The effect of agitated drying on the morphology of l-threonine (needle-like) crystals

Lekhal, Azzeddine; Girard, Kevin P.; Brown, Matthew A.; Kiang, San; Khinast, Johannes G.; Glasser, Benjamin J.

doi:10.1016/j.ijpharm.2003.10.022

Cited by 90 publications

(78 citation statements)

References 21 publications

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“…The breakage was shown to be minimal at the early stages of drying, however once a particular moisture content was reached, around 2% in this case, the attrition rate increased dramatically. Lekhal et al (2004) showed that the attrition of needle-like lthreonine in the same vessel followed a similar trend, though the increase in attrition began at a moisture content of 4%. This moisture content, below which attrition is greatly increased, could relate to the time when the surfaces of the particles in the bed no longer have complete moisture coverage.…”

Section: Introductionmentioning

confidence: 85%

Prediction of attrition in agitated particle beds

Hare

Ghadiri

Dennehy

2011

Chemical Engineering Science

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The majority of pharmaceutical powders produced through crystallisation are dried in agitated dryers. The rotation of the impeller causes shear deformation of the bed, which enhances the drying rate, but also leads to particle breakage. A method of predicting the extent of breakage occurring due to agitation is described and applied for Paracetamol in a small-scale dryer. The distributions of stresses and strains in the bed are estimated using the Distinct Element Method (DEM). The information obtained here is then coupled with the measured attrition of Paracetamol in an annular shear cell in order to predict the attrition in the agitated bed. The experiments are carried out on dry material so as to establish purely the effect of stresses and strains on attrition, whilst keeping moisture content and temperature constant.The shear cell provides uniform condition for stresses and strains so that the breakage taking place under relatively well-defined conditions is quantified. In contrast, the prevailing shear stresses and strains in the agitated bed have wide distributions, as little shearing takes place near the impeller shaft, whilst there are considerable shearing stresses near the impeller tip. Therefore the bed is divided into a number of segments for which the extent of attrition can be evaluated for each segment, based on the shear cell data. A good quantitative agreement is found between the predictions and experimental results obtained for the attrition of Paracetamol in the small scale dryer. The resulting prediction also suggests that, for a given number of impeller rotations, the extent of breakage is independent of impeller speed in the range tested (20 -78 rpm). This is expected as the prevailing strain rates are too low for the inertial effects to be dominating and the shear stresses are independent of shear rates within the range investigated. The attrition prediction suggest that over half of the attrition occurs in the bottom third of the bed, with increased attrition at greater radial distances. The attrition is also predicted to occur predominantly 30 o in front of and behind the impeller.

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Section: Introductionmentioning

confidence: 85%

Prediction of attrition in agitated particle beds

Hare

Ghadiri

Dennehy

2011

Chemical Engineering Science

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“…The experimental studies are presented first followed by the computational studies. Lekhal et al (2003Lekhal et al ( , 2004 examined the change in mean particle size within an AFD for two types of material: cubic crystals of KCl and needle-shaped crystals of L-threonine. They found that for both materials, attrition only became significant when the moisture content in the materials was below a threshold value, i.e., when the material was nearly dry.…”

Section: Particle Attrition In Vertical Axis Mixersmentioning

confidence: 99%

The internal loads, moments, and stresses in rod-like particles in a low-speed, vertical axis mixer

Xia

Curtis

Guo

et al. 2015

Chemical Engineering Science

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“…When the moisture content is low the shear resistance of the bed increases, thus resulting in a significant increase in the extent of breakage [7,8]. For this reason a completely dry bed is utilized in this work, which represents the end of the drying process.…”

Section: Introductionmentioning

confidence: 99%

Prediction of bulk particle breakage due to naturally formed shear bands

Hare¹,

Ghadiri²

2013

AIP Conference Proceedings

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Abstract. In many particle processes there exist both stagnant and flowing regions. Shear bands naturally form between these regions as a transition from zero to full velocity. Particles within shear bands are prone to breakage through surface erosion, chipping and fragmentation when exposed to substantial shear stresses. The precise nature and extent of breakage is difficult to describe mechanistically, due to the distribution of particle strengths and forces acting upon them. Consequently the breakage of particles within shear bands is typically defined empirically using an attrition shear cell. The use of such empirical relationships to predict breakage in larger processes is severely limited in the literature.This paper describes a method of predicting bulk breakage in an agitated vessel by establishing particle breakage caused by applied stress and strain in a shear cell. The Distinct Element Method (DEM) is utilised to estimate the distribution of stresses and strains within the agitated bed, this distribution is then applied to the breakage relationship to predict total attrition. The DEM analysis requires a number of measurement cells to be considered within the bed. It is imperative that the dimensions of these cells are comparable to that of a naturally occurring shear band. With this measure in place the method outlined here describes the experimental breakage well. This method can be applied to any system where particles break due to shear deformation.

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The effect of agitated drying on the morphology of l-threonine (needle-like) crystals

Cited by 90 publications

References 21 publications

Prediction of attrition in agitated particle beds

Prediction of attrition in agitated particle beds

The internal loads, moments, and stresses in rod-like particles in a low-speed, vertical axis mixer

Prediction of bulk particle breakage due to naturally formed shear bands

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