Masters Degrees (Chemical Engineering)

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    Techno-economic analysis of 1,3-propanediol, sorbitol, itaconic acid, and xylooligosaccharides production from sugarcane-based feedstocks
    (Stellenbosch : Stellenbosch University, 2023-03) van Heerden, Cara; Görgens, Johann Ferdinand; Farzad, Somayeh; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: In an effort to reduce the human impact on the environment and reduce our reliance on finite fossilbased resources, the need for cost-competitive bioproducts is increasing. One of the potential feedstocks for the establishment of a successful green economy is sugarcane. The South African sugar industry is facing several economic challenges that can be lessened by adding an annexed biorefinery producing high-value bio-products to existing sugarcane mills, to diversify their revenue streams. Both first generation (1G) (molasses) and second generation (2G) (bagasse and trash) feedstock can be used in a biorefinery. This project aimed to evaluate the feasibility of a biorefinery annexed to a South African sugar mill, based on a selection of potential products. Four different bio-products were considered for production from sugarcane-based feedstocks. 1,3- Propanediol (PDO) is a useful building block used in producing the popular textile iiolytrimethylene terephthalate. It can be used as a precursor in polyurethane production among several other uses. Xylooligosaccharides (XOS) is an emerging prebiotic with the outlook of giving value to the hemicellulose portion of lignocellulose. Sorbitol is a well-known sugar alcohol predominantly used in the food industry and for vitamin-C production with the potential of replacing several fossil-based chemical products. Itaconic acid (IA) can become an important commodity biochemical with applications in several polymers, detergent builders, and surfactants if the market price of this product can become competitive with its fossil-based equivalents. These selected 4 products were assessed for economic and environmental feasibility in their own right, together with comparison to alternative products reported elsewhere. Following the development of the process scenarios from literature, the economic feasibility of the scenarios was evaluated using discounted cash flow sheets. PDO production from 1G, 1G2G, and crude glycerol was compared. It was found that a direct fermentation strategy utilising 1G feedstock had the lowest energy and capital investment costs. Consequently, it was the most profitable PDO scenario with an internal rate of return (IRR) of 75.36%. XOS production was found to have the highest IRR of all the investigated scenarios at 87.94%. However, despite having a lower IRR of 77.73%, PDO and XOS co-production better utilized the available 1G and 2G feedstock and would likely be a safer investment. Sorbitol production with green hydrogen resulted in one feasible scenario for the 1G sorbitol and mannitol co-production, while two other scenarios were found to have an IRR within 2% of the preferred IRR of 20%. The low market price and energy-intensive fermentation process for IA production led to unfeasible IA scenarios. The environmental performance of investigated scenarios has been compared by determining their greenhouse gas (GHG) emissions with a GHG calculator tool. Sorbitol production, direct PDO production, PDO and XOS co-production, and 1G IA production were found to have low GHG emissions while indirect PDO production, XOS production, and 1G2G IA production had high GHG emissions. Two scenarios, direct PDO production and PDO and XOS co-production from 1G2G feedstock were consequently identified as biorefinery scenarios that will both have economic and environmental benefits and are therefore recommended as possibly feasible biorefinery scenarios.
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    UV degradation of bioplastics and conventional plastics in the marine environment
    (Stellenbosch : Stellenbosch University, 2023-03) Mzabane, Ondela; Akdogan, Guven; Chimphango, Annie Fabian Abel; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT:In the modern era, there has been a significant increase in the production of and demand for conventional plastics. Increased plastic use is a serious concern for the world. This is because of the accumulation of plastic in the marine environment, which leads to negative impacts on the marine ecosystem. In the marine environment, plastics are exposed to ultraviolet (UV) radiation, temperature changes, physical stress, salinity, and oxidation. Therefore, a key strategy to address this issue is to actively promote and develop biodegradable plastics in efforts to address and alleviate plastic pollution in the marine environment. The study aimed to investigate and compare physical and chemical degradation between bioplastics and conventional plastics to micro-plastics in the marine environment, with little or no microorganism effects. Three different plastics were investigated: polypropylene (PP), polyethylene terephthalate (PET), and polylactic acid (PLA). All plastics were 4 cm × 10 cm in size. Plastics were exposed to two treatments in different environments: (i) a dry UV pretreatment (in air) of neat plastics at two UV irradiances (65 and 130 W/m2 ), and (ii) artificial seawater tests under the same UV conditions. Each run commenced for four weeks, during which UV radiation was cycled for a total of 24 hours: 12 hours on, and 12 hours off. Sampling took place every seven days for further analysis. For signs of degradation, changes in mass loss, carbonyl index, percentage crystallinity, hardness, and morphology were tracked. Results from UV pre-treatment tests showed that in air, high UV irradiation (130 W/m2 ) resulted in more degradation compared to low UV irradiation (65 W/m2 ). Polypropylene was more susceptible to degradation than polyesters (PET and PLA). Degradation in seawater was slow for all plastics. There was a decrease in most properties of seawater compared to the pretreatment tests. This is because, in seawater, the degraded surfaces from the pre-treatment may have been washed away, exposing new surfaces. This investigation showed that the degradation rate is temperature-dependent, and processes in the ocean are slowed down because seawater is a good heat sink. Polylactic acid was the least responsive plastic to UV degradation in both environments.
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    Phase behaviour and thermodynamic modelling of carbon dioxide + (1-octanol or 1-decanol) + large n-alkanes
    (Stellenbosch : Stellenbosch University, 2023-03) Momoh, Chris Favour Ojonugua; Schwarz, Cara Elsbeth; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Large alcohols are useful in the synthesis of detergents, surfactants and plasticisers. However, the product stream of these processes often contains an impurity of large n-alkanes. Azeotropic distillation has been the mainly used process to separate the 1-alcohol product from the n-alkanes. However, due to the process requirements, health and safety risks and extreme operating conditions, investigations have been carried out into utilising an alternative process for this separation. Supercritical CO2 fractionation has been proven to be a feasible method for the separation of detergent range n-alkanes and alcohols because it serves as a cheaper, less toxic method to azeotropic distillation. However, there are notable solute-solute interactions exhibited by ternary systems containing CO2, n-alkanes, and 1-alcohol. The main aim of this work was to experimentally identify and analyse the solute-solute interactions that occur in ternary systems containing supercritical CO2 as a solvent and a solute mixture of 1-alcohols (C8 or C10) and n-alkanes (C10 – C20). The secondary aim of the study was to evaluate the capability of the RK-Aspen thermodynamic model to predict the phase transition data. The solute-solute interactions exhibited in these systems were identified and characterised by analysing their high-pressure bubble- and dew-point (HPBDP) data. Literature revealed that there is, however, a lack of data for the systems relevant to this study. HPBDP data for the ternary systems containing CO2 + 1-alcohols + n-alkanes were thus measured at temperatures ranging from 308 K to 358 K. The phase behaviour data were measured at solute mass fractions ranging from 0.015 to 0.65, with pressures up to 30 MPa, using the synthetic visual phase detection method. The experimental HPBDP data revealed that co-solvency was present in all the ternary systems measured. The systems exhibiting the most significant co-solvency effects were CO2 + 1-octanol + n-hexadecane and CO2 + 1-decanol + n-octadecane. The measured data highlighted that the influence of the n-alkanes on the solute mixtures decreased with an increase in molecular mass. The experimental data revealed that co-solvency occurs in alkane-rich mixtures for systems containing C10 – C14 alkanes while the co-solvency effects for systems containing larger alkanes are more prominent for alcohol-richer mixtures. The range of solute mixture compositions and temperatures investigated did not reveal the effects of other complex phase behaviour phenomena such as miscibility windows and l-l-g holes; these may be present at other conditions for these systems. The evaluation of the RK-Aspen model revealed that the model correlated well with the HPBDP data at higher temperatures (T > 328 K). Furthermore, the addition of experimentally determined solute-solute binary interaction parameters improved the ability of the RK- Aspen model to predict the data. The %AADP for the ternary systems investigated improved from a range of 3.95 % - 17.41 % to a range of 1.65 % - 5.99 % when the solute-solute BIPs were included. The RK-Aspen method also struggled to predict the phase behaviour for systems containing larger n-alkanes as these systems exhibited significant temperature inversions.
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    Optimality assessment with optimality recovery for multi-modal process operations
    (Stellenbosch : Stellenbosch University, 2023-03) Meyer, Tanya; Louw, Tobias Muller; Bradshaw, Steven Martin; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The field of optimality assessment (OA) is a recent development within data-driven process monitoring. OA is a plant-wide approach to real-time optimisation that aims to minimise nonoptimal online operation caused by (1) disturbances that cannot be rejected by the regulatory control system, or (2) inevitable controlled variable setpoint drift. The distinguishing design factor of OA, as opposed to fault- and quality-related process monitoring, is the incorporation of the comprehensive economic index to quantify overall plant optimality or performance. Since optimality is only available in retrospect, the estimation of optimality during real-time operation allows for prompt intervention when nonoptimal conditions arise, so as to prevent prolonged conditions of deteriorated Performance. This work proposes an alternative to the conventional latent variable model-based OA workflows, which employ monitoring charts founded on Shewhart- or similarity-based statistics. The proposed OA workflow is designed to account for continuous and multimodal industrial process data without transition states. The workflow is developed under the framework of a novel optimality landscape which captures various stable modes in the historical process dataset as well as their associated optimality grade. In addition, the proposed optimality landscape captures the cause for the historical operating point shifting from one mode to another, which is termed a modal shift. Two types of modal shifts are captured, namely those that are caused by disturbances, or by SP change(s) that are implemented by the control system or operational team. The offline phase of the proposed OA workflow constructs a holistic reference tool called the optimality graph. The nodes of the optimality graph are discovered by 𝑘-means clustering in the latent variable space, whereas the edges are discovered by the proposed TASLA (time-based alignment of modal shifts and plant log algorithm) technique. Furthermore, metrics are developed for selecting hyperparameters that result in an optimality graph which best reflects the optimality landscape. The online phase of the proposed workflow essentially projects online conditions onto the historical optimality graph using latent variable techniques, such that the closest reference mode is identified. Consequently, real-time conditions are assigned the optimality grade of the identified reference mode. The online optimality graph reveals which actions can be implemented to shift the online operating point toward modes of differing optimality. A key outcome of this work is the utility of the online optimality graph as an optimality advisory that provides the operational team with historically substantiated decision support for implementing optimality recovery. The performance of the proposed OA workflow is tested using a simulated Tennessee-Eastman Process dataset. The proposed workflow captures the historical optimality landscape of the pseudo-industrial dataset and estimates real-time optimality well upon comparison to the ground truth. The performance of each latent variable extraction technique – namely PCA, PLS, ICA – is evaluated by considering the spread of optimality within each mode of the related optimality graph. PLS is deemed the most suited to extracting features that are reflective of the optimality landscape. The holistic nature of the proposed PLS-based OA workflow offers a good alternative to existing latent variable model-based OA workflows.
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    Investigation of characteristics of polyacrylamide-based additives for copper electrowinning
    (Stellenbosch : Stellenbosch University, 2023-03) Masoga, Ivory; Tadie, Margreth; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The formation of nodulations during copper electrowinning remains a major operational concern because it increases the energy consumption of the operation and decreases the quality of the copper plates. Smoothing agents are introduced to mitigate the formation of nodulations and improve the efficiency of the process. One of the emerging prospects is polyacrylamides, research has shown that polyacrylamides have the potential to compete with the industry's leading additives, furthermore, it was also reported that molecular characteristics of polyacrylamides such as molecular weight can influence their grain refining efficacies i.e., reduce nodulation. This study investigated the structurally different polyacrylamide additives in terms of their viscosity in the solution and their percentage ionic charge. Their characteristics are compared in electrochemical methods and then related to electrowinning. Moreover, this study also investigated the compatibility of the additives with solvent extraction, an upstream process of copper electrowinning. This study was divided into three phases. The first phase was the fundamental study using cyclic voltammetry and involved three sections. The first section involved the investigation of the effect of additive concentration on polarisation as well as the investigation of additive molecular characteristics on polarisation. Furthermore, the results obtained were used to select the concentrations that were used for the rest of the work. Cyclic voltammetry experiments were conducted at 35 g/L copper at 40°C. The potentiostat was swept between 0.4 v vs Standard Hydrogen Electrode (SHE), and 0.07 v vs SHE, the scan rate was 5 mv/s and the electrolyte was stirred at 560 rpm using the rotating disk (cathode). The effects of additive concentrations of 1, 2, 10 and 20 mg/L on polarisation were investigated. For electrolytes at an additive concentration of 1 mg/L, polarisation was negligible. At 2 mg/L, polarisation increased with the decrease in molecular weight of the additive. However, at 10 and 20 mg/L, the inverse was reported. There was no significant difference between 10 and 20 mg/L, which prompted using 2 mg/L and 10 mg/L additive concentrations for further experiments. The second section involved the investigation of the effect of residence time on the polarisation behaviour of the additives as well as determining the additive dosage. At 2 mg/L, additive concentration polarisation decreased with time, and the additives were infective within 24 hours; however, at 10 mg/L, there was evidence of a degree of polarisation, prompting 10 mg/L as dosage for electrowinning experiments. The third section involved investigating the effects of copper at 35 g/L and 45 g/L and temperature at 40°C and 50°C. Copper concentration depolarised (increased) the copper reduction current density. However, introducing additives resulted in a less pronounced polarisation effect for 2 mg/L and 10 mg/L. Similar results were observed for the effect of temperature, where temperature depolarised the copper reduction process, however, the introduction of the additives resulted in a pronounced polarisation behaviour. The second phase involved bench electrowinning experiments with an electrolyte containing; 25 mg/L Cl- ,330 mg/L Mn2+, 2.0 g/ Fe3+, Co2+ 760 mg/L, H2SO4 170 g/L and 10 mg/L polyacrylamide. Effects of copper concentration (45-55 g/L) and temperature (40°C-50°C) were evaluated for current efficiency and specific energy consumption. Both factors positively affected efficiency and energy; this was evident by the increase in current efficiency and a decrease in specific energy consumption. Increasing temperature affected the molecular weight of the additives while increasing copper concentration affected the ionic charge of the additives. All the additives reduced the formation of nodulations. The last phase was stripping experiments which investigated the impact of the additives on phase disengagement time. were conducted at 25 mg/L Cl- , 330 mg/L Mn2+ , Fe3+ 2.0 g/L, Co2+ 760 mg/L, Cu2+ 40 g/L, 170 g/L H2SO4, and 10 mg/L PAM, the solutions were stirred at 700 rpm at A/0 ratio of 2.5. Phase disengagement time increased with the increase in additive concentration and molecular weight. The additive concentrations were varied from 2.5 mg/L to 10 mg/L. The results from this work can serve as a basis for polyacrylamide design for either direct electrowinning or the SX-EW integrated process. The molecular weight of the additives has a significant impact on the additives as well as their % ionic charge during electrowinning. It can be concluded that optimisation with respect to additive concentration is required for each operation to obtain an efficient process.