Uniform Gas Flow Research Articles

Numerical simulations of CO2 absorption by MgO-based sorbent in a gas–solid fluidized bed

Given that carbon dioxide (CO2) constitutes the predominant proportion of greenhouse gas emissions, its capture, utilization and storage have always been a subject of great concern. Gas-solid fluidized bed has become one of the commonly applied equipment for the removal of CO2 from flue gases owing to its advantages such as exceptional mixing effect and substantial interfacial area between the phases. However, there is a lack of attention to the mechanism of CO2 absorption by individually-tracked moving solid sorbents as well as their effects on the decarbonization performance in a gas–solid fluidized bed. In this study, CO2 removal model by solid particles based on MgO adsorbent was established with the implementation of the shrinking-core model and each particle was tracked individually. Results validated the accuracy of the absorption model and revealed that smaller sizes of the solid absorbent, lower inlet gas velocities and larger inlet CO2 mass fractions exhibit relatively high CO2 removal efficiency. In addition, the decarbonization performance of fluidized and packed beds was compared. The packed bed exhibits more uniform gas flow and higher CO2 removal efficiency, whereas the fluidized bed provides larger interphase contact area, which facilitates the heat transfer process. Through the analysis of a series of parameters, the results provide recommendations for improving the CO2 removal efficiency and help to explore the optimal protocol for the design of bed reactors for CO2 removal.

Design of an economic, portable, compact, and indigenous instrument setup for measuring sensing characteristics of thin film gas sensors.

Thin film gas sensor characterization is very demanding for various applications because of technical design trade-offs in commercially available gas sensors. For gas sensing characterization, a suitable gas-testing experimental setup is very much needed in this context. Various factors in the experimental setup can affect a thin film gas sensor's response beyond gas exposure. These factors include the test chamber's volume, relative humidity, uniform operating temperature, uniform pressure, uniform gas density, uniform gas distribution, uniform gas concentration in the gas chamber, and uniform relative gas flow velocity over the surface of the sensor. All these environmental parameters, not being so predictive in nature, induce an inherent design trade-off in the experimental setup design. Although all the commercially available gas testing experimental setups are equally good considering the dedicated purpose for which they are made. However, all of them are for generic applications but not for specific applications because of their inherent trade-offs in their usability features. Those trade-offs always provide an opportunity to introduce a new setup with its own unique advantages. Hence, in this article, we have presented a portable, compact, indigenous gas sensing experimental setup for studying the performance of gas sensors. We have characterized and tested the setup using a ZnO based thin film gas sensor when exposed to CO2 gas at concentrations ranging from 1445 to 4631 ppm. The proposed gas sensing setup's compact size offers unique advantages, including portability and compatibility for uniform environmental conditions.

Effect of elemental substitution on transition threshold behaviours of Ge-As(Sb)-Se glasses

In this paper, We prepare two groups of glasses: one is Ge<sub><i>x</i></sub>As<sub>20</sub>Se<sub>80–<i>x</i></sub> with x ranging from 5% to 32.5%, the other is Ge<sub><i>x</i></sub>Sb<sub>20</sub>Se<sub>80–<i>x</i></sub> with x spanning from 5% to 25%, by using the conventional melt-quench method, and investigate the effect of the elemental substitution of Sb for As on the threshold behaviors in Ge<sub><i>x</i></sub>As(Sb)<sub>20</sub>Se<sub>80–<i>x</i></sub> glasses. We are to understand to what extent the topological model and chemical order model can explain the correlation between physical properties and glass compositions, and how the chemical composition can affect the glass transition threshold. Glass transition temperature is measured by the differential scanning calorimeter (Mettler-Toledo, DSC1) with different scanning rates ranging from 5 K/min to 30 K/min under a uniform nitrogen gas flow of 50 mL/min, the glass density is measured by a Mettler H20 balance with a MgO crystal used as a reference. Samples of each glass composition are weighed five times and the average density is recorded. The refractive index of the glass at 1.5 um is measured by a Metricon Model 2010 prism coupler. Raman spectra are measured by a T64000 Jobin-Yvon-Horiba micro-Raman spectrometer equipped with a liquid-nitrogen-cooled CCD detector. The 830 nm laser line is used as an excitation source, and the laser power is kept as small as possible to avoid any photo-induced effects. The resolution of the spectrometer is about 0.5 cm<sup>–1</sup>. The systematic measurements of these physical parameters show that while the transition thresholds at MCN = 2.4 and 2.67 are verified in the Ge-As-Se glasses with ideal covalent network, these two transitions represent the covalent network structure inside the glass from an under-constrained “floppy” network to an over-constrained “rigid” phase and from the two-dimensional to the three-dimensional “stressed rigid” phase respectively. However, when As is substituted by Sb, the the resulting Ge<sub><i>x</i></sub>Sb<sub>20</sub>Se<sub>80–<i>x</i></sub> glass with non-ideal covalent network will change its transition threshold, changing into the chemically stoichiometric composition. We further deconvolve Raman scattering spectra into different structural units and the change of their respective intensity shows the same behavior, which is ascribed to the chemical effect induced by a large difference in atomic radius between As and Sb, and a relatively strong ionic feature of element Sb.

Effect of the Injection Structure on Gas Velocity Distribution in a 3D Vertical Oven

Gas injection structures were designed for a vertical oven to improve the gas–solid flow countercurrent structure. This work measured the wall temperature distribution of the vertical oven to reflect gas velocity distribution, and simulated the basic gas–solid flow field. The effects of the number of gas orifice layers and the injection angle on the gas velocity distribution were examined. The results showed that number of gas injection layers had a significant effect on the gas velocity distribution in the lower zone. Compared with the distributions with one or three injection layers, two injection layers produce more uniform gas flow. A small particle size of 6–15 mm increased the bed resistance and solid fraction standard deviation. A nozzle angle of 45° was conducive to increase the gas velocity in the upper zone and forming a more uniform gas distribution.

Enhancement of Doppler spectroscopy to transverse direction by using optical vortex

Tunable diode laser absorption spectroscopy (TDLAS) is a valuable method for measuring particle flow velocities in plasma. However, conventional TDLAS using a plane-wave beam is sensitive only to the laser propagation direction. This limitation is particularly unfavorable for the observation of the particle transportation perpendicularly incident on the material in the plasma–material interaction. In this paper, we show for the first time that flow measurements perpendicular to the beam direction are possible by replacing the probe beam with an optical vortex beam. Because an optical vortex has a helical wavefront, particles moving in its field experience an azimuthal Doppler shift in addition to the translational Doppler shift. Assuming a uniform gas flow across the optical vortex, the azimuthal Doppler shift of the absorption spectrum observed in the beam cross-section varies sinusoidally in the azimuthal direction. The transverse flow velocity is derived from the amplitude of this sinusoidal variation. At transverse velocities above 70 m/s, the measurement errors are found to be less than 15%, with a mean absolute percentage error of less than 8%.

Scaling-Up of Thin-Film Photoelectrodes for Solar Water Splitting Based on Atomic Layer Deposition.

Atomic layer deposition (ALD) is an established method to prepare protective layers for Si-based photoelectrodes for photoelectrochemical (PEC) water splitting. Although ALD has been widely used in microelectronics and photovoltaics, it remains a great challenge to design simple and effective ALD systems to deposit large and uniform protective films for Si-based photoelectrodes with industrial sizes. This paper describes the design and realization of a simple ALD chamber configuration for photoelectrodes with large sizes, in which the influence of a gas redistributor over the gas flow and heat transfer during film growth was revealed by computational fluid dynamics simulations and experimental investigations. A simple circular baffle-type redistributor was proposed to establish a uniform gas flow field throughout the ALD reactor, resulting in a uniform temperature profile. With this simple baffle redistributor, the large-area Al2O3 monitor film (46 nm thickness) reached a good nonuniformity (Nu %) of 0.88% over a large area of 256 cm2. This design enables the fabrication of large-scale photocathodes from standard industrial-grade 166 mm Si(100) wafers (276 cm2) by depositing 50 nm TiO2 protective films with Nu % less than 5%. The obtained photocathode achieves a saturation current of 6.45 A with a hydrogen production rate of 43.2 mL/min under outdoor illumination. This work elucidates how flow pattern and heat transfer may influence the deposition of protective layers over large photoelectrodes, providing guidance for future industrial applications of PEC water splitting.

Research on the gas migration trend and mechanism of the transition flow regime in coal based on MRT-LBM simulation

In order to reveal the process and mechanism of gas flow in a low-permeability coal seam, a new multiple-relaxation-time lattice Boltzmann method (MRT-LBM) model of gas migration in coal micro/nanopores based on Langmuir monolayer adsorption theory, the slip boundary scheme and Bosanquet effective viscosity was established. The software MATLAB was used to carry out the simulation study of uniform pore gas flow based on the MRT-LBM model, and the results were compared and verified with the porous anodic alumina membrane gas flow experimental results. On this basis, the gas flow in coal pores with different micro/nanopore sizes considering adsorption was simulated. The results show that the dimensionless permeability coefficient increases with decreasing pore size under the same pressure, which reflects the subsequent enhancement of the microboundary constraint effect and reveals that the pore system becomes the main controlling factor of coal seam permeability within the coal matrix in the middle and late stages of coal seam gas extraction, while the role of the microboundary constraint effect needs to be considered. The gas adsorption layer weakens the pore gas flow capacity, but for pores with a radius greater than 16 nm, the apparent change in permeability caused by the adsorption layer is less than 5%, and the adsorption effect can be ignored. N2, CH4, and CO2 enter the transition flow regime under different pressure conditions; with gas extraction, the gas pressure decreases, and the difference in the gas flow characteristics of the three gases increases.

Investigation of the RF Sputtering Process and the Properties of Deposited Silicon Oxynitride Layers under Varying Reactive Gas Conditions.

In a single process run, an amorphous silicon oxynitride layer was grown, which includes the entire transition from oxide to nitride. The variation of the optical properties and the thickness of the layer was characterized by Spectroscopic Ellipsometry (SE) measurements, while the elemental composition was investigated by Energy Dispersive Spectroscopy (EDS). It was revealed that the refractive index of the layer at 632.8 nm is tunable in the 1.48–1.89 range by varying the oxygen partial pressure in the chamber. From the data of the composition of the layer, the typical physical parameters of the process were determined by applying the Berg model valid for reactive sputtering. In our modelling, a new approach was introduced, where the metallic Si target sputtered with a uniform nitrogen and variable oxygen gas flow was considered as an oxygen gas-sputtered SiN target. The layer growth method used in the present work and the revealed correlations between sputtering parameters, layer composition and refractive index, enable both the achievement of the desired optical properties of silicon oxynitride layers and the production of thin films with gradient refractive index for technology applications.

Experimental Characterization of Anisotropic Mechanical and Thermal Properties of Gas Diffusion Layers

Gas diffusion layer (GDL) is a vital component in proton exchange membrane fuel cells (PEMFC) due its main functions to conduct electrons and heat between the adjacent fuel cell components, provide preferential pathways for product water removal and to provide uniform reactant gas flow distribution to the electrode surface. Because of the anisotropic GDL microstructure, the transport properties vary in the through-plane and in-plane direction. Furthermore, during fuel cell stack assembly pressures exerted on the flowfield land compress the GDL under land cause changes of the GDL microstructure. While moderate compression increases GDL thermal conductivity due to increased fiber-fiber contacts, excessive compression may impede diffusive and liquid water transport due to loss of GDL pore volume. Detailed knowledge of how thermal conductivity is affected by the anisotropic nature of gas diffusion layers under compression is imperative in order to provide a better understanding on how thermal gradients influence two-phase transport during PEMFC operation. Previous research efforts have focused on steady-state methods for measuring effective through-plane GDL thermal conductivity [1][2][3][4] and effective in-plane GDL thermal conductivity [5][6] but to the best of our knowledge no studies have been conducted to measure effective in-plane GDL thermal conductivity for a variety of different GDL types under compression.This study attempts to fill that gap by using specially designed in-house tools to characterize the influence of GDL anisotropy on effective thermal conductivity as a function compression for different GDL types. Furthermore, a comprehensive ex-situ mechanical study is conducted to characterize the compliance matrix for different GDL types. Early results indicate a highly non-linear compressive behaviour in the GDL through-plane direction with large variations for the different GDL types. Moreover, the flexural modulus is found to be highly anisotropic where stiffness in the GDL machine direction (MD) is consistently larger compared to stiffness in GDL cross machine direction (CMD). This work will provide a foundation for a numerical study to couple an anisotropic GDL structural model with a non-isothermal two-phase model to investigate the effects of inhomogenous compression on two-phase transport. Keywords: GDL, PEMFC, ex-situ, anisotropy, modulus, microstructure, mechanical, compression, characterization, MD, CMD, through-plane, in-plane, thermal conductivity[1] G. Karimi, X. Li, P. Teertstra, Electrochim. Acta (2010).[2] R. Bock, A.D. Shum, X. Xiao, H. Karoliussen, F. Seland, I. V. Zenyuk, O.S. Burheim, J. Electrochem. Soc. 165 (2018) F514–F525.[3] G. Unsworth, N. Zamel, X. Li, Int. J. Hydrogen Energy (2012).[4] E. Sadeghi, N. Djilali, M. Bahrami, J. Power Sources (2011).[5] E. Sadeghi, N. Djilali, M. Bahrami, J. Power Sources (2011).[6] N. Alhazmi, D.B. Ingham, M.S. Ismail, K.J. Hughes, L. Ma, M. Pourkashanian, Int. J. Hydrogen Energy (2013).

SEDS: A Flexible Standard EEG Data Structure for Large-Scale EEG Applications

Coal beneficiation technology using gas-solid fluidized bed is a hot topic in recent decade, however its proper design and scale-up remain a great challenge due to the very complex gas-solid flow involved. To this end, an Eulerian-Lagrangian-Lagrangian (ELL) model (Zhang et al., Renewable Energy, 2019, 136:193-201) was used to intensify the coal beneficiation process, by systematically studying the effects of particle property, operating gas velocity, initial bed height, design of gas distributor and the size of fluidized bed. It was found that (i) coal beneficiation process can be improved significantly by using coarser beneficiated coal particles, adopting smaller initial bed height (or shallow fluidized bed) and/or operating at a properly selected gas velocity, all findings indicate that the process intensification can be realized via proper selection of operating conditions; (ii) the design of gas distributor was critical for process intensification and it should be designed properly to make the gas flow as uniform as possible; and (iii) there will be no scale-up effect, if ideally uniform gas flow at the inlet can be achieved. Present study demonstrates that computational fluid dynamics is a cost-effective tool for process intensification of gas-solid fluidized beds.

Elucidating the operating behavior of PEM fuel cell with nickel foam as cathode flow field

Metal foam material, which serves as an alternative replacement of the conventional flow distributor of proton exchange membrane (PEM) fuel cell, has been attracting much attention over last few decades. In this work, three-dimensional modeling work for PEM fuel cell containing metal foam as cathode flow distributor has been carried out. The fuel cell performance and operating characteristics of metal foam flow field and conventional parallel flow channel have been compared and discussed. The cell performance has been reasonably validated based on the corresponding experimental tests conducted in this study. The superior performance of PEM fuel cell with metal foam as cathode flow field benefits a lot from the uniform gas flow. The porous metal foam material provides more pathways for the water delivery at the interface of metal foam flow field and gas diffusion layer (GDL), accelerating water removal capability of cathode. Because of the significant oxygen transfer loss in diffusion limited parallel channel, the operation of PEM fuel cell with parallel channel is found to be more sensitive to cathode humidification and oxygen supply at inlet. Due to the more uniform and effective electron transport though the porous electrode, it is possible to use thinner GDL in metal foam PEM fuel cell. It is expected that this study could give a good baseline of operating behavior of PEM fuel cell with metal foam flow distributor.

A review of technological improvements in laser-based powder bed fusion of metal printers

Additive manufacturing (AM) is an emerging process that has been extremely improved in terms of technology and application in recent years. In this technology review, new industrial improvements in laser powder bed fusion (LPBF) of metals are discussed. LPBF has the lowest build rate among all AM processes that produce metals such as electron beam powder bed fusion, direct energy deposition, binder jetting and sheet lamination. The findings of the current research show that the most innovations and future directions of LPBF printers are toward increasing the speed of the process by using interchangeable feedstock chamber, closed-loop control powder handling, automated powder sieving, multi-layer concurrent printing, 2-axis coating and multi powder hoppers. To increase the speed of the process, the new improvements for transferring time and using fast lasers are presented. Another innovation in the building of LPBF printers is enhancing part quality by improving lasers with the shorter beam diameter, multi-lasers, uniform inert gas flow, accurate positioning systems, using high vacuum systems and using sensors and automation.

CFD intensification of coal beneficiation process in gas-solid fluidized beds

Coal beneficiation technology using gas-solid fluidized bed is a hot topic in recent decade, however its proper design and scale-up remain a great challenge due to the very complex gas-solid flow involved. To this end, an Eulerian-Lagrangian-Lagrangian (ELL) model (Zhang et al., Renewable Energy, 2019, 136:193-201) was used to intensify the coal beneficiation process, by systematically studying the effects of particle property, operating gas velocity, initial bed height, design of gas distributor and the size of fluidized bed. It was found that (i) coal beneficiation process can be improved significantly by using coarser beneficiated coal particles, adopting smaller initial bed height (or shallow fluidized bed) and/or operating at a properly selected gas velocity, all findings indicate that the process intensification can be realized via proper selection of operating conditions; (ii) the design of gas distributor was critical for process intensification and it should be designed properly to make the gas flow as uniform as possible; and (iii) there will be no scale-up effect, if ideally uniform gas flow at the inlet can be achieved. Present study demonstrates that computational fluid dynamics is a cost-effective tool for process intensification of gas-solid fluidized beds.

Comparison of structured activated carbon and traditional adsorbents for purification of H2

RPSA technology can offer a more compact, cost-effective and energy-efficient solution for H2 recovery when compared to conventional PSA. The adsorption rate of adsorbents has become a key factor affecting the development of RPSA. Activated carbon, widely used as adsorbent for CO2 in H2 purification is in urgent need for further development to suitable for RPSA system. In this study, a structured activated carbon adsorbent was prepared using the dip-coating method, with nickel foam as a framework. The adsorption characteristics of the material for CO2 adsorption at different adsorption pressures, volume flow rates, and lamellar spacing parameters were investigated, and the kinetic behaviour was studied by the breakthrough curve. The results showed that the activated carbon was bonded to the nickel foam skeleton via an organic-inorganic binder with excellent mechanical strength. Structured adsorbents bed has uniform gas flow distribution, the pressure drop is about 1/3–1/7 of particles packed bed, and the breakthrough curve was sharper than particles. It indicated that the mass-transfer resistance in the structured adsorbents is low and has high effective diffusivity for CO2. The adsorption rate constant was calculated using the Yoon-Nelson model, and the correlation between the model calculation data and experimental data was greater than 0.98. Structured activated carbon adsorbent manufactured by dip-coating method, has a higher mass transfer coefficient and a shorter mass transfer zone length. It is a potential alternative adsorbent for RPSA.

The analysis of distribution of the reaction mixture in ammonia oxidation reactor

Abstract This article describes the influence of various design modifications of the ammonia oxidation reactor operating in nitric acid plant TKIV in Kędzierzyn-Koźle on flow distribution of an air-ammonia mixture. The CFD (Computational Fluid Dynamics) simulations of turbulent flow were carried out with SST k-ω turbulence model to close the system of RANS (Reynolds Averaged Navier-Stokes) equations. The simulation results show that the properly selected perforated plate screen and the conical diffuser ensure uniform flow of gas on the ammonia oxidation catalysts and on the catalysts for nitrous oxide decomposition. It was proved experimentally achieving uniform temperature of nitrous gases in different locations under the catalytic gauzes and high efficiency of ammonia oxidation and nitrous oxide decomposition

Global hydrodynamic of hybrid external loop airlift reactor: Experiments and CFD modelling

In this study, the mixing and mass transfer of air-water system were characterized in hybrid external loop airlift reactor using the computational fluid dynamics (CFD). This hybrid configuration involves the advantages of both airlift and torus reactors. To predict the flow in hybrid geometry, an Eulerian-Eulerian approach has been adopted with per-phase interaction of the standard k–ε turbulence model. To simplify the solution, the bubbles were considered to have approximately the same size. The global flow characteristics were experimentally determined by a conductimetric method. Gassing-out method was applied to determine gas-liquid mass transfer coefficients. The findings highlighted a uniform gas flow distribution in the riser tube, despite the riser included two different sections. It was noticed that beyond the superficial gas velocity, 0.0253 m/s, the fine rotational bubbles were dragged away by the liquid recirculation in the downcomer section. The STD k–ε turbulence model with per-phase effect, estimated correctly the hydrodynamic of homogenous and transition regimes except for high superficial gas velocity, Ug = 0.0703 m/s. For superficial gas velocities ranging from 0.0253 to 0.0703 m/s, there was a good agreement between simulated and experimental values; the error was less than 6%. Optimal conception of this hybrid geometry and its industrial scale use need further research and testing.

Asymptotic far-field behavior of macroscopic quantities in a problem of slow uniform rarefied gas flow past a sphere

The asymptotic far-field behavior of macroscopic quantities in a problem of slow uniform rarefied gas flow past a sphere is investigated for the purpose of acquiring a function occurring in the second-order drag. The thermal force in the problem of thermophoresis of a sphere is also discussed.

Influence of operational parameters on the performance of gas clean-up technology with a moving granular bed filter

This paper describes new design and test procedures of an inlet gas system that is part of a moving granular bed filter. A series of experiments is conducted to demonstrate the collection efficiency of this method under different operating conditions. In addition, the dynamic model and the flow analysis of gas in the inlet gas system and filter bed are discussed. Several parameters are considered, including uniform flow of gas, geometry of inlet gas system, and position of flow-corrective insert in filter bed, to understand the performance of the new filter system. The filtration superficial velocity, mass flow rate of filter granules, and baffle length and baffle angle of inlet gas system are controlled during the test process. The pressure drop, collection efficiency, flow velocity profile of gas, and size distribution of dust particulates are measured simultaneously. The trend in uniform gas flow behaviors can be useful for application in different cross-flow filter systems for gas clean-up. The results of this study are expected to serve as a basis for future research.

Process intensification and environmental consideration of sodium bicarbonate production in an industrial soda ash bubble column reactor by CO2 recycling

In the present study, a mathematical model is developed for a new configuration of sodium bicarbonate production in an industrial bubble column reactor. In the conventional configuration, the CO2outlet stream from the reactor is vented to the atmosphere while in the new configuration, a portion of the outlet CO2 stream is recycled and mixed with the inlet CO2flow of the reactor in order to reduce the greenhouse gas emission and also achieve a high purity level of the desired product. A gas sparger is installed at the bottom of the bubble column to provide a uniform gas flow distribution. A three−phase mass balance equation is considered. Liquid thermal behavior, gas pressure profile, CO2 conversion and mole fractions in the gas phase which mainly affect the production and size distribution of sodium bicarbonate crystals, are investigated. The results of the proposed model are compared with the conventional bubble column reactor (without recycle stream). The results show 314mol/m2.s CO2 saving and 50% increase in solid production as well as significantly reduction of air pollutants are also achieved with the new configuration. The optimized operating temperature for sodium bicarbonate production which was determined theoretically is reported to be 323K.

Study of the motion and deposition of micro particles in a vertical tube containing uniform gas flow

In this study, effects of a gaseous jet, formed in a vertical tube containing a uniform gas flow, on the injected micro particles have been investigated. A CFD model has been developed to simulate the particle motion in the tube. This simulation is very close to the experimental data. The results show that, increasing the flow rate of carrier gas or decreasing the flow rate of surrounding gas increases the effect of gaseous jet and also increases trapping rate of the particles by the tube wall. The minimum and maximum residence times of particles approach together with increasing the size of solid particles. Particles larger than 60 μm have a certain and fixed residence time at different flow rates of the carrier or surrounding gas. About 40 μm particle size has minimal trapping by the tube wall at various experimental conditions.