10 Acid Hydrolysis
Hydrolytic treatment of lignocellulosic biomasses with acid at higher temperatures
will improve the efficiency in producing higher yield of fermentable sugars. Sulfuric
acid is the mostly used acid, and it is operated under high temperature and low acid
concentration. Higher acid concentration makes it highly corrosive and dangerous.
But as a disadvantage, they produce large amounts of gypsum during the neutralization process, thereby increasing the investment and operational costs. Dilute acid
hydrolysis is a commonly used method among the chemical pretreatment methods.
This method can be used either as a pretreatment of lignocelluloses for the subsequent enzymatic hydrolysis or as a method of hydrolyzing into fermentable sugars.
At a higher temperature (140–190
C) and lower acid concentration (0.1–1% sulfuric
acid), they can result at high reaction rates and thus improve cellulose hydrolysis and
along with it 100% hemicellulose removal. It can be performed in short retention
time of 5 min at higher temperature of 180
C or in longer retention time of
30–90 min but at lower temperatures of 120
C. In olive tree biomass when used
as the biomass, 75% of maximum total sugars were obtained when it was pretreated
by dilute acid at 180
C with 1% sulfuric acid concentration and maximum hemicellulose recovery of 83% [40]. The major drawback is the formation of different types
of inhibitors such as carboxylic acids, furans, and phenolic compounds which will
inhibit the microbial growth during the fermentation process and results in lesser
amount of yield and productivity of ethanol [41].
Dilute acid hydrolysis usually occurs in two stages. The first stage being
performed at a lower temperature to maximize the hemicellulose yield, and the
second stage involves higher temperature at optimized conditions for the cellulose
hydrolysis. Mild process conditions (0.7% H 2 SO 4 , 463 K) are opted in the first stage
to recover the five carbon sugars, whereas in the second stage, the remaining solids
containing more resistant cellulose undergo harsher conditions (488 K, but a milder
0.4% H 2 SO 4 ) that help to recover the six carbon sugars. In order to allow adequate
acid penetration, the reduction of the size of the feedstocks is necessary where the
maximum particle dimension is in the range of a few millimeters. When considering
concentrated acid hydrolysis process, it involves an acid (dilute or concentrated)
pretreatment to help liberate the hemicellulosic sugars present in the biomass, while
the subsequent stage requires the pretreated biomass to be dried followed by the
addition of concentrated sulfuric acid (70–90%). The concentration of the acid used
in concentrated acid hydrolysis process ranges between 10 and 30%. Reaction times
are found to be much longer than the dilute acid process. This process provides a
complete and rapid conversion of cellulose to glucose sugars and hemicelluloses to
five carbon sugars with degradation level being very less. The critical factors needed
to be considered to make this process economically viable are the optimization of the
sugar recovery and cost-effectiveness during the acid recovery taken for recycling.
The concentrated acid process offers more potential for cost reductions and leads to
little sugar degradation than the dilute acid process. However, environment and
Lignocellulosic Sugarcane Tops for Bioethanol Production: An Overview
99
Hydrolytic treatment of lignocellulosic biomasses with acid at higher temperatures
will improve the efficiency in producing higher yield of fermentable sugars. Sulfuric
acid is the mostly used acid, and it is operated under high temperature and low acid
concentration. Higher acid concentration makes it highly corrosive and dangerous.
But as a disadvantage, they produce large amounts of gypsum during the neutralization process, thereby increasing the investment and operational costs. Dilute acid
hydrolysis is a commonly used method among the chemical pretreatment methods.
This method can be used either as a pretreatment of lignocelluloses for the subsequent enzymatic hydrolysis or as a method of hydrolyzing into fermentable sugars.
At a higher temperature (140–190
C) and lower acid concentration (0.1–1% sulfuric
acid), they can result at high reaction rates and thus improve cellulose hydrolysis and
along with it 100% hemicellulose removal. It can be performed in short retention
time of 5 min at higher temperature of 180
C or in longer retention time of
30–90 min but at lower temperatures of 120
C. In olive tree biomass when used
as the biomass, 75% of maximum total sugars were obtained when it was pretreated
by dilute acid at 180
C with 1% sulfuric acid concentration and maximum hemicellulose recovery of 83% [40]. The major drawback is the formation of different types
of inhibitors such as carboxylic acids, furans, and phenolic compounds which will
inhibit the microbial growth during the fermentation process and results in lesser
amount of yield and productivity of ethanol [41].
Dilute acid hydrolysis usually occurs in two stages. The first stage being
performed at a lower temperature to maximize the hemicellulose yield, and the
second stage involves higher temperature at optimized conditions for the cellulose
hydrolysis. Mild process conditions (0.7% H 2 SO 4 , 463 K) are opted in the first stage
to recover the five carbon sugars, whereas in the second stage, the remaining solids
containing more resistant cellulose undergo harsher conditions (488 K, but a milder
0.4% H 2 SO 4 ) that help to recover the six carbon sugars. In order to allow adequate
acid penetration, the reduction of the size of the feedstocks is necessary where the
maximum particle dimension is in the range of a few millimeters. When considering
concentrated acid hydrolysis process, it involves an acid (dilute or concentrated)
pretreatment to help liberate the hemicellulosic sugars present in the biomass, while
the subsequent stage requires the pretreated biomass to be dried followed by the
addition of concentrated sulfuric acid (70–90%). The concentration of the acid used
in concentrated acid hydrolysis process ranges between 10 and 30%. Reaction times
are found to be much longer than the dilute acid process. This process provides a
complete and rapid conversion of cellulose to glucose sugars and hemicelluloses to
five carbon sugars with degradation level being very less. The critical factors needed
to be considered to make this process economically viable are the optimization of the
sugar recovery and cost-effectiveness during the acid recovery taken for recycling.
The concentrated acid process offers more potential for cost reductions and leads to
little sugar degradation than the dilute acid process. However, environment and
Lignocellulosic Sugarcane Tops for Bioethanol Production: An Overview
99