Doctoral Degrees (Chemical Engineering)
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Browsing Doctoral Degrees (Chemical Engineering) by browse.metadata.advisor "Chimphango, Annie F. A."
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- ItemDesigning and evaluating the technical, economic and environmental performance of an adsorption cooling system operating using bioresources from waste streams of mango processing(Stellenbosch : Stellenbosch University, 2019-12) Dzigbor, Aaron; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: This study sought to improve the technical performance (coefficient of performance (COP) and specific cooling power (SCP)), environmental impacts and economic viability of employing the adsorption working pairs produced from waste streams of mango processing in the adsorption cooling system (ACS). The specific objectives were: to produce and characterize mango seed husk activated carbon (AC) using NaCl as the activation agent and compared with commercial AC; assess the performance (in terms of COP and SCP) of the mango seed husk AC (with commercial AC as the control) paired with both high-grade and low-grade ethanol as refrigerants; improve the heat and mass transfer performance of commercial AC paired with both high-grade and low-grade ethanol as refrigerants through composite formation; and evaluate the environmental and economic impacts of integrating adsorption cooling system (ACS) in dried mango chips processing in both grid and off-grid power conditions. Mango seed husk AC was produced through slow pyrolysis method using NaCl as the activation agent. About 100 g of dried mango seed husk was soaked in 250 ml of NaCl solution of concentrations (10 w/v%, 20 w/v%, and 30 w/v%) to obtain impregnation ratios of 0.25, 0.5 and 0.75 at 25 °C. The carbonization temperatures were 400 °C, 450 °C, and 500 °C. The experimental design was based on a 33 (impregnation ration, soaking time, and carbonization time) Box-Behnken fractional factorial optimization method with three center runs, giving total runs of 15. The responses analyzed were bulk density, ash content, and surface area. The optimized mango seed husk AC produced was tested in an ACS constructed in-house and its performance compared with commercial AC. The composite AC were also formed by soaking commercial AC in NaCl solution at varying concentrations of 10 w/v %, 15 w/v %, 20 w/v %, 25 w/v %, 30 w/v % and 35.7 w/v %, for 24 hours at 25 °C, dried at 105 °C for 24 hours and then tested in ACS constructed in-house with high purity (99.7%) and low-grade (60%) ethanol to evaluate the effect of ethanol grade on the performance of the composite formed. Finally, three scenarios for each power setting (on-grid and off-grid) were studied, on-grid: coal as boiler fuel and conventional chiller for cooling (Scenario 1), mango seed as boiler fuel and adsorption chiller for cooling (Scenario 2) and mango seed as boiler fuel and ACS for cooling (Scenario 3). Off-grid scenarios 4, 5 and 6 corresponded to on-grid scenarios 1, 2 and 3, respectively. Environmental impacts and economic viability for each scenario were based on material and energy balances and South African economic conditions, respectively. The results showed that mango seed husk AC had comparable ash content (6.92%) to the commercial AC. The SCP, COP and temperature drop recorded in ACS for mango seed husk AC when paired with high purity (99.7%) ethanol reduced from 40 W/kg, 0.050 and 4.46 °C to 37.3 Wkg-1, 0.048, and 4.5 °C, respectively, when paired with low-grade ethanol (60%). Moreover, the COP and SCP of commercial AC paired with high purity ethanol were 0.099 and 84.5 Wkg-1, which reduced to 0.091 and 75.5 W/kg, respectively, when paired with low-grade ethanol. In addition, the COP of the composite AC containing 20%, 25% and 30% NaCl paired with low-grade ethanol were 0.121, 0.160 and 0.146, respectively, which were higher than when paired with high purity ethanol, thus 0.082, 0.080, and 0.076, respectively. In terms of environmental and economic impacts, on-grid scenario 3 showed the greatest potential for reducing emissions and improving economic viability by emitting 7.10×105 kgCO2eq/yr and internal rate of return (IRR) of 25.33% compared to scenario 1 that had the GHG emission of 7.89×105 kgCO2eq/yr and IRR of 17.48%. In off-grid, scenario 6 had the least GHG emission of 6.90×105 kgCO2eq/yr and IRR of 24.84%while scenarios 4 had the highest GHG emission of 7.67×105 kgCO2eq/yr and IRR of 16.09%. Overall, it is possible to improve the heat and mass transfer of activated carbon paired with low-grade ethanol. The improvement in heat and mass transfer when AC + NaCl was paired with low-grade ethanol suggests that low-grade ethanol can be used as an alternative refrigerant. However, in areas where silica gel is accessible, forming composite with silica gel + NaCl paired with pure water as refrigerant would eliminate the mass transfer challenges associated with using AC+NaCl composites paired with ethanol. Furthermore, the replacement of vapour compression cooling technology with ACS and boiler fuel with mango seed has led to the reduction in GHG emission and improvement in the economic viability of dried mango chip processing. Thus, the study has improved the technical, economic and environmental performance of ACS in terms of temperature maintenance, resource consumption, and emissions.
- ItemDevelopment of integrated processes for the coproduction of inulin, protein, and ethanol from Jerusalem artichoke tubers in a biorefinery(Stellenbosch : Stellenbosch University, 2021-03) Maumela, Pfariso; Gorgens, Johann F.; Chimphango, Annie F. A.; Van Rensburg, Eugene; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Biorefining is an attractive approach to simultaneously address food supply, energy security and global warming. Furthermore, biorefining offers a sustainable strategy to utilise biomass, for energy production, thereby, reducing overreliance on fossil resources. The principal aim of the study was to evaluate the feasibility of a Jerusalem artichoke (JA) tuber-based biorefinery by integrating protein extraction to the conventional inulin extraction process, and subsequently hydrolysing the tuber residues with an enzyme cocktail of crude inulinases and commercial Cellic® CTec3 and Pectinex, for ethanol fermentation. The tuber mash was pressed for protein extraction from the juice and water extraction from the solid residues. Sequential water-extraction was used for protein and inulin, in the first and second step, respectively. The resulting tuber residues from the sequential extraction was hydrolysed and fermented into ethanol. Fed-batch culture was used to optimise the bioprocess conditions for recombinant endoinulinases production by Aspergillus niger. Comparison of sequential extraction sequences demonstrated that protein extraction in the first and second step, respectively, maximised the selectivity of the extraction and product yields. Both extraction steps utilised water as a solvent, and were optimised with respect to pH, solids loading and temperature for the selective extraction from each dedicated step from tubers. The soluble protein fraction contained a cumulative 71.8% of the protein present in tubers, while 17.1% was present in the inulin extracted in the subsequent step. The inulin yield was 67.6% of the inulin in the tubers, while 11.8% was co-extracted with the protein product. The protein extract was augmented by protein present in the press juice, obtained from tubers prior to the water extraction steps. High cell density fermentation of Aspergillus niger for recombinant endoinulinase production, was achieved through an exponential fed batch method. Endoinulinase production was growth associated at higher growth rates, achieving the highest volumetric activity (670 U/ml) and biomass concentration (33 g/L) at a growth rate (μ) of 0.07 h-1. Moreover, the significant decrease in enzyme activity (506 U/ml) and biomass substrate yield (0.043 gbiomassDW/gglucose) at low μ (0.04 h-1) was due to the high maintenance energy requirement. High biomass concentrations resulted in broth viscosity, which necessitated increased agitation for mixing and oxygen transfer. However, this led to pellet disruption and biomass growth in mycelial. Moreover, enzyme production profiles, product (Yp/s) and biomass (Yx/s) yield coefficients were not affected. High gravity simultaneous saccharification and fermentation (SSF) of the extraction residues, enriched in cellulose and inulin, was achieved with an optimised cocktail of enzymes. A combined inulin and cellulose conversion yield of 74% was achieved during fermentation at 21% w/v solids loading. The optimised enzyme cocktail improved the saccharification and fermentation of the residues, with an ethanol concentration and yield of 38 g/L and 83%, respectively, compared to an unoptimized cocktail with the same protein dosage, with 32 g/L and 59%, respectively, both at the maximum attainable solids loading of 21% w/v. Therefore, the current data demonstrated the potential of integrating a protein extraction with conventional inulin extraction from JA tubers and fermenting the residues into ethanol with an optimised enzyme cocktail.
- ItemFractionation of agro-waste to producebiopolymers and bioactive compounds for active food packaging(Stellenbosch : Stellenbosch University, 2020-12) Mugwagwa, Lindleen Runyararo; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: The food packaging industry is faced with a need to find alternative raw materials to replace the non-renewable petroleum-based polymers and active compounds used in developing active food packaging material (packaging film capable of maintaining food quality by releasing antioxidants into food in response to temperature and time) and temperature-time indicators. Agricultural residues are a promising one-stop feedstock for biopolymers (hemicellulose, nanocellulose, pectin) and active compounds (polyphenols, anthocyanins) that can be used to develop renewable, biodegradable and non-toxic active food packaging and temperature-time indicators. However, the sequential or co-extraction of these raw materials from agricultural residues is hindered by the differences in the optimal extraction conditions of the products. Furthermore, the coexistence of these products with other plant material in biomass affects their effective recovery, downstream processing and end-application. On the application side, biocomposite films such as hemicellulose-based films made from agro-derivatives, completely disintegrate when in contact with aqueous solutions, hence, they cannot be used for packaging wet food, limiting their application only to dry foods packaging. In addition, the poor mechanical properties (tensile strength and Young‘s modulus) of these films hinder their application as alternative packaging to petroleum-based films such as low density polyethylene (LDPE). Manipulating and optimising the biopolymers and antioxidants extraction processes taking into consideration downstream modification processes and end-application, can be a strategy in tailor-making the properties of the agro-derivatives with the aim of developing active food packaging films with properties similar or better than LDPE-based active packaging. Furthermore, modifying and blending the biopolymers to form films and applying hydrophobic coatings to the films can further enhance film properties. Therefore, the aim of the study was to develop processes for fractionating wheat straw and mango peels to recover biopolymers and bioactive compounds and then integrate the products into bio-composite films for active food packaging applications, using LDPE films as benchmark. In addition, the application of bioactive compounds from agro-residues as temperature-time indicators was evaluated. To fulfill the aim, the first part of the project focused on optimising organosolv pre-treatment of wheat straw [ethanol (50–80 %)/sodium hydroxide (NaOH) (1–13%)/2–6 h] prior to hemicellulose extraction (10% NaOH/25 °C/24 h) so as to enhance hemicellulose acetylation, hydrophobicity, and the mechanical properties of hemicellulose-based films. A three-stage sequential extraction process was developed and optimised to recover anthocyanins (50–80% ethanol/0.1–2% acetic acid/25 °C/60–150 min), polyphenols (65–85% ethanol/75 °C/20–60 min) and pectin (0.25% ammonium oxalate-oxalic acid/85 °C/60 min) from mango peels. The second part involved the development and evaluation of hemicellulose-based films as active packaging material. The utilisation of a combination of nanocellulose (25% w/w) and pectin (hemicellulose/pectin ratio from 0% to 100%) as reinforcement and filler in hemicellulose-based films was investigated. Furthermore, the effects of modifying hemicellulose by acetylation, reinforcing the acetylated hemicellulose (AH) with acetylated nanocellulose (ACNC) of varying degrees of acetylation (DS 0–2.34) and loading (10–50%) and coating the AH/ACNC films with polycaprolactone (PCL) (0.3 g/L) on the films‘ mechanical properties, hydrophobicity and solubility in wet food simulants were assessed. Hemicellulose-based active packaging material was then formulated by doping the films with mango peel polyphenols. Antioxidant release by the hemicellulose-based active packaging material to different food simulants, was evaluated both experimentally (temperature range 5–40 °C for two days) and by using Migratest Exp. LDPE films doped with polyphenols were used as a benchmark. Lastly, the utilisation of anthocyanins as a time-temperature indicator was investigated by evaluating colour change of the anthocyanin/hydrogen phosphate buffer solution incubated at temperatures ranging from 5 °Cto 40 °C for three days. Organosolv pre-treatment optimal conditions [(1% NaOH/50% ethanol/6 h/75 °C)] of wheat straw enhanced the acetylation of the recovered hemicellulose (DS increased from 1.2 to 1.7) and in turn improved water resistivity (water contact angle (WCA) increased from 34.21° to 39.90°) of hemicellulose-based films. Increasing the reinforcing nanocellulose DS to 2.34, loading to 50% and coating the AH/ACNC films with PCL increased the films‘ Young‘s modulus from 55.23 to 335.33 MPa, tensile strength from 1.79 to 6.72 MPa, and WCA from 39.90° to 82.40°. There was no significant difference (p > 0.05) between the aforementioned results and the LDPE properties (Young‘s modulus 244.46 MPa, tensile strength 11.07 MPa and WCA 84.91°). The solubility of the hemicellulose-based films in the food simulants was reduced from 100% to 3.25% by the aforementioned modifications. Blending hemicellulose with pectin and nanocellulose improved the mechanical properties and reduced the solubility of the films in the fatty food simulant by up to 84%. Hemicellulose-based films released up to 98.54% whereas LDPE released 6.89% of the encapsulated polyphenols into food simulants, thereby increasing the antioxidant activity of the food simulant by up to 60% and 8.97% respectively. AH-based films were least soluble and had the highest antioxidant release in the fatty simulant when compared to the other simulants. Therefore, hemicellulose-based films can be utilised as alternative active packaging for short term storage (0–2 days) of fatty foods. Mango anthocyanins changed colour from orange to dark green with an increase in temperature from 0 °C to 40 °C and time 0 to 2 days, hence exhibiting temperature-time indicator properties. Overall, the study generated novel methods for manipulation of hemicellulose to increase its applications as a functional food packaging material. New information was generated when hemicellulose films were tested as active packaging in a food environment. This work also contributed to the development of agro-residue biorefineries for multiple product production and their utilisation for sustainable food packaging material development.
- ItemSystems approach in developing a model for sustainable production of bioenergy in Malawi(Stellenbosch : Stellenbosch University, 2018-03) Chitawo, Maxon Lexon; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH SUMMARY: Bioenergy production from primary forest and rice residues can contribute to modern energy supply, such as electricity, liquid biofuels and gas, to rural communities in Malawi. These bioresources can be utilised for bioenergy production without alienating land from cultivation of other crops. However, sustainability of forest and rice residues-based bioenergy systems is complex owing to the dependency of availability of the residues on timber and rice production. Alterations in process operations in timber and rice production systems can cause variations in production and supply of the residues to a bioenergy conversion plant over time. For instance, forest management systems have evolved from sustainable yield management, which promotes clear cutting of mature forest stand to maximise the yield of wood products to sustainable forest management that promotes partial harvesting of mature forest stand to allow for ecosystem balance. Switching the harvesting regimes from clear cut to partial harvesting of mature forest stand, can influence variation in yield of forest residues in forest plantations over time. Variations are also evident in rice residues production and supply chains, emanating from seasonal production of rice and demand of the rice residues for competing uses. Stability in production and supply of the residues over a long time horizon can promote availability of the residues-based bioenergy systems and reliability of bioenergy supply to end use processes over time. Systems approach modelling based on systems thinking and system dynamics modelling methodology, was used in this study to develop a model for sustainable production of bioenergy (SAS-Biopro model). The model demonstrates state limiting processes to resilience of primary forest and rice residues supply chains for bioenergy production. Simulation results of the model show that variations in primary forest residues value chain over time result from variations in stocks of mature stand caused by over-exploitation for timber production, delayed replanting, high death (mortality) rate of replanted trees and underinvestment in plantations management. Results from scenario testing show that an integrated framework for forest plantations management and forest residues-based bioenergy production, can promote synchronised operation and management of the forest plantations and bioenergy production as a unit (whole) system. The framework entails setting an annual allowable cut for harvesting mature forest stand, synchronizing harvesting and replanting 100% of the annual allowable cut immediately after harvesting, reducing tree mortality fraction to less than 0.1, and sizing the scale of operation of bioenergy conversion plants based on the amount of residues generated from the annual allowable cut. The framework can promote stability of residues production and supply to bioenergy conversion plants. Similarly, modelling sustainability of rice residues-based bioenergy production has shown that a synergetic integration of bioenergy and rice production can simultaneously increase bioenergy and rice production over time. Thus, synergetic integration of bioenergy and rice production can promote stability, availability and reliability of rice (food) and rice residues supply for bioenergy production. This research has filled a significant gap in strategic information such as dynamics in residues-based bioresource flow and consumption rates that create a transient state, which can guide formulation of strategies for synchronising the scale of operation of the residues-based bioenergy conversion plants and operation processes in the primary systems that generate the residues. The research outputs provide innovative whole systems and synergetic integration, for production and deployment of residues-based bioenergy, to promote resilience of the residues supply chains to bioenergy conversion plants. These concepts can promote uptake and diffusion of small-scale bioenergy conversion technologies in primary systems that generate the residues. Matching the scale and rate of operation of the bioenergy conversion plants with the annual rate of production of the residues can provide opportunity for incremental uptake of small-scale residues-based bioenergy systems. Therefore, the concepts, although approached from the technology and process point of view, are flexible to respond to policy and societal changes in the value chain of bioenergy production. The concepts can be adopted in forest plantations and rice farms management systems to promote sustainability of bioenergy production from forest and rice residues.
- ItemTechno-economic and sustainability models for integration of cassava waste-based biorefineries into cassava starch processes based on process simulation and a systems modelling approach(Stellenbosch : Stellenbosch University, 2021-03) Padi, Richard Kingsley; Chimphango, Annie F. A.; Roskilly, Anthony Paul; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Cassava crop high starch yields, accompanied by its tolerance to drought/low soil nutrients, have increased research attention towards the crop’s adoption as a potential food security and economic empowerment crop for South Africa. Widely consumed as food and livestock feed, cassava starch also has potential industrial applications in pharmaceuticals, specialty chemicals (e.g. succinic acid), ethanol, adhesive, and food derivatives (e.g. glucose syrup). Commercialization of industrial cassava starch facilities (CSF) depends on profitability and sustainable energy supply for operations. Residues generated by CSFs [cassava starch wastewater (CWW), bagasse (CB)], and cassava stalks (CS) could generate the requisite energy for cassava starch industries (CSI), thus there is potential to integrate waste-based bioenergy developments with CSFs. Cassava waste biorefineries (CWBs) for co-producing energy and high-value bio-products have been proposed as potential solutions to energy and cost limitations in CSFs. Attributed to knowledge gaps on the techno-economic feasibility (TEF) and long-term sustainability (economic + environmental + social) of such CWBs, conventional waste management schemes involve the burning of CS and anaerobic digestion of CWW & CB to produce biogas for starch drying heat, with the digestate being disposed into watercourses. This research, through Aspen Plus® process/economic modelling and SimaPro simulation, investigated the TEF and sustainability of CWBs in the South African socio-economic context, with an overall objective of contributing to knowledge towards the commercialization of CWBs. The investigated CWB scenarios include: (i) enhanced waste resource recoveries (energy, biofertilizer, water) through integrating CS into CSF waste treatment, and (ii) advanced CWBs [(I) combined heat & power, with (II) hexose-bioethanol, (III) pentose & hexose-bioethanol, (IV) pentose-bioethanol + glucose syrup, and (V) pentose-bioethanol + succinic acid)]. The results showed that combined treatment of CS (14.32 t/h) with CSF wastes (7.29 t/h DM CB + 377.83 t/h CWW) could ensure further resource recoveries, including bioelectricity (up to 31.96 MW), liquid/solid biofertilizer, and usable water, with potential energy self-sufficiency and economic enhancements for CSIs. Co-conversion of 450.89 t/h CS and CSF waste could ensure sufficient energy supplies for both CWBs and CSFs, plus 300 MW electricity (I), or 287 MW + 1.48 t/h bioethanol (II), or 121 MW + 8.95 t/h bioethanol (III), or 164 MW + 5.72 t/h bioethanol + 9.29 t/h glucose syrup (IV), or 161 MW + 5.72 t/h bioethanol + 6.9 t/h succinic acid (V). However, only scenarios (I)-(II) demonstrated economic viability, while (III)-(V) favor environmental sustainability. Revitalizing the CSI’s via integrations with the resource recovery schemes, where the recoveries are re-used in the CSFs and crop cultivations, could ensure viable circular economy strategies that may enhance sustainable industrial developments. Hence, integrating CSFs with resource recoveries or CHP (I) or CHP + hexose-bioethanol (II) represent viable strategies for the synergetic advancement of food-energy security and low-carbon economies.