High Solids Flux Research Articles

Solids flux profiles of Geldart Group A particles in high‐velocity circulating fluidized bed risers

AbstractCirculating fluidized bed (CFB) risers using Group A particles have traditionally, mostly, been considered to operate in the fast fluidization regime, which consists of a core‐annulus flow profile with solids refluxing in the annulus layer. High gas and solids flow riser studies, however, suggest the existence of additional types of flow behaviours. Therefore, more studies are still needed to help clear uncertainties of local solids flow patterns in CFB risers of Group A particles, especially at gas and solids flow rates at or near those in commercial units. In this study, riser density and local solids flux profiles were measured in 0.3 m diameter risers of three CFB units at gas velocities of 9–16 m/s and solids fluxes of up to 700 kg/s · m2. A variety of radial solids flux profiles were obtained, including a parabolic profile with a peak flux at about the radial centre, a nearly flat profile across the riser cross‐section and an inverted parabola with peak upwards flux near the wall. At high gas velocity and solids flux, risers have no solids downflow at the wall. Multiple fluidization regimes were found to exist in the riser. The bottom dense part of the riser was in the dense suspension upflow regime, and the dilute upper part was in the dilute pneumatic transport regime. With commercial fluid catalytic cracking (FCC) risers operating at nearly similar conditions as tested here, it is likely that they also have one or both fluidization regimes and not the traditional fast fluidization regime. The data in this study fitted well on the Kim et al. fluidization regime map.

Long term operation performance and membrane fouling mechanisms of anaerobic membrane bioreactor treating waste activated sludge at high solid concentration and high flux

High solid anaerobic membrane bioreactor (HSAnMBR) is widely applied in biomass treatment and energy regeneration, while membrane operation performance and membrane fouling control remain critical issues. In this study, a HSAnMBR was utilized for waste activated sludge (WAS) treatment at organic loading rates of 3.69–3.72 gCOD/L·d and biogas yield was ranged in 0.38–0.39 L/gVSfed with the COD conversion efficiency of 40 %. The membrane operated stably when the average flux was 9.6, 4.5 and 1.2 L/m2/h at mixed liquor total solid of 25, 30 and 40 g/L with a filtration: relaxation of 4:1, 1:1 and 1:2, respectively. The distinctive characteristics of membrane fouling at high solid condition were that the polysaccharides and proteins had high fouling propensity and were the main composition of the foulant layer. Furthermore, phosphorus and magnesium were the predominant causes of inorganic fouling. The minerals precipitated on the membrane and were embedded into membrane pores, contributing to cake layer formation and pore blocking. This research provided a comprehensive analysis of the membrane operation characterization and fouling mechanisms of HSAnMBR, which was expected to push forward HSAnMBR applications to WAS treatment.

Flow characteristics in a pilot-scale circulating fluidized bed with high solids flux up to 1800 kg/m2 s

Comprehensive study on hydrodynamics is carried out in a pilot circulating fluidized bed riser of 18 m in height with high solids flux ranging from 800 to 1800 kg/m2s and superficial gas velocity of 5, 7 and 9 m/s. Results show that with such high solids flux, solids holdup can be up to 20–30% in the whole riser. When solids flux reaches up to1800 kg/m2s, both solids holdup and particle velocity become more uniform. Few down-flowing particles suggest reduced back-mixing at such operating conditions. Dynamic investigation shows that with high solids flux up to 800 kg/m2s, gas-solids fluctuation is almost the same with the circulating turbulent fluidized bed indicating good gas-solids contact. Therefore, riser reactors with high solids flux and gas velocity have great application potential for processes which usually need short reaction residence time with high capacity of both gas and solids.

Long Term Operation Performance and Membrane Fouling Mechanisms of Anaerobic Membrane Bioreactor Treating Waste Activated Sludge at High Solid Concentration and High Flux

Long Term Operation Performance and Membrane Fouling Mechanisms of Anaerobic Membrane Bioreactor Treating Waste Activated Sludge at High Solid Concentration and High Flux

The materials flow and membrane filtration performance in treating the organic fraction of municipal solid waste leachate by a high solid type of submerged anaerobic membrane bioreactor

The anaerobic digestion of leachate from organic fraction of municipal solid waste (OFMSW) is a long-standing challenge. A submerged anaerobic membrane bioreactor (AnMBR) embedding three flat sheet membrane was therefore continuously operated for 63 days to investigate the materials flow and membrane performance. The results obtained show that approximately 90% COD was removed and 86% was converted into methane under an OLR of 5.6 kgCOD/m3·d corresponding to a HRT of 10 days. Under the high solid condition (34.5–61.1 g/L total solids in AnMBR) and flux of 5 and 6 LMH, the membranes was operated practically at constant trans-membrane pressure (TMP). When the membrane was operated at a high flux of 7 LMH the TMP rapid increase occurred in 22 h resulting in a non-recoverable permeability. A sustainable flux was thus identified. This study demonstrated the feasibility of AnMBR treating OFMSW leachate under high solid condition with high flux.

Two new magnesium and magnesium-lead fluorogermanates and revision of the Mg28Ge7.5O38F10 phase

A new compound, Mg3Ge1-δO4(1-δ)F2(1+2δ) was obtained by high temperature solid state reactions and flux method and its structure was characterized by single crystal X-ray diffraction. The structure relates to the silicate mineral norbergite, Mg3SiO4(OH)F, a member of the humite mineral group. The diffraction experiment revealed a deficiency of the Ge site in the new structure, which can be compensated by the substitution of either OH− groups or F− anions for O2− anions. The combination of the NMR, neutron scattering, and IR spectroscopy experiments suggest that the F− substitution is most likely; thus, the Mg3Ge1-δO4(1-δ)F2(1+2δ) formula is suggested. The F− substitution is present for the side product of the Mg3Ge1-δO4(1-δ)F2(1+2δ) synthesis, the compound known as Mg28Ge7.5O38F10. Refinement of the twined Mg28Ge7.5O38F10 crystal showed a slightly higher Ge content and the final composition was assigned as Mg28Ge8O40F8. The PbF2 flux method yielded a new compound, Mg2Pb2Ge2O7F2. The structure of this digermanate is related to the group of naturally occurring and synthetic silicates containing R3+ cations. High-temperature powder XRD experiments showed that the Mg3Ge1-δO4(1-δ)F2(1+2δ) phase cannot be synthesized from Mg2GeO4 and MgF2 in air due to the transformation of magnesium fluoride into MgO.

Experimental Study of Solids Motion in an 18 m Gas–Solids Circulating Fluidized Bed with High Solids Flux

A comprehensive experimental study of solids motion is conducted in an 18-m high circulating fluidized bed with extremely high solids circulating rate (Gs) up to 1000 kg/(m2 s), which is never achieved in such a tall riser in previous researches. The axial distributions of particle velocity show exponential types and slight changes in the fully-developed zone with Gs higher than 400 kg/(m2 s). The radial profiles of particle velocity are the parabolic shapes and become steeper with increasing Gs. The development of gas-solids flow structure becomes slower with increasing Gs or decreasing Ug. When Gs reaches 800 kg/(m2 s), the full development is achieved until to the axial height higher than 12 m. Better correlations between the local particle velocity and solids holdup and less down-flowing particles indicate better reactor performance with the reducing solids back-mixing and uniform residence times at extremely high Gs.

Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enhanced calcium looping gasification process

This paper studies the mixing and movement of char in a dual fluidized bed for an enhanced calcium looping gasification process. The gasifier is a compact fluidized bed, consisting of a lower bubbling fluidized bed and an upper riser. The solid recirculation flux can be controlled by three loop seals. Quartz sand was used as a calcium-based sorbent and Chinese rice as the char. No flotsam behavior was observed in the bubbling fluidized bed. Experimental results reveal that the higher fluidization rate in the gasifier and the larger solid recirculation flux promoted char mixing. An increasing system char concentration decreased char mixing but led to higher char concentrations in the recirculation stream. The char holdup in the upper riser was primarily dependent on the fluidization rate in the gasifier. High solid recirculation flux with a low fluidization rate in the gasifier would benefit hot rig operation.

Effects of Operating Parameters on Solids Flux in a High-Density/-Flux Circulating Fluidized Bed Riser Reactor

High-flux/-density circulating fluidized bed (CFB) technology has been widely applied to oil and coal processing with high solids flux (up to 500–2000 kg/(m2 s)) and high solids holdup (up to 10%)....

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.

MP-PIC modeling of CFB risers with homogeneous and heterogeneous drag models

In this paper, the MP-PIC (multiphase particle-in-cell) approach is used for three-dimensional (3D) modeling of the gas-solid flows in two types of circulating fluidized bed (CFB) risers with Geldart group A and B particles by incorporating the homogeneous and heterogeneous drag force models in the MP-PIC method, respectively. First, the effects of the three important simulation parameters, namely, the grid cell number, numerical particle-parcel size and time step, are investigated. Having determined the appropriate values for the three parameters, the hydrodynamic characteristics predicted by different drag force models are rigorously analyzed. The homogeneous drag models considered are the six models, the Wen-Yu, Wenyu-Ergun, Syamlal-O’Brien, Gidaspow, HKL, and BVK models, while the four heterogeneous models considered are Sarkar and EMMS-based models (EMMS-Yang, EMMS-Matrix and EMMS-QL). For the riser 1 with the Geldart A particles, all the six homogeneous models predict extremely high solid fluxes and inconsistent void fraction distributions compared with experimental results. The heterogeneous Sarkar and EMMS-based models can effectively improve the simulation accuracy and predict a typical core-annulus flow structure. The lately-developed EMMS-QL model produces the most accurate solid flux. For the riser 2 with the Geldart B particles, both the heterogeneous and homogeneous drag force models can predict a reasonable flow structure. Further, there are no significant differences in the void fraction and velocity profiles due to the choice of a drag force model over the other. These drag force models also successfully capture the meso-scale local particle clusters. Of these drag-force models, the Wenyu-Ergun drag-forec model predicts comparatively accurate solid flux. Generally, MP-PIC combined with heterogeneous Sarkar and EMMS-based drag force models reasonably improve the simulation accuracy for the Geldart A particles, while these heterogeneous models have no superiority over the homogeneous drag models for the Geldart B particles.

Hydrodynamic simulation and optimization of the feeding system of a bubbling fluidized-bed gasifier in a triple-bed circulating fluidized bed with high solids flux

A bubbling fluidized bed (BFB) with a continuous feed/discharge of solids was used as a gasifier in a high-density triple-bed circulating fluidized bed (TBCFB). To carry the heat from the combustor to the pyrolyzer/gasifier effectively, a high-density and high solids-mass flux circulating system is necessary. In this study, the gas-solid flow behavior of a BFB with high solids continuous feeding/discharging mass flux was simulated by an Eulerian–Eulerian model incorporating the Lagrangian (discrete phase model) properties of particles as a tracer to analyze particle motion and determine the optimal feeding tube location and diameter. It was observed that a U-shaped track of particles from the inlet to the outlet and double vortices resulted in particle back-mixing and extended particle residence time. According to the results, some feeding-tube design principles were proposed to achieve smooth solids flow and improve the particle residence time in the BFB in solids mass circulating fluxes ranging from 800 to 1200kg/m2s.

Optimization of in Situ Gasification Chemical Looping Combustion through Experimental Investigations with a Cold Experimental System

In situ gasification chemical looping combustion (iG-CLC) is a promising coal combustion technology for implementing CO2 capture with a low energy penalty. A novel iG-CLC cold experimental system was developed in the authors’ previous work (Ind. Eng. Chem. Res. 2013, 52, 14208). It mainly consists of a high-flux circulating fluidized bed (HFCFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. As an extension of that work, in this study, we further optimized the iG-CLC system by redesignung the air reactor to enhance the carrying capacity of the gas flow and developing a two-stage separation system by adding a second-stage cyclone to the original first-stage inertial separator. Stability in operation and flexibility in adjusting operating parameters were achieved with the improved system. In the riser (fuel reactor), higher solids fluxes and solids holdups were achieved, which should enhance the gas–solid contact and promote the complicated heterogeneous reactions. In the moving bed (air reactor), the carrying capacity of the gas flow was significantly enhanced, which should lead to a great increase in the system power capacity. The confirmation of the ability to control the gas flow directions in the two reactors means that the gas bypassing between the two reactors can be restrained so as to ensure a high CO2 concentration in the exhaust of the fuel reactor. The high global separation efficiency and selective separation efficiency of the new two-stage separation system for fine particles indicate that a high combustion efficiency of coal can be achieved with a hot iG-CLC system.

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.

Flux Synthesis and Crystal Structure Determination of the Colored Skutterudite Ir2Sn3Se3

AbstractThe skutterudite (CoSb3) related crystal structure of Ir2Sn3Se3 = IrSn1.5Se1.5 is determined from single crystal data for the first time. High temperature solid state and flux reactions were applied to prepare powder and single crystals samples. Its crystal structure was refined from data of a fourfold twin crystal in space group R$\bar{3}$ (no. 148). It contains 24 formula units per unit cell [a = 12.6510(3) Å, c = 15.5236(6) Å, V = 2151.67(8) Å3] and represents the first single crystal determination of a colored skutterudite Ir2A3X3 = Ir(A, X)3 with heteroatomic [Sn2Se2] units and [Sn3Se3] coordination for Ir sites. The distorted Sn2Se2 rings show short and long Sn–Se distances. The obtained structure is isotypic to Ir2Sn3Te3 but differs from the cubic model estimated earlier for Ir2Sn3S3 (homoatomic Sn4 and S4 units). According to electronic band structure calculations Ir2Sn3Se3 is a low gap semiconductor (ΔE = 0.4 eV).

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.

Detailed measurements of particle velocity and solids flux in a high density circulating fluidized bed riser

A comprehensive study of particle velocity and solids flux is conducted in a high density circulating fluidized bed with extremely high solids circulation rate up to 1000kg/m2s, for the first time in the lab-scale settings. Results show that the axial distribution of particle velocity becomes more uniform in such extremely high solids flux conditions than cases with lower solids flux. Radial profiles of particle velocity and solids flux become increasingly steep with increasing solids circulation rate. No net downward flow near the wall is detected, which is considered an important advantage of the high density riser leading to reduced solids backmixing. Correlations of particle velocity against solids holdup are stronger for low density conditions than for high density cases, suggesting that gas–particle interaction dominates in low density risers, while particle–particle interaction plays a key role for the motion of particles in the extremely high density ones.

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.

Experimental Investigation on Flow Behaviors in a Novel In Situ Gasification Chemical Looping Combustion Apparatus

A novel cold reactor apparatus for in situ gasification chemical looping combustion (iG-CLC) is proposed and investigated. It is mainly comprised of a circulating fluidized bed (CFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. The direct hydraulic link between the two reactors brings inherent simplicity and stabilization of the whole system. The CFB fuel reactor provides favorable gas–solids contacts over the whole reactor height. The realization of high solids flux operation conditions greatly enhances the solids holdups, and the gas–solids contacts and reactions in the riser. The moving bed air reactor has advantages in terms of having a low pressure drop, continuous solids flow, and large gas–solids contact area except for the risk of plugging and particle leakage caused by excessive high cross-flow gas velocity. Independent pressure adjustments between the two reactors could control the gas flow direction and restrain the gas bypassing to ensure high CO2 concentration wit...

Flow Patterns Identification in a High-flux Circulating Fluidized Bed with Shannon Entropy Increment Rate

Experiments of gas-solid suspension were carried out in a high-flux circulating fluidization bed with the solid mass flux up to 536 kg/(m2·s). Differential pressure fluctuation time series were obtained at different locations in the riser. Shannon entropy increment rate analysis was developed to identify the flow pattern and transformation and characterize the dynamic behavior. Effect of superficial gas velocity on the Shannon entropy increment rate was examined under high solid mass flux. It was demonstrated that a circulating fluidization bed at high solid mass flux was a deterministic chaos system and the sensitivity of the system's chaotic characteristic to operating parameters at different flow regimes was different. Shannon entropies increment rate of different flow regimes were distinct. Shannon entropy increment rate analysis method results in high degree of recognition for flow pattern and transformation.