Ultrasonic Monitoring of Particulate Suspensions In-process: Physics, Technology and Applications

Ultrasonic compression wave propagation is sensitive to the physical state of particulate liquid suspensions – colloids and emulsions. It is possible to estimate dispersed particle size distribution (PSD) from measurements of wave attenuation and/or phase velocity as functions of frequency. It is also possible to detect and characterise dynamic phenomena such as flocculation, network formation and crystallisation. Despite these possibilities the take-up of ultrasonic techniques has been slow, even though the methods have considerable advantages over competing techniques such as laser light scattering in that they can be applied to optically opaque liquids contained in opaque metal vessels and pipe work. In this paper we discuss the physical principles underlying the ultrasonic characterisation of particulate mixtures covering both hydrodynamic and scattering theories, and showing how PSDs are obtained from wave propagation data. On the basis of these theories combined with experimental results from real materials we review the ultrasonic signal conditions that are required for the successful characterisation of suspensions, particularly in relation to the performance of the electronic system. We show how the available signal bandwidth determines the complexity of the information that can be gained about the test material. We conclude with examples of ultrasonic systems that we have built for both laboratory bench top use and for application in-line to pipe work on process plant. Introduction: Ultrasonic compression wave attenuation and phase velocity exhibit a frequency dependence that depends on the physical properties of the material through which the wave propagates. In the case of particulate mixtures they depend on the physico-thermal properties of both phases and on the dispersed phase particle size distribution (PSD). Changes in the state of the mixture due to, for example, flocculation or crystallisation can be recognised and quantified from measurements of the ultrasonic attenuation spectra [1,2]. Most ultrasonic systems for such measurements are custom assembled from commercial components and do not lend themselves to in-process applications. A few quite sophisticated commercial instruments are available, such as the Ultrasizer (Malvern Instruments Ltd, UK), but again these are not primarily designed for process use. Ultrasound is highly suited to process applications because it can be used to probe optically opaque materials through the opaque walls of process pipe-work and vessels. In this paper we outline the underlying physics of the interactions between ultrasonic waves and particulate mixtures, showing how measured ultrasonic propagation variables are sensitive to PSD. We next consider how the signal to noise performance of the electronic-ultrasonic system, combined with the dynamic range of measured attenuation data, sets limits on the available measurement bandwidth and ultimately the amount of information which can be gained in a measurement on any given colloidal mixture. The ideas are then considered in the context of the constraints on the sizes of pipes and vessels on which measurements are to be made, leading to a simple procedure for the assessment of the feasibility of any given measurement. Physics of ultrasonic wave propagation in particulate mixtures: In a homogeneous liquid mixture ultrasonic compression wave propagation can be represented by the following expression: