Mixing characteristics of binary mixtures in a spout-fluid bed

Mixing characteristics of binary mixtures in a flat-bottom cylindrical spout-fluid bed using glass beads and air are reported in this work. Experiments were carried out to investigate the mixing characteristics for binary mixtures at three flow conditions, i.e., only spouting, only fluidization and spout-fluidization. The experiments were performed at different gas velocities, diameter ratios of binary mixtures and three different bed arrangements. Mixing index was determined for fluidized bed and static bed conditions. It was found that, in all cases, lowest-diameter ratio mixture gave good mixing index values. For all flow conditions, mixing index for large–small bed arrangement was increasing with time, whereas for small–large bed arrangement, the mixing index deteriorated with time. However, in both cases, the mixing index reached almost a constant value. For well-mixed bed arrangement and spout-fluidization flow condition, segregation and re-mixing were observed.     

Study on the flow behavior of irregular plastic particles in a spout-fluid bed with a draft tube

This work presents an experimental investigation on the hydrodynamic performance of a draft tube spout-fluid bed with irregular particles. Nonmetal particles from waste printed circuit boards (NPCBs) were used as a spouting solid, and polypropylene (PP) particles were selected as an assistant to fluidization particles. The flow pattern, minimum spouting velocity (U_ms), and minimum spout-fluidization velocity (U_msf) were investigated under different operating conditions. The irregular cohesive particles from NPCBs showed poor flowability and channeling, which restrained stable spouting in the spout-fluid bed. The quality of fluidization and spouting improved when greater than 40?wt.% PP particles were added into the NPCB/PP mixtures. The mechanism was that the PP particles accelerated the movement of NPCB particles. Meanwhile, lower density differences between NPCB and PP particles decreased the segregation of the mixtures. The minimum spouting velocity decreased with an increase in fluidization gas velocity and the ratio of NPCB particle in the NPCB/PP mixtures. Two flow patterns, unstable spouting and unstable spouting fluidization, were observed over a large range of gas velocity. The ranges of gas velocity in these two flow patterns enlarged with the increase in mass fraction of NPCB particles within the NPCB/PP mixtures.     

Fluidization behavior of binary mixtures of nanoparticles in vibro-fluidized bed

The fluidization behavior of different mixed SiO₂, TiO₂ and/or ZnO nanoparticles under the application of vibrated fields of constant vibrated frequencies (40 Hz) and amplitude (3.0 mm) is studied. The single nanoparticles experiments show that SiO₂ nanoparticles have a better fluidization quality than TiO₂ and ZnO nanoparticles. For binary mixtures of the nanoparticles, the amount of SiO₂ nanoparticles generally has a beneficial effect on the fluidization quality of the binary mixtures. Using the linear regression, the Richardson–Zaki exponents of three kinds single and their binary mixture of nanoparticles are calculated. The Richardson–Zaki analyses indicate that the particulate fluidization degree of mixed nanoparticles can be greatly improved in agglomerate particulate fluidization (APF) behavior.     

Prediction of segregation behavior of binary mixture in a pulsed fluidized bed

A three-dimensional simulation is carried out to investigate the impact of pulsation flow on segregation behavior of binary mixture in a fluidized bed via the multi-fluid model. The simulated results are compared against the experimental data. The impacts of pulsation frequency and operating condition on flow and segregation behavior of binary mixture are discussed. The results reveal that an excessive increase of pulsation frequency and operating temperature is not beneficial to the enhancement of segregation efficiency. The pulsation frequency plays a more important role in segregation efficiency under a small size discrepancy of binary mixture. Meanwhile, the effect of pulsed air flow waveform on segregation efficiency of particles is also analyzed.     

CFD-DEM study on the mixing characteristics of binary particle systems in a fluidized bed of refuse-derived fuel

The fluidization of quartz in the fluidized bed has great influence on the combustion and gasification of refuse-derived fuel (RDF). The combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was used to explore the gas-solid hydrodynamics and mixing characteristics in a three-dimensional fluidized bed. All numerical analyses were performed referring to the experiments (Goldschmidt, Beetstra, and Kuipers 2004 Goldschmidt, M. J. V., R. Beetstra, and J. A. M. Kuipers. 2004. Hydrodynamic modelling of dense gas-fluidised beds: Comparison and validation of 3D discrete particle and continuum models. Powder Technology 142 (1):23–47. doi:10.1016/j.powtec.2004.02.020[Crossref], [Web of Science ®] , [Google Scholar]). The simulation results indicated that the quartz volume fraction agrees well with the experimental data. Furthermore, the cylinder-shaped RDF particles can mix well with the quartz particles as they were added from upside. For binary systems, it is necessary to investigate solid flow characteristics as well as pressure drops and examine the influence of superficial gas velocity on the solid mixing. Two main parameters are discussed: mixing degree and the time required to reach the steady state. It is also found that inlet gas velocity and particle properties (particle density ratio, shape and size) are significant factors on particle mixing in a fluidized bed.     

A study of gas and solid mixing behaviors in a three partitioned fluidized bed

A previously unknown partitioned fluidized bed gasifier (PtFBG) has been developed for improving coal gasification performance. The basic concept of the PtFBG is a fluidized bed divided into two parts, a gasifier and a combustor, by a partitioned wall. Char is burnt in the combustor and the generated heat is supplied to the gasifier along with the bed materials. During that time, highly concentrated CO₂ is inevitably generated in the combustor. Therefore, vigorous solid mixing is an essential precondition as well as minimizing horizontal gas mixing. In this study, gas and solid mixing behaviors were verified in a cold model three partitioned fluidized bed (3-PtFB). Glass beads with an average diameter of 150 μm and a particle density of 2500 kg/m³ were used as bed materials. For the gas mixing experiments, CO₂ and N₂ were introduced into the beds through each distributor. Then, outlet gas flow rates and concentrations were measured by gas flow meters and an IR gas analyzer respectively. The calculated gas exchange ratios ranged from 3% to 10% with varying gas flow rates. For the solid mixing experiments, 1000 μm polypropylene particles with a density of 883 kg/m³ were continuously fed into the reactor. Then, the polypropylene particles were distributed to the entire beds evenly. Solid mixing behaviors were very analogous to liquid mixing behaviors in a continuous stirred tank reactor (CSTR).     

Experimental investigation on hydrodynamic behavior in a spouted bed with longitudinal vortex generators

A spouted bed with longitudinal vortex generator (LVG) of sphere was built to enhance radial movement of particles. Particle Image Velocimetry (PIV) was applied to explore effects of longitudinal vortex flow and physical properties of particles on their radial velocity in a 152-mm-diametered spouted bed. The results show that, Compared with the conventional spouted bed, the existence of longitudinal vortex generator gives rise to a large amount of secondary fine vortex flow in the cross section of spouted bed. The enhancement factors of particles movement η with different particle densities are all greater than 1. The smaller the particle density, the more significant the effect of the longitudinal vortex on the radial velocity of the particles. The single-row LVGs can produce a good radial enhancement effect of particle movement when the particle handling capacity is small (H₀ = 165 mm). With the increase of the height of the static bed (H₀), the enhancement of the radial velocity of particles in the spouted bed by multi-row LVGs (three rows) increases gradually, which indicates that the multi-row LVGs have a better overall effect on the enhancement of particle motion in the spouted bed with more particle handling capacity (H₀ = 195 mm, 225 mm).     

Bed expansion ratio of mono-sized and binary mixtures in fluidized,spouted, and spout-fluid beds

Studies on bed expansion ratio were carried out in fluidized, spouted, and spout-fluid beds. A single column has been used to compare the characteristics of fluidized, spouted, and spout-fluid beds. Experiments were carried out using air and glass beads under fluidized, spouted, and spout-fluid bed conditions separately to study the effect of gas velocity, bed mass, and particle size on bed expansion ratio. Glass beads of different sizes (0.75, 1.2, 1.7, and 3.075?mm) have been used as solid bed material. Bed expansion ratio was determined for mono-size particles and binary mixtures (different diameter ratios and composition). It was found that the bed expansion ratio decreases with increase in bed mass for only spouting condition and spout-fluidization conditions. The bed expansion ratio increases with increase in bed mass for only fluidization condition.     

Hydrodynamic behavior of binary mixture of solids in a triple-bed combined circulating fluidized bed with high mass flux

Flow behaviors of binary mixture of silica sand and nylonshot (coal char substitute) were investigated in a triple-bed circulating fluidized bed (TBCFB) as a cold model of coal gasifier. The TBCFB consisted of a downer (ϕ 0.1 m × 6.5 m), a bubbling fluidized bed (0.75 m × 0.27 m × 1.9 m), a riser (ϕ 0.1 m × 16.6 m) and a gas-sealing bed (GSB, ϕ 0.158 m × 5 m). The initial fraction of the nylonshot in the solid mixture (X_nylon,i) was 15.4 and 30% on mass and volume bases, respectively, or otherwise, 30.7 and 50%. The maximum solids mass flux (G_s) at X_nylon,i of 15.4 and 30.7 wt% were 394 and 349 kg/m² s, respectively, when the gas velocity in the riser (U_gr) was 10 m/s. Apparent solids holdups of silica sand and nylonshot were calculated separately from the static pressure gradient across the riser and the downer. The results showed possibility of large-mass-flux circulation of char in the gasifier, which plays a significant role in decomposition of tar from pyrolysis as the primary step of gasification. A newly developed pressure balance model successfully predicted G_s of the binary mixtures in TBCFB.     

Experimental and numerical studies on a bubble-induced inverse gas-liquid-solids fluidized bed

Hydrodynamics in a newly invented bubble-induced inverse gas–liquid-solids three-phase fluidized bed has been studied via both experimental and numerical methods. With experiments in a 3.0 m column of 0.153 m in diameter, four fluidization regimes including a fixed bed regime, a bed expansion regime, a complete fluidization regime, and a freeboard regime have been identified with the increase in the superficial gas velocity. A three-phase Eulerian-Eulerian CFD model was developed to simulate the hydrodynamics in the inverse three-phase fluidized bed and the simulation results have a good agreement with the experimental data. The effects of the particle property and solids loading on the transitions across the flow regimes were numerically studied. A higher solids loading and/or a larger particle density are reported to contribute to an easier fluidization and a faster flow development to the complete fluidization regime. The radial flow structure becomes less uniform with increased inner circulation of the liquid after introducing more bubbles into the column.     

CFD-DEM study of segregation and mixing characteristics under a bi-disperse solid-liquid fluidised bed

In this work, a recent smoothed CFD-DEM model is employed to study the segregation and mixing behaviours in a bi-disperse solid–liquid fluidised bed (SLFB) with different superficial liquid velocities and wide size ratios. Here, the poly-disperse drag model is applied, and its validity and advantage are confirmed. Besides, the qualitative and quantitative characteristics are systematically studied using a variety of indices, while the intermixing behaviour in the transition zone is investigated. The results show that segregation degree is well quantified by the mixing index, and the circulation motion of particles and liquid is confirmed in the transition zone. The more drastic motions can be observed in the transition zone under lower superficial velocities or size ratios. Furthermore, the force analysis in terms of the drag force and collision force indicates that the combined contribution of hydrodynamics and collision is the main underlying mechanism of segregation behaviour in the bi-disperse SLFB.     

Numerical investigation of binary particle mixing in gas-solid fluidized bed with a bubble-based drag EMMS model

Heterogeneous flow structure of bubbling deeply affects gas–solid momentum transfer in a binary gas–solid fluidized bed. This work presented a binary particle bubble-based Energy Minimum Multi-scale (EMMS) model, and an assumption that bubble-emulation drag force acting on solid 1 and solid 2 depending on each solid volume ratio in the emulation phase was applied to simplify the force balance of the binary particle-phase. The bubble-based EMMS drag was incorporated in the Eulerian multi-fluid model to predict the mixing behaviors of two binary particle systems. The simulation results agree well with the experimental observations in terms of binary solid mixing, bed expansion, and bubble diameter. Compared with the prediction results by the Gidaspow drag model, the jetsam solid fraction and bubble size predicted by the present drag model is in more agreement with the measured results, which indicate the EMMS drag model is an alternative choice for modeling binary gas–solid bubbling system.     

Experimental investigation of bubble behavior in gas-solid fluidized bed

In this work, to investigate the source of pressure fluctuations, behavior of a single bubble in a two-dimensional gas–solid fluidized bed was studied. Pressure sensors located at different heights of the bed measured presure fluctuations, and simultaneously a high speed camera was used to pursue all steps from formation to eruption of bubbles. Two types of particles were applied with different sizes and densities. Experiments showed that the maximum amplitude of formation was independent of the bubble diameter. But, it depended on density of particles, velocity of injection and the distance from bed surface. When injection stopped, there was a minimum in pressure profile related to the higher dense phase voidage for a higher superficial gas velocity after injection. Also, the maximum pressure fluctuation of bubble eruptions was related to the bubble diameter, density and size of particles. It was concluded that pressure fluctuations of formation, passing and eruption of bubbles in fluidized beds are originated due to changes in dense phase voidage, bed voidage and movement of particles during bubble eruption.     

Experimental study on the discharge flow rate of binary mixture in a two-dimensional silo

In this work, the discharging process of the binary mixture composed of sphere and sphere-paired particles in a two-dimensional silo was studied. High-speed camera and self-developed particle tracking velocimetry (PTV) program were used to capture the flow behaviors of all particles. The key parameters of mixed flow, including coordination number, horizontal displacement and mechanical energy loss in the discharge process, were highlighted. It was found that the increase of sphere-paired particles can decrease the average coordination number of particles during the discharging process. The analysis about the loss of mechanical energy and the horizontal displacement of particles indicated that sphere-paired particles preferentially squeezed out sphere particles from fast flow field above the outlet. Moreover, an empirical formula was proposed to assess the influence of the proportion of sphere-paired particles on the discharge flow rate. Sphere-paired particles tended to hinder the discharging process, which was caused by the rotation around their centroids and the angular deflection close to angle of the hopper.     

鼓泡床内双组分颗粒流动行为的数值模拟

为了模拟预测粒径和密度同时存在差异的双组分颗粒体系的分级混合行为,基于欧拉-欧拉方法建立多流体模型,采用颗粒动力学理论描述颗粒相性质,分别通过Gidaspow和Syamlal曳力模型描述气-固相曳力和固-固相作用力。结果表明,模拟得到的轴向和径向颗粒浓度分布与实验数据吻合较好;当在较小气流速度下出现分级行为时,床层底部富沉积组分层中沉积组分的运动十分有限,而在较大气流速度下处于完全混合状态时,床层内部颗粒运动较为剧烈。     

Experimental analysis of overtaking behavior in disks falling in a low-density particle bed

Cooperative behavior displayed by five steel disks falling in a low-density particle bed involves the formation of upward and downward convex configurations, which resembles the flying pattern of a flock of birds. In this study, we focused on overtaking behavior in two falling disks, which causes the cooperative behavior, and we investigated the effects of differences in the disk release time and the initial disk separation distance on the falling behavior of the disks experimentally. Expanded polystyrene (EPS) particles (diameter 5.08 mm, mass 1.45 mg) were used as the bed particles and steel disks (diameter 25.4 mm, thickness 5.22 mm, mass 20.2 g) were used as the falling disks. We released one to five disks with various disk release time differences (0–0.154 s) and initial separation distances (0–100 mm). We recorded the disk falling behavior in the particle bed with a high-speed video camera (500 fps) and analyzed the behavior with image analysis software. Five-disk cooperative behavior similar to that reported in the literature occurred in our experimental setup. In the two-disk experiments, we observed overtaking behavior for an initial separation distance of 10 mm and release time difference of ≤ 0.076 s, and for an initial separation distance of ≤ 60 mm and release time difference of 0.02–0.03 s. The overtaking behavior arose from the decrease in the falling velocity of the first disk released. The EPS particle packing fraction in the area above the disk one disk diameter wide and a quarter of the disk diameter high determined the disk falling velocity. This mechanism was explained by the displacement behavior of EPS particles around the disks as the disks fell.     

Conversion air velocity at which reverse density segregation converts to normal density segregation in a vibrated fluidized bed of binary particulate mixtures

It is well known, when binary mixtures of different-density particles of the same size are vertically vibrated or fluidized by airflow through the bottom, the particles segregate by density. Reverse density segregation occurs in the vibrated bed; heavier particles move upward and lighter ones move downward, and normal density segregation occurs in the fluidized bed; lighter particles move upward and the heavier ones move downward. In this study, we investigated the particles’ behavior in a vertically vibrated fluidized bed at various air velocity using two types of particulate mixtures of glass beads (GB) and stainless steel powder (SP) or iron powder (IP) of same size. We found that reverse segregation converts to normal segregation at a certain air velocity; here we call it “conversion air velocity”. Then, we investigated the likely origin of the conversion air velocity considering the minimum fluidization air velocity u_mf determined for the three monocomponent particles (GB, SP and IP) with and without vibration. We found that the conversion air velocity is close to the u_mf of the lower density particles (GB) with vibration, indicating that the conversion from reverse segregation to normal segregation occurs around u_mf of lighter particles with vibration.     

Prediction of thermal diffusion in binary mixtures of nonelectrolyte liquids by the use of nonequilibrium thermodynamics

A derivation based on nonequilibrium thermodynamic leads to this expression for the thermal diffusion factor, _T=M ₂(h ₂ ^xs -h ₁ ^xs )/RT ₂, where M ₂ is the molecular weight of the lighter of the two components, h ₁ ^xs is the partial excess enthalpy of component i, J/g, R is 8.314 J · K ^–1 · mol ^–1, T is temperature in K, and ₂ is the thermodynamic correction factor (1 + d ln ₂/d ln X ₂), where ₂ is an activity coefficient and X ₂ is the mole fraction. The correctness of this theoretical prediction is verified for the liquid system ethanol-water at 298 K.Paper submitted to the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

橡胶连续混炼粉料混合均匀性仿真及实验研究

为了提高橡胶连续混炼中混炼胶质量稳定性,实现炭黑等粉体物料精确配比和均匀性混合问题,针对粉体物料在混合和输送过程存在复杂的物理性质,建立了炭黑等混合粉料的球体颗粒模型和Hertz接触力-位移模型,采用EDEM对典型粉体物料混合均匀性进行模拟仿真和粉体物料混合实验,对炭黑等粉体物料在橡胶连续混炼工艺中的混合均匀性进行分析,探求橡胶粉料连续混合和输送机理.研究发现:粉体物料混合仿真与实验测试结果具有较高的拟合性,表明在橡胶连续混炼工艺中可以在保证混合均匀性的前提下实现多粉体混合物的连续称量和输送,同时也验证了运用EDEM数据模拟仿真粉体物料混合的可行性.     

Correlation and extrapolation scheme for the composition and temperature dependence of viscosity of binary gaseous mixtures: Carbon dioxide + ethane

Experimental viscosity data of ethane, carbon dioxide, and three mole fractions of the binary system carbon dioxide + ethane in the temperature range 293.15<T633.15 K and in the density range 0.010.05 mol·L^–1 reported earlier were evaluated simultaneously to find out a useful correlation and extrapolation scheme for the viscosity of binary systems in the range of moderate densities. A procedure based on the ideas of the modified Enskog theory has been found to give the best results. Dependent on temperature, the collision diameters related to the equilibrium radial distribution function at contact are fitted to viscosity values of the pure substances and of at least one mixture. The results are compared with experimental data from the literature. A recommendation is given concerning the density range in which the first density contribution to the viscosity coefficient of the system carbon dioxide + ethane is sufficient to be included.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.