253
Hydrolysis and Fermentation Technologies for Alcohols
9.4.3.3 Comparison between ssF and shF Processes
Due to low end-product inhibition of the cellulase enzyme complex, the SSF systems
offer many advantages over the SHF processes. The SSF process shows higher yields
(88% vs. 73%) compared to the SHF process [2] and greatly improves the product
glucose concentration (10% vs. 4.4%). The most significant advantage of the SSF
process is the enzyme loading that is reduced from 33 to 7 IU/g cellulose, which
considerably cuts down the cost of ethanol production. A comparative study of the
approximate cost of the two processes reported in the literature [47] showed that
based on the ethanol selling price from a production capacity of 25,000,000 gallons
per year, the SSF process is found to be more cost effective than the SHF process
by a factor of 1.49. These estimates may change with new developments on enzymes
and yeasts.
A hybrid hydrolysis fermentation (HHF) process may also gain some acceptance.
This process will begin with a separate prehydrolysis step and ends with a simultaneous saccharification and fermentation step. In the first step of hydrolysis, higher
temperature enzymatic cellular saccharification takes place, whereas in the second
stage of the SSF process, mesophilic (moderate temperature) enzymatic hydrolysis
and sugar fermentation take place simultaneously. The optimized process scheme
may have to change if a specific enzyme that is proven to be highly efficient and cost
effective but also found to be intolerant against certain inhibitors that are associated
with any of these processing steps.
9.4.3.4 Xylose Fermentation
For certain feedstock such as hardwood and herbaceous biomass, xylose amounts
to 30%–60% of fermentable sugars. The efficient fermentation of xylose is therefore very important for the overall economics of ethanol from these feedstock.
Co-fermentation of both glucose and xylose is most desirable. xylose fermentation using pentose yeasts is difficult due to (1) the requirement of O 2 during ethanol
production, (2) the acetate toxicity, and (3) the production of xylitol as byproduct.
xylitol is a naturally occurring low-calorie sugar substitute with anticarcinogenic
properties. Other approaches to xylose fermentation include conversion of xylose to
xylulose using xylose isomerase prior to fermentation by S. cerevisiae and the development of genetically engineered strains [50].
A method of integrating xylose fermentation into the overall process is illustrated
in Figure 9.7. In this method, dilute acid hydrolysis is adapted as a pretreatment step.
The liquid stream is neutralized to remove any mineral or organic acid liberated
in the pretreatment process, and is then sent to the xylose fermentation. Water is
added before the fermentation, if necessary, so that organisms can make full use of
the substrate without having the yield limited by end-product inhibition. The dilute
ethanol stream from xylose fermentation is then used to provide the dilution water
for the cellulose–lignin mixture entering the SSF process. Thus, the water that enters
during the pretreatment process is used in both the xylose fermentation and the SSF
process. The conversion of xylose to ethanol using E. coli in pH-controlled batch
fermentation was investigated [51]. The results showed high concentrations of ethanol (56 g/l) produced from xylose with good efficiencies. Recombinant E. coli also
gave good conversions of glucose, mannose, arabinose, and galactose to ethanol.
Hydrolysis and Fermentation Technologies for Alcohols
9.4.3.3 Comparison between ssF and shF Processes
Due to low end-product inhibition of the cellulase enzyme complex, the SSF systems
offer many advantages over the SHF processes. The SSF process shows higher yields
(88% vs. 73%) compared to the SHF process [2] and greatly improves the product
glucose concentration (10% vs. 4.4%). The most significant advantage of the SSF
process is the enzyme loading that is reduced from 33 to 7 IU/g cellulose, which
considerably cuts down the cost of ethanol production. A comparative study of the
approximate cost of the two processes reported in the literature [47] showed that
based on the ethanol selling price from a production capacity of 25,000,000 gallons
per year, the SSF process is found to be more cost effective than the SHF process
by a factor of 1.49. These estimates may change with new developments on enzymes
and yeasts.
A hybrid hydrolysis fermentation (HHF) process may also gain some acceptance.
This process will begin with a separate prehydrolysis step and ends with a simultaneous saccharification and fermentation step. In the first step of hydrolysis, higher
temperature enzymatic cellular saccharification takes place, whereas in the second
stage of the SSF process, mesophilic (moderate temperature) enzymatic hydrolysis
and sugar fermentation take place simultaneously. The optimized process scheme
may have to change if a specific enzyme that is proven to be highly efficient and cost
effective but also found to be intolerant against certain inhibitors that are associated
with any of these processing steps.
9.4.3.4 Xylose Fermentation
For certain feedstock such as hardwood and herbaceous biomass, xylose amounts
to 30%–60% of fermentable sugars. The efficient fermentation of xylose is therefore very important for the overall economics of ethanol from these feedstock.
Co-fermentation of both glucose and xylose is most desirable. xylose fermentation using pentose yeasts is difficult due to (1) the requirement of O 2 during ethanol
production, (2) the acetate toxicity, and (3) the production of xylitol as byproduct.
xylitol is a naturally occurring low-calorie sugar substitute with anticarcinogenic
properties. Other approaches to xylose fermentation include conversion of xylose to
xylulose using xylose isomerase prior to fermentation by S. cerevisiae and the development of genetically engineered strains [50].
A method of integrating xylose fermentation into the overall process is illustrated
in Figure 9.7. In this method, dilute acid hydrolysis is adapted as a pretreatment step.
The liquid stream is neutralized to remove any mineral or organic acid liberated
in the pretreatment process, and is then sent to the xylose fermentation. Water is
added before the fermentation, if necessary, so that organisms can make full use of
the substrate without having the yield limited by end-product inhibition. The dilute
ethanol stream from xylose fermentation is then used to provide the dilution water
for the cellulose–lignin mixture entering the SSF process. Thus, the water that enters
during the pretreatment process is used in both the xylose fermentation and the SSF
process. The conversion of xylose to ethanol using E. coli in pH-controlled batch
fermentation was investigated [51]. The results showed high concentrations of ethanol (56 g/l) produced from xylose with good efficiencies. Recombinant E. coli also
gave good conversions of glucose, mannose, arabinose, and galactose to ethanol.