Ignition peninsula

A marked contrast is observed in the behaviour of the simplest aromatic hydrocarbon-air mixtures at high pressures. No cool-flame phenomena or an ignition peninsula in the (p-Ta) diagram are observed. These are found only when sufficiently reactive aliphatic side-chains are associated with the aromatic ring. Burgoyne et al. [129] showed this to be the case for n-propylbenzene in a closed vessel (Fig. 6.18). The ortho- and meta-isomers of the xylenes also showed a similar reactivity. Benzene, toluene and ethylbenzene were found to undergo spontaneous ignition at temperatures only above 700 K. 

Critical (extreme) phenomena are the specific features of the branching chain reactions namely, qualitative transfer from a slow reaction regime to an intensive (self-accelerated) one at minor changes in concentration of the initial reagents, kinetic parameters of the reaction system or ambient conditions. The region of the intensive reaction in the coordinates of temperature vs. pressure forms the so-called ignition peninsula [2,3] demonstrated in Figure 1.3. 

Fig. 53. Ignition peninsula of a stoichiometric hydrogen-oxygen mixture [407] (pi denotes the pressure at the lower and pg the pressure at the upper explosion limit)...

The induction period is one of the specific features of chain reactions. It has been studied in detail for the hydrogen combustion inside the ignition peninsula [239]. 

Inside the ignition peninsula, of great importance is also the process O -f OH -> Og + H, not included into the simplifed mechanism. 

Due to this opposite temperature nm, both limits converge at the temperature Tm ( cape of the ignition peninsula, Fig. 12.1) when p, = p2- This temperature is determined by the equality... 

Ignition peninsula of the stoichiometric mixture of dihydrogen and dioxygen. 

Since 2 is strongly temperature-dependent ( 2 = 21 kJ/mol), the region in which the reaction occurs in the non-stationary regime represents in the p-T coordinates the ignition peninsula (see Fig. 12.1). 

In the non-stationary regime, i.e., inside the ignition peninsula, the reaction proceeds with self-acceleration. Near the lowest ignition limit p] in the kinetic regime of chain termination on the wall and a small depth, the reaction kinetics is described in the first approximation by the system of only two differential equations... 

Figure 3.5 reveals that the low-pressure ignition of CO—02 is characterized by an explosion peninsula very much like that in the case of H2—02. Outside this peninsula one often observes a pale-blue glow, whose limits can be determined as well. A third limit has not been defined and, if it exists, it lies well above 1 atm. 

It has already been noted that the presence of small quantities of hydrogenous impurities expands the explosion peninsula. Such sensitization allows easier experimentation and provides for more reproducible results. The effect of hydrogen on the ignition limits is shown in Fig. 61. As was observed when considering the effect on the first limit, addition of sufficient hydrogen causes the reaction system to behave in essentially the same way as the H2 /O2 reaction. Dixon-Lewis and Linnett [30] found that, on replacing more than about 10 % of the CO by H2 in a KCl coated vessel at 510—570 °C, the second limit pressure could be extrapolated... 

The region of development of chain branched reaction is temperature-dependent. At an unchanged composition of the mixture and p = const, there is a temperature above which the fast reaction is observed and below which the reaction does not occur. In the / - r coordinates the ignition region for the chain branched reaction looks like a peninsula with a cape (T jn), so that at T < the reaction does not... 

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