Industrial-scale Fluidized Bed Research Articles

Numerical Investigation into Gas-Solid Fluidized Bed Reactors for Activating the Iron-based Fischer-Tropsch Synthesis Catalyst

Abstract The present work conducted experimental and computational fluid dynamics simulation studies on the hydrodynamic characteristics of gas-solid fluidized bed reactors for activating the iron-based Fischer-Tropsch synthesis catalyst. In order to provide reliable guidance for the design and scale-up of the reactor, the present study applied experimental data from a pilot fluidized bed device to verify the accuracy of the drag force model used in the two-fluid model and further used the validated drag force model to simulate an industrial-scale fluidized bed activation reactor. It was found that the bubbling-EMMS drag model can accurately simulate the pilot-scale fluidized bed in the flow regimes ranging from bubbling to turbulent fluidization. Simulation studies on the industrial-scale fluidized bed activation reactor showed that the reactor was in a turbulent fluidization regime under the designed operating gas velocity, with high gas-solid dispersion efficiency, reasonable utilization of reactor space, and low gas-solid separation load.

Numerical investigation on the influence of immersed tube bundles on biomass gasification in industrial-scale dual fluidized bed gasifier

Immersed tubes play a pivotal role in enhancing fluidization quality within fluidized beds by effectively disrupting bubbles. However, the effect of such internals within a large-scale gasification apparatus remains unclear. Consequently, this study focuses on numerically simulating biomass steam gasification within an industrial-scale dual fluidized bed gasifier that features immersed tube bundles using the multiphase particle-in-cell method. In this method, particles with identical properties are grouped, and the interparticle collisions are model via the solid stress to improve computational efficiency. The numerical model has been well verified with experiment in terms of bed pressure drop, solid recirculation and chemical reactions. Some main conclusions are shown as follows. Immersed tubes significantly alter the motion of gas and particles in the gasifier. The presence of immersed tubes can ameliorate solid entrainment and avoid excess particles escaping from the gasifier. The asymmetrical structure of the gasifier results in asymmetrical distribution of gas temperature and gas species. Increasing the number of immersed tubes slightly enhances particle temperature near the loop seal outlet. The radial biomass dispersion coefficient and axial sand dispersion coefficient decrease about 40.9 % and 28.9 % when increasing the immersed tube number from three to nine. The results provide enhanced understanding of the role of immersed tubes in altering gas-particle interactions and fluidization dynamics in industrial-scale fluidized bed gasifier.

Feedstock recycling of cable plastic residue via steam cracking on an industrial-scale fluidized bed

The use of plastic materials in a circular way requires a technology that can treat any plastic waste and produce the same quality of product as the original. Cable plastic residue from metal recycling of electric wires is composed of cross-linked polyethene (XLPE) and PVC, which is a mixture that cannot be mechanically recycled today. Through thermochemical processes, polymer chains are broken into syngas and monomers, which can be further used in the chemical industry. However, feedstock recycling of such a mixture (XLPE, PVC) has been scarcely studied on an industrial scale. Here, the steam cracking of cable plastic was studied in an industrial fluidised bed, aiming to convert cable plastics into valuable products. Two process temperatures were tested: 730 °C and 800 °C. The results show that the products consist of 27–31 wt% ethylene and propylene, 5–16% wt.% other linear hydrocarbons, and more than 10 wt% benzene. Therefore, 40%–60% of the products are high-value chemicals that could be recovered via steam cracking of cable plastic.

On the Applicability of the Coarse Grained Coupled CFD-DEM Model to Predict the Heat Transfer During the Fluidized Bed Drying of Pharmaceutical Granules.

Fluidized bed dryer often used in the pharmaceutical industry for drying of wet granules. Coupled computational fluid dynamics (CFD) - discrete element method (DEM) is frequently used to model the drying process because of its ability to obtain the relevant information at the particle level. However, it becomes almost impossible to model the industrial scale fluidized bed dryer using the coupled CFD-DEM method because of the presence of large number of particles [Formula: see text]. To reduce the number of particles to be tracked in the simulation, coarse grained coupled CFD-DEM method was developed by researchers where a certain number of particles of the original system was represented by a relatively bigger particle in the coarse-grained system. The appropriate scaling of the particle-particle and particle-fluid interaction forces is necessary to make sure that the dynamics of the coarse-grained particles/parcels accurately represent the dynamics of the original particles. The coarse-graining of the drying process of pharmaceutical granules during fluidization needs systematic coarse-graining of the momentum, heat, and solvent vapor transfer process. A coarse grained coupled CFD-DEM method was used to model the momentum and heat transfer during the fluidization of pharmaceutical granules. It was shown that the heat transfer during the fluidization of large number of particles could be predicted by simulating a smaller number of bigger particles with appropriate scaling of particle-particle heat and momentum transfer, and particle-fluid heat and momentum transfer at significantly smaller computational time. This model can be further extended by including a coarse-grained moisture transport model in future.

Partitioning of a wide bubbling fluidized bed with vertical internals to improve local mixing and bed material circulation

Industrial scale fluidized bed reactors are characterized by limited mixing rates, either local or global, especially when using low-pressure drop gas distributors to reduce operational costs. In this work, partitioning of wide beds using vertical internals is proposed as an effective technique to improve local mixing in large reactors, i.e., mixing in specific zones of the bed. The effect of the vertical internals height on local solids mixing within partitions was experimentally evaluated in a pseudo-2D bed by analyzing the velocity and flow structure of the solids and the circulation time within individual partitions. In the presence of internals, global mixing, i.e., mixing between neighboring partitions and across the entire reactor, may be reduced as vertical internals compartmentalize the bed. Thus, the effect of the internals height on global mixing was also quantified while using bed materials with the same properties, but differing in color, in the different partitions, and analyzing the time evolution of the concentration of solids. Furthermore, the effect of internals on bubbles was also evaluated for different internal heights. It was found that internals with a height between the gulf-stream height and the fixed bed height promote the appearance of vortex pair structures in each partition of the wide bed. These structures substantially improve local mixing within each partition, while global mixing between partitions is practically unaffected by the presence of these short internals. • Pseudo-2D bed was used to characterize the effect of vertical partitions on mixing. • Solids with different colors were used to evaluate local and global mixing. • A model is developed to determine the exchange efficiency between partitions. • Vortex pair structures appear with internals above the gulf-stream height. • Vortex structures improve local mixing, global mixing remaining nearly unaffected.

Development of data-driven filtered drag model for industrial-scale fluidized beds

Simulations of large-scale gas-particle flows using coarse meshes and the filtered two-fluid model approach depend critically on the constitutive model that accounts for the effects of sub-grid scale inhomogeneous structures. In an earlier study (Jiang et al., 2019), we had demonstrated that an artificial neural network (ANN) model for drag correction developed from a small-scale systems did well in both a priori and a posteriori tests. In the present study, we first demonstrate through a cascading analysis that the extrapolation of the ANN model to large grid sizes works satisfactorily, and then performed fine-grid simulations for 20 additional combinations of gas and particle properties straddling the Geldart A-B transition. We identified the Reynolds number as a suitable additional marker to combine the results from all the different cases, and developed a general ANN model for drag correction that can be used for a range of gas and particle characteristics.

Modeling VOC adsorption in lab- and industrial-scale fluidized bed adsorbers: Effect of operating parameters and heel build-up

Scale-up and optimization of fluidized beds are challenging due to the difficulty in accounting for the interrelated effect of various phenomena, which are typically described by empirical and/or semi-empirical equations. In this study, a two-phase model was introduced to simulate the adsorption of VOCs on beaded activated carbon (BAC) in a lab-scale fluidized bed adsorber. The model assumes the presence of a bubble phase free from adsorbent particles, and an emulsion phase composed of the adsorbent particles and interstitial gas. The versatility of the proposed model was then evaluated using data from an industrial scale adsorber with different operating conditions, adsorbent properties, and bed geometry. The response of the model to the operating conditions (adsorbent feed rate, air flow rate and initial concentration) showed better agreement with the experimental lab-scale data when the emulsion gas in two-phase model was considered in plug flow than in perfectly-mixed flow (R2 = 0.96 compared to 0.91). To simulate the performance of BACs with different service lifetimes (degree of exhaustion as a result of heel developed inside their pores), the main characteristics of the BACs (pore diameter, porosity, and adsorption capacity) were first correlated to their apparent densities. The model could accurately predict the experimental lab-scale VOC concentrations in each stage (R2 = 0.92) as well as overall removal efficiencies (R2 = 0.99) for BACs ranging from virgin to fully-spent. Finally, the model was used to predict the performance of an industrial-scale fluidized bed adsorber for VOC removal at different operating conditions and apparent densities. Predicted and measured VOC removal efficiencies were in good agreement (R2 = 0.94). Although the model was verified for adsorption of VOCs on BAC, the modeling approach presented in this study could be used for describing adsorption in different adsorbate-adsorbent systems in multistage counter-current fluidized bed adsorbers.

Adsorption characteristics and mechanism of Pb(II) by agricultural waste-derived biochars produced from a pilot-scale pyrolysis system

The objective of this study was to investigate the feasibility of removing Pb2+ by pilot-scale fluidized bed biochar, and then to put forward an industrial-scale fluidized bed pyrolysis progress of cogeneration of biochar and high-temperature gas. Corn stalk biochars (CSBs) were prepared at 400–600 °C, in which the maximum Pb2+adsorption capacity (Qm) of CSB450 is 49.70 mg⋅g−1 at the optimal condition. Adsorption isotherms, kinetics, and thermodynamics were determined, and Pb2+-loaded biochar was analyzed by fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and scanning electron microscope with energy dispersive spectrometer (SEM-EDS). Ion exchange, complexation and mineral precipitation together contributed to Pb2+ adsorption on CSBs. For high-temperature CSBs with fewer oxygen functional groups (OFGs) and stronger aromatization, Pb2+ adsorption by ion exchange and functional group complexation was reduced. The mineral precipitationwas formed during the adsorption process. Using the pilot-scale fluidized bed in this study, the carbon yield per year would achieve 31.79 t, and about 1.58 t of Pb2+ would be adsorbed according to the adsorption capacity at the pyrolytic temperature of 450 °C.The results are beneficial to screen for effective biochar as a cost-effective industrial adsorbent to remove Pb2+ in contaminated water.

Fluidization characteristics of cohesive powders in vibrated fluidized bed drying at low vibration frequencies

Mechanical vibration is often applied in industrial scale fluidized bed dryers for food and pharmaceutical powders to overcome operational problems caused by the cohesiveness of the products. However, the understanding of the process regarding detailed modeling, apparatus design, up-scaling and process optimization is still incomplete.Almost all of the experimental research in the field of vibrating fluidized beds is conducted on a lab-scale. Within the current project, experiments are conducted in a pilot plant scale unit with a cross section of 250 × 500 mm2 and a total height of 3 m. The influence of several process parameters, such as gas velocity, vibration intensity, powder moisture content and bed mass, on the fluidization characteristics of whole milk powder is studied. In order to characterize the lower limit of fluidization from an operational point of view, the velocity of complete fluidization (ucf) is used and quantified. This is defined as the lowest superficial gas velocity at which the entire bed is fully fluidized. It could be observed that an increase in moisture content of the powder results in a significant increase of ucf. The introduction of mechanical vibration into the bed results in the reduction of ucf, the expansion of the bed and the reduction of the bubble volume fraction. This expansion of the suspension phase explains the increase in heat and mass transfer in vibrated fluidized beds, reported in the literature.Adding the effect of vibration intensity to well established correlations allows for the accurate prediction of gas hold-up and bed porosity of fluidized beds of cohesive whole milk powder under mechanical vibration at low vibration frequencies.

Development and verification of anisotropic drag closures for filtered Two Fluid Models

Over the past decade, filtered Two Fluid Models (fTFMs) have emerged as a promising approach for enabling fluidized bed simulations at industrially relevant scales. In these models, the filtered drag force is considered to be the most important quantity that requires closure. To date, such closures have typically relied on an isotropic interphase momentum exchange coefficient by applying a drag correction factor to the microscopic drag closures commonly used in resolved simulations. In the present study, both isotropic and anisotropic closures are developed for predicting the filtered interphase forces. The relative performance of these two approaches is then evaluated by means of an a priori assessment, considering data obtained from simulations in which all flow variables are resolved, which were also used for closure derivation. Also, an a posteriori assessment, which compares coarse grid simulation results to a benchmark resolved simulation of a bubbling fluidized bed, is presented. The primary conclusion from the present study is that it is essential to account for the anisotropy of the filtered momentum exchange coefficient. It is shown that this can be done by employing a drift velocity formulation of the filtered drag force and by considering a gravitational contribution that only acts in the vertical direction. Furthermore, it is found that for the large computational grid sizes that are typically required in industrial scale fluidized bed simulations, a closure for the meso-scale interphase force is essential. Finally, also for coarse grids, a non-linearity correction factor, which accounts for assumptions in deriving the drift velocity-based form of the filtered drag force, requires closure. The present study therefore highlights multiple avenues for improving drag closures used in fTFMs. Hence, these results may critically strengthen the predictive capabilities of fTFMs, as well as guide future modelling efforts.

Transient Computational Fluid Dynamics/Discrete Element Method Simulation of Gas–Solid Flow in a Spouted Bed and Its Validation by High-Speed Imaging Experiment

A coupled computational fluid dynamics (CFD)/discrete element method (DEM) is used to simulate the gas–solid two-phase flow in a laboratory-scale spouted fluidized bed. Transient experimental results in the spouted fluidized bed are obtained in a special test rig using the high-speed imaging technique. The computational domain of the quasi-three-dimensional (3D) spouted fluidized bed is simulated using the commercial CFD flow solver ANSYS-fluent. Hydrodynamic flow field is computed by solving the incompressible continuity and Navier–Stokes equations, while the motion of the solid particles is modeled by the Newtonian equations of motion. Thus, an Eulerian–Lagrangian approach is used to couple the hydrodynamics with the particle dynamics. The bed height, bubble shape, and static pressure are compared between the simulation and the experiment. At the initial stage of fluidization, the simulation results are in a very good agreement with the experimental results; the bed height and the bubble shape are almost identical. However, the bubble diameter and the height of the bed are slightly smaller than in the experimental measurements near the stage of bubble breakup. The simulation results with their experimental validation demonstrate that the CFD/DEM coupled method can be successfully used to simulate the transient gas–solid flow behavior in a fluidized bed which is not possible to simulate accurately using the granular approach of purely Euler simulation. This work should help in gaining deeper insight into the spouted fluidized bed behavior to determine best practices for further modeling and design of the industrial scale fluidized beds.

Modeling the hydrochlorination reaction in a laboratory-scale fluidized bed reactor

In the manufacturing process of high purity, electronics-grade silicon, the hydrochlorination reaction step for converting silicon tetrachloride (STC) to trichlorosilane (TCS) is usually carried out in a fluidized bed reactor. However, the design and operation of industrial-scale fluidized bed reactors for the conversion of STC to TCS is currently based on ad hoc rules of thumb and operator experience rather than on reaction engineering principles. In this paper, a fluidized bed model was developed by using the Kunii–Levenspiel fluidization framework. The predictive capabilities of this model were tested on laboratory-scale experimental data from the literature. It is shown that the modified Kunii–Levenspiel model accurately predicts the no catalyst addition and copper catalyst addition experiments of the hydrochlorination fluidized bed with an average percent difference of less than 6%. By making a suitable adjustment to catalytically active iron content, the model prediction is in excellent agreement with experimental data with hydrochloric acid in the feed stream. The modified Kunii–Levenspiel model is well suited for use in scale-up calculations for an industrial reactor as all the parameters are physically reasonable and can be predicted for larger reactors.

Numerical Study of a Bi-disperse Gas-solid Fluidized Bed Using an Eulerian and Lagrangian Hybrid Model

In this paper, we present a hybrid model for the numerical assessment of poly-disperse gas-solid fluidized beds. The main idea of such a modeling strategy is to use a combination of a Lagrangian discrete phase model (DPM) and a kinetic theory based TFM to take advantage of the benefits of those two different formulations. On the one hand, the local distribution of the different particle diameters, which is required for the gas-solid drag force, can be obtained by tracking statistically representative particle trajectories for each particle diameter class. On the other hand, the contribution from the inter-particle stresses, i.e. inter-particle collisions, can be deduced from the TFM solution. These then appear as additional body force in the force balance of the DPM. Note that in a first step we solely consider diameter averaged solids stresses since the drag force is at least on order of magnitude higher than the solids stresses in fluidized beds. Finally, the numerical model is applied to a fluidized bed of a bi-disperse mixture of glass particles (0.5mm and 2.5mm particles) and with a cross-section of 0.15 m x 0.02 m. The results are then analyzed with respect to experimental data of Puttinger et al [1]. This comparison demonstrates that the computed bed hydrodynamics is in fairly good agreement with the experiment. However, the results also suggest that sub-grid drag corrections [2–4] for poly-disperse fluidized beds are required to make the numerical investigation of industrial scale fluidized bed units accessible.

Application of simulation in determining suitable operating parameters for industrial scale fluidized bed dryer during drying of high impurity moist paddy

A systematic approach has been developed for selecting the suitable drying parameters to be used for drying of high moisture and high impurity paddy with an industrial fluidized bed paddy dryer (10–20 t h−1 capacity) based on targeted specific air flow rate and residence time during two typical paddy drying seasons. A mathematical model was developed by modifying an existing model and was simulated and validated with observed industrial drying data as well as data reported in the literature. Comparison between the observed and simulated results showed that the mathematical model is capable of predicting outlet paddy moisture content and air temperature well. Suitable operating parameters were determined for reducing any initial paddy moisture content (mc) down to 24–25% dry basis (db), the safe mc level after fluidized bed drying to maintain rice quality, to achieve maximum possible throughput capacity of the dryer with corresponding energy consumption. Based on these criteria, bed thickness at 10 cm, specific air flow rate of 0.05 kg kg−1 s−1 (for corresponding bed air velocity of 2.3 m s−1), air temperature of 150 °C and residence time of 1.0 min were found to be suitable drying conditions for reducing paddy mc from 30 to 24.30% (db) in one season while the maximum throughput capacity of 15.7 tonne per hour (t h−1) might be achieved. The specific electrical and thermal energy were 0.48 and 6.15 MJ kg−1 water evaporated, respectively. On the other hand, the dryer capacity was found to be limited to 7.4 t h−1 during drying paddy of higher initial mc (35% db). This approach might provide easy and comprehensive guidelines for selecting suitable sets of operating parameters for any industrial fluidized bed dryer at its possible maximum throughput capacity for drying of freshly harvested high moist paddy with a high level of impurities.

Production of amorphous rice husk ash in a 500 kW fluidized bed combustor

A 500kW rice husk-firing fluidized bed combustor was designed for obtaining low carbon and high silicon ash in the amorphous form. The influence of operating condition (temperature, fluidizing velocity) on combustion efficiency and rice husk ash quality was experimentally investigated. Furthermore, the quality of rice husk ash collected from inner bed, two-stage cyclones and bag-house was analyzed using XRD, TEM and SEM instruments, in terms of amorphous silicon dioxide and unburnt carbon content. Comparative study was carried out between the rice husk ash produced in an electronic oven and that produced in the above-mentioned fluidized bed combustor. The results show that quality of ash from the industrial-scale fluidized bed is even better than that from electronic oven. Temperature of 660–720°C and fluidization velocity of 1.0–1.2ms−1 appear to be the favoring parameters for producing high quality rice husk ash with amorphous structure of silicon dioxide content over 93.8wt.% and unburnt carbon context less than 4.5wt.%. The fluidized bed combustor also assures combustion efficiency ∼90% in all test runs. This research indicates amorphous structure of silicon dioxide may be economically promisingly produced from the self-designed rice husk combustor.

CFD modeling and simulation of industrial scale olefin polymerization fluidized bed reactors

A two-fluid model for the numerical simulation of industrial scale olefin polymerization fluidized bed reactors is presented. However, a fully resolved simulation of industrial scale reactor is still nearly unfeasible. We, therefore, use sub-grid models (Schneiderbauer and Pirker, 2014) for the interphase drag and the solids stresses to account for the effect of the small unresolved structures on large resolved scales when using coarse grids. The sub-grid correction for the drag force is modified to consider the wide particle size distribution of high density polyethylene (HDPE). Furthermore, the sub-grid modification for the solids stresses is adapted to include the rheological properties of the polymer. On the one hand, the presented model is validated in the case of the coarse grid simulation of lab-scale bubbling fluidized bed by comparing bed expansion, bubble size and bubble rise velocities with experimental data. On the other hand, the model is applied to the coarse grid simulation of an industrial scale fluidized bed – moving bed reactor assembly. The numerical results demonstrate that our model reveals fairly good agreement with experimental data of average bed voidage, bubble diameters and bubble rise velocities. Finally, the impact of a barrier gas injection is studied, which is aimed to separate the fluidization gas from the gas in the moving bed reactor.

CFD simulations of gas–solid flow in an industrial-scale circulating fluidized bed furnace using subgrid-scale drag models

Mesoscale flow structures such as clusters and streamers of particles are characteristic features of gas–solid flow in fluidized beds. Numerical simulations of gas–solid flows for industrial-scale fluidized beds are often performed using the Eulerian description of phases. An accurate prediction of this type of flow structure using the Eulerian modeling approach requires a sufficiently fine mesh resolution. Because of the long computational time required when using fine meshes, simulations of industrial-sized units are usually conducted using coarse meshes, which cannot resolve the mesoscale flow structures. This leads to an overestimation of the gas–solid drag force and a false prediction of the flow field. For these cases, a correction must be formulated for the gas–solid drag. We have simulated a large-scale circulating fluidized bed furnace using different gas–solid drag models and compared the model results with measurements.

Multiscale Simulation of Agglomerate Breakage in Fluidized Beds

In this contribution a multiscale simulation strategy is proposed which is able to simulate an industrial scale fluidized bed spray agglomeration process considering the breakage of agglomerates. A set of novel simulation approaches and hierarchically distributed models were developed and implemented into the multiscale simulation environment for solids processes. The population balance model (PBM) was used for the simulation of the global production process on the macroscale. On the microscale the coupling between discrete element method (DEM) and the computational fluid dynamics (CFD) system was employed to calculate the particle dynamics in the granulator. The material-based parameters for the PBM, such as breakage probability and breakage function, were derived from the process description on the lowest hierarchical scale, where the agglomerate was described as a system of primary particles bonded by a solid binder.

Dehydrogenation of propane–isobutane mixture in a fluidized bed reactor over Cr 2O 3/Al 2O 3 catalyst: Experimental studies and mathematical modelling

An influence of propane addition to the inlet feed on the performance of industrial fluid bed reactor for isobutane dehydrogenation was experimentally and numerically studied. Experiments on dehydrogenation of propane–isobutane mixture in a pilot fluidized and in a lab fixed bed reactors were performed over Cr 2O 3/Al 2O 3 industrial catalyst. Adding C 3H 8 to the reactor inlet was found to increase experimental conversion of C 3–C 4 mixture and the total process selectivity to olefins. Results of the mathematical modelling of the industrial-scale fluidized bed reactor show some benefits of C 3H 8 addition. Selectivity to i-C 4H 8 was found to be high enough and grows slightly from 86 to 89% on increasing inlet C 3H 8 fraction from 0 to 60 wt%. Inlet concentrations of C 3H 8 up to 20 wt% lead to the apparent selectivity to C 3H 6 exceeding 100%. Coke yield rises slowly allowing safe industrial fluid bed reactor operation.

Steam hydration–reactivation of FBC ashes for enhanced in situ desulphurization

Bed and fly ashes originating from industrial-scale fluidized bed combustors (FBCs) were steam hydrated to produce sorbents suitable for further in situ desulphurization. Samples of the hydrated ash were characterized by X-ray diffraction analysis, scanning electron microscopy and porosimetry. Bed ashes were hydrated in a pressure bomb for 30 and 60 min at 200 °C and 250 °C. Fly ash was hydrated in an electrically heated tubular reactor for 10 and 60 min at 200 °C and 300 °C. The results were interpreted by considering the hydration process and the related development of accessible porosity suitable for resulphation. The performance of the reactivated bed ash as sulphur sorbent improved with a decrease of both the hydration temperature and time. For reactivated fly ash, more favourable porosimetric features were observed at longer treatment times and lower hydration temperatures. Finally, it was shown that an ashing treatment (at 850 °C for 20 min) promoted a speeding up of the hydration process and an increase in the accessible porosity.