Bioethanol Production from Sequential Acidic-Alkaline Pretreated Sorghum Straw Hydrolysate
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Date
2019-07
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Addis Ababa University
Abstract
The depletion and environmental problems associated with fossil fuels as main energy source
motivates to look for alternative energy sources that do not compromise both food security and
the environment. Sorghum is a fast-growing crop that can be harvested twice a year and produce
both food (grain) and straw that can be utilized for ethanol production. The study explores the
production of bioethanol from sequential acidic-alkaline pretreated sorghum straw hydrolysate.
Sequential acid-alkaline pretreatment method was employed to obtain most intact
monosaccharides from cellulose and hemicellulose. Dilute sulfuric acid (1%) pretreatment at 125
℃ for 10 minutes was used at the first stage mainly for hemicellulose removal. In the second stage
pretreatment, dilute sodium hydroxide (1.25 %) at 90 ℃ for 10 minutes was employed for
delignification. The residues after the second pretreatment were hydrolyzed using dilute sulfuric
acid of concentrations (2 %, 3%, and 4%), at temperatures (110 ℃, 125 ℃, and 140 ℃) and times
(20, 40, and 30 minutes). The sugar concentration of the hydrolysates was determined using phenol
sulfuric acid method. Three hydrolysates having sugar levels 30.42 g/L, 31.79 g/L, and 32.9875
g/L were selected for fermentation. Response surface methodology, Box-Behnken experimental
design with design expert version 11.1.0.1 was applied to randomize and analyze the effects of the
variables encountered in fermentation experiments. The experiments were done using the selected
hydrolysates, varying size of inoculum (5 %, 10 %, and 15 %), and pH (4.5, 5, and 5.5) at 30 ℃
for 72 hours. The fermented samples were finally centrifuged and evaporated for purifying the
ethanol. The yield of ethanol was determined after determining the percentage ethanol using
digital density meter. FTIR analysis was done to determine the functional group of the product
obtained and the result showed O-H, C-O, and C-H peaks similar to the standard ethanol. The
results of the statistical analysis showed that all the three independent factors and the interaction
between sugar level and pH affected the ethanol yield and a maximum of 0.617 mL/g yield was
obtained at sugar level of 3.29875 g, pH of 5, and 15 % inoculum size. Generally, controlling
fermentation parameters during the production of ethanol from sequential acidic-alkaline
pretreated sorghum straw is a good choice in view of increasing yields of sugar and ethanol,
decreasing dose and cost of chemicals, and minimizing waste generation.
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Keywords
Yield, Fermentation, Sequential Pretreatment, Sorghum Straw