Print page

Email page

Ultrasonics makes sound progress

By Wayne Genck, Genck International

ChemicalProcessing.com

Learn about how ultrasonics can enhance crystallization, as well as other plant operations.

Ultrasonics has been used extensively to clean mechanical parts, weld plastic materials, upgrade ores and create slurries in the mining industry. Additional applications of ultrasonic irradiation or sonication have been rather slow to develop, but several now are emerging. This article will look at some of these newer uses, including in crystallization and crude oil cracking.

First, though, let’s start with the basics. Ultrasonic waves can be produced by either piezoelectric or magnetostrictive transducers. Piezoelectric transducers have quartz crystals that oscillate at ultrasonic frequencies when exposed to alternating current; a piezoelectric sandwich with aluminum or titanium end masses leads to the face where sound is emitted. The magnetostrictive transducers are made of a piece of iron or nickel surrounded by an electric coil; current passed through the coil generates a magnetic field, causing the metal to expand and contract at ultrasonic frequencies.

The transducers can be bonded on the bottom or sides outside of a tank or placed in a probe submerged inside a tank. Sound waves delivered by a probe will be of high intensity at the tip, falling off dramatically by attenuation as the distance from the probe increases. Path lengths for emitted ultrasonics from conventional probes typically are 0.1 m; different designs provide various trade-offs between intensity at the tip and active area.

Scale-up considerations

Achieving a fairly uniform intensity distribution in commercial-size vessels usually requires multiple transducers. However, a single probe can be used in a flow cell to process large volumes, especially when the required sonication is relatively short and well-defined.
Probe systems operating at typical face intensities of 5 x 10⁴to 10⁶W/m² at frequencies of 20–60 kHz have a disadvantage in that the intense cavitation field cannot be transmitted for more than a few centimeters beyond the end of the probe. Even clusters of probes have been found incapable of transmitting cavitation through distances of more than 100 mm–700 mm.

To achieve high-density fields in large volumes, it is often preferable to operate at a lower face intensity such as 100 W/m² over an extended area. However, a single transducer will only transmit a maximum total power of around 50 W and, so, multiple transducers are required to attain high power density.

Agitation is essential to ensure that all of a solution is exposed to the ultrasonics.
The use of direct transducer coupling, that is, bonding onto a surface, and a large number of transducers extends the scale-up possibilities. The only fundamental limitation is the path length from the transducer tip and the required cavitational intensity.

The mechanism

The velocity of sound in liquids is typically about 1,500 m/sec. Frequencies range from 15 kHz to 10 MHz, with associated acoustic wavelengths of 10 cm to 0.01 cm, respectively.

Ultrasonics imposes an oscillating pressure on a fluid. At low intensity, the wave will induce motion and mixing, a process called acoustic streaming. At higher intensities, ultrasonics propagates by oscillating pressure waves that alternately stretch and compress the liquid, thereby creating during expansion tiny microbubbles or cavities which then collapse during compression.

The local pressure during the expansion phase falls below the vapor pressure of the liquid, thus causing microbubbles to form. During this step, gases dissolved in the bulk liquid can enter the microbubbles and then react during collapse. The compression stage leads to the release of significant energy that is both short lived (nanoseconds) and concentrated on microvolumes. The result can be extremely high shear forces for micromixing and the appearance of reactive species such as free radicals within the bubbles.

The conditions within the cavitating bubbles can be extreme, with temperatures as high as 5,000 K–7,000 K and pressures up to 500 atm–10,000 atm. (The exact parameters continue to be debated.) The energy is dissipated in microvolumes with little influence on the temperature of the bulk liquid. Rapid cooling, estimated by some individuals to be on the order of 10⁶–10⁹ °C/sec, follows the collapse of the bubbles.

Cavitation occurs over a definite threshold of ultrasonic intensity. The bubbles may be stable, resulting in acoustic micromixing plus increased compressibility.
For heterogeneous systems, bubbles near a solid/liquid or solid/gas interface collapse asymmetrically.

Liquid quickly travels from the bulk into the bubble, producing a jet of liquid moving at around 100 m/sec. These high velocity jets can be used for applications such as cleaning, removing impurities from spent catalyst, and pushing dyes into fabrics.

Sonocrystallization

The role ultrasonics can play in influencing crystallizations is of particular interest to the chemical and pharmaceutical industries. Sonication can impact both nucleation and growth. It can induce nucleation in the metastable zone without seeding, while reducing the extent of primary nucleation. The non-seeding aspect is an advantage for contained, sterile environments. Cavitational collapse may create nucleation similar to heterogeneous nucleation from trace particle impurities in a liquid or a surface imperfection.