Kinetic study on pyrolysis of C6H5Cl and m-C6H4Cl2 diluted in H2 : detailed reaction kinetics and thermodynamic property estimation techniques

Thermal reaction studies of dilute mixtures (0.37%) of C6H5Cl in hydrogen and m-C6H4C12 (0.4%) in hydrogen have been performed in tubular flow reactors at various surface to volume ratios, and 1 atm total pressure. Residence times range from 0.02 to 2.5 seconds,with temperatures between 1050 to 1275 K. HCl, C6H5Cl, C6H6, and carbon solids (C(s)) are observed as the major products; minor products include methane, cyclopentadiene, toluene, naphthalene and biphenyls. Pyrolysis in helium yields significantly less conversion, but more C(s) for similar residence times. A detailed chemical mechanism is developed to describe this reaction system. Our modeling calculations incorporate Energized Complex QRRK analysis for accurate inclusion of temperature and pressure effects in radical addition reactions. This is a straightforward method to estimate rate constants and branching ratios as a function of both T and P. The detailed mechanism, based upon fundamental thermodynamic and kinetic principles, describes the overall reaction remarkably well. We also propose a plausible kinetic scheme describing formation of minor products. The observed reagent loss can be explained in terms of energized adduct formation, followed by unimolecular dissociation to low energy exit channels. Phenyl, chlorophenyl, and Cl radicals produced by the initial unimolecular decay of C6H5Cl or m-C6H4Cl2 react with H2 to form C6H6, C6H5Cl, and HCl + H respectively. Hydrogen atom addition to C6H5Cl and m-C6H4Cl2 via ipso attack, with subsequent loss of Cl from the energized complex, is required to explain the faster reaction in H2 than He. QRRK Rate constant analysis is presented for the addition of H atom, phenyl and chlorophenyl radicals to C6H5Cl and m-C6H4Cl2, temperature and pressure ranges of 300 to 1900 K and 10-3 to 1.0 atm, respectively. Calculations were performed for both nitrogen and hydrogen dilution gases. Computer simulations show that initial chloro-phenyl and phenyl addition reactions cannot, by themselves, explain the rapid formation of solid carbon in these systems. Computer simulation and QRRK analyses on these reaction systems require accurate thermodynamic property data for all radicals and stable molecules considered. A computer code called THERM was developed to estimate these properties using the group additivity method of Benson. Accurate heat capacity estimates for radicals and molecules are obtained only to 1000 K using Benson\u27s method. Two methods for the accurate extrapolation of this low temperature heat capacity data (T \u3c 1000 K) to higher temperatures were developed in the present study. The first method uses an exponential function to interpolate and extrapolate heat capacity data, with a typical maximum error of 4%. The second fitting method is based upon the harmonic oscillator model for heat capacity of an ideal gas described by statistical mechanics. A 5 parameter harmonic oscillator equation (HOE) was developed and used to estimate heat capacity data from 300 - 5000 K, with maximum error less than 0.1% for simple polyatomic molecules having no hindered internal rotations. Less than 4% maximum error is obtained for molecules which have hindered internal rotations. The harmonic oscillator equation parameters were determined by a least square regression of heat capacity data using the method of Marquardt. These parameters were used to calculate various molecular properties such as geometric and arithmetic mean frequencies, vibrational partition functions, or the sum and density of vibrational quantum states. Estimates are found to compare favorably to those calculated from actual vibrational frequency data. A polynomial fitting method was developed to create thermodynamic property functions in the NASA polynomial format which is required by the CHEMKIN reaction simulation computer code. Two polynomials were generated (one for low temperature and one for high temperature) and a point of tangency was determined using the Newton-Raphson method. This polynomial fitting method differs from those developed at NASA and Sandia National Laboratory, where the point of tangency is fixed at 1000 K. The heat capacity fitting/extrapolation method, polynomial fitting, and property change for chemical reaction procedures were coupled with the group additivity method of Benson to form the THERM computer package. This code may be used to estimate the required ideal gas thermodynamic properties for radicals and molecules considered in a detailed reaction mechanism used to model kinetic data