Acoustic Optimization of a Flow Through Ultrasonic Cavitation Reactor for Treatment of Cellulose Fibers

In pulp and paper manufacturing cellulose fibers are modified in various process steps to obtain an end product with certain specifications. In this chain of interlinked processes, the mechanical refining stage has a significant impact on modifying the properties of the fiber wall. However, refining is identified as the most energy intensive process in paper manufacturing and in order to reduce the energy use for a more sustainable process, the development of alternative refining methods are of increasing interest. Ultrasound cavitation has proved to be a efficient refining process but so far it is limited to small volumes and batch process’s. The developed ultrasonic reactor is capable of handling larger fiber volumes in a continuous, scalable, flow process. The proposed method could therefore be an alternative and efficient way of processing fibers to minimize energy usage and waste. In this study, both numerical simulations and experiments are combined and analysed to establish a design of a scalable, flow-through ultrasound reactor. The implemented model is based on the linearized wave equation in the frequency domain with appropriate addition of nonlinear attenuation by cavitation bubbles. The influence of ultrasound cavitation was experimentally verified by characterization of the modification of CTMP softwood cellulose fiber properties. Results shows that the proposed method using ultrasound cavitation can modify the fiber’s mechanical properties at energy levels of 804 kWh/bdt. However, there was no significant change in paper strength properties as per ISO 1924-3.