Pyrolysis halogenated hydrocarbons

Electrical and electronic waste can be successfully recovered by pyrolysis following a suitable amount of separation. It has been demonstrated that by selecting the correct processing parameters, any halogenated hydrocarbons present are satisfactorily destroyed. Oil and gas from the pyrolysis are best used to heat the pyrolysis kiln. The metal containing pyrolysis coke can be treated in a secondary copper smelter without need for further processing. 

There are a large number of chain reactions that are significant in industrial processes or play an important role for the environment. Classes of chain reactions that are relevant industrially include hydrogen/halogen reactions and pyrolysis of hydrocarbons, which are both discussed below, and free-radical polymerization discussed in many textbooks on kinetics. As an example of a chain reaction of significant environmental consequence, we will discuss formation of nitric oxide from fixation of atmospheric nitrogen. 

Gas phase free radical reactions are used in industry for pyrolysis, halogenation and combustion reactions. Nowadays, and probably for a long time to come, the thermal cracking of hydrocarbons constitutes the main production route for olefins, which are the basic feedstocks of the chemical industry around the world. Hydrocarbon pyrolysis is thus of considerable economic interest, as is shown by the very large amount of effort dedicated both to fundamental and applied research in this field (see, for example, refs. 35—37). 

Apart from the molecular and the radical-chain mechanisms for the decomposition of halogenated hydrocarbons, the study of allyl bromide pyrolysis " revealed a third possibility, namely a radical non-chain mechanism, a type originally proposed by Daniels and Veltman for ethyl bromide pyrolysis. Its application to allyl bromide pyrolysis is shown in the following scheme as proposed by Maccoll ... 

Stepanenko and Kolodii [171] used a flamedonization detector for the detection of halogenated hydrocarbons, these compounds are converted to hydrocarbons by pyrolysis at 1000°C in the presence of hydrogen. 

As for transformers, the choice is more difficult. Halogenic hydrocarbons (Ugilec T, FORMED, etc.) still offer the greatest safety in terms of fire resistance. However, they have the disadvantage of releasing corrosive vapors under pyrolysis. 

The presence of halogen additives substantially increases the tendency of all fuels to soot under diffusion flame conditions [69], The presence of H atoms increases the soot pyrolysis rate because the abstraction reaction of H + RH is much faster than R + RH, where R is a hydrocarbon radical. Halogenated compounds added to fuels generate halogen atoms (X) at modest temperatures. The important point is that X + RH abstraction is faster than H + RH, so that the halogen functions as a homogeneous catalyst through the system... 

An example is trimethylenemethane (TMM), one of the first hydrocarbon biradicals whose triplet ESR spectrum was recorded (cf. Chapter 5 in this volume), but which eluded 25 years of efforts (among others by the present author) to obtain its UV-vis or IR spectrum, because the predominant product of photolysis or pyrolysis of different precursors was invariably methylenecyclopropane (MCP). Eventually, Maier et al. successfully generated a sufficient quantity of TMM, by irradiation of MCP in halogen-atom doped Xe matrices, to record the IR spectrum of this elusive compound. 

These different mechanisms and prodnct distribntions, to some extent, are related to the bond dissociation energies, the chain defects of the polymers, and the degree of aromaticity, as well as the presence of halogen and other heteroatoms in the polymer chains. Large amount of styrene monomers can be obtained by pyrolysis of PS, while a wide range of hydrocarbons are prodnced by random degradation of PE and PP [3, 59, 60]. 

Pyrolysis of chlorinated unsaturated polyhydrocarbons is in some respects similar to that of the parent polyhydrocarbon, and in some others similar to halogenated saturated hydrocarbon type polymers. The elimination of the hydrohalogenated acid takes place relatively easily, and the polymers from this class are not resistant to heating. In practice, additives that enhance the resistance to heating frequently are added. Since the elimination of the acid seems to accelerate thermal decomposition, similarly to the case of PVC, metallic oxides that scavenge the acid increase the resistance to heating. The main pyrolysis products of each polymer are not modified by the addition of these additives. 

Halogenated compounds Hydrocarbons Metals and inorganic compounds Nitrogen compounds Pesticides Pyrolysis Sugars... 

More recently other methods have been reported for the synthesis of hexafluoropropylene. One technique involves the pyrolysis of a mixture of tetrafluoroethylene and carbon dioxide at atmospheric pressure at 700-900°C.P ] Conversions of 20-80% and HFP yields of better than 80% were obtained. The unreacted tetrafluoroethylene and carbon dioxide were distilled from the product and recycled. HFP can be synthesized from hexachloropropylene via a multistep process beginning with fluorination.t ] Later steps convert the initial products to CF3-CFCI-CF3 which is dehalogenated to HFP. Other techniques report on the synthesis of hexafluoropropylene from the mixture of a variety of linear and cyclic three-carbon hydrocarbons with a partially halogenated three-carbon acyclic hydrocarbon.t " ]... 

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