Measurement principle and schematic drawing of the FBRM probe (Kail et al., 2006). Empirical correlations between a measured CLD and moments of the PSD are often used (Li et al., 2005).įigure 1. However, it is complicated to transform a measured CLD into its corresponding PSD accurately due to the lack of a theoretical analysis of the measurement principle. The instrument has benefits for on-line and in-situ measurements in systems with high solid concentrations (Hobbel et al., 1991). The product of rise-time and tangential velocity of the rotating laser beam records the chord length – the straight line between two points on the edge of a particle (Yu et al., 2008). When the laser beam intersects the edge of a particle, some of it backscatters to the detector installed in the same probe, and generates a rise in signal in the circuit until it reaches the opposite edge of the particle. It uses a focused beam of laser light that scans across a particle passing in front of the probe window to take measurements. The measurement principle of FBRM is based on backward light scattering. The operating principle of FBRM is shown in Figure 1. However, a significant challenge in applying LD for on-line particle size measurement is to accommodate the multiple scattering that takes place at the high particle concentrations encountered in some processes (Malvern Instruments Ltd).Ī large community of users successfully applies FBRM technology for monitoring, fault detection and quality control of dynamic processes (Kail et al., 2009). It is one of the most widely used techniques because of its range of applicability, ease of implementation, broad dynamic range, high reproducibility and speed of measurement (Blott et al., 2006 Li et al., 2005 Tinke et al., 2008). LD estimates volume-based PSD by measuring the forward light-scattering (diffraction) of light from the laser. Furthermore, the sieving time for SA is one of several factors which can significantly affect the final results of measurement (Chapeau et al., 2008). However, sieving is very labour-intensive and involves long measurement times. The significant advantage of SA is that the sieves can test large amounts of particles without being very expensive. The sieve defines a particle diameter as whether the particle can pass through the particular mesh size or not (Konert and Vandenberghe, 1997). Samples are sieved through several square meshes. Sieving is the most common method for measuring size distributions of many types of materials because of its simplicity of use and preparation. LD, FBRM and IA are among the most widely used in-process or off-line techniques (Li et al., 2005).Įach technique defines a size of particle in a different way. A variety of characteri- zation techniques are used for the determination of PSD in current chemical process industry such as Sieve Analysis (SA), Laser Diffraction (LD), Ultrasonic Attenu ation Spectroscopy (UAS), Image analysis (IA) and focused beam reflectance measurement (FBRM). Within the pharmaceutical industry, the PSD characterization of particles is generally of great concern for evaluating the quality of pharmaceutical products (Tinke et al., 2008). Particle size distribution (PSD) is an important parameter of many particulate products and it is critical for controlling process efficiency such as filtration rates, flow properties or dissolution rates (Kempkes et al., 2008 Hareland, 1994). The results obtained from FBRM were complex as its measurements depend on many factors such as its PSDs, particle optical properties and shape. It was confirmed that the square weighted Chord Distribution gives most similar results to those obtained by other techniques. The use of different statistics was evaluated to find the most suitable statistics which resemble the PSD results obtained from other techniques. It is very complicated to convert the CLDs obtained from FBRM to their corresponding PSDs as it requires complex calculations. The number based PSDs obtained by IA were converted to equivalent spherical volume based PSDs. For spherical particles, the PSDs obtained by SA, IA and LD agreed well but there was a less consistent result among different particle measurement techniques for non-spherical particles. It was observed that the results obtained by different techniques were affected by the particle shape and the type of particle. The two compounds selected for the study were spherical glass beads and non-spherical Sodium Chloride. Particle size distributions (PSDs) measured by different techniques, including Sieve Analysis (SA), Image Analysis (IA), Laser Diffraction (LD) and Focused-Beam Reflectance Measurement (FBRM), were investigated.
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