Explosion limits third

In all the work on the third limit it has been found that salt coating of the reaction vessel is essential in order to obtain reproducible results. Normally a thick coating of potassium chloride has been used for this purpose, but by altering the thickness of the coating Heiple and Lewis [35] were able to alter the surface efficiencies for chain breaking and 

Dependence of third explosion limit on diameter and temperature with KCl coated vessels [35] 

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GP 11] [R 19] The third explosion limit is discussed in detail in [9] as it is important from both practical and mechanistic viewpoints (230-950 °C 10-10 Pa). This limit is normally responsible for the occurrence of explosions imder ambient pressure conditions. In addition, these explosions are known to be kinetically induced by radical formation. The formation of these species is sensitive to size reduction of the processing volume owing to the impact of the wall specific surface area on radical chain termination. It turns out that the wall temperature has a noticeable, but not decisive influence on the position of the third limit The thermal explosion limit lies below the kinetic limit for all conditions specified above (Figure 3.50) [9]. 

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

The upper (third) explosion limit is due to a reaction that overtakes the stability of the H02 and is possibly the sequence... 

A detailed mechanistic investigation of the explosion limits revealed [59] that the first and the third explosion limits of the reaction are dependent on the reactor dimensions (see Figure 2.27). The first explosion limit is reached when the mean free path of the molecules becomes smaller than the reactor dimensions, which... 

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).

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. 

The form of Ecj. (XIV.7.7) is sufficient to fit the second and third explosion limits. However, again the quantitative fitting of the constants is practically meaningless because of the appearance of the quite variable surface terms. For this reason also we shall not attempt to discuss the various numerical results which have been obtained for the different rate constants or the assigned activation energies. Extensive studies of the effects of various surfaces on the different explosion limits have been made by Warren, who was not able to assign a specific chemical role to the behavior of the different surfaces studied or to detect significant differences in effects of the wall on the different radicals present. 

The balance of the two processes accounts for the characteristics of the steady reaction. Moreover, calculation shows that as the pressure increases further the diffusion of the HO2 itself will be so much impeded that a third explosion limit is reached. 

Of course, the coefficient B involves a combination of the activation energies of the reactions, (a)-(c), which determine , together with the temperature dependence of log which brings in reaction (0) as well. Also, it is clear that the neglect of reaction (/) and its higher-order dependence upon gas density makes equation (2.15) totally unsuitable for extension to conditions which approach the second or third explosion limits. 

Figure 4.1 Variation of the rate of reaction in a system containing a 2 1 mole ratio of hydrogen to oxygen as a function of the total pressure. First, second, and third explosion limits are labeled a, b, and c, respectively. Displacement of the first and second limits to lower pressures occurs on addition of an inert gas or enlarging the volume of the reactor (dashed lines). (Adapted from J. W. Moore and R. G. Pearson, Kinetics and Mechanism, 3rd ed., p. 409. Copyright 1981 by John Wiley Sons, Inc.)...

No general theory of the third explosion limit has been advanced however, it is generally assumed that such explosions are thermal in nature and reflect the system s inability to dissipate heat. However, in the Hg-Og system a new kinetic pathway may become important. As diffusion of HOg is retarded surface reaction is less important. The reaction... 

The seven elementary steps of (7.5) and (7.11) account for kinetics in the H2-O2 system below the third explosion limit. 

The isothermal model, constituted by eqs. (16) and (18) alone, does not predict the occurrence of the third explosion limit. In particular, when applying the generalized criterion to this model, it is found that the predicted values of the second explosion limit increase continuously as the initial pressure increases. For values larger than about 100 Torr the curves representing the normalized objective sensitivities become flatter and the maximum tends to vanish. Using the non-isothermal model, given by eqs. (15) to (17), the explosion limits predicted by the... 

Above such a limiting condition one reaches the limit of thermal explosion. This corresponds to the third explosion limit of the reaction O2/2H2. The explosions in the reactive system CI2/H2 are also primarily of a thermal nature, because the chain is not a branching one and owing to the high activation energy, E = 141 kJ mol the rate of formation of the chain carriers is sufficiently low to be the source of any isothermal explosion. 

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