Materials and Methods
Fermenter
Experiments were performed in a pilot scale Plexiglas reactor with an
inner diameter (T) of 0.44 m and a working volume of 0.168 m³. The
geometrical details of the vessel are displayed in Figure 1. Airflow was
introduced through a ring sparger with 36 holes, each diameter of 2 mm,
and controlled by a mass flow controller (EL-Flow Select F-203AV,
Bronkhorst AG, Netherlands). A 0.065 mol/L
Na2SO4 solution was used as model fluid
to simulate the non-coalescent behavior present in most fermentation
media (Prince & Blanch, 1990).
Experimental methods
The flow regimes were determined by means of the conductivity method
described by Bombac et al. (1997). In order to detect the gas-filled
cavities behind the impeller blades, and thus, the respective flow
regime, a self-made 2-pole conductivity probe with small tip size was
applied (Figure 1). The probe was mounted on an in height-adjustable
holder, which was moved to the respective impeller level for
measurements (Witz, Treffer, Hardiman, & Khinast, 2016). The cavity
size and type behind each blade were identified by a drop in the voltage
pulse response of the probe. For this, the raw signal was further
analyzed by discrete Fourier transformation using PicoScope software
(Version 6.9.14.16, Pico Technology Ltd., United Kingdom).
In a first set of experiments, the flow regimes were recorded covering
aeration rates between 2 and 18 m3/h and gas
superficial velocities (\(u_{g})\) between 0.004 and 0.033 m/s and
impeller speeds from 100 to 440 min-1. In a second set
of experiments, each impellers transition lines from VC to LC and from
LC to RC were identified. For this, the impeller speed was set to a
constant value and the gas flow rate was increased in small steps of 0.1
m3/h until a change of the flow regime was recognized.
This procedure was repeated at other impeller speeds to be able to draw
the transition line. For further analysis of the flow regimes, the
transition lines were plotted as a flow map (Warmoeskerken & Smith,
1985), which employs in the dimensionless Froude (Fr) and Flow number
(Fl) and allows a coherent classification of the flow regimes.
The gas hold-up and power measurements were conducted in separate
experiments with the conductivity measurement setup removed to avoid an
influence on the measurements. The gas hold-up and power input were
determined for vessel filling levels covering the impeller levels 1, 1
and 2, 1 to 3 and 1 to 4 (Figure 1). The calculation of the difference
between values of the reactor filled with \(n\) levels and \(n-1\)levels yielded the value of level \(n\).
The gas hold-up was therefore determined by calculation of the
difference of the measured total filling level for aerated conditions
for \(n\) impeller levels, \(H_{t,(n)}\), and for the lower \(n-1\)levels, \(H_{t,(n-1)}\), as described by Fujasova, Linek, and Moucha
(2007) for \(n=1,\ 2,\ 3,\ 4\) and the relation