Biomedical Engineering Reference
In-Depth Information
of 10-30
m, the enzymatic hydrolysis yields are almost up to 80 %. In addition,
the ground powder of lignocellulose has small size and no swelling property, which
can increase the substrate concentration and obtain high-concentration saccharified
liquid. But, mechanical grinding technology also has some disadvantages, such
as limited improvement of the saccharification rate, high energy consumption,
and high cost of grinding. The grinding cost accounts for 50-60 % of the total
energy consumption of this process. For example, if the material was reduced to
a particle size of 200 mesh, the grinding costs reached as high as 53-110 dollars/t,
applicability of which was not high, and the grinding treatment is not appropriate for
some materials. If the material was decreased to particle size of 270 meshes, which
required nearly 48 h of milling, the energy consumption accounted for 50-60 % of
the total energy consumption of the saccharification process [
13
]. This method is
also not suitable for a variety of materials processing. If particle size decreased to
200 meshes, the grinding cost was as high as 70-100 dollars/t raw material.
In recent years, many scholars found that the enzymatic hydrolysis rate of cellu-
lose will increase exponentially when enzymatic hydrolysis is combined with wet
grinding. In addition, some scholars also found that the yield of sugar will be greatly
improved if there is some minor grinding in the process of solvent pretreatment of
cellulose. The combination of multiform methods will be a development direction in
the future for lignocellulosic material pretreatment because they will give full play
to the advantages of each method.
4.3.1.2
Steam Explosion
A maiden attempt of steam explosion for the pretreatment of lignocelluloses was
begun in the early 1980s. Raw materials were heated to 180-235
ı
C by steam and
maintained for a certain time. Under high temperature and pressure, the acetyl of
hemicellulose generates organic acids and then participates in the catalytic process
of hemicellulose and lignin depolymerization. Hemicellulose is hydrolyzed into
soluble polysaccharide, and lignin is softened because of the breakage of the
'
-
propylene ether bond and parts of the
-propylene ether bond. As a physical and
chemical method, steam explosion pretreatment makes the three main components
of lignocellulosic substances be utilized respectively because it can effectively
isolate the active fiber without chemicals or with slight use of chemicals. Therefore,
steam explosion pretreatment technology has the advantages of no environmental
pollution and low energy consumption, which makes it one of the most widely used
methods for lignocellulose pretreatment.
Steam explosion utilizes high-temperature and high-pressure steam to treat the
lignocelluloses, and component separation and structural change of lignocelluloses
are realized through the process of instant decompression. Generally, raw materials
are treated with high-pressure steam in the pressure resistance reactor, under
conditions of 16-34 kg
“
cm
2
, 200-240
ı
C, holding for 30 s-20 min, and then
decompressed rapidly, forcing the material released into the atmosphere. The
earliest study of the Lotech Company indicated that, at any given pretreating
