Acoustic design aspects of megasonic reactors for oils and fat separation

Abstract

The performance of ultrasound reactors developed to enhance oil and fat droplet separation and fractionation rely on a number of factors. These factors include the selection of transducers, the reactor geometry and dimensions, and the physical properties of the product to be processed. Ultrasound standing wave separation reactors operating in the MHz frequency range are also referred to as megasonic reactors. The separation principle is based on acoustic radiation forces following Gorkov's theory. Depending on density and compressibility, fluid droplets are splitting between nodes and antinodes in a standing wave field. The interactions between primary and secondary acoustic radiation forces and acoustic streaming determine the reactor's performance. The efficiency of the megasonic reactor strongly depends on the attenuation of the sound waves by the fluid. The sound pressure levels obtained in a large scale (up to 1.2 m) trough attached with a 600 kHz transducer were measured in water or oil bearing materials using a novel hydrophone system. Significant attenuation was observed from the midpoint of the trough through to the end when sound transmission was investigated in water, while further attenuation was observed when using palm oil sludge. Following these results, a methodology to validate megasonically enhanced oil separation from avocado and palm oil at minimum sound penetration levels at laboratory scale was developed. The combination of sound penetration data in sludges or paste supported by benchtop results will provide the information to design large scale megasonic reactors.