(82c) Ultrasonically Enhanced Chemical Processes

Ultrasonically induced cavitation (UIC) can improve chemical processes like wastewater treatment, nanomaterials synthesis, and hydrothermal liquefaction. UIC is a physical phenomenon that entails the formation and subsequent fate of vapor-filled cavities in a liquid medium irradiated by a source of acoustic waves. Bubbles' collapse generates severe local conditions in the continuous medium, accelerating the reactivity of the system.The collapse of a bubble results in the formation of a nano-sized jet and a series of shock waves inducing optimal mixing of heterogeneous mixtures. Another relevant feature of UIC is the physical damage that small particles receive when exposed to nano-sized bubbles' collapse. Despite the beneficial effect of UIC being well known, the implementation at an industrial scale is yet to be achieved. The challenge related to UIC lies in the complexity of the phenomena involved in the process such as homogeneous and heterogeneous nucleation, formation of radicals, and bubbles' collapse. The systems are also generally three-phase systems with a variety of timescales involved.The knowledge gained from either UIC mathematical or experimental activity allowed the design and optimization of UIC reactors for process intensification in three different, but equally relevant, fields: (1) generation of stable water in oil (w/o) emulsions to reduce emissions during combustion. (2) ultrasonically assisted oxidative desulfurization of marine fuels. (3) ultrasonically assisted acid leaching of valuable metals from waste lithium-ions (Li-ion) batteries. A more in-depth discussion of the processes mentioned above is presented in the following paragraphs.

Water in oil emulsions: Heavy fuel oils (HFO), a blend of residue from refineries and lighter petroleum cuts, are widely adopted for power generation and marine transportation. It is well known that water-in-HFO emulsions help achieving greener combustion, increasing carbon efficiency and reducing the emissions of pollutants. The quality of the emulsion is mainly a function of three parameters: water content, droplet size distribution, and the stability of the emulsions. UIC induces the formation of nano-scale emulsions, which are stable without the addition of any surfactant. This is possible thanks to two factors. First, asphaltenes in HFO act naturally as a surfactant, disposing at the oil-water interface and creating a rigid layer that stabilized the emulsions. Secondly, emulsions with small droplets are generally more stable than those with large droplets because smaller droplets can resist flocculation, creaming, and coalescence. Thus, the droplet breakup is a key factor determining the droplet size distribution of w/o emulsions. Droplets can be broken by applying energy to overcome the pressure difference across the interface, known as the Laplace pressure. This can be accomplished by using high shearing forces or fluctuating velocities and pressures. Here is where UIC comes into play, allowing tiny droplets and very fine emulsions to form. We successfully applied UIC to generate stable w/o emulsions.

Ultrasonically assisted oxidative desulfurization: Oxidative Desulfurization (ODS) is a process that allows removing sulfur-containing molecules from heavy petroleum fractions. The process involves mixing an oxidizing agent with the hydrocarbon mixture, eventually using acid as a catalyst. The oxidizing agent reacts with the sulfur and generates sulfones. The oxidized molecules are then removed via solvent extraction. The solvent, commonly acetonitrile or methanol, removes preferentially sulphones because of their higher polarity. Despite the capability of attacking a wide range of sulfur molecules, the ODS process is more suitable for removing thiophenes, which are generally larger molecules than sulfides. Marine Fuel Oils, generally, present a much larger content by mole of thiophenes, thus making the process appropriate for these types of mixtures. The process operates at low temperatures (330-350 K) and atmospheric pressure. Conversely, hydrodesulfurization, an high temperature and pressure process, is currently adopted to reduce the amount of sulfur in heavy residues. Despite the advantages of the ODS process, various problems affected its performance in the past, precluding any commercialization. Some of those are:

• Low yield
• Difficult sulphones separation
• Extensive usage of oxidizing agent
• Formation of polymers

These limitations were overcome using ultrasonically induced cavitation, resulting in the following beneficial effects:

1. The surface area between droplets of oxidizer and the continuous phase made of oil increases because of the formation of nano-scale emulsions, maximizing the contact between the reactants.
2. The bubbles’ formation induces a second reactivity pathway as gas-liquid reactions.
3. The jets induced by the bubbles’ collapse destroy the asphaltene aggregates, increasing the probability of exposing sulfur molecules to the bulk. Also, the smaller size of asphaltenes results in better emulsification as they act as a surfactant.
4. Bubbles collapse induces hotspots with consequent radical formation. These radicals enhance the reaction rate.

This allowed us to build a pilot plant to test the ultrasonically enhanced oxidative desulfurization on an semi-industrial scale, with a maximum capacity of 10 tons/day

Metals leaching from waste Li-ions batteries: A smooth and rapid green transition to net zero carbon emission requires reducing the use of fossil fuels as energy sources, and creative, evidence-based solutions for more sustainable ways to live, work and travel. Today, one of the most significant gaps in the energy puzzle is the cost-effective and scalable storage of renewable energy to maintain a continuous and reliable energy supply. Lithium-ion (Li-ion) batteries offer the current best-in-practice method for energy storage. Precious metals, such as Nickel, Cobalt, and Manganese, constitute the cathode of Li-ion batteries. However, mining such valuable metals involves high costs and elevated carbon footprint. The limited availability and the controversial geographical location of the known reserves make recycling such metals essential for the near future. Two main industrial processes are currently applied for recycling Li-ion batteries, (1) pyrometallurgical and (2) hydrometallurgical. The pyrometallurgical process is generally characterized by high energy consumption and low recovery efficiency but is easily implementable as a single-step process. On the other hand, hydrometallurgical ensures a higher recovery rate of valuable metals and lower energy consumption. Still, it is more complex as it involves multiple steps and is characterized by using harsh solvents and generates wastewater. According to the current state of the art, the bottleneck of the hydrometallurgical recycling process lies in the leaching step. An in-house developed and optimized ultrasonic reactor allowed to speed up the leaching process significantly, moving its time scale from hours to a few minutes (8 to 10 min). Moreover, the correct application of ultrasound allows moving from solid inorganic acid, currently used in industry, to milder and greener weak organic acid, without mining the performance and the advantages of no UIC-based leaching processes.

The computational tool and the conducted experimental activity allowed us to apply UIC to intensify different relevant industrial processes effectively. Our work focused on the design, optimization, and translation to the industrial scale of UIC reactors. Three test cases where UIC was successfully applied for the process intensification are reported here. The diverse technologies mentioned above take advantage of a particular UIC reactor design that allows modulating the residence time and parameters like time-dependent particle size distribution. The design was guided by a computational analysis performed as a first step of each process scale-up. The computational fluid dynamics (CFD) algorithm was described in previous publications from the group. UIC's potential could be extended further to many different processes, especially those limited by mass transfer phenomena that our ultrasonic technology can easily enhance.