Sound assisted fluidization of nanoparticle agglomerates

This paper presents some preliminary observations on sound-assisted fluidization of hydrophobic fumed silica nanoparticles (Degussa Aerosil® R974, having a primary particle size of 12 nm) in the form of large 100–400 μm agglomerates. The effect of sound on the fluidization behavior of the nanoparticle agglomerates, including the fluidization regime, the minimum fluidization velocity, the bed pressure drop and the bed expansion has been investigated. It is shown that, with the aid of sound wave excitation at low frequencies, the bed of nanoparticle agglomerates can be readily fluidized and the minimum fluidization velocity is significantly reduced. For example, the minimum fluidization velocity is decreased from 0.14 cm/s in the absence of sound excitation to 0.054 cm/s with the assistance of the sound. In addition, under the influence of sound, channeling or slugging of the bed quickly disappears and the bed expands uniformly. Within a certain range of the sound frequency, typically from 200 to 600 Hz, bubbling fluidization occurs. Both the bed expansion and the bubble characteristics are strongly dependent on the sound frequency and sound pressure level. However, sound has almost no impact on the fluidization, when the sound frequency is extremely high, above 2000 Hz. A relatively high sound pressure level (such as 115 dB) is needed to initiate the fluidization.