Article

The influence of the riser exit on the particle residence time distribution in a circulating fluidised bed riser

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Abstract

This paper reports measurements of the influence of riser exit geometry upon the particle residence time distribution in the riser of a square cross section, cold model, circulating fluidised bed. The bed is operated within the fast fluidisation regime. The fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002) 127–142) was used to measure the residence time distribution.The geometry of the riser exit is shown to have a modest but consistent influence upon the particle RTD; the influence of operating conditions, i.e. superficial gas velocity and solids flux is more significant.Increasing the refluxing effect of the riser exit increases the mean, variance and breakthrough time and decreases the coefficient of variation of the residence time distribution. Changes in reflux do not have a systematic effect upon the skewness of the RTD.

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... The excited phosphorescent particles emit light and the afterglow is captured by a detector and collected by a computer data acquisition system as a function of time. This optical method has been perfected by Harris et al. [19]- [21]. In these phosphorescent tracer experiments, phosphorescent particles were used as both bulk and tracer particles. ...
... The phosphorescent particle tracer technique was perfected by Harris et al. [17]- [21]. They measured the particle RTD in a circulating fluidized bed riser under experimental closed boundary conditions. ...
... In order to improve the phosphorescent tracer method, Huang et al. [22] proposed a pneumatic injection phosphor tracer technique (PIPTT) based on the phosphorescent tracer technique developed by Wei et al. [23], [60], Harris et al. [17]- [21] and Ran et al. [62]. In this approach, small amount of phosphorescent tracer particles with similar property to the bed material were injected into the fluidized bed resulting in the reduction of the amounts of used phosphorescent tracer particles. ...
Thesis
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The aim of the present thesis is to develop a novel experimental technique for determining the residence time distribution (RTD) of solid particles in solid unit operations as well as model development. Initially, a novel optical method was developed to measure the particle RTD. Experiments are carried out with Silicon Carbide (SiC) and the pigment phosphorescent (Lumilux® Green SN-F50 WS) as tracer particle. A preliminary experimental study was conducted in a simple bubbling fluidized bed in order to validate the proposed RTD measurement methodology. In the second step, the developed technique of the concentration measurement was applied to measure the RTD of a deep fluidized bed. The particle RTD curves are determined experimentally in different operating conditions. Finally, a model consisting of the combination of the ideal reactors is proposed to predict the particle residence time distribution in the studied fluidized bed. The predicted output values are then compared with the experimental data to establish a good model fitting data
... The extent of the influence of the riser exit down the riser column can be predicted by the following equation (Harris, Davidson, & Thorpe, 2003): ...
... No differences in the solids flow were noticed when switching between smooth, abrupt and T-exit. Glicksman et al. (1993); Van der Meer et al. (2000); Harris et al. (2003) 0.14 m I.D. and 0.14 × 0.14 m 2 , 5.1 m and 4.5 m high ...
... a v p,down is the particle downward velocity in the annulus which is generally 0.5-1.5 m/s (Harris et al., 2003) but was assumed to be 1.1 m/s by Senior (1992). ...
Article
Laboratory and industrial risers are equipped with exits of many different layouts, and numerous publications discuss the influence of riser exit geometry on local and overall solids hydrodynamics in the riser. The present paper reviews literature findings—mostly based upon indirect experimental techniques and often somewhat contradictory. Direct measurement of particle velocity and particle occupancy near and in the riser exit provide a better indication of the effect of riser exit geometry. Positron Emission Particle Tracking (PEPT) was used in this work for the first time to investigate the exit region of the riser. An abrupt or sharp exit causes particles to be knocked out of the gas flow, so forming a recirculation or reflux region in the upper part of the riser. This is much less pronounced with a curved or gradual exit.
... Residence time distribution (RTD) Ambler et al. (1990), Patience and Chaouki (1993), Smolders and Baeyens (2000), Harris et al. (2003), Mabrouk et al. (2007), Van de Velden et al. (2007) and Chan et al. (2010), have reported RTDs from stimulus response experiments at various flow conditions. In the stimulus response experiments, a tracer is injected at bottom of riser and its concentration at outlet is measured at different times after injection. ...
... In the stimulus response experiments, a tracer is injected at bottom of riser and its concentration at outlet is measured at different times after injection. Harris et al. (2003) have measured RTD in a square riser by using a small portion of the solids particles as tracers, and investigate the effect of three types of exit geometries, namely smooth, abrupt and highly refluxing, on RTDs at various flow conditions. Fig. 5(a) shows variations in the RTDs caused by variations in the gas phase velocity for the smooth exit riser. ...
... Fig. 5(a) shows variations in the RTDs caused by variations in the gas phase velocity for the smooth exit riser. Harris et al. (2003) have observed that an increase in the superficial gas velocity decreases both mean residence time and variance of distribution over a range of investigated solids fluxes. At a higher gas velocity, the RTD curves are narrow, relatively unskewed and having a significant long tail. ...
Article
Design and scale-up of fluid catalytic cracking (FCC) riser is still largely empirical, owing to limited understanding of inherent multiphase flow in this equipment. The multiphase flow of FCC riser has therefore been extensively investigated both experimentally and computationally. The experiments have provided significant insight into gas-solid flow patterns inside cold-flow risers, but simultaneous observations on flow and performance parameters (conversion and yields) in FCC riser are rarely found in literature. Consequently, computational fluid dynamic (CFD) models of FCC riser that can simultaneously account for flow, interphase interactions, droplet vaporization and cracking kinetics have been developed. The CFD modelling of FCC riser, despite several efforts, has still remained a challenge as it requires careful consideration of governing equations and closure models. This review presents state-of-the-art in CFD modelling and experimental analysis of gas-solid hydrodynamics and reactive flow of FCC riser. The CFD models are explained in greater detail with governing equations, constitutive relations, and physical significance of all the terms. A brief review of DNS studies on cluster formation, gas-solid drag, and turbulent interactions is also presented. Impact of important closure models such as drag models, viscous stress models, boundary conditions, droplet vaporization models, and kinetic models on predictions is critically examined. The review identifies major shortcomings of current CFD models and makes detailed recommendations for future work.
... Widely utilized in many chemical and power industries, circulating fluidized bed (CFB) has attracted more and more attention because of its fuel flexibility, excellent efficiency, high heat transfer rates and low pollutant emissions [2]. Therefore, plenty of experiments concerned with various aspects of the CFB have been carried out in the past decades [3][4][5][6]. However, it exhi-bits many limitations of experiments on the full-loop CFB due to high costs and complicated operations. ...
... The solid cycle time distribution and the SRT distribution are mostly illustrated as probability histogram in the experiments [3,4]. In order to investigate the influence of operating conditions on the solid cycle time (or residence time), the averaged solid cycle time (or residence time) is chosen as a criterion, which reads: ...
... Via fitting the distribution profiles of the solid cycle time in the whole system and the SRT in the riser, it is noted that the solid cycle time distribution and the SRT distribution in the riser both show a log-normal distribution probability profile with an early peak and an extended tail. This feature agrees well with the experiments [3,4] and the simulations [22,23] in the literature. Moreover, the lognormal probability plots show that there are some discrepancies between the percentages and the reference line of the solid cycle time distribution and the SRT distribution with a little deviation, thus good fittings are achieved both for the solid cycle time distribution and the SRT distribution in the riser. ...
Article
A parallel simulation of the gas-solid motions in a 3-D full-loop dual-side refeed circulating fluidized bed (DRCFB) is carried out by using Large-eddy Simulation coupled with Discrete Element Method (LES-DEM), in which the gas and the solid motions are solved under the Eulerian and the Lagrangian frames, respectively. The start-up flow patterns, the time-related solid phase properties about solid cycle time and solid residence time (SRT) distributions, and the improvements of gas-solid flow uniformity are explored. Furthermore, the influences of different operating conditions on the time-related solid phase properties are discussed. The results demonstrate that the transient pressure drop is a criterion for determining steady state in the start-up process and the fluctuations under different superficial velocities differ. The solid cycle time and the SRT distribution curves show a feature of early peak with an extended tail, which indicates that the solid flow exhibits a back-mixing phenomenon. The averaged solid cycle time in the whole system and the averaged SRTs in the three components (i.e., riser, cyclone, and dipleg) show distinct characteristics under different operating conditions. Moreover, nearly 70% of the solid cycle time elapses in the riser and 25% of that elapses in the dipleg while only 5% of that elapses in the cyclone. Finally, the dual-side refeed structure remarkably improve the gas-solid flow uniformity (i.e., homogenous) in the DRCFB riser, especially in the inlet and outlet regions.
... The influence of riser exit geometry on the hydrodynamics of gas-solid circulating fluidized beds (CFB) has been in-vestigated in many studies [1][2][3][4][5][6]. Although the results reported in these studies apparently conflict quantitatively concerning the influence of riser exit, some common aspects can be observed: (1) the design of the exit has a large effect upon the reflux of solids; (2) abrupt exits cause an increase in the solids holdup and a large backmixing at the top of the riser; (3) increasing the refluxing effect of the exit has proved to increase the mean particle residence time; (4) larger and denser clusters are formed at the walls in the risers with abrupt exits. ...
... Four different kinds of configurations for the riser outlet ( Figure 3) are proposed in the present study, based on the works of Cheng et al. [1], Gupta and Berruti [2], Van Der Meer et al. [3], Harris et al. [4], Chan et al. [5], Pugsley et al. [9], and Van Engelandt et al. [10]. These configurations can be classified in two main groups: abrupt exits, in which there are sharp changes in the flow direction, inducing solids recirculation in the top section of the riser, and smooth exits, in which the flow changes according to the outlet bend, following a natural path. ...
Article
Full-text available
Fluid catalytic cracking (FCC) riser reactors have complex hydrodynamics, which depend not only on operating conditions, feedstock quality, and catalyst particles characteristics, but also on the geometric configurations of the reactor. This paper presents a numerical study of the influence of different riser outlet designs on the dynamic of the flow and reactor efficiency. A three-dimensional, three-phase flow model and a four-lump kinetic scheme were used to predict the performance of the reactor. The phenomenon of vaporization of the liquid oil droplets was also analyzed. Results showed that small changes in the outlet configuration had a significant effect on the flow patterns and consequently, on the reaction yields.
... There have been many reports of investigations of the RTD of solids in various CFB risers because of its importance in at the exit as a function of time. To improve the measurement of the RTD of the solid, various tracer particles have been used since 1980, including ferromagnetic particles [23,24] , radioactive sand [25] , sodium chloride [26][27][28] , and phosphorescent particles [29][30][31] . These studies mainly focused on the effect of the operating conditions; i.e. , the superficial velocity, solid circulation rate, and the diameter of the riser [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] . ...
... To improve the measurement of the RTD of the solid, various tracer particles have been used since 1980, including ferromagnetic particles [23,24] , radioactive sand [25] , sodium chloride [26][27][28] , and phosphorescent particles [29][30][31] . These studies mainly focused on the effect of the operating conditions; i.e. , the superficial velocity, solid circulation rate, and the diameter of the riser [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] . ...
Article
The authors investigate the effects of the direction of the gas jet on the solid residence time distribution in a CFB riser. Tracer technique was employed to calculate the RTD of solids. A Eulerian–Eulerian model with kinetic theory of granular flow and species transport was used to simulate the motion of tracer particles in a CFB riser. For a comparative analysis of the direction of the gas jet, simulations of vertical, horizontal and hybrid jets were carried out. The direction of the gas jet significantly influenced the axial and radial structure of bed, and hence affected the RTD for solid particles. The mean residence time of solids was changed, and the results showed that 16.3 s, 14.8 s, and 11.4 s with vertical, horizontal and hybrid jets nozzles, respectively.
... In this context it should be noted that particles of same size and density have not in reality the same residence time in the riser, but there is a residence time distribution (RTD) (see e.g. [82][83]). The gas does not flow in plug-flow manner, but there is a velocity profile affecting also to the particle velocities in the riser. ...
... Therefore, RTD measurement can be an efficient tool for better understanding the hydrodynamic condition in the reactor. This information can be applied for designing reactor as well as scale-up, operation, and optimization [7,8]. ...
Article
Full-text available
This study aims to determine the performance of the jet clarifier for turbidity removal and its mechanisms for proposing the optimal operating conditions and design criteria. The experiment were performed continuously using a pilot scale jet clarifier with the volume of 243 L. Effects of liquid flow rates, types of liquid phase, and sludge blanket heights on turbidity removal efficiency were investigated. Moreover, the residence time distribution (RTD) study was carried out to investigate the flow pattern. The results indicated that the jet clarifier can effectively reduce the turbidity of the synthetic water with the efficiency of 80% under the optimal condition. The RTD results suggested that the flow pattern in the jet clarifier corresponded to the design as the plug flow and mixed flow conditions were found in the coagulation and the flocculation/sedimentation zones, respectively. The presence of the sludge blanket can reduce the bypass and recirculated flows. Besides, the increase of flow rate resulted in the increase recirculation in the tank. It can be suggested that the jet clarifier can be used for removing turbidity in the water treatment. The hydrodynamic in the reactor, which relates to flow pattern in the reactor, is one among the important factors in a jet clarifier.
... The riser exit geometry of a circulating fluidized bed (CFB) has been reported to have a significant impact upon the hydrodynamics of the whole unit. In particular, the effect upon the particle residence time distribution (RTD) in the riser can influence the temperature profile for a CFB combustor or conversion for a CFB reactor [76]. ...
... In the fast fluidisation regime, an increase in the superficial gas velocity or a decrease in the solid circulation rate decreases the mean residence time of solids, decreases the variance and increases the coefficient of variation [40]. A change in the geometry of the bed, e.g. by the addition of baffles, by modification of the aspect ratio, by adding stages, or by modifying the exit has been found to alter the mixing behaviour [21,23,41]. Increasing the size of particles can lead to a reduction in mixing [42,43]. ...
Article
Full-text available
A model for chemical looping combustion has been developed to allow the effect of different residence time distributions of oxygen carrier particles in the air and fuel reactors to be investigated. The model envisages two, coupled fluidised bed reactors with steady circulation of particles between them. The results show that the process is sensitive to the residence time distributions, particularly when the mean residence time of particles in the reactors is similar to the time required for them to react completely. Under certain operating conditions, decreasing the variance of the residence time distribution, leads to a greater mean conversion of the particles by the time they leave the reactors and higher mean rates of reaction in the beds. In this way the required inventory and circulation rate of solids could be reduced, which would lower the capital and operating costs of a CLC process. Since the residence time distribution of solids is important, it should be taken into account when modelling or designing a chemical looping combustion process, e.g. by using a tanks-in-series model. This work indicates that if the number of tanks, N ≤ 5, knowing N to the nearest integer is generally sufficient, unless a high degree of accuracy is needed. As N increases, the sensitivity of the coupled system decreases, so for N > 5, knowing the value to the nearest 5 or 10 tanks is sufficient. This is valid whether N is the same or different in the two reactors. Chemical looping combustion is one example of a reactor-regenerator system, so the results are also relevant for other processes of this type, such as fluidised catalytic cracking.
... It is clear that a well designed bed exit has the function of a preliminary separator which may improve the performance improvement, for example increasing the bed temperature control ability, extending the bed material residence time (Harris et al., 2003), reducing the roof erosion by the suspended solid layer near the top of the bed and reducing the external circulation rate, consequently reducing the energy consumption and the erosion rate in the back pass of CFB boilers. Figure 5 shows the special designed bed exit structures featured with narrowed cross section and a projected roof. ...
Article
The exit configurations of CFB strongly influence the bulk density profile and the internal circulation of the bed material, which is called the end effect. This study analyzes the influence of three exit geometries and two narrowed exit geometries on hydrodynamics. Experiments indicate that the exit with the projected roof in CFB may be used as a separator and the projected height has a maximum. Narrowing the bed cross section near thebed exit zone is a simple and effective way to enhance the internal circulation and reduce the circulation of bed material simultaneously.
... are the dimensionless relative location and time in the continuous mixing system with l and τ representing the total length of the continuous mixer and the mean residence time, respectively. v z and E z are the axial velocity and the dispersion coefficient of the system) is the ratio of the rate of advection caused by bulk flow to cross-sectional dispersion.One common solution of this equation, known as the Taylor dispersion model[16] (Eq. 6.3), calculates RTD [E(t)] obtained from experimental data by estimating the concentration as a function of dimensionless location (ε) and dimensionless time (θ) with known and measured parameters. ...
Chapter
Continuous powder mixing has been the subject of much research in past few years since it offers several advantages over the conventional batch mixing methods still used in the pharmaceutical industry. These advantages include the opportunity for meaningful process analytical technologies (PAT) and modeling techniques to be implemented for automated process control leading to finished products with improved quality. With continuous manufacturing, the same equipment used for process development can often be used in manufacturing. Therefore, product scale up is eliminated or reduced, accelerating the introduction of new products to the market. The advantages also include significant reductions in equipment size and the ability for integration with other continuous processes already in use. This chapter provides an overview of continuous powder mixing and its mechanisms. It describes the theoretical and experimental characterization, modeling, and process control efforts. It also provides recommendations for future directions both in research and within industry.
... The annular-wall layer first forms at the top of the riser generated by solids that do not exit into the cyclone, but instead impact on the cover and sides of the riser and flow back downwards at the wall. According to Lackermeier et al. [6], Harris et al. [7], and Mabrouk et al. [8], this annular wall flow increases down the height of the riser as a portion of the solids from the upwards core flow is carried radially outwards by turbulence and entrained into the downwards wall flow. In CFB combustors or incinerators, water is converted to steam inside heat-exchanger tubes that line the walls. ...
Article
Design of a fluidized bed gasification plant requires a good understanding of how operating parameters influence solids circulation, because this affects the heat transfer that sustains the reactions in the system. In this paper, the effect of various operating parameters, such as primary and secondary fluidizing airflows, on solids circulation within a scaled-down cold model of a circulating fluidized bed (CFB) was investigated. An operational map was developed to show regions of stable CFB operation and boundaries of unsatisfactory CFB operation, when fluidization becomes highly unsteady or inefficient due to gas bypass. Under stable operation, it was observed that solids circulation increased with an increase in primary and secondary fluidizing airflows and solids inventory of the plant. Provided the loop seal was fluidized properly, any changes in operating parameters resulted in the solids flow adjusting accordingly to maintain a proper gas seal and ensure stable operation. The operating parameters also affected the pressure drop across various points in the CFB, essentially due to redistribution of solids within the system. A new semi-empirical model was developed to estimate the ratio of solids that exit the CFB riser to solids that recirculate back into it. This model was based on considerations of the outlet geometry and solids inertia, in particular the ability of the solids to resist the change in direction of the airflow as it curves towards the exit of the CFB riser.
... The basic definitions and systematic descriptions of RTD were introduced by Wei et al. (2013). The RTD of primary phase (fluid phase) or second phase (solid phase) can be determined via physical experiments (Christensen, Nijenhuis, Ommen, & Coppens, 2008;Rodríguez-Rojo, López-Valdezate, & Cocero, 2008) and numerical simulations (Harris, Davidson, & Thorpe, 2003;Harris, Davidson, & Thorpe, 2002;Wei et al., 2013), respectively. For SCWFBR, experimental study of the hydrodynamics is extremely difficult due to the restrictions of physical measuring techniques at high temperature and pressure working conditions. ...
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... The biogas flow is measured at standard temperature and pressure conditions and then calculated at normal conditions [11,12] based on the gas laws. The datapoints are then presented with a 5-points median signal filter [13]. Additionally, days and time are also adjusted to present the daily periods in realistic times, while the experimental activities run on times to facilitate the monitor and control of the peak time feeds during regular working hours. ...
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The power system needs flexible electricity generators. Whilst electricity generation from anaerobic digestion (AD) of sewage sludge has traditionally been baseload, transforming the generation capacity into a modern flexible operator is an opportunity to further valorise the resource. This work aims to demonstrate that AD of sewage sludge can support flexible generation and be operated dynamically in a relevant operational environment, to promote full scale implementation. A demonstration scale plant (20 m ³ conventional AD reactors) was used to test several feeding regimes designed to return a biogas production rate that matches the demand. Two demand profiles are defined, either by common corporate power purchase agreements or by the main balancing mechanism used by the grid operator in UK. Demand-driven biogas production is demonstrated in this relevant operational environment, and the flexibilisation performance is positive in all scenarios. The value of the biogas increases by up to 2%, which outperforms the results obtained at pilot scale. Additionally, an increase in biogas yield is observed. Whilst transitional imbalances are recorded, they last for few hours and the overall stability is not affected. In conclusion, these trials demonstrate demand-driven biogas production is a feasible operational solution and full-scale implementation is possible. Graphical Abstract
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Looking for a way to measure residence time distributions of an 80 micron fluidized cracking catalyst (FCC) powder, a simple measurement technique was discovered that quantifies tracer color concentration. Using a color spectrophotometer that measures percent reflectance as a function of wavelength, a calibration curve can be constructed for standard mixtures of dyed and un-dyed powder. This calibration curve can then be used to determine the color concentration of an unknown sample by measuring its reflectance. The effects of operating parameters such as dye strength, aperture size, surface roughness, sample volume and depth, and continuous flow were all evaluated. This spectrophotometric technique was found to be a quick and simple way to measure colored mixture concentrations. In addition to being ideal for residence time distribution applications, it has the potential to easily quantify mixing in any unit operation, batch or continuous.
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The solids mixing in a riser with a height of 10 m and 0.186 m inner diameter was investigated by using pneumatic phosphor tracer technique. Considering the shielding effect of the bed material on the light emitted from the phosphor tracer particle, a modified method for the phosphor tracer measurement is proposed. And then the curves of particle residence time distribution were obtained. The experimental results show that the particle diffusion mechanism can be explained by the dispersions of dispersed particles and particle clusters in the axial direction, and as well the core–annulus nonuniform distribution of the solids fraction in the radial direction of the riser. Moreover, based on the experimental results, a two-dimensional dispersion model was established to predict the solids axial and radial diffusion. Furthermore, the effects of superficial gas velocity and solids circulating flux on the axial and radial Peclet number of the particles were discussed; two empirical correlation formulas about the axial and the radial Peclet numbers were given; the calculated values agree well with the experimental results.
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A comprehensive two-dimensional transient Eulerian model combined with the kinetic theory of granular flow was developed to obtain the hydrodynamic and chemical reaction behaviors in tapered circulating fluidized bed reactor risers. In this study, the focus was on the chemical reactions and its behaviors inside three different riser geometries. The model was verified by using an experimental dataset from the literature, and was then used for both predicting the hydrodynamic behaviors and computing the system turbulent properties. The tapered-out riser improves the system turbulence or mixing which can be explained by the dispersion coefficients. On the other hand, the tapered-in riser increases the solid particle residence time and gives a more uniform temperature distribution, because it does not have sufficient force to support the weight of the particles. The same riser geometries but with the addition of the chemical reaction were then used for evaluating the previously proposed criteria that the riser geometry should be chosen with respect to the characteristics of the reactions. Reactions with a medium reaction rate were best suited to the typical riser, whilst reactions with a fast and slow reaction rate best fitted the tapered-out and tapered-in risers, respectively.
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Circulating fluidized bed reactor (CFBR) has been widely used for combustion, gasification and cracking. Each reaction has its own characteristics and requires different reactor responses. In this work, novel designs of the riser geometries were studied using a 2D transient Eulerian approach with kinetic theory of granular flow in a commercial simulation package. The new designs were based on the improvement of main factors that have an effect on reaction characteristics. It was found that the tapered-out riser improves the turbulence or mixing while the tapered-in riser enhances particle residence time and gives uniform temperature distribution in the system.
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The Circulating Fluidized Bed (CFB) boilers have emerged as flexible, environment-friendly equipment in recent years. Though less than 20 years old, the technology has already proved its maturity and has started replacing the Pulverized Fuel (PF) combustion in specific areas. Retrofitting and scaling-up of CFB boilers require, among other things, a clear understanding of the complex flow patterns observed in various zones of the boiler and formulating them into suitable computational models. This paper attempts to get an overview of the different modeling approaches found in literature in this regard, so that an appropriate model could be arrived at for designing large commercial-scale boilers.
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While continuous powder mixing has been an area of active research in recent years, effects of operating conditions on the mixing performance has not been well understood yet. Based on our previously developed periodic section modeling (Gao et al., 2012), this paper examines the effects of operating conditions on two significant parameters of the continuous blending processes: axial velocity and local mixing rate of the mixture. Four mixing cases differing in particle size, density and cohesiveness are simulated. Results show that when the local mixing rate improves at low fill levels and high blade speed, particles also move faster in the axial direction and reside for a shorter time inside the mixer. This trade-off between ascending local mixing rate and descending residence time indicates a non-optimal overall blending performance even when the best operating condition is applied. Based on these results, strategies that can further improve the blending performance are performed, which are proposed by increasing the blade speed while keeping a constant axial velocity. These strategies guarantee that the variance decay rate along the mixing axis is proportional to the blade speed in continuous blending processes. Dramatic improvements are observed when these strategies are applied, which shows the merit of this work on design and optimization of continuous power blending processes.
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Riser is the key component of circulating fluidized bed reactors, in which gas–solid or catalytic reactions occurs. The flow hydrodynamics and Geldart B type solids' concentration profiles in the riser, under higher superficial gas velocity, larger solids circulation rate, and wide ranges of operating conditions, were the focuses of this experimental study. The typical core-annulus structure with clusters and a small amount of particles' back-mixing phenomena near the wall of the riser was observed. The axial solids concentration distributions in the riser with an abrupt T-shape exit were characterized by a dense region near the bottom, a dilute region toward the up-middle zone, and a dense region near the top exit. Solids became dilute, and the back-mixing phenomena weakened with the increase of superficial gas velocity. And the solids concentration at the same height position increased with the increment of solids circulation rate. Furthermore, the effects of two exit configurations, namely smooth C-shape and abrupt T-shape, on the flow characteristics were investigated. In order to compare the influence of particles' density and diameter, four single Geldart D particles and a soybean and millet two-phase mixture are chosen as bed materials to study the bubbling bed flow character. Copyright © 2013 Curtin University of Technology and John Wiley & Sons, Ltd.
Article
The paper presents a mechanistic model to predict bed-to-wall heat transfer coefficient in the top region of a circulating fluidized bed (CFB) riser column by considering the riser exit geometry effects on bed hydrodynamics. With abrupt riser exit geometry, some solids will reflect back in to the riser column, thereby increasing the solids concentration in the top region of the riser column of a CFB. This in turn results in higher bed-to-wall heat transfer coefficients in the top region. At present, not much information exists in the literature to predict bed-to-wall heat transfer coefficient in the top region of a riser column with riser exit geometry effects. In the present work, a mechanistic model is proposed to estimate bed-to-wall heat transfer coefficient with riser exit geometry configurations. The length of influence of gas–solid flow structure from the riser exit due to various riser exit geometries is also presented. The solids reflux ratio is an important parameter, which influences the heat transfer rate in the top region. For the same operating conditions the bed-to-wall heat transfer coefficient increases with the abrupt riser exit geometry configuration compared to a smooth riser exit in the top region. The proposed model predictions are compared with the published experimental data for right angle exit configuration and a reasonable agreement is observed.
Article
The local solid-particle volume fraction distributions were studied in a plexiglas rectangular CFB cold model with the size of 0.35 m ×0.48 m×4.9 m by using a reflective-type fiber-optic particle concentration probe (PC6D) in order to analyze the effects of the geometry structure on the gas-solid flow properties. The transparent glass bead with 366.2 μm diameter was used as bed materials during the experiment. Time series analysis and numerical interpolation methods were adopted to analyze the experimental data. Results showed that: in transition region, the expanding cross-section of wear wall structure significantly affected the gas-solid flow and the radial distribution of solid volume fraction was asymmetry; the local solid volume fraction near the wear wall was higher than the solid volume fraction near the front wall. In the dilute region, the offset-exit and the corner effect induced the defluxion of the particles movement and the core-annular distribution was broken. Time series analysis of the transient signals indicated that the two-phase structure in the bottom was unstable, and in the transition region the solids fluctuation was acute near the boundary between the core and annular section.
Article
NOx generated from coal combustion brings about severe environmental problems. Among many combustion technologies, coal self-preheating combustion technology is valued for its great potential of low NOx combustion, while in-depth studies on the structure of the self-preheating device (SPD) have not been carried out, which makes it difficult to further reduce NOx. In addition, disposal of coal gasification fly ash (CGFA), a kind of solid waste, is still mainly dissipated in landfills, resulting in the pollution of soil, water and wastes energy. In order to optimize the SPD structure for further reducing NOx and provide technical support for the engineering utilization of CGFA, the properties of preheated fuel (coal gas and coal char), the behavior characteristics of combustion and NOx emission under different He/Hr (the ratio of projected height to the riser height) were investigated on a 30 kW bench-scale test rig. Stable and high efficient MILD combustion was achieved without manifest flame front. The results revealed that appropriate increase of He/Hr ratio was conducive to the release of fuel-N during preheating, facilitating the conversion of fuel-N to N2. For BC and CGFA, the pre-removal effect of fuel-N in the SPD reached the best when He/Hr ratio was 3/8 and 1/4, respectively. The increase of He/Hr ratio was not salutary to improving the combustion reactivity of BC, while for CGFA, the modification effect on the combustion reactivity of coal char could reach the best when He/Hr ratio = 1/4. Both NOx emission and the conversion ratio of fuel-N to NOx of BC and CGFA could reach the optimal value when He/Hr ratio was 1/4.
Article
On a φ0.205 m×7 m high density circulating fluidized bed cold-state test rig, studied was the influence of three types of outlet structure (C type smooth outlet, L type and T type abrupt change outlet) on the main bed axial particle concentration distribution under different circulating flow rates and apparent air speeds respectively. The high density circulation research work was mainly performed on the C type outlet with the maximum circulating flow rate Gs being 220 kg/(m2·s) and the particle concentration in the area of the bottom below 3 m being greater than 0.1. The test results show that the C type outlet can make the particle concentration inside the bed assume an exponential attenuation law and the L and T type outlet can both exercise their influence on the axial particle concentration distribution in the whole bed height. The concentration enhancement effectiveness of the T type outlet is better than that of L type and a local concentration enhancement phenomenon emerges in the zone 2 meters away from the centerline of the outlet.
Article
The aim of this work was to study the potentials and benefits of dynamic biogas production from Anaerobic Digestion (AD) of sewage sludge. The biogas production rate was aimed to match the flexible demand for electricity generation and so appropriate feeding regimes were calculated and tested in both pilot and demonstration scale. The results demonstrate that flexibilization capability exists for both conventional AD and advanced AD using Thermal Hydrolysis Process (THP) as pre-treatment. Whilst the former provides lower capability, flexible biogas production was achieved by the latter, as it provides a quick response. In all scenarios, the value of the biogas converted into electricity is higher than with a steady operational regime, increasing by 3.6% on average (up to 5.0%) in conventional and by 4.8% on average (up to 7.1%) with THP. The process has proven scalable up to 18m³ digester capacity in operational conditions like those in full scale.
Article
This work presents the development of a reduced order compartment model for a counter-current spray dryer. The compartment model is formulated using adaptable compartments and introducing the use of correlations based on dimensionless groups. These correlations can capture the mean residence time they but are unable to reproduce the variance of the entire residence time distribution (RTD). Limitations are also observed in the evaluation of internal fluxes. The application of these correlations to a specific zone requires the inclusion the geometrical modifications in any part of the unit. A small internal modification in one geometry not only modifies a zone, but also influences the remaining regions so that the zones cannot be independently scaled-up. The methodology is complemented with an analysis of the RTD showing that most of the dispersion generated takes place in the bottom cone.
Article
Solid mixing process plays a crucial role in evaluating the reaction performance of multiphase reactor. However, there still lacks the systematic research on solid mixing for the multistage circulating fluidized bed with enlarged sections. Solid mixing mechanism in a multistage riser is investigated numerically in this study. Some individual characteristics of solid mixing are found. It is shown by analyzing residence time distributions that solid mean residence time and the degree of solid dispersion increase with the increasing solid flux or the decreasing superficial gas velocity. Nevertheless, particle diffusion coefficient increases as solid flux decreases or superficial gas velocity increases. Moreover, the diffusion coefficient in enlarged sections decreases with the increasing height, which is contrary to the variations in traditional risers. Compared with the traditional riser, particle diffusion is weak, but overall solid mixing is enhanced in the multistage riser. Furthermore, two mixing patterns that gas-solid flow develops towards the plug-type flow in the center region of riser and the mixed-type flow in the enlarged section are identified. In addition, the dominant recirculation time of solids in the enlarged section is predicted as 1.5 s under examined cases.
Article
This work presents a novel technique with fast response for Residence Time Distribution (RTD) measurements in gas-solid unit operations (e.g., fluidized bed reactors). This technique is based on an optical method which eliminates the requirement of knowing the velocity and concentration profiles at the exit section of the system. Experiments were carried out with SiC particles and a phosphorescent pigment used as a tracer. A concentration measurement system was developed to measure the tracer concentration in SiC/pigment mixtures. The corresponding pigment concentrations were evaluated at the bottom of this system using a photomultiplier. The pigment concentration was derived from the integral of the signal intensity received by the photomultiplier. Then, a calibration curve was established which provided the empirical relationship between the integral and pigment concentration. In order to validate this RTD measurement technique, a series of experiments was performed in a bubbling fluidized bed and the effect of the bed height was studied. It was shown that the experimental RTD curves were in good agreement with the theoretical RTD of bubbling fluidized beds. This solids RTD measurement technique can be used to provide a better understanding of the hydrodynamics of complex solids unit operations.
Article
Circulating fluidized bed (CFB) has been commonly operated with polydisperse particles with a wide particle size distribution (PSD), which significantly affects solid transportation and the resulting thermochemical performance. Dense gas-solid flows in a pilot-scale CFB riser are simulated via a multiphase particle-in-cell (MP-PIC) coupled with a heterogeneous drag model. Gas-solid hydrodynamics together with the particle-scale information with a wide PSD is discussed. The results show that in the riser, the non-uniformly spatial distribution appears with the presence of a core-annulus structure and a dilute-upper dense-bottom profile. The wide PSD gives rise to the axial segregation phenomenon with the accumulation of large particles in the bottom region. Enlarging the superficial gas velocity and PSD width respectively decreases and increases the segregation intensity of solid phase. As compared with gas velocity, the PSD width performs limited impacts on solid concentration, particle-scale information along the axial direction, and solid residence time. The horizontal solid dispersion coefficient is at the scale of 10⁻³ m²/s, while the vertical dispersion is at the scale of 1 m²/s. For a wide PSD, particles with larger sizes have larger slip velocities but smaller dispersion coefficients in the riser.
Article
Powder flow and heat transfer in rotary calciners are difficult to predict, which can result in reduced product quality. This work attempts to improve our understanding of powder flow in a rotary calciner. We compare experimental results to existing models. In this study, a moderately cohesive fluid catalytic cracking (FCC) powder was passed through two pilot plants rotary calciners with and without dams. Residence time distributions were measured, and the resulting mean residence times and axial dispersion coefficients were compared to predictive models. It observed that increasing the calciner incline and speed of rotation resulted in decreased mean residence time (MRT) and feed rate only had a small effect on MRT and axial dispersion. Increasing the height of dams increases the mean residence time and lowers the dispersion coefficient. Bed depth profiles at various operating conditions and geometries, with and without dams, were also determined and compared to available models.
Chapter
In the last 20 years, significant improvements in the computational fluid dynamics (CFD) modeling have been made that allow the simulation of large-scale, commercial CFBs. Today, commercial codes are available that can model some of this behavior in large-scale, commercial units in a reasonable amount of time. However, the hydrodynamics in a riser or fluidized bed are complex with both micro and macroscale features. From particle clustering to large streamers to the core-annulus profile, the particle behavior in these unit operations rarely behaves as a “continuous fluid.” Even the role of particle size distribution is often neglected and models that do consider particle size distribution don’t always consider the role of particle size on granular temperature. Many models use insufficient boundary conditions by assuming uniform or symmetric profiles, which is rarely the case. Furthermore, grid sizing is usually based on computer limitations instead of model limitations, and many models of commercial systems extend beyond the capability of the constitutive equations being used. Successful application of today’s CFD models requires a good understanding of the equations behind the code, the assumptions used for those equations and the capability or limitations of the code. CFD is nothing more than a guess without an understanding of the fundamentals, underlying assumptions and code limitations that are part of every model.
Article
The understanding of gas–solid flow is of great importance for various high-density circulating fluidized bed (HDCFB) practices. In this work, the effects of exit geometry on the gas–solid flow behavior in an HDCFB riser were numerically studied by using the computational particle fluid dynamics (CPFD) method. The simulated results were compared against experimental data on bed pressure and solid volume fraction distributions along the riser and a satisfactory agreement was found between them, which indicated the capability of the CPFD for the HDCFB riser simulation. Thereafter, simulations were conducted for the HDCFB risers with six different exit geometries and comparisons of hydrodynamics characteristics between each other were made. It was found that exit geometry significantly affected the axial solid volume fraction distribution through the whole riser. The abrupt exits gave rise to higher SRT than the smooth ones due to the solids back-mixing and packing effects. The angle between the exit duct and the horizontal plane also has a great influence on the gas–solid hydrodynamics and the abrupt exit with 15° was found to be appropriate for HDCFB riser due to its promotion on solid dispersion and reduction in solids back-mixing and packing.
Article
In this paper, a three-dimensional compressible mathematical model of circulating fluidized bed (CFB) combustion was established, by using the Euler-Lagrangian method, based on the Computational Particle Fluid Dynamics (CPFD). The model considers heat and mass transfer, the phase chemical reaction. The distribution of temperature, gas composition, particle residence time and velocity of the co-combustion of oil shale retorting solid waste with cornstalk particles in the CFB were obtained, by using simulation calculation under a variety of operating parameters. The results of the model are established and analyzed. The optimal operating conditions of co-combustion of oil shale retorting solid waste and cornstalk particles in the CFB were obtained, which provide meaningful values for the experiment, design and working condition of the CFB.
Article
Rotating drums are encountered in numerous industrial applications, including blenders, rotary calciners, impregnators, coaters, granulators, and cement mixers. In all of these devices, the rotation of the drum is used to engender mixing of the granular material in the radial direction. Axial mixing, because of its significantly lower rate, can also have an impact on the process performance, especially when control of residence time is important. Typically, the particle dynamics in rotating drums are quantified as a function of process conditions, such as rotation speed, fill level, and cylinder size. Particle properties are also important, but previous studies have largely been limited to the effects of particle size. In this work, the quantification of the axial particle dynamics has been expanded to include the effect of bulk flow properties by studying a number of cohesive powders. Fick's second law was found to describe the axial dispersion behavior of cohesive particles. Therefore, changes in behavior can be characterized using the axial dispersion coefficient. The effect of material flow properties was found to be statistically significant; the flowability of the material (as measured using bulk flow properties) correlated significantly to the axial dispersion coefficient. Partial least squares was used to determine that 95% of the variation observed in the axial dispersion coefficient measurement can be explained using particle size, compressibility, and shear cell measurements.
Article
The rotary kiln is an essential device in chemical and metallurgical industries, with applications in a wide range of solids manufacturing processes. In particular, in the preparation of industrial chemical catalysts, the kiln has become a popular reactor for continuous calcination of catalysts ranging from millimeter-sized extrudates to micron-sized powders. As granular and powder flow behaviors are difficult to characterize, the design and scale-up of rotary calcination processes are often performed empirically. The goal of this research is to improve the fundamental understanding of powder flow in rotary kilns to aid in optimization of the continuous calcination process. For successful calcination to occur, the residence time of the particles must exceed the time required for heating and subsequent treatment. For uniform treatment of the feed, the particles must also exhibit low axial dispersion. In this work, the mean residence time and axial dispersion coefficient for a cohesive fluid catalytic cracking powder were determined in a pilot plant kiln by measuring the residence time distribution. This study utilized a pulse test developed by Danckwerts. Results were fit to the Taylor solution of the axial dispersion model and compared to the Sullivan prediction for mean residence time. It was found that the mean residence time decreases as the feed rate, kiln incline, and rotation rate increase. It was also found that the axial dispersion coefficient increases with speed of rotation and angle of incline. However, the axial dispersion coefficient decreases as the feed rate is increased.
Article
Solids Residence Time Distribution (RTD) in Circulating Fluidized Bed (CFB) risers has received increasing attention due to its vital role in determining the operating condition, particle property, and reactor geometry in industrial CFB applications. In recent years, various solids RTD experimental techniques and theoretical models have been utilized and proposed to study CFB risers. Some controversial issues, however, also arose in the open publications. By means of exploring the advantages and disadvantages of each available RTD experimental technique and model when they are applied to particles in CFB risers, this study discussed the primary causes leading to the huge discrepancy in magnitude of solids dispersion coefficient and Peclet number, which can achieve 4 orders or span from 1 to 100. On the basis of the massive experiment data collected from the literature, the variations of average residence time, Peclet number and dispersion coefficient of solids with superficial gas velocity, solids mass flux and solids concentration were presented. By applying the transition of flow regime in CFB mode, we provided a helpful way to explain some existing contradictions in the reported effects of operating conditions on solids RTD. The possible reasons were also summarized to clarify why some researchers measured a double- or multi-peak solids RTD curve and the others could not in a similar situation.
Article
A study of the hydrodynamics of two-phase stirred tanks is presented. The hydrodynamics in large-scale reactors is shown to be mainly a function of the superficial gas velocity and the stirring in the system. Six configurations for the stirrer have been tested: two single-stage and four two-stage configurations. The three hydrodynamic regimes (short circuit, load and flood) were observed for these configurations. The results show that the gas holdup, measured using the difference in level between aerated and non-aerated states, achieves a maximum value for the mixed two-stage combination. For this configuration, a study of the residence time distribution was carried out by employing the tracer (pulse injection) method, thus allowing the determination of the dead volume and the modeling of the flow in the reactor, corresponding to a perfectly mixed reactor. In stirred tank reactors, the study of the bubble size distribution has a great importance on the flow dynamics, the dimensions of bubbles are measured photographically; this investigation shows the presence of fine bubbles (d < 10 mm) with the experimental bubble size distribution curves exhibiting classical log-normal function traits within ± 3%. The characterization of the hydrodynamics and the flow regimes in the stirred reactor permits to optimize the operating parameters (stirrer type and configuration, stirring speed, gas velocity) within the reactor in order to treat the water contaminated by persistent pollutants.
Conference Paper
Efficiency of a photoreactor depends on the irradiation dose. Fluid residence time distribution (RTD) reflects hydrodynamic behavior of the flow. A computational model was built on a base and fitting a previous radiotracer experiment. Results of three simulations for three different configurations and height flow rate are presented and discussed below. This paper shows usefulness of CFD modeling as an imaging tool, which can be used to retrieve detailed, local information about the flow.
Article
The hydrodynamic characteristics and operational conditions to produce biodiesel by the ethanolysis of babassu oil catalyzed by immobilized lipase (Novozym® 435) were established in a fluidized bed reactor coupling with a column to simultaneous remove glycerol formed as byproduct. Hydrodynamics was determined by means of pulse tracer trials and results showed that the flow pattern can be described as an ideal continuous flow stirred tank. The best performance was obtained by running the reactor with biocatalyst loading of 12% and a space-time of 8 h, attaining an average yield of 98.1% and productivity of 9.9 molester/gcat/min.
Article
For nearly a century, fluidized beds have been the backbone of the processing industries. However, the complex hydrodynamic behavior aggravated, by the diversified solids handling has largely hindered its scaling up. The flow pattern may be influenced by the solid particles' feeders, and thus can have an impact on the system performance. In the first part of the present work, the hydrodynamic behavior of a 2D circulating fluidized bed is simulated under different superficial gas velocities to investigate their effect on the overall flow patterns. The numerical results are validated against available experimental data and a good agreement is achieved. In the second part, a special concern was dedicated to the effect of the gas/solid feeding configuration over the established flow structure. For the matter, five different configurations were tested and the corresponding results showed the high relevance of the feeding configuration, particularly in the bottom zone and on both phases. This impact decreases as we progress higher in the riser.
Chapter
Full-text available
The performance of two different geometries of riser exit on pressure drop and solid behavior inside the internal circulating fluidized bed (CFB) was investigated. The exit configurations were made in two forms of abrupt shapes( L-shape, T-shape ) and a smooth shape (C-shape). The particles of Iron oxide were used to study the fluidization behavior based on the scaling laws. The concentration of solids were measured through the pressure drop. Experiments were conducted using gas superficial velocity in the range of 2 to 5 m/s. As a results, during the circulation process with abrupt exit geometries, some solid particles were reflecting back into the riser column. Consequently the particle concentration at top of the riser was increasing. But in performance of smooth shape, the riser achieved the maximum mass flow rate (Gs) due to a high flux ratio. In comparison between smooth and abrupt shapes, the smooth one showed a better efficiency and a better profile of circulation rate. It was suggested in a calcium looping process either C-shape or T shape (50mm) can be used for a better efficiency.
Article
This is the first time an extensive investigation has been carried out regarding the effects of riser exit geometry on pressure drop and solid behaviour inside the Internal Circulating Fluidized Bed (ICFB) riser, using different riser exit geometries at several operating conditions.The Radioactive Particle-Tracking (RPT) technique was used for solid concentration measurements and solid residence time distribution at the exit zone. Experiments were conducted using Geldart B particles, in the gas superficial velocity range of 4 to 10 m/s. Axial solid hold-up, solid residence time distribution in the exit zone, and the reflux ratio factor km, (defined earlier by [E.H. Van der Meer, R.B. Thorpe, J.F. Davidson, Flow patterns in the square cross-section riser of a circulating fluidized bed and the effect of riser exit design, Chem. Eng. Sc. 55 (19) (2000) 4079–4099]), were the main criteria used to investigate the impact of gas–solid separator devices implemented at the ICFB riser exit.Solid residence time distribution results and axial solid hold-up profiles provided clear evidence that the separator device at the riser exit strongly influences the hydrodynamic structure of the ICFB riser. The V-shaped riser exit geometry was found to be the optimum of all the configurations studied.
Article
A unique phosphor tracer technique was used to allow experimental measurement of residence time distribution (RTD) of solids in the circulating fluidized bed. The instantaneous input of light impulse and the detection of the light emission by the excited phosphor particles permit on-line continuous determination of solids tracer concentration with time at different radial and axial positions within the riser and downer. The measured RTD curves in the downer show that less axial solids mixing takes place in the downer than that in the riser. The two-peak distribution of RTD curves in the riser is clearly seen in the experiment.
Chapter
Compared with conventional bubbling and turbulent fluidized beds, circulating fluidized beds have many advantages including better gas—solids contacting and reduced backmixing (Lim et al., 1995). However, due to the core-annulus structure, particle backmixing along the wall can still be significant. Gas—solids contacting is not optimal given the non-uniform distribution of gas and particle flow in the riser. Significant solids backmixing also occurs in the bottom solids-acceleration zone, often covering a significant portion of the riser. Backmixing can also occur at the riser exit if a strong constriction is used. For fast reactions such as fluid catalytic cracking where the intermediate is the product, uniform residence time distributions of gas and solids are very important. To further reduce backmixing and to improve gas—solid contact, internals can be installed inside CFBs to modify the flow pattern and many novel inlet and exit configurations can be employed. The first two sections of this chapter discuss the various available internals and novel geometry structures and their effects on gas and solids flow. The third section describes two special configurations where solids upflow and downflow are accommodated within a single vessel containing concentric upflow and downflow regions and the N-shape CFB loop.
Article
The relative influence of smooth and abrupt exit configurations on the axial pressure drop profile in Circulating Fluidized Bed (CFB) risers has been investigated. Experiments were performed with sand and FCC catalyst in two cold model CFBs of 0.1 metre and 0.2 metre riser diameter. The influence of the abrupt exit on the profile was observed to extend all the way to the base of the larger diameter riser for sand. However, the abrupt exit influenced only the final 1.5 to 2 metres of the riser length for the case of sand in the narrow diameter riser. When FCC catalyst was circulated, the effect of the abrupt configuration was seen only in the upper portion of the riser, irrespective of the riser diameter. A simple empirical modification to the model of Pugsley and Berruti (1996) provides a good description of the axial pressure drop profiles for the abrupt exit configurations, with the exception of the case of sand in the 0.2 metre diameter riser. This discrepancy is attributed to the lack of fully developed flow in the larger riser.
Article
Work published on gas-solid fluidization since 1986 is reviewed, with emphasis on findings that appear to be new or to represent significant steps forward in advancing the understanding of fluidization phenomena, or which have potential practical implications. Hydrodynamic regimes ranging from bubbling to fast fluidization are addressed. Mixing phenomena and circulating fluidized beds are given special attention.
Article
An experimental investigation of the gas and solids flow in a pilot-scale circulating fluidized bed (CFB) cold model with two different abrupt exit configurations (L-shape and extended top) has been carried out. Measurements of axial pressure profiles, high-speed video images of the flow phenomena at the wall as well as local optical probe measurements inside the exit zone are presented. Contrary to published results obtained in bench-scale CFB risers the axial profiles of the apparent solids volume concentration obtained by pressure measurements showed no indication of an increased solids hold-up in the vicinity of the exit, which confirms the conclusion by Pugsley et al. (Can. J. Chem. Eng. 75 (1997) 1001) that this is a scale effect. The local probe measurements showed the well-known core–annulus flow structure prevailing until the riser top. In the vicinity of the exit this flow structure is superimposed by a strong horizontal velocity component directed to the exit duct. In comparison to the conventional L-shaped abrupt exit the extended top does not increase the solids inventory in the riser.
Article
The results of a simple model of vacuum gasoil cracking are presented:the radial profiles of superficial gas velocity, solid concentrations and axial profiles of conversion and yields are predicted by a plug flow model with a gas velocity profile and a radial dispersion coefficient. The catalyst to gas slip coefficient is fitted to industrial results. The core annulus structure of the bed results in a substantial reduction of gasoline yield.
Article
The determination of mean residence time is a powerful method for studying the hold-up in a cement mill, and hence whether the mill is operating in the required steel to clinker ratio. To determine the mean residence time accurately it is necessary to correct for the recycle of tracer, and a method of correction is given. Analysis of experimental residence time distributions from two large mills, (one with a circulation ratio of 9.4 the other with a circulation ratio of 1.5) indicates that the results agree with the one dimensional diffusion/mixing model, with a controlling parameter D/uL equal to about 0.07. In the first case the velocity of flow was about 6.5 ft/min and D was 16 ft/min; in the second, 5.7 ft/min and 14 ft2/min.RésuméDie Bestimmung der Hauptverweilzeit ist eine ausgezeichnete Methode for das Studium der Verzögerung in einer Zementmühle, und somit, ob die Mühle im verlangten Stahl zu Zementklinker Verhältnis arbeitet. Zur genauen Bestimmung der Hauptverweilzeit muss diese um den Rücklauf des Indikators korrigiert werden. Eine Methode fur diese Korrektion wid angegeben. Die Untersuchung von experimentellen Aufenthaltszeitverteilungen zweier grosser Mühlen (eine mit einem Umlaufverhältnis von 9.4, die andere mit einem solchen von 1.5) zeigt, dass die Ergebnisse mit dem eindimensionalen Diffusions-Mischungs-Modell ubereinstimmen mit einem kontrollierenden Parameter D/uL von ungefähr 0.07 Im ersten Fall war die Strömungs-geschwindigkeit ungerfähr 6.5 ft/min und D=16 ft2/min, im zweiten Fall 5.7 ft/min und 14 ft2/min.
Article
The influence of exit geometry on the riser axial pressure profile in circulating fluidized beds (CFB) is examined. Experiments were performed in an academic-scale CFB operating at conditions chosen to give dimensional similarity with an industrial CFB combustor. The influence of riser exit geometry was investigated to compare smooth and abrupt exits in terms of three different measures: (1) a dimensionless length of influence, Ω, of the exit along the riser, where Ω = (length of influence)/(riser height); (2) the exit reflection coefficient, Rf, defined by Senior in 1992 where Rf = (downward solids flow in riser)/(upward solids flow in riser) just below the riser exit; and (3) the increase in cross-sectionally averaged solids concentration at the riser exit. Analysis of the new data and results from the literature, lead to the description of solids flow at the exit in terms of a riser exit Froude number (FrR) where FrR = ust/gR. A correlation between the riser exit Froude number and the exit reflection coefficient is presented. A serious correlation between the dimensionless length of influence of the exit bend, measured down the riser, and operating conditions, particle properties, gas properties and riser dimensions is also presented. Both correlations predict the influence of exit geometry (the exit effect) on the riser axial pressure profile. The relationship between operating parameters and the increase in cross-sectional averaged solids concentration at the riser exit is also examined.
Article
Solids flow patterns were observed and measured in the square cross-section riser of a laboratory circulating fluidised bed (CFB). Measurements in the 0.14 m square riser used a sampling probe, of internal diameter 3.4 mm, to measure upwards and downwards solids fluxes throughout the cross-section. Interpolation gave complete solids flux profiles over the entire 0.14×0.14 m cross-section. Integration of the solids flux profile gave the external solids circulation rate, in reasonable agreement with measurements external to the riser. Special features of the solids flux profiles are (i) much higher down flux in the corners of the square section and (ii) slightly higher down flux half-way between the corners, near the walls. These features may be due to secondary flow in the square section duct and to enhanced sheltering provided by the two side walls. There is qualitative similarity between these solids flux profiles in the 0.14 m square riser and the profiles measured by Werdermann ((1992, Feststoffbewegung und Warmeubergang in zirkulierenden Wirbelschichten von Kohlekraftwerken. Dr-Ingenieur dissertation, Technical University Hamburg-Harburg) for large industrial combustors. But it is clear that there is a scale-up effect: the film, i.e. the cross-sectional area of duct containing the downflow (adjacent to the walls), is a much smaller proportion of the total area in the large units as compared with the laboratory units. The detail design of the riser exit or outlet bend at the top of the riser has a profound effect upon solids flux profiles in the riser and in particular on the reflux ratio defined as =(solids downward flow)/(external circulation rate). Measured values of varied by a factor of about 20 according to outlet bend design. These measurements were mostly near the top of the riser, but indications are that is affected, throughout the riser, by the design of outlet bend, a phenomenon previously observed in a CFB of circular cross-section by Brereton and Grace ((1994). In A. A. Avidan, Circulating Fluidized Bed Technology IV (pp. 137–144). New York: AIChE).
Article
The inventory function is the quantity of tracer remaining in a continuous-flow system at elapsed time t when steady flow of the tracer is replaced by untraced flow at t = 0. The relations between residence-time distributions, moments and changes in inventory when a tracer is flushed from a system are established. It is shown that inventory measurements could be an attractive way of measuring moments. In particular, the mean residence time is given by the intercept on the baseline of the initial tangent to the inventory curve, and the variance by the area between the inventory curve, the initial tangent and the baseline. It is proposed that dispersion be defined in terms of the variance of the residence-time distribution. This would allow experimentalists to record their results independently of models or theories in addition to comparing their results with the predictions of theories. Methods based on inventory measurements are potentially more accurate than the traditional step- and pulse-response methods. Ways in which inventory measurements might be made are suggested. It is timely that the theory should be presented now because tomographic methods that could be used to measure inventory are starting to appear.
Article
Gas and solid hydrodynamics have been studied in dilute circulating fluidized beds under conditions occurring in catalytic cracking risers. Gas radial velocity profiles and dispersions were established by a tracer technique in a cold set-up. The gas axial dispersion was determined in an industrial riser. The local concentrations of the solid phase were measured by a tomographic technique. This has allowed an assessment of the core—annulus structure of the bed and an estimate of the solid radial and axial dispersions. The axial solid concentration profiles were determined in pilot and industrial scale beds. These show an important accumulation upstream of the abrupt exit. The overall conclusion is that the gas flow can be considered to be plug flow with a radial velocity profile and a radial dispersion; the solid flow is slightly more dispersed due to the core—annulus structure and a high radial mixing.
Article
There is increasing interest in high-velocity fluidized bed reactors operated in the turbulent and fast fluidization regimes. Understanding of the hydrodynamics of these fluidization regimes has improved greatly in recent years, and there are prospects for applications beyond those practiced in industry at this time. Reactors operated in these regimes offer some unique features for gas-solid contacting. However, considerable work is required to achieve a better understanding of these high-velocity systems and to allow them to be optimized with respect to reactor geometry and operating conditions.
Article
A novel, non-intrusive method is described for measuring the particle residence time distribution (RTD) in a system with a short mean residence time. The method uses phosphorescent tracer particles activated by a high intensity pulse of light at the inlet. Tracer is detected using a light sensitive photomultiplier unit. Appropriate boundary conditions are maintained by using an annular feeder fluidised bed at the entrance boundary and an inline jet mixer installed at the exit boundary. This well defined arrangement of experimental boundary conditions represents a significant advance in the measurement of unbiased particle RTDs in these systems. The method was developed for measuring the particle RTD in a circulating fluidised bed (CFB) riser, but is applicable to other particle–fluid systems where a fast response measurement is required.
Article
Based on experimental results from two fast fluidization columns (D = 0.186 m × 8 m and D = 0.140 m × 10 m) and the results reported in the literature, the effects of gas velocity, solids circulation rate, particle diameter and density, bed diameter and solids inlet and exit restriction on the axial voidage distribution in fast fluidized beds are studied. The axial voidage distribution is found to have a basic profile described by a simple exponential function, with increasing voidage from the bottom to the top of the bed. With special bed structures, the basic axial voidage distribution profile may change into a C-shape with strong exit restriction or into a S-shape with weak solids inlet restriction.
Article
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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Residence time distributions of solids in a gas–solids downnow transported reactor Circulating bed technology IV (pp. 555 –559) Riser exits and scaling of circulating beds Flow patterns in the square cross-section riser of a circulating bed and the eeect of riser exit design
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Solids flow pattern in the exit region of a CFB-furnace-influence of exit geometry
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Johnsson, F., Vrager, A., & Leckner, B. (1999). Solids ow pattern in the exit region of a CFB-furnace-in uence of exit geometry. 15th international conference on uidized bed combustion, Savannah, Georgia, ASME.
Flow patterns in the square cross-section riser of a circulating uidized bed and the e ect of riser exit design
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Riser exits and scaling of circulating fluidized beds
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Hold-up and dispersion
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Catalytic cracking in riser reactors
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