Cobalt Fischer–Tropsch Catalyst Deactivation Modeled Using Generalized Power Law Expressions

Ten sets of deactivation data from five previously reported studies of cobalt Fischer–Tropsch synthesis (FTS) catalysts were found to be modeled well using concentration-independent first and second order generalized power law expressions (GPLEs) which predict that activity approaches a non-zero asymptote. Concentration dependencies of reactants and products were generally not addressed in the model regressions, although selected simulations which incorporated CO, H2, and/or H2O concentrations in deactivation rate equations showed very little or no dependence on concentrations of these species. For reaction temperatures in the range of 220–230 °C, pressures of 15–30 bar, and H2/CO ratios of 1.6–2.6, first order and second order deactivation rate constants average 0.12 ± 0.06 and 0.11 ± 0.05 day−1, respectively. Limiting (asymptotic) activities are largely in the range of 30–40 % of initial activity based on the generally superior extrapolations of second order GPLE. This consistency is impressive considering significant differences among catalyst properties and operating conditions in the five studies that apparently involve different mechanisms of deactivation, including sintering, carbon formation, and/or cobalt aluminate formation. Second order models predict significant longevity for cobalt FTS catalysts; for example, based on the 2nd order models, normalized activities for commercial catalysts in two different pilot slurry reactor facilities are projected to be 56 and 45 % of initial activity after 200 days on stream. For two of the previous studies providing data over periods of 40–55 days, it was possible to identify two different causes of deactivation, one rapid (reaching completion in 10–20 days) and one slow (apparently continuing beyond 40–50 days). A method was developed for calculating first and second order model parameters for the two regions of operation. Rapid activity loss (path 1) is observed for either sintering or Co surface aluminate formation, while poisoning/fouling by deposited carbon or coke (path 2) occurs relatively slowly over the entire process run of 40–55 days and is the dominant mechanism after 10–20 days for both sets of data. The results show that simple GPLE models are surprisingly generally useful for predicting activity versus time behavior of supported cobalt FTS catalysts under typical process conditions.