Fluid Mechanics and Transport Phenomena zyxw Fractionation of Suspensions Using Synchronized Ultrasonic and Flow Fields zy Zenon I. Mandralis and Donald L. Feke zyxw Dept. of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106 zyx A fractionation method for fine-particle suspensions using resonant ultrasonic fields coordinated with bidirectional fluid flow fields is described. The basis f o r the separation is differences in the speed zyxwvu of response zyxw of particles to the imposition of a resonant acoustic field. As such, the method is sensitive to the particle size and the acoustic contrast between the solid particles and their suspending fluid. Both batch and continuous fractionation processes can be developed from a two-step acoustic-flow cycle. An analytical model was constructed from equations that de- scribe the trajectories of particles as they respond to the acoustic and flow fields. Model predictions indicate how the fractionation can be controlled through choice of cycle parameters. The method was implemented experimentally. Results for the fractionation of 325-mesh polystyrene spheres indicate that sharp fractionations can be achieved. The experimental results are generally in agreement with the model predictions. Introduction The ability to fractionate dispersions or to manipulate phys- ical characteristics such as the size distribution within fine- particle suspensions can have important applications in many industries. Many separation and reaction processes (extraction from solids, adsorption onto solids, or fluidization of catalysts, for example) could benefit by operating with only small par- ticles. Small particles have large interfacial area per unit volume of solids as well as decreased internal resistances to heat and mass transfer. In other cases, it is useful to have a very narrow particle-size distribution. In the medical sciences, for example, sharply fractionated particles could have many potential ap- plications in diagnosis and treatment. Ceramic particles having narrow size distribution may lead to improved material prop- erties. Also, in chromatographic separations, monodisperse packings lead to better resolution. During the last decade, there has been a growing interest in the manipulation of particles, droplets or bubbles suspended in liquids or gases by forces associated with resonant acoustic fields. The response of particles subjected to various combi- nations of acoustic forces with hydrodynamic, gravitational or diffusion forces has been experimentally studied (Higash- itani et al., 1981; Mandralis et al., 1990; Mandralis and Feke, Correspondence cobicemine this article should be addressed 10 D. L. zyxwvutsrq Feke. 1992; Tolt and Feke, 1991) and modeled (Haar and Wyard, 1978; Weiser and Apfel, 1982; Mandralis et al., 1990; Tolt and Feke, 1988; Collas et al., 1989). Suspended particles respond to resonant fields if there is a nonzero acoustic contrast between the particles and their suspending fluid. The acoustic force that acts on particles is sensitive to the size of the particles as well as the density and compressibility of both the solids and their suspending fluids. In many of the previously developed separation methods, acoustic forces can be used to drive particles to and hold them at certain positions within the acoustic standing wave field. The ultimate separation is achieved by either removing the fluid while the particles are held fixed (Tolt and Feke, 1992; Schram and Rendell, 1989) or by transporting the particles through the fluid using “drifting” resonant acoustic fields (Tolt and Feke, 1992; Hager et al., 1991; Whitworth et al., 1991; Schram, 1988; Schram and Rendell, 1989; Peterson, 1988). In all of these methods, the principal direction of prop- agation of the acoustic field coincides with the flow direction. During the harvesting of particles, the acoustic force must exceed the drag forces produced. To increase separation ef- ficiency and the speed of recovery of particles, it is possible to increase the acoustic force through inducing particle ag- glomeration with secondary acoustic forces. This practice, AIChE Journal February 1993 Vol. 39, No. 2 197