Environmental Engineering Reference
In-Depth Information
et al. 1999; Kocher et al. 2008). In our studies in shake flasks, maximal filter paper, CMCase, and
β-glucosidase activities were optimized as 1.05, 4.62, and 0.42 U/mL, respectively, that were used
for saccharification (Sharma et al. 2002b).
30.6 saccharIFIcatIon
A major step in the conversion of cellulose to ethanol or other useful chemicals is the “breakdown”
of cellulose to glucose. Two methods are currently suggested as economically feasible: acid or
enzymatic hydrolysis. Each method has its advantages and disadvantages, but the overriding factors
in the long run must be a low energy requirement and low pollution. Generally, acid hydrolysis
procedures give rise to a broad range of compounds in the resulting hydrolysate, some of which
might negatively influence the subsequent steps in the process. In addition, expensive corrosion-
proof equipment and high temperature and acid concentration are needed for hydrolysis, resulting
in high capital cost (Kosaric et al. 1980; Olsson and Hahn-Hägerdal 1996). Enzymatic hydrolysis is
not only energy sparing but also avoids the use of toxic and corrosive chemicals. Projected selling
prices for ethanol produced from cellulose by acid hydrolysis are currently comparable to those for
enzyme-based processes. Enzymatic processes are at a much earlier state of technological maturity;
however, in the absence of unforeseen breakthroughs for acid-based processes, research is likely to
result in enzyme-based processes that are significantly cheaper than acid-based processes (Wyman
1994; Olsson and Hahn-Hägerdal 1996).
30.6.1 a
cid
S
accharification
Acid hydrolysis of cellulosic biomass is now commercially viable. Two acid-catalyzed processes
are known: (1) dilute acid hydrolysis at elevated temperatures and pressures and (2) concentrated
acid hydrolysis at low temperature and ambient pressure. In the case of concentrated acids,
prehydrolysis and hydrolysis are carried out in one step. However, a weak acid hydrolysis is often
combined with a weak acid prehydrolysis (Olsson and Hahn-Hägerdal 1996). The major problems
associated with the dilute acid hydrolysis of lignocellulosic biomass are the low sugar yield and the
poor fermentability of the produced hydrolysate. The latter is due to the presence of various toxic
substances liberated from the structure of lignocellulosics during the hydrolysis process, such as
decomposition products of carbohydrates, lignin breakdown products, extraneous materials from
biomass, and metal ions from equipment corrosion. Among the identified toxins are furfurals,
hydroxymethyl furfural, levulinic acid, acetic acid, formic acid, and various phenolic compounds
originating from lignin (Chung and Lee 1985; Wyman 1994; Larsson et al. 1999). Strong acids
(e.g., concentrated sulfuric acid or halogen acids) hydrolyze cellulose and hemicellulose at
moderate temperatures of 40-50°C with little sugar degradation (Chandel et al. 2007). As a
result, concentrated acid processes achieve the high yields of ethanol critical to economic success
(Goldstein and Easter 1992; Chandel et al. 2007). However, one of the major problems is the
recovery of the expensive acids (Wyman 1994).
The acid saccharification of untreated sunflower stalks in our laboratories under optimized
conditions of 5% sulfuric acid at 1 bar for 30 min yielded 30.23 g/100 g of reducing sugars with
33.8% saccharification (Sharma 2000). A two-step sulfuric acid (H
2
SO
4
) hydrolysis of sunflower
seed husks was used by Eklund et al. (1976). The Pentosan fraction was removed using low-
temperature and mild acid conditions in the first step and the cellulose fraction was subsequently
hydrolyzed under severe reaction conditions. The Pentosan fraction was quantitatively hydrolyzed,
but for the cellulose hydrolysis, yield was only 79% of the theoretical yield. Bonillaa et al. (1990)
used hydrochloric acid (HCl) between 0.5 and 0.6% at a temperature range of 110-140°C for acid
hydrolysis of sunflower stalks. Elsewhere, acid hydrolysis of sunflower stalks has been optimized at
15% solids, 2.5% acid, and 30 min (Tosun 1997). Okur and Saracoglu (2006) degraded sunflower
seed hulls using 0.7 M H
2
SO
4
at 90°C and achieved 37 g/L of reducing sugars. Iranmahboob
