3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item An experimental and thermodynamic study of iron catalyst activation and deactivation during Fischer Tropsch synthesis(2016) Gorimbo, JoshuaOne gram amounts of a commercial iron based catalyst were loaded into three reactors and reduced with syngas, hydrogen and carbon monoxide respectively. Fischer Tropsch experiments on the three reactors in parallel with the same operating conditions, namely 60 mL(NTP)/min, 1 bar gauge and 250 °C, were then conducted for extended periods and the gaseous products analysed. Initially (for about 150 hours) the three catalysts had quite different carbon monoxide conversions. After this until about 1000 hours the conversions were similar. However the distribution of products for the differently reduced catalyst was significantly different. This suggested that permanent changes had been done to the catalysts by the different reducing conditions. To try to understand what the differences during the reduction process might be, a thermodynamic analysis of the solid phases after reduction was done. Unfortunately because all the thermodynamic data for the possible carbides was not available this analysis was of limited value. However it did suggest that hydrogen reduced catalyst might contain more oxides and the carbon monoxide reduced catalyst might contain more carbides. Some electron microscope and XRD experiments supported these ideas and might account for the different selectivities of the differently reduced catalysts. Runs after about 5000 hours were done at different flowrates (60, 30 and 15 mL(NTP)/min) of syngas and again the big effects were on differences between the selectivities, the big effects being when going to the lowest flowrate. After about 12000 hours regeneration of the catalysts was then done by oxidation and then the same syngas reduction on all the catalysts. Runs were then done at different pressures (1, 10 and 20 bar gauge) and again selectivities were the biggest effects that remained, clearly showing the initial reduction had made permanent changes. In the final section some novel plots were used to try to make more sense of the results. It was shown that for all the catalysts the Olefin to Paraffin ratios were tied to each other under all conditions and that they were mainly a function of the conversions with much higher values at low conversions.Item Effect of the homoionic form of clinoptilolite on Ni2+ adsorption isotherms: A thermodynamic study(2012-07-06) Gorimbo, JoshuaThis study investigates the effect of zeolite modification on Ni2+ removal from synthetic wastewaters. In this study sorptive removal of nickel ions from solutions containing 1-100mg/L using natural and pretreated clinoptilolite (Na+, K+, and Ca2+ homoionic forms) was investigated. The natural clinoptilolite (from Kwa-Zulu Natal, South Africa) was pretreated with 2M metal chlorides for 24 hrs to yield near homoionic Na, K, and Ca-forms. Experiments were carried out as a function of solute concentration and different zeolite masses. The equilibrium distribution was determined by contacting tared amounts of clinoptilolite (2g batches to 100mL of solution) with a known concentration of Ni2+ until equilibrium was reached. The natural and pretreated clinoptilolite samples were structurally studied using X-ray fluorescence (XRF), X-ray diffraction (XRD), BET, FTIR and scanning electron microscopy (FIB/SEM). The cation exchange capacity of this zeolite was determined to be 2.45×10-3 eq/g from XRF major elemental analysis. The equilibrium concentration of the solution was analysed for Ni2+ using Atomic Absorption Spectroscopy and results presented by plotting ion exchange isotherms. A comparison of the isotherms for the Na+-Ni2+, K+-Ni2+, Ca2+-Ni2+ and natural-Ni2+ systems gave us insight into how the displaced ion affects the selectivity of the clinoptilolite for Ni2+.The Na, K and natural forms presented highly selective convex isotherms whereas the Ca-form has a concave graph suggesting that the selectivity series is Ca2+> Ni2+> (Na+, K+, Natural). Fitting of the Langmuir and Freundlich isotherms to experimental data gave good fits, R2 values ranging from 0.9 – 0.99. Thermodynamic parameters revealed that the Ni2+ sorption capacity increases as the values of Keq and ΔG° increase with increasing temperature from 25 to 75°C. The ΔG° values were all negative except for calcium exchanged clinoptilolite at 25 and 35°C, where the ΔG° values were positive 3.098kJ/mol and 0.527kJ/mol respectively. ΔH° was positive for all forms of clinoptilolite and ranged from 18.72 to 42.05 kJ/mol, this provides an indication that the sorption reaction is endothermic for Ni(II). The values of ΔS° for Ni2+ sorption were positive and ranged from 0.08 to 0.14 kJ/mol.K. These positive values indicate increased randomness at the solid-solution interface during the adsorption of Ni2+ on clinoptilolite. As expected from theory, the enthalpy obtained from the Van‟t Hoff plot is dependent not only on the metal ion being adsorbed, but also on the ion being displaced.