Particle characterisation in highly concentrated dispersions using ultrasonic backscattering method

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Determining particle size and concentration in highly concentrated suspensions and emulsions is challenging, especially under process conditions. In general, ultrasound therefore can be used for particle characterisation due to the ability of sound waves to pass opaque dispersions, whereas optical detection principles mostly are limited to low particulate contents. An established acoustic method, the ultrasonic attenuation spectroscopy, uses a transmission setup for measuring the attenuation of a dispersion. A major drawback of this measurement method is caused by the fact, that the measuring gap tends to plug, which again limits the inline capability. To overcome this limitation, an ultrasonic reflection setup is used for gathering the sound waves, which are reflected, respectively backscattered by the dispersion. Statistically analysing the corresponding backscattering signal yields the sound attenuation as well as a scattering intensity equivalent. Both measurement parameters can be shown to be sensitive against particle size and concentration. Based on a single scattering theory, a semi-empirical approach is presented for interpretation of measurement results with respect to particle size and concentration. Measurements, performed on a glass beads in water dispersion, show good agreement with theory for dimensionless wave number 0.1 < ka ≤ 1, even for concentrations up to 30 vol.%.

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