Civil Engineering Reference
In-Depth Information
method is being extensively studied as a promising non-chemical process: the steam
explosion method [36, 37]. h e steam explosion process was invented by Mason in 1927
as a process to produce i bers for board production. h e steam explosion treatment of
lignocellulosic resources involves i lling a cylinder with wood chips, sealing and pres-
surizing it with saturated steam at pressures up to 1000 psi. h e chips are permeated
by the saturated steam and develop high internal pressures. When the pressure is sud-
denly released the chips are dei brilated by the sudden decompression [38]. During the
steam explosion signii cant amounts of sugars and phenolic compounds are obtained
due to partial hemicellulose hydrolysis and lignin depolymerization. h ese degrada-
tion reactions (autohydrolysis) are catalyzed by acetic acid formed when the acetyl
groups present in hemicelluloses are exposed to high temperatures [39]. Researchers
attempted to avoid i ber degradation by the application of dif erent chemicals. Steam
was i rst replaced with ammonia and later with liquid sulfur dioxide, but the com-
mercial application was precluded by some technical and economic problems. Finally,
steam explosion of aqueous systems at moderate temperatures and higher pressures led
to non-degraded cellulosic pulps, overcoming the problem of oxidation and hydrolytic
degradation [40] .
9.1.2.1.3 Chemical Pulping
Obtaining pure cellulose from dif erent types of plants and bioresources by chemical
methods has been extensively studied. h e i rst stage is usually a delignii cation pro-
cess, since lignin removal can signii cantly facilitate the extraction of hemicellulose
fractions. h e most common chemical pulping method still used by more than 75%
of the commercial paper making industries is the Krat method, which utilizes sodium
hydroxide (NaOH) and sodium sulphide (Na
2
S), followed by a bleaching stage usually
involving hydrogen peroxide (H
2
O
2
), chlorine dioxide (ClO
2
), ozone (O
3
) or peracetic
acid [41]. However, researchers are constantly developing improved pulping processes
in order to maximize cellulose yields and purity, obtain a specii c type of cellulose, or
reduce the environmental impact (reduce energy consumption and generation of toxic
byproducts). Several chlorine and/or sulphide-free treatments have been proposed in
order to reduce the ecological impact of the pulping process [42].
h ere are various alternative chemical pulping methods, some of them are: alkali
extraction [43], alkaline and sodium chlorite treatment [30], alkaline and peroxide
extraction [44], organic solvent extraction [45], acidic method [46], liquid-phase oxi-
dation or a combination of these treatments [42, 47].
9.1.2.1.4 Enzymatic Production
Traditional chemical bleaching processes produce hazardous toxic by-products that
are released into the environment together with waste waters. In the last few years,
there is an increasing trend to replace classical processes with biodegradable pro-
cesses. Biobleaching involves the use of microorganisms and enzymes to bleach pulp.
Xylanases are xylan-degrading enzymes which attack hemicellulose and alter the inter-
face between the cellulose and lignin, thereby facilitating the removal of the lignin-
associated hemicellulosic fraction. A requisite for the isolation of purii ed cellulose
with minimal damage (depolymerization) is to obtain cellulose-free xylanases capable
of exclusively attacking the non-cellulose components of the lignocellulosic materials.
