Research Articles (Chemical Engineering)
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- ItemEnhancing the functional properties of acetylated hemicellulose films for active food packaging using acetylated nanocellulose reinforcement and polycaprolactone coating(Elsevier, 2020-02) Mugwagwa, Lindleen R.; Chimphango, Annie F. A.Acetylated hemicellulose (AH)-nanocellulose (ACNC) films coated with polycaprolactone (PCL) films, were evaluated as active packaging for aqueous, alcoholic, fatty and acidic food. The effects of nanocellulose loading (0–50 %), degree of acetylation (DS) (0–2.34) and polycaprolactone coating (0.3 g/mL) on hydrophobicity and solubility of AH films in food simulants, were investigated. In addition, AH-CNC/PCL films were doped with polyphenols and their antioxidant release (temperature 5 °C–40 °C, time - 48 h) into food simulants was evaluated experimentally and by modelling (Migratest software). Increasing ACNC DS and loading, combined with a PCL coating increased films’ hydrophobicity (24.59° to 82.48°) and reduced film solubility in all the simulants (∼82.8 %). The release of polyphenols by the films was highest and best predicted using Migratest software for the fatty food simulant. Therefore, these films can be used as active packaging for fatty foods. Furthermore, Migratest modelling can be used to predict film performance during film design.
- ItemCanary in the coliform mine : exploring the industrial application limits of a microbial respiration alarm system(Public Library of Science, 2021-03-04) Stone, Wendy; Louw, Tobi M.; Booysen, Marthinus J.; Wolfaardt, Gideon M.; Zhang, DaweiFundamental ecological principles of ecosystem-level respiration are extensively applied in greenhouse gas and elemental cycle studies. A laboratory system termed CEMS (Carbon Dioxide Evolution Measurement System), developed to explore microbial biofilm growth and metabolic responses, was evaluated as an early-warning system for microbial disturbances in industrial settings: in (a) potable water system contamination, and (b) bioreactor inhibition. Respiration was detected as CO₂ production, rather than O₂ consumption, including aerobic and anaerobic metabolism. Design, thresholds, and benefits of the remote CO₂ monitoring technology were described. Headspace CO₂ correlated with contamination levels, as well as chemical (R² > 0.83–0.96) and microbiological water quality indicators (R² > 0.78–0.88). Detection thresholds were limiting factors in monitoring drinking water to national and inter- national standards (0 CFU/100 mL fecal coliforms) in both open- (>1500 CFU/mL) and closed-loop CO₂ measuring regimes (>100 CFU/100 mL). However, closed-loop detection thresholds allow for the detection of significant contamination events, and monitoring less stringent systems such as irrigation water (<100 CFU/mL). Whole-system respiration was effectively harnessed as an early-warning system in bioreactor performance monitoring. Models were used to deconvolute biological CO₂ fluctuations from chemical CO₂ dynamics, to optimize this real-time, sustainable, low-waste technology, facilitating timeous responses to biological disturbances in bioreactors.
- ItemPrediction of gas holdup in a column flotation cell using computational fluid dynamics (CFD)(The Southern African Institute of Mining and Metallurgy, 2019-01) Mwandawande, I.; Akdogan, G.; Bradshaw, S. M.; Karimi, M.; Snyders, N.ENGLISH ABSTRACT: Computational fluid dynamics (CFD) was applied to predict the average gas holdup and the axial gas holdup variation in a 13.5 m high cylindrical column 0.91 m diameter. The column was operating in batch mode. A Eulerian-Eulerian multiphase approach with appropriate interphase momentum exchange terms was applied to simulate the gas-liquid flow inside the column. Turbulence in the continuous phase was modelled using the k- realizable turbulence model. The predicted average gas holdup values were in good agreement with experimental data. The axial gas holdup prediction was generally good for the middle and top parts of the column, but was over-predicted for the bottom part of the column. Bubble velocity profiles were observed in which the axial velocity of the air bubbles decreased with height in the column. This may be related to the upward increase in gas holdup in the column. Simulations were also conducted to compare the gas holdup predicted with the universal, the Schiller-Naumann, and the Morsi-Alexander drag models. The gas holdup predictions for the three drag models were not significantly different.
- ItemInvestigation of flow regime transition in a column flotation cell using CFD(Southern African Institute of Mining and Metallurgy, 2019-02) Mwandawande, I.; Akdogan, G.; Bradshaw, S. M.; Karimi, M.; Snyders, N.ENGLISH ABSTRACT: Flotation columns are normally operated at optimal superficial gas velocities to maintain bubbly flow conditions. However, with increasing superficial gas velocity, loss of bubbly flow may occur with adverse effects on column performance. It is therefore important to identify the maximum superficial gas velocity above which loss of bubbly flow occurs. The maximum superficial gas velocity is usually obtained from a gas holdup versus superficial gas velocity plot in which the linear portion of the graph represents bubbly flow while deviation from the linear relationship indicates a change from the bubbly flow to the churn-turbulent regime. However, this method is difficult to use when the transition from bubbly flow to churn-turbulent flow is gradual, as happens in the presence of frothers. We present two alternative methods in which the flow regime in the column is distinguished by means of radial gas holdup profiles and gas holdup versus time graphs obtained from CFD simulations. Bubbly flow was characterized by saddle-shaped profiles with three distinct peaks, or saddle-shaped profiles with two near-wall peaks and a central minimum, or flat profiles with intermediate features between saddle and parabolic gas holdup profiles. The transition regime was gradual and characterized by flat to parabolic gas holdup profiles that become steeper with increasing superficial gas velocity. The churn-turbulent flow was distinguished by steep parabolic radial gas holdup profiles. Gas holdup versus time graphs were also used to define flow regimes with a constant gas holdup indicating bubbly flow, while wide gas holdup variations indicate churn-turbulent flow.
- ItemRational engineering of Saccharomyces cerevisiae towards improved tolerance to multiple inhibitors in lignocellulose fermentations(BMC (part of Springer Nature), 2021-08-28) Brandt, Bianca A.; Garcia‑Aparicio, Maria D. P.; Gorgens, Johann F.; Van Zyl, Willem H.Background: The fermentation of lignocellulose hydrolysates to ethanol requires robust xylose-capable Saccharomyces cerevisiae strains able to operate in the presence of microbial inhibitory stresses. This study aimed at developing industrial S. cerevisiae strains with enhanced tolerance towards pretreatment-derived microbial inhibitors, by identifying novel gene combinations that confer resistance to multiple inhibitors (thus cumulative inhibitor resistance phenotype) with minimum impact on the xylose fermentation ability. The strategy consisted of multiple sequential deltaintegrations of double-gene cassettes containing one gene conferring broad inhibitor tolerance (ARI1, PAD1 or TAL1) coupled with an inhibitor-specific gene (ADH6, FDH1 or ICT1). The performances of the transformants were compared with the parental strain in terms of biomass growth, ethanol yields and productivity, as well as detoxification capacities in a synthetic inhibitor cocktail, sugarcane bagasse hydrolysate as well as hardwood spent sulphite liquor. Results: The first and second round of delta-integrated transformants exhibited a trade-off between biomass and ethanol yield. Transformants showed increased inhibitor resistance phenotypes relative to parental controls specifically in fermentations with concentrated spent sulphite liquors at 40% and 80% v/v concentrations in 2% SC media. Unexpectedly, the xylose fermentation capacity of the transformants was reduced compared to the parental control, but certain combinations of genes had a minor impact (e.g. TAL1 + FDH1). The TAL1 + ICT1 combination negatively impacted on both biomass growth and ethanol yield, which could be linked to the ICT1 protein increasing transformant susceptibility to weak acids and temperature due to cell membrane changes. Conclusions: The integration of the selected genes was proven to increase tolerance to pretreatment inhibitors in synthetic or industrial hydrolysates, but they were limited to the fermentation of glucose. However, some gene combination sequences had a reduced impact on xylose conversion.