We investigate effects of catalyst
activity, catalyst particle
shape (sphere, slab, 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 the catalyst pellet. In numerical
simulations we utilize kinetic parameters for CO consumption rate
and CH4 formation rate determined from experiments with
a highly active Co/Re/γ-Al2O3 catalyst.
It is found that the use of small spherical particles (0.2–0.5
mm) or eggshell distribution for larger spherical particles with catalyst
layer thickness less than approximately 0.13 mm is needed to avoid
negative impact of diffusional limitations on CH4 selectivity
under typical Fischer–Tropsch synthesis operating conditions.
For monolith reactors with wash-coated catalyst, diffusional limitations
can be avoided by using a catalyst layer thickness less than 0.11
mm at base case conditions (473 K, 25 bar, and H2/CO molar
ratio of 2).
Please cite this article in press as: B. Todic, et al., Fischer-Tropsch synthesis product selectivity over an industrial iron-based catalyst: Effect of process conditions, Catal. Today (2015), http://dx.
a b s t r a c tThe effect of process conditions on product selectivity of Fischer-Tropsch synthesis (FTS) over industrial iron-based catalyst (100 Fe/5 Cu/4.2 K/25 SiO 2 ) was studied in a 1-L stirred tank slurry reactor. Experiments were performed over a range of different reaction conditions, including three temperatures (T = 493, 513 and 533 K), four pressures (P = 0.8, 1.5, 2.25 and 2.5 MPa), two synthesis gas feed molar ratios (H 2 /CO = 0.67 and 2) and gas space velocity from 0.52 to 23.5 Ndm 3 /g-Fe/h. The effect of process conditions on reaction pathways of FTS and secondary 1-olefin reactions was analyzed by comparing product selectivities, chain growth probabilities and ratios of main products (n-paraffin, 1-and 2-olefin). Reduction of methane production and increase of C 5+ products was achieved by decreasing temperature, inlet H 2 /CO ratio and/or increasing pressure. Overall selectivity toward methane and C 5+ did not show significant changes with variations in residence time. All of the product selectivity variations were shown to be related to changes in chain length dependent growth probabilities.
In this review we discuss the reasons responsible for higher than expected methane selectivity during Fischer–Tropsch synthesis (FTS) over cobalt-based catalysts and describe novel microreactors for use in FTS.
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