Synthèse de catalyseur de Fischer-Tropsch assistée par ultrasons

The oil extraction industry flares annually 140 billion cubic meters of natural gas. This gas exits oil wells together with liquid hydrocarbons. Its recovery for oil wells in remote places is challenging, because the cost of pipeline facilities is prohibitive. In order to avoid direct venting in the atmosphere, this gas is burned (flared) at the oil wells exit. Gas flaring accounts for 300 million tons of the annual CO2 emissions (close to 1 % of global emissions). It also pollutes the local environment and wastes valuable energy resources. An alternative to gas flaring is to convert natural gas (NG) to paraffins and olefins with a mobile, integrated Gas-To-Liquid unit (GTL): NG is first converted to synthesis gas (H2+CO) by CH4 catalytic partial oxidation, followed by the Fischer-Tropsch (FT) reaction at 300 °C. This second step is a polymerization reaction as well as an heterogeneous catalytic reaction; it proceeds on the surface of metallic sites, mostly iron and cobalt. In the FT synthesis, the conversion of synthesis gas increases with metallic loadings. Traditional catalyst synthesis methods require several steps of impregnation to reach a high metal loading, which significantly increases the duration of the catalyst synthesis. This master project studies the design of a Fischer-Tropsch catalyst synthesized by ultrasound-assisted impregnation, within the context of a mini-GTL reactor. The specific objectives of the project are to synthesize, characterize and test a catalyst active for the Fischer-Tropsch reaction within the conditions of a mini-GTL unit. Ultrasound-assisted impregnation takes advantage of the cavitation phenomena: the collapse of gas bubbles inside a liquid solution. This process (sonication) increases active phase dispersion on supported catalysts, and increases surface area. It also improves mass transfer, which could shorten impregnation durations. In this project, for the first time, Fischer-Tropsch catalysts are sonicated during the whole impregnation step of the synthesis. The effect of argon bubbling during impregnation is also studied. After impregnation, drying and calcination, the FT catalysts are characterized by SEM, XPS, BET, PSD and TGA. These analyses show efficient dispersion of the active phase on the support. Argon bubbling dampens the intensity of cavitation, which directly controls surface area and median diameter of the catalyst. After characterization, FT catalysts are tested in fixed bed reactor. A first set of experiments studies the catalytic behavior of iron catalysts synthesized with argon bubbling, with and without cobalt promotion, at 220 °C and 270 °C. These tests show rapid deactivation of the catalysts through coking, as a result of high inlet flow. The 15%(Fe)/Al2O3 catalyst shows the highest activity of this experiment set. It is hence tested at lower reagents flow rate. This time, reagents conversions are stabilized for more than 120 h on stream. CO conversion reaches 79 %, the yield toward C2+ products reaches 39 %. The catalyst selectivity towards light hydrocarbons is high, the chain growth probability being 0.66 which is typical of iron catalysts at high temperature. These results are obtained with an impregnation shorter than traditional methods by a factor of six. Hence the interest of applying sonochemistry techniques to the synthesis of the Fischer-Tropsch catalyst is confirmed. The temperature reaction being 270 °C, the catalyst is also active for the water-gas-shift reaction. A modification in the activation procedure, as well as the use of CO2 as a reagent, could allow an increase in the operating temperature, and would make the catalyst suitable for the FT reaction in the context of a mini-GTL reactor.