The influence of phosphorus containing compounds on steam cracking of n-hexane

In steam cracking of hydrocarbons P-containing compounds are used as additives to control coke formation and/or CO production. However, information concerning the influence of P-containing compounds on the decomposition of hydrocarbons during steam cracking is scarce. In this study, the influence of three P-containing compounds, hexamethyl phosphoric triamide (HMPA), tripiperidinophosphine oxide (TPyPO) and dioctyl phenylphosphonate (DOPP), on the conversion of n-hexane during steam cracking was investigated in a continuous flow micro-reactor with complete mixing. All these compounds accelerate the thermal decomposition of n-hexane. The effect of a P-containing additive on the conversion of n-hexane is quantitatively evaluated using the dimensionless group method proposed by LaMarca et al. [C. LaMarca, C. Libanati, M.T. Klein, Chem. Eng. Sci. 45 (1990) 2059]. To interpret the accelerating effect, the potential elementary radical reactions involved in the steam cracking of these P-containing compounds are proposed. Their rate coefficients are estimated based on thermochemical data. For all of these P-containing compounds the main coupling with n-hexane decomposition occurs through H-abstraction reactions. The accelerated effect can be traced back to the introduction of new initiation steps. The evaluated values are consistent with the experimentally measured values.