Characteristics of high-sulfate wastewater treatment by two-phase anaerobic digestion process with Jet-loop anaerobic fluidized bed

A new anaerobic reactor, Jet-loop anaerobic fluidized bed （JLAFB）, was designed for treating high-sulfate wastewater. The treatment characteristics, including the effect of influent COD/SO42 ratio and alkalinity and sulfide inhibition in reactors, were discussed for a JLAFB and a general anaerobic fiuidized bed （AFB） reactor used as sulfate-reducing phase and methane-producing phase, respectively, in two-phase anaerobic digestion process. The formation of granules in the two reactors was also examined. The results indicated that COD and sulfate removal had different demand of influent COD/SO4^2- ratios. When total COD removal was up to 85%, the ratio was only required up to 1.2, whereas, total sulfate removal up to 95% required it exceeding 3.0. The alkalinity in the two reactors increased linearly with the growth of influent alkalinity. Moreover, the change of influent alkalinity had no significant effect on pH and volatile fatty acids （VFA） in the two reactors. Influent alkalinity kept at 400-500 mg/L could meet the requirement of the treating process. The JLAFB reactor had great advantage in avoiding sulfide and free-H2S accumulation and toxicity inhibition on microorganisms. When sulfate loading rate was up to 8. 1 kg/（m^3.d）, the sulfide and free-H2S concentrations in JLAFB reactor were 58.6 and 49.7 mg/L, respectively. Furthermore, the granules, with offwhite color, ellipse shape and diameters of 1.0-3.0 mm, could be developed in JLAFB reactor. In granules, different groups of bacteria were distributed in different layers, and some inorganic metal compounds such as Fe, Ca, Mg etc. were found.

Experiment and simulation on the pyrite removal from the recirculating load of pulverizer with a dilute phase gas-solid fluidized bed

In order to reduce the energy consumption and subsequent air pollution of coal-fired power station, based on the analysis to size and density distribution of particles from the recirculating load of the classifier of pulverizer, the separation experiment on sampling material from power plant with a dilute phase fluidized bed to remove pyrite and other minerals and numerical simulation on the separation process were done. The results show that the minimum fluidization velocity is 1.62 cm/s. Pyrite and other minerals in the material are separated. Ash of the upper and bottom layer material account for 33.34% and 73.42% respectively and sulfur content occupy 1.12% and 8.96% respectively. Scanning electron microscopy and spectroscopy tests show that sulfur in the bottom material exist in the form of pyrite. Numerical simulation on the flow field form of the dilute phase separation bed with gas-solid two phase and particle motion verifies the experimental results.

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

PHASE HOLDUP AND LIQUID DISPERSION IN GAS-LIQUID-SOLID CIRCULATING FLUIDIZED BEDS

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.

Recovery of alumina from circulating fluidized bed combustion Al-rich fly ash using mild hydrochemical process

To utilize CFBC Al-rich fly ash, a mild hydrochemical extraction process was investigated for recovery of alumina. An alumina extraction efficiency of 92.31%was attained using a 45%NaOH solution, an original caustic ratio （molar ratio of Na2O to Al2O3 in the sodium aluminate solution） of 25, a molar ratio of CaO to SiO2 in the fly ash of 1.1, a liquid volume to solid mass ratio of 9, a reaction temperature of 280 ℃, and a residence time of 1 h when treating fly ash with an alumina to silica mass ratio （A/S） of 0.78 and an alumina content of 32.43%. Additionally, the alumina leaching mechanism was explored via structural and chemical analysis, which revealed that after alkaline digestion, the main solid phase containing silica was NaCaHSiO4 with a theoretical A/S of zero.

STUDIES OF THE EVAPORATION AND SCALING MITIGATION IN A RECIRCULATING THREE-PHASE FLUIDIZED BED

Heat transfer of liquid evaporation was studied in a recirculating three-phase fluidized bedin which an inert gas serving as'carrying gas'was introduced.The gas velocities,particle sizes,par-ticle densities and particle concentrations in the liquid were examined.Heat fluxes were measured aswell.Significant enhancement in heat transfer was resulted when an inert gas and solid particles wereintroduced into the flow boiling liquid.Scaling mitigation was to be expected in the process.

Photocatalytic oxidation of Rhodamine B in a three-phase internal circulating fluidized bed with TiO_2/SiO_2 as photocatalyst

A novel photoreactor of three-phase internal circulating fluidized bed was applied to the degradation of Rhodamine B with TiO2/SiO2 catalyst and TiO2 powder,respectively.The experimental results showed that the photocatalytic activity of TiO2/SiO2 catalyst was much higher than that of TiO2 powder under the same condition,and the half life of Rhodamine B using TiO2/SiO2 was 9.5 min,much lower than 63 min when using TiO2 powder.Moreover,TiO2/SiO2 had a good adsorption capacity of Rhodamine B,which played an important role on degradation.In addition,it was found that the degradation kinetics of Rodamine B with TiO2/SiO2 catalyst did not follow the first order reaction.The degradation kinetics model in terms of the adsorption process of catalyst and the analytic solution of reactant degradation rate in liquid phase could be deduced,which consisted of two parts.The first part was due to the adsorption,while the second part was due to the photocatalysis.In the beginning of the reaction,the adsorption process was dominant.However,when the adsorption achieved a balance,the degradation of Rhodamine B in liquid phase and solid phase was mainly caused by photocatalysis and the degradation kinetics model conformed to the first order reaction.

Implementation of a Demoisturization and Devolatilization Model in Multi-Phase Simulation of a Hybrid Entrained-Flow and Fluidized Bed Mild Gasifier

A mild gasification process has been implemented to provide an alternative form of clean coal technology called the Integrated Mild Gasification Combined Cycle (IMGCC), which can be utilized to build a new, highly efficient, and compact power plant or to retrofit an existing coal-fired power plant in order to achieve lower emissions and significantly improved thermal efficiency. The core technology of the mild gasification power plant lies on the design of a compact and effective mild gasifier that can produce synthesis gases with high energy volatiles through a hybrid system: utilizing the features of both entrained-flow and fluidized bed gasifiers. To aid in the design of the mild gasifier, a computational model has been implemented to investigate the thermal-flow and gasification process inside this mild gasifier using the commercial CFD (Computational Fluid Dynamics) solver ANSYS/FLUENT. The Eulerian-Eulerian method is employed to model both the primary phase (air) and the secondary phase (coal particles). However, the Eulerian-Eulerian model used in the software does not facilitate any built-in devolatilization model. The objective of this study is therefore to implement a devolatilization model (along with demoisturization) and incorporate it into the existing code. The Navier-Stokes equations and seven species transport equations are solved with three heterogeneous (gas-solid) and two homogeneous (gas-gas) global gasification reactions. Implementation of the complete model starts from adding demoisturization first, then devolatilization, and then adding one chemical equation at a time until finally all reactions are included in the multiphase flow. The result shows that the demoisturization and devolatilization models are successfully incorporated and a large amount of volatiles are preserved as high-energy fuels in the syngas stream without being further cracked or reacted into lighter gases. The overall results are encouraging but require future experimental data for verification.

Dynamic Simulation of Volume Fraction and Density Solid Phase Effect on Phase Hold-Up in 3 Phase Fluidize Bed Column through CFD

The gas-liquid-solid fluidized bed has emerged in recent years as one of the most promising devices for three-phase operation. Selection and design is one of them in parameter in the performance of three phase system. This paper focuses on volume fraction and density effect on the phases hold-up in a 3 phase fluidize bed column containing liquid phase with 100 cm height and 20 cm diameter, in this case the solid phase with 0.15, 0.25, 0.35 volume fraction and density 2470, 3000, 4000, 5000 m3 dispersion into liquid phase and the gas phase enter the column through a sparger of 2 cm diameter with 0.02 m/s velocities. The results show as the solid phase volume fraction increases from 0.02 to 0.08 m/s. The gas hold-up decreases and solid hold-up increases. Solid phase density increases, so solid phase hold-up decreases and gas hold-up decreases.

Surface-particle-emulsion heat transfer model between fluidized bed and horizontal immersed tube

A mathematical model, surface-particle-emulsion heat transfer model, ispresented by considering voidage variance in emulsion in the vicinity of an immersed surface. Heattransfer near the surface is treated by dispersed particles touching the surface and through theemulsion when the distance from the surface is greater than the diameter of a particle. A film withan adjustable thickness which separates particles from the surface is not introduced in this model.The coverage ratio of particles on the surface is calculated by a stochastic model of particlepacking density on a surface. By comparison of theoretical solutions with experimental data fromsome references, the mathematical model shows better qualitative and quantitative prediction forlocal heat transfer coefficients around a horizontal immersed tube in a fluidized bed.

液-固流化床最小流态化速度的数值模拟与验证

为使用计算颗粒流体力学(CPFD)的数值模拟法代替实验法,建立单管的液-固两相流化床实验系统;实验测量5种颗粒的最小流化速度;应用CPFD方法对单管液-固流化床模型进行数值模拟,比较5种颗粒的最小流态化速度的实验值与模拟值,并进行误差分析,验证数值模拟方法的正确性。结果表明:5种实验颗粒的最小流态化速度实验值分别为0.0216、0.0367、0.0293、0.0555、0.0845 m/s;2种公式验算值与实验值的最大相对误差小于10%,平均相对误差小于5%,证明实验结果是可靠的;5种模拟颗粒的最小流态化速度模拟修正值分别为0.024、0.044、0.041、0.069、0.062 m/s;颗粒S_(1)、S_(2)、S_(4)、S_(5)的模拟修正值与颗粒E_(1)、E_(2)、E_(4)、E_(5)的实验值之间的最小流态化速度的误差分别为11.1%、19.9%、24.3%、26.6%,均为正向偏差且在工程允许范围内,证明CPFD数值模拟方法是可靠的;最小流态化速度随固体颗粒的密度和粒径的增大而增大。

Numerical simulation of dense particle-gas two-phase flow using the minimal potential energy principle

A simulation method of dense particle-gas two-phase flow has been developed. The binding force is introduced to present the impact of particle clustering and its expression is deduced according to the principle of minimal potential energy. The cluster collision, break-up and coalescence models are proposed based on the assumption that the particle cluster are treated as one discrete phase. These models are used to numerically study the two-phase flow field in a circulating fluidized bed （CFB）. Detailed results of the cluster structure, cluster size, particle volume fraction, gas velocity, and particle velocity are obtained. The correlation between the simulation results and experimental data justifies that these models and algorithm are reasonable, and can be used to efficiently study the dense particle-gas two-phase flow.

Some hydrodynamic aspects of 3-phase inverse fluidized bed

Hydrodynamics of 3-phase inverse fluidized bed is studied experimentally using low density particles for different liquid and gas velocities. The hydrodynamic characteristics studied include pressure drop, minimum liquid and gas fluidization velocities and phase holdups. The minimum liquid fluidization velocity determined using the bed pressure gradient, decreases with increase in gas velocity. The axial profiles of phase holdups shows that the liquid holdup increases along the bed height, whereas the solid holdup decreases down the bed. However, the gas holdup is almost uniform in the bed.

Comparative study of two fluid model and dense discrete phase model for simulations of gas-solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics(CFD)has become a valuable tool to study the complex gas-solid hydrodynamics in the circulating fluidized bed(CFB).Based on the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,this work conducts the comparative study of the gas-solid hydrodynamics in a CFB riser by these two different models.Results show that DDPM could be used to predict gas-solid hydrodynamics in the circulating fluidized bed,and there are differences between TFM and DDPM,especially in the radial distribution profiles of solid phase.Sensitivity analysis results show that the gas-solid drag model exhibits significant effects on the results for both the two models.The specularity coefficient and the restitution coefficient in the TFM,as well as the reflection coefficient and the parcel number in the DDPM,exhibit less impact on the simulated results.

Experimental and numerical investigation of liquid-solid binary fluidized beds： Radioactive particle tracking technique and dense discrete phase model simulations

Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid-solid fluidized beds of the same density but dif- ferent sizes is investigated using radioactive particle tracking （RPT） technique and a dense discrete phase model （DDPM）. Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads （0.6 and I mm） and a binary fluidized bed of the same particles for vari- ous bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid-solid binary fluidized beds. The mean velocity predictions of DDPM are in good agree- ment with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle-particle interac- tions are found to be critical for predicting the flow behavior of solids in liquid-solid binary fluidized beds.

CFD-DEM模型并行化及其在流化床气固流动中的应用

提出广义区域覆盖法用于定位计算节点区域,利用Fluent软件的UDFs实现颗粒信息在计算节点间高效、准确地传递,建立了适用于非正交网格的CFD-DEM模型并行求解方法,在探讨并行模型计算效率的基础上对埋管流化床和单孔射流流化床内的气泡行为及双支腿流化床半床间的颗粒交换行为进行了数值模拟研究并对比实验结果。结果表明:CFD-DEM模型并行求解方法能够快速传递及收集计算节点内的颗粒信息;广义区域覆盖法降低了颗粒在计算节点内的定位时间,提高了模型的并行计算效率;能够捕获与Rong等一致的埋管周围气泡行为(其频率约为3 Hz),获得了与Bokkers等结果接近的单孔射流气泡尺寸及颗粒速度矢量分布,双支腿流化床半床间颗粒交换行为与实验结果一致,第一主频约为1 Hz,第二主频大于2 Hz。

汽-液-固三相循环流化床蒸发器中固体颗粒浓度和速度的研究

应用电荷耦合器件CCD(charge coupled devise)测量和图像处理技术,定性和定量研究了热模汽-液-固三相自然循环流化床蒸发器中固体颗粒浓度和速度的轴、径向分布以及其随热通量、固体颗粒加入量等操作参数的变化规律。热通量的变化范围为2179.8W穖-2到6624.2W穖-2,固体颗粒的体积分率的变化范围为0.5%到2%。结果表明:在液-固两相区,固体颗粒浓度在轴向和径向的分布较为均匀;随着热通量的增加,加热管中固体颗粒的浓度逐渐降低,循环管中固体颗粒的浓度逐渐升高,并且两者均随固体颗粒加入量的增加而升高;固体颗粒的速度在轴向上较为接近,其在循环管中的速度高于在加热管中的速度,并且两者均随热通量的增加而升高;固体颗粒加入量对其速度有增强和阻碍两方面的作用。

化工多相流图像分析系统及其在液-固循环流化床研究中的应用

开发了一套用于多相流流场测试的化工多相流分析系统.该系统应用简便,功能强大,并已成功用于竖直管内液-固循环流化床中固体颗粒运动特性的研究.试验结果表明:1)该系统能直观显示被测流场的运动状态,满足一定流场的测量要求;2)颗粒在管内的浓度分布是不均匀的,随颗粒的体积分率和流型的变化而变化;3)所得的试验结果能够解释液-固流化床换热器的强化传热和防除垢机理.

气-液-固三相循环流化床局部相含率分布

应用自行开发的微电导探针测试技术,以玻璃珠(dp=0·48mm,ρs=2460kg·m-3)和苯乙烯颗粒(dp=1·45mm,ρs=1264kg·m-3)为固相,空气为气相,水及0·05%、0·20%(质量)SCMC(羧甲基纤维素钠)水溶液为液相,对三相循环流化床(TCFB)的各相局部含率进行了同时测定.考察了不同表观液体速度、辅助液体速度、液体黏度及颗粒密度对局部相含率轴径向分布的影响.在不同操作条件下,获得了1286套局部相含率实验数据.给出了局部固含率和局部气含率与操作条件、流体物性及床层轴径向位置的关联式,关联式的计算值与实验值吻合较好.