No abstract

The molecular architecture of asphaltenes is still a matter of debate. Some literature reports provide evidence that the contrast of petroleum asphaltenes versus coal-derived asphaltenes is useful for understanding the governing principles of asphaltene identity. Coal-derived asphaltenes provide an excellent test for understanding the relationship of asphaltene molecular architecture with asphaltene properties. Diffusion measurements have shown that coal-derived asphaltenes are half the size of many crude oil asphaltenes, but there are relatively few studies comparing coal-derived and petroleum asphaltenes using liquid state 13C NMR. 13C NMR confirms that the molecular sizes of these coal-derived asphaltenes are smaller than virgin petroleum asphaltenes. DEPT-45 experiments were performed in order to determine the relative amount of nonprotonated and protonated carbon in the aromatic region of the spectrum. In contrast to previous NMR work on asphaltenes that ignored interior bridgehead carbon, we show this is an important component of asphaltenes and that correctly accounting for this carbon enables proper determination of the number of fused rings. XRS data supports interpreting the NMR data with a model that weighs circularly condensed structures more heavily than linearly condensed structures. Significantly more carbon exists in chains at least 9 carbons long in petroleum asphaltenes (≥7%) compared to coal-derived asphaltenes (≥1%).

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.

No abstract

Steam gasification of chars from the pyrolysis of a Japanese bamboo and cedar was studied using a reactor that enabled experimental definition of the gas composition in the vicinity of gasifying char particles. Intraparticle diffusion of neither steam nor the product gases influenced the kinetics of gasification. The chars underwent noncatalytic and catalytic gasification in parallel. The noncatalytic gasification, in which kinetic parameters were successfully defined by those for the gasification of the acid-washed char, was first-order with respect to the amount of residual carbon over the entire range of char conversion. In consequence of this, contribution of the catalytic gasification was quantified as a function of the char conversion. Among the inherent alkali and alkaline earth metallic species, potassium (K) played the major catalytic role and its overall activity changed via a maximum in the course of gasification, suggesting the presence of optimum sizes of clusters or particles of K catalyst. The noncatalytic and catalytic reactions obeyed respective Langmuir−Hinshelwood mechanisms that involved inhibition by H2.

No abstract

No abstract

No abstract