Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by browse.metadata.advisor "Cheng, H."

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    Selection of preferred sweet sorghum cultivars and their pretreatment optimisation for bio-ethanol production
    (Stellenbosch : Stellenbosch University, 2013-03) McIntosh, Paul Andrew; Gorgens, Johann F.; Cheng, H.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Currently the world is facing a global energy crisis due to rising energy demands, dwindling fossil fuels and increased greenhouse gas emissions. Countries are therefore looking at reducing fossil fuel reliability, mitigating emissions and meeting sustainable development goals. One such initiative has been the replacement of fossil fuel with bio-ethanol, which is both renewable and sustainable. Locally in the South African context, one of the potential bio-ethanol feed stocks under investigation is sweet sorghum. This feedstock is high in both soluble and non-structural sugars, it is drought resistant, has low input requirements, has high yields and can be grown over a wide range of climatic conditions. Thirty-six sweet sorghum cultivars were therefore collected from the University of Kwa-Zulu Natal, where they were grown, before being transported to the University of Stellenbosch where they were they underwent a selection process coupled with a pretreatment optimization process aimed at maximizing potential bio-ethanol yields. The initial thirty-six cultivars were screened at a small scale with dilute sulfuric acid pretreatment at 170⁰C, 15 minutes and 0.7% H2SO4 combined with enzymatic hydrolysis which was carried out at an enzyme loading of 15FPU/g water insoluble solids (WIS), a pH of 4.8, a temperature of 50⁰C and a residence time of 72 hours Results showed statistically that increasing lignin and ash content negatively affected pretreatment response resulting in a range of combined sugar yields for the initial thirty-six cultivars of between 32.64g/100g raw material and 44.04g/100g raw material. From these results, the thirty six cultivars were reduced to ten by inclusion of pretreatment response yields and agronomic factors into total estimated ethanol yields from the whole plant. The top ten ranking ethanol producing sweet sorghum cultivars selected were SS27, AS254, AS246, AS103, AP6, AS106, MSJH13, AS245, AS248 and AS79. Utilising two low severity pretreatments namely, 190⁰C, 5 minutes, 0.25% (w/w) H2SO4 and 200⁰C, 5 minutes, 0.07% (w/w) H2SO4 combined with two enzyme loadings of 3.75FPU/g WIS and 15FPU/g WIS, the previously selected ten cultivars were further evaluated and reduced to five through ranking of the average total potential ethanol yields for the two pretreatment conditions and their corresponding enzyme loadings. The five cultivars which performed well under these conditions were AP6, SS27, AS103, MSJH13 and AS246 which subsequently underwent further optimization. Conditions investigated were 180 - 190⁰C, 5 – 15 minutes and 0.25% H2SO4 which resulted in an increase in combined sugar yields to between 48.83 and 54.5g/100g raw material on an oven dry basis. Selection of three preferred cultivars was based on the best average total potential ethanol yields calculated for the two seasons at each cultivars optimum pretreatment condition. Further one of the selected cultivars had to have poor pretreatment response at a small scale to allow for the effect of chemical composition to be evaluated in the steam explosion pilot plant. The three selected cultivars were AP6, SS27 and AS246. Of the five previously selected sweet sorghum cultivars, AP6, SS27 and AS246 underwent optimization in a steam explosion reactor under air dried, water soaked and SO2 catalysed conditions. Cultivars performed similarly under air dried and water soaked conditions but varied under SO2 catalysed conditions. A 3% SO2 catalysed steam explosion was most promising with yields of 87.2% to 91.48% of theoretical sugar content in the native biomass for the 3 preferred Sweet Sorghum cultivars. Combined with agronomic data this translated into potential bio-ethanol yields from the whole plant of between 7131 and 8678 L/ha grown under South African conditions. While these results are promising, further development of the three preferred sweet sorghum cultivars, AP6, SS27 and AS246, is necessary to implement these as dedicated bio-energy feed stocks.