Biomedical Engineering Reference
In-Depth Information
simple. Raw material cost is relatively reduced. But, because a low concentration of
acid is used, the hydrolysis process requires a high-temperature and high-pressure
environment, and its initial development is slow. With the development of chemical
industry equipment and materials and the emergence of various temperature- and
pressure-tolerant equipment as well as acid-resistant material, the advantages of
dilute acid hydrolysis became more apparent. However, dilute acid hydrolysis will
produce formic acid, acetic acid, HMF, phenolic compounds, and other by-products.
So, various methods are studied, including a variety of physical, chemical, and
biological methods to remove the fermentation-inhibiting substances in cellulose
hydrolysis products to improve fermentation yield.
Factors of dilute acid hydrolysis include the pulverized degree of lignocellulosic
material, liquid-to-solid ratio, reaction temperature, reaction time, the concentration
of acid, and type and concentration of cocatalyst. The greater the cellulosic raw
material grinding degree is, the greater the contact area of the raw material with
acid catalyst will be, leading to better hydrolysis. Especially when the reaction rate
is fast, the generated monosaccharide can be removed from the solid surface in a
timely manner, which is helpful for carrying out the reaction. The liquid-to-solid
ratio refers to the ratio of hydrolysate volume and cellulosic raw material quantity.
Generally, the sugar yield of a unit of cellulose solid raw materials increases
with the increase in the ratio of liquid to solid. But, hydrolysis costs rise and the
concentration of the obtained sugar liquid is reduced, increasing the cost of the
follow-up fermentation and distillation processes. Generally, the liquid-solid ratio
is 5-20 mL
g
1
. Temperature is an important impact factor for the hydrolysis rate.
The hydrolysis rate will increase by 0.5-1 times when the temperature increases
10
ı
C. A high temperature causes the decomposition of monosaccharide. So, if
a higher hydrolysis temperature is used, the reaction time should be reduced. In
contrast, reaction time should increase appropriately. Theoretically, when the other
conditions remain unchanged, when the acid concentration doubles, hydrolysis time
can be shortened to a third or half. The acid concentration increase leads to high acid
costs. At the same time, corrosion resistance requirements of the equipment will
increase, resulting in high equipment costs, which is not helpful for industrialization.
Generally, the acid concentration will not exceed 10 %.
Previous research for the dilute acid hydrolysis method were mainly concentrated
in the field of inorganic acids, including hydrochloric acid, sulfuric acid, and so on.
Compared to sulfuric acid, hydrochloric acid hydrolysis has higher efficiency, but
also high difficulty of wastewater treatment. Hydrochloric acid can be taken into
consideration if there is cheap source because it has problems in that it is expensive
and more corrosive and requires high equipment cost. Researchers investigated
phosphoric acid, nitric acid, and maleic acid hydrolysis of cellulose, and preliminary
results were obtained [
132
].
(2) Enzymatic hydrolysis
Enzymatic hydrolysis is a new kind of cellulose hydrolysis technology; it began
in the 1950s. With the development of biology, it attracts increased attentions. It
obtains sugar by enzymatic hydrolysis with cellulase at atmospheric pressure at
