Temperature distribution in endothermic gas reactions: nitrosyl chloride pyrolysis

Abstract

Very fine thermocouples (made from 0.025 mm Pt and Pt-13% Rh wire) have been used to follow temperature changes and to map temperature-position profiles during an endothermic reaction, the pyrolysis of nitrosyl chloride NOCl→NO+0.5 Cl₂, ΔH/kJ mol^–1= 37.44. Self-cooling is always observed. Temperatures are lowest at the center of the reactant, i.e., at the axis of the cylindrical reaction vessel, typically falling 5 K below that (T_a) of the vessel walls for an initial pressure ca. 50 mmHg. If conduction is the only important mode of heat transfer, the temperature distribution in a truly stationary state is governed solely by the size of the dimensionless group (QE/λ)(RT²_a)wr²₀(the δ of Frank-Kamenetskii), and satisfies the relationship [graphic ommitted] Here, z is the fractional distance from the axis and C is an integration constant related to δ by 8C=δ(1+C)².

In size and shape the observed temperature profiles agree well with expectation. This confirms the dominant role of heat conduction in determining the magnitude of the cooling. Discrepancies between theory and experiment are insignificant except at the highest densities when the onset of convection is possible. The errors in k, A and E that may arise if self-cooling is unsuspected or ignored are discussed briefly.