Fluidized Bed Spray Research Articles

Estimation of Particle Rotation in Fluidized Beds by Means of PTV

Fluidized bed granulation is a widely used process to produce pharmaceuticals, food and fertilizer. The achievable product quality, for example the uniformity of the formed layer or the size of the agglomerates, inherently depends on the particle dynamics in the bed. Generally, the solid phase velocity field is used to determine characteristic particle times (turn-over and residence time) in different zones of a spray fluidized bed. This information can be acquired using particle image velocimetry (PIV) together with digital image analysis (DIA) as for example in Börner et al. (2011, 2013). In order to derive certain quantities, such as particle mass flow rates, the measurement results are averaged in space and time, which is directly associated with a loss of information. However, an alternative is available when using the Lagrangian particle description (particle trajectories and local particle velocities) in terms of particle tracking velocimetry (PTV). Particle velocities are estimated on the basis of identifying individual particles, leading to exact particle mass flow rates. There are known shortcomings with this method for dense particulate flows, in particular ambiguities during particle identification. These problems have been fixed to a certain extent, for example using colored tracer particles (Natarajan et al., 1995, Bendicks et al., 2011) or recognition of flow pattern (Capart et al., 2002). The former approach yields a low resolution, but ensures reasonable results for particle velocities and associated trajectories. Additionally, it is possible to obtain information concerning the particle rotation, when using traces with an adequate color pattern (e.g. Zimmermann et al., 2011). Therefore, we used half-page blackened tracer particles in order to estimate particle velocities and particle rotation simultaneously and unambiguously within 2D-fluidized bed. Solid mass flow rates and particle residence times can be derived from the local particle velocities. At the same time, the observation of discontinuous particle tracks together with the quantification of the particle rotation can be used to foster the insight and understanding of particle-particle contact and the associated momentum transfer in dense gas-solid multiphase flows. Eventually, we can obtain local particle collision rates and corresponding changes of normal and angular momentum of individual tracer pairs.

Micro-Macro Transition of Population Balances in Fluidized Bed Granulation

The objective of this study is to introduce a generalized population balance model for simulating spray fluidized bed agglomeration of non-porous particles by extending the recent works of Hussain et al. [9,10]. In this context, the focus of this work is to model the average wet surface coverage fraction per wet particle and average success fraction concerning the dissipation of kinetic energy on a macroscopic scale. Average wet surface coverage fraction per wet particle is modeled by assuming the steric hindrance to be constant and discretizing the surface area of primary particles into smaller areas. Moreover, an algorithm is presented which takes into account Stokes criterion to predict an average success fraction concerning the dissipation of kinetic energy on a macroscopic scale.The proposed model with these variables is capable of predicting the particle size distribution, the total number of agglomerates, the total number of droplets and the total number of wet particles present in the system. The population balance model is validated by comparing its results with the experimental data. All comparisons show, at least qualitatively, good agreement, and the ability and generality of the proposed model to predict system dynamics with adequate accuracy.

Experimental investigation of process stability of continuous spray fluidized bed layering with internal separation

The present work investigates spray fluidized bed layering to continuously generate sodium benzoate granules from a solution. Product granules are discharged from the process through a classifying tube installed centrally in the distributor plate of the fluidized bed. To ensure a constant product quality, such as particle size, process conditions realizing a stable stationary operation are vital. At the same time process stability strongly depends on the size and amount of new seed particles for the layering process. In all presented experiments seed particles are solely produced internally by overspray. Thus process stability is investigated according to a variation of separation and drying parameters, namely classifying gas velocity, spray mass flow rate, fluidizing gas inlet temperature and fluidizing gas mass flow rate. Different classifying gas velocities yield different bed hold-ups. Small and high hold-ups lead to a stationary particle size distribution in the fluidized bed for long process times and thus to a stable continuous layering growth process. Moderate hold-up yields sustained oscillations of the particle size distribution of the granules in the bed and thus a marginally stable process. Changing drying conditions also influence the stability of continuous layering processes due to their influence on overspray generation. Intense drying conditions lead to an emptying of the fluidized bed and thus an unstable process. Moderate drying conditions lead to damped oscillations of the particle size and thus a nearly stationary process behavior for long process times. Drying conditions in between yield sustained oscillations of the particle size distribution of the granules in the bed, turning the process marginally stable again.

A new framework for population balance modeling of spray fluidized bed agglomeration

Previous work (Hussain et al. (2013). Chemical Engineering Science, 101, 35) has pointed out that the conventional, one-dimensional population balance equation for aggregation can be expanded to accurately reproduce the results of discrete simulations of spray fluidized bed agglomeration. However, some parameters had to be imported from the discrete simulation (Monte-Carlo). The present paper shows how the expanded population balance can be run without importing parameters from the Monte-Carlo simulation. The expanded population balance still reproduces the results of Monte-Carlo simulations accurately, taking into account key micro-scale phenomena (sessile droplet drying, efficiency of collisions), but with much lower computational cost. Required input parameters are just the drying time of sessile droplets (calculated in advance), and the pre-factor of an equation that correlates particle collision frequency with fluidized bed expansion. In this way, the expanded population balance is, apart from autonomous, also (nearly) predictive. Its performance is demonstrated by comparisons with both Monte-Carlo results and experimental data for various operating conditions (binder mass flow rate, gas temperature). Despite formally being a one-dimensional expression, the expanded population balance captures additional properties, such as the number of wet particles and the number of droplets in the system, which are even difficult to measure in experiments.

Two-compartmental population balance modeling of a pulsed spray fluidized bed granulation based on computational fluid dynamics (CFD) analysis

In this work a two-compartmental population balance model (TCPBM) was proposed to model a pulsed top-spray fluidized bed granulation. The proposed TCPBM considered the spatially heterogeneous granulation mechanisms of the granule growth by dividing the granulator into two perfectly mixed zones of the wetting compartment and drying compartment, in which the aggregation mechanism was assumed in the wetting compartment and the breakage mechanism was considered in the drying compartment. The sizes of the wetting and drying compartments were constant in the TCPBM, in which 30% of the bed was the wetting compartment and 70% of the bed was the drying compartment. The exchange rate of particles between the wetting and drying compartments was determined by the details of the flow properties and distribution of particles predicted by the computational fluid dynamics (CFD) simulation. The experimental validation has shown that the proposed TCPBM can predict evolution of the granule size and distribution within the granulator under different binder spray operating conditions accurately.

Modeling aggregation kinetics of fluidized bed spray agglomeration for porous particles

The present work develops a comprehensive one-dimensional pure agglomeration population balance model for simulating spray fluidized bed agglomeration of porous particles. The effect of micro-processes on aggregation kinetics is analyzed using a sophisticated constant number Monte-Carlo (CNMC) approach. The CNMC is used as a virtual spray fluidized bed granulator and takes into account several micro-processes such as wetting, convective drying, imbibition, agglomeration etc. so that it is a model for the real granulator. The main components influencing the agglomeration kinetics, namely the total number of particles, the number of wet particles, the collision frequency per particle and the average wet surface coverage fraction per wet particle are modeled and incorporated in the conventional one-dimensional population balance model. Finally, the proposed population balance model is validated against the results of CNMC.

Study of the morphology of solidified binder in spray fluidized bed agglomerates by X-ray tomography

Properties of agglomerated products depend on the materials used (primary particles, binder) and on their internal structure. Previous studies have characterized the internal structure of spray fluidized bed agglomerates in terms of the placement of primary particles, but binder morphology has not been investigated until now. Therefore, solidified binder morphology in spray fluidized bed agglomerates is discussed in the present work on data gained by X-ray micro-computed tomography. The main results refer to glass beads as the primary particles and to HPMC with contrast agent as the binder. Gas temperature and drying rate are shown to have a very strong influence not only on the morphology of the bridges that connect primary particles, specifically on the macroscopic porosity (hollowness) of such bridges, but also on the microscopic porosity of seemingly full bridge parts. Individual bridge volume, bridge clustering, binder volume distribution and agglomerate filling ratio with binder are discussed, and a classification is proposed for the various solidified binder elements observed. A critical view to systematic errors stemming from limitations in spatial resolution of the applied imaging technique is provided.

Design and development of a combined calcium-iron-folic acid prenatal supplement to support implementation of the new World Health Organization recommendations for calcium supplementation during pregnancy.

Hypertensive diseases of pregnancy are important causes of maternal and perinatal mortality. Based on meta-analyses of efficacy trials of prenatal calcium supplementation to reduce the risk of hypertensive diseases of pregnancy, the World Health Organization recommends 1.5 to 2.0 g of elemental calcium per day for pregnant women with low dietary calcium intakes (as well as 60 mg of iron and 400 microg of folic acid). However, implementation of this recommendation is challenged by the size and number of calcium tablets required and the need to avoid concurrent ingestion of calcium and iron due to intraintestinal interactions. We developed a novel micronutrient powder containing microencapsulated pH-sensitive calcium in addition to iron and folic acid, designed to facilitate early intestinal iron release and delayed calcium release. Two pharmaceutical companies were contracted to develop a prototype, one of which was chosen for clinical testing. Calcium carbonate granules were coated with a trilayer pH-sensitive enteric coating using a fluid-bed spray coater. Iron and folic acid granules were encapsulated with a time-release coating. Iron and calcium dissolution profiles were assessed during exposure to acidic (pH 1.2) and/or basic (pH 5.8) media using a modified USP apparatus 1 (basket) method. At pH 1.2, calcium and iron release was < or = 10% and > 90% after 120 minutes, respectively. At pH 5.8, > 80% of total calcium was released after 90 minutes. Based on in vitro criteria, the supplement may be a promising approach for delivering calcium, iron, and folic acid as a single daily dose to pregnant women in settings of low dietary intake of calcium.

Population balance modelling and multi-stage optimal control of a pulsed spray fluidized bed granulation

In this work, one-dimensional population balance models (PBMs) have been developed to model a pulsed top-spray fluidized bed granulation. The developed PBMs have linked the key binder solution spray operating factors of the binder spray rate, atomizing air pressure and pulsed frequency of spray with the granule properties to predict granule growth behaviour in the pulsed spray fluidized bed granulation process at different operating conditions with accuracy. A multi-stage open optimal control strategy based on the developed PBMs was proposed to reduce the model mismatch, in which through adjusting the trajectory of the evolution of the granule size distribution at predefined sample intervals, to determine the optimal operating variables related to the binder spray including the spray rate of binding liquid, atomizing air pressure and pulsed frequency of spray. The effectiveness of the proposed modelling and multi-stage open optimal control strategies has been validated by experimental and simulation tests.

Influence of process variables on internal particle structure in spray fluidized bed agglomeration

In order to enhance and control properties of aggregates and be able to predict the structure of final products, it is essential to establish a link between the product properties and the operating conditions. Experimental investigations have been conducted on agglomerates produced in spray fluidized beds using different primary materials (non-porous glass, porous ceramic) and HPMC (hydroxyl-propyl-methyl-cellulose) as the binder, under varying operating conditions, inlet gas temperature and binder concentration. X-ray μ-computed tomography, μ-CT, was utilized to evaluate the three dimensional micro-structure of the agglomerates and spatial distribution of each constituent primary particle in the aggregate. Values and the behavior of evaluated morphological descriptors are presented and discussed for different agglomerates.

Modeling of aggregation kernel using Monte Carlo simulations of spray fluidized bed agglomeration

The present work attempts to consider the microscopic mechanisms of spray fluidized bed agglomeration while modeling the macroscopic kinetics of the process. A microscale approach, constant volume Monte‐Carlo simulation, is used to analyze the effects of micro‐processes on the aggregation behavior and identify the influencing parameters. The identified variables, namely the number of wet particles, the total number of particles, and the number of droplets are modeled and combined in the form of an aggregation kernel. The proposed kernel is then used in a one‐dimensional population balance equation for predicting the particle number density distribution. The only fitting parameters remaining in the population balance system are the collision frequency per particle and a success fraction accounting for the dissipation of kinetic energy. Predictions of the population balance model are compared with the results of Monte‐Carlo simulations for a variation of significant operating parameters and found to be in good agreement. © 2014 American Institute of Chemical Engineers AIChE J, 60: 855–868, 2014

On two-compartment population balance modeling of spray fluidized bed agglomeration

The present work focuses on the modeling and analysis of a spray fluidized bed granulation (SFBG) process based upon the concept that particles are communicating between the two compartments at some steady state mass flow rate. A numerical technique for solving the proposed two-compartment model (2CM) is developed and validated against some newly derived analytical solutions. Moreover, the inverse technique for extracting the rate constant of one-compartment model (1CM) is extended to 2CM. A correlation of aggregation rate constant of 2CM with the rate constant of conventional 1CM under some restrictions is investigated and it is found that the 1CM cannot be used, in general, to predict results of 2CM. Furthermore, it is observed that the existence of two zones in SFBG is responsible to certain extent for time dependent behaviour of aggregation rate constant. Finally, influence of compartment sizes and particles residence times on particle size distribution is investigated.

Application of Three Alternative Technologies (Spray Drying, Fluid Bed Drying And Freeze Drying) to Obtain Powdered Formulas from Plants with Antimicrobial Potential

This study aims the application of three alternative drying technologies (spray drying, fluid bed and freeze drying) to evaporate and immobilize the bioactive components of plant hydrophilic extracts on different natural matrices. Three different powdered formulas were obtained from watery extracts , e.g. 15% leaves of Persian walnut (Juglans regia ), European Mistletoe (Viscum album) and white willow bark (Salix alba ),  mixed with three different matrices (maltodextrin, lactose and salt) in a weight ratio 90:10. The mixes were evaporated by three alternative methods (spray drying, fluid bed and freeze drying) to obtain powdered formulas. By UV-Vis spectrometry and FTIR spectroscopy there were monitored the phenolic derivatives and their stability comparing the concentration in the initial extract and after drying. The concentration of phenolic derivatives decreased differently after drying, depending on the technology and the matrix. The best results were obtained for willow extract on salt, using spray drying technology, with recovery rates above 50%. The behaviour of the matrices was different during the drying process: the most reliable was maltodextrin used in spray drying and salt, used in fluid bed drying. By freeze drying, lactose and salt gave best results, but this technology was considered more expensive. Finally, the best procedure was found to be spray drying, due to its rapidity and the continuity of drying for a long period of time, being the most economic, while freeze drying assured reliable composition by slow vaporization process at low temperatures. The willow formula proved to be the most concentrated in phenolics and reccomended to be used as biodesinfectant.

Modeling spray fluidized bed aggregation kinetics on the basis of Monte-Carlo simulation results

The kernel of a one-dimensional pure aggregation population balance equation is modeled based on the results of simplified micro-scale constant number Monte-Carlo (MC) simulations of spray fluidized bed agglomeration. The effects of micro-processes such as particle collision, droplet deposition and droplet drying are incorporated in the aggregation kernel by taking into account the probabilities of wet collision and adhesion. The aggregation kernel is found to be directly correlated with the total collision frequency per particle, the total number of particles, the number of wet particles, the fraction of wet surface on such particles, and a success factor concerning the dissipation of kinetic energy. The derived expression of the aggregation kernel is computed using information provided by the MC and compared with the aggregation efficiency extracted directly from the MC for validation. Furthermore, a population balance model involving the derived expression for the aggregation kernel is also presented to compute the total number of particles and the number of wet particles independently from the MC. The results of this model concerning particle numbers and particle size distribution are found to be close to the results of corresponding MC simulations.

A priori prediction of aggregation efficiency and rate constant for fluidized bed melt granulation

This paper presents a predictive aggregation rate model for spray fluidized bed melt granulation. The aggregation rate constant was derived from probability analysis of particle–droplet contact combined with time scale analysis of droplet solidification and granule–granule collision rates. The latter was obtained using the principles of kinetic theory of granular flow (KTGF). The predicted aggregation rate constants were validated by comparison with reported experimental data for a range of binder spray rate, binder droplet size and operating granulator temperature. The developed model is particularly useful for predicting particle size distributions and growth using population balance equations (PBEs).

Quality risk management of top spray fluidized bed process for antihypertensive drug formulation with control strategy engendered by Box-behnken experimental design space

Introduction:Lacidipine (LCDP) is a very low soluble and highly biovariable calcium channel blocker used in the treatment of hypertension. To increase its apparent solubility and to reduce its biovariability, solid dispersion fluid bed processing technology was explored, as it produces highly dispersible granules with a characteristic porous structure that enhances dispersibility, wettability, blend uniformity (by dissolving and spraying a solution of actives), flow ability and compressibility of granules for tableting and reducing variability by uniform drug-binder solution distribution on carrier molecules.Materials and Methods:Main object of this quality risk management (QRM) study is to provide a sophisticated “robust and rugged” Fluidized Bed Process (FBP) for the preparation of LCDP tablets with desired quality (stability) and performance (dissolution) by quality by design (QbD) concept.Results and Conclusion:This study is principally focusing on thorough mechanistic understanding of the FBP by which it is developed and scaled up with a knowledge of the critical risks involved in manufacturing process analyzed by risk assessment tools like: Qualitative Initial Risk-based Matrix Analysis (IRMA) and Quantitative Failure Mode Effective Analysis (FMEA) to identify and rank parameters with potential to have an impact on In Process/Finished Product Critical Quality Attributes (IP/FP CQAs). These Critical Process Parameters (CPPs) were further refined by DoE and MVDA to develop design space with Real Time Release Testing (RTRT) that leads to implementation of a control strategy to achieve consistent finished product quality at lab scale itself to prevent possible product failure at larger manufacturing scale.

Influence of Drying Kinetics on Particle Formation: A Personal Perspective

Taking the special opportunity of an anniversary issue of Drying Technology, a topic that may be expected to gain much importance in the future, namely, the influence of drying on particle formation processes and, thus, on the morphology and properties of the resulting product, is outlined. This article is not an in-depth review; it is intended as a capsule discussion based on recent experimental and computational results from the author's group on agglomeration, layering granulation, and coating in spray fluidized beds and spray drying. Reference is made to recent book chapters for more details. Thematically relevant articles from the 2011 volume of Drying Technology are quoted as a frame of reference for the placement of the discussion in the broader landscape of current drying research activities.

Characterization of the internal morphology of agglomerates produced in a spray fluidized bed by X-ray tomography

The internal structure of granules produced by spray fluidized bed agglomeration was investigated by means of X-ray micro-computer-tomography. The X-ray tomography delivers three-dimensional volume images, which are first used to count primary particles and derive center coordinate position, radius and volume for each of them. Further analysis by add-on algorithms results in a variety of morphological descriptors: radius of gyration, porosity, fractal dimension and pre-factor, coordination number distribution, and coordination angle distribution. Values and the behavior of these descriptors are presented and discussed for agglomerates consisting of a different number of primary particles, from 4 up to 310. The primary particles were made of porous γ-Al2O3, and they were nearly mono-dispersed and spherical. The distribution of solidified binder (hydroxyl-propyl-methylcellulose, originally as dilute solution in water) was not investigated. The agglomerates were produced in a top-spray fluidized bed at constant operating parameters.

Attrition and abrasion resistance of particles coated with pre-mixed polymer coating systems

Abstract Nowadays many particulate products are coated to avoid dust formation and breakage when subjected to forces with different magnitude and direction. In order to distinguish the influence of each force, different testers are needed. In this work Purox-S and Cellets 1000 core particles were coated in a top spray fluid bed coater by three different pre-mixed polymer coating systems (Opadry®, Kollicoat® IR and Instacoat AQUA). The repeated impact tester (RIT) was used to measure their strength toward attrition caused by low magnitude normal forces. Tangential forces may also cause rupture to the particles, so an abrasion tester was used to measure this amount of damage caused to the particles. This work provides information on the resistance of the core particles coated with different mixed polymers against attrition and abrasion.

Assessing the Explosion Risk of Plants for Dusty, Granular and Combustible Products Example of a Risk Assessment Procedure for a Fluid Bed Spray Granulator

Risk management is specific to the organization and its external and internal context. In this paper risk management and some concepts focusing specifically on such plants for dusty, granular and combustible products were introduced. Some examples were presented here for practical uses, which can be applied in dusty and complicated surroundings.