Thermal effects accompanying spontaneous ignitions in gases II. The slow exothermic decomposition of diethyl peroxide

Abstract

An experimental investigation has been made of the self-heating that accompanies the (exothermic) decomposition of gaseous diethyl peroxide. Temperature histories have been recorded at 19 positions in a 1.21 spherical vessel by means of a very fine (13 μ m diam.) platinum/platinum + rhodium thermocouple. Temperature-position profiles constructed from these records show that the reactant is hotter than the vessel and that the excess is greatest in the centre. The absence of any detectable temperature excess at the walls is in agreement with a conductive theory of heat transfer and except at high pressures convective heat transfer is unimportant. To test quantitative agreement between theory and experiment, determinations of the thermal conductivity of the peroxide (alone and in mixtures) and of the stoichiometry and exothermicity of the decomposition have been made. The thermal conductivity is 6.4 cal (27 J) km -1 s -1 K -1 and the heat of reaction ( – ∆ H 298 ) is 47.0 kcal (197 kJ) mol -1 . The dependence of temperature on position differs significantly from that predicted by the simplest theory which ignores the non-uniformity of heat release arising from non uniform temperatures. Such a theory (Benson 1954) leads one to expect a parabolic temperature distribution: the true profiles are less steep at the edges and possess more curvature at the vessel centre. Critical phenomena are clearly displayed and it is found that the maximum stable centre temperature and the maximum stable edge-gradient are in excellent agreement with thermal theory. Around 200 °C, the course of slow reaction or pre-explosive decomposition may be represented by the equation: EtOOEt = 0.84EtOH + 0.40CH 2 0 + 0.385CO + 0.375CH 3 CHO + 0.272 5 C 2 H 6 + 0.24CH 4 + 0.032 5 H 2 , ∆ H = –47 kcal ( –197 kJ) mol -1 . Explosive decomposition is more extensive (but less exothermic): EtOOEt = 0.96CH 2 O + 0.49C 2 H 6 + 0.42CO + 0.40CH 4 + 0.32CH 3 CHO +0.30C 2 H 5 OH + 0.23H2 ∆ H = –38.3 kcal ( –160 kJ) mol -1 . The pattern of product yields in both slow and explosive decompositions can be interpreted in terms of the formation and subsequent reactions of the ethoxyl radical: ethanol formation by hydrogen abstraction is strongly favoured at low temperatures, while decomposition to yield formaldehyde, methyl radicals and some hydrogen atoms is favoured under explosive decomposition.