Hydrolysis and Fermentation Technologies for Alcohols
241
Step 4: The fourth step is called simultaneous saccharification fermentation.
Once the mixture of milled kernel and water, now known as mash, is
inserted in the fermentation tank, the glucoamylase enzyme breaks down
the dextrins and oligosaccharides to form simple sugars that are monosaccharides. Yeast is added to convert sugar into ethanol and carbon dioxide.
The mash is allowed to ferment for 50–60 h, resulting in a mixture that
contains about 15% ethanol as well as solids from the grain and added yeast
[16,20].
Step 5: The fermented mash is pumped into the distillation system to separate ethanol from water at a concentration of up to 95% ethanol by volume
(a level of azeotropic mixture). The residue from this process called stillage
contains nonfermentable solids and water, and is pumped out of the bottom
of the distillation columns into the centrifuges.
Step 6: The near-azeotropic binary mixture of 95% ethanol and 5% water is
dehydrated by a molecular sieve that physically separates the remaining
water from the ethanol based on the size difference between the two molecules [16]. The process produces nearly 100% ethanol.
Step 7: The produced ethanol is stored up to 7–12 days. The ethanol is appropriately used as a fuel blend with gasoline.
Step 8: Ethanol production process creates two coproducts: carbon dioxide
and distillers grains. These coproducts are captured and sold as dry ice and
animal feed, respectively, to improve the overall economics of the process.
9.4 CellUlOsiC ethanOl
While starch and sugar produce grain ethanol, the feedstock obtained from this
method is also used as food [2,21–54]. In the recent years, more efforts are made to
convert all lignocellulosic materials such as hardwood, softwood, agricultural waste,
and energy crops into ethanol. Unlike corn, this material is not useful for food purposes. The ethanol produced from lignocellulosic material is called cellulosic ethanol.
Lignocellulosic materials are composed of four ingredients: cellulose, hemicellulose, lignin, and extractives. As shown in Figure 9.4, a generalized plant cell wall
structure is like a composite material in which rigid (and crystalline) cellulose fibers
are embedded in a cross-linked matrix of lignin and hemicellulose that binds the
cellulose fibers. Generally, the dry weight of a typical cell wall consists of approximately 30%–50% cellulose, 20%–35% hemicellulose, and 10%–25% lignin [12]. The
exact percentages vary with the nature of the feedstock. For example, for woody biomass, cellulose accounts for 40%–50%, and lignin and hemicellulose each account
for about 20%–30%. Lignin is aromatic in nature and provides higher heating value
than cellulose or hemicellulose. The chemicals in the biomass matrix include extractives such as resins, phenols, and other chemicals and minerals such as calcium,
magnesium, and potassium. These extractives are left behind in ash when biomass
is combusted. The trace minerals and major elements in lignocellulosic materials
display a high degree of variability for most of the elements between different species and between different organs within a given plant, depending on the growing
conditions including the soil characteristics [21].
241
Step 4: The fourth step is called simultaneous saccharification fermentation.
Once the mixture of milled kernel and water, now known as mash, is
inserted in the fermentation tank, the glucoamylase enzyme breaks down
the dextrins and oligosaccharides to form simple sugars that are monosaccharides. Yeast is added to convert sugar into ethanol and carbon dioxide.
The mash is allowed to ferment for 50–60 h, resulting in a mixture that
contains about 15% ethanol as well as solids from the grain and added yeast
[16,20].
Step 5: The fermented mash is pumped into the distillation system to separate ethanol from water at a concentration of up to 95% ethanol by volume
(a level of azeotropic mixture). The residue from this process called stillage
contains nonfermentable solids and water, and is pumped out of the bottom
of the distillation columns into the centrifuges.
Step 6: The near-azeotropic binary mixture of 95% ethanol and 5% water is
dehydrated by a molecular sieve that physically separates the remaining
water from the ethanol based on the size difference between the two molecules [16]. The process produces nearly 100% ethanol.
Step 7: The produced ethanol is stored up to 7–12 days. The ethanol is appropriately used as a fuel blend with gasoline.
Step 8: Ethanol production process creates two coproducts: carbon dioxide
and distillers grains. These coproducts are captured and sold as dry ice and
animal feed, respectively, to improve the overall economics of the process.
9.4 CellUlOsiC ethanOl
While starch and sugar produce grain ethanol, the feedstock obtained from this
method is also used as food [2,21–54]. In the recent years, more efforts are made to
convert all lignocellulosic materials such as hardwood, softwood, agricultural waste,
and energy crops into ethanol. Unlike corn, this material is not useful for food purposes. The ethanol produced from lignocellulosic material is called cellulosic ethanol.
Lignocellulosic materials are composed of four ingredients: cellulose, hemicellulose, lignin, and extractives. As shown in Figure 9.4, a generalized plant cell wall
structure is like a composite material in which rigid (and crystalline) cellulose fibers
are embedded in a cross-linked matrix of lignin and hemicellulose that binds the
cellulose fibers. Generally, the dry weight of a typical cell wall consists of approximately 30%–50% cellulose, 20%–35% hemicellulose, and 10%–25% lignin [12]. The
exact percentages vary with the nature of the feedstock. For example, for woody biomass, cellulose accounts for 40%–50%, and lignin and hemicellulose each account
for about 20%–30%. Lignin is aromatic in nature and provides higher heating value
than cellulose or hemicellulose. The chemicals in the biomass matrix include extractives such as resins, phenols, and other chemicals and minerals such as calcium,
magnesium, and potassium. These extractives are left behind in ash when biomass
is combusted. The trace minerals and major elements in lignocellulosic materials
display a high degree of variability for most of the elements between different species and between different organs within a given plant, depending on the growing
conditions including the soil characteristics [21].