In-bed corrosion of alloys in atmospheric fluidized bed combustors

A series of tests, each of 250 h duration, which is designed to examine the effect of changes in operational parameters on in-bed corrosion, is in progress using a 0.3 m-square atmospheric fluidized bed combustor (AFBC). A 2000-h test has been conducted on a 1.1 mdiameter AFBC, partly to determine the comparability between the results of the tests in the small unit and those in more realistically sized beds, and partly to assess the performance of candidate materials of construction in a longer-term test. The results indicatethat the smaller unit is indeed a satisfactory representation of large AFBCs. Variables such as coal type, coal sulphur content, acceptor type, excess combustion air and bed temperature appear to have relatively little effect on in-bed oxidation/ sulphidation corrosion, certainly within the limits likely in normal operation. Several materials which appear to be suitable for construction of a fluidized bed boiler are identified, although it is emphasized that extrapolation of short-term tests is particularly dangerous for this form of corrosion.     

Segregation of char and silica sand particles in a hot-fluidized-bed steam gasifier

Understanding the behavior of coal char particles in a hot fluidized bed is essential for the design and operation of low-temperature gasifiers. The segregation of char as flotsam is the main focal point of this work. Segregation is generally unfavorable; however, it is attractive if the char holdup can act advantageously as a strong promoter of in-bed decomposition of tar and its conversion to syngas. This study first demonstrates the segregation of char during its gasification in a hot fluidized bed at a relatively low U₀/U_mf and at a steady state under continuous feeding and discharge of lignite and char, respectively. The distribution of char and its conversion from the bottom to the top of the bed are obtained from the motion of the char particles and the distribution of the gas components.     

Heat teansfer between a surface and a fluidized bed with packing

Heat transfer coefficients were measured under the following conditions: a fluidized bed with a spiral packing and a vertically immersed cylinder; a fluidized bed with a bundle of smooth tubes and with a bundle of lengthwise finned tubes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 3, pp. 397–402, March 1972.     

Formation of Sustained Release Wax Matrices Within Hard Gelatin Capsules in a Fluidized Bed

Sustained release wax matrices were formed within hard gelatin capsules during fluidization in a hot air stream. The capsules were filled with drug (propranolol HC1 or dieophylline) - wax (Precirol ATO-5 or Gelucire 50/13) powder blends and suspended in a fluidized bed to induce fusion of the wax. Upon cooling, wax matrices with embedded drug were formed in the ends of die capsules. The use of blends of waxes with different HLB values allowed good control over the drug release pattern. The drug release from the matrices was independent of the pH of the dissolution medium. Differential scanning calorimetry was used to study the physical state of the drugs in the matrices. Propranolol HC1 was insoluble and completely dispersed in the wax matrix while theophylline was partially dissolved in the wax.     

循环流化床锅炉内颗粒速度分布的实验研究

为了研究操作参数对循环流化床锅炉内颗粒速度分布的影响,在冷态模型装置上使用PV6A型颗粒速度光纤测量仪、毕托管对循环流化床锅炉炉膛内的颗粒速度进行了测量与分析。结果表明:在流化风速为2.57 m/s、循环质量流率为0.58 kg/(m2.s)、一次风量为2 300 m3/h、二次风量为1 500 m3/h时,循环流化床锅炉炉膛内颗粒浓度分布、压降、流化状态、质量循环都达到一个较稳定的水平,呈现典型的环核分布特征。     

Rate of CO2 adsorbent attrition induced by gas jets on perforated plate distributors in bubbling fluidized beds

Particle attrition is a major challenge when handling bulk solid materials with fluidized beds due to its ability to cause particle loss. Herein, the particle attrition induced by the gas jets on a perforated plate distributor in a bubbling fluidized bed was investigated for CO₂ adsorbent particles. An attrition tube, which used air as the fluidizing gas, was used as the fluidized bed. At a constant fluidizing velocity, the initial static bed height and orifice gas velocity were considered as variables. It was confirmed that abrasion dominated the particle attrition. The trend indicating the change in the maximum size of the particles (d_pm,a) formed by attrition followed that of the attrition rate (i.e., the formation rate of fine particles via attrition). A new stirring factor that combined the model developed by Werther and Xi with the original stirring factor adequately explained the effect of the static bed height on both the attrition rate and d_pm,a when the initial static bed height was greater than the length of the orifice gas jet that penetrated the bed. The attrition rate increased linearly with the new stirring factor. However, d_pm,a increased exponentially with the new stirring factor. Relationships were successfully proposed to enable the estimation of the attrition rate and d_pm,a for the CO₂ adsorbent particles. This study provided the evidence indicating the significance of the effect of bed height on particle attrition induced by the gas jet on the distributor. Moreover, proper models for correlating the attrition rate and the maximum size of the fine particles formed by attrition in the bubbling fluidized bed were provided.     

ELECTROSTATIC SEPARATION OF PLASTICS FOR RECYCLING

The authors describe some successful experiments for electrostatic separation of various two-component mixtures of plastic waste. Two methods of triboelectric charging of plastics are described. One consists of a fluidized bed system with a central feeding tube. The second is a novel form of a rotating tube charger. Separation takes place in an electrostatic separation tower equipped with a set of nine sampling bins at the bottom for collection of the components. Numerous experiments have shown the feasibility of obtaining extract contents in excess of 99%. The rotating tube charger is preferred over the fluidized bed because of its simplicity and energy savings. The process has recently been adapted to an industrial scale capable of processing 1000 kg/hour and is currently being used to separate post-industrial plastic waste.     

Gas flow influences on bubbling and flow characteristics of fluidized bed with immersed tube under electrostatic effects

Industrial bubbling fluidized beds are used to fluidize particles. When particles are fluidized, electrostatic effects will cause the particles to form obvious agglomerates, thus reducing fluidization performance. For better fluidization performance, internal component immersed tubes are usually placed in fluidized bed to limit the bubble size and reduce particle agglomerates. Meanwhile, pulsed gas flow can increase particle disturbance, which is also an effective method to reduce particle agglomerates. In this paper, the CFD-DEM model under electrostatic effects is constructed to research the bubbling and flow characteristics in fluidized beds. Firstly, particle mixing qualities with and without the immersed tube are compared. Then, the effects of different superficial gas velocities are investigated with an immersed tube. Finally, different frequencies are applied to study the energy loss and flow characteristics around the immersed tube. The results show that the addition of the immersed tube can reduce bubble size to facilitate particle mixing. Due to the obstruction of the immersed tube, the bubbles are generated near the wall. As the superficial gas velocity increases, the larger bubbles are generated. Moreover, the electrostatic force applied to the particles varies periodically with the frequency of incoming pulsed gas flow, with fluctuations maximal at 2.5 Hz.     

Effect of flow pulsation on fluidization degree of gas-solid fluidized beds by using coupled CFD-DEM

In this paper, the effect of inlet flow type on fluidization of a gas-solid fluidized bed was studied by using numerical simulations. Gas-solid fluidized beds are widely used in processes such as heating, cooling, drying, granulation, mixing, segregating and coating. To simulate the gas-particle flows, the unresolved surface CFD‐DEM was used considering Eulerian–Lagrangian approach. The fluid phase was modeled by computational fluid dynamics (CFD) while the solid phase was solved by discrete element method (DEM), and the coupling between gas and solid phases was considered to be four-way. The uniform and pulsed flows were injected through three nozzles located at the bottom of a rectangular bed. Three types of pulsed flow were considered: sinusoidal, rectangular and relocating. The fluidized bed behavior was discussed in terms of minimum fluidization velocity (MFV), pressure drop, bubble formation, bed expansion, particles velocity and, gas-solid interaction and particle contact forces. The results of different simulations indicated that the minimum fluidization velocity of the beds fluidized by pulsed flows was decreased by up to 33%. The influence of the pulsation amplitude on the minimum fluidization velocity was more significant than that of the pulsation frequency. The bed expansion and particles average velocity were increased by the pulsed flows, while the pressure drop and interaction force were decreased. As the pulsation frequency increased, the pressure drop and gas-solid interaction force increased, although size of the bubbles and bed expansion decreased. It was also observed that in large vibration frequencies, the bubbles became more regular. In the sinusoidal flow, the velocity and contact force between the particles were initially increased by frequency and in larger frequencies they were decreased.     

Photocatalytic degradation of formaldehyde in a fluidized bed with sound-magnetic assistance

The photocatalytic degradation of formaldehyde in a sound-magnetic assisted fluidized bed (SMFB) was investigated using Degussa P25 catalyst and Fe₃O₄ particles as the fluidized media. The effects of magnetic induction, sound pressure level, and sound frequency on the degradation rate of formaldehyde were investigated. Compared with fluidized bed (FB), sound assisted fluidized bed (SFB), magnetic assisted fluidized bed (MFB), SMFB could enhance the degradation efficiency of formaldehyde effectively. Experimental results showed that the degradation efficiency of formaldehyde increased with increasing magnetic induction and sound pressure level. The degradation efficiency of formaldehyde increased with increasing sound frequency ranging from 50 to 100 Hz, further decreased with increasing sound frequency from 100 to 150 Hz. Moreover, it was found that the reaction kinetic fitted the pseudo-first-order kinetic equation, which provided a detailed kinetic understanding of photocatalytic degradation of formaldehyde in SMFB.     

Enhanced fluidization of solid particles in an oscillating acoustic field

Compared with an ordinary fluidized bed, the fluidization quality of solid particles can be effectively improved by vibration induced by appropriate acoustic fields. The effects of sound on the hydrodynamic behavior of fluidized bed have been investigated under the application of acoustic fields of different intensities (110–130 dB) and frequencies (50–500 Hz). The obtained results show that the perturbation effect of the sound field on fixed-bed pressure drop becomes more significant with increasing sound pressure level, exerting a larger pressure than present under ordinary conditions, due to the change in particle arrangement induced by the acoustic field. Except for a particular frequency, the minimum fluidization velocity in the bed decreases gradually with the increase in the ratio of bed height to bed diameter. The rising velocity of the bubble and the average overflow velocity of residual gas in collapse tests are reduced by the acoustic field.     

The Effect of Agglomeration Methods on the Micromeritic Properties of a Maltodextrin Product, Maltrin 150™

Maltrin M150 is a fine powder of maltodextrin which is a carbohydrate product made by controlled hydrolysis of corn starch. Agglomerated Maltrin was prepared using a fluidized bed granulation process and a roller compaction method, respectively. The micromeritic properties of these two granular products were compared. Three different sizes of granules (20/30, 40/50 and 80/100 mesh size) were used in the evaluation. Granules produced by the fluidized bed method showed a relatively low bulk density as compared to the roller compacted granules. As the granule size was reduced, the roller compacted granules showed a decrease in bulk density while an increase in bulk density was seen in the fluidized bed granulated product. A better flowability of the roller compacted granules was demonstrated by a higher flow rate and a lower compressibility index. For a given compression pressure, roller compacted granules produced compacts with a lower tensile strength. A significant work-hardening effect was exhibited by the roller compacted product.     

Investigating the hydrodynamics of gas–solid bubbling fluidization using recurrence plot

Hydrodynamics of gas–solid fluidized bed was investigated using time series of pressure fluctuations by evaluation of the corresponding recurrence plot (RP). Patterns within RP of the fluidized bed were classified into two groups of local white areas (LWA), showing macro structures, and local bold areas (LBA), showing meso and micro structures. These patterns showed that the fluidized bed system has three different hydrodynamic behaviors as superficial gas velocity increases; at low gas velocities, macro structures become more dominant, further increase in gas velocity empowers influence of finer structures on the hydrodynamic and finally the fluidization regime changes. Additionally, these results were confirmed by recurrence rate (RR) and average cycle frequency. Comparison of RP of the fluidized bed with Lorenz and complete stochastic systems showed that the fluidized bed is more complex than Lorenz system, however, it’s hydrodynamic has not stochastic nature.     

Experimental and CFD-DEM numerical evaluation of flow and heat transfer characteristics in mixed pulsed fluidized beds

Pulsed fluidized beds can make gas-solid mix and contact more uniform, therefore obviously improving heat transfer efficiency. The mixed pulsed fluidized bed, whose total gas flow is composed of stable gas flow and pulsed gas flow, is proposed in this research. Firstly, the experimental device for drying particles in a mixed pulsed fluidized bed is established. Pressure signals with different frequencies and gas flow ratios are collected, and flow pattern diagrams are obtained through a high-speed camera. Secondly, the CFD-DEM parallel numerical simulation method is constructed to research the mixed pulsed fluidized bed performance. Particle mixing, motion and heat transfer characteristics under different pulse frequencies and flow ratios are studied. Results show that particles in the mixed pulsed fluidized bed exhibit regular periodic motion, thereby promoting the mixing effect of particles. Moreover the bubble nucleation point moves to the bottom of the bed with the increasing pulse frequency. When the total gas velocity is relatively low, particle mixing effect can be enhanced by increasing the proportion of pulsed gas. However, when the velocity is relatively high, particle mixing effect will be enhanced by decreasing the proportion.     

中心气升式气-固环流反应器中床层密度的实验研究

采用光纤测量方法,在不同的操作条件下,对一套中心气升式气-固环流反应器冷模装置内4个区域中不同轴向、径向位置的局部床层密度进行测量,得到沿轴向、径向局部床层密度分布的规律。结果表明,在导流筒区,沿轴向高度h=512~1 112 mm内,各截面平均床层密度分布比较均匀;在环隙区,由于颗粒夹带气泡量的不同,沿轴向不同截面平均床层密度降低的大小程度不同;局部床层密度在环隙区沿径向分布比较均匀,而在其它区域(导流筒区、底部分布器影响区和气-固分离区)呈中心区大边壁区小的不均匀分布形态。     

The effect of ash and filter media characteristics on particle filtration efficiency in fluidized bed

The phenomenon of filtering particles by a fluidized bed is complex and the parameters that affect the control efficiency of filtration have not yet been clarified. The major objective of the study focuses on the effect of characteristics of ash and filter media on filtration efficiency in a fluidized bed. The performance of the fluidized bed for removal of particles in flue gas at various fluidized operating conditions, and then the mechanisms of collecting particles were studied. The evaluated parameters included (1) various ashes (coal ash and incinerator ash); (2) bed material size; (3) operating gas velocity; and (4) bed temperature. The results indicate that the removal efficiency of coal ash increases initially with gas velocity, then decreases gradually as velocity exceeds some specific value. Furthermore, the removal of coal ash enhance with silica sand size decreasing. When the fluidized bed is operated at high temperature, diffusion is a more important mechanism than at room temperature especially for small particles. Although the inertial impaction is the main collection mechanism, the "bounce off" effect when the particles collide with the bed material could reduce the removal efficiency significantly. Because of layer inversion in fluidized bed, the removal efficiency of incinerator ash is decreased with increasing of gas velocity.     

The effects of frequency and amplitude on the powder coating of fluidizing particles in a vibro-fluidized bed

Glass beads were coated in a vibro-fluidized bed by atomizing a fine silica powder and polyvinyl alcohol solution. The coating efficiency and weight fraction of the agglomerated particles were measured under various experimental conditions, and their dependencies on the amplitude and frequency of vibration were investigated. When the humidity of the outlet gas was high, high coating efficiency and agglomeration of core particles were observed. The efficiency of the powder coating decreased as the amplitude and the frequency increased, while the agglomeration was prevented by the addition of vibration. It was confirmed that high-quality and high-efficiency coating, where few agglomerates were produced and silica powder was utilized efficiently, was possible using the vibro- fluidized bed with adequate amplitude and frequency.     

Influence of air velocity and powder bed height on local density and float–sink of spheres in a gas–solid fluidized bed

Depending on their density, large objects will either float or sink in a gas–solid fluidized bed due to the liquid–like properties and density of the fluidized bed. The float–sink technology has been applied to dry density separations in industry. It is important for optimized industrial application to understand how the air velocity and the powder bed height affect the float–sink as the key operating factors. In this study, we investigated the float–sink of spheres of various density by varying the air velocity and the powder bed height. Also, we obtained the local fluidized bed density and the buoyancy force working on the sphere at various heights. We used the weight of a stainless-steel sphere in the fluidized bed to estimate the local fluidized bed density and the buoyancy force based on Archimedes principle. We found that the spheres float–sink behavior changes dramatically with the air velocity and the powder bed height and that the spheres float–sink behavior is correlated to ΔF = F_b – F_g, where F_b is the buoyancy force and F_g is the gravity force acting on the sphere. We also found that the fluidized bed density is not constant as a function of height when the air velocity is relatively large; the local fluidized bed density is interestingly either minimal at approximately mid-height or surprisingly, gradually increases with height within the fluidized bed at higher air velocities. The possible reasons are discussed by considering the local variation of the motion of air bubbles and the fluidized medium which affect the fluid force acting on the sphere in the fluidized bed.     

Numerical prediction of particle erosion of pipe bends

In the present study the Euler/Lagrange approach in combination with a proper turbulence model and full two-way coupling is applied for erosion estimation due to particle conveying along a horizontal to vertical pipe bend. Particle tracking considers both particle translational and rotational motion and all relevant forces such as drag, gravity/buoyancy and transverse lift due to shear and particle rotation were accounted for Laín and Sommerfeld (2012). Moreover, models for turbulent transport of the particles, collisions with rough walls and inter-particle collisions using a stochastic approach are considered Sommerfeld and Laín (2009). In this work, the different transport effects on spherical solid particle erosion in a pipe bend of a pneumatic conveying system are analysed. For describing the combined effect of cutting and deformation erosion the model of Oka et al. (2005) is used. Erosion depth was calculated for two- and four-way coupling and for mono-sized spherical glass beads as well as a size distribution of particles with the same number mean diameter (i.e. 40?μm). Additionally, particle mass loading was varied in the range from 0.3 to 1.0. The erosion model was validated on the basis of experiments by Mazumder et al. (2008) for a narrow vertical to horizontal pipe system with high conveying velocity. Then a 150?mm pipe system with 5?m horizontal pipe, pipe bend and 5?m vertical pipe with a bulk velocity of 27?m/s was considered for further analysis. As a result inter-particle collisions reduce erosion although the wall collision frequency is enhanced Sommerfeld and Laín (2015); additionally, considering a particle size distribution with the same number mean diameter as mono-sized particles yields much higher erosion depth. Finally, when particle mass loading is increased, bend erosion is reduced due to modifications of particle impact velocity and angle, although wall collision frequency grows.     

Comparative study of propane oxidative dehydrogenation in fluidized and fixed bed reactor

Supported vanadia catalyst was successfully synthesized using wet impregnation of γ-alumina to study propane oxidative dehydrogenation (POD). The prepared catalysts were characterized with X-ray diffraction and BET tests. To investigate performance of fluidized bed reactor in comparison to fixed bed micro-reactor, propane conversion and propylene selectivity were measured in temperatures of 480 to 550°C and GHSV of 48000, 60000, and 78000 cc/gr.hr at equimolar feed ratio of propane to oxygen. The results indicated that at constant temperature propane conversion obtained in the fixed bed reactor was higher. But fluidized bed is superior to fixed bed reactor in terms of propylene selectivity leading to considerably higher propylene yields over the fixed bed. It is suggested that pre-filtering effect induced by hydrodynamics of fluidized bed reactor has major effect in the enhancement of propylene selectivity beyond that of fixed bed micro-reactor.