Bryan Ennis

Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review

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

Fundamental studies of granule consolidation Part 1: Effects of binder content and binder viscosity

Granule consolidation was studied experimentally using a 0.3 m diameter laboratory granulation drum with fine glass ballotini as the model powder and glycerol-water mixtures as model liquid binders. Granule consolidation during tumbling was found to be a complex process controlled by the balance between the different mechanisms that resist granule deformation: interparticle friction and viscous dissipation. The rate of consolidation decreased with decreasing particle size. As liquid content increased, interparticle friction effects decreased but viscous losses became more significant. Thus, the effects of binder viscosity and liquid content were highly interactive. Unless the balance between the two mechanisms is accurately known for a given system, the effect of changes to binder parameters on granulation behaviour cannot be predicted, even qualitatively. To overcome these difficulties a new methodology for relating formulation properties to granulation behaviour is suggested based on bulk powder properties measured by triaxial consolidation tests and the development of a new granulation criterion for deformable granules. A procedure for testing critically the proposed methodology is presented.

Population balance modelling of drum granulation of materials with wide size distribution

Abstract A population balance model is developed to describe the drum granulation of feeds with a broad size distribution (e.g. recycled fertiliser granules). Granule growth by coalescence is modelled with a sequential two-stage kernel. The first stage of granulation falls within a non-inertial regime as defined by Ennis et al. ( Powder Technol., 65 (1991) 257–272), with growth occurring by random coalescence. The size distribution is observed to narrow and quickly reach an equilibrium size distribution. Further growth then occurs within a second inertial stage of granulation in which the granule size distribution broadens and requires a size-dependent kernel. This stage is much slower and granule deformation is important. Non-linear regression is used to fit the model to the experimental data of Adetayo et al. ( Chem Eng. Sci., 48 (1993) 3951–3961) for granulation of ammonium sulfate, mono-ammonium phosphate and di-ammonium phosphate for a range of moisture contents, granulation times and initial size distributions. The model accurately describes the shape of the granule size distributions over the full range of data. The extent of granulation occurring within the first stage is given by k 1 t 1 ; the extent of growth k 1 t 1 is proportional to the fractional liquid saturation of the granule, S sat , and increases with binder viscosity. Here, k 1 represents the rate constant for the first stage of growth and t 1 represents the time required to reach the final equilibrium size distribution for the first stage. Changes to the initial size distribution affect k 1 t 1 by changing granule porosity and, therefore, liquid saturation. A critical saturation, S crit , is necessary for the second stage of granulation to occur, leading to further growth. For S sat ≤ S crit , a final equilibrium size distribution is reached before 5 min of granulation time. For S sat > S crit , granules are sufficiently deformable to continue growing for up to 25 min. S crit decreases with increasing binder viscosity. This model is suitable for use in dynamic simulation of granulation circuits where both moisture content and recycle size distribution may vary significantly with time.

Model-based control of a granulation system

Abstract Granulation, the process by which granules are made from powdered, slurried, solution or molten feed material, is an important process in many industries. The main objective in the granulation process is to produce granules with consistent product quality, as indicated by various industry standard variables which can be related to two fundamental process quantities: particle size distribution and bulk density. While it is customary to specify a desired setpoint value for bulk density, the specifications on particle size distribution typically take the form of an upper limit (dU) and a lower limit (dL) determined by the screen sizes used in product classification. The paper discusses the peculiar control problems arising from such a combination of product quality specifications and then develops a model-based control scheme which systematically addresses the problems. Simulation studies illustrate the control system implementation and performance for various situations of practical significance.

Agglomeration and size enlargement : session summary paper

Abstract This paper provides a summary of the Agglomeration and Size Enlargement session of the First International Particle Technology Forum held in Denver, CO, USA, August 17–19, 1994. The conference was sponsored by the recently formed Particle Technology Forum of the American Institute of Chemical Engineers. This first session on agglomeration and size enlargement brought together a cross-section of industries dealing with agglomeration processes. Represented were the areas of mineral processing, consumer products, ceramics, industrial chemicals, agricultural chemicals and fertilizers, biological materials, and pharmaceutical processing. The session focused on agitative granulation techniques including pan, fluidized bed, drum, and mixer granulation as well as compaction processes including tabletting, uniaxial compaction, and roll pressing. Two critical themes running throughout the session were (i) relating particle and formulation properties such as friction, plasticity, interfacial energy, and binder viscosity to bulk agglomeration behavior, and (ii) the incorporation of these relationships into process scale simulations. The aim of this endeavor was to share our different approaches to agglomeration processes and through this sharing achieve a cross-fertilization of ideas. It is hoped that this effort, and similar sessions to follow, will increase our understanding of agglomeration phenomena which can be utilized for the rational design and optimization of size enlargement processes as well as product design. This review paper summarizes the technical papers presented at this session as well as the subsequent discussion which followed (B.J. Ennis (ed.), Proc. 1st Int. Particle Technology Forum, Vol. 1, American Institute of Chemical Engineers, New York, pp. 155–286).

6 – Fluidized Bed Coating and Granulation

This chapter discusses the fluidized bed coating and granulation. Fluidized beds, both with and without internals, offer many advantages over conventional granulation and coating equipment such as pans, drums, and mixers. Fluidized beds, by virtue of the air or gas required to fluidize the solids, typically have high rates of heat and mass transfer leading to uniform temperature distribution within the bed and relatively short processing times. Moreover, the shearing forces exerted in such beds help to control the formation of agglomerates, and the movement of fluidizing gas, including bubbles, which causes solids to circulate within the equipment providing a constant flow of bed particles through the spray zone, which is essential for uniform product quality. The main goal of this chapter is to provide a technical basis or framework for analyzing fluidized bed coating and granulation operations.

On wear as a mechanism of granule attrition

Abstract Mechanisms of granule or agglomerate attrition are studied in the light of the principles of fracture mechanics. Fracture properties are measured for agglomerated systems including both glass beads bound by polymeric binders and pesticide products. In the present work, the sizes of typical granules are less than the critical specimen sizes capable of gross fracture, as calculated from fracture theory. This implies that the mode of granule attrition is primarily one of erosion of abrasive wear and not gross fracture. Previous abrasive wear research is reviewed, with the aim of establishing the dependence of agglomerate bar wear on material properties. Idealized bar agglomerates are studied, as they allow convenient characterization of the dependence of granule erosion on material properties. Bar wear rates are foud to parallel results from the ceramics wear literature. In particular, wear rate is found to have a similar but somewhat different dependence on fracture toughness than the work of Mullier et al.4, where both fluid-bed granule erosion rate and wear of bar agglomerates were found proportional to 1/Kc.

Breakage and Attrition

This chapter considers the last of the three classes of granulation processes that control granule attributes — breakage and attrition. There are really two separate phenomena to consider here: 1. Breakage of wet granules in the granulator; and 2. Attrition or fracture of dried granules in the granulator, drier or in subsequent handling.

Wetting, Nucleation and Binder Distribution

In Chapter 1, we divided granulation rate processes into three classes: wetting and nucleation, consolidation and coalescence, and breakage and attrition. Chapters 3 to 5 address each of these classes in turn beginning with wetting and nucleation.

Particle and Granule Morphology

Good particle design means understanding particle-particle and particle-fluid interactions and using this knowledge to properly design processes and products. It is the properties of the particle that dictate these interactions so particle characterisation is at the heart of particle design. Good characterisation of both the feed powders and the product granules is essential.