Fast Fluidization Regime Research Articles

Electrostatic characteristics in a large-scale triple-bed circulating fluidized bed system for coal gasification

Electrostatics charge generation by triboelectrification has significant implications for the proper design and operation of a circulating fluidized bed. In this study, electrostatics in the fully developed regions of both the riser and downer of a large-scale triple-bed combined circulating fluidized bed was characterized in terms of the equivalent currents over the cross section of the developed region. The average equivalent currents and solids holdup were measured under different superficial velocities in the riser, downer and gas-sealing bed. It was found that in the fast fluidization regime in the riser, the negative equivalent currents were comparable with the positive equivalent currents due to the typical core-annulus flow pattern. With increasing superficial velocities in the riser or in the gas-sealing bed, the flow pattern would approach dilute phase transport or dense suspension upflow regime. Thus, the positive equivalent currents became dominant because the backflow of sand particles were greatly suppressed. Some dominant frequencies for the equivalent currents in the riser were almost identical regardless of the magnitude of superficial velocities in the riser and the gas-sealing bed, indicating that they were determined by the inherent characteristics of electrostatics and/or signal noise, and were not affected by gas–solids flow behaviors. The frequencies in the downer were focused on the low value range, and the dominant frequencies were nearly zero. Both the solids mass flux and solids holdup had significant influence on the average equivalent currents. With increasing superficial velocities in the gas-sealing bed, the average equivalent currents first increased and then approached a constant. This led to the same trend in variation of solids mass flux, as well as solids holdup. With increasing air superficial velocities in the riser, the average equivalent currents in the riser increased, while the average equivalent currents decreased in the downer with increasing air superficial velocities in the downer.

Hydrodynamic Characteristics of a Novel Circulating Fluidized Bed Steam Reformer Operating in the Fast Fluidization Regime

Circulating Fluidized Bed Steam Reformers (CFBSRs) represent an important alternative for the production of syngas for the Fisher-Tropsch (FT) process and for hydrogen production. Most research regarding this novel CFBSRs did not consider its hydrodynamic characteristics. In this work, the riser Computational Fluid Dynamics (CFD) simulations were investigated using two phase Eulerian-Eulerian approach coupled with kinetic theory of granular flow with k-epsilon model to describe the turbulence of each phase. The model equations were solved via the commercial CFD package FLUENT, which uses the finite volume numerical approach. Cold flow simulations were carried out under the fast fluidization regime and results were validated qualitatively against available experimental data. The radial segregation of the catalyst, the velocity distribution of both phases and other characteristics of the flow were captured by the simulation. This work showed that for operation under high density and high flux conditions, solids flux should be higher than 300 kg/m2s and inlet void fraction lower than 85%.

Solids behavior in dilute zone of a CFB riser under turbulent conditions

The behavior of the solid phase in the upper zone of a circulating fluidized bed riser was studied using a phase Doppler anemometer. Glass particles of mean diameter 107 μm and superficial gas velocities U g covering the turbulent and the beginning of the fast fluidization regime were investigated. Three static bed heights were tested. Ascending and descending particles were found co-existing under all operating conditions tested, and at all measurement locations. Superficial gas velocity proved/happened to have a larger effect on descending particles at the wall and on ascending particles in the central region. Transversal particle velocities in both directions (toward the center and toward the wall) behaved relatively equivalently, with only slight difference observed at the wall. However, observation of the number of particles moving in either transversal direction showed a change in bed structure when increasing U g. Furthermore, a balance was constantly observed between the core zone and the annulus zone where the mutual mass transfer between these two zones occurred continuously. Transition from a slow to a fast particle motion was accompanied by a transition to high levels of velocity fluctuations, and was found corresponding to the appearance of significant solid particle flow rate.

Evaluating the Probabilities of Fluidization Regimes

Probabilities of fluidization regimes were determined experimentally using frequency domain analysis of pressure fluctuation data. It was assumed that regime transition does not occur at a distinct gas velocity and fluidization regimes coexist at each gas velocity. Three different flow structures (macro-, meso-, and microstructures) were recognized in a fluidized bed. Macrostructure is represented by bubbles, mesostructures are represented by clusters, and microstructure is represented by particles. These structures are the characteristics of bubbling, turbulent, and fast fluidization regimes. Regime probabilities were determined using power spectrum energies of these structures. Experiments were carried out in a 1 m high and 5 cm inner diameter glass column. Pressure fluctuations were measured using a piezoresistive pressure sensor. Sand particles with various diameters (150, 280, and 490 μm) and fluid catalytic cracking (FCC) particles (73 μm) were used as solids. Probabilities of fluidization regimes were calculated at various gas velocities. A new procedure was proposed for evaluating the regime probabilities versus gas velocity. It was shown that the proposed correlation is in good agreement with the experimental data and is noticeably better than the method proposed in the literature.

Design study of a 150 kW th double loop circulating fluidized bed reactor system for chemical looping combustion with focus on industrial applicability and pressurization

Nowadays the lab scale feasibility of the chemical looping combustion technology has been proved. This article deals with many of the design requirements that need to be fulfilled to make this technology applicable at industrial scale. A design for a 150 kW th chemical looping combustion reactor system is proposed. In the base case it is supposed to work with gaseous fuels and inexpensive oxygen carriers derived from industrial by-products or natural minerals. More specifically the fuel will be methane and a manganese ore will be the basis for the oxygen carrier. It is a double loop circulating fluidized bed where both the air reactor and the fuel reactor are capable to work in the fast fluidization regime in order to increase the gas solids contact along the reactor body. High operational flexibility is aimed, in this way it will be possible to run with different fuels and oxygen carriers as well as different operating conditions such as variation in air excess. Compactness is a major goal in order to reduce the required solid material and possibly to enclose the reactor body into a pressurized vessel to investigate the chemical looping combustion under pressurized conditions. The mass and heat balance are described, as well as the hydrodynamic investigations performed. Most design solutions presented are taken from industrial standards as one main objective is to meet commercial requirements.

Local solid particle behavior inside the upper zone of a circulating fluidized bed riser

The solid phase behavior is studied in the upper zone of a circulating fluidized bed riser with glass particle of mean diameter 107 μm, using a Phase Doppler Anemometer. Superficial gas velocities U g > U c are investigated covering the turbulent and the fast fluidization regimes and this for three static bed heights ( H s = 50 mm, H s = 100 mm and H s = 150 mm). The results show that the mean axial particle velocity lateral profile shapes found parabolic for H s = 50 mm, devolve to a concave shape for H s = 100 mm and H s = 150 mm, creating a particles speeding zone between the core and the annulus zones. For both axial and transversal standard deviations of particle velocities in the core region, the values for the three static bed heights unite to form two stages of evolutions with U g, where the transition velocity between these stages is found associated to the appearing of a significant entrainment of solid particles. At this transition velocity, the transversal movement originally directed toward the center riser, undergo a change toward the wall beginning near the wall and spreading into a large part of the section riser with increasing U g. A discussion on the boundary between the turbulent and fast fluidization regimes is made based on these results.

Flow structures inside a large-scale turbulent fluidized bed of FCC particles: Eulerian simulation with an EMMS-based sub-grid scale model

Turbulent fluidized bed reactors are widely used in industry. However, CFD simulations of the hydrodynamic characteristics of these reactors are relatively sparse, despite the urgent demand from industry. To address this problem, Eulerian simulations with an EMMS-based sub-grid scale model, accounting for the effect of sub-grid scale structures on the inter-phase friction, are performed to study the hydrodynamics inside a large-scale turbulent fluidized bed of FCC particles. It is shown that the simulated axial and radial solid concentration profiles, entrained solid fluxes and standard deviation of the solid concentration fluctuation agreed well with experimental data available in the literature. In-depth analysis of time-averaged particle velocity and solid concentration shows that a dense-suspension upflow regime coexists with fast fluidization regime in this bed, which is reminiscent of the hydrodynamic characteristics in high-density circulating fluidized bed (CFB) risers, even though they are operated in different fluidization regimes. The Reynolds stresses in turbulent fluidized beds are anisotropic, but the degree of anisotropy is far less pronounced than the reported values in CFB risers. It was also found that the solid concentration fluctuation and axial particle velocity fluctuation are strongly correlated.

Studies on segregation of binary mixture of solids in a continuous fast-fluidized bed. Part IV. Total solids holdup, axial solids holdup and axial solids concentration

The segregation pattern of a binary mixture of particles is studied in a continuous fast-fluidized bed of 3.65 m height and 69 mm ID. The influence of operating variables such as gas velocity, solids feed rate and feed composition on total solids holdup and axial solids holdup is studied. Empirical correlations are proposed for the total pressure drop in the fast fluidization regime when density and/or size segregation occur. The influence of the operating variables on the axial concentration of solids in both the dense and dilute phases is analyzed.

Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: A simulation study

An important decision in the design of fluidized bed reactors is which of several flow regimes to choose. Almost all fluidized bed reactor models are restricted to a single flow regime, making comparison difficult, especially near the regime boundaries. This paper examines the performance of fluidized bed methane reformers with three models—a simple equilibrium model and two kinetic distributed models, based on different assumptions of varying sophistication. Membranes are incorporated to improve reactor performance. Eighteen cases are simulated for different flow regimes and membrane configurations. Predictions for the fast fluidization and turbulent flow regimes show that the rate-controlling step is permeation through the membranes. Bubbling regime simulations predict somewhat less hydrogen production than for turbulent and fast fluidization, due to the effects of interphase crossflow and mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. Practical considerations also affect the advantages, shortcomings and ultimate choice of flow regime.

Drift flux modelling of entrained gas–solids suspensions

Circulating solids at high mass flux continues to be an area of industrial interest and academic relevance. We report experimental data for entrained gas–solids suspensions at high solids mass flux in three fluidization regimes: bubbly, fast and “dilute” phase transport. In the bubbly regime (below about 0.5 m/s), the gas velocity and mass flux have only a minor influence on the suspension density and most of the variance in the data is attributable to the change in particle size distribution: the suspension density increased with a decrease in the percentage of particles with a diameter below 44 μm (fines). In the dilute phase transport regime (above a gas velocity of 4 m/s), the fines appear to have little effect on the suspension density; the two most significant factors are the gas velocity and solids mass flux. In the fast fluidization regime, the void fraction increased with an increase in the fines content to the same degree as in the bubbly regime. In addition, increasing the solids mass flux increased the suspension density although increasing the gas velocity decreased it but to a lesser extent when compared to the dilute phase transport regime. To characterize the void fraction as a function of gas velocity and particle velocity, we modified the drift flux model used in gas–liquid systems to gas–solids systems and derived the following expression ɛ = ɛ m f + ( U g − U m f ) ( 1 − ɛ m f ) C o ( U g − U m f + U p 1 − ɛ m f ) + V a j where U represents superficial velocities; C o is the distribution coefficient that characterizes the deviation from ideal flow conditions; and, V aj is the drift velocity. In the bubbly regime, V aj represents bubble velocities but in the dilute phase transport regime, it approaches the particle terminal velocity.

Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions

A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas–solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian–Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas–solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas–solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes.

Behavior of the Dense-Phase Transportation Regime in a Circulating Fluidized Bed

The dense-phase transportation regime is noted as a flow regime in a gas-solid circulating fluidized bed with large solids throughputs, high bed density, and low gas and solids back mixing. The quasi-three-dimensional images obtained by the ECT with the NN-MOIRT reconstruction technique reveal a significantly different solids concentration distribution in the dense-phase transportation regime from that in the dense-phase fluidization regimes, i.e., the bubbling, the turbulent, and the slugging regimes. The transient and time-averaged flow behavior in the dense-phase transportation regime is compared to that in the dense-phase fluidization regimes based on the statistical properties including the standard deviation, power spectra, and intermittency index of the local and cross-sectional averaged solids concentration fluctuations. Examining the statistical properties indicates a less segregated solids concentration distribution over the bed cross section in the dense-phase transportation regime than in the fast fluidization regime. In the dense-phase transportation regime, at a given gas velocity below U tr , choking takes place upward along the CFB riser at a decreasing solids concentration as the solids circulation rate increases.

Solids flow mapping in a gas–solid riser: Mean holdup and velocity fields

In this study, solids flow dynamics has been investigated in a 6 in. “cold-flow” circulating fluidized bed riser using non-invasive flow techniques. Gamma ray Computed Tomography (CT) has been used to measure the time-averaged cross-sectional solids holdup distribution. The time-averaged mean and fluctuating solids velocity fields have been quantified using the Computer Automated Radioactive Particle Tracking (CARPT) technique. Details of the experimental techniques, protocol of implementation and data analysis are discussed. The experimental studies examine operating conditions in fast fluidization and dilute phase transport regimes. Comparative and symbiotic analyses of the results obtained from CARPT and CT have been used to develop a coherent picture of the solids flow field. In addition, this work also demonstrates the power of CARPT and CT as flow mapping techniques in studying highly turbulent and opaque multiphase systems.

New description of fluidization regimes

AbstractTwo pressure probes and an intrusive bioptical probe provide experimental data in the bubbling and turbulent regimes to improve our understanding of the transition from the bubbling regime to the fast fluidization regime. Assessment of pressure, voidage, and bubbling properties allows for a new description of turbulent fluidization hydrodynamics. Appreciable changes in the hydrodynamics appear well below the transition criterion Uc, determined based on pressure fluctuations: bubbles are fast bubbles below Uc, in bubble–emulsion equilibrium state at Uc, and then become slow bubbles for higher fluidizing velocity; the maximum of the total fluidizing gas fraction in the bubble phase is reached below Uc and never exceeds 75–80%. The fluidizing velocity at which major modifications in hydrodynamics are observed is lower than Uc. This critical velocity is detected with pressure drop assessment or frequency analysis of voidage fluctuations. Moreover, it can be theoretically calculated from the two‐phase modeling correlations supplemented with a new criterion deduced from our experiments. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Quantification of solids flow in a gas–solid riser: single radioactive particle tracking

Solids in risers of circulating fluidized beds (CFB) exhibit local backflow and recirculation. Measurement of the concentration-time response to an impulse injection of tracer, even at two elevations cannot determine the residence time distribution (RTD) of solids uniquely. Hence, evaluation of RTD in risers from conventional tracer responses is difficult and often not possible. In addition, estimating the solids circulation rate in these closed loop systems, is a non-trivial problem. In this work, a single radioactive particle in the CFB loop is tracked during its multiple visits to the riser and, by invoking ergodicity, solids circulation rate, accurate solids RTD and additional information on the solids flow pattern in the riser are estimated. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity. A second peak in the probability density function (PDF) of the solids RTD curve in the riser was observed for operating conditions in the fast-fluidization regime.

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles ( d p = 70 μ m , ρ p = 1700 kg / m 3 ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered ( U g = 5 – 8 m / s ; G s = 22 – 345 kg / m 2 s ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.

Spanning the flow regimes: Generic fluidized‐bed reactor model

AbstractProbabilistic averaging is used to model fluidized‐bed reactors across the three fluidlization flow regimes most commonly encountered in industry (bubbling, turbulent, and fast fluidization), extending earlier work, which introduced this approach to bridge the bubbling and turbulent regimes of fluidization. In extending this concept to the fast fluidization regime, the probabilities of being in each of the three regimes are represented as probability density functions derived from regime boundary transition data. The three regime‐specific models—a generalized version of a two‐phase bubbling bed model at low gas velocities, a dispersed flow model for turbulent beds at intermediate velocities, and a generalized version of a core‐annulus model at higher velocities—are employed, leading to improved predictions compared with any of the individual models, while avoiding discontinuities at the regime boundaries. Predictions from the new integrated model are in good agreement with available ozone decomposition data over the full range of applicability covered elsewhere.

Operating concept of circulating fluidized bed gasifier from the kinetic point of view

Circulating fluidized bed gasifiers used in integrated gasification and combined cycle (IGCC) possess unique design and operational features: high gas velocity, high solid loading, and a large amount of circulating chars. As a consequence, their performance is noticeably different from the conventional fluidized bed gasifier operating at lower gas velocity. At the same time, limited information has been published to show how to operate the circulating fluidized bed from the kinetic point of view.In order to study these aspects, kinetic models are developed using kinetic data from previous literature and two concepts of operating the circulating fluidized bed gasifier from the kinetic point of view are presented in this paper. The results of the kinetic analysis using Johnson's kinetic data of a constant specific gasification rate of char (0.001 1/s) show that all species approach their equilibrium values after travelling a distance of 2.5 m, which is set as the height of the gasifier. The resident time of the char in the bed is 0.86 s and the required circulation rate of char and lime are 11.8 and 4.5 kg/s, respectively. In addition to Johnson's kinetic data, Wen's kinetic model is also applied in the kinetic analysis. It shows a way to operate the gasifier in the fast fluidization regime with a superficial velocity between 3.6 and 4.9 m/s and a solid loading between 50 and 100 kg/m3, these operating conditions are recommended by industrial experience. It points out the necessity to predict the performance of the gasifier from the kinetic point of view in order to design the optimal geometry and operating methods that will allow the gasifier to play a successful role in the clean coal technology system such as IGCC, high-performance power system (HIPPS), etc.

Novel Circulating Fluidized-Bed Membrane Reformer for the Efficient Production of Ultraclean Fuels from Hydrocarbons

An integrated novel fluidized-bed membrane steam reformer for the efficient production of hydrogen is proposed and investigated. It consists of two connected subsystems, viz., a circulating fluidized-bed membrane reformer and a reactor−regenerator for dry reforming. The steam-reforming part is an extension of the previous successful work regarding a bubbling fluidized-bed membrane reformer. The fast fluidization regime is more efficient and productive than the bubbling regime. Hydrogen-permeable membranes to remove hydrogen are used together with a CO2 acceptor to remove carbon dioxide and enhance the performance. An in situ supply of heat through oxidative steam reforming is also suggested. The deactivated catalyst is regenerated and recirculated back to the reformer. The CO2-rich stream from the efficient fast fluidized membrane steam reformer is used in a reactor−regenerator dry reforming subsystem, and additional syngas is produced. A reliable reaction engineering model is utilized to investigate this novel configuration. A comparison between the predicted performance of the fast fluidized-bed reformer and the industrial data for a fixed bed as well as the pilot-plant data of the bubbling fluidized-bed configurations investigated earlier shows that the fast fluidized bed is more efficient and productive.

Variable-gas-density fluidized bed reactor model for catalytic processes

A generalized fluidized bed reactor model which covers the three fluidization flow regimes most commonly encountered in industry (bubbling, turbulent and fast fluidization) is proposed. The model is based on probabilistic averaging shown previously (Thompson, Bi, & Grace, Chem. Eng. Sci. 54 (1999) 2175; Grace, Abba, Bi, & Thompson, Can. J. Chem. Eng. 77 (1999) 305) to be applicable over a range of superficial gas velocities. In this paper, we extend the model to cases where the volumetric gas flow changes appreciably due to variations in molar flow, pressure and temperature. For the air-based oxy-chlorination process as a case study, it is shown that the volume change affects both the hydrodynamics and the reactor performance. Because the reactions are rapid, almost complete conversion of ethylene is attained immediately above the distributor resulting in an ∼25% reduction in volumetric flowrate. Using the probabilistic averaging technique, the model tracks the probability of being in the bubbling, turbulent and fast fluidization regimes along the reactor height. The impacts of temperature and pressure variations are also examined. The variable density model gives predictions which compare well with commercial data; ignoring density variations leads to significant underprediction.