Page 96 - Biofuels Refining and Performance

Bioethanol: Market and Production Processes 79

Depending on the substrate used and the conditions applied, up to 95%
of the hemicellulosic sugars can be recovered by dilute-acid hydrolysis
from the lignocellulosic feedstock [2, 13]. Of all dilute-acid processes, the
processes using sulfuric acid have been the most extensively studied.
Sulfuric acid is typically used in 0.5–1.0% concentration. However, the
time and temperature of the process can be varied. It is common to use
one of the following conditions in dilute-acid hydrolysis:

Mild conditions, i.e., low pressure and long retention time
Severe conditions, i.e., high pressure and short retention time
In dilute-acid hydrolysis, the hemicellulose fraction is depolymerized at
temperatures lower than the cellulose fraction. If higher temperature or
longer retention times are applied, the monosaccharides formed will be fur-
ther hydrolyzed to other compounds. It is therefore suggested that the
hydrolysis process be carried out in at least two stages. The first stage is
carried out at relatively milder conditions during which the hemicellulose
fraction is hydrolyzed, and a second stage can be carried out by enzymatic
hydrolysis or dilute-acid hydrolysis, at higher temperatures, during which
the cellulose is hydrolyzed [13]. These first and second stages are some-
times called “pretreatment” and “hydrolysis,” respectively.
Hydrolyzates of first-stage dilute-acid hydrolysis usually consist of
hemicellulosic carbohydrates. The dominant sugar in the first-stage
hydrolyzate of hardwoods (such as alder, aspen, and birch) and most agri-
cultural residues such as straw is xylose, whereas first-stage hydrolyzates
of softwoods (e.g., pine and spruce) predominantly contain mannose.
However, the dominant sugar in the second-stage hydrolyzate of all lig-
nocellulosic materials, either by enzymatic or dilute-acid hydrolysis, is glu-
cose, which originates from cellulose.
Detoxification of acid hydrolyzates. In addition to sugars, several by-products
are formed or released in the acid hydrolysis process. The most impor-
tant by-products are carboxylic acids, furans, and phenolic compounds
(see Fig. 3.6).

Mannan Mannose HMF Acids
Hemicellulose
Xylan Xylose Furfural Acids
Cellulose Glucan Glucose HMF Acids

Lignin Phenolic Compounds
Acetyl groups Acetic acid
Figure 3.6 Formation of inhibitory compounds from ligno-
cellulosic materials during acid hydrolysis.