Flow visualisation and modelling of solid soap extrusion

Ram extrusion of a solid granular soap was studied using three geometrically identical but differently-scaled extruders. The experimental design revealed deviation from the Benbow and Bridgwater (1993) extrusion model due to nonideal, scale-dependent effects. Typically these effects, linked to the shear rate in the extruder, are absorbed into the model's material pseudo-properties. The data were able to be represented using the Basterfield et al. (2005) model for extrusion flow which does include a shear rate as a variable. Flow visualisation in conjunction with fluid dynamics-based simulations showed, however, that the assumptions underlying the Basterfield et al. model are not appropriate for soap extrusion, despite the good agreement of the model with the experimental extrusion data. This highlights a need for care in interpretation of extrusion data, in that the limited information gathered about any given experiment, typically just the extrusion pressure, can lead to the generation of spurious parameters if the wrong model is applied.This work was funded by Ceratizit GmbH Austria and Sandvik Hyperion U

Experimental validation of a dimensional analysis of spheronisation of cylindrical extrudates

Extrusion–spheronisation is a widely used technique for the manufacture of pellets with high sphericity and narrow size distribution. A dimensionless framework for describing the evolution of pellet shape with spheronisation time is presented for the first time and is validated using new experimental data obtained with two families of materials: (i) microcrystalline cellulose (MCC)/water-based pastes with loadings of up to 15 wt.% calcium carbonate representing a ‘hard’ active pharmaceutical ingredient, and (ii) a lactose/MCC/water paste. The dimensional analysis of the pellet rounding stage identified the paste density and bulk yield strength, σY, as scaling quantities: σY was measured in separate extrusion tests and found to increase with increasing carbonate content. Larger paste strength gave longer spheronisation times and less spherical pellets for a given set of spheronisation conditions. The pellet aspect ratio was found to increase in a linear manner with the logarithm of spheronisation time, progressing towards an asymptotic final value. This behaviour, which is evident in old data sets but has not been discussed previously, is compared with two simple models. High speed imaging was also used to examine the collision behaviour of pellets during the breakage and rounding stages in spheronisation. This confirmed that the rounding phase was the rate-determining step. The velocities of a number of tracked pellets were consistently lower than the tip speed of the rotating friction plate, confirming previous findings in studies of beds of pellets.Microcrystalline cellulose for this final year student research project was kindly provided by MSD Devlab, Hoddesdon, UK. Support for M.P. Bryan from CERATIZIT GmbH and Sandvik Hyperion is gratefully acknowledged.This is the author accepted manuscript. The final version is available from Elsevier via https://doi.org/10.1016/j.powtec.2016.05.00

Non-aqueous formulations for ram and screen extrusion-spheronisation.

The use of non-aqueous cellulose-based formulations for extrusion-spheronisation (E-S) is investigated. A 10 wt% hydroxypropyl cellulose/isopropyl alcohol solution (HPC/IPA) was identified as a suitable sticky liquid binder for preparing non-aqueous pastes. Preliminary tests were performed on a series of pastes using a ram as well as a laboratory roller screen extruder, since the former is commonly used in batch testing and the latter replicates the shear range in a manufacturing screen extruder. Pellets with acceptable size and shape distributions were obtained with Avicel® HFE-102 NF/HPC/IPA for ram E-S, and with Avicel® RC-591/HPC/IPA for screen E-S. Further investigation was performed with calcium carbonate added as a model active pharmaceutical ingredient. Both formulations were able to generate pellets with acceptable size and shape characteristics at up to 50 wt% carbonate loading: further work is required to optimise yields.EPSRC Dorothy Hodgkin studentship for Min Zhang supported by MS

Influence of plate surface protuberance size and shape on the production of pellets by extrusion-spheronisation

The influence of spheroniser plate surface protuberance geometry on pellets produced from spheronisation of paste extrudates was investigated using a model paste material (45 wt% microcrystalline cellulose/water). Four cross-hatched pattern plates of different dimensions and/or shape of the surface protuberances were studied in terms of their effect on pellet water content, spheronisation yield, pellet size and shape distributions, and surface morphology. In addition, the effect of the relative size of extrudates to protuberances was investigated by comparing pellets generated from 1 and 2 mm diameter extrudates. In this study, all the plates have the same groove line arrangement but differ in surface protuberance dimensions and/or shape, and so the focus differs from that of Michie et al. (2012) who investigated plates of different groove line arrangement.This work was finically supported by the China Postdoctoral Science Foundation (Grant No.: 2014M550497).This is the author accepted manuscript. It is currently embargoed pending publication

Measurement of the wall slip behaviour of a solid granular soap in ram extrusion

The wall slip behaviour of a solid granular soap was investigated in the context of ram extrusion, with particular focus on determining the sensitivity of the wall shear stress to the pressure within the soap, as well as the slip velocity. Two novel slip measurement devices were used to infer the shear stress: a two stage extrusion die with inbuilt pressure gradient measurement; and a combined compression-translation assembly to measure the frictional force between the soap and the wall. Both devices provided differing measurements of the pressure sensitivity, and gave shear stress estimates in agreement with a Benbow and Bridgwater (1993) analysis of the extrusion behaviour, which cannot gauge pressure dependence. The influence of the wall material on the slip and extrusion behaviour was also investigated, using three geometrically identical extruders constructed from polycarbonate, stainless steel and tungsten carbide. There was found to be a non-negligible relationship between wall material, wall surface roughness, and the Benbow-Bridgwater extrusion parameters, in which the wall shear stress was greatest against the smoothest, cemented tungsten carbide wall in contrast to a rougher stainless steel and rougher-still polycarbonate wall. Keywords: extrusion, wall slip, soft solid

The evolution of pellet size and shape during spheronisation of an extruded microcrystalline cellulose paste

The process by which cylindrical rods of soft solid paste extrudate are converted into round pellets on a spheroniser (Marumeriser™) plate was studied by interrupting spheronisation tests and measuring the size and shape of the pellets. Batches of 20 identical rods (20 mm long, 3 mm diameter) generated by ram extrusion of 47 wt% microcrystalline cellulose/water paste were spheronised at rotational speeds, ω, between 1200 rpm and 1800 rpm on a laboratory spheroniser. The time to complete spheronisation was found to scale with ω−3.6, which was close to the ω−3dependency predicted by a simple collision model. Breakage occupied the first 10% of the process duration: rounding off was the rate-determining step. The evolution of pellet shape was classified into five stages, the duration of which was found to scale with spheronisation time. Pellet shape, quantified by aspect ratio, circularity, shape and angularity factors presented by Sukumaran and Ashmawy (2001), showed similar behaviour for all ω studied. A phenomenological model is proposed which identifies different routes for small and large rod breakage products.This is the accepted manuscript of an article originally published in Chemical Engineering Research and Design. The final published version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0263876214000379

Measurement and modelling of wall friction in the ram extrusion of stiff microcrystalline cellulose-based pastes

Simulation of paste extrusion requires knowledge of the material–wall interactions and bulk deformation behaviour. A systematic approach to decoupling these is presented for a stiff paste, comprising microcrystalline cellulose/calcium carbonate/water, employing three different experimental configurations to separate paste-wall friction from bulk rheology. The bulk yielding behaviour was identified from ram extrusion testing with orifice dies, while wall friction was characterised using the twin-die extrusion apparatus reported by (Bryan et al., 2018). Further extrusion tests with square and conical entry dies featuring various die land lengths provided data for comparison with simulations. The wall friction exhibited non-linear Navier slip with negligible pressure dependency: this relationship was included within a solid mechanics-based simulation of ram extrusion with ABAQUS. The estimate of the plastic yield stress from orifice die testing was refined using simulations of flow through square-entry dies and predicted the extrusion pressure for conical-entry dies with reasonable accuracy.Ceratizit Austria Gmb