Estimation of particle size distributions from focused beam reflectance measurements based on an optical model

A popular in situ particle characterization technique, which can be applied without dilution, is the focused beam reflectance measurement (FBRM^®). The FBRM probe measures a chord length distribution (CLD) which is different from a particle size distribution (PSD). In order to compare results obtained by an FBRM probe with other measurement technologies such as laser diffraction, it is necessary to reconstruct the PSD from a measured CLD. For this reconstruction a measurement model and an inversion procedure are required. Most FBRM models presented in the literature assume that an FBRM records a geometric chord which can be deduced from a two-dimensional projection of the particle silhouette. In previous work [Kail, N., Briesen, H., Marquardt, W., 2008. Analysis of FBRM measurements by means of a 3D optical model. Powder Technology 185 (3), 211-222] it has been demonstrated that FBRM data show significant deviations from this geometric model. Consequently, an estimation of a PSD using such a geometric FBRM model will fail. A novel FBRM model is developed in this work. This model imitates the chord discrimination algorithm used in a Lasentec D600L FBRM system and takes the intensity profile of the laser beam and the optical aperture of the probe into account. The model is ideally suited for the estimation of a PSD from a measured CLD using a sequential, linear inversion routine, as proposed in this work. The novel FBRM model and the inversion procedure are evaluated using small, mono-disperse polystyrene beads, large ion-exchanger beads, and α-lactose-monohydrate particles. The applicability of the FBRM for PSD measurements is discussed on the basis of these results.     

Measuring the particle size of a known distribution using the focused beam reflectance measurement technique

The accuracy of the focused beam reflectance measurement (FBRM) probe, which measures a chord length distribution, from Mettler-Toledo Lasentec^® has been explored. A particle video microscope (PVM) probe, which provides in situ digital images, was used as a direct visual method to test the reliability of the FBRM results. These probes can provide in situ particle characterization at high pressures. The FBRM has been used to study emulsions and ice and clathrate hydrate formation. The ability of the FBRM to accurately characterize unimodal and bimodal distributions of particles and droplets and to measure agglomeration events was investigated. It was found that while the FBRM can successfully identify system changes, certain inaccuracies exist in the chord length distributions. Particularly, the FBRM was found to oversize unimodal distributions of glass beads, but undersize droplets in an emulsion and was unable to measure full agglomerate sizes. The onset of ice and hydrate nucleation and growth were successfully detected by the FBRM, but quantitative analysis of the particle and agglomerate sizes required simultaneous PVM measurements to be performed.     

Monitoring of cell growth using the focused beam reflectance method

In this paper the use of a focused beam reflectance measurement (FBRM) particle characterisation probe was investigated as a tool for monitoring the growth of the filamentous bacteria Streptomyces natalensis. The optimum operating parameters of the probe were identified, and it was found that the sample agitation rate had the largest effect on the mean value of a number of statistics of the chord length distribution (CLD) measured by the FBRM probe. A series of fractions of the particle size distribution was generated by sieving. It was found that the mean chord length measured by the probe responded to changes in the size distribution examined, increasing from 57.2 µm to 69.9 µm with an increase in the upper particle size from 180 µm to 300 µm. It was also found that the average total counts measured by the probe also increased with increasing sample concentration. This relationship was further investigated and it was found that in the normal range of biomass levels experienced in a fermentation (up to 4.0 g dm^?3) there was an apparently linear relationship between counts and concentration. However, as concentration was increased further the relationship became increasingly non‐linear. Copyright © 2004 Society of Chemical Industry     

Geometrical triple phase boundary length measurement using focused ion beam tomography

In multi‐component materials, triple phase boundary (TPB) is the location where reactions occur. A typical example is the TPB encountered in solid oxide fuel cells at the cathode–electrolyte interface. We proposed a tomographic approach that was developed based on serial sectioning using a focused dual ion beam (FIB) system. For image capture, FIB tomography was coupled with scanning electron microscopy, and differentiation of the composite cathode materials was possible through image contrast adjustment. An algorithm, built on the Hoshen–Kopelman theory, was then applied to measure TPB length. The percentage of the connected TPB line was also calculated with the algorithm for 3D computation. © 2011 Canadian Society for Chemical Engineering     

Effect of solvent type on preparation of ethyl cellulose microparticles by solvent evaporation method with double emulsion system using focused beam reflectance measurement

The purpose of this study was to investigate the effect of solvent type on the solidification rate of ethyl cellulose (EC) microparticles and particle size/distribution of emulsion droplets/hardened microparticles during the solvent evaporation process using focused beam reflectance measurement (FBRM). EC microparticles were prepared with a water‐in‐oil‐in‐water solvent evaporation method using various solvents, including dichloromethane, dichloromethane–methanol (1:1), ethyl acetate and chloroform. The particle size/distribution of the emulsion droplets/hardened microparticles was monitored using FBRM. The morphology of EC microparticles was characterized using scanning electron microscopy (SEM). The transformation of the emulsion droplets into solid microparticles for all solvents occurred within the first 10–90 min. The square weighted mean chord length of EC microparticles prepared using chloroform was smallest, but the chord count was not the highest. The chord length distribution (CLD) measured by FBRM showed that a larger mean particle size gave longer CLD and a lower peak of particle number. SEM data revealed that the morphology of microparticles was influenced by the type of solvent. FBRM can be employed for online monitoring of the shift in the microparticle CLD and detect transformation of emulsion droplets into solid microparticles during the solvent evaporation process. The microparticle CLD and transformation process were strongly influenced by solvent type. © 2017 Society of Chemical Industry     

Determination of non-spherical particle size distribution from chord length measurements. Part 1: Theoretical analysis

In this paper, extensive theoretical studies are described on two important issues in translating a chord length distribution (CLD) measured by FBRM instrument into its particle size distribution (PSD) including PSD-CLD and CLD-PSD translation models for general non-spherical particles. Analytical solutions to calculate the PSD-CLD models for spherical and ellipsoidal particles are developed. For non-spherical particles, a numerical method is given to calculate the PSD-CLD model. The iterative non-negative least squares (NNLS) method is proposed in the CLD-PSD model, because of its many advantages converting measured CLD into its PSD, such as insensitivity to measurement noise and particle shape. The effectiveness of the proposed methods is validated by extensive simulations.     

Determination of non-spherical particle size distribution from chord length measurements. Part 2: Experimental validation

In this paper, the theory on the translation of a measured chord length distribution (CLD) into its particle size distribution (PSD), which was developed in the first part of this study [Li and Wilkinson, 2005. Determination of non-spherical particle size distribution from chord length measurements. Part 1: theoretical analysis. Chemical Engineering Science 60, 3251-3265], has been validated using experimental results. CLDs were measured using the Lasentec focused beam reflectance measurement (FBRM) with three different materials, spherical ceramic beads and non-spherical plasma aluminium and zinc dust particles. Meanwhile, the particle shape and PSD of each material were also investigated by image analysis (IA). Comparison of the retrieved PSDs with the measured PSDs by IA shows that the PSD can be retrieved from a measured CLD successfully using the proposed iterative nonnegative least squares (NNLS) method based on the PSD-CLD model.     

Modelling particle disruption of an organic fine chemical compound using Lasentec focussed beam reflectance monitoring (FBRM) in agitated suspensions

The focussed beam reflectance monitoring (FBRM) instrument developed by Lasentec is a ‘powerful’ tool used as an ‘in-situ’ particle monitoring technique for in-line real-time measurement of particle size. This technique was successfully used to monitor particulate attrition and breakage of an organic fine chemical in a turbulently agitated suspension. The great advantage of using the FBRM technique is that the change in the crystal size distribution (CSD) for different particle size classes (fine, intermediate and coarse) can be monitored as a function of time. The attrition rates can be calculated to produce a model for the disruption kernel for the organic compound. The shift in the CSD that was observed with an increase in the specific power input was found to be largely due to micro-attrition effects rather than particle breakage (splitting).     

Effect of particle concentration and turbidity on particle characterization using digital holography

In this paper, the effect of particle concentration and turbidity on the performance of inline transmission based digital holography (DH) for particle characterization is studied. The results are analyzed based on the two metrics, i.e., detection efficiency and mean size of the particle population. Two sample depths are analyzed to quantify the effect of particle concentration. Considering 50% detection efficiency as a threshold, it is concluded that DH works well up to 0.1% (v/v) and 0.2% (v/v) particle concentrations for 10 mm and 5 mm sample depths, respectively. From the turbidity tests, it is found that DH works well up to 150 nephelometric turbidity unit (NTU) turbidity level for 10 mm sample depth.     

Focused beam reflectance measurement for monitoring the extent and efficiency of flocculation in mineral systems

Focused beam reflectance measurement (FBRM), where a scanning laser focused through a sapphire window measures real‐time reflected chord distributions without solids dilution, is attractive for characterizing flocculation performance. An enhanced measurement principle in new FBRM instruments has implications for flocculation studies, demonstrated using hematite in synthetic Bayer liquor. Comparisons of previous (M500) and new (G400) instruments were complicated by the impact of their different physical dimensions upon flocculation hydrodynamics, but the G400 clearly measured larger chords. The original measurement principle based on a reflected intensity threshold counts large low‐density aggregates as multiple chords; in contrast, the change to “edge detection” (very low threshold) is more likely to see a single chord, an advantage for studying mineral systems (aggregates often >500 µm). The G400 also captures bimodal character in unweighted chord distributions, producing distinct peaks for aggregates and fines after suboptimal flocculation; such peaks are rarely well resolved in older FBRM. © 2013 American Institute of Chemical Engineers AIChE J, 60: 251–265, 2014     

Comparison of dielectric constant meter with turbidity meter and focused beam reflectance measurement for metastable zone width determination

This work demonstrates a detailed process analytical technology (PAT) comparison study of dielectric constant measurement with turbidity measurement and focused beam reflectance measurement (FBRM) in detecting phase transitions during crystallization of three model solutions, namely stearic acid–ethyl acetate, paracetamol–ethanol and carbamazepine–methanol. The cloud and clear points determined by the dielectric constant measurement are found to be in close agreement with those obtained from the other two well-established PAT tools. A calibration technique can be further applied on the dielectric constant to improve the accuracy of the cloud point detection. The results have shown that the dielectric constant meter can be reliably used for metastable zone width (MZW) determination. This study opens new opportunities for the use of the dielectric constant meter as a simple and inexpensive alternative PAT tool for process monitoring of solution crystallization.     

On‐line monitoring of chord distributions in liquid–liquid dispersions and suspension polymerizations by using the focused beam reflectance measurement technique

The on‐line monitoring of the droplet/particle size distributions is very important to ensure the quality and applicability of various products in heterogeneous systems. For this reason, the main objective of the present work was to study the usage of the focused beam reflectance measurement (FBRM) technique for monitoring of liquid–liquid dispersions (styrene dispersion in aqueous solutions) and suspension polymerization of styrene. To do better understand the FBRM technique in these systems, the effects of surfactant concentrations, agitation speed and ambient light were evaluated during the in‐line monitoring of average chord lengths and chord‐length distributions (CLD) at different operation conditions in batch experiments. In addition, a preliminary investigation of the optimal probe position was conducted in the polymerization experiments. It is shown that the FBRM technique is sensitive to variations of particle sizes in the characteristic ranges of particle diameters of typical styrene suspension polymerizations, being useful for monitoring and also control applications that require the on‐line characterization of CLD in real time in liquid–liquid dispersions and polymerization systems. POLYM. ENG. SCI., 56:309–318, 2016. © 2015 Society of Plastics Engineers     

Comparison of external bulk video imaging with focused beam reflectance measurement and ultra-violet visible spectroscopy for metastable zone identification in food and pharmaceutical crystallization processes

The purpose of the paper is twofold: it describes the proof of concept of the newly introduced bulk video imaging (BVI) method and it presents the comparison with existing process analytical technologies (PAT) such as focused beam reflectance measurement (FBRM) and ultra-violet visible (UV/vis) spectroscopy. While the latter two sample the system in small volumes closely to the probe, the BVI approach monitors the entire or large parts of the crystallizer volume. The BVI method is proposed as a complementary noninvasive PAT tool and it is shown that it is able to detect the boundaries of the metastable zone with comparable or better performance than the FBRM and UV/vis probes.     

Control of fines suspension density in the fines loop of a continuous KCl crystallizer using transmittance measurement and an FBRM® probe

The control of crystal size distribution (CSD) is investigated in a 1.5 L laboratory cooling KCl crystallizer using fines dissolution rate as the manipulated variable. The controlled variable was either the fines suspension density in the fines withdrawal loop, measured by an innovative double‐sensor turbidity meter manufactured in‐house, or the chord length distribution (CLD) measured by the Focused Beam Reflectance Measurement (FBRM®) probe (model: Par‐Tec® 100, Lasentec®, Redmond, WA). It was shown that effective control of mean crystal size and fines suspension density in the presence of setpoint and disturbance changes is feasible with both control schemes. The double sensor turbidity sensor proved to be very rugged even in the presence of insoluble clay background particles. The FBRM® probe was more sensitive and capable of detecting particle breakage and flocculation.