Residence time distribution in spouted bed drying of maltodextrin solutions on a bed of inert particles

Residence time distributions (RTD) for aqueous maltodextrin solutions were determined in two kinds of spouted bed dryers: (1) conventional spouted bed (CSB) 0.305 m diameter with a bed of polypropylene beads and (2) spout‐fluid bed 0.143 m diameter with draft tube submerged in a bed of FEP® pellets (S‐FBDT). RTD, mean residence time t_m, and spread of the distribution σ², were determined at different drying temperatures, spouting velocities, bed depths, spraying pressures, and feed concentrations. Average values of t_m and σ² were 6.5 min and 26.6 min² for the CSB and 6.9 min and 36 min² for the S‐FBDT, respectively, for all operating conditions except spraying pressure. RTD curves were well represented by the response of an ideal stirred tank with a superimposed bypass of 15% on average for the CSB and 7% on average for the S‐FBDT dryer for all operating conditions. Increase in spraying pressure produced a reduction of t_m and an increase in the bypass fraction of the product in both dryers. © 2011 Canadian Society for Chemical Engineering     

Continuous on-line measurement of solid moisture content during fluidized bed drying using triboelectric probes

Wet granulation and drying of solids in fluidized beds are widely used in the pharmaceutical, food and fertilizers industries. Although the moisture content of fluidized solids is the key parameter for on-line process monitoring, reliable, accurate and economical moisture sensors are lacking. The aim of this work was, therefore, to develop a new technique using triboelectric probes for real-time measurement of moisture content in fluidized beds, and to validate its applicability to fluid bed drying of glass beads (Sauter-mean diameter of 171 μm) and silica sand particles (Sauter-mean diameter of 190 μm) wetted by water. Several triboelectric probes, installed at different locations throughout the bed, monitored the bed moisture content during both the liquid spraying and the following drying process. The measuring technique developed in this study makes use of inexpensive probes that do not require any maintenance. The triboelectric signals were continuously recorded by a data acquisition system and, at selected times, samples of bed solids were taken and analyzed for their moisture content using Karl Fischer titration. The triboelectric signals were correlated with the moisture contents determined by titration to calibrate the technique, which was found to be sensitive to moisture contents below 100 ppm. For most of the experiments the bed was operated in the bubbling regime with a superficial gas velocity of 0.4 m/s. The relationship between triboelectric signal and moisture content was unaffected by changes in the fluidization gas velocity of up to 25%, and could be easily automated for direct control of industrial granulation and drying processes.     

基于LSTM神经网络的流化床干燥器内生物质颗粒湿度预测

生物质作为一种储量丰富、环境友好且易于获取的可再生能源,日渐成为能源研究利用领域的热点。生物质湿度是影响生物质利用效率的关键因素,因此干燥是生物质利用之前的必要步骤。流化床由于其良好的传热传质特性,在干燥过程中得到了广泛的应用。为了实时监测生物质颗粒的干燥过程,利用弧形静电传感器阵列,结合用于时间序列建模的长短期记忆（LSTM）神经网络,实现了流化床干燥器内生物质颗粒湿度的预测。在实验室规模的流化床干燥器上进行了多工况实验获取训练和测试数据,通过模型参数优化确定了LSTM模型。通过与标准循环神经网络（RNN）模型的预测结果的对比表明,LSTM神经网络模型的平均相对误差较小,能够较为准确地预测流化床干燥器内生物质颗粒的湿度。     

流化床颗粒团聚物水分分布测量新技术

在流化床中团聚物主要以宏观团聚物、微小团聚物、自由颗粒3种形式存在,通过TEB雾化喷嘴制造流化床颗粒团聚物,其中宏观团聚物沉积在流化床的底部位置,微小团聚物和自由颗粒均匀分布在整个流化床中。研究发现本文自主研制开发的多通道电导电路实验装置测量所得电导信号与流化床内水分含量存在线性比例的关系,利用流化床的持续流化效应造成团聚物破裂引发电导信号回升,实现了颗粒团聚物水分分布状态的测量。利用该装置测量了TEB雾化喷嘴气液比在0～4%范围内变化时流化床内颗粒及团聚物的水分分布状态,结果表明随着气液比的增加,微小团聚物、自由颗粒所包含的水分有所增加,宏观团聚物所包含的水分有所减小,即使在气液比增加到4%时,大量的水分仍然被宏观团聚物所吸收。     

旋流振动流化床干燥性能的研究

文章以聚四氟乙烯粒子和蔗糖为物料研究了旋流振动流化床的流体力学及干燥特性,并与振动流化床的各种特性进行了对比。实验在一个干燥室直径为240 mm的小型圆筒振动流化床中进行,采用斜孔分布板使床内产生旋流。实验结果表明:在空气分布板开孔率相差不多的情况下,物料流化以前,旋流振动流化床的床层压降要大于普通振动流化床,一旦物料达到正常流化状态以后,2种振动流化床的床层压降相差不多;旋流振动流化床可以降低物料的临界含湿质量分数。     

非均匀布风流化床内大颗粒停留时间特性

试验考察了非均匀布风配置对大颗粒在倾斜布风板流化床内停留时间分布的影响规律。结果表明,排渣管风速增加,E(t)曲线变平、波动增加,平均停留时间(MRT)呈指数增长;高风速区风速升高,先是分离过程占优,后转变为返混能力占优,MRT先减后增,存在一个适宜于分离的运行风速窗口;低风速区风速加快,混合能力增强,MRT变长。此外,还探讨了物性参数的影响。示踪物球形度越高、表面越光滑,越有利于分离,MRT变短。而尺寸的影响则与密度相关,对于重质示踪物,尺寸增加,MRT变短;而对于轻质示踪物,尺寸变大,MRT变长。试验所得停留时间分布(RTD)曲线形状、波峰时刻、峰值、波动等信息能够充分反映床内的颗粒流动与混合过程;对于内循环流动机理的探索、多组分流化床的工程应用具有一定的参考意义。     

卧式沸腾干燥床在PVC生产中的应用

简单介绍了卧式沸腾干燥床干燥工艺的工艺流程及特点,比较了目前PVC生产中的几种干燥工艺,分析了卧式沸腾干燥床在实际生产中存在的问题及改进措施。     

From hygroscopic single particle to batch fluidized bed drying kinetics

A new normalization concept for convective drying of hygroscopic particulates is introduced. Both, intraparticle drying kinetics and sorption equilibrium are considered separately, and integrated into a new heterogeneous fluid bed model for coupled heat and mass transfer. Experiments were carried out using spherical γ-Al₂O₃ particles. Sorption isotherms, as well as drying curves, for single particles and fluidized beds have been measured. Batch fluid bed drying curves appear to be predictable on the basis of single particle and material equilibrium data and with the help of the model. All model parameters are directly taken, or estimated from fluidization literature, without any fitting.     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

高密度液固循环流化床流动特性研究及数值模拟

在φ80 mm×8000 mm的液固循环流化床提升管中,利用实心玻璃珠和常温水,采用实验和数值模拟相结合的方法对高密度液固循环流化床的流动特性进行了考察。实验发现,高密度液固提升管中,颗粒固含率和颗粒速度径向分布均为抛物线分布,轴向平均固含率分布总体上为下浓上稀的波动形式分布,颗粒在提升管中的流动表现出加速-减速-再加速直至充分发展的过程。这种分布特征与较高颗粒浓度、较高表观液速和颗粒循环速率及喷管式液体分布器的影响有关。液固提升管中固体颗粒的停留时间分布曲线均为尖而窄、较对称且没有明显拖尾的单峰分布,这表明颗粒基本是以弥散颗粒形式存在,颗粒停留时间分布较为均匀。通过将数值模拟结果与实验结果进行比较发现,模拟值与实验值吻合较好,说明所建立的数学模型较为合理,进一步通过数值模拟实验对颗粒密度和颗粒粒径对流动特性的影响规律进行了考察。     

The effect of adsorption on mass transfer in fluidized bed catalytic reactors

Tracer gas residence time distributions (RTD) in a laboratory scale fluidized bed system have been measured for pulses of three different tracer gases (methane, ethane and propane) at different temperatures in the range 323 to 435 K. The fluidized solid was a commercial zeolite based FCC catalyst (CBZ‐2), and measurements were carried out in a superficial air velocity range of 0.01 to 0.04 m/s. The data were interpreted with two‐phase dense phase dispersion models for adsorptive tracers, available in the literature. In addition, modified models were considered by assuming a stationary dense phase and neglecting axial dispersion in this phase. Mean residence time, μ₁, and the variance of the residence time, σ², of RTD data were calculated for each experimental run. Applying the moment technique in the Laplace domain, the differential equations for all models considered were analytically solved. Mass transfer coefficients obtained from dynamic experiments were compared with the values estimated from the relations available in the literature. It was found that methods considering convective flux alone between the bubble and emulsion phases give closer values to the experimental ones than the methods also including the diffusive flux.     

An optimization model of the operating costs of a fluidized bed steam‐drying plant

A model is developed to determine an optimum apparatus geometry and, for given apparatus dimensions, a financially optimal fluidized bed height. The parameters that effect the operating costs are the bed mass, the apparatus diameter and the gas mass flow rate. To implement such cost optimization, a physics‐based mathematical model for describing the thermodynamic processes in fluidized bed steam‐drying is briefly explained and presented. The most important conclusion is not to operate the fluidized bed for a drying process below a certain minimum cost, calculated with the help of the modelling. The problem, when describing the drying process and consequently the mass transfer, is that in the superheated steam drying case studied here, water is evaporated as moisture and withdrawn into an atmosphere of vapor water.     

Residence time distribution in a packed bed bioreactor containing porous glass particles: Influence of the presence of immobilized cells

An experimental investigation of the liquid phase residence time distribution (RTD) in a packed bed bioreactor containing porous glass particles is presented. For Re < 1, intraparticle forced convection is negligible and only diffusion, characterized by an effective diffusion coefficient, must be considered to describe the mass transfer process between the extraparticle and the intraparticle fluid phase. For Re > 1, the mass transfer rate becomes dependent on the liquid flow rate, indicating the existence of intraparticle convection. A model including axially dispersed flow for the external fluid phase and an ‘apparent’ effective diffusivity that combines diffusion and convection, predicts experimental RTD data satisfactorily. Yeast cells immobilized inside the porous glass beads did not affect the mass transfer rate at low biomass loading. At high biomass loading (0·02 g yeast cells g^?1 carrier), the mass transfer rate between the extraparticle and intraparticle fluid phase was significantly decreased. Comparison of the RTD data from experimets performed in the presence and absence of cells in the external fluid phase revealed that the mass transfer rate is influenced by the cells immobilized inside the porous particles and not by the cells present in the external fluid phase.     

小型气-液-固流化床液相的停留时间分布

采用脉冲示踪技术,研究了3 mm床径的小型气-液-固流化床内液相停留时间分布。以KCl为示踪剂,液相为去离子水,气相为空气,固相为平均粒径0.123~0.222 mm的玻璃微珠和氧化铝颗粒,测量流化床出口液相的电导率,得到其停留时间分布曲线。结果表明,增大表观液速和表观气速,分布曲线变窄,平均停留时间缩短,Peclet数增大;固相的存在使液相的平均停留时间增长。表观液速1.96~15.70 mm×s^-1,表观气速1.18~1.96 mm×s^-1的条件下,流动接近层流;平均停留时间的范围为（19.6±0.34）s~（48.0±0.92）s,建立的Pe经验关联式对实验结果有较好的预测,偏差在±25%以内。研究结果对于小型三相流化床的设计放大具有指导意义。     

惰性粒子流化床干燥钻井废泥浆实验研究

实验考察了惰性粒子流化床干燥钻井废泥浆,关联了干燥器的体积传热系数表达式,可定量计算进风速度、进风温度、废泥浆体积流量和含水量等参数的影响。结果表明,惰性粒子流化床干燥热效率达55％,体积传热系数可达6kW·m^-3·K^-1,可用于钻井现场柴油机废气干燥钻井废泥浆。激光粒度分析仪测试干燥产物粒度平均值为12／μm,且分布较为集中,其密度为3．2g·cm^-3,可以回用于钻井泥浆的加重材料。浸毒试验表明,干燥产物CODCr值大幅度下降。     

Modulating the mean residence time difference of wide-size particles in a fluidized bed

For non-catalytic gas-solid reaction, it is desirable to match the mean residence time(MRT) of particles and complete conversion time(t_c) in a fluidized bed. In this study, the MRT differences(MRT ratios) between the coarse particles and the fine particles were investigated in a continuous fluidized bed with a side exit by varying the superficial gas velocity, feed composition and particle size ratio. The results show that the MRT ratio increases firstly and then decreases with increasing the gas velocity. By controlling the gas velocity and the feed composition of coarse particles, the MRT ratio can be modulated from 1.8 to 10.5 at the gas velocity of 1.0 m·s~(-1) for the binary mixture with the size ratio of 2.2. The MRT ratio can reach to ~ 12 at the gas velocity of 1.2 m·s~(-1)for the particle size ratio of 3.3. The present study has endeavored to obtain fundamental data for an effective plant operation to meet the need of synchronously complete conversion of particles with different sizes during the film diffusion controlling reaction.     

洗衣粉悬浮液在惰性粒子流化床中干燥的研究

针对气体分布板开直孔的惰性粒子流化床,开展了洗衣粉悬浮液在床内的干燥性能研究。测定了流化床的床层压降曲线,考察了进料量、进风温度、进风速度及惰性粒子直径对于流化床传热性能的影响,且与气体分布板开斜孔的传热性能进行了初步比较。结果表明,流化床的床层压降主要是由惰性粒子的流化阻力所致;适当增加进料量和进风速度,或减小惰性粒子直径,以及将气体分布板的孔道由直孔改为斜孔,均可提高流化床的传热性能,但过高的进风温度则可能导致传热性能的下降。     

Residence Time Distribution of Solids in a Fluidized Bed

Experimental investigation on RTD of solids is carried out in a single‐stage fluidized bed provided with an internal, using uniformly sized particles and a binary solid mixture, varying gas flow rate, solids rate, bed height, dilution and the bed geometry. The effect of these variables on first and second moments as well as on F‐curves has been determined. Using a binary solid mixture or an internal inside the bed is found to reduce backmixing of solids. The data is fitted to FTEM and the values of N obtained were compared for different variables.     

流化床稀相区淤浆停留时间和蒸发时间的计算

以Hypol聚丙烯工艺第三反应器为例,计算了淤浆液滴在气相流化床反应器稀相区的平均蒸发时间和平均停留时间,并讨论这2个计算值之间的关系,结果表明,淤浆液滴在流化床稀相区,液相丙烯已完全蒸发,因此适量增加淤浆量,进一步提高反应器产率是可能的,扬子石油化工公司聚丙烯装置B线扩容改造后,丙烯循环量增加,而流化床流化态稳定,本文提供了理论依据。