Using a multi-frequency acoustic backscatter system as an in situ high concentration dispersion monitor

Timothy N. Hunter, Lucy Darlison, Jeff Peakall, Simon Biggs

Research output: Contribution to journalArticle

21 Citations (Scopus)

Abstract

This paper demonstrates the potential of an in situ acoustic backscatter system (ABS) to measure particle dispersion concentrations in small and large scale mixing vessels. The ABS unit employs 1, 2, 4 and 5. MHz transducers that emit ultrasonic pulses and receive the resultant echo backscatter signals, with the strength of the return being related to particle concentration. In small scale studies (where the effect of depth-wise attenuation is effectively ignored), a peak is measured in the strength of the echo responses at intermediate concentrations, due to a balance of the backscatter and attenuation components on the overall signal. The average measured responses were then compared to backscatter theory, which suggested that such analysis is invalid for systems with particle levels greater than ~2.5. g/L and qualitative approaches may be necessary to correlate concentration. More detailed analysis is undertaken in a larger-scale system, where the deviation between expected depth-wise theoretical response and real experimental echo decays are quantified for individual frequencies. It is shown that theoretical estimations heavily over-predict the strength of backscatter echoes at higher particle concentrations, likely due to increasing inter-particle scattering, and such effects are most evident for the highest frequency tested. Because of these limitations, dispersion concentration is correlated using qualitative approaches. For the 4 and 5. MHz responses, which had approximated linear depth-wise decays (on a dB scale), it is found that the gradient of the attenuation decay slope (in dB/m) increases linearly with respect to particle concentration, which allowed the formation of a direct correlation relationship. For the 1 and 2. MHz responses, the interpolated differential is calculated for specific depth points. Again, a linear correlation is established between the gradient of attenuation and particle concentration, where importantly, it was found that this gradient is independent of dispersion depth. This result highlights the possibility of measuring concentration variation in larger scale systems, simply from the associated differential attenuation changes, and indicates the potential of acoustic techniques for the monitoring and characterisation of industrial multiphase systems.

Original languageEnglish
Pages (from-to)409-418
Number of pages10
JournalChemical Engineering Science
Volume80
DOIs
Publication statusPublished - 1 Oct 2012
Externally publishedYes

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