Biomedical Engineering Reference
In-Depth Information
inhibitor for enzymatic hydrolysis. The addition of this kind of reagent can increase
the proportion of monosaccharides in the total hydrolysate and decompose some
of the acid-soluble lignin. The removal of lignin and degradation of hemicellulose
increase the pore size of cellulose, contributing to the enhancement of the hydrolysis
efficiency of cellulose. However, the addition of chemical reagents causes environ-
ment pollution, and the equipment requirements also increase. Unpolluted steam
explosion technology proposed by Chen [
19
] does not require adding any chemicals,
just controlling the water content of the straw. It can isolate more than 80 % of the
hemicellulose, and the enzymatic hydrolysis rate of straw is more than 90 %.
An intermittent steam explosion device in China was produced in 1989 by the
Inner Mongolia Boiler Plant. However, rapid reaction and heating and uneven
heating of materials of the intermittent steam explosion reaction still need to be
solved. Currently, the most successful commercial application is the Staketech
Biomass Conversion process in the Canada Staketech Company. This technology
uses a continuous steam explosion process, and the heating time and temperature
are strictly controlled.
The cost of steam explosion pretreatment is low; generally, a ton of material
consumes 0.5-1.0 t steam. The enzymatic hydrolysis yields of steam-exploded
lignocellulose would be improved markedly. But, in this process, lignin is still
not removed, and the material is extruded at the same time, resulting in a low
concentration of sugar even with a high substrate concentration; for example, 10 %
of the substrate concentration can only obtain about 5 % sugar.
4.3.1.3
Microwave and Ultrasonic Pretreatment
A microwave is an electromagnetic wave ranging from 300 MHz to 300,000
MHz. The temperature of microwave treatment is closely related to the enzymatic
hydrolysis yields of cellulose. Microwaves can make the internal molecular structure
of material produce heat by collision, resulting in heating of the material. Generally,
the appropriate processing temperature of microwave pretreatment is 210-220
ı
C;
a temperature above this range will cause decomposition of material and decrease
the enzymatic hydrolysis rate.
The mechanism of microwave processing is the temperature effect. The
microwave processing temperature must be above 160-180
ı
C, which is just
consistent with the thermal softening temperature of hemicellulose, lignin,
and cellulose, 167-181, 127-193, and 231-253
ı
C, respectively. In addition,
microwave pretreatment has advantages such as short treatment time, simple
operation, and highly efficient saccharification. Jian et al. [
20
,
21
] studied the
supramolecular structure changes of cellulose and the reaction performance of
microwave- and ultrasonic-pretreated materials, which indicated that microwaves
and ultrasound can accelerate these two types of chemical reactions of cellulose. In
particular, they can greatly improve the reaction conditions of cellulose highly
selectively oxidized by periodic acid. Therefore, microwave pretreatment has
significant meanings for improving the chemical reactivity of cellulose, opening
