Self-heating and spontaneous ignition in the oxidation of gaseous hydrazine

Abstract

The spontaneous ignition of gaseous mixtures of oxygen and hydrazine has been investigated in a 1 l. spherical vessel over the temperature range 690–770 K. Emphasis is placed on direct detection of pre-explosive self-heating in the reacting gas. Very fine thermocouples (made from 0.025 mm Pt and Pt-Rh wires) are used to follow temperature changes, and to map temperature-position profiles during oxidation: N₂H₄+ O₂→ N₂+ 2H₂O, ΔH=–138 kcal mol^–1.

Unusually among gas-phase oxidations, this spontaneous ignition is clearly thermal in character. Strong self-heating is always observed. In accordance with a conductive theory of heat losses, temperatures are not uniform throughout the reactant but depend on the distance from the vessel centre, being greatest at the centre and least at the walls. The (approximately) parabolic form and the absence of any significant temperature-step at the walls confirm the absence of convection at the pressures concerned.

A critical centre temperature rise exists above which stable reaction is impossible and ignition inevitable. Thermal theory predicts a critical value of 1.61 RT²_a/E, i.e., about 70 K, under circumstances where reactant consumption can be ignored, and a value of 116 K when it is taken into account: the value observed here is 120 K.

The hydrazine + oxygen system is more difficult to handle than some systems previously investigated and catalysis at the thermocouple is harder to eliminate. These effects, and the consequences of employing thermocouples of finite heat capacity, are also discussed.