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Sonochemical reaction with microbubbles generated by hollow ultrasonic horn
Accepted Manuscript Short communication Sonochemical reaction with microbubbles generated by hollow ultrasonic horn Toshinori Makuta, Yuta Aizawa, Ryodai Suzuki PII: DOI: Reference:
S1350-4177(12)00278-7 http://dx.doi.org/10.1016/j.ultsonch.2012.12.004 ULTSON 2242
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
11 June 2012 20 November 2012 18 December 2012
Please cite this article as: T. Makuta, Y. Aizawa, R. Suzuki, Sonochemical reaction with microbubbles generated by hollow ultrasonic horn, Ultrasonics Sonochemistry (2012), doi: http://dx.doi.org/10.1016/j.ultsonch.2012.12.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Sonochemical reaction with microbubbles generated by hollow ultrasonic horn
2
Toshinori MAKUTA*, Yuta AIZAWA, Ryodai SUZUKI
3 4
Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
5
Yamagata 992-8510, Japan
6
*Corresponding author. Tel.: +81 238 26 3258; Fax: +81 238 26 3258.
7
E-mail address: [email protected] (T. Makuta)
8 9
Abstract
10
A microbubble generator with a cylindrical hollow ultrasonic horn (HUSH), gas flow path, and an
11
orifice inside it can produce high ultrasonic pressure around the generated microbubbles. We used
12
this microbubble generator with a HUSH as a sonochemical reactor for the degradation of indigo
13
carmine and evaluated the sonochemical reaction by simply inserting the horn end into a liquid. The
14
experimental results revealed that the ultrasonic irradiation around ultrasonically generated
15
microbubbles effectively degraded indigo carmine in water. In addition, degradation experiments
16
performed by varying the ultrasonic power and gas flow rates indicated that a continuous gas supply
17
and ultrasonic pressure were required for generating the microbubbles, without the generation of
18
millimeter-scale bubbles, to enhance the sonochemical reaction in water.
19 20
Keywords:
Microbubble,
21
Sonochemical Reactor
Degradation,
Ultrasonic
Irradiation,
Hollow
Ultrasonic
Horn,
22 23
1. Introduction
24
In sonochemical processes, the high pressure and temperature field due to a bubble’s volumetric
25
oscillation induced by an ultrasonic pressure oscillation for a physical or chemical reaction is used.
26
Ever since Suslick reported a novel chemical reaction with ultrasonic irradiation [1], many studies
27
about sonochemistry have been conducted in a number of different fields; in particular, in the
28
environmental field, many techniques for carrying out the degradation of environmental pollutants
29
by the sonochemical process have been studied [2-4]. In a conventional sonochemical process, an
30
acoustically optimized reaction tank, a high-power ultrasonic transducer, and pretreatment of the
31
liquid by degassing or bubbling are generally required [5]. However, these conventional methods are
32
generally poorly reproducible [6] and are sensitive to the geometries of the ultrasonic transducer and
33
reaction tank, the liquid depth, and the substance present in liquid [3]. Moreover, most of these
34
methods adopt the batch processing system and not the continuous processing system [7]. Therefore,
35
the sonochemical reaction yields are likely to be low with a high energy input and long processing
36
time, and improving the yield is necessary for practical application of sonochemistry.
HUSH: hollow ultrasonic horn
1
Further, Makuta et al. reported [8] that microbubbles were stably generated by the acoustically
2
induced disturbance of the gas–liquid interface near the gas-supplying orifice. Makuta et al. also
3
reported [9] that a hollow ultrasonic horn that has a gas flow path and an orifice inside it can
4
generate microbubbles and produce a high ultrasonic pressure around the generated microbubbles
5
simultaneously and continuously. In this study, we used this microbubble generator as the
6
sonochemical reactor for the degradation of indigo carmine and evaluated the sonochemical reaction
7
by employing a simple system by just inserting the horn end into the liquid.
8 9
2. Experiments
10
2.1 Sonochemical Reactor with HUSH
11
In the conventional sonochemical process, the ultrasonic transducer attached to a tank generates
12
ultrasonic oscillations that radiate into the liquid containing the reactant in the tank or a solid
13
ultrasonic horn is inserted in the liquid. We developed a novel and simple sonochemical reactor
14
consisting of a hollow cylindrical ultrasonic horn (HUSH), an ultrasonic power source, a gas supply
15
source, and a container for holding the liquid, as shown in Fig. 1. The HUSH is shaped in the form
16
of a hollow step cylinder—the smaller section is 20 mm in diameter and 33 mm in length, whereas
17
the larger section is 29 mm in diameter and 45 mm in length. The tapered part joining the two
18
sections is 42 mm in length. An orifice with a diameter of 3 mm is located at the smaller end of the
19
HUSH. The gas flow path penetrates into the side surface of the tapered section and then passes
20
through the orifice. The gas supplied through this path in HUSH creates a gas–liquid interface
21
around the orifice in water. The gas flow rate is controlled by the regulator attached to the gas
22
cylinder containing argon (Ar) or oxygen (O2). The larger end of HUSH is connected to an ultrasonic
23
transducer (Transducer 6281A, Kaijo Co. Ltd., Japan) controlled by an oscillation circuit (Generator
24
model 6271, Kaijo Co. Ltd., Japan). The HUSH oscillates at a frequency of 19.5 kHz when electrical
25
power is supplied to the ultrasonic transducer. The relationship between the electrical power input
26
(Pin) to the transducer and the peak-to-peak oscillatory displacement of the smaller end of HUSH can
27
be expressed by the following approximate equation obtained on the basis of observations made
28
using an ultra-high-speed camera [9]:
29
y = 53.24 {1 – exp (-0.01785Pin)}
(1)
30
where y = the peak-to-peak oscillatory displacement of HUSH [m] and Pin is the electrical power
31
input [W]. When the smaller end of HUSH oscillates over certain oscillation amplitudes, many
32
microbubbles are generated at the orifice due to the flowing gas stream. The HUSH has the
33
advantage of generating microbubbles as well as radiating strong ultrasonic-pressure oscillations to
34
the microbubbles simultaneously in the same region.
35 36
2.2 Experimental Methodology
2
1
In this study, we evaluated the sonochemical reaction using HUSH by carrying out the degradation
2
of indigo carmine, which is a blue dye used in textiles and food colorants. It has a peak absorbance
3
of 610 nm because of a double-bonded carbon, and it decomposes as a result of heating to over
4
300°C, UV irradiation, and ozone exposure. The concentration of indigo carmine can be determined
5
by measuring the absorbance of the sample at a wavelength of 610 nm using a UV–Vis spectrometer
6
(SEC2000 Spectrometer, ALS Co. Ltd., Japan). In this study, the degradation of the dye was
7
calculated from the concentration that was determined from the absorbance by using the standard
8
curve for indigo carmine.
9
For the evaluation of the sonochemical reaction, an aqueous solution of indigo carmine was
10
decomposed by the sonochemical process. The default sonochemical process consisted of the
11
following three steps: (1) 100 mL of 20 ppm dye solution was placed in a 150-mL glass beaker and
12
maintained at 13 °C using a water bath cooled by a cooling water circulator (LTC-450, AS-ONE Co.
13
Ltd., Japan). The 3-mm outgassing orifice of HUSH was adjusted at 12 mm below the surface of the
14
solution. (2) Ar gas in a cylinder was supplied to the orifice through the flow path at the rate (Q) of
15
10 mL/min. (3) The orifice oscillated when 100 W electrical power input (Pin) was supplied to the
16
ultrasonic transducer. This resulted in the generation of many microbubbles from the orifice that
17
further oscillated volumetrically. The indigo carmine aqueous solution was gradually degraded
18
because of microbubbles generation and ultrasonic irradiation from the HUSH. We stopped this
19
processing in 30 minutes because the temperature of the ultrasonic transducer non-negligibly rose by
20
the continuous ultrasonic oscillation over 30 minutes.
21
In this paper, we clarified the effects of the ultrasound power input and the gas flow rate on
22
degradation of indigo carmine. As for the comparative investigation of the ultrasound power inputs,
23
we did a set of experiments for different ultrasound power inputs, 0, 10, 20, 40, 60, 80, and 100 W,
24
under the same conditions as the default process except for the ultrasound power input. The
25
ultrasound power input was controlled by the ultrasound generator and an electrical power meter
26
(Power HiTester 3332, Hioki. E. E. Co. Ltd., Japan). In addition, we also did three sets of the
27
Pin-changing experiments for the O2-bubbling sample, Ar-saturated sample without gas supply, and
28
the degassed sample without gas supply. Ar-saturated sample was prepared by bubbling for 10
29
minutes by a porous gas disperser, and degassed sample was prepared by a vacuum degassing over
30
18 minutes. As for the comparative investigation of the gas flow rates, we did a set of experiments
31
for different gas flow rates, 0, 10, 30, 50, 100, and 200 mL/min under the same conditions as the
32
default process except for the gas flow rate. The gas flow rate was controlled by a flow meter and a
33
regulator attached to the gas cylinder.
34 35
3. Results and Discussion
36
3.1 Indigo Carmine Degradation by Ultrasonic Irradiation with/without Microbubbles
3
1
Figure 2 shows the optical microscope images of bubble generation from HUSH for different
2
ultrasound power inputs Pin = 0, 10, 20, 40, 60, 80, and 100 W with an Ar flow of Q = 10 mL/min.
3
When the gas was passed without ultrasonic irradiation (Pin = 0 W), a flattened bubble with a width
4
of more than 8 mm was released from the orifice, as shown in Fig. 2 (a). However, when the gas was
5
supplied with the ultrasonic irradiation (Pin = 100 W), many microbubbles were generated (Fig. 2
6
(g)).
7
Figure 3 shows the amount of indigo carmine degraded by the sonochemical reaction using
8
HUSH after 30 min for different ultrasound power input Pin = 0, 10, 20, 40, 60, 80, and 100 W with
9
Ar gas supply at Q = 10 mL/min. The degraded amount increased linearly as a function of Pin in the
10
range from 0 W to 40 W, but it increased moderately as Pin increased in the range from 60 W to 80 W.
11
When Pin was over 80 W, the amount of degraded indigo carmine sharply increased again as Pin
12
increased. When Pin was below 40 W, the frequency of generation of millimeter-scale bubbles and
13
submillimeter-scale bubbles decreased and that of microbubbles increased (Fig. 2(b), 2(c), and 2(d));
14
thus, the simultaneous increase in the area of the sonochemical reaction site and in the ultrasonic
15
pressure around the bubbles caused a significant increase in the amount of degraded indigo carmine.
16
However, the microbubble generation using the HUSH remained unchanged for Pin in the range from
17
60 W (Fig. 2(e)) to 80 W (Fig. 2(f)). As Makuta et al. [9] reported, there were two phases in the
18
microbubble generation from HUSH: first was the direct generation of microbubbles from the
19
fragmentized gas–liquid interface formed on the orifice (primary breakup) and the secondary
20
generation due to the collapse of harmonically oscillated bubbles with diameters around 300 m
21
(secondary breakup). Therefore, the amount of microbubble generation from primary breakup was
22
saturated and remained unchanged in the range from 60 W to 80 W, and increasing the ultrasonic
23
pressure alone caused a slight rise in the amount of degraded indigo carmine. When Pin was 100 W,
24
many microbubbles with diameters less than 100 m were generated by the secondary breakup, as
25
shown in Fig. 2(g). This was because bubbles with diameters around 300 m, which is the resonance
26
bubble diameter corresponding to 19.5 kHz ultrasonic frequency, were harmonically oscillated, and
27
they collapsed especially at high ultrasonic amplitude. Thus, a sharp increase in the amount of the
28
degraded indigo carmine was observed as Pin increased over 80 W. Makuta et al. [9] also reported
29
that the number of microbubbles with diameters less than 100 m increased with an increase in Pin,
30
in two steps due to primary and secondary breakup (Fig. 3). Therefore, these results indicate that the
31
degradation of indigo carmine was enhanced by using HUSH and that the ultrasound power input
32
increased due to a simultaneous increase in the number of microbubbles generated by primary
33
breakup and secondary breakup and the ultrasonic pressure around the generated bubbles.
34
Figure 4 shows the time evolution of the amount degraded by the different sonochemical
35
processes using HUSH under the following conditions: with Ar gas supplied at Q = 10 mL/min and
36
Pin = 100 W (Fig. 4(a)), with O2 gas at a Q = 10 mL/min and Pin = 100 W (Fig. 4(b)), in an Ar
4
1
saturated sample without gas supply at Q = 0 mL/min and Pin = 100 W (Fig. 4(c)), and in a vacuum
2
degassed sample without gas supply at Q = 0 mL/min and Pin = 100 W (Fig. 4(d)). Figure 4 reveals
3
that the sonochemical reactions with gas supply (Figs. 4(a) and (b)) decomposed a significantly
4
greater amount of indigo carmine than those without gas supply did (Figs. 4(c) and (d)). In the case
5
of the sonochemical reaction with O2 microbubbles instead of Ar microbubbles, the amount of
6
degraded indigo carmine with O2 microbubbles was 0.978 mg (48.9%), which is slightly lower than
7
that with Ar microbubbles. This is because of the shorter reaction time at a bubble temperature of
8
over 300 °C, which in turn is attributed to the specific heat ratio of O2 gas being lower than that of
9
argon. [10]. The rate of degradation in the sonochemical reaction in the Ar-saturated sample without
10
gas using ultrasonic irradiation alone, was approximately the same as those in reactions that generate
11
Ar microbubbles ultrasonically within 3 min, but it gradually decreased after 6 min and finally
12
became the same as the rate in the case of the vacuum degassed sample. As indicated by the
13
difference between the initial degradation with and that without gas supply, a certain number of
14
bubbles existing in the sample enhanced the degradation, and the Ar-saturated sample initially
15
fulfilled the microbubble-rich condition by the release of Ar bubbles under the ultrasonic pressure
16
oscillation. However, ultrasonic irradiation also induced the trap in acoustic field and coalescence of
17
bubbles [11], i.e., ultrasonic degassing; therefore, the rate of degradation in the Ar-saturated sample
18
finally became the same as that of the reaction in the vacuum degassed sample. Thus, the ultrasonic
19
irradiation with a continuous gas supply to the sample enhances the degradation of indigo carmine.
20 21
3.2 Effect of Flow Rate of Microbubbles on Indigo Carmine Degradation
22
Figure 5 shows the extent of degradation of indigo carmine by the sonochemical reaction using
23
HUSH at different Ar flow rates (Q = 0, 10, 30, 50, 100, and 200 mL/min) at Pin = 100 W. In all
24
reactions with gas supply, the amount of indigo carmine decomposed was more than twice that of
25
indigo carmine decomposed in reactions without gas supply. Moreover, the amount of indigo
26
carmine decomposed by sonochemical reactions at Q ≤ 50 mL/min is more than twice the amount
27
decomposed at Q ≥ 100 mL/min. Figure 6 shows the optical microscope image of microbubble
28
generation near the orifice for different Ar flow rates (Q = 0, 10, 30, 50, 100, and 200 mL/min) at Pin
29
= 100 W. Figure 7 shows photographs of laser-illuminated bubbles under the same conditions as
30
Fig.6. Figure 7 was the cross-sectional image around the HUSH end illuminated by the sheet laser
31
(PIVLaser G50, KATOKOKEN Co. Ltd., Japan) whose thickness was less than 1mm and exposure
32
time was 2 milliseconds. A microbubble cloud of consisting mainly of microbubbles with diameters
33
less than 20 m was also generated just below the HUSH end in the absence of gas supply, as shown
34
in Fig. 6(a) and Fig. 7(a). Figures 6(b)–(d) and 7(b)–(d) show that the microbubble clouds at Q = 10,
35
30, and 50 mL/min were clearly denser than that without gas supply (Q = 0 mL/min). These results
36
indicate that the microbubbles which keep the spherical shape under the strong oscillation by intense
5
1
surface tension increase the degradation rate of indigo carmine since most of sonochemical reaction
2
occurs just below the HUSH end where the ultrasound pressure amplitude is the highest. Further, in
3
the case at Q = 100 and 200 mL/min, the bubble plume which consisted by millimeter-scale bubbles
4
and submillimeter-scale bubbles was formed below the center of the HUSH end when Q exceeded
5
50 mL/min (Fig. 7(e) and Fig. 7(f)). As shown in Fig.5, the degradation amounts of indigo carmine
6
at Q = 100 and 200 mL/min were lower than those at Q = 10, 30 and 50 mL/min because the bubble
7
plume formation decreased the sonochemical reaction field especially below the center of the HUSH
8
end. Meanwhile, in case of Q = 100 mL/min and Q = 200 mL/min, both microbubble clouds just
9
below the outer rim of the HUSH end were approximately-same while the bubble plume at Q = 200
10
mL/min extended further than that at Q = 100 mL/min. Thus the degradation amounts of indigo
11
carmine at Q = 100 mL/min and Q = 200 mL/min were nearly unchanged. This millimeter-scale
12
bubble and submillimeter-scale bubble generation near the end of HUSH was caused by incomplete
13
breakup of excessive supply gas. These millimeter-scale bubbles and submillimeter-scale bubbles,
14
whose surface tension is smaller when compared to the inertial force, utilize the ultrasonic pressure
15
not only for compression and expansion of bubbles but also for the transformation of the
16
non-spherical bubble shape. Moreover, the relatively large surface area of millimeter-scale bubbles
17
and submillimeter-scale bubbles muffled or reflected the pressure propagation [12]. As a result, the
18
generation of millimeter-scale bubbles and submillimeter-scale bubbles brought about a reduction in
19
the high-pressure oscillation region where the sonochemical reaction occurred and decreased the rate
20
of degradation of indigo carmine. In the case of the Ar-saturated sample without the gas supply, the
21
degradation rate was initially the same as that at Q = 10 mL/min, and therefore, the most important
22
requirements for effective indigo carmine degradation are the following: continuous gas supply near
23
the orifice in order to generate as many microbubbles as possible and simultaneously minimizing the
24
generation of large bubbles whose diameters are over the submillimeter scale.
25 26
4. Conclusion
27
In this study, we used a microbubble generator with a hollow ultrasonic horn (HUSH) as a
28
sonochemical reactor for the degradation of indigo carmine, and we evaluated the sonochemical
29
reaction. The experimental results revealed that continuous microbubble generation and
30
simultaneous ultrasonic irradiation around generated microbubbles were effective in enhancing the
31
degradation of indigo carmine. Degradation experiments and optical microscope observations of
32
microbubble generation for different ultrasonic power indicated that an improvement in the
33
degradation required the following: (1) an increase in the number of microbubbles generated by high
34
ultrasonic amplitude, which is directly generated at the gas–liquid interface at the horn end and (2)
35
the secondary generation effected by the collapse of the generated bubbles. Further, we found that
36
continuous gas supply is necessary for the enhancement of degradation; however, excess gas supply
6
1
results in the generation of large bubbles that prevent the propagation of ultrasound waves.
2
Consequently, we have shown that a continuous gas supply and ultrasonic pressure required for
3
generation of the microbubbles without the generation of millimeter-scale bubbles increased the
4
sonochemical reaction in water. This sonochemical reactor with a HUSH can be used easily and
5
continuously by simply inserting the horn end in the liquid containing the reactant. Therefore, it is
6
expected to have widespread applications in the degradation of environmental pollutants, particle
7
synthesis, and so on.
8 9
Acknowledgment
10
This work was partly supported by a Grant-in-Aid for Scientific Research (B) from the Japan
11
Society for the Promotion of Science (24360066), TEPCO Memorial Foundation, and Ono Acoustic
12
Research Fund.
13 14
References
15
[1]K.S. Suslick, Sonochemistry, Science, 247 (1990) 1439-1445.
16
[2]J. Gonzalez-Garcia, V. Saez, I. Tudela, M.I. Diez-Garcia, M.D. Esclapez, O. Louisnard,
17
Sonochemical Treatment of Water Polluted by Chlorinated Organocompounds. A Review, Water 2
18
(2010) 28-74.
19
[3]D. Kobayashi, K. Sano, Y. Takeuchi, K. Terasaka, Effect of irradiation distance on degradation of
20
phenol using indirect ultrasonic irradiation method, Ultrason. Sonochem., 18 (2011) 1205-1210.
21
[4]Y.G. Adewuyi, Sonochemistry: Environmental science and engineering applications, Ind. Eng.
22
Chem. Res., 40 (2001) 4681-4715.
23
[5]Y. Kojima, T. Fujita, E. P. Ona, H. Matsuda, S. Koda, N. Tanahashi, Y. Asakura, Effects of
24
dissolved gas species on ultrasonic degradation of (4-chloro-2-methylphenoxy) acetic acid (MCPA)
25
in aqueous solution, Ultrason. Sonochem., 12 (2005) 359-365.
26
[6]A. Weissler, H.W. Cooper, S. Snyder, Chemical effect of ultrasonic waves: Oxidation of
27
potassium iodide solution by carbon tetrachloride, J. Am. Chem. Soc., 72 (1950) 1769-1775.
28
[7]L. Paniwnyk, H. Cai, S. Albu, T.J. Mason, R. Cole, The enhancement and scale up of the
29
extraction of anti-oxidants from Rosmarinus officinalis using ultrasound, Ultrason. Sonochem.,
30
16 (2009) 287-292.
31
[8]T. Makuta, F. Takemura, E. Hihara, Y. Matsumoto, M. Shoji, Generation of Micro Gas Bubbles of
32
Uniform Diameter in an Ultrasonic Field, J. Fluid Mech., 548 (2006) 113-131.
33
[9]T. Makuta, R. Suzuki, T. Nakao, Generation of Microbubbles from Hollow Cylindrical Ultrasonic
34
Horn, Ultrasonics, 53 (2013) 196-202.
35
[10]D. Drijvers, R. de Baets, A. de Visscher, D. van Langenhove, Sonolysis of trichloroethylene in
36
aqueous solution: Volatile organic intermediates, Ultrason. Sonochem., 3 (1996) S83-S90.
7
1
[11]D. Kobayashi, Y. Hayashida, K. Sano, K. Terasaka, Agglomeration and rapid ascent of
2
microbubbles by ultrasonic irradiation, Ultrason. Sonochem., 18 (2011) 1191-1196.
3
[12]M. Ida, T. Naoe, M. Futakawa, Suppression of cavitation inception by gas bubble injection: A
4
numerical study focusing on bubble-bubble interaction, Phys. Rev. E, 76 (2007) 046309.
5 6
Figure Captions
7 8
Figure 1 Schematic illustration of experimental setup.
9 10
Figure 2 Photographs showing typical bubble generation for different ultrasonic power input (Pin),
11
Pin = 0, 10, 20, 40, 60, 80, and 100 W at Q = 10 mL/min.
12 13
Figure 3 Graph of ultrasonic power input (Pin) to the transducer versus the degradation amounts of
14
indigo carmine at Q = 10 mL/min.
15 16
Figure 4 Time evolution of the amount of indigo carmine degraded with and without gas supply at
17
Pin = 100 W.
18 19
Figure 5 Graph of Ar gas flow rate (Q) versus the amounts of indigo carmine degraded at Pin = 100
20
W.
21 22
Figure 6 Photographs showing typical bubble generation for different Ar flow rate, Q = 0, 10, 30, 50,
23
100 and 200 mL/min at Pin = 100 W.
24 25
Figure 7 Photographs of laser-illuminated bubbles showing typical bubble generation around the
26
horn end for different Ar flow rate, Q = 0, 10, 30, 50, 100 and 200 mL/min at Pin = 100 W.
8
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
>A cylindrical hollow ultrasonic horn can easily generate microbubbles. >This US horn can produce high ultrasonic pressure around the generated microbubbles. >We evaluated the sonochemical reaction by inserting this US horn end into a liquid. >This reactor using this US horn effectively degraded indigo carmine in water. >A continuous gas supply and high US pressure enhanced the sonochemical reaction.
S1350-4177(12)00278-7 http://dx.doi.org/10.1016/j.ultsonch.2012.12.004 ULTSON 2242
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
11 June 2012 20 November 2012 18 December 2012
Please cite this article as: T. Makuta, Y. Aizawa, R. Suzuki, Sonochemical reaction with microbubbles generated by hollow ultrasonic horn, Ultrasonics Sonochemistry (2012), doi: http://dx.doi.org/10.1016/j.ultsonch.2012.12.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Sonochemical reaction with microbubbles generated by hollow ultrasonic horn
2
Toshinori MAKUTA*, Yuta AIZAWA, Ryodai SUZUKI
3 4
Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
5
Yamagata 992-8510, Japan
6
*Corresponding author. Tel.: +81 238 26 3258; Fax: +81 238 26 3258.
7
E-mail address: [email protected] (T. Makuta)
8 9
Abstract
10
A microbubble generator with a cylindrical hollow ultrasonic horn (HUSH), gas flow path, and an
11
orifice inside it can produce high ultrasonic pressure around the generated microbubbles. We used
12
this microbubble generator with a HUSH as a sonochemical reactor for the degradation of indigo
13
carmine and evaluated the sonochemical reaction by simply inserting the horn end into a liquid. The
14
experimental results revealed that the ultrasonic irradiation around ultrasonically generated
15
microbubbles effectively degraded indigo carmine in water. In addition, degradation experiments
16
performed by varying the ultrasonic power and gas flow rates indicated that a continuous gas supply
17
and ultrasonic pressure were required for generating the microbubbles, without the generation of
18
millimeter-scale bubbles, to enhance the sonochemical reaction in water.
19 20
Keywords:
Microbubble,
21
Sonochemical Reactor
Degradation,
Ultrasonic
Irradiation,
Hollow
Ultrasonic
Horn,
22 23
1. Introduction
24
In sonochemical processes, the high pressure and temperature field due to a bubble’s volumetric
25
oscillation induced by an ultrasonic pressure oscillation for a physical or chemical reaction is used.
26
Ever since Suslick reported a novel chemical reaction with ultrasonic irradiation [1], many studies
27
about sonochemistry have been conducted in a number of different fields; in particular, in the
28
environmental field, many techniques for carrying out the degradation of environmental pollutants
29
by the sonochemical process have been studied [2-4]. In a conventional sonochemical process, an
30
acoustically optimized reaction tank, a high-power ultrasonic transducer, and pretreatment of the
31
liquid by degassing or bubbling are generally required [5]. However, these conventional methods are
32
generally poorly reproducible [6] and are sensitive to the geometries of the ultrasonic transducer and
33
reaction tank, the liquid depth, and the substance present in liquid [3]. Moreover, most of these
34
methods adopt the batch processing system and not the continuous processing system [7]. Therefore,
35
the sonochemical reaction yields are likely to be low with a high energy input and long processing
36
time, and improving the yield is necessary for practical application of sonochemistry.
HUSH: hollow ultrasonic horn
1
Further, Makuta et al. reported [8] that microbubbles were stably generated by the acoustically
2
induced disturbance of the gas–liquid interface near the gas-supplying orifice. Makuta et al. also
3
reported [9] that a hollow ultrasonic horn that has a gas flow path and an orifice inside it can
4
generate microbubbles and produce a high ultrasonic pressure around the generated microbubbles
5
simultaneously and continuously. In this study, we used this microbubble generator as the
6
sonochemical reactor for the degradation of indigo carmine and evaluated the sonochemical reaction
7
by employing a simple system by just inserting the horn end into the liquid.
8 9
2. Experiments
10
2.1 Sonochemical Reactor with HUSH
11
In the conventional sonochemical process, the ultrasonic transducer attached to a tank generates
12
ultrasonic oscillations that radiate into the liquid containing the reactant in the tank or a solid
13
ultrasonic horn is inserted in the liquid. We developed a novel and simple sonochemical reactor
14
consisting of a hollow cylindrical ultrasonic horn (HUSH), an ultrasonic power source, a gas supply
15
source, and a container for holding the liquid, as shown in Fig. 1. The HUSH is shaped in the form
16
of a hollow step cylinder—the smaller section is 20 mm in diameter and 33 mm in length, whereas
17
the larger section is 29 mm in diameter and 45 mm in length. The tapered part joining the two
18
sections is 42 mm in length. An orifice with a diameter of 3 mm is located at the smaller end of the
19
HUSH. The gas flow path penetrates into the side surface of the tapered section and then passes
20
through the orifice. The gas supplied through this path in HUSH creates a gas–liquid interface
21
around the orifice in water. The gas flow rate is controlled by the regulator attached to the gas
22
cylinder containing argon (Ar) or oxygen (O2). The larger end of HUSH is connected to an ultrasonic
23
transducer (Transducer 6281A, Kaijo Co. Ltd., Japan) controlled by an oscillation circuit (Generator
24
model 6271, Kaijo Co. Ltd., Japan). The HUSH oscillates at a frequency of 19.5 kHz when electrical
25
power is supplied to the ultrasonic transducer. The relationship between the electrical power input
26
(Pin) to the transducer and the peak-to-peak oscillatory displacement of the smaller end of HUSH can
27
be expressed by the following approximate equation obtained on the basis of observations made
28
using an ultra-high-speed camera [9]:
29
y = 53.24 {1 – exp (-0.01785Pin)}
(1)
30
where y = the peak-to-peak oscillatory displacement of HUSH [m] and Pin is the electrical power
31
input [W]. When the smaller end of HUSH oscillates over certain oscillation amplitudes, many
32
microbubbles are generated at the orifice due to the flowing gas stream. The HUSH has the
33
advantage of generating microbubbles as well as radiating strong ultrasonic-pressure oscillations to
34
the microbubbles simultaneously in the same region.
35 36
2.2 Experimental Methodology
2
1
In this study, we evaluated the sonochemical reaction using HUSH by carrying out the degradation
2
of indigo carmine, which is a blue dye used in textiles and food colorants. It has a peak absorbance
3
of 610 nm because of a double-bonded carbon, and it decomposes as a result of heating to over
4
300°C, UV irradiation, and ozone exposure. The concentration of indigo carmine can be determined
5
by measuring the absorbance of the sample at a wavelength of 610 nm using a UV–Vis spectrometer
6
(SEC2000 Spectrometer, ALS Co. Ltd., Japan). In this study, the degradation of the dye was
7
calculated from the concentration that was determined from the absorbance by using the standard
8
curve for indigo carmine.
9
For the evaluation of the sonochemical reaction, an aqueous solution of indigo carmine was
10
decomposed by the sonochemical process. The default sonochemical process consisted of the
11
following three steps: (1) 100 mL of 20 ppm dye solution was placed in a 150-mL glass beaker and
12
maintained at 13 °C using a water bath cooled by a cooling water circulator (LTC-450, AS-ONE Co.
13
Ltd., Japan). The 3-mm outgassing orifice of HUSH was adjusted at 12 mm below the surface of the
14
solution. (2) Ar gas in a cylinder was supplied to the orifice through the flow path at the rate (Q) of
15
10 mL/min. (3) The orifice oscillated when 100 W electrical power input (Pin) was supplied to the
16
ultrasonic transducer. This resulted in the generation of many microbubbles from the orifice that
17
further oscillated volumetrically. The indigo carmine aqueous solution was gradually degraded
18
because of microbubbles generation and ultrasonic irradiation from the HUSH. We stopped this
19
processing in 30 minutes because the temperature of the ultrasonic transducer non-negligibly rose by
20
the continuous ultrasonic oscillation over 30 minutes.
21
In this paper, we clarified the effects of the ultrasound power input and the gas flow rate on
22
degradation of indigo carmine. As for the comparative investigation of the ultrasound power inputs,
23
we did a set of experiments for different ultrasound power inputs, 0, 10, 20, 40, 60, 80, and 100 W,
24
under the same conditions as the default process except for the ultrasound power input. The
25
ultrasound power input was controlled by the ultrasound generator and an electrical power meter
26
(Power HiTester 3332, Hioki. E. E. Co. Ltd., Japan). In addition, we also did three sets of the
27
Pin-changing experiments for the O2-bubbling sample, Ar-saturated sample without gas supply, and
28
the degassed sample without gas supply. Ar-saturated sample was prepared by bubbling for 10
29
minutes by a porous gas disperser, and degassed sample was prepared by a vacuum degassing over
30
18 minutes. As for the comparative investigation of the gas flow rates, we did a set of experiments
31
for different gas flow rates, 0, 10, 30, 50, 100, and 200 mL/min under the same conditions as the
32
default process except for the gas flow rate. The gas flow rate was controlled by a flow meter and a
33
regulator attached to the gas cylinder.
34 35
3. Results and Discussion
36
3.1 Indigo Carmine Degradation by Ultrasonic Irradiation with/without Microbubbles
3
1
Figure 2 shows the optical microscope images of bubble generation from HUSH for different
2
ultrasound power inputs Pin = 0, 10, 20, 40, 60, 80, and 100 W with an Ar flow of Q = 10 mL/min.
3
When the gas was passed without ultrasonic irradiation (Pin = 0 W), a flattened bubble with a width
4
of more than 8 mm was released from the orifice, as shown in Fig. 2 (a). However, when the gas was
5
supplied with the ultrasonic irradiation (Pin = 100 W), many microbubbles were generated (Fig. 2
6
(g)).
7
Figure 3 shows the amount of indigo carmine degraded by the sonochemical reaction using
8
HUSH after 30 min for different ultrasound power input Pin = 0, 10, 20, 40, 60, 80, and 100 W with
9
Ar gas supply at Q = 10 mL/min. The degraded amount increased linearly as a function of Pin in the
10
range from 0 W to 40 W, but it increased moderately as Pin increased in the range from 60 W to 80 W.
11
When Pin was over 80 W, the amount of degraded indigo carmine sharply increased again as Pin
12
increased. When Pin was below 40 W, the frequency of generation of millimeter-scale bubbles and
13
submillimeter-scale bubbles decreased and that of microbubbles increased (Fig. 2(b), 2(c), and 2(d));
14
thus, the simultaneous increase in the area of the sonochemical reaction site and in the ultrasonic
15
pressure around the bubbles caused a significant increase in the amount of degraded indigo carmine.
16
However, the microbubble generation using the HUSH remained unchanged for Pin in the range from
17
60 W (Fig. 2(e)) to 80 W (Fig. 2(f)). As Makuta et al. [9] reported, there were two phases in the
18
microbubble generation from HUSH: first was the direct generation of microbubbles from the
19
fragmentized gas–liquid interface formed on the orifice (primary breakup) and the secondary
20
generation due to the collapse of harmonically oscillated bubbles with diameters around 300 m
21
(secondary breakup). Therefore, the amount of microbubble generation from primary breakup was
22
saturated and remained unchanged in the range from 60 W to 80 W, and increasing the ultrasonic
23
pressure alone caused a slight rise in the amount of degraded indigo carmine. When Pin was 100 W,
24
many microbubbles with diameters less than 100 m were generated by the secondary breakup, as
25
shown in Fig. 2(g). This was because bubbles with diameters around 300 m, which is the resonance
26
bubble diameter corresponding to 19.5 kHz ultrasonic frequency, were harmonically oscillated, and
27
they collapsed especially at high ultrasonic amplitude. Thus, a sharp increase in the amount of the
28
degraded indigo carmine was observed as Pin increased over 80 W. Makuta et al. [9] also reported
29
that the number of microbubbles with diameters less than 100 m increased with an increase in Pin,
30
in two steps due to primary and secondary breakup (Fig. 3). Therefore, these results indicate that the
31
degradation of indigo carmine was enhanced by using HUSH and that the ultrasound power input
32
increased due to a simultaneous increase in the number of microbubbles generated by primary
33
breakup and secondary breakup and the ultrasonic pressure around the generated bubbles.
34
Figure 4 shows the time evolution of the amount degraded by the different sonochemical
35
processes using HUSH under the following conditions: with Ar gas supplied at Q = 10 mL/min and
36
Pin = 100 W (Fig. 4(a)), with O2 gas at a Q = 10 mL/min and Pin = 100 W (Fig. 4(b)), in an Ar
4
1
saturated sample without gas supply at Q = 0 mL/min and Pin = 100 W (Fig. 4(c)), and in a vacuum
2
degassed sample without gas supply at Q = 0 mL/min and Pin = 100 W (Fig. 4(d)). Figure 4 reveals
3
that the sonochemical reactions with gas supply (Figs. 4(a) and (b)) decomposed a significantly
4
greater amount of indigo carmine than those without gas supply did (Figs. 4(c) and (d)). In the case
5
of the sonochemical reaction with O2 microbubbles instead of Ar microbubbles, the amount of
6
degraded indigo carmine with O2 microbubbles was 0.978 mg (48.9%), which is slightly lower than
7
that with Ar microbubbles. This is because of the shorter reaction time at a bubble temperature of
8
over 300 °C, which in turn is attributed to the specific heat ratio of O2 gas being lower than that of
9
argon. [10]. The rate of degradation in the sonochemical reaction in the Ar-saturated sample without
10
gas using ultrasonic irradiation alone, was approximately the same as those in reactions that generate
11
Ar microbubbles ultrasonically within 3 min, but it gradually decreased after 6 min and finally
12
became the same as the rate in the case of the vacuum degassed sample. As indicated by the
13
difference between the initial degradation with and that without gas supply, a certain number of
14
bubbles existing in the sample enhanced the degradation, and the Ar-saturated sample initially
15
fulfilled the microbubble-rich condition by the release of Ar bubbles under the ultrasonic pressure
16
oscillation. However, ultrasonic irradiation also induced the trap in acoustic field and coalescence of
17
bubbles [11], i.e., ultrasonic degassing; therefore, the rate of degradation in the Ar-saturated sample
18
finally became the same as that of the reaction in the vacuum degassed sample. Thus, the ultrasonic
19
irradiation with a continuous gas supply to the sample enhances the degradation of indigo carmine.
20 21
3.2 Effect of Flow Rate of Microbubbles on Indigo Carmine Degradation
22
Figure 5 shows the extent of degradation of indigo carmine by the sonochemical reaction using
23
HUSH at different Ar flow rates (Q = 0, 10, 30, 50, 100, and 200 mL/min) at Pin = 100 W. In all
24
reactions with gas supply, the amount of indigo carmine decomposed was more than twice that of
25
indigo carmine decomposed in reactions without gas supply. Moreover, the amount of indigo
26
carmine decomposed by sonochemical reactions at Q ≤ 50 mL/min is more than twice the amount
27
decomposed at Q ≥ 100 mL/min. Figure 6 shows the optical microscope image of microbubble
28
generation near the orifice for different Ar flow rates (Q = 0, 10, 30, 50, 100, and 200 mL/min) at Pin
29
= 100 W. Figure 7 shows photographs of laser-illuminated bubbles under the same conditions as
30
Fig.6. Figure 7 was the cross-sectional image around the HUSH end illuminated by the sheet laser
31
(PIVLaser G50, KATOKOKEN Co. Ltd., Japan) whose thickness was less than 1mm and exposure
32
time was 2 milliseconds. A microbubble cloud of consisting mainly of microbubbles with diameters
33
less than 20 m was also generated just below the HUSH end in the absence of gas supply, as shown
34
in Fig. 6(a) and Fig. 7(a). Figures 6(b)–(d) and 7(b)–(d) show that the microbubble clouds at Q = 10,
35
30, and 50 mL/min were clearly denser than that without gas supply (Q = 0 mL/min). These results
36
indicate that the microbubbles which keep the spherical shape under the strong oscillation by intense
5
1
surface tension increase the degradation rate of indigo carmine since most of sonochemical reaction
2
occurs just below the HUSH end where the ultrasound pressure amplitude is the highest. Further, in
3
the case at Q = 100 and 200 mL/min, the bubble plume which consisted by millimeter-scale bubbles
4
and submillimeter-scale bubbles was formed below the center of the HUSH end when Q exceeded
5
50 mL/min (Fig. 7(e) and Fig. 7(f)). As shown in Fig.5, the degradation amounts of indigo carmine
6
at Q = 100 and 200 mL/min were lower than those at Q = 10, 30 and 50 mL/min because the bubble
7
plume formation decreased the sonochemical reaction field especially below the center of the HUSH
8
end. Meanwhile, in case of Q = 100 mL/min and Q = 200 mL/min, both microbubble clouds just
9
below the outer rim of the HUSH end were approximately-same while the bubble plume at Q = 200
10
mL/min extended further than that at Q = 100 mL/min. Thus the degradation amounts of indigo
11
carmine at Q = 100 mL/min and Q = 200 mL/min were nearly unchanged. This millimeter-scale
12
bubble and submillimeter-scale bubble generation near the end of HUSH was caused by incomplete
13
breakup of excessive supply gas. These millimeter-scale bubbles and submillimeter-scale bubbles,
14
whose surface tension is smaller when compared to the inertial force, utilize the ultrasonic pressure
15
not only for compression and expansion of bubbles but also for the transformation of the
16
non-spherical bubble shape. Moreover, the relatively large surface area of millimeter-scale bubbles
17
and submillimeter-scale bubbles muffled or reflected the pressure propagation [12]. As a result, the
18
generation of millimeter-scale bubbles and submillimeter-scale bubbles brought about a reduction in
19
the high-pressure oscillation region where the sonochemical reaction occurred and decreased the rate
20
of degradation of indigo carmine. In the case of the Ar-saturated sample without the gas supply, the
21
degradation rate was initially the same as that at Q = 10 mL/min, and therefore, the most important
22
requirements for effective indigo carmine degradation are the following: continuous gas supply near
23
the orifice in order to generate as many microbubbles as possible and simultaneously minimizing the
24
generation of large bubbles whose diameters are over the submillimeter scale.
25 26
4. Conclusion
27
In this study, we used a microbubble generator with a hollow ultrasonic horn (HUSH) as a
28
sonochemical reactor for the degradation of indigo carmine, and we evaluated the sonochemical
29
reaction. The experimental results revealed that continuous microbubble generation and
30
simultaneous ultrasonic irradiation around generated microbubbles were effective in enhancing the
31
degradation of indigo carmine. Degradation experiments and optical microscope observations of
32
microbubble generation for different ultrasonic power indicated that an improvement in the
33
degradation required the following: (1) an increase in the number of microbubbles generated by high
34
ultrasonic amplitude, which is directly generated at the gas–liquid interface at the horn end and (2)
35
the secondary generation effected by the collapse of the generated bubbles. Further, we found that
36
continuous gas supply is necessary for the enhancement of degradation; however, excess gas supply
6
1
results in the generation of large bubbles that prevent the propagation of ultrasound waves.
2
Consequently, we have shown that a continuous gas supply and ultrasonic pressure required for
3
generation of the microbubbles without the generation of millimeter-scale bubbles increased the
4
sonochemical reaction in water. This sonochemical reactor with a HUSH can be used easily and
5
continuously by simply inserting the horn end in the liquid containing the reactant. Therefore, it is
6
expected to have widespread applications in the degradation of environmental pollutants, particle
7
synthesis, and so on.
8 9
Acknowledgment
10
This work was partly supported by a Grant-in-Aid for Scientific Research (B) from the Japan
11
Society for the Promotion of Science (24360066), TEPCO Memorial Foundation, and Ono Acoustic
12
Research Fund.
13 14
References
15
[1]K.S. Suslick, Sonochemistry, Science, 247 (1990) 1439-1445.
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[2]J. Gonzalez-Garcia, V. Saez, I. Tudela, M.I. Diez-Garcia, M.D. Esclapez, O. Louisnard,
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Sonochemical Treatment of Water Polluted by Chlorinated Organocompounds. A Review, Water 2
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[3]D. Kobayashi, K. Sano, Y. Takeuchi, K. Terasaka, Effect of irradiation distance on degradation of
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phenol using indirect ultrasonic irradiation method, Ultrason. Sonochem., 18 (2011) 1205-1210.
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[4]Y.G. Adewuyi, Sonochemistry: Environmental science and engineering applications, Ind. Eng.
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Chem. Res., 40 (2001) 4681-4715.
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[5]Y. Kojima, T. Fujita, E. P. Ona, H. Matsuda, S. Koda, N. Tanahashi, Y. Asakura, Effects of
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dissolved gas species on ultrasonic degradation of (4-chloro-2-methylphenoxy) acetic acid (MCPA)
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in aqueous solution, Ultrason. Sonochem., 12 (2005) 359-365.
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[6]A. Weissler, H.W. Cooper, S. Snyder, Chemical effect of ultrasonic waves: Oxidation of
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potassium iodide solution by carbon tetrachloride, J. Am. Chem. Soc., 72 (1950) 1769-1775.
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[7]L. Paniwnyk, H. Cai, S. Albu, T.J. Mason, R. Cole, The enhancement and scale up of the
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extraction of anti-oxidants from Rosmarinus officinalis using ultrasound, Ultrason. Sonochem.,
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[8]T. Makuta, F. Takemura, E. Hihara, Y. Matsumoto, M. Shoji, Generation of Micro Gas Bubbles of
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Uniform Diameter in an Ultrasonic Field, J. Fluid Mech., 548 (2006) 113-131.
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[9]T. Makuta, R. Suzuki, T. Nakao, Generation of Microbubbles from Hollow Cylindrical Ultrasonic
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Horn, Ultrasonics, 53 (2013) 196-202.
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[10]D. Drijvers, R. de Baets, A. de Visscher, D. van Langenhove, Sonolysis of trichloroethylene in
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[11]D. Kobayashi, Y. Hayashida, K. Sano, K. Terasaka, Agglomeration and rapid ascent of
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microbubbles by ultrasonic irradiation, Ultrason. Sonochem., 18 (2011) 1191-1196.
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[12]M. Ida, T. Naoe, M. Futakawa, Suppression of cavitation inception by gas bubble injection: A
4
numerical study focusing on bubble-bubble interaction, Phys. Rev. E, 76 (2007) 046309.
5 6
Figure Captions
7 8
Figure 1 Schematic illustration of experimental setup.
9 10
Figure 2 Photographs showing typical bubble generation for different ultrasonic power input (Pin),
11
Pin = 0, 10, 20, 40, 60, 80, and 100 W at Q = 10 mL/min.
12 13
Figure 3 Graph of ultrasonic power input (Pin) to the transducer versus the degradation amounts of
14
indigo carmine at Q = 10 mL/min.
15 16
Figure 4 Time evolution of the amount of indigo carmine degraded with and without gas supply at
17
Pin = 100 W.
18 19
Figure 5 Graph of Ar gas flow rate (Q) versus the amounts of indigo carmine degraded at Pin = 100
20
W.
21 22
Figure 6 Photographs showing typical bubble generation for different Ar flow rate, Q = 0, 10, 30, 50,
23
100 and 200 mL/min at Pin = 100 W.
24 25
Figure 7 Photographs of laser-illuminated bubbles showing typical bubble generation around the
26
horn end for different Ar flow rate, Q = 0, 10, 30, 50, 100 and 200 mL/min at Pin = 100 W.
8
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
>A cylindrical hollow ultrasonic horn can easily generate microbubbles. >This US horn can produce high ultrasonic pressure around the generated microbubbles. >We evaluated the sonochemical reaction by inserting this US horn end into a liquid. >This reactor using this US horn effectively degraded indigo carmine in water. >A continuous gas supply and high US pressure enhanced the sonochemical reaction.