BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 413

of evaporated water and ethanol. Almost all ethanol remains in solution and achieves a
concentration of about 8.5 vol.% (=7.8 wt%) at the end of the fermentation.
The yeast cells require the ATP produced upon ethanol formation for maintenance.
If additional ATP is available, the yeast uses it for converting part of the sugars into
new cells (cell growth). On the one hand, that growth is favorable for ethanol produc-
tion, because a higher yeast concentration leads to faster ethanol production. On the
other hand, we want to minimize sugar consumption for anything else than ethanol
production, and growth decreases the yield of ethanol on sugars. The solution is to
try to achieve a steady state in which the yeast is recycled and reaches a concentration
level dictated by the available sugar amount (and hence ATP) and the requirement for
maintenance (and hence ATP). Some yeast cells will lyse, and a small portion of the
recycle flow is purged as the aforementioned to prevent accumulation of

dead cells mass. The portion of lost cells is compensated by growth in the steady state
that is achieved. The yeast recycle is possible because some tricks are used to prevent
bacterial infections. Otherwise, fast-growing bacteria that might barely produce
ethanol would take possession of the fermentor. The tricks involve centrifugation
of yeast cells while minimizing sedimentation of the (smaller) bacterial cells and
acidifying the yeast recycle stream to a pH not tolerated by harmful bacteria.

“ yeast bleed ”

13.2.6 Energy Flows

Many material flows in the overall process need heating or cooling. All hot streams
leaving the factory must have a temperature below 35 C. Heat integration is
performed to maximize exchange of heat between streams that need cooling and
streams that need heating. After optimization of the heat exchange network, it turns
out that still 91,423 kW extra cooling duty and 7,281 kW heating duty are required.

13.2.7 Equipment

Some major equipment items are treated here.

13.2.7.1 Fermentors For designing the fermentors, the reaction kinetics for the
growth of the yeast cells and for formation of ethanol and side products should be
known, ideally in detail. So, information is required on the reaction rates as a function
of the concentrations of yeast, sucrose, ethanol, ammonia, CO 2 , acetic acid, etc. In
fact, not only the extracellular concentrations but also all intracellular concentrations
should be known, and quantitative relations describing all mechanisms inside and out-
side the cells as functions of these concentrations are required. Only a few years ago,
large groups of researchers have started to measure the dynamic course of hundreds of
intracellular concentrations and describe these using fundamental relations. This
approach is too complicated for our design purpose, and even basic models that
can approximate the course of a fermentation are not treated here.
Instead, based on experience, we simply assume an average volume-specific
ethanol productivity of 8 kg ethanol per hour per m 3 fermentation liquid. The mass
balance then leads to 4529 m 3 fermentation volume.