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.     

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。     

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。     

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.     

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.     

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.     

Bulk coarse particle arching phenomena in a moving bed with fine particle presence

Bridging or arching of flowing solids particles is a serious hazard in the operation of moving bed systems. The mechanics of the arching has been extensively analyzed in the context of particle discharge from a hopper with conical geometry by considering the particulate layer stress distribution. However, bridging can also occur in a moving bed system with cylindrical geometry during the continuous mass flow of solids particles. Experimental work conducted in this study reveals that the appearance of solids bridging is normally accompanied by the presence of fine particles in the coarse moving particles as well as by the countercurrent interstitial gas flow. In this study, a stress analysis of the layered particles distributed in a cylindrical, vertical moving bed that flows downward opposing to upward flow of the interstitial gas is developed to quantify the bridging phenomenon. The analysis takes into account of the effects of presence of fine powder in the coarse particle flows and properties, such as particle‐size distribution, bed voidage, and interstitial gas flow rate. The experimental validation of the present stress analysis for moving bed systems with varied fine and coarse particle concentration distributions, and interstitial gas velocities is also conducted. The stress distributions of the particles under flowing and arching conditions are obtained. An arching criterion is formulated, which indicates that the critical radius of the standpipe to avoid arching phenomenon is only related to the property of the bulk solids in the present geometric configuration of the flow system. © 2014 American Institute of Chemical Engineers AIChE J, 60: 881–892, 2014     

流化床密相区内固体颗粒对流换热模型研究进展

为深入理解流化床密相区内活性颗粒表面换热过程,论述了乳化相模型、单颗粒和双颗粒模型、变物性模型以及接触热阻模型等流化床密相区内固体颗粒对流换热模型的研究进展,提出了模型发展方向。当流化风速较低时,固体颗粒对流换热系数可采用乳化相模型进行预测。针对流化风速较大的工况,有学者提出了单颗粒和双颗粒模型、变物性模型以及接触热阻模型,并采用适当假设简化模型,然而模型的可靠性仍需进一步验证。最后提出现应加强固体颗粒对流换热过程的模型研究,提高模型在较宽风速范围内对固体颗粒对流换热系数预测的适用性;重点研究活性颗粒在密相区内的运动形式及活性颗粒表面固体颗粒对流换热关键参数测量方法。     

Determination and comparison of rotational velocity in a pseudo 2‐D fluidized bed using magnetic particle tracking and discrete particle modeling

Modeling of dense granular flow has been subject to a large amount of research. Particularly discrete particle modeling has been of great importance because of the ability to describe the strongly coupled dynamics of the fluid and the solids in dense suspensions. Many studies have been reported on the validation of the translational particle velocities predicted by using these models. The rotational motion however has received far less attention, mainly because of the spherical nature of the particles under investigation and the lack of techniques with the capability to study the rotational behavior of the solid phase. In this study, we will for the first time present experimental data on the rotational behavior of particles in a pseudo two‐dimensional fluidized bed setup using Magnetic Particle Tracking. In addition the experimental results are compared to data obtained from discrete particle simulations. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3198–3207, 2015     

流化床气化炉内煤颗粒的升温过程及影响

根据热传递理论,对夹套高温烟气加热的流化床气化炉内煤颗粒的升温过程进行了分析与计算,结果表明,颗粒的总体升温速率越高,颗粒升至床温所需的时间就越少;提高流化速度,有利于颗粒的升温,但是影响较小;颗粒粒径对颗粒的升温影响很大,随着颗粒粒径的减小,煤颗粒达到床温所需时间急剧下降。当粒径小于1mm,床温为700～1100℃时,颗粒内部能够在1s左右达到等温,3～5s后颗粒温度能升至99%床温。     

A combined experimental and computational study of wet granulation in a Wurster fluid bed granulator

A methodology combining theoretical and experimental techniques for analyzing the growth of granules in a fluidized bed granulator was developed. The methodology combines several key features of this complicated process: (i) population balancing (PB) of growth of different granules; (ii) hydrodynamic modeling of the gas-solid mixture flow using Computational Flow Dynamics (CFD); (iii) modeling of contact mechanics and granule formation; (iv) the Stokes number analysis for calculation of successful collisions; (v) well-controlled experimental study of the wet granulation. First, a detailed CFD model of the gas-solid flow and agglomeration (Model CFD-PB) within the Wurster type granulator was developed. Second, a simplified PB model of agglomeration in a homogeneous system (Model H-PB) was developed. The quadrature method of moments (QMOM) was used for solution of PB equations in both models. The kinetic theory of granular flow (KTGF) was employed in both models for calculation of the number of collisions between solid particles of different classes. Success factors, based on the Stokes number analysis, were calculated using results of extensive mesoscale simulations of the formation of realistic three-dimensional virtual granules. Comparison of simulation results of CFD-PB vs. H-PB models allowed evaluation of the KTGF kernel functionality to be used in H-PB model. Next, fluid bed granulation experiments were conducted for typical pharmaceutical excipients (microcrystalline cellulose, mannitol and dicalcium phosphate) with 15% HPC binder solution in a pilot plant Wurster granulator. The observed granule growth was a function of the surface roughness of excipients. Finally, the H-PB model was fitted to the experimental data. The only adjustable parameter of the H-PB model was an effective agglomeration rate constant, which we expect to be mostly related to the binder wetness on the surface of colliding granules.     

Magnetic effects in particle adhesion. Part II. The influence of particle composition and size on deposition in a magnetic field

The effects of a magnetic field on the deposition of particles of various compositions, sizes, shapes (spherical and rod-like) on steel beads of different kinds and sizes in an aqueous environment are described. In the systems studied, the particles and the collector bear a negative charge. If both interacting bodies have a sufficiently high magnetic moment, the magnetic force causes an enhancement in the particle attachment. The process is very sensitive to the size of the depositing solids; larger particles adhere much faster. Interpretation of the results is based on the shape of the total interaction energy function consisting of electrostatic, dispersion, and magnetic contributions. The major influence of the magnetic field is in the formation of a deep secondary minimum in which the particles, moving toward the surface, are accumulated. The magnetic force enhances the flux of these particles and deepens the minimum, causing an increase in the retention efficiency.     

Mass transport models for a single particle in gas-phase propylene polymerisation

Olefin polymerisation on heterogeneous catalysts is gaining importance due to widening of the polymer properties window. The supported active catalyst on the heterogeneous particle reacts with the monomer and produces polymer. Polymeric flow (PF) model is relatively simple and assume that particle morphology develops as the catalyst active sites move outwards along with the polymer formed thus leading to PF morphology. The multigrain (MG) model describes the particle morphology incorporating a more detailed picture of the underlying phenomena. The heterogeneous support fragments into tiny pieces by the polymer formed, while keeping the fragments together thus forming the MG morphology. The both state of the art models describes the monomer transport by Fick's diffusion (FD) alone and hence polymeric flow Fick's diffusion model (PF_FDM) and Multigrain Fick's diffusion model (MG_FDM). Both these models does not account for the monomer convection through interconnected pores. Single particle models that account for the convection through the pores, created by the pressure gradient due to the monomer consumption within the particle are discussed in the present work. The so-called advection-dispersion model (ADM) and the dusty gas model (DGM) is used to describe the convective flow. The PF morphology is retained in the advection-dispersion model (PF_ADM) and dusty gas model (PF_DGM) and similarly multigrain morphology is retained in the advection-dispersion (MG_ADM) and dusty gas model (MG_DGM). Comparison of convective flow models with state of the art models which use Fick's diffusion alone (PF_FDM, MG_FDM) has been made and published convective flow models are briefly discussed.