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Subject: search.filters.subject.Fischer-Tropsch

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Now showing 1 - 5 of 5
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    Graphical methods for the representation of the Fischer-Tropsch reaction: towards understanding the mixed iron-cobalt catalyst systems
    (2011-04-14) Musanda Mukenz, Thierry
    Fischer-Tropsch is a process that converts synthesis gas (especially H2 and CO) into hydrocarbons by the mean of metal catalysts (such as Fe, Co, Ru, and Ni). Its success depends strongly on the catalyst used for the reaction, the reactor where the reaction is taking place, and some parameters such as the operating temperature, the reactor pressure, and the gas purity, composition (ratio H2:CO) and flow rate. Besides the above parameters, other factors, such as the degree of reduction of the catalyst, also play an important role for a successful FT reaction. Water can deactivate (by re-oxidation) the catalyst and carbon deposit can reduce the catalyst’s activity. It is well known that FT is a complex reaction because of the range of products that it produces as well as the reactions that occur during the process. A good choice or combination of catalysts, reactor and operating conditions can help to control the product spectrum. 2 In this thesis we develop a simple graphical technique to represent the mass, energy balance and thermodynamic constraints that affect both the catalyst and the reactor. This graphic model is shown to be capable of opening up insights into reactor operations and indicating preferred operational regions. The diagrams make it possible to visualize operations and understand the interactions between the catalysts and the reactor. The mass and energy balances also provide information about the best possible region in which the FT reactor system can be designed and operated. A few catalysts (Fe/TiO2, Co/TiO2 and Fe:Co/TiO2) were prepared for the completion of this work. Some of them were tested separately and others were mixed in the same reactor. The results showed that the physical mixture (of Fe/TiO2 and Co/TiO2) and bimetallic catalysts behave differently from one another. The addition of Fe Fe/TiO2 to a constant amount of Co/TiO2 results in an increase of CO hydrogenation activity, WGS activity and CH4 selectivity. However, the position of the two catalysts in the reactor (one followed by another) shows little effect on the rate of hydrogenation of CO and the CO conversion.
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    Fischer Tropsch synthesis over supported cobalt catalysts: effect of ethanol addition, precursors and gold doping
    (2008-06-06T10:17:09Z) Jalama, Kalala
    The effect of the addition of ethanol (2% and 6%) during the Fischer-Tröpsch (FT) synthesis has been investigated using a 10%Co/TiO2 catalyst in a stirred basket reactor (T = 220°C, P = 8 bar, H2/CO = 2). The transformation of ethanol vapour (2% and 6% in nitrogen) over the Co/TiO2 catalyst was also studied in the absence of the synthesis gas under FT reaction conditions. Ethanol was observed to be incorporated in the growing chain and was found to (i) increase the selectivity to light products, (ii) increase the olefin to paraffin ratio and (iii) significantly decrease the catalyst activity. These effects were almost completely reversed when the ethanol in the feed was removed. Thermodynamic predictions, TPR and XRD analysis have shown that cobalt metal particles were oxidised to CoO by ethanol but that re-reduction to Co metal was possible when ethanol was removed from the feed stream allowing the catalyst to recover most of its initial performance, in particular when high flow rates were used. The effect of the cobalt carboxylate chain length (C2, C5 and C9) used in the preparation of alumina supported cobalt catalysts has been studied by TPR, XRD and hydrogen chemisorption techniques. The activity and selectivity of the prepared catalysts have been evaluated for the Fischer-Tröpsch (FT) reaction in a stirred basket reactor. It is shown that for catalysts with Co content of 10 wt.% the activity increases as the carboxylate chain length increases while the selectivity towards methane and light hydrocarbons decreases with the carboxylate chain length. The catalyst prepared using cobalt acetate was found to present the highest metal-support interaction and the poorest performance for the Fischer-Tröpsch reaction. When the metal content was increased to 15 wt.% Co and 20 wt.% Co respectively, the metal-support interaction for the catalyst prepared from cobalt acetate significantly decreased making it a better catalyst for the FT reaction compared to the catalysts prepared from C5 and C9 cobalt carboxylates. The effect of the addition of Au to a Co FT catalyst supported on titania, alumina and silica respectively, has been investigated by varying the amount of Au (0.2 to 5 wt.%) added to the catalyst. The catalysts were characterized by atomic absorption spectroscopy, XRD, XPS and TPR analysis. The catalyst evaluation for the Fischer- Tröpsch reaction activity and selectivity was achieved in a fixed bed micro-reactor (H2:CO = 2; 20 bar; 220°C). Addition of Au to supported Co catalysts improved the catalyst reduction and the cobalt dispersion on the catalyst surface. The catalyst activity for the FT reaction and the methane and light product selectivity increased with Au loading in the catalyst.
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    Synthesis and use of carbon nanotubes as a support for the Fischer-Tropsch Synthesis.
    (2008-02-29T10:56:20Z) Bahome, Munga Christian
    Abstract Carbon nanotubes (CNTs) were grown catalytically by a chemical vapor deposition method and characterized by a range of techniques. Fe, Ru and Co catalysts supported on the carbon nanotubes were prepared and investigated for their performances in the Fischer-Tropsch synthesis. CNTs were synthesized in a quartz tubular reactor at atmospheric pressure and at temperatures of 700°C over iron supported on CaCO3 using C2H2 as carbon source. Prior to CNT synthesis, the iron catalyst was first reduced under the same conditions (700°C and atmospheric pressure) in a flow of 5% H2 balanced in Argon. The catalyst, for the preparation of the CNTs, was prepared by the incipient wetness impregnation. The purification of the CNTs was performed with 30 wt % HNO3. Characterization of the CNTs using TEM, SEM, HRTEM, BET and TPR revealed that the crude product contained solely CNTs, catalysts particles and support, while no amorphous carbon was observed. The purified product is comprised of an interwoven matrix of tubes that were shown to be multi-walled (MWCNTs). CNT supported FT based catalysts were also prepared by an incipient wetness impregnation method and tested in a plug flow reactor in Fischer-Tropsch synthesis. The TEM images of the different FT catalysts supported on CNTs revealed that the catalyst particles are well dispersed on the surface of the CNTs. The catalyst particles were very iii small, and some residual Fe catalyst material, not removed by the acid treatment, could clearly be seen on the surface of the CNTs. The reduction and metal dispersion properties of the catalysts were investigated through TPR and chemisorption techniques. A TPR study showed three reduction steps for Co catalysts, and addition of Ru to the catalyst decreased the reduction temperature of the catalysts. Gasification of the CNTs was noted to occur at temperatures higher than 600°C. The effect of metal catalyst loading and promoters on the activity and selectivity of CNT supported FT synthesis catalysts was studied under condition of 275°C, 8 bar, CO/H2 = 1/2 and different flow rates. The FT catalysts supported on carbon nanotubes displayed a high CO conversion and excellent stability with time on stream in the Fischer-Tropsch synthesis. Fe catalysts displayed the lowest methane selectivity compared to all other FT synthesis catalysts used in this study.
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    Applications of mesostructured carbonaceous materials as supports for fischer-tropsch metal catalyst
    (2007-02-21T12:32:55Z) Mbileni, Charity Nonkululeko
    Mesoporous MCM-48 was synthesized and used as a template to synthesize mesoporous carbon (MC) materials. Polystyrene, the carbon source, together with sulfuric acid and toluene were added to the template (160 oC for 6 h) and this procedure generated a low surface area carbon supported/MCM-48 material. A repeat addition and carbonization step was needed to form the precursor carbon/MCM-48 material that was pyrolysed at 900 oC to generate graphitic mesoporous carbon materials. After removal of the silica template, mesoporous carbons were characterized by XRD, HR-TEM, Raman spectroscopy and surface area analysis. The effect of the amount of polystyrene as well as the role of the pyrolysis temperature on the final product was investigated. This synthesis methodology can readily be controlled to produce partially ordered graphitic mesoporous carbon supports with predictable pore width and surface area. The methane selectivity was low (below 6%) and stable, and the overall olefin fraction was found to be good for all the supported catalysts studied. The potassium promoter increased the hydrocarbon chain growth to C68 giving α-1 and α-2 both between 0.79 and 0.90 for all supported catalysts with an exception of MCM-48 supported Fe catalyst that selectively produced hydrocarbons up to C28.
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    Synthesis and use of Silica materials as supports for the Fischer-Tropsch reaction
    (2006-11-17T09:30:54Z) Mokoena, Emma Magdeline
    The objective of the study was to prepare novel silica materials and then use them as supports/binders for the Fisher-Tropsch (F-T) reaction. Hence the thesis is divided into two parts - (i) the synthesis of silica materials (ii) use of silica materials as supports. PART I The studies that were carried out in this thesis evaluated the effect of templates and synthesis conditions on the nano- and microstructure and properties of silica materials that are obtained by the sol-gel method. The studies with DL-tartaric acid and citric acid as templates revealed that synthesis conditions (temperature, NH4OH concentration, water/ethanol concentration, time before NH4OH addition, static versus stirred conditions, stirring rate and solvent) all have an effect on the microstructure of the silica and influence the formation of particular silica morphologies. DL-tartaric acid produced longer and more uniform tubes when compared to citric acid. Tubes that are formed by DL-tartaric acid are hollow and open ended; however the ones formed in citric acid are a mixture of filled and hollow but closed tubes. Hollow spheres are exclusively formed when citric acid is used under certain conditions while only filled spheres are formed when DL-tartaric iii acid is used. The surface areas of the silicas formed from DL-tartaric acid are lower that the surface areas obtained for materials produced by citric acid. The nitrogen adsorption-desorption isotherms of silica materials obtained from both templates showed that the materials were mesoporous with some microporosity present in them. Studies with mucic and tartronic acids as templates also showed that the template as well as the synthesis conditions (such as solvent, temperature and stirring) affect the resulting silica morphology. Mucic acid produced silica materials with high surface areas, mesopores and a morphology that reveals fragmented tubes. Tartronic acid produced hollow tube materials with low surface areas and a combination of micro- and mesopores. The yield of the tubes was higher at lower temperatures for both templates. When sugars (e.g. glucose) were used only spherical particles were obtained and some sugars gave particle sizes that are smaller than the ones that are normally obtained by the sol-gel method. PART II Catalysts (Fe/Cu/K) supported on a range of silica materials with different morphologies (hollow nanotubes, hollow spheres, Stöber/closed spheres) were evaluated in the Fischer-Tropsch reaction (8 bar, 250 °C, 400 h-1 GHSV). The supported iron catalysts modified the physico-chemical properties and activity of iv the catalysts but not the catalyst selectivity. A Ruhrchemie catalyst (known F-T catalyst standard) was also evaluated under the same reaction conditions as the new catalysts for comparison purposes. The Ruhrchemie catalyst was found to be the most active catalyst followed by the catalyst supported on nanotubes, Stöber spheres and hollow spheres respectively. Catalysts containing 18% silica showed the best activity compared to the 9% and 27% silica catalysts. The product distribution and WGS activity were largely influenced by the potassium that is present in the samples and not the support type. Mössbauer spectroscopy showed that some active catalysts contained χ' – Fe2.5C and some superparamagnetic iron oxides or carbides while other catalysts also contained α – Fe and Fe3O4 in addition to χ' – Fe2.5C and some superparamagnetic iron oxides or carbides species. This finding supports the hypothesis that carbide formation is a requirement for active F-T catalysts. It also suggests that metallic iron is necessary for carbiding to occur, hence the need for a reduction pre-treatment.