Production of ethanol from tropical sugar beet
Brandling, Janine Ellen
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The concern over depleting fossil fuel resources and increasing greenhouse gas emissions has prompted the research into alternative and renewable energy resources. Bioethanol is seen as a potential alternative to petroleum fuels and is mainly produced from sugar and starch containing crops such as sugar cane and maize. In South Africa the use of maize for ethanol production has been prohibited due to food security concerns; therefore, alternative feedstocks need to be investigated. Tropical sugar beet, a new variety of sugar beet, is a potential alternative as it is able to grow in tropical and subtropical climates using much less water than sugar cane. The main objective of this study was to determine the potential of using tropical sugar beet for ethanol production. The study focused on the effects of dilution ratio, pH, yeast concentration and the addition of a nitrogen supplement on the ethanol yield. The maximum ethanol yield of 0.47 g.g–1 which is a conversion efficiency of 92% and a glycerol yield of 0.08 g.g–1 was obtained when no additional water was added to the juice. The best dilution ratio was found to be 1:4 which gave a maximum ethanol yield of 0.48 g.g–1 which is a conversion efficiency of 94% and a glycerol yield of 0.07 g.g–1. An ethanol yield of 0.48 g.g–1 which is a conversion efficiency of 94% was achieved at a yeast concentration of 5 g.L–1 after four hours of fermentation. Nitrogen supplements such as urea, peptone, yeast extract and ammonium sulphate were added during fermentation. The addition of a nitrogen supplement to fermentation had a positive effect on the ethanol yield. The maximum ethanol yield of 0.47 g.g–1 which is a conversion efficiency of 92% was achieved when urea was added to the fermentation. The addition of a nitrogen supplement also decreased the amount of glycerol formed from 0.15 g.g –1 to 0.08 g.g–1. Ammonium sulphate was chosen as the preferred nitrogen source as it is a simple component that can enter the cell directly. A maximum ethanol yield of 0.45 g.g–1 which is a conversion efficiency of 88%, was achieved when 750 mg N.L–1 ammonium sulphate was added. Adjusting the pH prior to fermentation had no real effect on the ethanol yield. The maximum ethanol yield of 0.45 g.g–1 was achieved at all the pH values investigated. Therefore the natural pH of the juice, or pH values between 4 and 5.5, could be used. Adjusting the pH was done to merely reduce the risk of contamination. The optimal fermentation parameters were found to be pH 4, yeast concentration 5 g.L–1 and a ammonium sulphate concentration of 750 mg N.L–1. At these conditions, a maximum ethanol of 0.45 g.g–1 was achieved. These results show that tropical sugar beet with a sugar content of approximately 21.8% (w.w–1) is a good feedstock for ethanol production in South Africa.
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