3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item The effect of metal support interactions between a cobalt catalyst and carbon dots (Cdots) in cobalt fischer-tropsch catalysts(2018) Mokoloko, Lerato LydiaThe aim of this study was to introduce functionalized carbon dots (Cdots) as potential support and reducing agents for cobalt-based Fischer-Tropsch catalysts. Cdots with different physicochemical properties were synthesized from natural-based product such as ascorbic acid, sucrose and chitosan, using microwave-assisted reactions. Two types of Cdots, namely N-free Cdots (or Cdots) and N-doped Cdots, with particle sizes < 10 nm were produced. Both Cdots and N-doped Cdots have an amorphous structure, containing both sp2 and sp3 type carbon. The surface of Cdots contained mainly hydroxyl and carboxyl functional groups, while N-doped Cdots contained hydroxyl, carbonyl, and nitrogen-based (mainly pyridinic and pyrrolic) functional groups. Spinel Co3O4 nanoparticles with 10 nm crystallite sizes were synthesized from the “benzyl alcohol route”. Both the spinel Co3O4 nanoparticles and functionalized Cdots were used to prepare the inverse Co3O4/Cdots and Co3O4/N-doped Cdots catalysts. The in-situ XRD results showed that under H2 gas reduction conditions, the Co3O4/Cdots and Co3O4/N-doped Cdots catalysts increased the reduction temperature of Co3O4 to CoO and metallic Co, and Co (fcc) is the predominant form of metallic Co formed. However, when N-doped Cdots were used as both support and reducing agents, Co3O4 was reduced to both Co (fcc) and Co (hcp). The increase in reduction temperature of Co3O4 on Co3O4/Cdots and Co3O4/N-doped Cdots suggest the presence of strong metal to support interactions (SMSIs) between Co3O4 and functionalized Cdots. Further studies on the activity and selectivity of these prepared catalysts will be conducted under F-T reaction process.Item Modelling of a slurry bubble column reactor for Fischer-Tropsch synthesis(2018) Mamabolo, BotangThe slurry bubble column reactor (SBCR) is of particular interest in Fischer-Tropsch (FT) reactor modelling because of its importance to gas-to-liquids processes and the technical challenges it poses. Being one of the most important and complex Fischer-Tropsch Synthesis (FTS) systems in use today, there is a need to improve the current knowledge and understanding of the SBCR at a fundamental level, particularly the hydrodynamics of the process. Accordingly, a mathematical model of a SBCR has been developed in this work. The model is based on mass balances into which hydrodynamic, mass transfer and kinetic parameters have been incorporated. The hydrodynamic model considers two distinct phases in the SBCR, namely the gas and slurry phases with the liquid and solid phases treated as a single pseudo-homogenous phase. The gas phase in the reactor was assumed to exist in the form of distinctly large and small bubbles with each bubble class moving predominantly upwards through the center of the reactor and down near the wall respectively. Material balances were accordingly performed over three compartments including the slurry, large bubbles and small bubbles compartments. Axial dispersion was assumed in both the slurry and gas phases. The overall superficial gas velocity decrease along the axial direction was taken into account using an overall gas balance. Species material balances, hydrodynamics, kinetics and gas/liquid physicochemical property models were all coupled into a single SBCR model. The model was able to produce simulations capable of describing the fate of the reactant species, in the axial direction, in all three phases. Notably, the CO and H2 concentrations dropped by 62.01% and 64.13% respectively in the large bubble phase. A sensitivity study revealed the negative dependence of syngas conversion on the superficial gas velocity. A positive effect on the syngas conversion was evident with an increase in reactor diameter, i.e., an increase in diameter between 6 m and 7.8 m resulted in an increase in the syngas conversion between 38.3% and 90.78%. An increase in catalyst loading (0.28 to 0.38) resulted in a decrease in the syngas conversion (93.57% to 0.704%) due mainly to the overall decrease in the bubble hold-up. The comparison of the model results with those from literature was favorable with some noticeable discrepancies resulting from the inherent differences between the models.Item Osmium as a potential Fischer-Tropsch catalyst using hollow carbon spheres as support(2018) Tshepo, MolefeLittle attention has been given to the use of osmium as a catalyst or promoter for Fischer Tropsch synthesis (FTS). In this study the synthesis and use of osmium nanoparticles supported or encapsulated inside hollow carbon sphere (HCS) as a potential FT catalyst is reported. Os is not expected to be a good FT catalyst and this study provided for an evaluation of the role of Os on carbon as a catalyst in the FT reaction. The Os was encapsulated inside hollow carbon spheres (HCSs) as this will eventually allow for a study of spillover effects in FT reactions. The Os@HCS was prepared using sacrificial Stӧber silica spheres as a template. The SiO2 spheres were loaded with Os ((NH4)2OsCl6 metal precursor) via two methods: homogenous deposition precipitation (HDP) and wetness impregnation (WI). The Os/SiO2 was encapsulated by a carbon layer deposited by CVD using toluene. Removal of the sacrificial template gave Os@HCS (dOs = 11-17.5 nm; dHCS = 450-560 nm). Numerous physicochemical properties were used to reveal that the Os@HCS catalyst had been successfully synthesized. These included electron microscopy, PXRD, TGA, BET, and TPR studies. In the FTS evaluation, the Os@HCS and Os/SiO2 (as reference) showed low activity compared to conventional FT catalysts. The Os supported catalysts showed a product selectivity mainly to C1 and C2 hydrocarbons with high selectivity towards methane (> ~65%).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 Hydrogenation of carbon monoxide over modified cobalt-based catalysts(1991) Colley, Saul EricA disadvantage of the Fischer-Tropsch synthesis is that a broad product spectrum is obtained. Economic considerations however require an improvement in the optimization of the reaction to maximize the production of high value commercial products, in·particular, short chain olefins and high molecular weight hydrocarbons. [Abbreviated abstract. Open document to view full version]Item Synthesis and performance evaluation of Co/H-ZSM-5 bi-functional catalyst for Fischer-Tropsch Synthesis(2016) Matamela, KhuthadzoThe motivation behind this study is the need to manage and reduce wastes, in particular waste tyre and biomass, while in turn recovering energy from these carbonaceous materials. These wastes were gasified to produce synthetic gas which served as a feed to the Fischer-Tropsch Synthesis process to produce hydrocarbons. The formed hydrocarbons can be used as fuels for different purpose like transportation, domestic and industrial heating systems. Cobalt supported on zeolite catalysts are used because of their high acidic sites present in the zeolite that can break the Anderson-Schultz-Flory polymerization kinetics and also because cobalt-based catalysts are preferred for low temperature Fischer-Tropsch (LTFT) synthesis process due to their negligible water and carbon dioxide formation as well as stability and life span. In this research, a bi-functional Co/H-ZSM-5 catalyst was synthesized, characterized and evaluated for direct production of hydrocarbons at different process conditions. The bi-functional catalyst was prepared by incipient wetness impregnation method of an aqueous cobalt solution as the source of cobalt metal onto an H-ZSM-5 zeolite support, thereafter dried at 120 °C and calcined at 400 °C to obtain the finished Co/H-ZSM-5 catalyst. Physicochemical analyses performed included, Nitrogen Physisorption at 77 K to determine the surface area, pore volume and size of the synthesized catalyst. Also the N2 adsorption was used to determine the adsorptive properties of the catalyst. X-ray diffraction at 2θ region between 10 to 90 ° by using Co-Kα radiation (λ=1.79026 Å) was used to determine the material crystallinity, structure and composition. For the morphology and elemental composition of the catalyst, a Scanning Electron Microscopy coupled with an Energy Dispersive X-ray Spectroscopy was used. Thermal stability of the catalyst was checked using a Thermal Gravimetric Analyzer to determine how the catalyst degraded with time when temperature was increased uniformly. Reducibility of the catalyst was determined by using Temperature Programmed Reduction equipment in a hydrogen environment from room temperature to 900 °C. Transmission Electron Microscopy was used to check the catalyst morphology, and the dispersion of the metal-oxide particles within the catalyst support. The bi-functional zeolite supported catalyst was found to possess a surface area of 292 m2/g, pore volume of 0.18 cm3/g and pore size of 2.83 nm. The catalyst morphology was found to be irregular and aggregated-circular shape with a particle size of about 2.5 ± 0.5 μm. The embedded cobalt-oxide particles were obtained to be about 8 ± 3 nm located closer to the surface of the support and were reduced to metallic cobalt of 25% composition, at 330 °C in a hydrogen rich environment with an expected hydrogen consumption of 133 %. The process conditions under study involved flow rate, pressure and temperature and synthetic gas of different H2/CO ratio. The Synthetic gas mixture was purchased from Afrox and prepared in a way to mimic or simulate the syngas mixture expected from gasification of the waste tyre and biomass. However the study mainly focused on Hydrogen, Carbon Monoxide and Carbon dioxide as the dominant constituents of a waste tyre produced syngas. The bi-functional, Co/H-ZSM-5 performance evaluation was compared to commercial Co/SiO2 catalyst under similar conditions. The performance evaluation and comparison was made based on conversion and selectivity at different conditions. The process conditions considered were a flow rate of 1200, 2400 and 3600 GHSV (ml/gcat.hr), a pressure of 2, 8 and 15 bar, Low Temperature Fischer-Tropsch (LTFT) process at 220 and 250 °C was used, with a syngas composition that included H2/CO ratio of 1.5, 2.5 and 2.5 with 5 % of CO2 present in the reactant feed. The combination of 2 bar, 1200 GHSV and temperature of 220 °C and 1.5 of ratio was considered as low process conditions. While the combination of 15 bar, 1200 GHSV, 250 °C and ratio of 2.5 was considered as high process condition. Three pre-calibrated GCs (two online and one offline) were used to analyze the reaction products and the feed and the integrated peak-data analyses was captured by the use of a Data Apex Chromatograph software package known as Clarity ® (v. 2.5). The captured and analyzed data was used to calculate conversion and selectivity according to the methods reported in literature. With regard to the effect of process conditions, at low process conditions, the bi-functional catalyst, Co/H-ZSM-5, resulted in a 3 % CO conversion, while the commercial Co/SiO2 catalyst, resulted in 15 % of CO conversion. However the bi-functional catalyst was more selective to gasoline range products and 16 % selectivity to C5 hydrocarbons was obtained and 79 % to C6+, as compared to selectivities of 4 and 75 % for C5 and C6+ respectively, for Co/SiO2 catalyst. Also Co/SiO2 was found to be more selective to Olefins, the undesired products, with a selectivity of about 91 % to C6+ hydrocarbons as compared to a selectivity of 87 % for C6+ hydrocarbon obtained by using the bi-functional Co/H-ZSM-5 catalyst. Methane production was high for the Co/SiO2 catalyzed reaction, (about 13 % selectivity) with some quantity of water produced, as compared to 3 % methane selectivity for Co/H-ZSM-5 catalyst with no water produced during the reaction. At low process condition, both catalysts were less prone to middle distillates hydrocarbon production. At high process conditions, a CO conversion of about 54 and 68 % was obtained by Co/H-ZSM-5 and Co/SiO2 catalyst respectively. At these conditions the H-ZSM-5 supported catalyst was observed to produce more methane, about 53 % selectivity while for Co/SiO2 catalyst it was obtained to be 35 % selective to methane, with 66 and 7 % of C6+ olefin and paraffin selectivity respectively. Co/H-ZSM-5 offered 9 % selectivity to C6+ per olefin and paraffin hydrocarbons. The commercial catalyst showed an orderly manner of distributing products at these conditions while the bi-functional catalyst randomly distributed the formed products with a high selectivity to middle olefin distillates. In terms of CO2 co-feeding in the reactant feed, both CO and CO2 were hydrogenated to hydrocarbons. A CO conversion of about 73 % was obtained by Co/H-ZSM-5 catalyzed reaction while for Co/SiO2 catalyzed reaction a conversion of 70 % was obtained. About 63 and 75 % of CO2 conversion was obtained by H-ZSM-5 and SiO2 supported catalyst. These results were obtained at high process conditions. No change in paraffin selectivity was observed when comparing a state in which CO2 was present and absent, however olefin selectivity is significantly affected by the presence of CO2. Thus, an increase in olefin selectivity is observed with Co/SiO2, achieving 76 % of C6+ Olefin from 66 % and Co/H-ZSM-5 increasing middle olefin distillated from 25 to about 30 % of selectivity. Based on the performance evaluation the bi-functional catalyst was proven to yield higher hydrocarbons from a simulated waste-tyre synthetic gas with no requirement of downstream hydrocracking, since the bi-functional catalyst cut-off higher hydrocarbons due to its acidic sites. While the metallic sites of the catalyst, catalyzes the reaction of synthetic gas to hydrocarbons. This type of catalyst with both metallic sites and acidic sites is a hybrid-catalyst commonly known as bi-functional catalyst (Kang et al., 2014). At low process conditions the bi-functional Co/H-ZSM-5 catalyst is found to be more preferred while at higher process conditions the commercial catalyst was found to be more preferred, however in the presence of CO2 co-feeding, either catalyst can be used, but if water elimination is required the bi-functional catalyst is more suitable for the process.Item Spherical carbons as model supports for Fe, Co and Fe-Co Fischer-Tropsch catalysts(2016) Dlamini, Mbongiseni WilliamThe production of liquid transportation fuels and chemicals by the Fischer-Tropsch (FT) synthesis continues to garner attention due to its economic and environmental benefits. This interest is also compounded by the flexibility to use readily available materials as feedstocks for synthesis gas production, with coal, natural gas, biomass and recently shale gas being used. Although this process is over 90 years old, challenges still remain. In this study, we have attempted to understand several FT synthesis challenges by exploring the use of carbon spheres as a model support for Co, Fe and Fe-Co FT catalysts. Thus the synthesis, characterization and application of carbon spheres with distinct architectures are described. The synthesis of solid carbon spheres using a sucrose precursor yielded materials that were mono-dispersed (600 nm) and adopted a necklace-like accreted conformation. Upon further investigation, it was demonstrated that annealing is useful for tuning the properties of the as-prepared materials to have high surface areas (> 500 m2/g), good thermal stability (>660 °C) and a mesoporous (> 2 nm) pore structure. Deposition of a Fe-Co bimetallic catalyst yielded oxides of the monometallic species with relatively small crystallites, with sizes in the range 7.9 – 14.4 nm. Reduction of the bimetallic samples was monitored by using in situ PXRD and TPR techniques, which revealed that a Co-Fe type-alloy is one of the phases formed on Co-rich samples at T > 450 °C. Interestingly, high relative abundances of this alloy did not correlate with high C5+ selectivities in Fischer-Tropsch synthesis; instead Co-rich/Fe-poor catalysts gave the best selectivity. The effect of the support morphology in heterogeneous catalysis was investigated by using high surface area solid and hollow carbon spheres (>560 m2/g) prepared from a resorcinol-formaldehyde precursor as support material. Loading the Co and Fe precursors on these two supports was shown by TEM and PXRD to result in smaller and well dispersed metal particles on the hollow support material. This corresponded with high activities and C5+ selectivities for the Co and Fe catalysts supported on the hollow carbon spheres. TEM studies revealed that the Co and Fe particles tended to sinter significantly when dispersed on a material with a solid architecture. iv Post-synthesis N-doping using a melamine precursor was shown by XPS to incorporate high quantities of nitrogen (up to 13%) on to the surface of the 30 nm thick shells of the hollow carbon spheres. On further investigation, N-doping by this method was shown to have minimal effects on the thermal stability and crystallinity of the materials. The N-doped HCSs were shown to be good anchors of Co particles as displayed by the good dispersion, activity and minimal sintering tendency of catalysts supported on N-doped HCSs. Studies conducted herein have demonstrated the versatility of carbon spheres as a model support, and how their properties can be tailored to suit the desired specifications by simply adjusting the synthesis parameters. We have also highlighted how the chemical inertness of these materials allows for studies on metal-metal interactions at elevated temperatures for bimetallic catalyst systems. The monodisperse, morphology-tunable aspects of carbon spheres were particularly useful in modelling the effect of the support morphology in Fischer-Tropsch synthesis. It is believed that the versatility of CSs demonstrated in this study can also be exploited in other heterogeneous catalytic systems.Item Study of the selectivity to light hydrocarbons in Fischer-Tropsch synthesis(2016) Muleja, Adolph AngaMany reports in the open literature have focused on Fischer-Tropsch (FT) kinetics, yet none of them appear to be able to explain FTS completely. Few of the FT models consider the production of olefins and paraffins separately. To study whether the selectivity to olefins and paraffins follows similar trends and if kinetics alone suffices to explain FT phenomena, a series of FT experiments were conducted in a fixed bed reactor loaded with 10% Co/TiO2. FT feeds were periodically switched from syngas to syngas + N2 by adjusting the total reactor pressure so that the reactant partial pressures (PCO and PH2) remained constant. During the initial deactivation (the first 1200 hours), it was found that the formation rates of olefins remained fairly constant (in some cases they increased) while those of paraffins decreased. This indicates the deactivation is mainly caused by the decrease in the paraffin formation rate. Currently, none of the published kinetic models can explain the phenomenon that the decay of the reaction rates of olefins and paraffins were not the same during the deactivation. At steady state (1055 to 2700 hours, overall reaction rate fairly constant), adding extra N2 decreased the selectivity to the light hydrocarbons. These results suggest that by feeding the extra N2 there could be an increase in selectivity and formation rates to long chain hydrocarbons (C5+). Plotting molar ratios of paraffin to olefin (P/O) with carbon number n+1 versus the ratio with carbon number n revealed linear relationships which are independent of feed gases, catalyst activity and reaction temperature. These results imply that product distributions might be determined by some sort of equilibrium. Another plot of normalised mole fractions of CnH2n, Cn+1H2n+2, and CnH2n+2 in ternary diagrams showed that after disturbances these product distributions tended to stable points. It is suggested that this could be due to slow changes in the liquid composition after the disturbances. Although not all the results are explained, the researcher emphasises that normal kinetics alone cannot explain these results completely. There might be factors, iii including vapour-liquid equilibrium or reactive distillation, which are worthy of consideration to explain FTS.Item The effect of low level sulfide addition and the performance of precipitated- iron Fischer-Tropsch catalysts(2016-08-31) Bromfield, Tracy CarolynPrecipitated-iron Fischer- Tropsch catalysts were sulfided in the range 500 - 20000 ppm S/Fe with an aqueous sulfide source (Na2S, (NB4)zS, (NB4)zS5) during the precipitation process. Sulfidation was performed at pH 10.75, 8.5 and 6.9. Sodium ions were removed by centrifugation, and atomic absorption analysis confirmed low sodium levels (0-51 ppm). Based on solution speciation models, ferrous sulfide (FeS) which formed from aqueous HS' species, was found to influence the iron-oxyhydroxide crystallite morphology. It is proposed that, when sulfide was added at pH 10.75, FeS molecules functioned as nuclei for crystallite growth, while a pH 6.9 they assisted 'with the aggregation of particles. The processes of nucleation and aggregation appeared to be in competition following sulfidation at pH 8.5, resulting in a composite morphology that produced an inactive catalyst. The bulk structure of the catalysts was elucidated using XRD, SEM and nitrogen porosimetry, All sulfided catalysts exhibited enhanced BET surface areas and total pore volumes with a maximum at 2000 ppm S (surface area = 166 m2/g,total pore volume = 0.254 cm3/g) compared to an unsulfided catalyst (surface area = 58 m2/g, total pore volume = 0.184 cm3/g), Furthermore, for any series of catalysts at the same level of sulfidation, the BET surface areas were observed to decrease as the pH of sulfide addition decreased. Increasing levels of sulfidation (to 20000 ppm) brought about an increase in crystallite size and therefore, improved crystallinity as determined by XRD measurements. Materials with larger crystallites possess smaller surface areas, and thus the crystallinity was found to increase as the pH of sulfidation decreased. Surface characterisation by XPS after calcination at 400°C and reduction (400°C), revealed sulfate species (169.4 eV) on catalysts sulfided with 500-2000 ppm, while sulfide species (162.O eV) emerged at higher sulfide content. No sulfates were observed on reduced catalysts following calcination at 200 C. [Abbreviated Abstract. Open document to view full version]Item The effect of microwave heating on manganese promoted iron based Fischer-Tropsch catalysts(2012-01-18) Mohiuddin, EbrahimA study was performed in order to investigate the effect of preparation method and the effect of microwave heating on a manganese promoted iron based Fischer-Tropsch catalyst. The effects of preparation method and microwave heating on the structure and morphology of the catalyst, its surface area and reduction behavior were investigated using various techniques such as Transmission electron microscopy (TEM), Powder x-ray diffraction (PXRD), surface area measurements (BET) and temperature programmed reduction (TPR). The FTS performance of the catalysts were also studied using a fixed bed reactor with Fischer-Tropsch Synthesis conditions (270 C, flow rate of 30 ml/min, H2/CO ratio = 2, pressure of 10 bar). Characterization of the catalysts calcined at 350 C revealed that manganese enriched the surface of impregnated Mn/Fe catalysts and suppressed the reduction of the iron catalyst. However, the Mn acted as a structural promoter in the co-precipitated catalysts and also promoted the reduction of Fe2O3 as the manganese content increased. The co-precipitated catalyst calcined at 650 C suppressed the reduction of iron. The impregnated catalysts showed similar conversion (~ 70%) for catalysts with Mn loadings 5%, 10% and 20%. This suggests Mn promotes the activity of the iron catalyst since less iron is present in the catalyst as the manganese loading is increased. The co-precipitated catalysts showed a 10 wt% Mn loading to be the optimum amount for increased activity and selectivity to C2 – C4 hydrocarbons, lower molecular weight olefins and a lower selectivity to heavier molecular weight hydrocarbons relative to Mn loadings of 5, 20 and 50 wt%. Mn loadings in excess of 10 wt% showed a slight increase in selectivity to heavier weight hydrocarbons. The impregnated catalysts showed very little difference in activity and selectivity but the co-precipitated catalyst showed a decrease in activity after the catalyst was microwave heated. A slight increase in selectivity to lower weight olefins and heavier molecular weight hydrocarbons was noted after microwave heating. The TPSR (Temperature programmed surface reaction) results revealed that this may be due to the stronger adsorption of CO on the surface of the catalyst after microwave heating. A similar trend was observed for catalysts promoted with 0.1 wt% potassium i.e. a slight increase in selectivity to heavier weight hydrocarbons after microwave heating.