Doctoral Degrees (Chemical Engineering)
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- ItemDevelopment 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.
- ItemThe 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.
- ItemContributions to theoretical developments and practical exploitation of mass transfer principles in separation technologies(Stellenbosch : Stellenbosch University, 2023-12) Nieuwoudt, Izak; Burger, Andries Jacobus; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.ENGLISH ABSTRACT: Our standard of living and quality of life rely on producing consumer products from chemicals. These chemicals typically have to be purified, and distillation, extraction and absorption are the primary technologies for affecting these separations. Such technologies are energy intensive, and further development and optimization can reduce their impact on greenhouse gas emissions. Furthermore, reduction in the capital consumed by these processes can significantly improve value creation. My contributions to these fields over the past 36 years are covered in detail in this dissertation under the sections highlighted below. It is conservatively estimated that these contributions generated more than two billion dollars of value for the companies who commercialized and use these technologies. In the process of making contributions to this field, I have been blessed to collaborate with brilliant people and their contributions to my endeavors are gratefully acknowledged. Improved separation processes My research into separation processes with reduced energy and capital consumption was focused on solvent-driven separations. Computer Aided Molecular Design (CAMD) methodologies for solventdriven separation processes were developed and, from this, improved extractive distillation, azeotropic distillation and liquid extraction processes were conceived. The energy and capital consumption of these processes were significantly lower than that of competing technologies, which also translates into lower greenhouse gas emissions. These processes were commercialized and create significant value for the chemical producers and the companies who produce consumer products. Improved separation tower internals My research into separation tower internals was focused on creating novel equipment that had higher separation efficiency and higher hydraulic capacity than the best equipment available at the time. The novel INTALOX® ULTRATM random packing exhibited higher efficiency and capacity than other random packing. This allowed chemical companies and refiners to debottleneck towers for capacity and/or separation efficiency. This reduced energy consumption and corresponding greenhouse gas emissions. The size of new towers could also be reduced. The PROFLUX® severe service packing allowed refiners to increase capacity, increase run lengths and increase the product yield in vacuum towers. I developed several tray configurations that improved the capacity and/or efficiency of separation towers. Other developments included improved liquid distributors and droplet separators. These new, improved separation tower products generated significant value for both the companies that commercialized it and the end-users who installed it in their separation towers. Separation technology education Although the separation processes and separation tower internals discussed in this dissertation generated significant revenue for the companies who commercialized and used these technologies, the lasting value lies in the underlying methodologies and knowledge that were developed. It is important that this knowledge be passed on to the next generation of engineers. To this end, I have developed university courses, the Koch-Glitsch Mass Transfer School and the FRI Distillation Academy. Under my guidance 14 Masters and 7 PhD students graduated in the field of separation technology. Almost 1000 students attended these courses and many commented that they have received unique knowledge that equipped them for the future. The details of my non-confidential contributions to the field of separation technology have been summarized in 410 patents (46 patent families), 48 papers and 80 conference contributions. At the 2022 Spring Meeting the Separations Division of the American Institute of Chemical Engineers (AIChE) gave special recognition to my “many outstanding achievements in distillation, extraction, 5 absorption, and troubleshooting” by dedicating an honors session to celebrate my “lifetime of contributions as an engineer, educator, inventor, and R&D leader in separations technology and engineering”.
- ItemA comparative investigation of the technoeconomic feasibility and sustainability of mango waste biorefineries in South Africa: a process modeling approach(Stellenbosch : Stellenbosch University,, 2023-03) Manhongo, Tariro Tecla; Chimphango, Annie Fabian Abel; Thornley, Patricia; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering.ENGLISH ABSTRACT: Fruit processing waste (FPW) is a suitable biorefinery feedstock for conversion into bioenergy, biofuels, and chemicals. However, information on which processing routes and product combinations are economically viable and sustainable is limited. Using South Africa as a base developing economy, the availability of FPW as biorefinery feedstocks, economic viability, and sustainability of FPW-based biorefinery systems were evaluated in this study using mango processing waste as a base feedstock. Six biorefinery scenarios were evaluated; (I) combined heat and power (CHP) generation, (II) co-production of pectin and CHP, (III) co-production of pectin, polyphenols and CHP, (IV) co-production of pectin, bioethanol, and CHP, (V) co-production of pectin, polyphenols, bioethanol, and CHP, and (VI) co-production of bioethanol and CHP. In scenarios II to VI, residues from pectin and/or polyphenols recovery and wastewater are anaerobically digested for biogas production and the biogas in all scenarios is co-combusted with mango seed for steam generation (for use within the biorefinery and export to the host dried mango chips facility) and power (for consumption within the biorefinery and export if excess is generated). Aspen Plus process simulation models were developed at a plant capacity of 1500 tonnes per day (1200 tonnes process wastewater + 133.33 tonnes peel + 166.67 tonnes seed), operating for 24 h/day, and 120 days/annum. A discounted cash flow analysis was employed in assessing the economic viability of the six biorefinery models using the mass and energy flows from the models and incorporated in SimaPro-based attributional life cycle analysis models to evaluate the environmental impacts of the biorefineries. Using systems thinking, results from the technoeconomic analysis were employed in estimating the socio-economic benefits of the biorefineries using input-output Jobs and Economic Development Impact assessment models adopted from the National Energy Renewable Laboratory. Indicators for economic, environmental, and socio-economic performances of the biorefineries were normalized, weighted, and aggregated in a multi-criterion decision analysis approach to compare sustainability performances of the biorefineries. Scenarios I and VI are economically unattractive with net present values (NPVs) of -$94.4 and -$120.9 million, respectively. NPVs of the biorefineries increase with the recovery of more products in the order Scenario IV
- ItemA thermosiphon photobioreactor for photofermentative hydrogen production by Rhodopseudomonas palustris.(Stellenbosch : Stellenbosch University, 2023-03) Bosman, Catharine Elizabeth; Pott, Robert William M.; Bradshaw, Steven Martin; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Hydrogen has widely been identified as a commodity chemical. Currently, however, hydrogen is primarily produced through non-renewable methods. Biological hydrogen production through microbial photofermentation offers an environmentally friendly and potentially economically feasible alternative. Although this technology is promising, the costs associated with photofermentation systems need to be reduced and hydrogen productivity increased, to make the technology a competitive alternative to non-renewable hydrogen production methods. This can potentially be realised through cost-reduction strategies in combination with bioremediation – purifying wastewater whilst simultaneously producing a valuable chemical. This work applied a combination of techniques to develop and evaluate a novel thermosiphon photobioreactor (TPBR) for photofermentative hydrogen production, using Rhodopseudomonas palustris (R. palustris). The TPBR implements the thermosiphon effect to passively circulate biomass – the first and currently the only photobioreactor with the potential of operating without any external energy inputs. The TPBR was successfully implemented for photofermentative hydrogen production using R. palustris, achieving maximum hydrogen production rates of up to 0.310 mol·m−3 ·h−1 in the growing state. The effects of light intensity, temperature and biomass concentration on hydrogen production and passive circulation of biomass were investigated. The effects of biomass concentration were found to be most pronounced (0.4 to 1.2 g·L−1 ), while light intensities of 400 to 600 W·m−2 and an internal operating temperature of 31 to 44 °C were found to be suitable for hydrogen production. Exploring the effects of geometry, two novel TPBR designs were proposed – a tubular loop TPBR and a flat-plate TPBR. Using computational fluid dynamics (CFD) simulations, these designs were characterized in terms of fluid flow patterns, temperature profiles and radiation fields. Both TPBR designs showed potential for hydrogen production, achieving temperature gradients sufficient to ensure adequate circulation and velocities to maintain biomass in suspension. CFD simulations indicated light distribution as a possible area for improvement in the existing TPBR. Consequently, a reflector system was developed and implemented for the enhancement of light distribution and hydrogen production in the experimental TPBR – achieving a more uniform light field and an associated 48% increase in hydrogen production. Evaluating the feasibility of outdoor operation, the effects of diurnal light cycles and the emission spectrum of light were investigated. R. palustris was able to produce hydrogen under a sunlight-mimicking light emission spectrum achieving maximum hydrogen production rates of 0.790 mol·m−3 ·h−1 , albeit slightly lower as compared to under near-infrared light where it reached production rates up to 0.891 mol·m−3 ·h−1 . Hydrogen production was found to cease during dark periods in the diurnal light cycles; however, continuing again in the presence of light and achieving maximum Stellenbosch University https://scholar.sun.ac.za iii hydrogen production rates of ~0.015 mol·m−3 ·h−1 . This demonstrated promising potential towards outdoor operation of the TPBR, circumventing the requirement for external energy inputs. This dissertation has successfully demonstrated the application of a novel thermosiphon photobioreactor for photofermentative hydrogen production with minimal external energy input. The research comprised determination of suitable operating conditions for hydrogen production, a CFD modelling method for the design of PBRs, two novel TPBR designs and characterization thereof, a light distribution strategy for the enhancement of hydrogen productivity in PBRs, and insight into the passive circulation of biomass in a TPBR and the behaviour of R. palustris under simulated outdoor conditions. Collectively, this research provides knowledge not only improving the TPBR, but which could also be extended to other systems in the biohydrogen field.