Advanced Granulation Theory at Particle Level (original) (raw)
Related papers
Coalescence of deformable granules in wet granulation processes
AIChE Journal, 2000
In this work, the coalescence of deformable granules in wet granulation processes is modelled. The model accounts for both the mechanical properties of the granules and the effect of the liquid layer at the granule surface. It is an extension to the model of ( ) Ennis et al. 1991 to include the possibility of granule plastic deformation during collisions. The model is written in dimensionless groups such as ®iscous and deformation Stokes numbers and the ratio of granule dynamic yield strength to granule Young's ( U ) modulus Y rE . These ®ariables are bulk parameters of the powder-binder mixture d and also functions of the process intensity. The model gi®es the conditions for two types of coalescence ᎐ type I and type II. Type I coalescence occurs when granules coalesce by ®iscous dissipation in the surface liquid layer before their surfaces touch. Type II coalescence occurs when granules are slowed to a halt during rebound, after their surfaces ha®e made contact. The model explains some of the trends obser®ed in the literature, are preliminary ®alidation of the coalescence criterion with drum granulation data is encouraging. An extension is also made to the case of surface dry granules, where liquid is squeezed to the surface during granule deformation.
Unifying approach to modeling granule coalescence mechanisms
AIChE Journal, 1997
A novel, physically based kernel for population balance modeling of granule growth by coalescence is presented. This kernel is size-independent in that all collisions with an effective average granule sue less than a critical value are successful. Simulations based on this kernel show that a variety of contradictory experimental observations can be modeled. In the limiting case of viscoelastic collisions, the kernel can be related to the governing group of the Stokes number (Ennis et al., 1991), representing the ratio of granule collisional kinetic energy to viscous dissipation brought about by the binder. In more general cases, material properties that control deformability, such as interparticle fiction, binder viscosiy, and liquid content, strongly affect this critical size. The kernel clearly demonstrates the three regimes of drum granulation originally proposed by and compares favorabb with the two-stage sequential kernel developed by for the drum granulation of fertilizers.
Powder Technology, 2020
This paper presents a novel approach to the determination of the coalescence kernel for population balance modelling of a general class of batch fluidised bed aggregation systems, using a continuous sprayed-in liquid binder. Coalescence requires inter-particle collisions, the wetting of the contact surfaces and the dissipation of particle kinetic energy by the viscous squeezing of the binder film. These three sub-processes in principle depend both on the size of the contacting particles and on time. A new time-dependent aggregation rate constant of the coalescence kernel has been formulated by considering the general evolution of inter-particle collision behaviour with time. The model is implemented in MATLAB and its numerical output compared to two sets of experimental data: the granulation of glass beads with polyethylene glycol as a binder and the granulation of semolina with water. The evolution of mean and standard deviation in diameter versus time are examined as is the state of the size distribution at different stages in the process. The time-averaged aggregation rate for the glass beads evaluated as 7.59 × 10 −9 m-0.5 s −1 while for the semolina it was 3.86 × 10 −9 m-0.5 s −1. The agreement between numerical predictions and experiment is shown to be good, demonstrating the validity of the approach. Whilst conceptually simple, the model generates realistic output and provides a powerful insight into the underlying mechanisms of granulation.
Balling and granulation kinetics revisited
International Journal of Mineral Processing, 2003
Balling of finely comminuted solids by random coalescence and granulation of iron ore fines and other minerals by autolayering are two major size enlargement processes. The existing kinetic model for random coalescence does not take into account the strong dependence of coordination number on the size distribution of agglomerating entities. We present a coordination number based coalescence model, which mimics the underlying physical process more realistically. Simulations show that in spite of highly diverse model structures, random and coordination coalescence models give remarkably similar results. Only static models of autolayering are available presently. These map the input size distribution of feed solids into steady state or terminal size distribution of granules, with little or no information on the path traversed by the process. We propose a continuous-time dynamic model of autolayering within the population balance framework. The model, which is based on the proportionate growth postulate of autolayering, agrees reasonably well with experimental data.
Chemical Engineering Research and Design, 2009
In this study, a dynamic model is presented for the granulation process, employing a three-dimensional population balance framework. The major focus of this work is the theoretical development and experimental validation of a novel mechanistic breakage kernel that is incorporated within the population balance equation. Qualitative validation of breakage kernel/model was first performed and trends of lumped properties (i.e., total particles, average size, binder content and porosity) and distributed properties (i.e., granule size and fractional binder content) show good agreement with the expected phenomenological behaviour. Successful high-shear mixer granulation experiments using glass-ballotini as the primary powder and poly-vinyl alcohol in water (PVOH-H 2 O) as the liquid binder were then carried out to mimic predominantly breakage-only behaviour whereby the rate of breakage was greater than the rates of nucleation and aggregation. Good agreement between experimental and simulation results were obtained for the granule size distribution under different operating conditions. In addition, accurate model predictions were obtained for the evolution of the lumped properties.
Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review
Powder Technology, 2001
Wet agglomeration processes have traditionally been considered an empirical art, with great difficulties in predicting and explaining observed behaviour. Industry has faced a range of problems including large recycle ratios, poor product quality control, surging and even the total failure of scale up from laboratory to full scale production. However, in recent years there has been a rapid advancement in our understanding of the fundamental processes that control granulation behaviour and product properties. This review critically evaluates the current understanding of the three key areas of wet granulation processes: wetting and nucleation, consolidation and growth, and breakage and attrition. Particular emphasis is placed on the fact that there now exist theoretical models which predict or explain the majority of experimentally observed behaviour. Provided that the correct material properties and operating parameters are known, it is now possible to make useful predictions about how a material will granulate. The challenge that now faces us is to transfer these theoretical developments into industrial practice. Standard, reliable methods need to be developed to measure the formulation properties that control granulation behaviour, such as contact angle and dynamic yield strength. There also needs to be a better understanding of the flow patterns, mixing behaviour and impact velocities in different types of granulation equipment. q S.M. Iveson . ( ) S.M. IÕeson et al.r Powder Technology 117 2001 3-39 6
Predicting granule behaviour through micro-mechanistic investigations
International Journal of Mineral Processing, 2003
A novel micro-manipulator device has been developed to observe and measure, directly, the behaviour of binder liquid bridges between pairs of solid particles. The objective has been to develop the fundamental understanding of the role of the liquid and solid properties in the growth and consolidation of granules, from the initial contact between the liquid and particles to the resultant multi-particle bodies. On the particle level, it is the liquid bridges that are responsible for the strength of ''wet'' agglomerates, since they hold the particles together. In this paper, results of experiments will be reported that identify the role of liquid surface tension, bridge Laplace pressure, liquid viscosity and, hence, wetting behaviour, in the axial strength of the bridges. In particular, the differences in bridge shape when particles of different surface properties come together (i.e. in mineral mixtures) provides a crucial insight into whether granules will grow successfully or not. A parabolic approach to describing the shapes adopted by the liquid bridges, from which parameters such as resistance to deformation can be calculated, will be shown. From the theoretical behaviour of individual bridges, determined through the direct experimental observations, a simple model has been constructed, which relates granule porosity, liquid content and the physicochemical properties of the materials to the agglomerate hardness. Some experimental measurements using spherical particles and powders commonly granulated in the pharmaceutical industry, such as lactose, will be compared to the model predictions and the role of interparticle friction and liquid surface tension and viscosity will be shown quantitatively.
The kinetics of the granulation process: Right from the early stages
2009
In this study, experimentation and modelling were carried out to understand the granulation process. This work assesses whether there is a significant difference in the aggregation rate of the wet granulation process between the very early stages and later stages of high shear granulation. Measurements of the size distribution and binder content from the beginning of the process, just after liquid binder addition, were carried out. A population balance model based on two different kernels, the Equi Kinetic Energy (EKE) kernel and two-dimensional population balance equations with a Size Independent (SI) kernel, was applied to the high shear granulation process. It was concluded that the population balance equations with SI kernel best described the aggregation in the high shear granulation process. The value of aggregation rate constant in the early stages is smaller than aggregation kernel in the later stages.
The importance of wet-powder dynamic mechanical properties in understanding granulation
Powder Technology, 2003
Granule impact deformation has long been recognised as important in determining whether or not two colliding granules will coalesce. Work in the last 10 years has highlighted the fact that viscous effects are significant in granulation. The relative strengths of different formulations can vary with strain rate. Therefore, traditional strength measurements made at pseudo-static conditions give no indication, even qualitatively, of how materials will behave at high strain rates, and hence are actually misleading when used to model granule coalescence. This means that new standard methods need to be developed for determining the strain rates encountered by granules inside industrial equipment and also for measuring the mechanical properties of granules at these strain rates. The constitutive equations used in theoretical models of granule coalescence also need to be extended to include strain-rate dependent components. D 2002 Elsevier Science B.V. All rights reserved.