Detailed Kinetic Modeling of Gas-Phase Reactions in the Chemical Vapor Deposition of Carbon from Light Hydrocarbons

Abstract

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 C 4 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.

Modeling and simulation of materials synthesis: Chemical vapor deposition and infiltration of pyrolytic carbon

“…The model is validated by comparison of the computational with experimental results for homogeneous gas-phase pyrolysis of ethylene, acetylene, and propylene as well as for heterogeneous CVD of pyrocarbon from those precursors [8,9]. Concentrations of 44 species including hydrogen and hydrocarbons ranging from methane to coronene are analyzed.…”

“…More details of the mechanism development can be found in our previous papers [8,9]; the mechanism can be downloaded [10].…”

“…Furthermore, in the present work methane is used as carbon precursor and the mechanism of methane pyrolysis is considerably less understood in the temperature regime of CVD/CVI than those of C 2 -pyrolysis [11][12][13][14][15][16], which were mainly applied in the validation of the above described mechanisms of elementary reactions [8,9]. Thus lumped gas-phase and surface reaction mechanism were developed to calculate carbon deposition from those gas-phase species that dominate CVI of pyrolytic carbon from methane.…”

“…For details of the experimental procedure it is referred to Ref. [8,9]. The mole fractions are plotted as a function of the effective residence time at the isothermal zone in the reactor.…”

Modeling of the Interactions Between Catalytic Surfaces and Gas-Phase

“…7 for methane pyrolysis [85]. Detailed reaction mechanisms have been developed to describe the molecular growth processes in the gas-phase by hundreds of elementary steps [86,87]. Even though sticking coefficients for the many potential coke precursors in the gas-phase are difficult to estimate simple models for either adsorption of individual species or the coking growth rate have been developed and applied in the prediction of coking propensity of the catalytic system [88].…”