James D. Litster

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.

Growth regime map for liquid-bound granules: further development and experimental validation

An attempt was made to quantify the boundaries and validate the granule growth regime map for liquid-bound granules recently proposed by Iveson and Litster (AlChE J. 44 (1998) 1510). This regime map postulates that the type of granule growth behaviour is a function of only two dimensionless groups: the amount of granule deformation during collision (characterised by a Stokes deformation number, St(def)) and the maximum granule pore saturation, s(max). The results of experiments performed with a range of materials (glass ballotini, iron ore fines, copper chalcopyrite powder and a sodium sulphate and cellulose mixture) using both drum and high shear mixer granulators were examined. The drum granulation results gave good agreement with the proposed regime map. The boundary between crumb and steady growth occurs at St(def) of order 0.1 and the boundary between steady and induction growth occurs at St(def) of order 0.001. The nucleation only boundary occurs at pore saturations that increase from 70% to 80% with decreasing St(def). However, the high shear mixer results all had St(def) numbers which were too large. This is most likely to be because the chopper tip-speed is an over-estimate of the average impact velocity granules experience and possibly also due to the dynamic yield strength of the materials being significantly greater than the yield strengths measured at low strain rates. Hence, the map is only a useful tool for comparing the granulation behaviour of different materials in the same device. Until we have a better understanding of the flow patterns and impact velocities in granulators, it cannot be used to compare different types of equipment. Theoretical considerations also revealed that several of the regime boundaries are also functions of additional parameters not explicitly contained on the map, such as binder viscosity

Liquid distribution in wet granulation: dimensionless spray flux

This study investigates binder distribution in wet granulation and focuses on the nucleation zone, which is the area where the liquid binder and powder surface come into contact and form the initial nuclei. An equipment independent parameter, dimensionless spray flux Psi (a), is defined to characterise the most important process parameters in the nucleation process: solution flowrate, powder flux, and binder drop size. Ex-granulator experiments are used to study the relationship between dimensionless spray flux, process variables and the coverage of binder fluid on the powder surface. Lactose monohydrate powder on a variable speed riffler passed under a flat spray once only. Water and 7% HPC solution at two spray pressures were used as binders. Experiments with red dye and image analysis demonstrate that changes in dimensionless spray flux correlate with a measurable difference in powder surface coverage. Nucleation experiments show that spray flux controls the size and shape of the nuclei size distribution. At low Psi (a), the system operates in the drop controlled regime, where one drop forms one nucleus and the nuclei size distribution is narrow. At higher Psi (a), the powder surface cakes creating a broader size distribution. For controlled nucleation with the narrowest possible size distribution, it is recommended that the dimensionless spray flux be less than 0.1 to be in the drop-controlled regime

Scale-up of mixer granulators for effective liquid distribution

There is considerable anecdotal evidence from industry that poor wetting and liquid distribution can lead to broad granule size distributions in mixer granulators. Current scale-up scenarios lead to poor liquid distribution and a wider product size distribution. There are two issues to consider when scaling up: the size and nature of the spray zone and the powder flow patterns as a function of granulator scale. Short, nucleation-only experiments in a 25L PMA Fielder mixer using lactose powder with water and HPC solutions demonstrated the existence of different nucleation regimes depending on the spray flux Ψa—from drop-controlled nucleation to caking. In the drop-controlled regime at low Ψa values, each drop forms a single nucleus and the nuclei distribution is controlled by the spray droplet size distribution. As Ψa increases, the distribution broadens rapidly as the droplets overlap and coalesce in the spray zone. The results are in excellent agreement with previous experiments and confirm that for drop-controlled nucleation, Ψa should be less than 0.1. Granulator flow studies showed that there are two powder flow regimes—bumping and roping. The powder flow goes through a transition from bumping to roping as impeller speed is increased. The roping regime gives good bed turn over and stable flow patterns. This regime is recommended for good liquid distribution and nucleation. Powder surface velocities as a function of impeller speed were measured using high-speed video equipment and MetaMorph image analysis software. Powder surface velocities were 0.2 to 1 ms−1—an order of magnitude lower than the impeller tip speed. Assuming geometrically similar granulators, impeller speed should be set to maintain constant Froude number during scale-up rather than constant tip speed to ensure operation in the roping regime.

Fundamental studies of granule consolidation part 2: Quantifying the effects of particle and binder properties

Abstract Glass ballotini of surface mean size 10, 19 and 37 μm were granulated in a 30 cm diameter tumbling drum. Water, glycerol and surfactant solutions used to vary the viscosity and surface tension of the binder liquid. The change of porosity with time was found to be well described by a simple, empirical, exponential decay equation: e−e min e O −e min = exp (−N) where ϵ is the average granule porosity after N drum revolutions, ϵ0 is the initial average porosity of the feed, ϵmin is the minimum porosity reached by the tumbling granules and k is the consolidation rate constant. ϵmin and k were complex functions of binder and solid properties—decreasing particle size and increasing liquid viscosity both reduced k; decreasing surface tension increased ϵmin; and the effect of binder content on ϵmin varied with binder viscosity. Where possible, these relationships were expressed quantitatively and compared with consolidation models in the literature. k was also related to the previously measured dynamic yield stress of the granules. The results show that capillary, viscous and friction forces all play a role in granule consolidation.

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.

Liquid-bound granule impact deformation and coefficient of restitution

Abstract The impact behaviour of liquid-bound granules and pellets was studied using a novel but simple experimental technique. Cylindrical pellets (20 mm diameter and 25 mm long) were made from 10, 19, 31, 37 and 60 μm glass ballotini with a range of binders (water, surfactant solutions and glycerol) and binder contents (0.40 to 0.55 m 3 binder/m 3 solid). These pellets were dropped from heights of 10 to 30 cm and the amount of impact deformation measured. Impacts were mostly plastic with a coefficient of restitution less than 1%. The energy conservation model of Hawkyard [J.B. Hawkyard, A theory for the mushrooming of flat-ended projectiles impinging on a flat rigid anvil, using energy considerations, Int. J. Mech. Sci. 11 (1969) 313–333] for rigid-plastic materials was used to calculate the pellets dynamic yield stress ( Y ) from the size of the deformed area. When water was used, Y increased exponentially with decreasing surface mean particle size. However, when glycerol was used, particle size had no measurable effect on Y in the range of conditions studied. Increasing binder viscosity and increasing binder surface tension both increased Y . The effect of binder content varied—increasing the amount of a viscous binder (glycerol) increased Y throughout the range of conditions studied, whereas when a low-viscosity binder (water) was used, Y passed through a maximum. Hence, there are (at least) three energy dissipation mechanisms that control impact deformation. These are due to interparticle friction, capillary and viscous forces. The effect of varying binder content in a particular system cannot be predicted a priori, unless the balance between these three mechanisms is known. Measurements of granule deformation must be made at high strain rates if dynamic effects are to be accounted for. The simple technique developed has the potential for being used to characterise different formulations in order to better predict their granulation behaviour.

Scaleup of wet granulation processes: science not art

Significant advances have been made in the last decade to quantify the process of wet granulation. The attributes of product granules from the granulation process are controlled by a combination of three groups of processes occurring in the granulator: (1) wetting and nucleation, (2) growth and consolidation and (3) breakage and attrition. For the first two of these processes, the key controlling dimensionless groups are defined and regime maps are presented and validated with data from tumbling and mixer granulators. Granulation is an example of particle design. For quantitative analysis, both careful characterisation of the feed formulation and knowledge of operating parameters are required. A key thesis of this paper is that the design, scaleup and operation of granulation processes can now be considered as quantitative engineering rather than a black art. Resume

Population balance modelling of granulation with a physically based coalescence kernel

It was previously published by the authors that granules can either coalesce through Type I (when granules coalesce by viscous dissipation in the surface liquid layer before their surfaces touch) or Type II (when granules are slowed to a halt during rebound, after their surfaces have made contact) (AIChE J. 46 (3) (2000) 529). Based on this coalescence mechanism, a new coalescence kernel for population balance modelling of granule growth is presented. The kernel is constant such that only collisions satisfying the conditions for one of the two coalescence types are successful. One constant rate is assigned to each type of coalescence and zero is for the case of rebound. As the conditions for Types I and II coalescence are dependent on granule and binder properties, the coalescence kernel is thus physically based. Simulation results of a variety of binder and granule materials show good agreement with experimental data