Temperature limit, cool flames

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. 

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. 

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... 

The only condition for cool-flame production with ethanol was that the temperature at the centre of the vessel rose above the ambient by a critical amount, about 20 °C. This suggests that thermal factors are important in cool-flame production, and this was confirmed by the effect of addition of inert gases. The special importance of thermal conductivity is exemplified by the differing effects of helium and xenon, two gases with identical heat capacities but very different thermal conductivities. Thus helium raised the limit, while xenon lowered it. 

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)]... 

Some data on the rates of oxidation of simple ketones are collected in Table 2. Further information on the effect of temperature of the maximum rate of reaction is given in Fig. 6, while Figs. 5 and 7 give the cool flame and hot ignition limits. 

The ignition di am for diethyl ether—oxygen mixtures has been determined [68] and the slow combustion has been studied in some detail. The low temperature oxidation becomes appreciable at 120 °C and Lemay and Ouellet [69], working at 160—175 °C which was below the lower limit of cool flame for in their pyrex reaction vessel, observed an initial pressure drop during which oxygen was consumed by a zero-order process with an activation energy of about 50 kcal. mole. Peroxides... 

The results using a static system [97] confirmed the existence of a region of negative temperature coefficient of the rate between 320 and 360 °C, and the limits for cool-flame formation for an equimolar mixture were determined. In contrast to the earlier work [96], methanol was found to be a major product at both 300 and 360 °C. Some iso-propanol and methane were detected, the yield of the latter being greater at the higher temperature. No carbon dioxide was observed, but carbon monoxide was a major product. 

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]. 

In the early years of flame photometry, only relatively cool flames were used. We shall see below that only a small fraction of atoms of most elements is excited by flames and that the fraction excited increases as the temperature is increased. Consequently, relatively few elements have been determined routinely by flame emission spectrometry, especMly j ew of those that emit line spectra (several can exist in flames as molecular species, particularly as oxides, which emit molecular band spectra). Only the easily excited alkali metals sodium, potassium, and lithium are routinely deterniined by flame emission spectrometry in the clinical laboratory. However, with flames such as oxyacetylene and nitrous oxide-acetylene, over 60 elements can now be determined by flame emission spectrometry. This is in spite of the fact that a small fraction of excited atoms is available for emission. Good sensitivity is achieved because, as with fluorescence (Chapter 16), we are, in principle, measuring the difference between zero and a small but finite signal, and so the sensitivity is limited by the response and stability of the detector and the stability (noise level) of the flame aspiration system. 

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. 

Refractory species (such as zirconium oxide) are incompletely atomized at the temperatures of a flame or a muffle furnace. If the radionuclide of interest may be in a refractory form, the material must be mixed with fusion reagents and melted at a high temperature. After cooling, the solidified melt is dissolved in a HNOj solution. Fusion mixtures that have been tested for many types of soil are listed in Table 4.2. Sample masses are limited by practical consideration of the final sample size, taking into consideration the large amounts of fusion reagents added to the sample. 

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