Biomedical Engineering Reference
In-Depth Information
4.3.2.4
Dilute Acid Pretreatment
In acid pretreatment, acid as a catalyst accelerates the autohydrolysis of lig-
nocellulose raw materials. For example, using dilute sulfuric acid treatment of
lignocellulosic raw materials to produce uronic, hemicellulose is hydrolyzed to
xylose and other sugars by dilute sulfuric acid until the formation of uronic. The
added sulfuric acid can remove hemicellulose and improve the digestion of cellulose
in residual solids. In acid pretreatment, the quality of lignocellulosic raw material
will certainly be lost, which is mainly caused by the hydrolysis of hemicellulose.
The raw materials become porous because of hemicellulose hydrolysis, increasing
the contact area of the cellulose and cellulase.
Hemicellulose can be dissolved in dilute acid (0.3-3 %) below 100
ı
C. In the
process of dilute acid hydrolysis, most of the hemicellulose is dissolved in the acid
solution; the reaction take places on the surface, and the reaction rate is fast. The
cellulose is insoluble, and the contact area of acid and cellulose is small, so the
reaction rate is slow. According to research, the speed of these two reactions differ
by about 100 times. Because of hemicellulose dissolution, the cellulose enzymatic
hydrolysis rate can be greatly improved. At the same time, hemicellulose sugar can
be recovered.
Dilute acid hydrolysis has been successfully used in the pretreatment of ligno-
cellulosic biomass. Dilute acid pretreatment and concentrated acid hydrolysis have
been industrialized. However, because the parcel of cellulose polymer lignin in the
acid treatment process has not been effectively removed, the matrix structure is still
relatively close. The concentrated acid can be used to process lignocellulosic raw
materials, but the concentrated acid is toxic and corrosive and requires acid-resistant
equipment. After concentrated acid pretreatment, recycling the acid is required
to minimize pollution of the environment, increasing the cost of production. The
remaining acid must be neutralized for subsequent hydrolysis and fermentation after
acid pretreatment, and then it will produce some inhibition for fermentation and the
need to remove the salt, which both increase production costs.
4.3.2.5
Oxidation Pretreatment
The oxidation pretreatment process uses hydrogen peroxide, ozone, or oxygen under
alkaline conditions to decompose and dissolute lignin, and cellulose can also be
partially oxidized, enhancing the enzymatic hydrolysis rate. Ozone can be used to
break down lignin and hemicellulose in the lignocellulosic material. In this method,
lignin can have a large degree of degradation, with hemicellulose only attacked in
a minor way; the cellulose is almost unaffected. In the ozone pretreatment process,
after removing 60 % of the lignin, the enzymatic hydrolysis rate of the cellulose
matrix increased by five times, the mass fraction of the lignin decreased from 29 to
8 %, and enzymatic hydrolysis yields increased by 57 % [
43
].
The wet oxidation method was proposed in the early 1980s; by heating and
pressurizing, water and oxygen both participate in response to pretreatment of
