The use of cement based materials could be widespread in the long term management of radioactive materials in the United Kingdom. In the Geological Disposal Concepts proposed by the Radioactive Waste Management Directorate of the Nuclear Decommissioning Authority (NDA), several cement based materials are used in the long-term management of intermediate-level wastes. Much of the waste will be immobilised within stainless steel containers using cement grouts based on ordinary Portland cement (OPC) blended with blast furnace slag (BFS) or pulverised fuel ash (PFA). The resulting waste packages will be placed underground in a Geological Disposal Facility (or Repository) after a period of storage at the waste producers’ sites. The repository will then be filled with cement based backfill. The encapsulation grouts and the backfill materials will perform as both a physical barrier and chemical barrier for confining the radioactive wastes. During storage and disposal, some wastes may generate carbon dioxide from the degradation of organic materials and this will react with the cement based materials. Therefore, carbonation of the cementitious encapsulation grouts and backfill materials is of interest because of the resulting changes to their physical and chemical properties and also because of its ability to remove carbon-14 labelled carbon dioxide from the gas phase. It is also important to understand the reaction kinetics under a range of conditions, due to the long-term nature of storage and disposal. In this work, the carbonation progress of one backfill material and of two encapsulation grouts used in the UK has been studied in batch reactors. These materials are known as Nirex Reference Vault Backfill (NRVB), 3:1 PFA/OPC and 3:1 BFS/OPC. Based on the single dimensional carbonation experiments, fundamental parameters affecting the rate of carbonation were investigated and the carbon dioxide uptake capacity of each material was determined. For these three materials, an increase in relative humidity (75% to 100%) decreases the carbonation rate. A higher reaction pressure can facilitate the carbonation, but its effect was less obvious than the effect of relative humidity. The progression of the carbonation fronts have also been observed by various techniques and the shape of carbonation front was proved to be influenced by the relative humidity. Special attention was given to the modelling of the kinetics and mechanism of the carbonation reaction of these materials. This work provides fundamental understanding of the carbonation reaction of NRVB, 3:1 PFA/OPC and 3:1 BFS/OPC of relevance to the future optimization of a geological disposal facility in the UK and to assessments of the performance of such a facility.

This thesis describes the development, fundamental extension and extensive testing (validation and verification) of mathematical models for predicting outflow following the failure of pressurised pipelines containing incompressible liquids. The models, for the first time, account for all the important sequential flow regimes taking place during the discharge process. These include full pipe flow, bubble formation and propagation, followed by open channel flow. The system configurations modelled include a draining pipeline connected to a storage tank and pipe with one closed-end. In the first part of this thesis, the development of outflow models to simulate the full-bore rupture of horizontal pipelines is presented. In order to model the full pipe flow in a pipe fed from an upstream tank, the published model by Joye & Barrett (2003) is employed in this study. Bubble propagation and open channel flow for both configurations (in the presence of upstream tank and pipe with one closed-end) are modelled by assuming critical flow condition throughout the pipe and in the tank (where applicable). Bubble propagation velocity is calculated based on Benjamin’s (1968) and Bendiksen’s (1984) proposed equations. The second part of this study focuses on the extension of the developed models to account for pipe inclination angle. Bubble propagation and open channel flow are modelled by replacing the critical flow equation with Darcy-Weisbach equation, applicable to downward-inclined pipes. The bubble propagation pattern in the pipe is determined based on the drift velocity method through the results obtained from parametric studies. The developed models are validated by comparing the predicted values against experimental measurements recorded using laboratory scale setups. Through sensitivity analysis based on comparing the results of the models to case studies representative of real events, the importance of accounting for post-full pipe flow on the total amount of inventory discharged is demonstrated.

Polymer electrolyte fuel cells (PEFCs) are a key technology as the world strives for a low carbon future. The main obstacles facing mass-market uptake are the high cost and the longevity of the units; as such, research is needed to enhance performance and understand the degradation mechanisms. In this study, dynamic compression is applied using a cell compression unit (CCU) to study the effect on performance of a membrane electrode assembly (MEA) and its individual components with dimension change. Electrochemical impedance spectroscopy (EIS) is used to delineate the effect of compression on contact resistance, membrane resistance and mass transfer losses. Derived parameters such as the ‘displacement factor’ are used to characterise a representative range of commercial gas diffusion layers (GDLs). Increasing compaction pressure leads to a non-linear decrease in resistance for all GDLs. Different GDLs have different intrinsic resistance; however, all GDLs of the same class share a common compaction profile (change in resistance with pressure). Cyclic compression of Toray GDL leads to progressive improvement in resistance and reduction in thickness that stabilises after ~10 cycles. During initial hydration of Nafion membranes there is a direct relationship between membrane conductivity and dimensional change (swelling) of MEAs. Electrode flooding is found to result in membrane hydration and an increase in stress or strain, depending on the compression mode of the fuel cell. Results suggest that hydration cycles and flooding events can lead to cell degradation due to the stresses imposed. With increasing compression, a significant reduction in net performance is observed, with the most significant differences occurring in the mass transport regions of the performance curves. As the compression increases, the high-frequency resistance reduces with the improvement in contact resistance between the GDL and bipolar plate material, concurrently the low frequency resistance increases with increasing compression.

Solid oxide fuel cells (SOFCs) are a promising power conversion technology that has the benefit of high efficiency and the ability to work on a range of fuels, including natural gas. However, if run on hydrocarbon fuel under non-optimised conditions the anode can undergo carbon deposition (coking) which leads to a loss in performance. Research is required to understand the complex nature of the carbon formation process in order to develop superior electrode materials and avoid its formation. To do this, a range of ex situ analytical techniques are available; however, these do not allow the process to be studied in real time within the studied environment of the SOFC. The gallium orthophosphate crystal microbalance (GCM) is a piezoelectric device capable of acting as a microbalance sensor up to 900 °C. This work describes the development of the GCM for studying coke deposition on nickel substrate, as a mimetic of the cermet electrodes used for SOFC anodes. A novel holder system was design and produced to allow operation of both the GCM and SOFC at high temperatures in a range of gas environments. Change in oscillation frequency associated with temperature and gaseous environment was studied and found to be conducive to the intermediate temperature SOFC operating environment. Surface development of the GCM to produce a nickel catalytic surface has shown the ability to detect coke formation for the application of SOFC anodes. The degradation of electrochemical performance due to deposition of carbon onto symmetrical SOFCs is measured using electrochemical impedance spectroscopy (EIS). Direct correlation is observed between the frequency shift of the sensor and the change in resistance to charge transfer of the SOFC anode. Evidence of an induction period following exposure to methane has been shown in SOFC anodes.

This thesis describes the fundamental extension and extensive testing of a robust CFD model for predicting outflow following the failure of pressurised hydrocarbon pipelines. The main thrust of the study involves the extension of the basic outflow model to account for complex pipeline systems, improvements of the theoretical basis and numerical stability. The basic model, based on the numerical solution of conservation equations using the method of characteristics, incorporates a suitable equation of state to deal with pipelines containing pressurised multi-component hydrocarbon mixtures. It utilises the homogeneous equilibrium flow (HEM) assumption, where the constituent phases in a two-phase mixture are assumed to be at thermal and mechanical equilibrium. The first part of the study focuses on the development of an outflow model to simulate the failure of multi-segment pipelines incorporating valves and fittings passing through terrains of different inclinations. In the absence of real data, the numerical accuracy of the model is assessed based on the calculation of a mass conservation index. The results of a case study involving the comparison of the simulated outflow data based on the failure of a multi-segment pipeline as opposed to an equivalent single segment pipeline containing gas, liquid or two-phase inventories are used to highlight the impact of pipeline complexity on the simulated data. The development and extensive testing of two models, namely the Hybrid Model and the Modified Homogeneous Equilibrium Model (MHEM) each addressing a principal limitation of the HEM are presented next. The Hybrid Model deals with the failure of the HEM in predicting post-depressurisation outflow for inclined pipelines containing two-phase mixtures or liquids through its coupling with a hydraulic flow model. The MHEM on the other hand addresses the failure of the HEM to accurately predict the discharge rates of flashing/ two-phase fluids discharging through an orifice. Finally, the dilemma of the appropriate choice of the size and duration of the numerical discritisation steps expressed in terms of the Courant, Friedrichs and Lewy (CFL) criterion on the stability and computational workload of the pipeline failure model is investigated for different classes of hydrocarbon inventories. These include gas, liquid (flashing and incompressible) and two-phase mixtures.

Enantiomer separation remains an important technique for obtaining optically active materials. Even though the enantiomers have identical physical properties, the difference in their biological activities make it important to separate them, in order to use single enantiomer products in the pharmaceutical and fine chemical industries. In this project, the separations of three pairs of diastereomer salts (Fig1) by crystallisation are studied, as examples of the ‘classical’ resolution of enantiomers via conversion to diastereomers. The lattice energies of these diastereomer compounds are calculated computationally (based on realistic potentials for the dominant electrostatic interactions and ab initio conformational energies). Then the experimental data are compared with the theoretical data to study the efficiency of the resolving agent. All three fractional crystallisations occurred relatively slowly, and appeared to be thermodynamically controlled. Separabilities by crystallisation have been compared with measured phase equilibrium data for the three systems studied. All crystallisations appear to be consistent with ternary phase diagrams. In the case of R = CH3, where the salt-solvent ternaries exhibited eutonic behaviour, the direction of isomeric enrichment changed abruptly on passing through the eutonic composition. In another example, R = OH, the ternaries indicated near-ideal solubility behaviour of the salt mixtures, and the separation by crystallisation again corresponded. Further, new polymorphic structures and generally better structure predictions have been obtained through out this study. In the case of R = CH3, an improved structure of the p-salt has been determined. In the case of R = C2H5, new polymorphic forms of the n-salts, II and III, have been both discovered and predicted. This work also demonstrates that chemically related organic molecules can exhibit different patterns of the relative energies of the theoretical low energy crystal structures, along with differences in the experimental polymorphic behaviour. This joint experimental and computational investigation provides a stringent test of the reliability of lattice modelling to explain the origins of chiral resolution via diastereomer formation. All the experimental and computational works investigated in this thesis are published (see APPENDIX 1).

This thesis describes the fundamental extension, development and testing of a mathematical model for predicting the transient outflow following the failure of pressurised pipelines. The above encompasses improvements to the theoretical basis and numerical stability, reduction in the computational runtime and the modelling of fracture propagation with particular reference to CO2 pipelines. The basic model utilises the homogeneous equilibrium model (HEM), where the constituent phases in two-phase mixtures are assumed to be in thermodynamic and mechanical equilibrium. The resultant system of conservation equations are solved numerically using the Method of Characteristics (MOC) coupled with a suitable Equation of State to account for multi-component hydrocarbon mixtures. The first part of the study involves the implementation of the Finite Volume Method (FVM) as an alternative to the MOC. In the case of gas and two-phase hydrocarbon pipeline ruptures, both models are found to be in excellent accord producing good agreement with the published field data. As compared to the MOC, the FVM shows considerable promise given its significantly shorter computation runtime and its ability to handle non-equilibrium or heterogeneous flows. The development, testing and validation of a Dynamic Boundary Fracture Model (DBFM) coupling the fluid decompression model with a widely used fracture model based on the Drop Weight Tear Test technique is presented next. The application of the DBFM to an hypothetical but realistic CO2 pipeline reveals the profound impacts of the line temperature and types of impurities present in the CO2 stream on the pipeline’s propensity to fracture propagation. It is found that the pure CO2 and the postcombustion pipelines exhibit very similar and highly temperature dependent propensity to fracture propagation. An increase in the line temperature from 20 – 30 oC results in the transition from a relatively short to a long running propagating facture. The situation becomes progressively worse in moving from the pre-combustion to the oxy-fuel stream. In the latter case, long running ductile fractures are observed at all the temperatures under consideration. All of the above findings are successfully explained by examining the fluid depressurisation trajectories during fracture propagation relative to the phase equilibrium envelopes. Finally, two of the main shortcomings associated with previous work in the modelling of pipeline ruptures are addressed. The first deals with the inability of Oke’s (2004) steady state model to handle non-isothermal flow conditions prior to rupture by accounting for both heat transfer and friction. The second removes the rupture plane instabilities encountered in Atti’s (2006) model when simulating outflow following the rupture of ultra high pressure pipelines. Excellent agreement between the new nonisothermal model predictions and the published data for real pipelines is observed.

Burning fossil fuels contributes largely to the release of CO2 emissions, which CO2 accounted for 84% of the total UK greenhouse emissions in 2009. Energy production can affect climate change because it is currently produced using non- renewable fuels. As a result the UK Government has set a target of 15% renewable energy use by 2020. Renewable energy is the production of energy using fuels that are produced and sourced sustainably. Maximising renewable energy by using alternative fuels to produce our heat and electricity can help decrease our emissions and reach Government targets. The main objective of this work is to investigate the techno-economic assessment and life cycle assessment of energy from different types of biomass in the UK context. Energy use in the UK and climate change is discussed to present a case for sustainable energy. An extensive review of the thermal treatment options, as well as the different types of biomass available in the UK has been presented. Issues related to energy from biomass such as food vs. fuel and land vs. fuel are also discussed. In this thesis two second generation types of biomass are individually investigated - solid recovered fuel (SRF) and forestry waste wood chips (FWWC). A techno-economic assessment was performed on small to medium scale combustion plants using SRF (50 ktpa and 100 ktpa) or FWWC (50 ktpa, 80 ktpa and 160 ktpa). These are waste forms of biomass one of which is a mixed waste source (SRF) and the other a single waste source (FWWC), of which we have a great untapped resource in the UK. Discounted cash flow analysis, internal rate of return and levelised cost for the plants are calculated. The techno-economic assessment for the SRF plants were done previously by Yassin et al., (2008) and updated in this study using new cost data, such as landfill disposal costs and the new banded ROC’s scheme. The techno-economic performance of the FWWC was devised in the same way as for the SRF plant to ensure consistency. The results showed that the small and medium scale SRF plants were technically and economically viable, whilst only the largest scale FWWC plant was economically viable. A sensitivity analysis on the economic assessment was also performed, to investigate changes in levelised cost when seven different parameters were changed by 10% and 30%. As a result of these investigations a life cycle assessment (LCA) was performed on the large scale plants to investigate environmental aspects of sustainability. Hot-spot analysis was conducted for both plants and landfill reference systems were investigated for the SRF plant, whilst the FWWC plant investigated the emissions associated with leaving wood in the forest. In addition, the plants were compared against fossil fuel alternatives at the same production scale. The results of the LCA showed that both types of biomass are more environmentally friendly than fossil fuel alternatives. The SRF hot-spot analysis showed that the Fairport Process releases the most CO2. The FWWC hot-spot analysis showed harvesting released most CO2. The work was developed further by investigating a first generation liquid form of biomass rapeseed oil (RSO) for the production of energy using internal combustion engines. RSO is grown in increasing amounts in the UK for bio-diesel production but can also be used crude to produce energy. A techno-economic assessment of energy from RSO was conducted at small (27 ktpa) to medium (40 ktpa) scale plants, using the identical methodology as above. The results found only the medium scale plant to be economically viable. A sensitivity analysis on the economic assessment was also performed using the same percentage changes as above. An LCA was performed for the 40 ktpa RSO plant. A base case was investigated and compared to the plant. A hot-spot analysis was investigated, which showed the harvesting and cultivating units released the most CO2. The effects of growing rapeseed oil and how we use our land is investigated. The results showed the plant releases least emissions when the rapeseed is grown on rotation, using reduced tillage methodology.

Pressure Swing Adsorption (PSA) is the most efficient option for middle scale separation processes. PSA is a cyclic process whose main steps are adsorption, at high pressure, and regeneration of the adsorbent, at low pressure. The design of PSA cycles is still mainly approached experimentally due to the computational challenges posed by the complexity of the simulation and by the need to detect the performance at cyclic steady state (CSS). Automated tools for the design of PSA processes are desirable to allow a better understanding of the the complex relationship between the performance and the design variables. Furthermore, the operation is characterised by trade-offs between conflicting criteria. A multi-objective flowsheet design framework for complex PSA cycles is presented. A suite of evolutionary procedures, for the generation of alternative PSA configurations has been developed, including simple evolution, simulated annealing as well as a population based procedure. Within this evolutionary procedure the evaluation of each cycle configuration generated requires the solution of a multi-objective optimisation problem which considers the conflicting objectives of recovery and purity. For this embedded optimisation problem a multi-objective genetic algorithm (MOGA), with a targeted fitness function, is used to generate the approximation to the Pareto front. The evaluation of each alternative design makes use of a number of techniques to reduce the computational burden. The case studies considered include the separation of air for N2 production, a fast cycle operation which requires a detailed diffusion model, and the separation of CO2 from flue gases, where complex cycles are needed to achieve a high purity product. The novel design framework is able to determine optimal configurations and operating conditions for PSA for these industrially relevant case studies. The results presented by the design framework can help an engineer to make informed design decisions.

Microreaction technology was conceived, thanks to the advances on microfabrication by the semiconductor industry. The first applications of microchannels used for performing reactions date back to the early nineties. Since then, many conferences dedicated to this topic are held worldwide such as the International Microreaction Technology Conference (IMRET) or the International Conference on Microchannels and Minichannels. The small dimensions of the microchannels lead to very high heat and mass transfer rates, reactions are therefore performed very efficiently on these devices. However, the small dimensions of the channels lead to high pressure drops. In addition, microchannels are very susceptible to clogging. This thesis studies the effect of different microchannel configurations in terms of mixing, mass transfer, residence time distribution and reaction. The objective is to design microreactors which incorporate different structures which make them efficient in terms of heat/mass transfer, but do not have the issue of high pressure drop and channel blockage.