Browsing by Author "Chikati, Roick"

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    Fischer–Tropsch synthesis: application of clinoptilolite (natural zeolite) as catalyst support
    (2024) Chikati, Roick
    In this study, clinoptilolite (a naturally occurring zeolite) was used as catalyst support in Fischer Tropsch Synthesis. Prior to its use, raw clinoptilolite was crushed sieved to yield different particle sizes (-212 to +150 µm; -150 to +106 µm; -106 to +75 µm; -75 to +53 µm; -53 to +38 µm; -38 to +25 µm; and less than -25 µm). Seven 10% wt cobalt catalyst supported on different size classes of clinoptilolite were prepared using the incipient wetness impregnation method. A thorough investigation was done on the characteristics of cobalt supported on clinoptilolite particles of different sizes using TPR, XRD, XRF, BET and SEM techniques. It has been demonstrated that these techniques provide insight on the effect of the support particle size, and this could be used as a quality control tool to evaluate the efficacy of the preparation method. Temperature-programmed reduction (TPR) was used to examine the non-isothermal reduction of cobalt oxide using 5% hydrogen in argon at three distinct heating rates (5, 10, 15 o C/min). When using the Kissinger model, it was discovered that the activation energy (Ea), varied from 102.45 to 254.01 kJ/mol, depending on the support particle size of the catalyst. The lowest activation energy being achieved with a support particle size in the 212 to 150 µm range. Reducing the catalyst reduction temperature has significance in FTS, since it drastically reduces the sum of money spent on the energy input required for reduction. XRD, XRF and SEM confirmed the phases making up the catalyst, loading of the catalyst and the particle size distribution respectively. It is worth noting that though differences exist between different size classes, no clear trend was obtained for any of the BET parameters. For this study, three size classes were investigated as the support for an FT catalyst: -75 to +53 µm; -53 to +38 µm; less than 25 µm. Using a fixed bed reactor at 220 o C and 10.85 bar(abs), the maximum CO conversion obtained was 44.97% when using the -53 to +38 µm size class (-78 to +53 µm size class giving 32.06 %, and < 25 µm µm giving 31.29% Co conversion). At the conditions studied, methane selectivity ranged between 14.95 and 16.97% for the support class size studied, while C2-C4 selectivity ranged between 14.55 and 19.01%, and C5+ selectivity ranged between 66.04 and 70.29%. The acquired product selectivity results are similar to those reported in the literature, which validates the use of this support. Statistical analysis done on the FT results obtained, one-way analysis of variance (ANOVA) and post-hoc Bonferroni adjustment indicated that utilization of different support size classes had an effect on CO conversion. An innovative data simulation technique based on response surface methodology (RSM) was used as part of the design of experiments (DOE) to thoroughly investigate the effect of the various operating FTS conditions for both cobalt and Iron based catalyst. These discoveries might be have valuable implications for the design of a catalyst that can be used in the coal/biomass to liquid process
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    Iron supported on Clinoptilolite (natural zeolites) as a low-temperature Fischer-Tropsch synthesis catalyst
    (2019) Chikati, Roick
    In the Fischer-Tropsch (FT) synthesis, CO and H2 (synthesis gas) are converted into plethora of hydrocarbons mainly paraffins and olefins and these can be further upgraded to high-quality fuels and chemicals. Different carbon sources such as natural gas, coal and biomass can be used as feed-stocks for the synthesis gas. In commercial applications, precipitated and fused iron catalysts are commonly used in the Fischer-Tropsch synthesis, especially when the synthesis gas emanates from coal or biomass where the CO/H2 needs adjustments via the WGS reaction and when the desired final products are mainly olefins. However, there was a problem associated with the catalyst’s mechanistic resistance; also, these types of Fe catalysts consume large amounts of iron resource. Development of cheap, efficient and robust support iron catalyst become an urgent task Zeolites and zeolite rocks are commonly used in different industrial applications. Natural zeolites present an attractive material as supports in FTS because of their high abundance, availability, low costs and their properties. Detailed mineralogical knowledge and profound characterization of natural zeolites are essential for fitting chemical composition to use. Si/Al ratios are very import as well as the other contaminates. A fundamental difference exists between commercial supports such as silica and alumina - with functional porous materials - and natural supports such as zeolites. In this study natural zeolite called clinoptilolite (a type of zeolite found in South Africa) was used as a catalyst support. This support proved to be promising for low-temperature Fischer–Tropsch synthesis (LTFTS) targeting liquid fuel production, as well as chemical feedstock. Synthesis of this highly active catalyst was by loading of iron on clinoptilolite through the wet impregnation method. The prepared catalyst was then characterized by XRF, BET surface area analyzer, XRD and SEM. The catalyst was then loaded into the reactor and reduced with hydrogen prior to FTS. The effects of its use as support in FTS were investigated in a fixed bed reactor. From the XRF results the molecular ratio SiO2/ Al2O3 of the Clino-support was 5.86. The average crystal size of the particles from both HRTEM and XRD ranged 9.8 -11.6 nm and around 10.10nm for used and fresh catalyst. It was found that the CO consumption rate of 1.02 x 10-4 mol/min.gcat of which 7.24 x 10-5 mol/min.gcat was the actual Fischer Tropsch rate with the remaining 2.93 x 10-5 mol/min.gcat consumed by the WGS reaction. The product distribution of the gaseous phase analysed were more olefinic than paraffinic. The product distribution for this condition follows a one alpha ASF distribution with an alpha value of 0.86. These findings may permit the development of new effective support materials, which are cost effective for clean fuel production via FTS process.