Measurement of particle concentration in a Wurster fluidized bed by electrical capacitance tomography sensors

It is essential to measure and monitor the particle flow characteristics in a Wurster fluidized bed to understand and optimize the coating processes. In this article, two electrical capacitance tomography (ECT) sensors are used to measure the particle concentration in different regions in a Wurster fluidized bed for the “cold” particle flows. One ECT sensor has a 12‐4 internal‐external electrodes and another has eight electrodes. The 12‐4‐electrode ECT sensor is used to measure the particle concentration in the annular fluidization region (outside of the Wurster tube) and the eight‐electrode ECT sensor is used to measure the particle flow in the central region (inside the Wurster tube). The effect of particle type, particle moisture, fluidization velocity, and geometrical parameters on the Wurster fluidization process is studied based on the two ECT measurements. The radial particle concentration profiles in the annular fluidization and central flow regions with different operation parameters are given. Fast Fourier Transform analysis of the particle concentration in the Wurster tube is performed with different superficial air velocities. The optimum operating ranges of the Wurster fluidization process for different particles are given. In the end of the article, computational fluids dynamics simulation results are given and used to compare with the measurement results by ECT for a typical Wurster fluidized bed. © 2014 American Institute of Chemical Engineers AIChE J 60: 4051–4064, 2014     

CFD modeling of the Wurster bed coater

In the Wurster bed coater, the wetting, drying, and circulation of particles are combined to produce a high quality coating. The drying and wetting conditions in a laboratory scale Wurster bed coater are modeled and compared with experimental data. A model combining multiphase fluid dynamics with heat and mass transfer is developed to model the particle and gas motion and the transport of thermal energy and moisture. A wetting region is defined, where a specified moisture content is set in the particle phase, above the jet inlet, to describe the injection of coating liquid. The simulation shows the characteristic circulation of particles in the equipment, as well as the behavior of the moisture in the system and agrees well with measurements. The simulation indicates how different process conditions influence the drying regions. The results show that most of the drying, under typical operating conditions, takes place in the Wurster tube. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Evaluation of the morphological and release characteristics of coated fertilizer granules produced in a Wurster fluidized bed

In the present study, coated fertilizer granules were produced in a Wurster chamber of a fluidized-bed equipment for controlled-release agrochemical applications. The coating material was a commercially available latex of poly(vinylidene-chloride). The influence of the process parameters (e.g., spray rate of the polymer latex, fluidization air velocity, distance of the partition from the air inlet and perforation percentage of the plate at the air inlet) on the morphology and quality of the coating was thoroughly investigated. The surface characteristics of the coated fertilizer granules were examined by scanning electron microscopy (SEM). The release rate of the fertilizer's nutrients from the coated granules was determined by kinetic-release experiments carried out in distilled water. Depending on the selected process conditions, the coating thickness could vary from 25 to 65 μm, while the surface of the coated granules changed from completely smooth to rough and porous. It was shown that by suitable selection of the coating conditions in the Wurster process, controlled-release fertilizers exhibiting prolonged release profiles could be produced.     

Monte Carlo simulations to determine coating uniformity in a Wurster fluidized bed coating process

This paper presents a coating model to predict the mass coating uniformity in a Wurster fluid bed coater using a Monte Carlo method. The velocity and voidage data obtained using imaging techniques on the same Wurster coater are used as inputs to the model. The semi-circular Wurster fluid bed used in this work was 22.9 cm in diameter. A batch of 3.6 kg tablets was used to conduct coating experiments and the coating weight gain distribution results were compared to predictions from the simulation. The model rigorously considers the sheltering effect of particles as they move in the spray zone. Good agreement was obtained when comparing the results with an analytical model.Spray shape and orientation of discretization were found to play an important role in predicting the coating uniformity. A simple spray experiment in a particle-free bed showed that the direction of spray material, in general, was vertically upward. Simulation results confirmed that an upward cylindrical spray model gives better agreement with experimental results compared to a solid cone spray model. Finally, the model was used to predict the changes in coating uniformity with bed operating conditions such as gas velocity and gap height. A wider coating distribution was found for the case with the lower gas velocity and gap height.     

PEPT study of particle cycle and residence time distributions in a Wurster fluid bed

Particle cycle and residence time distributions are critical factors in determining the coating quality in the Wurster process. Positron emission particle tracking experiments are performed to determine the cycle and residence times of particles in different regions of a Wurster fluid bed. The results show that particles tend to recirculate in and sneak out below from the Wurster tube. The experiments also show that a larger batch size leads to a shorter cycle time and a narrower cycle time distribution (CTD). It is possible to avoid recirculations and obtain a shorter cycle time and a narrower CTD by selecting the operating conditions appropriately or via equipment design. Experiments using binary mixtures of particles with a diameter ratio of 1.5 show that large particles have a longer cycle time than small particles and that the cycle time is shorter for mixtures with approximately equal amounts of small and large particles. © 2014 American Institute of Chemical Engineers AIChE J, 61: 756–768, 2015     

Measurement of voidage in the fountain of a spouted bed

A γ-ray technique is described for measuring the distribution of fractional voidage in the fountain of an air spouted bed. What appear to be the first experimental data on fountain voidage are presented. For the relatively shallow, 160 mm diameter, conical spouted bed studied, fountain fractional voidage is quite high, 0.88 and upwards. Generally voidage increases with height up the fountain, apart from a plateau or slight fall part way up. This trend differs from that proposed by Grace and Mathur in a theoretical model for a deeper cylindrical spouted bed. Fountain voidage is shown to depend on air flow rate and on particle size distribution.     

Measurement of particle movement in a spouted bed using a new microprocessor based technique

A new experimental technique for measuring particle cycle times, times of occurrence of particles at different levels, and particle velocities in the spout of a spouted bed is described. A magnetized marker particle and search coil or coils are employed together with a microprocessor system for ease of data collection and processing. The magnetic particles have the same diameter, density and similar surface texture to bed particles to ensure proper representation. Data is presented on distributions of cycle time and particle velocity and implied particle mass circulation rates at different levels in a spouted bed. Comparisons are made with photographic and laser–doppler anemometry techniques.     

The influence of particle size and density on the radial gas mixing in the dilute region of the circulating fluidized bed

A cold model of a CFB, having a 0.120m ID, 5.75m high riser, was operated at a superficial gas velocity of 6m/s and at average solids mass fluxes varying in the range between 16 and 55kg/sm². A stream of carbon dioxide was injected continuously at a fixed position along the riser axis and used as tracer gas. Three solids, belonging to groups A and B of Geldart's classification, were selected in order to investigate separately, the effect of particle size and density on the radial mixing of the gas phase. The intensity of radial gas dispersion reduced when larger or heavier particles were used. The differences in the values of radial dispersion coefficient and Peclet number were explained, taking into account the role of the particle size with respect to the scale of gas turbulence.     

新型固定床颗粒层除尘器的研究

介绍了新型固定床颗粒层除尘器的结构、组合式流化床清灰机构的工作原理以及除尘器的特点。实验表明,新型固定床颗粒层除尘器操作简单,组合流化床清灰装置能满足颗粒层清灰要求。     

Capturing the effects of particle heterogeneity on adsorption in a fixed bed

To efficiently design new adsorption systems, industrial scale fixed beds are often scaled down to bench-top experiments and/or modeled using computational fluid dynamics (CFD). While there has been considerable work exploring adsorption of volatile organics onto activated carbon fixed beds in the literature, this article attempts to reckon with the high variability of adsorption capacities observed at small scales and improve small-scale experiments for industrial scale reactor design. This study integrates experimental results with CFD simulations, which can explicitly model system heterogeneities and their influence on adsorption by resolving local packing densities and flow paths. Activated carbon physical properties were determined through surface area analysis, proximate analysis, and toluene adsorption (measured via mass spectroscopy). Variability in the small-scale systems was not attributed to surface area or carbon content, as is often stated, but instead was due to local packing density variations and the heterogeneity of particle size distributions.     

Experiments on particle cluster behaviors in a fast fluidized bed

A three-dimensional (3D) fast fluidized bed with the riser of 3.0 m in height and 0.1 m in inner diameter was established to experimentally study the cluster behaviors ofGeldart B particles.Five kinds of quartz sand particles (dp =0.100,0.139,0.177,0.250 and 0.375 mm and ρp =2480 kg·m-3) were respectively investigated,with the total mass of the bed material kept as 10 kg.The superficial gas velocity in the riser ranges from 2.486 to 5.594 m·s-1 and the solid mass flux alters from 30 to 70 kg· (m 2· s)-1.Cluster characteristics and evolutionary processes in the different positions of the riser were captured by the cluster visualization systems and analyzed by the self-developed binary image processing.The results found four typical cluster structures in the riser,i.e.,the macro stripe-shaped cluster,saddle-shaped cluster,U-shaped cluster and the micro cluster.The increasing superficial gas velocity and particle sizes result in the increasing average cluster size and the decreasing cluster time fraction,while the solid mass flux in the riser have the reverse influences on the cluster size and time fraction.Additionally,clusters in the upper region of the riser often have the larger size and time fraction than that in the lower region.All these effects of operating conditions on clusters become less obvious when particle size is less than 0.100 mm.     

Using stereo XPTV to determine cylindrical particle distribution and velocity in a binary fluidized bed

Nonspherical particles are commonly found when processing biomass or municipal solid waste. In this study, cylindrical particles are used as generic nonspherical particles and are co-fluidized with small spherical particles. X-ray particle tracking velocimetry is used to track the three-dimensional particle position and velocity of a single tagged cylindrical particle over a long time period in the binary fluidized bed. The effects of superficial gas velocity (u _f), cylindrical particle mass fraction (α), particle sphericity (Φ), and bed material size on the cylindrical tracer particle location and velocity are investigated. Overall, the cylindrical particles are found in the near-wall region more often than in the bed center region. Increasing the superficial gas velocity u _f provide a slight improvement in the uniformity of the vertical and horizontal distributions. Increasing the cylindrical particle mass fraction α causes the bed mixing conditions to transition from complete mixing into partial mixing. © 2018 American Institute of Chemical Engineers AIChE J, 65: 520–535, 2019     

Microencapsulation of water-soluble macromolecules with acrylic terpolymers by the Wurster coating process for colon-specific drug delivery

The aim of this work is to explore a microcapsule formulation applicable for a peroral colon-specific delivery of water-soluble macromolecules such as peptide drugs, based on our previous delayed-release formulations using self-made colloidal terpolymers of ethyl acrylate, methyl methacrylate and 2-hydroxyethyl methacrylate (poly(EA/MMA/HEMA)). Peptides are thermally unstable and less permeable through a polymeric membrane in general. Thus, the present work was intended to establish the acrylic terpolymers having the following characteristics: an excellent film-formability even at a low process-temperature of 40 °C to preserve the stability of heat-sensitive drugs and a desired permeability allowing a delayed-release profile of macromolecular drugs. Using various types of poly(EA/MMA/HEMA) synthesized with different monomer molar ratios, microencapsulation of the particles consisting of a lactose core and a surface-layer of water-soluble fluorescence-labeled dextran (FITC-Dex, Mw 9500), a model of macromolecular solute, was carried out by the Wurster process. The release profiles of FITC-Dex were highly dependent on the terpolymer compositions. The increased EA content made the poly(EA/MMA/HEMA) films flexible and less permeable, leading to significant decrease in release rate of FITC-Dex. Contrary, it was found that the increase in HEMA content enhanced the FITC-Dex release due to the enhanced hydration of the films, as expected from its higher hydrophilicity than the other monomers. Among a series of poly(EA/MMA/HEMA) prepared here, the poly(EA/MMA/HEMA) with the molar ratio of 95:85:40 exhibited a good film-formability in the post-thermal curing process at 40 °C. The established coating films allowed delayed-release of the microencapsulated FITC-Dex; the profile was characterized by a lag time and subsequent rapid release. This delayed-release was probably due to time-dependent change of the semi-permeable nature of the hydrated coating films as a consequence of anomalous film-deformation arising from the osmotic pressure inside the microcapsules. Moreover, the release profiles of FITC-Dex were found to be pH-independent. Based on our findings, the use of poly(EA/MMA/HEMA) with molar ratio of 95:85:40 could be proposed as a feasible way to prepare a delayed-release microcapsule containing water-soluble macromolecules by the Wurster coating process, toward the future application to colon-specific delivery of peptide drugs.     

微细物料颗粒大小的测定(续)

4.2 显微镜法 显微镜法包括使用光学显微镜和电子显微镜,是唯一可以观察和测量颗粒的方法。几种显微镜在测定颗粒大小时的性能如表3。     

Measurement of the floating particle size distribution by a buoyancy weighing-bar method

The particle size distribution of cylinder-shaped solid particles was measured using a buoyancy weighing-bar method where the liquid phase density was adjusted to settle or float the particles. The particle size distribution obtained in our experiment agrees with the particle size measured by other method. The present study demonstrates that a buoyancy weighing-bar method, a novel method for measuring the particle size distribution, is suitable for measuring the particle size distribution of a floating solid. The precision of the resulting particle size distribution is comparable to that of a laser diffraction/scattering method as well as a direct measurement with a micrometer. Moreover, this buoyancy weighing-bar method can measure the particle size distribution even in a mixture of two particles with different sizes.     

Hydrodynamics and particle collection in turbulent bed contactor

Perforated spheres with 10% and 25% free area, oblate spheroids and the conventional plain spheres were tested as packing medium in a Turbulent Bed Contactor (TBC). Hydrodynamics and particle collection experiments were carried out in a column having 0.264 m diameter, at 3 static bed heights and several gas and liquid flow rates. Alumina (1.5 to 5.5 μm) was used as test powder for particle collection experiments. The higher efficiency of particle collection, the lower bed expansions and the higher fluidization velocities of perforated spheres, indicates that this packing type is suitable as contacting medium for particle collection in TBCs.     

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.     

带纵向涡发生器喷动床内颗粒流动特性PIV实验

针对喷动床内环隙区颗粒缺少横/径向运动的特点,通过实验将纵向涡流发生器及纵向涡流效应引入喷动床。采用粒子图像测速技术研究了在内径为152 mm的喷动床内纵向涡流及颗粒设计参数对喷动床内喷射区及环隙区颗粒相径向速度的影响,研究结果表明,纵向涡发生器在扰流元件上方横截面内颗粒相运动出现大量二次涡流,相比较于无纵向涡流扰流件情况,喷动床内的颗粒径向速度得到了显著增加,表明纵向涡发生器能够增强颗粒相在喷射区及环隙区的径向运动能力。在喷动床稳定喷动范围内,颗粒直径及颗粒密度越小,纵向涡流对颗粒相径向运动的强化效果越佳。