Fischer-Tropsch synthesis (FTS) is a heterogeneous reaction used to convert synthesis gas into a range of hydrocarbon products. This reaction is a key step in gas-to-liquid (GTL) process in which natural gas is converted into liquid fuels and value-added chemicals. Low temperature FTS is often conducted commercially in multi-tubular fixed bed reactors, where the overall design goal is to maximize productivity and enhance selectivity to liquid hydrocarbons while minimizing pressure drop and costs. The latter requirements can lead to use of larger catalyst particles, which results in intraparticle diffusional limitations, i.e. decrease of catalyst effectiveness and selectivity to liquid hydrocarbons. In this study we investigate effects of particle shape (sphere, slab, solid cylinder and hollow cylinder), size (i.e. diffusion length), catalyst distribution (uniform vs. eggshell type distribution for a spherical particle) and process conditions (temperature, pressure, syngas composition and conversion level) on catalyst effectiveness factor and methane selectivity inside a particle using mathematical modeling. With increase in Thiele modulus (i.e. particle size at a fixed set of process conditions) we observe increasing H2/CO ratio profile towards the center of the particle resulting in increase of local and average CH4 selectivity. For each catalyst particle shape we determined values of Thiele modulus, i.e. characteristic length of diffusion, corresponding to the upper limit of the kinetic region, and investigated how it changes with operating conditions. We found that the use of small spherical particles (dp = 0.2 – 0.5 mm) or eggshell distribution with a larger spherical particle (dp = 2 mm) and catalyst layer thickness less than approximately 0.13 mm at base case conditions (473 K, 25 bar, H2/CO = 2 and low CO conversion) is needed to avoid negative impact of diffusional limitations on CH4 selectivity for a highly active cobalt catalyst. Furthermore, simultaneous increase of pressure and the use of syngas with H2/CO feed ratio of 1.4 - 1.7 seems to be the best strategy for mitigating the negative impact of intraparticle diffusional limitations on CH4 selectivity. This allows one to use larger particles while maintaining the desired CH4 selectivity (dp = 1 mm at 50 bar, 473 K and H2/CO = 1.4 results in CH4 selectivity of 5.6%).

Influence of intraparticle diffusion on effectiveness factor and methane selectivity of a cobalt based Fischer-Tropsch catalyst

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.