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The Sono-Hydro-Gen process (Ultrasound induced hydrogen production): Challenges and opportunities

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Producing hydrogen using ultrasonic waves offers tremendous opportunities, which could lead to a clean, affordable and reliable energy source. Introducing high-frequency ultrasonic waves to liquid water could provide an efficient way to produce efficient and clean hydrogen. This particular review makes a focus on the application of power ultrasound in hydrogen production and discusses the challenges, opportunities and future directions. This new, ultrasonic based hydrogen production technology is given the name of “Sono-Hydro-Gen”. It is well known that hydrogen can be formed from the dissociation of water molecules subjected to ultrasound via the so-called sonolysis process. Factors affecting the hydrogen production rate and the theory beyond these effects are described herein. The average hydrogen production-rate reported from the Sono-Hydro-Gen process is 0.8 μMol per minute at an acoustic intensity of 0.6 W cm ⁻² . This review also compares the Sono-Hydro-Gen technology with the most commonly used technologies and it is found that this technology could lead to a prosperous and secure hydrogen energy for the future. Recent numerical and experimental investigations on the hydrogen production pathways have been reviewed showing various numerical simulations for different experimental configurations. Finally, performance and efficiency criteria are discussed along with the challenges associated with the Sono-Hydro-Gen process.
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Review Article
The Sono-Hydro-Gen process (Ultrasound induced
hydrogen production): Challenges and
opportunities
Sherif S. Rashwan
a,*
, Ibrahim Dincer
a
, Atef Mohany
a
, Bruno G. Pollet
b
a
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON L1G 0C5, Canada
b
Hydrogen Energy and Sonochemistry Research Group, Department of Energy and Process Engineering, Norwegian
University of Science and Technology (NTNU), NO-7491 Trondheim, Norway
article info
Article history:
Received 7 March 2019
Received in revised form
5 April 2019
Accepted 11 April 2019
Available online 10 May 2019
Keywords:
Ultrasonic hydrogen production
Hydrogen energy
Sono-hydro-gen
Sonochemistry
Power ultrasound
abstract
Producing hydrogen using ultrasonic waves offers tremendous opportunities, which could
lead to a clean, affordable and reliable energy source. Introducing high-frequency ultra-
sonic waves to liquid water could provide an efficient way to produce efficient and clean
hydrogen. This particular review makes a focus on the application of power ultrasound in
hydrogen production and discusses the challenges, opportunities and future directions.
This new, ultrasonic based hydrogen production technology is given the name of Sono-
Hydro-Gen. It is well known that hydrogen can be formed from the dissociation of water
molecules subjected to ultrasound via the so-called sonolysis process. Factors affecting the
hydrogen production rate and the theory beyond these effects are described herein. The
average hydrogen production-rate reported from the Sono-Hydro-Gen process is 0.8 mMol per
minute at an acoustic intensity of 0.6 W cm
2
. This review also compares the Sono-Hydro-
Gen technology with the most commonly used technologies and it is found that this
technology could lead to a prosperous and secure hydrogen energy for the future. Recent
numerical and experimental investigations on the hydrogen production pathways have
been reviewed showing various numerical simulations for different experimental config-
urations. Finally, performance and efficiency criteria are discussed along with the chal-
lenges associated with the Sono-Hydro-Gen process.
Crown Copyright ©2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publica-
tions LLC. All rights reserved.
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14501
Hydrogen era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14502
Potential of ultrasonic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14502
Review objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14502
*Corresponding author.
E-mail address: Sherif.Seifeldin@uoit.ca (S.S. Rashwan).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy 44 (2019) 14500e14526
https://doi.org/10.1016/j.ijhydene.2019.04.115
0360-3199/Crown Copyright ©2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. All rights reserved.
Hydrogen production processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14503
Sonochemistry technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14505
Sonoelectrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14505
Benefits from sonochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14505
Sono-Hydro-Gen system illustration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14505
Factors affecting the Sono-Hydro-Gen process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14507
Ultrasonic frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14508
Dissolved gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14508
Acoustic power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14509
Bulk liquid temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14509
Bubble temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14510
Recent numerical modeling studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14510
Hydrodynamic modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14510
Sonoreactor modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14511
Experimental configurations and analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14512
Different experimental configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14513
Recent experimental reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14514
Performance assessment criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14516
Hydrogen production quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14519
Ultrasound and energy consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14520
Challenges for Sono-Hydro-Gen reactor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14520
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14522
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14522
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14522
Introduction
Different carbon-cutting options are under the spotlight, and
arguably, one of the most promising is the use of hydrogen as
an energy carrier [1]. Highlighting the need for drastic cuts in
carbon emission, hydrogen is one among the most powerful
fuels and highly suitable for clean energy production [2].
Furthermore, hydrogen is an excellent high capacity and long-
term energy storage medium that can be connected to the
intermittent renewable energy technologies like wind and
solar. Producing hydrogen that can be kept in gaseous or
liquid form for an undefined periods without affect its energy
content adversely can be a challenge. At times of peak power
demand, the stored hydrogen can then be used to generate
grid electricity using Combined-Cycle Gas Turbines (CCGTs) or
distributed energy supplies using fuel cells, for example;
alternatively, it can be harnessed to heat households, to fuel
vehicles, and in many additional applications. In recent years,
the idea of harnessing hydrogen as a mainstream energy op-
tion has been given a great importance. Many research studies
are probing the opportunities and challenges of hydrogen-for-
future-energy considering an important and yet a very basic
question: how might we produce sufficient hydrogen, produce it
sustainably enough, and clean to meet the needs for a low-carbon
economy [3,4]?
Answers to this very frequent question are provided in this
review. The importance of hydrogen to power our economies
and societies and the potential use of power ultrasound for
producing hydrogen is also highlighted herein.
Nomenclature
cSpeed of sound [m/s]
CpUs Specific heat of water
h Energy input
Mus Mass of water [kg]
paAcoustic pressure amplitude [kPa]
rBubble radius from the center [m]
tus Time [s]
DTTemperature difference [K]
uParticle velocity
xPosition in xdirection [m]
zAcoustic impedance
Acronyms
HHV Higher Heating Value
GHG Greenhouse gases
KI Potassium iodide
PZT Piezoelectric transducer
SE Sonochemical efficiency
Greek symbols
rDensity [kg/m
3
]
mDynamic viscosity [kg/m/s]
n0Velocity amplitude of the probe
lWavelength [m]
international journal of hydrogen energy 44 (2019) 14500e14526 14501
Hydrogen era
Hydrogen is known to have the highest heating value per unit
volume as a fuel. Demand for energy has grown substantially
over the past century and continues to grow at a staggering
pace. One of the primary sources of energy is fossil fuels.
However, the combustion of fossil fuels (mainly hydrocar-
bons), in internal combustion engines is usually incomplete,
resulting in reduced efficiency and the emission of various
pollutants into the atmosphere [5,6]. In this regard, it is
known that using hydrogen as fuel increases the efficiency of
the internal combustion engines (ICE) and reduces drastically
GHG emissions. Hydrogen is considered as one of the key
parameters for a clean and environmental energy source. In
addition, it is a renewable source of energy. Hydrogen fuel is
one of the most powerful fuels because it has a very high
energy content of 141.8 MJ/kg which, can be defined as the
amount of energy released while burning 1 (one) kg of the
fuel [7,8]. Its heating value is approximately 3 (three) times
higher than that of the natural gas [9]. Many countries are
mainly using fossil fuels for energy production, which leads
to a tremendous amount of pollutants. Consequently,
hydrogen is considered highly suitable for clean energy.
Recently, hydrogen is used in the Integrated Gasification
Combined Cycles (IGCC) as a fuel blend because it is char-
acterized by lower GHG emissions such as carbon dioxide
(CO
2
) and nitrous oxides (NO
x
)[9,10]. In the following sec-
tions, different methods of hydrogen production are briefly
presented.
The benefits of using hydrogen as a fuel can be summarized
as follows: it is an environmentally friendly, non-toxic, efficient
fuel and a renewable source of energy, emitting very low levels
of greenhouse gases when burnt. However, the challenge lies in
an energy-hungry, low-carbon age, is to manufacture enough
hydrogen and to do it cleanly and cost effectively.
Potential of ultrasonic applications
Ultrasound has the "power"to explore and destroy. The ul-
trasound frequency is the sound frequency beyond which the
human ear can react. In other words, it is a sound of frequency
after which human will not hear (>16 kHz). The ultra-power of
sound has been widely used in the medical and clinical ap-
plications. However, it has many applications in the engi-
neering field as well. The ultrasound method offers a potential
option in routinely engineering applications for monitoring
and diagnostics processes [11e13].
In Fig. 1, a summary of several applications is presented
with their corresponding ultrasound frequency ranges speci-
fied in previous studies. Ultrasound can be used, either by it-
self or in combination with emerging technologies; and it has
been used for different applications including, thermoacous-
tic heat engines, thermoacoustic refrigerators, axial and
circumferential crack detection, detecting piping corrosion/
notch, pipe wall thinning monitoring, ultrasonic viscometer,
monitoring food materials, monitoring food processing and
preservation, car parking sensors, mapping of lubricant film
thickness along piston skirt, monitoring machining prcoesses
[160]. Mohany et al. [14] have reviewed the development and
recent patents on thermoacoustic devices.
On the other hand, however, engineering ultrasound has
taken a new form of life as productive applications to produce
useful gases (e.g. hydrogen) by either sonication [15] or trans-
esterification. One example is the Sono-Hydro-Gen process,
which is the main topic of this review article. Each application
in Fig. 1 is associated to more than one literature report as
summarized in Table 1 providing an overview of the ultra-
sonic frequency condition, a couple of references and a short
description for different ultrasound applications but does not
report on the recommended ultrasonic frequency or fre-
quency range for each application. Other important parame-
ters including, acoustic power, intensity, pressure amplitude
and irradiation durations are not listed for the sake of clarity.
In closing, ultrasound has a wide history of use in routinely
engineering applications for monitoring and diagnostic as
summarized in Table 1. However, ultrasound is now consid-
ered a potential method for producing useful gases, such as
hydrogen [25]. The ultrasonic power can produce acoustic
cavitation-bubbles; the implosion of these bubbles after
several successive growths will result in a tremendous
amount of energy enough to produce hydrogen from liquid
water via the sonolysis process. This Sono-Hydro-Gen process
has a significant implication that takes place in the frequency
range between 20 and 40 kHz. Producing hydrogen using the
power of ultrasound is a significant challenge, although ul-
trasound offers an eco-friendly way to produce hydrogen by
introducing ultrasound waves to liquid water.
Review objectives
Knowledge about hydrogen production using sonoreactors is
insufficient for describing the best operating conditions.
Recently, limited studies consider the Sono-Hydro-Gen
approach for hydrogen production. However, factors affecting
the hydrogen production rate is still unclear [25]. The obvious
advantage of this technique is to highlight the tremendous
opportunity and the new venue that will be opened to many
research studies for hydrogen production via ultrasonication.
This challenge lies in advancing our fundamental under-
standing of the novel approach and probing the different
influential factors to obtain the optimum H
2
-production rate.
With the ability of ultrasound and with the potential of
applying ultrasound waves to liquid water, ultrasound forms a
powerful tool for the future of hydrogen generation.
This review article is a follow-up review by Pollet et al. in
Ultrasonic Sonochemistry [25] and is to provide further a
comprehensive review on the Sono-Hydro-Gen technology. In
this review, an intensive introduction to the use of the ultra-
sonic power in the engineering applications is carried out.
Different methods of hydrogen production are demonstrated
and compared to assess whether these methods are effective
and environmentally favorable. Furthermore, the article pro-
motes our understanding of the ultrasonic power on the Sono-
Hydro-Gen technology along with enhancing the knowledge of
the mechanism associated with hydrogen production as the
mechanism is not yet understood and the most reported sug-
gestions are controversial [50]. Factors affecting the hydrogen
production rate and the theory beyond these effects are well
analyzed and reported. Recent numerical and experimental
investigations on the hydrogen production scheme are
international journal of hydrogen energy 44 (2019) 14500e1452614502
intensively reviewed showing different numerical simulations
and different experimental configurations. Finally, perfor-
mance and efficiency criteria are reviewed along with the
challenges associated with the Sono-Hydro-Gen design.
In the next section, different hydrogen production
methods will be briefly reviewed and tabulated to compare
whether these methods are environmentally friendly and
economically feasible.
Hydrogen production processes
Hydrogen is not only very powerful and efficient but also it is a
renewable source of energy, as it can be produced via five (5)
main categories of technology, namely, thermochemical [51],
(ii) electrochemical [52], (iii) photobiological [53], (iv) photo-
electrochemical [54], and (v) Sono-Hydro-Gen [55], which are all
summarized in Fig. 2. A brief overview is made to describe
each of these technologies stating the advantages and disad-
vantages of each of them and a comparison is drawn among
all methods in terms of the process, the chemical reaction, the
advantages and disadvantages, and the H2- production rate
and cost.
(i) Thermochemical technology. This mean of hydrogen
production involving the steam gas reforming (SMR)
[56], which is considered one of the widely used means
of hydrogen production from a gas material such as
methane, ethanol and methanol. However, the gasifi-
cation processes are used when the raw material is solid
such as: coal or biomass [57e59]. This technique has a
wide sustainability problems, therefore, Dincer and
Acar [4] reviewed and evaluate different hydrogen pro-
duction methods for enhancing sustainability of such a
technique. Stream gas reforming is not an environ-
mentally friendly way for hydrogen production as re-
ported by Haryanto et al. [60].
(ii) Electrochemical technology. This technique is in charge
with water electrolysis (WE) to produce hydrogen [61].
This technique is a high-energy demanding with an
overall efficiency of 60%. In fact, it can be very efficient if
the electricity cost is below 2 cents/kWh.
(iii) Photobiological technology. The photobiological tech-
nology uses the natural photosynthesis activity of bac-
teria and green algae to produce hydrogen [62]. One
main problem is that the production rate is very slow.
Detailed reviews associated with this technique can be
found in Refs. [63,64].
(iv) Photoelectrochemical technology. The photo-
electrochemical technology is producing hydrogen in
only one-step using the water-splitting phenomenon via
illuminating a water-immersed semiconductor with
sunlight [65]. A better technique to produce more cleaner
hydrogen is the so-called Photocatalytic water splitting
which can decompose oxygen and hydrogen by utilizing
sunlight with the aid of photo-catalyst [66e69]. One
obstacle of this methods is that the instability of the
semiconductor materials in the aqueous phase. Other
disadvantages is provided by Haryanto et al. [60].
(v) Sonochemical technology. Sonochemistry is defined as
how the power of ultrasound can be utilized in chem-
istry. In fact, it has been well recognized that, hydrogen
Fig. 1 eA summary of ultrasound applications at different corresponding ultrasonic frequencies.
international journal of hydrogen energy 44 (2019) 14500e14526 14503
can be produced by introducing ultrasonic waves to
liquid water. As compared to the other non-renewable
energy sources, hydrogen can be produced infinitely
by simple means of separation from water molecules.
This can be provided by the Sono-Hydro-Gen approach.
A summary given in Table 2 presents a brief illustration
and a comparison between the five main categories of
hydrogen production in terms of fundamental theory and
remarks. The thermochemical technology is associated with
the steam gas reforming [70] which is not environmentally
friendly method, whereas, the electrochemical technology
related to water electrolysis requires high electrical power.
The photobiological technology problem is that the produc-
tion rate is very slow. The photoelectrochemical technology is
producing hydrogen in only one-step using the water splitting.
In all cases, though, the need to drive costs and carbon down
and to drive efficiency and production capacity up provides a
striking solution right to the heart of the problem confronting
hydrogen-for-energy.
Table 1 eA summary of ultrasound applications in the engineering field.
Application Frequency Description References
Thermoacoustic
refrigeration
400 Hz An eco-friendly refrigeration technology, which triggered
energy to transfer heat from one side to another side.
Wetzel et al. [16];
Newman et al. [17];
Thermoacoustic heat
engines
4e20 kHz A thermos-acoustic technology that utilizes the power of
ultrasound waves to pump heat from one side to another and
contrariwise. This application uses the temperature difference
to produce high amplitude sound waves.
Backhaus and swift [18,19];
Ghazali [20]
Ultrasonic in Extraction 20 kHz The process of transferring a substance from any matrix to an
appropriate liquid phase, assisted by sound waves (>20 KHz in
frequency) that propagate through the liquid media.
Alzorqi and Manickam [21,22];
Chemat and Ashokkumar [23]
Sono-Hydro-Gen 20e1,000 kHz A new method for H
2
-production from a mechanical source
such as ultrasound. The power from ultrasound used to
dissociate the water molecules into OH and H radicals, then
these radicals recombine together to produce H
2
and O [24].
Pollet et al. [25]
Son et al. [26]
Ultrasonic crack inspection 40e48 kHz Ultrasonic crack inspection has the following procedure:
Introduce ultrasound pulses to piping system, reflection and
refraction at the inner wall surface, corner reflection at external
crack, receiving surface echo and finally receiving crack echo.
Glushkov et al. [27];
Komura et al. [28];
Burrows et al. [29]
Detecting Piping corrosion 70 kHz A new technology for detecting of corrosion in piping system of
chemical plants using cylindrical waves.
Alleyne and Cawely [30];
Detecting notch 70 kHz Detecting notch by using ultrasound waves Lowe et al. [31]
Novel Ultrasonic
Viscometer in Engines
100 kHz To obtain the viscosity of the mixture it is mandatory to choose
a lower operating frequency at 100 kHz.
Schirru et al. [32];
Markova et al. [33]
Monitoring of food
materials
100 kHz Quality control and monitoring of different food materials
throughout food industry to guarantee and maintain high
quality and safety food production.
Awad et al. [34];
Mason et al. [35]
Food processing,
preservation and safety
500 kHz The power of ultrasound is implemented at higher frequencies
to induce physical, mechanical and biomedical effects on the
foods properties and considered very promising in food
preservation.
Juarez et al. [36]
Chandrapala et al. [37]
Axial and circumferential
Crack detection
500 kHz Detecting axial and circumferential cracks in piping systems of
nuclear power plants via the analysis of the short-time Fourier
transformation.
Cheong et al. [38];
Reber et al. [39]
Piping inspection 3.1e5 MHz Development of ultrasonic solutions for sewer inspection. Pipe
deformation and anomalous conditions can be simulated.
Gomez et al. [40]
Liu and Kleiner [41]
Pipe wall thinning
monitoring
2e4 MHz It is a non-destructive type evaluation of wall thinning in power
plants for continuously monitoring the plant while under
operation.
Kosaka et al. [42]
Lee et al. [43]
Mapping of the film
lubricant thickness used
in a piston skirt
10 MHz To outline the applicability of the ultrasonic methodology to
both piston skirt film thickness measurement and show the
possibility of deducing some piston secondary motions.
Mills et al. [44]
Measuring Wheel/Rail
Contact Stresses
10 MHz To investigate how rail components contact by reflection of
ultrasound. The acoustic wave is emitted and bounced back
from an incomplete interface. The higher the interaction load,
the more reasonable will be the contact and consequently more
wave will be transmitted.
Marshall et al. [45,46]
Monitoring Car Engines 40e80 MHz Ultrasound scans have been widely used in medical application,
but it has never been used in testing the performance of a
modern combustion engines. It is used to measure the engine
performance parameters. Such as monitoring the piston oil
film, piston ring oil and lubricant film.
Dwyer Joyce [47,48]
Avan et al. [49]
international journal of hydrogen energy 44 (2019) 14500e1452614504
The next section will focus on the sonochemistry tech-
nology, benefits of sonochemistry, Sono-Hydro-Gen theory and
the system design. The originality of this interesting topic
goes back to the sonochemistry field.
Sonochemistry technology
In this section, sonochemistry approach is will be reviewed
presented as well as the Sono-Hydro-Gen process. The sono-
chemistry approach is defined as when ultrasound waves are
introduced into a liquid medium to bring an unusual chemical
environment [76]. Ultrasound waves are introduced to the
sonication medium by ultrasonic transducers. Piezoelectric
transducers selection and applications in sonoelec-
trochemistry are reported by Pollet [77]. The Sono-Hydro-Gen
approach is one of several benefits beyond the sonochemistry,
and it will be illustrated in a later section in details. The power
of ultrasound can generate an acoustic cavitation bubble
within a liquid. Pollet [15] summarized the main benefits of
the acoustic cavitation bubble. Highlighting that it can pro-
duce a tremendous amount of energy, which can enhance a
range of chemical reactions and can enhance the electro-
chemical diffusion processes.
Sonoelectrochemistry
Sonoelectrochemistry is defined as a combination of three
fields including electrolysis, ultrasound and electrochemistry
which is initially reported by Morigushi in 1930s. In the elec-
trolysis process, hydrogen is produced at the decomposition
cell voltage in the molecular form which is taking place on the
surface of electrodes via electrochemical reaction. Then the
molecular hydrogen gas nucleate at the cavity of electrode
surface to hydrogen gas bubbles at the cathode active sites.
The hydrogen gas bubbles start to enlarge at the surface of the
electrode. Early in 1990s, Sheng-De Li et al. [78] and Richard
et al. [79] reported that the effect if introducing ultrasonic
waves to an electrolysis process increases the energy effi-
ciency considerably.
In next sub-sections, fundamental aspects, many benefits,
the Sono-Hydro-Gen production approach, acoustic cavitation
bubbles and important factors affecting the hydrogen pro-
duction rate are coherently reviewed.
Benefits from sonochemistry
Ultrasound is widely used for several applications in different
fields including hardening by immersed metals [80], several
medical and clinical applications, for example: drug delivery
and other therapeutic applications [81], enhanced electro-
spinning [82], enhanced bladder cancer therapy [83] and
accelerating chemical reactions and processes [84]. The ul-
trasonic waves and irradiation are associated with efficient
chemical and physical effects for driving enhancing the
chemical reactions and yields. The idea beyond using ultra-
sound is to use less hazardous chemicals and solvents and to
reduce energy consumption. There are several benefits
beyond the sonochemistry approach such as it can enhance
the electrochemical diffusion processes.
Ultrasonic waves used to enhance the chemical reactions
and to provide an unusual chemical environment. For
example, organic syntheses can be greatly improved by the
use of ultrasound. A comprehensive review on the use of ul-
trasound in synthetic organic chemistry concentrating on the
applications in organic synthesis was written by Mason [85].
Many other researchers e.g. Cravotto and Cintas [86] and Bang
and Suslick [87] have performed successfully synthetic
organic reactions using ultrasound. Production of nano-
materials, environmental treatment, purifying water, corro-
sion of metals, cleaning of polymeric membranes, food
processing, cavitation bubble dynamics and hydrogen pro-
duction. Chen [88] performed a comprehensive review on
the applications of ultrasound in water and wastewater
treatment.
A summary of the recent different research disciplines
using the benefits of the sonochemistry technology is sum-
marized in Table 3 including the area of research, recent or old
references and a short description of each discipline.
Sono-Hydro-Gen system illustration
When sound waves of high frequency passing through a liquid
such as water, it leads to vibration of liquid water mechani-
cally, it is so-called Water Sonolysisor Water Sonication.
Fig. 3 shows and illustrates schematic of the sonoreactor
model. The ultrasonic probe immersed in a water container
emits sound waves through the water in the frequency range
between 20 and 40 kHz. Ultrasound also generates acoustic
cavitation bubbles within the liquid that are generated at the
tip of the ultrasonic probe. The typical ultrasonic wave has
compression and rarefaction acoustic pressures that accu-
mulate energy inside the acoustic cavitation bubble. This en-
ergy is in the form of several thousand of kelvins in
temperature and several hundreds of atmospheres in pres-
sure which is enough to dissociate the water vapour trapped
inside the bubble, the so-called sonolysis process [117].
Fig. 2 eHydrogen production through different sources of
energy.
international journal of hydrogen energy 44 (2019) 14500e14526 14505
These bubbles so-called acoustic cavitation bubbles that
take place when ultrasound is introduced to liquid water;
the medium goes through a series of compression and
rarefaction cycles. As rarefaction and compression high-
frequency sound waves travel through water, the expan-
sion will push apart the water molecules and give the
strong negative pressure to overcome, the intermolecular
forces while the compressions push the molecules together
through the strong positive pressure. If the sound waves
strong enough and in succeeding cycles this will lead to a
sudden pressure drop at which the cavitation phenomenon
occurs and creation of gaseous bubbles in liquid takes
place. Sequence and dynamics of acoustic cavitation bub-
bles. The mechanism has 4 (four) consecutive and instan-
taneous stages as seen in Fig. 4; (a) bubble formation, (b)
successive growth, (c) collapse [13],(d)microjets[81] and
as reported by Lee et al. [118,119].Thefirststageisthe
acoustic cavitation bubble formation due to the mechanical
vibration of water when ultrasonic waves introduced. The
second stage is the bubble enlarges and growth in succes-
sive cycles after which the bubble reaches the unstable
mode at which it is about to collapse. The third stage is the
acoustic cavity implosion at which a violent bubble collapse
leading to release high energy. However, a detailed system
description can be found in the recent perspective article by
Rashwan et al. [11].
The reaction mechanism inside a single-bubble satu-
rated with water vapour during a water sonolysis experi-
ment has a great interest. The rapid heating phase is
described as heat generated from the cavity implosion is
enoughfordissociatethewatermolecule(H
2
O) into highly
reactive hydrogen radicals H* and hydroxyl radicals OH*.
While the quick cooling process is responsible for recom-
bining the highly reactive radicals H* and OH* to form
hydrogen H
2
.Merouaniet al. [120] reported the most two
important reactions that 99.9% of the hydrogen is produced
from the gas phase recombination reaction, the reaction
can be given as follows:
H*þOH*4H2þO(1)
However, another recombination reaction takes place at
the surface of the bubble shell with a minor impact in H
2
-
production can be given as follows [121]:
H*þH*4H2(2)
Merouani et al. [122] performed a water sonolysis (water
dissociation to OHþH). They reported that the sonolysis pro-
cess of water by low ultrasound frequencies result in thermal
dissociation of water into hydrogen radicals H* and hydrogen
oxide radical OH*, this process is driven by a tremendous
amount of heat accumulated inside the bubbles due to a very
high temperature and high pressure resulted from cavitation
bubbles collapse. Ultrasonic cavitation of water has a subse-
quent collapse of microbubbles. This is considered a unique
phenomenon leads to hydrogen production during the water
sonolysis process. Water sonolysis is a promising and clean
technique to produce hydrogen, particularly if water is used as
the hydrogen source. The effect of the Sono-Hydro-Gen pa-
rameters is not clarified yet.
Table 2 eA conceptual illustration of different H
2
-production methods.
H
2
-production methods Theory beyond each method Remarks H
2
-Production rate and
cost
Thermochemical (steam reforming) CH
4
þ2H
2
Oþh
thermal
/4H
2
þCO
2
Not environmentally friendly as carbon
dioxide is produced.
9e12 tons of CO
2
/1 ton H
2
[71].
Electrochemical (water electrolysis) H
2
Oþh
electrical
/H
2
þ1
2O
2
Not environmentally friendly as it
requires high electrical energy.
53.4e70.1 kWh/1 kg of
hydrogen [72].
Photobiological 2H
2
OþCO
2
þAlgae/Cyanobacteria þh
solar
/O
2
þ4e
þ4H
þ/2H
2
Environmentally friendly, however, it has
very low and slow production.
0.07e96 mmol H
2
L
1
h
1
[73]
Photoelectrochemical H
2
Oþh
solar
/H
2
þ1
2O
2
Environmentally friendly, however, it has
limited durability because of the
instability of the semiconductor material.
This technology is still under
development. The challenge lies on the
durability material and the steady
operation. This technology is very
expensive.
17.3 $/kg of H
2
[74].
Sonochemical H
2
Oþh
sound
/OH* þH*
OH* þH*/H
2
þO[50]
Environmentally friendly, sustainable,
durable and low-energy consumption.
0.8 mM min
1
at acoustic
intensity of 0.6 W cm
2
[75].
international journal of hydrogen energy 44 (2019) 14500e1452614506
In the next section, several factors affecting the H
2
-pro-
duction rate during the Sono-Hydro-Gen process will be inten-
sively discussed.
Factors affecting the Sono-Hydro-Gen process
As a matter of fact, the rate of hydrogen production is gov-
erned by several important parameters as shown in Fig. 5,
foremost the acoustic frequency, acoustic intensity, dissolved
gas and the water bulk temperature [120]. However, a way to
quantify the hydrogen production rate has not yet been fully
developed and still in need of many numerical and experi-
mental investigations.
In the following sub-sections, major factors govern the rate
of hydrogen production are well illustrated including the
theory of how each factor affects the production rate and a
summary table is added at the end of different sections to
summarize the effect of each parameter on some useful gases
produced such as hydrogen peroxides H
2
O
2
and hydrogen H
2
.
Table 3 eSummary of the recent available area of research using the sonochemistry.
Area of research Description References
Organic syntheses The ultrasound in synthetic organic chemistry. Luche et al. [89];
Einhorn et al. [90];
Mason [85]
Production of
nanomaterials
The ultrasound technology is used for preparing nanomaterials by the
means of pulsed sonoelectrochemistry. Application of nanoparticles in
electrochemical is also reported by Luo et al. [91].
Saez et al. [92];
Luo et al. [91];
Pollet [13]
Muthoosamy and Manickam [93,94]
Environmental
treatment
It can be used for water and wastewater treatment by using the
advanced oxidation processes for the remediation of water,
wastewaters, odors and sludge.
Simon Parsons [95];
Oller et al. [96];
Poyatos et al. [97]
Water disinfection or
purifying water
The ultrasound is used also for purifying water Esclapez [98,99]
Panda and Manickam [100]
Corrosion of metals The corrosion behavior of these coating on some metal studied by the
electrochemical methods
Ashasssi and Bagheri [101,102];
Mason [103]
Cleaning of Polymeric
membranes
The ultrasound waves are also used for cleaning of polymeric
membranes for water treatment
Chai et al. [104]
Howell and Velicangil [105]
Ultrasound in food
processing
Ultrasound is promising for food processing because it has a significant
effect on enhancing several food processes.
Chemat et al. [106];
Mason [107];
Chandrapala et al. [108];
Knorr et al. [109]
Cavitation bubble
dynamics
The sonoelectrochemistry approach is used to investigate the
dynamics of cavitation bubbles and flow velocities
Pollet et al. [77,110];
Ashokkumar et al. [111,112];
Lee et al. [113]
Ultrasound in
separation
In recent years the use of high frequency ultrasound standing waves
for droplet or cell separation from biomass has emerged beyond the
microfluidics scale into the liter to industrial scale applications
Spotar et al. [114]
Manickam et al. [115]
Sono-Hydro-Gen The process is firmly illustrated in details Merouani et al. [24,50]
Son et al. [26,116]
Fig. 3 eSchematic of an ultrasonic probe vibrating in a liquid and thecorresponding acoustic pressurewaves adapted from [11].
international journal of hydrogen energy 44 (2019) 14500e14526 14507
Ultrasonic frequency
It is noticed that the amount of hydrogen produced from such
a process is considered a highly frequency dependent as it is
the most important parameter in sono-hydrogen generation.
The hydrogen generation rate increases with the increase of
applied frequency [123]. Several dynamic factors govern the
hydrogen production rate with frequency, namely, maximum
bubble core temperature and pressure, the amount of water
vapour trapped and the collapse time. At low frequencies, the
bubble will have more time to expand and enlarge this would
allow more water vapor to be trapped inside the bubble core.
As a result, the bubble collapse will be very strong and will
generate a higher pressure and temperature, which will pro-
mote the chemical reaction producing more radicals.
Whereas, at higher frequencies, the collapse time will be very
short and the bubbles will not have enough time to generate
radical as the reaction inside the bubbles will be very fast.
Combining all these factors together, we figure out why the
applied ultrasound frequency has a significant impact on the
hydrogen production rate. In Table 4 a summary of the con-
ducted studies on H
2
O
2
production using the ultrasound
waves is presented while comparing different studies at
different ultrasonic frequencies. It can be seen that the H
2
O
2
production rate is increasing while increasing the frequency
until it reaches an optimum point, then the rate goes down
back, this can be attributed to the formation of bubble clouds
that attenuate the acoustic intensity, which in turn will
reduce the production rate of H
2
O
2
.
Dissolved gas
The effect of dissolved gas on the hydrogen production per-
formance lies between two major physical properties; (1)
specific heat capacity ratio (g¼C
p
/C
v
) and (2) thermal con-
ductivity (k). The dissolved gas that has higher heat capacity
could accumulate higher temperature. Whereas, dissolved
gases with low thermal conductivity will have low heat
dissipation, which will allow more temperature to be trapped
inside the bubble. Consequently, selecting a dissolved gas
with high heat capacity and low thermal conductivity will be
the optimum selection for enhancing the dissociation process
of water vapor, hence, more hydrogen generation in return.
Summary of the numerical work carried out on the hydrogen
production using ultrasound is presented in comparison on
the hydrogen production rate at different frequencies and
different dissolved gases from the available literature review
and presented in Table 5.
Fig. 4 eThe sequence of acoustic cavitation bubble collapse. Note that the transducer probe is represented in a rectangular
shape underneath the bubble.
Fig. 5 eFactors affecting hydrogen production rate through
the sono-hydrogen approach.
Table 4 eA summary of the conducted studies on H
2
O
2
production using the ultrasound waves.
Frequency
[kHz]
Production rate of H
2
O
2
(mMole/min)
Petrier and
Francony [124]
Jian et al. [125] Merouani
et al. [24]
20 0.7 1.1 e
200 5 5.2 e
300 ee2.5
500 2.1 3 e
585 ee4.2
800 1.4 2 e
860 ee3.4
1140 ee2.1
international journal of hydrogen energy 44 (2019) 14500e1452614508
Acoustic power
The hydrogen production rate is highly dependent upon the
acoustic intensity. This is attributed to the fact that during the
collapse the acoustic bubble is acting as a micro-combustor in
which high-temperature chemical reaction takes place and
highly reactive radicals are the product of such chemical re-
action. The chemical reaction is governed by 3 (three) factors:
bubble temperature, collapse time and the bubble size, which
correspond to the amount of water vapor, trapped in the
bubble. With the increase of the acoustic intensity, the
expansion ratio of the bubbles will increase allowing more
water vapor to be trapped in every single bubble. Similarly, the
compression ratio increases leading to a higher bubble tem-
perature. As a result, the increase in expansion and
compression ratios of the bubbles will promote an unusual
chemical reaction leading to produce more free radicals from
the dissociation of the water molecules inside the bubbles.
Furthermore, increasing the acoustic intensity will increase
the collapse time, so the chemical reaction will have more
time to produce more reactive-radicals. Combining all of these
factors together leading to higher H
2
generation. Kerboua and
Hamdaoui [127] performed a numerical estimation of
hydrogen production at different operating conditions of
acoustic power and frequencies. They confirmed the theory of
increasing the acoustic intensity lead to an increase in the
hydrogen production rate. Their results are extracted and re-
ported in Table 6.
Bulk liquid temperature
The cavitation is considered a dynamic phenomenon, which
is strongly affected by the operating parameters such as bulk
liquid temperature, static pressure, and geometry of sonor-
eactor. The reaction mechanism of the sonochemical process
is influenced by the bulk temperature as pointed out by Sutkar
and Gogate [128]. Any tiny changes in the temperature will
alter the conditions of pressure and acoustic intensity of the
liquid medium that may yield a dramatically different cav-
itational effect [129]. Therefore, studying the temperature
change with the ultrasound irradiation is considered consid-
erably important to understand the characteristics of the flow
and acoustic fields inside the sonoreactor. Few studies have
considered the quantitative determination of the parameters
such as temperature and pressure field over an entire range of
operation as a function of different operating parameters by
Marangopoulos et al. [130] and Zeqiri et al. [131,132]. Kim et al.
[133] studied the effects of ultrasound irradiation on the
temperature and pressure distribution inside a sonoreactor.
Four (4) different solvents have been investigated at various
ultrasonic power for predicting energy density. Fig. 6 shows
the temperature change with respect to the sonication time at
different ultrasonic power namely 300 and 450 W. In all liquid
media, the temperature increases with time. However, the
differences in the physical and thermodynamic properties of
all liquid media are the reason behind the variation of the
temperature trends with respect to time.
The effect of the liquid bulk temperature is scarce in the
literature and the precise mechanism of this effect remains
unclear. The liquid bulk temperature has a significant effect
on bubble temperature and the hydrogen production rate in
return. The liquid bulk temperature is very important as it is
considered the surrounding medium of the acoustic cavita-
tion bubbles. In fact, when bulk liquid temperature increases,
the bubble temperature increases leading to liquid-vapor
pressure increases and more vapor is trapped inside the
bubble. However, increasing the bulk temperature will make
Table 5 eA summary of the conducted studies on H
2
-production using the ultrasound waves.
Frequency Dissolved Gas H
2
-production
Rate
References
20 kHz Argon 0.8 to 5 mM/min Venault [75], 1997.
1000 kHz Argon 13.6 mM/min Margulis and Didenko [126], 1984.
1000 kHz Air 0.22 mM/min Margulis and Didenko [126], 1984.
1100 kHz Argon 10
17
M/s Merouani [122], 2014
1100 kHz Argon 10
13
M/s Merouani [120], 2016
Table 6 eH
2
-production (Mole) at different acoustic power
and frequencies. Data extracted from Ref. [127] by
Kerboua and Hamdaoui.
Acoustic Amplitude
Acoustic
frequency
1.5 [atm] 2.0 [atm.] 2.5 [atm.] 3.0 [atm.]
200 [kHz] 1.33 10
19
2.53 10
17
7.35 10
17
1.30 10
16
1,000 [kHz] 2.98 10
33
5.67 10
24
1.64 10
21
2.91 10
19
Fig. 6 eTemperature rise in various solutions with
different ultrasonic power, reprinted from Ref. [133].
international journal of hydrogen energy 44 (2019) 14500e14526 14509
the bubbles collapse less violent in turns affecting the
decomposition process of water molecules causing fewer
active radicals. Then the combination of these two important
factors should lead to an optimum liquid bulk temperature at
which the maximum hydrogen production rate is achieved.
Bubble temperature
The bubble temperature is one of the important parameters
that affect the mole fraction of the produced hydrogen. The
maximum bubble temperature is associated with two opera-
tional conditions such as the frequency and the acoustic
amplitude. Merouani et al. [122] reported the amount of H
2
production with respect to bubble temperature with different
gas based models. It can be seen that there is an optimum
hydrogen rate recorded in the range 5,000e7,000 K. The higher
the bubble temperature, the higher the amount of hydrogen
production as per Table 7. The results revealed that at a low
acoustic amplitude and high frequency, the amount of
hydrogen production is higher, while the hydrogen produc-
tion is lower at a high acoustic amplitude and low frequency.
On the other hand, to attain the maximum bubble tempera-
ture at the end of bubble collapse, higher acoustic amplitude
and low frequency should be applied [122].
Merouani et al. [120] performed a comprehensive numeri-
cal study of hydrogen production using acoustic cavitation
bubbles in water. Fig. 7 (a) presents the effect of ultrasound
frequency on the hydrogen production rate in case of air as a
dissolved gas. While Fig. 7 (b) depicts the production rate of
hydrogen with respect to ultrasound frequencies at different
acoustic intensities [W/cm
2
].
To sum up this section, the overall generation of H
2
is
controlled by the amount of water vapour trapped inside the
bubbles. To quantify this amount of water vapour, a series of
preliminary numerical and experimental studies need to be
performed. A large number of parameters, including
frequency and intensity, need to be explored to develop a
sufficient understanding of the phenomena. Furthermore,
optimization and regression/statistical analysis need to be
conducted to examine the optimum point and the most sig-
nificant parameter that would give the maximum H
2
pro-
duction rate.
In the next section, recent numerical modeling and simu-
lations concerning sono-reactors are presented.
Recent numerical modeling studies
In this section, recent numerical modeling and solution for
the Sono-Hydro-Gen approach is presented starting from the
hydrodynamic modeling for the gas inside bubble in liquid
medium considering simulation about bubble behavior at
different ultrasonic frequencies. Then we will be turning to
some numerical simulation of the sono-reactor for charac-
terizing the flow and the acoustic fields within the
sonoreactor.
Hydrodynamic modeling
Hydrodynamic modeling and solution for the gas inside a
bubble in a liquid medium is subjected to ultrasound waves
triggers solving the Navier-Stokes equations for the gas inside
the bubble. The conservation of mass for the gas inside the
bubble assuming that the bubble has a symmetrical and
spherical shape and the governing equations associated with
the gas trapped inside a bubble subjected to ultrasound waves
are introduced including mass, momentum, and energy are
given by Kim et al. [134]. Numerical simulation of a near wall
bubble collapse is performed by Osterman et al. [135] in an
ultrasonic pressure field.
This numerical simulation has considered a 2-Dand
axisymmetric model. A pressure field is generated with the
Table 7 eH
2
-production [mol] at bubble temperature [K] by Merouani et al. [122].
Bubble Temperature [K] 1500 2000 2500 3000 3500 4000 4500 5000
H
2
Production [mol] 2 10
33
110
25
2.1 10
24
3.5 10
21
0.5 10
19
710
18
6.3 10
18
5.1 10
15
Fig. 7 e(a) The effect of the ultrasound frequency on hydrogen production rate, (b) the effect of acoustic intensity on
hydrogen production rate by Merouani et al. [120].
international journal of hydrogen energy 44 (2019) 14500e1452614510
bottom of a container oscillating at 33 kHz. In this study, a
validation of the model is successfully achieved by comparing
a bubble collapsing near the oscillating wall as compared to
the experimental work done by Philipp and Lauterborn [136].
Results considering the pressure contour oscillation and the
pressure fluctuation are reported in Fig. 8 (a) and (b), respec-
tively. The comparison is made in terms of the dynamics
sequence of the cavitation bubble collapse with respect to
time. A sequence of the acoustic cavitation bubble captured in
an experiment that is conducted by Philipp and Lauterborn
[136] and a numerical work done by Osterman et al. [135]. The
differences between the experimental and numerical simu-
lation is that at the end of the collapse, it can be noticed that
the differences lie on the bubble shapes and the bubble posi-
tion. This can be attributed to that the numerical simulation
did not consider the phase changes and the experimental
work has some uncertainties due to the gravitational effects.
Another difference can be found between both experimental
and numerical work is that the counter-jet resulted from the
bubble collapse is not captured by the numerical simulation,
this is also attributed to that the phase change has not
considered in the numerical simulations.
Many research studies conducted to investigate the
acoustic cavitation bubbles. The cavitation bubbles can be
characterized by the dynamics of oscillations and maximum
pressure and temperature inside the bubbles before the
collapse. Rooze et al. [137] performed an overview for charac-
terization of acoustic cavitation bubbles reporting some
recent experimental reports for characterization of the bub-
bles. In the textbook by Yasui [138], a comprehensive illus-
tration is included for helping readers to understand the
phenomenon of the acoustic cavitation and bubble dynamics.
Sonoreactor modeling
CFD simulation is performed on the acoustic cavitation in a
crude oil upgrading sono-reactor and prediction of collapse
temperature and pressure of a cavitation bubble by Niazi et al.
[139]. In this study, ultrasonic waves introduced to liquid
water contained in a sonoreactor via an ultrasound probe to
investigate the pressure distribution.
The experimental data is utilized from the Hielscher Ul-
trasound Technology website for a sono-reactor filled with
water at 20 kHz and 2 kW. In the same study, CFD analysis of
Fig. 8 e(a) Oscillating pressure field in the domain; (b) the pressure fluctuation at the center of the bottom (blue) and the
bottom displacement (pink) by Osterman et al. [135]. (For interpretation of the references to colour in this figure legend, the
reader is referred to the Web version of this article.)
Fig. 9 eActive cavitation zones simulated by CFD technique for the reactor filled with saturated crude oil at temperature of
25 C by Niazi et al. [139].
international journal of hydrogen energy 44 (2019) 14500e14526 14511
acoustic cavitation in a crude oil sono-reactor and prediction
of collapse temperature and pressure of the cavitation bubble
is conducted as well. Fig. 9 presents the numerical results
simulated to show the active cavitation bubbles zones in the
sono-reactor filled with saturated oil at a bulk temperature of
25 C. The acoustic pressure threshold for acoustic bubbles is
estimated to be 0.153 MPa with an initial oil bubble size of
10 mm. On the other hand, the figure is also showing the
collapse pressure and temperature of the generated acoustic
cavitation bubbles while crude oil is the working medium. The
collapse pressure and temperature may go up to several
thousands of Pa and Kelvins, respectively. The temperature
and pressure fields due to the collapsing of bubbles under
ultrasound conditions are predicted by Kim et al. [134] via the
solution of Navier Stokes equations for the gas trapped inside a
bubble. They compared the pressure profile of four different
liquid mediums. The pressure profiles were different from
each other; this can be attributed to the difference of the
sound velocity and the density of each medium. They re-
ported the pressure profile and temperature contours of water
as seen in Fig. 10.
Generally, the hot spot zone usually takes place near to the
tip of the ultrasound probe. From the results of pressure
profile, it can be concluded that the pressure profile oscillates
starting from the probe tip to the way down to the bottom of
the container. At an acoustic power of 300 W, the maximum
and minimum pressure are recorded at 8.838 and 9.533 Pa,
respectively. The temperature increased while increasing the
acoustic power or the irradiation time. Sutkar et al. [140] per-
formed a numerical analysis of the theoretical prediction of
cavitational activity distribution in a sono-reactor. Numerical
simulation is carried out and compared with experimental
investigations. A 2 cm diameter ultrasonic probe with
a maximum power of 240 W and a frequency of
20 kHz is been immersed in a cylindrical water bath
(D ¼13.5 cm H¼17.5 cm). The results presented the vari-
ation of the pressure distribution inside the sono-reactor as
seen in Fig. 11. The pressure contours of the vertical trans-
ducer and its corresponding pressure amplitude in the axial
direction of the ultrasound probe are shown in Fig. 11(a) and
(b), respectively. It is well recognized that the maximum
pressure amplitude is close to the tip of the transducer probe
and the pressure tremendously decreases in the way to the
bottom of the reactor. The pressure contours of the longitu-
dinal transducer and the corresponding pressure amplitude in
the axial direction of the ultrasound probe are shown in
Fig. 11(c). Pressure fluctuation is observed along the length of
the probe in the x-direction at z¼0.095 m as presented in
Fig. 11(d).
Many research studies considered the improvement of the
acoustic and flow fields of the sono-reactor. Wei [141] who
performed a numerical simulation to design and characterize
an ultrasonic transducer to overcome disadvantages of
traditional transducers. Wang et al. [142] and Memoli et al.
[143] performed characterization studies and improvement of
a cylindrical-type sono-reactor.
In the next section, different experimental configurations
have been summarized and reported. Analysis of the most
important findings is quantified coherently.
Experimental configurations and analyses
An ultrasound-induced cavitation bubbles can be a source of
acoustic waves due to bubble oscillation. The production of
these sound pressure waves can be attributed to two reasons;
Fig. 10 ePressure profile (a), pressure contour (b), and temperature contour (c) with ultrasonic power of 300 W, the liquid
medium is water by Kim et al. [134].
international journal of hydrogen energy 44 (2019) 14500e1452614512
the first reason is that these pressure waves is a result of the
bubbles collapse, whereas the second reason is that these
pressure waves are produced from the interaction between
the bubbles, the wall and the reflected ultrasound waves from
the walls. It is not yet clear that the production of these sound
pressure waves is due to which of these reasons. Therefore,
further experimental investigations should be carried out. An
overview of different experimental configurations and recent
experimental work procedure and their significance in un-
derstanding the Sono-Hydro-Gen production approach will be
presented.
Different experimental configurations
There are three main configurations of sono-reactors as
shown in Fig. 12, the ultrasonic transducer horn or probe
(Type-A), the ultrasonic transducer bath (Type-B) and the in-
direct irradiation ultrasonic bath (Type-C). In case of the ul-
trasonic horn, the transducer is immersed inside the liquid
container, the ultrasound waves are introduced from the horn
tip with a diameter smaller than the acoustic wavelength,
consequently, the acoustic cavitation bubbles generated.
While in case of the ultrasonic bath, it is mainly used for
cleaning purposes, where the ultrasonic waves are introduced
at the bottom of the liquid container. The generated bubbles
are strongly affecting the ultrasound waves and the acoustic
intensity. The decrease in the number of bubbles causes an
increase in acoustic intensity in a tight liquid container.
An indirect irradiation of ultrasound waves is also possible
as shown in Fig. 12(c). This configuration consists of an ul-
trasonic bath within which a small water container. The ul-
trasonic bath is filled with degassed water so as not to form
bubbles [144]. The degassed water can be obtained by
reducing the ambient pressure using a vacuum pump or by
boiling. The benefits of this configuration are that the number
of bubbles decreases with time because of degassing of bub-
bles leading to a decrease in gas concentration of the liquid
inside the bath. The transducer emitting ultrasonic waves to
travel through the degassed water until reaching the small
liquid container. As known, the presence of bubbles in the
bath will attenuate the acoustic intensity travels to the small
liquid container. Therefore, the decrease in the number of
bubbles in the bath will increase the acoustic intensity in the
small container. Recently, Yasui et al. [138] recommended
that the liquid surface inside the small water container
should be aligned with the same level of the degassed water
in the liquid bath in order to obtain the same irradiation
condition.
Traveling and standing waves governing equations asso-
ciated with the ultrasonic transducer immersed in the sono-
reactor are summarized and well-illustrated by Kinsler et al.
[145]. They provided an intensive illustration of different wave
shapes such as a plane sound wave traveling through a liquid
medium. Furthermore, a spherical wave can be formed if the
acoustic wave source is point source that emits an acoustic
wave into a liquid medium. The authors gave also an example
considering a circular plane disc emits an acoustic wave into a
liquid medium. In fact, the circular disc is acting similarly to
the tip of the ultrasonic probe that emits ultrasound waves to
the liquid medium inside the sono-reactor. The equation
governing the spatial distribution of the pressure amplitude of
a circular disk emits an acoustic wave into a liquid medium;
the acoustic pressure distribution can be given as follows on
the axis of symmetry:
Fig. 11 ePressure field distributions inside the sonoreactor; (a) pressure contours of the vertical transducer, (b) Axial
pressure amplitude distribution, (c) pressure field of longitudinal transducer (d) radial pressure amplitude and the direction
of transducer at z ¼0.095 from the bottom by Sutkar et al. [140].
international journal of hydrogen energy 44 (2019) 14500e14526 14513
PaðxÞ¼2r0cn0
sinp
lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x2þa2
px
(3)
where n0is the velocity amplitude of the vibrating disc, lis the
wavelength of the acoustic wave, xis the position from the
disc to the point of measurement on the axis of symmetry and
ais the radius of the circular disc. The disc is acting similar to
the tip of the ultrasound horn or probe that emits ultrasound
waves to water inside the sono-reactor. The generated
acoustic cavitation bubbles by ultrasound are significantly
affecting the density and sound velocity in the medium. In
general, the density, the sound velocity, and the acoustic
pressure amplitude decrease because of the generation and
presence of bubbles under an ultrasound probe. As a matter of
fact, the decrease in acoustic pressure amplitude has been
studied and can be found in Ref. [146].
Koda et al. [147] compared 3 (three) different experimental
setups for the sake of calibrating the sonochemical efficiency
of different sono-reactor. The experimental setup (a) is built,
operated and tested in the National Institute of Advanced
Industrial Science and Technology (AIST). An ultrasound
transducer of 45 mm is mounted at the bottom of a water bath
to sonicate a sample of a volume of 50 cm
3
as seen in Fig. 13(a).
While the experiments of Nagoya University and Shiga Uni-
versity of Medical Science are presented in Fig. 13(b) and (c).
All experiments are used to create a standard method to
calibrate the efficiency of the sono-reactor and sonication
efficiency and results of this comparison study are presented
in the next section.
Recent experimental reports
An optimization of a sono-reactor (Type-A) subjected to a
frequency of 20 kHz was investigated numerically by Klima
et al. [148] and compared with experimental results. The sec-
ond significant important parameter affecting the sono-
reactor performance is the acoustic power intensity, which
comes after the ultrasound frequency. In this study, the effect
of acoustic power intensity on the sono-reactor characteris-
tics and the ultrasound fields are studied. In case of low
intensity, the prediction of the intensity distribution is well
simulated, while the opposite in case of high intensity, the
estimation of the intensity distribution is more complex.
Fig. 14 (b) presents the intensity distribution around the tip of
the ultrasound probe. It is found that the higher intensity
takes place close to the probe tip. Fig. 14 (b) presents a com-
parison between the experimental results and the numerical
analysis and it can be seen that both are fitted closely.
Son et al. [26] performed an experimental investigation on
the acoustic emissions spectral using Type-B experimental
configuration. They considered different experimental pa-
rameters including the liquid height and transducer power.
Fig. 15 (a) presents their experimental setup of the sono-
reactor, which is consisted of an acrylic cylindrical sono-
reactor (11 cm diameter and 110 cm height) with a trans-
ducer type piezoelectric transducer PZT. A 36 kHz frequency
transducer mounted at the bottom of the sono-reactor. The
water container is filled up with water at different liquid
heights such as 100 cm. A power meter is mounted at the exit
of the ultrasonic transducer controller to control the power
input. A hydrophone is used to record the acoustic emission
spectra and it is fixed at the mid-point of the sono-reactor.
They investigated 3 (three) different electrical input power
and they reported the total relative sono-chemiluminescence
H
2
O
2
generation and the calorimetric heat power. They re-
ported that the H
2
O
2
generation is 10.7, 30.6 and 25.6 mM at 30,
60, 90 W, respectively. Fig. 15 (b) shows the effect of increasing
the transducer power on the Sono-chemiluminescence im-
ages. The water inside the sono-reactor is mechanically
vibrated. The results showed that at low acoustic input power
bright zone appeared as an indication for a traveling wave and
a standing wave is observed in the remaining part of the sono-
reactor. At an average acoustic power, the bright conical zone
is reduced and the stripes are concentrated along the sono-
reactor axis. Whereas, at high acoustic input power, the
bright zone is reduced because of a cloud of bubbles is formed
near to the transducer, note that the transducer is mounted at
the bottom of the sono-reactor. They reported a very impor-
tant observation that higher acoustic power is not signifi-
cantly affecting the hydrogen production yield, but it can be a
Fig. 12 eDifferent experimental configurations of the sono-reactor to generate acoustic cavitation bubbles.
international journal of hydrogen energy 44 (2019) 14500e1452614514
reason of emitting acoustic emissions at harmonic fre-
quencies. The more the bubbles are generated, the more the
acoustic energy will be attenuated.
A recent experimental work performed by Merouani et al.
[24] studied different methods for estimating the active bub-
bles in a type-B sonoreactor. Experiments involving Sono-
Hydro-Gen and H
2
O
2
production was carried out using an ul-
trasonic reactor containing 300 ml of distilled water. They
conducted the experiments in cylindrical water-jacketed glass
reactors. They reported some experimental procedures
including that water temperature should be kept at 25 Cbya
water jacket recirculation around the cylinder. They reported
that the bubble radius and H
2
O
2
production rate are highly
ultrasonic frequency dependent.
A different configuration has been suggested by Cotana
et al. for studying water photo-sonolysis for hydrogen pro-
duction. H
2
O
2
and H
2
are the main products of this sono-
chemical reaction mechanism because of the recombination
Fig. 13 eDifferent experimental configuration of sonoreactors in three different laboratories (a) AIST 96k, (b) Nagoya 130k
and (c) Shiga 200k by Koda [147].
Fig. 14 e(a) Detail normalized ultrasonic intensity distribution at the ultrasonic horn tip, (b) comparison between the
experimental sonoreactor (water, 20 kHz, ultrasonic power ¼10 W) and the predicted intensity distribution for the same
geometry by Klima et al. [148].
international journal of hydrogen energy 44 (2019) 14500e14526 14515
of the highly reactive radicals, produced from the dissociation
of the water molecules at the first chemical reaction. They
reported the sonochemical reactions steps in Table 8, corre-
sponding to H
2
and H
2
O
2
production. The experimental photo-
sonolysis reactor consisted of a rectangular reactor with one
glassed side to introduce photonic energy into the reactor. It
had 3 (three) main ducts as highlighted in the figure with two
piezoelectric transducers mounted in the bottom of the
reactor. In order to generate the ultrasonic field in water, two
piezoelectric transducers connected to the ultrasonic trans-
ducer controllers were attached to a power meter. The
transducer generated the ultrasonic waves at a frequency of
22.5 kHz with a minimal input power of 50 W only. The Sono-
Hydro-Gen experiments were carried out using the following
procedure; first, the sono-reactor was filled with 0.1L of
distilled water, second, the water above the water surface was
injected with an inert gas, thirdly, the water was subjected to
the ultrasonic actions at different pressure conditions
namely, 1.0, 1.5, 2.5 atm. They performed a parametric study
analysis to investigate the effect of the sono-reactor pressure
on the hydrogen production rate.
Fig. 16 presents the hydrogen production in mM with
respect to time (in minutes). The results show a linear rela-
tionship between the produced hydrogen and time. Further-
more, the highest production rate took place for 1.0 atm
pressure condition and the production rate of hydrogen
decreased as the pressure inside the sono-reactor increased.
This could be attributed to as the sono-reactor is pressurized,
the acoustic cavitation bubbles cannot be oscillating freely
which in turns reducing the amount of heat absorbed by the
bubbles and affecting the hydrogen production rate.
The performance and efficiency assessment criteria of the
sonoreactors are presented in the next section. Detailed
description in view of the calculation procedure of the sono-
chemical efficiency based upon the energy density, ultrasonic
power dissipated in the liquid medium inside the sono-reactor
and cavitational energy. A comparison is made regarding the
sonochemical efficiency from previous reports and studies in
the literature.
Performance assessment criteria
In order to scale-up sonochemical reactors for industrial use,
one needs to investigate the efficiencies and the factors
affecting the sonochemical process. The most important two
parameters for developing a performance and efficiency
criteria are the energy density and the ultrasonic power
dissipation. In fact, energy density EUS is related to the tem-
perature change of the liquid medium during irradiation time
tUS that can be measured in a flask filled with the required
liquid to estimate the realpower of ultrasound. This
Fig. 15 eA redrawn experimental schematic of water sonoreactor (a); The Sono-chemiluminescence images under different
input power for 30, 60 and 90 W (b) by Son et al. [26].
Table 8 eSonochemical reaction steps by Cotana et al.
[149].
H
2
O/H* þOH* (1)
H*þOH* /H
2
þO (2)
OH*þOH* /H
2
O
2
(3)
H
2
O
2
/H
2
Oþ½O
2
(4)
H*þOH* /H
2
O (5) Fig. 16 eHydrogen production versus time for different
pressure conditions reprinted from Ref. [149].
international journal of hydrogen energy 44 (2019) 14500e1452614516
experiment was primarily suggested by Zarzycki et al. [150].
The energy density with the irradiation time and reaction
volume is calculated using the following equation as follows:
EUS ¼PUS:tUS
VI
(4)
A standard method to calibrate the sonochemical effi-
ciency of sono-reactor was initially reported by Mason et al.
[151]. They carried out the sonication experiment using
distilled water at 25 C. They performed a calibration analysis
and concluded the ultrasonic power dissipation into a liquid
can be calculated by the following equation:
PUS ¼CpUS
DT
Dt(5)
where mUS is the mass of water in kg, C
pUS
is the specific heat
capacity of water at a constant pressure of 4.19 kJ/kg/K and
(DT/Dt) is the temperature rise per second. For the study,
50 cm
3
of water is used at the initial room temperature. The
temperature rise is monitored by immersing a thermocouple
in a solution and is held at the half height of the solution. The
results of the ultrasonic power obtained by the ultrasonic
power-dissipation equation. Continuous introduction of ul-
trasound waves results on a temperature increase in the range
from 5 to 10 K. The potassium iodide solution (KI) is used to
quantify the amount of energy absorbed and it can be defined
as when ultrasound is irradiated into an aqueous KI solution,
the I
is oxidized to I
2
then they recombine together to form I
3
as following: I2þI4I
3, the concentration of KI solution is
(0.1 mol dm
3
). In a later study by Asakura et al. [152], the ef-
ficiency of the sonochemical process is quantified and it can
be given by the ratio of the number of reacted molecules m
1
over the ultrasound energy as follows:
hsonochemistry ¼mI
EUS
¼CI
PUStUS =VI
(6)
where C
I
,tUS and V
I
refers to the concertation of the ion I
3of a
(0.1 mol dm
3
) KI solution, the irradiation time of ultrasonic
energy and the solution volume, respectively. The production
yield of CI
3against the ultrasonic energy (kJ) which is calcu-
lated by multiplying the calorimetric ultrasonic power in (W)
by the sonication time (s). The production yield of CI
3corre-
sponding to the sonochemical effect when ultrasonic power of
1 W is introduced to a liquid for 1 s can be obtained by Fig. 17
(a). Actually, the CI
3values correspond to different ultrasound
frequencies can be obtained by Fig. 17 (b). In case of different
water volumes, Fig. 17 (c) shows that the CI
3is independent of
water volume. Here we draw attention that the sonochemical
efficiency is not laboratory dependent and not a volume or
quantity dependent. On the other hand, in case of the
sonoelectrochemical process, the sonoelectrochemical effi-
ciency can be described as the ratio of the theoretical energy
consumption of the electrolysis process (WTheo) over the
summation of the electrochemical energy consumption aided
with ultrasound (WUsA) and the amount of energy consumed
by the ultrasonic transducer (WUs) which is can be described as
follows [25]:
hSonoelectrochemical ¼WTheo
WUsA þWUs
100% (7)
The sonochemical efficiency has been also quantified by
Asakura et al. [152] as a function of ultrasonic frequency and
the liquid height inside the sono-reactor. They examined 4
(four) different frequencies including 45, 129, 231 and 490 kHz
while varying the liquid height from 10 to 700 mm, which is
corresponding to the different water volumes. Distilled water
was used in this experiment saturated with air at 298 K. The
sonochemical efficiency of the reactor was evaluated by po-
tassium iodide (KI) dosimetry and calorimetry. The sono-
chemical efficiency results were reported with respect to the
ultrasonic frequency and the liquid height inside the sono-
reactor. Fig. 18(a) and (b) present the effect of the ultrasonic
frequency and the liquid height of the sono-reactor on the
sonochemical efficiency, respectively. At different liquid
heights, the efficiency is reported with respect to the
different ultrasonic frequencies. It can be seen that for the
height of 100 mm, the findings are in good agreement with
the experimental work performed by Koda et al. [147] at
different ultrasonic frequencies. The sonochemical efficiency
was also examined with respect to the liquid height, which
behaved as one or two peaks for each ultrasonic frequency.
The ultrasonic irradiation time varied in the range of
60e1,800 s, based upon the liquid height and the maximum
temperature rise of the KI aqueous solution after introducing
the ultrasonic waves.
Another method to map out the ultrasonic energy in the
sonoreactor is to use electrochemistry as a tool. This method
consists of recording currents generated from an electro-
chemical quasi-reversible redox couple in the vicinity/at the
ultrasonicated electrode surface by using well-known elec-
trochemical techniques and using the Pollet-Hihn equation to
quantify with precision the acoustic power at several loca-
tions in the sonoreactor [153-154].
Sonochemical luminescence was also investigated in the
purpose of visualizing the sonochemical reaction field. In
Fig. 18 sono-reactor images were taken at different ultrasonic
frequencies and liquid heights. In (c) high intensity is visual-
ized near the top of the acrylic glass pipe and the highest in-
tensity is found at the 400 liquid height. In case of a lower
frequency of 129 kHz, the ultrasonic standing wave is
observed in the way traveling from the transducer at the
bottom to the top of the sono-reactor. Furthermore, the
standing wave is elongated while increasing the liquid height.
A comparison was made between two experimental
studies conducted by Koda et al. [147] in 2003 and Asakura et al.
[152] in 2008 on the sonochemical efficiency using the KI
dosimetry method. They reported the ultrasonic frequency
dependence of the sonochemical efficiency (SE-value) for KI-
solution oxidation as shown in Table 9. As shown, the
maximum sonochemical effects were recorded at an ultra-
sonic frequency range of 200e500 kHz. Both studies gave very
similar values to the SE.
Here it is emphasized and after comparing the sono-
reactor efficiencies from several studies that, the sono-
reactor efficiency is not affected either by the laboratory
scale experiments or by the sample volume. It can only give
some information and predict the average ultrasonic energy
induced by the evaluation of KI oxidation.
Investigation of acoustic cavitation bubble energy in a
large-scale sono-reactor was performed by Son et al. [116].
international journal of hydrogen energy 44 (2019) 14500e14526 14517
Fig. 17 eThe chemical effect per unit power (mol.dm
3
.W
-1
). The frequency dependence of chemical effects per unit power
concentration CI
3by Asakura et al. [152].
Fig. 18 eThe effect of ultrasound frequency and the sonoreactor's liquid height on the sonication efficiency by Asakura et al.
[152].
international journal of hydrogen energy 44 (2019) 14500e1452614518
They investigated cavitational energy distribution inside a
sono-reactor consisting of an acrylic glass container with di-
mensions of (W ¼0.6 m L¼1.2 m H¼0.4 m). The ultra-
sound PZT transducer with a diameter of 5 cm was placed at
the center-side of the container. The maximum transducer
capacity was 400 W. The sono-reactor was filled with 250 L of
tap water. The calorimetry method was not used here to
calculate the input power because of the large heat dissipation
due to the large surface area of the container; consequently,
the input power was measured using a multi-meter. An ul-
trasonic cavitation meter was used for measuring the average
cavitation energy in the sono-reactor. The maximum power
capacity was 240 W while examining different ultrasonic fre-
quencies namely 35, 72, 110 and 170 kHz. The maximum
cavitation energy was recorded at an ultrasonic frequency of
72 kHz at varying ultrasonic probe locations corresponding to
different irradiation distances as seen in Fig. 19(a). At a higher
ultrasonic frequency of 170 kHz, the cavitation energy was
approximately 1.0 W.
Son et al. carried out experimental analyses to quantify the
relationship between the average cavitation energy and the
ultrasonic frequencies as shown in Fig. 19(b). They reported
the average cavitation energy corresponding to different ul-
trasonic frequencies. It can be noticed that the cavitation
energy is much higher in case of low frequencies. The results
in this study showed that at higher frequencies more than
100 kHz, the ultrasonic waves could not propagate efficiently.
Furthermore, they developed an empirical correlation to
quantify the cavitation energy and sonochemical efficiency
which can be quantified by the following equations:
Cavitation energy ¼0:0008 ðinput energyÞ2
þ0:4699 ðinput energyÞ(8)
Sonochemical efficiency ¼0:0003 ðcavitation energyÞ2
þ0:0140 ðcavitation energyÞ(9)
To correlate the relationship between the cavitation energy
and the sonochemical efficiency, the input energy is used. In
Fig. 19 (c) and (d), the empirical correlation between the
cavitation energy against the input power and the sono-
chemical efficiency against the cavitation energy are pre-
sented, respectively.
Next section gives a snapshot on how previous reports
quantified the hydrogen production rate in case of water
electrolysis.
Hydrogen production quantification
This area of research also requires a great attention as a very
few studies quantified hydrogen generation via Sono-Hydro-
Gen process. Hydrogen production quantification plays an
essential role to understand the effect of several factors
affecting operating conditions and for the sake of upgrading
the sono-reactor to a large or an industrial scale.
In the case of water electrolysis, it is well known that this
process is a costly and highly energy demanding technology
with a consumption rate of 4.5e5 kWh per m
3
of H
2
. The
overall efficiency of the process is in the range of 30e40%. One
solution to reduce energy consumption is to use an ultrasound
aided-water electrolysis system. In a combined study of water
electrolysis with ultrasound, Symes [155], a previous
researcher at the Pollet's Birmingham University PEM Fuel Cell
Research group (UK), performed an experimental investiga-
tion of water electrolysis (under acidic and alkaline condi-
tions) in the presence of an ultrasonic field. They used a
graduated glass column filled with water to collect gas bub-
bles. The hydrogen gas formed was measured from the water
level change in the column. The equation of the efficiency of
hydrogen gas production is illustrated as follows:
hð%Þ¼ Vreal
Videal
100% (10)
where Vreal is the actual hydrogen gas production per unit of
time read by a digital hydrogen flow meter (Red-y compact
flowmeter (L/min)) [156]. While the ideal hydrogen production
rate for the water electrolysis process can be given as follows:
Vrealcm3¼SIt
nF RT
P(11)
where Sis the stoichiometric coefficient, Iis the applied cur-
rent, tis the electrolysis time, nis the number of electrons
transferred, Fis the Faraday constant (96,484 C/mol) and Pis
the pressure in Pascal. R and T are the universal gas constant
and the temperature in Kelvins, respectively. The results
revealed that the production efficiency of hydrogen is
increased at a range of 5e18% while the energy efficiency is
increased by 10e25%. In another combined study in the same
research group by Zadeh [156], the ultrasound aided water
electrolysis technology is used and the results showed an
improvement in production efficiency by 4.5% and energy ef-
ficiency by 1.3%.
To sum up, few studies considered hydrogen production
quantification only in case of ultrasound-aided water elec-
trolysis. However, methods to quantify the amount of
hydrogen from the Sono-Hydro-Gen process is not yet clarified.
In the next section, challenges associated with the Sono-
Hydro-Gen process are clarified and reported. These challenges
related to each of the following: the complexity of the acoustic
field, enhance understanding of the chemical reaction
mechanism, intensity distribution inside the sono-reactor,
attenuation of ultrasound waves, factors affecting hydrogen
Table 9 eUltrasonic frequency dependence of the
sonochemical efficiency (SE-value) for KI oxidation.
G[kHz] Koda et al. [147], 2003 Asakura et al. [152], 2006
SE
KI
(mol J
1
)10
10
SE
KI
(mol J
1
)10
10
20 0.6 ±0.02 e
40 0.6 ±0.02 e
45 0.67 ±0.06 5.5 ±0.6
96 4.5 ±0.2 e
96 4.1 ±0.2 e
130 5.6 ±0.4 6.4 ±0.3
200 8.3 ±0.6 6.7 ±0.6
400 7.8 ±0.2 e
500 7.1 ±0.2 7.1 ±0.1
1200 0.64 ±0.3 e
international journal of hydrogen energy 44 (2019) 14500e14526 14519
production rate, energy conversion and scaling-up the sono-
reactor.
Ultrasound and energy consumption
In case of the ultrasound efficiency, the overall ultrasound
power reaches the liquid is approximately 80e90% due to the
sequence of losses in the way between the power plug to the
liquid as illustrated in Fig. 20 [157].
On the other hand, the energy consumption of the sono-
chemical (Sono-Hydro-Gen) process is compared with the
sonoelectrochemical process in terms of the ultrasound
frequencies and the energy consumption and they are all
summarized in Table 10.
Challenges for Sono-Hydro-Gen reactor design
In this section, the need for further research studies is pre-
sented. The main challenges associated with the efficient
design and operation of sono-reactor are summarized in
Fig. 21.
The challenges are revealed from previous and ongoing
studies considering sono-reactor experiments and given in
details as follows:
Fig. 19 eEmpirical relationships (a) Cavitation energy vs. irradiation distance at different ultrasonic frequencies, (b) Average
Cavitation Energy vs. Ultrasonic Frequency, (c) Cavitation energy and input energy (W) and (d) Sonochemical efficiency and
cavitation energy (W) by Son et al. [116].
Fig. 20 eOverall ultrasound power attenuation from the plug to liquid and ultrasound efficiency.
international journal of hydrogen energy 44 (2019) 14500e1452614520
1) The acoustic field: has not been fully understood both
numerically and experimentally. The acousticfield is highly
complex due to several reasons such as, for example, the
inhomogeneous spatial distribution of bubbles. Conse-
quently, the speed of sound is time and position dependent.
Furthermore, the liquid container's walls are vibrating due
to the pressure oscillation of the liquid medium. These vi-
brations emit acoustic waves back to the liquid medium,
which will significantly affect the acoustic field.
2) Mechanism of H
2
production: this challenge lies in
advancing our fundamental understanding of the mecha-
nism of hydrogen production as the mechanism is not yet
understood and the most reported suggestions are
controversial. Water-sonication experiments are still
under investigation. Some research studies hypothesized
that most of the H
2
production is generated inside the
bubble during the gas phase. Whereas others contradict
this hypothesis by reporting that, H
2
is formed on the
bubble shell by the recombination of the generated radicals
from the water dissociation.
3) Intensity distribution: many difficulties are found in the
determination of the intensity distribution inside the sono-
reactor. Determination of ultrasonic intensity is well inves-
tigated in case of low-intensity ultrasound. However, the
challenge comes in when high power ultrasound is used
when exceeded the cavitation threshold. Other difficulties in
the determination of ultrasonic intensity distribution lie on
that ultrasound is mainly characterized by the power deliv-
ered to the system determined by calorimetry. Furthermore,
the ultrasound fieldinside the sono-reactor is known to be a
non-uniform sound field because most of the ultrasonic
energy is consumed at the tip of the ultrasonic probe.
4) Attenuation of the sound waves in the liquid medium. As
ultrasound waves are emitted and propagate through the
liquid medium, the acoustic intensity of the sound de-
creases along the distance from the ultrasonic probe to the
bottom of the liquid container. The attenuation takes place
due to reflection, refraction, and absorption of the sound
waves by the generated bubbles. As a result, active and
passive acoustic zones will exist inside the sono-reactor. It
is essential to understand the effect of these changes
within the sono-reactor with probing the possibilities to
reduce these attenuation effects.
5) Factors affecting the ultrasonic production of hydrogen: a
number of factors affecting ultrasonic production
including, the frequency of ultrasound, type of dissolved
gas, acoustic power, and the bulk liquid temperature. Data
concerning the effect of ultrasound frequency and the
water bulk temperature are very limited and scarce in the
literature and need to be furtherly investigated.
6) Energy conversion: one of the most important factors in
the development of an industrial process is the energy
conversion from ultrasound waves to the required effect.
The importance lies in the change of liquid properties as
Table 10 eEnergy consumption of hydrogen production by different technologies.
Technology Process Frequency [kHz] Energy consumption References
Electrochemical technology Electrolysis process
0.1 M KOH
N/A 6.3 kWh/m
3
H
2
Zadeh [156]
Sonoelectrochemical technology Electrolysis process aided with ultrasound
0.1 M KOH
20 kHz 5.12 kWh/m
3
H
2
Zadeh [156]
Sonochemical Technology Sono-Hydro-Gen process 20 kHz 0.8 mM/min Venault [75]
Sonochemical Technology Sono-Hydro-Gen process 300 kHz 0.83 mM/min/W Fischer et al. [158]
Sonochemical Technology Sono-Hydro-Gen process 1000 kHz 0.42e0.68 mM/min/W Buettner et al. [159]
Sonochemical Technology Sono-Hydro-Gen process 1000 kHz 0.22 mM/min Margulis et al. [126]
Fig. 21 eChallenges associated with sonoreactors.
international journal of hydrogen energy 44 (2019) 14500e14526 14521
per the ultrasonic characteristics, which is considered very
limited in the literature.
7) Large-scale sono-reactor: some researchers highlighted
that the implementation of industrial sono-reactors have
not reached fully commercialization yet because most of
the research studies are considered the lab-scale sonor-
eactors and they do not provide enough information about
the optimum design and optimum operating conditions of
the sonoreactors. It is necessary to understand the sono-
reactor characteristics including the cavitational energy
and acoustic intensity distribution.
8) Quantification of the produced hydrogen: few studies have
considered quantification of the hydrogen production rate.
Detailed quantification is essential to understand clearly
the effects of different operating conditions, which is
necessary for upgrading sono-reactors from the lab-scale
to industrial or conventional scale.
Conclusions
In this article, a comprehensive review is provided on the
background and principles associated with Sono-Hydro-Gen.
This approach is an interesting and expanding field as it is
considered one of the benefits from utilizing the power of
ultrasound using specially designed sono-reactor.
To obtain the optimum design of the sonoreactor at a range
of ultrasonic frequencies, comprehensive numerical and
experimental investigations should be performed to address
lab-scale and industrial concerns to achieve the optimum
design of the Sono-Hydro-Gen reactor. Some insightful lab-
scale results are reported on the effect of some important
factors influences the hydrogen production rates from the
Sono-Hydro-Gen reactor. These factors are crucial to initiate a
strong database to establish an industrial sono-reactor.
Promising results are obtained to date from both lab-scale
experiments and numerical tests, however, it worth noting
that the Sono-Hydro-Gen sonoreactor are still in the stage of
testing, development and application commercialization. The
key challenges, which need to be addressed for the develop-
ment and application for a durable reactor for industrial
application, include:
Characterizing the flow field, the acoustic field of the
acoustic cavitation bubble
Characterizing the cavitational energy intensity distribu-
tion inside the sono-reactor
Enhancing and characterizing the Sono-Hydro-Gen process
Developing a low cost but effective sono-reactor geometry
Optimizing the operating modes of the sono-reactor
Acknowledgment
The first author would like to express gratitude to the Gov-
ernment of Ontario, Canada for providing the Ontario Trillium
Scholarship (OTS).
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