The effect of bed materials on the solid/bubble motion in a fluidised bed

Gas fluidisation provides good mixing and contact of the gas and particle phases as well as good heat transfer. These attractive features are achieved by the high degree of bubble-induced particle circulation within the bed. Bubble and particle motion vary with bed materials and operating conditions, as investigated in the present study, by the use of the non-intrusive positron emission particle tracking (PEPT) technique. The selected materials were spherical polyethylene and glass particles.The data obtained by the PEPT technique are used to determine the particle velocities and circulation pattern. Bubble rise velocities and associated sizes can be inferred from the particle velocity data, since particles travel upwards mostly in the bubble wake. The results indicate that the flow structure and gas/solid motion within the fluidised beds were significantly different, even at the same value of the excess gas velocity, U-U_mf. The solid circulation pattern within the beds differ: if for glass beads, a typical UCDW-pattern existed (upwards in the centre of the bed, downwards near the wall), the pattern in the polyethylene bed is more complex combining a small zone of UWDC movement near the distributor and a typical UCDW-pattern higher up the bed. Transformed data demonstrate that at the same value of excess gas velocity, U-U_mf, the air bubbles in the polyethylene fluidised bed were smaller and rose more slowly than in the fluidised bed of glass beads, thus yielding a longer bubble residence time and improved gas/solid contact. For polyethylene beads, the size and rise velocity of air bubbles did not increase monotonically with vertical position in the bed as would be predicted by known empirical correlations, which however provide a fair fit for the glass beads data. Bubble sizes and solid circulation patterns are important parameters in the design of a fluidised bed reactor, and vary with the bed material used.     

Fluidization of moist sawdust in binary particle systems in a gas–solid fluidized bed

The fluidized behavior of binary mixtures of moist sawdust and glass spheres has been investigated. The sawdust alone was observed to fluidize poorly, with extensive channelling occurring. The addition of 0.322 and 0.516 mm glass spheres to the fluidized bed of sawdust improved the fluidization characteristics. The mixtures of sawdust and 0.322 mm spheres were completely mixed when fluidized. Mixtures of sawdust and 0.516 mm spheres were either partially or completely mixed, depending upon gas velocity in the fluidized bed. As the moisture content of the sawdust was increased, the minimum fluidization velocity of the binary mixture also increased. There was an upper limit to the moisture content of the sawdust at which fluidization could be achieved. When the moisture content of the sawdust exceeded 33 and 54 wt% on a dry basis, agglomeration and channelling occurred in the mixtures of sawdust and glass spheres, with sizes 0.322 and 0.516 mm, respectively. The moisture likely contributes to interparticle liquid bridging forces. Binary mixtures of larger 0.777 and 1.042 mm glass spheres and up to 82% moisture sawdust did not readily agglomerate, but the two components completely segregated during fluidization.     

Mathematical modelling of a solid particle motion in a re‐circulatory fluidised bed unit

Two fundamental properties of the particles are their velocity and consequent displacement versus time history during the granulation process. Knowledge of the particle velocity might supply much information about the sub‐processes. In this paper, an analytical model of a solid particle motion in an internal re‐circulatory fluidised bed unit is developed and validated against experimentally obtained data. The model predictions show good correspondence with the experimental results for the spherical particles. For the case of non‐spherical granules, the agreement between the model and the experiments is not equally convincing but still adequate.     

Investigation into gas-solid heat transfer in a cryogenic vibrated fluidised bed

Experiments were carried out in a cryogenic vibrated fluidised bed to investigate the heat transfer between gas and rubber particles obtained from discarded tyres. The effects of parameters such as bed layer thickness and gas flow rate on the gas-solid heat transfer were investigated, and a heat transfer correlation obtained by regressing the experimental data. Theoretical analysis based on radial thermal conductivity indicated that higher heat transfer efficiency could be obtained by the use of a fluidised bed rather than a fixed bed or a moving bed, especially for rubber particles having low thermal conductivity under cryogenic conditions. A numerical modelling was developed, based on assumptions of the movement of the particles and the vibrating bed plate, using a unique method of regarding particles as the source term in the energy equation. Computational results from the modelling showed good agreement with the experimental data.     

Experimental study on solid circulation in a multiple jet fluidized bed

Particle image velocimetry was used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two‐dimensional fluidized bed setup. Results were used to estimate the solid circulation rate of the fluidized bed as well as particle‐entrainment into the individual jets. The effects of fluidization velocity, orifice diameter, orifice pitch, particle diameter, and particle density were studied. It was determined from this study that the solid circulation rate linearly increased with an increase in the fluidization velocity until the multiple jet system transitioned from isolated to an interacting system. In the interacting system of jets, the solid circulation increased with fluidization velocity but at a much lower rate. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3003–3015, 2012     

The impacts of standard wall function and drag model on the turbulent modelling of gas–particle flow in a circulating fluidised bed riser

A transient turbulence model was applied to simulate the gas–particle system in a circulating fluidised bed riser. The k–epsilon turbulent equations coupled with the fluctuating energy equation were used to simulate the gas–particle system in a riser. The simulation results were validated by the experimental data of a CFB system. A grid study was implemented to examine the impact of grid discretisation. A comparison between the conventional drag models and the EMMS model was also conducted. Other factors, like the restitution coefficient particle to particle, was also found to have a significant impact on the turbulence model. © 2013 Canadian Society for Chemical Engineering     

Growth and breakup of a wet agglomerate in a dry gas–solid fluidized bed

Using CFD‐DEM simulations, a wet agglomerate of particles was placed in a void region of a dry vigorously fluidized bed to understand how wet agglomerates grow or breakup and how liquid spreads when agglomerates interact with dry fluidized particles. In the CFD‐DEM model, cohesive and viscous forces arising from liquid bridges between particles were modeled, as well as a finite rate of liquid bridge filling. The liquid properties were varied between different simulations to vary Bond number (surface tension forces/gravitational forces) and Capillary number (viscous forces/surface tension forces) in the system. Resulting agglomerate behavior was divided into regimes of (i) the agglomerate breaking up, (ii) the agglomerate retaining its initial form, but not growing, and (iii) the agglomerate retaining its initial form and growing. Regimes were mapped based on Bo and Ca. Implications of agglomerate behavior on spreading of liquid to initially dry particles were investigated. This article identifies a new way to map agglomerate growth and breakup behavior based on Bo and Ca. In modeling both liquid forces and a finite rate of liquid transfer, it identifies the complex influence viscosity has on agglomeration by strengthening liquid bridges while slowing their formation. Viewing Ca as the ratio of bridge formation time to particle collision and separation time capture why agglomerates with high Ca struggle to grow. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2520–2527, 2017     

Numerical and experimental study on multiple-spout fluidized beds

In this paper we study the effect of multiple spouts on the bed dynamics in a pseudo-2D triple-spout fluidized bed, employing the discrete particle model (DPM) and non-intrusive measurement techniques such as particle image velocimetry (PIV) and positron emission particle tracking (PEPT). A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agree very well for all the three cases, showing the good performance of these techniques. The DPM simulation results slightly deviate from the experiments which is attributed to particle–wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds.     

Development of a multi-fluid model for poly-disperse dense gas-solid fluidised beds, Part I: Model derivation and numerical implementation

We have derived a new set of closure equations for the rheologic properties of a dense gas-solid fluidised bed consisting of a multi-component mixture of slightly inelastic spheres, using the Chapman-Enskog solution procedure of successive approximations, where the particle velocity distribution of all particle species is assumed to be nearly Maxwellian around the particle mixture velocity with the particle mixture granular temperature. In this theory, differences in the mean velocities (i.e. particle segregation) and granular temperatures of the particle species result from higher order perturbation functions. Special attention is paid to assure thermodynamic consistency between the radial distribution function and the chemical potential of a hard-sphere particle specie appearing in the diffusion driving force when applying the revised Enskog theory, which is often overlooked. In the resulting closure equations, the rheologic properties of the particle mixture are explicitly described in terms of the particle mixture velocity and granular temperature, and the diffusion velocity and granular temperature of the individual particle phases can be computed from the mixture properties, which is a major advantage with respect to the numerical implementation. A new numerical solution strategy has been devised, which is an extension of the well-known SIMPLE algorithm and takes the compressibility of the solids phase directly into account, which allows for much larger time steps (about a factor of 10 larger). In Part 2 the simulation results obtained with the new model are compared with experimental data and discrete particle model (DPM) simulations.     

Particle‐resolved direct numerical simulation of gas–solid dynamics in experimental fluidized beds

Particle‐resolved direct numerical simulations (PR‐DNS) of a simplified experimental shallow fluidized bed and a laboratory bubbling fluidized bed are performed by using immersed boundary method coupled with a soft‐sphere model. Detailed information on gas flow and individual particles’ motion are obtained and analyzed to study the gas–solid dynamics. For the shallow bed, the successful predictions of particle coherent oscillation and bed expansion and contraction indicate all scales of motion in the flow are well captured by the PD‐DNS. For the bubbling bed, the PR‐DNS predicted time averaged particle velocities show a better agreement with experimental measurements than those of the computational fluid dynamics coupled with discrete element models (CFD‐DEM), which further validates the predictive capability of the developed PR‐DNS. Analysis of the PR‐DNS drag force shows that the prevailing CFD‐DEM drag correlations underestimate the particle drag force in fluidized beds. The particle mobility effect on drag correlation needs further investigation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1917–1932, 2016     

Experimental and numerical investigation of liquid circulation induced by a bubble plume in a baffled tank

Bubble induced liquid circulation is important in applications such as bubble columns and air-lift reactors. In this work, we describe an experimental and numerical investigation of liquid circulation induced by a bubble plume in a tank partitioned by a baffle. The baffle divides the tank into two compartments. Liquid can flow from one compartment to the other through openings at the top and the bottom of the baffle. Gas (air) was injected in the riser section in the form of bubbles at one corner of the tank. The temporal and spatial variation of velocity field in the liquid as a function of the gas flow rate was measured using particle image velocimetry (PIV). At a constant gas flow rate, the liquid flow field is unsteady due to the interaction with the bubbles. The time scales associated with the velocity-time series and the bubble plume thickness variation were calculated. The time averaged-velocity field was used to quantify the variation of the liquid circulation rate with gas flow rate. The turbulence in the liquid was measured in terms of turbulent intensities. These were calculated from the experimental data and were observed to be less than 3 cm/s. A 2-d Euler-Euler two-fluid model with buoyancy and drag as the interaction terms was used to simulate the flow. The parameters chosen for the simulations were selected from literature. It is shown that inclusion of turbulence model such as k-ε is necessary to capture the overall flow behavior. Good agreement was observed between experimentally obtained velocity profiles and the recirculation rates with the simulation results.     

Experimental and numerical research for fluidization behaviors in a gas–solid acoustic fluidized bed

The effects of sound assistance on fluidization behaviors were systematically investigated in a gas–solid acoustic fluidized bed. A model modified from Syamlal–O'Brien drag model was established. The original solid momentum equation was developed and an acoustic model was also proposed. The radial particle volume fraction, axial root‐mean‐square of bed pressure drop, granular temperature, and particle velocity in gas–solid acoustic fluidized bed were simulated using computational fluid dynamics (CFD) code Fluent 6.2. The results showed that radial particle volume fraction increased using modified drag model compared with that using the original one. Radial particle volume fraction was revealed as a parabolic concentration profile. Axial particle volume fraction decreased with the increasing bed height. The granular temperature increased with increasing sound pressure level. It showed that simulation values using CFD code Fluent 6.2 were in agreement with the experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Flow patterns of solids in a two-dimensional spouted bed with draft plates: PIV measurement and DEM simulations

Particle flow behaviors in a two-dimensional spouted bed (2DSB) with draft plates were studied using both the particle image velocimetry (PIV) and the combined technique of discrete element method and fluid dynamic computation (DEM-CFD) while considering the gas turbulence effect. The bed consisted of a rectangular column, 152 mm wide and 15 mm deep, a conical section with an included 60° angle and two draft plates with a distance of 15 mm. Images of particle flow were recorded by a high speed CCD camera and analyzed using a self-developed PIV algorithm to obtain a time-averaged particle velocity field. Experiments predict that the addition of draft plates not only makes the streamline of particles in the annulus steeper, but the velocity magnitude is made smaller as well. DEM results predict well the longitudinal profile of the particle vertical velocity along the bed centerline, especially during the rapid acceleration stage at the lower part of the spout. Finally, the distributions of drag forces and net forces are introduced in this paper to explain the particle velocity profiles by PIV measurement.     

Mixing characteristics of irregular binaries in a promoted gas–solid fluidized bed: A mathematical model

The vertical as well as horizontal dispersion for the gas–solid fluidization has been analyzed in the present work. The fluidization and solids mixing characteristics of large irregular particles (Geldart‐BD type) for both homogeneous and heterogeneous materials (size‐variant and density‐variant respectively) have been investigated in a 15 × 100 cm² cylindrical column. A theoretical model for concentration of jetsam particles has been developed as a function of height of any layer of particles (measured from the grid) by considering the counterflow of solids and circulation model together with the dispersion model for the vertical and the horizontal displacement of the particles in the fluidized bed. The mixing index at any position in the bed has thereby been expressed as a function of the concentration of the jetsam. The values of mixing index calculated from the above model as well as the experimentally measured ones for both the homogeneous and the heterogeneous systems have been compared with each other. Attempt has also been made to develop correlations for the mixing index using various system parameters based on the dimensional analysis approach for the unpromoted and the promoted beds. The mixing index values thus obtained through the dimensional analysis approach have been compared with those obtained from the developed theoretical model. The values of the mixing index calculated for promoted beds have also been compared with the corresponding values for the unpromoted beds. Segregation effect is found to be stronger for the heterogeneous systems than the homogeneous ones. The density difference is the major factor affecting the segregation.     

High resolution experimental measurement of turbulent flow field in a high pressure homogenizer model and its implications on turbulent drop fragmentation

Particle image velocimetry is performed on a model of a high pressure homogenizer, scaled for qualitative similarity of the one phase turbulent flow field in a production scale homogenizer. Flow fields in gap entrance, gap and gap outlet chamber are obtained with high resolution. The measurements show gap flow development and formation of a turbulent wall adherent jet when exiting into the outlet chamber. Turbulent kinetic energy spectra show how the turbulent energy available for fragmentation is transported over distance along the jet centre axis.The high resolution images are also used together with a Kolmogorov–Hinze theory framework for discussing drop fragmentation together with a direct evaluation of disruptive stresses from measurements. For the turbulent inertial mechanism large drops experience high fragmenting force close to eight gap heights downstream of the gap exit whereas this occurs closer to 20 gap heights for smaller drops. The turbulent viscous mechanism is most efficient at a downstream distance of eight gap heights into the outlet chamber for all drops sizes.     

Experimental and computational study of gas–solid fluidized bed hydrodynamics

The hydrodynamics of a two-dimensional gas–solid fluidized bed reactor were studied experimentally and computationally. Computational fluid dynamics (CFD) simulation results from a commercial CFD software package, Fluent, were compared to those obtained by experiments conducted in a fluidized bed containing spherical glass beads of 250– in diameter. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas–solid flow. Momentum exchange coefficients were calculated using the Syamlal–O’Brien, Gidaspow, and Wen–Yu drag functions. The solid-phase kinetic energy fluctuation was characterized by varying the restitution coefficient values from 0.9 to 0.99. The modeling predictions compared reasonably well with experimental bed expansion ratio measurements and qualitative gas–solid flow patterns. Pressure drops predicted by the simulations were in relatively close agreement with experimental measurements at superficial gas velocities higher than the minimum fluidization velocity, U_mf. Furthermore, the predicted instantaneous and time-average local voidage profiles showed similarities with the experimental results. Further experimental and modeling efforts are required in a comparable time and space resolutions for the validation of CFD models for fluidized bed reactors.     

Flow regimes in a spout-fluid bed: A combined experimental and simulation study

Spout-fluid beds find a widespread application in the process industry for efficient contacting of large particles with a gas. However, detailed understanding of the complex behavior of these systems is lacking, which leads to significant scale-up problems in industry. In this paper we report results of a combined experimental and simulation study on the various regimes, which can be encountered during spout-fluid bed operation.A regime map for a 3D spout-fluid bed was composed employing spectral analysis of pressure drop fluctuations and fast video recordings. In addition 3D Euler-Lagrange computations were performed to assess the capability of the model to reproduce the experimentally observed flow regimes.The influence of the drag closure on the model results was assessed and the influence of the computational grid was studied using a new method for the implementation of the two-way coupling, which is proposed in this paper.For most regimes our model is able to predict the appropriate regime. The frequency, at which the largest power is found, is overpredicted by the model. Contrary to the experimental observations, our model did not predict any large slugs in the slugging bed regime.The remaining differences between the simulated and experimentally observed bed behavior is most likely related to the representation of the effective fluid-particle interaction in our model, which relies on local spatial homogeneity.     

Transient multi gas—solid reactions in a bubbling fluidized bed

A “multimodel” for gas‐solid reactions in a reacting particle has been applied to a bubbling fluidized bed reactor. The particle is tracked and bed and particle variables are determined continuously. The conservation equations of mass and heat with auxiliary relations are solved in an accelerating particle, which may rise or fall. The effects of bulk pressure, velocity and temperature, and particle diameter are studied. Heat and mass transfer coefficients may fluctuate up to 75% and 148% respectively. Doubling the pressure changes h_c by 75% and k_c by ?45%. Increase in pellet diameter reduces both h_c and k_c.