Thermal Cracking of Fischer−Tropsch Waxes

Abstract

Thermal cracking of Fischer−Tropsch waxes at mild conditions (430−500 °C) has been reconsidered as a possible refining technology for the production of fuels and chemicals. In this study, the description of paraffin cracking, which was historically developed using material no heavier than C40, was extended to C20−C120 wax. In this carbon number range, cracking rate increased with increasing carbon length and thermal cracking resulted in a bimodal product distribution. Using the Rice−Kossiakoff mechanism as a basis, a relationship could be derived between carbon number distribution and conversion that described the observed behavior. The application of thermal cracking to fuels production was evaluated, and it was shown to be less efficient than hydrocracking, although it requires no hydrogen co-feed and has a lower gas make. Thermal cracking is better suited for the production of chemicals such as candle wax and linear α-olefins. The study also investigated the formation of deposits during wax cracking. The deposits were inorganic in nature and originated from material produced in the Fischer−Tropsch process. It was found that stripping gases (steam, H2, and N2) were ineffective in reducing these deposits.