The Effects of Transport and Intrapellet Liquids on Fischer-Tropsch Synthesis Rate and Selectivity
Enrique Iglesia, Dept. of Chemical Engineering, University of California, 103 Gilman Hall, Berkeley, CA 94720-1462
The voids within porous pellets fill with liquid hydrocarbons during Fischer-Tropsch synthesis on cobalt-based catalysts. In the absence of H2 and CO fugacity gradients within such pellets, this liquid phase increases C5+ selectivity by increasing the probability that primary α-olefin products will readsorb and initiate surface chains that continue to grow and ultimately desorb as larger paraffins. The greater readsorption tendencies of larger olefins lead in turn to non-Flory product distributions and to increasing paraffin content with chain size. Transition state theory is applied here to this thermodynamically non-ideal reacting system to show that the higher olefin readsorption rates for larger olefins do not reflect their higher solubility within intrapellet liquids. Solubility is irrelevant for readsorption when β-hydrogen abstraction from alkyl chains controls olefin formation. When the desorption process itself control olefin formation rates, higher solubility actually favors desorption by stabilizing the desorbed olefins within the liquid phase; thus, readsorption rates would be lower for larger olefins in spite of their higher concentrations within the liquid phase. The liquid hydrocarbon phase within catalyst pores merely introduces an intraparticle transport restriction that leads to an olefin fugacity gradient within pellets, but it is not otherwise relevant to the rate of chemical reactions of dissolved species. These gradients are most severe for larger olefins and lead to the observed changes in olefin content and termination probability with chain size. These conclusions are consistent with the effects of the structural parameters defining the Thiele modulus (active site density, pellet size, void fraction) on olefin content and molecular weight distribution. The relevance of fugacity, instead of concentration, as the driving force for chemical reactions in thermodynamically non-ideal systems is apparent from the constant reaction rates for CO hydrogenation as small pellets fill with liquid during the early stages of reaction, even though CO and H2 are initially present as gas phase molecules but ultimately access sites as solvated species within the liquid phase. In contrast, pore filling within large pellets causes marked changes in rates and selectivities, because the liquid imposes diffusional constraints as it fills previously empty catalyst pores.