Utilization of FBRM in the Control of CSD in a Batch Cooled Crystallizer

Abstract

Controlling crystal size distribution (CSD) is important to downstream processing and to product quality. It is well-recognized that selective removal functions can be used to influence CSD, for example by manufacturing a product with a larger dominant size or narrower distribution. Early work on the use of feedback control to manipulate the residence time distribution functions of fines in a continuous crystallizer demonstrated the utility of such an approach in handling process upsets and cycling that resulted from system instability. These efforts were extended to batch crystallization, although there remained significant difficulty associated with on-line analysis of the size distribution.

The development of new technologies, such as Focused Beam Reflectance Measurement (FBRM), provides a methodology for on-line monitoring of a representation of the crystal population in either batch or continuous crystallization systems. The FBRM technology is based on laser light scattering; properly installed, it allows on-line determination of the chord length distribution (CLD), which is statistically related to the CSD and depends on the geometry of the crystal. The purpose of the present study is to use the FBRM to monitor the evolution of CSD characteristics and to implement a feedback control scheme that provides the flexibility to move the CSD in a preferred direction. Cooling batch crystallizations of paracetamol has been chosen to investigate implementation of the control scheme. The work will show how fines removal and varying cooling rates provide reliable and practical control of crystal size distribution.