Investigation of catalytic conversion of Fischer-Tropsch wax on Pt/ALSBA-15 and Pt/beta zeolite catalysts

Abstract

Because of the more serious problems with the environment (e.g. greenhouse effect) and the crude oil supply (e.g. import dependence) the use of fuels and lubricants produced from renewable feedstocks have come to the front nowadays. The Fischer-Tropsch wax (60–80% of the Fischer-Tropsch products) which is produced on synthesis gas from different sources (biological or waste), is a mixture of high molecular weight (C20-C60) n-paraffins, which are in solid state (high pour point) at normal conditions. The products (fuels and base oils) which can be produced from this paraffin mixture have high quality and have fewer negative effects on the environment (practically zero sulphur- and nitrogen content, low aromatic content, excellent application properties) thus theydo not demand changes in the fuel supply infrastructure and in the engine constructions. The isomerization of high molecular weight n-paraffins can be effectively carried out on bifunctional catalysts. There are only a few indications about the application of metal catalysts on mesoporous carrier in the literature. Consequently our objective was to investigate some Pt/AlSBA-15 (SBA: Santa Barbara Amorphous) catalysts which have not been investigated in detail in this reaction system yet, and compare its properties with a Pt/beta zeolite catalyst which has been recommended for this reaction earlier. The applicability and catalytic activity of Pt/AlSBA-15 and Pt/beta zeolite catalysts of 0.5% platinum content for the selective isomerization of Fischer-Tropsch wax was investigated in the present experiment. The experiments werecarried out in a high-pressure microreactor system in continuous operation and on a catalyst with steady-state activity. The main properties of the feedstock which was a mixture of paraffin produced by Fischer-Tropsch synthesis (the synthesis gas was produced from biomass) were: n-paraffin content (C18-C57): 97.4%, sulphur content: 5 mg/kg, pour point: 72 °C. In the experiment the following process parameters were applied: T=275–375 °C, P=40–80 bar, LHSV=1.0–3.0 h-1, H2/hydrocarbon ratio: 400–800 Nm3/m3. The composition of the products was determined by gas chromatography. From the catalysts with different support the best results were gained on the catalysts with AlSBA-15 support, and with increasing temperature the yield of liquid products (C5+) decreased, but until 325 °C this value was above 93% in every case. Incase of the beta zeolite high amount of cracking took place. Increasing the pressure shifted back the hydrocracking reactions(with the increasing number of moles) so it had a decreasing effect on the volume of the gas products and the lower contact time (higher liquid hour space velocity) had the same effect. In the gas products mainly branched isobutane was identified, which indicated that the cracking enacted partly after the isomerization reactions. The isoparaffin contents of the liquid products in the function of process parameters increased with increasing temperature and decreased with increasing pressure and LHSV in every case while other parameters were kept constant. Based on the isoparaffin contents of the different fractions it can be concluded that on Pt/AlSBA-15 catalyst at advantageous process parameter combinations (T=300–325 °C (C11-C20)/ 275–300 °C (C21-C30), P=40–80 bar, LHSV=1.0–2.0 h-1) the catalyst was applicable to produce C11-C20 and C21-C30 fractions with high isoparaffin content (63.5–85.6% and 34.1–58.7%) with adequate yields (29.9–36.6% and 46.2–58.8%). We experienced that the gas oil fractions having the lowest pour point were obtained in the case of high concentrations of 5-methyl isomers. The C21-C30 fraction is a high viscosity index (VI ≥125) base oil. The selectivity of the target product fractions was high, and based on these facts the selective isomerization of the Fischer-Tropsch wax can be a new application area of the Pt/AlSBA-15 catalyst.