Table 1.

Summary of findings on the demulsification of oil emulsions with US.

No	Probe Description/Assessment	Experimental Parameters	Key Findings	Reference
1	-4 various Piezoelectric Transducers at different frequencies used --Frequencies: 25.8, 39.4, 90.0, 126.4 kHz --Intensity: 60 W --Transducer diameters: 55, 46, 42, 35 mm	-Silicone oils (Dow Corning Co) -Emulsion properties: viscosities 52.19, 106.00, 169.50, 282.17 mPa s at 20 °C; water content in oil: 2.5, 5, 10, 20% used -Emulsion volumes: 201.6, 176.4, 172.8, 180.0 mL	-Mechanical oscillation is the main cause of emulsion separation for low frequency US (25.8 and 39.4 kHz), whilst droplet aggregation and banding is the main route for high frequency US (90.0 and 126.4 kHz) -High frequency US effective for emulsions with low dispersed phase content, less energy density compared to low frequency US and small droplet size but low frequency US are suitable for emulsion with high viscosity and interfacial strength	Luo et al, 2020 [53]
2	-Horn type piezoelectric transducer -Frequency: 20 kHz -Power: 25, 50, 75, 100 W	-Crude oil 020, 030, 040 with viscosities 19, 20, 21 mm2 s−1, respectively and initial water content (10, 15, 20, 25%) were used -Effect of ultrasonic field intensity (0.25, 0.5, 0.75, 1 W cm−3) and irradiation time (0–5 min) on the dewatering process was studied	-Although higher initial water content and irradiation time provided better de-emulsification, there is less effectiveness at 20 and 25%; and separation reduced at irradiation time>5 min -The more the salt content in crude oil, the better the separation of the water from the oil	Sadatshojaie et al, 2021 [41]
3	--Piezoelectric Transducers used --Frequencies: 18.96, 126.34 kHz --Intensity: 60 W	-Silicone oils (Dow Corning Co) with viscosities 52.19, 106.00, 169.50, 282.17, 378.52, 530.20 mPa s and different water contents in oil (2.5, 5, 10, 20, 25%) were evaluated -Total oil–water emulsion volume = 100 mL -Interfacial tension: 5.12–11.18 mN m−1	-Excessive acoustic intensity is undesirable in both low (I > 2.03 W m−2) and high (I > 2.5 W m−2) frequency US -US performed better than gravitational sedimentation under the water content (5–25%), interfacial conditions (5–11 mN m−1) and irradiation time (0–30 mins) conditions studied. -US performed better than gravitational sedimentation at viscosities lower than 200 mPa s, after which the difference in demulsification was reduced until they become the same at ∼ 535 mPa s	Luo et al, 2019 [36]
4	-Two piezoelectric transducers -Individual power: 40 W -Ultrasonic coaleser chamber of 1 MHz was achieved with the transducers at the side of the chamber. The chamber has a volume of 264 cm3	-Oil composed of 70% of oil 29 API and 30% oil 13 API (Vol) with viscosities of 24, 15 cP at 60, 70 °C, respectively -Water content of 30, 50% -Amount of demulsifier: 25, 50, 100 ppm	-Ultrasonic coalescence showed potential to lower separation time as compared to gravitational segregation -Under integrated US and chemical demulsifier conditions, the US reduced the consumption of chemical demulsifiers to 25 ppm as compared to 50 ppm in standalone chemical demulsification. Hence, it lowers production costs -The demulsification efficiency was similar at 60 and 70 °C	Atehortua et al, 2019 [45]
5	Piezoelectric transducer at 100 W	-SAGD watery crude oil used -Type SD demulsifier at 250 ppm -Effect of US irradiation time and temperature on demulsification, and comparative evaluation US assisted technologies were assessed -Temperature of 40, 50, 60 and 70 °C used	-Usage of US in demulsification can lower the demulsifier concentration in chemical demulsification and settling time as compared to natural sedimentation -Better demulsification was observed at low temperature, high water content and power lower than a critical value of 100 W	Xu et al, 2019 [37]
6	-Three different non contacting meter transducers used -Power: 50, 100 and 150 W -Frequency: 40, 30 and 20 kHz -Chamber volume: 100 mL	-Watery crude oils emulsion from Daqing Oilfield with water content of 25.23%, density of 0.9247 g cm−3, viscosity of 145000 mPa s, and salt content of 95367 mg L-1 was used	--Dehydration via sonochemistry (∼45%) performed better than standalone ultrasound (20%) and chemical (34%) methods --The equilibrium demulsification efficiency with sonochemistry increased with rising temperature and power	Yi et al., 2017 [38]
7	-Ultrasonic water bath at adjustable frequency 25, 45 and 135 kHz used -Power: 100, 200 W	-Heavy crude oil with water content of 0.41%, density of 0.9574 g cm−3 at 20 °C, kinematic viscosity of 1195 mm s−2 at 60 °C and salt content of 0.41% utilized for the emulsion synthesis -Effect of Raschig rings in the acoustic chamber, ultrasound frequency and temperature on the separation of the oil–water emulsion was observed	--Usage of Raschig rings in the acoustic chamber enhanced the separation of oil from water. --Metallic rings such as copper and steel was described as having better demulsification performance as compared to organic rings such as polyvinyl chloride and poly propylene --Demulsification was improved by increasing the ultrasonic waves irradiation time and frequency. However, the effect of time stabilized after 15 min for 35 and 45 kHz --The optimum irradiation time at 135 kHz was 20 min	Ronchi et al, 2020 [39]
8	-Vessel dimensions: -Length 25 cm -Diameter 4.8 cm -Two transducers at each end of vessel with frequency of 20 kHz -Power: 25, 50, 75, 100 W	-Three crude oils Cheshmeh Khosh, Gachsaran 1 and Gachsaran 2 with kinematic viscosities between 19.4 and 21.4 mm2 s−1 at 25 °C were used --Effect of water cut of emulsion (10–25%), ultrasonic irradiation time (0–5 min) and intensity (0–1 W cm−3) on demulsification effectiveness investigated	--Increment in ultrasonic irradiation time provided better dehydration than increment in the intensity --For the crude oils studied, the authors suggested that the utilization of chemical demulsifiers could be reduced by 50% at suitable US intensity and time	Khajehesamedini et al, 2018 [40]
9	-Ultrasonic bath -Power: 100, 160 W -Frequency: 35 kHz	-Crude oil with API density of 19.0 and viscosity of 122.9 mPa.s at 45 °C used -Effect of parameters such as water content (12, 35, 50%), time (5–60 min) and temperature (25, 45, 60 °C) on dewatering efficiency evaluated	--Dewatering efficiency attains 65% at droplet size of 10 µm at water cut of 50% --Good separation efficiency (51.7%) was achieved in 15 min at US power of 160 W and US temperature of 45 °C	Antes et al, 2015 [51]
10	-Ultrasonic bath -US Intensity Amplitude: 20, 60, 100% -Frequency: 35 kHz	Emulsion contains crude oil with viscosity of 279 cP and API density of 19.7; water droplet of median size (5 µm) and NaCl concentration of 250 g L-1	-Through 3D transitioning hydrophones, the mapping of the US bath was achieved across its width (240 mm), depth (150 mm) and length (300 mm) -The acoustic intensities across the US bath ranges from 0.1 to 0.6 W cm−2	Pedrotti et al, 2018 [17]
11	-Ultrasonic bath -Power: 100, 200 W -Frequency: 25, 35, 45, 130, 582, 862, 1146 kHz	Emulsion with crude oil of API density of 19 and water cut of 12, 35, 50%Water droplets of median sizes (5, 10 and 25 µm) were used	-About 65% separation efficiency after 15 min at 10 µm droplet diameter, 50% water content and f = 45 kHz achieved -No apparent demulsification at frequencies greater than 45 kHz	Antes et al, 2017 [21]