EFFECT OF DRYING ON POWDER COATING EFFICIENCY AND AGGLOMERATION IN VIBRO-FLUIDIZED BED

Glass beads of 43 μm were coated in a vibro-fluidized bed by atomizing a fine silica powder together with polyvinyl alcohol aqueous solution. The coating efficiency and weight fraction of the agglomerated particles were measured under various experimental conditions, and their dependencies on the frequency and the direction of vibration were investigated. The coating efficiency and the degree of the agglomeration among core particles correlated well with an index R. The index R was introduced to evaluate quantitatively the drying conditions in a fluidized bed.

Application of vertical vibration on the fluidized bed lowered the coating efficiency somewhat, while it prevented agglomeration. From the experimental results it was confirmed that coating with high quality and high efficiency, where few agglomerates were produced and silica powder was utilized efficiently, was possible in a vibro-fluidized bed with adequate vibration frequency and orientation of the vibration vector.     

Powder coating efficiency of small particles and their agglomeration in circulating fluidized bed

The coating efficiency of fluidizing small particles and their agglomeration were investigated to evaluate the possibility of powder coating by the use of a circulating fluidized bed. Glass beads, whose mean diameter was 43 Μm, and silica powder of 1 Μm were used as a core and a coating material. Polyvinyl alcohol was used as a binder and its solution was supplied together with silica powder from a spray nozzle equipped in the circulating fluidized bed. Glass beads of 43 Μm, which had been impossible to coat in a conventional fluidized bed coater, were successfully coated with silica powder in a circulating fluidized bed, and agglomeration among core particles was prevented. From this result, it was confirmed that a circulating fluidized bed performs excellently as a coater, especially for fine core particles, so a circulating fluidized bed coater has bright prospects for particle coating.     

Effect of solid circulation rate on coating efficiency and agglomeration in circulating fluidized bed type coater

Circulating fluidized bed was proposed to be used as a coater, and coating experiments of glass beads with silica powder were performed in a circulating fluidized bed. Glass beads and silica powder were chosen as model particles, because their shape was almost spherical. The respective effects of gas flow rates supplied from a distributor and from an air nozzle for solid circulation, feed rate of powder suspension and particle content in the bed on coating efficiency and agglomeration are mainly discussed. Coating efficiency in circulating fluidized bed coater was correlated well with solid circulation time rather than with gas flow rates or solid circulation rate, while the agglomeration among core particles was mainly governed by solid circulation rate.     

振动流化床中磷石膏颗粒流态化性能研究

在振动流化床中进行了磷石膏颗粒流态化性能的冷模实验研究。结果表明,磷石膏颗粒在普通流化床中流化性能很差,难以实现正常的流化态,不适合在普通流化床中进行流态化分解反应。而振动的引入极大地改善了磷石膏颗粒的流态化性能。并考察了振幅和进料高径比对磷石膏颗粒的床层压降、膨胀比、最小流化速度的影响。为热模实验条件的选择提供了理论基础。     

Experimental study and modeling of fluidized bed coating and agglomeration

This work deals with the fluidized bed coating and agglomeration of solid particles. The effect of particle size on coating criteria was investigated using sand particles as the coating support and aqueous solutions containing NaCl as coating liquid. The results showed that both growth rate and efficiency increase with decreasing the particle size. The growth was mainly governed by layering for particles larger than 200 μm, whereas for finer particles it occurred by agglomeration. As the particle size became less than 90 μm, the coating operation led to uncontrolled growth and bed quenching. However, the coating of the same particles was successfully achieved by adding some coarser particles. In addition, a mathematical model based on the population balance concept, taking into account the simultaneous growth by layering and agglomeration, was established to predict the time evolution of the particle size distribution. The comparison between experimental and calculated data permitted the establishment of a law for the size dependency of the agglomeration kernel.     

Drying of Pastes in Vibro-Fluidized Beds: Effects of the Amplitude and Frequency of Vibration

Abstract

The dimensionless vibration number (Γ) is the parameter most widely applied to quantify the vibration energy of a vibro-fluidized bed. It is defined as a function of the amplitude (A) and of the frequency of vibration (f). Most works in literature present their results as a function of Γ, which is adopted as a single parameter to explain the results on fluid dynamic behavior of vibro-fluidized beds, independent on the magnitudes of A and f. From its definition, however, it is possible to obtain identical values of Γ just by combining different values of A and f, and further studies are necessary for a better analysis of fluid dynamic behavior of vibro-fluidized beds concerning their vibration energy. Therefore, in this work the effects of dimensionless vibration number on the fluid dynamic behavior of a vibro-fluidized bed will be investigated by choosing different combinations of A and f values arranged to provide identical values of Γ. The tests were carried out for dry and wet beds, for values of Γ = 0.0 (fluidized bed), 1.0, 2.0, and 4.0. The results indicate that Γ must be used carefully when applied as the only parameter to characterize the vibration effects in a vibro-fluidized bed since very different fluid dynamic behavior was detected at a fixed Γ at different amplitudes and frequencies.     

带内置水平管的大颗粒振动流化床流体力学研究

以大颗粒MAP（磷酸-铵）为试验物料对带内置水平管的大颗粒振动流化床流体力学进行了试验研究,结果表明,振动的引入对床层临界压降有明显的降低作用;同样床层条件下振动频率越大床层临界压降越低,振幅的改变对床层压降影响不大;低气速下振动使床层孔隙率降低而导致压降高于普通流化床;在带内置水平管的大颗粒振动流化床中,振动对大粒径颗粒的影响小于小粒径颗粒;由试验拟合出经验公式,与试验数据吻合良好。     

Drying of Pastes in Vibro-Fluidized Beds: Effects of the Amplitude and Frequency of Vibration

The dimensionless vibration number (Γ) is the parameter most widely applied to quantify the vibration energy of a vibro-fluidized bed. It is defined as a function of the amplitude (A) and of the frequency of vibration (f). Most works in literature present their results as a function of Γ, which is adopted as a single parameter to explain the results on fluid dynamic behavior of vibro-fluidized beds, independent on the magnitudes of A and f. From its definition, however, it is possible to obtain identical values of Γ just by combining different values of A and f, and further studies are necessary for a better analysis of fluid dynamic behavior of vibro-fluidized beds concerning their vibration energy. Therefore, in this work the effects of dimensionless vibration number on the fluid dynamic behavior of a vibro-fluidized bed will be investigated by choosing different combinations of A and f values arranged to provide identical values of Γ. The tests were carried out for dry and wet beds, for values of Γ = 0.0 (fluidized bed), 1.0, 2.0, and 4.0. The results indicate that Γ must be used carefully when applied as the only parameter to characterize the vibration effects in a vibro-fluidized bed since very different fluid dynamic behavior was detected at a fixed Γ at different amplitudes and frequencies.     

A new fluidized bed coating process via photo‐initiated cationic polymerization

An ultraviolet (UV) photo‐polymerization particle coating process was developed by coupling the photo‐initiated cationic polymerization with the fluidized bed coating techniques. Unlike the conventional air‐suspension coating in the fluidized bed, the new process employs a UV curable composition instead of a solvent/water‐borne system as a coating material, which has a rapid curing rate and virtually no inhibition to oxygen and moisture. A modified fluidized bed coater equipped with a quartz window allows UV light to penetrate and to initiate the curing of photo‐sensitive polymerizable chemicals coated on the particles. A UV‐curable liquid composed of cycloaliphatic epoxide, oxetane, and triarylsulfonium cationic photo‐initiator was specifically formulated for the fluidized bed particle coating process. A systematic experimental approach including photo‐Differential Scanning Calorimetry, Fourier Transform Infrared Spectroscopy, and tackiness measurements has been developed to characterize the curing mechanism of the cationic UV curable formulations and to optimize the chemical compositions. The effects of the UV curable chemicals, viscosity of coating liquid, and the fluidization operating conditions on the physical properties of coated particles have been thoroughly investigated. Under optimized conditions, this novel process is very efficient as follows: particles can be coated very rapidly with ultra‐thin films of the cured chemicals, with little, if any, formation of particulate agglomeration. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Fluidized bed spray granulation: Investigation of the coating process on a single sphere

The coating and granulation of solid particles in a fluidized bed is a process which converts pumpable and atomizable liquids (solutions, slurries, melts) into granular solids in one step by means of drying. The solution to be processed is sprayed onto a fluidized bed. Particle growth can take place either via surface layering or agglomeration. In the case of surface layering the atomized droplets deposit a thin layer of liquid onto the seed particles. The solvent is then evaporated by the hot fluidizing, leaving behind the dissolved material on the surface. Although fluidized bed spray granulation and film coating have been applied in industry for several years, there is still a lack of understanding of the physical fundamentals and the mechanisms by which spherical granules are formed. Hence a new method was developed which allows the direct observation of the subsequent particle-forming mechanisms such as droplet deposition, spreading, wetting and drying. The authors present a laboratory scale apparatus in which a single freely suspended particle can be coated under well defined and constant coating and drying conditions. With this device, particle-growth-rate and the development of particle morphology were measured and investigated under various experimental conditions.     

Agglomeration of biomass fired fluidized bed gasifier and combustor

Agglomeration is a major problem in biomass fired fluidized bed combustors and gasifiers. Mechanism, reduction options and detection techniques of agglomeration are reviewed. Agglomeration may be classified broadly into three types: defluidization induced agglomeration, melt‐induced agglomeration and coating‐induced agglomeration. Sodium and potassium content of the biomass are the major contributors to the agglomeration in biomass fired fluidized beds. Higher temperature, lower fluidizing velocity and coarser bed particles also increase the risk of agglomeration. Alternative bed materials, additives or the co‐combustion of biomass with other fuels can reduce agglomeration potential of a fluidized bed. Two agglomeration detection techniques are discussed: controlled fluidized bed agglomeration and early agglomeration recognition system.     

A new and rapid method for the evaluation of the liquid-solid contact resulting from liquid injection into a fluidized bed

Liquid feeds are injected into fluidized bed reactors such as fluid cokers, fluid catalytic crackers and gas-phase polymerization reactors. In these industrial processes, it is of crucial importance to optimize the contact between the injected liquid and the bed solids to minimize agglomeration, to ensure good reactor operability, and to allow reactions to proceed under minimum heat and mass transfer limitations in order to maximize the yields of valuable products. It has been shown [P. House, M. Saberian, C. Briens, F. Berruti, E. Chan, Injection of a liquid spray into a fluidized bed: particle-liquid mixing and impact on fluid coker yields. Industrial & Engineering Chemistry Research 43 (2004) 5663-5669., S. Bruhns, J. Werther, An investigation of the mechanism of liquid injection into fluidized beds. AIChE Journal 51 (2005) 766-775] that the nozzle technology and the operating conditions have a significant effect on the quality of the liquid-solid interaction resulting from the injection of gas-atomized liquid feed. The goal of this study was to develop a rapid and reliable experimental technique to assess the liquid-solid contact efficiency resulting from the injection of a liquid feed into a fluidized bed. Air-fluidized silica sand particles were first charged by triboelectrification as a result of their random collisions with the inner walls of the fluidized bed. Immediately after the injection of water through an aerated nozzle, the fluidization air was stopped and the wetted bed solids were allowed to settle. While the bed was defluidized, the triboelectric charges accumulated on the particles migrated to a grounded electrode through the low-resistance paths offered by the conductive liquid. A stronger electric current flowing through the electrode indicated that the liquid was more evenly distributed on the solid particles. The intensity of the current flowing through the electrode was, therefore, used to define a spray nozzle performance index. This technique was used to examine the effect of increasing the nozzle aeration, and, specifically, the gas-to-liquid mass ratio (G/L) through the nozzle on the liquid-solid contact efficiency. The results showed that changing the nozzle geometry can change how the contact efficiency between atomized liquid and fluidized solids varies with (G/L), especially at relatively high G/L ratios. A model of the time-evolution of the electric current generated during defluidization of the bed solids is also presented.     

声波的多尺度分解与气固流化床故障检测

引言 在流化床乙烯气相聚合过程中,由于传热速率较低(与淤浆工艺相比),聚合物小颗粒容易聚集在一起形成结块,若不及时采取措施,将影响流化状态.而不良的流化状况经常导致装置不能满负荷、长周期地运转,严重时会"熔床",继而停产.为了保证流化床长期稳定的运行,聚合物颗粒结块的监测就变得十分重要.目前工业生产中主要根据流化松密度、反应温度、床高等参数的突变和γ射线法来检测结块.前者多依赖人为经验的判断,漏报、误报的情况时有发生.而γ射线法则对人体存在致命的伤害.因此,发展一种简单快捷、灵敏准确、安全环保的结块测量技术成为当务之急.     

DESIGN,TESTING, AND OPTIMIZATION OF VIBRO-FLUIDIZED BED PADDY DRYER

The objectives of this research were to design, construct and test a prototype of vibro-fluidized bed paddy dryer with a capacity of 2.5–5.0 t/h and develop a mathematical model that determines optimum operating parameters. Experimental drying conditions were: air flow rate, 1.7 m³/s; bed velocity, 1.4 m/s; average drying air temperature, 125–140°C; residence time of paddy approximately 1 minute; bed height, 11.5 cm; fraction of air recycled, 0.85 and vibration of intensity, 1 (frequency, 7.3 Hz and amplitude, 5 mm). Moisture content of paddy with a feed rate of 4821 kg/h was reduced from 28 to 23% d.b. Specific primary energy consumption (SPEC) was 6.15 MJ/kg-water evaporated. Electrical power of blower motor and vibration motor was 55% as compared to electrical power of blower motor used in fluidized bed drying without vibration. Comparison between the experimental and simulated results showed that the mathematical model could predict fairly well. To find out optimum operating parameters, the grid search method was employed with criteria based on acceptable moisture reduction and quality and minimum energy consumption.

     

基于魏格纳分布的鼓泡流化床中结焦早期诊断

An experimental verification is reported on the early predicting index of agglomeration in bubbling fluidized bed. Coarse quartz sand, which has the same density but larger diameter than the bed material, was used to simulate the initial agglomerated particle. Wigner distribution was used to analyze the pressure fuctuation of the tested bed, and the average amplitude of local domain frequency （LDF） and local peak weighted average frequency （LPWA） under different operating conditions were measured and compared. The results showed that the LDF is sensitive to the agglomeration phenomena and had quick response to the incipient agglomeration in fluidized beds. It can be concluded from the results that these two parameters could be taken as the characteristic indexes to the agglomeration in fuidized beds.     

Particles agglomeration in a conical fluidized bed in relation with air temperature profiles

Particles agglomeration is obtained by spraying liquid over solid particles fluidized by hot air. The growth mechanism depends on the operating parameters (geometry and process conditions) and initial materials, influencing drying conditions and agitation, leading either to agglomeration or coating or wet quenching. It is linked to air temperature and/or humidity distributions appearing in the well-mixing system of the fluidized bed due to the penetration of the sprayed liquid jet.In this study, air temperatures distributions in a conical fluidized bed of model particles (glass beads) top sprayed with water were measured varying the initial particles load (250, 500, 750 g), the fluidizing air inlet temperature (60-70-80 °C), the liquid feed rate (2.65, 5.33, ) and the relative air spraying pressure (1,2,3 bars). Three thermal zones were identified (heat transfer, isothermal, wetting-active), with sizes and shapes related to particles circulation patterns and drying and spraying conditions influenced by the operating parameters. Subsequent agglomeration trials, were carried out with glass beads and soluble maltodextrin particles agglomerated, respectively, with an acacia gum solution and water. They showed a relationship between the air temperatures distribution and the resulting growth mechanism. Particularly, controlled agglomeration was obtained for a wetting-active zone occupying 18-30% of the fluidized bed.     

Identification of thermal zones and population balance modelling of fluidized bed spray granulation

Air temperature measurements in a fluidized bed of glass beads top sprayed with water showed that conditions for particles growth were fulfilled only in the cold wetting zone under the nozzle which size and shape depended on operating conditions (liquid spray rate, nozzle air pressure, air temperature, and particles load). Evolution of the particle size distribution during agglomeration was modelled using population balance and representing the fluidized bed as two perfectly mixed reactors exchanging particles with particle growth only in the one corresponding to the wetting zone. The model was applied to the agglomeration of non-soluble glass beads and soluble maltodextrin particles spraying respectively an acacia gum solution (binder) and water. Among the three adjustable parameters, identified from experimental particle size distributions evolution during glass beads agglomeration, only one describing the kinetics of the size distribution evolution depended on process variables. The model allowed satisfying simulation of the evolution of the particle size distribution during maltodextrin agglomeration.     

Simulation of bubbling fluidized bed of fine particles using CFD

Computational fluid dynamics (CFD) simulation for bubbling fluidized bed of fine particles was carried out. The reliability and accuracy of CFD simulation was investigated by comparison with experimental data. The experimental facility of the fluidized bed was 6 cm in diameter and 70 cm in height and an agitator of pitched-blade turbine type was installed to prevent severe agglomeration of fine particles. Phosphor particles were employed as the bed material. Particle size was 22 μm and particle density was 3,938 kg/m³. CFD simulation was carried by two-fluid module which was composed of viscosity input model and fan model. CFD simulation and experiment were carried out by changing the fluidizing gas velocity and agitation velocity. The results showed that CFD simulation results in this study showed good agreement with experimental data. From results of CFD simulation, it was observed that the agitation prevents agglomeration of fine particles in a fluidized bed.     

BATCH VIBRATING FLUID BED DRYER FOR SAWDUST PARTICLES: EXPERIMENTAL RESULTS

In a batch experimental equipment, the behavior of a sawdust dryer in a vibrating fluidized bed is analyzed. Empirical data concerning fluidization velocities, pressure drops and drying kinetics was obtained, and advantages of using vibration in the drying chamber, relative to a conventional fluidized bed, are shown. This technique is presented as an alternative to solve problems of solid agglomeration and bed defluidization. Results show that it is possible to dry sawdust with more than 2 kg/kg moisture, in a vibrating bed keeping a high degree of bed homogeneity and high quality of fluidized state.     

BATCH VIBRATING FLUID BED DRYER FOR SAWDUST PARTICLES: EXPERIMENTAL RESULTS

ABSTRACT

In a batch experimental equipment, the behavior of a sawdust dryer in a vibrating fluidized bed is analyzed. Empirical data concerning fluidization velocities, pressure drops and drying kinetics was obtained, and advantages of using vibration in the drying chamber, relative to a conventional fluidized bed, are shown. This technique is presented as an alternative to solve problems of solid agglomeration and bed defluidization. Results show that it is possible to dry sawdust with more than 2 kg/kg moisture, in a vibrating bed keeping a high degree of bed homogeneity and high quality of fluidized state.