Cooling Flame

Cool flames Cooling crystallizers Cooling towers... 

Chemical Factors. Because knock is caused by chemical reactions in the engine, it is reasonable to assume that chemical stmcture plays an important role in determining the resistance of a particular compound to knock. Reactions that produce knock are generally free-radical chain-type reactions which are different from those that occur in the body of the flame the former occur at lower temperatures and are called cool flame reactions. 

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. 

Below a certain critical temperature, which varies with pressure and stoichiometry, cool flames for several hydrocarbons propagate from the wall inward above this temperature, they propagate from the center of the vessel (78). This transition is interpreted as evidence for a changeover from a predominantly heterogeneous preflame mechanism to a homogeneous one. 

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

The composition of an oxidizing mixture is altered extensively by the passage of a cool flame (66,83,84). Before passage of the flame, oxygenated materials are present. In the case of hexane oxidation, ROO radicals are reportedly displaced by HOO radicals above 563 K (85), in concordance with previous work (86,87). After the passage of a cool flame, olefins, some conjugate and others of lower molecular weight, are observed. 

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. 

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

The reported characteristics of methane oxidation at high pressures are interesting. As expected,the reaction can be conducted at lower temperatures eg, 262°C at 334 MPa (3300 atm) (100). However, the cool flame phenomenon is observed even under these conditions. At high pressures. 

Ethane. Ethane VPO occurs at lower temperatures than methane oxidation but requires higher temperatures than the higher hydrocarbons (121). This is a transition case with mixed characteristics. Low temperature VPO, cool flames, oscillations, and a NTC region do occur. At low temperatures and pressures, the main products are formaldehyde, acetaldehyde (HCHOiCH CHO ca 5) (121—123), and carbon monoxide. These products arise mainly through ethylperoxy and ethoxy radicals (see eqs. 2 and 12—16 and Fig. 1). 

Propane. The VPO of propane [74-98-6] is the classic case (66,89,131—137). The low temperature oxidation (beginning at ca 300°C) readily produces oxygenated products. A prominent NTC region is encountered on raising the temperature (see Fig. 4) and cool flames and oscillations are extensively reported as compHcated functions of composition, pressure, and temperature (see Fig. 6) (96,128,138—140). There can be a marked induction period. Product distributions for propane oxidation are given in Table 1. 

Isobutane shows the usual NTC and cool flame phenomena (78,154,157,158). As the pressure is iacreased, the expected iacrease ia oxygenated products retaining the parent carbon skeleton is observed (96). Under similar conditions, isobutane oxidizes more slowly than / -butane (159). There are stUl important unresolved questions concerning isobutane VPO (160). 

Below the NTC region, iatramolecular abstraction appears to generate P-dicarbonyl iatermediates that are consumed duriag cool flames (161—164). Secondary attack on nonradical monofunctionals does not appear to be a significant source for these difunctional iatermediates. 

Gas-phase chemiluminescence is illustrated by the classic sodium—chlorine cool flame (174) ... 

Potassium chlorate is used mainly in the manufacture of matches (qv) and pharmaceutical preparation. In pyrotechnics, chlorate salts may be mixed with certain organic compounds such as lactose to give a relatively cool flame, so that certain dyes may be incorporated in the mixture to give colored flares. 

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. 

Cool Flames. Under particular conditions of pressure and temperature, incomplete combustion can result in the formation of intermediate products such as CO. As a result of this incomplete combustion, flames can be less exothermic than normal and are referred to as cool flames. An increase in the pressure or temperature of the mixture outside the cool flame can produce normal spontaneous ignition (1). 

Coffee, R. D., Cool Flames and Autoignition, presented AIChE Loss Prevention Symposium, Houston, Texas, April 2, 1979. 

For the sake of brevity, the so-called cool flame techniques based upon the use of an oxidant-lean flame such as hydrogen/nitrogen-air, have not been included. 

Oxalate Blasting Powders. Mining safety expls invented in Eng) by Greaves and Hann in 1897— 98 and manufd by the Oxalate Blasting Powder Co at Gatebeck (Westmoreland), which later became the Nitrates Explosives Co, Ltd. These expls were a modification of BlkPdr in which sulfur was partially or entirely replaced by one or more of the following oxalic acid, oxalates of Aram, K or Na (simple or double), borax, boric acid, etc, each of which could contain w of hydration. The purpose of these substitutions was to obtain expls with a cool flame, so that they could be safely used in gaseous mines Refs 1) Daniel (1902), 592-3 2) Cond-... 

All joints, no matter how they are made, shoidd be annealed with a relatively cool flame (p. 170). 

The seal is made by cutting 1-cm diameter tubing to give one piece with a square end. Near the end of the other piece a spindle is drawn. The shoulder of the spindle is heated in a small hot flame so that it thickens, and then, with a fairly cool flame, the spindle beyond the shoulder is drawn out to a fine capillary about 1 mm diameter. This is heated in a very cool flame and bent first one way and then the other, as m Figure 47, II. The end is finally sealed off. The tip thus prepared is inserted into the square end of the other piece of tubing, as in Figure 47, /, and a joint is made by directing a small flame at... 

While no 1,2-alkyl shifts have been observed in solution, the cool flame oxidation of Me3CH (in the gas phase at 480°) is found to... 

Cool flame behaviour of acetaldehyde is apparently eliminated by tert-butyl bromide, and reduced by methyl iodide. 

The nature of the reaction and the products in methylal-oxygen mixtures during cool flame or explosive oxidation were studied. 

The relationship between the cool flames observed at 210° C and subsequent explosions were studied. 

When fractional distillation at 150°C/27 mbar of a mixture of the ester with acetic acid (36% mol) was interrupted by admission of air, smoking and gas evolution occurred. This was attributed to cool flame auto-ignition and detailed investigation confirmed this. Nitrogen should always be used to break vacuum, and further precautions are recommended. 

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