Effect of mass transfer limitations on catalyst performance during reduction and carburization of iron based Fischer-Tropsch synthesis catalysts

Abstract

Existence of intraparticle mass transfer limitations under typical Fischer-Tropsch synthesis has been reported previously, but there is no suitable study on the existence of intraparticle diffusion limitations under pretreatment steps (reduction and activation) and their effect on catalytic performance for iron based catalysts. In this study, Fe-Cu-La-SiO2 catalysts were prepared by co-precipitation method. To investigate the intraparticle mass transfer limitation under reduction, activation and reaction steps, and its effect on catalytic performance, catalyst pellets with different sizes of 6, 3, 1 and 0.5 mm have been prepared. All catalysts were calcined, pretreated and tested under similar conditions. The catalysts were activated in hydrogen (5% H2 in N2) at 450 °C for 3 h and exposed to syngas (H2/CO = 1) at 270 °C and atmospheric pressure for 40 h. Afterwards, FTS reaction tests were performed for approximately 120 h to reach steady state conditions at 290 °C, 17 bar and a feed flow (syngas H2/CO= 1) rate of 3 L/h (STP). Using small pellets resulted in higher CO conversion, FT reaction rate and C5+ productivity as compared with larger pellets. The small pellets reached steady state conditions just 20 h after starting the reaction. Whereas for larger pellets, CO conversion, FT reaction rate and C5+ productivity increased gradually, and reached steady state and maximum values after 120 h of operation. The results illustrate that mass transfer limitations exist not only for FTS reaction but also for the reduction and carburization steps which lead to various phase formation through catalyst activation. Also the results indicate that some effects of mass transfer limitations in activation step, can be compensated in the reaction step. The results can be used for better design of iron based catalyst to improve the process economy.