Page 111 - Biofuels Refining and Performance

94 Chapter Three

of centrifuge (which is expensive) in order to avoid deactivation of the
cells [47, 73].
Application of an encapsulated cell system in continuous cultivation
has several advantages, compared to either a free-cell or traditionally
entrapped cell system, e.g., in alginate matrix. Encapsulation provides
higher cell concentrations than free-cell systems in the medium, which
leads to higher productivity per volume of the bioreactor in continuous
cultivation. Furthermore, the biomass can easily be separated from the
medium without centrifugation or filtration. The advantages of encap-
sulation, compared to cell entrapment, are less resistance to diffusion
through beads/capsules, some degree of freedom in movement of the
encapsulated cells, no cell leakage from the capsules, and higher cell con-
centration [74].

3.12.4 Series-arranged continuous
flow fermentation
Ethanol can be produced by using continuous flow fermentors arranged
in a series with complete sugar utilization or high ethanol concentra-
tion. With two fermentors arranged in a series, the retention time can
be chosen so that the sugar is only partially utilized in the first, with
fermentation completed in the second. Ethanol inhibition is reduced in
the first fermentor, allowing a faster throughput. The second, lower-
productivity fermentor can now convert less sugar than if operated
alone. For high product concentration, productivity of a two-stage system
has been 2.3 times higher than that of a single stage [47, 75].
A two-stage continuous ethanol fermentation process with yeast recir-
culation is used industrially by Danish Distilleries Ltd., Grena, for
molasses fermentation (see Fig. 3.8). Two fermentors with 170,000-L
volume produce 66 g/L ethanol in 21-h retention time [76].

Yeast
Feed settler Mash

Fermentor I Fermentor II

Figure 3.8 Two-stage continuous ethanol fermentation process with yeast recircu-
lation [76, 77]. (A seven-fermentor-series system (70,000-L volume each fermentor)
was also used in the Netherlands to produce 86 g/L ethanol in 8-h retention time
[78]. AJapanese company used a six-fermentor-series system (total volume 100,000 L)
with 8.5-h retention time to produce 95 g/L ethanol [79].)