Gas-solid Model Research Articles

Three-dimensional particle-scale investigation of transient gas–solid flow characteristics in the propulsion system with a complex charge structure

This work is focused on researching the particle behavior of the transient gas–solid flow in the propulsion system with the modular charge structure, which has a great effect on the temporal-spatial distribution of energy release. Based on the coupling method of computational fluid dynamics and the discrete element method, an efficient three-dimensional unsteady gas–solid model is developed to provide a detailed means of capturing particle behavior in the propulsion system with a complex structure. Comparisons of the particle distribution of simulation results are done with experimental research, and a reasonable match has been obtained. Furthermore, the particle behavior characteristics can be divided into three stages. In the first stage, the particle behavior is dominated by the high-pressure, high-speed gas from the igniter. In the other two stages, the particle behavior is dominated by the complex gas flow generated by the opening of the module and the groove structure of the vehicle. The results show that the distribution of particles consists of slope accumulation and horizontal accumulation. The slope accumulation with a large gradient contains 84% of the total particles, and the height of the slope accumulation is increased exponentially along the axis.

Random pore model insights into structural and operational parameters for hydrogen-based iron oxide reduction

The hydrogen-based direct reduction of iron oxide (DRI) provides a sustainable approach to cleaner ironmaking. In this research, for analyzing hydrogen-based DRI in pellet scale, the random pore model (RPM), the most sophisticated non-catalytic gas-solid model, is applied to determine the effects of structural parameters, including pore-size parameter (ψ), Thiele modulus (ϕ), and product layer resistance (β), along with operational parameters such as pressure, temperature, and gas composition. The PDE equations of RPM were solved numerically using the method of lines together with a finite-difference discretization in space. The effects of parameters ψ and ϕ were investigated, revealing that the reaction rate increases with larger pores and decreases with higher values of ϕ over length and time. Kinetics resemble the unreacted shrinking core model (USCM) for small particles, while pore structure dominates for large particles. Parameter determination is influenced by ϕ changes, shifting control from chemical kinetics to diffusion, with operating conditions impacting reaction rate enhancement. Investigations revealed that as ϕ increases from 0.454 to 1.454, a 6 % conversion gradient forms from the pellet center to the surface. Additionally, at 700°C, reducing the temperature by 100°C slows conversion by 15.45 minutes while increasing it accelerates conversion by 4.55 minutes.

Liquid phase exfoliation of metal–organic frameworks in aqueous quaternary ammonium hydroxide solution enables highly efficient photocatalytic CO2 reduction

Metal−organic framework nanosheets (MONs) show promising applications in many fields, but the production of large amounts of high-quality MONs remains a challenge. Here, we report a general liquid-phase stripped approach to exfoliate a benzoic acid linked MOF (Al2(OH)2TCPP-Co) in water by using quaternary ammonium hydroxides as deprotonation reagent, intercalator and stabilizer simultaneously. It is shown that the stripped yield is high up to 27.5%, the thickness of MONs is mainly 1–3 nm, and the horizontalsize is less than one hundred nanometers. The molecular geometry and alkalinity of quaternary ammonium hydroxides play vital roles in the exfoliation of MONs. Moreover, the MONs have been applied as photocatalysts for the conversion of CO2 to CO in a gas–solid model under UV–visible light illumination without using anyphotosensitizer and sacrificialagents. The productivity rate of CO is 118.7 μmol g−1 h−1, which outperforms most of the photocatalysts reported previously. Thus, the liquid phase stripping method reported in this work has great potential in the large-scale production of MONs for efficient photocatalysis of CO2 reduction in the gaseous phase.

Probing Contact Electrification between Gas and Solid Surface

Contact electrification exists everywhere and between every phase of matter. However, its mechanism still remains to be studied. The recent triboelectric nanogenerator serves as a probe and provides some new clues about the mechanism present in solid–solid, solid–liquid, and liquid–liquid contact electrification. The gas–solid model still remains to be exploited. Here, we investigated the contact electrification between gases and solids based on the single-electrode triboelectric nanogenerator. Our work shows that the amount of transferred charges between gas and solid particles increases with surface area, movement distance, and initial charges of particle increase. Furthermore, we find that the initial charges on the particle surface can attract more polar molecules and enhance gas collisions. Since ions in gas–solid contact are rare, we speculate that gas–solid contact electrification is mainly based on electron transfer. Further, we propose a theoretical model of gas–solid contact electrification involving the gas collision model and initial charges of the particle. Our study may have great significance to the gas–solid interface chemistry.

Euler multiphase‐CFD simulation on a bubble‐driven gas–liquid–solid fluidized bed

AbstractAn integrated flow model was developed to simulate the fluidization hydrodynamics in a new bubble‐driven gas–liquid–solid fluidized bed using the computational fluid dynamic (CFD) method. The results showed that axial solids holdup is affected by grid size, bubble diameter, and the interphase drag models used in the simulation. Good agreements with experimental data could be obtained by adopting the following parameters: 5 mm grid, 1.2 mm bubble diameter, the Tomiyama gas–liquid model, the Schiller–Naumann liquid–solid model, and the Gidaspow gas–solid model. At full fluidization state, an internal circulation of particles flowing upward near the wall and downward in the centre is observed, which is in the opposite direction compared with the traditional core‐annular flow structure in a gas–solid fluidized bed. The simulated results are very sensitive to bubble diameters. Using smaller bubble diameters would lead to excessive liquid bed expansions and more solid accumulated at the bottom due to a bigger gas–liquid drag force, while bigger bubble diameters would result in a higher solid bed height caused by a smaller gas–solid drag force. Considering the actual bubble distribution, population balance model (PBM) is employed to characterize the coalescence and break up of bubbles. The calculated bubble diameters grow up from 2–4 mm at the bottom to 5–10 mm at the upper section of the bed, which are comparable to those observed in experiments. The simulation results could provide valuable information for the design and optimization of this new type of fluidized system.

Research on the Extraction Technology of Gas-Integrated Working-Face in Close Coal Seam Mining in Permafrost Area

In order to understand the mechanical deformation characteristics of the permafrost layer and the stress-fracture-gas seepage law of the superimposed mining of the proximity coal seam group, a coupled gas-solid model of the proximity coal seam in the permafrost area was constructed based on the theoretical model of small deflection of thin plate and the deformation control equation, gas transport equation, porosity evolution equation and permeability evolution equation of the coal rock body, and the analysis of the dependent project. The analysis was carried out at the NMT coal mine. The results show that the multiyear permafrost seam in the NMT coal mine has a tendency to move to the left and will produce extensive pull damage near the right end. The initial and periodic collapse steps are 192.87 m and 43.24 m, respectively; the pore and fissure structural characteristics of the NMT coal seam are not conducive to gas transport, and the stress and fissure fields have a nonlinear increasing effect. The study also proposed the “protected seam mining and bottom extraction lane directional drilling group comining technology” to increase the daily production of working face by 25.8%. The results of the study can be used as a reference for similar projects.

Improving Hydrothermal Carbonization (htc) Processes by Hydrochar Gasification

Hydrochar is produced by means of hydrothermal carbonization at relatively low temperatures (180-260°C) in sub-critical water. While, in many respects similar to biochar, its physical and chemical properties differ significantly (Basu, 2018). Of particular interest for this work are the higher energy density and lower ash content, that make hydrochar a possible feedstock for gasification plants with energy generation purposes. A gasification step is used instead of direct combustion to convert solid fuels in energy to achieve cleaner combustion and higher efficiency (Wang and Stiegel, 2016). In this work a 400 kg/h hydrochar gasification plant was modelled to identify optimal conditions and energetic yield of hydrochar obtained from municipal sewage. The fixed bed, updraft, gasification reactor was modeled in detail using a multi-scale, multiphase methodology already widely tested on biomass (Corbetta et al., 2015; Ranzi et al., 2014). The gas solid kinetic model was coupled with a detailed gas-phase kinetic scheme with over 200 species, including reaction intermediates, and 2000 reactions for reliable product yield prediction. Using Visual Basic Application as an interface, the predictions from the detailed simulation package were delivered to a commercial simulation package to model the energy generation section of the plant. Aspen HYSYS V10 was used for this purpose for the simplicity of integration and its widespread use in similar industrial plants. The gasification was carried out with air, air enriched in oxygen to 28%, air enriched in oxygen to 35% and pure oxygen with different amounts of steam to control the temperature in the chamber and at different values of equivalence ratio. The gasification performance was evaluated in terms of lower heating value of the generated fuel gas while the H2S formation was accounted for only in a superficial manner using rules of thumb derived from previous experimental experience.

Relaxation Resonance Properties of the Dielectric Response of Water

Broadband terahertz infrared absorption spectra of liquid water are described using the nonstandard Frenkel gas–solid model for water, supplemented by the interconversion of molecules and ions due presumably to the mixed translational–vibrational motion of molecules in liquids. The periods of water molecule retention by hydration and ionic shells are calculated.

A Mathematical Model of Desiccant Wheel in Desiccant Cooling

The desiccant wheel performance of desiccant cooling system components is very important whose function is to regulate air humidity levels as well as to the capability, size and cost of the entire system. Mathematical models for predicting the performance of desiccant wheels in the development of mathematical models is one of the effective methods for analyzing the performance of wheel this moisture-reducing. This mathematical model can also be used in guiding system operation, delivering experimental results and automation in designing this cooling system. The purpose of this paper is to provide an overview of efforts to mathematically model the process of heat transfer and mass transfer that occurs in the moisture-reducing wheel. The desiccant wheel model built here is a gas and solid system including basic principles, heat and mass transfer mechanism and model building. The model is based on ideal assumptions, equations, additional conditions and the method of solution and also the main results. The gas-solid model is a more precise and more complex model than the other models. From these results the evolutionary process of the mathematical model is obtained and the aspects of calculation of pressure loss, air leakage, and optimal rotation speed of the drying wheel / dehumification.

Effect of Temperature on the Initial Oxidation Behavior and Kinetics of 5Cr Ferritic Steel in Air

The oxidation behavior of 5 wt pct Cr (5Cr) ferritic steel in air at high temperatures has been investigated. A connected Cu-rich layer adjacently forms in inner oxidation layer (IOL) at 500 °C to 600 °C in addition to the formation of a continuous Cr-rich and Si-rich ribbon. Their synergistic effects imped growth of the Fe-rich oxide film towards the outer oxidation layer (OOL) and thus improve the oxidation resistance of 5Cr steel. With increasing oxidation temperature, the Cr, Si, and Cu oxide scales are gradually damaged and spallation of the oxide scale occurs at 1000 °C. Regarding the reaction kinetics, oxidation is controlled by oxygen inward diffusion. A gas–solid model is used to describe the oxidation behavior of 5Cr steel at 500 °C to 900 °C and there is good agreement with the experimental data. In particular, this model can predict the oxidation behavior and the typical results at 750 °C and 850 °C are provided.

Modeling chemical looping syngas production in a microreactor using perovskite oxygen carriers

Synthesis gas, a mixture of hydrogen and carbon monoxide, could be produced in a chemical looping process. The objective of this work is the modeling of syngas production in a fixed bed microreactor by chemical looping reforming. A perovskite oxygen carrier was used for the reduction of methane to syngas. Twenty one gas-solid kinetic models were applied to the experimental data in which their parameters were estimated using an optimization code. The results show that among all models, reaction order model is the most preferable choice with satisfactory fitting criteria. The gas-solid model was coupled with a catalytic scheme to predict not only the conversion of perovskite oxygen carrier, but also the catalytic performance of the solid particles for syngas production. The kinetic parameters of the unified model were evaluated based on the experimental data of a fixed bed reactor. Analysis of both perovskite and nickel oxide, oxygen carriers shows that perovskite particles could convert 50 times slower than those of nickel oxide. A H2/CO ratio of below 10 was obtained in a period of time. A large amount of hydrogen was produced after completing gas-solid reactions which was due to cracking of methane to carbon and hydrogen. Although hydrogen was the main outlet product afterwards, corresponding carbon formation is a problem which should be avoided. The reduction of methane was proposed before 500 s with a carbon formation of below 0.04 kg carbon per one kg of perovskite carrier. Solid reduction conversion, methane consumption and product distribution were analyzed inside the microreactor.

Morphological evolution of porous silicon nitride ceramics at initial stage when exposed to water vapor

The reaction behavior of porous silicon nitride (Si3N4) ceramics at initial stage when exposed to water vapor ranging from 5 to 20 vol% at 1200 °C was investigated using online thermogravimetric (TG) method. The effect of water vapor on the surface morphology and corrosion interface of porous Si3N4 ceramics was characterized using scanning electron microscopy (SEM). The results show that water vapor can aggravate the reaction and the reaction rate accelerates with the water vapor content increasing. As for the morphological evolution, water vapor promotes the formation of interconnected pores, which is possibly caused by the breakage of amorphous SiO2 by inner massive gas product and gas releasing through crystalline SiO2. Based on these, a gas-solid model is adopted to describe the effect of different water vapor partial pressure on the reaction kinetics and obtains a good agreement.

Comparison of the Reaction Behavior of Hexagonal Silicon Carbide Powder in Different Atmospheres

The reaction behavior of hexagonal silicon carbide (h-SiC) powder under different atmospheres including dry air, air containing 20 vol pct water vapor, and Ar containing 20 vol pct water vapor at 1373 K to 1773 K (1100 °C to 1500 °C) for 10 hours has been investigated and compared. The SiC powder exhibits good oxidation resistance up to 1373 K (1100 °C) and its reaction behavior depends on both the temperature and atmosphere. Below 1773 K (1500 °C), water vapor can promote the formation of pores and enhance the oxidation of SiC powder. However, it tends to aggravate the surface sintering and intensify the volatilization reaction at 1773 K (1500 °C). This phenomenon is further verified with the increasing water vapor content. Based on this, a gas–solid model is adopted to deal with the corresponding reaction kinetics, and especially both oxidation and volatilization reactions are considered when dealing with the reaction kinetics of SiC powder in water-containing atmospheres.

Numerical Validation of Analytical Estimates for Propagation of Thermal Waves Generated by Gas-Solid Combustion

Gas-solid combustion appears in many applications such as in situ combustion, which is a potential technique for oil recovery. Previous work has analyzed traveling wave solutions and obtained analytical formulas describing combustion wave temperature, velocity, and gas velocity for one-dimensional gas-solid combustion model using geometrical singular perturbation theory. In the present work these formulas are generalized. Using numerical simulation we show that they can be adapted and then applied to describe more general two-dimensional models for in situ combustion in a nonhomogeneous porous medium.

Comparison of numerical simulations and experiments in conical gas–solid spouted bed

Flow behavior of gas and particles in conical spouted beds is experimentally studied and simulated using the two-fluid gas–solid model with the kinetic theory of granular flow. The bed pressure drop and fountain height are measured in a conical spouted bed of 100mm I.D. at different gas velocities. The simulation results are compared with measurements of bed pressure drop and fountain height. The comparison shows that the drag coefficient model used in cylindrical beds under-predicted bed pressure drop and fountain height in conical spouted beds due to the partial weight of particles supported by the inclined side walls. It is found that the numerical results using the drag coefficient model proposed based on the conical spouted bed in this study are in good agreement with experimental data. The present study provides a useful basis for further works on the CFD simulation of conical spouted bed.

A conductive heat transfer model for particle flows over immersed surfaces

A fundamental continuum model for conductive heat transfer between an immersed boundary and flowing particles is developed. The model is derived for systems where conduction through the interstitial gas between nearby surfaces is the dominant heat transfer mechanism. Conductive heat transfer depends on the thermal properties of the solids and gas phases, particle size and morphology, and packing structure. The new model incorporates both particle size and arrangement effects using first principles, and is applicable to flows spanning dilute to dense regimes. Specifically, a novel particle–wall distribution function is employed to capture the effects of particle arrangement over a range of solids concentrations. Discrete element method (DEM) simulations are used to close the model in terms of continuum variables and to generate constitutive relations for the Nusselt number and local heat transfer coefficient. The resulting expression is implemented into a continuum gas–solid model and tested against DEM data for particle flow down a ramp, flow around a hexagon, and crossflow around a cylinder. The model accurately predicts the local heat transfer coefficient over a range of flow parameters, and is valid for the full range of solids concentrations.

Study on Gas-solid Second-order Interaction Model in Fluidized Bed Reactors

Considering the fluctuating energy transfer between the SGS (Sub-Grid Scale, SGS) turbulent kinetic energy of gas and the second-order moment of particles, the gas-solid interaction model is present. The second-order term is studied, and the simulation results of second-order interaction are analyzed and compared with the simulation results by Koch. The effects of second-order interaction on concentration, velocity and second-order moment of particles and the SGS turbulent kinetic energy of gas are all researched.

Continuum model validation of gas jet plume injection into a gas–solid bubbling fluidized bed

A continuum gas–solid model that includes descriptions for solid frictional stress and a turbulent gas phase is evaluated against published experimental measurements of mean and fluctuating velocity inside the jet plume region of a bubbling fluidized bed with a high‐speed vertical jet injection. The main uncertainties in closure relations necessary in the continuum model are first identified and then determined using available experimental data. The overall model shows good agreement with both the gas and particle experimental velocity profiles. The trends in the centerline mean and fluctuating velocity with change in the fluidized state of the emulsion are also captured favorably. Main deviations between the model and experiment are noted and possible reasons for the mismatch are discussed. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3247–3264, 2013

Asymptotic Analysis in a Gas-Solid Combustion Model with Pattern Formation

The authors consider a free interface problem which stems from a gas-solid model in combustion with pattern formation. A third-order, fully nonlinear, self-consistent equation for the flame front is derived. Asymptotic methods reveal that the interface approaches a solution to the Kuramoto-Sivashinsky equation. Numerical results which illustrate the dynamics are presented.

A One-Dimensional Two-Fluid Gas–Solid Model Applied to Fluidized Bed Reactors: The SMR and SE-SMR Processes

A one-dimensional two-fluid Eulerian–Eulerian model is adopted for numerical simulations of gas–solid flows in fluidized bed reactors. The simulation results obtained with the one-dimensional model are compared with the results of a two-dimensional model for the SMR and SE-SMR processes. For the species concentrations and temperature predictions, the one-dimensional and two-dimensional models are in good agreement. Deviations in the simulation results are observed between the models in the phase area fractions and gas phase velocity. The one-dimensional model should therefore be further extended to include the effect of the gas bubbles in the dense bed zone. Moreover, it is necessary to compensate for the radial convective flow pattern by extended conductive fluxes.