Acoustic Separation of Moving Fibers in a Pulp Flow, 1999 Pulping Conference Proceedings

J.H. Jong, M.H. Choi, J.P. Gerhardstein & P.H. Brodeur

Institute of Paper Science and Technology

The use of an acoustic force to deflect moving wood pulp fibers in a pipe flow and, hence, separate fibers from water, was investigated. The goal is to devise a continuous process to increase the consistency of a pulp stream (pulp thickening). Experiments were performed using a prototype ultrasonic separation system capable of processing fiber suspensions at consistencies up to 2% and flow rates up to 180 L/min. In this setup, ultrasonic transducers mounted on one of the walls of a rectangular cross-section pipe are used to apply an acoustic force normal to the flow direction. Fibers deflect toward the opposite side wall as they interact with the ultrasonic field. At least three nonlinear effects are responsible for the acoustic force: acoustic radiation pressure, acoustic streaming, and acoustic cavitation. A CDD camera was used to record the deflection effect as a function of pulp consistency, flow velocity, transducer frequency, and acoustic intensity. Observations using softwood fibers show that as the acoustic intensity increases at constant flow velocity, the level of fiber deflection increases. On the other hand, higher flow velocity at constant acoustic intensity level decreases the deflection effect. Also critical is the change of pulp consistency as the attenuation of ultrasound increases with higher consistency pulp. Results were used to establish a preliminary analytical model to predict fiber deflection.