Process modelling in production of biobutanol from lignocellulosic biomass via ABE fermentation

Naleli, Karabo (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.


ENGLISH ABSTRACT: The interest in production of biobutanol as a fuel has increased significantly in the last two decades. The main reason is that biobutanol is recognised as superior biofuel than ethanol, which is already being blended with gasoline in USA and Brazil. In addition, biofuels have potential to reduce greenhouse gas (GHG) emissions when they are used as replacement of gasoline in transportation industry. A major drawback in Acetone-Butanol-Ethanol (ABE) fermentation is the low final product concentration, due to limited biomass growth and product inhibition. Low concentrations of butanol in the fermentation broth have severe disadvantage of high energy requirement during downstream processing. Fermentation technology improvement like in situ gas stripping for butanol recovery during the fermentation has potential to provide a more concentrated feed to downstream purification. For downstream product recovery and purification, alternative methods to double effect distillation (DD), which may be more energy efficient, have been investigated, including liquid-liquid extraction and distillation (LLE&D). The main objective of this study was to develop six conceptual process model scenarios for production of biobutanol from lignocellulosic biomass, from the literature data available, using ASPEN Plus® V8.2 software. These include: (1) Batch Simultaneous Saccharification and Fermentation (SSF) integrated with Gas Stripping and double effect distillation used as recovery and purification method(SSF-GS/DD). (2)Batch SSF integrated with Gas Stripping and liquid-liquid extraction and distillation used as recovery and purification method(SSF-GS/LLE&D). (3) Continuous Separate Hydrolysis and Fermentation (SHF) and double effect distillation used as recovery and purification method(CONT-SHF/DD). (4) Continuous SHF and liquid-liquid extraction and distillation used as recovery and purification method(CONT-SHF/LLE&D). (5) Batch SHF and double effect distillation used as recovery and purification method(B-SHF/DD). (6) Batch SHF and liquid-liquid extraction and distillation used as recovery and purification method(B-SHF/LLE&D). The impacts of different fermentation methods, fermentation technology improvements and products recovery/purification methods on the energy demand, energy efficiency and economics of the various process scenarios were investigated. Furthermore, the best performing process scenario was compared to previously process model on biobutanol production from molasses on the basis of same butanol capacity, in terms of energy demand and efficiency and economic feasibility. Among the six scenarios modelled for a plant capacity of 1 million dry tonnes feedstock per year, the economic assessment showed that only Batch SSF-GS/DD and SSF-GS/LLE&D scenarios were viable under current market conditions. These scenarios gave net present values (NPV) of US$140million and US$47million and internal rates of return (IRR) of 16% and 11% respectively. Sensitivity analysis showed that change in the feedstock price from US$30/tonne to US$150/tonne has greatest impact in minimum butanol selling price (MBSP) (US$0.41/kg – US$1.76/kg) with the market price at US$0.78/kg. The total capital investment (TCI) of butanol production from molasses (US$187million) was significantly lower than the TCI of US$585million for scaled best performing SSF-GS/DD process scenario; on the basis of equal annual butanol production of 118800 tonnes. The comparison further showed that molasses based butanol had higher IRR and NPV of 36% and US$958million compared to 14% and US$112million of SSF-GS/DD. With regards to energy demand and efficiency, energy demand was met in all of the scenarios by combustion of solid residues after fermentation-purification together with 10% of the lignocellulose feedstock. Onsite electricity production was in excess to process demands, providing surplus electricity that could be sold for additional revenue. SSF-GS/DD and SSF-GS/LLE&D scenarios gave highest liquid fuel energy efficiencies of 26% and 23% respectively, and overall energy efficiencies of 36% and 30% respectively.Butanol production from lignocellulose required more process energy per unit of butanol produced, compared to butanol production from molasses. This was evidenced by lower energy demand of23MJ/kg for molasses based butanol compared to 58MJ/kg of the best selected scaled up SSF-GS/DD process scenario.

AFRIKAANSE OPSOMMING: Die belangstelling in die produksie van biobutanol as brandstof het tydens die laaste twee dekades baie vergroot. Die hoofrede hiervoor is dat biobutanol herken word as ‘n beter brandstof as etanol wat alreeds in die VSA en Brasilië kombineer word met petrol. Biobrandstowwe het ook die potensiaal om kweekhuisgas-emissies te verminder as dit in plaas van petrol of diesel in die vervoerindustrie gebruik word. ‘n nadeel wat betref die fermentasie van Aseton-Butanol-Etanol is die lae konsentrasie van die finale produk weens die beperkte groei van die biomassa en die inhibisie van die produk. Die lae konsentrasie van butanol in die fermentasie vloeistof beteken dat baie energie later in die proses benodig word. Verbeterings in die fermentasie tegnologie soos die in situ stroop van gas vir die herwin van butanol het die potensiaal om ‘n meer gekonsentreerde voer te verskaf vir latere suiwering. Vir die latere herwinning en suiwering is alternatiewe metodes om die effek van distillasie te verdubbel (DD) ondersoek, sook vloeistof-vloeistof ektraksie en distillasie (LLE&D). Dit mag meer doeltreffend wees. Die hoofdoel met hierdie studie is om ses konseptuele proses model scenario‘s vir die produksie van biobutanol vanaf lignosellulose biomassa, in die beskikbare literatuur met die gebruik vanASPEN Plus® V8.2 sagteware, te ondersoek. Die sluit in: (1) Groep Tegelyke Sakkarifikasie en Fermentasie (SSF) geïntergreer met die Stroop van Gas en dubbel-effek distillasie wat gebruik word vir die herwin- en suiwerings metode. (2)Groep SSF geïntergreer met die Stroop van Gas en vloeistof en distillasie wat gebruik word as herwin- en suiwerings metode(SSF-GS/LLE&D). (3) Aanhoudende Aparte Hidrolise en Fermentasie (SHF) en dubbel-effek distillasie gebruik as herwin- en suiweringsmetode(CONT-SHF/DD). (4) Aanhoudend SHF en vloeistof ekstraksie en distillasie gebruik as herwin- en suiweringsmetod(CONT-SHF/LLE&D). (5) Groep (Batch) SHF en dubbel-effek distillasie gebruik as herwin- en suiweringsmetode (B-SHF/DD). (6) Groep (Batch) SHF en die vloeistof-vloeistof ekstraksie en distillasie gebruik as herwings- en suiweringsmetode(B-SHF/LLE&D). Die impakte van die verskillende fermentasie metodes, verbeterings in fermentasie tegnologie en herwin-/suiwerings metodes op die vraag na, doeltreffendheid en ekonomiese scenario‘s is ondersoek. Die proses wat die beste presteer het is met die proses wat voorheen vir die produksie van biobutanol van molasse gebruik is, vergelyk wat betef butanol kapasiteit, die energie wat benodig word, die doeltreffendheid en die ekonomiese lewensvatbaarheid. Van die ses scenario‘s vir ‘n aanleg met ‘n kapasiteit van 1 miljoen droë ton voer per jaar het die ekonomiese ondersoek bewys dat net die Groep SSF-GS/DD en SSF-GS/LLE&D scenario‘s onder die huidige marktoestande lewensvatbaar is. Die huidige netto waardes (NPV) is US$140million en US$47million en die interne opbrengs (IRR) is 16% and 11% onderskeidelik. Ontledings het getoon dat veranderings in die prys van voer vanaf US$30/tonne na US$150/tonne die grootste impak het op die minimum butanol verkoopprys. (US$0.41/kg – US$1.76/kg) met ‘n markprys vant US$0.78/kg. Die totale kapitale belegging van butanol produksie vanaf melasse (US$187miljoen) was heelwat laer as die totale kapitale belegging van US$585miljoen vir die beste SSF-GS/DD proses scenario, op die basis van ‘n gelyke jaarlikse butanol produksie van 118800 tonnes. Die vergelyking het verder aangetoon dat die melasse wat op butanol baseer ‘n hoër IRR en NPV van of 36% en US$958 miljoen in vergeleke met 14% en US$112 miljoen van SSF-GS/DD, het. Wat betref die vraag na krag en die doeltreffendheid, is die vraag na krag in al die scenario‘s bevredig deur die verbranding van die vaste oorblyfings van die fermentasie-suiwerings proses tesame met 10% van die lignosellulêre voer. Die aanbod van elektriese krag was groter as die vraag daarna en die oortollige krag kon teen ‘n wins verkoop word. Die. SSF-GS/DD en SSF-GS/LLE&D scenario‘s het die beste gevaar wat betref ‘n vloeistof/brandstof energie doeltreffendheid van 26% and 23% onderskeidelik, en ‘n algehele doeltreffendheid van 36% en 30% onderskeidelik. Butanol wat uit lignosellulose vervaardig is, het meer prosesenergie per eenheid vervaardig, benodig in vergelyking met butanol wat uit melasse vervaardig is. Minder krag 23MJ/kg word vir benodig vir butaneol wat op melasse baseer is as vir die beste geselekteerde opgeskaalde SSF-GSD/DD proses scenario. Stellenbosch University

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