内构件流化床内颗粒停留时间分布及压降的研究

在一内径100 mm设有数块水平挡板、底部连续加料与上部溢流排料的流化床内,采用粒径为0.04～1.0 mm的精钛矿颗粒,进行了加料速率、流化气速以及内构件间距等不同因素对物料在床内停留时间及压降影响的实验研究,发现在气固并流向上的流化床内增设水平挡板且d/H=1时,可形成稳定流化状态.研究结果表明流化床中的持料量主要取决于流化气速,平均停留时间随流化气速和进料速率增加而降低.平衡时床内颗粒平均粒径大于进料颗粒的平均粒径,使得床内小颗粒停留时间减少,大颗粒停留时间延长,可满足各种粒度颗粒所需的反应时间.通过简化经验公式计算表明,大而重的颗粒趋于沉积床底,小而轻的颗粒趋于漂浮床顶.实验数据可为此类床型用于气固相加工提供有益参考.     

工艺条件对内旋式移动床停留时间的影响

为了给热解过程中颗粒在内旋式反应器内的停留时间和混合效果提供理论指导,在内旋式移动床试验装置上,以煤颗粒作为填充物料和示踪剂,利用脉冲法研究了转轴长度、转轴转速、粒径及给料速率等工艺条件对物料停留时间的影响。结果表明:60°倾角安装的桨叶式传动叶片测得的平均停留时间分布曲线存在较大的"拖尾",曲线尾峰较多,表明反应器内存在返混;反应器内煤料流动流型介于平推流与全混流之间;转轴长度与平均停留时间呈线性关系;转轴转速与平均停留时间呈指数关系,增大转速会加剧轴向返混,同时缩短物料在反应器内的平均停留时间;随着煤料粒径的增大,反应器内平均停留时间和轴向返混程度均呈减小趋势,且粒径大的煤颗粒在输送过程中会出现一定程度的破碎,5~6 mm煤颗粒的破碎率为35.39%;给料速率主要影响煤料在反应器内的填料量,从而影响停留时间分布,平均停留时间随着给料速率的增加而降低。     

回转窑内滚动状态下颗粒横向运动的影响因素的实验研究

回转窑内物料颗粒的横向运动使物料的传热和传质得到加强,其研究对于回转窑的优化设计和运行是十分重要的。本文在数学模型的基础上采用数码摄像的方法对多种物料颗粒在旋转筒体内的横向运动进行了实验研究,考察了筒体直径、旋转速度、料床内物料的动态休止角等参数对于泻落层厚度、颗粒运动速度、流量以及物料在料床表面停留时间的影响,并通过实验验证了数学模型。     

新颖环形喷动床颗粒喷动特性实验研究

一种新颖的环形喷动床由内外两个不同内径、同心的垂立圆筒组成,在环形空间底部设置多个喷口,在喷口两侧布置倾斜的导流板.研究颗粒在这种喷动床内的流动特性,探讨喷口结构、颗粒种类以及床内载料量对环形喷动床颗粒喷动特性的影响.实验结果表明:颗粒在环形喷动床内分为三个明显不同的区域,即颗粒填充移动区、密相喷动流化区以及稀相夹带区.当颗粒出现分区喷动后,随床内载料量的增多,填充移动区的高度维持不变,始终等于导流板的高度,而密相喷动区的高度不断增加.风量和颗粒种类对床层最大喷动量、密相喷动高度以及床层压力分布规律有着十分重要的影响.采用不同的喷口结构时,在相同的载料量下,直向喷口的密相喷动区高度更大,而且床内各测点的平均压力大于采用斜向喷口时的相应测点压力.     

新型内构件填充床反应器数值模拟-I.气相流场和停留时间分布

针对含新型内构件的复杂填充床内部结构，建立了包含颗粒填充床、气体通道、气体挡板的几何模型下气液两相流动的数学模型，采用计算流体力学(CFD)技术首先对气流在复杂结构下的流动分配、流型和停留时间分布进行了详细的模拟，并考察了操作参数和结构设计对流场和停留时间分布的影响. 通过压降实验数据在宏观尺度上验证了CFD模拟的正确性. 详细的内部流场展示了气体在颗粒床和气体通道内的曲折流动行为，增加了气体的平均停留时间；停留时间分布预测表明气相流动没有短路发生，平均停留时间与表观气速成反比. 内构件结构参数对气体流场和停留时间分布产生重要的影响.     

图像法用于流化床颗粒混合特性的研究

利用基于相关的图像法速度测量技术,测量了小型二维气固流化床密相区颗粒运动速度分布.并提出了用颗粒速度不均匀指数来衡量流化床内颗粒的速度波动与混合.速度波动对于物料扩散与对流混合均十分重要.在流化床下部,颗粒运动速度往往低于床层表面,但速度不均匀指数则高于床层表面.在床层底部颗粒速度不均匀指数随流化床气泡产生而变化.床层内部混合以对流及扩散为主,而在床层表面,混合以对流为主.床层下部速度波动有利于颗粒在床内的扩散及对流混合.流化风量的增加可以增强颗粒的混合.针对这些结论,在流化床垃圾焚烧炉的设计中,对不同的组分选用不同的给料方式,可以促进物料在床内的混合.     

三维整体式喷动-流化床结构优化与流动数值模拟

为了对环隙区内的颗粒堆积层产生局部流化作用,提出了一种在喷动床锥体处开一定数量侧喷嘴的整体式多喷嘴喷动-流化床结构,并采用双流体模型(TFM)对三维整体式多喷嘴喷动-流化床内的气固两相流动行为进行了数值模拟。通过计算流体力学(CFD)模拟获得了喷动床内颗粒体积分数、颗粒速度及流场均匀度分布情况,并将模拟结果与传统喷动床进行了对比,同时对锥体处开孔直径等关键参数进行了优化分析。结果表明:与常规喷动床相比,三维整体式多喷嘴喷动-流化床结构能有效增强喷动床环隙区与喷射区颗粒的径向混合,特别是流化了喷动床环隙区底部颗粒的流动死区。颗粒流场均匀度(CV)值随着床层高度的增加而上升,表明多喷嘴对颗粒流场的均匀化效应主要体现在喷动床柱锥区,当A_i/A_z=0.67时,侧喷嘴对喷动-流化床内整体的颗粒流化作用达到最佳。     

内置螺旋挡板流化床颗粒停留时间分布

采用脉冲示踪法在内置螺旋挡板冷态鼓泡流化床上研究了螺旋挡板、加料速率、流化风速、颗粒粒径和床料高度对颗粒在流化床内停留时间分布的影响. 结果表明,颗粒停留时间的无量纲方差从无螺旋挡板时的0.558减小到有螺旋挡板时的0.085,螺旋挡板可有效抑制颗粒返混,增大颗粒运动的平推流趋势;加料速率增大为约2倍时,停留时间减小为约50%,流动更趋向于平推流;床料高度增加,颗粒返混加剧,颗粒平均停留时间及无量纲方差均增大,颗粒运动向全混流靠近;随流化风速增大,颗粒平均停留时间变长;实验范围内,颗粒粒径对颗粒停留时间分布影响不大.     

连续进出料鼓泡流化床颗粒停留时间分布

针对双流化床气化或双床热解气化工艺中鼓泡床反应器的设计,采用脉冲法研究了Geldart B类固体颗粒在连续颗粒进料和出料的矩形流化床内的停留时间分布(RTD),考察了气速、床料高度、粒径、物料流率等操作参数对RTD的影响. 结果表明,物料流率、床料高度、粒径是影响颗粒RTD的主要因素,而气速则是次要因素. 随物料流率和粒径增加,鼓泡床内颗粒流动向平推流靠近;随床料高度增加,物料在床内的混合更加充分,颗粒流动向全混流靠近. 根据实验结果,推荐采用比理想平推流时间低9%~18%计算平均颗粒停留时间.     

整体式多喷嘴喷动-流化床内气固两相流动规律数值模拟

采用双流体模型(TFM)对一种新型整体式多喷嘴喷动-流化床内气固两相流动进行了数值模拟,在喷动床锥体两侧开若干侧喷嘴形成辅助多喷嘴结构,使其在喷动床锥体处产生喷动-流化效果,从而对环隙区锥体边界处堆积颗粒层产生扰流作用。通过CFD数值模拟获得了喷动床内颗粒速度及浓度的分布情况,并与单喷嘴喷动床模拟结果进行对比。研究并优化分析了不同侧喷嘴数量以及侧喷嘴宽度等关键参数对喷动床气固两相流动的影响规律。研究表明,与常规喷动床相比,整体式多喷嘴喷动-流化床结构能有效增强喷动床内环隙区颗粒相运动,特别是强化了喷动床环隙区底部流动死区的颗粒运动,使得锥体边界层颗的粒体积分数显著下降,颗粒体积分数沿径向分布变得更为均匀,同时省略了旁路供气辅助设备。     

Scale-up of the axial profile of material hold-up in horizontal rotating cylinders

Scale-up of the axial profile of the hold-up of material flowing through grate-discharge horizontal rotating cylinders is presented. parameters investigated were drum length, rotational speed and material feed rate. Equations useful for engineering design for relating material hold-up in the cylinder to the feed rate, cylinder dimensions and speed of rotation are given.     

Prediction of axial velocity profiles and solids hold-up in a rotary kiln

Axial bed depth profiles were experimentally measured in a rotary kiln containing ilmenite particles under steady state and transient conditions. The variables include feed rate of solids, inclination and rotational speed of the kiln. and dam height. The variation of the axial velocity with kiln axis was estimated. The semi-experimental model proposed by Perron and Bui (1990) was modified to include the effect of the variables of the present study. The mean residence time of solids was estimated from the fractional hold-up and expressed in terms of the process variables. The transients induced by a step change in any of the operating conditions were measured as variation of discharge rate of solids with time.     

Longitudinal mixing in horizontal rotary drum reactors

The longitudinal mixing of sodium carbonate (soda) (mean particle size 137 μm) has been investigated in a laboratory horizontal rotary drum reactor 250 mm in diameter and 600 mm long. The distributions of residence times have been estimated by means of a pulse of a small amount of sodium bicarbonate. The concentration of the tracer at the reactor discharge has been evaluated by thermal decomposition of the samples and by measuring their weight loss.The hold-ups, the mean residence times and the variances of the distribution of residence times were evaluated as a function of the rotational speed of the reactor and the feed rate of the particles. By applying a dispersion model, the Peclet numbers were evaluated from the standardized variances and plotted as a function of the feed rate and rotational speed. The mean residence time of the particles was calculated by means of an extended model of Vahl and Kingma. They correspond with the experiments.     

Numerical study of twin-screw extruders by three-dimensional flow analysis—development of analysis technique and evaluation of mixing performance for full flight screws

We have developed a method for predicting the three-dimensional flow field in the melt conveying zone in counter-rotating and co-rotating twin-screw extruders. We applied this technique to the full flight screws with thin flight width and open C-shaped channels in both rotating type extruders having the same screw configurations. We compared the details of velocity and stress fields, the flow rates of transportation, and various kinds of leakage flows for both rotating type extruders. Also, we obtained the spatial distribution of tracer particles and residence time distribution using a numerical tracer experiment. The flow rate in the transport direction in the co-rotating twin-screw was larger than that in the counter-rotating twin-screw, and this suggested that the latter has higher transport performance when the screws have thin flight width and open C-shaped channels as used in this study. As for the distributive mixing, it was found that the co-rotating twin-screw excels in the area of fluid rearrangement between the two screws and distribution in the rotational direction, while the counter-rotating twin-screw has the desirable characteristic of wide distribution in the axial direction. With regard to dispersive mixing, there was no considerable difference between calculated stress fields in both rotating type extruders.     

Process intensification of gas–liquid downflow and upflow packed beds by a new low‐shear rotating reactor concept

In the present work, a new low‐shear rotating reactor concept was introduced for process intensification of heterogeneous catalytic reactions in cocurrent gas–liquid downflow and upflow packed‐bed reactors. To properly assess potential advantages of this new reactor concept, exhaustive hydrodynamic experiments were carried out using embedded low‐intrusive wire mesh sensors. The effect of the rotational velocity on liquid flow patterns in the bed cross‐section, liquid saturation, pressure drop, and regime transition was investigated. Furthermore, liquid residence time and Péclet number estimated by a stimulus‐response technique and a macro‐mixing model were presented and discussed with respect to the prevailing flow patterns. The results revealed that the column rotation induces different flow patterns in the cross‐section of the packed bed operating in a concurrent downflow or upflow mode. Moreover, the new reactor concept exhibits a more flexible adjustment of pressure drop, liquid saturation, liquid residence time, and back‐mixing at constant flow rates. © 2016 American Institute of Chemical Engineers AIChE J, 63: 283–294, 2017     

Drying of Slurries in Fluidized Bed of Inert Particles

A fluid bed dryer with inert particles was used for drying of slurries. Experiments were performed in a cylindrical column 215 mm in diameter and 1200 mm in height with glass spheres as inert particles. Slurries of Zineb fungicide, copper hydroxide, calcium carbonate, and pure water were used as the feed material. The effects of operating conditions on dryer throughput and product quality were investigated. Main performance criteria, i.e., specific water evaporation rate, specific heat consumption, and specific air consumption, were quantified. Temperature profile along the bed was mapped, and nearly isothermal conditions were found due to thorough mixing of the particles. The material hold-up and residence time were determined. Simple heat and mass balances predict the dryer performance quite well.     

基于能量等效的双转子连续混炼机混炼段相似放大研究探讨

根据双转子连续混炼机自身特性和等效平均停留时间、等价转子顶部剪切速率的原则,以能量等效为衡量混合效果的准则,提出双转子连续混炼机混炼段相似放大设计的流程及其理论模型。在该模型确定的转子转速和喂料速率下,利用Polyflow软件模拟了机筒内径分别为30 mm和100 mm的双转子连续混炼机混炼段和卸料段内的流场特征,以能量等效为指导准则,通过对流场特征和混合效果进行分析,对所提出的相似放大设计理论模型进行了验证。     

Scale-up of particulate hold-up in rotary drums

This paper presents a group of dimensionless parameters which are useful in scaling-up the material hold-up in cylindrical drums of different geometry. These parameters include feed rate, drum rotational speed and discharge opening diameter. It also presents a semi-empirical model which can be used to predict the material hold-up in rotating drums for a wide range of operating and design variables. The values of hold-up predicted with this model are slightly lower than the corresponding experimental values, possibly due to end effects.     

Flow of particulate solids through tumbling mills

The transport behavior of particulate solids flowing through tumbling mills depends strongly on the mill operating conditions. This paper presents the results of a detailed study to delineate the effect of the important operating variables on the hold-up, mean residence time and residence time distribution of particulate solids flowing through ball mills and rod mills. The effects of feed rate, media load, and mill speed on these transport parameters are discussed, emphasizing the fundamental differences between particulate transport in ball mills and rod mills. Under identical dimensionless operating conditions over the range of the investigation, the material hold-up and the Peclet number of the flow regime in the rod mill were always higher than those in the ball mill. Mechanistic interpretations of the observed transport phenomena are presented, and their implications in the context of tumbling mill analysis and design pointed out.     

A Model for Residence Time Distribution of Solids in a Rotary Kiln

A five‐parameter model for residence time distribution of low‐density particles in a bed of high‐density particles in a rotary kiln is derived based on combinations of flow regimes in several sub‐regions. The parameters of the model are the number of turnover stages, the cascade volume fraction, the Peclet number, the dead volume fraction in cascade region, and the cross‐flow ratio. The effect of feed rate of solids, rotational speed and inclination of the kiln on the model parameters is evaluated.