ECT studies of the choking phenomenon in a gas–solid circulating fluidized bed

Du, Bing; Warsito, W.; Fan, Liang‐Shih

doi:10.1002/aic.10168

Cited by 51 publications

(84 citation statements)

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Section: Shape Of Local Psdmentioning

confidence: 42%

“…In another kinetic-theory-based effort, Benhayia et al (Benyahia, Syamlal et al 2007) reported that wall roughness leads to a reverse core-annulus too, but the reversal is not as exaggerated as found in the current work. In an experimental study, Du et al (Du, Warsito et al 2004) observed deviation from the traditional core-annulus profile (coined a double-ring structure) during the transition to choking, which is at much higher solids concentration than in this work.One explanation for the new finding of reverse core-annulus profile involves the Stokes number (St) of the material and roughness elements along the upper wall of the rounded elbow exit. Specifically, particles are re-directed from the rounded exit elbow and back into riser due to large-scale roughness along the elbow.…”

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confidence: 42%

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Development, Verification, and Validation of Multiphase Models for Polydisperse Flows

Hrenya¹,

Cocco²,

Fox³

et al. 2011

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This report describes in detail the technical findings of the DOE Award entitled "Development, Verification, and Validation of Multiphase Models for Polydisperse Flows." The focus was on high-velocity, gas-solid flows with a range of particle sizes. A complete mathematical model was developed based on first principles and incorporated into MFIX. The solid-phase description took two forms: the Kinetic Theory of Granular Flows (KTGF) and Discrete Quadrature Method of Moments (DQMOM). The gas-solid drag law for polydisperse flows was developed over a range of flow conditions using Discrete Numerical Simulations (DNS). These models were verified via examination of a range of limiting cases and comparison with Discrete Element Method (DEM) data. Validation took the form of comparison with both DEM and experimental data. Experiments were conducted in three separate circulating fluidized beds (CFB's), with emphasis on the riser section. Measurements included bulk quantities like pressure drop and elutriation, as well as axial and radial measurements of bubble characteristics, cluster characteristics, solids flux, and differential pressure drops (axial only). Monodisperse systems were compared to their binary and continuous particle size distribution (PSD) counterparts. The continuous distributions examined included Gaussian, lognormal, and NETLprovided data for a coal gasifier.FINAL TECHNICAL 4 DE-FC26-07NT43098 5 EXECUTIVE SUMMARYThis report is the final technical report for the award DE-FC26-07NT43098 entitled "Development, Verification, and Validation of Multiphase Models for Polydisperse Flows." Below is a summary of work completed under the grant for each of the major goals. Goal I: Continuum Theory for Solid PhaseTwo separate and complementary continuum theories were derived to model polydisperse solids: the kinetic theory of granular flow (KTGF) and the discrete quadrature method of moments (DQMOM). Both theories are based on first principles with no adjustable parameters. The polydisperse KTGF and DQMOM were then encoded in MFIX, and underwent a wide range of verification testing to ensure, to the best possible extent, that no coding errors were present. These verification tests included both granular and gas-solid flows, including cases where an analytical solution and/or DEM (discrete element method) data and/or simple test cases. For the case of KTGF, another set of constitutive equations were derived which rigorously incorporated the gas phase for a simplified case of monodisperse systems at low Reynolds numbers. This derivation used the acceleration model developed in Task 2 from direct numerical simulations (DNS) in the starting kinetic equation, and indicated the effect of the fluid phase on the resulting constitutive relations. Goal II: Improved Gas-Particle Drag Laws -effect of particle size distributionIn this portion of the effort, two types of direct numerical simulations (DNS) were used to extract the drag force experienced by particles in a polydisperse suspension. These two methods, na...

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Section: Shape Of Local Psdmentioning

confidence: 42%

mentioning

confidence: 42%

Development, Verification, and Validation of Multiphase Models for Polydisperse Flows

Hrenya¹,

Cocco²,

Fox³

et al. 2011

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“…The riser is an integrated part of a circulating fluidized bed system used in our previous studies, 10 as shown in Figure 6, with an I.D. of 0.1 m and a height of 6.32 m. The solids circulation rate is measured by timing the falling distance of tracer particles in the standpipe.…”

Section: Experimental Approachmentioning

confidence: 99%

“…10 As shown in Figure 1, in the dense and acceleration regimes (represented by Planes 1 or 2), the solids concentration is much denser than that in the annulus (except in the wall region). This core-annulus-wall heterogeneous structure, significantly different from the commonly known ''core-annulus (wall)'' two-zone structure in riser flows, becomes unstable with the increase in solids loading, which eventually leads to the occurrence of choking 10 . In the dilute transport regime (represented by Planes 3 or 4), the solids concentrations, however, follow very much the ''core-annulus (wall)'' two-zone structure.…”

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confidence: 96%

Heterogeneous structure in gas–solid riser flows

You¹,

Zhu²,

Du³

et al. 2008

AIChE Journal

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in Wiley InterScience (www.interscience.wiley.com).Recent measurements by electric capacitance tomography reveal that, under certain operating conditions, the solids concentration distribution in the dense and acceleration regimes of gas-solid riser flows exhibits a strong heterogeneous structure, with the solids concentration near the centerline much higher than that in the annular surrounding flows (except in the wall region) within the same cross-section. This coreannulus-wall heterogeneous structure, significantly different from the commonly known ''core-annulus (wall)'' two-zone structure in riser flows, becomes unstable with the increase in solids loading, which eventually leads to the occurrence of choking. In this study, we propose a mechanistic model to investigate the formation and distribution of these interesting heterogeneous structures in riser flows. The intrinsic mechanisms include (1) counterflow between back mixing solids and upward moving solids from entrance, (2) the radial migration of solid flow from the wall toward the centerline of riser, and (3) the nonuniform acceleration of solids across the cross-section in the dense and acceleration regimes. The model, assuming one-dimensional two-phase flow in each zone along the riser, consists of a set of coupled ordinary-differential equations developed from the conservation laws of mass, momentum, and energy of both gas and solids phases. The solving algorithm is based on the Runge-Kutta method. Our model calculations show that the commonly known ''core-annulus (wall)'' flow structure occurs at low solids loadings and/or high gas velocities, whereas the coreannulus-wall flow structure is formed at moderate solids loading and/or at low gas velocities. With a high solids loading at low gas velocity, severe backflow results in a solid concentration peak formed in the core, which not only causes the flow instability but also possibly triggers the choking. Model simulations are validated by direct comparisons against measurements in solids concentrations as well as in the pressure drop along the riser, which shows a fairly good agreement.

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“…Under these circumstances, process tomography (PT) finds their unique role in studying the multiphase flow phenomena encountered in chemical engineering fields, involving a number of tomographic techniques, such as X-ray, -ray, optical, ultrasonic, electrical, and nuclear magnetic resonance imaging (Beck and Williams, 1996;Du et al, 2004;Dyakowski et al, 1997;George et al, 2000;Kumar et al, 1997;Rowe, 1971;Seville et al, 1986;Warsito and Fan, 2003a,b,c;Warsito et al, 1999). Among them, X-ray computed tomography (XCT) and electrical capacitance tomography (ECT) are regarded as the two types of the most attractive techniques (Warsito and Fan, 2003c;Werther, 1999).…”

Section: Introductionmentioning

confidence: 99%

A novel X-ray computed tomography method for fast measurement of multiphase flow

Cheng

Ding

et al. 2007

Chemical Engineering Science

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ECT studies of the choking phenomenon in a gas–solid circulating fluidized bed

Abstract: Choking is commonly defined as a phenomenon where a sudden change in the solids holdup occurs in a gas

Cited by 51 publications

References 37 publications

Development, Verification, and Validation of Multiphase Models for Polydisperse Flows

Development, Verification, and Validation of Multiphase Models for Polydisperse Flows

Heterogeneous structure in gas–solid riser flows

A novel X-ray computed tomography method for fast measurement of multiphase flow

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