Accumulation of liquid hydrocarbons in catalyst pores during the initial phase of cobalt-catalysed Fischer-Tropsch synthesis

Title data

Rößler, Stefan ; Pöhlmann, Ferdinand ; Kern, Christoph ; Jess, Andreas:
Accumulation of liquid hydrocarbons in catalyst pores during the initial phase of cobalt-catalysed Fischer-Tropsch synthesis.
2016
Event: DGMK Conference "Catalysis - Novel Aspects in Petrochemistry and Refining" , 26.-28.09.2016 , Berlin, Deutschland.
(Conference item: Conference , Poster )

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The reduction of carbon dioxide emission by enhanced use of renewable energy is set as the main goal of the energy revolution within the next decades. For this, an economically and ecologically feasible method for energy storage needs to be found. A promising way utilize emitted CO2 is the Power-to-liquid-process, converting CO2 to CO hydrogenating this via Fischer-Tropsch synthesis (FTS), using H2 produced by renewable means. During the FT synthesis step, i.a. catalyzed by Cobalt, syngas containing only H2 and CO reacts into H2O and hydrocarbons (HC), ranging from methane to high molecular compounds. The resulting HC products are described predominantly by the Schulz-Flory distribution using single chain growth probability α. Industrially as well as for this article relevant are reaction conditions of less than 250°C (low-temperature-synthesis) at a typical pressure of 1-2 MPa in a wall-cooled fixed-bed reactor, using mm-sized catalyst-particles to reduce pressure drop. It is widely claimed in the literature that during the initial synthesis phase liquid HC products (waxes) accumulate in the catalyst’s pores, a complex process, which leads to completely filled particles in just a few days time [1, 2, 3]. Due to this accumulation the diffusivity of H2 and CO inside the catalyst pores is reduced severely resulting in a strong decline of catalyst effectiveness and, often, to changes in selectivity.
However, our own recent studies have shown that complete catalyst filling with liquid HCs depends on the respective reaction conditions, and thus, even after 2 weeks of time on stream the catalyst particles were not yet filled completely. This phenomenon was up to today only discussed by Huff and Satterfield (1985), although based on numerical simulations only, without experimental proof [4].
Further research regarding HC synthesis, HC vapor output and consequently HC accumulation, its influence on the product distribution and mutual dependence is conducted using in-situ gravimetric analysis (magnetic suspension balance, thermogravimetric balance) and particle extraction experiments. In addition a complex mathematical model based on intrinsic kinetic data, mass transport limitations and HC multilayer adsorption was developed, in order to identify the most influential parameters and to describe the experimental data.
References
[1] A. Jess, P. Wasserscheid, Chemical Technology, WILEY-VCH, Weinheim, 2013.
[2] M.F.M. Post, A.C. Van’t Hoog, J.K. Minderhoud, S.T. Sie, AIChE J., 35, 1989, 1107–1114.
[3] H. Raak, K. Hedden, Erdöl Erdgas Kohle, 114, 1998, 251–256
[4] G.A. Huff, C.N. Satterfield, Ind. & Eng. Chem. Proc. Des. & Dev., 24, 1985, 986-995.