Particle Recirculation Research Articles

Control of spout deflection in dry and wet spouted beds using vibration

Spout deflection is a common instability phenomenon observed in spouted beds which decreases control over the spouted bed and worsens particle recirculation efficiency. Prior work has mitigated spout deflection using electromagnetic fields, inlet gas treatment, and baffles. Here, we demonstrate experimentally that spout deflection can be mitigated through the use of vertical vibration, which can be applied in circumstances which prior methods may not be possible. Spout deflection was mitigated for cases of non-uniform gas flow in a dry spouted bed and particle agglomeration in a wet spouted bed. Furthermore, the particle recirculation rate was found to increase significantly with increasing vibration strength in a wet spouted bed. Thus, we show that vertical vibration can significantly enhance the spout stability in both dry and wet spouted beds, without compromising recirculation rate.

Experiments and modelling of biomass pulverisation in swirling and non-swirling bluff body-stabilised turbulent annular flows

Deeper insights into particle loading have practical significance in applications particularly when this affects fuel-oxidiser mixing and flame stabilisation, as occurs in pulverised fuel (biomass) combustion systems. The underlying particle flow and dispersion characteristics not only play a crucial role in controlling overall combustion performance but can also impact emissions. A fundamental understanding of particle flow dynamics and dispersion behaviour for raw pulverised biomass, under non-swirling and swirling conditions need further systematic investigation to better understand the interplay between swirling and non-swirling bluff-body stabilised recirculation zones and particles emitted from a centralised jet. This work uses Particle Image Velocimetry (PIV) with three-dimensional multi-phase simulations based on the Discrete Phase Model (DPM) and Reynolds Stress Model (RSM) to investigate the flow and dispersion characteristics in confined flows typical of many practical combustors. Raw pulverised biomass-laden is introduced through a central turbulent jet (Rej = 4500 and 7800) and also subjected to turbulent annular flows (Res = 35,500), both non-swirling (S = 0) and swirling conditions (S = 0.3). Simulations are first validated against Constant Temperature Anemometry (CTA) resolved inlet boundary conditions and flow field PIV data under similar conditions.Results show that when a pulverised biomass-laden central jet interacts with a surrounding turbulent annular flow (non-swirling, swirling) the presence of a bluff-body based recirculating zone (BB-RZ) leads to biomass particle entrainment (pick up) and their recirculation over the bluff-body before being dispersed further downstream. Under non-swirling conditions, a significant 35% decrease in the mean particle axial velocity is measured coupled with an even more substantive 177% increase in turbulent fluctuation along the centreline. These findings are indicative of intense upstream (x/D ≈ 0.64) turbulent mixing and more intense particle dispersion into a BB-RZ. For swirling annular flow conditions near the end of the BB-RZ, the interaction between a biomass-laden central jet and the annular flow is comparatively weaker in the upstream region relative to non-swirling conditions. However, swirl significantly enhances downstream particle dispersion and lateral spread as reflected by a 254% hike in the mean particle radial velocity. Numerical predictions show that for the same particle loading ratio, but different (higher and lower) Reynolds number of a central particle-laden jet (i.e., the carrier gas), the conditions at relatively lower central jet Reynolds number allow better particle recirculation in BB-RZ as well as enhancing downstream lateral particle dispersion and entrainment, compared to a higher Reynolds number in the central jet. Outcomes from this investigation may have implications on the design and operation of pulverised (solid) fuel combustors if operated on renewably sourced biomass rather than traditional fossil fuels.

A solid-liquid mixing reactor based on swirling flow technology

Solid-liquid mixing is often applied in the chemical industry to keep solids in suspension for, amongst others, crystallization, adsorption, heterogeneous catalysis, etc. The most widely used technology to obtain a good degree of mixing is the mechanically stirred tank reactor. However, this technology is limited to small reactor volumes in case of high pressure/temperature processes due to sealing issues. An alternative for solid-liquid mixing is the application of swirling flow. This work studies the mixing capacities of a specially designed reactor geometry, the Swirling Flow Reactor (SFR). Computational Fluid Dynamics (CFD) is used to simulate the flow field and particle distribution in the SFR, where the RNG k−ϵ model and the Eulerian-Eulerian (E-E) multi-phase approach with a Kinetic Theory of Granular Flow (KTGF) model are applied for their proven effectiveness in the study of dense solid-liquid mixing problems. The effectiveness of mixing in the SFR is assessed by the axial particle distribution and homogeneity H. The mechanism of particle recirculation in the SFR is also identified, in which a Coanda jet flow (CoJF) formed by a specially designed inlet nozzle plays an important role in washing the reactor bottom surface. Additionally, Spectral Proper Orthogonal Decomposition (SPOD) is used to identify and reconstruct the coherent structures in the swirling flow. The results indicate that precessing coherent flow structures are present, which include the dominant double layer double helical vortex core and its second order harmonic, the double layer quadruple helical vortex core.

Numerical investigation and dimensionless erosion laws of solid particle erosion in plugged tees

In the process of oil and gas production, particle erosion is an important cause of pipe fitting failure. It is generally believed that the plugged tees have the potential to mitigate erosion. A computational fluid dynamics (CFD)-based erosion prediction numerical model is employed to investigate the gas-solid flow and erosion behavior in the plugged tees. The trajectories of particles released from different positions on the inlet cross-section are tracked. The phenomenon of particle recirculation is found in the plugged pipe through particle trajectory analysis. Combined with particle impact characteristics, the erosion morphology at the end region is analyzed. Erosion mitigation performance of the plugged tees is evaluated by comparison with elbows. Additionally, four important dimensionless groups are derived through dimensional analysis. Self-similarity behavior between these groups is extracted to establish two new dimensionless numbers that are suitable for the erosion prediction of the end region and the corner region, respectively.

Real-time diagnosis of circulation stability for CFB combustion optimization using a novel image trajectory method

A novel particle image trajectory method was developed to determine the velocity of particles entering a cyclone particle separator in a typical circulating fluidized bed (CFB) incinerator burning biomass and industrial waste. Based on particle velocity, the mass recirculation ratio was calculated to characterize the fluidization stability, and its key factors were quantitatively discussed. The results indicated that the proposed method can determine the particle velocity and recirculation ratio rapidly and accurately, and the relative errors of velocity calculation were 4.07 % and 0.04 %. The time lag between the recirculation ratio and the cyclone negative pressure, furnace temperature, and secondary air was 20–70s. We observed that the cyclone negative pressure was most likely related to circulation stability after eliminating the time lag. The longest time lag was between the recirculation ratio and boiler load, which was 230 s. The correlation between the recirculation ratio and boiler load improved (0.220) after the time lag was eliminated. Furthermore, the recirculation ratio could characterize changes in the boiler load in advance and provide predictive information for boiler load feedback control, which can be useful for monitoring and controlling incinerator combustion.

Particle dynamics in a multi-staged fluidized bed: Particle transport behavior on micro-scale by discrete particle modelling

This work studies the particle exchange rates in horizontal fluidized beds equipped with different weir designs between compartments. These particle exchange rates provide information on the axial dispersion of the solid material within the process. For this purpose discrete particle modelling (DPM) was used to determine the particle exchange on microscopic level. This method uses a coupled CFD-DEM approach to observe particle dynamics in a fluid field. The model was validated against exchange rates in a lab-scale setup as determined by Particle Tracking Velocimetry (PTV) with very good quantitative agreement, showing the suitability of the method for the evaluation of weir designs. Simulations were performed for different weir designs and under variation of the hold-up mass, the feed rate and gas velocity to predict their transport behavior in a pilot-scale 3D horizontal fluidized bed. The results indicate that the solids transport behavior is strongly dependent on the used weir design and the main driving force for the particle transport that can be influenced by the process conditions. The installation of weirs between two compartments induces a transport resistance, while the base type without the installation of a weir between the two chambers represents the fastest possibility for mixing the particles of a two-compartment system. It has been observed that the general trend shows higher particle recirculation rates for the overflow weir and base configuration (no weir), whereas the underflow and sideflow weir applications improve the solids transport through the horizontal fluidized bed.

A hydrodynamic model for biomass gasification in a circulating fluidized bed riser

This study presents a three-dimensional Computational Fluid Dynamic (CFD) model and experimental measurements of the hydrodynamics in the riser section of a Circulating Fluidized Bed (CFB) biomass gasifier consisting of a binary mixture of polydisperse particles. The model is based on multi-fluid (Eulerian-Eulerian) approach with constitutive equations adopted from the Kinetic Theory of Granular Flow (KTGF). The study first presents an assessment of the various options of the constitutive and closure equations for a binary mixture followed by sensitivity analysis of the model to the solution time step, cell size, turbulence and the alternative formulations of the granular energy equation. Accordingly, a robust and reliable hydrodynamic model is recommended and validated using conventional pressure measurements and Positron Emission Particle Tracking (PEPT) technique. Furthermore, the model predictions and experiments revealed evidence of the particle re-circulation within the lower part of the riser, which is an important feature contributing to rapid mass and heat transfer in a CFB gasifier. The present hydrodynamic model can be further developed; by incorporating appropriate reactions and heat transfer equations, in order to fully predict the performance and products of a CFB biomass gasifier.

CPFD study of a full-loop three-dimensional pilot-scale circulating fluidized bed based on EMMS drag model

For the purpose of process optimization and scale-up, coupling energy-minimization multi-scale (EMMS) drag model with computational particle fluid dynamics (CPFD) is a good option for large scale three dimensional full-loop circulating fluidized bed (CFB) simulation. In this study, such integration is applied for a complex CFB with six cyclones, based on Barracuda platform, to investigate the gas-solids flow dynamic characteristics in scaled-up system, especially in the dense flow region, and to characterize different fluidization regimes. The simulation results are compared with those obtained from Wen-Yu model, in terms of axial pressure distribution, solid volume fraction, particle recirculation, etc., and are evaluated against available measurement data. The result demonstrates that EMMS drag model can predict the axial pressure distribution well, especially in the dense region, which gives an S-shape solid volume fraction in the axial direction and two-core-annulus structures (‘M’ shape) in the radial direction. Meanwhile, EMMS drag model provides smaller drag force between the gas phase and the solids that the particle recirculation fluxes inside and outside the riser is relatively smaller. With the increase of superficial gas velocity, three different fluidization regimes, including multiple bubble regime, exploding bubble regime and turbulent fluidization regime, are identified based on EMMS drag model. The results show that the dense region is clear at the multiple and exploding bubble regimes; while the particle volume fraction and mass flow rate in the riser top are larger than those in the middle at the turbulent fluidization regime, which gives a guidance that the riser height is limited that particles do not have enough space to distribute.

Granular-front formation in free-surface flow of concentrated suspensions.

A granular front emerges whenever the free-surface flow of a concentrated suspension spontaneously alters its internal structure, exhibiting a higher concentration of particles close to its front. This is a common and yet unexplained phenomenon, which is usually believed to be the result of fluid convection in combination with particle size segregation. However, suspensions composed of uniformly sized particles also develop a granular front. Within a large rotating drum, a stationary recirculating avalanche is generated. The flowing material is a mixture of a viscoplastic fluid obtained from a kaolin-water dispersion with spherical ceramic particles denser than the fluid. The goal is to mimic the composition of many common granular-fluid materials, such as fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to separate due to particle settling. However, through the shearing caused by the rotation of the drum, a reorganization of the phases is induced, leading to the formation of a granular front. By tuning the particle concentration and the drum velocity, it is possible to control this phenomenon. The setting is reproduced in a numerical environment, where the fluid is solved by a lattice-Boltzmann method, and the particles are explicitly represented using the discrete element method. The simulations confirm the findings of the experiments, and provide insight into the internal mechanisms. Comparing the time scale of particle settling with the one of particle recirculation, a nondimensional number is defined, and is found to be effective in predicting the formation of a granular front.

Behavior of air particles associated with atmospheric recirculation over complex coastal area

Several numerical experiments were carried out over Gwangyang Bay area, which is located southwestern part of the Korean Peninsula, to clarify the characteristics of the recirculation potential and the impact of the spatial size of the model domain on air pollutant dispersion. The numerical models used in this study were Weather and Research Forecasting (WRF) model to assess the atmospheric circulation and FLEXPART to estimate the level of pollutant dispersion. Regardless of the synoptic conditions, the variation in the recirculation potential based on the transported distance of air pollutants agreed well with the change in ozone concentration. Weak synoptic wind and strong regional flow results in the highest recirculation potential in both inland and coastal areas. The concentration in limited areas is strongly associated with the pollutants recirculated by regional circulation. The persistence of synoptic wind often prevents particle recirculation but intensified regional circulation around coastal area is favorable for increasing the returning particles. Therefore, the determined domain size should be large enough to include the trace of recirculated pollutants when weak synoptic wind occurs.

Characterization of an entrained flow reactor for pyrolysis of coal and biomass at higher temperatures

Abstract A laboratory-scale entrained flow reactor for gasification/pyrolysis of coal and biomass has been designed and constructed at the Pennsylvania State University. The pre-experimental numerical simulations have been used as an aid in the design of the reactor as well as understanding and explaining the experimental results. Post experimental modeling of the reactor has been carried out using the CFD package ANSYS-Fluent. Results from experiments conducted with the reactor are here presented. These initial characterization activities of the entrained flow reactor are carried out at atmospheric pressure. Modeling and experiments are conducted at three different temperatures: 1573 K, 1673 K and 1773 K. The CFD models show some particle and gas recirculation at the inlet of the reactor. The calculated residence time in the reactor is 0.5 s for biomass and 0.4 s for coal when the particles traveling distance is 0.65 m. Tar and CO are the dominant species at 1573 K in both coal and biomass conversions, however while tar reduces as the temperature increases, the CO formation increases. Fuel conversion varies significantly between coal and biomass. The minimum conversions observed during experiments were 86.7% for biomass and 56.8% for coal at 1573 K. Conversion rates as high as 90.5% were observed for biomass at 1773 K, while the maximum coal conversion observed was 64.0% at 1773 K. The BET surface area of coal chars obtained at 1573 K and 1673 K was similar and higher than that of the char obtained at 1773 K. This drop of surface area at 1773 K has been attributed to pore coalescence, following observation of the SEM images. The surface area of biomass chars does not vary significantly. The reactivity studies conducted on the chars reveal some thermal annealing at higher temperature for coal; this occurrence is observed to be less pronounced for biomass chars.

Cluster Formation and Drag Reduction—Proposed Mechanism of Particle Recirculation within the Partition Column of the Bottom Spray Fluid-Bed Coater

Bottom spray fluid-bed coating is a common technique for coating multiparticulates. Under the quality-by-design framework, particle recirculation within the partition column is one of the main variability sources affecting particle coating and coat uniformity. However, the occurrence and mechanism of particle recirculation within the partition column of the coater are not well understood. The purpose of this study was to visualize and define particle recirculation within the partition column. Based on different combinations of partition gap setting, air accelerator insert diameter, and particle size fraction, particle movements within the partition column were captured using a high-speed video camera. The particle recirculation probability and voidage information were mapped using a visiometric process analyzer. High-speed images showed that particles contributing to the recirculation phenomenon were behaving as clustered colonies. Fluid dynamics analysis indicated that particle recirculation within the partition column may be attributed to the combined effect of cluster formation and drag reduction. Both visiometric process analysis and particle coating experiments showed that smaller particles had greater propensity toward cluster formation than larger particles. The influence of cluster formation on coating performance and possible solutions to cluster formation were further discussed.

ChemInform Abstract: Properties of Circulating Fluidized Beds (CFB) Relevant to Combustion and Gasification Systems

AbstractReview: 83 refs.

Performance of a Bench-Scale Fast Fluidized Bed Carbonator

The carbonate looping process is a promising technology for CO2 capture from flue gas. In this process, the CO2 capture efficiency depends on the performance of a carbonator that may be operated as a circulating fluidized bed (CFB). In this paper, the carbonator performance is investigated by applying a new experimental method with accurate control of the particle recirculation rate. The experimental results show that the inlet calcium to carbon molar ratio is the main factor on the CO2 capture efficiency in the carbonator, that is, increasing the inlet Ca/C from 4 to 13 results in increasing the CO2 capture efficiency from 40 to 85% with limestone having a maximum CO2 capture capacity of only 11.5%. Furthermore, a reactor model for a carbonator is developed based on the Kunii–Levenspiel’s model. A key parameter in the model is the particle distribution along the height of the reactor, which is estimated from experiments under stable operating conditions with constant bed inventory, reactor temperature and exit CO2 concentration. The validated CFB carbonator model was used to simulate different operating conditions relevant for CO2 capture from a power plant and from a cement plant. The results show that particle recirculation rates of 2–5 kg/(m2s) or ratio of bed inventory to recirculation rates of 70–176 s are sufficient for attaining 90% CO2 capture efficiency depending on the inlet Ca to C ratio.

Experimental spray zone characterization in top-spray fluidized bed granulation

In fluidized bed granulation wetting and coating of particles depends on the atomization of a liquid binder agent. Droplet distribution and the subsequent micro-scale processes of droplet deposition on particle surfaces as well as drying of layers and liquid bonds between aggregated particles lead to a subdivision of the process space into two major compartments, a spray zone and a drying zone. By using a self-constructed, simple conductivity probe spray patterns inside the fluidized bed are located. The spatial demarcation of the compartments, which is dependent on the fluidization and spray conditions, is deduced. Particularly, nozzle height and nozzle gas flow rates influence the expansion of the spray zone and its intrusion into the bed. The presented results show by means of the particle residence time for the two considered compartments that an increased nozzle mass flow rate leads to significantly accelerated particle flow in the spray zone, and the fluidization velocity of the gas forces a faster particle re-circulation behavior in the entire fluidized bed. Consequently, process time for wetting and drying is reduced. By using a flat fluidized bed with rectangular cross section in combination with image-based acquisition techniques, particle velocities and solid volume fractions have been acquired. Comparing the results of particle circulation patterns with data obtained in cylindrical equipment shows that information can be transferred from the quasi-2D configuration to real 3D geometries.

Modeling of particle transport and combustion phenomena in a large-scale circulating fluidized bed boiler using a hybrid Euler–Lagrange approach

The constantly developing fluidized combustion technology has become competitive with a conventional pulverized coal (PC) combustion. Circulating fluidized bed (CFB) boilers can be a good alternative to PC boilers due to their robustness and lower sensitivity to the fuel quality. However, appropriate engineering tools that can be used to model and optimize the construction and operating parameters of a CFB boiler still require development. This paper presents the application of a relatively novel hybrid Euler–Lagrange approach to model the dense gas–solid flow combined with a combustion process in a large-scale industrial CFB boiler. In this work, this complex flow has been resolved by applying the ANSYS FLUENT 14.0 commercial computational fluid dynamics (CFD) code. To accurately resolve the multiphase flow, the original CFD code has been extended by additional user-defined functions. These functions were used to control the boiler mass load, particle recirculation process (simplified boiler geometry), and interphase hydrodynamic properties. This work was split into two parts. In the first part, which is referred to as pseudo combustion, the combustion process was not directly simulated. Instead, the effect of the chemical reactions was simulated by modifying the density of the continuous phase so that it corresponded to the mean temperature and composition of the flue gases. In this stage, the particle transport was simulated using the standard Euler–Euler and novel hybrid Euler–Lagrange approaches. The obtained results were compared against measured data, and both models were compared to each other. In the second part, the numerical model was enhanced by including the chemistry and physics of combustion. To the best of the authors’ knowledge, the use of the hybrid Euler–Lagrange approach to model combustion is a new engineering application of this model. In this work, the combustion process was modeled for air-fuel combustion. The simulation results were compared with experimental data. The performed numerical simulations showed the applicability of the hybrid dense discrete phase model approach to model the combustion process in large-scale industrial CFB boilers.

Technology Advancements for Next Generation Falling Particle Receivers

The falling particle receiver is a technology that can increase the operating temperature of concentrating solar power (CSP) systems, improving efficiency and lowering the costs of energy storage. Unlike conventional receivers that employ fluid flowing through tubular receivers, falling particle receivers use solid particles that are heated directly as they fall through a beam of concentrated sunlight for direct heat absorption and storage. Because the solar energy is directly absorbed by the particles, the flux limitations associated with tubular central receivers are mitigated. Once heated, the particles may be stored in an insulated tank and/or used to heat a secondary working fluid (e.g., steam, CO2, air) for the power cycle. Thermal energy storage costs can be significantly reduced by directly storing heat at higher temperatures in a relatively inexpensive, stable medium. This paper presents an overview of recent advancements being pursued in key areas of falling particle receiver technology, including(1) advances in receiver design with consideration of particle recirculation, air recirculation, and interconnected porous structures; (2) advances in particle materials to increase the solar absorptance and durability; and (3) advances in the balance of plant for falling particle receiver systems including thermal storage, heat exchange, and particle conveyance.

A Preliminary Research on a Plasma Spout-Fluid Bed Reactor

A laboratory-scale plasma spout-fluid bed reactor with a 10 kW DC plasma torch was developed and tested using quartz sand particle and rice hull. The preliminary experimental results including particle recirculation and attrition, bed temperature distribution and stability, as well as biomass gasification system energy balance were presented in this paper. Research results indicated that plasma spout-fluid bed reactor may be a technically feasible reactor for carbonaceous organic material gasification.

Particle Residence Times in Fluidized Bed Granulation Equipments

AbstractVarious types of fluidized bed equipments used in granulation were experimentally investigated in order to gain a more detailed insight in particle recirculation behavior and particle residence times in the two considered compartments, namely the spray compartment and the drying compartment. The investigated fluidized bed technologies comprise a top‐spray process, a spout fluidized bed without and with internal riser, and a spouted bed. The measurements were conducted by using image analysis techniques on an almost 2D particle flow. From the results, conclusions about particle wetting and drying can be extracted, subject to fluidization velocity, nozzle mass flow rate, and facility/riser construction.

Development of a visiometric process analyzer for real-time monitoring of bottom spray fluid-bed coating

Particle recirculation within the partition column is a major source of process variability in the bottom spray fluid-bed coating process. However, its locality and complex nature make it hidden from the operator. The aim of this study was to take snapshots of the process by employing a visiometric process analyzer based on high-speed imaging and ensemble correlation particle image velocimetry (PIV) to quantify particle recirculation. High-speed images of particles within the partition column of a bottom spray fluid-bed coater were captured and studied by morphological image processing and ensemble correlation PIV. Particle displacement probability density function (PDF) obtained from ensemble correlation PIV was consistent with validation experiments using an image tracking method. Particle displacement PDF was further resolved into particle velocity magnitude and particle velocity orientation histograms, which gave information about particle recirculation probability, thus quantifying the main source of process variability. Deeper insights into particle coating process were obtained and better control of coat uniformity can thus be achieved with use of the proposed visiometric process analyzer. The concept of visiometric process analyzers was proposed and their potential applications in pharmaceutical processes were further discussed. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:346–356, 2010