Coupled CFD-PBE Simulation of Nucleation in Fluidized Bed Spray Granulation

Fluidized bed spray granulation is used to produce porous granular particles from suspensions, solutions, and melts. It is the general aim of our work to provide a physics-based simulation tool for this process. For this the process-relevant mechanisms such as droplet deposition, dust integration, and drying are implemented using computational fluid dynamics (CFD) to generate growth kinetics of the granules and of the dust particles. The latter gives rise to the nucleation rate. This kinetics is applied to simulate the development of the particle size distribution of granules for a continuously operated fluidized bed spray granulation process using population balance equations of the granules.     

Kinetics of fluidized bed melt granulation—II: Modelling the net rate of growth

Previous work [Tan, H.S., Goldschmidt, M.J.V., Boerefijn, R., Hounslow, M.J., Salman, A.D., Kuipers, J.A.M. (2004a). Building population balance for fluidized bed granulation: lessons from kinetic theory of granular flow. Powder Technology, 142, 103-109] shows that we can derive an aggregation kernel (equi-partition of kinetic energy (EKE)) on the basis of the kinetic theory of granular flow and use it effectively to describe the net granule growth in fluidized bed melt granulation (FBMG). In this paper, we incorporate the EKE kernel into a population balance model to extract the effective aggregation rate constant that accounts for the net granule growth for the series of FBMG experiments shown in Tan, et al. [(2004b). Kinetics of fluidized bed melt granulation I: effect of process variables, Chemical Engineering Science, to be submitted]. These extracted rate constants are subsequently expressed as a function of different operating condition. The results consistently show that the aggregation rate constant increases in direct proportion with binder spray rate, from where we conclude that the rate of granule formation is directly dependent on the amount of binder available for aggregation per unit time. The aggregation rate constant was also observed to increase with higher bed temperature when a higher viscosity binder was used, but showed a maximum value for a less viscous binder as a function of temperature. The aggregation rate was also seen to be faster when granulating using a larger droplet size and at a lower fluidizing air velocity. The observations in the rate constant plot can be effectively explained by the physical parameters in the EKE model and the sequence of rate events proposed in Tan, et al. [(2004b). Kinetics of fluidized bed melt granulation I: effect of process variables, Chemical Engineering Science, to be submitted].     

Population balance modelling for a flow induced phase inversion based granulation in a two-dimensional rotating agglomerator

A novel two-dimensional rotating agglomerator was developed to carry out the flow induced phase inversion (FIPI) based granulation. The process in this agglomerator shows that a continuous paste flow (mixed with liquid binder and primary particles) is extruded into the interstice of two relatively rotating disks, as the paste becomes solidified due to the loss of heat to the disks, it is then broken into granules by the shearing force imposed by the rotating disk. Experimental measurements have shown that the size of these granules is enlarged along the positive radial direction of the disks. It is also found that these granules contain approximately the same quantity of binder in terms of its volume fraction. The paper thus proposes a population balance (PB) model to describe the growth of the granules by considering a size independent agglomeration kernel. The PB simulated results are found to be well capable of describing the change of the particle size distribution (PSD) of the granules in the radial direction. This study also proposes a velocity profile for the paste flow and attempts to establish a quantitative relationship between the granulation rate and the deformation rate as this would help us understand the mechanism of the agglomeration. It is hoped that this study would be used to improve the design of the agglomerator and to assure the control of the process and the granular product quality.     

Stochastic Modeling of Fluidized Bed Agglomeration: Determination of Particle Moisture Content

The present study describes the theoretical modeling of particle formation by agglomeration in fluidized beds. The model is mathematically solved by applying a stochastic Monte Carlo method. It takes into account the complete droplet history (predrying during flight, spreading, solidification and penetration after deposition on a particle) so that thermal effects such as the increase in fluidization gas mass flow or gas temperature have a physically based impact on process kinetics. Contradictions between measured and simulated particle moisture contents were found. Thus, we present a modified drying model and compare results with experimental data.     

Agglomeration on drying of yttria-stabilised-zirconia slurry on a metal substrate

Drying behaviour of aqueous yttria-stabilised-zirconia (YSZ) slurry on a metal substrate has been investigated by examining drying kinetics, shrinkage rate and cracking behaviour as a function of agglomeration degree. The agglomeration behaviour of the slurry was varied by changing its pH value, and evaluated by measuring both zeta-potential and viscosity. The drying kinetics of the slurries on substrates was characterized with weight loss and classified as an initial constant rate period (CRP) followed by a falling rate period (FRP). Uneven thickness reduction, measured using optical microscope, occurred in the CRP stage due to lateral flow, and became more uniform with increase in agglomeration degree. Tensile stresses developed within the coating because of the substrate constraint and resulted in crack formation and propagation. Cracking behaviour was controlled by the agglomeration degree of the slurry.     

The Use of a Population Balance Model in the Study of Inoculation of Soybean Seeds in a Spouted Bed

The coating of soybean seeds with bacteria and micronutrients favours the vigorous growth of the plant, dispensing with the use of ammoniacal fertilizers. The optimum thickness of the coating should allow the fundamental gaseous interchanges for the germination and the ideal conditions for the activity of the bacteria. The objective of this work was to simulate dynamic mass distribution of soybean seeds covered with bacteria and macronutrients in spouted bed through a specific program developed in Maple V^®. The validation of the model is done through 16 experiments carried out in a spouted bed with spray at the top.     

Drying and Heat Transfer Characteristics for a Novel Fluidized Bed Dryer

Abstract

The use of a fluidized bed dryer with a lateral air flow and mechanical agitation to the drying of sludge from a wastewater treatment plant was investigated. Experimental curves of moisture content vs. drying time, as well as heat transfer coefficients and the size characteristics of the products, were determined at temperatures between 80°C and 110°C, a stirring rate of 55 rpm and air velocity of 0.9 m/s for 3 kg sludge batches with initial moisture contents of 0.55 and 0.65 (d.b.). Experimental drying kinetics were compared with values derived from three models based on Fick's second law, namely: the constant diffusivity model, the simplified variable diffusivity model, and the modified quasi-stationary model.     

Agglomeration and breakage of nanoparticles in stirred media mills—a comparison of different methods and models

The increasing industrial demand for nanoparticles challenges the application of stirred media mills to grind in the sub-micron size range. It was shown recently [Mende et al., 2003. Mechanical production and stabilization of submicron particles in stirred media mills. Powder Technology 132, 64-73] that the grinding behavior of particles in the sub-micron size range in stirred media mills and the minimum achievable particle size is strongly influenced by the suspension stability and thus the agglomeration behavior of the suspension. Therefore, an appropriate modeling of the process must include a superposition of the two opposing processes in the mill i.e., breakage and agglomeration which can be done by means of population balance models. Modeling must now include the influence of colloidal surface forces and hydrodynamic forces on particle aggregation and breakup. The superposition of the population balance models for agglomeration and grinding with the appropriate kernels leads to a system of partial differential equations, which can be solved in various ways numerically. Here a modified h-p Galerkin algorithm which is implemented in the commercially available software package PARSIVAL developed by CiT (CiT GmbH, Rastede, Germany) and the moment methodology according to [Diemer and Olsen, 2002a. A moment methodology for coagulation and breakage problems: Part I—analytical solution of the steady-state population balance. Chemical Engineering Science 57 (12), 2193-2209; Diemer and Olsen, 2002b. A moment methodology for coagulation and breakage problems: Part II—moment models and distribution reconstruction. Chemical Engineering Science 57 (12), 2211-2288] are used and compared to explicit data on alumina. This includes a comparison of the derived particle size distributions, moments and its accuracy depending on the starting particle size distribution and the used agglomeration and breakage kernels. Finally, the computational effort of both methods in comparison to the prior mentioned parameters is evaluated in terms of practical application.     

Process Design of a Multistage Drying Process via Flowsheet Simulation

Multistage drying processes including spray drying and fluidized bed unit operations are challenging to design due to the dependencies of the preceding process steps. Flowsheet simulation offers the possibility to simulate a complete process by the application of suitable short-cut models. In this contribution a novel model for a spray dryer based on a population balance approach is presented. Furthermore, a dynamic model for a fluidized bed dryer is introduced. Experimental data is used for validation and parameter optimization. The calibrated models are then used to design an industrial drying process of multiple drying stages.     

Multidimensional Population Balance Modeling for Pathological Calcium Pyrophosphate Dihydrate Crystals in the Presence of Amino Acids

A multidimensional population balance model was applied to elaborate the effects of amino acids on the crystallization mechanisms of calcium pyrophosphate dihydrate crystals. Breakage and growth terms defined in the model were expressed in two dimensions depending on the sizes. The exponential hyperbolic function based on the particle sizes was selected for expressing the growth. The model was resolved by using the finite differences method. It was detected that in the non-additive media, the growth along the length-1 is extremely dominant against both the growth along the length-2 and the breakage. However, it was discovered that the amino acids made the crystals more fragile, and the breakage at certain size ranges and certain concentrations of amino acids was the dominant mechanism. In any case, it was proven that both growth and breakage are suppressed considerably in the amino acid-containing media.     

CFD Simulation on Drying and Dust Integration in Fluidized Bed Spray Granulation

Fluidized bed spray granulation is used to produce porous granular particles from suspensions, solutions, and melts. In this work, the drying of the spray and the drying of the particles in the fluidized bed are simulated using the Two Fluid Model (TFM) with the Eulerian-Eulerian approach. Using the wetted surfaces of the particle, the dust integration is implemented. The model is used to simulate the particle growth for a semi-batch fluidized bed spray granulation process.     

A Grade Transition Strategy for the Prevention of Melting and Agglomeration of Particles in an Ethylene Polymerization Reactor

To satisfy the diverse product quality specifications required by the broad range of polyethylene applications, polymerization plants are forced to operate under frequent grade transition policies. During the grade transition, the reactor temperature must be kept within the narrow range between the gas dew point and the polymer melting point, otherwise the particles melt or agglomerate inside the reactor. In the present study, a dynamic well‐mixed reactor model is used to develop a grade transition strategy to prevent melting and agglomeration of particles in an ethylene polymerization reactor. The model predicts the conditions under which the temperature of the reactor is outside the allowable range in continuous grade transition. Manipulation of feed flow and cooling water flow rates has shown that the reactor temperature cannot be maintained within the allowable range. Hence, a semi‐continuous grade transition strategy is used for this case so that the temperature is maintained within the allowable range. In addition, several continuous and semi‐continuous grade transition strategies for the production of linear low‐density polyethylene (LLDPE), medium density polyethylene (MDPE), and high‐density polyethylene (HDPE) are compared.     

Particle population modeling in fluidized bed-spray granulation—analysis of the steady state and unsteady behavior

Owing to its intensive mass and heat transfer ratios and its coupling of the process stages of drying, shaping and homogenization as well as classification, continuous fluidized bed-spray granulation drying has gained acceptance as a thermal treatment process for granular solids. In this study, a balance of the particle populations is completed for a continuous fluidized bed-spray granulation with external classification. Thus, it ought to be possible to describe the particle size distributions changing over time in the fluidized bed and in the product flow [Powder Technol. 82 (1995) 37; H. Uhlemann, L. Mörl, Wirbelschicht-Sprühgranulation, Springer Verlag, 2000, ISBN 3-540-66985-X.].     

Modeling Particle Formation at Spray Drying Using Population Balances

There have been several efforts to simulate the physical processes in a single droplet during spray drying in the last several years, but most of the models do not describe the solid formation in detail. In this work, the development of the microscopic solid structure in a droplet during spray drying is simulated. A radial-symmetric model of the droplet is used to simulate the mass and heat transport. The solid formation at every radial discretization point is obtained by the solution of population balances. This way, the distribution of the particle number density in the droplet depending on the macroscopic process parameters can be predicted. The model equations are solved in a normalized coordinate system to be able to describe the shrinkage of the droplet. The suitability of these population balances will also be discussed. For the validation, monodisperse single droplets consisting of a solution or suspension are dried with constant boundary conditions.     

Modeling Particle Formation at Spray Drying Using Population Balances

There have been several efforts to simulate the physical processes in a single droplet during spray drying in the last several years, but most of the models do not describe the solid formation in detail. In this work, the development of the microscopic solid structure in a droplet during spray drying is simulated. A radial-symmetric model of the droplet is used to simulate the mass and heat transport. The solid formation at every radial discretization point is obtained by the solution of population balances. This way, the distribution of the particle number density in the droplet depending on the macroscopic process parameters can be predicted. The model equations are solved in a normalized coordinate system to be able to describe the shrinkage of the droplet. The suitability of these population balances will also be discussed. For the validation, monodisperse single droplets consisting of a solution or suspension are dried with constant boundary conditions.     

Co-Combustion of Oil Sludge Char and Brown Coal in a Continuous Fluidized-Bed Combustor

For developing technologies for innocent disposal of/energy recovery from high-ash oil sludge char (OSC), it was continuously combusted in a 10 kg h⁻¹ fluidized-bed combustor. Addition of brown coal (BC) improved its combustion, but excessive BC was prone to cause slagging. Fine coke particles in OSC and volatiles in BC led to staged temperatures along the fluidized bed through air-staging co-combustion, enabling excellent combustion performance as well as efficient NO_x reduction, especially when coupled with selective non-catalytic reduction. Thus, there is a potential to cleanly recover energy from OSC by its co-combustion with BC.     

Formation of bed agglomeration in a fluidized multi-waste incinerator

A case study was carried out to investigate the bed agglomeration observed in a fluidized bed incinerator when burning blends of three wastes (carbon soot, biosludge and fuel oil). Several instrumental approaches were employed (i.e. XRF, SEM, XRD, and ICP-AES) to identify the bed materials (fresh sand and degrader sand) and clinkers formed in the full-scale incinerator tests. Several elements (V, Al, S, Na, Fe, Ni, P, and Cl), which normally are associated with the formation of low melting point compounds, were found in the waste blends at high content levels. The clinker bridges were identified to be associated with Al, Fe, V, K, Na, S, Ni, and Si elements.The effects of temperature and blending ratio were investigated in a muffle furnace. Carbon soot is believed to be more susceptible to the clinker formation than the other two fuels. Thermodynamic multi-phase multi-component equilibrium calculations predict that the main low melting point species could be Al₂(SO₄)₃, Fe₂(SO₄)₃, Na₂SO₄, NaCl, Na₂SiO₃ and V₂O₅. This information is useful to understand the chemistry of clinker formation. Also, it helps to develop methods for the control and possible elimination of the agglomeration problem for the design fuels.     

Population Balance Model (PBM) for flocculation process: Simulation and experimental studies of palm oil mill effluent (POME) pretreatment

The present study is intended for the first time to completely replace the inorganic coagulants with organic polymers in palm oil mill effluent (POME) pretreatment by using direct flocculation of single and dual polymer systems under applied shear. The efficiency of direct flocculation of POME was investigated by using the Population Balance Model (PBM) which considered the charge neutralization and bridging attraction under applied shear. The collision efficiency was calculated based on the Derjaguin Landau Verwey Overbeek (DLVO) theory which considered the effect of adsorbed polymer layers on van der Waals attraction and bridging attraction. This is the first attempt to correlate the floc size distribution from PBM to the indirect indicators of COD, suspended solids, oil and grease. The model predictions are in close agreement with the experimental results for both single and dual polymer systems. The interaction energy curves based on PBM shows that the flocculation using cationic polymer is by charge neutralization and bridging attraction whereas flocculation using anionic polymer is by only bridging attraction. At the optimum flocculation conditions, 99.66%, 55.79%, 99.74% and 80.78% of suspended solids, COD, oil and grease removal and water recovery are achieved, respectively. The direct flocculation process significantly reduced the treatment cost by a factor of 3.6 compared to the conventional coagulation–flocculation process.