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).
A multiphase fixed-bed reactor (FBR) model for Fischer−Tropsch Synthesis has been developed. A high level of details is considered for description of the phenomena on the reactor and particle scale. Detailed kinetics is used, with parameters estimated from experiments with a cobalt-based catalyst. Model robustness has been validated using literature data. Performance analysis was made for a conventional scale FBR with egg-shell distribution of catalyst and a millimeter-scale FBR with small particles and uniform distribution. In both cases, diffusion limitations are almost eliminated due to use of small diffusion lengths. For similar qualitative results, a milli-scaled design would result in a significantly lower reactor volume, but the capital costs could be high due to large wall area and a vast number of tubes. Heat removal is efficient in both cases, and pressure drop in the milliscale reactor is low due to the use of a shorter bed and lower velocity.
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