Potable ethanol production from raw corn using simultaneous saccharification and fermentation

Pieters, Ruhardt Jacobus (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Corn starch is one of the most widely used substrates for the production of potable ethanol, such as Scotch grain whisky or South African single grain whisky. High energy demands in these processes led to extensive research on the development of more cost-effective production methods with lower energy demands and higher corn-to-ethanol efficiency. Therefore, finding and optimising less energy intensive methods are of utmost importance. In this study 30 South African corn cultivars were used as substrate to perform a comprehensive process comparison in 1 L shake flask cultures between cooked starch hydrolysis (CSH) and raw starch hydrolysis (RSH) ethanol production processes, where STARGENTM 002 was used as a raw starch hydrolysing enzyme (RSHE). Information based on optimisation experiments were used in an Aspen Plus® process simulation to predict the energy requirements and cost per litre ethanol for both the CSH and RSH processes. Furthermore, the RSH process was investigated to establish weather bacterial contamination had a significant impact on process performance. Similar final ethanol concentrations and ethanol yields as fraction (%) of theoretical maximum were observed in both methods, with final ethanol concentrations of 9.82% and 9.63% (v/v) for the CSH and RSH processes, respectively. Ethanol productivity for the RSH process was beyond any doubt higher than that of the CSH process, with the highest RSH process productivity of 1.3 g/L.h, which was 20% higher than the highest productivity of the CSH process. The absence of starch gelatinization during the pre-treatment section of the RSH process led to the opportunity for very higher gravity fermentations. Small-scale optimisation of the RSH process showed a maximum solids loading of 40% during pre-treatment, due to the inability to obtain homogenously mixed slurries. Surface response models with final ethanol concentration, ethanol productivity and ethanol yield as fraction (%) of theoretical maximum as dependent variables, were successfully used to find an optimum solids loading (37.5%) and an enzyme dosage (1.4 g/kg corn) for the RSH process. Scale-up of the preferred RSH process to pilot-scale achieved a final ethanol concentration of 13.12% (v/v) at a productivity of 1.23 g/L.h, with a solids loading not higher than 37.5% and at an enzyme dosage of 1.4 g/kg, indicating that the process may be applicable under industrial conditions. Aspen Plus® simulations, based on the industrial ethanol production process at the James Sedgwick distillery, together with optimum process parameters for the RSH process, were used to predict and compare the energy requirements for the CSH and RSH processes. The Aspen Plus® simulation predicted an energy requirement of 1.97 kg steam per litre ethanol produced for the RSH process, while the value of 2.8 kg steam per litre ethanol was predicted for the CSH process. The RSH process was more energy efficient, due to the lower pre-treatment temperatures, when compared to the CSH process. A cost model developed for each process, based on the performance fixtures of the Aspen Plus® simulations, showed that the RSH process had higher enzyme costs, when compared to the CSH process, which was due to high STARGENTM 002 dosage requirements and high STARGENTM 002 price. However, the lower energy requirements and lower water consumption by the RSH process outweighed the drawbacks of STARGENTM 002 dosage and price. The cost models predicted a total cost of R 7.70 per litre ethanol produced for the RSH process, while the CSH process had a predicted value of R 8.97 per litre ethanol. All the experimental and simulation work show that the STARGENTM 002 is ready to be tested and as a raw RSHE at an industrial ethanol production process, such as the James Sedgwick distillery. It is recommended that the industrial-scale testing should be at solids loading not higher than 37.5% and at an STARGENTM 002 dosage of 1.4 g/kg.

AFRIKAANSE OPSOMMING: Mieliestysel is een van die algemeenste substrate wat gebruik word tydens die produksie van drinkbare etanol soos Skotse graan whisky of Suid-Afrikaanse enkelgraan whisky. Die hoë energie vereistes wat benodig word tydens hierdie prosesse het gelei tot ʼn aanvraag vir navorsing om sodoende hierdie proses meer koste-effektief te maak. Dit kan bereik word deur meer energie effektiewe metodes te implementeer, wat ook ʼn hoër mielie-na-etanol opbrengs het. Dit is daarom uiters belangrik om energie effektiewe metodes te optimaliseer. In hierdie verslag was 30 Suid-Afrikaanse mieliekultivars as substrate gebruik om ʼn omvattende vergelyking tussen verskillende prosesse te tref. ʼn Een liter skudfles was gebruik tydens die vergelyking tussen gaar stysel hidrolise (GSH) en rou stysel hidrolise (RSH) met STARGEN™ 002 as die rou-stysel-hidroliserings-ensiem (RSHE). Inligting aangaande die optimalisering van die eksperimente was gebruik in ʼn Aspen Plus® proses simulasie om die energie behoeftes en koste per liter etanol, vir die GSH en RSH prosesse, te voorspel. Soortgelyke finale etanol konsentrasies en etanol opbrengs as funksie (%) van die teoretiese maksimum was in beide GSH en RSH metodes waargeneem. Die etanol konsentrasie vir die GSH en RSH prosesse was onderskeidelik 9.82% en 9.63% (v/v). Die etanol produktiwiteit vir die RSH proses was, sonder twyfel, aansienlik hoër in vergelyking met die GSH proses. Die hoogste produktiwiteit vir die RSH proses was 1.3 g/L.h wat 20% hoër was as die hoogste GSH waarde. Die afwesigheid van stysel gelatinisasie tydens die behandelings aspek van die RSH proses het die geleentheid geskep vir baie hoë gravitasie fermentasie. Die optimalisering van die RSH proses op klein skaal het aangedui dat ʼn maksimum vaste stof hoeveelheid van 40% gedurende die behandeling gebruik moet word, aangesien ʼn homogene mengsel nie verkry kan word met ʼn hoër persentasie vaste stof nie. Reaksie oppervlak modelle met ʼn finale etanol konsentrasie, etanol produktiwiteit en etanol opbrengs as funksie (%) van die teoretiese maksimum was as afhanklike veranderlikes gebruik. Die modelle het die ʼn optimale vaste stof hoeveelheid bepaal (37.5%), asook die ensiem hoeveelheid van 1.4 g/kg mielies, vir die RSH proses. Tydens die uitvoering van die RSH proses op ʼn 150 L skaal was ʼn finale etanol konsentrasie van 13.12% (v/v) teen ʼn produktiwiteit van 1.23 h/L.h bereik. Die vaste stof hoeveelheid was egter nie hoër as 37.5% nie en die ensiem hoeveelheid was 1.4 g/kg wat aandui dat die proses wel tydens industriële omstandighede ʼn noemenswaardige opsie kan wees. Aspen Plus® simulasies was gebaseer op die industriële etanol produksie by die James Sedgwick distilleerdery, asook optimale proses parameters was gebruik om die energie vereistes van die GSH en RSH prosesse te voorspel en te vergelyk. Die Aspen Plus® simulasie het ʼn energie vereiste van 1.97 kg stoom per liter etanol voorspel tydens die RSH proses, waar die waarde van die GSH proses 2.8 kg stoom per liter etanol was. Die RSH proses was dus meer energie effektief, aangesien laer behandelings temperature gebruik was in vergelyking met die GSH proses. ʼn Koste model wat saamgestel was vir elke proses, na aanleiding van die Aspen Plus® simulasie, het aangedui dat die RSH proses ʼn hoër ensiem koste het in vergelyking met die GSH proses. Dit was as gevolg van die hoë STARGENTM 002 hoeveelhede en koste. Die laer energie vereiste en laer water verbruik van die RSH proses dui egter aan dat die voordele van die RSH proses steeds die nadele van STARGENTM 002 oortref. Die koste model voorspel ʼn totale koste van R 7.70 per liter etanol wat geproduseer word, terwyl die GSH proses ʼn voorspelde waarde van R 8.97 per liter etanol het. Hierdie simulasie en eksperimentele resultate dui aan dat STARGENTM 002 gereed is om getoets te word as ʼn RSHE tydens ʼn industriële etanol produserings proses, soos by die James Sedgwick distilleerdery. Dit word aanbeveel dat die vaste stof hoeveelheid op industriële skaal nie 37.5% oorskry nie en dat ʼn STARGENTM 002 hoeveelheid van 1.4 g/kg gebruik moet word.

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