Effect of viscosity on homogeneous–heterogeneous flow regime transition in bubble columns

Abstract

Experiments were performed in a cylindrical 0.14 m diameter bubble column with a metal perforated plate. Air and aqueous solutions of glycerol with viscosity 1-22 mPa s were the phases. Gas holdup was measured and plotted against the gas flow rate. The critical point where the homogeneous-heterogeneous regime transition begins was determined by the drift-flux plot of the primary data. The homogeneous regime stability was expressed by the critical values of the gas holdup and gas flow rate. The results show that moderate viscosity (3-22 mPa s) destabilizes the homogeneous regime and advance the transition. The results indicate that low viscosity (1-3 mPa s) could stabilize the homogeneous regime. The destabilizing effect of the column height proved previously for air-water system applies also to viscous batches.

The dual effect of viscosity on bubble column hydrodynamics

“…The destabilization of the homogeneous flow regime is caused by the increased coalescence as discussed and experimentally observed by Wikinson et al (Wilkinson et al, 1992) and, more recently, by Yang et al (Yang et al, 2010). The increased coalescence may also suppress the homogeneous flow regime and, for μ L > 8 mPa·s, it may not exist even with a 'fine sparger' (Kuncová and Zahradník, 1995;Ruzicka et al, 2003;Zahradnik et al, 1997). Other authors discussed the effect of liquid viscosity on the flow regime transition, indicating that many basic questions about this effect remain unanswered.…”

“…However, the coupling between the phases causes deviations from linearity (see ref. (Ruzicka et al, 2003)). In the homogeneous flow regime, the hindrance reduces the bubble velocity, thus increasing the gas holdup, whereas in the transition flow regime, the presence of large bubbles along with the enhanced circulations (which increase bubble velocity) result in a decrease of the gas holdup and cause the gas holdup to decrease less than proportionally to the gas flow rate.…”

“…Khare and Joshi (Khare and Joshi, 1990) (d c =0.20 m, H c =3.0 m, sieve plate, d 0 =2.0 mm) studied the influence of viscosity on the gas holdup using aqueous solutions of glucose: as a result, they observed that the gas holdup first increases up to μ L =4 mPa s and then it decreases for 4 < μ L < 10 mPa·s. Ruzicka et al (Ruzicka et al, 2003) (d c =0.14 m, H 0 =0.2 -0.8 m, perforated plate, d 0 =0.5 mm) summarized the influence of the viscosity over the gas holdup as follows: with increasing liquid viscosity the gas holdup increases for μ < 3 mPa·s and it decreases for 3 < μ < 22 mPa s. Olivieri et al (2011) (d c =0.12 m, H c =2 m, H 0 =0.8 m, needle sparger, d 0 =0.4 mm) investigated the effect of the liquid viscosity using several aqueous solutions of Alginate within the viscosity range 1-117 mPa s: they reported an increase in the gas holdup up to μ L =4.25 mPa s and, then, a decrease at higher viscosities.…”

“…When increasing the viscosity, the tendency to coalescence prevails, creating large bubbles rising the column at a higher velocity, thus reducing the gas holdup. In particular, the "dual effect of the viscosity" was clearly observed in some experimental investigations (Bach and Pilhofer, 1978;Eissa and Schügerl, 1975;Godbole et al, 1982;Khare and Joshi, 1990;Olivieri et al, 2011;Rabha et al, 2014;Ruzicka et al, 2003), but it is not clear whether it also applies to larger bubble columns (see, for example, the fluid dynamics phenomena behind Eq. (1)) and what the reasons are behind this behavior.…”

“…Other authors discussed the effect of liquid viscosity on the flow regime transition, indicating that many basic questions about this effect remain unanswered. Ruzicka et al (2003) encountered this effect when using aqueous solutions of glycol. They found out that the homogeneous flow regime is stabilized at low viscosities (μ L =1-3 mPa s) and that U G, trans increases with the viscosity in this range, whereas it is destabilized at moderate viscosities (μ L =3-22 mPa s).…”