Page 110 - Biofuels Refining and Performance

Bioethanol: Market and Production Processes 93

When lignocellulosic hydrolyzates are added at a low feed rate in con-
tinuous fermentation, low concentration of bioconvertible inhibitors in
the fermentor is assured. In spite of a number of potential advantages
in terms of productivity, this method has not developed much yet
in fermentation of the acid hydrolyzates. One should consider the
following points in continuous cultivation of acid hydrolyzates of lig-
nocelluloses:

Cell growth is necessary at a rate equal to the dilution rate in order
to avoid washout of the cells in continuous cultivation.
Growth rate is low in fermentation of hydrolyzates because of the
presence of inhibitors.
The cells should keep their viability and vitality for a long time.
The major drawback of the continuous fermentation is that, in contrast
to the situation in fed-batch fermentation, cell growth is necessary at a
rate equal to the dilution rate, in order to avoid washout of the cells in
continuous cultivation [21]. The productivity is a function of the dilu-
tion rate, and since the growth rate is decreased by the inhibitors, the
productivity in continuous fermentation of lignocellulosic hydrolyzates
is low. Furthermore, at a very low dilution rate, the conversion rate of
the inhibitors can be expected to decrease due to the decreased specific
growth rate of the biomass. Thus, washout may occur even at very low
dilution rates [18]. On the other hand, one of the major advantages of
continuous cultivation is the possibility to run the process for a long time
(e.g., several months), whereas the microorganisms usually lose their
activity after facing the inhibitory conditions of the hydrolyzate. By
employing cell-retention systems, the cell-mass concentration in the
fermentor, the maximum dilution rate, and thus the maximum ethanol
productivity increase. Different cell-retention systems have been inves-
tigated by cell immobilization and encapsulation, and cell recirculation
by filtration, settling, and centrifugation. A relatively old study [72]
shows that the investment cost for a continuous process with cell recir-
culation has been found to be less than that for continuous fermentation
without cell recirculation.
®
Biostil is the trade name of a continuous industrial process for
ethanol production with partial recirculation of both yeast and waste-
water. The fermentor works continuously; the cells are separated by
using a centrifuge, and a part of the separated cells is returned to the
fermentor. Most of the ethanol-depleted beer including residual sugars
is then recycled to the fermentor. In this process, besides providing
enough cell concentration in the fermentor, less water is consumed and
a more concentrated stillage is produced. Therefore, the process has a
lower wastewater problem. However, the process needs a special type