OCR Text |
Show - 4 - well as partially oxidized chlorocarbons and hydrocarbons formed in the first stage. Table I lists some measured and estimated rate constants currently used in detailed kinetic modeling studies. The reactions include radical attack by Cl, H, 0, and OH on both methane and ethylene and their chlorinated counterparts. The rates of reactions for OH and ° radical attack are observed to be relatively insensitive to the WCl ratio. In contrast, reactions of H-atoms with chlorinated hydrocarbons tend to be greater than H-atoms with the corresponding hydrocarbon, whereas reactions of Clatoms with the hydrocarbon are generally greater than Cl-atoms with the corresponding chlorinated hydrocarbons. These trends can be explained by thermochemical considerations. Like hydrocarbon oxidation, radical attack on chlorinated hydrocarbons is considerably faster than the rates of H-atoms reacting with molecular oxygen or hydroxyl radicals reacting with carbon monoxide. Table II shows equilibrium compositions and adiabatic flame temperatures for mixtures of the homological series of chlorinated-ethylenes combined with molecular chlorine, molecular hydrogen, and air. In the first set of calculations, the chlorine atoms bonded to the carbon atoms were sequentially replaced with atomic hydrogen while the amounts of molecular chlorine and molecular hydrogen in the mixture were adjusted such that the total number density of each element was conserved. The adiabatic flame temperatures are all nearly the same for the first four mixtures varying only 63 K between the mixture with trichloroethylene to the mixture with ethylene. Assuming the structure of the two-stage trichloroethylene flame as mentioned above, calculations with the same mixtures were perfonned again, but with CO2 eliminated from the data base to simulate a partially equilibrated mixture between the two stages. An intermediate zone flame temperature of about 1600 K is obtained. Comparison of mixture compositions between the simulated intermediate rone and full chemical equilibrium show that the H20 and HCl mole fractions remain nearly constant. Neglecting the changes in the CO, 02 and CO2 mole fractions, the most significant differences are in the decrease in ~ mole fractions and corresponding increase in the Cl mole fractions and the increase in the ~ mole fraction. An interesting similarity is found with the adiabatic flame calculations for ethylene with molecular chlorine as also shown in Table II. This similarity suggest the possible presence of double flames even with hydrocarbons seeded with appropriate concentrations of chlorine. |