Reaction and Deactivation Rates of Methane Catalytic Cracking over Nickel

The kinetics of methane catalytic cracking over nickel supported on porous and nonporous aluminas was modeled using a separable kinetics approach in order to develop initial rate and activity decay equations. The model parameters were estimated using a set of experiments conducted in an electrobalance. The experimental work covered the 500–650 °C temperature range, using pure methane, as well as different partial pressures of CH₄/N₂ and CH₄/H₂ mixtures at atmospheric pressure. The model results showed a good match with the experimental data, and the estimated kinetic parameters agreed well with those reported in the literature. The morphology of the support affected the initial reaction rate and catalyst deactivation. The methane cracking activation energy was estimated to be 88 and 75 kJ/mol for the porous and nonporous catalysts, respectively. The activation energy for the encapsulating carbon formation was estimated to be 147 and 149 kJ/mol for the porous and nonporous catalysts, respectively. The deactivation reaction was found to be half-order in surface carbon. The model was expanded to include cracking/regeneration cycles. The model showed good agreement with the experimental data at different experimental conditions and up to 39 cycles. Cracking/regeneration cycles suggest that the porous catalyst can be used for conducting continuous methane cracking

Effect of Metal–Support Interface During CH₄ and H₂ Dissociation on Ni/γ-Al₂O₃: A Density Functional Theory Study

Methane and hydrogen dissociation are important reactions in carbon nanotube (CNT) and hydrogen production. Although there is extensive literature on theoretical studies for CH₄ and H₂ dissociation on Ni, little is known about the effect of the oxide support, especially the metal–support interface, on the dissociation properties of CH₄ and H₂. In this study, the dissociations of CH₄ and H₂ on Ni cluster supported on γ-alumina were investigated using density functional theory (DFT) calculations. Two systems: Ni₄ cluster supported on the spinel model of γ-Al₂O₃ (100) surface, S­(Ni₄), and on the nonspinel model of γ-Al₂O₃ (100) surface, NS­(Ni₄), have been used to model Ni₄/γ-Al₂O₃. For both models, it was found that CH₄ and H₂ dissociations are kinetically preferred at the Ni₂ site located at the nickel–alumina interface when compared with the top of the Ni cluster. Also, the study of CH₃ and H adsorption on different sites of the S­(Ni₄) and NS­(Ni₄) show that CH₃ and H bonded with the Ni₂ atom at Ni₄/γ-Al₂O₃ interface are more stable than at the top site adsorption. Moreover, the calculation of the metal–support interaction indicates that molecular adsorption on the Ni particle weakened its interaction with the oxide. Hirshfeld charge analysis showed that the surface Al atom works primarily as a charge donation partner when CH₃ and H are bonded with the Ni₂ atom at the interface. This also resulted in an upshift of the d-orbital around the Fermi energy of the Ni₂ atom, which finally stabilized the interface adsorption by this Al (donor)–Ni–adsorbates (acceptor) effect. The results obtained from the DFT calculations indicate that the metal–oxide interface plays an essential role in the dissociation of CH₄ and H₂

Dynamic Modeling and Evaluation of an Industrial-Scale CO₂ Capture Plant Using Monoethanolamine Absorption Processes

This paper presents a step-by-step method to scale-up an MEA (monoethanolamine) absorption plant for CO₂ capture from a 750 MW supercritical coal-fired power plant. This CO₂ capture plant consists of three absorbers (each 11.8 m in diameter and with 34 m of height) and two strippers (each 10.4 m in diameter with 16 m of height); the plant has been designed to achieve 87% CO₂ recovery at 95% CO₂ purity. A dynamic mechanistic model of a commercial-scale CO₂ capture plant with a control scheme was developed in gPROMS and evaluated under several scenarios. The analysis revealed that this plant is able to reject various disturbances and switch between different operating points displaying prompt responses in the key controlled variables. Also, this study highlights that poor wetting in strippers can be avoided if the CO₂ capture removal set point is scheduled based on the periodic operation of the power plant

Synthesis and Characterization of γ‑Fe₂O₃ for H₂S Removal at Low Temperature

The performance of γ-Fe₂O₃ as sorbent for H₂S removal at low temperatures (20–80 °C) was investigated. First, γ-Fe₂O₃/SiO₂ sorbents with a three-dimensionally ordered macropores (3DOM) structure were successfully prepared by a colloidal crystal templating method. Then, the performance of the γ-Fe₂O₃-based material, e.g., reference γ-Fe₂O₃ and 3DOM γ-Fe₂O₃/SiO₂ sorbents, for H₂S capture was compared with that of α-Fe₂O₃ and the commercial sorbent HXT-1 (amorphous hydrated iron oxide). The results show that γ-Fe₂O₃ has an enhanced activity compared to that of HXT-1 for H₂S capture at temperatures over 60 °C, whereas α-Fe₂O₃ has little activity. Because of the large surface area, high porosity, and nanosized active particles, 3DOM γ-Fe₂O₃/SiO₂ sorbent shows the best performance in terms of sulfur capacity and utilization. Moreover, it was found that moist conditions favor H₂S removal. Furthermore, it was found that the conventional regeneration method with air at high temperature was not ideal for the composite regeneration because of the transmission of some amount of γ-Fe₂O₃ to α-Fe₂O₃. However, simultaneous regeneration by adding oxygen in the feed stream allowed the breakthrough sulfur capacity of FS-8 to increase up to 79.1%, which was two times the value when there was no O₂ in the feed stream

Design of a Sorbent to Enhance Reactive Adsorption of Hydrogen Sulfide

A series of novel zinc oxide–silica composites with three-dimensionally ordered macropores (3DOM) structure were synthesized via colloidal crystal template method and used as sorbents for hydrogen sulfide (H₂S) removal at room temperature for the first time. The performances of the prepared sorbents were evaluated by dynamic breakthrough testing. The materials were characterized before and after adsorption using scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that the composite with 3DOM structure exhibited remarkable desulfurization performance at room temperature and the enhancement of reactive adsorption of hydrogen sulfide was attributed to the unique structure features of 3DOM composites; high surface areas, nanocrystalline ZnO and the well-ordered interconnected macroporous with abundant mesopores. The introduction of silica could be conducive to support the 3DOM structure and the high dispersion of zinc oxide. Moisture in the H₂S stream plays a crucial role in the removal process. The effects of Zn/Si ratio and the calcination temperature of 3DOM composites on H₂S removal were studied. It demonstrated that the highest content of ZnO could reach up to 73 wt % and the optimum calcination temperature was 500 °C. The multiple adsorption/regeneration cycles showed that the 3DOM ZnO–SiO₂ sorbent is stable and the sulfur capacity can still reach 67.4% of that of the fresh sorbent at the fifth cycle. These results indicate that 3DOM ZnO–SiO₂ composites will be a promising sorbent for H₂S removal at room temperature