Browsing by Author "Fletcher, Jack C Q"

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    Open Access
    Gas phase heterogeneous catalyst performance testing in laboratory fixed-bed reactors
    (2019) Kruger, Dawid; Fletcher, Jack C Q; Roberts, Stephen J
    Activity, selectivity and stability are invariably among the key factors of the performance of a catalyst. In the development of catalysts these properties are often screened for a range of materials and formulations. Interpretation of these key performance indicators are prone to various confounding effects. Here, performance testing of solid, porous catalysts for gas phase reactions in tubular fixed bed reactors is considered. Transport limitations and particularly internal mass transfer limitations are often cited in this case. Many have given general discussions and guides for effective catalyst performance testing, reviewed or put forward theoretical descriptions for transport phenomena and have measured and correlated associated transport coefficients. Some quantitative requirements and the relative importance of different effects have been found to remain unclear. Here, some of these aspects are addressed by the development of 3 catalyst testing criteria. Specifically, an upper limit is derived for the chemical conversion in a firstorder reaction such that differential rate conditions are established, a lower limit on the chemical conversion is applied to limit the loss of precision in conversion measurements, and an expression is derived to limit the effect of pressure drop across a catalyst bed on the observed rate of a first-order reaction. The prevalence and sensitivity of these and other transport limitation criteria were investigated theoretically in the context of the low-temperature (LT) water-gas shift (WGS) reaction over a Cu/ZnO/Al2O3 catalyst in a laboratory scale performance test. Factorial combination of some commonly manipulated experimental parameters (reactant feed rate, temperature, catalyst particle size, catalyst loading, dilution fraction and reactor tube size) was employed in this regard. The upper conversion limit, the internal mass transfer criterion and the radial heat transfer criterion were found to be particularly severe. So too, to a lesser extent, were the axial dispersion and pressure drop criteria, and the lower conversion limit. The sensitivity analysis indicated optima in the varied experimental parameters and yielded insights into effective control of different effects by selection of process conditions. Application of the set of criteria in an experimental performance test was demonstrated using a proprietary medium-temperature (MT), WGS catalyst under reaction at temperatures of 275 °C, 300 °C and 375 °C, 1 atm total pressure, dry feed composition of 10% CO, 10% CO2, 70% H2, 10% N2, steam-to-dry gas ratio of 0.5 and 158 h-1 weight hourly space velocity (WHSV). The catalyst was found to have near total selectivity towards the WGS reaction with activities of 12.2 ± 1.1, 17.1 ± 0.5 and 24.9 ± 1.5 µmol/s.gcat at 275 °C, 300 °C and 375 °C respectively. This corresponds to an activation energy of 39 ± 2 kJ/mol; a value within range of what is reported in literature for similar catalysts. This experiment also served to compare experimental and predicted internal mass transfer limitations by testing catalyst particles of different mean sizes. This catalyst as well as a CuO/ZnO/Al2O3 catalyst precursor was characterised in respect of their pore size distributions (N2 physisorption and mercury intrusion porosimetry (MIP)), particle size distributions (by photo- and microscopic analysis), bulk and particle densities and product gas compositions (by gas chromatography) to enable evaluation of the various criteria employed. Evaluation of the various criteria indicated that, theoretically, the considered confounding effects had a negligible effect on the measured catalytic activities for the catalyst sample with the smallest mean particle size, while the larger particles experienced only internal mass transfer limitations. Different models considered for effective diffusivities all under-predicted values when compared to the effective diffusivities inferred from the reaction-diffusion experiments. Predictions ranged to within factors of 3 – 20 of the experimental values, depending on whether pore size distribution data were derived from MIP or physisorption data. Here, the lack of characterisation of the macro-porosity by N2 physisorption resulted in more severe under-estimations of the effective diffusivities than the equivalent estimations made with MIP data. The best prediction was made by the ‘parallel-path pore’ model by Johnson & Stewart (1965) using MIP data. Predictions of internal mass transfer limitations varied in a similar manner. It is noted that the simplifications of the highly complex porous catalyst by these model combinations introduce large sources of error in the prediction of internal mass transfer limitations.
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    Open Access
    III-V OMVPE growth and characterisation of graded AlₓGaₓ₋₁As-GaAs layers and heterointerfaces for the development of GRIN-SCH lasers and Gunn diodes
    (1994) Thavar, Rajan; Fletcher, Jack C Q
    AlGaAs-GaAs graded index single confinement heterostructure single quantum well (GRIN-SCH SQW) lasers and both 35 GHz and 94 GHz Gunn diodes have been satisfactorily grown by organometallic vapour phase epitaxy (OMVPE). This work is based on the material development of such device structures and systematically shows the steps taken to achieve the final goal of repeatedly producing high quality devices geared towards small-scale production. The key elements of the process are the realisation of high quality AlGaAs compositionally graded layers, abrupt GaAs-AlGaAs as well as dopant heterointerfaces and silicon-dopant spikes. A consistently high quality of epitaxial GaAs and AlGaAs is achievable with controllable silicon, tellurium and zinc doping on both material systems. The OMVPE system is sufficiently calibrated to grow sharp transitions in GaAs doping interfaces and quantum wells in the order of 2.5 nm. SIMS measurements showed almost square doping profiles in the 35 GHz Gunn diode structure and was able to resolve a 5 nm Si-spike doping layer in GaAs. The SIMS results of the 94 GHz Gunn diode material clearly indicates the presence of all layers with certain measured values evolving exactly as designed. These achievements are attributed to the fine pressure and flow control implemented on the reactor system by using automated steps to control the growth process.
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    Kinetic models for the Pt/CeO₂ catalysed water-gas shift reaction
    (2018) Brown, Darryl Edward; Fletcher, Jack C Q
    As the global population grows, so does the world's demand for energy. Consequently, there exists an increased interest in the development of fuel cells for power generation due to their low greenhouse gas emissions. For fuel cells to be a successful power source, a reliable hydrogen source is required. Ultimately, the goal is for hydrogen to be supplied from renewable energy technology however, this type of technology is currently not mature enough to meet the continuous demand of the world's energy systems. Producing hydrogen from fossil fuels can be seen as a temporary solution while further advances are made in developing renewable hydrogen infrastructure. A fuel processing train, therefore, remains an important alternative to producing hydrogen. A fuel processing train converts fossil fuels into hydrogen for use in fuel cells and eliminates the need for hydrogen storage as hydrogen is produced on demand. Currently, the water-gas shift (WGS) reactor is one of the largest components in a fuel processing train and thus opportunity exists to reduce the size of this reactor. To design future WGS catalysts and an optimised fuel processor, the reaction kinetics taking place must be understood and quantified. In this study, kinetic measurements were conducted at 2 bar(a) and across a temperature range of 270 - 300 °C using 16 parallel fixed bed reactors (high throughput experimentation) over a 0.5 wt% Pt/CeO₂ catalyst. The feed composition was varied over the ranges 2 - 12 mol% CO, 20 - 45 mol% H₂O, 4 - 15 mol% CO₂ and 25 - 55 mol% H₂. An online micro gas chromatograph (μGC) was used to analyse the dry gas composition. Fitting of experimental data to various kinetic models was accomplished with the gPROMS software package. An initial evaluation of several Langmuir-Hinshelwood (LH) type mechanisms to two data sets obtained from literature was undertaken to evaluate the strengths and weaknesses of different kinetic expressions. The results of the initial evaluation indicate that a dual-site mechanism with an intermediate species results in the best fit for reducible supports, while a single site mechanism offers a better fit for non-reducible supports. For both kinetic models, the formation of the intermediate species is most likely to be the rate determining step. A power-rate law empirical rate expression and a LH type rate expression were both found to predict the WGS outlet composition well within 10 % error at 2bar(a). The apparent activation energy of the reaction was determined to be 110 kJ/mol. This value was confirmed to be constant, throughout the range of conditions evaluated, by means of a classical Arrhenius analysis. Simulations of increasing total system pressure, using both the empirical and "best fitting" LH model, indicate a significant pressure effect for the LH type equation, whereas the power-rate law empirical equation predicts a small, negative effect on the reaction rate with increaseing pressure. Consequently, further experiments were conducted to determine the true effect of pressure. It was found that increasing system pressure increased the WGS reaction rate, which has also been reported by Twigg (1989:288). Only the LH type rate expression was able to predict this. It is therefore recommended that either the power-rate law empirical rate expression or the LH type rate expression be used to predict the WGS outlet composition when operating below 2 bar(a). Furthermore, when predicting reaction rates outside of the window in which the rate equations were derived, it is recommended that the LH model be used as it is expected to give a better prediction as it is based on fundamental steps.
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    Monolith Catalyst Supports for Hydrogen Production
    (2021) Mzolo, Ntokozo; Fletcher, Jack C Q; Roberts, Stephen J
    Due to increasing energy demands and environmental concerns, more research studies arc conducted on alternative energy sources. Hydrogen fuel cells arc one of the desired alternatives. The CCT Catalysis Institute has demonstrated the entire hydrogen production process from propane (LPG) reforming. This is aimed for off-grid and backup pOver supply to rural areas and telecommunication towers respectively. The steam reforming unit employs ruthenium supported on gamma-alumina (Ru/ -AbOo) catalyst.. However, it has been observed that. upon increasing the sea.le, pressure-drop and mass transfer limitations become significant issues. It is, therefore, desired to shift from packed bed reactors to monolithic reactors to overcome these issues. :Vfonoliths arc attractive due to their high open frontal area., stability, good mass and heat transfer, as well as easy sea.ling up. The main focus of this study is to coat. monoliths ,vith a. homogeneous and stable layer of Ru/ AbO3. Viscosity, pH and slurry solid content are investigated as these are the main factors affecting washcoat loading, morphology and adherence. A slurry coating technique was used to deposit. the alumina. layer onto t he monolit hs which were then subjected to ult.ra.sonica.t ion and thermal shock treatment. to test. for adherence. The active phase ,vas deposited by ion exchange and dry impregnation of the alumina coated monoliths by making use of a RuCb.xH"O solution. SEivI was employed to analyse the morphology of the layer, as well as its metal content and distribution. An experimental design soft,vare, Design Expert. was used to generate statistically significant and robust models for the results. It was found that washcoat loading, stability and morphology arc highly dependent on slurry solid content betvveen 20 and 45 v,rt.%, slightly affected by viscosity bet,veen 20 and 45 mPa.s and almost insensitive to pH beluw isoelectric point. Increasing slurry solid content results in an increase in washcoat loading which, in turn, results in thick and inhomogeneous layers which, in turn, arc more prone to cracking and to lo,v mcclmnical and thermal stability. The optimal point. that. gives a. st.able, homogeneous layer a.t the desired loading was identified to be at pH of 4, viscosity of 20 mPa.s and slurry solid content of 20 wt%. The resulting washcoat. layer is 'crack-free', uniform, and characterised by a washcoat loading of 10.8 wt%, an average thickness of 30 m, and mechanical and thermal weight losses of 0.79 and 1.42 wt%., respectively. The ion exchange techniques resulted in small and poorly distributed metal loading while dry impregnation resulted in sufficient metal loading and good distribution throughout t.he monolith channels.
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    Synthesis, characterization and high pressure propene oligomerization over heteropoly acids, SAPO-11 molecular sieves and MeAPSO-11 molecular sieves
    (1993) Vaughan, James Stuart; O'Connor, Cyril T; Fletcher, Jack C Q
    Heteropoly acids, HPAs, and SAPO-11 and MeAPSO-11 molecular sieves have been proposed to be active for the oligomerization of propene to high quality distillate fuels. In this study these catalysts have been synthesized, characterized, modified and screened for the oligomerization of propene to distillate fuels. Heteropoly acids consist of high molecular weight poly-oxoanions, counter cations and water of hydration and are strongly acidic. The following salts of 12-tungstophosphoric acid, a 12-type HPA with the Keggin anion structure, were synthesized: AI, Fe, Ce, Ni, Co, Cu, K and NH₄, as well as the AI and NH₄ salts of 12-tungstosilicic acid. Physicochemical characterization of the HPA salts revealed that, depending on the counter cation, these salts exhibited different thermal characteristics and surface areas. The salts were classified into two types, viz.: Type A:- low surface area salts, typically less than 15m²/g, with multiple endothermic mass losses (e.g. AI, Fe, Ce, Ni, Co, Cu) and; Type 8:- high surface area salts, typically 35-150 m²/g, with a single endothermic mass loss (e.g. K and NH₄). The Type A salts were multi-crystalline, consisting of more than one phase, and exhibited a non-uniform morphology. The Type B salts, on the other hand, were uniformly crystalline and consisted of small rounded cubic crystallites. Ammonia desorption, monitored by both TCD and FT-IR, revealed that both the Type A and B salts were strongly acidic with the majority of the ammonia being released only upon decomposition of the salt. However, as ammonia and other polar molecules are known to readily absorb into the bulk of HPAs, the ammonia TPD results did not correlate with catalytic activity for surface type reactions such as alkene isomerization and oligomerization. The Type B salts were active for cracking and skeletal isomerization reactions which suggested strong acid sites were present on their surface. The Type A salts only exhibited double bond shift isomerization activity, suggesting sites of only weak to medium acid strength.