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.
The practice is claimed to allow precise production of high-purity organic and inorganic crystalline substances, including intermediates and active pharmaceutical ingredients (APIs). The technology has been validated in a good manufacturing practice (GMP) environment.
Reproducibility of the point of nucleation is difficult in an industrial environment. Ideally, ultrasonics can create a defined nucleation starting point with the batch then proceeding with a controlled growth profile.
Ultrasonics can reduce the induction time and metastable zone, thereby lessening the tendency of a solution to undergo extensive nucleation, which would result in a large number of undesirable fines. Figure 1 depicts a typical cooling and nucleation profile with and without sonication. The metastable zone is reduced, the temperature of nucleation is increased, and the supersaturation at the point of nucleation is decreased in the presence of sonication.
Ultrasonics also can be used to generate secondary nuclei via the impact of the large shear forces originating from the collapse of cavitation bubbles on or near the crystal surfaces. The technology can influence growth by enhancing mass transfer near the crystal surface. Crystal purity may improve as crystal surface impurities are preferentially dissolved in the temporary local under-saturated solution resulting from highly localized heat regimes near the crystal surfaces.
It may be possible to dictate the initial polymorph that is crystallized by controlling the supersaturation at the point of nucleation. Thus, ultrasonics may enable forming a kinetically favorable unstable polymorph that is thermodynamically unfavorable. On the other hand, this technique may be able to ensure the creation of a desired stable form.
Continuing sonication of a slurry does not appear to cause extensive mechanical damage, but may foster further nucleation and hinder growth. The nucleation is most likely secondary. Sonication can give a controlled primary nucleation point and, at the end of the batch, be used to break up agglomerates. A short burst (5 sec–10 sec) may minimize the nuclei, resulting in a larger particle, while continuous application may yield a smaller crystal size distribution (CSD) due to continuing nucleation. Pulsing with ultrasonics can serve to control the CSD, filtration characteristics, and bulk density.
Accentus, Didcot, U.K., for one, has installed several of its C3 sonocrystallization units at pharmaceutical plants to produce APIs, as well as at fine and agricultural chemical sites. Figure 2 shows a C3 system installed at Aughinish Alumina in Ireland. Accentus calls it the world’s first large-scale continuous system and says that Aughinish will use it to produce 1.8 million metric tons of alumina this year.
Commercial units typically operate at around 20 kHz and have multiple transducers, each with relatively low power output (approximately 0.1 W/cm–1.0 W/cm2 at the point of delivery) coupled to the wall of the crystallizer. Average power densities for the multiple transducers are in the 75-W/L–80-W/L range.
Ultrasonics has been used successfully for batch crystallization in vessels with approximately 0.5 m–1.0 m3 active volume. Seeding requires a short sonication time. This can be accomplished in situ, via a transducer and probe in a circulating loop, or by the use of vacuum to draw out a part of the batch which, following sonication, is returned to the vessel.
For more on crystallization, consult Refs. 1–3.
Sonocracking
SulphCo, Sparks, Nev., is promoting ultrasonics for upgrading heavy sour crude oils into lighter sweeter material. Its technology reportedly increases gravity and reduces sulfur and nitrogen levels and viscosity, thus providing more product per barrel. A typical Middle East crude oil contains 40–45% residuum and 0–5% asphaltenes. The ultrasonic process converts a portion of these undesirable components to lighter, more desirable fractions. Sulfur and residuum content is claimed to be reduced by up to 80%.
A test unit with a throughput of 2,000 bbl/d of petroleum products was recently installed at OIL-SC in South Korea (Figure 3). It has demonstrated a 5-degree rise in gravity on Arab Medium crude oil.
The process was originally designed to enable low-cost desulfurization of diesel fuel, obviating capital-intensive investments in high-pressure, high-temperature hydrodesulfurization units. It also can be applied to upstream or downstream feeds, along with product streams in the refinery.
SulphCo relies on a low-pressure, low-temperature procedure using high-power ultrasonic reactors containing an oxidizer and proprietary catalyst. The sound waves alter naturally occurring molecular structures in hydrocarbons and water. The high temperature created during cavitation breaks molecular bonds in water vapor, creating hydrogen and hydroxide radicals. While most of the free radicals quickly reform into water vapor, some free hydrogen radicals displace sulfur in hydrocarbons. The sulfur is transformed into sulfones and sulfates that can be removed by solvents.
The cavitation also breaks up bonds in the complex hydrocarbons. This frees up some of the sulfur, plus cracks some of the bonds in the residuum fraction, thereby improving crude quality.
The company plans to install seven 30,000 bbl/d-units at Fujairah Oil Technology (FOT), U.A.E., by the beginning of June. FOT is a 50/50 joint venture of SulphCo and Trans Gulf Petroleum, which is owned by the government of Fujairah.
Other applications
The unique characteristics of sonication can enhance the chemistry of a variety of other systems.
For instance, organic pollutants in contaminated water can be reduced by the pyrolysis of organics and reaction with free radicals formed by water decomposition. (The primary products of sonicating water are H2 and H2O2, along with high energy radicals, including HO2-3,H+ and OH-.)
The technique can precipitate calcium carbonate in cooling-tower water systems, eliminating the need for pretreatment via ion exchange. A 500-W cell with only a 0.2-sec. exposure can degas carbon dioxide, causing the precipitation of calcium carbonate, which can be removed by cyclones. Reduction of algae and bacterial growth may also occur.
Amorphous metals can be produced due to the quick cooling effect of ultrasonics. Improved catalysts have been made by removing passivating oxides and creating a porous form.
Sonication also can be used for cell disruption of enzymes by shear forces and cavitation — however, temperature must be controlled and free radical formation may present a problem.
Many other applications of sonochemistry are emerging.
Ultrasonics long has had a role for specialized cleaning tasks. Now, it is getting broader interest as companies strive to eliminate or reduce requirements for solvents. The goal is to cut the need for hazardous, regulated solvents while minimizing air and water emissions.
The vibrating and collapsing bubbles create a scrubbing mechanism on metal surfaces, thereby removing contaminants. The efficiency of the process depends upon the application time, part/vessel shape, temperature and additives. A bath typically includes water or aqueous cleaning solutions, plus cleaning agents such as ethyl lactate, alcohols, n-methyl pyrrolidene, dibasic esters or glycol ethers.
The technique is flexible, with efficient particle removal, the capacity to reach the desired degree of cleanliness, and the ability to be combined with other techniques such as spray rinses. Potential erosion of metal surfaces should be considered during the design and operation of these systems.
Applications continue to expand as researchers investigate harnessing the unique chemical and physical characteristics developed by ultrasonics.
Wayne Genck is president of Genck International, Park Forest, Ill. E-mail him at [email protected].
References
1. Genck, W.J., “Crystal Clear — Part I,” Chemical Processing, Vol. 66 (10), p. 63 (Oct. 2003).
2. Genck, W.J., “Crystal Clear — Part II,” Chemical Processing, Vol. 66 (12), p. 37 (Dec. 2003).
3. Genck, W.J., “Cooling Crystallization,” Pristine Processing, p. 28 (Oct. 2004).
