(652e) Spray Dried Iron Catalysts for Slurry Phase Fischer-Tropsch Synthesis

Three catalysts of the same nominal composition (100 Fe/3 Cu/5 K/16 SiO2 in parts by weight) prepared by spray drying (using three different sources of silica) were tested in a stirred tank slurry reactor (STSR) and their attrition properties were determined by SEM and particle size distribution (PSD) measurements. They are designated according to the silica source employed as: CS (Colloidal Silica); TO (Tetraethyl Orthosilicate - TEOS) and PS (potassium silicate).

Spray dried catalysts were prepared from wet precursors and had excellent sphericity and smooth external surfaces (SEM). The particle size distribution was rather broad ranging from 5 μm to 40 μm in diameter, regardless of the source of silica used (PSD measurements). The observed morphology of spray dried catalysts (APV Anhydro Lab spray drier) is ideally suited for use in slurry reactors, but the particles were smaller than intended (30-70 μm) due to limitations of small-scale equipment used for spray drying. All three catalysts were impregnated with potassium by incipient wetness impregnation after spray drying, and were tested in STSR runs SB-30702, SB-33802 and SB-09703 (300-364 h on stream).

SEM micrographs of catalyst samples at TOS = 0 h (TOS = time on stream) showed that all three catalysts are formed of mostly spherical particles. However, TO catalyst showed the presence of cracks and surface irregularities, which had an adverse effect on its attrition resistance. After testing in the STSR TO and PS catalysts lost their sphericity due to attrition. Also, the formation of large agglomerates was observed after reaction tests. In contrast, the CS catalyst had an excellent attrition resistance. After 345 hours of testing in the STSR the catalyst's morphology remained practically unchanged relative to catalysts sample at TOS= 0 h. Results from PSD measurements with CS catalyst confirmed its excellent attrition resistance. Reductions in the volume moment and Sauter mean diameters (5.4 and 6.8 %, respectively) were the smallest among all catalysts tested in our Laboratory. A small increase in the fraction of fine particles (0.7 %) shows that this catalyst also did not experience a significant attrition by erosion. PSD results from the STSR test of TO and PS catalysts showed increase in the average particle size. This is attributed to the presence of residual wax.

Catalytic performance of spray dried catalysts at the baseline process conditions (260°C, 1.5 MPa, H2/CO = 2/3 and 4 NL/g-Fe/h) was excellent. Syngas conversion was between 71 and 76%, whereas methane selectivity was between 2.2 and 3.5% and that of C5+ hydrocarbons was between 78 and 86% (all selectivities are on C-atom basis). Catalyst productivities were 5-12% lower than that of precipitated (not spray dried) catalyst of the same composition, whereas hydrocarbon selectivities were similar. Spray dried CS catalyst (run SB-30702) had the most superior attrition strength but it produced more light hydrocarbons than the other two spray dried catalysts.