In bubble column reactors there are two principal flow regimes [1,2,3], the
homogeneous (HoR) and the heterogeneous (HeR). These reactors have different behaviour in
HoR and HeR, thus the dependences of the rates of mass, heat and momentum transfer on the
design and operating parameters (such as reactor geometry, gas and liquid flow rates and
properties of the contacting phases) are also very different. Therefore, for rational reactor
design and operation it is of crucial importance to know the range of parameters over which a
certain regime prevails and the regime transition conditions [4].
The effect of the presence of solids on gas holdup in bubble columns has been
extensively investigated. Most of the published work reports that the gas holdup decreases
with increasing solid concentration [5-14]. However, a favourable effect of solids on gas
holdup has been also observed by Douek et al. [15]. In spite of all these efforts, information
on the effect of solids on homogeneous-heterogeneous regime transition is very scarce.
Krishna et al. [16] found that the transition gas holdup was significantly reduced due to the
presence of silica particles. Xie et al. [17] reported that as pulp consistency is increased, the
regime transition is delayed.
The goal of this work was to examine the influence of solid particles on homogenous
regime stability and regime transition in a three phase bubble column. For that, two studies
were done, one focused on the regime transition and the other on visualization of bubbleparticle
interaction.
The regime transition experiments were performed in a cylindrical bubble column of
0.14 m diameter. Air, distilled water and calcium alginate beads (deq=2.1mm) at
concentrations up to 30%(v/v) were the phases. The dependence of the gas holdup (e) on the
gas flow rate (q) was measured. At low solid loading, the experimental data e(q) show a slight
increase of gas holdup with solid concentration, which is not normally observed. At higher
solid loading, a significant reduction of gas holdup with solid concentration is observed. This
suggests that solid concentration could play a dual role in the homogeneous regime stability:
low concentrations stabilize, while high concentrations destabilize. The critical point, where
the HoR loses stability and the transition begins, was evaluated using the drift flux plot. The
critical values of gas flow rate and voidage were the measures of the homogeneous regime
stability. For low solid volume fraction, a stabilizing effect of the presence of solids is shown
by an increase in the critical values, while for higher solids volume fraction ( φ
s>3%) the
critical values decrease, witnessing a destabilizing effect of the solids.
The visualization study was performed in a 0.07 m diameter cylindrical bubble
column, using a standard and high-speed cameras. This work shows important phenomena
that result from the bubble-particle interactions, which partly explains the regime transition
results. At low solid loading, bubble-particle collisions lead to reduction of bubble rise
velocity that results in higher gas holdup and a delay of the transition. At higher solid loading,
the bubble coalescence rate increases with the solid content that decreases the gas holdup and
advances the transition.Commission of the European
Communities for the Marie Curie Training Site - contract No. HPMT-CT-2000-0074
