Flames hydrocarbon-oxygen

For sufficiently large energy of activation such as that for hydrocarbon-oxygen mixtures where E is of the order of 160kJ/mol, (E/RT) > 1. Thus most of the energy release will be near the flame temperature, 7] will be very near the flame temperature, and zone II will be a very narrow region. Consequently, it is possible to define a new variable a such that... 

The low-temperature oxidation represents a complex system and can be better interpreted when the elementary reactions are firmly established. We arc inclined to assign formaldehyde only a minor role in the low-temperature regime. Further experimental work is required to clarify the interactions between formaldehyde and peroxides, the radical-induced formaldehyde oxidation, and the effect of formaldehyde addition in the low-temperature hydrocarbon-oxygen systems. It has been established that mercury vapor is effective for the destruction of peroxides. Mercury vapor addition to systems in the cool-flame zone would perhaps be of value in assessing not only the role of peroxides, but also that of formaldehyde in this interesting region. 

Provided (f - g) > 0, the chain carrier concentration, and hence, the reaction velocity will increase exponentially with time. However, (/ - g) may be small enough so that t, corresponding to the induction period, r, may be very long. If (f — g) < 0, a true explosion never develops. A slow change from — to + values of (/ - g) has been observed for hydrocarbon-oxygen systems. These phenomena are sometimes referred to as degenerate chain-branching explosions or cool flames (44, ) ... 

The cool flame phenomenon seems to be closely tied to the formation of aldehydes and peroxides in oxidation systems. In Fig. XIV. 10 is shown a typical example of the explosion limits for a hydrocarbon-oxygen mixture. The explosion region, except for a region of positive slope, resembles the limit curve for a thermal explosion. The transition between slow combustion and explosion is characterized by an intense luminous blue flame which appears after a short induction period and is followed by explosion. The induction periods are not more than a few seconds. 

Further support for the attainment of a critical concentration of hydroperoxide prior to the passage of a cool flame at temperatures corresponding to the Lq and L, lobes has been obtained by Taylor [131], and more recently by Pollard and co-workers [68,132], who determined the maximum concentrations of tert-butyl hydroperoxide found during the cool-flame oxidation of isobutane. Again, the concentration of hydroperoxide increased prior to the cool flame and it was almost entirely consumed during its passage (Fig. 12). Also, in common with other hydrocarbon + oxygen systems, (e.g. refs. 55, 65, 78,133) the induction period to the first cool flame (r,) was related to the initial reactant pressure (po) by the expression... 

Notwithstanding the obstacles, however, some absorption studies of combustion processes have been made. Molecular intermediates, such as aldehydes and acids, have been identified in the slow combustion of propane . Hydroxyl radicals can be observed in the absorption spectra of several flames . The greatest success in the application of absorption spectroscopy to flame studies has been in investigations of diffusion flames. Wolfhard and Parker studied the diffusion flames in oxygen of hydrogen, ammonia, hydrocarbons and carbon monoxide. In every case they were able to observe absorption by hydroxyl radicals, and they observed also the absorption of NH in the ammonia flame (NH2 appeared in emission only). Molecular oxygen, and in suitable cases the reactants, could be detected by their absorption spectra, so that a clear picture of the structure of the diffusion flame... 

The hydrocarbon (methane or naphtha) and oxygen are preheated before introduction into a combustion chamber where, after passing through a venturi, they enter the burner block fitted with a hundred or so channels. Small amounts of oxygen introduced in countercurrent flow enhance the stability of the flame. The oxygen to hydrocarbon ratio is regulated s o that pan (about one-third) of the hydrocarbon is burned, mid the remainder cracked. The gas formed is quenched with water at a level of the combustion chamber corresponding to the maximum acetylene production- The coke formed is withdrawn and separated. 

Kilham, J. K., and Purvis, M. R. I. "Heat Transfer from Hydrocarbon-Oxygen Flames." Combustion and Flame 16 (1971) 47-54. 

As written, these reactions are exothermic and, as ion-molecule reactions, can be expected to be very rapid, with rate coefficients 10 ml sec . In the temperature range 2000-2500°K, the equilibrium constant for reaction (46) is between 3 and 5 and for reaction (47) is between 27 and 42. Thus, for the reverse reactions, the rate coefficients will be > 10 ml sec These ion-molecule reactions are therefore faster than the reactions governing the neutral species concentrations H, H2, OH, O, and O2. The ions then are in equilibrium with the neutral species whether or not equilibrium among the neutral species has been established. Miller has measured the ratios of O , OH , and O2 downstream of low-pressure hydrocarbon-oxygen flames, from which he calculated, assuming equilibrium, the ratios of OH to H and of H2 to H. Comparison of the measured and equilibrium... 

M.E. Morrison and K. Scheller, The effect of burning velocity inhibitors on the ignition of hydrocarbon-oxygen-nitrogen mixtures. Combustion and Flame, 18, 3-12, (1972). 

Cool Flames. An intriguing phenomenon known as "cool" flames or oscillations appears to be intimately associated with NTC relationships. A cool flame occurs in static systems at certain compositions of hydrocarbon and oxygen mixtures over certain ranges of temperature and pressure. After an induction period of a few minutes, a pale blue flame may propagate slowly outward from the center of the reaction vessel. Depending on conditions, several such flames may be seen in succession. As many as five have been reported for propane (75) and for methyl ethyl ketone (76) six have been reported for butane (77). As many as 10 cool flames have been reported for some alkanes (60). The relationships of cool flames to other VPO domains are depicted in Figure 6. 

Effect of Pressure. The effect of pressure in VPO has not been extensively studied but is informative. The NTC region and cool flame phenomena are associated with low pressures, usually not far from atmospheric. As pressure is increased, the production of olefins is suppressed and the NTC region disappears (96,97). The reaction rate also increases significantly and, therefore, essentially complete oxygen conversion can be attained at lower temperatures. The product distribution shifts toward oxygenated materials that retain the carbon skeleton of the parent hydrocarbon. 

In summary, the bad features of partial combustion processes are the cost of oxygen and the dilution of the cracked gases with combustion products. Flame stability is always a potential problem. These features are more than offset by the inherent simplicity of the operation, which is the reason that partial combustion is the predominant process for manufacturing acetylene from hydrocarbons. 

In the heating and cracking phase, preheated hydrocarbons leaving the atomizer are intimately contacted with the steam-preheated oxygen mixture. The atomized hydrocarbon is heated and vaporized by back radiation from the flame front and the reactor walls. Some cracking to carbon, methane, and hydrocarbon radicals occurs during this brief phase. 

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