Dehydrogenation of n-Butane to Butadiene-1,3 on an Alumina-Chromium Catalyst. Part 1: Kinetics of Dehydrogenation and Reactions of Coke Formation

Abstract

The kinetics of n-butane dehydrogenation to butadiene is studied with temperature (T) variation of 550–625°C, duration of dehydrogenation stage (t) of 5–30 min, and space velocity (V) of 4400–35 200 h⁻¹ on industrial catalyst K-CrO_x/γ-Al₂O₃ at a fraction of 56–94 μm. The catalyst is stabilized before studies. The granulated catalyst in a reduction–dehydrogenation–regeneration cycle at 593°C, and then as a fraction of 56–94 μm in dehydrogenation–regeneration cycle at 650°C. The maximum selectivity toward butadiene of ~25 mol % is achieved with n-butane conversion of 26–30% (V = 35 200 h⁻¹), T = 600 °C, and t = 5 min, while the maximum yield of butadiene ~10 mol % is obtained with an increase in conversion up to ~50% (V = 8800 h^–1) under the same conditions. Raising T to 625°C and t to 30 min and lowering V to ~4400 h^–1 increases the selectivity toward by-products to ~50 mol %. It is found that the energy of activation for the rates of product formation falls in the order by-products > butylene > butadiene. A kinetic model is proposed that describes the formation of butadiene via butylene, the formation of ethane/ethylene and methane/propylene by-products during butylene hydrocracking, and secondary conversions of by-products, plus the formation of coke and its effect on catalyst activity. In the model, the inhibition of dehydrogenation reactions by components of reaction mixture is described by a mechanism in which the limiting stage is a surface reaction on two active centers. The adequacy of the kinetic model is confirmed by good agreement between the calculated and experimental results.