CONTINUOUS SUSPENSION FRACTIONATION USING ACOUSTIC AND DIVIDED-FLOW FIELDS

A new method is described for fractionating suspensions based on the transient response of fine particles (0.1-100 mum) to planar, ultrasonic standing wave fields applied across the thin dimension of a long rectangular channel. Under the influence of the sound field, particles migrate towards the midplane of the channel at rates that depend on their size and the ratio of their density and compressibility to that of their suspending fluids. A transverse laminar flow field is used to carry the suspension along the channel. Thin stream splitters divide the flow into two product streams. Particles that respond quickly to the sound field are enriched in one product stream, while particles with slow response exit from the other. The fractionation process can be modeled through calculation of the particle trajectories due to the combined effects of ultrasonic standing wave and laminar flow field. Model predictions indicate the potential sharpness of the fractionation that can be achieved with this method. The method was implemented experimentally. Experimental results for the continuous fractionation of 325 mesh polystyrene particles in aqueous suspension are generally in agreement with the model predictions.