Flame decomposition

Fig. 1. Pressure required for propagation of decomposition flame through commercially pure acetylene free of solvent and water vapor in long horizontal pipes. Gas initially at room temperature ignition by thermal nonshock sources. Curve shows approximate least pressure for propagation (0), detonation,...

The calculated detonation velocity in room temperature acetylene at 810 kPa is 2053 m/s (61). Measured values are about 1000-2070 m/s, independent of initial pressure but generally increasing with increasing diameter (46,60—64). In a time estimated to be about 6 s (65), an accidental fire-initiated decomposition flame in acetylene at ca 200 kPa in an extensive piping system traveled successively through 1830 m of 76—203-mm pipe, 8850 m of 203-mm pipe, and 760 m of 152-mm pipe. 

The predetonation distance (the distance the decomposition flame travels before it becomes a detonation) depends primarily on the pressure and pipe diameter when acetylene in a long pipe is ignited by a thermal, nonshock source. Figure 2 shows reported experimental data for quiescent, room temperature acetylene in closed, horizontal pipes substantially longer than the predetonation distance (44,46,52,56,58,64,66,67). The predetonation distance may be much less if the gas is in turbulent flow or if the ignition source is a high explosive charge. 

In certain exceptional cases, a specially designed deflagration arrester may be mounted in-line without regard to run-up distance. This can be done only where the system is known to be incapable of detonation. An example is the decomposition flames of ethylene, which are briefly discussed under Special Arrester Types and Alternatives. ... 

Decomposition Flame Arresters Above certain minimum pipe diameters, temperatures, and pressures, some gases may propagate decomposition flames in the absence of oxidant. Special in-line arresters have been developed (Fig. 26-27). Both deflagration and detonation flames of acetylene have been arrested by hydrauhc valve arresters, packed beds (which can be additionally water-wetted), and arrays of parallel sintered metal elements. Information on hydraulic and packed-bed arresters can be found in the Compressed Gas Association Pamphlet G1.3, Acetylene Transmission for Chemical Synthesis. Special arresters have also been used for ethylene in 1000- to 1500-psi transmission lines and for ethylene oxide in process units. Since ethylene is not known to detonate in the absence of oxidant, these arresters were designed for in-line deflagration application. 

There is a need in many chemical processes for protection against propagation of nnwanted combnstion phenomena snch as deflagrations and detonations (inclnding decomposition flames) in process eqnipment, piping, and especially vent manifold systems (vapor collection systems). 

A number of other gases can undergo reactions that produce decomposition flames—for instance, ethylene, ethylene oxide, methyl nitrate, ethyl nitrate, and hydrazine (CCPS 1993). 

Acetylene may propagate decomposition flames in the absence of any oxidant above certain minimum conditions of pressure, temperature, and pipe diameter. Acetylene, unlike most other gases, can decompose in a detonative manner. Among the different types of flame arresters that have proven successful in stopping acetylene decomposition flames are hydraulic (liquid seal) flame arresters, packed beds, sintered metal, and metallic balls (metal shot). 

Flame A region in which chemical interaction between gases occurs, accompanied by the evolution of light and heat (see Decomposition Flames). 

Addnl Ref A.R. Hall R.A.M. St raker, The Methyl Acetylene (Propyne) Decomposition Flame at Pressures of 10, 20 and 40 Atmospheres , RPE Tech Note 192, RPE, Westcott (Engl) (1960)... 

The vivid interest in hydrazine as a powerful propellant has stimulated many investigations both of its thermal decomposition and of its oxidation. Although hydrazine decomposes much more readily than ammonia, the study of its homogeneous decomposition by classical means using a static system is complicated considerably by wall catalysis. Thus, other experimental techniques have had to be applied, e.g. decomposition flames, flash photolysis, studies of explosion characteristics and the shock-tube technique. 

This is the dominant overall reaction for the decomposition on platinum or tungsten at 200 and 380 °C, respectively40. Most workers on hydrazine decomposition flames41-44, in which the reactions are homogeneous, report a stoichiometric equation similar to (b) for final flame temperatures up to 1900 °K. Measurements of MacLean and Wagner45 on decomposition flames and of Husain and Norrish37 on the flash photolysis of hydrazine indicate the contribution of the overall reaction... 

Section 2 deals with reactions involving only one molecular reactant, i.e. decompositions, isomerisations and associated physical processes. Where appropriate, results from studies of such reactions in the gas phase and condensed phases and induced photochemically and by high energy radiation, as well as thermally, are considered. The effects of additives, e.g. inert gases, free radical scavengers, and of surfaces are, of course, included for many systems, but fully heterogeneous reactions, decompositions of solids such as salts or decomposition flames are discussed in later sections. Rate parameters of elementary processes involved, as well as of overall reactions, are given if available. 

Rogg, B., A. Linan, and F. A. Williams. 1986. Deflagration regimes of laminar flames modeled after the ozone decomposition flame. Combustion Flame 65 79-101. 

Compared with the AP decomposition flame thickness, the fuel-oxidant redox flame extends a much greater distance from the propellant surface and depends on the rate of both chemical reaction and diffusional mixing. 

The behavior of decomposition flames of acetylene (50, 73) indicates a chain mechanism, and there is evidence, from data on paramagnetism, for the presence of free radicals in the solid carbon itself (34). However, these effects could result with either type of mechanism, and do not furnish any clear indication with regard to dehydrogenation alone. 

A most successful application of the approximate theory has been performed for the ozone decomposition flame, where the flame velocity has been estimated over the entire 02-03 composition range in which the flame is propagated (V2). A comparison of the calculated flame velocities and the experimental values is made in Fig. 4. 

The simplest example of a flame-supporting medium is a pure chemical compound which decomposes exothermically. The widespread interest in such flames is due to their possibilities as monopropellants. Many studies are motivated by purely fundamental considerations, since a decomposition flame can be a kinetically simple flame. The most widely used and studied combustion reactions are those between hydrocarbons or hydrocarbon derivatives and air or oxygen. However, many other chemical substances may be substituted for the common fuels and/or oxidizers. Flames of uncommon fuels and oxidizers are most important because of their possibility of surpassing ordinary hydrocarbon oxidation as a source of energy. Some unusual flames are discussed in reference (PI). 

A list of substances which have been used or considered to support decomposition flames is shown in Table I. Almost all of these substances have been studied at one time or another to provide fundamental information for the evaluation of the theory of flame propagation. As previously mentioned, the ozone decomposition has proved most useful as the basis of a flame which is amenable to both theoretical and experimental study. The NO decomposition flame provided a situation where a clear-cut prediction was made possible by flame theory (P2). On the basis of a flame calculation it was predicted that a strong preheat would permit the stabilization of this flame at a measurable flame velocity, since it was known that a flame would not propagate into the gas at room temperature. Subsequent experimental work confirmed the prediction by stabilizing a flame with approximately the predicted value. This places a great deal of... 

Decomposition flames Special fuels Special oxidizers... 

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