Chemical and Biomolecular Engineering - Theses
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ItemNitrogen Removal from Wastewater Treatment Using Filamentous Algae( 2023-09)Nitrogen removal from wastewater using freshwater filamentous algae has advantages over conventional bacterially-dominant systems, especially for reducing greenhouse gas emissions. Further, filamentous algae provide easier harvesting compared to microalgal processes. However, there are large research gaps that must be addressed before this technology can be implemented at large scale. This thesis aims to understand how filamentous algae can be utilised in wastewater treatment for nitrogen removal alongside traditional treatment processes, with key research objectives being to understand the ammonia tolerance and organic carbon uptake of filamentous algae, and physical design parameters and practical considerations needed for implementation in outdoor conditions. Key experimental methods specific to the study of filamentous algae were also developed in the thesis. The ammonia tolerance of four filamentous algae species from the Oedogonium, Tribonema, Spirogyra and Cladophora genera was investigated, by cultivation under different combinations of pH, temperature and ammonia concentrations. At 60 mg-N/L initial TAN, the critical pH range was found to be approximately 8.0–8.6, 7.5–8.3 and 7.5–8.0 for Tribonema, Oedogonium and Spirogyra respectively. The critical threshold calculated based on the initial amount of free NH3 was 1.5–3 mg-N/L for Tribonema and Oedogonium and approximately 1 mg-N/L for Spirogyra. Although Oedogonium cultivated with TAN at pH 7.5 and 15 degree Celsius showed stable growth and capability to utilise TAN under controlled conditions, it was concluded that the use of filamentous algae for downstream wastewater treatment rather than direct TAN removal may be a more practical option. In terms of organic carbon utilisation, the filamentous alga Tribonema was found to have increased productivity and enhanced photosynthesis by direct utilisation of glucose. In contrast, acetate could only be indirectly utilised in the presence of bacteria, whereas ethanol could not be utilised under any conditions. Despite the positive results with respect to glucose utilisation, it was also found that bacteria can easily outcompete Tribonema in terms of organic carbon utilisation, implying that the algae-bacteria interactions require further understanding and optimisation especially for the complex outdoor environments. It was therefore concluded that Tribonema is more suitable for treatment of wastewater with low organic carbon concentrations, such as secondary-treated wastewater effluent. The performance of filamentous algae in real wastewater was also investigated during a series of outdoor trials. The effects of key parameters that require consideration for scale-up were investigated, including aeration, wastewater strength, weather, tank setup and operations, and the filamentous algae’s adaptability to strong wastewater. Overall, Tribonema was found to be able to remove nitrogen from diluted anaerobically treated wastewater at the Western Treatment Plant (Melbourne) under outdoor conditions, with the use of undiluted wastewater also possible following a lengthy adaptation period for the algae. However, the nitrogen recovery rate into the algal biomass was relatively low compared to the overall nitrogen removal rate required. Therefore, it was concluded that applying filamentous algal systems to remove nitrogen from partially treated wastewater containing reduced nitrogen levels is a more practical option, since the operation under low wastewater toxicity improves the robustness of the system without compromising nitrogen recovery and CO2 capture by filamentous algae. Combining the findings from all chapters, it was concluded that filamentous algae can be used for nitrogen recovery and CO2 capture in partially-treated, low-strength wastewater, with Tribonema identified to be the most suitable algal genera among the four isolated candidates. Despite the promising results presented in this thesis, more future studies focusing on the variations and complexity of large industrial scale operations need to be performed before the process can be implemented.
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ItemCompatibility of Plastic Piping with Future Fuels( 2023-10)The future fuels, methanol, ammonia (NH3) and dimethyl ether (DME), have gained significant attention as a substitute for conventional fossil fuels, owing to their high volumetric energy density and the ability to be synthesised from hydrogen. Utilising existing natural gas infrastructure to transport these future fuels would offer notable advantages from both economic and operational standpoints. However, these future fuels are highly condensable, and have strong interactions with HDPE (high-density polyethylene) pipelines and associated elastomers that may lead to high leakage and material failure. Hence, this thesis investigated the compatibility of commercial HDPE piping samples and common elastomers used in natural gas distribution pipelines with methanol, NH3 and DME. Methanol compatibility was investigated for HDPE, as well as poly(styrene-co-butadiene) (SBR) and poly(acrylonitrile-co-butadiene) (NBR) as the base elastomer material, with the inert polytetrafluoroethylene (PTFE) as the comparison gasket material. Commercial elastomers that incorporated additives were also studied to evaluate the interaction and impact of methanol when additives were present. Methanol solubility and diffusion in those polymeric materials, as well as changes to the mechanical properties, were determined through sorption measurements. Methanol exhibited higher solubility and diffusivity in HDPE than methane, thereby raising concerns about potential inventory losses in practice. Only minor changes in mechanical properties were observed upon exposure of HDPE to methanol, indicating that HDPE pipelines can safely transport methanol. In contrast, the two base elastomers -SBR and especially the more polar NBR- showed orders of magnitude higher solubility of methanol than methane, attributed to methanol’s high condensability and its similar polarity with these elastomers. In comparison, the inert gasket material PTFE was unaffected by exposure to methanol. Commercial elastomers demonstrated significant leaching of additives and mass change after methanol exposure. Subsequent mechanical testing revealed that the methanol significantly impacted the performance of the elastomers by reducing yield strain and therefore lowering flexibility, a key characteristic of elastomers. The compatibility of HDPE and associated elastomers -SBR and NBR- were investigated with gas phase NH3 and DME. A constant volume variable pressure (CVVP) method was employed for measuring transport properties. Both future fuel penetrants exhibited much higher solubility in these materials than methane, especially in the elastomers, due to the similar polar nature between these fuels and elastomers. The permeability of NH3 in HDPE was independent of pressure, while DME permeability displayed significant pressure dependence, which was also associated with a notable loss of structural integrity. DME caused swelling within HDPE, leading to a significantly higher permeability than methane. Over time, the strong interactions reduced the permeability to 52% of its initial value through a process known as anti-plasticisation. This outcome revealed that NH3 can be safely transported within HDPE pipes, but DME cannot be safely transported. DME significantly affected the elastomers, with an almost 70% decline in permeability observed for NBR systems, attributed to the combination of densification of the polymer structure and anti-plasticisation by the penetrant fuel. This indicated that elastomers undergo significant morphology change as a result of exposure to DME and therefore have a higher likelihood of failure when exposed to this fuel. Ammonia-resistant elastomers with low permeance already exist but are based on expensive fluorinated polymers, such as PTFE, which have several health and environmental impacts associated with their fabrication. Hence, methodologies to decrease the NH3 permeance in a common and cost-effective elastomer were developed here. Three kinds of additives were incorporated into the elastomer NBR, deploying three different approaches: increasing the basicity of the elastomer environment; chemically reacting with NH3 to limit transport; and incorporating barrier additives to prevent diffusion. Additives utilised included organic molecules, polymers, metal oxide, metal hydrides, metal organic frames and nanocomposites. The mechanical properties were also assessed, as these are fundamental to an elastomer’s role. The creation of a basic environment through amines, especially putrescine, resulted in NH3 permeability losses of 50%. Chemical additives lowered the permeability from 10% to 20% with limited impact on the tensile properties. The most significant decrease was observed for the addition of 3% graphene oxide to NBR, with NH3 permeability decreasing by 80%, but a negative consequence was the production of a more rigid material. As a result of this investigation, NH3 permeability can be significantly reduced in inexpensive NBR elastomers through tailored additives. This represents a relatively straightforward approach to future-proof natural gas infrastructure for NH3 transportation. This thesis has demonstrated the compatibility of transporting the future fuels methanol and NH3 through pipeline material HDPE, as well as the lack of compatibility for DME transportation. Importantly, the polar nature between elastomers and future fuels results in strong interactions and high sorption, leading to higher permeability than observed for methane, which places a greater safety risk on using these materials with these future fuels.
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ItemChemical Dynamics of reactive intermediates in extreme environments from cold till combustion( 2023-09)Reactive gases are present in a wide range of P, T conditions ranging from interstellar medium operating at near vacuum and ultracold conditions to combustion engines functioning at 100’s of atmospheric pressures and temperatures reaching more than 2000 K. Much of the reactive gases are placed in intermediate conditions, such as emission formation in diesel engines. Generally, reaction rates are influenced by the operating conditions in multiple ways and are measured experimentally in a narrow space. Quantum chemical and statistical reaction rate theory calculations are the theoretical tools that provides the platform to validate the experimental rates and further explores the rates in untrodden conditions. This thesis models chemical reactivity of varied species such as cations, anions, neutral molecules, and radicals in a wide range of conditions. The thesis opens by modelling ion – molecule reactions of CCl+ addition to CH3CN in collaboration with the experimental researchers using linear Paul ion trap with a time-of-flight mass spectrometry to measure the overall and product specific rate coefficients along with the branching fractions. The theoretical calculations calculated rates and found that both the products HNCCl+ and C2H3+ were equally probable equivalent to the experimental predictions. This work showed reaction pathway to product channels is as important as the product’s energies. The work was further extended to study the acetylene cation (C2H2+) addition to acetonitrile. Same theoretical techniques were utilised to predict the rates and product splits. The predicted products based on theory were C3H4+ (m/z 40) and C2NH3+ (m/z 41) aligned with the experimentally detected ones. The only variation to the experiments were the branching fractions. The potential energy surface developed for the bimolecular reaction (C2H2+ + CH3CN) had the most stable isomer pyrrole cation (C4H5N+). To further validate the developed theoretical model, reaction model developed for the bimolecular reaction (C2H2+ + CH3CN) were utilized to calculate the decomposition rates of pyrrole cation (C4H5N+). Experiments on pyrrole cation decomposition were present in the literature. The dissociation experiments on pyrrole cation measured the appearance energies of two ionic masses (m/z 40 and 41) and identified the species would be C3H4+ (m/z 40) and C2NH3+ (m/z 41). Theoretically, the threshold to product channels were experimentally measured appearance energies. The threshold values were 3.95 eV and 3.5 eV for C3H4+ (m/z 40) and C2NH3+ (m/z 41) respectively, aligns reasonably well with the literature measurements. In addition to the threshold calculation, unimolecular decomposition rates of pyrrole cation were calculated, aligned relatively well with the literature experimental values. This work highlights the fact that reaction model developed for a bimolecular reaction (C2H2+ + CH3CN) could predict the unimolecular decomposition rates of pyrrole cation (C4H5N+). The remaining works focused on kinetic modelling of carbon-based compounds, especially polyaromatic hydrocarbons (PAH) which are present in interstellar media as well as in combustion engines. Chapter 3 focuses on the decomposition chemistry of the indenyl radical (C9H7), a PAH. A recent literature photolysis study on indenyl radical discovered a new product channel in addition to existing chemistry. Theoretical calculations identified that the new product channel was c- C5H5 + C4H2. Rate coefficients and branching ratios to the product channels were computed. The rates for other product channels (o-C6H4 and C7H5) matched with the existing literature chemistry. These theoretically computed rate coefficients were then incorporated into detailed chemical kinetic models. The results showed that indenyl decomposition chemistry comes into play only at high temperatures or at longer residence times. Chapter 4 of this thesis discusses the oxidation chemistry of phenylvinyl radical using theoretical kinetic modelling. In collaboration with the experimental researchers, photoionization mass spectrometry (PIMS) studies were conducted at 600 K identified peaks at m/z 106. At the experimental condition, this theoretical study found two products, i.e. benzaldehyde and o-Quinone methide with their molecular mass being 106 and validated the experimental results. This study was further extended to calculate rates and product splits till 2000 K. At temperatures greater than 1200 K, the new product phenylvinoxy started to form and occupied major flux around 2000 K. Without the aid of theoretical work, this new product phenylvinoxy would have been unidentified. The developed oxidation chemistry was then incorporated into detailed styrene chemistry and found an increase in the mole fraction profile of benzaldehyde and thereby moved closer to the experimental value. In this thesis, chapter 5 concentrates on the decomposition kinetics of phenylvinoxy, a product identified from the phenylvinyl oxidation work. In the literature, phenylvinoxy decomposition chemistry were estimated from vinoxy and benzyl chemistry due to their structural similarity. Theoretical decomposition kinetics rectified the overpredicted phenylketene literature rate coefficients and introduced a new pathway of benzofuran + H. This study showed phenylvinoxy connects combustion intermediates such as benzyl and benzofuran and led us to calculate the rate coefficients for the bimolecular reaction: benzofuran + H. These calculated rate coefficients were then included in the styrene chemistry and found that reverse reaction of benzofuran + H forming phenylvinoxy was more favourable.
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ItemA mathematical, digital and experimental investigation of the freezing, thawing and storage of Mozzarella cheese( 2023-09)Mozzarella cheese is a popular cheese product, resulting in widespread markets and demand for cheese distribution. The limited shelf-life of this cheese makes long-distance shipping more challenging. This short shelf-life is due to relatively high moisture content, which promotes bacterial and enzymatic activity that accelerates proteolysis, impacting functional attributes, such as stretchability and meltability. Managing proteolysis is therefore important for maintaining Mozzarella cheese quality throughout storage and shelf-life. Freezing has been proposed as a potential way to retain the functional shelf-life of Mozzarella cheese by arresting proteolysis. Yet there are limited tools available for prediction of freezing and thawing times as a function of composition and processing variables. Specifically, thawing of Mozzarella cheese has not been investigated in detail and there is lack of a model that can predict thawing of Mozzarella cheese. This thesis aimed to develop robust models for prediction of freezing and thawing processes for Mozzarella cheese. To do this, six different Mozzarella cheese blocks, differing significantly in block size and composition, were assessed using freezing and thawing experiments. Temperature profiles of freezing and thawing were obtained for these different Mozzarella cheese samples. The enthalpy method was then used to develop a novel robust heat and mass transfer model for freezing and thawing in Mozzarella cheese. The presence of salt (NaCl) affected both freezing and thawing, depressing the freezing point. The freezing point depression was predicted for Mozzarella cheese with different salt contents, providing useful information that was not available elsewhere in the literature. Salt content had a significant impact on freezing, with a decrease in salt from 1.34% w/w to 0.07% altering the temperature of phase change from ~-4.5 C to -3 C. Additionally, simulations showed salt migration was restricted to the first ~1-2 centimeters from the surface during freezing, with a slight increase of 8-10% salt in free moisture at the block center. Another aim was to develop a response surface methodology (RSM) model based on the simulated data to create a computationally efficient tool for predicting freezing and thawing times for single cheese blocks, using a range of industrially relevant conditions. The proposed models can increase the current understanding of impact of cheese composition and size, as well as storage settings in the case of thawing, on freezing and thawing of Mozzarella cheese. The approach presented here can also be used to determine freezing and thawing times, which are important information for manufacturers in the design of storage facilities, the choice of transportation and process optimization to improve Mozzarella shelf-life and quality during storage. For instance, the interaction effects provided by the RSM model can be used to find the optimum thawing conditions including the air velocity, thawing temperature and thawing time for the frozen cheeses. The next aim was to predict the level of proteolysis in Mozzarella cheese during the storage at 4 C, using data available in the literature from extensive prior studies on Mozzarella cheese proteolysis. Considering the complex nature of cheese proteolysis (i.e., presence of correlated variables, such as cheese composition, and nonlinear relationships between these variables and proteolysis levels) there are no generalizable tools that can predict proteolysis accurately. The predictive performance of a multilinear regression model and three different machine learning techniques, namely, gradient boosting, support vector regression and random forest, were explored using various input features including chemical composition, storage time and well-known manufacturing factors, such as milling pH, acidification type and coagulating enzyme. Machine learning techniques outperformed the multilinear regression due to their ability to handle the presence of correlated input features and nonlinearity between input features and the level of proteolysis. The collected data from the literature had not been previously used for predicting cheese proteolysis and the performance of machine learning techniques for this purpose has not yet been assessed. The proposed approach was also applied to predict the proteolysis in Cheddar, which shares some similar manufacturing steps with Mozzarella cheese, using a relevant set of data collected from the literature. The gradient boosting method could more accurately predict proteolysis than the other two machine learning algorithms for both Mozzarella (R2 = 92%) and Cheddar (R2 = 97%) cheese, possibly due to the iterative sequential training of the gradient boosting algorithm. Storage time was the most important input feature for both cheese types, followed by coagulating enzyme concentration and calcium content for Mozzarella cheese as well as fat and moisture content for Cheddar cheese. Manufacturers can use this information for distribution of Mozzarella cheese within an optimal window of functionality. Information on proteolysis is also important for Cheddar cheese, as it can be correlated with Cheddars quality attributes, such as flavor and texture. Finally, freezing and thawing of Mozzarella cheese was investigated under industrial conditions using a pallet containing 96 cheese blocks, each weighing 10 kg. This was the first time that effects of freezing and thawing processes under industrial conditions were evaluated on properties of Mozzarella cheese during a storage period of five months post-thaw. Additionally, the effects of the placement of cheese blocks at inner and outer positions of the pallet were investigated, as placement could lead to different properties during storage, which has not previously been examined. Heat and mass transfer simulations predicted temperature profiles at different pallet locations, achieving a maximum root mean square error of 3.6 C for both freezing and thawing processes. Significant differences were observed in the cooling rates between blocks located at inner (0.7 C day-1) and outer (0.87 C day-1) positions during freezing, as well as in the warming rates for inner (0.81 C day-1) and outer (5.9 C day-1) locations during thawing. Freezing simulations showed salt migration during the freezing of the samples, with more migration observed on the sides of the outer blocks. While the frozen-thawed samples had slightly lower urea-PAGE intact casein levels after 5 months of storage at 4 C compared to the control unfrozen sample, the confocal microstructure, expressible serum obtained by centrifugation and select functional properties including the hardness, stretchability and meltability measured by the transition temperature method were only different between frozen-thawed and control samples during the first month. Despite varied heat transfer rates, pallet block locations during freezing and thawing did not significantly affect most properties, ensuring that freezing and thawing pallet-stacked products under industrial conditions of this study led to a consistent good quality (i.e., stretchability measured via texture profile analysis and meltability measured via rheological analysis) after thawing and during the subsequent storage. These results can help manufacturers make informed decisions about inventory management and distribution of their products. Additionally, a gradient boosting algorithm, which was shown to outperform other algorithms in earlier sections, successfully predicted proteolysis levels in the frozen-thawed samples (R2 = 88%), which can be potentially linked to the functionality of industrially frozen-thawed products. This thesis addressed the challenges posed by the short shelf-life of Mozzarella cheese and freezing and thawing processes. Well-established principles, previously employed in other disciplines, have been applied to offer innovative insights into the freezing and thawing processes of Mozzarella cheese as well as its proteolysis. A series of recommendations were also made for future studies in the field, including steps that could be taken to increase the accuracy and the applicability of the current simulations and models to a wider range of other dairy products. This thesis not only addressed the challenges in the dairy manufacturing industry but also improved research on dairy storage and quality control by introducing a range of new modelling approaches.
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ItemSeparation of Hydrogen from Natural Gas Using Adsorption Technologies( 2023-05)There is growing global interest in hydrogen as an eco-friendly energy carrier due to the environmental concerns associated with global warming. One potential application is injecting hydrogen produced from various sources into the natural gas grid for efficient transportation and use. After co-transportation in the natural gas pipeline, either pure hydrogen or pure natural gas should be extracted from the mixture at desired locations for special end-users’ needs. This thesis deals with separation of hydrogen from natural gas mixtures to produce either high purity hydrogen or high purity natural gas for various applications. Natural gas is a complex mixture consisting of CH4, C2H6, CO2, N2, and trace amount of C3+ hydrocarbons. To produce high-purity hydrogen from a hydrogen + natural gas mixture, a separation technology is needed that not only operates at low capital and energy costs but also provides hydrogen of sufficient purity and recovery. A literature review on different gas separation technologies for deblending of low hydrogen concentration from natural gas mixture is presented. Adsorption technologies such as pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) have shown promising performance for gas separation due to design flexibility, high separation efficiency, low operational costs, and high-purity product. Accordingly, adsorption processes were selected for current separation. In this study, according to the natural gas network pressure levels in Australia, adsorption process performances were investigated over a range of feed pressures from 1 to 30 bar with different levels of feed hydrogen concentrations from 5 to 50%. First, to produce high purity hydrogen, a three-layered adsorption column was designed to capture specific groups of mixture components in each layer. Silica gel (to capture C3+), activated carbon (to capture CH4, C2H6, and CO2), and LiLSX zeolite (to capture N2) were chosen as the pre-layer, main-layer, and top-layer, respectively, based on the measured equilibrium isotherm data. Afterwards, a customized pressure swing adsorption (PSA) cycle was created and simulated using a six-bed PSA system with 12 steps. The PSA system achieved a high-purity hydrogen product (>99%) with a high recovery rate (>85%) for various hydrogen concentrations (5-30%) in a 30 bar feed stream. A comparison was made between the PSA system and an onsite electrolyzer for hydrogen production. The findings indicated that the PSA system was a viable alternative, even at low hydrogen recovery rates (40%), when the PSA plant is built at a pressure reduction station. Afterwards, in order to investigate the separation performance for low-pressure applications, vacuum swing adsorption (VSA) was investigated. The experiments were conducted using binary mixtures of H2 + CH4 in the lab. CH4 was the representative of natural gas in VSA experiments since it constitutes the major portion of natural gas. A four-bed VSA apparatus was filled with Norit RB4 activated carbon and achieved high purity hydrogen (>99%) through the VSA process for hydrogen feed concentrations of 30% and 50% at 102 kPa feed pressure. After validating the experimental results through simulations, a six-bed three-layered pressure vacuum swing adsorption (PVSA) system at a pilot scale was developed in Aspen Adsorption software to separate hydrogen from a representative natural gas mixture, and achieved high purity hydrogen (>99%) and high purity natural gas (>98%) simultaneously for different hydrogen concentrations (10 to 50%) in the feed at a feed pressure of 4 bar. An important outcome from these sets of experiments was that hydrogen could be replaced with helium in the low-pressure experiments showing similar adsorption behavior in vicinity of CH4, which is valuable for safer laboratory operations. The experimental and simulation results showed that as the H2 feed concentration and desorption vacuum pressure increased, the required power to achieve hydrogen purity above 99% decreased, and bed productivity increased with increasing H2 feed concentration. Since the H2 concentration in natural gas pipelines will be low at early implementation phases; in the final section, a double VSA system for industrial scale was developed and simulated using Aspen Adsorption to produce high hydrogen purity from a representative 10% hydrogen and natural gas mixture. The experiments were carried out in a laboratory using a four-bed VSA plant filled with activated carbon and a binary mixture of 10% H2 + CH4. After validating the simulation tool with the experimental results, Aspen Adsorption software was utilized to evaluate the performance of a double-stage VSA process at a small scale to achieve 99% hydrogen purity. The results revealed that upgrading H2 from 10% to 50% in the first stage and then to 99% H2 in the second stage leads to better separation performance and less energy consumption than a single-stage VSA for the same separation target. Subsequently, a five-bed double VSA system in industrial scale was developed to produce high hydrogen purity from a representative 10% hydrogen and natural gas mixture. The results showed that high purity hydrogen (>99%) and high recovery (>82%) were achieved at a feed pressure of 110 kPa by implementing higher vacuum (40 kPa) in the first stage and lower vacuum (10-20 kPa) in the second stage. These findings suggest that it is possible to achieve high purity hydrogen product from a low hydrogen feed concentration, 10% H2, using a double-stage VSA process with less energy consumption and higher recovery than a single-stage VSA. A range of future work was also recommended. A sensitivity analysis based on the level of different impurities available in natural gas like N2, CO2, CH4 and heavy hydrocarbons should be investigated. It is recommended to conduct high pressure PSA experiments at small scale using low H2 concentration in feed (10%-20%) to confirm the simulation results that achieving 99% H2 purity is possible using a single stage PSA. To determine the economic feasibility of implementing the technology on a large scale, a comprehensive economic analysis should be conducted. This analysis should consider both technical and financial factors. Once the evaluation is completed, an engineering map can be developed based on the findings. This map should indicate the areas where adsorption technologies such as PSA and VSA are economically viable. Finally, it is advisable to explore hybrid processes, such as combining membrane and PSA technologies, to produce ultra-pure hydrogen products as a potential solution to improve the overall efficiency of hydrogen production processes.
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Item3D printing meets colloidal processing: Hierarchically porous ceramics for bone scaffolds( 2023-08)Hierarchically porous structures in nature, such as bamboo and bone, have remarkable strength and stiffness-to-density properties. Direct Ink Writing (DIW) or robocasting, an Additive Manufacturing (AM) material extrusion technique, is able to synthetically create near-net-shaped complex geometries. In this research, DIW and colloidal processing are synergized to fabricate multiscale porous ceramic scaffolds with tailorable properties. 100-micron scale porosity is produced by the 3D printed scaffold architecture. By applying particle-stabilized emulsions or capillary suspensions as feedstock for printing, a secondary level of smaller porosity is created within the scaffold filaments via soft templating of the oil phase. A third scale of porosity of sub-micron pores is formed by partial sintering of ceramic particles. Several ceramic systems are investigated in this work, namely clay, alumina, as well as hydroxyapatite and beta-tricalcium phosphate (bio-compatible and bio-resorbable ceramics). In Chapter 3, possible criteria for obtaining suitable ceramic paste feedstocks for DIW are developed by studying the relationships between paste formulation, rheological properties and print parameters. The study demonstrates that storage modulus and apparent yield stress from visco-elastic rheological measurements could be indicators of “printability” of the feedstock. This is further strengthened by the results in Chapter 4 of a different material. The particle size, surfactant concentration, oil fraction, and mixing speed are shown to influence the rheological properties, which can be adjusted to improve printing. In Chapter 4, the primary focus is to understand how to control the pore sizes within the filament microstructure. The study shows that by increasing the oil fraction and particle size, but reducing the surfactant concentration and mixing speed, produces larger micropore sizes within the filaments. The pore morphology is also determined to morph from sphere-like pores typical of Pickering emulsions, to more elongated pores of the granular phase-inverted emulsions (referred to as capillary suspensions in later studies), by increasing the oil and/or surfactant concentrations. Next, a potential application of this customizable multi-scale porous structure is explored – as a scaffold for bone tissue regeneration purposes. By varying the two levels of macro- and microporosities, scaffolds with different properties are produced. A comprehensive study of the influence of the different levels of porosity and two micropore morphologies is undertaken. Additionally, the macropore effects from variations in print nozzle diameter and inter-filament spacing are investigated. In Chapter 5, the viability and proliferation of primary human osteoblasts (bone cells) on the scaffolds are studied in vitro, firstly on slip-cast scaffolds for their micropore morphology, then as a 3D construct for cellular interactions with both micro- and macro-porosities. In Chapter 6, the scaffolds’ physical properties, such as strength and elastic modulus under compression and bending, are investigated, as well as how they change after undergoing degradation (simulating resorption in the body). Finally, in Chapter 7, all the results from the studies are discussed as a whole, concluding that this reported process of DIW of colloidal ceramic feedstocks is a promising strategy for highly porous bone tissue engineering scaffolds. Additionally, it offers a high level of customization of mechanical as well as biological properties.
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ItemWetting of Chemically Homogeneous Rough Surfaces( 2023-05)Miniaturization has emerged as a trend in the last few decades across multiple areas. Lab-on-a-chip devices (for example) are preferred over traditional macroscopic techniques due to their high throughput, fast operation and ease of automation. In general, miniaturization means the dominance of surface forces, therefore, surface wetting is important to these applications; and many others like novel oil separation techniques, processing of metal ores, and the self-cleaning superhydrophobic surfaces. However, our ability to design surfaces tailored to specific applications is hampered by the lack of a quantitative model for the critical wetting parameter of Contact Angle Hysteresis (CAH), being defined as the difference between the advancing and receding (static) contact angles. CAH largely determines liquid mobility when surface forces dominate. Previous wetting models are unable to quantitatively predict CAH on general rough surfaces: The kinematic model suits smoothly undulating surfaces only; the energy minimization model offers qualitative insights; and the energy dissipation model is con- strained by an incomplete understanding of energy dissipation during liquid spreading. The well-known Young, Wenzel and Cassie-Baxter equations only predict a single contact angle instead of a range of stable contact angles as observed experimentally. In this thesis, a predictive wetting model is developed that can be used for predicting CAH on naturally occurring substances as well as for designing functionalised wetting surfaces for specific applications. Numerical simulations of an advancing fluid interface are performed using the open-source program Surface Evolver, with the results interpreted via a mechanical energy balance (MEB) framework to predict CAH from the input parameters of the surface topology and Young’s angle. A complementary experimental study is also performed to measure CAH on surfaces having both periodically distributed and randomly distributed cylindrical pillars, using a number of different liquids and surface treatments. The numerical model was successfully used to predict CAH on surfaces having: a) a structured array of square cross-section pillars and b) sinusoidally varying smooth undulations. Results were successfully validated against available experimental and analytical results. Based on the experimental study a predictive equation for CAH is developed for surfaces with randomly distributed cylindrical pillars, valid over a range of Young’s angles. The experimental study showed that surfaces having periodically distributed features (cylinders) have a slightly higher CAH compared to surfaces having randomly distributed features, for a given roughness area fraction. Further, the validity limits of the proposed model were predicted using the MEB framework.
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ItemSynthesis of DNA-inspired polymers with nucleotide-pendant functionalities( 2023-04)Deoxyribonucleic acid (DNA) is the most important hereditary material. This natural polymer possesses an outstanding ability to store and translate genetic information. However, its use in the biosciences has been restricted by the high cost involved in producing long DNA sequences with more than 200 base pairs, and the limited shelf life of these sequences once produced. To tackle the stability and cost related issues, developing DNA-like materials, such as DNA-inspired polymers, is of increasing interest. Furthermore, combining the DNA substituents and synthetic polymers provides special features that are not seen in either material alone. For example, synthetic polymers typically do not display defined structures and functions as DNA. One of the major challenges in generating DNA-inspired materials is the inclusion of the entire DNA building block into the system, namely the nucleotide, consisting of the nucleobase, deoxyribose and phosphate groups. This project aims to combine all three DNA components into a polymeric system. Exploring the synthetic routes and studying the specific interactions and behaviour of the produced system are the major aims of the project. To accomplish these aims, first adenine (A), thymine (T), cytosine (C) and guanine (G) protected nucleotide functionalised propyl methacrylate monomers were synthesised. This was achieved by coupling 2-hydroxy propyl methacrylate (HPMA) to commercially available protected phosphoramidites, 5'-dimethoxytrityl-N-benzoyl-2'- deoxyadenosine, 3'-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite (hereafter denoted as dA(Bz)), 5'-dimethoxytrityl-N-benzoyl-2'-deoxycytidine, 3'-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite (hereafter denoted as dC(Bz)), 5'-dimethoxytrityl-2'-deoxythymidine, 3'-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite (hereafter denoted as dT) and 5'-dimethoxytrityl-N-isobutyryl-2'-deoxyguanosine, 3'-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite (hereafter denoted as dG(ib) via phosphite triester linkages. By designing bio-functional monomers, containing protecting groups, side-reactions were minimised and increased solubility in a broad range of solvents was realised for subsequent polymerisation reactions, i.e., free radical polymerisation (FRP) and reversible addition fragmentation (RAFT). This is the first time phosphoramidite-HPMA monomers have been synthesised via a room temperature approach, and isolated for polymerisation reactions. Secondly, the seminal synthesis of A and T phosphoramidite-functionalised propyl methacrylate polymers was realised, using post- and pre-functionalisation methods via RAFT polymerisation. The RAFT method was used to achieve a higher control over the molecular weight of the produced polymers. Using the post-polymerisation functionalisation method (addition of phosphoramidite functionalities to already polymerised HPMA polymer), a RAFT-made poly(HPMA) with a narrow dispersity (1.29) and controlled DP (58), was functionalised with dA(Bz) and dT protected phosphoramidites via phosphite triester linkages. Although the polymer synthesis was successfully performed, the degree of phosphoramidite functionalisation was low, particularly for the A containing polymer (45 %). Pre-polymerisation functionalisation (direct polymerisation of the HPMA nucleotide-based monomers) was then used to investigate if there was any improvement in the degree of phosphoramidite functionalisation along the polymeric backbone. Applying this approach, A and T phosphoramidite-HPMA monomers were directly polymerised using the RAFT technique. The attempts to improve the degree of functionalisation was successful in case of A containing polymer (66%), however, the degree of functionalisation remained approximately the same as the T containing polymers for both methods (57% for post- and 50% for pre-functionalisation processes). These polymers offer some unique features which are currently not seen in literature including relatively high degree of protected nucleotide functionality, ready dispersion in solvents appropriate to a broad range of polymerisation and protection of the amine group on the nucleobases, ideal for RAFT polymerisations. In addition, the RAFT polymers, still with pendant RAFT agent, have future potential as macro-RAFT agents for subsequent block extension through the addition of other monomers for potential sequence control. This is not explored within the scope of this thesis. The final part of the thesis the noncovalent interactions of deprotected A and T nucleotide-functionalised propyl methacrylate polymers in solution was studied. To achieve this, the removal of the phosphoramidite protecting groups was required to achieve free hydroxyl and amino functionalities which could further interact via noncovalent bonding. The resulting polymers were potentially capable of noncovalent interactions with their complementary polymer (A with T). The noncovalent interactions between the A and T deprotected nucleotide-functionalised propyl methacrylate polymers were studied using ultraviolet (UV) and circular dichroism (CD) spectroscopies at various temperatures. These studies revealed that the pi-pi stacking between the complementary A and T nucleobases was the dominant interaction in the polymer mixture. Performing the studies in three different solvents (dichloromethane (DCM), cyclohexane (CY) and isopropanol (IPA)) showed that increasing the solvent dielectric constant promotes the pi-stacking interactions. For the A and T deprotected nucleotide-functionalised propyl methacrylate polymer mixture in IPA, the enhanced pi-stacking led to spherical and worm-like polymeric assembled structures with average diameters of 100 nm (confirmed by transmission electron microscopy (TEM)). The morphology of the particles was controlled by the degree of functionalisation, where the post-functionalised polymers formed worm-like particles and the pre-functionalised polymers formed spherical particles. In summary, this thesis showed the RAFT synthesis of nucleotide functionalised propyl methacrylate polymers, including complementary nucleobases A and T. The inclusion of hydrophobic and hydrophilic groups along the polymeric backbone led to the formation of self-assembled polymeric nanostructures. The specific structure and behaviour of the produced DNA-like materials makes them attractive for a broad range of biomedical applications such as delivery of drugs and genetic materials. Moreover, the inclusion of phosphate linkages into these materials paves the way for the synthesis of polymers with improved helicity levels.
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ItemUltrasound Assisted RAFT Polymerization( 2023-04)Polymers are widely used in modern society, with applications ranging from materials science to biotechnology. As a result, developing new and versatile polymer synthesis methods is critical to meeting the diverse demands of these applications. One such method is RAFT (Reversible Addition-Fragmentation chain Transfer) polymerization, a controlled radical polymerization technique that allows for the production of well-defined polymers with precise control over molecular weight, distribution, and end-group functionality. RAFT polymerization is commonly used in the synthesis of stimuli-responsive polymers for drug delivery systems, biodegradable polymers for tissue engineering and medical implants, and functional polymers for surface coatings and adhesives. Power ultrasound refers to sound waves with frequencies ranging from 20 kHz to 1 MHz that generate microbubbles through a process known as acoustic cavitation. This method has numerous applications, such as drug carrier synthesis, sewage treatment, food dehydration, dairy processing, and medical imaging. Ultrasound-assisted RAFT polymerization (sono-RAFT) is an eco-friendly and exogenous initiator-free polymerization technique that generates radicals uniformly throughout the reaction mixture. This method offers unique benefits for the synthesis of functional polymers with tailored structures and properties. The objective of this thesis is to investigate the synthesis of various block copolymers and star polymers via sono-RAFT, analyze the chain-end fidelity of the polymers and analyze the impact of sono-RAFT in batch and continuous processes on the final monomer conversion. The investigation in Chapter 2 involves the examination of the livingness and chain-end fidelity of polymers that were synthesized through the sono-RAFT technique in an aqueous medium. This was done through the performance of chain extension studies and the preparation of multi-block copolymers. Additionally, the polymers synthesized by this method were compared to those produced via the photo-iniferter method. The sono-RAFT method has been demonstrated to be an effective alternative to the photo-iniferter method for achieving high conversion rates. Compared to the photo-iniferter method, quantitative monomer conversions were observed in a fraction of the time using the sono-RAFT method. These results suggest that sono-RAFT has potential as an efficient method for polymer synthesis and could be a valuable addition to the current range of polymerization techniques. Following on from the analysis of livingness of polymer chains synthesized via the sono-RAFT method, in Chapter 3, the synthesis of core crosslinked star (CCS) polymers via the sono-RAFT technique were investigated. The star polymers were prepared in a fast and efficient two-step approach at room temperature. Moreover, the optimum conditions for both arm and star formation by varying the applied power, crosslinker ratio and degree of polymerization were studied. This method is significantly faster than traditional methods of CCS polymer synthesis for high arm to star conversion. Although sono-RAFT polymerization produces polymers with higher chain-end fidelity in a fraction of the time, it has two major drawbacks. The batch size is affected by the size of the transducer, and polymerization can be carried out only at lower monomer concentrations as the cavitational effects are lower at higher viscosities. In Chapter 4, we explored the role of various degassing methods and the influence of microreactor material on the final monomer conversion. Furthermore, we investigated the synthesis of polymers with different monomer concentrations in batch and continuous sono-RAFT processes. Using a continuous process, we successfully demonstrated the synthesis of polymers at higher monomer concentrations (up to 5 M ). This is a significant advance in making this process more desirable for industrial applications. The sono-RAFT method presents a promising strategy for the effective synthesis of functional polymers with tailored structures and properties. While sono-RAFT has some limitations, recent advances have made it a viable option for industrial applications, with the successful synthesis of high-concentration polymers using continuous processes. These findings represent a significant step forward in the field of polymer synthesis and pave the way for new and exciting applications of sono-RAFT.
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ItemCO2 CONVERSION TO VALUABLE CARBONACEOUS PRODUCTS THROUGH LIQUID-PHASE INTERMEDIATES UNDER MILD CONDITONS( 2023-04)The emission of carbon dioxide, CO2,presents a critical economic and environmental challenge due to its significant contribution to global warming. To address this challenge, carbon capture and utilization, CCU, strategies have been developed to mitigate CO2 emissions and replace fossil fuels. Effective CO2 conversion technologies hold promise in reducing the impact of global warming, promoting the production of renewable fuels such as methanol, and offering new opportunities for CO2 utilization in various industries. This study aims to investigate advanced CO2 conversion processes towards achieving sustainable and cost-effective utilization of this ubiquitous gas. While CO2 is known to be a thermodynamically stable molecule , its reactivity can vary significantly depending on the chemical environment. With efficient catalysts or reactants, CO2 could be converted to the demanded products at low temperatures, significantly saving energy consumption. CO2 conversion to methanol is a popular choice since methanol is a building block for many other platform chemicals. However, though the catalytic system has been thoroughly studied during the past 20 years, the development of catalysts has fallen into a bottleneck and needs new breakthroughs. As for CO2 reduction to solid carbon, it is a new trend for CO2 conversion and utilization. Though various types of carbonaceous material could be utilized in a wide range of industrial fields, the research in this area is quite limited and has not yielded satisfactory results. Here, for the first time, several efficient methodologies that could successfully achieve CO2 conversion at mild reaction conditions are presented in this thesis. Various attempts have been made to improve the catalytic performance of conventional copper-based catalysts, and a new catalyst preparation route has been proposed, which gives new inspiration to the entire catalyst preparation field. The amazing results obtained from CO2 reduction to solid carbon at room temperature with the help of liquid metal alloy are also demonstrated, which marks a new chapter for CO2 conversion and utilization. This thesis comprehensively describes the CO2 catalytic conversion to useful chemical products with novel material and provides an inspiring strategy for catalyst design and CO2 conversion process: Chapter 1 is an extensive literature review on the emerging technologies for CO2 conversion under mild conditions, which inspires researchers to develop efficient pathways for CO2 conversion without extensive energy consumption. Chapter 2 elucidates the catalytic behavior and synthesis methods of novel catalysts for CO2 hydrogenation to methanol under mild conditions in 1 butanol solvent. The addition of alcohol solvent, 1 butanol, altered the reaction route of CO2 conversion to methanol in the gas phase, which significantly reduces the reaction requirements for CO2 conversion. Various promoters such as Zn and Zr and supporting materials were added to the copper based catalysts to enhance the reaction of CO2 conversion to methanol under 170 degrees and 30 bar. The CuZnOZrO2 catalyst with the addition of 60 weight percent supporting material hydrotalcite, HTC, MG 50, showed a higher methanol selectivity compared with the commercial methanol catalyst. The catalyst synthesizing process presented in this chapter involved the participation of a stabilizer to prevent particle agglomeration during the preparation process and improved the efficiency of methanol synthesis compared with the traditional precipitation method. The introduction of Zn promoter through ion exchange process based on the special structural properties of hydrotalcite not only enhanced the catalytic performance but also inspired researchers in a new way of introducing promoters. Chapter 3 presents a novel approach for reducing CO2 to carbon, utilizing liquid metal MgGa at near room temperature. Liquid metals have garnered considerable attention in the field of catalysis due to their exceptional properties, and Mg based materials have shown great promise for driving CO2 conversion reactions. The results of this study highlight the potential of MgGa as a viable catalyst for reducing CO2 and suggest avenues for further exploration of liquid metal based catalytic systems. Therefore, Mg was chosen as the main catalytic component for CO2 conversion at room temperature. During the reaction period, Mg atoms dissolved in Ga-based liquid metal gradually diffuse to the gas liquid interface, facilitating the reduction of CO2 to carbon while undergoing an oxidation reaction to MgO at near ambient temperature. The utilization of an electrochemical method allows for a sustainable cyclic process by effectively reducing Mg and Ga ions back to their metallic state. The study presented in this chapter provides new breakthroughs for carbon utilization without extensive energy consumption. Chapter 4 investigates the reduction of CO2 through the surface of a LiGa liquid metal alloy. Lithium has been identified as a promising catalyst due to its strong affinity to CO2 and highly reactive nature. In the reaction process, Li atoms exhibit a diffusion process similar to that of Mg atoms discussed in Chapter 3, migrating to the surface of the LiGa alloy and forming a crust layer. The reaction mechanism and final products at different reaction conditions, 50, 200, 500, were explored. Carbon was observed at all the reaction temperatures as the final product of CO2 reduction, and the high-temperature reaction condition made it possible to obtain nanofibers through CO2 conversion using a liquid metal alloy. The final obtained carbonaceous mixture on the surface of the alloy could be utilized directly as one kind of supercapacitor material with excellent performance and stability. Chapter 5 summarizes the findings and perspectives of this project.