Page 115 - Biofuels Refining and Performance

98 Chapter Three

On the other hand, enzymatic hydrolysis has its own problems in
comparison to dilute-acid hydrolysis. Hydrolysis for several days is nec-
essary for enzymatic hydrolysis, whereas a few minutes are enough for
acid hydrolysis. The prices of the enzymes are still very high, although a
new development has claimed a 30-fold decrease in the price of cellulase.

3.13 Ethanol Recovery
Fermented broth or “mash” typically contains 2–12% ethanol. Further-
more, it contains a number of other materials that can be classified into
microbial biomass, fusel oil, volatile components, and stillage. Fusel oil
is a mixture of primary methylbutanols and methylpropanols formed
from -ketoacids and derived from or leading to amino acids. Depending
on the resources used, important components of fusel oil can be isoamylal-
cohol, n-propylalcohol, sec-butylalcohol, isobutylalcohol, n-butlyalcohol,
active amylalcohol, and n-amylalcohol. The amount of fusel oil in mash
depends on the pH of the fermentor. Fusel oil is used in solvents for
paints, polymers, varnishes, and essential oils. Acetaldehyde and trace
amounts of other aldehydes and volatile esters are usually produced
from grains and molasses. Typically, 1 L of acetaldehyde and 1–5 L of
fusel oil are produced per 1000 L of ethanol [9, 47].
Stillage consists of the nonvolatile fraction of materials remaining
after alcohol distillation. Its composition depends greatly on the type of
feedstock used for fermentation. Stillage generally contains solids, resid-
ual sugars, residual ethanol, waxes, fats, fibers, and mineral salts. The
solids may be originated from feedstock proteins and spent microbial
cells [9].

3.14 Distillation
Mash is usually centrifuged or settled in order to separate the micro-
bial biomass from the liquid and then sent to the ethanol recovery
system. Distillation is typically used for the separation of ethanol, alde-
hydes, fusel oil, and stillage [9]. Ethanol is readily concentrated from
mash by distillation, since the volatility of ethanol in a dilute solution
is much higher than the volatility of water. Therefore, ethanol is sepa-
rated from the rest of the materials and water by distillation. However,
ethanol and water form an azeotrope at 95.57 wt% ethanol (89 mol%
ethanol) with a minimum boiling point of 78.15 C. This mixture
behaves as a single component in a simple distillation, and no further
enrichment than 95.57 wt% of ethanol can be achieved by simple distil-
lation [9, 47, 81]. Various industrial distillation systems for ethanol purifi-
cation are (a) simple two-column systems, (b) three- or four-column barbet
systems, (c) three-column Othmer system, (d) vacuum rectification,