Gas-solid Slip Velocity Research Articles

Hydrodynamic CFD-DEM model validation in a gas–solid vortex unit

Process intensification in gas–solid fluidization processes can be achieved by working in a centrifugal rather than a gravitational field. In this regard, the gas–solid vortex unit (GSVU) is an ideal candidate for heterogeneously catalyzed processes. A four-way coupled CFD-DEM model describing the hydrodynamics in the GSVU with an unprecedented level of detail is validated using 2D particle image velocimetry (PIV) experimental data on both azimuthal and radial particle velocity components. It captures high and low velocity regions, both qualitatively and quantitatively. Gas-solid slip velocities several times higher than those obtainable in a gravitational field are achieved, greatly enhancing heat and mass transfer rates. Furthermore, the gas-phase residence time distribution in the GSVU is shown to be narrow. This developed model presents a powerful tool for a better understanding and a detailed design aimed at enhancing the non-reactive and reactive process intensification capabilities of the gas–solid vortex technology.

Characteristics of Gas–Solid Flow in an Intermittent Countercurrent Moving Bed

The calculation equations of pressure drop and solid flow rate are given in an intermittent countercurrent moving bed with multiple optimized structures. The flow fields are investigated by experimental methods under different conditions, e.g., ratio of position of the gas distributor to the bed width (rp = 0.5~1.5), gas superficial velocity (ug = 0~0.1591 m/s), solid outlet diameter (Do = 5~25 mm) and cone angle (α = 0~60°). It is found that when rp ≥ 1.0, the flow field in the bed is little affected by rp. Flow patterns are divided into three modes: continuous discharging, intermittent discharging (synchronous, asynchronous) and particle bridging. During continuous discharging, the calculation formula of the solid flow rate, which is closely related to Do and gas–solid slip velocity vslip, is established by referring to the modified De Jong and Beverloo formulas, and its error ≤ ±12.5%. The pressure drop of the total bed consists of the pressure drops of the granular bed and the solid outlets, which are affected by Do, vslip and α; it is built by referring to the Ergun formula, and its error ≤ ±17.0%. As the solid flow rate and pressure drop influence each other through vslip, an iterative algorithm is proposed to enhance the computation accuracy.

Three-dimensional modelling of the multiphase hydrodynamics in a separated-gasification chemical looping combustion unit during full-loop operation

Separated-gasification chemical looping combustion (SG-CLC) is a clean utilization technology for fossil fuels, which can separate CO2 during high-performance combustion process with low degradation of oxygen carrier and zero-energy penalty. As an important part of the optimization, modelling and prediction of the multiphase hydrodynamics in the SG-CLC unit were conducted in this work. This unit consists of a gasifier (GR), a reduction reactor (RR) and an air reactor (AR). For thorough understanding of the flow behaviors of three phases (gas, oxygen carrier and sand phases), a three-dimensional computational fluid dynamics (CFD) model for SG-CLC system was first implemented. The main purpose is to predict multiphase hydrodynamics at the fuel side of this SG-CLC system due to its significant effects on combustion efficiency. The axial pressure profile predicted by the CFD model was validated by experimental data to demonstrate its feasibility. Then, based on this model, the hydrodynamics of different phases at fuel side (GR and RR) were investigated under fundamental condition, which indicated the gas-solid flow mechanism in circulation establishment process. Furthermore, effects of the operation condition on the hydrodynamics properties were investigated. The local gas-solid slip velocity in RR was fitted as the function of superficial fluidizing number Nr and axial position, which showed high accuracy for the prediction in this system.

Effect of elevated pressure on gas-solid flow characteristics in a circulating fluidized bed

Pressurized circulating fluidized beds are applied in a range of applications because of intensified heat and mass transfer and chemical reaction. The gas density increases linearly with the pressure, which influences the gas-solid flow characteristics. In this work, the effects of operating pressure (0.1 MPa–0.6 MPa) on the distribution of particle volume fraction and gas-solid slip velocity in the riser are studied under different conditions. The particle belongs to Geldart B group, the solids circulation rate ranges from 20 to 50 kg/(m2.s), and the fluidizing gas velocity ranges from around 2 to 8 m/s. With an increase in pressure from 0.1 to 0.6 MPa, in order to achieve the same solids circulation rate, the fluidizing gas velocity decreases approximately to a half. Under pressurized conditions, the profile of particle volume fraction along the axial direction shows a similar trend, while the particle volume fraction in the bed central zone at the upper riser increases slightly. Under different pressures, the plot of the nondimensional gas-solid slip velocity against the particle volume fraction converges on a power correlation. The present study is a first step towards the aim of correlating the solids circulation rate and gas-solid slip velocity with operating parameters under pressurized conditions.

Cold-state experimental study on discharge characteristics of solid particles in a gravity driven moving bed solar receiver

A small-scale multi-tube experimental system was established to pretest the workability of a gravity driven moving bed solar receiver with its absorbing section containing an insert in each tube. Discharge characteristics of solid particles with various particle sizes range from 149 μm to 1359 μm in mean diameter are studied at ambient temperature. In multi-tube experiments, the discharge rate in each tube is almost uniform. In single-tube experiments, the packed height in the particle dispenser exhibits negligible influence on the mass flow rate. Three models have been set up to predict the mass flow rate in the without-insert case of particle flow experiments. Among them, Model-3 is shown to obtain the minimum mean error of 1.54% compared with the experimental results, and proved that the influence of particle size and orifice diameter on the gas-solid slip velocity should be considered. Effect of the insert on particle flow characteristics was also investigated. The added insert in the tubes is proved to increase the mass flow rate of the finest particles. However, for particles coarser than 642 μm in mean diameter, this effect is insignificant and the mass flow rate in with-insert case is consistent with predicted results by Beveloo equation and Model-3. Besides, the experimental results of particles finer than 297 μm in with-insert case are larger than predicted results by Model-3 and less than that by Beveloo equation. It is noteworthy that two types of flow instability are observed with specific experimental variables, indicates that particle layer thickness, particle size and orifice diameter are relevant parameters to affect the granular flow state. Hence, optimizing structural parameters and selecting suitable size of particles are necessary to avoid flow instability in this type of solid particle solar receiver.

Process Intensification in a Gas–Solid Vortex Unit: Computational Fluid Dynamics Model Based Analysis and Design

The process intensification abilities of gas–solid vortex units (GSVU) are very promising for gas–solid processes. By working in a centrifugal force field, much higher gas–solid slip velocities can...

Neural-network-based filtered drag model for gas-particle flows

Filtered two-fluid model (fTFM) for gas-particle flows require closures for the sub-filter scale corrections to interphase drag force and stresses, the former being more significant. In this study, we have formulated a neural-network-based model to predict the sub-grid drift velocity, which is then used to estimate the drag correction. As a part of the neural network model development effort, we derived a transport equation for drift velocity and then performed a budget analysis to conclude that an algebraic model for drift velocity in terms of the filtered variables that are resolved in a fTFM simulation is adequate, and the model should include the filtered gas-phase pressure gradient as a marker in addition to the filtered particle volume fraction and the filtered gas-solid slip velocity. Both a priori and a posteriori analyses reveal that the present model for drift velocity when used in a fTFM simulation is able to capture the fine-grid simulation results quite well.

Design and cold flow testing of a Gas-Solid Vortex Reactor demonstration unit for biomass fast pyrolysis

Innovative gas-solid fluidized beds with process intensification capabilities are among the most promising alternatives for the current state of the art in the chemical industry. In the present work the advantages of such a reactor that sustains a rotating fluidized bed with gas-solid slip velocities much higher than those in conventional fluidized beds are illustrated computationally and experimentally. A Gas-Solid Vortex Reactor (GSVR) demonstration unit is designed to operate at typical biomass fast pyrolysis conditions targeting the production of chemicals and fuels from renewable feedstocks. For the demonstration unit preheated N2 supplies the thermal energy required by the fast pyrolysis process but alternative sources can also be evaluated. A broad range of operation conditions in the 80mm diameter and 15mm height GSVR can be evaluated: N2 mass flow rates of 5–10gs−1 and biomass feed mass flow rates of 0.14–1.4gs−1. Particle-free and particulate flow experiments confirmed that the carrier gas is evenly distributed around the GSVR cylindrical chamber as anticipated by computational fluid dynamic simulations. The latter also supported the inclusion of a profiled bottom end wall and a diverging exhaust. Cold flow experiments with biomass confirmed that the GSVR sustains a rotating fluidized bed with average bed height of 10mm and solids azimuthal velocities of 6–7ms−1.

A generalized drag Law for heterogeneous gas-solid flows in fluidized beds

An accurate drag model is key to simulating the fluidization process in circulating fluidized beds. Existing drag models only apply well to homogeneous gas solid flows or to some heterogeneous flows, but lack generalization. The present work generalizes the heterogeneous QC-EMMS drag model [12]. The particle heterogeneity was represented in the sub-model of the QC-EMMS model by introducing a local heterogeneity factor Ψ so that the meso-scale drag model varies with the operating conditions. The overall macroscopic heterogeneity is characterized by the Reynolds number (Re*) of the overall gas-solid slip velocity, which represents the fluidization state variations with the operating conditions. The relationship between the local heterogeneity, Ψ, and the overall heterogeneity, Re*, indirectly relates the local drag force to the operating conditions via the cluster sub-model to generalize the QC-EMMS model. The model is incorporated into the two-fluid method to simulate the gas-solid flow behavior in a riser, with the results for various working modes verifying the model accuracy with relative differences all less than 10%.

Impact of particle properties on gas solid flow in the whole circulating fluidized bed system

The effect of particle properties on gas solid flow in the whole circulating fluidized bed (CFB) system was investigated. Two Geldart-B particles with different sizes were used and the differences in pressure balance and material balance of the whole CFB system with a loop seal were described. The results show that coarse particles have a larger transport velocity Utr to fall into the fast bed regime, which means accumulation in the riser is easier for the coarse particles at the same fluidization velocity Ur. The coarse particles also have a smaller solid saturation carrying rate Gs,max and a lower height of the acceleration region in the fast fluidization regime. Considering the pressure balance of the system, a higher pressure head should be supplied by the standpipe at the aeration tap of the loop seal under the same Gs when coarse particles are used. The pressure drop through the cyclone is lower for coarse particles under the same solid concentration at the entrance of it. In the loop seal, coarse particles need more aeration gas to reach the same Gs because of its higher minimum fluidization velocity. Coarse particles show a higher pressure drop through the weir part and a lower pressure drop through the horizontal part of the loop seal. In the standpipe, coarse particles have a lower drag coefficient and need a higher gas solid slip velocity to build up the same pressure gradient, so fine particles suffer less bypassing gas into the cyclone and have higher material seal capability. However, because of the same density of the two particles, the maximum pressure gradient built up in the standpipe is similar.

Parametric study of the time-averaged gas–solid drag force in circulating fluidized bed conditions

Steady state modeling based on time-averaged transport equations is a computationally efficient method for CFD simulation of large industrial-scale circulating fluidized beds. The feasible alternative for steady state modeling is to carry out transient simulations in a coarse mesh, which would require mesh resolution dependent closure laws and lead to long simulations in order to characterize the average process behavior. These closures could be avoided by simulations with adequate spatial and temporal resolutions, but the required computational effort renders such simulations unfeasible in industrial scale applications in near future. Equation closures for steady state modeling can be developed by time-averaging results from transient simulations. One of the largest terms to be modeled is the time-averaged drag force. In the present work, parameters that need to be accounted for in a model for the time-averaged drag force were studied by analyzing time-averaged results from a number of transient 2D simulations carried out with fairly fine spatial resolution. The analysis was limited to Geldart B particles. In the literature, the solid volume fraction, distance to the wall and the gas–solid slip velocity have been included as parameters in drag correction functions developed for transient coarse-mesh simulations. In the present work, the same parameters are found to have significant effects on the time-averaged gas–solid drag force. Additionally, solid density, particle size and gas phase laminar viscosity are shown to have significant effects on the average drag force. Thus process conditions, which significantly vary inside a CFB, need to be accounted for in the model.

Generalized maxwell velocity slip boundary model

For the gas flow of micro gas bearings in the near continuum and slip flow region where the squeezing velocity exists,a generalized Maxwell velocity slip boundary model is derived in detail using the velocity distribution function from the Grad thirteen moment approximation based on the conservation of momentum and energy fluxes in the Knudsen layer.The research shows that the generalized Maxwell velocity slip boundary model is consistent with the typical Maxwell velocity slip model when the temperature gradient along the surface and the squeezing velocity of gas flow are neglected.By applying the model in the control equation for micro gas journal bearings,the mathematical model for the gas-solid surface slip velocity boundary in the micro gas journal bearings has been obtained.

Drying of Biomass Particles: Experimental Study and Comparison of the Performance of a Conventional Fluidized Bed and a Rotating Fluidized Bed in a Static Geometry

The performance of a conventional fluidized bed and a rotating fluidized bed in a static geometry (RFB-SG) for the drying of woody biomass is experimentally studied. The performance with respect to the specific biomass drying rate, product uniformity, and air utilization is evaluated. RFB-SGs are shown to intensify the drying process by easily one order of magnitude, due to a combination of increased gas–solid slip velocities and improved particle bed density and uniformity. The cost in terms of increased and less efficient air utilization can be minimized by optimizing the RFB-SG drying chamber design.

Numerical investigation of gas-solid heat transfer in rotating fluidized beds in a static geometry

Gas–solid heat transfer in rotating fluidized beds in a static geometry is theoretically and numerically investigated. Computational fluid dynamics (CFD) simulations of the particle bed temperature response to a step change in the fluidization gas temperature are presented to illustrate the gas–solid heat transfer characteristics. A comparison with conventional fluidized beds is made. Rotating fluidized beds in a static geometry can operate at centrifugal forces multiple times gravity, allowing increased gas–solid slip velocities and resulting gas–solid heat transfer coefficients. The high ratio of the cylindrically shaped particle bed “width” to “height” allows a further increase of the specific fluidization gas flow rates. The higher specific fluidization gas flow rates and increased gas–solid slip velocities drastically increase the rate of gas–solid heat transfer in rotating fluidized beds in a static geometry. Furthermore, both the centrifugal force and the counteracting radial gas–solid drag force being influenced by the fluidization gas flow rate in a similar way, rotating fluidized beds in a static geometry offer extreme flexibility with respect to the fluidization gas flow rate and the related cooling or heating. Finally, the uniformity of the particle bed temperature is improved by the tangential fluidization and resulting rotational motion of the particle bed.

Perspectives for Fluidized Bed Nuclear Reactor Technology using Rotating Fluidized Beds in a Static Geometry

The new concept of a rotating fluidized bed in a static geometry opens perspectives for fluidized bed nuclear reactor technology and is experimentally and numerically investigated. With conventional fluidized bed technology, the maximum attainable power is rather limited and maximum at a certain fluidization gas flow rate. Using a rotating fluidized bed in a static geometry, the fluidization gas drives both the centrifugal force and the counteracting radial gas-solid drag force in a similar way. This allows operating the reactor at any chosen sufficiently high solids loading over a much wider fluidization gas flow rate range and in particular at much higher fluidization gas flow rates than with conventional fluidized bed reactor technology, offering increased flexibility with respect to cooling via the fluidization gas. Furthermore, the centrifugal force can be a multiple of earth gravity, allowing radial gas-solid slip velocities much higher than in conventional fluidized beds. The latter result in gas-solid heat transfer coefficients one or multiple orders of magnitude higher than in conventional fluidized beds. The combination of dense operation and high fluidization gas flow rates allows process intensification and a more compact reactor design.

Filtered gas–solid momentum transfer models and their application to 3D steady-state riser simulations

To account for meso-scale phenomena in coarse grid simulations, correlation terms appearing in the filtered gas–solid flow equations have to be modeled. Possible approaches to describe filtered gas–solid momentum transfer are evaluated via a mixture speed of sound test. In contrast to the effective drag coefficient closure model for the filtered drag force, the generalized added mass closure model for the correlation between the solid volume fraction and the gas phase pressure gradient shows an acceptable behavior. Furthermore, the generalized added mass may become determining in describing filtered gas–solid momentum transfer when sufficiently coarse grids are used. 3D steady-state riser simulations demonstrate the impact of the generalized added mass based description of filtered gas–solid momentum transfer on the calculated gas–solid flow behavior, in particular on the gas–solid slip velocity profile.

Simultaneous solution algorithms for Eulerian–Eulerian gas–solid flow models: Stability analysis and convergence behaviour of a point and a plane solver

Simultaneous solution algorithms for Eulerian–Eulerian gas–solid flow models are presented and their stability analyzed. The integration algorithms are based on dual-time stepping with fourth-order Runge–Kutta in pseudo-time. The domain is solved point or plane wise. The discretization of the inviscid terms is based on a low-Mach limit of the multi-phase preconditioned advection upstream splitting method (MP-AUSMP). The numerical stability of the simultaneous solution algorithms is analyzed in 2D with the Fourier method. Stability results are compared with the convergence behaviour of 3D riser simulations. The impact of the grid aspect ratio, preconditioning, artificial dissipation, and the treatment of the source terms is investigated. A particular advantage of the simultaneous solution algorithms is that they allow a fully implicit treatment of the source terms which are of crucial importance for the Eulerian–Eulerian gas–solid flow models and their solution. The numerical stability of the optimal simultaneous solution algorithm is analyzed for different solids volume fractions and gas–solid slip velocities. Furthermore, the effect of the grid resolution on the convergence behaviour and the simulation results is investigated. Finally, simulations of the bottom zone of a pilot-scale riser with a side solids inlet are experimentally validated.

Local slip velocity in a downer

Based on the EMMS model, the local slip velocity between gas and solid is systematically analyzed and a theoretical correlation of local slip velocity with local voidage for a downer is derived as follows: u s ( r ) u t = D 8 / 7 ( 1 - ɛ mf ) - 2 / 7 [ 1 - ɛ ( r ) ɛ ( r ) ] 8 / 7 ɛ ( r ) 47 / 14 ɛ ( r ) - ɛ mf ɛ ( r ) . Using this correlation, the local gas-solid slip velocity in a downer is calculated. The calculated results are well consistent with experimental data. In addition, the variation of the local slip velocity with its corresponding solid holdup is also discussed.

The clustering annular flow model of circulating fluidized beds.

The Clustering Annular Flow Model (CA model) is presented to describe the annular flow structure in the main column of a circulating fluidized bed (CFB). With the present model the flow parameters for a CFB including the dimensionless radius of the core α can be estimated, given the values of both the diameter and the voidage of the cluster phase.The model was successfully validated by comparison of values of α experimentally determined with those predicted from the model and observed cluster parameters. With consideration of the cluster formation it became possible to give a quantitative explanation of the annular flow structure and the high gas-solid slip velocity so far observed in circulating fluidized beds.

Particle flow pattern in the freeboard of a vortexing fluidized bed

A new coal-firing technique known as Vortexing Fluidized Bed Combustion (VFBC) was recently developed to improve the combustion intensity and the turndown capability, and to reduce fines elutriation and the freeboard height of conventional FBC boilers. This paper presents a simplified mathematical model for particle flow patterns in the freeboard of a vortexing fluidized bed. The governing differential equations for solid flows were formulated and solved numerically. Effects of particle properties (particle inverse relaxation time), axial and tangential air velocities, and particle weight loss due to combustion on particle trajectories in the freeboard were examined. Two types of air velocity profiles representing two practical designs of injection nozzles were compared. The particle inverse relaxation time, primary and tangential velocities were found to characterize the dynamic behavior of particles in the freeboard. Results also indicated that the VFBC process can increase the overall gas—solid slip velocity by 4 to 5 times, and the design of protrusive injection nozzles in the freeboard is superior to the design of peripheral air injection at the wall.