Gas-solid Heterogeneous Reactions Research Articles

Effect of Hyperthermal Hybrid Gas Composition on the Interfacial Oxidation and Nitridation Mechanisms of Graphene Sheet.

Graphene is one of the most promising thermal protection materials for high-speed aircraft due to its lightweight and excellent thermophysical properties. At high Mach numbers, the extremely high postshock temperature would dissociate the surrounding air into a mixture of atomic and molecular components in a highly thermochemical nonequilibrium state, which greatly affects the subsequent thermal chemical reactions of the graphene interface. Through establishing a reactive gas-solid interface model, the reactive molecular dynamics method is employed in this study to reveal the influences of the thermochemical nonequilibrium gas mixture on the thermal oxidation and nitridation mechanisms of graphene sheet. The results show that three distinctive stages can be recognized during bombardment of various nonequilibrium gas components toward the graphene sheet: (i) collision and surface adsorption stage, (ii) gas-solid heterogeneous reaction stage, and (iii) gas phase homogeneous reaction stage. The surface catalysis effect is found to be dominant during the first two stages, which can influence the following ablation behavior of graphene significantly at high-temperature conditions. Moreover, surface catalysis, oxidation, nitridation, and ablation mechanisms between nonequilibrium gas and graphene interface are revealed, which is of high relevance for future interfacial design and application of graphene as a thermal protection material.

A mesoscale model for heterogeneous reactions in porous media applied to reducing porous iron oxide

A mesoscale model based on the lattice gas cellular automata (LGCA) is proposed for heterogeneous reaction in porous media. This model features an entirely discretized reaction system, including discrete space, time, gas phases, and solid particle. To describe gas diffusion at solid nodes and gas–solid heterogeneous reaction on the phase boundary, two local probabilities are proposed, which are verified by the analytical solutions for gas diffusion and gas–solid reaction problems. This model was then applied to the hydrogen reduction of porous Fe3O4. The effects of the porosity, reaction rate constant (kl), and initial H2 concentration on the reduction were analyzed. The results indicate that the reaction rate increased with increasing porosity, kl, and H2 concentration. The solid product Fe was randomly distributed on the framework of Fe3O4 at smaller kl and concentrated around the unreacted Fe3O4 at higher kl, which is consistent with our experimental results.

Optimisation of Solid Fuel In-furnace Blending for an Opposed-firing Utility Boiler: A Numerical Analysis

Continuous research on the clean and effective use of coal is still necessary as coal will continue to play a key role in global energy supply for the foreseeable future. Hence, in the current study, the optimisation of in-furnace coal blending for one of Malaysia's opposed-firing utility boilers was numerically executed on the basis of hydrodynamics and combustion characteristics. The predicted FEGT from the numerical model was validated against the actual FEGT from the coal-fired power plant, revealing a difference of less than 10 %. Four (4) coal blended cases were tested, which included both bituminous (bit) and sub-bituminous (sub-bit) coals. The findings demonstrate that due to the difference in density between bit and sub-bit coals, the hydrodynamic performance is predicted to significantly improve when sub-bit coal is injected at the bottom burner as opposed to the upper burner. In terms of kinetics, the higher volatile matter (VM) of sub-bit coal in contrast to bit coal has been postulated to release a substantial amount of volatiles and improve the combustibility of bit coal. Furthermore, enhanced oxygen release from sub-bit coal volatiles can aggravate the gas-solid heterogeneous reaction during bit coal char combustion. As a result of the bottom burner's high temperature, it has been discovered that introducing sub-bit coals into those burners speeds up VM release and char combustion, which increases the rate of combustion. Thus, when combustibility rises, the peak temperature position moves downward, reducing the likelihood of delayed combustion and, consequently, the risk of heat exchanger pendant failure and ash deposition. In a furnace with a relatively long coal residence time, a considerable fraction (>20 %) of high gross calorific value (GCV) sub-bit coal (>5800 kcal/kg) is predicted to produce two peak flame temperatures exceeding 1600°C owing to the likelihood of enhanced char which created delayed combustion. Therefore, a furnace condition with a comparatively shorter coal residence time may aid in the rapid evacuation of residual char from the combustion/burner zone and minimise the potential for delayed combustion. Nonetheless, residual char escape may exacerbate the emission problem by releasing considerable unburned carbon. Overall, the current numerical model has the potential to be a reliable and cost-effective tool for investigating the combustion characteristics of coal blends in a power plant boiler.

Enhanced gasification effect from discharges and the formation of centimeter-scale plasma during microwave combustion of coke

Microwave technology has been demonstrated as a promising candidate for utilizing fossil fuel in a carbon-efficient manner to ensure energy supply security during the transition to the future green energy economy. Microwave plasma enhancement on gas fuel combustion has been demonstrated, which is attributed to the production of reactive radicals and fuel cracking. However, for solid fuels with low volatilities and ignition and combustion efficiency problems, such as coke; it is still unknown whether the discharge plasma can assist in destroying its aromatic structure, thereby producing extra combustible gases. This process may be a common concern associated with various gas–solid heterogeneous reactions using coal as a reactant. In this study, a centimeter-sized plasma was generated during the combustion of coke under microwave radiation, which significantly promoted the flame burning. Two main factors accounting for the formation of centimeter-scale plasma are discussed: the carbonization temperature and particle size of coke. Additionally, the correlation between CO production and microwave discharge was also examined. Finally, the synergistic promotion mechanism of flame combustion and microwave discharge is proposed. This paper provides new insights into the microwave enhancement mechanism in gas–solid heterogeneous reactions.

Facile preparation of hierarchical heterostructured TiO2/Bi/Bi2MoO6 photocatalyst for phocatalytic degradation of toluene gas

AbstractBACKGROUNDIn recent years, the emission of volatile organic compounds (VOCs) from different sources has caused huge environmental problems. As a ‘green’ terminal treatment technology, photocatalytic technology is a competitive technology that has practical application prospects for the emission reduction of VOCs. The core problem of photocatalysis technology is constructing a photocatalyst with higher activity.RESULTSIn this work, a new hierarchical heterostructured photocatalyst of TiO2/Bi/Bi2MoO6 was facilely fabricated by solvothermal method with the TiO2 nanoparticles uniformly distributed on the surface of flower‐like Bi2MoO6 nanospheres. Compared with the bare flower‐like Bi2MoO6 nanospheres, the band gap width of TiO2/Bi/Bi2MoO6 is wider, which helps to improve the Fermi level‐derived oxidation–reduction capacity of the catalyst. N2 adsorption–desorption experiments show that the composite material has a larger specific surface area, which is beneficial to improve the mass transfer efficiency in the gas–solid heterogeneous reaction. Photocatalytic data display that, under irradiation by Xenon lamp for 2 h, the degradation rate of toluene gas over TiO2/Bi/Bi2MoO6 reached 26.08%, which is 17.92% higher than that of the bare Bi2MoO6 nanospheres.CONCLUSIONThese results indicate that coupling TiO2 could significantly enhance the photocatalytic degradation effect of toluene over the hierarchical heterostructured TiO2/Bi/Bi2MoO6, and this has practical application prospects for the use of photocatalytic technology to treat VOCs pollution. © 2021 Society of Chemical Industry

Quaternization of a Polystyrene-block-poly(4-vinylpyridine) Isoporous Membrane: An Approach to Tune the Pore Size and the Charge Density.

Isoporous integral asymmetric membranes derived from the self-assembly of block copolymers combined with the non-solvent-induced phase separation (SNIPS) have gained great attention. To extend their utility, good control over pore size and surface functionality in a facile manner is highly desirable. Here, an approach is proposed to achieve this by quaternization of the poly(4-vinylpyridine) moiety of a polystyrene-block-poly(4-vinylpyridine) SNIPS membrane using alkyl iodides via a scalable gas-solid heterogeneous reaction. By changing the size of the alkyl groups of the quaternization agent and the degree of quaternization, the effective pore size of the membrane is tailored in a wide range from the ultrafiltration to the nanofiltration regime. A quaternization of approximately half of the 4VP repeating units of the membranes with methyl iodide, ethyl iodide, or 1-propyl iodide leads to a retention of methylene blue from a 10mg L-1 aqueous solution of 96%, 87%, and 83%, respectively.

Numerical and experimental study of La–Ni hydriding kinetics based on the varying-size model

Particle size often varies during gas–solid heterogeneous reactions. For example, metal hydrides (MH) can reversibly absorb and desorb H2, accompanied by the expansion and shrinkage of the alloy particles, respectively. Although the traditional shrinking-core model (SCM) can be used to describe the H2 absorption/desorption process, the calculation results from SCM show relatively large deviations from the experimental data without considering the factor of particle expansion/shrinkage. Therefore, we proposed a shrinking-core–varying-size model (SC-VSM, henceforth VSM) for MH particles that accurately determines the kinetics equation with particle deformation. Three types of control mechanism including H2 dissociative chemisorption, H internal diffusion, and surface reaction were studied extensively, and the rate-controlling step for both VSM and SCM was determined to be the internal diffusion of H. Hydriding experiments on the systems of LaNi5 and LaNi4.5Al0.5 were performed under quasi-isothermal and variable-pressure conditions. The simulation results indicated that the new VSM was highly consistent with the experimental data for both alloys, evidently providing higher precision than the traditional SCM. The simplified forms of the VSM diffusion equations for LaNi5 and LaNi4.5Al0.5 were also developed to facilitate their practicability during the hydriding process: LaNi5:1.426-1.419(1-X)23-0.046(1-X)53-0.95X+0.038(1-X)2=tτdiff′LaNi4.5Al0.5:1.362-1.35(1-X)23-0.069(1-X)53-0.90X+0.057(1-X)2=tτdiff′

Study on Gasification Characteristics of Petroleum Asphalt in a Two-Stage Gasifier

The gasification performance of petroleum asphalt in a two-stage gasifier was investigated through experiments and simulations. The analysis results of asphalt samples show that the asphalt has characteristics of high volatile content, high calorific value, and low ash content. Coal pyrolysis model, gas-solid heterogeneous reaction model, gas homogeneous reaction model and Gibbs free energy minimization method were employed to build the two-stage entrained flow gasification model and simulate the asphalt gasification process using Aspen Plus software. The results indicate that the cold gas efficiency of asphalt is 3% higher than that of bituminous coal, while the consumption of coal and oxygen is 40% lower than that of bituminous coal. The gasification performance of asphalt is superior to that of coal. In addition, the cold gas efficiency of asphalt can be increased by about 3% when the two-stage gasification process is employed. The utilization rate of coal in the two-stage gasifier is much higher than that in traditional one-stage gasifier.

Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid

BackgroundThe catalytic performance of supported heteropolyacids in various liquid–solid and gas–solid heterogeneous reactions may be affected by the choice of support and/or the method of heteropolyacid deposition. Main methodsRadioligand binding assays were used to quantify affinity, density and ratio of ET receptors. Design/methodology12-tungstophosphoric acid (HPW) was dispersed throughout three matrixes: Na, H-exchanged montmorillonites (Na-Mt, H-Mt) derived from Polish bentonite and commercially available K10 montmorillonite (Mt-K10). Two series of catalysts were prepared via conventional and modified impregnations. Conventional procedure based on the incipient wetness impregnation method involved stirring of the aqueous dispersion of HPW and montmorillonite, followed by drying at elevated temperature. Modified preparation method employed ultrasonication as a means of the dispersion homogenization, followed by freeze-drying. The catalysts were characterized by XRD, N2 sorption, TG analysis, FT-IR and 31P solid-state MAS-NMR spectroscopy. The acidic properties of HPW/montmorillonites were examined in the conversion of ethanol, whereas their catalytic performance was evaluated in the transesterification of castor oil with methanol. FindingsIntact Keggin anions were preserved on H-Mt and Mt-K10, while lacunary Keggin anions dominated on Na-Mt (especially after deposition aided by ultrasonication). Appropriate choice of the montmorillonite support and/or pretreatment allowed tailoring of the acidic function of the heteropolyacid supported catalysts. The use of H-Mt prepared under mild conditions resulted in enhanced acidity and high catalytic performance of acid catalysts, whereas the use of Na-Mt, aided by ultrasonication diminished their acidic function and catalytic activity. The most important achievement was to obtain HPW/montmorillonite system more active than unsupported HPW, via spreading heteropolyacid on H-Mt derived from Polish bentonite.

A generalized kinetic model for heterogeneous gas-solid reactions

We present a generalized kinetic model for gas-solid heterogeneous reactions taking place at the interface between two phases. The model studies the reaction kinetics by taking into account the reactions at the interface, as well as the transport process within the product layer. The standard unreacted shrinking core model relies on the assumption of quasi-static diffusion that results in a steady-state concentration profile of gas reactant in the product layer. By relaxing this assumption and resolving the entire problem, general solutions can be obtained for reaction kinetics, including the reaction front velocity and the conversion (volume fraction of reacted solid). The unreacted shrinking core model is shown to be accurate and in agreement with the generalized model for slow reaction (or fast diffusion), low concentration of gas reactant, and small solid size. Otherwise, a generalized kinetic model should be used.

Effect of structural, thermal and flow parameters on steam reforming of methane in a catalytic microreactor

Steam reforming of methane in microchannels, embedded in a monolith is numerically modelled. Horizontal heating layers at equal intervals within the monolith are maintained at constant temperature. The channels are coated internally with catalyst to enhance gas–solid heterogeneous reaction. The numerical method combines the analytical solution for heat transfer through a fin, extended to a stack of fins, and the reactive flow of gases through an iterative procedure. The method offers a tool for quick design of a micro-structure, without considering detailed CFD-based model. In addition, the method can be suitably modified to address thermal management in electronic chip. The temperature within a stack between two heating layers drops near the centre of the stack, in case of an endothermic reaction. This drop, signifying the deviation from isothermal behaviour is found more near the heating layer, and tapers off near the centre of the stack. When the feed temperature is significantly less than the temperature of the heating layer, the portion of the reactor, away from the heating layer remains at a substantially lower temperature, particularly when the number of channels between two heating layers is large. Accordingly, the conversions in the individual channels at the outlet are affected. If the channel wall becomes thicker, the drop in fluid temperature away from the heating layer is more. The increase in feed velocity leads to larger drop in temperature and overall conversion. The decrease in thermal conductivity and the increase in number of channels between two heating layers enhance the temperature drop. None of these functionalities appears to be linear.

SIMULATION AND OPTIMIZATION OF COIL DECOKING IN ETHANE PYROLYSIS FURNACE

A detailed model of coupled heat transfer in the firebox and coke oxidation in the coils of a commercial ethane pyrolysis furnace was developed to simulate the decoking process. The model was created by combining a plug flow reactor model, the shrinking core model, and the zonal approach method to approximately describe the fluid flow in the coils, the gas-solid heterogeneous reactions of coke gasification, and the heat transfer in the furnace. The simulation accurately modeled the decoking time and CO2 mole fraction at the coil outlets. The model may be used to evaluate existing decoking processes, and new decoking procedures can be investigated by iterative approximation. The model can be used to validate the following steady-state operating conditions: coil outlet temperature, coil inlet temperature, decoking time, coil outlet CO2 mole fraction, and the maximum external tube wall temperature, resulting in lower consumption of steam and air and a reduction in residual coke. This work has provided essential information for the development of advanced on-line coke-burning procedures for ethylene production.

HeI photoelectron spectroscopy and quantum chemical investigation on trichloromethanesulfenyl benzoate, C 6H 5C(O)OSCCl 3

Trichloromethanesulfenyl benzoate, C 6H 5C(O)OSCCl 3 was prepared in the gas phase through gas–solid heterogeneous reaction between CCl 3SCl vapor and powdered C 6H 5C(O)OAg. The electronic structure of C 6H 5C(O)OSCCl 3 has been studied by photoelectron spectroscopy (PES), together with quantum chemical calculations. The S–O bond length ( r SO) and the torsional angle around S–O bond ( δ CSOC) of the gauche conformer are determined to be 1.691 Å and 104.4° at the MP2/6-311++G(d,p) level, respectively. The first vertical ionization energy of C 6H 5C(O)OSCCl 3 is 9.46 eV, as revealed by PES. The ionization process of the first ionization potential comes from the π orbital at benzene ring.

A study on sulphidation and regeneration of Z-Sorb III sorbent for H 2S removal from simulated ELCOGAS IGCC syngas

The sulphidation and regeneration of the sorbent Z-Sorb III has been investigated at bench scale in a high-temperature, high-pressure fixed-bed unit, over a range of conditions relevant to the Puertollano IGCC plant (Spain). The effect of gas hourly space velocity (1000– 5000 h - 1 ), temperature (673–773 K) and pressure (1–2 MPa) on sulphidation is studied. As predicted by thermodynamics, the sorbent is capable of reducing the H 2S concentration from 10,000 ppmv to near 0 ppmv. The analysis of the experimental results shows that gas residence time has a strong effect on sorbent utilization. Below 2000 h - 1 , the sorbent seems to promote the Boudouard reaction. As a result of this, CO content in the gas stream increases with simultaneous carbon formation in the solid phase. At higher gas velocities ( 3500 h - 1 ) , CO disproportionation does not occur and the sulphidation reaction proceeds isothermally to complete conversion of ZnO. Further increase in space velocity to 5000 h - 1 leads to lower sorbent utilization, which is typical of diffusion-limited gas–solid heterogeneous reactions. On the other hand, sulphidation temperature has very little effect on sulphur loading uptake which is slightly higher at 773 K than at 673 K. Regarding the effect of pressure, the sorbent is most effective at 2 MPa, almost reaching the theoretical saturation capacity. The study of the durability of the sorbent is investigated through multiple sulphidation–regeneration cycles. Oxidative dry mild regeneration using an oxygen concentration of 2% v/v results in complete regeneration of the sulphided sorbent without severe structural property changes or activity loss. The primary reaction during regeneration of the sulphided sorbent is direct oxidation of the metal sulphide with SO 2 release.

Formation of Mutagenic Nitrofluoranthenes in the Gas-Solid Heterogeneous Reaction of Particle-associated Fluoranthene in NO2-O3-O2 System

Abstract In the gas-solid heterogeneous reactions of particle-supported fluoranthene with ppm levels of nitrogen dioxide and ozone, dinitrofluoranthenes and 3-nitrofluoranthene (3-NF) were formed only in the dark and 2-NF was formed only under photoirradiation while 1-, 7-, and 8-NFs were formed under both conditions. Yield and distribution of the products varied depending on the kind of supports employed.

Isobaric and isothermal DSC measurements of metal-hydrogen systems

The reversible formation reaction of hydrides from metals and hydrogen is used as a prototype of a gas-solid heterogeneous reaction in order to show that a sufficient thermodynamic and kinetic characterization of such reactions can be achieved by isothermal and isobaric DSC measurements. The isobaric measurements essentially provide thermodynamic data such as reaction enthalpy, yield and the pressure-temperature relation. On the other hand, the isothermal measurements can be used for kinetic evaluation due to the small number of parameters. Results are given for the PdH and LaMgH systems.

Calorimetric bead techniques for measurement of kinetic data for gas-solid heterogeneous reactions

A critical assessment has been made of present experimental methods using calorimetric bead systems for the measurement of gas-solid catalytic kinetic data. Two distinct methods, the isothermal and nonisothermal, are identified and their relative merits are discussed.

The measurement of transport coefficients in gas-solid heterogeneous reactions

If reasonably uniformly sized spherical pellets are packed in a tube only slightly larger in size, so that the tube/pellet dia ratio is from 1·1 to 1·4, and the tube contains 50 or more pellets, then the flow behaviour is very similar to that of a conventional packed bed. Pressure drop results and axial dispersion values obtained for non-porous pellets using pulse techniques can be predicted from existing packed bed correlations. Magnetite pellets of different porosities were used as porous packing, also, and, by Fourier analysis of outlet tracer pulse dispersion, external mass transfer and effective diffusion coefficients were obtained, assuming the validity of the Kubin-Kucera model. The use of this ‘single pellet string reactor’ (SPSR) appears to give a convenient laboratory simulation of a packed bed reactor, which would be particularly useful for larger solid particles such as pelletized ores. It also offers a convenient laboratory method for obtaining average effective diffusion coefficients over a range of temperatures and pressures for a representative sample of large porous particles.

The Distribution of Impurity in a Semi-infinite Solidified Melt

The Distribution of Impurity in a Semi-infinite Solidified Melt O W Memelink The Theory of Gaseous Arcs: I The Fundamental Relations for the Positive Columns K S W Champion Calorimetric bead techniques for measurement of kinetic data for gas-solid heterogeneous reactions A Jones, J G Firth and T A Jones The Theory of Gaseous Arcs: II The Energy Balance Equation for the Positive Columns K S W Champion Analysis of satellite and undulation structure in the spectrum of Na+Hg continuum emission J P Woerdman, J Schlejen, J Korving et al. A collisional-radiative model applicable to argon discharge over a wide range of conditions. IV. Application to inductively coupled plasmas J Vlcek and V Pelikan Jets In Heavy Ion Collisions with CMS Sevil Salur Radiation from Gaseous Arcs