Determination of Particle Size Distribution by Par-Tec 100: Modeling and Experimental Results Abdolsamad Tadayyon, Sohrab Rohani* (Received: 20 June 1997; resubmitted: 23 February 1998) Abstract Some particle size analyzers, such as the Par-Tec 100 (Laser Sensor Technology, Redmond, WA, USA), measure the so-called cord length distribution (CLD) as the laser beam emitted from the sensor randomly crosses two edges of a particle (a cord length). The objectives of this study were to develop a model that can predict the response of the Par-Tec 100 in measuring the CLD of a suspension for spherical and ellipsoidal particles and to infer the actual particle size distribution (PSD) using the measured CLD output. The model showed that the measured CLD is reasonably accurate for the spherical particles. However, this measurement progressively deteriorates as the shape of particles changes from spherical to ellipsoidal with large ratios of major to minor diameters. Experimental results obtained with spherical particles having a normal and a non-normal PSD indicated that the Par- Tec 100 measurements deteriorate as the PSD deviates from a normal distribution. The information obtained from these experi- ments also showed that the model can reasonably predict the Par- Tec response. Use of the inferred PSD rather than the measured CLD made a major improvement in estimating the actual PSD. Mean particle size analysis revealed that the Par-Tec 100 volume-weighted mean particle size is closest to the unweighted mean particle size measured by sieve analysis. 1 Introduction Particle size determination is of prime importance in many processes. Polymerization, precipitation, homogenization and crystallization are examples of processes in which the quality indices of a product depend on the measurement and control of the particle size distribution. Several techniques are available for particle size determination. Sieve analysis is considered as an accurate sizing method for relatively large particles. However, the technique is time con- suming and therefore is not suitable for on-line monitoring and control. The Coulter Counter is based on the electrical properties of the particles and the solution. Its use is limited owing to frequent plugging of the orifice. Transmittance measurements can be used to infer the mean particle size [1, 6]. However, the use of this inexpensive device is restricted by the need for prior knowledge of the concentration of the particles and also by multiple particle blockage at high particle concentrations which require dilution of the sample. Forward light scattering has been used extensively to obtain the particle size distribution in a suspension. Illuminating small particles with a beam of light results in scattering of light in all directions. The scattering pattern measured in terms of light intensity as a function of the angle of light diffraction contains information regarding the particle size distribution [2]. Similarly to transmittance measurements, this technique also requires sample dilution to avoid multiple scattering. Both transmittance and forward light scattering techniques require isokinetic removal of a representative sample for analysis. The emergence of laser back-scattered particle size analyzers, patented by Laser Sensor Technology (Redmond, WA, USA), has made the in-line monitoring of particle size distributions possible. This instrument is differentiated from other angular scattering laser-based devices by its time measurement rather than diffracted light intensity for particle size determination [3]. The unit includes a probe that can be inserted in the process stream, eliminating the need for sample withdrawal and pumping through the sensing zone. Compared with other optical-based devices, the instrument can be operated at much higher concentrations, meaning that sample dilution is no longer required. Particle size determination using this technique has been the subject of several studies [4, 5], which were mainly focused on the experimental measurement of particle size and counts under various operating conditions. The results obtained are valuable, but in some cases, owing to the lack of theoretical analysis, the observations have not been properly interpreted. This paper attempts to present a model to interpret experimental results obtained with the Par-Tec 100 analyzer. 2 Basic Principles of Measurement in Using the Par-Tec 100 Analyzer The Par-Tec 100 analyzer uses the so-called Focused Beam Reflectance Measurement (FBRM  ) technique. The FBRM  instrumentation is composed of three parts: a probe which can be inserted directly in the process stream, the electronic measurement unit and a computer for data acquisition and analysis. A laser beam is generated in the electronic unit and is sent to the probe via a fiber optic system. Inside the probe, using a set of lenses, the beam is focused on a focal point. A mechanical system inside the probe rotates the focal point in a circular path. As the focal point intersects a particle, the particle scatters light back 127 Part. Part. Syst. Charact. 15 (1998) 127–135  WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0934-0866/98/0306-0127 $5.00þ.25/0 * A. Tadayyon, Ph.D. student; S. Rohani, Professor (to whom corres- pondence should be addressed), Department of Chemical Engineering, 110 Science Place, University of Saskatchewan, Saskatoon, S7N 5C9 (Canada).