Measurement of thermodynamic data at elevated pressure and temperature conditions with a microfluidic setup

With this thesis, I present an experimental study focusing on the provision of thermodynamic data of fluids at elevated pressure and temperature conditions. Hereby a microcapillary setup that is equipped with an in situ Raman Spectroscopy unit as well as with a high-speed camera, was further improved within the scientific employment of the author. The setup consists in principle of a fused-silica microcapillary embedded in a heating block, which is furthermore connected to high pressure syringe pumps. Pure compounds and mixtures were studied with the microfluidic setup and different thermodynamic properties were determined. For instance, vapor pressures of Poly(oxymethylene) Dimethyl Ethers (OME3 and OME4), a potential class of renewable diesel fuels, were the first time measured for temperatures exceeding the atmospheric boiling temperature. Hereby the regarded compound is pressurized at constant temperature, from what the vapor pressure is determined optically by detecting bubble or film formation, indicating the transition from vapor to liquid state. The main results of this thesis were however the vapor-liquid equilibria (VLE) of fuel/air-systems that were determined by in situ Raman Spectroscopy, whereby the Stokes-scattered Raman signal can be successfully separated phase-dependently by light barrier technology. A further task was the determination of saturated mixture densities of the validation system ethanol/CO2. With this study, I intend to contribute to the scarce literature data for the studied systems and properties. Therewith I want to help to enhance the understanding of microprocesses such as the evaporation and mixing formation in diesel combustion engines

Development of an improved group contribution method for the prediction of vapour pressures of organic compounds.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2007Vapour pressure is an important property in the chemical and engineering industries. There ar

A new computer representation of the properties of water substances

This thesis forms a record of the investigation carried out by the author on the correlation of the thermodynamic properties of ordinary water substance. The background to this problem was thoroughly explored and is discussed together with details of the latest international developments. Attention is drawn to the fact that the derivation of so-called 'Thermodynamic Temperatures' by the addition of the quantity, 273.15, to temperatures referred to the International Practical Temperature Scale leads to discrepancies in the values of the thermodynamic properties calculated from equations of state by means of the thermodynamic relations. These differences are shown to be significant when compared with the tolerances in the 1963 International Skeleton Tables. Equations in the form of Chebyshev polynomials are presented which enable the thermodynamic properties of saturated water and steam to be calculated in a systematic manner. In the equation defining the pressure-temperature relationship allowance has been made for certain unpublished measurements of the National Bureau of Standards. Accurate tables of saturation properties for regular intervals of temperature are included. A new equation for compressed water from 1 to 1000 bar and 0 to 15

Reduced order modeling of distillation systems

The concept of distillation separation feasibility is investigated using reduced-order models. Three different models of nonequilibrium rate-based packed distillation columns are developed, each with progressive levels of complexity. The final model is the most complex, and is based on the Maxwell-Stefan theory of mass transfer. The first and second models are used as building blocks in the approach to the final model, as various simplifying assumptions are systematically relaxed. The models are all developed using orthogonal collocation. The order reduction properties of collocation are well documented. A low order model is desirable as the subsequent generation of data required for assessing the separation feasibility is fast. The first model is the simplest as constant molar overflow is assumed. This assumption is relaxed in the subsequent models. The second and third models differ in their respective mass and energy transfer. The second model uses a constant bulk phase approximation for an overall gas phase transfer coefficient. The third model uses rigorous Maxwell-Stefan mass transfer coefficients, which vary throughout the column. In all models, the bootstrap equation for the energy balance across the two-phase film is used after the appropriate modifications are made based on the system assumptions. Starting point solutions and minimum height and flows analysis are presented for all models. The first model is used to develop an azeotropic methodology for identifying and characterizing pinches. Different numerical techniques are also compared, and the accuracy of orthogonal collocation is verified. Ternary and pseudo McCabe-Thiele diagrams are used to represent the result$ for the multicomponent models 2 and 3. The results for models 2 and 3 are similar. This is expected as they differ only in the mass and heat transfer definitions. An argument is made for a specific definition of an objective function for models 2 and 3, which is subsequently used to generate separation surfaces. This function is defined such that there will always be a solution and for this reason is deemed superior to any alternatives. Feasible regions are identified using a grid projection of the relevant sections of the separation surfaces. The data set contained within the feasible region will be used in an optimizer in future work. In general, this work involves an understanding and application of the collocation mathematics to distillation systems. A further understanding of distillation systems, the associated mathematics and degrees of freedom is essential. A large section of this work is devoted to explaining and manipulating the available degrees of freedom, such that the desired end result of a feasible region for a specific separation can be obtained. Other complicating factors include the use of the collocation boundary conditions, and the relationship between these and the overall degrees of freedom for the system. In the literature, collocation is largely applied to staged columns. The resulting feed stage discontinuities are smoothed out using various interpolation routines. Both of these approaches are incorrect. It is shown that the use of collocation in staged columns is fundamentally flawed due to the underlying theory of staged distillation and the implications of collocation assumptions. Further, the feed discontinuities present in all the results are intrinsic features of the system and should be preserved. It is further concluded that Models 2 and 3 were correct in comparison with each other. Finally it was shown that the separation feasibility was successfully determined using the optimal objective function. This success was based on the accuracy and order reduction achieved through the use of collocation. Further work will involve optimizing the data found in the feasible region using Non-Linear Programming

Transient Optical Characterisation of Donor-Acceptor Block Copolymers for Use in Solar Cells

This thesis presents a study of photo-active, semiconducting block copolymers for use in molecular solar cells. Current state-of-the-art organic devices utilise blends of two (or more) materials that are co-deposited from a common solution; the resulting structures formed are determined by material properties and deposition conditions, but often result in configurations that are detrimental to device performance. An answer to this problem comes in the form of the block copolymer; using these materials, devices can be formed from a single material active layer. In addition, the counterbalance of forces within films of block copolymer can lead to nano-scale self-assembly that allows for a strong degree of control over layer equilibrium morphology. Such control will be an important step forward in the evolution of molecular solar cells. The main body of this work is concerned with the study of the photo-physics of photo-conductive block copolymers, especially the generation of free charge. First, an investigation is made into the inherent structure-function relationship in block copolymers. A varying chain length is seen to drastically affect the photoluminescence quenching and yield of long-lived charges. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with the spectroscopic results, subject to a trade off between charge generation and transport/collection. In a second investigation, the effects of post-fabrication annealing on block copolymer films are considered; studies on annealed samples lead to the conclusion that domain crystallinity is a significant factor in determining the yields of long-lived charge carriers. It is found that these yields are comparable with those of a standard blend (that achieve 75% photon to electron conversion efficiency). Annealing leads to increases in photovoltaic device performance over unannealed samples, although additional control over active layer morphology is necessary for these materials to attain their potential. Following this, a comparative study is made between a block copolymer and a similarly composed blend formed from well studied polyfluorene copolymers. Further advantages of block copolymers are highlighted, including the stability of morphologies generated under different deposition conditions. Finally, a novel tool set is introduced using a block copolymer sample to emphasise the experiments potential with regard to studying interfacial photophysical effects

Organic fluid mixtures as working fluids for the trilateral flash cycle system

The requirements for power generation systems have been reviewed together with the various energy sources available for them. Geothermal energy has been examined in more detail and the principal methods of recovering power from it which are currently employed are discussed. A novel method for improved power recovery from geothermal sources called the Trilateral Flash Cycle (TFC) system is described which has the special requirement of an efficient two-phase expander. Optimum results are obtained from this cycle if a working fluid is used which leaves the expander as dry saturated vapour. A binary mixture of hydrocarbons was therefore sought which by variation of the constituent proportions, would satisfy this requirement for a range of inlet temperatures when the condensing temperature is constant. Methods of estimating mixture properties are reviewed and the chosen thermodynamic model, as well as a computational procedure for evaluation of vapour-liquid equilibria of organic binary mixtures at high pressures, are described. This is based on the Redlich-Kwong- Soave cubic equation of state. By this means a mixture of n-pentane and 2,2-dimethylpropane (neopentane) was found to be the most suitable for the TFC system for expander inlet temperatures between 150-180'C. Temperature-entropy (T-S) diagrams of this organic binary mixture were obtained for several compositions. Bubble and dew pressures of (n-pentane + 2,2-dimethylpropane) have been determined experimentally for five different compositions at six different temperatures, (333.15 K, 353.15 K, 373.15 K, 393.15 K, 413.15 K, and 433.15 K). Vapour pressures of pure n-pentane and pure neo-pentane were also determined at these temperatures. The critical point of neo-pentane was measured to assess the accuracy of the isothermal compression apparatus used. Theoretical predictions were found to be in good agreement with experimental measurements

Investigations of Metal/Organic Interfaces and Metalation Reactions of Organic Semiconductors

Modern electronic devices are increasingly based on organic semiconductors. The performance of such devices crucially depends on the properties of the interface between the organic semiconductors and the metal contacts. Understanding the influence of the topology of the organic semiconductor’s conjugated pi-electron system on the interface interaction could greatly improve the device’s performance. Furthermore, the knowledge about reactions of heteroatomic organic semiconductors with metal atoms during electrode fabrication may lead to enhanced lifetimes of such devices. This cumulative dissertation comprises several publications and a number of so far unpublished results, addressing metal/organic interface interactions and metalation reactions of heteroatomic organic semiconductors. The properties of the interfaces are tailored by investigating the alternant aromatic molecules naphthalene and pyrene as well as the nonalternant aromatic molecules azulene and azupyrene on different metallic singlecrystal surfaces. Investigations by means of temperature-programmed desorption reveal stronger desorption energies for the non-alternant molecules on both Ag(111) and Cu(111). The biggest difference is observed on Cu(111), on which azulene and azupyrene are chemisorbed, whereas naphthalene and pyrene are physisorbed. The enhanced interface interaction of the non-alternant molecules is associated with the formation of surface dipoles that lead to stronger intermolecular repulsion between the adsorbed molecules. These results are supported by additional surface science methods, such as X-ray photoelectron spectroscopy or near-edge X-ray absorption fine structure spectroscopy, as well as density functional theory calculations conducted by group members and external collaboration partners. Detailed quantitative analysis of temperature-programmed desorption data of benzene on Cu(111) and Ag(111) yields experimental desorption energies that can be used as a benchmark for theoretical adsorption energies derived by density functional theory calculations. The interactions of metal/organic interfaces are compared with organic/inorganic interfaces in the case of pentacene and its fluorinated derivative perfluoropentacene on Au(111) as well as on bulk and two-dimensional MoS2 in a collaboration project. Organic semiconductors often interact weakly with inorganic surfaces, e.g., the thermal desorption of the first molecular layer is indistinguishable from multilayer desorption. No monolayer desorption peaks are observed as is mostly the case on metal surfaces. However, monolayer desorption of pentacene and perfluoropentacene on MoS2 occurs at significantly higher temperatures than the multilayer desorption. Detailed analysis reveals that the monolayers of both molecules are entropically stabilized. Codeposition of both molecules results in strong attractive intermolecular interactions on MoS2, while these interactions are weaker on Au(111). Metalation reactions of organic semiconductors with metal atoms, e.g., Co on tetraphenylporphyrin and Ca on alpha-sexithiophene, during interface preparation were investigated by means of hard X-ray photoelectron spectroscopy and temperature-programmed desorption mass spectrometry. The thickness of the reaction zone is changed by variation of experimental properties during interface formation. It is found that only the sample temperature during metal atom deposition and the metal atom flux in the case of Ca have an impact on the reaction depth, which is usually limited to few nanometers. In contrast to Co and Ca, Li atoms readily diffuse into the organic bulk and react with tetraphenylporphyrin over several tens of nanometers, forming dilithium tetraphenylporphyrin or monolithium monohydrogen tetraphenylporphyrin depending on the deposited Li amount. Furthermore, the transmetalation reaction of lead(II) tetraphenylporphyrin with Cu atoms on the Cu(111) surface was proven by temperature-programmed desorption. In addition, the Ullmann coupling reaction of bromo- and iodobenzene on Cu(111) was examined. While bromobenzene molecules desorb intact from the Cu(111) surface, iodobenzene molecules dissociate into iodine atoms and phenyl radicals. The latter form biphenyl that desorbs in three distinct desorption peaks at different temperatures. In a collaborative project, the oxidation state and electronic structure of Pb atoms in the newly synthesized Pb3F8 were studied by hard X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy giving evidence for the presence of Pb(II) and Pb(IV) species. The experimental results are complemented by constructional work to improve the temperatureprogrammed desorption setup. Moreover, two Igor Pro 8 scripts were written to quickly import data from different experimental setups and speed up the data treatment