Particles settling studies using ultrasonic techniques A. Shukla, A. Prakash ⁎ , S. Rohani Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9 Received 14 August 2006; received in revised form 12 December 2006; accepted 7 February 2007 Available online 15 February 2007 Abstract Effects of acoustic velocity and attenuation measurements during settling of 43-, 110- and 168-μm glass beads in water are reported. Ultrasonic waves were generated at a frequency of 3.2 MHz. An abrupt increase in acoustic velocity and a sharp peak in the attenuation characterized the onset of settled bed. The observed attenuation peak at the transition between suspended and settled bed was attributed to dissipation caused by viscous absorption losses. The critical concentration at which increase in acoustic velocity and attenuation peak occurred was estimated for these particle sizes. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultrasonic techniques; Acoustic velocity; Attenuation; Settling 1. Introduction Sedimentation and settling of particles in liquid is important in various chemical engineering operations such as fluidized beds, hydrosizers, thickeners, and hydraulic conveying. Fluid- ized beds are commonly used in physical, chemical and bio- chemical processes. These systems provide good heat transfer, mass transfer and mixing characteristics. An important require- ment for such systems is to keep the particles in suspensions for achieving the desirable properties. Similarly, efficient suspen- sion of particles is also desired in hydraulic conveying. On the other hand, hydrosizers rely on controlled settling for separating particles of different sizes and/or densities by utilizing the difference between their settling velocities. Thickeners rely on maximization of sediment rate for efficient operation. Hence, a real time measurement technique is required for the online detection and control of settling in industrial processes. Experimental techniques available for monitoring sedimen- tation can be broadly summarized in the following categories based on the underlying measurement principle [1]: 1. External radiation e.g. Neutrons, X-ray, γ-ray, microwaves 2. Emitted radiation e.g. Radioactive tracers, NMR, magnetic tracers 3. Electrical properties e.g. Sedimentation potential, capaci- tance, conductance 4. Physical properties e.g. Sedimentation balance, pressure measurements 5. Direct methods e.g. Sampling, physical interruptions A comprehensive review of these measurement principles have been carried out by Williams et al. [1]. However, most of these techniques can have limited industrial application due to either stringent safety requirements (NMR, radioactive tracers, neutron, X-ray, γ-ray) or need for specific properties of solids under investigation (magnetic, electrical) or are highly intrusive. Direct measurement techniques require sampling and hence are not suitable for real-time online measurement. Measurement of physical properties such as pressure and viscosity can only be used to obtain macroscopic properties such as net solid flux. Measurement principles of techniques, which are non/less intrusive and capable of online application in dense suspensions, are discussed in brief. These include techniques based on the measurement of external radiation, electrical and physical properties. Measurement of electrical properties (capacitance/ permittivity, conductance) of the system has been extensively used for online monitoring of sedimentation in optically opaque and dense systems. The relationship between effective permittivity Powder Technology 177 (2007) 102 – 111 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel.: +1 519 661 2111x88528. E-mail address: aprakas2@uwo.ca (A. Prakash). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.02.003