and non-Newtonian shear thinning fluids some of which also exhibited a yield stress and some of which were viscoelastic. For the unaerated case. the power number-Reynolds number plot is in good agreement with the literature. For the aerated case, the resull can conveniently be divided into three Reynolds number regions. At Re > -900. the data obtained for the solutions is not markedly different [0 that for water except that a higher impeller speed is required to achieve complete gas dispersion. For -10 < Re < -900. the power drawn is independent of gassing rate and greater levels of elasticity and the presence of a yield stress leads to the lowest power numbers. For Re < --10. the gassed and ungassed power numbers are equal.

Effect of rheological properties on power consumption with helical ribbon agitators

“…The fact that the results for the highly shear thinning and viscoelastic xanthan solution in glycerol conform the Newtonian behavior up to rather large values of the Weissenberg number (-0.1) suggests that shear thinning delays the onset of viscoelastic effects. Qualitatively similar trends have been documented by Brito et al (1991) for a helical ribbon agitator and by Nienow et al (1983) for a Rushton type agitator. However, there does not appear to be a simple method of predicting a priori the onset of departure from the Newtonian curve in this complex flow configuration.…”

“…For instance, both Chavan and Ulbrecht (1973) and Yap et al (1979) concluded that the shear thinning effects completely overshadowed viscoelastic effects for helical ribbon impellers. Whereas Nienow et al (1983) reported a slight increase in power consumption for turbine impellers in viscoelastic xanthan gum solutions. In most of the aforementioned studies, aqueous polymer solutions (exhibiting both shear dependent viscosity as well as varying levels of viscoelasticity) have been used as model test fluids.…”

Agitator speed and dissolved oxygen effects in Xanthan fermentations

“…(2) is valid for D < D c < T, and the value of H c /D c is 0.4 for Rushton turbines . The calculations assume that each impeller consumes the same power because Po g is independent of the air flow through the impeller at Re < ∼1000 (Nienow et al, 1983). Detailed calculation procedures for estimating the well-mixed regions in Xanthan fermentations can be found in Amanullah et al (1996bAmanullah et al ( , 1997.…”

“…The air was dispersed by the impeller into a strongly bimodal distribution of bubble sizes as described earlier in mixing studies on fluids of similar rheological properties (Nienow et al, 1983). There were a few large bubbles (>∼1 cm) that passed through the fluid quickly and a myriad of small ones (<∼1 mm).…”

Mixing in the Fermentation and Cell Culture Industries

“…Aeration of these fluids in stirred tanks results in the formation of stable equi-sized cavities behind each impeller blade. Increases in the aeration rate do not change the cavity size significantly and hence the power draw (Nienow et al, 1983). In viscous shear thinning fermentation broths (whether a yield stress exists or not is debatable; see Amanullah et al, 1998a), cavern formation can occur.…”