Biomedical Engineering Reference
In-Depth Information
acid hydrolysis gradually became the main technology for enzymatic hydrolysis
pretreatment [
39
]. The dilute acid hydrolysis technology mainly includes a high-
temperature dilute acid hydrolysis process and a low-temperature, two-stage acid
hydrolysis process. As traditional technology, the high-temperature dilute acid
hydrolysis mainly has two modes of operation: percolation hydrolysis and the bolt-
flow high-temperature dilute acid method.
The dilute high-temperature hydrolysis method has the following problems: (1)
The long retention time of products in the reactor results in serious sugar degrada-
tion. (2) Under high-temperature and acidic conditions, the monosaccharides further
degrade to generate uronic and 5-(hydroxymethyl) furfural and lower the sugar
yield (40-48 %). On the other hand, detoxification treatment is necessary because
furfural will also inhibit the yeast alcoholic fermentation. (3) Acid used in dilute acid
hydrolysis is difficult to recover because of its low concentration. So, a large number
of alkalis are used to neutralize acid, leading to huge consumption of acid and
alkali and serious pollution of the neutralized products. (4) The sugar concentration
after dilute acid hydrolysis of lignocellulose is relatively low (2.5-4 %), so a huge
fermentation vessel is necessary, and the concentration of fermentation products is
low. (5) Distillation is associated with costs.
The low-temperature, two-step concentrated acid hydrolysis process is the more
advanced acid hydrolysis technology. Its principle is the hydrolysis of hemicellulose
and cellulose according to their different hydrolysis conditions. Namely, first
hemicellulose is separated under conditions of lower temperature and weak acid
(or no acid), and then separate cellulose is separated at high-temperature and strong
acidic conditions, thus avoiding the overlong retention time of the sugar obtained
in the reactor and reducing sugar degradation [
40
]. The hydrolysis of hemicellulose
and cellulose was conducted in different conditions twice with a low hydrolytic
temperature, resulting in a low concentration of by-products. Thus, the sugar yield
obtained from the hydrolysis of hemicellulose was up to 75-90 %, while part of
the cellulose was dissolved with sugar to yield 50-70 % [
41
]. By further increasing
the solid concentration, the monosaccharide concentrations in the prehydrolysate
and hydrolysate can reach 12 % and 38 %, respectively. A high sugar concentration
can greatly minimize the size of the fermentation equipment and reduce energy
consumption.
At present, there still are many difficulties in the industrialization of the two-
stage, low-temperature acid hydrolysis process: (1) Only about 75 % of the
acid used can be recovered, and large consumption of sulfuric acid and lime
causes environmental pollution. (2) This technology requires a huge engineering
investment because of the large number of highly corrosion-resistant materials and
acid recovery equipment. (3) This technology requires a larger production scale,
which is contradict to the scattered distribution characteristics of lignocellulose
resulting in high cost of transport and storage and consequent high investment in
process equipment.
Thus, acid hydrolysis technology still has many problems (such as acid recovery,
equipment corrosion, construction costs, etc.). In addition, acid hydrolysis has
considerable inhibition effects on subsequent fermentation because of the generation
