Kinetic modeling of titania reduction by a methane-hydrogen-argon gas mixture

Abstract

This article describes kinetic modeling of titania reduction and carburization by methane-containing gas, based on experimental data reported previously by Zhang and Ostrovski. A sequence of titania reduction to titanium oxycarbide,

$$TiO_2 \to Ti_5 O_9 \to Ti_4 O_7 \to Ti_3 O_5 \to Ti_2 O_3 \to TiC_f O_{1 - f} $$

which was observed experimentally, is represented by the following two reactions:

$$TiO_2 + (H_2 + CH_4 ) \to {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}Ti_2 O_3 + H_2 O + CO$$

([1])

$${1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}Ti_2 O_3 + ({1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2} + f)CH_4 \to f(TiC)_{ss} + (1 - f)(TiO)_{ss} + ({1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2} + f)CO + (1 + 2f)H_2 $$

([2])

where ss designates a solid solution and f is the molar fraction of TiC in the solid solution. A two-interface shrinking-core model and a crackling-core model are employed for the kinetic modeling of the reduction and carburization process. The rates of Reactions [1] and [2] are both controlled by the chemical-reaction stage. For the intrinsic chemical-reaction control, the extent of the reaction as a function of reaction time is well described analytically. The two models give close results that are consistent with experimental data obtained at 1473 to 1773 K and a methane partial pressure up to 8 kPa. Reaction [1] is of the first order with respect to methane and of one-half to first order with respect to hydrogen. The apparent activation energy of reaction [1] is 124 kJ/mol for the two-interface shrinking-core model and 126 kJ/mol for the crackling-core model. Reaction [2] is of the first order with respect to methane and is independent of hydrogen concentration. Nevertheless, hydrogen plays an important role in the reduction/carburization process, as it suppresses the decomposition of methane and deposition of solid carbon. The apparent activation energy of the reaction is 161 kJ/mol for the two-interface shrinking-core model and 191 kJ/mol for the crackling-core model.