Instabilities when using a standard (T/3) rushton turbine for stirring as foam disruption (SAFD)

Abstract

n the fermentation industry, the formation of foam during the fermentation process can lead to blockage of the outlets and reduce I the productive volume. Foam can be broken mechanically, through the installation of a foam disrupter, but the installation and operation of such a system can be costly. Another much used method is the addition of anti-foam, which however leads to a reduction in kp and consequently in oxygen transfer capacity (van't Riet and Tramper, 1991). A novel approach is the use of the upper impeller of a set of impellers for the elimination of foam. This technique has been described by Hoeks et al. (1997), who called it "stirring as foam disruption" (SAFD). Since the process of foam formation is a dynamic one (Pahl and Franke, 1999, the aim of SAFD is to use agitation to enhance foam disruption mechanisms. Hoeks et al. (1997) tested the concept by measuring, for a given combination of impellers, the thickness of the foam layer under different operating conditions and unaerated liquid level. They then gradually lowered the level until the agitation eliminated the foam layer. Altogether they tested five different upper impellers; two sizes of Rushton turbine, two downward pumping pitched blade impellers and a hydrofoil Lightnin' A31 5 pumping downwards, using a low viscosity fermentation broth. The "best" impeller is one that is able to disrupt foam at relatively high unaerated liquid volumes and low specific power inputs. Of the impellers tested, the Lightnin' A31 5 gave the best foam disruption performance since, although it did so with much the same amount of unaerated liquid volume as the other impellers at the same aeration rate, it consumed less power. However, all the impellers tested were of rather large impeller diameter (0) to tank diameter (T) ratio, i.e. 0.49Tto 0.64T. The Rushton turbines were of D/T = 0.49 and 0.62 whilst the "standard geometry" for Rushton turbines is a diameter of T/3 (Harnby et al., 1997; Oldshue, 1983). As SAFD is a new technique, its assessment with such "standard" impellers is important. Therefore, the use of the T/3 Rushton turbine for SAFD is the subject of this study.For multiple Rushton turbine systems, the impellers act independently in unaerated conditions provided that the inter-impeller clearance, AC > T (Hudcova et al., 1989). Under aerated conditions with such a spacing, the bottom impeller behaves like a single impeller whilst all the upper impellers behave the same but differently from the bottom one.The bottom impeller handles all of the sparged gas and gives the greatest power drop upon aeration provided N > N, , since the upper impellers handle less than the gas feed rate, as some gas will bypass them. This consequently leads to less drop in power in comparison to the bottom impeller (Hudcova et al., 1989; Abrardi et al., 1990 The concept of using the upper stirrer for foam disruption in a bioreactor agitated by multiple impellers has recently been published by Hoeks et al.(1 997). Thi s concept, stirring as foam disruption (SAFD), was shown...

Scale-up of stirring as foam disruption (SAFD) to industrial scale

“…In previous work, the tip speed ranged from 2.5 to 5 m/s, as occurs on a large scale [6]. Boon et al [2] suggested that the D/T ratio was fairly high, i.e. up to 0.6, for the laboratory experiments on SAFD by Hoeks et al [6].…”

“…However, P g /P u values of 0.2 are low for upper Rushton turbines as they are normally in the range of 0.5 and higher [16]. These very low values are probably due to increased gas entrainment when applying SAFD with the standard Rushton turbine (D/T=0.33) [2]. The observed range of P g /P u (see Table 4) related well to the measured range in gas hold-up (see Table 6 and below), i.e.…”

“…up to 0.6, for the laboratory experiments on SAFD by Hoeks et al [6]. Therefore, for each of the vessels used in the experiments, the lowest rotational speed was calculated from the tip speed of 2.5 m/s and an impeller diameter given by a D/T ratio of 0.33, which is ''standard'' for the Rushton turbine [2]. This rotational speed is referred to as the ''lower rotational speed''.…”

“…The model medium used is a combination of two surfactants [0.2 g/l Tween 40 (polyoxyethylene-sorbitan-monopalmitate) and 0.4 g/l polypropylene glycol P 2000 (PPG)] and 7.5 g/l salt (NaCl) in deionised water. The preparation of this medium has been described previously [2]. The temperature of the medium during the experiments was controlled at 25±0.5°C.…”

“…However, some impellers-particularly hydrofoil impellers-are more effective than others in foam disruption with respect to power draw [3,6,12]. The standard Rushton turbine (ratio of impeller diameter over tank diameter (D/T)=0.33) shows a rather poor performance with regard to SAFD [2]. Therefore, retrofitting is an attractive alternative when applying the SAFD technique to an existing bioreactor.…”