Experimental validation of a dimensional analysis of spheronisation of cylindrical extrudates

Abstract

Extrusion-spheronisation (E-S) 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.The experiments employed a 45 wt% microcrystalline cellulose (MCC)/water-based paste, with up to 15 wt% calcium carbonate added to represent a 'hard' active pharmaceutical ingredient (API). The bulk yield strength of the paste was measured and found to increase with increasing carbonate content, which in turn led to higher extrusion pressures, longer spheronisation times and less spherical pellets for a given set of operating parameters. 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 is compared with two simple models.The dimensional analysis of the pellet rounding stage identified the paste density and bulk yield strength as scaling quantities. The model borne from the dimensional analysis was validated and was found to be applicable to a second material (MCC/lactose/water).High speed imaging was also used to examine the collision behaviour of pellets during the breakage and rounding stages in spheronisation. The results confirmed that the rounding phase was the rate-determining step. The velocities of a number of tracked pellets were also quantified, and were consistently lower than the tip speed of the rotating friction plate. Bulk yield strength velocity factor, Equation (16) Pa (s m -1 ) m  Wall shear stress velocity factor, Equation (16) Pa (s m -1 ) n  p Friction coefficient, pellet-pellet - s Friction coefficient, pellet-spheroniser surface - Density kg m -3  Y0 Bulk yield strength at zero extrudate velocity, Equation (16) Pa  Y Bulk yield strength Pa  w  Wall shear stress Pa  0 Wall shear stress at zero extrudate velocity, Equation (16) Pa  Spheroniser rotational speed s -1 Acronymns API Active pharmaceutical ingredient E-S Extrusion-spheronisation MCC Microcrystalline cellulose PEEK Polyether ether ketone PIV Particle image velocimetry

Modeling the breakage stage in spheronization of cylindrical paste extrudates

“…Lau et al 6 interpreted this finding in terms of the number of revolutions of the plate and thus number of collisions, which pellets experience. Parkin et al 7 measured pellet velocities in spheronization tests similar to those reported here and found that the rotational speeds were smaller than Rω, but this value provided an upper limit to the range observed. Since the energy in collisions will scale with kinetic energy, and thus (Rω) 2 , and the number of collisions is likely to relate to the number of revolutions (ωt s ), Lau et al postulated that the work required to round a pellet would be proportional to R 2 ω 3 t s .…”

Influence of the dimensions of spheroniser plate protuberances on the production of pellets by extrusion-spheronisation

“…Video analyses (e.g. [20]) confirmed that pellet velocities were of order R, and usually significantly below this value. 4 By comparison, there is little in the published literature on screen extrudate-spheroniser interactions and their effect on pellet quality.…”

“…There is no consistent trend for plates with θ  45 o : plates A, B and E give relatively even surfaces for D  2 mm, whereas pellets generated using C and F from similar extrudates were dimpled. 20 Visual inspection suggests that pellets with D/d* > 1.5 tended to be smoother: in this case, the pellet bed would be more likely to roll over the plate, promoting collisions from many contacts and thus rounding. More energetic collisions are expected to result in dents and cracks.…”

“…They experience collisions with the wall, with the plate, and with each other in a complex flow pattern. Rough and co-workers [18][19] showed that ram extruded extrudates undergoing spheronisation passed from a breakage to a rounding stage and presented a dimensional analysis of the process [20].…”

A comparative study on the nucleation, growth, and agglomeration kinetics of potassium dihydrogen phosphate crystal in circulating fluidized‐bed crystallizer

“…In our research, we can obtain the dimensional matrix according to dimensionless variables. Dimensional matrix consists of two parts: Kernel matrix and residual matrix . The physical variables are first lined up, and the basic dimensions are arranged in a row.…”