Page 212 - Biofuels Refining and Performance

Ethanol and Methanol as Fuels in Internal Combustion Engines 195

TABLE 7.2 Crops Area Required for Growth
Sweet Sugar
Crop Sugarcane sorghum beet Cassava Potatoes Wheat

Area (ha) 0.49 0.38 0.5 1.43 1.2 2.52

Sugarcane. The present method adopted to obtain alcohol for energy
purposes requires three stages: (1) extracting the juice from sugarcane,
(2) fermentation of the juice, and (3) distillation into 90–95% alcohol.

Molasses. The black residue remaining after the sugar is extracted
from sugarcane is called molasses. It contains mostly invert sugars and
some sucrose. This sucrose also undergoes hydrolysis to produce invert
sugar by a catalytic action of acids in molasses.

C H O H O → C H O (D-Glucose) C H O (D-Fructose)
12
6
12
6
22
6
12
11
11
2
This mixture product is not crystallizable. Yeast organisms in the pres-
ence of oxygen oxidize sugars into CO and H O and convert sucrose
2
2
mostly into ethyl alcohol.
C H O → 2C H OH 2CO 2
5
12
6
2
6
Process adopted. Molasses is mixed with water so that the concentra-
tion of sugar in it is 10–18% (optimum is 12%). If the concentration is
high, more alcohol may be produced and may kill the yeast. Then, a
selected strain of yeast is added (it should not contain any wild yeast).
For some nutrient substances like ammonium and phosphates, the pH
value is kept between 4 and 5, which favors the growth of yeast organ-
isms. H SO is used for lowering pH. The temperature of the mixture
4
2
is kept at 15–25 C. The fermentation takes place as follows:
1. First, the yeast cells multiply at an optimum temperature (30 C).
2. Rapid fermentation takes place at the boiling temperature, and
oxygen is given off. The optimum temperature (50 C) is maintained,
and the process is continued for 20–30 h.
3. The fermentation rate is reduced, and alcohol is produced slowly.
Total time for fermentation is 36–48 h, depending upon the temper-
ature and sugar content. Last, the formed ethanol is distilled.
Starch. In this process, starchy materials are first converted into fer-
mentable sugars. This is done by enzymatic conversion (by means of
malt process) or by acid hydrolysis.
Starch → C H O (Maltose) C H O (Dextrose)
6
22
11
12
12
6