Effect of Process Conditions on Olefin Selectivity during Conventional and Supercritical Fischer−Tropsch Synthesis

Abstract

A precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25 SiO2 on mass basis) was tested in a fixed-bed reactor under a variety of process conditions during conventional Fischer−Tropsch synthesis (FTS) and supercritical Fischer−Tropsch synthesis (SFTS). In both modes of operation it was found that: total olefin content decreases whereas 2-olefin content increases with either increase in conversion or H2/CO molar feed ratio. Total olefin and 2-olefin selectivities were essentially independent of reaction temperature. The effect of conversion was more pronounced during conventional FTS. Comparison of olefin selectivities in the two modes of operation reveals that total olefin content is greater while the 2-olefin content is smaller during SFTS. Also, both the decrease in total olefin content and the increase in 2-olefin content with increase in carbon number (i.e. molecular weight of hydrocarbon products) was significantly less pronounced during SFTS in comparison to the conventional FTS. The obtained results suggest that 1-olefins, and to a smaller extent n-paraffins, are the primary products of FTS. Secondary reactions (isomerization, hydrogenation, and readsorption) of high molecular weight α-olefins occur to a smaller extent during SFTS, due to higher diffusivities and desorption rates of α-olefins in the supercritical propane than in the liquid-filled catalyst pores (conventional FTS).

Heterogeneous Catalysis in Supercritical Media, Part 3: Other Media

“…Again, an increased total content of olefins, but less 2-olefins were detected. This result was also attributed to a reduction of the secondary reactions [51,52].…”

“…Local overheating of the catalyst causes deactivation and an increased formation of methane [50]. Propane [51,52], pentane [49,53], and hexane [54] were applied as supercritical media. Due to the increased solubility, also additives like olefins are dissolved easily and transported to the catalyst surface at high diffusion rates.…”

“…For instance, 1-hexane was added to shift the product spectrum towards higher hydrocarbons [53]. The catalysts used were Co-La and Co catalysts on SiO 2 support [50,53], Ru catalysts on Al 2 O 3 carriers [50], and Fe catalysts [54], such as copper-containing Fe silicates [51,55]. According to studies with supercritical hexane and a Ru catalyst, the reaction rate ranges between those of the gas-phase process and the liquid-phase process.…”

Heterogene Katalyse in überkritischen Medien – Teil 3: Sonstige

“…Auch bei Fischer-Tropsch-Synthesen in überkritischem Propan mit einem gefällten Fe-Cu-Silikat als Katalysator wurde eine andere Produktzusammensetzung festgestellt als in der Gasphase: ein höherer Gesamtgehalt an Olefinen, aber weniger 2-Olefine. Auch dieses Ergebnis wurde auf eine Verminderung der Sekundär-reaktionen zurückgeführt [51,52] …”

Selective Synthesis of Higher Linear α-olefins over Cobalt Fischer–Tropsch Catalyst

“…For example, C 5 -C 8 a-olefins are used as comonomers in polyethylene products and the longer chain a-olefins provide premium value synthetic lubricants. Significant economic benefit, therefore, can be achieved by increasing the a-olefin selectivity in Fischer-Tropsch synthesis (FTS) products [3,4].…”