Detailed kinetic modeling of gas-phase reactions in the chemical vapor deposition of carbon from light hydrocarbons

The chemical kinetics of the pyrolysis of the hydrocarbons ethylene, acetylene, and propylene are modeled in detail under conditions relevant to the chemical vapor deposition of pyrolytic carbon. A mechanism that consisted of 227 species and 827 reactions (most of which are reversible) is developed and computed using a software package designed for computing time-dependent homogeneous reaction systems. Experimental results used for model validation are obtained using a vertical flow reactor at 900°C, pressures of 2-15 kPa, and residence times of up to 1.6 s. The products are analyzed using on-line and off-line gas chromatography. Computational and experimental results are compared for more than 30 products, including hydrogen, small hydrocarbons (ranging from methane to C₄, species), and aromatic hydrocarbons (ranging from benzene to coronene), The resulting reaction model predicts the profiles of the major pyrolysis products (mole fractions of > 10^-2) of the three hydrocarbons, as a function of both residence time and pressure, with satisfactory accuracy. It also predicts the mole-fraction profiles of minor compounds, ranging from polycyclic aromatic hydrocarbons (PAHs) to naphthalene, fairly well; however, it significantly underpredicts the larger PAHs. The deviation increases as the molecular mass of the PAHs increases. Sensitivity and reaction-rate analyses were also conducted to identify crucial reaction steps.