Characterization of organic and inorganic chemicals formed by batch-cooling crystallization: Shape and size

Batch-cooling crystallization is an important industrial unit operation often carried out from aqueous solutions. Measurement of crystal size and shape plays a major role in crystallizer control in order to improve product quality. Certain crystals grown from solution may exhibit polymorphism, which can significantly affect product properties such as bioavailability as well as impinging on downstream operations such as filtration and drying L-Glutamate was considered as a model compound for this study because it crystallizes from aqueous solution into two polymorphic forms, alpha and beta, which are rhombic and acicular respectively. In this study, cooling rate and initial solution concentration were chosen as manipulated variables to control polymorph formation. Rapid cooling from low solution concentration favors the formation-of the a form, while slow cooling with high concentration favors the beta form. Oxalic acid dihydrate and copper sulfate pentahydrate, which crystallize into monoclinic and triclinic systems, respectively, are also used in this study. Crystal morphology interacts with other product quality measurements, particularly crystal size. Currently, acquiring crystal size along with shape measurement is not readily achieved. Image analysis measures shape with size distribution but only for very small samples, and it is not currently practical for control in the process industries for small particle sizes using simple equipment. In this paper, size distributions obtained by ultrasonic attenuation spectroscopy (UAS). laser diffraction spectroscopy (LDS), focused-beam reflectance measurement (FBRM), and microscopic image analysis (MIA) are compared and the influence of crystal shape on size measurements is investigated. Data obtained from UAS are combined with information on shape factor (circularity) from imaging to obtain equivalent crystal-size distribution (CSD). Data obtained from LDS are converted with a shape factor (sphericity) from imaging to obtain CSD. In this paper, the influence of shape on size measurement is observed for these organic and inorganic chemicals.