Particle size is a key parameter in many solids-handling processes, and can have a strong influence on product performance and quality, as well as for effectiveness of downstream operations. One important approach to particle-size measurement is conducting real-time, inline measurement of particle size using techniques that do not require manual grab-sampling (Figure 1). This one-page reference outlines some of the prominent technologies used for real-time particle-size monitoring.
FIGURE 1. Real-time measurement of particle size can be accomplished by a number of different technologies, depending on the needs of the application
Laser diffraction
Laser diffraction (LD) is the most commonly used technology in particle characterization. A light (laser) beam is passed through a sample of the particles of interest and their sizes are calculated from the diffraction signal. Specifically, the intensity of scattered light at different angles is measured. Large particles scatter light at smaller angles relative to the laser, while smaller particles scatter light at larger angles. Applications with high particle concentrations, like in many industrial processes, lead to multiple scattering behavior phenomena, which is still not well understood. Therefore, LD must be applied in a bypass line if applied to an industrial production process [1]. LD technology is considered a one-dimensional (1D) technology, as it does not provide any information about the shape of the particles.
Focused beam reflectance
In focused beam reflectance measurement (FBRM), a probe with a sapphire window at the end is inserted into a process stream at an angle that allows particles to flow across it. A rotating laser inside the probe passes across particles in the stream and the individual particles backscatter the laser light to a detector. The pulses of backscattered light are counted and the duration of each pulse is multiplied by the speed of the laser scan to determine the chord length of passing particles. While FRBM doesn’t give a true particle-size distribution, it is excellent for tracking real-time changes in particle count and chord distribution. This is useful for crystallization, granulation and milling. The challenge is to correctly differentiate background scattering signals from any specific particle signal, and then to interpret the chord length into a particle size. As the laser beam might interact only with a short edge of the particle, the chord-length distributions are usually undersizing the actual particle-size distribution. FRBM technologies are also considered 1D.
Ultrasound attenuation
Ultrasound attenuation spectroscopy (UAS) can be used to measure particle-size distribution in colloids, dispersions and emulsions. UAS works by transmitting high-frequency sound waves through the suspension of particles and measuring the attenuation or weakening of the signal as it passes through the suspension. An ultrasonic transducer is placed in the reaction vessel, and a pulse of sound is transmitted through the suspension of particles. The signal received by the transducer is then analyzed to determine the attenuation caused by the particles. From these data, the particle-size distribution can be calculated using mathematical models. In cases where the measurement gap (distance between emitter and sensor) is affected by strong fouling, a bypass solution is often necessary to successfully apply UAS. UAS technology is considered a 1D technology, because it does not provide any information about the shape of the particles.
Image analysis
Image analysis involves high-speed cameras capturing particle images and accompanying software that extracts size, shape and morphology data. This approach is particularly valuable when particle shape is important (needles versus spheres, for example). Automated image analysis can be combined with inline microscopy (IM), allowing for real-time monitoring of particle size and shape during production. The cameras can identify individual particles as oversized particles, agglomerates or other unwanted characteristics that may impact product quality. Image analysis is less precise for fine particles (<50µm). In addition to size measurements, inline microscopy and image analysis can provide detailed information on the shape of the particles, which can be useful in understanding how the particles will behave during downstream processing.
Selecting the most reliable particle-size-monitoring technology for a particular application depends on several factors, including the specific requirements of the production process, the size of the particles being measured, the operating conditions, and the desired level of accuracy and precision of the measurements.
Editor’s note: Parts of this column were adapted from Ref. 1.: Maass, S., Real-Time Particle-Size Analysis Technologies: Overview and Case Study, Chem. Eng., July 2023, pp. 37–42.