OCR Text |
Show temperature for chlorobenzene decomposition is extremely high. Certainly, it is not expected to be reached in a municipal solid waste incinerator except in the flame front where residence time are probably of the order of milliseconds. The general conclusion from this is that unimolecular decomposition is not the prime channel for decomposition for such compounds. It should be noted that this does not necessarily hold for other molecules for which there exist low energy channels for decomposition. b. Bimolecular Attack by OH: In high temperature oxidative environments, an important agent for the destruction of organic compounds is by OH attack. This is due to the high OH concentrations that may be attained and the large rate constants for OH attack on organics. For aromatic compounds, the decomposition process involves at some stage the formation of the phenoxy radical and its decomposition to form cyclopentadienyl type radical and C07. Cyclopentadienyl radical will decompose to smaller unsaturated species. Until one reaches the highest temperatures where the aromatic ring is decomposed unimolecularly this is the principal means of destroying the aromatic structure. In Table I we summarize some rate constant data on OH reactions. It should be noted that there are two fundamental processes for radical attack on aromatics, displacement and abstraction. The former is simply the substitution of OH for another group, while the latter involves the pulling off of a reactive group from the aromatic structure. For OH radical, the thermochemistry is extremely unfavorable for abstraction of a Cl atom and for our purposes it does not occur. The thermochemistry is much more favorable for hydrogen abstraction and the rate expression for OH + benzene can be regarded as characteristic for such a process. OH displacement of hydrogen is 8 |