CFD SIMULATION OF THE GAS-SOLID FLOW IN THE RISER OF A CIRCULATING FLUIDIZED BED WITH SECONDARY AIR INJECTION

A comprehensive investigation was carried out to study hydrodynamics aspects of secondary air injection in circulating fluidized beds. This article presents modeling and results of computational fluid dynamics simulations of gas-solid flow in the riser section of a laboratory-scale (ID = 0.23 m, height = 7.6 m) circulating fluidized bed with a radial secondary air injector. The gas-solid flow model is based on the two-fluid (Eulerian-Eulerian) approach, where both gas and solids phases are treated as interpenetrating continua. A granular kinetic theory model is used to describe the solids phase stresses. The simulation results are compared with measured pressure drop and axial particle velocity profiles; reasonable agreement is obtained. Qualitatively, excellent agreement is obtained in predicting the increase in solids volume fraction below secondary air ports, the accumulation of solids around the center of the riser due to momentum of secondary air jets, and the absence of the solids down-flow near the wall above the secondary air injection ports, which are the prominent features of secondary air injection observed in the experiments.     

Numerical investigation on the number of active surface sites of carbon catalysts in the decomposition of methane

The number of active sites on the surface of carbon catalysts is an important factor in determining their activity in the decomposition of methane. Although several studies have been performed to identify the nature of these sites, no method has been established to estimate their number. A method is presented to estimate this value, and its effect on hydrogen production is evaluated, along with that of temperature and residence time. For this purpose, the thermocatalytic decomposition of methane is modeled with the inclusion of the number of active sites of the catalyst in the kinetics. The results of the model indicate the high influence of variations of small residence times in this process, and the reduction of this effect at high temperatures. Also, the effect of the number of surface sites is shown to be more prominent at low residence times and temperatures. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2228–2234, 2014     

1D modeling of a flowing electrolyte-direct methanol fuel cell

In this study, the performance characteristics of a flowing electrolyte-direct methanol fuel cell (FE-DMFC) and a direct methanol fuel cell (DMFC) are evaluated by computer simulations; and results are compared to experimental data found in the literature. Simulations are carried out to assess the effects of the operating parameters on the output parameters; namely, methanol concentration distribution, cell voltage, power density, and electrical efficiency of the cell. The operating parameters studied include the electrolyte flow rate, flowing electrolyte channel thickness, and methanol concentration at the feed stream. In addition, the effect of the circulation of the flowing electrolyte channel outlet stream on the performance is discussed. The results show that the maximum power densities that could be achieved do not significantly differ between these two fuel cells; however the electrical efficiency could be increased by 57% when FE-DMFC is used instead of DMFC.     

Effect of gasification agent on the performance of solid oxide fuel cell and biomass gasification systems

In this paper, an integrated solid oxide fuel cell (SOFC) and biomass gasification system is modeled to study the effect of gasification agent (air, enriched oxygen and steam) on its performance. In the present modeling, a heat transfer model for SOFC and thermodynamic models for the rest of the components are used. In addition, exergy balances are written for the system components. The results show that using steam as the gasification agent yields the highest electrical efficiency (41.8%), power-to-heat ratio (4.649), and exergetic efficiency (39.1%), but the lowest fuel utilization efficiency (50.8%). In addition, the exergy destruction is found to be the highest at the gasifier for the air and enriched oxygen gasification cases and the heat exchanger that supplies heat to the air entering the SOFC for the steam gasification case.     

Development of a reaction mechanism for predicting hydrogen production from homogeneous decomposition of methane

In this paper, a reaction mechanism is developed to model the kinetics of hydrogen production from decomposition of methane. The pyrolysis of hydrocarbons from several combustion mechanisms is compared with experiment to obtain the elementary reactions of this mechanism. Some modifications are then made to reduce the large errors observed at a high residence time. Sensitivity analysis is performed to find the reactions with the highest effect on hydrogen production and their rate constants are changed by using other mechanisms to obtain the lowest error in hydrogen production compared to experimental data. This study shows that modifying the rate constants of the reactions of dissociation of methane to hydrogen and methyl radicals, and the formation of benzene from propargyl radicals have the highest effect on improving the results. The new mechanism reduces the error introduced from existing models for predicting the amount of hydrogen production up to 15%, depending on residence time and temperature levels.     

Drop-weight impact of plain-woven hybrid glass–graphite/toughened epoxy composites

The progressive damage behaviors of hybrid woven composite panels (101.6 mm × 101.6 mm) impacted by drop-weights at four different velocities were studied by a combined experimental and 3-D dynamic nonlinear finite element approach. The specimens tested were made of plain-weave hybrid S2 glass-IM7 graphite fibers/toughened epoxy (cured at 177 °C). The composite panels were damaged using a pressure-assisted Instron-Dynatup 8520 instrumented drop-weight impact tester. During these low-velocity simpact tests, the time-histories of impact-induced dynamic strains and impact forces were recorded. The damaged specimens were inspected visually and using ultrasonic C-Scan methods. The commercially available 3-D dynamic nonlinear finite element (FE) software, LS-DYNA, incorporated with a proposed user-defined damage-induced nonlinear orthotropic model, was then used to simulate the experimental results of drop-weight tests. Good agreement between experimental and FE results has been achieved when comparing dynamic force, strain histories and damage patterns from experimental measurements and FE simulations.     

Kinetic model of homogeneous thermal decomposition of methane and ethane

In this paper, the homogeneous decomposition of methane and ethane is modeled in a well stirred flow reactor. The kinetics of this process is represented by a reaction mechanism of 242 reactions and 75 species, based on a mechanism developed for hydrocarbon combustion and soot formation. It is shown that this model correctly predicts the hydrogen yield from pyrolysis in a temperature range of 600–1600 °C, and pressure range of 0.1–10 atm. Furthermore, the effect of temperature, pressure and residence time on the amount of hydrogen produced from the decomposition of methane, ethane, natural gas, and a mixture of methane and argon is studied. The model predicts that the use of ethane or its addition to methane increases the speed of hydrogen production at low temperatures and pressures. The addition of a noble gas like argon also increases the yield of hydrogen at high pressures.     

Periodic autoregressive model identification using genetic algorithms

Periodic autoregressive (PAR) models extend the classical autoregressive models by allowing the parameters to vary with seasons. Selecting PAR time‐series models can be computationally expensive, and the results are not always satisfactory. In this article, we propose a new automatic procedure to the model selection problem by using the genetic algorithm. The Bayesian information criterion is used as a tool to identify the order of the PAR model. The success of the proposed procedure is illustrated in a small simulation study, and an application with monthly data is presented.     

Micro‐modeling of porous composite anodes for solid oxide fuel cells

A new anode micromodel for solid oxide fuel cells to predict the electrochemical performance of hydrocarbon‐fuelled porous composite anodes with various microstructures is developed. In this model, the random packing sphere method is used to estimate the anode microstructural properties, and the complex interdependency among the multicomponent mass transport, electron and ion transports, and electrochemical and chemical reactions is taken into account. As a case study, a porous Ni–YSZ composite anode operated with biogas fuel is simulated numerically and distributions of the current density, polarization, and mole fraction and rate of flux of the fuel components along the thickness of the anode are determined. The effect of the anode microstructural variables including the porosity, thickness, particle‐size ratio, and particle size and volume fraction of Ni particles on the anode electrochemical performance is also studied. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1893–1906, 2012     

Regeneration of catalyst clay soils (Tonsil CO 610 G) by supercritical carbon dioxide

Extraction of deactivatived materials from contaminated clay soils (Tonsil CO 610 G) by supercritical carbon dioxide was investigated. Effect of different conditions including extraction temperature (308.15–338.15 K) and pressure (100–330 bar) (thermodynamic conditions), flow rate (4.2–42.6 cc/min), and static time (45–85 min) were investigated to find the optimum conditions for extraction of deactivatived materials. Based on the different experiments, optimum conditions for flow rate (4.2 cc/min), static time (85 min) and extraction pressure and temperature (330 bar and 313.15 K) were obtained. In addition, the GC-MS analysis and Bromine index (BI) analysis were revealing that the clay soil is activated and could be used as catalyst again.