Sintayehu, Nibret (PhD)Fseha, Gebremichael2020-03-072023-11-102020-03-072023-11-102019-07http://etd.aau.edu.et/handle/12345678/20986The 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.en-USYieldFermentationSequential PretreatmentSorghum StrawThesis