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Abstract
Crystal shape distribution, i.e. the multidimensional size distribution of crystals, is of great importance to their down-stream processing such as in filtration as well as to the end-use properties including the dissolution rate and bioavailability for crystalline pharmaceuticals. Engineering crystal shape and shape distribution requires knowledge about the growth behavior of different crystal facets under varied operational conditions e.g. supersaturations. Measurement of the facet growth rates and growth kinetics of static crystals in a crystallizer without stirring has been reported previously. Here attention is given to study on real-time characterization of the 3D facet growth behavior of crystals in a stirred tank where crystals are constantly moving and rotating. The measurement technique is stereo imaging and the crystal shape reconstruction is based on a stereo imaging camera model. By reference to a case study on potash alum crystallization, it is demonstrated that the crystal size and shape distributions (CSSD) of moving and rotating potash alum crystals in the solution can be reconstructed. The moving window approach was used to correlate 3D face growth kinetics with supersaturation (in the range 0.04 - 0.12) given by an ATR FTIR probe. It revealed that {100} is the fastest growing face, leading to a rapid reduction of its area, while the {111} face has the slowest growth rate, reflected in its area continuously getting larger.
Highlights
On-line microscopic imaging and image analysis for real-time characterization of crystallization processes has attracted much attention in recent years
A novel device was designed for direct measurement of face specific growth kinetics of growing crystals based on 3D online imaging technique through fixing a seed onto the pipeline in a flowcell rather than a stirred tank [2]
Compared to the previous work mentioned above, the current work has made progress in two aspects: 1) 3D facet growth kinetics of moving and rotating crystals rather than static crystals was measured in a stirred tank; 2) A more effective 3D reconstruction method, stereo imaging camera model, incorporated with a moving window approach was used to reconstruct the crystal size and shape distributions (CSSD)
Summary
On-line microscopic imaging and image analysis for real-time characterization of crystallization processes has attracted much attention in recent years. The main limitation of the triangulation of 3D reconstruction algorithm is that it relies on accurate identification of all the corners of each crystal on the 2D images, which proved to be challenging For this reason, a new reconstruction method named stereo imaging camera model was presented and applied to static crystals growing in crystallizer without a stirrer to obtain a crystal’s growth kinetics [4]. Kovačević et al [16] reported a new algorithm for obtaining 3D crystal shape from microcomputed tomography [17] They further extended this method for asymmetrical crystals and applied it for studying the disorientation angle distribution (DAD) of potash alum crystals, and could measure the DAD using 3D imaging and the proposed image processing routines [18].
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