循环流化床气固传质实验研究

This study is devoted to gas-solid mass transfer behavior in heterogeneous two-phase flow. Experiments were carried out in a cold circulating fluidized bed of 3.0m in height and 72mm in diameter with naphthalene particles. Axial and radial distributions of sublimated naphthalene concentration in air were measured with an online concentration monitoring system HP GC-MS. Mass transfer coefficients were obtained under various operating condition, showing that heterogeneous flow structure strongly influences the axial and radial profiles of mass transfer coefficients. In the bottom dense region, mass transfer rate is high due to intensive dynamic behavior and higher relative slip velocity between gas and clusters. In the middle transition region and the upper diluter region, as a result of low mass transfer driving force and the influence of flow structure, mass transfer rate distribution becomes non-uniform. In conclusion, among the operating parameters influencing mass transfer coefficients, the superficial gas velocity is the most important factor and the solid circulation rate should be also taken into account.     

耦合反应器提升管段颗粒速度分布及约束特性

A large-scale cold model experimental setup of a riser-fluidized bed coupled reactor was established according to the olefin reduction technology with an auxiliary reactor for FCC naphtha upgrading.Distributions of particle velocity in the riser section were experimentally investigated in the setup.Furthermore,the restriction index of particle velocity was defined to quantitatively show the restriction effects of the riser outlet lotus-shaped distributor and the upper fluidized bed on the particle flow behavior in the riser.The experimental results showed that the riser could be divided into two regions in the longitudinal direction,i.e.,lower traditional transport region and upper restriction region.In the longitudinal direction,the averaged cross-sectional particle velocity in the traditional transport region increased firstly,and then tended to be smooth,while decreased in the restriction region.With the increase of static bed height in the upper fluidized bed,the local particle velocity decreased,and the tendency of change in the core region is more than that in the wall region.Restriction effects of the lotus-shaped distributor and the upper fluidized bed on particle flow behavior enhanced with the increases of superficial gas velocity,solids flux and static bed height in the upper fluidized bed.In the same cross-section,outlet restriction effects enhanced with the increase of the dimensionless radial position r/R,and would not change when r/R≥0.5.     

整体式床层中液体分布与气-液传质的关系

With a particular focus on the connection between liquid flow distribution and gas-liquid mass transfer in monolithic beds in the Taylor flow regime, hydrodynamic and gas-liquid mass transfer experiments were carried out in a column with a monolithic bed of cell density of 50 cpsi with two different distributors (nozzle and packed bed distributors). Liquid saturation in individual channels was measured by using self-made micro-conductivity probes. A mal-distribution factor was used to evaluate uniform degree of phase distribution in monoliths. Overall bed pressure drop and mass transfer coefficients were measured. For liquid flow distribution and gas-liquid mass transfer, it is found that the superficial liquid velocity is a crucial factor and the packed bed distributor is better than the nozzle distributor. A semi-theoretical analysis using single channel models shows that the packed bed distributor always yields shorter and uniformly distributed liquid slugs compared to the nozzle distributor, which in turn ensures a better mass transfer performance. A bed scale mass transfer model is proposed by employing the single channel models in individual channels and incorporating effects of non-uniform liquid distribution along the bed cross-section. The model predicts the overall gas-liquid mass transfer coefficient with a relative error within ±30%.     

MODEL FOR PREDICTING THE LATERAL DISPERSION COEFFICIENT OF SOLIDS IN GAS-SOLIDS FLUIDIZED BEDS

The lateral mixing of solids in a gas-solids fluidized bed is very complicated.It can be caused by:(a)bubble movement through the bed,(b)bubble burst at the bed surface,and(c)gross particle circulation in thebed.However,experiments show that the major factors effected the lateral mixing of solids are the bubblemovement through the bed and the bubble burst at the bed surface.Thus a model with two mixing re-gions,i.e.mixing in bubble rising region and mixing in bubble breaking region,was proposed.Based on thismodel,an equation for predicting the lateral dispersion coefficient of solids in gas-solids fluidized beds wasderived without any adjustable parameter.The calculated values by this equation are well comparable withthe observed data including the present work and the other investigations.     

A CFD model for predicting the heat transfer in the industrial scale packed bed

Compared to the traditional lumped-parameter model,computational fluid dynamics (CFD) attracted more attentions due to facilitating more accurate reactor design and optimization methods when analyzing the heat transfer in the industrial packed bed.Here,a model was developed based on the CFD theory,in which the heterogeneous fluid flow was resolved by considering the oscillatory behavior of voidage and the effective fluid viscosity.The energy transports in packed bed were calculated by the convection and diffusion incorporated with gaseous dispersion in fluid and the contacting thermal conductivity of packed particles in solids.The heat transfer coefficient between fluid and wall was evaluated by considering the turbulence due to the packed particles adjacent to the wall.Thus,the heat transfer in packed bed can be predicted without using any adjustable semi-empirical effective thermal conductivity coefficient.The experimental results from the literature were employed to validate this model.     

循环流化床的沸腾传热

A model is proposed to predict boiling heat transfer coefficient in a three-phase circulating fluidized bed (CFB), which is a new type of evaporation boiling means for enhancing heat transfer and preventing fouling. To verify the model, experiments are conducted in a stainless steel column with 39 mm ID and 2.0 m height, in which the heat transfer coefficient is measured for different superficial velocities, steam pressures, particle concentrations and materials of particle. As the steam pressure and particle concentrations increase, the heat transfer coefficient in the bed increases. The heat transfer coefficient increases with the liquid velocity but it exhibits a local minimum.The heat transfer coefficient is correlated with cluster renewed model and two-mechanism method. The prediction of the model is in good agreement with experimental data.     

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.     

反渗透过程的浓度极化及其传质系数测定

The relationship between osmotic pressure difference across the membrane and mass transfer coefficient is developed in this paper. On the basis of this relationship, a method for measuring mass transfer coefficient by using experimental data on reverse osmosis is established. Pitzer's equations are used to calculate osmotic pressure differences in order to assure accuracy of results. Under the conditions of constant operating pressure and bulk flow, mass transfer coefficient is scarcely affected by membrane structure, but decreases slightly with increasing feed concentration. The solute concentration in the polarization layer is calculated by using the measured values of mass transfer coefficient. Polarization layer concentration increases with augmentation in bulk concentration. However, their difference increases with increasing bulk concentration until a maximum difference is reached, and then decreases. Mass transfer coefficient increases with higher velocity of bulk flow. If mass transfer coefficient is so large that the ratio(PwΔπσ/k)becomes very small, polarization could be neglected.     

超细粉在导向管喷动床中的固体循环速率

Ultra-fine powders are difficult to be fluidized due to the strong particle to particle cohesiveness.However, the authors‘ experiments showed that the ultra-fine powder CaCO3 could be stably fluidized in a spouted bed with a draft tube. The effects of geometric and operating parameters on solid circulation rate of ultra-fine powder CaCO3 were investigated in a 120 mm diameter transparent semicircular spouted bed with a draft tube. Three draft tubes with different sizes were used in this study. It was found that the solids circulation rate was mainly dependent on the drawing rate of the gas jet from the nozzle, then on the gas transport capacity in the draft tube. With increasing gas feed rate, distance between the nozzle and the draft tube inlet and draft tube diameter, the solids circulation rate could be increased. Based on the jet theory, a quantitative correlation was proposed for predicting the solid circulation rate of ultra-fine powders in a spouted bed with a draft tube by taking into account the gas transport capacity in the draft tube.     

Effect of Adsorbent Diameter on the Performance of Adsorption Refrigeration

Adsorbents are important components in adsorption refrigeration. The diameter of an adsorbent can af-fect the heat and mass transfer of an adsorber. The effect of particle diameter on effective thermal conductivity was investigated. The heat transfer coefficient of the refrigerant and the void rate of the adsorbent layer can also affect the effective thermal conductivity of adsorbents. The performance of mass transfer in the adsorber is better when pressure drop decreases. Pressure drop decreases with increasing permeability. The permeability of the adsorbent layer can be improved with increasing adsorbent diameter. The effect of adsorbent diameter on refrigeration output power was experimentally studied. Output power initially increases and then decreases with increasing diameter under different cycle time conditions. Output power increases with decreasing cycle time under similar diameters.     

提升管-流化床耦合反应器颗粒约束返混区的流动特性及约束作用分析

针对催化汽油辅助反应器改质降烯烃工艺,在一套提升管-流化床耦合反应器大型冷态实验装置上,系统研究了提升管出口段的颗粒流动特性,通过定义约束指数R_i（R_i为颗粒约束返混区实际截面平均固含率与理论截面平均固含率之比）定量反映提升管出口分布器及流化床层的约束作用。结果表明,与常规提升管相比,耦合反应器提升管出口存在一个颗粒约束返混区,其长度主要受表观气速、颗粒循环强度及上部流化床内颗粒静床高度影响;由于出口设置了倒锥形分布器,使得颗粒约束返混区靠近提升管出口区域在表观气速较低和颗粒循环强度较大时,局部固含率最大值出现在量纲 1半径Φ＝0.7处;颗粒约束返混区的约束指数在靠近出口的过程中逐渐增大,气固流动受到分布器及上部流化床层的约束作用亦逐渐增强。     

A new correlation for predicting solids concentration in the fully developed zone of circulating fluidized bed risers

The present work focuses on developing a new comprehensive correlation for better prediction of the solids concentration in the fully developed region of co-current upward gas-solid flow in circulating fluidized bed (CFB) risers. Systematic experiments were carried out in two risers (15.1 m and 10.5 m high with the same 0.1 m i.d.) with FCC and sand particles. The results obtained from about 200 sets of operating conditions show that the average solids concentration in the fully developed region is more than just a function of the corresponding terminal solids concentration, as most previous correlations are based on. Operating conditions, particle properties and riser diameters also have significant effects on the solids concentrations in the fully developed region of CFB risers. Based on our experimental data and those reported in the open literature from CFB risers up to 0.4 m in diameter and 27 m in height with superficial gas velocities and solids circulation rates up to 11.5 m/s and 685 kg/m²·s, a new empirical correlation for predicting the average solids concentrations in the fully developed region of CFB risers is proposed. The correlation works well for a wide range of operating conditions, particle properties and riser diameters.     

A multiscale mass transfer model for gas-solid riser flows: Part 1 — Sub-grid model and simple tests

The gas-solid mass transfer in circulating fluidized bed (CFB) riser flow is both structure-dependent and dynamic in nature. Recent progress in multiscale computational fluid dynamics (CFD) allows fresh insight into the dynamic flow structure, yet its influence on the mass transfer remains to be settled. To this end, a multiscale mass transfer model is established in this paper based on the extended framework of the energy-minimization multiscale (EMMS) model. The relevant algorithm named EMMS/mass is proposed for CFD-coupled mass transfer computation. Two testing cases accounting for sublimation of naphthalene and decomposition of ozone, respectively, are presented to demonstrate the characters of the model. It is shown that structural consideration can have significant effects on the model prediction. The normally used Reynolds number is not adequate to characterize these effects, while the combination of gas velocity and solids flux seems to capture the structural effects and allows to explain the variation of Sherwood number reported for CFB risers in the literature. Sub-grid coupling of this multiscale mass transfer model and CFD approach can be expected to provide a promising tool to probe the dynamic and structure-dependent nature of mass transfer in CFB risers.     

颗粒相壁面条件对非球形颗粒流动影响的数值模拟

采用双流体模型对设置竖直隔板的气固密相流化床中非球形颗粒的运动进行了模拟,颗粒形状的影响由相间曳力模型考虑,重点考察壁面处颗粒边界条件的影响。同时进行了实验室规模三维流化床的流化实验,以验证模型的有效性。通过压降轴向分布、颗粒浓度径向分布以及物料出口处颗粒质量流率功率谱估计等定量分析,结果表明：对不设置内构件的自由床,壁面反射系数对系统宏观流动特性影响较小,而对壁面处局部颗粒运动影响较大;对壁面面积大幅增加的内构件床,壁面反射系数可显著改变气体和颗粒的运动特征,取值需控制在适当范围内。     

Monitoring the particle-wall contact in a gas fluidized bed by RPT

Particle-wall contact behavior of the solids in a gas-solid fluidized bed was experimentally studied using the radioactive particle tracking (RPT) technique in which the position of a radioactive tracer is monitored when moving freely in the bed. The solids were sand particles, fluidized by air at room temperature and atmospheric pressure at various superficial velocities, covering both bubbling and turbulent regimes of fluidization. The motion of individual particles near the wall of the bed was studied based on the position of the tracer. The contact time, contact distance and contact frequency of the particles at the wall were evaluated. It was found that the distribution functions of these three parameters become wider by increasing the superficial gas velocity. Axial profiles of contact time and contact distance were also studied in this work. Axial profiles of the overall heat transfer coefficient in the fluidized bed were estimated based on the formulas reported in the literature and the experimental particle-wall contact time evaluated in the present study. Based on such profiles, in order to benefit from the maximum heat transfer coefficient along the bed, it is recommended to place the heat exchanging surface in the middle of the bed, i.e., not very close to the gas distributor as well as far from the top of the dense bed.     

快速流化床内床层与浸润表面间的传热研究——传热系数的径向分布规律

本文对低温快速流化床内传热系数的径向分布规律进行了实验研究.推荐以下关联式来计算床内各轴、径向位置处的传热系数:式中a、n_1、n_2均为径向位置的函数,(?)为轴向位置的函数.     

非均匀气固两相系统中多尺度传质模型

建立了适用于气固循环流化床的多尺度传质模型 .从过程与尺度的角度出发 ,将非均匀气固两相流中的传质过程分解为静态与动态的过程 ,并将前者分解为稀相内、密相内以及稀密相间 3个尺度下的传质 ;在用多尺度能量最小 (EMMS)模型求解已知物系性质和操作条件的非均匀气固两相流体动力学参数的基础上 ,借助于前人的研究结果 ,利用相对滑移速度、空隙率等参数求解传质系数 ,求得轴向的浓度场分布 ,并讨论非均匀两相流动结构对传质效率的影响     

离心流化床中强制对流换热的实验研究

对离心流化床干燥器中气体与被干燥颗粒物料之间的强制对流换热进行了实验研究,获得了各主要运行参数对气固换热系数的影响规律,并利用场协同原理分析了对流换热强化的机理. 实验证明,在一定转速范围内,在气流速度方向和热流方向（温度梯度方向）一致时,换热的准则关联式具有Nu＝CRePr的形式. 获得了满足Nu～RePr呈直线关系的Pe(Pe=RePr)数变化范围和临界Pe数,当Pe数大于临界值后,离心流化床中对流换热强度随Pe数增加而增大的趋势会明显减缓并偏离线性区.     

Hydrodynamics in airlift loop section of petroleum coke combustor

Based on the combustion characteristics of petroleum coke, a coupled gas-solid fluidized bed combustor is proposed in this work. The overall circulating system of the fluidized bed mainly consists of a dense-phase airlift loop section and a dilute-phase riser section. In different operating conditions, the particle flow behaviors in the airlift loop section were investigated systematically by using optical fiber probe. The experimental results show that the airlift loop section can be divided into four regions, namely, the draft tube, the annulus, the bottom region and the particle diffluence region, in which the average cross-sectional solids fraction and the particle velocity are different. The overall solids fraction difference between the draft tube and the annulus provides a driving force for particle circulation flow in the airlift loop section, and the driving force increases with increasing the superficial gas velocity in the draft tube. The ratio of the particle mass flux in the annulus to that in the riser ranges from 8 to 16. The particle circular velocity in the annulus also increases with increasing the superficial gas velocity in the draft tube. Moreover, a model about the particle circular velocity is established on the basis of energy equilibrium principle.