There is great need for new ways to effectively purify and desalinate water. There are protein channels in cell walls that act as very efficient water desalination membranes by rapidly and selectively transporting water. The mechanism of these biological membranes could be implemented in the design of a synthetic membrane by creating a size and electrostatic barrier suitable for efficient water desalination. Ordered mesoporous silica is a particularly attractive material, with ordered arrays of uniform nanochannels of a controllable size that have a wide range of applications, including separations. The silica surface can easily be functionalized to create the required steric and electrostatic effects. The synthesis of mesoporous silica films typically leads to pores that are poorly accessible from the film surface, hindering membrane applications. This thesis explores the formation of mesoporous silica structures within vertically aligned channels of anodic alumina membranes, so that an externally accessible pore orientation is obtained. These mesoporous structures are grown via an aspiration method. The thesis examines the influence of experimental conditions on the confined growth of the silica structures. Surfactants P123 and F127 act as structure directing agents, yielding circular, columnar or lamellar structures, such as 2D hexagonal (P123) and 3D cubic (F127), within the alumina channels. Ordered mesoporous silica in the confined space is achieved by controlling ethanol content. Transitions between the mesophases and changes in textural properties are observed with variations in experimental parameters. These experiments reveal that obtaining a defect-free composite structure where the alumina channels are tightly filled with the mesoporous silica material is not as trivial as literature precedent would suggest. Challenges remain, which include filling the alumina channels uniformly and homogeneously, and detachment of silica from the alumina walls. Consequently, fabrication of a porous silicon membrane is explored as an alternative robust matrix for growing aligned ordered mesoporous silica.

The thesis presents investigations on the process intensification behaviour in small scale separators operating in plug (segmented) flow. The continuous extraction behaviour, as well as the hydrodynamic characteristics of the liquid-liquid flow in microchannels is numerically and experimentally studied. The interphase mass transfer process using the extractant species Europium (III) as tracer was observed and quantified. The Eu(III) microfluidic extraction from nitric acid solutions was carried out in 0.2 mm and 0.5 mm channels using an ionic liquid solution (0.2M n-octyl(phenyl)-N,Ndiisobutylcarbamoylmethyphosphine oxide (CMPO) -1.2M Tributylphosphate) (TBP)/1-butyl- 3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide; ([C4min][NTf2]) as the extraction phase. Distribution and mass transfer coefficients were found to have maximum values at nitric acid concentration of 1M. Mass transfer coefficients were higher in the small channel, where recirculation within the plugs and interfacial area are large, compared to the large channel for the same mixture velocities and phase flow rates. Within the same channel, mass transfer coefficients decreased with increasing residence time indicating that significant mass transfer takes place at the channel inlet where the two phases come into contact. The experimental results were compared with previous correlations on mass transfer coefficients in plug flows. To better characterize the microfluidic flow, bright field Micro-Particle Image Velocimetry and high speed imaging were employed to measure velocity profiles and to obtain the geometric characterstics of the plug flow for the 1M HNO3 solution that was used in the extraction experiments. Correlations regarding film thickness, plug velocity and plug length are developed based dimensionless parameters. It was found that the liquid film surrounding the plug is largely affected by the changes in the front cap for the range of Capillary numbers studied (0.0224< Ca <0.299), while the droplet volume is highly dependent on the channel diameter as well as the mixture velocity. The volume-of-fluid (VOF) method is then used to model the velocities and pressure distribution in the plug flow in the channel and shows good agreement with experimental results and previous models. These features will help in optimizing the microfluidic plug flow for mixing, as well as mass and heat mass transfer enhancement. The mass transfer profiles of the extractant species Eu(III) are also studied using Laser Induced Fluorescence. Recirculation patterns appear in the dispersed aqueous phase from the plug formation stage, and 30-50 % of the mass transfer occurs during plug formation, where new interfaces are formed and mixing is enhanced from the recirculation pattern, especially at high Umix. After experiencing convection and normal diffusion extraction equilibrium achieved, the fluorescent signal in the ionic liquid phase is very strong as Eu(III) transfers into it. The correlations proposed on the hydrodynamics and observations of the mass transfer characteristics during plug flow will contribute to the development of microfluidic devices.

Managing hydrates is one of the most important hurdles in flow-assurance applications. Among various technological alternatives, the use of low-dosage hydrate inhibitors is attracting attention for both economic and environmental reasons. However, it is not well understood how these chemicals function. Molecular simulations were conducted to identify some of the molecular mechanisms that can help prevent growth and agglomeration of hydrates using anti-agglomerants. It has been suggested that the structure of the anti-agglomerants film at the hydrate-oil interface is important in determining performance. Therefore, the structure of anti-agglomerants adsorbed at the interface between methane hydrates and a liquid hydrocarbon was firstly investigated. The anti-agglomerants considered were surface-active compounds with three hydrophobic tails and a complex hydrophilic head that contained both amide and tertiary ammonium cation groups. The tail length, the surface density of the compounds as well as the composition of the liquid hydrocarbon were changed systematically. Some of the surfactants helped maintain a disordered structure at the interface, while others packed tightly, yielding a film that resembled a frozen interface. Comparing qualitatively to experimental data indicates that anti-agglomerants for which a frozen interface was observed exhibited better performance in rocking cell experiments. The results showed that anti-agglomerants films can yield an effective resistance to the agglomeration as well as the growth of gas hydrates, provided their density is sufficiently high. Using non-equilibrium simulations, the anti-agglomerants with short n-butyl tails were found to be able to stabilize hydrate cages and promote growth. However, the penetration of the alkyl tails into the growing hydrate structure helps anti-agglomerants firmly adsorb at the hydrate surface, which is expected to retard hydrate particles agglomeration. These results are crucial for quantifying the hydrate growth mechanism in the presence of anti-agglomerants, which could be exploited for designing new substances with better hydrate-promoting or hydrate-inhibiting characters for different applications.

After decades of declining interest, nuclear energy is poised for a comeback in the UK, driven primarily by pledges and binding agreements on limiting greenhouse gas emissions, but also by increasing energy security concerns. However, the industry has yet to tackle some of its most crucial challenges regarding management of used nuclear fuels, and especially of highly radioactive wastes. Life Cycle Assessment (LCA) - indeed the most mature and also the only standardised life-cycle methodology - represents a widely accepted tool for quantifying the environmental impacts associated with goods or services and supporting decision-making processes. This Thesis aims at operationalising the use of LCA to nuclear waste management. After introducing the LCA standard methodology, the Thesis proceeds with a comprehensive review of methodologies for assessing radiological impacts - the lack of an appropriate approach for radiological impacts in LCA is in fact identified as the crucial barrier for its application to the industry, especially with respect to waste management. Building upon the main findings of the review, the Thesis presents an overarching framework and two practical methodologies - namely UCrad and the Critical Group Methodology (CGM) - for assessing radiological impacts of direct discharges, and crucially, of nuclear waste disposed of in a geological repository. The LCA and the methodologies for radiological impacts are then applied to two case studies. The first is a prospective attributional study that examines the current procedure for managing used nuclear fuels and the UK Government policy for disposal of nuclear waste in the UK. The objective is to identify hot-spots and suggest potential improvements. The study shows that the highest impacts are due to the production of chemicals required by the reprocessing process and the materials used for High Level Waste canisters rather than the construction and decommissioning of a final repository for nuclear waste. The second study focuses on future scenarios for managing used nuclear fuels in the UK, including direct disposal and four reprocessing options, and clearly demonstrates how LCA can be used to support decisions. Reprocessing of uranium, but especially of plutonium, is shown to be of crucial importance from an environmental perspective.

One of the greatest challenges currently faced by humanity is the evolution of global environmental trends. Evidence points to the increased greenhouse gas emissions generated by humans as the main cause of these changes. One possible solution to the reduction of CO2 emissions is the production of value-added products or precursors which can be used as the feedstock for the former. In this project, several methods are used to facilitate the hydrogenation of CO2 via reaction with isopropyl alcohol, which acts as a hydrogen source, for the production of formic acid/formate and acetone. Since CO2 is thermodynamically stable, microbubble technology and plasmolysis were hypothesised to reduce the activation energies, even though the literature has only the precedent of catalysis.

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