Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by browse.metadata.advisor "Chimphango, Annie Fabian Abel"

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    Advancing Lignocellulosic Biorefineries through Co-Production of Hemicellulosic Biopolymers and Bioenergy
    (Stellenbosch : Stellenbosch University, 2022-04) Mihiretu, Gezahegn Teklu; Görgens, Johann Ferdinand; Chimphango, Annie Fabian Abel; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: This research project was conceived in the context of advancing a lignocellulosic biorefinery for co-production of xylan biopolymers, bioethanol and electricity from two agro-industrial materials, namely sugarcane residues (SCT) and aspen wood (AW). The research was primarily designed to include two full-fledged experimental studies and one techno-economic case study. Accordingly, two biomass pretreatment approaches, namely: microwave-assisted pressurised hot water (MWA-PHW) and alkalinised steam explosion pretreatment (ASEPT) methods were experimentally investigated for their effect on the extraction of xylan from SCT and AW. Extraction experiments (via MWAPHW and ASEPT) were conducted by varying temperatures between 165 – 205 ℃ and retention times 3 - 22 min at test points identified using Central Composite Design (CCD) as response surface methodology (RSM). Pretreatment conditions were intended for a dual purpose: maximizing xylan extraction yield while simultaneously enhancing cellulose digestibility. Experimental results on xylan yield and cellulose digestibility were analysed using ANOVA method to establish optimal conditions for significantly enhanced values. Accordingly, under the MWA-PHW method, maximum xylan yields of 66 and 50%, and highest cellulose digestibility of 78 and 74%, were respectively attained for AW at (195℃, 20 min) and SCT at (195 ℃, 15 min). Whereas maximum xylan yields of 51 and 24%, and highest cellulose digestibility of 92 and 81%, were attained for SCT and AW respectively, following their pretreatment under ASEPT at (204 ℃, 10 min). Under both methods, the xylan extracts were predominantly non-monomeric with insignificant formation of degradation products. This strongly suggested both MWA-PHW and ASEPT were viable approaches for xylan extraction purposes. ANOVA results also revealed that temperature was the dominant factor influencing the xylan yield and cellulose digestibility. The techno-economic case study was aimed at evaluating the economic viability of the biorefinery for co-production of xylan biopolymers, bioethanol and electricity (i.e. main-case scenario, MCS) against two benchmark processes, i.e. Base-case (BCS) and Intermediate-case (ICS) scenarios, where only bioethanol and electricity are produced from sugarcane residues (Basis: daily capacity of 1000 tons of dry biomass subjected to ASEPT condition of 204 ℃ and 10 min). The study results showed that co-production of xylan biopolymers substantially improved the economic performance of the main biorefinery case (i.e. MCS) by lowering the selling price of ethanol against higher values under the benchmark processes. A minimum hemicellulose selling price (MHSP) of 809 USD/ton of xylan co-product was determined by fixing ethanol selling price at 0.70 USD/L (market price of ethanol in 2019); higher MHSP values certainly lead to further lower prices. Minimum ethanol selling prices (MESP) under the MCS, BCS and ICS were respectively estimated at 0.61, 0.95 and 0.81 USD/L, where the xylan price was assumed at 1000 USD/ton (=> MCS). Even though the economic viability of the main biorefinery case was significantly enhanced with co-production of xylan than without, this multiproduct biorefinery complex was rendered rather energy-intensive as a result of such coproduction scheme where the recovery of xylan biopolymers necessitated substantial thermal and electrical energy demands. From environmental point of view, the coproduction of xylan biopolymers along with bioethanol and electricity was shown to have a positive contribution towards mitigating GHG emissions from fossil sources. The GHG emissions savings under the MCS, BCS and ICS were estimated around 69, 64 and 65% against gasoline as fossil baseline of 90 gCO2eq/MJ (RSB-Global), but there was only marginal difference between the savings under the main biorefinery case and that under the benchmark processes.
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    A 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
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    Development of multi-step biorefinery schemes for the production of nanocellulose and high value-added bioproducts from mango seed
    (2022-04) Bello, Fatimatu; Chimphango, Annie Fabian Abel; Görgens, Johann Ferdinand; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Due to environmental and health concerns associated with non-renewable and synthetic-based packaging materials, there is an upward demand for bioproducts from renewable sources, particularly for food packaging. Nanocellulose is one such bioproduct and is generally produced from woody biomass due to its fibrous nature, however, woody biomass is in high demand. Mango (Mangifera Indica L.) seed is a major lignocellulosic waste from mango fruit processing, whose management is limited to disposal by incineration, burning, or landfilling with no current commercial use, resulting in environmental and economic burdens such as green-house gas emissions and land use concerns. However, mango seed has potentials for various bioproduct exploits including nanocellulose, due to the high cellulose content comparable to woody biomass and valuable co-products (hemicellulose, starch, polyphenols, and lignin). Thus, compared to the woody biomass, biorefinery exploits for the mango seed focused on the comprehensive fractionation and conversion into bioproducts and the nanocellulose offers additional advantages of enhancing the economic exploits through the additional product recoveries. However, the conventional nanocellulose production processes involving controlled sulphuric acid treatment of bleached cellulose-rich Kraft pulp, degrades the valuable components of polyphenols, lignin, and hemicellulose, and the sulphuric acid applied lowers the thermal stability of the nanocellulose thereby limiting its application in thermo-processing. In addition, due to the different biomass components responding differently to process conditions and the specificity of optimal extraction conditions for each product, challenges of low product yields and qualities could be envisaged for non-optimized extraction processes. Thus, establishing appropriate and optimal multi-step fractionation/conversion processes for the multi-product biorefinery schemes is essential for the sustainable recovery of the bioproducts. Therefore, this study aimed at developing multi-step biorefinery schemes for fractionating mango seed into high-value products such as hemicellulose, lignin, starch, cellulose, and polyphenols. Furthermore, produce nanocellulose from the cellulose fibers using organic acids and compare with the classical sulphuric acid-based process. Additionally, evaluate the potential of the hemicellulose extract as a material for biocomposite film development for food packaging without external additives. A multi-step biorefinery process consisting of sequential organosolv extraction (OE) [ethanol concentration (50–80% v/v), temperatures (20–60°C)], enzymatic hydrolysis (EH) [Termamyl®SC; (300 mL of Termamyl SC per ton starch in MSK) and Saczyme®Plus; (800 ml Saczyme Plus/ ton starch in MSK)] and alkaline pretreatment (AP) [(40–90 °C), NaOH concentrations (1–2 M), time (2–4 h)] for the recovery of polyphenols, starch, and bioactive hemicellulose from mango seed kernel (MSK) while minimizing the degradation of cellulose and lignin in the residual solids was developed. In addition, a multi-step sequential AP and rotor-stator high shear homogenization-assisted organosolv (HSHO) [ethanol concentration (50–70%), temperature (130–150 °C), homogenizing time (10–20 min)] process to recover hemicellulose (xylan/xyloglucan), lignin and cellulose-rich fibers from mango seed husk (MSH) was developed. The feasibility of the MSH hemicellulose extract to form self-supporting biocomposite film without external additives as applicable with xylan-based films was assessed. In addition, the feasibility of producing nanocellulose from the cellulose-rich fibers obtained from the multi-step process via optimized non-catalyzed formic acid-based treatment was investigated. Effect of an alternative CNC production process involving acetic acid treatment [pulp-to-acid ratio (1:20–1:40), reaction time (6–12 h)] on the preparation and acetylation of CNCs from the cellulose-rich fibers was also evaluated versus the combined acetic acid plus high shear homogenization and the classical sulphuric acid-based process. Results from the optimized multi-step sequential OE, EH, and AP process route for the fractionation of MSK led to the recovery of polyphenols, starch, bioactive hemicellulose, and solid residue enriched in cellulose and lignin. The optimized OE process (64.99%w/w ethanol, 54.18 ºC) resulted in an extract with total polyphenol content (TPC) and antioxidant activity (AA) of 95.21 mg GAE/g and 84.69% respectively with >90% cellulose, lignin, starch, and hemicellulose retention in the residual solids. The EH of the OE process resulted in >90% starch removal (as simple sugars). The optimized conditions for the subsequent AP of the destarched OE solids resulted in >50% hemicellulose recovery with 34.96 mg GAE/g TPC and 55.05% AA, and solids having 88.3% and 91.12% cellulose and lignin retention respectively that could be further valorized to increase the product range from the MSK. At the optimized AP process conditions (1.92 M NaOH, 86.0 °C, and 3.84 h), the recovered MSH hemicellulose extract possess suitable properties for thermally stable biocomposite film for food packaging. Subsequently, at the optimized HSHO pretreatment conditions (60% ethanol, 148.41 ºC, 15 min homogenization), over 70% lignin dissolution with high purity (>95%), 3247 g/mol molecular weight (Mw), and 298 ºC maximum thermal degradation temperature (Tmax) were obtained based on the AP MSH. The recovered solids post-HSHO process had >77% cellulose with fibers separated into individual strands of diameters <1 to 10 μm, and >55% crystallinity suitable for CNC production. Thus, the multi-step sequential AP and HSHO biorefinery route is promising for multi-products recovery in addition to the cellulose-rich material for CNC production.