Explosion peninsula

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. 

Such reactions have been used to explain the three limits found in some oxidation reactions, such as those of hydrogen or of carbon monoxide with oxygen, with an "explosion peninsula between the lower and the second limit. However, the phenomenon of the explosion limit itself is not a criterion for a choice between the critical reaction rate of the thermal theory and the critical chain-branching coefficient of the isothermal-chain-reaction theory (See Ref). For exothermic reactions, the temperature rise of the reacting system due to the heat evolved accelerates the reaction rate. In view of the subsequent modification of the Arrhenius factor during the development of the reaction, the evolution of the system is quite similar to that of the branched-chain reactions, even if the system obeys a simple kinetic law. It is necessary in each individual case to determine the reaction mechanism from the whole... 

The reaction mixture is initially outside the explosion peninsula. However, a sensitizer, which shifts the peninsula, may be produced at a relatively slow rate. Explosion would then occur when the shift was sufficient to enclose the mixture temperature and pressure within the peninsula. The induction period would then have the same temperature dependence as above. 

Fig. XIV.3. Over-all rate of reaction of a branching chain reaction as a function of pressure at fixed temperature. Pl is the lower explosion limit and P is the upper explosion limit of the explosion peninsula Pz corresponds to third explosion limit (Fig. XIV.26). For purposes of comparison, the dotted curve TR illustrates the rate of a normal rea( tion up to the thermal explosion limit Pte (see Fig. XlV.2a).

If we are to construct a kinetic model to explain the preceding behavior, it seems most reasonable to start by saying that in the explosion peninsula,... 

It is difficult to postulate without considerable strain a branching process of higher kinetic order in total pressure and first or higher order in radicals which could account for the third explosion limit. Much attention has been given to this problem without a decisive answer appearing. The chief reason lies in the appearance of the thermal explosion limit in this region of pressures above the explosion peninsula. In the absence of specific evidence to the contrary it is usually safer to interpret the third explosion limit as a thermal explosion. 

It seems to be necessary, in order to account for the influence of the wall on the first explosion limit, to postulate termination of chains on the wall. From other evidence (which we shall discuss later in a few specific cases) on the slow rates outside the explosion peninsula it appears equally necessary to postulate chain initiation at the walls. Where surfaces such as the walls play an important role in chemical reactions, we must expect to find that in a certain range of reaction conditions the diffusion to and from such surfaces may exert a limiting effect on the rates of chemical reactions. If the reactions arc taking place in the volume of the vessel in competition with reactions at the wall, then we may expect to find concentration gradients within the volume of the vessel. ... 

The features of the explosion peninsula are still at issue because of the difficulties associated with minute amounts of H2O and irreproducible surface effects. In common with most explosions, light is produced in fact... 

W. L. Garstang and C. N. Hirishclwood, Proc. Roy. Soc. Lorulon)y A130, 640 (1931). Organic iodides and HI are also effective bromides and Br2 somewhat less so. As little as 0.002 per cent of I2 can completely eliminate the explosion peninsula. 

First, there is an inhibiting effect of the additive on the low pressure explosions, so that the second limit pressure is reduced and the first limit is raised on addition of the hydrocarbon [329—331]. Secondly, there may be an increase in the maximum rate (of decrease of pressure) in the slow reaction [330]. Thirdly, induced explosions may occur in some cases (not with methane) at pressures outside the + O2 explosion peninsula. In most cases such induced explosions appear as one sharp explosive reaction. However, they are sometimes characterized (e.g. with CaHg at 560 °C) by an induction period during which there is a rapid pressure increase, and this is followed immediately by a very rapid pressure decrease in the system. It is probable that all the induced explosions follow this two-stage pattern. This type of explosion does not occur in H2—Oj—CH4 mixtures because methane is not as reactive as propane in... 

Because of the extreme sensitivity of the position of the explosion peninsula to small amounts of hydrogenous impurity, it is not possible to separate the attempts to measure the position of the explosion region for dry mixtures on a P-T diagram from those in which impurities or additives were definitely present. 

Similar promoting and inhibiting effects have been observed by Hoare and Walsh [357], who found that the addition of 6 % methane considerably expanded the explosion peninsula, but that with 10 % methane the explosion regime was less extensive. Ammonia had a similar effect, with additions above 6 % causing the first limit pressure to increase. Methanol also lowers the first limit [364], as shown in Fig. 57. ESR studies of CO/O2 flames containing deuterated methanol indicated that hydrogen... 

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... 

Outside the explosion peninsula, particularly in the region above the second limit, the oxidation of CO can take place at a speed convenient for normal kinetic measurements. As with the explosive combustion, the kinetics are very sensitive to the surface and to the purity of the reacting gases. 

The reaction is promoted by small additions of hydrogenous materials, the kinetics being radically altered with the rate of oxidation. Near the tip of the explosion peninsula [374] the rate is proportional to [CO] [H2]/... 

Critical explosion limits for a typical branching chain explosion showing explosion pe-ABCD represents explosion limits. The region ABC is called the explosion peninsula. 

Fig. 12, Explosion limits for a typical branched-chain reaction. The region ABC is known as the explosion peninsula. The curve ABCD is the boundary between explosive and bounded reaction rates.

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