Browsing by Author "Robus, Charles Louis Loyalty"

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    Production of bioethanol from paper sludge using simultaneous saccharification and fermentation
    (Stellenbosch : Stellenbosch University, 2013-03) Robus, Charles Louis Loyalty; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Whereas fuel used for transport and electricity production are mainly fossil–derived, there has recently been an increased focus on bio-fuels due to the impact of fossil derived fuel on the environment as well as the increased energy demand worldwide, concomitant with the depletion of fossil fuel reserves. Paper sludge produced by paper mills are high in lignocellulose and represents a largely untapped feedstock for bio-energy production. The aim of this study was to determine the composition, fermentability and optimum paper sludge loading and enzyme dosage for producing ethanol from paper sludge. This information was used to develop a model of the process in Aspen Plus®. The mass and energy balances obtained from the Aspen Plus® model were used to develop equipment specifications which were used to source equipment cost data. A techno-economic model was developed from the equipment cost data to assess the economic viability of the simultaneous saccharification and fermentation (SSF) process utilising paper sludge as feedstock. Nine paper sludge samples obtained from Nampak Tissue (Pty) Ltd. were evaluated in terms of ethanol production and those samples yielding the highest and lowest ethanol titres were selected for optimisation. This allowed for the determination of a range of ethanol concentrations and yields, expressed as percentage of the theoretical maximum, which could be expected on an industrial scale. Response surface methodology was used to obtain quadratic mathematical models to determine the effects of solid loading and cellulase dosage on ethanol production and ethanol yield from paper sludge during anoxic fed-batch fermentations using Saccharomyces cerevisiae strain MH1000. This approach was augmented with a multi response optimisation approach incorporating a desirability function to determine the optimal solid loading and cellulase dosage in fed-batch SSF cultures. The multi response optimisation revealed that an optimum paper sludge loading of 21% (w/w) and a cellulase loading of 14.5 FPU g-1 be used regardless of the paper sludge sample. The fact that one optimal enzyme dosage and paper sludge loading is possible, regardless the paper sludge feed stock, is attractive since the SSF process can be controlled efficiently, while not requiring process alterations to optimize ethanol production when different batches of paper sludge are processed. At the optimum paper sludge loading and cellulase dosage a minimum ethanol concentration of 47.36 g l-1 (84.69% of theoretical maximum) can be expected regardless of the paper sludge used. An economic assessment was conducted to ascertain whether ethanol production from paper sludge using SSF is economically viable. Three scenarios were investigated. In the first scenario revenue was calculated from the ethanol sales linked to the basic fuel price, whereas in the second and third scenarios liquefied petroleum gas (LPG) consumption at the paper mill was replaced with anhydrous and 95% ethanol respectively. In all the cases, paper sludge feed rates of 15, 30 and 50 t d-1 were used. The production of ethanol from paper sludge for ethanol sales (scenario 1) resulted in higher IRR and NPV values, as well as shorter payback periods, compared to replacement of LPG at the paper mill (scenarios 2 and 3). At an assumed enzyme cost of $ 0.90 gal-1 (R 2.01 litre-1), IRR values of 11%, 22% and 30% were obtained at paper sludge feed rates of 15, 30 and 50 t d-1. A sensitivity analysis performed on the total capital investment and enzyme cost revealed that the SSF process is only economically viable at a paper sludge feed rate of 50 t d-1 irrespective of the variation in capital investment. For the SSF process to be economically viable the enzyme costs must be lower than $ 0.70 gal-1 (R 1.56 litre-1) and $ 1.20 gal-1 (R 2.68 litre-1) for paper sludge feed rates of 30 and 50 t d-1 respectively. The SSF process at a paper sludge feed rate of 15 t d-1 was not economically viable even assuming a zero enzyme cost. A Monte Carlo simulation revealed that the SSF process is economically viable at a paper sludge feed rate of 50 t d-1 as a mean IRR value of 32% were obtained with a probability of 26% to attain an IRR value lower than 25%. The SSF process at lower paper sludge loadings is not economically viable as probabilities of 70% and 95% were obtained to attain IRR values lower than 25% at paper sludge feed rates of 30 and 15 t d-1 respectively. From this study it can be concluded that paper sludge is an excellent feedstock for ethanol production for the sales of ethanol at a paper sludge feed rate in excess of 50 t d-1 with the added environmental benefit of reducing GHG emissions by 42.5%.