Cool flame limits

The increasing reactivity as the homologous series ascends is illustrated in Figure 1. All three ketones show cool flames the cool-flame limits (35) of acetone are shown in Figure 2. 

Figure 2. Cool flame limits for an equimolar acetone-oxygen mixture (35)...

A similar sharp transition has been noted [43] when formaldehyde is the additive. Furthermore, there is an accompanying shift of the cool flame limits to higher pressures and/or temperatures. Formaldehyde also interferes with the cool flame combustion of acetaldehyde in flow systems [7]. Its addition retards the development of the cool flame but promotes the second stage this promotion may be caused by the occurrence of... 

Fig. 5. The cool flame limit curves of equimolar ketone—oxygen mixtures in an HF treated spherical pyrex vessel, diameter = 10.1 cm [45], (1) Acetone (2) methyl iso-propyl ketone (3) methyl ethyl ketone (4) methyl n-propyl ketone (5) diethyl ketone.

Diethyl ketone gives rise to a single and multiple cool flames under relatively mild conditions, and the cool flame limit diagram [45] is shown in Fig. 5. The cool flames are accompanied by considerable temperature rises [37(a)]... 

The Arrhenius graph for the combustion of this compound possesses a region of negative temperature coefficient, and the cool flame limits have been determined [45]. 

We are not aware of any significant study of autoxidation of polypropylene at elevated temperatures, i.e. oxidative pyrolysis. On the other hand, much is known about gas-phase oxidation of hydrocarbons at high temperatures and the cool-flame limit (25,26,27,28). The reactions are recognized as free radical chain reactions propagated by peroxy radicals and hydroperoxides which was essentially a development of Backstrom s scheme for the oxidation of aldehydes (29). These mechanisms may be adapted to the oxidative pyrolysis of polypropylene. 

Dichloroethylene is usually shipped ia 208-L (55 gal) and 112-L (30 gal) steel dmms. Because of the corrosive products of decomposition, inhibitors are required for storage. The stabilized grades of the isomers can be used or stored ia contact with most common constmction materials, such as steel or black iron. Contact with copper or its alloys and with hot alkaline solutions should be avoided to preclude possible formation of explosive monochloroacetylene. The isomers do have explosive limits ia air (Table 1). However, the Hquid, even hot, bums with a very cool flame which self-extiaguishes unless the temperature is well above the flash poiat. A red label is required for shipping 1,2-dichloroethylene. 

The complex and incompletely understood phenomena of cool flames and then-close relationship with autoignition processes is discussed in considerable detail. As the temperature of mixtures of organic vapours with air is raised, the rate of autoxidation (hydroperoxide formation) will increase, and some substances under some circumstances of heating rate, concentration and pressure will generate cool flames at up to 200° C or more below their normally determined AIT. Cool flames (peroxide decomposition processes) are normally only visible in the dark, are of low temperature and not in themselves hazardous. However, quite small changes in thermal flux, pressure, or composition may cause transition to hot flame conditions, usually after some delay, and normal ignition will then occur if the composition of the mixture is within the flammable limits. 

FIGURE 3.9 General explosion limit characteristics of stoichiometric hydrocarbon-air mixture. The dashed box denotes cool flame region. 

The cool-flame phenomenon [15] is generally a result of the type of experiment performed to determine the explosion limits and the negative temperature coefficient feature of the explosion limits. The chemical mechanisms used to explain these phenomena are now usually referred to as cool-flame chemistry. 

The preflame reactions include slow oxidation and cool flame reactions (110). Slow oxidation threshold temperatures and reaction rates have been considered important factors in controlling knock resistance (43, 75, 133). Knock ratings have been related to cool flame intensities and temperature limits (36, 43, 153). Recently, Barusch and Payne (9) have found striking correlations between octane number and the position of the cool flame in a tube (a parameter which should be a function of Ti). The heat evolved during cool flame reaction may also be a vital factor in determining the occurrence of knock (106,156,179). 

Levedahl (106) noted that aliphatics from acetylene to octane all gave hot flames at 600° C. He suggested that some reaction, such as thermal decomposition, which was common to all aliphatics, became important. Benzene showed a very high, inconsistent hot flame limit, while cyclohexane was low and variable. Levedahl believed the cool flame and subsequent reaction, along with compression, served to raise the mixture temperature to the critical value. Acetylene was believed to play a major role in the ignition reaction. 

Radiation and Ignition Studies. The existence of cool flame radiation prior to the occurrence of autoignition in a motored engine was discovered early by Peletier, van Hoogstraten, Smittenberg, and Koojman (109). Since then there has been a constant effort to define the engine conditions limiting the occurrence and extent of cool flame radiation with various hydrocarbons (25, 26, 37, 71, 72, 77, 107). 

Cool flame behaviour is the result of highly limited degenerate branching, where the branching coefficient, a, is small, e.g. is of the order of 1.1, compared with being of the order of two or three in normal branching. 

H2/02 reaction - an example of a reaction with explosion limits, 249-252 Cool flames in oxidation of hydrocarbons - an analysis of the chemical behaviour and reactions involved, 254—259... 

Fig. XIV. 10. Ignition limits for equimolar mixtures of CjHs and O2. Limits after work of D. M. Newitt and L. S. Thornes, J, Chem. Soc., 1656, 1669 (1937). The thin dark-hatched boundary between slow combustion and explosion is the region of intense blue flames. The contoured regions designated as cool flame regions arc numbered after the number of successive cool flames which can be observed in them.

This delayed decomposition of the metastable peroxides has been termed degenerate chain branching by Semenoff and has been used by him to account for the ignition limits of hydrocarbon oxidations, in particular for the long induction periods preceding the cool flames. 

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. 

The region of cool flames, which also has a region of positive slope (and is in this sense analogous to the second explosion limits for H2 and CO), has been the subject of much interest. The cool flames are most arresting because of the very long induction periods that precede them. Andrew" found that, in n-butane + O2 mixtures, the induction period decreased ex-... 

The form of the solutions to the simplified model were analysed by examining the existence and types of the pseudo-stationary points of the equations for d0/dr = d 3/dr = 0 and values of e in the range 0—1 (r = Figure 29 shows the oscillation of a multiple-cool-flame solution about the locus of such a pseudo-stationary point, Sj. The initial oscillation is damped while Si is a stable focus. The changing of Si into a unstable focus surrounded by a stable limit cycle leads to an amplification of the oscillation which approaches the amplitude of the limit cycle. When Si reverts to a stable focus, and then a stable node, the solution approaches the locus of the pseudo-stationary point. In this way an insight may be gained into the oscillatory behaviour of multiple cool flames. 

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