Effect of intraparticle diffusion on Fischer-Tropsch synthesis with a cobalt catalyst

Titelangaben

Pöhlmann, Ferdinand ; Jess, Andreas:
Effect of intraparticle diffusion on Fischer-Tropsch synthesis with a cobalt catalyst.
2015
Veranstaltung: ESCRE 2015 - European Symposium on Chemical Reaction Engineering , 27.-30.10.2015 , Fürstenfeldbruck, Deutschland.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung , Vortrag )

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Abstract

Modeling of reaction-diffusion in a single catalyst particle
The Fischer-Tropsch synthesis (FTS) is an excellent way to convert synthesis gas (H₂ and CO) from various carbon sources (e.g. natural gas, coal, biomass, carbon dioxide) into high value fuels, diesel oil and petrochemicals. Under low temperature FT conditions (LT-FT) between 200-230°C and 2.0-3.0 MPa, cobalt produces predominantly long-chained paraffinic hydrocarbons, which are hydrocracked downstream into valuable liquids. Next to slurry phase reactors multi-tubular fixed bed reactors are used in industrial application. However, those reactors tend to exhibit a significant pressure drop. Hence, catalyst articles between 1 and 5 mm are to be chosen. This onevitably leads to diffusion effects in the particle and, thus, the optimum catalyst performance and selectivity decrease, as expressed by the catalyst effectiveness actor. Thereby, the H₂/CO-ratio concentration profile is expected to change towards the particle center, affecting the local reaction rate and selectivity. In the literature, many reports simulating this behavior are available, but the results have never been evaluated by experimental data.
Thus, we investigated the FT reaction under intrinsic conditions using a cobalt catalst supported on alumina, developed a mathematical model to describe the system from that, and verified our model by measurements with catalyst particles where pore diffusion effects occur. The experiments were carried out in a fixed bed reactor at temperatures between 200 and 240 °C and a total pressure of 3.0 MPa. For the intrinsic data, the synthesis gas ratio (H₂/CO) gradient inside a pellet was simulated experimentally by partial pressure variation (H₂/CO = 0.5-40) at a CO conversion of 10%.
In order to characterize the effect of pore diffusion on catalyst efficiency, experiments with entire particles were conducted at H₂/CO-ratios of 2 to 6. By stepwise reduction of the CO and thus increasing H₂/CO-ratio in the synthesis gas the methane selectivity rises and the chain growth probability α decreases. The catalyst effectiveness factor decreases if temperature rises or the CO feed gas concentration drops. The effective reaction rate calculated from intrinsic data using a modified Thiele modulus agrees
quite nicely with the measured data set. The results for both intrinsic reaction rate and chain growth probability are well described by the Langmuir-Hinshelwood approach proposed by Yates and Satterfield and the modified chain growth probability α-model of Vervloet et al [1, 2]. However, more accurate modeling is possible by numerical calculation of the concentration profiles inside a catalyst pellet using kinetic parameters from intrinsic experiments. The derived mathematical model delivers regions of favorable process operating conditions, maximizing the C₅₊-productivity from the perspective of intra-particle diffusion limitations.
1] I.C. Yates, C. N. Satterfield, Energy & Fuels, 1991, 5 (1), 168-173.
[2] D. Vervloet, Catalysis Science & Technology, 2012, 2 (6), 1221-1233.