Solids Mass Flux Research Articles

New closure models for CFD modeling of high-density circulating fluidized beds

Experimental results from a high-density circulating fluidized bed (CFB) riser have been used to develop new closure models for the drag coefficient and the gas–solid mixture viscosity. The models predict a rapid increase in both viscosity and drag associated with the high solids concentrations near the riser wall. These new models have been incorporated into the commercial Computational Fluid Dynamics software FLUENT and the predictions of FLUENT have been compared with experimental data from the literature. With the inclusion of the new closure models, FLUENT was able to predict the radial distribution of solids concentration and solids mass flux found experimentally in three different cold-flow CFB risers operated at solids mass fluxes between 148 kg/m 2·s and 455 kg/m 2·s and superficial gas velocities between 4.7 and 10 m/s. These conditions lead to average solid concentrations in excess of 10 vol%, which corresponds to high-density CFB operation.

Numerical simulations of gas–solid flow in tapered risers

Hydrodynamic behavior of gas–solid flow in tapered risers was simulated using the two-fluid model based on the kinetic theory of granular flow representing the constitutive relations of the solid phase. Present numerical model was verified by comparing with experimentally measured solid mass fluxes, particle concentrations and velocities in column risers. Computed results showed that the core-annular flow structure existing in the column riser may disappear in the tapered risers. The distributions of particle concentration tend to be more uniform in the tapered riser than that in the column riser under the same operating conditions. The uniform particle distribution can be achieved by changing the inclined angle of the tapered riser under specific operating conditions.

Characterization of the flow patterns in a pressurized circulating fluidized bed

To investigate the influence of the solid/gas density ratio ρ S/ ρ F on the flow pattern in a riser reactor, a pressurized circulating fluidized bed with an inner diameter of 0.194 m and 9 m height was used. The experiments have been carried out from ambient conditions to a static pressure p abs of 5 MPa in the regime of fast fluidization and/or pneumatic conveying. With the use of glass beads ( ρ S = 2500 kg/m 3, d P = 85 μm), density ratios ρ S/ ρ F from 2094 to 41 and solids net fluxes G S,Net from 65 to 200 kg/m 2s ( U G = 0.8 to 4.5 m/s) were found. Local information about solids concentrations, solids velocities, and solids mass flows has been obtained by a capacitance probe system. The results show a more uniform axial and radial solids distribution with increasing pressure p abs or decreasing density ratios ρ S/ ρ F, indicating flow conditions which are different to a well known core-annulus flow structure. This flow structure is defined by a dilute upflow in the core region and a dense downflow in the annulus region which is situated immediately near the wall. In agreement, the radial profiles of axial solids mass flux show flow patterns which are untypical for fast fluidization operating conditions. The results depict a distinct wall region that covers almost half of the riser cross-sectional area. Therefore, significant solids downflow occur, pointing out massive solids backmixing even at low solid/gas density ratios ρ S/ ρ F. Finally, the obtained information is used to develop an easy correlation for the prediction of internal solids reflux in a riser reactor as a function of solids/gas density ratios ρ S/ ρ F and the dimensionless superficial gas velocity Fr P.

Time-dependent behavior of the ash particle size distribution in a circulating fluidized bed system

In the present work, a particle population balance for a circulating fluidized bed combustor (CFBC) is developed that allows a description of the fate of individual particles in terms of attrition and transport effects. In the modeling of abrasion and shrinkage of particles, we consider besides operating conditions and particle size distribution also the residence time of particles in the system. The model takes account of the fact that fresh particles have a higher attrition rate than particles that have stayed for some time already in the system. The model aims at the description of the dynamic adjustment of the particle size distribution in a given system. The model is validated with experimental data from a pilot plant scale combustor, i.e., a CFBC with an inner diameter of 0.1 m and height of 15 m. A comparison between experimental and calculated data shows the suitability of the model. The model gives furthermore the opportunity for studying the effects of changes in operating conditions on the steady state solids particle size distribution and solids mass fluxes. The results of modeling a load change of the CFBC are presented and discussed in this work.

Measurement of overall solids mass flux in a gas–solid Circulating Fluidized Bed

It is important to know the overall solids flux and solids Cycle Time Distribution (CTD) to properly characterize Circulating Fluidized Bed (CFB) systems. Although conceptually simple, estimating the solids mass flux in these closed loop systems, either via experiments or by using a model, is a non-trivial problem. In the present work, a non-invasive in situ technique is applied, which is sensitive, and can be used on a large-scale circulating flow systems. The technique is based on estimating the solids velocity and volume fraction distribution in a section (downcomer) of the CFB loop. Solids velocity is obtained by tracking a single radioactive particle as tracer, using a two detector setup, and the volume fraction distribution is obtained by γ-ray densitometry measurements. In the downcomer section of a 2-in.-diameter column, the solids flow was found to be close to a moving packed bed condition, with minimal random fluctuations in the solids volume fraction. Hence, only a negligible error is incurred by using the cross-sectionally averaged solids velocity and holdup values for the estimation of the solids mass flow. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity, and the mass flux values compared with those obtained from a timing and weighing method were within 4%.

Quantification of solids flow in a gas–solid riser: single radioactive particle tracking

Solids in risers of circulating fluidized beds (CFB) exhibit local backflow and recirculation. Measurement of the concentration-time response to an impulse injection of tracer, even at two elevations cannot determine the residence time distribution (RTD) of solids uniquely. Hence, evaluation of RTD in risers from conventional tracer responses is difficult and often not possible. In addition, estimating the solids circulation rate in these closed loop systems, is a non-trivial problem. In this work, a single radioactive particle in the CFB loop is tracked during its multiple visits to the riser and, by invoking ergodicity, solids circulation rate, accurate solids RTD and additional information on the solids flow pattern in the riser are estimated. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity. A second peak in the probability density function (PDF) of the solids RTD curve in the riser was observed for operating conditions in the fast-fluidization regime.

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles ( d p = 70 μ m , ρ p = 1700 kg / m 3 ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered ( U g = 5 – 8 m / s ; G s = 22 – 345 kg / m 2 s ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.

NUMERICAL SIMULATION OF THE GAS-PARTICLE FLOW IN RISERS I: THE MACRO FLOW BEHAVIOR

The steady gas-particle flow in risers under variant operating conditions was predicted based on a k-ϵ-kp-ϵp-Θ two-fluid model, which describes the dense turbulent gas-particle flow by the Eulerian method, considering the turbulence of both gas and particles as well as the particles' collisional effects. Much information on the macro flow behavior in risers, such as the profiles of local particle axial velocity, solids volume fraction, and solids mass flux in different operating regimes was obtained, and the predicted results show satisfactory agreement with experimental data. Further analysis of the predictions gives the comprehension of flow characteristics on the macro scale.

Flow structure and thickness of annular downflow layer in a circulating fluidized bed riser

Flow structures were determined in a circulating fluidized bed (CFB) riser (0.203 m i.d.×5.9 m high) of FCC particles ( d p=70 μm, ρ s=1700 kg/m 3). A momentum probe was used to measure radial momentum flux profiles at several levels and to distinguish between upward and downward flow regions. Time–mean dynamic pressure (Δ P m) decreases towards the wall in the range U g=5–8 m/s, G s=10–340 kg/m 2 s. The thickness of the annular downflow layer based on Δ P m=0 reaches a maximum with increasing height. The annular downflow layer disappears locally with increasing solids mass flux ( G s) at a constant gas velocity, with achievement of the dense suspension upflow (DSU) regime. A new correlation is developed to predict the time–mean thickness of solids down-flowing layer based on solids mass flux and momentum flux. It successfully accounts for the variation of the annular layer thickness with height and G s, and covers a wide G s range right up to near the onset of the DSU regime.

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO2) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.

Studies on hydrodynamics of annular circulating fluidised bed drier with millet ragi particles mixture

Millet and ragi particles mixture with the millet particles having average diameter 2250 μm and density 1163.9 kg m - 3 and ragi particles of average diameter 1500 μm and density 1310.4 mg m - 3 , with the mass fraction 0.5 each of millet and particles were used to study the hydrodynamics of an annular circulating fluidised bed drier. Gap openings were varied from 12 mm to 72 mm and it was found that mass flux of solids remained constant for all air rates for gap openings greater than 60 mm. Dilute phase flow and dense phase slugging flow regime models were verified. A correction factor, K has been incorporated for the dense slugging flow regime. The error in the use of the model for pressure drop calculation was 28%. As there was smooth flow of particle mixture and little variation in pressure drop in dilute phase regime, drying could be done in this regime.

CFD simulation of dilute phase gas–solid riser reactors: Part I—a new solution method and flow model validation

A three-dimensional simulation of a dilute phase riser reactor (solid mass flux: 2 kg m −2 s −1 ) is performed using a novel density based solution algorithm. The model equations consisting of continuity, momentum, energy and species balances for both phases, are formulated following the Eulerian–Eulerian approach. The kinetic theory of granular flow is applied. The gas phase turbulence is accounted for via a k– ε model. An extra transport equation describes the correlation between the gas and solid phase fluctuating motion. The solution algorithm allows a simultaneous integration of all the model equations in contrast to the sequential multi-loop solution in the conventional pressure based algorithms, used so far in riser simulations. The simulations show an unsteady behaviour of the flow, but a core-annulus flow pattern emerges on a time-averaged basis. The abrupt nature of the T type outlets causes a significant recirculation of gas and solid from the top of the riser. The flow near the outlets is highly non-symmetric and has a three-dimensional character. A significant decrease of the gas phase turbulence and particle granular temperature across the riser length is attributed to the presence of small particles, which is qualitatively consistent with the experimental data from literature.

Two-dimensional transient numerical simulation of solids and gas flow in the riser section of a circulating fluidized bed

A computational fluid dynamics software (CFX) was modified for gas/particle flow systems and used to predict the flow parameters in the riser section of a circulating fluidized bed (CFB). Fluid Catalytic Cracking (FCC) particles and air were used as the solids and gas phases, respectively. Two-dimensional, transient, isothermal flows were simulated for the continuous phase (air) and the dispersed phase (solid particles). Conservation equations of mass and momentum for each phase were solved using the finite volume numerical technique. Two-dimensional gas and particle flow profiles were obtained for the velocity, volume fraction, and pressure drop for each phase. Calculations showed that the inlet and exit conditions play a significant role in the overall mixing of the gas and particulate phases and in the establishment of the flow regime. The flow behavior was analyzed based on the different frequency of oscillations in the riser. Comparison of the calculated solids mass flux, solids density and pressure drop with the measured pilot-scale PSRI data (reported in this paper) showed a good agreement.

Radial solid mass flux profiles in a high-suspension density circulating fluidized bed

For external solid mass fluxes between 148 and 264 kg/(m 2·s), at a fixed superficial gas velocity of 4.7 m/s, a dense bed was formed at the base of a 0.14-m ID circulating fluidized bed (CFB) riser. The apparent solid concentration in the dense bed was approximately 17 vol%. Radial solid mass flux profiles were measured at axial locations of 1.6 and 2.1 m. The net flow of solids was always upward at 1.6 m, and the maximum occurred at the riser wall. Net downflow was observed for 148 kg/(m 2·s) at 2.1 m. Net flow at the wall was still upward for the other flux conditions, but the maximum shifted to an intermediate radial location near r/ R=0.3. A qualitative description of the developing flow at the base of the riser is presented and used to interpret the profiles from previous studies.

Characterization of nonlinear dynamics in a circulating fluidized bed by rescaled range analysis and short-term predictability analysis

In the present study, nonlinear hydrodynamic behavior of a circulating fluidized bed with a riser 0.10 m in inner diameter and 10 m in height was characterized in terms of the Hurst exponent and V statistic obtained by the R/ S analysis, as well as Mann–Whitney statistic determined by short-term predictability analysis. Hurst exponent, the maximum V statistic and Mann–Whitney statistic were estimated for time series of the local heat transfer rate measured by a hot-wire probe in three radial positions at the height of 6.0 m from the riser bottom. The effects of superficial gas velocity and circulating solid mass flux on the gas–solid flow behavior in the CFB were examined based on the statistics. The Mann–Whitney statistic estimated for the time series was lower than the 99% rejection threshold for the null hypothesis, indicating that the gas–solid flow behavior in the CFB is statistically distinguishable from linearly correlated noise. Radial distributions of the Hurst exponent and Mann–Whitney statistic were found to become flatter as circulating solid mass flux increased. This result indicates that the gas–solid flow structure becomes more uniform and deterministic with increasing circulating solid mass flux. The variation of the maximum V statistic toward circulating solid mass flux appeared to be similar to that of the Hurst exponent.

An experimental study of fluid catalytic cracking in a downer reactor

First results of an experimental study of the fluid catalytic cracking (FCC) process in a new type of reactor, the downflow gas–solids reactor or ‘downer’, are presented. A bench-scale experimental set-up has been constructed, the downer with an inner diameter of 25 mm and a length of 800 mm being the central part of it. As a first step in the development and characterisation of this reactor, catalytic cracking of cumene has been chosen as a model reaction. In a temperature range of 743–823 K, catalyst-to-oil (CTO) ratios up to 25 and residence times of 60–350 ms, conversions were between 10 and 70%. Comparison to modelling results, on the basis of a plug flow model, showed that the measured conversions were lower than expected on the basis of the calculations. The higher the solids mass flux, the greater the difference between the two, indicating that incomplete break-up of the solids stream entering the downer — with particle clusters negatively affecting transfer processes to and from the individual particles — might be the cause for these reduced conversions.

Solid Recycle Characteristics of Loop-seals in a Circulating Fluidized Bed

Solid recycle characteristics through a conventional and a newly developed loop-seal (0.08 m i.d.) system are determined in a circulating fluidized bed of FCC or silica sand particles. In the loop-seal developed here, gas was injected downward tangentially to the wall of the loop-seal to increase solids mass flux with stable flow. For conventional loop-seal, solids mass fluxes increase linearly with increasing aeration rate but it reaches a maximum value. At the same aeration rate with different aeration locations (0.1 – 0.6 m) in a conventional loop-seal, a maximum solids mass flux is seen at a height to diameter ratio of 2.5. For the newly developed loop-seal, mass fluxes of FCC and sand particles are higher and more stable than in conventional loop-seal at the same aeration rate. The solid mass fluxes obtained have been correlated with the aeration rate and Archimedes number.

DESCRIBING MASS TRANSFER IN CIRCULATING FLUIDIZED BEDS BY OZONE DECOMPOSITION

The objective of this work was lo improve understanding of the mass transfer between gas and solids in the riser section of a gas-solids circulating fluidized bed (CFB). In order to experimentally investigate gas-solids mass transfer in a CFB, a suitable low-temperature mass transfer process was applied: the decomposition of ozone (O3) using particles as catalyst. Particles used were angular cast steel with an average diameter (mass mean) of 134μm. The experiments were carried out in a semi-industrial sized CFB. Operating conditions were varied, with superficial gas velocity of up to 7.2 ms−1 at 60°C riser temperature, and riser net solids mass flux up to 53kgs−1m2. The overall solids concentration was around 2-2.5% and about 10% in the lower part of the riser. Ozone concentration profiles, axial pressure profiles, solids net mass flux as well as local particle velocities were measured. Gas velocity profiles were assumed in order to calculate the total transport of ozone at different heights in the riser. The importance of the lower part of the riser with respect to mass transfer is shown.

Hydrodynamic studies of a Annular Circulating Fluidised Bed Drier with sand particles

The hydrodynamics of Annular Circulating Fluidised Bed Drier (ACFBD) has been studied with sand particles of diameter 928 m and density 2650 kg/m3. The gap opening was varied from 12 to 48 mm and the mass flux of solids was found to be constant for the gap openings greater than 44 mm. Two regimes, slugging dense flow and dilute phase flow were identified and models were proposed for pressure drop calculation for the two regimes. An erratic factor has been introduced for the slugging dense flow regime. The models agreed with the experimental data with an error of less than 20%.

The hydrodynamics of the high-density bottom zone in a CFB riser analyzed by means of electrical capacitance tomography (ECT)

The hydrodynamics of the bottom region of a CFB riser have been investigated. Experiments were performed in a 0.14-m I.D. CFB riser using FCC catalyst. The riser solids mass flux was varied between 148 and 302 kg/m 2 s and the superficial gas velocity between 3.7 and 4.7 m/s. Pressure transducers were employed to evaluate the axial profile, especially the height of the bottom zone in the CFB riser. The bottom zone itself was analyzed by means of electrical capacitance tomography (ECT). Radial solids concentration profiles, probability density distribution profiles, standard deviation profiles and frequency spectra were determined in order to quantify the bottom zone hydrodynamics. The different investigation methods are in a good agreement and indicate a core-annulus structure in the bottom zone of the CFB riser. A lean core region with a narrow probability density distribution and a small standard deviation of the solids concentration exists, while the wall region exhibits a high solids concentration with a wide probability density distribution and a large standard deviation of the solids concentration. Within the bottom zone the hydrodynamics vary little with a change in height or external solids mass flux.