The application of acoustics to the monitoring of gas-solid processing systems

Abstract

This dissertation describes an investigation into the potential use of acoustic signals for measurement of various flow properties in industrial gas-solid flow systems. The field of acoustic based instrumentation is one in which there have been many applications developed for use in single phase and in liquid-solid flow systems. The development of applications in gas-solid systems, however, appears to have lagged behind, despite the undeniable need for reliable measurement instrumentation in these systems. It is in an attempt to alleviate this situation that the work described here was undertaken. To develop a better understanding of the properties of acoustic signals in gas-solid systems a series of measurements were taken of the velocity and attenuation of acoustic signals through dilute suspensions and through packed and fluidised beds. The measurements covered frequencies typically between 50 Hz and 20,000 Hz, and particles ranging in mean size from 100 μm to 10,000 μm. Calculations based on available theory generally under predicted the effect of the presence of solids on the acoustic propagation for dilute suspensions, and simple theories for the packed bed systems were valid only for low frequencies. To account for higher frequency signals through dense systems, a modified theory originally developed for dilute phase systems is presented. The modified theory uses terms more appropriate for representing the viscous interactions between the phases in dense particulate systems and gives consideration to the changes in the thermodynamic properties of the system that occur with changes in the frequency. The acoustic measurements demonstrated that measurements of this type are sensitive to the particle size and concentration of the solids in both dilute and dense systems, and that the measurements can be taken non-invasively in a range of particulate flow systems. A number of potential applications are discussed in .this dissertation, but most of the development work is focused on two applications; in-line non-invasive measurement of the solids flow rate in pneumatic conveying lines, and particle flow velocity measurement in dense phase gravity flows through a conduit. The solids flow measurement instrument for pneumatic conveying lines makes use of measurements of the axial propagation of acoustic signals through the pipeline to measure independently the conveying velocity and the solids concentration. The result is an instrument that is demonstrated to be largely unaffected by variations in solids distribution across the pipeline. The solids concentration also correlates linearly with the measured acoustic attenuation and simplifies the calibration of the instrument. To assist in a better understanding of the effects of particle flow properties on the solids flow rate measurement, the flow, distribution, and dispersion of particles through pneumatic conveying lines was investigated, in particular after the exit of horizontal 90° bends. The effect of interactions between closely spaced horizontal bends was also investigated and it was found that the interactions can contribute significantly to the formation of poor solids distributions, low solids velocities, and an increase in the overall pressure drop through the system. Solids flow measurement in a confined dense flow of solids under gravity is effected using acoustic signals to detect the passage of the small inhomogeneities inherently present in randomly packed beds of materials. Cross correlation of these signals leads to a measure of the solids flow velocity that correlates well with the centreline velocity. The operating limits of this approach are explored, as are the effects of velocity profiles near the wall of the vessel.