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 RT2a/E. These values do not provide support for the theoretical treatment by Adler and Enig, which requires stable temperature excesses greater than 2.41 RT2a/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.