4 - Analysis and measurement of crystallization utilizing the population balance

Abstract

The size and shape of particles in a particulate product are important for several reasons. The efficiency of any process for production of a particulate material relies on the size, shape, and size distribution. Therefore, any comprehensive discussion of crystallization must include the genesis of size and size distribution, as well as its possible measurement and control. The population balance technique parallels other balance approaches—such as material and energy balances—which are familiar to process engineers. The population balance is used to account for both the size and number of particles. Crystals do not occur in single sizes but have distributions. Two types of crystal size distributions are normally considered, the density distribution and the cumulative distribution. There are two major classes of sizing techniques commonly used in practice: mass-based and number-based. The mass-based techniques include classical sieving procedures. A second class of measurement techniques, based on particle number, overcomes some of the problems associated with mass-based techniques. The electronic zone-sensing technique has been widely used both as a research and quality control tool. Focused beam reflectance measurement and microscopy are also used sometimes. The power of the population balance for analysis of crystallizers is demonstrated by its application to a continuous crystallizer.