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Overall Solids Movement and Solids Residence Time Distribution in a CFB-Riser
Abstract
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.
... Baeyens [15] Riser of CFB NaCl tracer ...
... Therefore, they must be chosen and applied with great care. 11 The chemically different tracers technique have been used extensively for fluidized bed and CFB studies [12]- [15], [51]. Bader et al. [52] obtained the solid residence time distribution in a fluidized bed cracking catalyst (FCC) riser from pulse injection of sodium chloride (NaCl) tracer. ...
... Smolders and Baeyens [15] also investigated the influence of operating conditions on RTD of solids in CFB reactors with NaCl tracer. After sampling, the salt concentration is then derived from the electrical conductivity of the solution (sample and 100ml water) by using a calibration curve. ...
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
... For solids RTD in CFB risers, some significant discrepancies arose among numerous publications, and even seemingly contradictory conclusions were drawn by various researchers. First of all for the shape of RTD, some researchers observed the occurrence of a double-peak (Ambler et al., 1990;Bhusarapu et al., 2004a;Kojima et al., 1989;Lin et al., 1999;Patience et al., 1991;Wei et al., 1998;Weinell et al., 1997) or even multi-peak (Bhusarapu, 2005;Wang et al., 1996), whereas more researchers only measured a typical single-peak curve (Andreux et al., 2008;Bader et al., 1988;Harris et al., 2003c;Rhodes et al., 1991;Smolders and Baeyens, 2000;Viitanen, 1993;Zheng et al., 1992). Secondly, as the two widely used parameters to quantitatively measure solids RTD, solids dispersion coefficient, D, and Peclet number, Pe, have a huge discrepancy in magnitude for the tests measured in risers. ...
... For example, the estimated axial or radial solids dispersion coefficients can differ by more than 4 orders of magnitude (Breault, 2006). And axial solids Peclet number can span from 1 to 100 (Andreux et al., 2008;Rhodes et al., 1991;Smolders and Baeyens, 2000;Wei et al., 1998Wei et al., , 1995Yan et al., 2009). Thirdly, the reported effects of operating conditions, like superficial gas velocity, U g , and solids mass flux, G s , on D and Pe are totally different. ...
... Most of them may be not adequate for a fast response system or a circulating particulate system like a CFB riser. The most striking contradictions among RTD experimental studies include: (1) tracer particles do not have the identical material properties to the rest particles in the bed (Ambler et al., 1990;Andreux et al., 2008;Chesonis et al., 1990b;Du and Wei, 2002;Godfroy et al., 1999;Guío-Pérez et al., 2014;Patience and Chaouki, 1995;Patience et al., 1991;Rhodes et al., 1991;Smolders and Baeyens, 2000;Wang et al., 1996;Weinell et al., 1997); (2) boundary conditions at the inlet and outlet of tracer cannot be guaranteed to be closed-closed type (Bhusarapu et al., 2004a;Harris et al., 2003c); and (3) the duration of tracer injection in the pulse-response RTD experiment, which should be like a delta function, is too long (Chesonis et al., 1990a;Guío-Pérez et al., 2014;Patience et al., 1991;Smolders and Baeyens, 2000). In addition, a diverse set of models are applied to interpreting the measurements, leading to a variety of parameters. ...
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.
... 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] . ...
... The authors compared the simulated RTD of solids calculated using the CFD method described in the previous section with the available experimental data to validate the approach. The experimental results reported by Smolders [28] were simulated in a CFB riser with a height of 6.5 m and a diameter of diameter 0.1 m. The solid circulation rate was 34.1 kg/m 2 s and the superficial velocity was 4.92 m/s. ...
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.
... 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: ...
... Many of the general modeling approaches developed for chemical reactors have been adapted for modeling particle RTDs in bubbling and circulating beds. Some relevant articles in the literature that discuss RTD modeling in this context include the following: Yagi and Kunii (1961a), Verloop et al (1968), Berruti et al (1988), Ambler et al (1990), Smolders and Baeyens (2000), Harris et al (2002), Bhusarapu et al (2004), and Andreux et al (2008). As with the modeling approaches used for the more general problem in chemical reactors, particle RTD models for bubbling and fluidized beds have adopted one of three basic approaches, listed below in increasing order of complexity: ...
... • The 1D continuum dispersion model [see for example Berruti et al ((1988), Levenspiel (1999), and Smolders and Baeyens (2000]; ...
... Besides theoretical investigations as discussed in Section 2.2.1, some experimental results have also been reported to measure the particle diffusion in the riser of CFB (Breault et al., 2008;Du and Wei, 2002;Koenigsdorff and Werther, 1995;Ran et al., 2001;Rhodes et al., 1991;Smolders and Baeyens, 2000;Wei et al., 1995Wei et al., , 1998Yan et al., 2009). However, the diffusion coefficient obtained from experiments frequently contains more effects which may come from the complicated internal and external surroundings. ...
... Meanwhile, the specific conditions of each experiment, such as the riser layout, particle properties, detecting technique of tracers, operating conditions as well as the analysis model, are also summarized. It should be noted that although the facility, the particles, the tracers, the range of operating conditions, and even the analysis models in Rhodes et al. (1991) and Smolders and Baeyens (2000) were very close, the correlations they proposed are quite different. Although the anisotropy of particle diffusion in the riser has been verified by experiments, we still used the same diffusion coefficient in axial and lateral directions for simplicity, i.e. an isotropy assumption was made. ...
... Some investigators (Rhodes et al., 1991;Smolders and Baeyens, 2000) regarded t as the median residence time (i.e. 50% value of the cumulative solids residence time distribution), whereas a majority of investigators (Harris et al., 2003;Patience et al., 1991) considered t as the mean residence time (i.e. the first moment of the RTD curve). ...
Solids residence time distribution (RTD) in circulating fluidized bed risers is a critical parameter for evaluating reactor performances, however, it is still very difficult to be predicted via computational fluid dynamics (CFD) simulation due to the complexity of particle clustering phenomenon. This paper tries to establish an effective CFD model to reasonably predict solids RTD of gas-solids riser flows by means of properly addressing the paramount role of particle clusters in determining solids RTD. The gas-solids hydrodynamic characteristics were solved by Eulerian-Eulerian model, where an energy minimization multi-scale (EMMS) drag model was applied to modify the gas-solids drag force to account for the influence of particle clusters. The motion of tracer particles was calculated using species transport equation, where the diffusion coefficient of particles, a vital parameter indicating particle diffusion capacity, was investigated thoroughly. The established CFD model was validated against the available experimental data in the literature. It was shown that axial profiles of solids volume fraction and radial profiles of solids mass flux can be well predicted with EMMS drag model, but not with homogeneous drag model. The proper prediction of bed hydrodynamics is also very crucial to the success of solids RTD simulation. On the other hand, the effect of the diffusion coefficient of particles, the magnitude of which can span a range from 10-5m2/s to 10m2/s, is minor when compared with the convective transport mechanism, at least for the specific cases we studied. In addition, the importance of the sampling time resolution and tracer injection time for a RTD curve was addressed. The simulation results showed that a low time resolution often results in the loss of some micro-scale information, i.e. drastically smoothing the fluctuations of the RTD curve, and an inappropriate assessment of the tracer injection time can lead to a significant change of the RTD curve.
... However, these results showed a different trend based on the experimental data reported by Smolder and Baeyens (12) and Harris et al. (13). At a constant velocity, the mean solid residence time in a riser increased with increasing solid mass flux based on the data reported by Smolder and Baeyens (12). and Harris et al. (13). ...
... and Harris et al. (13). This is because the back-mixing of solids in a riser is enhanced by the increased solid mass flux, which results in an increase in solid residence time (12). These contrasting results can be explained by the apparatus adopting a 0.009m and 0.0254m-ID, which is much smaller than that used by Smolder and Baeyens (12) and Harris et al. (13). ...
... This is because the back-mixing of solids in a riser is enhanced by the increased solid mass flux, which results in an increase in solid residence time (12). These contrasting results can be explained by the apparatus adopting a 0.009m and 0.0254m-ID, which is much smaller than that used by Smolder and Baeyens (12) and Harris et al. (13). Therefore, back-mixing, which is the down-flow of particles near the wall of the riser in our apparatus (0.009 m-ID x 1.9 m-high CFB), wasn't detected. ...
The effects of the riser inlet velocity, solid mass flux and particle size on the axial solid holdup profile and decay factor were investigated using two circulating fluidized beds (CFBs) with FCC (Geldart A) particles as the bed materials. Based on the experimental results from the two-CFBs, the axial solid holdup in the two CFBs were compared with the correlations of previous studies. Also, an empirical correlation was proposed for decay factor that exhibited a good agreement with experimental data.
... For experimental measurements, a certain amount of tracers were introduced into the apparatus, and then the solid residence time was determined via detecting tracer signals at the system exits [26,27]. Using this incentive-response technology of the tracer, researchers carried out many studies about solid residence time in the CFB riser [28][29][30][31][32][33][34][35]. The results showed that the solid residence time distribution could be descript by a plug flow with dispersion [34], and the solid residence time distribution curves had a sharp peak with a long tail [31]. ...
... Using this incentive-response technology of the tracer, researchers carried out many studies about solid residence time in the CFB riser [28][29][30][31][32][33][34][35]. The results showed that the solid residence time distribution could be descript by a plug flow with dispersion [34], and the solid residence time distribution curves had a sharp peak with a long tail [31]. Besides, the Positron Emission Particle Tracking (PEPT) of single radio-active tracer particles was also used to study solid residence time under different operating regimes [36]. ...
The effect of superficial gas velocity (Uf) on solid behaviors in a full-loop circulating fluidized bed is numerically studied using computational fluid dynamics-discrete element method (CFD-DEM). Specifically, the solid mixing and dispersion, solid residence time, solid force and velocity, and particle granular temperature are comprehensively explored. The results show that increasing Uf slows the solid mixing. The solid axial dispersion in the riser is dominated among that in the three regions. Besides, increasing Uf decreases the solid cycle time and leads to a more uniform solid residence time (SRT) distribution in the riser and dipleg. The fluid force is an order of magnitude smaller than the collision force. Increasing Uf shows distinctive influences on the rotational speed and translational velocity in the three regions. Moreover, increasing Uf suppresses the particle granular temperature. The average particle granular temperatures in the riser at four superficial gas velocities (i.e., 5.5 m/s, 6.0 m/s, 6.5 m/s, and 7.0 m/s) are 0.09326 m²/s², 0.08040 m²/s², 0.06531 m²/s², and 0.05047 m²/s², respectively.
... Meanwhile, it is well known that the residence time distribution (RTD) of solids is a major issue in the fluidizing process because it characterizes the fluid mixing quality that represents the main advantage of a fluidized bed over the gas-solid reactors, such as rotary kiln, shaft furnace and so on. Therefore, it is essential to understand and analyze the RTD to better design and operate BFBs (Smolders & Baeyens, 2000;Zhang & Xu, 2015). ...
... This approach can give much more information about the local values of phase holdups and their spatial distributions, especially in regions where physical measurements would be either difficult or impossible. Previous research has thus far concentrated on the RTD of circulating fluidized beds (CFBs) and has found some factors that impact on the RTD, such as particle diffusion capacity and back-mixing (Chan, Seville, Parker, & Baeyens, 2010;Shi et al., 2015;Smolders & Baeyens, 2000). However, to our knowledge, few studies have attempted to simulate the RTD of a BFB. ...
The residence time distribution (RTD) of solids and the fluidized structure of a bubbling fluidized bed were investigated numerically using computational fluid dynamics simulations coupled with the modified structure-based drag model. A general comparison of the simulated results with theoretical values shows reasonable agreement. As the mean residence time is increased, the RTD initial peak intensity decreases and the RTD curve tail extends farther. Numerous small peaks on the RTD curve are induced by the back-mixing and aggregation of particles, which attests to the non-uniform flow structure of the bubbling fluidized bed. The low value of t50 results in poor contact between phases, and the complete exit age of the overflow particles is much longer for back-mixed solids and those caught in dead regions. The formation of a gulf-stream flow and back-mixing for solids induces an even wider spread of RTD.
... Traditionally, the impulse stimulus response technique is usually adopted to obtain the solid RTD in the CFB riser. 20,27 On the other hand, a series of empirical models in this area have been developed: one-and two-dimensional diffusional models, the stochastic model, and the core−annulus tracer balance model. 28 However, the majority of these models are based on simplification or depend heavily on lots of semiempirical correlations, thus they are physically unrealistic. ...
... These results indicate that the radial motion of the solid phase is mainly from the central region to the near-wall region of the riser, forming the solid internal circulations, which is consistent with the model and experiments. 27,72 Considering the gas−solid intensive interaction, the gas−solid velocities show fluctuations in the radial direction while they are smooth in the axial direction. In addition, the solid radial velocities inside the CFB are smaller than their counterparts of the gas flow, while they are about 1 order of magnitude smaller than the solid axial velocity. ...
... Different simulations with different residence times and number of reactors have been conducted. The former have been varied from 3 to 12 s, which are expected residence times for CFB reactors (Smolders and Baeyens, 2000), and the latter from 2 to 7 reactors. The objective was to determine the variation in CO2 capture efficiency between the simulation and the corresponding one with one reactor less: ...
... The global Péclet numbers of solids in the two virtual systems are also estimated using empirical correlations in literature to demonstrate the different combined effects of solids convection and mixing. For FFB, a correlation by Smolders and Baeyens (2000) was adopted, i.e. Pe = 2.4 + 18.3/ . ...
Cross-correlation algorithm has been widely used in experimental measurement of solids velocity. However, its reliability and validation range remain unquantified. To this end, a CFD-based “virtual error quantification” method was proposed to quantify the reliability of cross-correlation algorithm in the measurement of solids velocity in two different gas fluidization regimes, i.e., bubbling and fast fluidization regimes (BFB and FFB). It was found that the cross-correlation algorithm works quite well in the core region of FFB, but its applications in the annulus region of FFB and in BFB should be considered with great caution. Finally, the findings were delineated by solids convection-mixing mechanism based on the relative standard deviation (RSD) of solids velocity signals and solids Péclet number. The present study highlights the deficiency of the cross-correlation based solids velocity measurement in fluidization systems and addresses the need to develop better measurement methods for solids mixing dominated systems.
... [15] To reduce errors in the experimental measurement of solid RTD, the injection duration of pulse tracer should be controlled as short as possible when compared with the particle mean residence time. While the tracer injection time is in the range of 0.1s to several seconds [16] due to the limitations of the current tracer injection techniques, which are not much shorter than or even close to the solids residence time in the FCC riser. ...
Computational fluid dynamics (CFD) is a powerful tool for prediction and analysis of complex multiphase flow hydrodynamics and residence time distribution (RTD) in chemical reactors. This study presented the validation and application of a filtered drag model for solid RTD prediction in a pilot-scale fluid catalytic cracking (FCC) circulating fluidized bed (CFB) riser with Geldart A particles. First, the filtered drag model implemented in the open-source MFiX-TFM solver was validated for flow hydrodynamics simulation in a FCC CFB riser. After that, the model was further employed to validate its ability for solid RTD prediction in the same riser by comparing the simulation results with the experimental data. The simulation with the filtered drag model well reproduced the tracer response experiment which is more accurate than that with the Gidaspow drag model. Simulations with both pulse and step tracer injection methods were compared, which reveals the limitation of solid RTD measurement using a pulse tracer injection method in the experiment.
... In order to take into account the effects of tracer injection, the measured curve of RTD at the end of the outflow pipe, E output (t), can be regarded as a convolution of the RTD functions of both the injection process, E input (t), and the bed system, E bed (t) (Sheoran et al., 2018;Smolders and Baeyens, 2000). Hence, the relationship of them is, ...
The residence time distribution (RTD) of particles in a gas-solid dense fluidized bed with a continuous solid flow operation and baffles needs much more focus for the purpose of the wide industrial applications. The available models of solid RTD are not adequate in this case due to the additional convective diffusion induced by the cross-flow of the solid feeding and the complex geometry caused by the baffles. To address this problem, this work applied a two-phase Eulerian-Eulerian model combined with the species transport equation to predict solid RTD in a dense fluidized bed. The effects of the continuous solid feeding and baffles were considered. The solid dispersion coefficient D s in the species transport equation was calculated by an analytical solution from kinetic theory of granular flow. D s only needs the value at a molecular level due to the fact that the computational fluid dynamics (CFD) model is able to reproduce the RTD procedure exactly same as the practical experiments. To validate the established CFD model, a series of 3D lab-scale cold flow experiments were conducted in the free and baffled beds for the bed material with various severe non-spherical shapes. The measurement included the solid hydrodynamic characteristics at the outlet of the outflow pipe and solid RTD of the system. The reasonable agreement between the CFD prediction and experimental data demonstrated the good performance of the CFD model. 1D plug flow with dispersion model and the tanks-in-series model were further used to fit the calculated RTD curves. The estimated lateral dispersion coefficient of solids locates in a rational range compared with the data collected extensively from the literature. The results showed that the lateral dispersion coefficient of solids decreases when the baffles are installed.
... As depicted in Fig. 10(b), with the increase of superficial velocity, the averaged SRT and its standard deviation in the riser gradually decease, which suggests that the gas-solid flow patterns gradually transform from the fast fluidization regime to the pneumatic transport regime. These results are consistent with plenty of experiments [3,4,[58][59][60][61][62]. Moreover, the averaged SRT in the riser is dominated, thus the profile of the averaged solid cycle time against the superficial velocity in Fig. 10(a) is similar with the SRT distribution profile in Fig. 10(b). ...
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.
... 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. ...
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.
... The term "fluidized bed" is unavoidably connected to the term "particulate solid material" [105][106][107][108][109][110]. The geometrical, physical and aerodynamical properties of the particles affect the onset of fluidization, the characteristics, behaviour and the main parameters of fluidized beds. ...
Matching the power supply with demand is a key issue for the development of renewable energy sources: the intermittence of wind or solar power can be overcome by integrating an energy storage system. In comparison with sensible and latent heat storage, thermochemical heat storage has the advantages of both a high energy density, so that reaction products can be stored at ambient temperature for later reuse, and a flexibility of transportation when off-site use is required. An initial thermodynamic and kinetic screening of thermochemical reactions in the temperature range of about 600 to 1200 K is presented. Thermodynamics and kinetics are used to characterize the reactions through the equilibrium temperature, the heat of reaction, the reaction rates, and recovery work. These aspects were studied theoretically and experimentally. The analysis defines the essential design parameters and provides guidance for the important assessment criteria of new candidate reaction systems.
... The term "fluidized bed" is unavoidably connected to the term "particulate solid material" [105][106][107][108][109][110]. The geometrical, physical and aerodynamical properties of the particles affect the onset of fluidization, the characteristics, behaviour and the main parameters of fluidized beds. ...
Thermal energy storage (TES) transfers heat to storage media during the charging period, and releases it at a later stage during the discharging step. It can be usefully applied in solar plants, or in industrial processes, such as metallurgical transformations. Sensible, latent and thermo-chemical media store heat in materials which change temperature, phase or chemical composition, respectively. Sensible heat storage is well-documented. Latent heat storage, using phase change materials (PCMs), mainly using liquid–solid transition to store latent heat, allows a more compact, efficient and therefore economical system to operate. Thermo-chemical heat storage (TCS) is still at an early stage of laboratory and pilot research despite its attractive application for long term energy storage.
... In the core/annulus mode on the contrary, extensive (back-) mixing occurs and particles are subject to a wide residence time distribution. Previous studies (17,18) mostly assess operations at moderate values of the solids circulation fluxes (≤ 100 kg/m²s). The present study extends this to > 600 kg/m²s, using Positron Emission Particle Tracking (PEPT, 19) to study the movement and population density of particles in the CFB-riser. ...
CFB biomass pyrolysis produces mostly bio-oil. Reaction rates are fast (k > 0.5 s-1). Yields exceed 60 wt% of bio-oil at 500 °C and at a residence time for oil and char t < 2.5 s. as achieved in plug flow CFB-mode, shown by PEPT to occur at U > (Utr + 1) m/s and G > 200 kg/m² s.
... Several studies of solids mixing and residence time have been published and were recently reviewed [3,4]. Almost all of these studies were performed at low superficial gas velocities (≤10 m/s) and low solids circulation rates (≤ 100 kg/m 2 s). ...
Circulating Fluidised Beds (CFB) are attracting increasing interest for both gas–solid and gas-catalytic reactions, although the operating modes in these two cases are completely different. In modelling CFBs as reactors, the solids residence time is an important parameter. Previous studies mostly assess operations at moderate values of the solids circulation rates (≤100 kg/m2 s), whereas gas-catalytic reactions and e.g. biomass pyrolysis require completely different operating conditions. In the current work, Positron Emission Particle Tracking (PEPT) is used to study the movement and population density of particles in the CFB-riser.The PEPT results can be used to obtain: (i) the vertical particle movement and population density in a cross sectional area of the riser; (ii) the transport gas velocity (Utr) required in order to operate in a fully established circulation mode; (iii) the overall particle movement mode (core flow versus core/annulus flow); and (iv) the particle slip velocity (Us).Only in a core flow mode can the particle slip velocity be estimated from the difference between the superficial gas velocity (U) and the particle terminal velocity (Ut). The slip velocity is lower than U−Ut outside the core flow mode. To operate in core flow, the superficial gas velocity should exceed Utr by approximately 1 m/s and the solids circulation rate should exceed 200 kg/m2 s.
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.
Olive oil production urges the energy companies to exploit the potential of the residues as biomass fuels for clean energy production. Circulating fluidized bed (CFB) gasifier gains global interest within these several years due to its promising solution for converting biomass material to renewable energy. However, the multi-physics processes and multiscale structures inside the gasifier impede the experimental measurements. As an alternative, a reactive multiphase particle-in-cell approach is developed in this work to simulate the gasification process of olive oil waste in a pilot-scale full-loop CFB gasifier. After the model validation, the heat transfer properties of solid phase and the effect of size-induced segregation on solid thermochemical properties are explored. The results show that the low-temperature biomass particles injected highly influence the spatial distribution of solid temperature and solid heat transfer coefficient (HTC). A larger particle size gives rise to a higher HTC. Moreover, the temperature and HTC of biomass are larger than that of sand. A wide and narrow residence time distribution of sand in the riser and cyclone can be observed. The scale of the biomass dispersion is respectively larger and the same as that of sand in horizontal and vertical directions. Particles in the dense region have a smaller HTC, Reynolds number, and temperature. Superficial gas velocity improves the uniformity of solid distribution. Elevating the gas velocity and initial bed temperature enhances solid dispersion intensity.
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.
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.
Fluidised bed technology is a very common industrial process and has a wide range of applications in process engineering. Though indirectly, a common way to infer the end product uniformity is through investigating the solid residence time distribution (RTD) in the system. A good understanding of the particle RTD is crucial for the proper design, optimisation, and scale-up of a fluidisation process. Flexible ribbon particles have a very different flow behaviour in a fluidised bed compared to that of spheres which in turn influences the RTD of the particles. However, this issue is still not very well addressed to date. In this paper, a double chain model is adopted to model cut tobacco strips in a fluidised bed riser. The spring constants of this chain model are calibrated, and the model is validated against experimental datum. Finally, the particle RTD as a function of different process parameters is investigated.
Accurate prediction of transport phenomena is critical for VPU reactor design, optimization, and scale-up. The current study focused on the validation and application of a multiphase CFD model within an open-source code MFiX for hydrodynamics, temperature field, and residence time distribution (RTD) simulation in a non-reacting circulating fluidized bed riser for biomass pyrolysis vapor phase upgrading (VPU). First, an Eulerian-Eulerian approach three-dimensional CFD model was employed to simulate the pilot-scale VPU riser on the supercomputer Joule. Excellent quantitative agreement between experimental and simulated results was achieved for pressure drops and temperature field in a range of operating conditions. Then the validated multiphase CFD model was applied to predict gas and solid residence time distributions (RTDs) since prediction and analysis of RTD is an important tool to study the complex multiphase flow behavior and mixing inside chemical reactors. The predictions show that solid mean residence time is 3.5 times the gas residence time; the solid RTD is more sensitive to the process gas flow rate than the solids circulation rate.
The raise of environmental concerns in the past decades consequently increased the need of obtaining cleaner sources of energy. Among the studied alternatives, second generation biofuels (produced from non-food resources) are one of the most promising solutions and nearly reached industrialization. The production of cellulosic bioethanol is one of the possibilities of second generation biofuels, and the studies involving the use of cellulosic compounds to produce bioethanol recently increased. Its production involves four dependent steps: pretreatment, enzymatic hydrolysis, fermentation and distillation. This work considered the pretreatment stage, aiming at modifying the structure of the lignocellulosic biomass so that cellulose becomes more accessible to enzymatic hydrolysis step.
This work particularly focused on the biomass flow characterization in the transport and compression screws involved in the pretreatment step of an industrial-scale process. The main challenge was firstly to perform measurements on an industrial-scale device working under harsh conditions. Residence time distribution (RTD) experiments were thus performed using a novel methodology adapted to these working conditions. Sodium carbonate was selected as a tracer. Due to the reaction with the acidified biomass, both electrical conductivity and pH were monitored at the exit of the screws. A chemical model was developed, allowing the determination of tracer concentrations from the measured data. The measurements obtained were compared with three optimized models: a combination of plug flow and continuous stirred tank reactor in series (PFR-CSTR), plug flow with axial dispersion (AD) and a model based on the Zusatz function. The results of this work pointed out the non-plug flow behavior of these screws in their standard working conditions. In accordance with the physical motion of the tracer inside the screws, the use of the PF-CSTR is recommended for representing RTD inside screws in conditions in which backflow is likely to occur.
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.
Fluidized particle-in-tube solar absorbers are increasingly investigated both as receiver in a solar power concept, or as receiver/reactor for the thermal decomposition of minerals or for gas-solid reactions such as CO2 looping concepts and thermo-chemical energy storage. Such applications require a high heat transfer rate from the tube wall to the upflowing suspension of particles, and a strict control of the particle residence time. Conversion and heat transfer depend upon the particle mixing, its residence time (RT) and residence time distribution (RTD). Both parameters are hence important in fluidized bed applications. The present research experimentally investigated the use of an Upflow Bubbling Fluidized Bed (UBFB) as particle-in-tube concentrated solar receiver and/or reactor. The RTD was determined by tracer response, and compared with predictions from different models. Both a cascade of stirred tank reactors and a plug flow with dispersion model provide a good fitting of the experimental results. The former is however preferred, with a number of cascade mixing cells between 3 and 4, slightly dependent on both the operating air velocity and the particle circulation rate. The commonly applied ratio of the superficial fluidization velocity and the minimum fluidization velocity of the particles is between 3 and 20, whereas solid circulation fluxes up to 100 kg/m²s are used. Design equations are derived. The results are moreover used in test cases of a concentrated solar power absorber, and in the use of a UBFB as solar limestone calcination furnace. The approach can also be applied to processes of e.g. heat absorption or drying, provided kinetic data are known.
It has been realized in recent decades that a proper investigation of gasification reactor requires the detailed information over the entire flow field, as well as time, at multiple scales. Such detailed information needs the use of sophisticated measuring techniques with capability to provide the required information over the entire flow field, as well as time, at multiple scales. Aside from the mean velocities and volume fractions, information about the flow fluctuations or dynamics (quantified in terms of cross-correlations and auto-correlations) is also desirable. In addition, it is preferable if such techniques are amenable to automation to reduce extensive human involvement in the data collection process. While such data are “stand-alone” sets of information, which can be used for design and scale-up strategies, it also provides information that is crucial to establish the validity of conventional models like phenomenological flow models describing residence time distribution (RTD), as well as more recent and sophisticated models like those based on computational fluid dynamics (CFD). In fact, it almost seems imprudent to validate CFD predictions on overall holdup and flow rates, because these spatial integrals of point properties are simply averages of a complete flow field that a CFD code is designed to and claims to compute. Thus, fair validation must involve validation at multiple scales, for which one needs experimental information also at multiple scales (and not just spatial and temporal averages). Several experimental techniques have been reported in past to quantify the flow field in gas–solid gasification reactors, with each technique having its own advantages and disadvantages. In this chapter, details of pressure, solid velocity, solid fraction, and RTD measurement techniques will be presented. Techniques will be divided majorly in two types, invasive and non-invasive. The postprocessing methods for each technique, advantages, and limitations will be discussed. Finally, some of the recent findings on gas–solids circulating fluidized bed using radioactive particle tracking (RPT) technique will be discussed in detail to explain the use of the experimental techniques for design and scale-up of these reactors.
Cross-flow bubbling fluidized beds (BFBs) have been widely used in dual fluidized bed systems such as chemical and heat looping. Understanding the residence characteristics of solids in such system is important for better design and optimization of reactors. Previously we experimentally measured the residence time distributions (RTDs) of sands in a cross-flow rectangular BFB by using coal particles as tracer. A computational investigation of solids RTD using multi-fluid Eulerian method combined with the species transport equation showed that the RTD of sands could be correctly represented by that of coal particles, and the simulation results well agreed with experimental data. Parametric studies demonstrated that, under the considered operation conditions, the influence of tracer injection time period on the predicted solid residence times was nearly ignorable. Simulation results revealed that in the investigated cross-flow BFB the solids RTD is closely related to solids inventory and solids flux. Through proper data processing, it was found that the descending part of solids RTD profile can be uniquely fitted by an empirical exponential function. A semi-empirical approach was thus developed and further validated, for the first time in the literature, to predict the entire profile of solids RTD, in which the ascending part of solids RTD profile is obtained through CFD simulation whereas the descending part is given by the fitted empirical exponential function.
Gas-solid fluidized beds have been used in CO2 capture processes because of their high mixing characteristic and heat and mass transfer. Sufficient residence time of solid particles in a reactor is required to capture CO2. However, a fraction of solid particles pass through a reactor without capturing CO2 due to normal reaction characteristics. Therefore, the objective of the present study was to increase the sorbent residence time using a diffuser in a reactor for CO2 capture. An Eulerian-Eulerian model in a commercial CFD program was employed to simulate gas-solid flow in the reactor. First, sensitivity analysis depending on operating conditions was conducted to predict the residence time of solid particles. The diffuser was located in the middle of the reactor and the angle of the diffuser was changed. Solid particles dispersed in the radial direction because of gas characteristics in the diffuser and increased the residence time. The results showed that the diffuser increased the sorbent residence time, so that the probabilities of gas-solid reaction would be also improved.
The authors brought forward composite fluidization structure against the problems in the CFB-FGD. Axial swirl vanes, which formed the composite fluidization of swirl-once through flow, were installed in the venturi. The characteristics of gas-solid flow at cold state were discussed through experiment and simulation. The comparison between the composite fluidization and the once-through fluidization was carried out. As a result, the composite fluidization mode is nonuniform air distribution mode. Its tangent velocity is larger than that of the once-through fluidization, which results in the longer particles residence time, higher particles concentration, stronger fluctuation, and better capability of diffusion and mixture. The uniformity is improved and can be achieved more quickly, which improves the flow field in the composite fluidization.
The flow behavior in the fluid catalytic cracking strippers with and without internals was simulated using computational fluid dynamics. The residence time distributions (RTD) of the catalyst solids were obtained at the catalyst outlet by coupling the tracer technique. The dead volume model was applied to analyze the RTD curves of the solids. The fractions of the dead, plug and well-mixed volumes in different strippers were compared. In the V-baffled stripper, the dead volume fraction is about 10%, the plug volume fraction is above 40% while the well-mixed volume fraction is below 50%. In the empty cylinder stripper, they are 15%-35%, 10%-25% and 50%-60%, respectively. The results demonstrate that in the V-baffled stripper, the stripping volume is utilized more sufficiently and the solid back-mixing is lowered, which will finally improve the stripping efficiency.
With the Computational Particle Fluid Dynamic (CPFD), which is based on the scheme of Multi-Phase Particle-In-Cell (MP-PIC), the present work studied the flow hydrodynamics and solids back-mixing behaviors in the riser with smooth and abrupt exit geometries. It was found that the flow hydrodynamics and solids back-mixing behaviors are quite different in the riser with different exit geometries. While particles can uniformly flow out of the riser with smooth exits with relatively lower residence time, part of the particles tend to be reflected back into the riser with abrupt exits. Different mechanisms were proposed to explain the distinct solids back-mixing behaviors in the riser with smooth and abrupt exit geometries. The moderate solids back-mixing in the riser with smooth exits is mainly due to the dynamic formation and dissolution of particle meso-scale structures/particle-clusters at the bottom part of the riser, while the intense solids back-mixing in the riser with abrupt exit geometries is the combined results of the particle downward flow from the riser top and the dynamic formation and dissolution of particle meso-scale structures at the riser bottom. The present work implied that the riser exit geometry should be carefully specified in the modeling study of the CFB riser.
Effect of fluidized bed geometry on solids mixing was investigated. Experiments were carried out in cylindrical and square fluidized beds. White and black particles with 850 µm average diameter were used in the experiments in which black particles were considered as tracer. In order to determine the concentration of tracers in each sample, the method of image processing was applied. Based on the final value of the mixing index, it was concluded that complete random mixing can be achieved in both beds. The results indicated that mixing is faster and mixing time is shorter in the cylindrical bed than in the square bed. This trend was attributed to the fact that the movement of solids in the cylindrical bed is faster compared to the squared bed due to the presence of dead zones in the square bed which reduce the cross-section area available for passage of bubbles. A one-dimensional dispersion model was used to predict the solids axial dispersion coefficient. It was shown that the axial dispersion coefficient in the case of cylindrical bed is greater than that in the square bed and that the axial dispersion coefficient increases by increasing the superficial gas velocity in both beds. 2016
With quartz sand and calcium hydroxide serving as the feed and return material respectively, studied were the particle mixing characteristics of a Φ0.6 m×15 m underfeed circulating spouted bed in a desulfurization tower. The mixing entropies at various locations and its mixing indexes of the underfeed and return material at various elevations were calculated by utilizing the solubility difference of the quartz sand and calcium hydroxide in water, and the influence of various operating parameters on the particle mixing behavior of particles in the tower was analyzed. The research results show that the mixing index can reflect very well the mixing quality of the feed and return material particles in the tower. With an increase of the fluidization speed, the mixing indexes at various elevations in the tower assume an ascending tendency. The spouting velocity and circulation ratio conspicuously affect the particle distribution characteristics in the tower, especially at its bottom. When a relatively high spouting speed and a comparatively high circulation ratio are adopted, the mixing index will go up accordingly, indicating that the mixing quality in the tower has been improved.
A Computational Particle Fluid Dynamics (CPFD) approach was applied to investigate the solids residence time distribution (RTD) and back-mixing behavior in a circulating fluidized bed (CFB) riser. The comparison between the simulation results and the experimental data indicates that the CPFD method was capable to predict the hydrodynamics and solids mean residence time. It was found that the solids residence time exhibited a non-uniform distribution both in the axial and in the radial directions of the riser. Even in the dilute phase transport (DPT) regime, the predicted solids RTD curves had the feature of an early peak and an extended tail, indicating that the solids flow deviates from plug flow and exhibits back-mixing. The solids back-mixing mainly occurred in the lower part of the riser, which provided significant implications for the industrial applications of the CFB reactors since most of the chemical reactions and heat/mass transport processes occur at the lower part of the riser. It is important to minimize the solids back-mixing at the lower part of the riser for the industrial CFB applications like the FCC.
The full-loop gas–solid motions in a three-dimensional circulating fluidized bed are numerically modeled using the computational fluid dynamics combined with the discrete element method. The time-averaged flow characteristics and the particle-scale details related to solid motion are discussed. The results show that solid motion shows an ‘S’ rising path in the riser but a spirally falling behavior in the cyclone. Typical core-annulus flow structure and non-uniform distribution of solid holdup appear in the riser. The inside and outside swirling flows of gas motion as well as the fishtailing phenomenon are captured in the cyclone. The solid residence time in the riser shows an early-occurred peak with a long tail. In the riser, details related to solid motion at the particle-scale level are obtained. The sudden changes of these particle-scale details when increasing the superficial velocity may be considered to be a new criterion of identifying the flow regime change.
Circulating fluidized beds are popular for large-scale combustion operations, and they are also gaining attention for gasification of carbonaceous feedstocks. Circulating fluidized beds utilize the entire height of tall vessels for gas-solids contacting, with no distinct interface between bed and freeboard. The corresponding flow regime is usually fast fluidization for combustion and gasification of solid fuels. Operating conditions, riser geometry, hydrodynamics, heat transfer, solids feed and particle recirculation all have significant influence on the operation and effectiveness of the system. This chapter reviews some of the most important hydrodynamic properties of circulating fluidized beds that are relevant in CFB gas-solid combustion and gasification reactions, with particular attention to results for columns of rectangular cross section. Gas and solid mixing are summarized also, and a brief survey of relevant heat transfer aspects is provided.
Solids back-mixing significantly influences the performance of circulating fluidized bed (CFB) risers. A better understanding of the solids back-mixing behavior is greatly essential for the design and operation of CFB risers. Computational particle fluid dynamics (CPFD) modeling based on the multiphase particle-in-cell (MP-PIC) method was applied to investigate the solids back-mixing behavior and its influencing factors in CFB risers operating in dilute phase transport (DPT) and fast fluidization (FF) regimes. The present work observed extremely long residence time and wide residence time distribution (RTD) curves for particles in the FF regime, indicating an extensive back-mixing of particles in the FF regime. The overall and local back-mixing behaviors of solids were further analyzed. The particles in the DPT regime have a little back-mixing in the lower part of the riser, while the particles in the FF regime experience a large-scale back-mixing throughout the riser. In addition, the factors influencing the back-mixing behavior of solids in different flow regimes were investigated. The results demonstrate that the solids back-mixing in the DPT regime is caused by the downward flow of particles near the wall region, while the severe back-mixing in the FF regime is due to the downflow particles and mesoscale structures (mainly particle clusters). The dynamic formation and dissolution of mesoscale structures could be the dominating factor, leading to the intense back-mixing for particles in the FF regime.
The gas and solid mixing in fluid catalytic cracking strippers with and without internals were investigated using computational fluid dynamics simulations. The Eulerian–Eulerian two-fluid model coupled with the modified Gidaspow drag model was used to simulate the gas-solid flow behavior. The grid independency study and the comparison of 2D and 3D simulations were carried out first. The residence time distribution model and axial dispersion model were utilized to obtain the parameters indicating the back-mixing degree, such as mean residence time, dimensionless variance and Peclet number of gas and solids. Moreover, the influence of bubble size and gas/solid flow distribution on the mass transfer between the bubble and emulsion phase were also analyzed. The results show that the baffles in the V-baffle stripper can efficiently enhance the gas and solids mixing, reduce the back-mixing degree of gas and solids, strengthen the mass transfer between the bubble and emulsion phase, and hence improve the stripping efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 2012
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.
Single radioactive particle tracking was used to measure the overall solids residence time (and its distribution) in the riser of a CFB, operating at superficial air velocities (U) of 1 to 9ms−1 and solids circulation fluxes (G) between 20 to 600kg m−2s−1.The results demonstrate that the particle motion and mixing differ according to the operating mode of the riser, with a fairly constant velocity throughout the riser achieved in the dilute or dense riser flow, but with a significant amount of back-mixing for intermediate values of U and/or G. This back-mixing is due to the core-annulus mode of particle flow. Whereas experimental results and theoretical predictions are in fair agreement for the dilute and dense riser flow, the core-annulus regime needs to account for a U and G dependent slip factor (φ), in excess of the commonly proposed value φ=2, especially at U–UTR
Both gas and solids mixing and dispersion in circulating fluidized beds have been studied using steady and unsteady state tracer techniques. Most studies, however, have been conducted in CFB risers with relatively low solids circulation rates. In recent years, the high density and high solids flux CFB riser has received increasing attention due to its applications in catalytic cracking of crude oil and potential applications in catalytic reactions requiring high catalyst-to-reactant loading ratios. In view of the significantly different flow structure in high-density and high solid flux risers where annulus solids downward flow is no longer present as revealed in some recent studies, this paper analyzed the similarity and differences of gas and solids mixing behaviour between low density and high density CFB risers based on the data recently collected in our lab and data reported in the literature. It is shown that there exists a clear transition of both gas and solids axial mixing behaviour when the flow structure changes from low density to high density operating conditions, in correspondence to the disappearance of solids downward flow near the wall region. In the low density CFB risers with solids flux lower than similar to 250 kg/m(2)s, both gas and solids axial dispersion and back-mixing increased with increasing solids flux and solids concentration. Beyond the transition to the dense suspension upflow regime, however, both gas and solids axial dispersions tend to decrease with increasing solids flux, indicating the development of gas and solids flow toward the plug flow resulting from the disappearance of solids downflow in the near wall region. Radial dispersion of gas and solids, on the other hand, showed a continuous decrease over a wide range of solids fluxes within both the low density and high density flow regions, indicating a reduced lateral exchange of gas and particles with increasing solids fluxes.
CFBs are increasingly used for both gas-catalytic and gas–solid reactions. The conversion is a function of the gas hydrodynamics, subject of the present research.Available literature on the gas mixing in the riser of a CFB shows contradictory results: some investigators neglected back-mixing of gas, whereas others report a considerable amount of back-mixing in CFB risers. The present paper reports experimental findings obtained in a 0.1m I.D. riser, for a wide range of combined superficial gas velocity (U) and solid circulation flux (G). The gas flow mode (plug vs. mixed) is strongly affected by the operating conditions, however with a dominant mode within a specific (U, G)-range. Sand was used as bed material. The superficial gas velocity was varied from 5.5 to 8.3m/s, the solids circulation flux was between 40 and 170kg/m2 s. A tracer pulse response technique was used with a pulse of propane injected at the bottom and detected at the riser exit. The cumulative response curves, F(t), define (i) an average residence time (t50) obtained for F(t)=0.5; and (ii) the slope of the curves (a steeper one corresponding with more pronounced plug flow) and expressed in terms of a span, σ. These parameters (t50 and σ) define the gas flow mode. A quantitative comparison of experimental results with literature RTD-models is inconclusive although the occurrence of both mixed flow and plug flow is evident, and (U, G)-dependent. The experimental results are expressed in empirical design equations, and the comparison of predicted and experimental results is fair: low values of σ determine the plug flow regimes, whereas back-mixing is more pronounced at higher value of σ. Experiments with similar systems might favor plug flow or mixing as function of the combined (U, G)-values. The introduction of the RTD-function in reaction rate equations can improve the prediction of the gas-conversion in a riser-reactor.
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.
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.
Tests were conducted in a 30.5-cm-diameter riser with 76 μm FCC catalyst to determine the degree of gas/solid contacting in the “fast-fluid bed” regime. Data were obtained at superficial gas velocities of 3.7 to 6.1 m/s and solid fluxes of 98 to 195 kg/s-m². The conveying gas was air at ambient temperature and 103 kPa. Data were obtained on axial pressure profiles, particle velocity and mass flux as a function of radial and axial position, radial and axial gas dispersion, and solids residence time distributions. Density and voidage profiles were calculated from these data.
In the turbulent fluidized bed, there is an upper bed surface though it is considerably more diffuse than in a bubbling fludized bed because of the greater freeboard activity attending operation at higher gas velocities. The turbulent regime extends to the so-called transport velocity. As the transport velocity is approached, there is a sharp increase in the rate of particle carry-over, and in the absence of solid recycle, the bed would empty in short order. Beyond the transport velocity, solid fed to the bottom of the column or vessel transverses it in fully entrained transport flow, and the concentration or density of the resulting suspension depends not only on the velocity of the gas but also on the solid-flow rate. Refs.
Similitude is a powerful tool which allows a small laboratory experiment at ambient conditions to simulate a much larger commercial fluidized bed. The scaling laws for bed dynamics and heat transfer are systematically developed from the fundamental relationships. Recent results demonstrate the validity of scaling and shed light on the key parameters which must be included in scaling. Simplified forms of the similitude relationships are identified.
A predictive model based on the core-annulus flow structure has been developed to evaluate axial voidage profile and interanal flow structure along the risers of Circulating Fluidized Beds (CFBs). Through comparison with experimental data, the proposed model is shown to be able to qualitatively and quantitatively predict the effect of operating variables such as superficial gas velocity and solids mass flux on the axial voidage profile.
The hydrodynamics in a riser can be explained in terms of the coexistence of a dense bed, a decaying entrainment and a dilute refluxing transport. The model equations previously developed by Rhodes et al. were tested against experimental data obtained in a 0.1 ID m riser. The predicted axial pressure drop is in agreement with the experimental data provided the entrainment decay constant a is taken equal to approximately 0.4. Experimental data were also used to predict the transport velocity UTR. Equation (1) predicts literature data to within approximately 20%. The ratio of the slip velocity to the particle terminal velocity largely exceeds the value of 1. Using a slip factor, as introduced by Patience et al., enables us to examine the variation of the slip velocity over the height of the riser.
Lateral and axial profiles of solids cross-flow flux were measured in a circulating fluidized bed riser of square cross-section with a sampling probe. The solids cross-flow flux was found to be higher near the wall, especially near the corners, and lower in the core of the riser. The net solids horizontal flux was outwards in the lower part of the riser and inwards near the top.
The lateral solids momentum flux, measured by means of a piezoelectric probe, increased with height and then decreased near the top of the riser. The lateral solids momentum flux first increased with distance from the axis and then decreased towards the wall. It was especially low near the corners.
Les profils latéral et axial de flux d'écoulement transversal de solides ont été mesurés à l'aide d'une sonde d'échantillonnage dans une colonne montante à lit fluidisé de section carrée. On a trouvé que le flux d'écoulement de solides transversal était plus élevé près de la paroi, en particulier dans les coins, et plus faible au centre de la colonne. Le flux de solides horizontal net est dirigé vers l'extérieur en bas de la colonne et vers l'intérieur dans la partie supérieure.
Le flux de quantité de mouvement de solides latéral, mesuré à l'aide d'une sonde piézoélectrique, augmente avec la hauteur puis diminue dans la partie supérieure de la colonne, Le flux de moment de solides latéral augmente d'abord avec la distance par rapport à l'axe pour diminuer ensuite vers la paroi; il est particulièrement faible dans les coins.
Longitudinal solids mixing was studied experimentally in circulating fluidized bed risers of internal diameter 0.152 m and 0.305 m. Superficial gas velocity and mean solids flux used were 2.8–5.0 m/s and 5.0–80 kg/m2·s, respectively, and the bed solids had a surface volume mean diameter of 71 μm and a particle density of 2,456 kg/m3. A sodium chloride tracer was used in impulse injection experiments. A simple, one-dimensional dispersion model describes measured solids mixing satisfactorily. Peclet numbers (UoL/Dz) found, in the range 1.0–9.0, were correlated with the riser diameter and mean solids flux. The modeling approach described here permits residence time distribution curves to be calculated directly from the knowledge of superficial gas velocity, mean solids flux, and riser diameter. Longitudinal solids mixing in the riser decreased with increasing riser diameter. The results are consistent with recent hydrodynamic studies.
A simple hydrodynamic model for Circulating Fluidized Beds has been developed. The mathematical model, based on the core-annulus flow structure, is shown to be able to predict the two-phase flow characteristics and it requires only two measurable steady-state parameters, namely the experimental average voidage profile along the riser, or equivalently the pressure distribution, and the net solids circulation rate. The model has been successfully tested using recently obtained literature data covering a variety of reactor configurations and operating conditions.Un modèle hydrodynamique simple a été mis au point pour des lits fluidisés circulants. On montre que le modèle mathématique, basé sur la structure d'écoulement coeur-espace annulaire, est capable de prédire les caractéristiques d'écoulement biphasique; en outre, il ne nécessite que deux paramètres à l'état permanent mesurables, soit le profil de désaturation moyen expérimental le long de la colonne, ou de manière équivalente la distribution de pression, soit la vitesse de circulation des solides nette. Ce modèle a été testé avec succès à l'aide de données publiées récemment couvrant une variété de configurations de réacteurs et de conditions de fonctionnement.
A class of high resolution multidimensional wave-propagation algorithms is described for general time-dependent hyperbolic systems. The methods are based on solving Riemann problems and applying limiter functions to the resulting waves, which are then propagated in a multidimensional manner. For nonlinear systems of conservation laws the methods are conservative and yield good shock resolution. The methods are generalized to hyperbolic systems that are not in conservation form and to problems that include a “capacity function.” Several examples are included for gas dynamics, acoustics in a heterogeneous medium, and advection in a stratified flow on curvilinear grids. The software packageCLAWPACKimplements these algorithms in Fortran and is freely available on the Web. One and two space dimensions are discussed here, although the algorithms and software have also been extended to three dimensions.
A mathematical model, based on the core-annulus flow structure, is proposed to characterize the two-phase flow and the residence time distribution of solids within the riser of circulating fluidized beds. The model is tested against experimental data obtained in a laboratory unit utilizing a tracer technique. While the accuracy of the experimental technique is shown not to be sufficient to allow conclusive results, the model is capable of a good prediction of the observed trends of the characteristic bimodal residence time distributions with an excellent match of the times of arrival of the two peaks. Simulation runs have been performed to study the effects of key operating variables on the residence time distribution.
Hydrodynamic modelling of circula-ting #uidized beds High velocity #uidization
- K Oxford
- J Smolders
- Baeyens Kuleuven
- Belgium
- K Smolders
- J Baeyens
Oxford: Pergamon Press. K. Smolders, J. Baeyens / Chemical Engineering Science 55 (2000) 4101}4116 dissertation, KULeuven, Belgium. Smolders, K., & Baeyens, J. (1998). Hydrodynamic modelling of circula-ting #uidized beds. Advances in Powder Technology, 9, 17}38. Yerushalmi, J., Avidan, A. A. (1985). High velocity #uidization. In J.F. Davidson, R. Clift, & D. Harrsion, Fluidization (2nd ed.), (pp. 226}293). New York: Academic Press.
Bed expansion and solids mixing in high velocity fluidized beds
- A A Avidan
Avidan, A. A. (1980). Bed expansion and solids mixing in high velocity
yuidized beds. Ph.D dissertation, City College of New York.
Contribution à l'étude de la déshydratation du carbonate de sodium monohydrate en lit fluidisé dense et mise en oeuvre en lit fluidisé circulant: étude expérimentale et modélisation
- S Diguet
Diguet, S. (1996). Contribution a % l 'e & tude de la de & shydratation du carbonate de sodium monohydrate en lit yuidise& dense et mise en oeuvre en lit
yuidise & circulant: e & tude expe& rimentale et mode & lisation. Ph.D. dissertation, L'Institut National Polytechnique de Toulouse, France.