Fluidization characteristics of printed circuit board plastic particles with different sizes

The experiments were carried out in a fluidized bed of 56 mm in diameter and 1 600 mm in height to determine the fluidization characteristics of four sizes of printed circuit board plastic (PCBP) particles. It indicates that the fluidization characteristics of PCBP particles depend on the average size and particle type. 123 µm PCBP particles (1#), belonging to Geldart A group with strong viscous force, whose fluidization behaviours was similar to those of Geldart C, was difficult to fluidize. Whereas, 275 µm (2#), 354 µm (3#), and 423 µm (4#) PCBP particles, belonging to Geldart B, were fluidized smoothly. The bed collapsing process is composed of three stages: the bubble escaping stage, the sedimentation stage, and the solid consolidation stage. The collapsing process of 1# PCBP particle lasts 6 s or long. 2#, 3#, and 4# PCBP particles, Geldart group B particles, collapse process consists of the bubble escaping stage and the solid consolidation stage. The minimum fluidization velocities from modified Ergun Equation were agreement with experimental data for 2#, 3#, and 4# PCBP particles.     

微型流化床反应动力学分析仪的研制与应用

本文首次研发了一种用于测定气固反应速度、求算反应动力学参数的微型流化床反应分析仪器（MFBK）。该仪器利用流化床强化反应过程的热量与质量传递过程,通过气体脉冲输送在给定温度下瞬时进样,根据在线监测的气体组分浓度变化,测试反应速度、推导反应动力学参数和分析反应机理。利用该仪器测定氩气气氛中碳酸钙的分解反应表明：其表观活化能与指前因子分别为142.73kJ.mol^-1和399777s^-1,活化能在文献报道范围之内,且小于热重分析仪测定的184.3kJ.mol^-1,并建立了反应模式函数f(α)=(1-α)0.86,对应的拟合线性相关系数达到0.99。测定煤和生物质热解反应过程表明：MFBK测试的反应完成时间在15s左右,且揭示了生成气关键组分具有不同的释放时间和生成量,为深入探讨热解反应机理提供了新的证据。     

废塑料制取液体燃料试验研究

利用热解法将废塑料制成高品质液体燃料是资源回收和避免二次污染的重要途径。为此,设计了一套连续给料的鼓泡流化床反应器,对废塑料在鼓泡流化床中热解的关键工艺参数进行了试验研究。通过对试验数据的分析,得到了热解温度对液体产率的影响规律、气体产物的主要成分及其所占份额,并计算了废塑料热解过程的能量收支关系,为废塑料制取液体燃料的工业装置的设计、运行提供了理论及试验依据。     

Pyrolysis characteristics of Oriental white oak: Kinetic study and fast pyrolysis in a fluidized bed with an improved reaction system

The kinetic parameters for the pyrolysis of Oriental white oak were evaluated by thermogravimetric analysis (TGA). The white oak was pyrolyzed in a fluidized bed reactor with a two-staged char separation system under a variety of operating conditions. The influence of the pyrolysis conditions on the chemical and physical characteristics of the bio-oil was also examined. TGA showed that the Oriental white oak decomposed at temperatures ranging from 250 to 400 °C. The apparent activation energy ranged from 160 to 777 kJ mol^− 1. The optimal pyrolysis temperature for the production of bio-oil in the fluidized bed unit was between 400 and 450 °C. A much smaller and larger feed size adversely affected the production of bio-oil. A higher fluidizing gas flow and higher biomass feeding rate were more effective in the production of bio-oil but the above flow rates did not affect the bio-oil yields significantly. Recycling a part of the product gas as a fluidizing medium resulted the highest bio-oil yield of 60 wt.%. In addition, high-quality bio-oil with a low solid content was produced using a hot filter as well as a cyclone. With exception of the pyrolysis temperature, the other pyrolysis conditions did not significantly affect the chemical and physical characteristics of the resulting bio-oil.     

Effect of bed depth on characterisation of slow pyrolysis products

The trend towards separation of waste for material recovery could assist with the implementation of waste pyrolysis and gasification processes. Following the previous work on slow pyrolysis of segregated waste materials, this study investigated the effect of bed depth on the yields and properties of pyrolysis products in a fixed bed pyrolyser. The results showed that the bed depth had a strong influence on the properties of pyrolysis products but a small effect on the yield of products. Differences in the liquid yield between the two cases of bed depth (17-57 vol% of the reactor) were observed at temperatures above 500 °C while the difference in the char yield was negligible. The aqueous fraction was 5-15% higher for the bed depth of 57 vol% than for the bed depth of 17 vol%. Increase in the bed depth increased the H/C ratio by a factor of 2-6 in the char product and the CO/CO₂ ratio by a factor of 1.74 in the gas product but slightly decreased in the H/C ratio in the liquid product.     

Limestone particle attrition and size distribution in a small circulating fluidized bed

Limestone particle attrition was investigated in a small circulating fluidized bed reactor at temperatures from 25 to 850 °C, 1 atm pressure and superficial gas velocities from 4.8 to 6.2 m/s. The effects of operating time, superficial gas velocity and temperature were studied with fresh limestone. No calcination or sulfation occurred at temperatures ?580 °C, whereas calcination and sulfation affected attrition at 850 °C. Increasing the temperature (while maintaining the same superficial gas velocity) reduced attrition if there was negligible calcination. Attrition was high initially, but after ∼24 h, the rate of mass change became constant. The ratio of initial mean particle diameter to that at later times increased linearly with time and with (U_g − U_mf)², while decreasing exponentially with temperature, with an activation energy for fresh limestone of −4.3 kJ/mol. The attrition followed Rittinger’s surface theory [Beke B. Comminution. Budapest: Akademiai Kiado, 1964; Ray YC, Jiang TS, Wen CY. Particle attrition phenomena in a fluidized bed. Powder Technol 1987a; 49:193-206]. The change of surface area of limestone particles was proportional to the total excess kinetic energy consumed and to the total attrition time, whereas the change of surface area decreased exponentially with increasing temperature. At 850 °C, the attrition rate of calcined lime was highest, whereas the attrition rate was lowest for sulfated particles. When online impact attrition was introduced, the attrition rate was about an order of magnitude higher than without impacts.     

Mass production of aligned carbon nanotube arrays by fluidized bed catalytic chemical vapor deposition

A parametric study investigating the impacts of loading amount of active phase, growth temperature, H₂ reduction, space velocity, and apparent gas velocity on the intercalated growth of vertically aligned carbon nanotube (CNT) arrays among lamellar catalyst was performed. A series of Fe/Mo/vermiculite catalysts with Fe/vermiculite ratio of 0.0075-0.300 were tested. Metal particles were dispersed among the layers of vermiculite after H₂ reduction. Uniform catalyst particles, with a size of 10-20 nm and a density of 8.5 × 10¹⁴ m⁻², were formed among the vermiculite layers at 650 °C. CNTs with high density synchronously grew into arrays among the vermiculites. With the increasing growth temperature, the alignment of CNTs intercalated among vermiculites became worse. Moreover, intercalated CNTs were synthesized among vermiculite layers in various flow regimes. The as-grown particles were with a size of 1-2 mm when the fluidized bed reactor was operated in particulate fluidization and bubbling fluidization, while the size of the as-grown products decreased obviously when they grown in the turbulent fluidized bed. Based on the understanding of the various parameters investigated, 3.0 kg/h of CNT arrays were mass produced in a pilot plant fluidized bed reactor.     

Wall-to-bed heat transfer coefficient in gas—liquid—solid fluidized beds

Wall to bed heat transfer has been studied in three-phase fluidized beds with a cocurrent up-flow of water and air. Six sizes of glass beads, two sizes of activated carbon beads and one size of alumina beads, varying in average diameter from 0.61 to 6.9 mm and in density from 1330 to 3550 kg/m³, were fluidized in a 95.6 mm diameter brass column heated by a steam jacket. Complementary heat transfer experiments have been performed also for a gas–liquid cocurrent column and liquid–solid fluidized beds. The wall-to-bed coefficient for heat transfer in the gas–liquid–solid fluidized bed is evaluated on the basis of the axial dispersion model concept. The ratio of the wall-to-bed heat transfer coefficient in the gas–liquid–solid fluidized bed to that in the liquid–solid fluidized bed operated at the same liquid flow rate is correlated in terms of the ratio of the velocity of gas to that of liquid and the properties of solid particles. A correlation equation for estimating the wall-to-bed heat transfer coefficient in the liquid–solid fluidized bed is also developed.     

Heat and mass transfer characteristics in a gas-slurry-solid fluidized bed

The gas-slurry-solid fluidized bed is a unique operation where the upward flow of a liquid-solid suspension contacts with the concurrent up-flow of a gas, supporting a bed of coarser particles in a fluidized state. In the present study we measured the gas holdup, the coarse particle holdup, the cylinder-to-slurry heat transfer coefficient, and the cylinder-to-liquid mass transfer coefficient at controlled slurry concentrations. The slurry particles were sieved glass beads of 0.1 mm average diameter and their volumetric fraction was varied at 0, 0.01, 0.05 or 0.1. The slurry and the gas velocities were varied up to about 12 and 15 cm/s, respectively. The coarse particles fluidized were sieved glass beads of average diameters of 3.6 and 5.2 mm. The individual phase-holdup values were measured and served for use in correlating the heat and mass transfer coefficients. The heat and mass transfer coefficients in the slurry flow, gas-slurry transport bed, slurry-solid fluidized bed and gas-slurry-solid fluidized bed operations can be correlated well by dimensionless equations of a unified formula in terms of the Nusselt (Sherwood) number, the Prandtl (Schmidt) number and the specific power group including the energy dissipation rate per unit mass of slurry, with different numerical constants and exponent values, respectively, to the heat and mass transfer coefficients. The presence of an analogy between the heat and mass transfer from the vertically immersed cylinder in these slurry flow, gas-slurry transport bed and gas-slurry-solid fluidized bed systems is suggested.     

煤程序升温与等温热解特性及动力学比较研究

采用热重分析仪和微型流化床分别考察了不同煤阶五个煤样的程序升温和等温快速热解的挥发分析出特性和反应动力学.程序升温实验揭示了热解气体的析出顺序依次为CO2,CO,CH4和H2,而等温热解实验证明CO2和CO的析出先于CH4和H2.微型流化床等温快速热解的挥发分气体总量析出活化能为17kJ/mol～35kJ/mol,小于程序升温热解的活化能.CO2和CO的反应级数与1接近,而CH4和H2的反应级数与1偏差较大,反映了这两类气体在生成机制上存在差异.     

Characterization of dynamic behaviour of the continuous phase in liquid fluidized bed

The dynamic behaviour of the continuous phase in liquid solid fluidized bed is characterized through velocity measurements by laser anemometry at the top of the bed. The experiments were conducted using glass particles of 2, 4 and 8 mm diameter fluidized by water. The root mean square (RMS) of axial velocity fluctuations presents a maximum value at porosity around 0.7 and increases with particle diameter. When compared to the fixed bed situation, we observe an enhancement of the agitation probably due to the added mass effect which plays the role of a turbulence promoter. The spectra analysis of the velocity time series has revealed a specific spectral dynamic of liquid fluidized bed for the higher frequency range which does neither follow strictly the Kolmogorov law nor a Brownian process power law. A time frequency-scale decomposition combined to an autocorrelation analysis of velocity signal was pertinent to capture the impact of porosity waves and cooperative movements of particles on the liquid phase dynamic, and to characterize these coherent structures by low frequency scales (below 1 Hz). The results compare well with the available data obtained directly from void propagation studies by light transmission techniques. Moreover, the high frequency scales have been found random and linked to the small scale movements of the particles. We have shown, when possible, the similarity of behaviour between the liquid and the dispersed phase dynamics through the comparison of some characteristics.     

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.     

Coal pyrolysis and gasification in a fluidized bed thermogravimetric analyzer

The kinetics and modelling of coal gasification were studied in the newly developed fluidized bed thermogravimetric analyzer. The total weight loss obtained from the fluidized bed reactor and the total gas product are in general agreement. The presented model for the micro‐fluidized bed reactor encompasses the kinetics of coal pyrolysis as well as the gasification reactions. For coal pyrolysis, the resulting activation energies for the individual gases were 34.7 to 59.8 kcal/mol. These values are 19 to 21 % lower than those found in the literature for similar coals. This decrease of the activation energies of the endothermic pyrolysis reactions is attributed to a gradient of temperature of 185 to 209 °C. The obtained activation energy for the CO shift reaction is 46.6 kcal/mol, increasing by 20 % from the one used in the literature. This increase of the activation energy of such a mildly exothermic reaction represents an equivalent of 170 °C gradient of temperature. The effects of temperature on the yield and the composition of the gas product are studied. Experimental results and equilibrium data are also compared. The model shows reasonably good agreement with the experimental results, except for the water gas shift reaction.

     

Heat transfer and bubble dynamics in a three-phase inverse fluidized bed

In this study, surface-to-bed heat transfer experiments were performed to gain insight on heat transfer and hydrodynamics in a three-phase inverse fluidized bed. Air, tap water or 0.5 wt.% aqueous ethanol, and polypropylene were, respectively, the gas, liquid and solid phases. The solid loading was varied from 0 to 30 vol.%, and the gas and liquid superficial velocities from 2 to 50 mm/s and 0 to 21 mm/s, respectively. Visual observations were associated with measured phase holdups and instantaneous heat transfer coefficients. Larger gas velocities lead to an increase in bubble size due to the transition to the coalesced bubble flow regime. The greater turbulence induced by the larger bubbles increases the average heat transfer coefficient. On the other hand, adding ethanol reduces the heat transfer coefficient. Solid concentrations up to ∼13 vol.% increase the average heat transfer coefficient whereas higher solid concentrations tend to lower it. The distribution of instantaneous heat transfer coefficient peak height is wider at higher gas and liquid velocities while the addition of a surfactant narrows it. Gas holdups and average heat transfer coefficients are both compared with existing correlations, which are then adjusted for a better fit.     

Pyrolysis of synthetic polymers and plastic wastes. Kinetic study

Thermal decomposition of natural polystyrene, recycled plastics, low density polyethylene, acrylonitrile butadiene styrene, polyenterophthalate of ethylene, and polypropylene has been carried out. Both isothermal and dynamic experiments at different heating rates have been performed in a thermobalance with the objective of determining the kinetic parameters. Also, a first set of experiments, performed in a cylindrical stainless-steel atmospheric pressure reactor, was designed to evaluate the products of the pyrolysis process. The effects of N₂ flow rate (200–300 cm³ min^− 1), initial mass fed to the reactor (15–75 mg), temperature (415–490 °C), and heating rate (5–30 K min^− 1) were studied. By application of a first order kinetic model, the activation energy for every plastic has been determined. The values of activation energy in isothermic and dynamic regimen are the following: natural polystyrene: 136, and 168 to 286 kJ/mol. Recycled plastic: 250, and 150 to 290 kJ/mol. Low density polyethylene: 285, and 220 to 259 kJ/mol. Acrylonitrile butadiene styrene: 118, and 104 to 251 kJ/mol. Polyenterophthalate of ethylene: 161, and 117 to 255 kJ/mol. Polypropylene: 169, and 153 to 265 kJ/mol.     

Development of a three-phase fluidized bed reactor with enhanced oxygen transfer

A three-phase fluidized bed reactor (TFBR) was developed in this study with the objective to achieve high rates of oxygen transfer from the gas to the liquid phase in the presence of fluidized solid particles. With 2.9 m height, 0.605 m diameter, and a short residence time of 8 h, the TFBR is particularly suitable for industrial applications such as aerobic biodegradation of high-strength wastewaters including refractory compounds. Experiments with tap water and air show that the TFBR enables complete fluidization. With the water and air superficial velocities in the respective ranges of 0.005–0.203 and 0.8–2.0 cm/s, the volumetric oxygen transfer coefficient is 2.3 × 10⁻² s⁻¹, which is higher than that obtained in similar experimental studies on oxygen transfer carried out in the past. These results suggest that the developed TFBR could be very effective in industrial applications where short hydraulic time and high gas-to-liquid mass transfer rates are desirable.     

Horizontal pneumatic conveying from a fluidized bed

In this paper the feasibility of feeding a horizontal pneumatic conveying line directly from a fluidized bed is explored by investigating the relationships governing the solids mass flow rate in such a pipe as a function of both pressure drop and pipe length. Three different materials were fluidized by air and discharged though a 25 mm internal diameter pipe. Materials used were turnip seeds of mean diameter 1.5 mm, carbon steel shot of mean diameter 0.73 mm and plastic pellets of mean diameter 3.76 mm. Several pipe lengths were used, from 0.75 to 1.77 m. The experiments showed that it is feasible to feed directly from a fluidized bed to a horizontal pneumatic conveying line. The flow regime in the pipe was that of dense phase conveying also called slug flow. The data collected show that there is a clear relationship between the pressure drop down the conveying line and the discharge rate of solids from the line. The discharge rate is also dependent on the pipe length.In previous studies of pneumatic conveying, the solids and gas mass flow rates have been independently set, which cannot be done if the conveying line is fed from a fluidized bed. For a pipe fed from a fluidized bed, the solid and gas mass ratio are coupled and this was modelled using the theory for air-augmented granular discharge through an orifice in a hopper or silo of Nedderman et al. [1983. The effect of interstitial air pressure gradients on the discharge from bins. Powder Technology 35, 69-81], but as modified by Thorpe [1984. Air-augmented flow of granular materials through orifices. Ph.D. Thesis, University of Cambridge] for horizontal discharge. This was then combined with a modification of the theory of Konrad [1981. Ph.D. Dissertation, University of Cambridge] to give a prediction of the total pressure drop and the gas and solid mass flow rates. This combined model for dense-phase conveying from a fluidized bed was found to give an excellent fit to the data using the standard values for the constants in every equation. The predictions of the combined model also agree well with the experiments of Konrad [1981. Ph.D. Dissertation, University of Cambridge] for discharge from a hopper into a horizontal conveying line.     

Solids mixing behavior in a nano-agglomerate fluidized bed

The nano-particles mixing behavior in a nano-agglomerate fluidized bed (NAFB) using R972, a kind of nano-SiO₂ powder, was investigated by the nano-particle coated phosphors tracer method. The axial and radial solids dispersion coefficients in this system were two orders of magnitude lower than those in fluid catalytic cracking (FCC) catalyst systems. The axial solids dispersion coefficient increased with increasing superficial gas velocities, and ranged between 9.1 × 10^− 4 and 2.6 × 10^− 3 m²/s. There was a step increase in the axial solids dispersion coefficient between the particulate fluidization regime and bubbling and turbulent fluidization regimes. As the superficial gas velocity increased, the radial solids dispersion coefficient increased gradually, from 1.2 × 10^− 4 to 4.5 × 10^− 4 m²/s. The much smaller D_a and D_r, compared to regular fluidized systems, is mainly due to the reduced density difference between the fluidized particles and fluidizing medium. To validate this, the solids dispersion coefficients in the NABF were compared with literature values for liquid-solid particulate systems in the particulate fluidization regime and FCC systems in the bubbling and turbulent fluidization regimes. The density difference between the fluidized particles and fluidizing medium and kinetic viscosity of the fluidizing medium, and other hydrodynamic factors like the superficial velocity of the fluidizing medium and the average diameters of the fluidized particles, were the key factors in the solids mixing in the fluidized beds. Empirical correlations are given to describe the results.     

废轮胎热裂解技术研究现状与进展

结合目前废旧轮胎资源化处理现状及研究成果,本文对热解机理、热解技术进行分析、对比,着重介绍了热解温度、升温速率、物料粒径、催化剂等工艺参数对热解产物产率的影响,分析表明Coast-Redfern积分法所得动力学模型较准确,平均反应活化能为129.5kJ/mol;现有的研究表明,热解温度对产物产率影响最大,气相产物与液相产物产率随温度升高而增加,其中液相产物产率相对较高的热解温度在500～550℃范围内,固相产物品质较高的热解温度在500～650℃范围内。其次对其固、液、气三相产物特性及应用和污染物(S、PAHs)的分布与控制方法做了归纳总结,为废旧轮胎热解技术向工业化发展提供技术依据。     

Particle fluctuations and dispersion in three-phase fluidized beds with viscous and low surface tension media

Particle fluctuations and dispersion were investigated in a three-phase (gas–liquid–solid) fluidized bed with an inside diameter of 0.102 m and height of 2.5 m. Effects of gas and liquid velocities, particle size (0.5–3.0 mm), viscosity (1.0–38×10⁻³ Pa s) and surface tension (52–72×10⁻³ N/m) of continuous liquid media on the fluctuating frequency and dispersion coefficient of fluidized particles were examined, by adopting the relaxation method base on the stochastic model. The fluctuations and dispersion of fluidized solid particles were successfully analyzed by means of the pressure drop variation with time, which was chosen as a state variable, based on the stochastic model. The fluctuating frequency and dispersion coefficient of particles increased with increasing gas velocity, due to the increase of bubbling phenomena and bed porosity in which particles could move, fluctuate and travel. The frequency and dispersion coefficient of particles showed local maximum values with a variation of liquid velocity. The two values of fluctuating frequency and dispersion coefficient of particles increased with increase in particle size, but decreased with increase in liquid viscosity due to the restricted movement and motion of particles in the viscous liquid medium. Both fluctuating frequency and dispersion coefficient of particles increased with decrease in surface tension of liquid phase, due to the increase of bubbling phenomena with decrease in σ_L. The values of obtained particle dispersion coefficient were well correlated in terms of dimensionless groups as well as operating variables.