下行气固两相流与管内壁间的摩擦压降

在较宽的操作条件范围内系统测试了下行床床层压力降,获得气固两相流与管内壁间的摩擦压降,提出了下行气固两相流与管壁间摩擦压降的计算模型。结果表明,在下行床的充分发展段,气固两相流与管壁间的摩擦导致表观颗粒浓度显著小于真实颗粒浓度;当表观气速大于8 m·s^-1时,气固两相流与管壁间的摩擦压降接近甚至超过气固两相流重力产生的静压降。在采用压差法测试下行床中的平均颗粒浓度时,如忽略气固两相流与管壁间的摩擦,则可能导致显著的偏差。下行气固两相流与管内壁间的摩擦压降主要来自于颗粒与管壁间的摩擦。颗粒直径对气固两相流与管壁间摩擦压降的影响随着操作气速的提高逐渐减弱。采用提出的摩擦压降模型对表观颗粒浓度进行修正后,预测值与实验值吻合较好。     

高密度下行床反应器的流体力学特性

在一套内径为80 mm,高5.6 m的新型下行循环流化床内,以硅胶、FCC催化剂以及玻璃珠等颗粒为实验物料,在颗粒循环流率最高达600 kg•m^-2•s^-1,床层颗粒平均浓度达14%的条件下,进行了低气速、高浓度下行床内气固流动特性的研究.实验结果表明：高浓度下颗粒浓度的波动特性与低密度的有所差异.在低浓度操作条件下,颗粒浓度的概率分布曲线为单峰,而在高浓度下,概率密度分布曲线近似为水平直线;床层颗粒浓度随固体颗粒循环流率的增加而提高,颗粒直径及密度小的物料容易达到高的床层浓度,密度大而流动性好的物料容易达到高的颗粒循环流率;在低密度操作条件下,下行床内气固沿轴向流动过程可分为两个区域：加速区以及恒速区;而在高浓度操作条件下,可分为3个区域：加速区、恒速区以及出口受限区.     

上行气固两相流充分发展段颗粒浓度关联及预测

在高度分别为15.1m和10.5m的两套实验装置上,对快速流态化到稀相气力输送流型下提升管内的轴向压力梯度进行了系统测试,以研究提升管充分发展段内不同颗粒的浓度变化及其与操作参数的关系。实验在其中175组操作条件下展现出明显的充分发展段(>2.8m)。结果表明,表观气速在3~8m?s-1之间变化时,对充分发展段颗粒浓度随终端颗粒浓度的变化关系影响显著,但当表观气速>8m?s?1或<3m?s?1时,其对充分发展段颗粒浓度随终端颗粒浓度线性增加的关系影响极弱;在此基础上提出的预测关联式更明确地反映了操作条件等因素对充分发展段颗粒浓度的定量影响关系,其计算结果与本实验和相关文献的实验数据吻合良好。     

矩形导流管喷动床下行区的床层压降

带导流管的矩形喷动床是传统喷动床的改进型式,矩形床内设置的与床同厚的垂直导流管,可以控制固体颗粒的内循环速率,同时使下行区中的气固移动床维持平推流.本文实验测定了不同表观气速、床层重量、不同固体颗粒与气体入口形式与尺寸时,矩形导流管喷动床下行区的床层压降,以考察其流动特征.实验结果表明,下行区存在床层压降的轴向分布,气固流动处于负压差下移上流区,且气固滑移速度自下而上是逐渐下降的.下行区颗粒床层的压降以及颗粒的移动下输,受到喷动床表观气速、床高、喷嘴尺寸、物料种类和颗粒直径的不同影响.     

高密度下行床内颗粒浓度径向分布的研究

在自行设计的一套下行循环流化床内进行了低气速、高浓度下行床内颗粒浓度的分布研究。实验分别采用了硅胶和FCC颗粒,在床层截面平均浓度最高到12％范围内进行了下行床不同截面上颗粒浓度径向分布的研究,并进一步分析了床层颗粒浓度对稀相中心区、环形浓相区浓度径向分布的影响。研究表明,在同一截面,随着床层颗粒平均浓度的增加,浓度分布趋于均匀;在截面平均浓度相近,浓度的径向分布沿轴向从上到下逐渐趋于均匀。颗粒的相对浓度的最大值随截面平均浓度的增加而减小,其在径向的位置基本不变。实验还发现,床层截面平均浓度的增加,浓相区内颗粒浓度的分布更均匀,而对稀相区内颗粒浓度分布没有明显的影响。     

气液固三相流化床层膨胀特性实验分析

为分析气液固三相流化床床层特性,选用两种粒径接近、密度不同的颗粒:塑料颗粒(湿堆积密度1 273 kg/m~3,平均直径750μm)和陶粒(湿堆积密度1 680 kg/m~3,平均直径800μm),以常温空气为气相、水为液相。实验装置内径0.13 m,全床高度4.75 m,实验液柱高度3.5 m,床内颗粒有足够的膨胀高度。分别测试了液速0—13 mm/s,气速0—12 mm/s条件下的床层高度,研究气液速对塑料颗粒和陶粒床层膨胀率的影响。研究结果显示液速较低时床层收缩,随着气速增加,收缩率增大;液速较高时,膨胀率大于0,随着表观气速的增加,膨胀率先降低,表观气速大于临界值后,膨胀率将增大;除高气速外,床层膨胀率总是随着液速的升高而增大。相同气速、液速条件下,大密度陶粒的床层膨胀(收缩)率大于塑料颗粒。     

上行气固两相流充分发展段的颗粒浓度

在Ф100mm× 16m循环床提升管实验装置上测试了134组操作条件下提升管 12个截面位置的平均颗粒浓度 ,其中 113组操作条件下提升管中的气固两相流展现出明确的充分发展段 .结果表明 ,充分发展段的颗粒浓度εs 随颗粒循环量G_s 的增加而增大且具有显著的线性关系 ,ε_s 随表观气速U_g 的增加以幂函数关系减小 .所提出的实验关联式不仅较好地拟合了本文所获得的充分发展段的平均颗粒浓度数据 ,而且诠释了以往有关提升管稀相段出口颗粒浓度预测结果之间的差异 ,更明确地反映了充分发展段颗粒浓度与操作参数之间的定量关系     

高密度下行床颗粒浓度与颗粒速度分布特性

为了考察下行床内颗粒浓度和速度的分布特性以确定高密度操作条件,在自建的气固下行床冷模装置(总高度18 m,下行床高8 m,直径80 mm)中,测定了不同表观气速(Ug)和颗粒循环速率(Gs)下床内颗粒浓度和颗粒速度的轴径向分布.结果表明,在Gs为700 kg/(m2·s)时,下行床内平均颗粒浓度达0.12,整个下行床内...     

利用光导纤维表征并流下行流化床中颗粒聚凝体的动态特征(英文)

利用光导纤维来表征并流下行流化床中颗粒聚凝体的动态特征。下行床的直径为0.1m、床高10m,操作气速为3.5～10m·s-1,颗粒流率为50～200kg·m-2·s-1。所用颗粒为催化裂化FCC颗粒,直径67μm,密度为1500kg·m-3。研究中首先用灵敏度分析方法建立起确认颗粒聚凝体的最佳条件,由此从所测得的瞬时颗粒浓度数据来获得颗粒聚凝体的各种特性(频率、时间分率、存在时间及平均浓度)。研究发现颗粒聚凝体的性质明显地受到操作条件的影响(气体速度与颗粒流率)以及局部颗粒浓度的影响。颗粒聚凝体的性质亦沿着下行床的轴向与径向发生很大变化。在充分发展段,下行床中心比近壁处有着更强的形成颗粒聚凝体的趋势。靠近下行床的底部,颗粒聚凝体性质沿轴向分布的变化比较缓和。     

大型快速流化床内颗粒浓度和速度分布的实验研究

采用压力巡检仪和光纤测量仪,对直径300 mm的快速流化床反应器内气固两相流动特性进行了研究,考察了操作条件对快速床轴、径向催化剂颗粒浓度、颗粒速度、筛分分布等的影响. 结果表明,当操作气速提高到2.0~2.6 m/s,相应的催化剂循环强度在60~160 kg/(m2×s),床层密度可保持在50~650 kg/m3;催化剂颗粒浓度在径向上呈中心低、边壁高的不均匀分布,轴向上各径向位置在颗粒加速区逐渐降低、在充分发展区趋于稳定、随表观气速增大或催化剂循环强度减小而减小,且径向均匀性变好,在r/R<0.7的中心区域趋于一致;颗粒速度在径向上呈中心高、边壁低的抛物线形分布,且随操作气速增大或催化剂循环强度增大而更加明显.     

Solids concentration in the fully developed region of circulating fluidized bed downers

To investigate solids concentration in the fully developed region of co-current downward gas-solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature.     

气固下行流化床反应器Ⅱ气固两相的流动规律

气固下行流化床反应器气固两相流动过程是比较复杂的，沿轴向气固两相运动可分为第一加速、第二加速和恒速３个运动段，沿径向局部气体速度、颗粒速度和颗粒浓度都具有不同程度的不均匀性。而这种不均匀性是由气固两相顺重力场湍动运动所决定的。和循环床提升管相比，下行管反应器气固两相沿径向分布的不均匀性得到有效地改善，气固可以实现超短接触操作，因而是一种新型高效气固超短接触反应器     

Catalytic reaction in a circulating fluidized bed downer: Ozone decomposition

Catalytic ozone decomposition reaction was used to study the performance of a 76 mm i.d. and 5.8 m high gas–solid circulating fluidized bed (CFB) downer reactor. Optical fiber probes and an ultraviolet (UV) ozone analyzer were used to obtain comprehensive information about local solids holdup and ozone concentration profiles at different axial and radial positions at superficial gas velocity of 2–5 m/s and solids circulation rates of 50 and 100 kg/m² s. Axial ozone concentration profiles significantly deviated from the plug-flow behavior, with most conversion occurring in the entrance region or flow developing zone of the downer reactor. Strong correlation was observed between the spatial distributions of solids and extent of reaction; higher local solids holdups cause lower ozone concentrations due to higher reaction rates. Radial gradients of the reactant (ozone) concentrations increased in the middle section of the downer, and decreased with increasing superficial gas velocity and solids circulation rate. Contact efficiency, a measure of the interaction between gas and solids indicated high efficiency in the flow developing zone and decreased with height in the fully developed region.     

Modeling of the hydrodynamics of the fully developed region in a downer reactor

A mathematical model is developed to describe the hydrodynamics of the fully developed region in a downer reactor. The model is based on the energy-minimization and multi-scale (EMMS) principle. The influence of the friction coefficient, the solid flow rate and the superficial gas velocity on the flow pattern in the downer is simulated. The local solid concentration and gas/solid velocities are predicted. The model results show a good agreement with experimental data in literatures. In addition, cluster size and local slip velocity are discussed.     

Flow development in a gas-solids downer fluidized bed

The development of gas and solids flow structure was studied in a 9.5 m high and 0.10 m diameter, gas-solids cocurrent downflow circulating fluidized bed (downer). Local solids concentration and particle velocity were measured using two separate optical fibre probes at different radial positions on several axial levels along the downer. The results show that the flow development is significantly influenced by the operating conditions. For most of the conditions under which the experiments were conducted, the gas-solids flow reaches its fully developed zone within 3 to 8 m away from the entrance. On the other hand, the development zone can extend as long as the downer itself, under certain conditions. When the solids circulation rate is over 100 kg/m²s, an increasing solids circulation rate largely extends the length of radial flow development. It is found that the flow developments in the core and at the wall are not quite simultaneous. For solids concentration, the core develops more quickly at low gas velocities and the wall region develops faster at high gas velocities. For particle velocity, higher gas velocity speeds up the development of the wall region but does not significantly affect the development of the core region. The wall region is much more sensitive to the change of superficial gas velocity than the core region. At high superficial gas velocities (> 7 m/s), a “semi-dead” region is observed in the fully developed zone adjacent to the wall where the dilute solids are moving at a very low velocity.     

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.     

气液固三相外环流反应器相含率分布与气液流动结构

结合电阻层析成像和差压法,以粒径3 mm的聚氨酯(密度1110 kg/m3)和粒径0.5, 1, 3 mm的玻璃珠(密度2460 kg/m3)为固相、空气为气相、水为液相,在直径0.09 m、高3.1 m的气液固三相气升式外环流反应器中考察了固相颗粒密度、粒径、固相含率对上升管和下降管中气含率的影响. 结果表明,当玻璃珠粒径为0.5和1 mm、表观气速ug>0.10 m/s时环流反应器可达到充分发展段. 以玻璃珠为固相时,气含率随固相含量和粒径增大而增大,从层析图像可见,在r/R=0~0.2处出现非导电相的极大值,且加入大颗粒的下降管中气固含率明显大于加入小颗粒的. 根据电阻层析成像的虚拟像,给出了时间序列的气固混合相的流动特征.     

Predicting axial pressure profile of a CFB

The numerical simulation of CFBs is an important tool in the prediction of its flow behavior. Predicting the axial pressure profile is one of the major difficulties in modeling a CFB. A model using a Particle Based Approach (PBA) is developed to accurately predict the axial pressure profile in CFBs. The simulation model accounts for the axial and radial distribution of voidage and velocity of the gas and solid phases, and for the solids volume fraction and particle size distribution of the solid phase. The model results are compared with and validated against atmospheric cold CFB experimental literature data. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05 to 0.305 m, bed height between 5 and 15.45 m, mean particle diameter from 76 to 812 μm, particle density from 189 to 2600 kg/m³, solid circulation fluxes from 10.03 to 489 kg/m² s and gas superficial velocities from 2.71 to 10.68 m/s. The computational results agreed reasonably well with the experimental data. Moreover, both experimental data and model predictions show that the pressure drop profile is affected by the solid circulation flux and superficial velocity values in the riser. The pressure drop increases along the acceleration region as solid circulation flux increases and superficial velocity decreases.     

INVESTIGATION ON THE INCIPIENT FLUIDIZATION AND HEAT TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Based on the continuum theory, a physical model of gas-solid two phase flow in a centrifugal fluidized bed has been proposed. A set of governing equations to describe the fluidization state are obtained and solved numerically after some simplifying. The quantitative experimental study on the characteristics of the incipient fluidization in the centrifugal fluidized bed is performed to examine the proposed model. Gas-solid two phase heat transfer in CFB during a drying process is also conducted. The influences of bed thickness, particle diameter, physical properties of particle, rotating speed of the bed and the gas superficial velocity on heat transfer characteristics are examined. A correlation that can be used to calculate the heat transfer coefficients in the drying process in CFB is obtained.