Lower ignition threshold

The same data permit the computation of the progress of these concentrations as a consequence of a flash evaporation at distances of 100 and 500 m from the emission source (Figure 7.27). Due to the rate of dispersion of the very rapidly released amount of ethylene, the ethylene/air mixture would remain ignitable for approximately 350 s at a distance of 500 m from the emission source, i.e., the lower ignition threshold would have been exceeded and the upper level would not yet have been reached. 

If we consider a certain temperature between point B and point C and the pressure is increased, after the slow combustion area we will reach a first pressure threshold Pi. This is also called the lower ignition pressure at pressures above this the mixture ignites. By further increasing the pressure we will reach the corresponding pressure threshold P2, above which there is no mixture ignition. By further increasing the pressme, we now reach threshold pressure P3, from which ignition of the mixture reappears. 

Thus the theory of branched chain reactions can account for two explosive thresholds restricting an area of lower ignition. Qualitative results are therefore interpreted correctly. 

The detailed, slow photolysis of azides by UV- or X-irradiation is described in Chapter 7, Volume 1. Direct ignition or initiation of azides by light flashes has normally been interpreted in terms of a thermal mechanism, perhaps preceded by a photochemical step. More sophisticated approaches involving the careful selection of the wavelength and the application of an electric field are described in Chapter 9, Volume 1. These indicate much lower thresholds for ignition than those predicated on a thermal mechanism. 

Other factors intervene to modify the values of critical pressures, such as the presence of inert gas which lowers the pressure at both lower and upper threshold pressures of ignition. The lower area of inflammation may eventually disappear. Some substances play a positive catalytic role, for example nitrogen pentoxide for oxidation. Others have a negative role, acting as an inhibitor, for example halogens in oxidation reactions. 

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