Masters Degrees (Chemical Engineering)

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Browsing Masters Degrees (Chemical Engineering) by Author "Arthur, Wellington"

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    Assessment of enzyme recycling schemes for paper sludge fermentation using kinetic modelling
    (Stellenbosch : Stellenbosch University, 2021-03) Arthur, Wellington; Gorgens, Johann F.; Van Rensburg, Eugene; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Ethanol for road transport is well established, but it also presents a “gateway molecule” in the bigger circular economy setting, where it is used for jet fuels, bioplastics, organic solvents, etc. These high-value products make ethanol production an attractive route compared to other biomass valorisation methods such as incineration, anaerobic digestion, and composting. Cheaper raw materials such as agricultural and industrial wastes have been found as suitable feedstock for bioethanol production. Paper sludge (PS) is one major waste stream from the pulp and paper industry that is mainly landfilled. Due to the extensive pulping processes, PS has substantial accessible cellulose content which makes it appropriate for bioethanol production. However, the cost of enzymes required during the process is one of the major bottlenecks to the commercialization of bioethanol from PS. This work sought to achieve significant savings on the enzyme dosage required in a simultaneous saccharification and fermentation (SSF) process by recycling enzymes in the clarified supernatant or whole fermentation broth, consisting of solids and liquid. To achieve this goal, kinetic models that predict the loss of cellulase activity and ethanol concentrations during the fermentation process were developed. The loss of cellulase activity was assumed to follow a first-order reaction for batch enzyme feeding in the reactor system. This model was used to predict the level of cellulase activity at any time in the fermentation. It was based on the overall decay of residual activities from the preceding reactor and fresh enzyme supplementations at the beginning of each round of fermentation. The model that describes the enzymatic hydrolysis of PS used the Avrami-Kolmogorov-Erofeev (AKE) equation to determine the kinetic conversion rates. Assuming a constant ethanol yield from sugars, the AKE equation was used to predict ethanol concentrations. Model predictions were validated experimentally by evaluating the performance of the recycling schemes over multiple recycling rounds. Approximately 48% and 40% of the initial activity of Cellic® CTec 3 remained in the supernatant and broth respectively after 72 h of SSF in shake flask. Although the recycling schemes showed no significant variation, contamination with lactic acid-producing bacteria decreased ethanol yields during successive recycling steps. During the scale-up of enzyme recycling in 5L bioreactors, close to 35% of the initial enzyme activity was measured in the supernatant after 168 h of fed-batch fermentation. With only 65% fresh enzyme supplementation, the final ethanol concentration in the fermentation broth increased from 40 to 70 g/L. However, the experimental data deviated from model predictions as the ethanol yields decreased from 280 to 200 kg ethanol/ton dry PS for the first and last fermentations respectively. The observed reduction in the overall enzymatic hydrolysis rate was possibly due to the effect of time-wise loss of enzyme potency/synergy. The kinetic models were able to predict the performance of PS fermentation with some reasonable level of accuracy. In spite of the inherent factors that affected PS fermentation, the advantages of high ethanol concentrations and significant reduction in enzyme dosages achieved can be very beneficial to the economic viability of the process.