Slow oxidation of ethane and ethylene in the gas phase. Part 1.—General features at 362°C

Abstract

The kinetic and analytical features of the oxidation of ethane and ethylene at 362" are described.In the early stages of the oxidation of ethane, 80 % of the hydrocarbon consumed gives ethylene.In the intermediate stages the ethylene is itself oxidized. In the initial stages of the oxidation of pure ethylene over 80 % of the hydrocarbon consumed gives formaldehyde, and in the intermediate stages the formaldehyde is oxidized to carbon monoxide. The main molecular stages in the oxidation are therefore ethane, ethylene, formaldehyde, carbon monoxide, with water also being formed at the appropriate stages.The kinetics of the ethane oxidation are similar to those of the propane oxidation and the autocatalysis of the reaction is probably due to the oxidation of formaldehyde. The slow start of the acceleration is in part due to the fact that the branching intermediate is not an initial product of the reaction.The general nature of hydrocarbon oxidation reactions is discussed. It is concluded that the main reaction of alkyl radicals with oxygen at 300-400" is usually an abstraction reaction However, there is strong evidence that the H02 radical is unable to abstract H from ethane or most other hydrocarbons, and it is proposed that a key reaction in alkane oxidation is the conversion of HO2 to OH. The reaction 2H02 = 20H+02 is suggested.CnH2n+1+ 0 2 = CnH2n+ HO2.

Evaluated and estimated kinetic data for phase reactions of the hydroperoxyl radical

“…These results show reasonable agreement with those of Baldwin et al [95]. In order to derive an approximate Arrhenius expression for this reaction, a value of 14 kcal/mole may be taken for the activation energy, as suggested by Benson [65], Fish [97],and Knox and Wells [98]. Combination with k19(C&) = 1 X lo8 cm3/mole-sec at 773°K gives k19(CzHs) = 1 X 1OI2 exp [-14,00O/RT] cm3/mole-sec Both A and E are probably upper limits.…”

Addition of ethane to slowly reacting mixtures of hydrogen and oxygen at 500°C

“…When a hydrocarbon such as C2H6 is introduced into the reacting system, it will be removed by reactions ( 21)- (24).…”

“…Reaction (8), which is important at the second limit, has been shown, both by mathematical analysis l4 and by computer methods,16 to play little part in the slow reaction, Studies l 7 of the H2-sensitized decoinposition of H202 give k14a/k14 = 0.16 at 500"C, so that reaction (14a) makes only a minor contribution which, for preliminary mathematical treatment, can be replaced by an equivalent contribution from reaction (14). Computer treatment * has shown that reaction (15) has little effect on the relative radical concentrations, and that reaction (11) can also be neglected in a preliminary treatment unless there is a major contribution from reaction (24), since its occurrence increases proportionately the concentrations of H, 0 and OH.…”

The thermal reactions of ethylene at 500°C in the presence and absence of small quantities of oxygen

“…Although there are some uncertainties in these estimates, reaction (29) appears to be the only pro- cess, other than reactions (251, (261, and (27), significantly faster than reaction (1). Formaldehyde, although not analyzed in these experiments, is known to be an intermediate in the oxidation of ethylene at lower temperatures [18,19]. The mechanism by which it is formed has not been established, and some possible reactions are discussed in the next section.…”