Self-heating and spontaneous ignition in the combustion of gaseous methylhydrazine

Abstract

The exothermic oxidation of gaseous monomethylhydrazine (CH₃NHNH₂) has been investigated in the gas phase over a temperature range 370–510°C in a 1-litre vessel. Critical conditions of pressure, temperature and reactant proportions for spontaneous ignition, and the effects of inert diluents have been examined. CH₃NHNH₂+ 2.5 O₂→N₂+ CO₂+ 3 H₂O; ΔH=– 1340 kJ mol^–1(–320 kcal mol^–1) Temperature changes have been followed by means of very fine thermocouples (13 µm diam. Pt–Pt/Rh) coated to prevent surface catalysis. The reactant is always hotter than the vessel, and the temperature excess is greatest at the centre, in accord with conductive theory. Uncommonly among gas-phase oxidations, the combustion of methylhydrazine is dominated by thermal effects. As was previously found for hydrazine, large central temperature excesses (about 80 and 100°C) are common and have to be exceeded for ignition to occur.

The critical temperature excesses are consistent with the effective activation energy (ca. 80 kJ mol^–1 or 19 kcal mol^–1) derived from critical pressure limits and taken together indicate that the critical increment in average temperature is between 1.1 and 1.2 RT²_a/E. These values do not provide support for the theoretical treatment by Adler and Enig, which requires stable temperature excesses greater than 2.41 RT²_a/E to be realisable for a second order reaction; they are consistent, however, with our recent analytical and computational predictions on the stability of such systems.