Development of alumina supported cobalt catalysts for Fischer Tropsch synthesis

Due to the run out of petroleum resources, the existence of external dependence and environmental effects, producing safe and sufficient quantities of cheap and clean fuel is one of the main problems of economic and social life for all countries. Therefore, using today's energy resources efficiently and improving the technologies used is one of the basic goals of all countries. Fischer-Tropsch (FT) synthesis, which emerged in Germany in the 1920s, is a process defined as the conversion of a synthesis gas consisting of carbon monoxide and hydrogen gases into various hydrocarbons and as an alternative to crude oil in recent years for the production of various fuels such as gasoline. Hydrocarbon products resulting from FT synthesis are high-quality fuels that are environmentally improved, producing less particulate matter during combustion, containing less sulfur and nitrogenous pollutants compared to petroleum-based fuels. Considering these reasons, FT synthesis is a very important application for non-oil resources to adapt to increasingly stringent environmental regulations and to convert valuable fuels or synthetic gas into valuable chemicals. Products resulting from FT synthesis, olefins, paraffins and oxygenated products (acids, alcohols, ketones and aldehydes). There are a number of factors such as the type of catalyst, promoter, temperature, synthesis gas composition and pressure that are effective on the distribution of the resulting products. Commercially available FT reactors are classified into two categories, high temperature and low temperature reactors. FT synthesis carried out at low temperature is more suitable for the formation of long chain hydrocarbons (C10-C18+) and multi-tube fixed bed reactors and slurry type reactors are used as reactors. In the synthesis of high temperature FT, while hydrocarbons with gasoline and low carbon number are formed as products, fixed fluid bed reactor and circulating fluid bed reactor are preferred as the reactor. The tests carried out in the thesis study are in the fixed bed reactor and in the low temperature FT synthesis range. One of the most important issues to consider in the synthesis of FT is selection of the suitable active metal for the appropriate catalyst. The choice of the suitable active metal depends on a number of parameters, including the carbon source used in the synthesis gas production, the active metal cost of the catalyst, and the final products desired to be synthesized. Cobalt and iron-based catalysts are considered to be the best catalysts for applications in industrial scale FT processes. Iron catalysts are preferred to form long chains of cobalt catalysts which are a more suitable alternative for the production of olefins. Iron catalysts have water gas shift reaction (WGS) activity, which is not observed in cobalt. Iron catalysts are suitable for use in low H2/CO conversion (0.5-2.5) synthesis gas conversion from biomass or coal. Cobalt has better catalytic performance at higher H2/CO ratios (2 and higher) and is a more suitable catalyst for synthesis gas conversion, which is natural gas. In addition to the active metal in the structure of the catalyst, auxiliary materials are present in order to regulate and improve certain properties of the catalyst. These are promoters and support materials. Promoters can increase activity in FT synthesis and regulate product selectivity by causing easier results such as easier reduction of active metal particles, increased metal distribution, inhibition of catalyst deactivation. Support materials provide positive effects such as high surface area on the catalyst, protection of mechanical strength, and improvement of heat and mass transfer during FT reactions. In this thesis, ruthenium-promoted and promoter-free alumina-supported cobalt catalysts were prepared and performance tests were carried out to determine the activity of prepared catalysts in FT synthesis and hydrocarbon product selectivity. The objective of the performance tests is to examine the effect of the active metal composition on the FT activity of reaction conditions such as support particle size, promoter loading, temperature and synthesis gas composition. Cobalt (Co) as the active metal, gamma aluminum oxide (γ-Al2O3) as the support material and ruthenium (Ru) as the promoter were selected to be used in experiments and catalyst synthesis was performed by impregnation method up to wetness. Firstly, it is aimed to determine the effect of the active metal (cobalt) composition and support particle size on the activity and selectivity of the catalyst for cobalt-free cobalt catalysts. For this purpose, non-promoter catalysts with three different particle sizes and three different cobalt percentages were synthesized. The ruthenium promoter was then added to the non-promoter-catalyzed synthesized to examine the promoter effect in Fischer Tropsh synthesis. The catalysts prepared after the synthesis process include: BET surface area analysis was performed to determine the effect of synthesis parameters and promoter addition on the surface area, coupled double plasma analysis (ICP) to determine the active metal and promoter composition, X-ray diffraction (XRD) characterization studies to determine the crystal structure of the catalysts, followed by FT performance tests. Then FT performance tests were carried out. During and after the performance tests, product analyzes were carried out using gas chromatograph (GC) and hydrocarbon distributions were determined. According to the results obtained, the activity of the catalysts in the FT synthesis was found to change with increasing cobalt and ruthenium percent and by changing the support particle size. Accordingly, the highest CO conversion activity among the non-promoter-free catalysts was obtained for the Co/Al2O3 catalyst of size 250-355 μ containing 20% cobalt. When the activity results of the promoter catalysts were examined, it was determined that the highest conversion was again at % 0.9Ru20Co/Al2O3 catalyst in the same particle size. Experiments were also carried out in order to determine the effect of certain working conditions on activity and selectivity. For this reason, activity tests were carried out in three different temperatures and three different synthesis gas compositions (H2/CO ratio). As a result, it has been found that increasing the temperature increases CO conversion. In addition to the FT activity, tests were also conducted to examine the hydrocarbon selectivity of the catalysts. Firstly, it has been observed how the cobalt composition influences hydrocarbon selectivity in catalysts without promoter. For this purpose, 250-355 μ Co/Al2O3 catalyst, the best result of the activity test, was chosen. As the cobalt composition increased, the selectivity of C5+ increased while the selectivities of CH4 and C2-C4 light hydrocarbons decreased. It has also been examined how the addition of the Ru promoter, in addition to the cobalt compound, changes the hydrocarbon selectivity of the catalysts. For this purpose, 250-355 μ %20Co/Al2O3 catalyst was selected and analyzed to observe the effect of 0.3%, 0.6% and 0.9% Ru addition on selectivity. As a result of the analysis, it was revealed that as the percentage of Ru increased, the C5+ selectivity increased, while the light hydrocarbon selectivities in the CH4 and C2-C4 range decreased. As a result of the performance tests, as the reaction temperature increased, the result of FT activity increased. The highest CO conversion was obtained at 240°C compared to the tests carried out at three different temperatures for %0.9Ru20Co/Al2O3 catalyst. The effect of H2/CO ratio on FT activity and hydrocarbon selectivity was investigated on this catalyst based on the highest activity of %0.9Ru20Co/Al2O3 (250-355μ) catalyst, which is generally the most active in the thesis. For this purpose, H2/CO ratio was tested at three different values of 1.0, 1.5 and 2.0. As a result of the tests it is revealed that the value which gives the most desired result is 2. When the H2/CO ratio was 2, both the CO conversion was the highest; and the C5+ hydrocarbon selectivity has reached its maximum value.