Gas Molar Fraction Research Articles

Evaluating the effectiveness of the modified multi-scale multi-physics coupled model for solid oxide fuel cells: A comparative analysis

A multi-scale multi-physics coupling numerical model serves as an effective tool to study the performance and improve the efficiency of solid oxide fuel cells (SOFCs) and provides the theory and data support for optimizing SOFC structure and operational strategy. To improve the accuracy and reliability of the model, this work established a modified simulation model that considers the effect of operational, structural, and physical parameters of SOFCs. First, the electrochemical kinetics equation, multi-component diffusion lattice Boltzmann (LB) model, and heat transfer LB model are modified by incorporating the reaction gas molar fraction, gas mixture concentration, temperature, and other parameters. Second, a regional hierarchical modeling strategy is proposed, and the multi-scale coupling model is modified by coupling mass and heat transfer. Third, considering the effect of carbon monoxide molar fraction, temperature difference, concentration change and porosity on electrochemical, mass, and heat transfer processes, the effectiveness of modification model is analyzed and validated. Results demonstrate that the modified model considerably improves the numerical simulation to handle the complex operational conditions of SOFCs. The modified model's predictions show better accuracy compared with experimental data, while maintaining consistency with the trends predicted by the original numerical models.

A multi-cycle pump for efficient separation of CO2 or other trace gas from a mixture of gases

An idealized gas separation pump is described for a membrane filtration system that conserves compressional energy by transferring it from one component to another operating at a different phase of the compression-decompression cycle. An example of the basic principle, we consider a pump that resembles an internal combustion engine with pistons in cylinders and three valves, one for mixed gas input, another for select gas output through a membrane, and a third for non-select gas output to the environment. With the transfer of compressional energy from the decompressing cylinder to the compressing cylinder, the energy required is the sum of frictional losses and the amount needed to compress only the trace gas that is to be separated. Compressional energy for the non-select is recovered. The mechanical energy required is the energy to compress the cylinder, NRT(lnC−1+1∕C)∼3NRT in the isothermal case for number of moles of select gas, N, gas constant R, temperature T, and compression factor C ∼ 50, multiplied by 1 + 2ϵ∕f ∼ 2 for frictional loss fraction ϵ ∼ 6% comparable to that in internal combustion engines and an input select gas molar fraction f ∼ 10% as for CO2 in flue gas. With T = 300 K and N = 2.3 × 104 per ton for CO2, this energy becomes ∼ 6NRT ∼ 94 kWh/tCO2. The advantages of a system like this are that the compression ratio through the membrane can be large and the timescale for compression can be adjusted to maximize the throughput for a given selectivity, overcoming the usual selectivity-permeance trade-off. A disadvantage is that the throughput is generally low for volumetric pumps at practical scales.

Evolution of the Enrichment Factor for an Iterative Scheme of Zirconium Isotopes Separation

In this paper evolution of the enrichment factor for zirconium isotopes separation by the laser assisted retardation of condensation has been found for specific choice of iterative policy of isotopes recovery. It has been evaluated for a set of gas flow temperatures, corresponding to its maximal value, provided all values of other important parameters of the system such as gas flow pressure, target gas molar fraction, laser intensity, laser beam radius, irradiation chamber length, and nozzle throat height are fixed. Enrichment factor has been found by solving a set of transport equations for molar fractions of four characteristic groups represented in the gas flow (monomers, dimers, excited monomers, and epithermals) in a static approximation on each iteration.

A new insight into implementing Mamdani fuzzy inference system for dynamic process modeling: Application on flash separator fuzzy dynamic modeling

In this work, a novel approach to model the dynamic behavior of the flash separation process (as a main building block of non-reacting stage-wise operations) based on Mamdani Fuzzy Inference Systems is proposed. This model surmounts the need to solve various types of mathematical equations governing the system and does not require thermodynamic properties which are either not available or computationally demanding. Hence it can be easily used in dynamic simulation of multi-phase flow in distributed systems. In the proposed approach the overall model is broken into several simple sub-models based on intuitive analysis of an expert. Moreover, a new fuzzy concept, named “Linguistic Composition Variable” is introduced to represent components mole fractions of each phase as a fuzzy variable. Accordingly, large number of rules which is the main shortcoming of the Mamdani Fuzzy system is significantly reduced.The performance of the proposed dynamic model is evaluated through comparing its results against their corresponding counterparts for a flash separator of crude oil. Overall MAPE (Mean Absolute Percentage Error) values of 7.17% for the gas molar fractions, 3.06% for liquid molar fractions, 10.16% for the temperature, 0.63% for the pressure and 16.44% for the liquid level of the separator have been achieved showing that the proposed fuzzy model can effectively capture the general trends of process data of the dynamic process.

Experimental study on phase behavior of heavy oil-solvent mixture system

Phase behavior research is of great significance for formulating oilfield development plans. In this paper, an experimental procedure was proposed to investigate the phase behavior of a solvent mixture(5 mol% CO2+45mol% CH4+55 mol% C3H8) withVenezuela Orinoco heavy oil. The measured physical properties of the heavyoil–gaseous solvent system include the bubble point pressure, swelling factor, density and viscosity of the liquid phase as a function of gas mole fraction. The experimental results show that as the gas molar fraction increases from 10% to 30%, the bubble point pressure and swelling factor of the heavy oil-solvent mixture system increase accordingly; as the dissolved gas content in the heavy oil increases, the viscosity and the densityof the system is reduced, but the reduction is getting smaller and smaller.

Anion Effect on Gas Absorption in Imidazolium-Based Ionic Liquids.

We performed classical Molecular Dynamics computer simulations to analyze solutions of the gases CO2, N2, and CH4 in four 1-n-butyl-3-methylimidazolium-based ionic liquids (1-n-butyl-3-methylimidazolium acetate, 1-n-butyl-3-methylimidazolium prolinate, 1-n-butyl-3-methylimidazolium bromide, and 1-n-butyl-3-methylimidazolium tetrafluoroborate). Typical experimental conditions (10 bar gas pressure and room temperature) have been chosen to study mixtures of the ionic liquids with the gases at a single gas molar fraction of 0.25. Structural aspects are discussed to judge the absorption capacities of the ionic liquids. We observed that CO2 coordinates preferentially within the polar domain of the ionic liquids with the bromide and tetrafluoroborate anions presenting the best performances. The other gases, N2 and CH4, remain in the less polar domains of the ionic liquids. Cluster size analysis indicates phase separation for these two gases. Considering both, the absorption tendency and gas separation capacity of the ionic liquids, the anion is desired to be small and possessing multiple coordination sites. In this aspect, the tetrafluoroborate anion accomplished the best results.

Thermodynamic Behavior of Olefin/Methane Mixtures Applied to Synthetic-Drilling-Fluid Well Control

Summary The presalt hydrocarbon discoveries in Brazilian deep water have been a challenging environment for drilling operations as a result of the technical demands and restrictions and the environmental regulations driving scientific and technological development. In this drilling context, an accurate estimation of the downhole pressure is mandatory to avoid drilling problems such as kicks, lost circulation, and wellbore instability. When considering kick prevention and control, understanding of the mixture behavior (drilling fluid and formation gas) is essential to improve the estimation of the downhole pressure, which would support efficient, safe, and economic drilling operations. The widespread application of synthetic-based drilling fluids in the Brazilian presalt polygon is justified by the technical performance offered by this kind of drilling fluid, such as reduced drilling time compared with water-based drilling fluids, increased lubricity in directional and horizontal wells, and shale-swelling inhibition. In addition, this is an environmentally friendly alternative to oil-based drilling fluids. However, those fluids are more sensitive to pressure and temperature variations than water-based drilling fluids. To obtain a better understanding of the behavior of one specific kind of synthetic-based drilling fluid, the olefin experimental research was conducted and the results and findings are presented in this technical article. The work involved pressure/volume/temperature (PVT) measurements for olefin/methane mixtures to investigate the effect of pressure, temperature, and mixture composition on thermodynamic properties such as density, formation volume factor (FVF), gas-solubility ratio, and saturation pressure. Those properties are important for knowledge of the mixture volumetric behavior at downhole conditions, especially when gas enters the wellbore. The experiments were conducted at isothermal conditions, and a gas-enrichment experimental procedure was applied. The temperature range was from 25 to 80°C (77 to 176°F), the gas molar fraction (GMF) ranged from 30 to 50%, and pressure was up to 69 MPa (10,000 psi). In addition, the measured data were compared with an existing database (Silva et al. 2004; Atolini 2008) for methane/n-paraffin mixtures, as well as for methane/ester mixtures (Kim et al. 2015). Pit-gain analyses were also performed according to the procedure of Monteiro et al. (2010), and a comparison of the results with other base fluids showed interesting findings.

The solvation inhomogeneity of sulfur dioxide in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid probed by Raman spectroscopy

AbstractThe different solvation states of SO2 in 1‐butyl‐3‐methylimidazolium thiocyanate ionic liquid were probed by the detailed analysis of the SO2 symmetric stretching mode, νs(SO2). The band position and linewidth were monitored in function of gas molar fraction, excitation wavelength, and temperature. As expected, the decreasing of the SO2 molar fraction, XSO2, from approximately 0.8 to 0.2, leads to a gradual downshift of the νs(SO2) band, attributed to a more effective charge transfer interaction from thiocyanate anion to SO2. Interestingly, at high gas concentrations (XSO2 = 0.8), a gradual downshift of the νs(SO2) band was noticed as the Raman excitation energy increases, an unexpected behavior because the change in the excitation wavelength in Raman spectroscopy should affect only the band intensities. Such νs(SO2) Raman dispersion can be understood as a consequence of the distinct charge transfer electronic transition energies associated to different SO2 solvation states, whose relative band intensities are modulated by resonance Raman conditions. Moreover, the lowering from room temperature to 100 K showed an increasing in the Raman dispersion effect. The temperature dependence was attributed to the strengthening of the cation–anion interaction, responsible for the increasing of SO2 solvation states distribution. The vibrational spectroscopic data regarding the SO2 solvation inhomogeneity in ionic liquids are showed for the first time, and it may contribute in the understanding of why such liquids are promising solvents for SO2 capture.

Fuel Composition in Pressurized SOFCs

Introduction Pressurized SOFC system combined with gas and steam turbine will achieve high power generation efficiency approaching 70% as the triple combined cycle power generation system. It has been reported that operating SOFCs under high pressure can make cell power output higher (1,2). However, there are only a limited number of studies available on the fuel composition for pressurized SOFCs. In general, fuel gas consists of carbon, hydrogen and oxygen, so that the C-H-O equilibrium composition determines anode performance. In this study, fuel gas composition on the anode side of pressurized SOCFs was derived by thermochemical equilibrium calculation for considering various operating conditions in terms of carbon deposition and each gas molar fraction affecting power generation characteristics, all described in the C-H-O diagrams. Calculation Procedure We calculated carbon deposition region and each gas molar fraction under various equilibrium conditions upon suppling fuels composed of carbon, hydrogen and oxygen. Thermochemical calculations were carried out using three approaches: (i) calculation with equilibrium constants, using (ii) HSC Chemistry (Version 9.0.5, Outotec Research Oy, Finland) and (iii) FactSage (Version 6.3.1, Thermfact Ltd., Canada). As the first approach, we defined main reactions within the fuel and solved the system of equations using relational expression of equilibrium constants and elements ratios by Mathematica (Version 10.4.1, Hulinks Inc., Japan). HSC Chemistry and FactSage are both thermochemical equilibrium calculation software based on Gibbs free energy minimization, containing various databases. It has been known that the major constituents of equilibrium products are H2(g), H2O(g), CO(g), CO2(g), CH4(g), and C(s) (graphite) (3). The line specifying the carbon deposition region means the equilibrium concentration of C(s) equal to 10-6 of the initial carbon content in the fuel. The C-H-O diagrams are described in such a way between 100 and 1000oC, and between 1 and 30 bar. Results and discussion Comparing Fig. 1 with Fig. 2, carbon deposition region boundaries vary with respect to temperature and pressure. The C-H-O diagrams clearly show that carbon deposition region expands on the oxygen-rich side and contracts on the hydrogen-rich side with decreasing temperature and/or increasing total pressure. It can be explained by the chemical equilibrium reactions (I) and (II): CH4↔C(s)+2H2 (I) 2CO↔C(s)+CO2 (II) Reaction (I) is an endothermal reaction; on the hydrogen-rich side, equilibrium reaction (I) shifts to the left side, preventing carbon deposition at lower temperatures. With increasing total pressure, the equilibrium also shifts to the left side due to the decrease in the number of molecules. In the same way, Reaction (II) is an exothermal reaction and the number of molecules is lower on the right side, so that the equilibrium reaction shifts to the right side to promote carbon deposition. The C-H-O diagrams of various gases and the corresponding theoretical open circuit voltage are also calculated. These results suggest, for example, a decrease in the fraction of H2and CO under higher total pressure.

Modelling of coal/biomass co-gasification in internal circulating fluidized bed using kinetic theory of granular mixture

A multi-fluid Eulerian model with the kinetic theory of granular mixture (KTGM) is employed for multi-component particles to simulate the coal/biomass co-gasification process in the internal circulating fluidized bed (ICFB). The hydrodynamic characteristic and chemical reaction kinetics are analyzed. The simulations with the KTGM model are in good agreement with experimental data. The instantaneous variables of volume fraction of particles, molar fraction of gas, molar concentration of carbon and temperature are described for the coal/biomass co-gasification process. The time-averaged distributions of volume fraction and velocity for both coal and biomass particles are given. The profiles of granular temperature with solid volume fraction and temperature are also discussed. The results reveal that the granular temperatures of both biomass and coal particles are obvious at a high temperature and a low solid volume fraction. Biomass particles have a higher granular temperature compared to coal particles.

Numerical modelling of gas dispersion using OpenFOAM

In the current work the rhoReactingBuoyantFoam solver was customised for performing gas leak and gas dispersion modelling. Using experimental data from gas leaks the proposed modelling was investigated for subsonic and sonic releases. The gas molar fraction and velocity decay along the jet centreline were calculated using the modified reacting solver, and the numerical findings were compared with available experimental data. Different approaches for the turbulence closure problem were considered using standard two-equation models. The numerical stability of the solver was also investigated varying the CFL number for a set of simulations. The work also considered the modelling of gas cloud volume in a real engineering case. Standard computational setup for ANSYS-CFX was applied, and the same set of scenarios were modelled in OpenFOAM using the modified rhoReactingBuoyantFoam solver. The analysis considered 5 different leak directions and 4 wind directions in a typical industrial site.For all scenarios simulated, very good agreement with experimental data and with the commercial CFD (computational fluid dynamics) tool considered in this study was observed. The results are within 10% tolerance intervals. Detailed information of the modelling is also provided, which enable any CFD user to reproduce the results and also apply it for future analysis.

Coupling compositional liquid gas Darcy and free gas flows at porous and free-flow domains interface

This paper proposes an efficient splitting algorithm to solve coupled liquid gas Darcy and free gas flows at the interface between a porous medium and a free-flow domain. This model is compared to the reduced model introduced in [6] using a 1D approximation of the gas free flow. For that purpose, the gas molar fraction diffusive flux at the interface in the free-flow domain is approximated by a two point flux approximation based on a low-frequency diagonal approximation of a Steklov–Poincaré type operator. The splitting algorithm and the reduced model are applied in particular to the modelling of the mass exchanges at the interface between the storage and the ventilation galleries in radioactive waste deposits.

Effect of reactions in small eddies on biomass gasification with eddy dissipation concept – Sub-grid scale reaction model

Large-eddy simulation (LES) approach is used for gas turbulence, and eddy dissipation concept (EDC)-sub-grid scale (SGS) reaction model is employed for reactions in small eddies. The simulated gas molar fractions are in better agreement with experimental data with EDC-SGS reaction model. The effect of reactions in small eddies on biomass gasification is emphatically analyzed with EDC-SGS reaction model. The distributions of the SGS reaction rates which represent the reactions in small eddies with particles concentration and temperature are analyzed. The distributions of SGS reaction rates have the similar trend with those of total reactions rates and the values account for about 15% of the total reactions rates. The heterogeneous reaction rates with EDC-SGS reaction model are also improved during the biomass gasification process in bubbling fluidized bed.

Performance Assessment of an Allothermal Auger Gasification System for On-Farm Grain Drying

Biomass gasification is a well-developed technology with the potential to convert agricultural residues to value-added products. The availability of on-farm gasifiers that can handle low-density agricultural wastes such as soybean residue, an underutilized feedstock, is limited. Therefore, the goal of this research was to install and assess an allothermal, externally heated, auger gasifier capable of converting agricultural wastes to combustible gas for on-farm grain drying. The system was used to convert soybean residues under different reactor temperature, i.e., 700°C, 750°C, 800°C, and 850°C. The results showed that increasing the reactor temperature from 700°C to 850°C increased the producer gas molar fractions of H2, CO, and CH4, from 1.1% to 1.5%, from 15.0% to 23.8%, and from 5.1% to 7.7%, respectively. The higher heating value of the producer gas reached 6.3 MJ/m3 at reactor temperature of 850°C. Specific gas yield increased from 0.32 to 0.58 m3/kgbiomass while char and particulate yield decreased from 41.7% to 33.6% by increasing the reactor temperature from 700°C to 850°C. Maximum carbon sequestration achieved, in the form of biochar-carbon, was 32% of the raw feedstock carbon. Gasification of collectable soybean residues from 1 acre would be sufficient to dry 1132 kg of soybean seeds (the average yield from one acre)

Performance analysis and parametric study of a solid oxide fuel cell fueled by carbon monoxide

A theoretical model of the solid oxide fuel cell (SOFC) fueled by carbon monoxide is adopted and validated, in which the activation overpotential, concentration overpotential, and ohmic overpotential are regarded as the main sources of voltage losses. Based on the thermodynamic-electrochemical analysis, mathematical expressions of some performance parameters such as the cell potential, power output, efficiency, and entropy production rate are derived. The effects of microstructure parameters such as the electrode porosity, tortuosity, pore size, grain size, etc. on the electrochemical performance characteristics of the SOFC are revealed. Moreover, the effects of some operation conditions such as the current density, anode inlet gas molar fraction, operating temperature, and operating pressure on some important performance parameters of the SOFC are also discussed. It is found that there exist some optimal values of microstructure parameters and operating conditions at which the better performance can be expected. The results obtained in the paper may provide some theoretical guidance for the design and operation of practical SOFCs fueled by coal-derived gases.

Electrochemical performance characteristics and optimum design strategies of a solid oxide electrolysis cell system for carbon dioxide reduction

An analytical model is developed to study the electrochemical characteristics of a solid oxide electrolysis cell (SOEC) for carbon dioxide reduction, in which the activation overpotential, concentration overpotential, and ohmic overpotential are considered as the main sources of voltage losses. The Bulter-Volmer equation, DGM model, and Ohm's law are employed to characterize the three overpotentials, respectively. The theoretical model is validated by comparing the simulation results with the experimental data from the literature. The anode-supported configuration SOEC is found to be the most favorable design. The effects of the cathode inlet gas molar fraction on the cathode overpotential and the cell potential are discussed in detail. It is found that there exists an optimum molar fraction for the cathode inlet gas at which the cathode concentration overpotential attains its minimum for given operation conditions. Moreover, the effects of some importantly operating parameters such as the current density, temperature and pressure on the cell potential are discussed. Thermal-electrochemical analysis shows that the Joule heat generated from the irreversibilities in the SOEC may be larger than, equal to, or smaller than the thermal energy needed for the carbon dioxide reduction reaction, and consequently, a system layout with five different design strategies to implement the carbon dioxide electrolysis is put forward.

An Innovative Acoustic Sensor for In-Pile Fission Gas Composition Measurements

In this article we propose a new method able to determine the fission gas composition using in situ ultrasonic waves measurements. To do so an acoustic resonator was connected to a fuel rodlet, in order to perform speed of sound measurements of gas mixture (Helium and fission gases) inside the plenum. By using a dedicated signal processing the peaks due to resonant frequencies inside the gas mixture were successfully extracted from the output signal. From these data, the variations of helium and fission gas molar fraction were calculated using an adapted virial state equation. It will be proved that these data provide important information about the kinetics of gas release and about the effects of high neutron and gamma irradiation on piezoceramic sensors.

Ripple Current Effects on PEMFC Aging Test by Experimental and Modeling

Polymer electrolyte membrane fuel cells’ (PEMFCs) systems usually require power conditioning by a dc-dc boost converter to increase the output fuel cell voltage, especially for automotive applications and stationary applications. The output fuel cell current is then submitted to the high frequency switching leading to a current ripple. The ripple current effects on fuel cell are studied by experimental ripple current aging test on a five cell stack (membrane electrode assembly (MEA) surface of 220 cm2) and compared with a reference aging test. The stack is run in nominal conditions but an ac component is added to the dc load. The ac component is a 5 kHz triangle, amplitude of which is ±20% of the dc component, in order to simulate a boost waveform. Fuel cell characterizations (polarization curves, impedance spectra, and voltammetry) provide information on the PEMFC aging and the performance evolution. Local conditions are computed through a dynamic stack model. The model takes into account transport phenomena, heat transfer, and semi-empirical electrochemical reactions and includes a meshing to calculate local conditions on the MEA surface (gas reactant pressures, local temperature, gas molar fractions, water activity, and local electronic current density). The consequences about performance and aging during high frequency ripple current are explained.

Investigation into a gas–solid–solid three-phase fluidized-bed carbonator to capture CO2 from combustion flue gas

A two-dimensional (2D) transient model was developed to simulate the local hydrodynamics of a gas (flue gas)–solid (CaO)–solid (CaCO3) three-phase fluidized-bed carbonator using the computational fluid dynamic method, where the chemical reaction model was adopted to determine the molar fraction of CO2 at the exit of carbonator and the partial pressure of CO2 in the carbonator. This investigation was intended to improve an understanding of the chemical reaction effects of CaO with CO2 on the CO2 capture efficiency of combustion flue gases. For this purpose, we had utilized Fluent 6.2 to predict the CO2 capture efficiency for different operation conditions. The adopted model concerning the reaction rate of CaO with CO2 is joined into the CFD software. Model simulation results, such as the local time-averaged CO2 molar fraction and conversion of CaO, were validated by experimental measurements under varied operating conditions, e.g., the fraction of active CaO, chemical reaction temperature, particle size, and cycle number at different locations in a gas–solid–solid three-phase fluidized bed carbonator. Furthermore, the local transient hydrodynamic characteristics, such as gas molar fraction and partial pressure were predicted reasonably by the chemical reaction model adopted for the dynamic behaviors of the gas–solid–solid three-phase fluidized bed carbonator. On the basis of this analysis, capture CO2 strategies to reduce CO2 molar fraction in exit of carbonator reactor can be developed in the future. It is concluded that a fluidized bed of CaO can be a suitable reactor to achieve very effective CO2 capture from combustion flue gases.

Adsorption of gas mixtures in TSA adsorbers under various heat removal conditions

The aim of this article is to highlight the behavior differences between an indirectly cooled temperature swing adsorption (TSA) adsorber and other adsorbers classically studied in TSA systems (adiabatic, near-adiabatic and isothermal) during adsorption of a mixture of gases. For this purpose, a numerical model is presented and validated from experimental results. By means of this model, the temperature, gas molar fractions and adsorbed concentration profiles are drawn at different times. The various zones of the adsorption patterns are identified and compared. The influence of the feed and initial bed temperatures is discussed too. From these results, the advantages of indirectly cooled adsorbers are pointed out. A new criterion for the possibility of omitting the cooling step in TSA cycles, based on the bed cooling efficiency, is then proposed for the indirectly cooled adsorbers.