Thermal decomposition of chlorobenzene reaction with hydrogen and oxygen mixture in an atmosphere

The thermal reactions of chlorobenzene in hydrogen and oxygen mixtures were studied in tubular flow reactors at 1 atmosphere total pressure. Experiments were carried out in flow reactors of varied diameter for determining effects of different surface to volume ratios. Residence times ranged from 0.03 to 2.5 seconds and temperature was varied over a range of 560 - 660 °C. The 02/H2 ratios ranged from 1% to 5%. It was found that the conversion of chlorobenzene in hydrogen and oxygen mixtures increased with both temperature and residence time. The oxidation of chlorobenzene also occurred more rapidly when oxygen concentraction was increased. A small amount of oxygen exhibited a significant effect on the decomposition of chlorobenzene, because equivalent decomposition without 02 (H2 only) required temperatures over 900°C for 2 seconds residence times. Complete decay (99%) for the chlorobenzene at 1 second residence time occurs at about 660°C for all the O2/H2 ratios studied. The major products were benzene, CH4, C2H6, and HCl. The minor products were toluene, cyclopentadiene, C2H2, C2H4, CO and CO2. The hydrocarbon products increased approximately linearly with temperature. An increase in surface to volume ratio of the reactor was observed to slow the chlorobenzene decomposition in mixtures of hydrogen and oxygen, but it had no effect on the relative distribution of major products. The oxygen concentration did, however, have significant effect on product distribution. The pseudo first order plug flow model was utilized to analyze the global experimental data. This study demonstrates that small amounts of oxygen ( 1% to 5% ) in excess H2 is a practical route to detoxification of chlorinated aromatics. Expressions for the pseudo-first order rate con stant were obtained as follows: For 16 mm ID reactor kexp = 6.9 * 1011 e(-48900/RT) (O2/H2: 1%) kexp = 1.5 * 1013 e(-53700/RT) (O2/H2: 2%) kexp = 7.0 * 1014 e(-60000/RT) (O2/H2: 3%) A detailed kinetic reaction mechanism was developed and used to model the experimental data. The detailed kinetic reaction mechanism includes 84 elementary reactions involving stable compounds and free radical species. The addition, beta scission and recombination type reactions were all analyzed by QRRK theory and sensitivity analysis on the model was performed to show the most important reactions in the mechanism. The three most important reactions were found to be: H\u27+ HO2H2 + O2 k=2.50*1013 C6H5C1 + HC6H6 + Cl k=1.50*1013*exp(-7500/RT) C6H5C1 + HC6H5.+ HC1 k=1.00*1013*exp(-11300/RT