Accumulation of liquid hydrocarbons during Fischer-Tropsch fixed-bed synthesis and process optimization based on periodical pore filling and pore draining

• Applicant: Professor Dr.-Ing. Andreas Jess
• Subject Area: Chemical and Thermal Process Engineering ; Technical Chemistry
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 318313257

In Fischer-Tropsch synthesis (FTS), syngas (CO and H2) is converted to higher hydrocarbons over heterogeneous catalysts based on Fe or Co. In case of the so-called low-temperature FTS (T < 250 °C), it was until now generally assumed that the pores of the catalyst are completely filled with liquid reaction products (wax) within a couple of days in the initial phase of the reaction. This leads to an unwanted but unavoidable decrease of the pore effectiveness factor and in return to a lower activity and a higher selectivity to methane, above all, if mm particle are used as in case of technical fixed-bed reactors (pressure drop).In the course of a DFG-project, which will be completed in autumn 2020, we could show that the pores of the catalyst (cobalt) are not always completely filled and the filling time may even last several months, above all, if the chain growth probability is high (> 0.9). The filling process depends on the H2/CO-ratio, pressure and temperature. All experiments were conducted with single catalyst particles in a magnetic suspension balance.Within the applied new research project (application for continuation of funding), this research should be continued and transfered to FTS in a fixed bed reactor. In addition, a promising new transient process based on pore filling by FTS and pore draining by hydrogenolysis should be investigated. Based on experimental and theoretical studies (modelling), the following two main questions should be answered: 1) Do local (axial) differences in CO conversion and gas phase composition, respectively, in a fixed bed reactor have an influence on the local pore filling degree during the initial phase of FTS, and how does this affect the effective activity and selectivity of the catalyst (here based on cobalt)? 2) Does a transient operation of a fixed-.bed reactor, i.e. a periodical switch from pore filling (FTS) to pore draining (hydrogenoloysis in case of a feedgas without CO) lead to a higher mean reaction rate and a higher rate of formation of liquid hydrocarbons?The following examinations are intended: The particles of a fixed-bed reactor are partly/completely filled with higher hydrocarbons at different degrees of CO conversion (up to 40%). The FTS is stopped after a certain time, and particles at different axial positions are sampled and analyzed (i.e. filling degree, composition of liquid phase in the pores). Experiments on the hydrogenolysis will gain the best reaction conditions with regard to a rapid pore draining process. The rate of pore filling should also be investigated directly by in situ measurements in a magnetic suspension balance with a fixed-bed reactor (forced flow). Finally, the accumulation of higher hydrocarbons in the FT fixed-bed reactor as well as the alternating transient process (filling/draining of pores) should be modelled with regard to an optimization of the proposed transient process.

• DFG Programme: Research Grants