Production of bioethanol from paper sludge using simultaneous saccharification and fermentation

Robus, Charles Louis Loyalty (2013-03)

Thesis (MScEng)--Stellenbosch University, 2013.

Thesis

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%.

AFRIKAANSE OPSOMMING: Aangesien dat brandstof vir vervoer en energie meestal vanaf fossiel afgeleide bronne kom, is daar onlangs ʼn groter fokus op bio-brandstowwe as gevolg van die impak van fossiel afgeleide brandstowwe op die omgewing en 'n verhoogde aanvraag na energie wêreldwyd, gepaardgaande met die uitputting van fossielbrandstof-reserwes. Papier slyk geproduseer deur papier meule is hoog in lignosellulose en verteenwoordig 'n grootliks onontginde grondstof vir etanol produksie. Die doel van die studie was om vas te stel wat die samestelling, fermenteerbaarheid, optimale papier slyk en ensiem ladings is vir die vervaardiging van etanol uit papier slyk. Die inligting was gebruik om 'n model van die proses in Aspen Plus® te ontwikkel. Die massa-en energiebalanse wat verkry is van die Aspen Plus® model was gebruik om toerusting spesifikasies te ontwikkel wat gebruik was om toerusting kostes te bereken. ‘n Tegno-ekonomiese model is ontwikkel om die ekonomiese lewensvatbaarheid van die gelyktydige versuikering en fermentasie proses “SSF” wat gebruik maak van papier slyk as grondstof te assesseer. Nege papier slyk monsters verkry vanaf Nampak Tissue (Pty) Ltd. is geëvalueer in terme van etanol produksie. Die monsters wat die hoogste en laagste etanol konsentrasies opgelewer het, is geselekteer vir optimalisering omdat dit toegelaat het vir die vasstelling van etanol konsentrasies en opbrengste, uitgedruk as persentasie van die teoretiese maksimum, wat verwag kan word in industrie. Reaksie oppervlak metodologie “RSM” is gebruik om wiskundige modelle te ontwikkel om die impak van papier slyk lading en sellulase dosis op etanol produksie en etanol opbrengs te assesseer. Die RSM is aangevul met 'n multi effek optimiserings benadering wat 'n wenslikheid funksie inkorporeer om die optimale papier slyk lading en sellulase dosis in gevoerde-enkellading SSF kulture te bepaal. Die multi effek optimalisering het getoon dat 'n optimale papier slyk lading van 21% (w/w) en 'n sellulase dosis van 14.5 FPU g-1 gebruik moet word, ongeag van die papier slyk monster. Die feit dat die optimale ensiem dosis en papier slyk lading dieselfde is ongeag die papier slyk monster, is aantreklik aangesien die SSF proses meer doeltreffend beheer kan word omdat proses veranderinge nie nodig is om die proses te optimaliseer nie. By die optimale papier slyk lading en sellulase dosis kan 'n minimum etanol konsentrasie van 47.36 g l-1 (84,69% van die teoretiese maksimum) verwag word ongeag van die papier slyk wat gebruik word. 'n Ekonomiese evaluasie is gedoen om vas te stel of etanol produksie vanaf papier slyk met behulp van SSF ekonomies lewensvatbaar is. Drie moontlikhede is ondersoek. In die eerste moontlikheid is die inkomste bereken vanaf etanol verkope gekoppel aan die basiese brandstofprys, terwyl in die tweede en derde moontlikhede, LPG by die papier meul vervang is met anhidriese en 95% etanol onderskeidelik. In al die gevalle was daar gebruik gemaak van papier slyk voer tempo’s van 15, 30 en 50 t d-1. Die produksie van etanol uit papier slyk vir verkope (moontlikheid 1) het gelei tot hoër IRR en die NPV waardes, sowel as korter terugverdien tydperke, in vergelyking met die vervanging van LPG by die papier meul (moontlikhede 2 en 3). Met ʼn ensiem koste van $ 0.90 gal-1 (R 2.01 litre-1) is IRR-waardes van 11%, 22% en 30% verkry teen papier slyk voer tempo’s van 15, 30 en 50 t d-1 onderskeidelik. 'n Sensitiwiteitsanalise uitgevoer op die totale kapitale belegging en ensiem koste het aan die lig gebring dat 'n SSF proses slegs ekonomies lewensvatbaar is op 'n papier slyk voer tempo van 50 t d-1 ongeag van die variasie in die kapitale belegging. Vir die SSF proses om ekonomies lewensvatbaar te wees, moet die ensiem kostes laer wees as $ 0.70 gal-1 (R 1.56 liter-1) en $ 1.20 gal-1 (R 2.68 liter-1) vir papier slyk voer tempo’s van onderskeidelik 30 en 50 t d-1. Die SSF proses was op 'n papier slyk voer tempo van 15 t d-1 nie ekonomies lewensvatbaar nie, selfs teen 'n ensiem koste van nul. 'n Monte Carlo-simulasie het getoon dat die SSF proses ekonomies lewensvatbaar is met 'n papier slyk voer tempo van 50 t d-1 omdat 'n gemiddelde IRR-waarde van 32% verkry is met 'n waarskynlikheid van 26% om 'n IRR-waarde laer as 25% te verkry. Die SSF proses teen papier slyk voer tempo’s van 30 en 15 t d-1 is nie ekonomies lewensvatbaar nie omdat waarskynlikhede van 70% en 95% onderskeidelik verkry is om IRR-waardes laer as 25% te kry. Daar kan van die studie afgelei word dat papier slyk 'n uitstekende grondstof is vir die produksie van etanol mits 'n papier slyk voer tempo van meer as 50 t d-1 bereik kan word. Die produksie van etanol vanaf papier slyk het die bykomende voordeel dat kweekhuis gasse (GHG) met 42.5% verminder word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/80251
This item appears in the following collections: