L.X. Liu

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

Spouted bed seed coating: The effect of process variables on maximum coating rate and elutriation

The effect of bed hydrodynamics, particle and feed slurry properties on maximum slurry feed rate and elutriation during spouted bed coating of seed was studied. Experiments were performed in a 0.15 m diameter laboratory batch spouted bed using 13 different seed types and two fertilizer coatings - monocalcium phosphate (MCP) and tricalcium phosphate (TCP) - with a methylcellulose binder. Gas superficial velocity, bed height and temperature, orifice size, coating strength and flow rate were varied during experiments. The slurry feed rate is limited by spout collapse caused by the formation of embryo agglomerates in the spray zone at the bottom of the spout. The formation of agglomerates is determined by a balance between binding forces which are related to seed and coating properties, and shearing forces which are determined by bed hydrodynamics. The spout collapses before exit air from the bed is completely saturated with water. The relative humidity of the exit air at spout collapse increases with increasing excess air velocity and particle size, decreases with increasing feed slurry viscosity and is independent of bed height and temperature. Attrition of newly formed coating is the main mechanism to generate elutriated fines. The elutriation rate increases linearly with feed slurry rate and with gas velocity to the third power. It is a strong function of coating strength. Two attrition models - the inlet gas kinetic energy model and the excess energy model - were examined. The inlet gas kinetic energy model best explains the elutriation data from these experiments at constant orifice size (25 mm). However, when the inlet orifice size is decreased, the increase in elutriation is less than predicted by the increase in inlet gas kinetic energy. The mechanistic models used in this work may also be relevant to other spouted bed granulation and coating processes.

Coating mass distribution from a spouted bed seed coater : experimental and modelling studies

The coating mass distribution from a 150 mm diameter spouted bed coater was studied. Three types of seed were coated with two types of fertilizer at various feed slurry rates, total coating mass added and spouting air velocity. The coating mass distribution was strongly dependent on the initial particle size distribution. The amount of coating on a particle was proportional to its mass. Over the range covered, spouted bed process parameters (slurry rate and spouting air velocity) had no effect on the coating mass distribution. A population balance model was developed to predict the coating mass distribution. This model, using a size dependent growth term, gave excellent agreement with experimentally measured coating mass distributions for all data sets. Model predictions were not as good using a size independent linear growth term. At this stage, it is not obvious why the larger particles are coated preferentially. The population balance model was extended to predict the coating mass distribution from a continuous spouted bed coater with both representative overflow and with overflow depending on particle mass.

Chapter 20 Granulation rate processes

Publisher Summary Wet granulation is a complex process with several competing physical phenomena occurring in the granulator, which ultimately leads to the formation of the granules. These phenomena are divided into three groups of rate processes: (1) wetting, nucleation, and binder distribution, (2) consolidation and growth, and (3) attrition and breakage. The physical phenomena that control these processes are the same, independent of the type of granulation used. Granule size, size distribution, and porosity, as well as many other key product attributes are controlled by the balance of the rate processes that occur in the granulator. This chapter examines the underlying physics behind each rate process, defines the controlling formulation properties and process parameters for each rate process, uses regime maps to establish the operating regime for the granulator; and provides quantitative relationships to predict the effect of changing operating parameters and formulation properties. This chapter focuses on developments in the past decade where substantial advances in quantitative understanding of granulation rate processes have been made with an emphasis on work done by our group at The University of Queensland and The University of Newcastle. The philosophy of this chapter is to characterize process parameters in generic terms that are equipment independent (collision velocity, powder surface flux, etc.).

The effect of particle shape on the spouting properties of non-spherical particles

Abstract The spouting properties of thirteen types of particles (twelve being agricultural seeds) were measured in a 150-mm diameter cylindrical column with a 60° conical base. The particles, with sphericities varying from 0.39 to 1 (one being 0.39, others being in the range of 0.66–1.0), were investigated to determine the effect of shape factor on the minimum spouting velocity and the fountain height of non-spherical particles. By using the effective particle size, ϕ d v , in the Mathur-Gishler correlation, the minimum spouting velocity was predicted within ±20%. Using the volume equivalent size d v , overpredicted the minimum spouting velocity for particles with sphericities less than 0.75. The fountain height is calculated by first solving the mass and momentum balance equations in the spout to calculate the particle velocity at the bed surface. By including the effect of particle shape on the particle drag coefficient in the spout, the fountain height could be predicted reasonably well (within ±40% for 12 of the 13 types of particles). If the effect of particle shape was ignored, the predicted fountain heights were much lower than the experimental values.

A direct current, plasma fluidized bed reactor: Its characteristics and application in diamond synthesis

In this study, the bed temperature profile of a conical/plasma fluidized bed without a distributor was studied. The same reactor was also used for growing diamond by chemical vapour deposition. It was found that the fluidized bed quenches the plasma gas quite significantly. The bed temperature increases with both the plasma gas flow rate and the plasma input power. A lateral temperature difference from the centre of the bed to the wall exists due to heat loss through the wall. The temperature profiles obtained are suitable for controlling of bed quenching conditions in particle processing and synthesis. The conical/plasma fluidized bed with a distributor on top of the plasma was then applied to diamond synthesis from a gas phase using a distributor between the bed and the plasma. The deposition mainly occurred in the plasma tail flame. Both the scanning electron micrographs and Raman spectra show the existence of diamond on seed particle surfaces, but with a rather low nucleation density. The limitations of the d.c. plasma fluidized bed system are discussed and modifications to the reactor are recommended.

Strength and attrition resistance of agglomerates and particulate coatings

The mechanical properties of a range of agglomerates and particulate coatings have been measured using a nanoindenter. The effect of formulation properties such as powder and binder properties on coating hardness is described. An attempt is also made to measure the fracture hardness with the nanoindenter. The use of indentation technology to measure fundamental agglomerate properties is critically analysed. Based on the indentation measurements and standard attrition test results, the coating hardness is found closely related to the attrition rate under standard conditions and can be used to screen different powder/binder formulations

A population balance model for high shear granulation

In a previous paper, Hoornaert et al. ( Powder Technol. 96 (1998); 116-128) presented data from granulation experiments performed in a 50 L Lödige high shear mixer. In this study that same data was simulated with a population balance model. Based on an analysis of the experimental data, the granulation process was divided into three separate stages: nucleation, induction, and coalescence growth. These three stages were then simulated separately, with promising results. It is possible to derive a kernel that fit both the induction and the coalescence growth stage. Modeling the nucleation stage proved to be more challenging due to the complex mechanism of nucleus formation. From this work some recommendations are made for the improvement of this type of model.

The Importance of Pore-Grain Boundary Evolution in Sintering Kinetics

This paper presents a theoretical analysis of the pore-grain boundary evolution during sintering. A random pore model is used to study the effects of initial pore structure of a powder compact on the boundary evolution behaviour. The pore size distribution is also studied by assuming a typical Gaudin-Meloy distribution. It is demonstrated that a compact with low initial porosity and high surface area would have fast densification rate. In terms of pore volume distribution, a narrow distribution or high exponent m would result in fast densification. In practical systems, a wide particle size distribution with small mean size of a powder compact is desired to enhance the sintering kinetics and achieve high densification.