| Summary: | 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. === Thesis (M.Sc. Engineering Sciences (Chemical Engineering))--North-West University, Potchefstroom Campus, 2011.
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