Kinetics of Fischer-Tropsch Synthesis Using 25%Co/Al2O3 Catalysts Calcined in Air or Nitric Oxide

Recently, researchers at CAER developed a kinetic approach to study the water effect during FTS. An empirical model, rCO = k PCOaPH2b/ (1 + m PH2O/PH2) was successfully developed for defining water effect over cobalt based catalysts, where k = k0 exp (-Ea/RT), k being the kinetic rate constant, a and b are the reaction orders for CO and H2, respectively, and m is a water effect constant. For supported cobalt catalysts, aside from support type playing a major role in determining the water effect during FTS, the size of cobalt particle may also be an important factor to affect catalyst intrinsic activity (including the water effect) and hydrocarbon selectivity. As a continuous study of the sensitivity of Co particle size to water during FTS on 15-25%Co/SiO2 catalysts, we employed two 25%Co/Al2O3 catalysts in this contribution to explore the effect of Co size of Al2O3-supported catalyst on the FTS (e.g. kinetic parameters and hydrocarbon distribution). Different sizes of cobalt on the Al2O3 support were produced by using the conventional air calcination and a patented nitric oxide (NO) calcination. The Co size was determined by hydrogen chemisorption/pulse reoxidation. The kinetic experiments on the 25%Co/Al2O3 catalysts were conducted in a 1-L continuously stirred tank reactor (CSTR) under a wide range of reaction conditions: 205-220 oC, 280 psig, H2/CO = 1.0-2.5 and 3-20 Nl/g-cat/h. The kinetic results showed that both the 25%Co/Al2O3 catalysts displyed a negative water effect on the FTS rate and the NO calcined 25%Co/Al2O3 catalyst with smaller Co particles (11.4 nm) relative to the air calcined one (13.3 nm) displayed a greater sensitivity to water. The NO calcined 25%Co/Al2O3 catalyst also exhibited a higher reaction rate constant, slightly higher C5+, more paraffinic products and a higher selectivity to internal olefins compared to the air calcined 25%Co/Al2O3 catalyst.