Page 107 - Biofuels Refining and Performance

90 Chapter Three

In fermentation of this hydrolyzate, the pentoses should be utilized in
order to increase the overall yield of the process and to avoid problems
in wastewater treatment. Therefore, it is still a challenge to use a
hexose-fermenting organism such as S. cerevisiae for fermentation of
the hydrolyzate.
When a mixture of hexoses and pentoses is present in the medium,
microorganisms usually take up hexoses first and produce ethanol. As
the hexose concentration decreases, they start to take up the pentose.
Fermentation of hexoses can be successfully performed under anaerobic
or microaerobic conditions, with high ethanol yield and productivity.
However, fermentation of pentoses is generally a slow and aerobic
process. If one adds air to ferment pentoses, the microorganisms will
start utilizing the ethanol produced as well. It makes the entire process
complicated and demands a well-designed and controlled process.

3.12.1 Batch processes
In batch processes, all nutrients required for fermentation are present in
the medium prior to cultivation. Batch technology had been preferred
in the past due to the ease of operation, low cost of controlling and moni-
toring system, low requirements for complete sterilization, use of
unskilled labor, low risk of financial loss, and easy management of feed-
stocks. However, overall productivity of the process is very low, because
of long turnaround times and an initial lag phase [9].
In order to improve traditional batch processes, cell recycling and
application of several fermentors have been used. Reuse of produced
cells can increase productivity of the process. Application of several
fermentors operated at staggered intervals can provide a continuous
feed to the distillation system. One of the successful batch methods
applied for industrial production of ethanol is Melle-Boinot fermen-
tation. This process achieves a reduced fermentation time and
increased yield by recycling yeast and applying several fermentors
operated at staggered intervals. In this method, yeast cells from pre-
vious fermentation are separated from the media by centrifugation
and up to 80% are recycled [9, 68]. Instead of centrifugation, the cells
can be filtered, followed by the separation of yeast from the filter aid
using hydrocyclones and then recycled [69].
In well-detoxified or completely noninhibiting acid hydrolyzates of
lignocellulosic materials, exponential growth will be obtained after inoc-
ulation of the bioreactor. If the hydrolyzate is slightly inhibiting, there
will be a relatively long lag phase during which part of the inhibitors
are converted. However, if the hydrolyzate is severely inhibiting, no
conversion of the inhibitors will occur, and neither cell growth nor fer-
mentation will occur. Aslightly inhibiting hydrolyzate can thus be detoxified