245
Hydrolysis and Fermentation Technologies for Alcohols
9.4.1.1 rapid steam hydrolysis
This process was recently described by Lee and Shah [2]. A typical autohydrolysis
process uses compressed liquid hot water at a temperature of about 200°C and pressure above the saturation pressure [48]. Thus, the liquid water can hydrolyze hemicellulose in minutes. While hemicellulose recovery is high in this noncatalytic process,
wet pyrolysis results in the production of inhibitory compounds. A well-controlled
process at high temperature with small particles, however, gives high xylose yields
and is desirable. Dekker and Wallis [24] showed that for bagasse, this process gave
90% solubilization of hemicellulose and the product that was highly susceptible to
hydrolysis by cellulases from Trichoderma reesei. In general, however, the xylose
yield in the RASH process is low (30%–50%).
Steam consumption in autohydrolysis strongly depends on the moisture content
of the starting material. An important advantage of autohydrolysis is that it breaks
down lignin into smaller fragments that can be easily solubilized in either base or
organic solvents. This process was first developed in 1925 for hardwood application
and more recently for aspen wood (in the 1980s). At a high pressure of 20–50 atm
and temperature of 210°C–290°C, the water molecules diffuse into the microporous
structure of lignocellulose and the steam condenses at high pressure, thereby wetting
the materials [23]. The wetted material is then driven out of the reactor by a small
nozzle using a pressure difference. The term “explosion” is used because of the process characteristics of the ejection driven by a sudden large pressure drop of steam.
9.4.1.2 dilute acid Prehydrolysis
The pretreatment process can be operated at a lower temperature with reduced sugar
degradation by adding a small amount of mineral acid in the pretreatment process.
The acid increases the reaction rates at a given temperature, and the ratio of hydrolysis rate to the degradation rate is also increased. The reaction rate can be optimized
between the temperature and the reaction time. Higher temperature (200°C) can take
10 s, whereas lower temperature (100°C) may take several hours. Generally, the acid
concentration (sulfuric acid) between 0.5 and 4 wt% is used. While sulfuric acid
gives the xylose yields of 70%–95%, it produces more condensed lignin. Sulfur dioxide is often used as a catalyst. Numerous reports have indicated good results using
this method [2,25]. While acid hydrolysis has been used for more than 100 years, the
replacement of dilute acid hydrolysis by more concentrated acid prehydrolysis was
found to be more expensive. While sulfuric acid is the most widely used catalyst in
this pretreatment, other mineral acids such as hydrochloric, nitric, and trifluoroacetic
(CF 3 COOH) acids have also been used.
9.4.1.3 Organosolv Pretreatment
This process is a pulping technique that uses an organic solvent to solubilize lignin
and hemicellulose. A process developed by Kraft pulping produces high-quality
lignin for added values and easy recovery and recycling of solvents used in the
process. The organic solvents such as ethanol, butanol, and methanol are added
to the pretreatment reaction to dissolve and remove lignin fraction. The internal
lignin and hemicellulose bonds are broken, and both fractions are solubilized while
cellulose remains intact as solid. Careful steps are taken such that the process
Hydrolysis and Fermentation Technologies for Alcohols
9.4.1.1 rapid steam hydrolysis
This process was recently described by Lee and Shah [2]. A typical autohydrolysis
process uses compressed liquid hot water at a temperature of about 200°C and pressure above the saturation pressure [48]. Thus, the liquid water can hydrolyze hemicellulose in minutes. While hemicellulose recovery is high in this noncatalytic process,
wet pyrolysis results in the production of inhibitory compounds. A well-controlled
process at high temperature with small particles, however, gives high xylose yields
and is desirable. Dekker and Wallis [24] showed that for bagasse, this process gave
90% solubilization of hemicellulose and the product that was highly susceptible to
hydrolysis by cellulases from Trichoderma reesei. In general, however, the xylose
yield in the RASH process is low (30%–50%).
Steam consumption in autohydrolysis strongly depends on the moisture content
of the starting material. An important advantage of autohydrolysis is that it breaks
down lignin into smaller fragments that can be easily solubilized in either base or
organic solvents. This process was first developed in 1925 for hardwood application
and more recently for aspen wood (in the 1980s). At a high pressure of 20–50 atm
and temperature of 210°C–290°C, the water molecules diffuse into the microporous
structure of lignocellulose and the steam condenses at high pressure, thereby wetting
the materials [23]. The wetted material is then driven out of the reactor by a small
nozzle using a pressure difference. The term “explosion” is used because of the process characteristics of the ejection driven by a sudden large pressure drop of steam.
9.4.1.2 dilute acid Prehydrolysis
The pretreatment process can be operated at a lower temperature with reduced sugar
degradation by adding a small amount of mineral acid in the pretreatment process.
The acid increases the reaction rates at a given temperature, and the ratio of hydrolysis rate to the degradation rate is also increased. The reaction rate can be optimized
between the temperature and the reaction time. Higher temperature (200°C) can take
10 s, whereas lower temperature (100°C) may take several hours. Generally, the acid
concentration (sulfuric acid) between 0.5 and 4 wt% is used. While sulfuric acid
gives the xylose yields of 70%–95%, it produces more condensed lignin. Sulfur dioxide is often used as a catalyst. Numerous reports have indicated good results using
this method [2,25]. While acid hydrolysis has been used for more than 100 years, the
replacement of dilute acid hydrolysis by more concentrated acid prehydrolysis was
found to be more expensive. While sulfuric acid is the most widely used catalyst in
this pretreatment, other mineral acids such as hydrochloric, nitric, and trifluoroacetic
(CF 3 COOH) acids have also been used.
9.4.1.3 Organosolv Pretreatment
This process is a pulping technique that uses an organic solvent to solubilize lignin
and hemicellulose. A process developed by Kraft pulping produces high-quality
lignin for added values and easy recovery and recycling of solvents used in the
process. The organic solvents such as ethanol, butanol, and methanol are added
to the pretreatment reaction to dissolve and remove lignin fraction. The internal
lignin and hemicellulose bonds are broken, and both fractions are solubilized while
cellulose remains intact as solid. Careful steps are taken such that the process