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Flow Reactor

Fiber development by ultrasound controlled cavitation

Published: 7 November 2017

The project aims to reduce the specific electrical energy in fibrillation processes required to modify and improve the properties of cellulose fibers. The idea is to create an alternative technology to today's refiners in the paper industry based on controlled cavitation. The design approach is to optimize an ultrasonic reactor concept combining FE and CFD modeling with experimental verification.

The proposed concept gives a new opportunity for precise fiber development. In the reactor concept the fibers are treated in a two-step process. First, the cavitation bubbles are initiated when the suspension flows through an adjustable nozzle. Second, the bubbles are collapsed by ultrasound in a subsequent resonant tube volume. The ultrasound is generated by sonotrode devices connected to the tube surface. The generated ultrasound is transformed to cause transient cavitation on the fiber surfaces. The goal is to develop a scalable reactor concept for energy efficient fiber development. Process intensification requires a multivariate optimization of the excitation frequency, concentration, temperature, static pressure, electrical power, and flow rate.

The hypothesis is that controlled ultrasonic cavitation function due to the transient asymmetric collapse of cavitation bubbles can cause extreme pressure on a small area. The principle is based on the small gas bubbles in water is excited by high-intensity ultrasound. At a certain critical size, the bubbles resonate and then grow quickly. The transient bubble collapse occurs when external pressure is about to reach its maximum. Ultrasound with constant frequency (e.g., 20 kHz) create lots of bubbles, with varying size coupled to harmonically related resonant frequencies. Bubble collapsing creates the jets in the micro scale and shock waves in macro scale. The asymmetric collapse of cavitation bubbles is believed to provide a mechanical action of cellulose fibers in the form of both internal and external fibrillation. The goal is to achieve a precise development of fiber quality with minimum electrical energy input, to achieve high-quality pulp with desired tensile strength.

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Partners: ÅF-Ljud & Vibration, SCA Paper, Holmen AB, Stora Enso, Innventia AB, MittUniversitetet, TU-Dresden