Turbulent Fluidized Bed Research Articles

Bubble Properties in Bubbling and Turbulent Fluidized Beds for Particles of Geldart’s Group B

Predicting bubble properties in fluidized beds is of high interest for reactor design and modeling. While bubble sizes and velocities for low velocity bubbling fluidized beds have been examined in several studies, there have been only few studies about bubble behavior at superficial gas velocities up into the turbulent regime. For this reason, we performed a thorough investigation of the size, shape and velocity of bubbles at superficial gas velocities ranging from 0.18 m/s up to 1.6 m/s. Capacitance probes were used for the determination of the bubble properties in three different fluidized bed facilities sized of 0.1 m, 0.4 m and 1 m in diameter. Particles belonging to Geldart’s group B (Sauter mean diameter: 188 µm, solid density: ρs = 2600 kg/m3) were used. Correlations for the determination of bubble phase holdup, vertical bubble length and bubble velocity are introduced in this work. The shape of bubbles was found to depend on superficial gas velocity. This implies that at large superficial gas velocities the horizontal size of a bubble must be much smaller in comparison to its vertical size. This leads to a decrease of pressure fluctuations, which is observed in the literature as a characteristic of transitioning into a turbulent regime.

A four-zone drag model based on cluster for simulating gas-solids flow in turbulent fluidized beds

Gas-solids drag model is crucial for the accurate prediction of gas-solids flow in turbulent fluidized bed (TFB). A four-zone drag model based on cluster was proposed to describe the gas-solids interactions in TFB. The model considers the cluster effect in the bottom dense bubbling zone while regards the particles as discrete ones in the top dilute dispersed zone. The structural parameter model was used to calculate the equivalent cluster diameter in the drag model. Validations against experiments show that the drag model can accurately simulate the gas-solids flow characteristics in TFBs with Geldart A and B particles. Besides, the non-uniform flow structures resulting from gas voids and particle clusters were successfully captured with the model. Compared with the bubbling fluidized bed, more small bubbles with a more uniform size distribution exist in TFB and the variation of operating conditions has little effect on gas voids characteristics in TFB.

Cold Model Study of a 1.5 MWth Circulating Turbulent Fluidized Bed Fuel Reactor in Chemical Looping Combustion

A circulating turbulent fluidized bed connected with a riser and an annular carbon stripper (CS) is proposed to be used as a fuel reactor (FR) in chemical looping combustion. The bottom section of ...

Simulation of Sorbent-Enhanced Steam Methane Reforming and Limestone Calcination in Dual Turbulent Fluidized Bed Reactors

This paper presents a steady-state simulation of sorbent-enhanced steam methane reforming in turbulent fluidized bed reactors. The effects of different operating conditions are assessed, including ...

Heat transfer in a pulsating turbulent fluidized bed

The multiphase Eulerian-Eulerian approach was applied to investigate heat transfer through an immersed tube in a turbulent fluidized bed computationally. Heat transfer occurred in the turbulent fluidized bed via four mechanisms: particle-tube conduction, particle-particle conduction, gas-tube convection and gas-particle convection. A strong correlation between heat transfer performance and particle distribution around the immersed tube was observed. Replacement of hot particles around the tube with cold particles promoted heat transfer. Three parametric studies on effects of fluidization regime, particle size and pulsed flow were conducted. It was observed that the heat transfer performance of a bubbling fluidized bed was superior to that of a turbulent fluidized bed. The size of gas pocket increased with increasing particle size. As the tube was enveloped by gas pockets for a longer period of time, particle-tube conduction was hindered and average heat transfer coefficient around the tube decreased. Two key weaknesses of a turbulent fluidized bed were identified to be formation of large gas pockets and presence of a stagnant zone above the tube. The novelty of the current study is the application of pulsed flow to address these weaknesses. It was observed that there existed an optimal pulsating frequency at which heat transfer in a turbulent fluidized bed was enhanced. When the pulsating amplitude was selected such that the fluidized bed oscillated between two different fluidization regimes, the heat transfer performance of the turbulent fluidized bed became comparable to that of a bubbling fluidized bed.

Machine learning to assist filtered two‐fluid model development for dense gas–particle flows

AbstractMachine learning (ML) is experiencing an immensely fascinating resurgence in a wide variety of fields. However, applying such powerful ML to construct subgrid interphase closures has been rarely reported. To this end, we develop two data‐driven ML strategies (i.e., artificial neural networks and eXtreme gradient boosting) to accurately predict filtered subgrid drag corrections using big data from highly resolved simulations of gas‐particle fluidization. Quantitative assessments of effects of various subgrid input markers on training prediction outputs are performed and three‐marker choice is demonstrated to be the optimal one for predicting the unseen test set. We then develop a parallel data loader to integrate this predictive ML model into a computational fluid dynamic (CFD) framework. Subsequent coarse‐grid simulations agree fairly well with experiments regarding the underlying hydrodynamics in bubbling and turbulent fluidized beds. The present ML approach provides easily extended ways to facilitate the development of predictive models for multiphase flows.

Preparation of potassium carbonate supported on gamma alumina using acid modification impregnation for carbon dioxide sorption in turbulent fluidized bed

The conventional impregnation was improved by acid modification as surface treatment to obtain the higher performance of solid sorbent, potassium carbonate supported on gamma alumina, for carbon dioxide (CO2) sorption. Acid modification affected on the shape of the sorbents. Weak acids, phenol and acetic, provided round shape with uniform size distribution. In contrast, strong acids, hydrochloric and sulfuric, obtained sharp edges irregular shape sorbent with broad size distribution. The CO2 capture capacity was improved by weak acid modification. All modifications could increase surface area of sorbent except for phenol modification. The highest CO2 sorption capacity was obtained from acetic acid modification due to the better crystallinity condition of the modified sorbent. Direct acetic acid modification with impregnation time of 24 h was suggested to be the suitable modification method. The modified sorbent showed a good regeneration property. The modified sorbent was compatible with the continuous mode of CO2 capture unit using circulating fluidized bed reactor, which consumed less energy than commercialized liquid amine absorber-stripper unit.

Investigation of a circulating turbulent fluidized bed with a fractal gas distributor by electrostatic-immune electrical capacitance tomography

In this paper, an electrostatic-immune AC-based electrical capacitance tomography (ECT) is developed to visualize the gas-solid flow inside the circulating turbulent fluidized bed (C-TFB) reactor. A fractal gas distributor based on fractal self-similarity theory is used to improve the particle distribution by re-distributing the intake gas. Experiments are carried out on a cold model of lab-scale C-TFB to investigate the effect of the fractal gas distributor using the electrostatic-immune ECT. Experimental results show that the particle concentrations with the fractal gas distributor are higher than those without the fractal gas distributor under identical operation conditions, indicating that the fractal gas distributor is favorable to enhance the upward movement of particles inside the C-TFB reactor. Moreover, the radial particle distribution with the fractal gas distributor is more even at a certain area of C-TFB, indicating that the fractal gas distributor can improve the radial uniformity of C-TFB within its active region.

Simulation of autothermal hydrogen-producing limestone calcination for calcium looping in turbulent fluidized bed reactors

This paper investigates the performance of combining limestone calcination with catalytic methane reforming and combustion in turbulent fluidized bed membrane reactors. The equilibrium performance of the process was evaluated using Aspen simulation, with a unique correlation to estimate the required gaseous feed composition for autothermal and complete sorbent regeneration at operating temperatures of 800–900 °C. A one-dimensional, isothermal, steady-state kinetic model is then employed to study the performance of the four-in-one (combustion + reforming + calcination + optional membrane separation) integrated process under turbulent fluidization conditions. The simulation results demonstrate the high potential of this novel sorbent regeneration technique for autothermal, rapid and hydrogen-producing limestone calcination. The performance is shown to depend strongly on the sorbent/total gas molar feed ratio, operating temperature, operating pressure and sorbent residence time.

A consolidated flow regime map of upward gas fluidization

AbstractA new flow regime map, resulting from more fundamental studies on the hydrodynamics and new flow regimes, is proposed in response to more practical reclassifications of the existing regimes with the development of upward gas‐solids fluidization systems. The previously reported flow regime maps and flow structures of some widely used fluidized beds are carefully examined. To better reflect the industrial applications, the fast fluidization regime is reclassified as high‐density and low‐density circulating fluidization regimes. A consolidated flow regime map is then proposed where the flow regimes of upward fluidization expand to include new types of fluidized beds such as circulating turbulent fluidized bed and high‐density circulating fluidized bed. The proposed flow regime map consists of six flow regimes: bubbling, turbulent, circulating turbulent, high‐density circulating and low‐density circulating fluidization, and pneumatic transport. The transitions between the regimes are discussed with new correlations proposed for fluid catalytic cracking type particles. Analysis on the dominating phase in the different types of fluidized beds reveals the dynamic changeover from solids phase continuous in conventional low‐velocity batch/“fixed” fluidization operations to gas phase continuous in high‐velocity continuous/“moving” fluidization operations and provides more insights to the transitions between the flow regimes for industrial design and practice.

A general EMMS drag model applicable for gas-solid turbulent beds and cocurrent downers

Eulerian-Eulerian models incorporated with the kinetic theory of granular flow were widely used in the simulation of gas-solid two-phase flow, while the effects of mesoscale structures such as particle clusters and gas bubbles could not be considered adequately if traditional homogeneous drag models were adopted in the coarse grid simulations. The energy minimization multiscale (EMMS) model has been proved to facilitate calculating a structure-dependent drag coefficient by considering particle clustering phenomena, which can be coupled with the two-fluid model (TFM) to improve the accuracy of coarse-grid simulation of gas-solid circulating fluidized beds. However, the original EMMS drag model cannot be further applied to the simulation of gas-solid fluidized beds with solids flow rate smaller than zero, e.g., turbulent fluidized beds and cocurrent downward flow, because the original cluster diameter correlation gives rise to a value smaller than single particle diameter or even negative value at extremely low solids fluxes or downward gas-solid flow. In this study, a new proposed cluster evolution equation is proposed by quantifying local clustering dynamics to replace the original cluster diameter correlation. The newly formulated EMMS drag model can be used to avoid a negative cluster diameter to be involved in calculating interphase drag force in the overall fluidization regime. The improved EMMS drag law is incorporated into the Eulerian-Eulerian model to simulate gas-solid turbulent fluidized beds and cocurrent downer reactors, since they both were widely used in many industrial processes. By analyzing local hydrodynamics as well as the axial and radial heterogeneous distributions in the two kinds of fluidized beds, it is clarified that the simulation using the improved EMMS drag model shows a better agreement with the experimental data than the computation using the homogeneous drag law.

Two- and three-dimensional hydrodynamic modeling of a pseudo-2D turbulent fluidized bed with Geldart B particle

This study presents a comprehensive comparision between 2D and 3D hydrodynamic modeling of a pseudo-2D turbulent fluidized bed with Geldart B particle. Based on the Euler-Euler approach and the EMMS-based drag model, 2D/3D CFD models are established, their sensitivities to the restitution coefficient and the specularity coefficient are analyzed, and the 2D/3D hydrodynamic simulations are performed and compared. The simulation results show that 3D simulations are more sensitive to the restitution coefficient and the specularity coefficient than 2D simulations. At the beginning of fluidization process, 2D simulation predicts greater bubble size and higher bed expansion than 3D simulation; as a complete fluidization is achieved, 2D model exhibits higher solid concentrations in the middle transition and the upper dilute-phase regions; the fluidization process in the 2D simulation develops more quickly than that in the 3D computation. Both the 2D and 3D models could capture the global flow behavior in the bottom dense-phase region of the turbulent fluidized bed reasonably. In the middle and upper regions, however, the 2D model overestimates the solid concentration and particle velocity while the 3D simulation gives better hydrodynamic prediction. For the present pseudo-2D turbulent fluidized bed with Geldart B particle, the bottom dense-phase region resembles 2D flow and 2D simulations may be adequate; however, the middle and upper regions exhibit 3D flow and full 3D simulations are needed.

Circulating turbulent fluidized bed regime on flow regime diagram

Circulating turbulent fluidized bed (CTFB) regime was recently found under a low superficial gas velocity (Ug) system and contained the advantages of both a conventional turbulent fluidized bed, with an excellent mixing property between the gas and solid phases due to the high solid volume fraction (SVF), and a fast fluidized bed (FFB) or circulating fluidized bed (CFB) regime, which could be employed in a continuous mode. To observe the new flow regime, it was conducted in a plexiglas two-dimensional CFB reactor with a riser of 2.00 m height, 0.15 m width and 0.05 m depth. The interpretation of the SVF profiles along the riser height were used to determine the appearance the tubulent bed front where the turbulent bed was terminated. The identification of the CTFB was a profile without the appearance of a bed front in the riser. In conclusion, the development of a CTFB regime was caused by the turbulent bed expansion, which was affected by the magnitude of Ug. There were three important criteria to perform a CTFB: a solid recirculating rate of at least 300 kg/m2.s, the regulated Ug should be between a bubbling fluidized bed (Uc) and FFB (Utr), and lastly, the most important design parameter, the riser height must be shorter than the maximum turbulent bed expansion, which depends on the choice of solid particles. The functions that showed a good agreement with the experimental results were applied to obtain the correlations that later became the compulsory part of addressing the boundaries for each fluidized bed regime. The advantage of the new flow regime diagram was that it included the aspects of operation and design if CTFB was the desire.

CFD simulations of tapered bubbling/turbulent fluidized beds with/without gas distributor based on the structure-based drag model

The hydrodynamic characteristics of tapered bubbling/turbulent fluidized beds with/without gas distributor are simulated based on the structure-based drag model. Besides the only parameter of gas voidage (εg) contained in the original drag model, the heterogeneous drag index (Hd) for the tapered fluidized bed (TFB) also includes the parameter of gas velocity (ug) for the axial velocity gradient along bed height. Both the simulations of tapered bubbling and turbulent fluidized beds achieve more accurate predictions than the traditional drag models. There exist dilute center and dense annular regions with solids cycle flow structure in the TFB. The simulation also gives reasonable prediction for TFB without gas distributor, the variation of gas voidage profile reveals gas converging toward bed center with axial location rising.

A probabilistic heat transfer model for turbulent fluidized beds

A probabilistic heat transfer model was developed that allows for the gradual evolution of bed-to-surface heat transfer coefficients with increasing superficial gas velocity when bubbling and turbulent flow structures co-exist. The model is based on the packet renewal theory and the probability of particle packets for the specific hydrodynamic regime adjacent to the heat transfer surface. Assuming distinctive two-phase flow, dense packets at minimum fluidization voidage and voids almost free of particles, causes the heat transfer coefficients to be underestimated in the turbulent flow regime. By allowing for contributions of structures of intermediate voidage that reflect typical behavior in the turbulent flow regime of fluidization, heat transfer predictions are improved significantly. The resulting model is shown to give good agreement with experimental heat transfer data for fluidized beds of FCC and Alumina particles, changing smoothly from one flow regime to another in columns of different diameters and from the core of the bed to the wall.

Gas solid contacting measurements in a turbulent fluidized bed by oxidation of carbon monoxide

The conversion rate of the mass transfer controlled oxidation of CO over a Pt/?-alumina catalyst (d = 65 ?m) has been studied in a fluidized bed (internal diameter = 0.05 m) p operated close to and in the turbulent fluid bed regime. The objectives were to investigate the gas-solids contacting efficiency to evaluate the conversion data in terms of overall mass transfer coefficients and define the apparent contact efficiency. At high superficial gas velocities, the concept of formation of particle agglomerates and voids is more realistic than the two-phase model considering discrete bubbles and a dense phase. The two-phase model is not useless but has hardly any relation with the real flow pattern in the turbulent regime.

One-dimensional modeling of a turbulent fluidized bed for a sorbent-based CO2 capture process with solid–solid sensible heat exchange

A one-dimensional model for a turbulent fluidized bed is proposed and applied to the analysis of a CO2 capture process using a polyethylenimine (PEI)-silica sorbent. A formula for the contact efficiency was derived from the kinetic equation of the sorbent and the Kunii–Levenspiel core–shell model. Axial profiles of the gas and solid phase CO2 concentrations and bed temperature were computed, and the energy demand for CO2 capture was assessed for different degrees of sensible heat recovery.

Computational Fluid Dynamics Study of Heat Transfer in Turbulent Fluidized Beds

AbstractThe fluidization and heat transfer behaviors of a turbulent fluidized bed were investigated using computational fluid dynamics (CFD). The effects of inlet superficial velocity on heat transfer behaviors in a turbulent fluidized bed were analyzed and compared with those operated in other fluidization regimes. The effects of using particles belonging to different Geldart groups in a turbulent fluidized bed on fluidization and heat transfer behaviors were evaluated. For both fluidization regimes investigated, the solids temperature distribution during the heat transfer process became less uniform when the particle size was reduced.

An effective three-marker drag model via sub-grid modeling for turbulent fluidization

The effect of meso-scale structures on hydrodynamic predictions is not considered in the classically uniform drag models when the coarse grid is used. To address this issue, this study tries to develop an effective three-marker drag correlation via straightforward sub-grid modeling, which accounts for a parabolic spatial concentration distribution within a computational grid. The reliability and accuracy of the developed model is then assessed in detail. How the uniform drag inputs affect the derived sub-grid correction is quantified for the first time. Besides, a comprehensive comparison between several typical sub-grid models and present work is implemented. Results reveal a systematic dependence of our drag modification on the concentration gradient as an additional marker. Coarse-grid hydrodynamic validation shows that the developed model yields a fairly improved agreement with experiments under various operating conditions in a 3D turbulent fluidized bed. Furthermore, results demonstrate that the present model using different uniform drag inputs still can exhibit satisfactory performance. The developed model is able to resolve the heterogeneous flow behavior both cheaply and adequately, which is potentially applied for industrial reactor design and optimization.

Void Properties in Dense Bed of Cold-Flow Fluid Catalytic Cracking Regenerator

Fluid catalytic cracking (FCC) processes have been used widely in petroleum refineries. FCC regenerators play important roles for maintaining catalyst activity and supply the reaction heat. The regenerator efficiency is mainly connected to the hydrodynamics of the fluidized bed, because the gas and solid behaviors are very important factors in mass and heat transfer. The void properties, such as chord length, rising velocity, frequency, and fraction, have been determined in a large cold flow model (0.48 m-ID × 6.4 m-high) of the FCC regenerator, which was geometrically scaled down from a commercial FCC unit. The local void chord length, rising velocity, frequency, and fraction exhibit their maximum value along the radial direction of the bed. The cross-sectional mean void chord length, rising velocity, and fraction increase and the cross-sectional mean void frequency decreases with height in the bed. The variation of void properties in the FCC regenerator with turbulent fluidized bed exhibit similar trends to those in a bubbling fluidized bed. The void properties in the FCC regenerator have been correlated with the experimental parameter on the basis of bubbling bed concept. The predicted void velocities based on the correlations agreed well with the experimental data from present and previous studies. A modified bubbling fluidized bed model could describe the void properties in the regenerator operated in turbulent fluidized bed regime.