Page 109 - Biofuels Refining and Performance

92 Chapter Three

The fed-batch processes with feedback control have been classified as
indirect-control and direct-control fed-batch processes [70, 71].
The fed-batch technique is one of the promising methods for fermen-
tation of dilute-acid hydrolyzates of lignocellulosic materials. The basic
concept behind the success of this technique is the capability of in situ
detoxification of hydrolyzates by the fermenting microorganisms. Since
the yeast has a limited capacity for conversion of the inhibitors, the
achievement of a successful fermentation strongly depends on the feed
rate of the hydrolyzate. By adding the substrate at a low rate in fed-
batch fermentation, the concentrations of bioconvertible inhibitors such
as furfural and HMF in the fermentor remain low, and the inhibiting
effect therefore decreases. At a very high feed rate, using an inhibiting
hydrolyzate, both ethanol production and cell growth can stop, whereas
at a very low feed rate, the hydrolyzate may still be converted, but at a
very low productivity rate, which has been experimentally confirmed.
Consequently, there should exist an optimum feed rate [15, 18, 21].
Similar to batch operations, higher optimum dilution rate in fed-
batch cultivation can be obtained by (a) high initial cell concentration,
(b) increasing the tolerance of microorganisms against the inhibitors,
and (c) choosing optimal reactor conditions to minimize the effects of
inhibitors. Productivity in fed-batch fermentation is generally limited
by the feed rate which, in turn, is limited by the cell-mass concentra-
tion [21].

3.12.3 Continuous processes
Process design studies of molasses fermentation have shown that the
investment cost was considerably reduced when continuous rather than
batch fermentation was employed, and that the productivity of ethanol
could be increased by more than 200%. Continuous operations can be
classified into continuous fermentation with or without feedback control.
In continuous fermentation without feedback control, called a chemostat,
the feed medium containing all the nutrients is continuously fed at a con-
stant rate (dilution rate D) and the cultured broth is simultaneously
removed from the fermentor at the same rate. The chemostat is quite
useful in the optimization of media formulation and to investigate the
physiological state of the microorganism [71]. Continuous fermenta-
tions with feedback control are turbidostat, phauxostat, and nutristat.
A turbidostat with feedback control is a continuous process to maintain
the cell concentration at a constant level by controlling the medium
feeding rate. A phauxostat is an extended nutristat, which maintains the
pH value of the medium in the fermentor at a preset value. A nutristat
with feedback control is a cultivation technique to maintain nutrient con-
centration at a constant level [71].