The catalytic performance of H‐ZSM‐5 samples prepared from different silica sources, namely, Ludox (sample A), waterglass (sample B) and tetraethyl orthosilicate (sample C) in aromatization of propane was studied. The catalysts were characterized by BET, SEM, XRD and NH3‐TPD techniques to analyze the structure and acidity of the active sites. The performance tests were carried out in fixed‐bed reactor at 550 °C. For a quantitative assessment, the long‐term propane conversion data were fitted with a second order kinetics for the main reaction and a second order independent kinetics for deactivation using integral method of analysis. Fair fits were observed. According to silica source, the order for activity and stability were A>B>C and B>C>A, respectively. The incorporation of Zn (0.4 wt. %) as a promoter showed beneficial effect on aromatic selectivity at the expense of propane conversion and catalyst stability. The introduction of Zn decreased the rate constant of aromatization from 9.4 to 8.7 m6·kg−1·mol−1·h−1 at 550 °C, while the rate constant of deactivation increased by a factor of five, that is, from 0.0056 to 0.025 h−1.
The thermodynamics of methane dehydroaromatization in the absence and presence of coke‐removing agents was studied using the Gibbs free energy minimization approach. Numerical results indicated that higher temperatures and lower pressures increase methane conversion as well as formation of olefins and aromatics but suppress that of paraffins. Higher H2/CH4 ratios enhance the selectivity of light hydrocarbons but reduce that of naphthalene. Benzene selectivity has a maximum at an H2/CH4 molar ratio of 0.26. Methane conversion exhibits a minimum at an H2O/CH4 molar ratio of 0.07. As the H2O/CH4 ratio increases, formations of heavier hydrocarbons decrease at the expense of CO, CO2, and hydrogen. These results may provide guidelines to improve the process performance.
The effect of propane addition to the feed of methane aromatization process was studied both thermodynamically and experimentally. Thermodynamic equilibrium calculations were performed within temperature, pressure, and CH 4 /C 3 H 8 molar ratio ranges of 673−873 K, 1−10 bar, and 0−20, respectively, considering different constraints according to two alternative reaction network models. Experimental evaluations were conducted in a fixed-bed reactor over Zn/HZSM-5 catalyst under different operating conditions. Methane formation showed an induction period of about 4 h on stream. Aromatic yield slightly increased with increase of methane concentration in the feed. Experimental results showed the best agreement with the thermodynamic model based on a proposed mechanism of methylation of propane-derived aromatics by methane, giving equilibrium methane conversions of about 1% under the above conditions. Both thermodynamic and experimental results revealed that in the presence of propane cofeed, the contribution of methane to the overall reaction is negligibly small on a net basis.
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