Environmental Engineering Reference
In-Depth Information
which are free and already available for fermentation. The sucrose is separated
from the sugarcane by pressing chopped and shredded material. The residual solid
left over after pressing is fibrous and is usually used as fuel in the sugar mill.
Several steps are involved in the isolation of pure solid sugar, including several
crystallizations steps. Sugarcane production requires a tropical climate and is very
popular in Brazil, which has the largest area of land devoted to sugarcane cultiva-
tion and was the first and biggest producer of bio-ethanol in the world. Brazil also
produces the cheapest bio-ethanol from sugarcane in the world today; the second
cheapest is made from corn in the USA.
Sugar beet (
Beta vulgaris
L.) is a plant whose roots contain large amounts of
sucrose. Sucrose consists of one glucose and one fructose unit that are linked by
a glycosidic bond, that is, it is a disaccharide. The bulk of the world's sugar beet is
produced in Europe, Russia, and the US. The 10 biggest beet-producing countries
in Europe produced 242 million metric tons of sugar beet in 2005.
7.6.2.2
From Starchy Feedstock
Ethanol is much easier to produce from cereal grains such as barley, wheat, and
corn than from cellulose-rich material. The process involves several steps: milling
of grains; hydrolysis of starch to sugar units; fermentation by yeast; distillation;
and removal of water from ethanol. After grinding the raw material, it is mixed
with water and enzymes to break down the starch into sugar units. The free sugar
can be fermented by yeast or bacteria, which convert it into ethanol and carbon
dioxide. As the concentration of ethanol increases to around 15%, the rate of fer-
mentation falls because high alcohol concentrations kill the yeast or bacteria. It is
then necessary to separate the ethanol from the other material in the fermentation
tanks by distillation. Distillation can increase the ethanol concentration of the
mixture to around 95%. In order to remove the remaining water from the ethanol
solution, it can be dried using different agents to a concentration of 99.9% etha-
nol, yielding so-called absolute ethanol.
The distillation and purification steps require significant amounts of energy and
absolute ethanol will reabsorb water from the air over time. In Sweden, an ethanol
company has been producing 50,000 m
3
a
-1
ethanol from grain since 2000. One
litre of absolute ethanol can be obtained from around 3 kg of wheat.
7.6.2.3
From Cellulose Feedstock
Ethanol production from cellulose feedstocks, which are often called “next-
generation feedstocks”, is depicted in Figure 7.7. It can be performed using
cellulose-rich biomass from many different sources including wood, fast-growing
plants such as switch grass or reed canary grass, and crop residues from food pro-
duction such as corn stover. Ethanol production from lignocellulose material
involves two processes: (1) hydrolysis of cellulose and hemicellulose into different
