Page 116 - Lignocellulosic Biomass to Liquid Biofuels

90 Lignocellulosic Biomass to Liquid Biofuels

where 0.51 kg of ethanol and 0.49 kg of carbon dioxide gas could be
produced from 1 kg of glucose [203]. However, the fermentative micro-
organisms consume some of the glucose for cell growth, and consequently
the actual yield is lower than 100% [204].
The fermentation process is performed by three main technologies:
batch, fed-batch, and continuous fermentation [205]. In the batch process,
regarded as a discontinuous fermentation technique, the starting materials
and sporadically nutritive supplements are loaded in the fermenter with
microorganisms only at the beginning of the fermentation. The fermenta-
tion is then carried out for a certain period of time, under optimal condi-
tions of pH, temperature, O 2 supply, agitation, etc., without further
addition of fresh culture medium. Successively, the fermenter contents are
taken out for processing. In the fed-batch fermentation, a technique
between batch and continuous fermentation, the substrate is progressively
added at different time intervals throughout the course of fermentations.
The continuous fermentation is an open system that involves the addition
of the substrate in the fermenter and the removal of fermentation products
continuously. Both addition and removal are done at the same rate, in
order to maintain a constant working volume. Therefore the percentage
of end product achieved in continuous fermentation is much higher than
in batch and fed-batch processes.
During the bioethanol production from lignocellulosic biomass, pre-
treated lignocelluloses are converted to simple sugars, in hydrolysis reac-
tors, by catabolic enzymes (cellulases); subsequently, the hydrolyzate is
fermented to ethanol by ethanologenic yeasts, in separate units [205].
Currently, this method, known as separate enzymatic hydrolysis and fer-
mentation (SHF), is still the main process configuration for the biofuels
production from lignocellulose [206].
Usually, in SHF method, the hexose and pentose fermentations are
carried out in independent reactors (Fig. 3.1): (1) the hydrolyzate obtained
from hydrolysis reactors first enters the glucose fermentation reactor. The
mixture, including unconverted xylose, is then distilled to remove the
bioethanol from fermentation broth; (2) successively, xylose is fermented
to bioethanol in a second reactor, and the bioethanol obtained is again
separated from fermentation mixture by distillation technique [201].
The major advantage of SHF method is the ability to perform the
saccharification and fermentation step at its own optimal conditions. For
instance, the optimum temperature for most of the fermenting organisms
is between 28°C and 37°C, while the optimum temperature for