Solids mixing kinetics and segregation in a vibrostabilized fluid bed

The vibration-induced reduction in internal mobility of a fluidized bed, which was reported in a recent paper (Beeckmans and MacWilliam, 1986), has been confirmed using a larger bed. It has now been found that above a particular fluidization velocity internal mobility is not affected by vibrations. Distributor grid geometry was found to have a small effect on the fluidizing gas velocity at which bubbles are first observed at the surface of the bed, but it had an insignificant effect on solids mixing rates. The extent of segregation at steady state was reduced by vibrations with both flotsam and jetsam tracer particles.     

Characterization of the devolatilization rate of solid fuels in fluidized beds by time‐resolved pressure measurements

The characterization of volatile matter (VM) release from solid fuel particles during fluidized‐bed combustion/gasification is relevant to the assessment of the reactor performance, as devolatilization rate affects in‐bed axial fuel segregation and VM distribution across the reactor. An experimental technique for the characterization of the devolatilization rate of solid fuels in fluidized beds is proposed. It is based on the analysis of the time series of pressure measured in a bench‐scale fluidized‐bed reactor as VM is released from a batch of fuel particles. A remarkable feature of the technique is the possibility to follow fast devolatilization with excellent time‐resolution. A mathematical model of the experiment has been developed to determine the time‐resolved devolatilization rate, the devolatilization time and the volume‐based mean molecular weight of the emitted volatile compounds. Devolatilization kinetics has been characterized for different solid fuels over a broad range of particle sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

流化床作为生物质气化反应器试验研究

在流化床生物质气化炉内 ,用空气进行气化生物质 (花生壳 )的试验研究 ,分析的参数是当量比ER 0 .2— 0 .4 5 ,气化床的温度 75 0— 85 0℃和加入二次风。当ER在 0 .2 5— 0 .33,气化燃气热值为 6 .2— 6 .8MJ/m3 ,气体产量在 2 6 0— 390m3 /h ,生物质燃烧时比气化产量在 1.2 8— 2 .0 3m3 /kg之间 ,炭转化率在 5 3%— 80 %。并对 7种农、林废弃物进行了初步气化试验研究 ,生成的燃气体积分数 :CO为 14 %— 18% ,H2 一般低于 6 % ,甲烷 4 %— 12 %。燃气热值在 4 70 0— 710 0kJ/m3 。试验结果表明 ,在流化床生物质气化炉中 ,通过在悬浮空间加入二次风 ,可使燃气热值得到提高。     

Mixing and segregation in a liquid fluidized bed of particles with different size and density

Experimental studies have been carried out on fluidization of irregular particle mixtures of different size and density. The mixing and segregation phenomena could be interpreted on the basis of the diffusion model of Kennedy and Bretton. The dependence of computed particle dispersion coefficient on liquid velocity, particle density and size has been discussed.     

Combustion modelling of coal volatiles in the freeboard of a calorimetric fluidized bed combustor

A simplified kinetic approach, based on functional groups of the parent coal, was coupled with the bed hydrodynamics and a volatiles evolution region within the bed to conduct a parametric study with the experimental results obtained from a calorimetric fluidized bed combustor (FBC). The model results revealed that, for high-volatile coals with particle diameters of 1–3 mm, the fraction of the original volatiles burnt above the bed may be as high as 0.44-0.20, 0.36-0.09 and 0.30-0.02 for excess air levels ranging from 0 to 40% and bed temperatures of 800, 850 and 900°C respectively. For a low-volatile coal, the computed fractions were found to be in the ranges 0.35-0.08, 0.29-0.01 and 0.25-0.00 for similar operating conditions to the above. Good agreement between the model and experimental data suggests that the evolution of volatiles for coal particle diameters <5 mm is mechanistically controlled by both diffusion and chemical kinetics, while their combustion is largely governed by the mixing of volatiles and oxygen in the bed region.     

气固流化床中颗粒的夹带和扬析特性

在内径0.5 m、总高6 m的流化床中对玻璃珠和白刚玉颗粒的夹带和扬析现象进行了实验研究,两种颗粒通过筛分配比得到不同粒径分布的实验物料,以0.25~0.76 m/s的速度分批进行流态化试验,考察了表观气速、床层物料粒径分布以及颗粒粒径对颗粒夹带和扬析速率的影响,得到了颗粒的扬析速率常数(Ki*)。实验结果表明,夹带量和扬析速率常数随着表观气速呈指数增加;扬析速率常数随着颗粒粒径减小先增加,达到一个临界粒径(dcrit)后,扬析速率常数会随着粒径减小趋于平缓或降低。提出了一组经验关联式,分别用于预测临界粒径两侧颗粒的扬析速率常数,关联式能很好地预测本工作实验条件下的数据,误差在30%以内,并且关联式能够对文献中的实验数据进行较好的预测,可以用于流化床装置的放大。     

蔗糖三相流态化结晶过程中离集与混合特性的研究

在一总高６５０ｍｍ，直径６０ｍｍ的三相流化床实验装置上，分别采用压降法和取样法测定了不同操作条件下二元混合物蔗糖晶体颗粒浓度沿轴向的分布，并建立了表征颗粒混合与分级程度的数学模型，得到不同条件下的混合系数。结果表明，混合系数即混合程度随气速增加而增大，但随液速增加略有减小；随大颗粒比率的增加，床中的离集程度有所增大。     

Catalytic upgrading of woody biomass derived pyrolysis vapours over iron modified zeolites in a dual-fluidized bed reactor

In this paper, the influence of the proton forms of beta, Y and ferrierite zeolites and their iron modified counterparts during upgrading of pine wood pyrolysis vapours under nitrogen atmosphere was investigated. A dual-fluidized bed reactor was used where in the first bed pyrolysis of pine wood occurred, and in the second upgrading of the pyrolysis vapours over zeolites was conducted. The temperature for pyrolysis and upgrading was 400 and 450 °C, respectively. De-oxygenation reactions over the proton form and iron modified zeolites increased compared to the non-catalytic pyrolysis. The increased selectivity towards organic compounds through de-oxygenation could be noticed as a higher water yield and CO formation.     

A two-dimensional model for gas mixing in the upper dilute zone of a circulating fluidized bed

Based on measurements in a circulating fluidized test unit with a riser of 0.4 m i.d., a two-dimensional two-phase model for gas mixing has been developed. Radial gas dispersion and gas backmixing caused by dense clusters falling countercurrently to the main flow of a lean gas/solid suspension are considered. The model has successfully been compared with experimental data over a wide range of operating conditions. The model accounts for the main mixing phenomena and may be applied to calculations of chemical reactions in CFB risers.     

灰熔聚气化炉内气体与固体混合特性的研究

气化炉内的气化剂与煤粉的混合特性直接影响着气化炉内的碳转化率和灰熔聚流化床气化炉的热效率。研究气化炉内气体和固体颗粒的混合特性,掌握炉内气体和固体颗粒的轴向和径向的运动规律,对认识床内传热和传质机理具有重要的意义。基于欧拉双流体模型,结合颗粒动力学理论,应用商用FLUENT软件,对太原化学工业集团有限公司运行的灰熔聚流化床气化炉内的气体与固体的混合特性进行了数值模拟。     

二维双支腿流化床内气体混合特性

Experiments of gas mixing between two half-beds were performed in a dual-leg circulating fluidized bed(DL-CFB)240 mm in width,40 mm in depth and 2000 mm in height by using glass beads with diameter of 0.25-0.28 mm as bed material.SO₂ and CO as the gas tracers entering the left and right distributors separately were used to simulate the gas mixing between the two legs.MSD(mean square displacement)model was adopted to calculate the gas dispersion coefficient(D_w)which was used to investigate the effect of fluidization velocity and bed material inventory on gas mixing in the DL-CFB.The experimental results showed that D_w was about 50-300 cm²穝^-1 at different fluidization velocities and bed material inventories.A higher fluidization velocity benefited particles exchange between two half-beds,which intensified the gas-solids interactions at the region with higher solids volume fraction.The gas mixing in the lower region of the DL-CFB was stronger than that in the upper region of the bed.A higher bed inventory was helpful to gas mixing at a lower fluidization velocity,while a higher fluidization velocity weakened gas mixing because of higher solids concentration in the center of the bed that prevented gas mixing.     

流化床反应器气固传热面积模型

研究了流化床反应器内发生气固反应时间的传热机理,认为气固间的传热面积包括两部分,即气泡内所含颗粒的表面积与一气泡和泡晕间的有效传热面积 ,据此首次导出了气固传热面积模型A＝u0-umf/22.26db0.5-umf[4.5unf ρgCpg/db 5.85(λgρgCpg)0.5g^0.25/db^1.25]LmS/a将该模型 应用于半间歇的裂化催化剂烧炭再生过程,与实验数据的比较表明,模型预测是可靠的。     

Modeling,Simulation, and Experimental Evaluation of Particle Size Distribution in Stabilized Fluidized Bed Adsorption System

Fluidized beds containing solid particles of a wide size distribution is of significant practical importance. In such systems, the overall behavior depends on the opposing effect of mixing and classification. In the present work, the mixing and segregation behavior of liquid fluidized beds containing particles of different sizes is described mathematically. Particle size distribution (PSD) is studied in a glass column of 5 cm internal diameter and 250 cm length. Ion exchange resins were used as a solid phase with a particle size range of 50 to 650 µm. The bed was fluidized at constant and low water flow velocity and the particle size measurement was carried out at different locations over the column length by analytical scanning electron microscope. Particle size fractionation data obtained by Malvern Mastersizer-2000, version 5.4, was utilized in the solution of developed model equations to obtain PSD. It is apparent that the mixing model along with the classification model represents better results than any other model given by various researchers in the literature. The proposed model is in good agreement with the PSD data given by Malvern particle size analyzer.     

生物质快速热解装置研究进展

当今化石能源日渐枯竭和环境压力日益加重是亟待解决的问题,而生物质热解液化技术被认为是解决能源紧张的潜在方法,尤其是生物质快速热解技术。随着生物质快速热解技术与工艺不断成熟,需要快速热解装置不断放大以提高处理量,以实现生物质快速热解的工业化。生物质快速热解装置复杂且多样化,在装置的放大过程中,各系统的合理选择是难点。本文首先对生物质热解机理、快速热解过程的粒径选择和前处理进行了简述,并对快速热解流程中的进料系统、供能系统、热解反应器和快速冷凝系统4个关键系统进行了综述,着重介绍了快速热解反应器的类型及其特点,提供了该4个关键系统的选择及研究趋势。流化床反应器具有易放大、可以较好地实现自热式快速热解的优点,本文总结出流化床式反应器是目前研究的热点。在保证产品品质下,设备易放大、稳定实现自热式、流程能耗低、运行稳定安全等是快速热解装置未来的研究方向。     

撞击流—旋转填料床反应器的微观混合性能研究

采用化学偶合法 ,对撞击流—旋转填料床反应器内微观混合进行了实验研究 ,得到了转速、喷嘴间距、射流速度及浓度等因素对离集指数Xs的影响规律 ,并对其进行了定性的分析。结果表明撞击流—旋转填料床反应器中微观混合能被极大地强化 ,可成为适合快速反应新型反应设备     

流化床与沸腾床内部微生物群落演变与分析

用流化床反应器和沸腾床反应器处理焦化废水,对两种反应器运行效能及微生物群落变化进行对比研究。结果表明,提高污泥负荷后流化床COD去除率优于沸腾床反应器。提高进水污泥负荷对沸腾床内微生物的冲击更大,致使其种群丰富程度下降明显。研究结果证明,流化床在宏观去除率、维持菌种丰富度方面具有较大优势。     

Numerical investigation of gas mixing in gas‐solid fluidized beds

Gas mixing in a tall narrow fluidized bed operated in the slugging fluidization regime is simulated with the aid of computational fluid dynamics. In the first part, a parametric study is conducted to investigate the influence of various parameters on the gas mixing. Among the parameters studied, the specularity coefficient for the partial‐slip solid‐phase wall boundary condition had the most significant effect on gas mixing. It was found that the solid‐phase wall boundary condition needs to be specified with great care when gas mixing is modeled, with free slip, partial slip and no‐slip wall boundary conditions giving substantial differences in the extent of gas back mixing. Axial and radial tracer concentration profiles for different operating conditions are generally in good agreement with experimental data from the literature. Detailed analyses of tracer back mixing are carried out in the second part. Two parameters, the tracer backflow fraction and overall gas backflow fraction, in addition to axial profiles of cross‐sectional averaged tracer concentrations, are evaluated for different flow conditions. Qualitative trends are consistent with reported experimental findings. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Size distribution of the particles entrained from fluidized beds: Gas humidity effects

Experimental results obtained with 65 μm sand and 69 μm cracking catalyst showed that the smallest particles were not the most easily elutriated from fluidized beds of mixed size particles. This is in contradiction with the assumption applied to existing models for the prediction of the flux and size distribution of particles elutriated from a fluidized bed. The smallest particles cannot be removed by elutriation from a mixture of particles of various sizes. The smallest particles may be agglomerated with larger particles. Increasing the gas humidity, which would presumably reduce electrostatic effects, did not greatly affect the size distribution of the elutriated particles.