Low Solids Fluxes Research Articles

CFD-DEM Simulation of Fast Fluidization of Fine Particles in a Micro Riser

In recent years, the discrete element method (DEM) has gradually been applied to the traditional fluidization simulation of fine particles in a micro fluidized bed (MFB). The application of DEM in the simulating fast fluidization of fine particles in MFB has not yet received attention. This article presents a drag model that relies on the surrounding environment of particles, namely the particle circumstance-dependent drag model or PCDD model. Fast fluidization in an MFB of fine particles is simulated using DEM based on the PCDD model. Simulations indicate that the local structure in an MFB exhibits particle aggregation, which is a natural property of fast fluidization, forming a structure where a continuous dilute phase and dispersed concentrated phase coexist. There exists a strong effect of solid back-mixing in an MFB, leading to relatively low outlet solid flux. The gas back-mixing effect is, however, not so distinct. The axial porosity shows a monotonically increasing distribution with the bed height but does not strictly follow the single exponential distribution. The solid volume fraction at the bottom of the bed is significantly lower than the correlated value in CFB. The axial heterogeneous distribution of the cross-sectional average porosity in the lower half of the bed is also weakened. The radial porosity shows a higher distribution pattern in the central region and a lower one in the sidewall region.

A general EMMS drag model applicable for gas-solid turbulent beds and cocurrent downers

Eulerian-Eulerian models incorporated with the kinetic theory of granular flow were widely used in the simulation of gas-solid two-phase flow, while the effects of mesoscale structures such as particle clusters and gas bubbles could not be considered adequately if traditional homogeneous drag models were adopted in the coarse grid simulations. The energy minimization multiscale (EMMS) model has been proved to facilitate calculating a structure-dependent drag coefficient by considering particle clustering phenomena, which can be coupled with the two-fluid model (TFM) to improve the accuracy of coarse-grid simulation of gas-solid circulating fluidized beds. However, the original EMMS drag model cannot be further applied to the simulation of gas-solid fluidized beds with solids flow rate smaller than zero, e.g., turbulent fluidized beds and cocurrent downward flow, because the original cluster diameter correlation gives rise to a value smaller than single particle diameter or even negative value at extremely low solids fluxes or downward gas-solid flow. In this study, a new proposed cluster evolution equation is proposed by quantifying local clustering dynamics to replace the original cluster diameter correlation. The newly formulated EMMS drag model can be used to avoid a negative cluster diameter to be involved in calculating interphase drag force in the overall fluidization regime. The improved EMMS drag law is incorporated into the Eulerian-Eulerian model to simulate gas-solid turbulent fluidized beds and cocurrent downer reactors, since they both were widely used in many industrial processes. By analyzing local hydrodynamics as well as the axial and radial heterogeneous distributions in the two kinds of fluidized beds, it is clarified that the simulation using the improved EMMS drag model shows a better agreement with the experimental data than the computation using the homogeneous drag law.

Mapping the Wall-Region Dynamics of High-Flux Gas-Solid Riser Using Scaling Regions from the Solid Concentration Time Series

An experimental study of the gas-solid flow dynamics in the high-flux CFB riser was accomplished by analysing the scaling regions from solid concentration signals collected from a 76 mm internal diameters and 10 m high riser of a circulating fluidized bed (CFB) system. The riser was operated at 4.0 to 10.0 m/s gas velocity and 50 to 550 kg/m2s solids flux. Spent fluid catalytic cracking (FCC) catalyst particles of 67 μm mean diameter and 1500 kg/m3 density together with 70% to 80% humid air was used. Solid concentration data were analysed using codes prepared in FORTRAN 2008 to get correlation integrals at different embedding dimensions and operating conditions and plot their profiles. Scaling regions were identified by visual inspection method and their location on planes determined. Scaling regions were analysed based on operating conditions and riser spatial locations. It was found that scaling regions occupy different locations on the plane depending on the number of embedding dimensions and operating conditions. As the number of embedding dimensions increases the spacing between scaling regions decreases until it saturates towards higher embedding dimensions. Slopes of scaling regions increases with embedding dimensions until saturation where they become constant. Slopes of scaling regions towards the wall decrease while the number of scaling regions for a particular profile increases. The span of the scaling region is wider at the initial values of hyperspherical radius than its final values. The scaling regions in some flow development sections show multifractal behaviour for each embedding dimension which manifests into visible basin which is defined in this study as multifractal basin. Further, the end points of the scaling region for each correlation integral profile differ from each other as the embedding dimension changes. This study suggests that identification of scaling region by visual inspection method is useful in understanding the gas-solid flow dynamics in the High-Flux CFB riser system. Further studies are recommended on risers of different diameters and heights operated at low and high solid fluxes and different gas velocities for comparison or usage of time series of different signal types like pressure fluctuations.

Quantifying cluster dynamics to improve EMMS drag law and radial heterogeneity description in coupling with gas-solid two-fluid method

Quantification of multiscale heterogeneity is the key to the simulation of gas-solid two-phase flow. The energy-minimization multiscale (EMMS) model can be used at different scales to describe local and global heterogeneities as well as steady-state fluid dynamics of gas-solid fluidization. However, the same cluster diameter correlation based on energy dissipation was used in all cases, which may give a smaller cluster diameter than single particle diameter at low solids fluxes. In this paper, a description of the solids mass transfer between the dilute and dense phases in heterogeneous gas-solid flow is proposed to replace the correlation of cluster diameter to avoid some problems associated with the empirical calculation of the cluster diameter. This cluster description can facilitate the prediction of the drag coefficient at the scale of numerical cells in computational fluid dynamics (CFD) at extremely low solids fluxes and the radial heterogeneity in large-scale circulating fluidized bed (CFB) risers with downward particle velocities near the wall. It is integrated with two-fluid model (TFM) to simulate the fluid dynamics in an industrial-scale CFB riser accurately, which is also accelerated significantly by using the predicted radial voidage distribution as the initial condition of CFD.

Effect of Fluxes on 60Sn-40Bi Solder Alloy on Copper Substrate

This paper investigated the effect of different types of fluxes on the wettability of a type of low temperature lead-free solder, 60Sn-40Bi alloy. The purpose of this paper is to investigate the effect of different types of fluxes on the wettability of 60Sn-40Bi solder (Tm : 138 -170 oC), so that the most compatible flux to be used with low temperature alloy can be determined.The results of this paper showed that the water soluble flux sample has the highest spread area and lowest contact angle. This meant that the solder has the highest wettability when water soluble flux is used, followed by RMA flux and low solids flux. Therefore, it was determined that water soluble flux is the most compatible to be used with the low temperature 60Sn-40Bi solder. The characteristic of this type of flux enables it to function well even at a low working temperature.

Ejector type solid circulation system analysis for circulating fluidized beds

Proper solid circulation is of crucial importance for the stable operation of circulating fluidized beds. There should be a pressure barrier between the riser and the downcomer to avoid reverse gas flow through the downcomer and to maintain a stable solid circulation. In conventional systems, loop seals, L, J, V, valves are mostly used as solid circulation valves. In this study, ejector (eductor) type solid circulation was investigated in an experimental CFB setup (94mm in diameter, 7950mm in height). The solid flux at the downcomer (34mm in diameter) and the motive gas flowrate of the ejector were investigated under different cold flow conditions. There are two different operation modes of the ejector type solid circulation depending on the solid flux through the downcomer. For low solid fluxes, there is no solid accumulation at the downcomer. As the solid flux increases, a solid column formation is observed above the ejector system. The desired pressure difference obtained with the ejector is higher for the latter mode of operation and requires less motive gas flow compared to the former for proper operation. The experimental results show that the ejector is capable to provide the required pressure barrier for the CFB loop in both cases. In addition to the experimental study, ejector system was analyzed with CFD model in order to investigate the effect of basic geometric and operational parameters on the pressure difference generated by the ejector. Furthermore a novel semi-empirical method was proposed as a tool for predicting the ejector performance comparable with CFD results.

Effect of Different Gas-Solid Drag Models in a High-Flux Circulating Fluidized Bed Riser

Circulating fluidized beds (CFBs) have applications in many industrial processes like fluid catalytic cracking, coal gasification, coal combustion, biomass gasification and chemical looping. Consequently, the process have been studied intensively for many researchers seeking to understand the complex gas-solid flow encountered in circulating fluidized beds. In this sense, the Computational Fluid Dynamics (CFD) tools are very useful to study gas-solid flows, but this approach directly depends on the correct choice of the mathematical models. There are many numerical studies on CFBs at low solids fluxes and only a few studies for high solids flux, which is very common in many applications, therefore, this regime was chosen to conduct the simulations in this work. The results were compared with experimental data on a riser with an internal diameter of 76 mm and a height of 10 m. Radial and axial profiles were computed using a two-phase 3-D computational fluid dynamics model, with four different correlations for the drag between the phases, Gidaspow, Syamlal-O’Brien, and other two models that account for particle clusters, the energy-minimization multi-scale (EMMS) and the Four-Zone model. The results indicates that all the correlations predicts the solids concentration, found experimentally, in the dilute and developed flow regions but, in the region of highest solids concentration, especially at the level close to the inlet, the Gidaspow and Syamlal-O’Brien correlations not properly represented the solids concentration. The EMMS and Four-Zone models improved the results on these regions showing the influence of these models especially at the bottom of the riser, where the solids concentration is higher.

Effect of closure models on Eulerian–Eulerian gas–solid flow predictions in riser

Gas–solid flows in a riser have often been investigated by using the Eulerian–Eulerian (E–E) model. The E–E model consists of mass and momentum conservations equations, along with equations for boundary conditions and several closure relations that dictate flow predictions. Despite several previous studies, there is still ambiguity in the selection of appropriate closure laws and boundary conditions for a given flow condition. In this study, the effects of specularity coefficients of partial-slip wall boundary condition, gas phase viscous stress models and interphase drag models on flow predictions from the E–E model have been investigated for both low (14.3kg/m2s) and high (489kg/m2s) solids flux riser flows. A lower specularity coefficient of 0.0001 gives a reasonable agreement between the predicted axial profiles and experimental data; however, in a dense bottom region (at a height of 3.5m) of low solids flux riser, a higher specularity coefficient of 0.1 shows a core–annulus radial profile as observed in the experiments. Both laminar and k-ε dispersed turbulent models are able to give qualitative predictions of the experimental values, whereas predictions from k-ε per-phase turbulent model result in wide discrepancies. For both the solids flux conditions, the conventional Gidaspow drag model fails to predict the experimental data; whereas the axial and radial profiles from both the EMMS and corrected-EMMS models are in reasonable agreement with the experimental data. Only a minor difference between the values predicted by the EMMS and corrected-EMMS is observed as the drag values calculated from the both drag models are similar for a range of solid volume fraction from 0 to 0.15. While the laminar or k-ε dispersed turbulent model and EMMS or corrected-EMMS drag model are found to be more appropriate for riser simulations with both flow conditions, the selection of the specularity coefficient requires more investigations at various flow conditions.

Hydrodynamics of gas–solid risers using cluster structure-dependent drag model

A modified cluster structure-dependent (CSD) drag model is proposed to improve drag predictions for heterogeneous gas–solid flows in risers. A micro-meso-grid scales (M2GS) equation set consists of six hydrodynamic equations and one stability criteria with bivariate extreme value (BEV) theory as a function of eight independent variables and four dependent parameters on the basis of grid parameters. The modified CSD drag model is further verified by CFD simulations by coupling with the two-fluid model for low and high solid fluxes in risers. The simulated results in 2D domain of the riser are compared with those using the Huilin–Gidaspow drag model and experimental data. The modified CSD drag model is verified by CFD simulations by coupling with a kinetic theory of granular flow based two-fluid model for low and high solid fluxes in risers. The comparison shows that the modified CSD drag model is able to capture the axial heterogeneity with the dense bottom and dilute top sections. The radial profiles using modified CSD drag models show only qualitative agreement with the experimental data. The results using the modified CSD drag model and Huilin–Gidaspow drag model show a reasonable agreement at the center. Thus, further improvements combining with wall friction effect are required to achieve quantitative agreement with experimental data.

Characterizing Corrosion Effects of Weak Organic Acids Using a Modified Bono Test

To meet environmental requirements and achieve benefits of cost-effective manufacturing, no-clean fluxes (NCFs) or low-solids fluxes have become popular in present electronic manufacturing processes. Weak organic acids (WOAs) as the activation ingredients in NCFs play an important role, especially in the current lead-free and halogen-free soldering technology era. However, no standard or uniform method exists to characterize the corrosion effects of WOAs on actual metallic circuits of printed wiring boards (PWBs). Hence, the development of an effective quantitative test method for evaluating the corrosion effects of WOAs on the PWB’s metallic circuits is imperative. In this paper, the modified Bono test, which was developed to quantitatively examine the corrosion properties of flux residues, is used to characterize the corrosion effects of five WOAs (i.e., abietic acid, succinic acid, glutaric acid, adipic acid, and malic acid) on PWB metallic circuits. Experiments were performed under three temperature/humidity conditions (85°C/85% RH, 60°C/93% RH, and 40°C/93% RH) using two WOA solution concentrations. The different corrosion effects among the various WOAs were best reflected in the testing results at 40°C and 60°C. Optical microscopy was used to observe the morphology of the corroded copper tracks, and scanning electron microscopy (SEM) energy-dispersive x-ray (EDX) characterization was performed to determine the dendrite composition.

CPFD simulation of circulating fluidized bed risers

This study investigated the applicability of computational particle fluid dynamics (CPFD) numerical schemes for simulating flows in circulating fluidized bed (CFB) risers. Gas–solid flows were simulated in CFB risers containing Geldart A particles with both low and high solid fluxes as well as in a CFB riser containing Geldart B particles for three flow conditions using CPFD. The results are compared to experimental data and previous two-fluid model (TFM) simulations. The time-averaged axial and radial distributions of the solid concentration show that the bottom-dense, upper-dilute and core-annulus heterogeneous structures were successfully captured by the CPFD calculations, but only qualitatively. The results differ from experimental data for Geldart A particles and high solid fluxes, although they were more accurate than two-fluid simulations with conventional drag models. Two-fluid modeling with the EMMS (energy minimization multi-scale) drag model gave more accurate results than the CPFD simulations. The results indicate that the drag force in the CPFD scheme is still overestimated, although the cumulative method used to compute drag force is more accurate than the proportional method in the two-fluid model. An EMMS drag model which takes into account the intrinsic heterogeneity in the CFB risers is needed for the CPFD scheme. The effect of the realistic particle size distribution was seen in the by CPFD results. CFB risers with Geldart A and B particles are calculated using CPFD scheme. The results are more accurate than two-fluid model simulations with the same drag model. A drag model accounting for heterogeneities in CFB risers is needed for CPFD simulation to agree well with experimental data. The effect of particle size distribution is detected by CPFD simulation. ► CFB risers with Geldart A and B particles are calculated using CPFD scheme. ► The capability of CPFD for calculating flows in CFB risers is promising. ► Comparing to TFM, the CPFD scheme is indicated to be more advanced. ► A drag model accounting for heterogeneities in CFB risers is needed for CPFD. ► The effect of particle size distribution is detected by CPFD calculation.

Desliming of dense minerals in fluidized beds

This paper applies Drift Flux theory for the first time to explore the relationship between the liquid and solids flow through fluidized beds in the context of desliming of dense minerals. At low solids fluxes the process is not flux curve constrained. Here continuity considerations indicate the net liquid flux through the lower bed is upwards, ideal for the removal of slimes from the underflow product. Moreover, the liquid split to the underflow is also lower, further reducing the slimes content of the underflow. At higher solids fluxes the net liquid flux becomes downwards and eventually the system becomes flux curve constrained. Under these circumstances slimes entrainment to the underflow increases significantly. In order to operate at these higher solids fluxes, and achieve efficient desliming, Split Fluidization should be used. Here additional fluidization liquid is added at a higher elevation, producing a net liquid flux in the upwards direction through the zone above the Split Fluidization entrance level. Desliming experiments covering a range of solids fluxes were conducted to investigate the effects of increasing solids flux and Split Fluidization. This study shows that a system of parallel inclined channels, a key feature of the Reflux Classifier, permits the introduction of this additional liquid while preventing the ultra fine product from being entrained to the overflow reject stream, thus permitting the efficient removal of slimes. Experiments were conducted using an iron ore feed with particles smaller than 0.50mm in diameter. Efficient desliming at a relatively high solids feed flux of 20t/m2h was achieved, but to efficiently deslime at 40t/m2h significant Split Fluidization was found to be essential.

Extending EMMS-based models to CFB boiler applications

Recently, EMMS-based models are being widely applied in simulations of high-throughput circulating fluidized beds (CFBs) with fine particles. Its use for low flux systems, such as CFB boiler (CFBB), still remains unexplored. In this work, it has been found that the original definition of cluster diameter in EMMS model is unsuitable for simulations of the CFB boiler with low solids flux. To remedy this, we propose a new model of cluster diameter. The EMMS-based drag model (EMMS/matrix model) with this revised cluster definition is validated through the computational fluid dynamics (CFD) simulation of a CFB boiler.

Hydrodynamics of an FCC riser using energy minimization multiscale drag model

In this study, a structured-based drag was derived using the energy minimization multiscale (EMMS) model, and used to carry out computational fluid dynamics (CFD) simulations for low and high solid flux fluid catalytic cracking (FCC) risers. The results were compared with those using the Gidaspow drag model, as well as experimental data and previous simulation results. Initially, the EMMS model was solved for two flow conditions and the correlations for the drag coefficients were derived, which were then used to simulate 2D domain of the risers. The time-averaged axial and radial profiles of voidages and pressured drop were compared with the experimental data. The comparison showed that only EMMS model was able to capture the axial heterogeneity with the dense bottom and dilute top sections. The radial profiles using both drag models showed only qualitative agreement with the experimental data. The results using the EMMS and Gidaspow drag model showed a reasonable agreement near the wall and the centre, respectively. Thus, it was concluded that the EMMS model was able to predict both axial and radial heterogeneity for both flow conditions, but only qualitatively; however, further improvements are required to achieve quantitative agreement with the experimental data.

Simulation of gas–solid flows in riser using energy minimization multiscale model: Effect of cluster diameter correlation

Reduced effective drag is observed in gas–solid riser flows due to formation of clusters. Thus cluster diameter correlation has direct impact on the calculated drag and the hydrodynamics predictions. However, its effect has not been studied. Therefore in this study, the effect of cluster diameter correlations on the drag coefficient and simulation predictions is evaluated. A structure-based drag is derived using the EMMS model, and is used to carry out computational fluid dynamics (CFD) simulations for low solid flux fluid catalytic cracking (FCC) risers. The results are compared with those using the Gidaspow drag model, as well as experimental data and previous simulation results. The time-averaged axial and radial profiles of voidages are compared with the experimental data. The comparison shows that only EMMS model is able to capture the axial heterogeneity with the dense bottom and dilute top sections. The radial profiles using both drag models shows only qualitative agreement with the experimental data. The results using the EMMS and Gidaspow drag model show a reasonable agreement near the wall and the centre, respectively. In order to improve the quality of the results obtained by the EMMS model, simulations are conducted using calculated drag coefficients from different cluster diameter correlations. The cluster diameter correlation proposed by Harris et al. (2002) gives reasonable qualitative and quantitative agreement with the experimental data for axial voidage profile, particularly in the dense bottom section; however, the quantitative disagreements in the radial profiles persists.

Circulation of Geldart D type particles: Part II—Low solids fluxes: Measurements and computation under dilute conditions

In Part I of this paper, solids slugging phenomena were studied for high solids fluxes. In this study, the solids slugging was eliminated for extremely low solids fluxes. Here, gamma ray densitometry and PIV techniques were used to measure radially averaged solids volume fractions, and solids laminar and turbulent properties near the wall, respectively. The PIV system measured turbulent properties for solids, such as stresses, granular temperatures, and dispersion coefficients, non-invasively near the wall, in the developing region. This study was the first known measurement of solids dispersion coefficients in the axial and radial directions, using the PIV technique. A two-dimensional kinetic theory based IIT CFD code was used to perform simulations. The measured and computed radially averaged total granular temperatures, axial and radial solids and axial gas dispersion coefficients, reasonably agreed with the literature. This study also showed the capabilities of the kinetic theory based CFD codes to reasonably match the solids dispersion coefficients measured using the PIV technique.

Simulation of sludge blanket height in clarifiers

Sludge blanket height (SBH) is an important parameter in the clarifier design, operation and control. Based on an overview and classification of SBH algorithms, a modified SBH algorithm is proposed by incorporating a threshold concentration limit into a relative concentration sharp change algorithm to eliminate the disturbance of compression interfaces on the correct simulation of SBH. Pilot-scale test data are adopted to compare reliability of three SBH algorithms reported in literature and the modified SBH algorithm developed in this paper. Calculated results demonstrate that the three SBH algorithms give results with large deviation (> 50%) from measured SBH, especially under low solid flux conditions. The modified algorithm is computationally efficient and reliable in matching the measured data. It is incorporated into a one-dimensional clarifier model for stable simulation of pilot-scale experimental clarifier data and into dynamic simulation of a full-scale wastewater treatment plant (WWTP) clarifier data.

Identification of Antarctic ablation areas using a regional atmospheric climate model

The occurrence of Antarctic ablation areas in Dronning Maud Land, the Lambert Glacier Basin, Victoria Land, the Transantarctic Mountains and the Antarctic Peninsula is realistically predicted by the regional atmospheric climate model RACMO2/ANT, with snowdrift‐related processes calculated offline. Antarctic ablation areas are characterized by a low solid precipitation flux in combination with strong sublimation, snowdrift erosion and/or melt. The strong interaction between atmospheric circulation and topography plays a decisive role in the precipitation distribution and hence that of ablation areas. Three types of Antarctic ablation areas can be distinguished, all occurring in dry regions: Type 1 is the erosion‐driven ablation area, caused by 1‐D and/or 2‐D divergence in the katabatic wind field at high elevations (2000–3200 m asl). Type 2 is the sublimation‐driven ablation area. This type occurs at lower elevations (<2000 m) preferably at the foot of steep topographic barriers, where temperature and wind speed are high and relative humidity low. Type 3 represents the melt‐driven ablation area, occurring in the northern Antarctic Peninsula. Combinations of types 1/2 and 2/3 are possible. Over the period considered here (1980–2004), no significant trend is found in the total area covered by Antarctic ablation areas, which equals about 2% of the total ice sheet surface (including ice shelves).

Experimental Study of the Solids Velocity Field in Gas−Solid Risers

Solids flow dynamics in gas−solid risers is inherently complex. Model refinement through experimental validation requires the acquisition of detailed nonintrusive measurements. In this study, noninvasive computer-automated radioactive particle tracking (CARPT) is employed to visualize and quantify in a three-dimensional domain the solids dynamics and mixing in gas−solid risers. This technique has the added advantage that, along with the derived Eulerian solids flow field (time-average velocity map and various turbulence parameters such as the Reynolds stresses, turbulent kinetic energy), it also provides directly the Lagrangian description of the solids motion. The solids velocity field data are obtained in two different risers at low and high solids fluxes at varying superficial gas velocity to span both the fast-fluidized (FF) and dilute phase transport (DPT) regimes. The effect of operating conditions on solids flow and mixing is studied. Comparative analysis of the results is presented to provide insi...

Steady state thickener modelling from the compressive yield stress and hindered settling function

An algorithm has been developed to predict steady state thickener operation from fundamental material properties, properly accounting for compression of the suspension network structure within the sediment bed. The material properties include the compressive yield stress, Py(ϕ), and the hindered settling function, R(ϕ). Py(ϕ) reflects the suspension network strength as a function of solids volume fraction ϕ, while R(ϕ) is inversely related to the permeability. The required inputs to the model include Py(ϕ) and R(ϕ) curve fits, thickener diameter as a function of height, solids density, liquid density and feed solids volume fraction. The model output is either solids throughput or solids flux as a function of underflow solids concentration, for a range of suspension bed heights. As a bonus, the solids residence time in the suspension bed can also be determined. The algorithm involves prediction of the solids throughput versus underflow solids concentration in two parts; free settling (clarification) and compression within the suspension bed (thickening). The free settling prediction utilises an adaption of the simple Coe and Clevenger method, while prediction of compression in the bed is achieved through integration of a differential equation developed from the fundamental dewatering theory of Buscall and White. The limiting steady state solids flux is the minimum of the two predicted values for each underflow solids concentration and bed height. In just minutes, this algorithm can produce tabulated and graphical results providing useful insights into the inter-relationship between solids throughput, bed height and underflow solids concentration. For steady state thickener operation, the outputs reveal three general modes of stable operation; permeability limited at high solids fluxes, compressibility and permeability dependant at intermediate solids fluxes and compressibility limited at very low solids fluxes. Knowledge of the conditions under which each of these modes is applicable enables process operators to understand the effect of variations in process conditions and assists in process optimisation.