Doctoral Degrees (Chemical Engineering)

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https://scholar.sun.ac.za/handle/10019.1/323

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Incorporating sustainable practise principles into development of solutions for mitigation of mine waste
(Stellenbosch : Stellenbosch University, 2024-03) Manjengwa, Evelyn Ruvimbo; Tadie, Margreth; Dorfling, Christie
ENGLISH ABSTRACT: Mining in its nature is a capital intensive, spatially invasive global undertaking characterised by negative environmental, political and socio-economic legacies. Increased regulatory compliance, institutional pressures and access to information has necessitated a proactive and holistic shift in business practices and, governance of the mining industry. Therefore, there is an increased urgency for more holistic frameworks contributing to consistency in measurement and monitoring of environmental, social & governance (ESG) performance in mining related projects. To this end, different researchers have endeavoured to develop such frameworks combining two or more sustainability pillars. In response to the existing pressures, threatening the viability and legitimacy of the industry in general, the goal of the PhD study sought to develop a holistic decision-making framework embodying sustainable practice principles useful for informing mining projects. In this regard the use of sustainable practice principles refers to the intentional engagement of economic, social and environmental sustainability metrics in the development of the decision-making framework. Such projects include for example, mine waste management, effluent reuse and mine waste valorization etc. Therefore, the framework development was contingent on the existence of demonstrated and sustained pressures located in the business ecosystem and broader macroeconomic environment. Secondly, the framework development was also contingent on the existent scope to improve the approach to decision making frameworks. As a result, the pillars supporting the framework’s development were based on a developed understanding of different stakeholder priorities. The stakeholder priorities were identified using two perspectives, chiefly the multi-stakeholder and company-based perspectives. Underpinning management theories and schools of thought that informed the framework’s development included, the stakeholder, shareholder, agency and institutional theories. At the same time, schools of thought, such as principle-principle conflicts, legitimacy and clientalism were also used to support the framework’s development. The management theories and schools of thought were useful in developing a frame of reference that enabled a connection between the observations made in literature and the attitudes and reactions observed across different stakeholder groups, to mining related projects. To understand the stakeholder priorities from a multi-stakeholder perspective, a detailed examination of literature on varied discourses associated with extractive industries and society was undertaken. The exploratory study ultimately led to the development of a quantitative method (Value Analysis Tree (VAT) framework) to assess stakeholder perceptions’ of the value of mining projects. To understand stakeholder priorities from a company perspective, a sustainability performance assessment was undertaken for a selected gold miner, Company X. Life cycle-based approaches Environmental Life Cycle Assessment, dynamic Societal Life Cycle Assessment and Social Organisational Life Cycle assessment (ELCA, dSLCC and SO-LCA) were used to examine the relevance of mine tailings reprocessing to the business. An interrogation of the mining company’s organic growth strategy supported by mine waste valorization, specifically tailings reprocessing, was undertaken. The sustainability performance was gauged against established industrial and normative metrics, the company's strategic objectives captured by its key performance indicators, contextual regulations and policies and, finally against the level of attainment of the sustainable development goals (SDGs). As a result, the implications on the company and its stakeholders, following the inclusion of mine waste valorization were documented. The output of the life cycle studies confirmed that motivators shaping decision making priorities across different stakeholder categories, are tied to the known and perceived costs, risks and benefits associated with a mining project. Using this understanding of stakeholder priorities from a multi-stakeholder (applying the VAT framework) and a company perspective, a Multicriteria-Multistakeholder Decision Making (MMD) framework capturing the sustainability and business metrics, was then developed and tested successfully in Microsoft excel. Different decision and management-based theories underpin the framework’s development. The MMD framework was developed considering normative, rationalist and intuitive approaches. Research novelties: Mining company growth strategy was interrogated using Life Cycle thinking. No literature documents the explicit use of dSLCC, ELCA and SO-LCA to interrogate organisational growth strategy supported by production operations in mining. Additionally, a holistic approach (MMD framework) was developed that consolidates the sustainability and business-based perspectives to support decision making associated with mine waste management projects. The MMD framework will need to be tested further using other case studies to validate its applicability.
Comparing the environmental impact of different hydrometallurgical processes for the recycling of lithium-ion batteries using a life cycle assessment approach
(Stellenbosch : Stellenbosch University, 2024-03) Maritz, Roelof Frederick; Dorfling, Christie; Akdogan, Guven; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
ENGLISH ABSTRACT: Lithium-ion batteries (LIBs) have become commonplace for everyday use in consumer electronics. These batteries have also gained a lot of popularity recently for usage in larger scale application such as electric vehicles (EV). The LIB market is projected to grow from 700 GWh in 2022 to 4.7 TWh in 2030 (Fleischmann et al., 2023). The consequence of this rapidly increasing demand for LIBs is the formation of a fast-growing end-of-life (EOL) LIB waste stream. This waste stream includes valuable metals such as lithium, cobalt, nickel, and manganese to potentially be recycled, thus providing benefits in terms of waste management and income from the sale of these recovered metals. There is thus a clear need for EOL LIB recycling and a necessity to find out what is the best process technology available to recycle EOL LIBs. Traditionally LIBs have been recycled using pyrometallurgy, but the recent industry focus has shifted towards alternative process technologies such as hydrometallurgy. There is, however, no clear consensus on how these hydrometallurgical flowsheets should be arranged. As such, the purpose of this study was to compare the environmental impacts of implementing different hydrometallurgical process flowsheets designed for the recovery of metals from EOL LIBs. This comparative environmental study was performed using the life cycle assessment (LCA) framework and considered the use of three lixiviants (hydrochloric-, sulphuric-, and citric acid) alongside the use of three flowsheet options (sequential metal precipitation, mixed metal precipitation, and hybrid sequential precipitation - solvent extraction systems). Lastly, the process was modelled based on a mixed feed of LiCoO2, LiFePO4, and NMC111 batteries. The potential environmental impacts of mineral acid-based processes were found to generally be lower than that of organic acid-based processes by 18 to 61 percentage points. Furthermore, mixed metal precipitation provided the greatest environmental benefit of the flowsheet options considered by 46 to 117 percentage points when compared to the closest competitor. The LCA system was subsequently subjected to multivariate uncertainty analysis and a discernability analysis regarding process feed sensitivity which served to confirm the trends already observed. The LCA system was also subjected to a weak point analysis, where the consumption of NaOH and electricity were listed as the main concerns for process improvement. The process solutions recommended to address both weak points involve the integration of membrane technology and antisolvent crystallisation. Furthermore, the LCA system was compared for a South African and a European context, where it was determined that South Africa’s overreliance on hard coal for energy generation is the main difference between the two regions. Finally, the hydrometallurgical EOL LIB recycling processes were subjected to an additional LCA study regarding the use of recycled metals for resynthesizing NMC cathode materials. This additional study showed that integrating the sequential precipitation recycling process with solid-state synthesis of NMC622 cathode could save up to 70% on energy consumption during cathode synthesis. Meanwhile, integrating the mixed NMC precipitation recycling process with the solid-state synthesis of NMC622 cathode could reduce the environmental impact of NMC cathode production by up to 67%.
Developing the s-SAFT-γ Mie equation of state toward nonaqueous alkanolamine-based carbon capture systems
(Stellenbosch : Stellenbosch University, 2024-03) Schulze-Hulbe, Alexander; Cripwell, Jamie Theo; Burger, Andries Jacobus; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
ENGLISH ABSTRACT: Decarbonizing industrial processes is imperative for mitigating the harmful effects of climate change. A promising route to decarbonization lies in developing nonaqueous alkanolamine-based carbon capture processes. However, there is a very wide range of nonaqueous formulations to choose from, and little available thermodynamic data. Accordingly, an apt starting point for assessment of nonaqueous alkanolamine-based carbon capture is the development of a predictive thermodynamic modeling tool which captures the salient phenomena of these systems. The Statistical Associating Fluid Theory (SAFT) equations of state (EoSs) present a fundamental approach to thermodynamic modeling. Combining these EoSs with the group-contribution (GC) approach provides these rigorous models with considerable predictive capabilities. This renders GC-approach SAFT EoSs particularly useful in the data-scarce context of nonaqueous alkanolamine-based carbon capture. Accordingly, the main aim of this work was to develop structural SAFT-γ Mie (“s-SAFT-γ Mie”), a stateof- the-art GC-approach SAFT EoS, toward a description of alkanolamine solvent/CO2/organic cosolvent systems. This presents the first instance in which the predictive capabilities of a GC-approach EoS are extended to nonaqueous alkanolamine-based carbon capture systems. However, myriad approaches can be followed in developing parameters for GC-approach EoSs. This renders parameterization challenging, thus presenting an obstacle to industrial implementation of these models. To facilitate use of GC-approach EoSs, a further aim of this work was to illustrate how GCapproach EoSs can be parameterized for nonaqueous alkanolamine-based carbon capture systems using a systematic and consistent approach. Transferable s-SAFT-γ Mie group interaction parameters were developed from the ground up for primary and secondary alcohols, as well as primary amines. The model exhibited robust capabilities in modelling these components as well as their mixtures with n-alkanes. However, results for linear alkanolamines indicate that s-SAFT-γ Mie’s generalizability comes at the expense of quantitative accuracy. In the process of developing these parameters, a novel and generalizable approach was devised to account for the effect of changing hydroxyl group position in secondary alcohols. This further developed s-SAFT- γ Mie’s capabilities in distinguishing between the properties of isomers, an important characteristic for solvent/cosolvent screening purposes. s-SAFT-γ Mie further provided qualitatively accurate descriptions for a wide range of organic cosolvents with a single parameter set. This broadly generalizable modeling approach can be extended to components for which little or no reliable data are available, highlighting its value to carbon capture process designers. The parameters thus developed were transferred to CO2-containing mixtures. Pertinently, s-SAFT-γ Mie provided qualitatively accurate descriptions of CO2 solubility in polyethylene glycols, which are important components for nonaqueous carbon capture. Regarding alkanolamine solvent/CO2/organic cosolvent systems, s-SAFT-γ Mie was capable of qualitatively reproducing the effects of temperature, liquid-phase composition as well as organic cosolvent chain length on CO2 solubility. This holds for lower pressures, where CO2 solubility is driven by chemical absorption, as well as higher pressures, where CO2 is dissolved by physical absorption. These robust predictive capabilities render s-SAFT-γ Mie well-suited to comparing CO2 solubility in several alkanolamine solvent/organic cosolvent formulations, highlighting its potential future use within the context of a solvent/cosolvent screening tool.
Development of a dynamic model for direct copper electrowinning operations
(Stellenbosch : Stellenbosch University, 2023-12) Grobbelaar, Suné; Tadie, Margreth; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
ENGLISH ABSTRACT: Innovation is essential for fostering sustainable and environmentally conscious growth in copper production, particularly for operations employing resource-intensive direct copper electrowinning. A dynamic model can be coupled with advanced control strategies in an innovative approach to addressing the control and optimisation challenges associated with copper electrowinning. Previous studies have primarily focused on steady-state models, and limited research has been conducted on dynamic models for copper electrowinning. Consequently, this project aimed to develop a dynamic model for copper electrowinning, with a specific focus on the direct electrowinning process. The main original contribution of this project is the validated semi-empirical dynamic copper electrowinning model. The model can be calibrated for a specific tankhouse, including direct electrowinning operations. An offline parameter-fitting approach was developed for fitting initial model parameters, and for use when limited data are available. The project also introduced an accompanying online parameter-fitting approach that uses moving horizon estimation to continuously adjust the model parameters based on evolving input data. The approach ensures the parameters remain up to date as process conditions change. The least-squares error objective function was selected for use in the online approach, with two types of system models investigated: fundamental and surrogate. The surrogate models were investigated mainly as a future-orientated strategy for online parameter-fitting using computationally intensive datasets. The model incorporated a conceptual resistance network, mass conservation equations, and reactionrate and mass-transfer kinetics. Key performance indicators (copper yield, current efficiency, and specific energy consumption) were used to quantify electrowinning performance. The model included input variables such as current, and the concentrations of copper, iron, nickel, cobalt, and sulfuric acid. The effect of nickel and cobalt were accounted for through existing empirical density and conductivity correlations, and a newly regressed limiting-current density correlation. Validation using dynamic industrial tankhouse data showed the credibility of the model for representing real-life systems. The average normalised residual mean square errors over the five 14-day validation cycles investigated (with the online approach activated) were 10.0%, 29.3%, 79.2%, and 3.9%, for the current efficiency, copper plating rate, specific energy consumption, and potential, respectively. The quantifier values for the specific energy consumption were consistently above the threshold for acceptable model fit. Caution was, therefore, advised in interpreting the model-predicted specific energy consumption values. Overall, the model's performance, particularly with inclusion of the online parameter-fitting approach, however, exhibited satisfactory agreement with the industrial data. The developed model has the potential to make a meaningful contribution to the field. The model's versatility and accuracy make it a valuable tool for use in operator training, process monitoring, and early-fault detection. It also opens avenues for exploration of advanced control strategies. By everaging these potential benefits, operations can enhance productivity, reduce costs, and minimise environmental impact. It is recommended that future work should focus on developing online data validation strategies to further enhance model fidelity, as well as exploring advanced surrogate model structures.
The impact of processing conditions on enzymatic protein hydrolysis performance from sardine (Sardina pilchardus) by-products using Alcalase 2.4L, and the influence on final spray dried hydrolysate powder properties
(Stellenbosch : Stellenbosch University, 2023-12) Chiodza, Godknows Kudzai; Goosen, Neill Jurgens; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
ENGLISH ABSTRACT: Sardine processing by-products are a valuable resource that is currently underutilised. Currently, they are being processed into low value fish meal, which is used as animal feed, despite their being food-grade material suitable for human consumptiom Enzymatic hydrolysis is one of the best methods for producing value-added products as it produces protein hydrolysates with bioactive, functional and physico-chemical properties. However, despite more than 60 years of research and development, some important information relevant to the chemical engineering discipline remain lacking. This study was aimed at determining the impact of processing conditions at different stages of enzymatic hydrolysis of sardine by-products using Alcalase to produce spray dried hydrolysates. This was achieved by (i) investigating the effect of mixing speed, solids concentration and enzyme dosage on dry solids yield and protein recovery, and emulsion formation during enzymatic hydrolysis of sardine processing by-products, (ii) determining the effect of solids concentration and emulsion formation on molecular weight distribution of protein hydrolysates, (iii) investigating the effect of mixing speed and solids concentration on the viscosity and mixing regime of material during enzymatic hydrolysis, (iv) establishing the role played by processing conditions (degree of hydrolysis (OH), maltodextrin addition and inlet air temperature) on powder recovery during spray drying, and handling and storage properties of the spray dried protein hydrolysates. A complex relationship was observed between variables, where the effect of one variable was dependent on the levels of the other processing variables. Low solids concentration at low mixing speed and high enzyme dosage were required to maximise protein recovery in the aqueous phase while minimising protein loss to emulsion and sludge. Simultaneously increasing solids concentration and mixing speed did not attenuate protein loss to emulsion. The peptides in the emulsion phase are of bw value, as they are removed from the hydrolysate that is finally dried. Their recovery requires additional processing, which adds to process complexity and cost At high solids concentration, energy consumption in the first 10 minutes of hydrolysis was significantly higher than in the last 10 minutes (hydrolysis time = 60 min). The reduction in energy consumption was due to liquefaction of solids. Although viscosity was expected to decrease due to liquefaction of solids and reduction in molecular weight of proteins, it increased in the first 10 minutes of hydrolysis and remained high thereafter. This was most likely due to gel formation caused by agglomeration of proteins when their concentration in solution increases, or emulsion formation, or both. It is shown that processing conditions affected powder recovery during spray drying, and handling and storage properties. DH had the strongest influence on powder recovery, with high DH resulting in bw powder recovery. The hygroscopic nature of protein hydrolysates caused a significant amount to stick to the chamber walls. This material can be recovered easily by using appropriate mechanisms or modifications to the drying equipment, increasing powder recoveries to over 80%. The protein hydrolysates at high DH had higher water affinity and addition of maltodextrin had little effect on their moisture adsorption capacity.

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