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DECOMPOSITION OF NaHCO//3 IN LABORATORY AND BENCH SCALE CIRCULATING FLUIDIZED BED REACTORS.

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Abstract

This paper summarizes several years of testwork in laboratory and bench scale circulating fluidized bed reactors (CFB) using the thermal decomposition of NaHCO//3 as model reaction. Selected results are presented concerning the gas and solids phase fluid dynamic behaviour and the influence of the chemical reaction on CFB operating conditions.

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... The experimental methods that have provided the most direct measurement of particle RTDs have relied on measurements of the exit times of distinctive particles (e.g., particles with some distinctive physical characteristic such as color, size, or chemical composition that can be readily detected) after they have been injected as pulses into experimental bubbling or circulating fluidized beds. Typically, the particles that were measured have been classified either as Geldart A or B type, which characterizes the flow patterns they tend to exhibit [Geldart (1973), Kunii and Levenspiel (1991)]. 1 Some of the most relevant articles on these experiments include: Yagi and Kunii (1961a), Helmrich et al (1986), Berruti et al (1988), Ambler et al (1990), Smolders and Baeyens (2000), Harris et al (2003a&b), Bhusarapu et al (2004), andAndreux et al (2008). All of these studies have reported some common features in the observed RTDs: ...
... Based on the background summarized above, we hypothesize that it should be possible to combine a simplified reactor modeling approach such as that proposed by Liden et al (1988) with appropriate global reaction kinetics (modified for particle size and moisture) and low-order particle RTD functions to simulate the expected trends for fast pyrolysis in both bubbling bed and circulating bed reactors. To increase our confidence in the reliability of the RTD functions, we evaluated the ability of each function above to fit the experimental RTD results for Group B particles in both bubbling and circulating fluidized beds reported by the following investigators: Helmrich et al (1986), Berruti et al (1988), Ambler et al (1990), Smolders and Baeyens (2000), Harris et al (2003a&b), Bhusarapu et al (2004), and Andreaux et al (2008). We observed that all of the above RTD functions can give reasonably close agreement to the reported RTDs using appropriate parameter values, especially considering the significant measurement errors that were explicitly reported or implied. ...
... Several authors have tried to describe the solids motion in the dilute zone of the riser. Helmrich et al. (1986) modeled the solids RTD data by using a loop reactor model, which combined plug flow and stirred tank compartments. However, such models do not physically represent the actual mechanism of solids mixing. ...
... The bimodal RTD as indicated in Figure 5-2a in FF regime was also reported by several other researchers (Helmrich et al., 1986;Kojima et al., 1989;Ambler et al., 1990;Lin et al., 1999). The bimodal PDF of the solids RTD indicates that two flow phenomena occur in the riser. ...
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Statement of the Problem: Developing and disseminating a general and experimentally validated model for turbulent multiphase fluid dynamics suitable for engineering design purposes in industrial scale applications of riser reactors and pneumatic conveying, require collecting reliable data on solids trajectories, velocities ? averaged and instantaneous, solids holdup distribution and solids fluxes in the riser as a function of operating conditions. Such data are currently not available on the same system. Multiphase Fluid Dynamics Research Consortium (MFDRC) was established to address these issues on a chosen example of circulating fluidized bed (CFB) reactor, which is widely used in petroleum and chemical industry including coal combustion. This project addresses the problem of lacking reliable data to advance CFB technology. Project Objectives: The objective of this project is to advance the understanding of the solids flow pattern and mixing in a well-developed flow region of a gas-solid riser, operated at different gas flow rates and solids loading using the state-of-the-art non-intrusive measurements. This work creates an insight and reliable database for local solids fluid-dynamic quantities in a pilot-plant scale CFB, which can then be used to validate/develop phenomenological models for the riser. This study also attempts to provide benchmark data for validation of Computational Fluid Dynamic (CFD) codes and their current closures. Technical Approach: Non-Invasive Computer Automated Radioactive Particle Tracking (CARPT) technique provides complete Eulerian solids flow field (time average velocity map and various turbulence parameters such as the Reynolds stresses, turbulent kinetic energy, and eddy diffusivities). It also gives directly the Lagrangian information of solids flow and yields the true solids residence time distribution (RTD). Another radiation based technique, Computed Tomography (CT) yields detailed time averaged local holdup profiles at various planes. Together, these two techniques can provide the needed local solids flow dynamic information for the same setup under identical operating conditions, and the data obtained can be used as a benchmark for development, and refinement of the appropriate riser models. For the above reasons these two techniques were implemented in this study on a fully developed section of the riser. To derive the global mixing information in the riser, accurate solids RTD is needed and was obtained by monitoring the entry and exit of a single radioactive tracer. Other global parameters such as Cycle Time Distribution (CTD), overall solids holdup in the riser, solids recycle percentage at the bottom section of the riser were evaluated from different solids travel time distributions. Besides, to measure accurately and in-situ the overall solids mass flux, a novel method was applied.
... Bai [177] concluded that while the degree of axial dispersion of the gas is small compared to that of the solids, it still is significant enough that it should not be ignored. On the other hand, early data from researchers [161,167,178] suggested that back-mixing of gas decreased substantially beyond the turbulent regime towards fast and pneumatic fluidisation regimes, so that gas may be assumed to have substantial plug flow in that regime. However, that claim was questioned by Li and Weinstein [170] who reported considerable gas back-mixing in fast fluidisation regime, clearly in contrast to the earlier work. ...
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Heavy petroleum industries, including the fluid catalytic cracking (FCC) unit, are useful for producing fuels but they are among some of the biggest contributors to global greenhouse gas (GHG) emissions. The recent global push for mitigation efforts against climate change has resulted in increased legislation that affects the operations and future of these industries. In terms of the FCC unit, on the riser side, more legislation is pushing towards them switching from petroleum-driven energy sources to more renewable sources such as solar and wind, which threatens the profitability of the unit. On the regenerator side, there is more legislation aimed at reducing emissions of GHGs from such units. As a result, it is more important than ever to develop models that are accurate and reliable, that will help optimise the unit for maximisation of profits under new regulations and changing trends, and that predict emissions of various GHGs to keep up with new reporting guidelines. This article, split over two parts, reviews traditional modelling methodologies used in modelling and simulation of the FCC unit. In Part I, hydrodynamics and kinetics of the riser are discussed in terms of experimental data and modelling approaches. A brief review of the FCC feed is undertaken in terms of characterisations and cracking reaction chemistry, and how these factors have affected modelling approaches. A brief overview of how vaporisation and catalyst deactivation are addressed in the FCC modelling literature is also undertaken. Modelling of constitutive parts that are important to the FCC riser unit such as gas-solid cyclones, disengaging and stripping vessels, is also considered. This review then identifies areas where current models for the riser can be improved for the future. In Part II, a similar review is presented for the FCC regenerator system.
... Figure 7 shows the gas velocity distributions simulated by use of Model A and Model B. It is noticed that, in Figure 7a, gas tends to flow uniformly upwards with a slightly lower velocity close to the wall, while in Figure 7b,c, the gas flows heterogeneously and even back flows near the wall or around the clusters. This indicates that gas back-mixing can also be captured in the simulations employing the structure-dependent drag Model B. Some researchers [28,29] argued that, essentially, a plug flow existed and, thus, backmixing was impossible to be present in fast beds, compared with low-speed fluidized beds. Yang et al. [15] reported that, in their simulations of FCC-air riser flow, the solid and gas velocities in the core region were upward and much higher than those in the annulus region, while the solid and gas velocities near the wall were downward. ...
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Nonlinear drag force has been a research frontier in complex gas-solid systems. The literature has reported that the commonly-used drag correlations often overestimate drag force and, thus, cause unrealistic homogeneous flow structures in gas-solid fluidized beds of fine particles. For solving this problem, the structure-dependent drag model, derived from energy-minimization multi-scale approach, is used in discrete simulations of fluid catalytic cracking particles in a small riser. The gas phase is dealt with by computational fluid dynamics. Particles are considered as a discrete phase and described by Newton’s second law of motion. Gas-particle phases are coupled according to Newton’s third law of motion. Simulations show that use of structure-dependent drag model results in drag reduction, the effect of which is not so apparent as that in simulations of the two fluid model. The particle clustering tendency, however, is more distinct and leads to more heterogeneous flow structures in riser flow with a much greater amplitude of outlet solid flux fluctuations. Moreover, the behaviors of particle and gas back-mixing can be captured in the present simulations, which was supported by past simulations and experimental data. The simulation time resolution is discussed. The spring constant can be artificially brought down for safe setting of larger time step when modelling the collision process between fine particles with a higher calculation load. To appropriately mimic the continuous decay of van der Waals force may, however, need a much smaller time step. There is also an obvious effect of space resolution on simulations. When using a grid size smaller than 3 times the particle diameter, the simulated clusters turn extraordinarily large, and the effect of gas-solid back-mixing turns insignificant.
... Avidan (1980) and Yerushalmi and Avidan (1985) made RTD-measurements using a ferromagnetic tracer and an inductance bridge detector. Helmrich, Schurgerl and Janssen (1986) reported a double-peak RTD in their experiments using a radioactive tracer. Bader, Findlay and Knowlton (1988) and Rhodes, Zhou, Hirama and Cheng (1991) used salt as tracer to be detected by dissolving it and by measuring the electrical conductivity of the resulting solution. ...
Article
The residence time distribution (RTD) of solids is essential for the design of CFB reactors where conversion proceeds with time. The residence time distribution for the solids is measured at different working conditions. The resulting average residence times are correlated as function of gas velocity and solids circulation rate and are compared with literature data. In order to predict the RTD of the solids, the solids/gas flow is firstly described by a plug flow with dispersion. A fitting procedure gave experimental Péclet numbers, which were correlated as function of gas velocity and solids circulation rate. A more fundamental approach based on the core/annulus flow structure is thereafter used to predict the residence time. The riser is divided in a dilute core with particles flowing upward and a denser film moving slowly downward along the wall. Particle interchange between the two regions is described by a convective interchange flow. The model is used to predict experimental RTD-curves. The interchange flux between core and annulus corresponds well with radial fluxes reported in literature. Although the RTD is qualitatively well described, experimental curves show a higher dispersion than the calculated ones. To improve the core/annulus approach, further research is necessary.
... Solids mixing at the entrance appears to be insensitive to particle properties in contrast to the fully developed region. Helmrich et al. (1986) modelled solids RTD data by using a loop reactor model which combined plug flow and stirred tank compartments. While such models accurately describe the experimental data and give an indication of the effects of riser operating conditions and geometry on axial solids mixing, they do not physically represent the actual mechanism of solids mixing observed in CFB risers. ...
Article
Any vessel in which solids are transported upward by a gas stream and then recycled to the bottom may be classified as a Circulating Fluidized Bed (CFB). We describe possible CFB operating regimes in the context of this broad classification and highlight commercial processes that employ CFB technology and potential applications. Process design and development require a fundamental understanding of gas and solids hydrodynamics - solids hold-up, mixing and velocity distribution. We discuss techniques used to measure solids mass flux, which is a critical parameter for both design and control. In the last decade, significant research efforts have been devoted to new experimental techniques to measure both gas and solids spatial and temporal distribution. We list these techniques and detail the different modelling approaches that have emerged based on the new data. Characterization of the data is still incomplete and the available models require further refinement to reliably predict the effect of scale, operating conditions and particle characteristics on hydrodynamics.
... For measurement of biomass residence time based on the relative movement of particles in the bed, different techniques are used. The most common of these techniques are based on single particle tracing [3] and on stimulus responses from chemical differences [27], radioactivity measurements [28] or phosphorescence [29]. The char yield during devolatilization is usually obtained by cooling and weighing method for a given measurement condition. ...
Article
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Gasification of biomass in bubbling fluidized beds can be limited by accumulation of unconverted char particles during the process. The amount of unconverted biomass depends on the residence time of the fuel particles. This study demonstrates a method for measuring the biomass residence time over the conversion period at a given air flowrate and a given amount of biomass in a bubbling bed using the variation of bed temperature and fluid pressure recorded over time. The results show that biomass conversion is characterized by the devolatilization and extinction times. The two biomass residence times increase with decreasing air flowrate and increasing amount of biomass charged in the bed. The amount of unconverted char between the two characteristic times also increases with decreasing air flowrate and increasing biomass load. The total heat loss during the devola-tilization is observed to increase with increasing air flowrate and amount of biomass in the bed. Correlations are proposed for predicting the mean biomass residence time, the amount of unconverted char particles and the devolatilization heat loss at a given operating condition. The results of this study can be used in determining the bubbling bed properties and solid circulation rate required to decongest the accumulated char particles in the bed.
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The fast-fluidized bed is characterized by intensive backmixing and nonhomogeneous flow patterns represented by two flow regions, i.e. a dense wall (or annular) region and a dilute core region. This segregated flow structure is undesired for certain chemical reaction applications. To reduce the backmixing and nonhomogeneity, superimposition of coarse particles onto a fine-particle circulating fluidized bed is attempted in this study. The effects of the coarse particles on the reactor performance are investigated in the light of a catalytic ozone decomposition reaction in the bed. The apparatus used consists of a riser, 102 mm in diameter and 6.32 m in height. FCC particles with a mean diameter of 58μm, impregnated with ferric oxide, are used as catalysts. Polyethylene particles of 4.4 mm diameter and glass beads of 2 mm diameter are employed in the experiments as coarse particles. Both the hydrodynamic parameters and the ozone concentrations are measured under various operating conditions. Comparison are made of the axial and radial distributions of ozone concentrations in the presence and absence of coarse particles. For a given Damkoeheler number, the presence of coarse particles is found to improve ozone conversions in a riser with polyethylene particles at a high gas velocity, and with glass beads at a low gas velocity. A reactor model based on the core—annulus flow structure is also proposed to account for the ozone conversion in the reactor.
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A model to predict the carbon combustion efficiencies in circulating fluidized beds (CFB) previously developed was validated with experimental data obtained in the combustion of different coals in a CFB pilot plant with a 20 cm internal diameter and 6.5 m height. The model gave a good fitting without using any adjustable parameters. A simulation, from the point of view of carbon combustion efficiencies, to analyze the behavior of circulating fluidized bed combustors depending on different design and operating variables was made. The variables most affecting the carbon combustion efficiencies were the pressure drop or the solid inventory in the combustor, the coal reactivity, and the particle size distribution through the coal particles of small size fed in the combustor. Likewise, a good design of the cyclone is necessary to obtain high carbon combustion efficiencies.
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This paper presents an experimental study on the hydrodynamics and mixing behaviors in a riser−downer−coupling reactor proposed for the fluid catalytic cracking (FCC) process. Special attention is placed on two parts:  the annular riser and the riser−downer junction. In comparison to the conventional riser, the annular riser has a more uniform radial flow structure, which can be observed from the radial distribution of particle velocity and solid concentration; however, the axial particle mixing seems to be similar. The carefully designed junction can serve as an inlet structure of the downer part to realize the uniform distribution of solids, while it does not increase the back-mixing of particles of the whole coupling reactor. The particle dispersion mechanism has also been investigated, and the differences in the microstructures can explain the characteristic particle mixing behaviors observed in the coupling reactor.
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An investigation of gas backmixing in fluidized beds spanning the entire range from the bubbling regime to the dilute transport regime was conducted using continuous injection of a helium tracer. The focus was to compare backmixing levels in the different regimes. While time averaging at a point averages over the phase heterogeneities in the bubbling and slugging regimes, it does not average these out in the high-velocity regimes of turbulent and fast fluidization where the heterogeneities are distributed in space rather than time. It is also shown that, in the bubbling and slugging regimes, the concentration of backmixed tracer is almost independent of the gas velocity while, in the turbulent and fast regimes, the concentration is strongly dependent on the gas velocity.
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The application of a specific model to coal combustion in circulating fluidized beds presents the initial difficulty of having to choose between the different submodels proposed in the literature, where some important discrepancies exist. Therefore, in this work a sensitivity analysis of the carbon combustion efficiency predictions to the different hypotheses, equations, and parameters defining the different submodels has been carried out. Thus, it has been found that the hypotheses related to the radial and axial solid distributions and the possible combustion of the descending char particles in the annulus exercise an important effect on the predictions when low-reactivity coals were considered, its importance being lower with high-reactivity coals. The same occurs when modifying parameters or equations referring to the reactivity of the char or to the mass transfer around particles. However, those corresponding to the devolatilization type have no significant effect on the efficiency predictions. On the other hand, however, it has been found that the hypotheses of the type of gas flow in the dense region has a medium sensitivity on the carbon combustion efficiencies.
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While gas dispersion has been determined in CFB risers operated at low solids fluxes, data are lacking with respect to the high-density conditions common in FCC risers and other catalytic processes. In this paper experimental results are reported showing gas residence time distributions obtained using helium tracer and FCC particles for a 76mm diameter riser of height 5.6m with net solids fluxes extending up to 480kg/m2s. A radially non-uniform dispersion model is proposed and compared with the axially dispersed plug flow model to determine the gas dispersion coefficients. The results show that gas backmixing becomes small for dense suspension upflow conditions where there is no downflow of solids near the riser wall.
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Commercial-scale circulating fluidized bed (CFB) reactors have been in use for several years for processing fine-grained solid materials in non-catalytic gas/solid reactions. Recently they have also been employed successfully for the combustion of carbon-containing fossil fuels and/or residues for power generation. Despite these numerous industrial applications, theoretical knowledge of CFB operation still lags behind.The present paper covers modelling and simulation methods for the description of steady- and unsteady-state operating conditions in CFBs. The validity of the simulation method is demonstrated by a comparison of simulation results with empirical data for two different reactions: the relatively simple thermal decomposition of NaHCO3, and the complex combustion of coal for power generation.Results such as reactor temperatures, flue gas analyses, pollutant emissions and heat flows are presented. Acceptable agreement between measured and calculated data was found.ZusammenfassungEs wird ein Simulationsverfahren zur Beschreibung einer expandierten zirkulierenden Wirbelschicht und einer entsprechenden Anlage vorgestellt. Dabei findet ein Zellenmodell für die Anlagenkomponenten Reaktor, Zyklon und externer Wirbelschichtkühler Anwendung. Unter Berücksichtigung der Reaktionskinetik werden für jede Zelle Stoff-, Massen- und Energiebilanzen aufgestellt. Das entstehende Bilanzgleichungssystem wird mit einem kombinierten Newton-Raphson-Relaxationsverfahren simultan iterativ gelöst. Am Beispiel des thermischen Zerfalls von NaHCO3 werden Simulations- und Versuchsergebnisse verglichen. Der Vergleich liefert eine gute Übereinstimmung von Simulation und Experiment. Daraufhin wird das Simulationsverfahren auf die komplexere Reaktion der Kohleverbrennung angewandt. Verschiedene Simulationsergebnisse, wie Reaktortemperaturen, Abgaszusammensetzung, Schadstoffemissionen und ausgekoppelte Wärmeströme werden vorgestellt.
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One problem associated with the use of Computational Fluid Dynamics(CFD) in reactor modeling is the proper validation of the models. Proper validation in this context means that the physical fluid dynamic model, the mathematical implementation and the data used for validation must be consistent. The present paper addresses this issue and to provide appropriate relations between experimental method and modeling approach A critical review of currently used measurement techniques for characterizing multiphase now systems is presented. The interpretation of the data obtained from the various techniques is discussed as well as how these data can be used for validation of various CFD model formulations Steady state models can be validated using time averaged data, making sure that the averaging time for the experimental data is long enough so that low frequency periodic oscillations also are evened out. If homogeneous systems are considered, then a volume average approach may be used for modeling, If the system cannot be considered homogeneous and steady, as is the most common case, then a dynamic ensemble averaging technique should be preferred. The validation of such models must be done with methods fast enough to resolve periodic fluctuating structures of interest. These methods are cumbersome and tedious to operate and the ergodic hypothesis may be invoked enabling the use of volume or time averaged data for the validation of ensemble averaged models.
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A new-style complex absorbent was prepared for simultaneous desulfurization and denitrification in this paper. The performance of the absorbent was tested in fixed bed. On the basis of the experimental results in fixed bed, the M substances were selected as the optimal additive. Furthermore the effects of various factors, such as additive content, flue gas temperature, humidity, Ca/(S+N), and NOx inlet concentration et al, on the removal efficiencies of SO2 and NOx were investigated. According to our simultaneous flue gas desulfurization and denitrification experiment results using the new-style absorbent in a circulating fluidized bed (CFB) system, High simultaneous removal efficiency, 94.9% of SO2 and 68.3% of NO, was achieved under the optimal experimental conditions. The work would offer important basic data for industrial application of simultaneous removal SO2 and NOx based on flue gas circulating fluidized bed.
Article
Based on the assumption of a core-annulus structure, radial gas mixing characteristics were experimentally studied in the core of the upper dilute zone of a pilot scale circulating fluidized bed (CFB), 0.4 m in diameter and 9 m in height. C02 as a tracer was injected continuously into the center of the bed by a point source. Radial tracer gas concentration profiles were measured in several planes downstream of the injection point. An analytical solution derived by Klinkenberg et al. (Ind. Eng. Chem., 6 (1953) 1202) [1] for the description of gas mixing in turbulent single phase flow was successfully applied to determine the Peclet number, Per,c, for radial gas mixing in the core zone. Per,c was found to be independent of the superficial gas velocity u, which is in agreement with the gas mixing behaviour in turbulent single phase flow. A mean value of Per,c=465, which is also in agreement with other authors' measurements in single phase flow, was calculated from measurements at different solids rates Gs, up to 70 kg m−2 s−1. The fact that Per,c was found to be independent of Gs is attributed to the low mean solids concentration in the core zone and to the size of the particles, which is too small to cause a significant influence on the turbulent intensity of the gas phase.
Chapter
Circulating fluidized beds pose considerable challenges to our understanding. Because it is not yet possible to produce ab initio predictions of their complex fluid dynamics, transport and chemical behavior, experimentation remains essential. Because many phenomenological models require empirical input, their quality depends on the accuracy of measurement techniques. Process control and monitoring also require precise experimental data.
Chapter
The dispersion of gas phase in the riser of a circulating fluidized bed exerts great, sometimes even crucial, influence on the performance of a CFB gas—solids reactor. Incomplete mixing of secondary air streams into the gas—solids suspension rising from the bottom region of a CFB combustor may lead to hydrocarbon emissions from sub-stoichiometric zones as well as NO x emissions from over-stoichiometric zones. Optimal axial and radial dispersion of the gas phase may be crucial in achieving high conversion and/or high selectivity in some gas-conversion processes such as catalytic cracking. An adequate understanding of the gas mixing process may shed light on other issues, such as the evolution of volatiles from fuel particles or the optimal distribution of feed points of a reactant. It is therefore essential for practical design as well as for successful modelling of reactor behaviour.
Article
An increasing interest arises in new applications of the circulating fluidized bed for gas catalytic reactions, in addition to the well established catalytic cracking (FFC). Gas mixing in the riser of the CFB becomes an important parameter. The present paper focuses on dilute phase risers. A short literature review on gas mixing is presented with experimental results. Tests at different working conditions of gas velocity and low solids circulating rate showed that only a minor amount of mixing is detected in the riser and that the residence time can be calculated assuming plug flow.
Chapter
Fast fluidized bed (FFB) reactors have been widely used in the petrochemical industry, coal combustion, and calcining processes. Large gas throughput, essentially plug flow of gas giving higher selectivity for intermediate products, and excellent interparticle and interphase heat and mass transfer are derived from the fineness of particles and a high mean relative gas–solids velocity. Chemical reactions may be carried out almost adiabatically, with the high solids flux providing either a heat source or a heat sink to control the riser temperature. FFB provides a practical approach to implementing periodic operation. The high heat transfer coefficient between the suspension and the heat exchange surface may also lead to a uniform temperature profile throughout the reactor. FAB/CFB boiler has advantages like fuel flexibility, high combustion efficiency, excellent contribution to pollution control, good turndown and load following, simpler fuel handling and feed system, and high availability. In the circulating fluidized bed, prereduction of finegrained iron ore concentrate together with powdered coal proceeds without sticking at temperatures above 950°C. Sticking is prevented by an excess of carbon in the bed material and by the turbulent mixing behavior of the CFB. Computer software for application collocation is in use to make the application collocation easier.
Thesis
Cette étude porte sur la technologie du Lit Fluidisé Circulant appliquée à l'adsorption pour éliminer des Composés Organiques Volatils (toluène) dans un courant gazeux. Dans ce procédé, l'adsorbant (grains sphériques millimétriques de carbone microporeux) est transporté par un fort débit gazeux dans un tube vertical sans garnissage. Les transferts de matière et de chaleur sont très efficaces, et des débits importants peuvent être traités. Le travail présente la détermination de l'équilibre d'adsorption, la description de l'installation expérimentale et des résultats d'expériences, le développement d'un modèle original de l'opération, combinant un modèle d'écoulement couplé au transfert de matière, une étude paramétrique de ce modèle, et enfin des extensions de principe du procédé à des opérations étagées, à des cycles à variation de pression (PSA) ou de température (TSA).
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Using a solid Na‐based sorbent is one potential option to decrease CO2 emission in coal‐fired power plants, and the CO2 sorption reactivity of Na2CO3/γ‐Al2O3 sorbent was improved by mechanically doping MgO into Na2CO3/γ‐Al2O3 in our previous study while the mechanism was not clear. In this paper, the CO2 sorption/desorption mechanisms of the promising MgO‐doped Na‐based sorbent prepared by the two‐step incipient wetness impregnation method were studied using a fixed‐bed reactor, together with characterizations of X‐ray fluorescence, nitrogen adsorption apparatus, field emission scanning electron microscopy, X‐ray diffraction, and thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG‐FTIR). Also, the sorption behaviors were well described with Avrami's fractional‐order kinetic model. Results demonstrated that MgO not only dispersed on γ‐Al2O3 but entered γ‐Al2O3’s lattice, leading to the formation of Mg‐Al mixed oxides for CO2 sorption. In addition, a new phase Mg6Al2CO3(OH)16·4H2O was produced during the CO2 sorption process, which plays a crucial role in facilitating the conversion of Na2CO3 to NaHCO3. The CO2 sorption capacity of MgO‐doped Na‐based sorbents is presumably determined by the trade‐off between microstructure and active component dispersion. The knowledge gained about the promotion mechanism of MgO provides fundamental direction for the synthesis of Mg–Al mixed oxides, supported with the developed microstructure for CO2 sorption enhancement of Na‐based sorbents. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.
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This paper gives experimental measurements of the particle residence time distribution (RTD) made in the riser of a square cross section, cold model, circulating fluidised bed, using the fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002a) 127). This technique depends upon all particles having phosphorescent properties. A small proportion of the particles become tracers when activated by a flash of light at the riser entry; the concentration of these phosphorescent particles can subsequently be detected by a photomultiplier. The influence of the solids circulation rate and superficial gas velocity on the RTD were investigated. The results presented are novel because (i) the experiments were performed in a system with closed boundaries and hence give the true residence time distribution in the riser and (ii) the measurement of the tracer concentration is exceedingly fast. The majority of previous studies have measured the RTD in risers with open boundaries, giving an erroneous measure of the RTD.Analysis of the results suggests that using pressure measurements in a riser to infer the solids inventory leads to erroneous estimates of the mean residence time. In particular, the results cast doubt on the assumption that friction and acceleration effects can be neglected when inferring the axial solids concentration profile from riser pressure measurements.An assessment of particle RTD models is also given. A stochastic particle RTD model was coupled to a riser hydrodynamic model incorporating the four main hydrodynamic regions observed in a fast-fluidised bed riser namely (i) the entrance region, (ii) a transition region, (iii) a core-annulus region and (iv) an exit region. This model successfully predicts the experimental residence time distributions.
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New industrial applications of the circulating fluidized bed. Strongly expanding fluidized beds in the boundary region between fluidized bed and pneumatic conveying, which can achieve a steady state only by recirculation of the solids are of considerable importance for environmental protection because of the high gas flow rates and the high relative velocities possible between gas and solids in performing reactions between gases and fines with an average particle diameter less than 300 μm. Lurgi has utilized the principle of the circulating fluidized bed industrially both in the high temperature regime for the non-pollutive production of process heat from low-grade coal in a plant of 84 MW thermal rating and in the low temperature regime for adsorptive, dry purification of fluorine-containing waste gases from aluminium electrolysis and for dry removal of pollutants from garbage-burning power plants. This paper reports on the new process.