Bed Hydrodynamics Research Articles

Particle-scale study of heat and mass transfer in a bubbling fluidised bed

In this study, the multiphase flow and thermochemical behaviours of char combustion in a bubbling fluidised bed (BFB) are simulated using CFD-DEM approach featuring particle size polydispersity and thermochemical sub-models. The model is first validated in terms of mixing index, particle temperature, and particle diameter. Then, it is applied to examine the contribution of each heat transfer mode and study particle-scale behaviours of char and sand comprehensively. The results show that the polydisperse drag model should be used to accurately reproduce bed hydrodynamics when simulating a BFB system with polydisperse particles. Under the simulation conditions, the particle-averaged heat fluxes to char particles through convection, conduction, radiation, and char reaction take 9.79%, 0.82%, 40.44%, and 48.95%, respectively; the particle-averaged heat fluxes to sand particles through convection, conduction, and radiation take 30.28%, 1.0%, and 68.72%, respectively. For reactive char particles, the radiation and heat of reaction are dominated, while for inert sand particles, the radiation and convection are dominated; and for both particle species, the conduction is negligible. Axial dispersion coefficient is one order of magnitude larger than the horizontal one, demonstrating the dominant role of the introduced gas flow in determining bed hydrodynamics.

Recent Advances in Fluidized Bed Hydrodynamics and Transport Phenomena—Progress and Understanding

Fluidization technology has found widespread applications for a variety of chemical and physical transformations since its introduction in the first half of the 1900s [...]

NSERC discovery grants and a tribute to Leo A. Behie

AbstractThe scope of chemical engineering continues to evolve and has become ever more multifaceted. It encompasses traditional fields like polymers, thermodynamics, transport processes, control, and environmental science, but has expanded to include biotechnology, biomedical, food technology, materials, and numerical techniques like artificial neural networks and discrete element methods. In fact, the Natural Sciences and Engineering Research Council of Canada (NSERC) combines chemical engineering and materials in the same evaluation group to award competitive funding for the Discovery Grant. Prof. Leo Behie's career reflected the change in the scope of chemical engineering. In the 1970s and early 1980s, Prof. Behie's research included hydrodynamics of fluidized beds and deriving the kinetics of the Claus reaction. In the late 1980s and into the 1990s, he contributed to biotechnology. In the 2000s and 2010s, his areas of expertise related to biomedical engineering comprising stem cell research and a focus on Parkinson's and Huntington's diseases. The common denominator for most of his research was reactor technology. In 2014, Prof. Behie quipped that chemical engineering was dead and many North American departments shared this tenet and added various scientific fields to recognize the emerging synergies. Here, we show that chemical engineering is a vibrant and growing field. We demonstrate in what way it is changing and describe how NSERC evaluates research dossiers for the Discovery Grants. We describe the relationship between the hindex and the number of citations and confirm that bibliometrics has only a minor role in the Discovery Grant award.

Numerical analysis of hydrodynamics and thermochemical property of biomass gasification in a pilot-scale circulating fluidized bed

This study presents a three-dimensional reactive multiphase particle-in-cell (MP-PIC) model to simulate the hydrodynamics and thermochemical property of the biomass gasification process in a pilot-scale circulating fluidized bed gasifier. The bed hydrodynamics and thermochemical characteristics are investigated, together with discussing the effects of operating conditions on the species distribution, lower heating value (LHV), and combustible gas concentration (CGC). The results show that solid particles present oscillation behaviors for the time elapsed at the turbulent fluidization regime. Solid flux and rising velocity have peaks of about 249 kg/(m2·s) and 6.2 m/s in the core region of the bed, respectively. Increasing the gasification temperature reduces the CGC from 15.85% to 15.16%. However, only the concentration of H2 increases with the enlargement of the steam biomass ratio. Comparatively, the equivalence ratio (ER) has the most direct impact on gasification efficiency, and the increase of the ER adversely affects the LHV and CGC. The effect of particle diameter on the LHV, the magnitude of which can cover a range from 3.87% to 3.98%, is negligible compared with the other operational parameters, at least for the specific cases in the present investigation.

Particle-scale study of gas-solid flows in a bubbling fluidised bed: Effect of drag force and collision models

Inter-particle and inter-phase interactions of gas-solid flows dominate macro-scale bed hydrodynamics in fluidised bed systems. In this work, typical fluidisation behaviours are presented. The effects of different drag models (i.e., Di Felice drag model, Gidaspow drag model) and collision models (i.e., hard-sphere model, soft-sphere model) on gas-solid behaviours in a bubbling fluidised bed are comprehensively compared. Besides, the influence of critical operating conditions (i.e., superficial gas velocity) is discussed. The simulation results show that Di Felice drag model shows better agreement with the experimental data than the Gidaspow drag model. Both the hard-sphere model and soft-sphere model can well predict particle velocity profiles. Larger superficial gas velocity and hard-sphere model respectively cause more vigorous inter-phase momentum exchange and more frequent collision events, indicating the more intense bubble evolution and the resulting fluctuations of void fraction. The rotational velocity obtained from the soft-sphere simulation is larger than that from the hard-sphere one, but the rotational velocity fluctuation is much weaker.

Investigation of cold flow hydrodynamics in a dual fluidized bed for gasification of high-ash coal

The hydrodynamics of a compact dual fluidized bed (DFB) gasifier, consisting of a short riser and a bubbling fluidized bed (BFB), designed for high-ash coal, has been investigated. The effects of fluidization numbers from various aeration ports, and secondary aeration (SA) in the riser have been examined on solids circulation rate and solids holdup in the DFB. The performance of solids recycle systems has been investigated for on-bed and in-bed solids discharge from the BFB to the riser resulting in a low- and high- bed density operation in the riser. For high superficial gas velocity and high-density operation of the riser, an intense solids backmixing is noticed near the riser-exit, which decreases when SA is used. SA is found to be more effective in maintaining a proper balance between solids holdup and solids circulation rate when the bottom riser bed is operated in the fast fluidized bed regime for in-bed solids discharge.

ПСЕВДООЖИЖЕНИЕ СЕМЯН ПРОСА И ГОРОХА

One of the convective drying methods used to intensify the process of drying granular materials is the use of a device with a fl uidized bed. The essential advantages of fl uidized bed dryers are the uniform drying of all the particles in the bed and the ease of loading and unloading the material. The study goal was to conduct an experimental study of the hydrodynamics of the fl uidized bed of millet and pea seeds and the determination of the fi rst critical velocity (the rate of the beginning of fl uidization) based on these data. The authors took small (millet) and large (pea) seeds for the experiment. Based on the experimental data obtained, they constructed pseudo-liquefaction curves – ΔP = f (v) for millet and peas to determine the fi rst critical fl uidization rates (0.73 m/s for millet and 1.68 m/s for peas). The experimentally found values of the fi rst critical fl uidization rate were compared with the values calculated by the equations of O.M. Todes, Wen and Yu, and Grace. The study showed that all the equations provide an acceptable accuracy of calculations, but the accuracy of using the equation of O.M. Todes is somewhat higher. Therefore it can be primarily recommended for engineering analysis. The fi rst critical fl uidization rates, calculated according to the formula of O.M. Todes, were, accordingly: 0.69 m/s for millet and 1.60 m/s for pea. The calculation and experimental data on the fi rst critical fl uidization rates for millet and peas agree satisfactorily (the relative error for millet is 5.5%, for peas – 4.8%). The suffi ciently high accuracy of the fi rst critical fl uidization rates is explained by the correct geometric shape of the studied seeds, which is close to spherical. Therefore, the fi rst critical fl uidization speed of the studied seed material can be determined using the formula of O.M. Todes.

Study of the effect of particle diameters on two-phase (air-solid) fluidization bed hydrodynamics

Computational fluid dynamics (CFD) is executed through a model of gas-particle two phase based on fluidized bed. The influences of particle size and inlet velocity on the fluidization process are discussed. The results showed that at the time of the simulation, the height of solid particles inside the pipe increased as the air velocity increased, the pressure drop for the three particle sizes during time is near and constant, Also showed the volume fraction increases as the velocity of the inlet air increases. As with the voidage, it decreases and then increases slightly as the velocity increases, but with a large diameter is the opposite. The volume fraction decreases with increasing bed height. Finally, the model can predict the shape of radial-direction particle patterns.

Numerical simulation of commercial scale autothermal chemical looping reforming and bi-reforming for syngas production

Autothermal Chemical Looping Reforming (a-CLR) is an emerging technology that facilitates CO2 capture and minimizes energy losses in syngas production. The dual-fluidized bed process uses a bubbling fuel reactor (FR) and a riser air reactor (AR). A 1-D multiscale model was developed that couples fluidized bed hydrodynamics with intrinsic reaction kinetics, including catalyst deactivation by oxidation and coke formation. An a-CLR unit with a capacity equivalent to 50 conventional reformer tubes was considered to demonstrate feasibility. The effects of the catalyst activity, main operation conditions and CO2 co-feeding on the performance were then analyzed. The results confirm a-CLR is feasible with realistic reactors dimensions and solids circulation rate. To avoid the risk of sintering and catalyst deactivation, the oxygen carrier should only be slightly oxidized in the AR. Autothermal operation then requires an oxygen-to-CH4 feed ratio of around 1.2. The high catalyst-to-gas feed ratio in the FR and bubble-emulsion phase mass transfer limitations result in a low sensitivity of the methane conversion to the catalyst activity. A low H2O-to-CH4 feed ratio introduces a risk for coke formation in the bottom of the FR, but part of the feed methane is fully oxidized, producing H2O and CO2 in the emulsion phase and mitigating this risk. Co-feeding CO2 with CH4 and H2O is seen to allow adjusting the H2/CO-ratio of the syngas, but increases the risk of coke formation. Finally, heat recovery from the flue gas and syngas from both reactors by preheating the feed gases further increases the energy efficiency.

In-situ quantification of the magnitude of interparticle forces and its temperature variation in a gas-solid fluidized bed

Interparticle forces (IPFs) are among the important parameters that can influence the hydrodynamics of a gas-solid fluidized bed and, hence, its overall performance. They are particularly critical at elevated temperatures, where their magnitude is typically greater than that at ambient conditions. Two simple and indirect approaches were developed in this study to quantify the magnitude of IPFs and its variation with temperature in a gas-solid fluidized bed. Quantification of thermally induced IPFs, i.e., the difference between the level of IPFs at a given temperature and that at ambient conditions, was attempted in the first approach. It was shown that the variation of IPFs with temperature is related to the difference between the experimental minimum fluidization velocity (Umf,exp) and that predicted by a correlation (Umf,pred) based purely on hydrodynamic principles. The magnitude of thermally induced IPFs at a given temperature was then quantified with the help of the Ergun equation and the difference between Umf,exp and Umf,pred. The second approach was developed for the quantification of absolute values of IPFs based on the difference between the loose bulk voidage (ε0) and the minimum fluidization voidage (εmf). It was shown that the magnitude of IPFs directly affects the ratio of εmf/ε0, which we called “dynamic Hausner ratio”. A phenomenological model was, hence, proposed where the magnitude of IPFs is expressed as a function of the dynamic Hausner ratio. The performances of these approaches were verified and qualitatively validated, while their limitations and opportunities were also discussed.

Influence of frictional packing limit on hydrodynamics and performance of gas-solid fluidized beds

The influence of frictional packing limit (FPL) on prediction of hydrodynamics and performance of fluidized bed reactors was studied. Dense gas-solid flows in non-reactive (under isothermal cold and at elevated temperatures) and reactive atmospheres (fluidized bed gasifier) were simulated using Eulerian-Eulerian methodology considering a range of values for FPL. Simulations under cold flow conditions were conducted to establish a range of FPL values that provides physically realistic predictions. It is noticed that bed pressure drop increases with increasing value of FPL when superficial gas velocity (U) is less than or equal to the minimum fluidization velocity. For larger values of U, predicted pressure drop is unaffected by the choice of value of FPL. However, in these cases, the distribution of particles, their velocities and bubbling behavior are significantly affected by FPL. Effect of FPL at elevated temperatures is similar to the one observed at cold flow conditions. It is further noticed that FPL not only affects the predictions on bed hydrodynamics but also has profound influence on reactive flow characteristics such as bed temperature and product gas composition. Sensitivity analysis under cold flow conditions could reveal better predictions when the ratio of FPL to close packing limit is chosen between 0.9 and 0.97.

Experimental and numerical investigations on the hydrodynamics of gas-solid fluidized bed with an inclined agitator

The effect of inclined agitation on the fluidization quality of a gas-solid fluidized bed is experimentally and numerically investigated. The pressure characteristics are characterized by the standard deviation of pressure drop and power spectra. The particle velocity, solid volume fraction and granular temperature distributions are investigated by numerical modeling. It is found that inclined agitation enhances the gross circulation of solid particles and enlarges the affect region of the agitator, improving the gas-solid fluidization quality. The increase in the agitation speed leads to higher frequency of collisions and more activity of particles, resulting in decreases of bubble sizes and a more homogeneous solid volume fraction distribution. Wide-blade agitator can also improve the fluidization performance.

Effect of particle size and shape on liquid–solid fluidization in a HydroFloat cell

The separation of coarse particles by flotation can be maximized by the support of fluidized bed hydrodynamics. Here, we investigated the effect of particle size and shape on the liquid-solid fluidization in a HydroFloat cell that was developed for the special separation process using CFD results and experimental data. Specifically, a 3D CFD model was developed to study the liquid and aspherical quartz particle flow behaviors in a fluidized bed in the cell. The Eulerian–Eulerian formulation with the RNG k-ε turbulence model was applied for the liquid phase, while the kinetic theory of granular flow was utilized for the particle phase. The Gidaspow with Haider and Levenspiel equations were used for the drag force coupling between the liquid and the aspherical particles. The CFD results agreed with the experimental data for the bed voidage and pressure drop. The CFD model was further applied to investigate the effect of particle size (dS) and particle shape factor (ψ) on the bed expansion, pressure drop, and voidage of the fluidization for Re (particle Reynolds number) < 11.6. The Richardson-Zaki equation was modified to predict the voidage, within 12% of relative errors. The prediction was further improved by our semi-empirical correlation that reduced the errors to 8%. The particle size and shape were also found to significantly affect the time-averaged radial distribution of the axial velocity and volume fraction of the particles, granular temperature, dissipation rate, and viscosity of turbulence. The outcomes of this paper are useful for optimizing the operation of the HydroFloat cell.

Effect of specularity co-efficient on bed hydrodynamics and heat transfer characteristics of a PCFB riser

Abstract The fluidized bed gasification and combustion process are getting attention from researchers due to its advantages such as low NOx emission and power production from waste materials. Laboratory scale study of fluidized bed gasifier and combustor is not possible due to its high instalment cost. The heat supply and temperature are also of higher magnitude. A simulation study of large-scale fluidized bed setup is a potential solution to such a problem. Hence this numerical study directs the multiphase simulation of 3D fluidized bed riser using ANSYS Fluent. The riser of 2000 mm in height and 54 mm of outer diameter is considered for numerical simulation. Air with a superficial velocity of 6 m/s and sand with 2300 kg/m3 and a mean particle size of 307 μm are considered as primary and secondary phase material for the numerical simulation. The effect of the specularity co-efficient volume fraction of air has been calculated numerically. Along with the riser height, the temperature contour indicates the distribution of temperature. When the air pressure at the inlet of the riser rises, the volume fraction of air is found to be declining. The solid mass flux increases with pressure at all values of the specularity coefficient. The maximum suspension density is found to be 892 kg/m3 for the operating pressure of 5 bar. The maximum solid mass flux is observed at 5 bar pressure for this simulation work.

Prediction and optimization of syngas production from a kinetic-based biomass gasification process model

This work presents a robust method for prediction and optimization of syngas production by taking advantage of the established kinetic-based process model and data analysis techniques for constructing a surrogate model. Compared with the widely-used equilibrium model, herein, we first develop a process model of biomass gasification by incorporating updated biomass reaction kinetics and dense bed hydrodynamics. A parallel comparison of model predictions and experimental results illustrates a good agreement under a wide range of operating conditions. The sensitivity results indicate that the initial volatile composition from the pyrolysis step is crucial for final gas product distribution and the gasification temperature is most sensitive to the syngas composition and yield, followed by the equivalence ratio (ER), steam-to-biomass ratio (S/B ratio), and biomass moisture content (MC). More importantly, a multivariable analysis is further carried out by running the model for 864 combinations of four input parameters. A surrogate model is established for predicting and optimizing the syngas yield under various operating conditions. A selective set of response surfaces illustrates the mutual effects of two parameters simultaneously and reveals optimal syngas yields ranging from 61.4 vol% to 78.5 vol% on a dry N2-free basis. The global optimization model demonstrates a maximum syngas yield of 78.6 vol% at a temperature of 900 °C, ER of 0.23, S/B ratio of 0.21, and MC of 30 wt%.

Hydrodynamics and heat transfer in a fluidized bed with liquid spray: Particle color-change based measurement and modelling

Fluidized beds with the liquid spray have been widely applied due to efficient heat transfer ability. In this work, the liquid-containing particles and the liquid dispersion pathway are visualized by a unique reversible thermochromic material (RTM) tracking method. The effects of evaporative liquid considering different nozzle modes on bubble density, particle motion and heat transfer are investigated combining PIV and DIA. The results indicate that the liquid injection decreases particle fluctuation and promotes bubble generation. A stable liquid-containing zone is observed under the top spray mode, whereas liquid disperses rapidly under the side-wall spray mode. A multi-zone heat transfer model (MHTM) is then developed based on the bubble growth and inter-layer exchange models. Experimental and calculated temperature profiles indicate that the side-wall spray mode leads to larger temperature differences. The methods proposed can be applied for the analysis of the complex multiphase system and optimization of the nozzle configuration.

Modelling of a fluidized bed carbonator reactor for post-combustion CO2 capture considering bed hydrodynamics and sorbent characteristics

This manuscript presents a general model for the fluidized bed carbonator reactor of the Calcium Looping (CaL) process based on bed hydrodynamics, kinetics and sorbent properties. The model proposed relies on the Kunii–Levenspiel theory for circulating fluidized beds in the fast fluidization regime and considers recent findings on the CO2 capture properties of CaO, including the effects of the diffusion-controlled carbonation stage and CaO modification by means of additives. By performing a sensitivity analysis, it is found that the solids inventory in the carbonator and superficial gas velocity are the main operational and hydrodynamic factors affecting the CO2 capture efficiency. Decay constant of the fraction of solids in the lean region and fraction of solids in the dense region are also identified as two important model parameters. Model predictions are shown to agree with experimental results retrieved from pilot-scale tests. The main novelty of the model is the consideration of a CaO/Al2O3 composite either as main CO2 sorbent used in the carbonator or as a make-up flow fed into the calciner. Using CaO/Al2O3 as the main CO2 sorbent leads to a 14.17% loss of capture efficiency after 100 cycles compared to 72.81% when using limestone derived CaO. It is demonstrated that feeding a very low ratio (~0.1) of the CaO/Al2O3 composite or a rather high ratio (~5) of limestone as makeup flow, leads to a significant improvement of the capture efficiency. The results obtained may be relevant for optimizing CaL operation conditions to be used in power plants.

CFD-DEM bidirectional coupling simulation and experimental investigation of particle ejections and energy conversion in a spouted bed

Fluidized bed has gained popularity in the renewable energy industry due to its superior thermochemical conversion efficiency. Investigating the flow patterns inside the fluidized bed is of great importance to obtain detailed knowledge of the energy conversion of biofuel and to further optimize the combustion performance. In this paper, the effect of different inlet gas flow rates on bed hydrodynamics parameters such as bed height, equivalent bubble diameter, and particle ejection were studied by the high-speed photography technology and numerical simulation methods. The results show that changes in the particle bed height and bubble area can be accurately predicted by numerical simulation. The mechanism of particle ejection is varied under different inlet flow conditions. At the low inlet flow rate of 300 L/min, the particles ejected into the freeboard region are only from the bursting bubbles. As the inlet flow rate increases to 400 L/min, the ejected particles are not only from the bursting bubble but also from the bubble wakes. Particle ejection is mainly caused by the bubble expansion rate. These findings could serve as the reference for the optimal design of fluidized beds to achieve higher combustion efficiency.

Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed

The processing of fine and ultrafine particles using a fluidized bed is challenging in view of their unpredictable hydrodynamic behavior due to interparticle forces. The use of assisted fluidization techniques in such cases can be effective in improving the bed hydrodynamics. This work investigates the dynamics of pulsed fluidized bed of ultrafine nanosilica subjected to square-wave flow pulsations. The pulse duration used in this study is sufficient to allow the complete collapse of the pulsed fluidized bed between two consecutive flow pulsations. The proposed pulsation strategy is carefully implemented using electronic mass flow controllers with the help of analog output signals from data acquisition system. Given that the different regions of the fluidized bed exhibit varying dynamics, which together contribute to overall bed dynamics, the bed transients in the upper, central, and lower regions of the fluidized bed are monitored using several sensitive pressure transducers located along the height of the bed. The effect of the flow pulsation on the hydrodynamics of the fluidized bed is rigorously characterized. A significant reduction in the minimum fluidization velocity was obtained and an increase in the bed homogeneity was observed due to flow pulsations. The frequency domain analysis of the signals clearly delineated the frequency of the various events occurring during the fluidization.

Gas flow distribution and solid dynamics in a thin rectangular pressurized fluidized bed using CFD-DEM simulation

Operating a fluidized bed at elevated pressure is typically used to intensify processes. The effects of pressure on bubble behavior have been extensively studied; however, the gas flow distribution and particle dynamics are much less studied due to difficulties in measurement. In this work, CFD-DEM simulations are conducted to study hydrodynamics in a thin rectangular fluidized bed at pressures increased from 1 bar to 10 bar. The important findings can be summarized as follows. With an increase of pressure, the bubble size and pressure fluctuations decrease. With increasing pressure, the percentage of visible flow of the rising bubbles increases, while the percentage of the gas throughflow through bubbles decreases. At elevated pressures, the solid flow flux and mixing rate are increased. Finally, the reasons of the influence of pressure on fluidization characteristics are explored in view of the increased inertia force on particles and the increased differential pressure on bubbles.