Biomedical Engineering Reference
In-Depth Information
high-temperature soaking, the content of pentosan, lignin, and inorganic impurities
in cellulose is reduced, which not only is conducive to the penetration of the reagents
but also provides the possibility of further changes of supramolecular structure of
cellulose. Lu et al. have shown that NaOH pretreatment has a great influence on the
chemical composition of lignocellulose. In pretreated material, cellulose swelled
significantly, the cellulose crystallinity degree was reduced, the cellulose crystalline
region was damaged, and the enzymatic hydrolysis became easier [
32
]. In addition,
calcium hydroxide, ammonia, and the like can be used to remove lignin [
33
,
34
].
In ammonia treatment, lignocellulose is soaked in 10 % ammonia solution (w/w)
for 24-48 h to remove most of the lignin. Ammonia solution can remove the acetyl
contained in raw materials, which is harmful to the fermentation. But, there will
be some loss of hemicellulose in the high ammonia concentration. The Laboratory
of Renewable Resources Engineering (LORRE) at Purdue University pretreated
lignocellulose with ammonia and then transformed the xylose in hydrolysates with
the breeding strains to xylitol, with a xylose conversion of 0.74 g
g
1
[
30
]. Kim and
Lee [
35
] studied component separation of corn stover by hot water and ammonia
two-step pretreatment; hot water pretreatment was used to degrade hemicellulose,
and ammonia pretreatment was used to remove lignin. In this method, the enzymatic
hydrolysis yields of xylan reached 92-95 %, the recovery yields reached 83-86 %,
and lignin removal yields reached 75-81 %. After two-step processing, the cellulose
content in the material was maintained at 78-85 %, and the enzymatic hydrolysis
rate was significantly increased.
There are three types of alkali pretreatment: batch process, semicontinuous
process, and continuous process [
36
]. In a batch process, materials and chemical
agents are mixed with a solid-to-liquid ratio of 1:20 and pretreated at a temperature
of 100-180
ı
C for about 20-45 min. In the semicontinuous process, materials and
the chemical treatment agent are premixed for 30 min with a solid-to-liquid ratio
of 1:1; then, the mixture is continuously added to the extruder and pretreated at
200
ı
C for a certain period of time. In the continuous process, dried materials are
continuously filled in the extruder through the feeding device, and chemicals are
injected into the extruder by an injection pump at an injection rate of 0.6 L
min
1
.
The pretreatment is conducted using the following conditions: solid-to-liquid ratio
of about 1:4 and processing temperature of 100-200
ı
C. A batch process device is
common equipment with a heating and stirring device. The semicontinuous process
device is a PCM30-25G-type extrusion machine with a double-helix axis having
the same rotation direction. The continuous process adopts the PCM65-30-type
extruder, which is similar to the semicontinuous process device but with opposite
rotation directions of the two axes. The batch process has the disadvantages of large
consumption of chemicals and energy. The amount of chemical agent used in the
semicontinuous process is 1/20 of the agent used in the batch process. The demand
of grinding raw materials (2 mm) is slightly lower, and the heat of reaction cannot be
fully utilized. In the continuous process, materials and chemical agents continuously
mix, and reaction time is short, just 2 min. The reaction heat can be effectively
utilized; raw material crushing requires a lower particle size of about 10 mm.
