On the Breakage of High Aspect Ratio Crystals in Filter Beds under Continuous Percolation
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RESEARCH PAPER
On the Breakage of High Aspect Ratio Crystals in Filter Beds under Continuous Percolation F. M. Mahdi 1
&
A. P. Shier 1 & I. S. Fragkopoulos 1 & J. Carr 2 & P. Gajjar 2 & F. L. Muller 1
Received: 17 July 2020 / Accepted: 16 October 2020 # The Author(s) 2020. This article is an open access publication
ABSTRACT Purpose This work details experimental observations on the effect of liquid flow percolating through packed beds of crystals to elucidate how the filtration pressure severely alters the size distribution and crystal shape. Pressure filtration is widely used in the pharmaceutical industry, and frequently results in undesired size distribution changes that hinder further processing. Methods The percolation methodology presented fixes fluid flow through a bed of crystals, resulting in a pressure over the bed. X-ray computed tomography (XCT) provided detailed observations of the bed structure. Detailed 2D particle size data was obtained using automated microscopy and was analysed using an in-house developed tool. Results Crystal breakage is observed when the applied pressure exceeds a critical pressure: 0.5–1 bar for ibuprofen, 1– 2 bar for β-L glutamic acid (LGA) and 2–2.5 bar for para amino benzoic acid (PABA). X-ray computed tomography showed significant changes in bed density under the applied pressure. Size analysis and microscope observations showed two modes of breakage: (i) snapping of long crystals and (ii) shattering of crystals. Conclusion LGA and PABA have a similar breakage strength (50 MPa), ibuprofen is significantly weaker (9 MPa). Available breakage strength data may be correlated to the volumetric Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11095-020-02958-x) contains supplementary material, which is available to authorized users. * F. L. Muller [email protected] 1
School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK
2
Henry Moseley X-ray Imaging Facility, Henry Royce Institute for Advanced Materials, Department of Materials, The University of Manchester, Manchester M13 9PL, UK
Gibbs free energy. Data from 12 and 35 mm bed diameters compares well to literature data in a 80 mm filter; the smaller, easy to operate percolation unit is a versatile tool to assess crystal breakage in filtration operations.
KEY WORDS crystal breakage . percolation . pharmaceutical materials . pressure filtration . X-ray computed tomography (XCT)
INTRODUCTION Needle-shaped crystals with a high aspect ratio are common in pharmaceutical and fine chemicals processes (1,2) and their breakage during crystallisation has been widely investigated (3–5). When considering that particle/powder properties strongly influence the effectiveness of downstream operations, the desire to control these properties through particle engineering is a key facet of the modern crystallisation process (4). Particle engineering involves regulating the super saturation, hydrodynamic conditions and/or addition of seed to impart control over
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