Real-Time Measurement of the Growth Rates of Individual Crystal Facets Using Imaging and Image Analysis: A Feasibility Study on Needle-shaped Crystals of L-Glutamic Acid

Abstract

Given that the fundamental process of crystal growth and its associated kinetic control is surface controlled, the use of a single scalar parameter, particle size, usually defined as a volume equivalent diameter, i.e., based on a spherical assumption of particle shape can be misleading for a number of practical crystallization systems, notably pharmaceutical products. Hence, measurement of the growth rate for each individual crystal surface in real-time and within processing reactors could open the way for the development of more effective process and concomitant product quality control. This paper presents the measurement of the growth rates of needle-shaped crystals in two dimensions using on-line imaging and image analysis techniques through a feasibility study of the batch crystallization of β form L-glutamic acid. The length and width of each needle-shaped crystal were measured every 60 s, ranging from 100 to nearly 180 μm in length and from 30 to 45 μm in width, and the values were used to estimate growth rates on both directions. The growth rate in length was found to be four to six times greater than for the width. The (101) plane was found to be the fastest growing surface of the morphology studied and an attempt has been made to estimate its growth-kinetics parameters from measurements of length, whilst it was harder to estimate kinetics from measurements of width for other crystal facets.