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Water for Energy and Fuel Production
feedstock, enzyme production, hydrolysis, and utilities. The feedstock and utility
costs are high because only about 73% of the cellulose is converted to ethanol in
48 h and the remainder of the cellulose, hemicellulose, and lignin is burned or gasified. Enzyme production is expensive due to a large amount of enzymes that are
used in the attempt to overcome the end-product inhibition and the slow reaction
rate. The hydrolysis step is also expensive due to the large capital and operating
costs. The most important parameters are the hydrolysis section yield, the product
quality, and the required enzyme loading, all of which are interrelated. Generally,
the process should be operated at the minimum required enzyme loading. Um and
Hanley [49] examined the effect of cellulose loading on the performance of the
SHF process.
Generally, hydrolysis is carried out at 50°C and fermentation requires a lower
temperature (around 30°C). The SHF process accommodates both of these requirements. The fermentation step takes about 48 h.
9.4.3.2 simultaneous saccharification and Fermentation
The operating cost of the SSF process is generally lower than that of SHF process
as long as the process integration is synergistically done. Since in the SSF process
both hydrolysis and fermentation are carried out in the same vessel, yeast ferments
glucose to ethanol as soon as the glucose is produced, thus preventing the sugars
from accumulating and causing end-product inhibition. Even in the SSF process,
cellobiose inhibition occurs to an appreciable extent. The enzyme loading for SSF is
only 7 IU/g of cellulose compared to 33 IU/g in SHF. The cost of energy and feedstock is somewhat reduced because of the improved yield and the increased ethanol
concentration, which also considerably reduce the cost of distillation and utilities.
The cost of the SSF process is slightly less than the combined cost of hydrolysis and
fermentation in the SHF process. The longer reaction time for SSF (about seven days)
versus two days for hydrolysis and two days for fermentation for SHF is offset by the
reactor volume and high ethanol concentration. In earlier studies, fermentation was
the rate-limiting step, but with recent advances in recombinant yeast strains that are
capable of effectively fermenting both glucose and xylose, the rate-liming step may
have changed to hydrolysis.
The hydrolysis is carried out at 37°C and an increase in temperature (up to 50°C)
increases the reaction rate. However, in the SSF process, the ceiling temperature
is usually limited by the yeast cell viability. The concentration of ethanol is also a
limiting factor (a periodic removal of ethanol improves the productivity up to 44%).
Recycling the residual solids may also increase the process yield. However, enzyme
recycling may be limited by the increase in lignin concentration causing handling
difficulties.
Two types of enzyme recycling schemes have been examined: in one scheme,
enzymes are recovered in the liquid phase and in the other, enzymes are recovered
by recycling unreacted solids [47]. The first scheme works well with the SHF process in which hydrolysis is carried out at higher temperatures (50°C). The increase
in temperature allows more enzymes to remain in the liquid phase. At lower temperature, more enzymes are adsorbed on the surface, and therefore, for SSF solids
recycling becomes a more effective option.