Introduction
Pilot- and large-scale fermentation processes are mostly carried out in
bioreactors equipped with multiple impellers. They provide better gas
utilization with higher gas phase residence times, an increased gas
hold-up, and thus, a higher volumetric mass transfer coefficient
(Gogate, Beenackers, & Pandit, 2000). Despite the variety of new
impeller models, the most common impeller for microbial fermentation and
for educational purposes is still the Rushton turbine. The bottom
impeller has been shown to behave differently from the upper impellers
regarding flow regime and the resulting oxygen mass transfer coefficient
and power input. For instance, several studies showed that the gas
hold-up in case of a multiple impeller reactor is higher in the upper
impeller compartments than in the bottom impeller compartment (Gogate et
al., 2000; Linek, Moucha, & Sinkule, 1996; Nocentini, Magelli,
Pasquali, & Fajner, 1988). Gogate et al. (2000) assumed that gas
hold-up in the second impeller compartment is about 30 ‒ 40 % higher
than in the bottom compartment and Linek et al. (1996) determined an
increase of about 15 % on average for the upper compartments, paired
with a higher total oxygen mass transfer coefficient of about 40 % in a
four-level reactor.
At the same time, numerous studies showed that the bottom impeller
exhibits a more pronounced power drop, when aerated than the upper
impellers (Hari-Prajitno et al., 1998; Linek et al., 1996; Middleton &
Smith, 2004; Warmoeskerken & Smith, 1988). Linek et al. (1996) measured
an about 50 % higher power draw by the upper impellers. Nienow and
Lilly (1979) assumed that the reason for this behavior is that the
number of air bubbles passing the upper impellers must be significantly
lower than those at the bottom impeller.
When aerated, Rushton impellers form cavities behind the impeller
blades. Extensive studies (Bombac, Zun, Filipic, & Zumer, 1997; Bruijn,
Vantriet, & Smith, 1974; Nienow, Warmoeskerken, Smith, & Konno, 1985)
analyzed the different forms and transitions of the cavities in relation
to impeller speed and aeration rate. In the so-called vortex cavity (VC)
regime, which appears at low aeration rates and high impeller speeds,
vortices behind the impeller blades disperse the air. In the loaded
state at higher aeration rates, the gas accumulates in the low-pressure
zone behind each impeller and forms large cavities (LC). With a further
increase in aeration rate, the large cavities grow until reaching the
next impeller blade resulting in the ragged cavity regime (RC), or also
called “flooding”. Typically, flooding occurs earlier at lower
impeller speeds. The transitions between these flow regimes have been
studied comprehensively for various operating conditions and reactor
geometries for reactors with one impeller (Bombac et al., 1997; Lee &
Dudukovic, 2014; Lu & Ju, 1989; Nienow, Wisdom, & Middleton, 1977;
Warmoeskerken & Smith, 1985). Only few studies investigated the flow
regimes in multiple impeller reactors (Abrardi, Rovero, Baldi, Sicardi,
& Conti, 1990; Bombac & Zun, 2006; Smith, Warmoeskerken, & Zeef,
1987; Warmoeskerken & Smith, 1988). These studies examined only a
narrow operational range and the flow regimes impact has been studied
only with regard to power input.
An increasing number of publications analyze the gas-liquid flow in
bioreactors by means of computational fluid dynamics (CFD), focusing for
example on flow patterns of the gas and liquid phase (Bakker &
Oshinowo, 2004; Guan, Li, Yang, & Liu, 2019; Khopkar, Rammohan, Ranade,
& Dudukovic, 2005; Khopkar & Tanguy, 2008; Scargiali, D’Orazio,
Grisafi, & Brucato, 2007; Wang et al., 2014). Khopkar and Tanguy (2008)
investigated the complex pattern in a dual impeller configuration and
simulated gas hold-up and the flow pattern of a VC and LC regime.
Furthermore, they were able to show a decreasing liquid pumping
efficiency of the bottom impeller. However, none of these studies
investigated the mechanisms leading to differences between the impeller
levels, which were described in the above-mentioned contributions.
The existing data on the flow regimes in multiple impeller reactors and
their impact on the gas dispersion behavior is incomplete. Therefore, in
this study, we analyze the power input and gas hold-up of each impeller
stage and the corresponding flow regime in a four-level reactor and over
a wide, industrially relevant operational range. The second objective of
this paper is to study mechanisms leading to the differences between the
reactor compartments. By means of a two-phase CFD simulation, it is for
the first time possible to analyze the bubble flow in each compartment
of a pilot-scale (0.15 m3) bioreactor applying high
gas hold-up and turbulent conditions, to determine the effective (local)
aeration rate and local gas density at each impeller.