Ignition diagram

Fig. 6. Schematic ignition diagram for a hydrocarbon+ O2 mixture, with appHcations. Region A, very rapid combustion, eg, a jet engine region B, low temperature ignition, eg, internal combustion engine, safety ha2ards regions C and D, slow oxidation to useful chemicals, eg, 0-heterocycHc compounds in C and alcohols and peroxides in D. Courtesy of Blackwell Scientific PubHcations, Ltd., Oxford (60).

Modern society probably depends more upon the combustion of hydrocarbons than upon any other chemical reaction. The various applications of hydrocarbon combustion were recently described [1] in terms of a typical temperature—pressure ignition diagram for a hydrocarbon + oxygen (or air) mixture, as shown in Fig. 1. 

Fig. 1. Ignition diagram for a typical hydrocarbon and oxygen mixture 1, conversion (by ignition) of chemical energy, e.g. turbo-jet engine 2, conversion (by ignition) of chemical energy to heat and mechanical energy, e.g. internal combustion engine 3, conversion of fuels to potentially useful chemicals, e.g. O-heterocycles and 4, controlled conversion of fuels to useful chemicals, e.g. alcohols, peroxides, aldehydes, ketones, etc. (From ref. 1.)...

A typical temperature—pressure ignition diagram is shown in Fig. 8 for mixtures of 3-ethylpentane and oxygen in the molar ratio 1 2. The low temperature region of the diagram which is associated with the propagation of multiple cool flames shows fine structure , i.e. there are several minima and maxima in the value of the initial pressure required for... 

Fig. 16. The pressure—temperature ignition diagram for propane—oxygen mixtures in the molar ratio 1 1. Cylindrical silica reaction vessel, volume = 30 cm. (1), (4) slow reaction (2), (5) slow reaction with pic d arret (3) normal flames (6) cool flames. (From ref. 147.)...

Fig, 17. The pressure—composition ignition diagram for propane—oxygen mixtures at 429 °C. (From ref. 147.)... 

Fig. 26. The temperature—pressure ignition diagram obtained from the model for acetaldehyde + oxygen + argon mixtures in the molar ratio 1 1 1. (From ref. 183.)...

Values of the coefficients for the set of differential equations were chosen to give cool flames at realistic initial pressures and temperatures. Further restrictions on the choice of coefficients were imposed by requiring that the fuel conversion should not exceed 25 % at the maximum of the temperature pulse, that the induction period should be between 15 and 20 sec, and that the thermal relaxation time should be 0,25 sec. To achieve this the rate coefficients of reactions (d), (f), (h) and (g) were varied about reasonable estimates of their likely values. The parameters chosen for the model are given in Table 24. The computer was used in a conversational mode to map out an ignition diagram (Fig. 26) which compares favourably with that found experimentally [191] (Fig. 27). 

Fig. 31. The temperature—pressure ignition diagrams for equimolar mixtures of acetaldehyde + oxygen + argon determined by the modified model and experimentally. ---, Complete diagram — — experimental. (From ref. 194.)...

The p-Ta ignition diagram obtained in the above manner shows an explosion limit. Fig. 5.19, similar in both form and location to that observed for identical mixture compositions in closed vessels and also to similar slow reaction behaviour for pressures above the limit. Use of a flow reactor, however, also allows the behaviour on the ignition side of the... 

Fig. 5.19. Thep-Ta ignition diagram for an equimolar H2 + O2 mixture with mean residence time tres = 5.2 0.7 s showing region of slow reaction separated by second limit from regions of oscillatory ignition and steady-ignited state. (Reprinted with permission from reference [33], Royal Society of Chemistry.)...

The basic global behaviour of a mixture of acetaldehyde vapour in O2 is illustrated by reference to the p-T ignition diagram. Fig. 5.39. Up to five regions of qualitatively different responses are characteristic [75]. At low ambient temperature and pressure, the system exhibits a steady dark reaction. Region I. This may support a measurable steady-state tem-... 

Cool-flames arise for some ranges of experimental conditions for many other hydrocarbon oxidation reactions. Chapter 6 presents many p-T ignition diagrams for different hydrocarbon fuels, all of which have a... 

Fig. 6.10. Idealized (p-T ) ignition diagram which is typical of that obtained for the combustion of many hydrocarbons in oxygen or air in closed vessels and well-stirred flow systems. Ignition, oscillatory cool-flame and slow reaction regions are shown.

Fig. 6.12. Ignition and cool flame boundaries (broken lines) in the (p-T ) ignition diagram for the combustion of propane in air as the proportion of propane (% by volume) in air is...

Fig. 6.13. A comparison of (p-Ta) ignition diagrams for n-butane + oxygen in the molar proportions (a) 1 1 and (b) 1 2 respectively. (After Bardwell [15].)...

Fig. 6.15. A comparison of the (p-T ) ignition diagrams for the oxidation of /i-butane and n-hexane in stoichiometric proportions in air in a closed vessel. Points a-d are discussed in the text. (After Kane et al. [138].)...

A complementary ignition diagram to that for n-C4Hio in an unstirred closed vessel was obtained in a jet-stirred flow reactor by Proudler et al. [58]. The reactor was a 500 cm stainless steel cylinder, operated at a mean residence time of 9.4 0.4 s with a reactant mixture [n-C4Hio] [O2] = 1.13 1 (Fig. 6.16). Oscillatory cool-flames and ignitions were detected within narrow temperature ranges, but comparable with those of the closed... 

Fig. 6 16. p-Ta) ignition diagram obtained for the combustion of n-butane + oxygen in a well-stirred flow reactor at a mean residence time of 9.4 0.4 s. Different phenomena are identified as follows L, low-temperature stationary states H, high-temperature stationary states CF, oscillatory cool-flames I, oscillatory ignitions. A hysteresis of the boundaries was measured as a result of increasing (solid lines) and reducing (broken lines) the vessel temperature. (After Proudler et al. [58].)... 

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