Fischer–Tropsch synthesis. Effect of alkali, bicarbonate and chloride addition on activity and selectivity

• Alkali compounds may be present in biomass-derived syngas and are a concern.
• Their effect on a typical 100Fe/5.1Si/2Cu/3K FT catalyst was studied using a CSTR.
• Up to 100 ppm KCl, NaCl, NaHCO3, and KHCO3 in syngas was examined at 260 and 270 °C.
• The impurities had low impacts on catalyst activity and selectivity at relevant concentrations.
• Catalyst uptakes of alkali compounds were found to be low based on ICP analysis.

The sensitivity of 100Fe/5.1Si/2Cu/3K Fischer–Tropsch synthesis (FTS) catalyst to the impurities of KCl, NaCl, KHCO3 and NaHCO3 in syngas (0.1–100 ppm) was studied in a slurry phase reactor at 533.2 or 543.2 K, H2/CO = 0.67–0.77, 1.31 MPa and 10 NL/g-cat/h. The impurities were co-fed with syngas, and the influence of each contaminant concentration on Fe catalyst behavior was examined for 72–144 h. The presence of up to 40 ppmw halide compounds (NaCl and KCl) or alkali bicarbonates (NaHCO3 and KHCO3) in syngas at 543.2 K or up to 100 ppm of NaCl and KCl at 533.2 K had little impact on the Fe catalyst activity and selectivities to CH4, C5+ and C4 olefin and 1-olefin during 400 h (543.2 K) or 1400 h testing (533.2 K). CO2 selectivity slightly increased after feeding the impurity-containing solutions, which was due to enhanced water gas shift (WGS). ICP results for the Fe catalysts at the end of the FTS reaction test and of the FTS products (i.e., water, oil or wax phase) indicate that the impurity ions (K+, Na+ or Cl−) introduced were present in all phases of the FTS products, the greater part being retained in the wax. Therefore, the contaminant ions (i.e., Na, K or Cl) in the water solution injected appear to not strongly adsorb on the Fe catalyst surface at typical FTS conditions in the slurry phase reactor. This is assumed to be responsible for the lack of change in FTS behavior for the Fe catalyst using the contaminants studied.

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