Spray Fluidized Bed Agglomeration Research Articles

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.

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.

Multiscale Simulation of Agglomerate Breakage in Fluidized Beds

In this contribution a multiscale simulation strategy is proposed which is able to simulate an industrial scale fluidized bed spray agglomeration process considering the breakage of agglomerates. A set of novel simulation approaches and hierarchically distributed models were developed and implemented into the multiscale simulation environment for solids processes. The population balance model (PBM) was used for the simulation of the global production process on the macroscale. On the microscale the coupling between discrete element method (DEM) and the computational fluid dynamics (CFD) system was employed to calculate the particle dynamics in the granulator. The material-based parameters for the PBM, such as breakage probability and breakage function, were derived from the process description on the lowest hierarchical scale, where the agglomerate was described as a system of primary particles bonded by a solid binder.

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.

A novel, structure‐tracking monte carlo algorithm for spray fluidized bed agglomeration

AbstractA stochastic modeling approach to fluidized bed agglomeration is presented. First, the fundamentals of a general Monte Carlo algorithm are discussed, namely the correlation of real with simulation time and the size of the simulation box. Two different approaches regarding the morphological description of particle structure development are then compared for different theoretical test cases. The first approach (concept of positions) is a fast and effective geometrical discretization to follow properties such as primary particle number per agglomerate throughout the simulation. The second approach is a new three‐dimensional (3‐D) structure algorithm, which allows a complete, unrestricted description of the spatial evolution of particles related to physical microprocesses. The 3‐D algorithm enables to apply distributed particle and droplet sizes, which is not possible in the concept of positions. In the perspective, the structural growth of particles can be connected to process conditions, opening new possibilities for process and product design. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3016–3029, 2012

Kinetics of fluidized bed spray agglomeration for compact and porous particles

The effect of the primary particle porosity during the formation of agglomerates in spray fluidized beds is presented in this study. The method is based on the single micro-interactions occurring within the fluidized bed such as inter-particle collisions, droplet spread on the particle surface, aging of the deposited droplets and particle coalescence. The porous character of the particles is expected to directly affect the aging process of the deposited binder layer by penetration into the pores of the substrate. The droplet penetration process is experimentally analyzed by single droplet deposition on spherical, porous alumina particles. The results indicate that the penetration process is mainly governed by the viscosity of the liquid and that at relatively low viscosities, droplet penetration is fast. For highly viscous liquids, the penetration velocity slows down and an additional mechanism, namely drying becomes important. A combined imbibition–drying model is developed and included into a comprehensive stochastic agglomeration model that allows the simulation of agglomerate formation in a batch process. Lab-scale agglomeration experiments with porous and non-porous particles are carried out in an attempt to validate the general tendencies predicted by the main agglomeration model. The results show that the agglomeration rate for porous particles is significantly reduced due to the losses of deposited droplets into the pores of the primary particles; this tendency is much more pronounced at low binder viscosities.

Investigation of the kinetics of fluidized bed spray agglomeration based on stochastic methods

AbstractParticle formation during fluidized bed spray agglomeration is modeled by a Monte Carlo method. The methodology is based on the micromechanisms occurring within the bed. The most important model parameters are identified as interparticle collision time, deposited droplet drying time and droplet addition time. It is found that a high number of collisions leads to a negligible role of the drying mechanism. In the real bed, however, the process is dependent on the gas inlet temperature. This indicates that the number of collisions relevant to agglomeration is relatively low. The accordance of the model with experimental results for variations of several process parameters demonstrates that the approach is a promising way to simulate the formation of agglomerates. In addition, the model is able to reproduce slower agglomeration at increased temperatures. This result is, for the first time, based on physical mechanisms rather than on the use of fitted agglomeration kernels. © 2011 American Institute of Chemical Engineers AIChE J, 2011

Stochastic simulation of agglomerate formation in fluidized bed spray drying: A micro-scale approach

A stochastic model that describes agglomerate growth during fluidized bed spray agglomeration is presented and numerically solved by constant volume Monte Carlo method. The methodology overcomes the difficulties of solving multivariate population balance equations and includes continuous binder addition and drying. Agglomerate formation is treated as a complex combination of consecutive and parallel micro-mechanisms. Due to the discrete nature of the approach, the individual role of the micro-mechanisms on the agglomeration behavior can be analyzed. The results suggest that the droplet capture mechanism governs the agglomeration speed while the maximum agglomerate diameter is ruled by the equilibrium reached between coalescence, rebound and breakage. The mechanism of deposited binder drying is found to play a negligible role on agglomerate formation because of an extremely rapid droplet consumption. The main process variables affecting each micro-mechanism have been identified showing that the liquid spraying rate affects directly the droplet capture mechanism whereas binder properties influence mainly the agglomeration and rebound interactions. The model presented in this study is able to predict qualitatively the experimentally observed response of the system as well as the general shape of the agglomerate size distribution under the variation of several process parameters, demonstrating the potential of the discrete micro-level approach.

A generic population balance model for simultaneous agglomeration and drying in fluidized beds

Fluidized bed spray agglomeration is a common process of particle size enlargement in process engineering. The objective of this study is to derive a general fluidized bed model, which takes the particle size enlargement by agglomeration and the simultaneous drying of particles into account. Therefore, a population balance approach is used to predict the size distribution, the moisture content and the temperature of the particles. The interaction of solid and gas phases is described by means of a heterogeneous fluidization model with active bypass. Selected simulation results are presented and discussed. The model is validated by various comparisons of simulations and batch agglomeration experiments.

A new technique to determine rate constants for growth and agglomeration with size- and time-dependent nuclei formation

Fluidized bed spray agglomeration is a particle size enlargement process where particles stick together with the help of spraying binder. High impact forces between particles lead to attrition. Attrition may be modeled as poly-disperse nucleation. Furthermore, particulate event like over-spray leads to the formation of particles in a wide range of volume. A new technique for the determination of agglomeration, growth and nucleation parameters is presented in this work. The model is based on a previous approach which takes mono disperse nuclei formation in the smallest class into account. Frequently in crystallization processes, nucleation is assumed to be mono-disperse. The technique presented here incorporates nuclei formation in a certain range of volume. It is quite general and applicable to consider size- and time-dependent nuclei formation. For two particular cases of growth and agglomeration including size-dependent nuclei formation, simulation data was generated by continuous feeding of nuclei in a certain range to demonstrate the capability of parameter extraction of the model. Further, the new technique is applied to extract rate constants from experimental data measured during fluidized bed spray agglomeration. This technique is also useful for the prediction of bimodal behavior of particle size distributions (PSD).

Investigation of coalescence kinetics of microcristalline cellulose in fluidised bed spray agglomeration: experimental studies and modelling approach

In this paper a model for fluidized bed spray agglomeration is presented. To describe the processes of heat and mass transfer, a physical based model is derived. The model takes evaporation process from the wetted particles as well as the effects of transfer phenomena between suspension gas and bypass gas into account. The change of particle size distribution during agglomeration, modeled by population balances, is linked to the heat and mass transfer model. A new technique is derived to extract agglomeration and nucleation rates from experimental data. Comparisons of experiments and simulations are presented.