Masters Degrees (Chemical Engineering)
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- ItemDecontamination strategies and enzyme dosages for the fermentation of food waste to produce ethanol(Stellenbosch : Stellenbosch University, 2024-03) van Rooyen, Jaybe; Van Rensburg, Eugene; Görgens, Johann Ferdinand; Coetzee, Gerhardt; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Innovative solutions are required to deal with an ever-increasing energy demand and generation of organic waste. Processes where energy or valuable chemicals can be produced from waste, will support the circular economy and a sustainable future. Ethanol production from biomass, such as maize or sugar cane, is a mature technology. However, ethanol production from food waste (FW) is far more challenging, given variable feed composition and microbial contamination. This research aimed to produce ethanol from food waste by evaluating (i) different decontamination strategies, (ii) different yeast strains, namely Saccharomyces cerevisiae Ethanol Red® and S. cerevisiae ER T12, which is an advanced strain engineered to secrete α-amylase, and (iii) increasing solids loading to produce higher ethanol concentrations. Ethanol fermentation using pre- and post-consumer FW was conducted in shake flasks at 30 °C for 72 h. Thermal sterilisation proved effective for post-consumer FW at a low liquefaction temperature of 55 °C with a significant (p < 0.05) increase in ethanol yield of 77.79% compared to 67.29% recorded after fermentation using an unsterilised feed. Similarly, at a liquefaction temperature of 55 °C after thermal sterilisation of pre-consumer FW, an ethanol yield of 92.2% was obtained, substantially higher than the 48.2% yield of the control where the feedstock remained unsterilised. This data suggested that thermal sterilisation effectively served simultaneously as a decontamination and gelatinisation strategy. Substantial decreases in enzyme dosages of up to 33% were achieved by using the consolidated bioprocessing (CBP) yeast S. cerevisiae ER T12 without affecting the ethanol yield of 80.87% ± 1.40 and productivity 1.51 g/L/h compared to 82.56% ± 2.81 and 1.54 g/L/h when using S. cerevisiae strain Ethanol Red as benchmark. The same enzyme reduction of https://scholar.sun.ac.za iii | P a g e 33% was observed for the post-consumer FW. The ethanol concentration of the pre-and postconsumer FW was increased by 96% to 74.11 ± 0.75 g/L and 85% to 48.52 ± 1.32 g/L without a significant effect on the ethanol yield when the solids loading was increased to 20.6% and 21.06% w/v. Although sterilisation proved to be an effective decontamination method, it remains energy intensive, which could affect the process's financial feasibility. Using the CBP yeast can substantially decrease operational costs due to the reduced requirements for commercial enzymes, which is crucialto developing a sustainable FW fermentation process. The CBP yeast strain proved effective even when the solids loading was increased, and the results are promising, showing that the usage of CBP yeasts can further improve the viability of the fermentations.
- ItemMeasurement, modelling and uncertainty propagation of low-pressure phase equilibrium data for 1-alcohols and n-alkanes(Stellenbosch : Stellenbosch University, 2024-03) Buitendach, Nadine; Schwarz, Cara; de Klerk, Danielle Lee; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.ENGLISH ABSTRACT: Phase equilibrium data are important to the design of chemical separation processes. Good thermodynamic models are required to determine the phase equilibrium data of the components being separated. The Non-Random Two-Liquid (NRTL) model is a popular activity coefficient model (ACM) used to model a wide variety of systems at low pressures. This is due to the inclusion of a non-randomness parameter and temperature in(dependent) parameters (TDPs). Experimental phase equilibrium data are required to parameterise the NRTL model. These data are associated with uncertainties and as a result the model parameters and process design outputs are associated with uncertainties. Accelerating research is being performed on the propagation of these uncertainties to the thermodynamic model and process design outputs. Out of the various uncertainty propagation (UP) techniques employed, Monte Carlo Simulation (MCS) is the most popular. However, in addition to experimental uncertainties, there are uncertainties inherent in the regression of model parameters. A few uncertain regression elements have been investigated in literature, but to the best of the author’s knowledge none of the UP studies have performed a thorough investigation on the comparison of these two types of uncertainties. The aim of this work is to investigate the effect of model parameterisation and experimental uncertainties in the thermodynamic modelling of low-pressure phase equilibrium data for 1-alcohol + n-alkane systems using the NRTL model. The systems are chosen to present a series of systems showing varying degrees of non-ideal phase behaviour. This work considers binary systems comprising of either ethanol, 1-propanol and 1-butanol, each paired with n-hexane, n-heptane and n-octane. Experimental vapour-liquid equilibrium (VLE) data are measured at 101.3 kPa for four systems: 1-propanol + n-hexane, 1-propanol + n-heptane, 1-butanol + n-hexane and 1-butanol + n-heptane with ethanol + n-heptane serving as the verification system. Experimental uncertainties are rigorously calculated for the systems measured and are used in the experimental UP performed using MCS. This work uses the isobaric PTxy data as the uncertain input variables in the MC approach. This work shows that neglecting correlation between the input variables can lead to the overestimation of the model output uncertainties. While the magnitude of the input uncertainties is important, the inclusion of vapour pressure data uncertainties is less important. For the model parameterisation, the effect of model parameter initial guesses is important. While the choice of objective function is less important in the modelling of the reported VLE data, it is important in the experimental UP. A temperature independent modelling approach can model the less non-ideal systems in this work well. However, for the most non-ideal system, ethanol + n-octane, a more flexible modelling approach is required to obtain a good enough model fit to the VLE data. The experimental UP results are specific to the TDP modelling approach and are system specific. It is thus encouraged that engineers investigate the model parameterisation before performing the experimental UP. The insight provided in this work can be used to extend the investigations to other thermodynamic models and systems.
- ItemThe effect of mixing on Rhodopseudomonas palustris growth and hydrogen production(Stellenbosch : Stellenbosch University, 2024-03) Grobler, Elzé; Pott, Robert William McClelland; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.ENGLISH ABSTRACT: Biological hydrogen production methods, such as photofermentation, offer a potentially environmentally friendly alternative to the non-renewable hydrogen production methods currently used in industry. However, a significant drawback of photofermentation is the mutual shading effect, as not all the cells are exposed to sufficient light intensities, which decreases light utilisation efficiency and, consequently, hydrogen production. Mitigating this effect is important to make photofermentation more economically viable. Various solutions have been proposed in literature, including designing photobioreactors with an increased surface area to volume ratio; however, this results in increased land and material costs. This study proposes controlled mixing as a solution, as it might increase light/dark (L/D) cycling frequencies. The effects of mixing on purple non-sulphur bacteria (PNSB) cells are still poorly understood, especially when a homogeneous substrate is used. This study aims to determine the effect of mixing on the PNSB, Rhodopseudomonas palustris, using glycerol as the substrate. Mixing would be facilitated via static mixers and varying the flow rate since static mixers have not yet been investigated as a mixing method in PNSB systems. The light distribution through the photobioreactor was modelled while considering light scattering effects, revealing that at a cell concentration of 0.7 g/L, 81% of the cells were exposed to insufficient light intensities for hydrogen production in this study. Therefore, increasing the L/D cycling frequency via mixing was expected to increase hydrogen production. Objectives two and three evaluated the impact of three static mixer designs and three flow rates on the growth and hydrogen production of R. palustris, revealing that R. palustris is sensitive to shear stress from the peristaltic pump, with the static mixers causing significantly less stress. The half-moon and spiral static mixers significantly increased the growth and hydrogen production at a flow rate of 0.15 L/min. Due to the static mixers alleviating the mutual shading effect and their corresponding lower degree of shear stress, they resulted in a higher specific hydrogen production than when the cells were circulated via the peristaltic pump. Mixing via the static mixers, therefore, warrants further investigation. Both the spiral and half-moon mixers resulted in a significant increase in growth and hydrogen production. However, the concentric mixer did not, potentially due to an ineffective flow path. The spiral mixer requires further investigation, as it resulted in the maximum specific hydrogen production, although a large degree of error persists with this static mixer. Interestingly, the static mixers only resulted in a significant increase in hydrogen production at 0.15 L/min, with no significant impact as the flow rate increased. R. palustris may already have been photosaturated when circulated through the peristaltic pump every 3.18 minutes (0.31 L/min). This highlights the importance of considering both the light distribution and the L/D cycling frequency when determining the required photobioreactor depth. Compared to microalgae, R. palustris could thus be recirculated much less frequently with no adverse effects on hydrogen production. This is advantageous as less pumping power would be required for optimal performance in PNSB systems.
- ItemThe effect of light intensity and reactor configuration on Rhodopseudomonas palustris growth and hydrogen production(Stellenbosch : Stellenbosch University, 2024-03) Ross, Brandon Sean; Pott, Robert William McClelland; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.ANGLISH ABSTRACT: Biological hydrogen production is a promising replacement for the current modes of hydrogen production seen in industry today. However, key issues with the process still need to be solved, such as its economic viability and whether environmentally sustainable production is possible. The use of photosynthetic bacteria such as the purple non sulphur bacterium Rhodopseudomonas palustris to produce hydrogen via photofermentation has shown to have a low environmental impact but the required energy input is greater than the energy derived from the hydrogen produced. To increase the efficiency of the hydrogen production process the ratio of energy consumed to energy produced must be improved. One method of enhancement is the design and development of photobioreactor (PBR) systems to improve the hydrogen production of the process. The majority of PBRs have been designed for microalgae systems, with very few reactors designed for photosynthetic bacteria. PBR designs are therefore required to improve the hydrogen production of bacterial systems. Outdoor operation has also been earmarked as a means to improve the economic viability of the process as the required light energy will be provided by sunlight, which is a free source of light energy. However, the effect of light intensity on the growth of R. palustris is fairly poorly understood. The aim of this study was to determine the effect of light intensity on the growth of R. palustris, and to design and construct a multipurpose PBR that can be used for the cultivation of planktonic and immobilised R. palustris to produce biological hydrogen. The first objective was to compare the effect of changing light intensity on the growth kinetics of R. palustris. The results of this comparison showed that R. palustris grows readily over the range of light intensity investigated (70 to 600 W/m2 ), with photo-limitation occurring at low intensities (30 and 70 W/m2 ), photosaturation occurring between 200 and 400 W/m2 , and photo-limitation occurring when the light intensity increased to 600 W/m2 . The highest maximum specific growth rate of 0.0420±0.014 h-1 was achieved at a light intensity of 400 W/m2 . A predictive model for the cell growth and substrate utilisation of R. palustris over the range of light intensity investigated (70 to 600 W/m2 ) was successfully developed with the model showing good fit to the experimental data. A PBR that facilitated hydrogen production by R. palustris either as a growing planktonic cell culture or immobilised in a PVA cryogel matrix was successfully designed and constructed. The performance of the PBR operating with planktonic cells was compared to immobilised cells operating as a fluidised bed PBR (FBPBR) and a packed bed PBR (PBPBR). The FBPBR achieved the highest maximum specific hydrogen production rate of 15.74±2.2 mL/g/h. The planktonic culture and PBPBR achieved lower production rates of 12.6±8.0 mL/g/h and 4.53±0.8 mL/g/h respectively. In terms of the substrate conversion efficiency, the FBPBR achieved a conversion of 43% which outperformed the PBPBR which only achieved a conversion of 26.7%. The conversion efficiency of the planktonic cell culture was between the two immobilised cell configurations with an efficiency of 32%.
- ItemInvestigating the feasibility of a chlorinated resin for water disinfection in rural and peri-urban areas(Stellenbosch : Stellenbosch University, 2024-03) Mtimuni, Tadala; Pillay, Visvanathan Lingamvrthi; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.ENGLISH ABSTRACT: Water is essential for humanity in daily household activities, but most water sources are contaminated and necessitate treating water before it can be used. Chlorine has been the predominant disinfectant in rural areas, but determining the right dosage has been challenging. Chlorinated resins are a promising option as they release chlorine in a controlled manner. However, control of chlorination during the synthesis of these resins is a major problem. Previous chlorine sources released reactive chlorine (Cl) species during chlorination, resulting in unreacted chlorine that did not bind on the specific site and over-chlorinated resins. Hence, this study aimed to synthesize a resin using sodium dichloroisocyanurate (NaDCC) as a chlorine source that releases less reactive Cl species and could improve the chlorination process. First, a chlorinated resin was developed using a two-step procedure. The first procedure was split into two: a hydantoin with amide functional groups was prepared and grafted onto the resin's surface. The modified resin was then chlorinated using NaDCC. The first procedure used a two-factor central composite design (CCD) to optimize temperature and reaction time. A three-factor central composite design was used in the chlorination reaction by varying temperature (20-30 °C), reaction time (40-50 mins), and pH (5.5-6.5). The optimum conditions were identified as a temperature of 25°C, a reaction time of 54 minutes, and a pH of 5. The developed resin contained 6.18% chlorine (by mass%), in contrast to the earlier chlorinated resins that had chlorine loadings of more than 20% (mass%). The performance of the developed resin was evaluated through chlorine release experiments and disinfection efficacy experiments using a 1-litre water sample with a bacterial concentration of 3.6 x106 cfu/ml. The first-order kinetic model was used to study the chlorine release behaviour of the resin. A resin mass of 485.43 mg (containing 30 mg of Cl) exhibited a chlorine release rate of 0.351 mg/minute. Experiments on disinfection efficacy demonstrated that a resin mass with over 30 mg of chlorine achieved a 99.99% reduction in bacteria concentration within 5 minutes. A lab-scale point-of-use (POU) water treatment system with a resin disinfecting column was designed using the parameters obtained from the first-order kinetic model for chlorine release. A resin column with a 3 cm diameter achieved a minimum flow rate of 0.89 L/min and demonstrated a disinfection efficacy of 99.99%. The resin was stable; it maintained a 99.99% disinfection efficacy and a residual chlorine concentration of 0.3 mg/L after flushing 1L of water through the resin column for 20 continuous cycles. By a small margin, the chlorine content in the resin decreased with increasing disinfection cycle A feasibility study was conducted to evaluate the disinfection efficacy, ease of implementation, and environmental impact of the resin POU system in comparison to the World Health Organization (WHO) standards for water treatment technologies. The chlorinated resin achieved 99.99% (4 log10) bacterial elimination, aligning with the WHO standards. The scalability advantages of the POU system were demonstrated, allowing easy implementation. In addition, the resin-based disinfection unit maintained a residual chlorine level of 0.3 mg/L, meeting the WHO standards on residual chlorine. With the improved chlorine loading, the chlorinated resin holds a promising prospect as a potential point of use water treatment device for peri-urban and rural areas. However, a techno-economic analysis will be necessary.