Hydrodynamic cavitation in oil, fuels and biogas

Three uses, one question

In oil, fuels and biogas, studies used cavitation for three jobs: to thin or partly crack heavy oil, to mix oil and alcohol in biodiesel production, and to break up digester feed so microbes can reach it. Almost all of the work is at laboratory or pilot scale. In each case the practical question is the same: does the gain outweigh the energy spent pumping liquid through the cavitating device? How cavitation forms and what its collapse does are covered in What hydrodynamic cavitation is and What cavitation does to water.

Heavy oil and residue

Cavitation lowered heavy-oil viscosity only modestly, and mostly when a hydrogen donor was added. Reviewers explain that hydrogen donors convert heavy molecules into lighter ones Cako et al., 2022; in one study, the donor supplied hydrogen that capped the hydrocarbon radicals formed by cavitation Wan et al., 2019.

• A 300 L rig. Heavy oil with a viscosity of 355 mPa·s at 50 °C was passed repeatedly through a cavitating jet, with a pump delivering about 75 L/min. Without a donor, viscosity fell by at most 5.35%. With 3% and 6% tetralin (tetrahydronaphthalene), it fell by 11.03% and 15.7% after 10 passes, measured against the oil after the donor had diluted it. The viscosity had not recovered a week or more later Wan et al., 2019.
• Reviewed studies. In one vortex-diode study, viscosity first rose during cavitation and then fell; with gasoline as a donor it fell by almost 19% in 15 minutes. An orifice lowered the viscosity of vacuum residue mixed with kerosene by 17.9% in 60 minutes Cako et al., 2022.
• A marginal result. In another study, cavitation raised the light and intermediate fractions of a crude oil by 3–7%, but changed its density and viscosity only marginally Ansari et al., 2015.

The review described the data on cavitation nozzles and vortex heat generators for heavy oil as insufficient Cako et al., 2022. Heavy-oil cavitation alongside nanobubbles in oil recovery is covered in Enhanced oil recovery (EOR).

Fuel desulfurization

Cavitation has been paired with oxidants to remove sulfur from fuels. In laboratory oxidative desulfurization of diesel, with hydrogen peroxide as the oxidant and formic acid as the catalyst, cavitation reached 95% desulfurization at a 4.2 bar pressure drop in 29 minutes Baradaran & Sadeghi, 2019. The 2022 review found that vortex diodes and single-hole orifices gave more than 95% desulfurization Cako et al., 2022. As in water treatment, cavitation intensified the chemistry rather than replacing it.

Biodiesel: mixing two liquids that barely mix

Biodiesel is made by reacting vegetable oil or fat with an alcohol such as methanol, and the two barely mix. In cavitation-assisted reactors, fine bubbles and fast alcohol jets act at the oil–alcohol interface Cako et al., 2022.

• Reviewed reactors. Multi-hole orifice reactors reached 99% biodiesel yield in 5 minutes at an estimated 4.80 USD/m³, against 95% in 90 minutes at 6.7 USD/m³ for an ultrasonic bath Cako et al., 2022. The reaction and these comparisons are covered in more detail in Produced water and fuels.
• A continuous reactor. A laboratory rotor reactor running continuously at 60–105 mL/min gave fatty acid methyl ester (FAME) yields above 96.5%, meeting EN 14103, from soybean and used cooking oils. It used less energy and reached a lower peak temperature than conventional transesterification Vera-Rozo et al., 2025.

What the cost models show

Two process simulations asked whether cavitation changes the economics of a biodiesel plant. Neither is a record from an operating plant.

• Energy versus cost. Replacing the conventional reactor with a cavitation reactor cut process energy by about 40%. Total costs barely moved: 820 €/t with cavitation against 830 €/t without, for virgin oil. The feedstock mattered far more: starting from waste cooking oil, costs fell to about 290–300 €/t Innocenzi & Prisciandaro, 2021.
• Capital and product cost. For a modelled plant producing 100 t of biodiesel a day from sunflower oil, cavitation had roughly 65% lower capital investment and 10% lower product cost than mechanical stirring. Net present value depended most on the price of sunflower oil, and the authors noted that few industrial biodiesel plants use cavitation Gholami et al., 2021.

Biogas: pretreating the feed

From an energy standpoint, pretreatment pays only if the extra methane outweighs the energy spent on it. The mechanism, cavitation breaking up flocs and cells so that hydrolysis runs faster, is covered in Biogas and sludge hydrolysis, and farm feedstocks in Hydrodynamic cavitation in agriculture and food.

At a full-scale agricultural biogas plant, cavitation at 470 kJ/kg of total solids raised specific methane production by about 10% over nearly six months of operation Garuti et al., 2018.

Laboratory work on aerobic granular sludge shows how the balance shifts with treatment time. Cavitation solubilized 37% of chemical oxygen demand and 42% of total organic carbon, and methane yield reached 496 mL per gram of volatile solids, 19.6% above untreated sludge, with no change in the methane content of the biogas. The best net energy, 2,890 kWh per megagram of total solids, came after 15 minutes of treatment, not after the longest Zieliński et al., 2024.

What is counted also matters. A study with a vortex cavitation device warned that net-energy claims often leave out the energy used to mill the feedstock. Its milled, cavitated bagasse matched the net energy of unmilled bagasse only if milling used no more than 700 kWh per ton Nagarajan & Ranade, 2022. Sludge disintegration results are in Hydrodynamic cavitation in water and wastewater treatment.

Counting the energy

The 2022 review of cavitation in fuels asked for pilot-scale studies to obtain reliable energy costs, a complete description of device geometry in every paper, and inspection of cavitation chambers for damage after treatment, because cavitation erodes equipment Cako et al., 2022. A site weighing cavitation would count the pump energy, any milling or heating, and the wear of the device, and set them against the gain in viscosity, yield or methane.