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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Solutions to some shape problems in electrochemical machining using the boundary element method

Narayanan, O. H. January 1985 (has links)
This work aims to show that the Boundary Element Method (BEM) is ideally suited for two-dimensionaL prediction of workpiece shapes for given tooling, and evaluation of tool profiles for known workpiece forms in Electrochemical Machining (ECM). A critical review of existing techniques for modelling ECM is presented which is followed by a resume of BEM. A few 'static' potential problems are analysed using BEM programs with constant, linear and quadratic elements before concentrating upon the two. ECM shape probLems. A number of complex-shaped tool profiles are used for simuLated ECM ceLLs. The effects of boundary mesh and time step size, eLement type and method of moving the work boundary upon accuracy of the resulting work profile are examined. The capabilities of the BEM based model to cater for over-potential and variable eLectrolyte conductivity conditions are also considered. Computed work shapes are shown to correlate well with exact and experimental solutions obtained from various sources. In some cases, BEM is shown to be superior to other established numerical techniques. Evaluation of tool profiles in this work is limited to fully conducting profiles only. It is iterative in nature and uses the approximate cos θ method to initiate the. procedure. Subsequent iterations employ formulations developed herein to correct tool profile to satisfy work boundary conditions. Effects of element type and merits of specifying different boundary conditions on the work surface are assessed. Computed tool profiles, when used in the machining model, are shown to yield desired workpiece shapes. Finally, general conclusions and a few suggestions for 'future research complete the thesis.

An evaluation study of models to predict erosion / corrosion in undersea oil and gas pipelines

Nicholson, Shaun January 2000 (has links)
The material documented in this thesis represents a contribution to the knowledge of the sponsoring company relating to assessment of the effects of two-phase flow on corrosion. Techniques available in the open literature for determining hydrodynamic characteristics of two-phase flow such as flow regimes and wall shear stresses were reviewed and compared with particular emphasis on stratified and slug flow regimes. The research work conducted improves the sponsoring company's knowledge and understanding of these areas providing a platform for undertaking two-phase flow related projects and developing / extending this business function. The work presented also enhances the company's qualitative understanding of momentum and mass transfer under two-phase flow providing a focus for driving future developments in laboratory based corrosion assessment. Initial work was undertaken to compare and validate hydrodynamic models studied using data available in the open literature a limited amount of which was from high pressure large diameter systems. This enhances confidence in models implemented and indicates the performance and accuracy expected. A feature of the work conducted of primary importance to the sponsoring company was generation of the first stages of a knowledge base supporting company projects whilst conforming to company QA procedures. In depth study of fundamental two-phase hydrodynamic models conducted during this research also provides a platform for future investment in commercially available technology. The material presented in this thesis enables investment in such technology with the benefit of detailed knowledge of models commonly used and critical understanding of expected performance and suitability.

Denitrification using immersed membrane bioreactors

McAdam, Ewan J. January 2008 (has links)
Nitrate is practically ubiquitous in waters abstracted for municipal potable water production in Europe due to decades of intensive agricultural practice. Ion exchange is principally selected to target abstracted waters with elevated nitrate concentrations. However, the cost associated with disposal of the waste stream has re-ignited interest in destructive rather concentrative technologies. This thesis explores the potential of membrane bioreactor (MBR) technology for the removal of nitrate from potable water. Two configurations are considered: an MBR to replace ion-exchange completely; and an MBR to treat the ion-exchange waste stream in-situ for re-use. For the replacement MBR, permeate quality can be affected by nitrite accumulation, micro-organism and carbon breakthrough. However, at steady-state and provided substrate addition was controlled, permeate quality was consistently high. Selection of an appropriate substrate was observed to improve permeability by a factor of three. Permeability was sustained within the MBR by adopting a dead-end filtration strategy having identified a relationship between filtered volume, flux and suspended solids concentration. Provided the filtered volume within a single filtration cycle did not exceed a set volume, the accumulated deposit was reversible. For the ion-exchange waste stream MBR, organic carbon breakthrough was considerable. However, the impact upon resin capacity was apparently limited when permeate was re-used for resin regeneration. Salt shocking did not induce permeability decline although some denitrification capacity was lost. Cost evaluation demonstrated that operating ion- exchange in parallel with MBR regenerant treatment was more cost effective than ion exchange with direct disposal.

Fault tree synthesis for real time and design applications on process plant

Martin-Solis, Gregorio Alberto January 1978 (has links)
A method of fault tree synthesis relevant to the generation of fault trees for use both in hazard assessment in the design of plant and in the analysis of fault conditions using a process control computer is described. List processing techniques and bit manipulation have been used to reduce computation time and computer storage require ments. The programming language used in the development of the algorithm was RTL/2. Several examples are presented to illustrate the methodology.

Enantioselective decarboxylativeprotonation of malonic acids using aryl malonate decarboxylase

Goodall, Mark John January 2012 (has links)
The demand for new methods through which important enantiopure compounds can be produced is increasing within the chemical industry. In this regard, one group of interesting compounds are enantiopure carboxylic acids as traditional methods towards producing these compounds that rely on a process known as enantioselective decarboxylative protonation (EDP) are often insufficient due to a combination of factors including low yields, low enantioselectivities, the need for environmentally deleterious reaction conditions and high costs. Due to the shortcomings of these synthetic techniques, the application of biological systems is an alternative method that has come to the forefront of current research, which is because biological systems often boast a number of advantages over traditional chemical methods. Described within this thesis is one such biological method towards the production of enantiopure carboxylic acid compounds via EDP, a method that utilises the enzyme aryl malonate decarboxylase (AMDase). This research follows on from previous work detailing the mechanism this enzyme utilises toperform decarboxylation reactions and specifically looks into the substrate specificity that thisenzyme possesses. This work allows us to see how flexible this method is with regards to theproduction of a range of different carboxylic acid products, which is important as previous chemical methods are often limited by a lack of diversity. It also allows us to see how the kinetic parameters of the enzyme catalysed reactions change dependent on the identity of the substrate, allowing us to better justify the currently proposed mechanism and to better direct attempts to improve the enzyme through mutagenesis experiments. Indeed, several mutagenesis experiments have also been performed in an attempt to producevariants of the enzyme that would have either an increased enzyme scope or better reactivity with currently known substrates, and this work is also detailed within this thesis.

Asymmetric Transfer Hydrogenation & Transfer Hydrogenation in Water

Feng Wu, Xiao January 2007 (has links)
Asymmetric transfer hydrogenation (ATH) has emerged as a practical, powerful alternative to asymmetric hydrogenation for the production of chiral alcohols, one of the most valuable intermediates in the synthesis of pharmaceuticals, flavour, aroma and agricultural chemicals, and speciality materials. However, the application of ATH has been hampered by its low TONs and TOFs in the past. On the other hand, enabling catalysis in water contributes to one of the most important and challenging fields of modem chemistry, green synthesis. As a solvent for organic reactions, water bears a number of attractive physical-chemical properties over traditional organic molecular solvents. A~ will be seen in the thesis, ATH in neat water has proved to be viable, affording chiral alcohols in fast rates, high productivity and high enantioselectivity. The reduction can be carried out with unmodified or tailor-made catalysts by using mild, readily available formate salt as reductant with no organic solvents required. A series of ligands (L =chiral diamine and its derivatives, amino alcohols) and metal complexes (those of Ru, Rh and Ir) have been explored for the ATH in water, and a wide range of ketones have been efficiently reduced to chiral alcohols (Scheme A-I.) Furthermore, for the PEG-supported catalyst, water as solvent, it can be very easy to separate the catalyst from the product by simple phase separation, which is a very important and ideal dream, especially in pharmaceutical applications. Aldehydes are hard to reduce successfully by TH catalysis in the solvent which is normally used for TH of ketones, such as refluxing 2-propanol, azeotropic formic acid and triethylamine mixture. However, this water-formate reducing system can also apply to the TH of aldehydes, furnishing highly chemoselectivity and remarkably fast reaction rate for the reduction of aldehydes. Thus, the in-water reduction provides a new method that is simple, economical and eco-friendly, and can be readily adapted for laboratory synthesis and commercial-scale production.

Fabrication of diamond-based microplasma devices

Allen, Monika Jozefa January 2014 (has links)
Plasmas and microplasmas in particular have a great potential for expanding their range of applications if improvements can be made to the lifetime of the devices housing them, and their operating pressure can be increased. Using a hollow-cathode geometry and scaling down the devices to micron size allows for their work in atmospheric pressure, while using diamond as the main material greatly increases their lifetime. Polycrystalline undoped diamond was used as the insulating layer for creating sandwich structured microplasma devices, with electrodes of either evaporated titanium/gold or thin films of polycrystalline boron-doped diamond created by chemical vapour deposition (CVD). The hollow cathode cavities and holes were micromachined using an Oxford laser system and the quality of the electrode thin films was tested using Raman spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. Successful ignition of several samples at below- and above-atmospheric pressure was achieved, thus proving that CVD diamond can be used as the material for microplasma devices. Three types of devices were created: diamond insulator with metal electrodes, diamond insulator with boron-doped diamond electrodes, and diamond insulator with one metal and one boron-doped diamond electrode. Varying the thickness of the insulating layer and the size of the cavity or hole allowed the micro plasma to ignite at different pressures. The devices showed no sign of reaching the end of its life in the time it took a comparable device made of previously used materials (metal and glass) to break down. This thesis describes the fabrication of these devices. as well as the refinement process of the architecture which eventually led to a successful ignition of a microplasma at above-atmospheric pressure.

PCP pincer compounds: synthesis and application as methanol carbonylation catalysts

Reynolds, Timothy James January 2011 (has links)
A range of 1,8-bis(PR2)anthracene - " Anthraphos " - compounds have been synthesised. Previously reported (where PR2 = -PPh2 (2.1) or -Pi-Pr2 (2.5)), and new (where PR2 = -PCYP2 (2.18), -PCyh2 (2.19) -Po-Tol2 (2.20) or -Po-Ans2 (2.21)) anthraphos structures have been prepared in 3 steps starting from 1,8-dichloroanthraquinone (2.7). Compound (2.7) was converted by chlorine- bromine substitution followed by reduction with Al(OCyhh into 1,8-dibromoanthracene (2.4), from which (2.1), (2.5) and (2.18)-(2.21) were prepared by double lithiation of (2.4) with n-BuLi and subsequent reaction with the corresponding ClPR2 reagent. 1,8-bis(di-tert-butylphosphino)anthracene (2.6) was prepared according to the literature procedure in 3 steps starting from 1,8-dichloroanthraquinone (2.7). On reaction of (2.5), (2.18) or (2.19) with [NiCl2(dme)l or [PtCl2(t-BuCNhl in the presence of i-Pr2NEt, cyclometallation of the C-H bond in the anthracene C(10)-position led to the new square-planar nickel and platinum pincer complexes (3.1)-(3.3) and (3.7)( 3.9), respectively. Reaction of (2.5), (2.18) or (2.19) with [PdCl2(CH3CNhl afforded the analogous new palladium pincer complexes (3.4)-(3.6) in the absence of i-Pr2NEt. On reaction of (2.5), (2.6), (2.18) or (2.19) with 0.5 eq. of [RhCl(COEhb cyclometallation of the C-H bond at the anthracene C(lO) position gave the corresponding new 5- coordinate Rh(llI) pincer complexes (3.10)-(3.13). Exposing solutions of (3.10)-(3.12) to CO gas led to coordination of CO, affording the 6-coordinate pseudooctahedral Rh (Ill) pincer complexes (3.14)-(3.16). Complexes (3.14)-(3.16) were also prepared by reaction of the diphosphines (2.5), (2.18) or (2.19) with 0.5 eq. of [RhCl(COhb at room temperature. Exposing a C6H6 solution of (2.6) to CO afforded the Rh(I) square-planar pincer complex (3.20) with elimination of HCl. Square-planar Rh (I) carbonyl pincer complexes (3.17)-(3.19) were prepared by dehydrochlorination of (3.14)-(3.16) with i-Pr2NEt at RT. The analogous complexes of diphosphines (2.20) and (2.21) were prepared by reaction of the diphosphines with 0.5 equivalents of [RhCl(CO)2b in the presence of i-Pr2NEt. 5-coordinate Ir(Ill) pincer complexes (3.25) and (3.26) were prepared under similar conditions to those in the literature, and the new complex (3.27) by reaction of (2.19) with 0.5 eq. of [IrCl(COEhb. Rh(I) complexes (3.17)-(3.19), (3.21) and (3.22) oxidatively add MeI at room temperature and subsequently undergo migratory insertion to give the corresponding 5-coordinate Rh(IH) acetyl species. Complex (3.20), with bulky tBu2P-substituents, was unreactive towards MeI under the same conditions. Kinetic studies with (3.17)-(3.19) and (3.21) and (3.22) showed that the second-order rate constants (kl ) for MeI oxidative addition varied by a factor of >850, depending on the structure of the diphosphine. Complex (3.22), with o-anisyl P-substituents, showed a second-order rate constant of 90.2 M-I S-1 (CH2Cl2, 23 CC) compared to 0.106-0.408 M-I S-1 for (3.17)-(3.19) and (3.21). The rate enhancement is . proposed to be due to neighbouring group participation of the methoxy groups. Rh(I) complexes (3.17)-(3.19) and (3.22) were used as pre-catalysts for HI-promoted methanol carbonylation under industrially relevant conditions, using CO and CO /H2 (95:5) as feed gasses. 31p{IH} NMR spectra of the exit solutions suggested the phosphines remained largely bound to the metal centres at the end of the reactions. Using pincer compounds as pre-catalysts, carbonylation rates over twice that of the Monsanto catalyst ([Rh(COhI2t) were observed using CO feed gas, and approaching 3 times using CO/H2. A mechanism for the formation of acetic acid and side-products from MeOH and CO catalysed by pincer compounds is proposed .

Manganese oxides and other adsorbents for oxidative capture of mercury

Wiatros-Motyka, Malgorzata January 2013 (has links)
Removal of mercury from flue gases (especially from coal - fired plants) continues to be a challenging task due to numerous factors. Firstly, the amount, chemical and physical forms of mercury in flue gases vary greatly within different plants, and depend on numerous factors making it impossible for there to be one solution or control technology which can be applied to all power plants and other coal burning facilities. Secondly, while conventional air pollution control devices can help to reduce oxidized and particulate bounded mercury they are usually inefficient in capturing its elemental form, which is very volatile and water insoluble, and requires specific technologies to be removed. Among these, sorbents are most commonly chosen solutions, although the use of existing commercial sorbents, whether via injections or as fixed beds, are not without problems. First of all, they are frequently deactivated by flu gas components and high temperatures, which reduce their efficiencies and means that the huge amounts of material must be used. They can also contaminate fly ash which not only prevents its subsequent sale, but also creates an additional problem of disposing it as hazardous waste. In addition most of the sorbents cannot be regenerated, so their total capital and operational costs are high. Therefore the search for new improved mercury sorbents is an important and active area of research, especially in the context of a new United Nation's convention on reduction of mercury emissions and exposure on a global scale. This study presents data on novel mercury sorbents: co-precipitated manganese oxides and synthetic zeolites derived from coal fly ash, and an evaluation of their suitability as alternative materials to currently commercially available.

Mass transfer to thin liquid films on rotating surfaces, with and without chemical reaction

Moore, Stephen Russell January 1986 (has links)
This investigation was concerned with the ability of perforated discs to act as the active mass transfer surface in rotary contact devices. Of particular interest were discs constructed of a woven stainless steel mesh. This material had a high fractional free area and an irregular surface, which is desirable if interfacial turbulence is to be promoted. The characteristics of three grades of mesh were compared with those of a plate perforated with punched holes and a smooth plane disc. A hydrodynamic study indicated that a stable film could be maintained on these flexible materials at speeds in excess of 200 RPM. However it was also demonstrated that at higher speeds dry areas tended to form around the periphary. The mass transfer performance was analysed in a specially designed rotary test rig using a carbon dioxide/water system. These experiments indicated that high transfer rates are attainable provided that operation is restricted to conditions where film breakdown does not occur. At 1500 RPM flowrates in excess of 200 cm 3 /sec must be utilised. The same apparatus was adapted so that the enhancement due to a chemical reaction in the liquid phase could be determined. Aqueous solutions of diethanolamine were used in these experiments. The results were compared with predictions based upon a penetration type theory. The results of the investigation were applied to the design of a scrubber for an underwater closed—cycle nitro diesel engine and conclusions as to the relative merits of physical and chemical systems made.

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