3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Development of a Non-Derivatizing Solvent System for the Pretreatment of South AfricanCorn Cob(2019) Ejekwu, OlayileDepleting fossil fuels and the increasing energy demand has necessitated the move to alternative renewable forms of energy. Lignocellulosic biomass is a renewable and sustainable source for highly valuable bio-based chemicals and material production in a biorefinery system. The effective fractionation of the main components of lignocellulosic biomass (cellulose, hemicellulose and lignin) into usable forms is a crucial step in unlocking an economically viable, high-value product producing biorefinery. The main concern associated with the conversion of lignocellulose is overcoming biomass recalcitrance using pretreatment while still maintaining a green, cost-effective and energy efficient process. Over the last decade, molten hydrate salts have been used for isolated cellulose dissolution, however very few studies have been done to check their ability in lignocellulosic biomass pretreatment. The aim of the study was to compare seven molten hydrate salt solvent systems including unary, binary and ternary mixtures of ZnCl2.4H2O, LiClO4.3H2O and Urea for the effective pretreatment of corncob in terms of physicochemical properties and pretreatment efficiencies and to optimise these efficiencies. The molten salt hydrate pretreatment systems used in this study are aimed at fractionating the corn cobs biomass into a solid fraction which mostly contains cellulose and lignin as the major components, while the liquid fraction contains hemicellulose as the main component. The pretreatment experiments were carried out at 70 for 60 minutes at a biomass: solvent ratio of 1:10. Physicochemical change after pretreatment was checked by FTIR, XRD and SEM. The most efficient solvent mixture was identified by gravimetric analysis for its ability to fractionate the biomass into a cellulose and lignin rich solid fraction and a hemicelluloserich liquid fraction. The effect of solvent pretreatment operating variables (temperature, time and solvent concentration) was investigated to maximize cellulose recovery, hemicellulose recovery in the liquid fraction and lignin recovery from the biomass by response surface methodology (RSM) approach using a central composite design (CCD). Physicochemical analysis showed a decrease in crystallinity and an increase in surface area after the pretreatment in all the MHS solvents tested. This work has successfully shown the use of ZnCl2.4H2O/ Urea, to pre-treat and fractionate corn cob with high recovery of cellulose (100%), low recovery of hemicellulose (42%) and lignin (44%) when compared to the other proposed systems. Through the RSM approach, optimum pretreatment conditions obtained Abstract were: 90 min, 120 oC and concentration of 71.32%/28.68 (w/w) ZnCl2.4H2O/ Urea. At these conditions, the predicted recovery for cellulose, hemicellulose and lignin 99.03%, 27.18% and 72.43% respectively with a desirability of 0.902. The actual recovery was 91%, 29% and 68% for cellulose, hemicellulose and lignin respectively at the same conditions. For a better understanding of the dissolution kinetics and thermodynamics of cellulose, hemicellulose and lignin dissolution in ZnCl2.4H2O/ Urea solvent system, a kinetic study was carried out. The results reveal the dissolution to be a 1st order kinetics and the obtained activation energy for cellulose, hemicellulose and lignin dissolution were 14.10 kJ.mol-1, 11.29 kJ.mol-1 and 7.606 kJ.mol-1 ,respectively. that the dissolution process for all three components are endothermic and endergonic. The -0.190; -0.195 kJ.mol-1) showed that the process of dissolution of hemicellulose occurred more rapidly and produced more stable products. It was concluded that ZnCl2.4H2O/ Urea pretreatment provided a potential way to fractionate lignocellulosic biomass which can improve the effective utilization of all feedstock fractions.Item Bioconversion of waste lignocellulosic biomass (South African corn cob) to succinic acid in molten hydrate solvent system(2018) Awosusi, Ayotunde AyokunleLignocellulosic biomass (LCB) resources are central to modern, sustainable development of economies worldwide. They are abundant, cheap and do not contribute to net CO2 emissions hence their refinement for energy and valuable industrial commodities production provides a necessary alternative to crude oil. LCB have been used as solid fuels since the paleolithic ages. Today, advances on bio-chemo catalysis and carbon chemistry have seen more efficient processes converting biomass to a wide range of valuable products e.g. chemicals, biofuels and other bio-based commodities. Likewise, increase in awareness of the dangers of the continuous use of non-renewable fossil have continued to drive and accentuate the potential of biomass as a sustainable source of organic carbon but the high cost of biomass processing, poses a major threat to proposals for further development and commercialisation. LCB is an abundant, cheap and sustainable resource but it is also naturally inert and recalcitrant to biocatalytic conversion. As such, biorefining processes generally require a feed activation step (pre-treatment) prior to efficient conversion. The high cost of pre-treatment have made the need for modern strategies targeting cheaper, effective, catalyst-compatible and sustainable pre-treatment techniques very imperative. Cheap, recyclable and sustainable nonderivatising zinc chloride molten hydrate salt systems was proposed in this work as a plausible alternative to acid and alkali pre-treatment by means of investigations probing a) pre-treatment efficiency at relatively mild conditions, and (b) influence on biocompatibility of fermentation medium. This work sought to establish a holistic approach for improved techno-feasibility of largescale biorefining by exploring the potentials of an intensified and integrated process. The proposed process involved mild pre-treatment by solvation of local waste LCB in inorganic ZnCl2.nH2O and whole-cell microbial conversion of liberated sugar to value-added products (e.g. succinic acid). Succinic acid (BSA) is a valuable, bio-based commodity with a growing demand and market value as a top-platform chemical. The biocommodity has a wide-range of application in several industries such as biopharmaceuticals, food and materials production. By targeting anaerobic fermentative synthesis of SA, a waste-to-value concept was explored, further contributing to the proposed strategy of cheaper, sustainable bioprocess. The efficiency and general potentials of the bio-chemocatalytic process for SA production was described by charting specific objectives such as, (a) feedstock analysis (organic and inorganic composition), (b) pre-treatment efficiency as described by biomass deconstruction in molten hydrate salt (MHS) (c) statistical optimisation of sugar yield, and (d) metagenomic analysis of media selectivity of microbial catalysts as correspondent to yield. The suitability of locally-sourced corn cob wastes as feeds in a bioconversion system was assessed by means of the characteristic organic and inorganic composition of the materials. The high (82.5 %) percent composition of sugars in the biomass was promising for anaerobic fermentation process as explored in this study. Using zinc chloride tetrahydrate ZnCl2.nH2O salt solvent system, this work demonstrated the efficient pre-treatment of locally sourced waste corn cobs at relatively mild conditions of 70 oC for 60 minutes. The solvation process resulted in microstructural modifications of the materials that were correlated for reduced crystallinity and improved fiber reactivity. In comparison to common techniques (such as dil. acid or alkali treatment at relatively harsher operating conditions) used in overcoming biomass recalcitrance, the effect of ZnCl2.nH2O treatment on biomass recalcitrance was promising especially from cost (energy and solvent) and sustainability perspective. Implications for biomass susceptibility to biological digestion of inherent saccharose was also assuring as approximately 85 and 95.1 % of cellulosic and hemicellulosic fractions were recovered upon enzymatic saccharification of treated samples. A predictive second-order polynomial model, developed using a 23 Box-Behnken design was used to describe the effect (main and interactive) of pre-treatment process variables (time, temperature, and solvent loading) on yield towards the parametric optimisation of the process. The experimental yield of reducing sugars (YE) was well-fitted into the resulting model according high f-value (> p-value) obtained from the regression analysis and ANOVA tests of the model. ANOVA tests also reiterated the significance of the response model (Reducing sugar yield) with a reliability of 92.5 %, (adjusted R2= 88. %), suggesting an error margin of < 8%. The optimal parameters (90 minutes, 107 oC , 18 g/g liquid-to-solid ratio) for the recovery of a total of 90 % of total sugars (based on biomass compositional data) were obtained by multi-objective numerical optimisation and cross-validated experimentally. The fermentability of the dissolved biomass sugars at varying concentrations of ZnCl2 salt was characterised by the observed synthesis of organic intermediates and target product; Succinic acid, at select salt concentrations. Anaerobic fermentation set-up was a batch simultaneous saccharification and co-fermentation (SScoF) bioprocess with feed of pretreatment slurry at varying solvent; ZnCl2.4H2O loading, catalysed by a mixed-consortia of rumen bacteria at standard anaerobic fermentation conditions. The mixed acid- fermentation process of the unconditioned ZnCl2 MHS pre-treatment slurry, resulted in the conversion (up to 2 %) of dissolved biomass sugars to SA (10.4 g L-1). SA production declined with increasing salt concentration but the original findings were promising especially considering the uncontrolled carbon channelling resulting from the use of mixed-consortium of microbial biocatalysts. Analysis of SA yield results were reconciled with bioactivity; as measured by DNA quantification, at the different salt concentration while microbial inventory analysis was done at constant slurry salt concentration. The choice of a mixed-consortia of rumen bacteria as to single-cell cultures was utilized to broaden the sampling range for SA-producing biocatalysts (using ZnCl2 salt concentration as a singular factor to model their micro-environments). Selectivity of media for microbes was profiled by characterizing the inoculum (cow dung) and fermenter samples using advanced next-generation sequencing (NGS) tools. The characteristic operational taxonomic units (OTUs) of the active species in raw cow dung were determined towards informing specific microbial activity in media. The microbiome of the cow-dung to consisted of approximately 1500 OTUs, with the detection of up to 265 genera (of which only 18 genera constituted over 1 % of relative abundance). The dominant phyla was Firmicutes and Actinobacteria both of which are notable for immense industrial applicability in organic acid and alcohol biosynthesis. The observed strains of phyla such as thermileophilia, Acidimicrobiia have potentials in metageneomic engineering for programmed cell-free catalysis and synthetic biomanufacturing designs which could find applicability in proposed process. This thesis is a careful documentation of proof of concept of SA biosynthesis in an intensified bio-chemocatalytic process and experimental proof of efficient biomass deconstruction; characterised by ultra-hemicellulosic solubulization and increased bioreactivity (saccharification and digestion) of residual cellulose in ZnCl2.nH2O solvent systems at relatively mild conditions. The findings as described in this thesis have the potentials to be fundamental to the design of advanced process for cheaper, efficient and integrated one-pot, bio-chemocatalytic pre-treatment and conversion of abundant waste LCB resources such as corn cob to valuable commodities such as bio-based succinic acid (BSA). Some of the findings as described in this dissertation have been communicated as scientific articles in reputable journals and as contributions to scientific conferences.Item Optimization and kinetics study of solvent pretreatment of South African corn cob for succinic acid production(2018) Mudzanani, Khuthadzo EdnaIncreasing concerns over environmental and geo-political issues on resources’ sustainability have driven the industries to shift their efforts to produce chemicals from renewable biomass. Amongst the lignocellulosic biomass, corncob contains cellulose, hemicellulose and lignin that are built in a compact structure which makes it difficult to access. Pre-treatment is then applied to make the content to be accessible to enzymatic hydrolysis which breaks down the polysaccharides to monomers. The sugar monomers can be converted to a wide range of bioproducts such as biofuels and bio-chemicals. The objective of the study was to determine, evaluate and optimize the best solvent system to pre-treat corn cob. In addition, the study evaluated the effect of pre-treatment parameters on the yield of cellulose and hemicellulose and attempt to develop a kinetic model to explain the dissolution. Lithium perchlorate, zinc chloride, phosphoric acid, sulphuric acid and sodium hydroxide were used during the pre-treatment, which was carried out at 70-80 ° C for 6 hours. Characterization of pre-treated samples showed a significant change in structure after pretreatment indicating disruption in cell wall of the lignocellulosic material. FTIR revealed a reduction in phenolic group; indicating that the lignin content has been reduced. The XRD patterns show that crystallinity was considerably reduced; this was shown by an increase in calculated crystallinity index (CrI) after LiClO4, ZnCl2, H3PO4 and NaOH pre-treatment. The CrI of raw corncob (CrI= 32.7%) increased to 46.2 %, 42.3 %, 55.6 % and 53.4 % of LiClO4, ZnCl2, H3PO4 and NaOH, respectively. The crystallinity index increased for pre-treated material, indicating that the amorphous cellulose is dissolved in the liquor, as well as lignin and hemicellulose removal This study has shown that LiClO4.2H2O pretreatment agent is an efficient solvent system to pretreat corncob which consecutively increase the accessibility of cellulose and hemicellulose from the solid fractions. The accessibility was confirmed by an ease hydrolysis of cellulose & hemicellulose to glucose & xylose respectively. An increase of nearly four times compared to the untreated corncob. The effect of reaction operating parameters i.e. Reaction time, temperature and solvent concentration was carried out and then optimized by response surface methodology (RSM) using Minitab 16. The target was to maximize the yield of cellulose and hemicellulose. It was discovered that the increase in temperature and reaction time increase the accessibility of cellulose and hemicellulose until an equilibrium is reached at 3 & half hours and 176 °c. The pretreatment solvent concentration was discovered to have an effect on the accessibility but not as much as temperature and time. The best pretreatment conditions to obtain high polysaccharides conversions to monomers were at 176°c for 3.5 hours using LiClO4.2H2O for 10 g of corncob. The results obtained from RSM were used to evaluate the temperatures profile, kinetic model for the corncob pretreatment as a function of temperature. The kinetics of pretreatment were studied by the amount of glucose, xylose and the lignin removed from the pretreated solids. The kinetic model of lignin removal and sugars accessibility was identified as a first-order reaction corresponding to the bulk phase for pretreatment time up to 24 hours. The rate constant results show that the kinetic rate increased with temperature. The activation energy for glucose, xylose and lignin were calculated to be 15.0 kJ/mol, 14.2 kJ/mol and 36.54 kJ/mol, respectively.