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Water for Energy and Fuel Production
9.2.1 STArCh hydrolySiS
Starch is regarded as a long-chain polymer of glucose (i.e., many glucose molecular units are bonded in a polymeric chain similar to a condensation polymerization
product) [18]. This starch is first broken down to simple sugar units by the hydrolysis
process. In this process, starch feedstock is ground and mixed with water containing
about 15%–20% starch. The mash is then cooked at or above its boiling point and
subsequently treated with two enzyme preparations. The first enzyme hydrolyzes the
starch into short-chain molecules and the second enzyme hydrolyzes the short-chain
molecules into glucose. The first enzyme is amylase, which liberates “maltodextrin”
by the liquefaction process. These maltodextrins are very sweet and contain a group
of low-molecular-weight carbohydrates called dextrins and oligosaccharides (a polymer of small number of simple sugars, monosaccharides). The dextrins and oligosaccharides are further hydrolyzed in the second step by an enzyme called pullulanase
and glucoamylase in a process known as saccharification. Complete saccharification converts all the dextrans to glucose, maltose, and isomaltose. The mash is then
cooled and subjected to yeast fermentation.
9.2.2 yeAST FermenTATion
Yeasts convert sugar into ethanol via a biochemical process called fermentation. The yeasts of primary interest to industrial fermentation of ethanol include
Saccharomyces cerevisiae, Saccharomyces uvarum, Schizosaccharomyces pombe,
and Kluyueromyces sp. Under anaerobic conditions, the yeasts metabolize glucose
to ethanol primarily via the Embden–Meyerhof pathway. This pathway for glucose
metabolism is the series of enzymatic reactions in the anaerobic conversion of glucose to lactic acid or ethanol, resulting in the energy in the form of adenosine triphosphate (ATP) [19]. Generally, the yield is about 90%–95% of the stoichiometric
relationship mentioned earlier. About 1716 kg of fermentable sugar is required for
the production of 1000 l of ethanol. When the fermentation is completed, the remaining solution is called distilled mash or stillage that contains a large amount of nonfermentable portions of fibers or proteins.
9.2.3 eThAnol PuriFiCATion And ProduCT SePArATion
Ethanol is separated from the mash by distillation. Unfortunately, conventional distillation process works only up to 95.63% ethanol because water and ethanol form
an azeotrope that will not allow any further concentration of ethanol. The minimum
boiling point temperature of 78.2°C is attainable at the azeotropic concentration
and not at the pure ethanol concentration. The additional concentration of ethanol
is carried out by dehydration by one of the two methods. In the first method, a third
component (such as benzene) is used to change the boiling characteristics of the
solution. The third component breaks azeotrope and allows conventional distillation
to be carried out in a tertiary system to achieve the desired separation. The second
method uses the molecular sieves that absorb water selectively and therefore concentrate ethanol further. There are different forms of molecular sieves that are based
Water for Energy and Fuel Production
9.2.1 STArCh hydrolySiS
Starch is regarded as a long-chain polymer of glucose (i.e., many glucose molecular units are bonded in a polymeric chain similar to a condensation polymerization
product) [18]. This starch is first broken down to simple sugar units by the hydrolysis
process. In this process, starch feedstock is ground and mixed with water containing
about 15%–20% starch. The mash is then cooked at or above its boiling point and
subsequently treated with two enzyme preparations. The first enzyme hydrolyzes the
starch into short-chain molecules and the second enzyme hydrolyzes the short-chain
molecules into glucose. The first enzyme is amylase, which liberates “maltodextrin”
by the liquefaction process. These maltodextrins are very sweet and contain a group
of low-molecular-weight carbohydrates called dextrins and oligosaccharides (a polymer of small number of simple sugars, monosaccharides). The dextrins and oligosaccharides are further hydrolyzed in the second step by an enzyme called pullulanase
and glucoamylase in a process known as saccharification. Complete saccharification converts all the dextrans to glucose, maltose, and isomaltose. The mash is then
cooled and subjected to yeast fermentation.
9.2.2 yeAST FermenTATion
Yeasts convert sugar into ethanol via a biochemical process called fermentation. The yeasts of primary interest to industrial fermentation of ethanol include
Saccharomyces cerevisiae, Saccharomyces uvarum, Schizosaccharomyces pombe,
and Kluyueromyces sp. Under anaerobic conditions, the yeasts metabolize glucose
to ethanol primarily via the Embden–Meyerhof pathway. This pathway for glucose
metabolism is the series of enzymatic reactions in the anaerobic conversion of glucose to lactic acid or ethanol, resulting in the energy in the form of adenosine triphosphate (ATP) [19]. Generally, the yield is about 90%–95% of the stoichiometric
relationship mentioned earlier. About 1716 kg of fermentable sugar is required for
the production of 1000 l of ethanol. When the fermentation is completed, the remaining solution is called distilled mash or stillage that contains a large amount of nonfermentable portions of fibers or proteins.
9.2.3 eThAnol PuriFiCATion And ProduCT SePArATion
Ethanol is separated from the mash by distillation. Unfortunately, conventional distillation process works only up to 95.63% ethanol because water and ethanol form
an azeotrope that will not allow any further concentration of ethanol. The minimum
boiling point temperature of 78.2°C is attainable at the azeotropic concentration
and not at the pure ethanol concentration. The additional concentration of ethanol
is carried out by dehydration by one of the two methods. In the first method, a third
component (such as benzene) is used to change the boiling characteristics of the
solution. The third component breaks azeotrope and allows conventional distillation
to be carried out in a tertiary system to achieve the desired separation. The second
method uses the molecular sieves that absorb water selectively and therefore concentrate ethanol further. There are different forms of molecular sieves that are based