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Study of intensification of sonochemical reactions using gaseous additives Page 1
Study of intensification of sonochemical reactions using
gaseous additives
Thesis submitted to for the degree of
MASTER OF CHEMICAL ENGINEERING
Suraj S. Shaha
Department of Chemical Engineering
Institute of Chemical Technology, Mumbai
Maharashtra, India
2012
Study of intensification of sonochemical reactions using gaseous additives Page 2
Study of intensification of sonochemical reaction using
gaseous additives
Thesis submitted to
Institute of Chemical Technology, Mumbai
For the award of the degree of
MASTER OF CHEMICAL ENGINEERING
by
Suraj S. Shaha
under the supervision of
Professor (Dr.) Parag R. Gogate
Department of Chemical Engineering
Institute of Chemical Technology, Mumbai
Maharashtra, India
May 2012
© 2012, Suraj S. Shaha, All rights reserved
Study of intensification of sonochemical reactions using gaseous additives Page 3
INSTITUTE OF CHEMICAL TECHNOLOGY, MUMBAI
Approval of the Research Supervisor and the External Examiner
Certified on , that the thesis titled “Study of intensification of sonochemical
reaction using gaseous additive” submitted by Mr. Suraj Sudhir Shaha to the Institute of
Chemical Technology, Mumbai, for the award of the degree “Master of Chemical
Engineering” has been accepted by the external examiners, and that the student has
successfully defended the thesis in the viva voce examination held today.
Signature:
Research Supervisor: Prof. (Dr.) Parag R. Gogate
Affiliation: Institute of Chemical Technology, Mumbai, Maharashtra, India
Signature:
External Examiner:
Affiliation:
Study of intensification of sonochemical reactions using gaseous additives Page 4
CERTIFICATE
This is to certify that the thesis titled titled “Study of intensification of sonochemical
reaction using gaseous additive” submitted by Mr. Suraj Sudhir Shaha to the Institute of
Chemical Technology, Mumbai, for the degree of “Master of Chemical Engineering” is a
bona fide record of the research work carried out by him in the Department of Chemical
Engineering, Institute of Chemical Technology, Mumbai, under my supervision. Mr.
Suraj Sudhir Shaha has worked on this topic from February 2007 till February 2011 and
the thesis, in my opinion, is worthy of consideration for the award of the degree “Master
of Chemical Engineering” in accordance with the regulations of this Deemed-to-be
University. The results embodied in this thesis have not been submitted to any other
University or Institute for the award of any degree, diploma, or certificate.
Signature:
Research Supervisor: Prof. (Dr.) Parag R. Gogate
Affiliation: Institute of Chemical Technology, Mumbai, Maharashtra, India
Date:
Study of intensification of sonochemical reactions using gaseous additives Page 5
Dedicated to my family and friends..!!
Study of intensification of sonochemical reactions using gaseous additives Page 6
ACKNOWLEGMENT
It is indeed a pleasure to get an opportunity to thank all those who have helped me to
complete my M. Chem. Eng course and for showering me their blessings. Words and
actions are not sufficient to show how thankful I am to my parents, sister (Sayali) and
mausi.
I am indeed proud to thank my research guide Dr.Parag Gogate who has put in all his
efforts and has helped me by giving suggestions on how to work and go about my project.
I will never forget his boundless passions and efforts to make students perfect in
everything. I am very much thankful to him for the freedom he has given to me in my
research work. He was a constant source of encouragement & motivation as well as a
principle cantered person. His overtly enthusiasm and integral view on research always
had a deep impression on me and gave me confidence to go about and complete my
project successfully.
I am indebted Dr.Amit Pratap for his due help and knowledge he imparted to me for my
future and career.
The friendly environment in the lab kept me cheered up during my work. I am really
obligated to my senior lab mates Pankaj, Bagal Madam, Kiran, and Chandu for their
support all throughout my research period. I would like to thank my lab mates
Ghanshyam, Sanket, Amar and Kavita for their support and help during critical
situations. My special thanks for Ghanshyam and Sanket for their support, help,
counselling and many other things. Also would like to thank Atchut, Tushar, Prashil,
Inder, Pallavi, Dhanashri and Kavita for their valuable help in lab.
Transition from B. Tech Oils to Chemical Engineering was indeed a very difficult job for
me but thanks to Patle, Viplav, Shefali, Southy and Gaurav, who gave their valuable time
during exam period & throughout the year and have helped me a lot in my study and
made my tedious journey easy.
I would want to thank Shraddha, Shefali, Southy, Gaurav, Patle, Ghanshyam, Ankush,
Rikku, Ankita, Shilpa, Rohini, Suma & Karan who had been there to entertain me with
their laughter and talks. There were many trips and outings with them which helped me
feel refreshed to work ahead.
Study of intensification of sonochemical reactions using gaseous additives Page 7
I would also like to thanks all my classmates Pratik, Aniket, Gaja, Avinash, Santosh,
Ritesh, Parag and all placement co-ordinators (Nikhil, Navneet, Sanket, Rohini, Shilpa
and Darshan who worked with me during whole placement cell. Without all of them my
social life would have been incomplete.
I would like to thank my roommates- Magar, Gund, Kulkarni, Kabade, Khandare, Ankur,
Dhumal, Raybole as well as all my hostel & trekking friends specially Bhogale, Mukesh,
Parag, Patil, Chavan, Sanket, Venu, Labade, Rathod, Sahare who added to the fun trips
and enjoyments to the research life.
I want to give my special thanks to Ghanshyam, Sanket, Ankur, Dhumal, Chandan,
Magar, Darade, Smita, Amruta, Purva, Pradnya who had been there to encourage &
help me in every tough situation; moreover they were always there with me in all the
good & bad times.
Last but not the least I am very much grateful to the non-teaching staff, watchmen, Mess
workers and the most important Munnaji without whom my 6 years of ICT would have
been impossible.
I wish my juniors all the very best for their future and regret if I have missed out anyone
in my acknowledgment. Thank you to everyone and above all God.
- Suraj Shaha
Study of intensification of sonochemical reactions using gaseous additives Page 8
DECLARATION
I certify that,
- the work contained in this thesis is original and has been done by me under the
guidance of my research supervisor / supervisors.
- the work has not been submitted to any other University or Institute for the award
of any degree, diploma, or certificate.
- I have followed the guidelines of the Institute in preparing the thesis.
- I have conformed to the norms and guidelines given in the Ethical Code of
Conduct of the Institute.
- whenever I have used materials (data, theoretical analyses, figures, text, etc.)
from other sources, I have given due credit to them by citing them in the text of
the thesis and giving their details in the references. Further, I have taken
permission from the copyright owners of the sources, whenever necessary.
- I hereby grant to the university and its agents the non-exclusive license to archive
and make accessible, my thesis, in whole in all forms of media, now or hereafter
known. I retain all other ownership rights to the copyright of the thesis. I also
retain the right to use in future works (such as articles or books) all or part of this
thesis.
Signature:
Research Scholar: Mr. Suraj S. Shaha
Study of intensification of sonochemical reactions using gaseous additives Page 9
List of Figures
Figure
No.
Figure Caption Page
No.
1.1 Growth collapse cycle of cavitation bubbles 22
1.2 The compression and expansion cycle of ultrasound 22
1.3 Classification of different types of cavitation 24
1.4 Schematic of acoustic cavitation 25
3.1 Calibration curve of Iodine on UV-spectrophotometer 45
3.2 Calibration curve of salicylic acid using HPLC 46
3.3 The schematic view of experimental setup for ultrasonic horn 48
3.4 The schematic view of experimental setup for ultrasonic longitudinal
horn of 36 kHz and 25 kHz
51
4.1 Sonication of 100 ppm KI on different reactors in absence of any
gases
57
4.2 Sonication of 100 ppm KI on different reactors in presence of air 58
4.3 Sonication of 100 ppm KI on different reactors in presence of oxygen 58
4.4 Sonication of 100 ppm KI on different reactors in presence of
nitrogen
59
4.5 Sonication of 100 ppm KI on different reactors in presence of carbon
dioxide
59
4.6 Sonication of 100 ppm S.A. on different reactors in absence of any
gases
60
4.7 Sonication of 100 ppm S.A. on different reactors in presence of air 61
4.8 Sonication of 100 ppm S.A. on different reactors in presence of
oxygen
62
4.9 Sonication of 100 ppm S.A. on different reactors in presence of
nitrogen
62
Study of intensification of sonochemical reactions using gaseous additives Page 10
4.10 Sonication of 100 ppm S.A. on different reactors in presence of
carbon dioxide
63
4.11 Effect of initial concentration of potassium iodide on sonolysis using
20 KHz horn in absence of any gases
68
4.12 Effect of initial concentration of potassium iodide on sonolysis using
20 KHz horn in presence of air
68
4.13 Effect of initial concentration of potassium iodide on sonolysis using
20 KHz horn in presence of oxygen
69
4.14 Effect of initial concentration of potassium iodide on sonolysis using
20 KHz horn in presence of nitrogen
69
4.15 Effect of initial concentration of potassium iodide on sonolysis using
20 KHz horn in presence of carbon dioxide
70
4.16 Effect of initial concentration of potassium iodide on sonolysis using
25 KHz longitudinal horn (1 Kw) in the absence of any gases
70
4.17 Effect of initial concentration of potassium iodide on sonolysis using
25 KHz longitudinal horn (1 Kw) in presence of air
71
4.18 Effect of initial concentration of potassium iodide on sonolysis using
25 KHz longitudinal horn (1 Kw) in presence of oxygen
71
4.19 Effect of initial concentration of potassium iodide on sonolysis using
25 KHz longitudinal horn (1 Kw) in presence of nitrogen
72
4.20 Effect of initial concentration of potassium iodide on sonolysis using
25 KHz longitudinal horn (1 Kw) in presence of carbon dioxide
72
4.21 Effect of gaseous additives on KI (100 ppm) oxidation (using 20
KHz horn)
74
4.22 Effect of gaseous additives on KI (300 ppm) oxidation (using 20
KHz horn)
75
4.23 Effect of gaseous additives on KI (500 ppm) oxidation (using 20
KHz horn)
75
4.24 Effect of gaseous additives on S.A. (100 ppm) oxidation (using 20
KHz horn)
76
Study of intensification of sonochemical reactions using gaseous additives Page 11
4.25 Effect of gaseous additives on KI (100 ppm) oxidation (on 25 KHz
reactor)
78
4.26 Effect of gaseous additives on KI (300 ppm) oxidation (on 25 KHz
reactor)
78
4.27 Effect of gaseous additives on KI (500 ppm) oxidation (on 25
KHzreactor)
79
4.28 Effect of gaseous additives on S.A. (100 ppm) oxidation (on 25 KHz
reactor)
79
4.29 Effect of air flow rate on sonolysis of 300 ppm KI (20 KHz horn) 86
4.30 Effect of air flow rate on sonolysis of 300 ppm KI (36 KHz
longitudnal horn)
86
4.31 Effect of air flow rate on sonolysis of 300 ppm KI (25 KHz reactor) 87
4.32 Effect of air flow rate on sonolysis of 100 ppm S.A. (20 KHz horn) 87
4.33 Effect of air flow rate on sonolysis of 100 ppm S.A. (36 KHz
longitudnal horn)
88
4.34 Effect of air flow rate on sonolysis of 100 ppm S.A. (25 KHz
longitudnal horn)
88
Study of intensification of sonochemical reactions using gaseous additives Page 12
List of Tables
Table
No.
Table Caption Page
No.
1.1 Overview of the applications of cavitation 27
2.1 Research work in cavitations field using potassium iodide and salicylic
acid dosimetry
33
3.1 Data obtained from UV- spectrophotometer 44
3.2 Data obtained from HPLC 46
4.1 Effect of reaction temperature on cavitational yield of sonolysis of
potassium iodide
53
4.2 Effect of reaction temperature on cavitational yield of sonolysis of
salicylic acid
53
4.3 Effect duty cycle percentage on cavitation yield of potassium iodide 54
4.4 Effect duty cycle percentage on cavitation yield of salicylic acid 54
4.5 Effect of power supply on sonolysis of potassium iodide 55
4.6 Effect of power supply on sonolysis of salicylic acid 56
4.7 Comparison study of three different reactors using different initial
concentrations of potassium iodide
64
4.8 Study of effect of initial concentration on iodine liberation using 36
kHz reactor in presence of different gases (all values approximate and
are in ppm)
67
4.9 Effect of gaseous additives on sonochemical reactions ( using 36 kHz
reactor)
80
Study of intensification of sonochemical reactions using gaseous additives Page 13
List of Abbrevation
CO2 Carbon Dioxide
HPLC High pressure liquid chromatography
KI Potassium iodide
N2 Nitrogen
O2 Oxyegn
SA Salicylic acid
UV Ultra-violate spectroscopy
Study of intensification of sonochemical reactions using gaseous additives Page 14
Abstract
Due to its large potential of intensification of physical as well as chemical processes,
cavitation can be used in many industrial applications. Tremendous research has been
done in finding its applicability in different fields. But it is observed that due to lack of
economical operation and reliable design of sonochemical reactors based on the use of
ultrasonic irradiation, industrialization of this phenomenon is nearly negligible. So to
make it industrially feasible, intensification of cavitation is required. To study the
intensification aspects of sonochemical reactors oxidation of potassium iodide and
salicylic acid degradation using ultrasonication have been studied in the present work.
Initially effect of different operating parameters such as temperature, power, duty cycle
and initial concentrations of reaction solution has been investigated with experiments in
the available different geometries and capacity of reactors. Intensification of these
sonochemical reactions using different gaseous additives such as air, oxygen, nitrogen
and carbon dioxide has been then investigated. Effect of air flow rate on sonochemical
reactions in different sonochemical reactors has also been examined.
The experimental results show that the cavitational yield is strongly influenced by the
operating parameters and type of the reactor. Also it is observe that presence of gases
increase the extent of oxidation of potassium iodide and degradation of salicylic acid.
This extent is different for different gases and depends on the nature of the gas and
physical properties of gases like polytropic index, vapor pressure etc. In the study of
effect of air flow rate on sonochemical reaction, increase in cavitational yield is observed
in the presence of air up to a certain flow rate and after that it has been observed to
decrease.
Study of intensification of sonochemical reactions using gaseous additives Page 15
CONTENTS
Page No.
Cover Page 1
Title page 2
Approval of supervisor(s) and external examiner 3
Certificate by the supervisor(s) 4
Dedication 5
Acknowledgements 6
Declaration by the student 8
List of figures 9
List of tables 12
List of abbreviations 13
Abstract and keywords 14
Contents 15
Page No.
Chapter 1 Introduction 19-31
1.1 History 19
1.2 Advantages using ultrasouns 20
Study of intensification of sonochemical reactions using gaseous additives Page 16
1.3 Cavitation 21
1.4 Types of cavitation 23
1.4.1 Acoustic cavitation 23
1.4.2 Hydrodynamic Cavitation 23
1.4.3 Particle cavitation 23
1.4.4 Optic Cavitation 23
1.5 Acoustic cavitation 24
1.6 Theory of cavitation 25
1.7 Factors affecting cavitation 26
1.8 Application of cavitation 27
1.9 Limitation of ultrasonic cavitation 29
1.10 Objective of present work 30
Chapter 2 Literature survey 33-41
Chapter 3 Experimental 42-52
3.1 Reaction scheme 42
3.2 Materials 42
3.3 Analytical procedure 43
Study of intensification of sonochemical reactions using gaseous additives Page 17
3.3.1 Iodine measurement 43
3.3.2 Calibration curve for salicylic acid 45
3.4 Experimental Set-up of ultrasonic horn reactor 47
3.5 Parameter optimization 48
3.5.1 Procedure for optimization of reaction temperature 48
3.5.2 Procedure for optimization of duty cycle 49
3.5.3 Procedure for optimization of power 49
3.6 Experimental procedure for sonolysis of reaction solution
in the presence of gases using horn reactor
50
3.7 Experimental Set-up of ultrasonic longitudinal horn
reactor
50
3.8 Experimental procedure for sonolysis of reaction solution
using longitudinal horn reactors of different capacity
52
Chapter 4 Results and Discussion 53-88
4.1 Effect of temperature 53
4.2 Effect of duty cycle 54
4.3 Effect of power 55
4.4 Comparison of different sonochemical reactors 56
4.5 Effect of initial concentration on sonochemical reactions 65
Study of intensification of sonochemical reactions using gaseous additives Page 18
4.6 Effect of different gaseous additives on sonochemical
reactions
73
4.7 Effect of air flow rate on sonochemical reactions 83
Chapter 5 Conclusion 89
Chapter 6 Future work 90
Chapter 7 References 91-97
Study of intensification of sonochemical reactions using gaseous additives Page 19
1. Introduction
1.1 History
Cavitation dates back to 1800s. During field tests of the first high-speed torpedo boats in
1894, Barnaby and Thornycroft I et al., (1895) discovered severe vibrations from rapid
erosion of the ship's propeller. It was postulated that the formation and collapse of
bubbles were the reason behind the observation of the formation of large bubbles (or
cavities) on the spinning propeller. By increasing the propeller size and reducing its rate
of rotation, this difficulty of "cavitation" could be minimized. However with increasing
ship speed, this became a serious concern and the Royal Navy commissioned Lord
Rayleigh (1917) to investigate this phenomenon. He confirmed that the effects were due
to the enormous turbulence, heat, and pressure produced when cavitation bubbles
imploded on the propeller surface.
Such phenomenon of cavitation is observed in liquids not only during turbulent flow but
also under high-intensity sound wave irradiation. Audible sound has a frequency between
10 and 18 kHz, but this is generally of too low an energy to have any influence to cause
cavitation. However, ultrasound with a frequency between 20 kHz and 2 MHz are
reported to show a remarkable effect to produce the same, leading to the physical and
chemical consequences. These effects of ultrasounds on chemical reactions are referred
as “sonochemistry”.
Application of ultrasound was first reported by Richards and Loomis in 1927. Ten years
later Brohult (1937) discovered that ultrasound led to the degradation of a biological
polymer. Research in this field of ultrasonics led to research in the area of degradation of
synthetic polymers by Schmid and Rommel in 1939. Since 1950 there have been several
new and exciting developments in the field of sonochemistry. Noltingk and Neppiras
(1950) performed the first computer calculations modeling on a cavitating bubble. Three
years later work on sonolysis of an organic liquid (Schultz and Henglein, 1953) was
reported. Elder et al. (1954) suggested that bubble-induced micro-streaming was one of
the factors leading to the well-known ultrasonic cleaning effects in heterogeneous
systems. Sonochemistry gained inportance in the 1980s (Caupin et al., 2006) and over
Study of intensification of sonochemical reactions using gaseous additives Page 20
these past few years, intense research has been going on for effective utilization of
acoustic energy for cavitationally induced transformations. Reactions involving solid
surfaces as reagents or catalysts were improved by the mechanical effects associated with
the collapse of a bubble. They are the cleaning and oxide removal from the surface
enhancing its reactivity enabling the molecules to be efficiently swept over the surface
(Caupin et al., 2006). But in case of homogeneous liquid phase, ultrasonic irradiation
induces the production of cavitational bubbles in the liquid medium through which it is
transmitted. These microbubbles act as micro-reactors, which produce very high
temperature and pressure on collapsing, leading to sonolysis (formation of hydroxyl
radical from water) (R. d’Auzay et al. (2010), Cravotto et al. (2006). The radical species
produced can react and recombine with other gaseous species present in the cavity, or
diffuse out of the bubble into the bulk fluid medium where they can react with the solute
molecules (Merouani et al., 2010). These effects can be effectively worked upon for the
intensification of physical and chemical processing applications such as chemical
synthesis, wastewater treatment, textile processing, biotechnology, crystallization,
polymer chemistry, extraction, emulsification and petrochemical industries, etc. (Sutkar
et al., 2009).
1.2 Advantages using Ultrasound:
Before using any technology it is desirable to know its benefits. The benefits of
ultrasound are many folds. It has been studied extensively in the field of chemical
reactions with respect to synthesis and kinetic aspects. To carry out chemical reactions
we require removal or addition of energy, in one form or another to proceed and
ultrasonic irradiation has number of advantages over traditional energy sources, such as
high heat, light, electricity or ionizing radiation. Some of the advantages are given below.
Use of ultrasound,
Accelerates the rate of reaction
Enhances radical reactions and catalyst efficiency
Permits the use of less forcing conditions
Study of intensification of sonochemical reactions using gaseous additives Page 21
Revitalizes older discarded synthetic techniques by enhancing the reactivity of
reagents
Reduces the number of steps required and induction period
Initiates stubborn reactions
1.3 Cavitation
Very high densities (~1018
kW/m3)
can be produced over a small location using the
cavitation phenomenon, which involves generation, growth and collapse of cavitites in a
liquid (Gogate et al. 2006). Ultrasound can be used to bring about cavitation events in
sonochemical reactors. In the case of sonochemical reactors, the cavitation events are
brought about by the passage of ultrasound. Ultrasound when transmitted through a
medium creates a time-varying pressure field which induces vibrational motion to the
molecules leading to compression and stretching of the molecular structure of the
medium. Thus, the molecules oscillate around their mean position leading to variation in
the distances among them. If the intensity of ultrasound in a liquid is increased, a point is
reached at which the intramolecular forces are not able to hold the molecular structure
intact. Consequently, it breaks down and a cavity is formed, shown in the figure 1.1. This
cavity is called cavitation bubble and the point where it starts is known as cavitation
threshold or the inception of cavitation. A bubble responds to the sound field in the liquid
by expanding and contracting, i.e. it is excited by a time-varying pressure, shown in the
figure 1.2. Such bubbles grow by a process known as rectified diffusion i.e. small
amounts of vapour (or gas) from the medium enters the bubble during its expansion phase
and is not fully expelled during compression. The bubbles grow over the period of a few
cycles to an equilibrium size for the particular applied frequency.
Study of intensification of sonochemical reactions using gaseous additives Page 22
Figure 1.1: Growth collapse cycle of cavitation bubbles, Suslick (1998)
Figure 1.2: The compression and expansion cycle of ultrasound, Suslick (1998)
In the subsequent compression cycles, the collapse phase of the bubbles is usually very
fast as compared to the growth phase of the bubbles. This rapid adiabatic collapse of the
cavities generates the energy levels suitable for significant intensification of many
chemical and processing applications as mentioned earlier.
Study of intensification of sonochemical reactions using gaseous additives Page 23
Cavitation events can occur at countless locations in the reactor simultaneously in an
actual reactor. It generates different physical and chemical effects suitable for process
intensification (Gogate and Pandit 2004; Gogate, 2002; Gogate, 2008; Gogate et al.
2006). Physical effects of local turbulence and liquid microcirculation are a result of
cavitation and for applications limited by transport processes; cavitation can be used to
enhance the rates of transport process, leading to process intensification. Cavitation also
results in chemical effects such as the generation of hot spots (conditions of very high
temperatures and pressures) and reactive free radicals, which can intensify the chemical
processing applications limited by intrinsic chemical kinetics (Gogate 2008). In actuality,
the combination of the physical and chemical effects of cavitation leads to net
intensification of the process.
In actual practice, depending on the operating conditions, two forms of cavitation are
observed, stable and transient. In stable cavitation the bubbles oscillate around their
equilibrium position over several refraction/compression cycles. In transient cavitation,
the bubbles grow over one (sometimes two or three) acoustic cycles to a maximum size
which can be in multiples of the initial size and finally collapse violently over a quick
time duration (Thompson and Doraiswamy, 1999).
1.4 Types of Cavitation:
1.4.1 Acoustic cavitation: In this case, passage of sound waves usually ultrasound (16
kHz – 100 MHz) causes pressure variation.
1.4.2 Hydrodynamic cavitation: Cavitation is produced by pressure variation, which is
obtained using geometry of system creating velocity variation. For example based on the
geometry of system, the interchange of pressure and kinetic energy can be achieved
resulting in the generation of cavities as in the case of flow through orifice, venturi etc.
1.4.3 Particle cavitation: It is generated by any type of elementary particle rupturing a
liquid, as in a bubble chamber.
1.4.4 Optic cavitation: It is produced by photons of high intensity light (laser) rupturing
the liquid continuum.
Study of intensification of sonochemical reactions using gaseous additives Page 24
Thus, the tensions prevailing in a liquid leads to acoustic and hydrodynamic cavitations
and local deposition of energy results in optic and particle cavitations. The classification
of the phenomenon of cavitation has been shown schematically in Figure 1.3.
Figure 1.3: Classification of different types of cavitation
1.5 Acoustic Cavitation:
The mechanism of acoustic cavitation consists of
Liquid being exposed to the acoustic field
Cavity formation due to pressure waves which contain small quantities of
dissolved gases and vapours from the surrounding medium
Expansion of bubble or cavity due to compression and rarefaction cycles
Attaining a maximum bubble size depending on operating conditions
Collapse of bubble releasing large amount of energy creating conditions with
extreme pressures and temperatures. (Patil et al., 2007).
Under these conditions, gas molecules entrapped in the cavitation bubbles are thermally
fragmented (by pyrolysis) to dissociate into a variety of short-lived energetic free radical
species. The impact of a collapsing bubble on the contents of the surrounding liquid
depends on the vibrational frequency of the applied field: if the frequency is low (20–100
kHz) mechanical effects are dominant, whereas in the medium to high range frequency
(300–800 kHz) chemical effects dominate (Kidak and Ince, 2006).
Study of intensification of sonochemical reactions using gaseous additives Page 25
Figure 1.4: Schematic of acoustic cavitation (Maikel M., 2008).
1.6 Theory of Cavitation:
Two theories have been reported to explain the observed sonochemical effects
Hot-spot theory (Flynn, 1964)
Electrical theory (Margulis, 1981).
Hot spot theory says that, a small gas bubble subjected to acoustic pressure amplitude of
more than a few bars, experience violent pulsation such that the wall velocity for
collapsing bubble approaches the velocity of sound. At this point, the bubble shatters
upon collapse and results in adiabatic heating. The conditions so induced are in thousands
of degrees temperature and thousands of atmospheres of local pressure and are described
as in hot spots. These hot spots can also result in the dissociation of the water molecules
or other chemical species entrapped in the cavitating bubble and result in the formation of
the free radicals. This is the most common accepted explanation for chemical effects
involving cavitation. (Flynn, 1964; Noltingk and Neppiras, 1950).
Margulis (1981) showed that some observations could not be completely explained by the
“hot-spot” theory and proposed an alternative ‘electrical theory’. This considers the
Study of intensification of sonochemical reactions using gaseous additives Page 26
charge distribution due to dipoles in water and their distribution around a bubble. It was
lso shown that during bubble formation and collapse, enormous electrical field gradients
in the region of 1011
V/m can be generated which are sufficiently high to cause bond
breakage and chemical activity.
1.7 Factors affecting cavitation
The magnitudes of collapse pressures, temperatures and the number of free radicals
generated are a measure of how optimized a cavitation event has been. They are strongly
dependent on the operating parameters of the equipment: intensity and frequency of
irradiation; geometrical arrangement of the transducers in the case of sonochemical
reactors and the liquid phase physicochemical properties, which affect the initial size of
the nuclei and the nucleation process.
Gogate P.R. (2010) had studied the influence of various parameters on cavitation and
operational parameters for optimal cavitation effects have been presented. It was
observed that the intensity of irradiation influenced the collapse pressure of a single
cavity and also the number of cavities generated. In order to enhance this effect it is
suggested to use a wide area of irradiation and also operate at optimum power dissipation.
The intensity study was done within the range of 1-300 W/cm2.
Effect of frequency was studied in the range of 20-200 kHz and it was observed that the
frequency of irradiation affected the collapse time of the cavity and the final pressure &
temperature pulse. Operation at optimum frequencies leads to desired effects. The liquid
vapor pressure (range: 40-100 mm Hg at 30 oC) influences the cavitation threshold,
intensity of cavitation, rate of chemical reaction. The viscosity of liquid governs the
transient threshold and hence liquids of low viscosity are preferred to lower the threshold.
Effect of surface tension was studied in the range of 0.03-0.72 N/m. It plays a crucial role
in determining the size of nuclei, in order to generate nuclei of lower sizes low surface
tension liquids must be used.
Bulk liquid temperatures used affect the intensity of collapse, rate of reaction, threshold
nucleation and other physical properties mentioned above. Optimum value of the bulk
Study of intensification of sonochemical reactions using gaseous additives Page 27
temperature varies with system used and has to be determined. It was studied in the range
of 30-70 oC.
The geometry of reactor is quite instrumental in determining the number of cavitational
events and the distribution of cavitational activity distribution. Shape and number of
transducers should be optimized for enhanced performance. The dissolved gas used
determines the gas content, nucleation collapse phase and intensity of cavitation events.
Gases with low solubility, high polytropic constant and low thermal conductivity
(preferable monoatomic gases) should be used.
1.8 Applications of Cavitations:
It is worthwhile to overview the different applications, where cavitation can be used
efficiently. Few of the important applications are mentioned in the table 1.2.
Table 1.1: Overview of the applications of cavitation
Area Application References
Physical Processes Degassing
Filtration
Emulsification
Crystallization
Surface cleaning
Particle Fusion
Agglomeration
Gandhi K. S. et al.,
(1994)
Martin (1993)
Mason, (1990)
Mechanical
Engineering
Cutting and drilling
Machining
Metal Tube Drawing
Mason, (1990)
Gandhi K. S. et al.,
(1994)
Waste water
treatment
To degrade compounds which are present
in waste water stream like,
P-nitrophenol [1]
1) Kotronarou et al.,
1991; Sivakumar et
al., (2001)
Study of intensification of sonochemical reactions using gaseous additives Page 28
Rhodamine B [2]
1,1,1 Trichloroethane [3]
Phenol [4]
CFC 11 and CFC 113 [5]
o-dichlorobenzene and
dichloromethane [5]
potassium iodide, sodium
cyanide, carbon tetrachloride [6]
2) Shivkumar and
Pandit., (2001)
3) Toy et al., (1990)
4) Petrier et al., (1994)
5) Bhatnagar and
Cheung , (1994)
6) Shirgaonkar and
Pandit, (1997)
Chemical
processing
applications
The different ways in which cavitation
can be used. Like,
Reaction time reduction
Reduction in the induction period
of the desired reaction
Increase in the cavitational yield
Use of less forcing conditions
(temperature and pressure) as
compared to the conventional
routes
Possible switching of the reaction
pathways resulting in increased
selectivity
Increasing the effectiveness of the
catalyst used in the reaction
Initiation of the chemical reaction
by way of generation of the
highly reactive free radicals
Ando et al.(1984)
Javed et al. (1995)
Lie Ken Jie and
Lam, (1995)
Li et al., (1996)
Thompson and
Doraiswamy,
(1999)
Biotechnology Cell disruption for recovery of
intracellular proteins. [1,2]
To selectively release the
intracellular enzymes or enzymes
1) Harrison and Pandit,
(1992).
2) Save et al., 1994,
1997.
Study of intensification of sonochemical reactions using gaseous additives Page 29
present in the cell wall. [3]
Retaining the activity of the
leached out enzymes. [2]
3) Balasundaram and
Pandit, (2001)
Miscellaneous
applications
In petroleum industry for refining
fossil fuels, determination of
composition of coal extracts,
extraction of coal tars, etc. [1]
In textile industry for enhancing
the efficacy of dyeing of clothes
and in wet processing. [2]
To synthesis of nanocrystalline
materials, preparation of high
quality quartz sand, preparation
of free disperse system using
liquid hydrocarbons and dental
water irrigator. [3]
For solvent extraction of herbs.
[4]
1) Patra and Das
(2006)
2) Gogate et al, (2001)
3) Vinatoru M. (2001)
1.9 Limitations of ultrasonic Cavitation
The factors causing hindrance to successful application of sonochemical reactors on an
industrial scale are multifold. Firstly (Mason, 2000), there is a lack of suitable large-scale
design strategies. Also, intense cavitational activity occurs very close to the transducer
which is the device used for generating ultrasound. This intense activity could prove
detrimental to the functioning of transducer. Secondly, substantial efficacy at larger
scales of operation is a challenge with the existing conventional designs. The operating
temperature of the equipment has to be below 70 °C. Thirdly, the frequent erosion of the
ultrasonic surfaces hinders the pilot plant scale operation. The whole process cavitation
occurs in extremely short period of time. Thus, in spite of extensive research, there is
hardly any chemical processing based on ultrasonic cavitation phenomenon carried out on
Study of intensification of sonochemical reactions using gaseous additives Page 30
an industrial scale .The lack of expertise required in diverse fields such as material
science, acoustics, chemical engineering, etc., for scaling up successful lab-scale
processes has also limited its application. .
Ultrasonic transducer, which works on the principle of magneto striction capable of
handling large scale volumes are now available in the commercial application. Theses
transducers are capable of being operated at relatively higher temperature and
continuously for 24 hrs. The recent past has seen the successful use of Multiple-
frequency multiple transducer reactors. These reactors are an improvement when
compared to the conventional designs such as ultrasonic horn or ultrasonic bath. Such
reactors have substantially higher processing capacities (in the range of 1–1000 L).
However, acoustic cavitation reactors still lack the continuous large scale industrial
operation in spite of all these available designs
1.10 Objectives of the present work:
As discussed in earlier section, there are large numbers of promising prospective
applications of sonochemical reactors, but there are very few numbers of applications
which are successful over industrial scale operation. One of the reasons for this lack of
successful applications is the lower rates of processing at large scale applications, which
can be overcome by the use of different additives as process intensifying parameters. The
present work is concentrated in evaluating the efficiency of different gaseous additives at
laboratory scale operation and also understands the dependency of the observed effects
on the scale of operation by performing experiments at three different scales of operation.
Specific hydroxyl radical dosimeters are adequate methods to standardize the
characterization of sonochemical processes. Monitoring the generation of OH radicals in
sonochemical reactors is essential to know the potential applicability as an advanced
oxidation process. In cavitation processes, the hydroxyl radicals are generated inside the
bubbles. Therefore, in order to react with the substances in the liquid phase the radicals
have to diffuse through the gas phase, the interface and the liquid phase. In this process,
the radicals tend to recombine, eventually leading to an underestimation of the •OH
radicals generation. Thus, dosimetry in cavitation systems has to guarantee the
Study of intensification of sonochemical reactions using gaseous additives Page 31
accessibility of the substrate to the •OH radicals. Taking into account the previous studies
(Sutkar and Gogate (2009), Martínez-Tarifa A. et al. (2010)), the present study is being
focused on salicylic acid and iodide dosimetry as the method to estimate the •OH radicals
generation in the cavitation process (Martínez-Tarifa A. et al. (2010)). These approaches
offer some advantages like,
1. Organic dosimetries quantify hydroxylated products that are obtained exclusively due
to the action of •OH radicals. No intermediate products affect the results.
2. The reaction products of salicylic acid/ Iodide dosimetry can be easily analyzed.
Different types of additives used in the present work include air, oxygen, nitrogen and
carbon dioxide. They are used to ease the process of cavity generation and intensify
cavitational activity in the reactor. The presence of additives in the sonochemical reactors
results in intensification due to the any of the following simultaneously acting
mechanisms:
Provide additional nuclei so that the number of cavitation events in the system
increases leading to enhanced effects.
Promote enhanced generation of free radicals or generation of additional
oxidizing species in the system.
Alter the physicochemical properties of the liquid medium thereby facilitating the
ease of generation of cavitation events.
Alter the distribution of the reactants at the site of cavity collapse.
Objectives:
1) Quantification of cavitational activity using salicylic acid and potassium iodide
dosimetry and optimizing the parameters to increase the overall yield of the process.
2) Understand the effect of the presence of air and effect of flow rate of air on the
reaction at optimized conditions.
Study of intensification of sonochemical reactions using gaseous additives Page 32
3) Understand the effect of presence of different gases on the sonochemical reaction in
the ultrasonic horn and ultrasonic bath.
So the present research topic focuses on the studies related to the intensification of
cavitational activity using gaseous additives like air, oxygen, nitrogen and carbon
dioxide. The cavitational activity has been quantified using salicylic acid and potassium
iodide dosimetry.
Study of intensification of sonochemical reactions using gaseous additives Page 33
2. Literature Survey:
As discussed earlier specific hydroxyl radical dosimeters are adequate methods to
standardize the characterization of sonochemical processes. A lot of research was done in
cavitation field using potassium iodide and salicylic acid dosimetry. Table 2.1 gives an
overview about the use of these dosimetries.
Table 2.1: Research work in cavitations field using potassium iodide and salicylic
acid dosimetry.
Research Description Reference
To standardize
ultrasonic power for
sonochemical reaction.
Potassium iodide dosimetry was used to
standardize the ultrasonic power of
individual ultrasonic devices. Results
showed that the potassium iodide
dosimetry, which can be regarded as a
chemical dosimeter for measuring acoustic
energy, was directly and linearly related to
the calorimetrically determined ultrasonic
power
Kimura T. et al.,
(1996)
To study the influence
of experimental
parameters on
sonochemistry using
dosimetries
The influence of several operational
parameters on the sonochemistry
dosimetries namely KI oxidation, Fricke
reaction and H2O2production using
300 kHz ultrasound was investigated. The
main experimental parameters which
showed significant effect in KI oxidation
dosimetry were initial KI concentration,
acoustic power and pH. The solution
temperature showed restricted influence on
KI oxidation.
Merouani S. et
al., (2010)
To enhance the An examination of the efficacy of power- Dominick J. et al.,
Study of intensification of sonochemical reactions using gaseous additives Page 34
sonochemical activity
in aqueous media
using power-
modulated pulsed
ultrasound
modulated pulsed (PMP) sonochemistry
was done by exploring the effects of pulse
type and pulse frequency on the oxidation
of potassium iodide.
(2005)
Modelling of a batch
sonochemical reactor.
A model has been developed for batch
sonochemical reactor using various
assumptions was verified by conducting
experiments with KI solutions of different
concentrations
Naidua D. V. P.
et al., (1994)
To investigate acoustic
cavitation energy in a
large-scale
sonochemical reactor.
Acoustic cavitation energy distributions
were investigated for various frequencies
such as 35, 72, 110 and 170 kHz in a large-
scale sonochemical reactor.
Younggyu Son et
al., (2009)
For comparison of
cavitational activity in
different
configurations of
sonochemical reactors
supported with
theoretical
simulations.
The work deals with evaluation of different
configurations of sonochemical reactors
using a model reaction, potassium iodide
oxidation, also known as the Weissler
reaction, with justification on the basis of
cavitational activity predictions of
theoretical simulations.
Levente C. et al.,
(2011)
To study the effect of
resonance frequency,
power input, and
saturation gas type on
the oxidation
efficiency of an
The sonochemical oxidation efficiency of a
commercial titanium alloy ultrasound horn
has been measured using potassium iodide
as a dosimeter at its main resonance
frequency (20 kHz) and two higher
resonance frequencies (41 and 62 kHz).
Rooze J. et al.,
(2011)
Study of intensification of sonochemical reactions using gaseous additives Page 35
ultrasound horn.
To evaluate
hydrodynamic
cavitation as an
advanced oxidation
process
The generation of OH radicals inside
hydrodynamic cavitation bubbles was
monitored using a salicylic acid dosimeter.
This method has been applied to study the
influence of the flow-rate and the solution
pH for a given cavitation chamber
geometry. The salicylic dosimetry has
proven especially suitable for the
characteristic time scales of hydrodynamic
cavitation (higher than those of ultrasonic
cavitation), which usually gives rise to
recombination of radicals before they can
reach the liquid-phase.
Arrojo S. et al.,
(2007)
To study the
intensification of
hydroxyl radical
production in
sonochemical reactors.
In the present work, the effect of different
operating parameters viz. pH, power
dissipation into the system, effect of
additives such as air, haloalkanes, titanium
dioxide, iron and oxygen on the extent of
hydroxyl radical formation in a
sonochemical reactor have been
investigated using salicylic acid dosimetry.
Chakinala A. G.
et al. (2007)
For the statistical
determination of
significant parameters
in a sonochemical
reactor
The most significant parameters were
determined by designing a 25 factorial set
of experiments and applying some
statistical tools to the results obtained with
the dosimeter. When operating within a
limited range (typical range of standard
sonochemical reactors) the statistical tools
Martínez-Tarifa
A. et al. (2010)
Study of intensification of sonochemical reactions using gaseous additives Page 36
showed that only the dosimeter
concentration and the reactor geometry
were mathematically significant in the
process.
For the chemical
oxygen demand
(COD) determination
assisted by ultrasonic
radiation
Ultrasound works successfully with easily
oxidative organic matter, like salicylic acid,
at optimised conditions. But the COD
values obtained with more difficult organic
matter are poor and still much more
research efforts must be done in order to
improve the instrumental set-up.
Canals A. et al.,
(2002)
The focus of the project is on the intensification of sonochemical reactions using gaseous
additives. A detailed literature survey has been done to analyze the existing data on the
intensification of different sonochemical reactions using gaseous additives. The objective
was to get the proper knowledge of the mechanism of intensification processes and also
to finalize the set of important operating parameters affecting the extent of conversion as
well as the extent of intensification.
Katekhaye and Gogate (2012) have investigated the effects of different additives such as
air, solid particles (cupric oxide and titanium dioxide), salts (sodium chloride and sodium
nitrite) and radical promoters (hydrogen peroxide, ferrous sulphate, iron metal, carbon
tetrachloride and t-butanol) on the degradation of potassium iodide. Combination of
additives has also been investigated for examining the possible synergistic effects in
comparison to the use of individual additions. They reported that the use of different
additives results in enhanced cavitational effects as quantified by an increase in the iodine
formation. Also the comparison between air and other solid additives has been done and
it was reported that some solid additives like titanium dioxide gives higher yield than air
at same conditions. It has been established that those additives which give additional
reactive capability to generate enhanced quantum of free radicals are more effective as
compared to those additives which merely enhance the cavitational activity by virtue of
Study of intensification of sonochemical reactions using gaseous additives Page 37
surface cavitation. They have also observed that using different additives in combination
for intensification of sonochemical oxidation would be dependent on the type of additive
and its mechanism of intensification.
Chakinala A. G. et al., (2007) have studied the intensification of hydroxyl radical
production in sonochemical reactor with different additives such as air, haloalkanes,
titanium dioxide, iron and oxygen. They have taken salicylic acid dosimetry as the model
reaction and found out that acidic condition under optimized power dissipation in the
presence of iron powder and oxygen resulted in maximum liberation of hydroxyl radicals
as quantified by the kinetic rate constant for production of 2, 5- and 2, 3-
dihydroxybenzoic acid. Also it was reported that the presence of oxidant in the form of
air in combination with fenton chemistry might also play some role in enhancing the
reaction rate.
Pang Y. L. et al., (2011) have studied the sonochemical methods in the presence ozone as
an additive for the treatment of organic pollutants in wastewater and observed the
improvement in degradation efficiency. Mechanism offered for this intensification as the
increased mass transfer of ozone from the gas phase to the bulk solution to react with
substrate by mechanical effects of ultrasound. The cavitation bubbles can more readily
induce O3 decomposition under mild conditions. Decomposition of O3 yields molecular
O2 and triplet atomic oxygen (•O(
3P)) state
•O atoms produced are un-reactive and may
react back with O3, they can also contribute to increase the formation of •OH. Therefore
it was reported that combined sonolysis and ozonolysis is an effective oxidation method
compared to its individual oxidation methods as two •OH are formed for every O3
molecule consumed. In bulk aqueous phase, the remaining dissolved O3 could be
decomposed by species originating from H2O molecules during sonolysis and ozonolysis
such as •OH,
•O−2 and
•O−3 to yield
•OOH and
•OH as shown in reactions. These reactive
radicals may react with the target substrates and their initial degradation by-products.
Also studies showed that decomposition using ozone as a additive was maximum at high
pH and at optimum feed rate.
Mohod and Gogate (2011) have worked on intensification of ultrasonic degradation of
polymers like carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) using air as
Study of intensification of sonochemical reactions using gaseous additives Page 38
an additive. Yield polymer degradation as reflected by a significant reduction in the
intrinsic viscosity or the molecular weight. The experimental results showed that the
viscosity of polymer solution decreased with an increase in the ultrasonic irradiation time
and approached a limiting value. Use of air helped in increasing the extent of viscosity
reduction. The extent of viscosity reduction increases by using aeration. This was
attributed to the fact that, presence of the dissolved gases in the liquid significantly
increases the cavitational effect by supplying nuclei for the process.
Kojima Y. et al. (2005) have studied the sonochemical degradation of MCPA ((4-chloro-
2-methylphenoxy) acetic acid) in dilute aqueous solutions using ultrasound. The effect of
gas atmosphere on MCPA degradation was investigated in nitrogen (N2), air (O2/N2),
oxygen (O2), argon (Ar) and Ar/O2 (60/40% v/v) atmospheres. For sonochemical
degradation of MCPA in N2, air (O2/N2), O2 and Ar atmospheres, and the rate
enhancement of MCPA decomposition by sonolysis was found to be more effective in an
O2- enriched atmosphere compared to Ar atmosphere. It was considered that a higher
amount of oxidants was formed in a higher O2 partial pressure, which accelerated MCPA
decomposition in a radical reaction system. On the other hand, both dechlorination and
total organic carbon (TOC) removal rates were higher in Ar atmosphere, compared to
those in O2/N2 atmosphere. It was found that, MCPA was most effectively decomposed
by sonication in Ar/O2 (60/40% v/v) atmosphere, with higher rates of decomposition,
dechlorination and TOC removal.
Wayment D. G. et al., (2002) have worked on the effect of ultrasonic frequency on the
sonochemical degradation of alachlor, which is widely employed herbicide that was used
to control most annual grasses and many broadleaf weeds. Effect of dissolved gases such
as air, oxygen and argon was also studied. It was observed that the rate of degradation
was increased by approximately 1.5 at 300 kHz and by a factor of two at 446 kHz for an
argon-saturated atmosphere compared to oxygen, while the rate did not change with air.
At 300 kHz, the rates were followed the order argon > oxygen > air = nitrogen, while at
446 kHz the rate appears to follow the order argon > air ~ oxygen.
Nagata Y. et al. (2000) have studied the sonochemical degradation of dilute aqueous
solutions of 2-, 3- and 4-chlorophenol and pentachlorophenol under air or argon
Study of intensification of sonochemical reactions using gaseous additives Page 39
atmosphere. Under specified experimental conditions the investigation of dependency on
time of the degradation of 2-, 3-, 4-chlorophenol and pentachlorophenol and liberation of
chloride anions during sonication under argon or air atmosphere was studied. It was
observed that the rate of degradation was faster in argon than in air. They justify this as
the temperature in a collapsing cavity was defined as Tfin= Tin[Pfin(γ−1)/Pin] where Tfin
and Pfin were the final temperature and pressure, and Tin and Pin were the initial
temperature and pressure in the cavities. γ =Cp/Cv was the ratio of the specific heat at
constant pressure to the specific heat at constant volume of the gas in the bubble. The γ
value was higher under argon (1.67) than in air (1.40); therefore, the cavitation effect was
observed higher under argon than under air and hence the acceleration of the reaction by
oxygen appears to be small.
Adewuyi and Oyenekan (2007) worked on optimization of a sonochemical process using
a novel reactor and Taguchi statistical experimental design methodology. For this study
they had selected the model reaction, the sonochemical oxidation of carbon disulfide, and
the response measured was the amount of sulfate (i.e., the predominant oxidation
product) formed during the ultrasonic irradiations. The effect of gaseous additives like
helium, air, oxygen, argon, and 75% argon/25% oxygen mixtures in the temperature
range of 5-50 °C were examined. It was observed that at 581 kHz, the optimum
conditions were 35 °C, 49 W, and oxygen, while the contributions of the temperature,
power, and gas were 5%, 20% and 75% respectively. At 611 kHz, the optimum
conditions were 35 °C, 39 W and helium while the contributions of the temperature,
power and gas were 6.5%, 58% and 35.5% respectively. At 1.3 MHz, the optimum
conditions were 35 °C, 90 W and oxygen while the contributions of the temperature,
power, and gas were 21%, 29% and 50% respectively.
Entezari and Kruus (1996) have studied the effects of different parameters on the
potassium iodide sonochemical oxidation. It was observed that with 20 kHz irradiation,
the reaction rate in a degassed solution is lower than that in an aerated solution. However,
the rate with argon present is greater than that with air, and independent of whether there
is continuous introduction of argon. The nature of the gas present does therefore affect
the reaction rate, and any degassing during ultrasonic irradiation does not seem to be
Study of intensification of sonochemical reactions using gaseous additives Page 40
significant. The gas affects the ratio of specific heats and the thermal conductivity of the
bubble contents. O2 can also participate in the secondary reactions that can occur around
the cavitation bubble. The order of the rate at 20 kHz was argon>air>degassed, whereas
at 900 kHz it is air>degassed>argon. No reasonable explanation can be offered for the
low rate of reaction in degassed solutions using 20 kHz ultrasound. It may be due to a
lack of nucleation centres.
Hart and Henglein (1985) have conducted experiments by irradiating aqueous solutions
of KI in a batch reactor with 300 kHz ultrasound under argon, oxygen and Ar-02 mixtures
of different compositions. The products formed were determined to be I and H2O2. The
study showed that the rate of formation of I2 increased with increasing KI concentration,
but reached a plateau value at very high concentrations of KI. Also the rate of I2
formation reached a maximum at a gas composition of 30% oxygen-70% argon. It was
observed that the ratio of specific heats (γ) for argon is higher than that of O2 and thus,
with the increase in O. content of the bubble, γ decreases, and the collapse temperature of
the bubble also decreases. This leads to the generation of fewer hydroxyl radicals. At the
same time, as the O2 concentration increases, the formation of hydroxyl radicals should
also increase. Thus, a composite effect exists at an intermediate point, resulting in a
maximum.
Rooze J. et al. (2011), have studied the effect of resonance frequency, power input, and
saturation gas type such as air, oxygen, nitrogen, carbon-dioxide and argon and helium on
the oxidation efficiency of an ultrasound horn. It was observed that cavitational yield
increases with increase in frequency for all saturation gases. At low frequency oxidation
efficiency was maximum under argon saturation than air and oxygen but at high
frequencies air saturation showed maximum cavitational yield than oxygen and argon.
Also it was mentioned that in presence of carbon dioxide, bubble grows faster due to
large solubility of carbon dioxide in potassium iodide solution. Hence higher oxidation
efficiency was observed under carbon dioxide saturation.
Sivasankar and Mohalkar (2009) have worked on intensification of sonochemical
degradation of phenol using four different gases as a additives viz. argon, oxygen,
nitrogen and air. It was observed that degradation of phenol was more than nitrogen,
Study of intensification of sonochemical reactions using gaseous additives Page 41
argon and air. Also combinations of gases were studied and observed that combination of
oxygen-argon gave the maximum degradation. Combination of gases with FeSO4 were
also studied and resulted in maximum degradation of phenol using combination with
oxygen. Reaction mechanisms were offered for all the gases.
Shimizu N. et al. (2008) have investigated the effect of dissolved gases on the generation
of OH radicals in the presence of TiO2 catalyst. It was observed that in a sonochemical
reactor of operating frequency of 36 kHz and power rating of 200W, maximum rate was
given by Xenon followed by Ar, O2 and N2.
Study of intensification of sonochemical reactions using gaseous additives Page 42
3. Materials and Methods:
3.1 Reaction scheme:
Reactions considered for the quantification of cavitational activity are oxidation of
potassium iodide and salicylic acid dosimetry. The reaction scheme for oxidation of
potassium iodide can be shown as follows.
Similarly, reaction scheme for salicylic acid docimetry can be shown as follows.
It can be seen that both the reactions are driven by hydroxyl radicals and hence are true
indicator of the cavitational activity.
3.2 Materials:
AR grade potassium iodide (KI) and salicylic acid (SA), sodium hydroxide and
phosphoric acid were procured from S.D Fine-Chem Pvt. Ltd., Mumbai, India. Potassium
iodide and salicylic acid were diluted to required concentrations using distilled water for
Study of intensification of sonochemical reactions using gaseous additives Page 43
experimental studies. Compressor was used for air sparging. Oxygen, Nitrogen and
Carbon Dioxide cylinders were obtained from Alchemi gases. All the chemicals were
used as received from suppliers.
3.3 Analytical Procedure:
Analysis of samples of potassium iodide oxidation was obtained using Thermo Scientific
SPECTROSCAN UV 2600 spectrophotometer and analysis of samples of salicylic acid
dosimetry was performed using high pressure liquid chromatography (HPLC). Column
used for the HPLC was C18 column having inner diameter of 4mm and length as 25 cm.
The mobile phase used was a mixture of phosphate buffer (pH=2.5) and methanol (45:55
%), isocratically delivered (constant composition and flow rate) by a pump at a flow rate
of 1 ml/min. The wavelength set for UV detection was 291 nm.
Calibration Curve:
In analytical chemistry, a calibration curve is a general method for determining the
concentration of a substance in an unknown sample by comparing the unknown to a set of
standard samples of known concentration. The concentrations of the analyte and the
instrument response for each standard can be fit to a straight line, using linear regression
analysis. This yields a model described by the equation y = mx, where y is the instrument
response and m represents the sensitivity. The analyte concentration(x) of unknown
samples may be calculated from this equation.
3.3.1. Iodine Measurements:
Concentration of iodine liberated was obtained by measuring the absorbance of standard
iodide solution with the help of the calibration curve.
Procedure:
1) Initially 500 ml of distilled water was taken in the glass beaker. Then 350 mg
iodine was added into it.
Study of intensification of sonochemical reactions using gaseous additives Page 44
2) From this solution, 5 ml was taken in the test-tube and 5 ml of distilled water
was added so that total volume in the test tube became 10 ml.
3) Using step 2 (with different volumes) different concentration of iodine over the
range of 50 ppm to 500 ppm were prepared.
4) Distilled water was used as a blank sample and the spectrophotometer was set at
auto zero.
5) Set the spectrophotometer at λ = 352 nm and analyze the samples which gives
the maximum absorbance for iodine.
6) From the readings taken, a graph of absorbance values against concentration was
plotted as shown in figure 3.1, while the obtained data have been given in table
3.1.
7) The extent of iodine liberation, which gives a net quantification of the
cavitational activity, can be estimated from the absorbance values using the
calibration equation.
Table 3.1: Data obtained from UV- spectrophotometer.
Concentration
(ppm)
Absorbance
0 0
50 0.06
100 0.094
150 0.135
200 0.181
250 0.205
300 0.257
350 0.278
400 0.317
450 0.363
500 0.394
Study of intensification of sonochemical reactions using gaseous additives Page 45
Figure 3.1: Calibration curve of Iodine on UV-spectrophotometer
3.2.2. Calibration curve for salicylic acid:
The procedure for salicylic acid calibration has been given as follows:
Procedure:
1) Initially 500 ml of distilled water was taken in the glass beaker. Then 50 mg
salicylic acid was added into it.
2) From this solution, 5 ml was taken in the test-tube and 5 ml of distilled water was
added so that total volume in the test tube became 10 ml.
3) Using step 2 (with different volumes) different concentration of salicylic acid over
the range of 10 ppm to 100 ppm were prepared.
4) Every sample prepared was analysed on HPLC to give corresponding peak area.
5) The areas obtained, shown in the table 3.2, were plotted versus the concentration
of salicylic acid solutions, as shown in the figure 3.2.
6) From the graph, we get a linear line passing through zero. The line is thus
described by equation y = mx in which ‘y’ is area of peak and x is concentration
of sample. ‘m’ represents slope of the line.
7) The extent of salicylic acid degradation, which gives a net quantification of the
cavitational activity, was estimated from the absorbance values using the
calibration equation.
y = 0.0008x R² = 0.9908
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 200 400 600
Ab
sorb
ance
Concentration (ppm)
Standerdization curve- Iodine
Linear (Standerdization curve- Iodine)
Study of intensification of sonochemical reactions using gaseous additives Page 46
Table 3.2: Data obtained from HPLC.
Concentration
(ppm) Area under Curve
10 61.23
20 262.177
30 613.45
40 904.87
50 1217.68
60 1543.67
70 1934.54
80 2202.85
90 2532.76
100 2876.25
Figure 3.2: Calibration curve of salicylic acid using HPLC
y = 27.044x R² = 0.9692
0
500
1000
1500
2000
2500
3000
3500
0 50 100 150
Are
a u
nd
er c
urv
e
Concentration (ppm)
Calibration on Agilent HPLC
Linear (Calibration on Agilent HPLC)
Study of intensification of sonochemical reactions using gaseous additives Page 47
3.4 Experimental Set-up of ultrasonic horn reactor:
Three different ultrasonic reactors were used for characterizing the effects of different
gaseous additives such as air, oxygen, nitrogen and carbon dioxide.
Ultrasonic horn reactor:
The unit consists of an ultrasonic horn equipped with generator and was procured from
Dakshin India Ltd., Mumbai. The reactor operates at a frequency of 20 kHz. The
maximum rated power output of the generator is 240 W. The tip diameter of the
transducer was 2.1 cm with an active acoustical vibrational area as 3.46 cm2. The reactor
was operated at optimized conditions and experiments were conducted in a 500 ml glass
reactor. Experiments were conducted using 300 ml reaction solution of 100ppm, 300
ppm, 500 ppm concentrations of potassium iodide and 100 ppm concentration of salicylic
acid. It was observed that during sonication reaction temperature increased due to heat
dissipation induced by cavitational events during. Hence to achieve a constant
temperature throughout the reaction time glass reactor was kept in the ice bath.
Temperature for sonication of potassium iodide was maintained at 150 C and for salicylic
acid reaction temperature was maintained at 200 C. L-shaped sparger was used having
inner diameter of 3.5 mm and hole diameter of 1 mm. Rota-meter of capacity 100 liter
per hour was used to measure the gas flow. The schematic view of experimental setup is
shown in figure 3.3.
Study of intensification of sonochemical reactions using gaseous additives Page 48
Figure 3.3: The schematic view of experimental setup for ultrasonic horn.
3.5 Parameter optimization:
The magnitudes of collapse pressures and temperatures as well as the number of free
radicals generated at the end of cavitation events are strongly dependent on the operating
parameters of the equipment. Hence for getting a sufficient cavitational yield using
ultrasonic horn reactor, operating parameters like temperature, power supplied and duty
cycle were optimized. One parameter at a time method was used for parameter
optimization.
3.5.1 Procedure for optimization of reaction temperature:
1) 300 ml of reaction solution (potassium iodide of concentration 300 ppm) was
prepared and taken in the glass reactor of capacity 500 ml.
2) The output power was adjusted at 60 W and 50% duty cycle was provided by
adjusting on and off timing as 20 seconds.
3) Initially for the first set temperature was maintained at 10 0C using ice bath.
4) Sonication was done for one hour and sample was withdrawn at the end of the
sonication.
5) Sample was analyzed on UV-spectrophotometer.
Study of intensification of sonochemical reactions using gaseous additives Page 49
6) Same procedure was carried out at different reaction temperature such as 15 0C,
20 0C, 25
0C and 30
0C.
3.5.2 Procedure for optimization of duty cycle:
1) 300 ml of reaction solution (potassium iodide of concentration 300 ppm) was
prepared and taken in the glass reactor of capacity 500 ml.
2) The reaction temperature was maintained at 15 0C using ice bath and the output
power was adjusted at 60 W.
3) The first set, 20% duty cycle was provided by adjusting on and off timing as 10
seconds and 40 seconds respectively.
4) Sonication was done for one hour and sample was withdrawn at the end of the
sonication.
5) Sample was analyzed on UV-spectrophotometer.
6) Same procedure was carried out at different percentage of duty cycles such as 40,
50, 60, and 80.
3.5.3 Procedure for optimization of power:
1) 300 ml of reaction solution (potassium iodide of concentration 300 ppm and
salicylic acid of concentration 100 ppm) was prepared and taken in the glass
reactor of capacity 500 ml.
2) The reaction temperature was maintained at 15 0C for potassium iodide and 20
0C
for salicylic acid using ice bath and the duty cycle was adjusted at 60 %.
3) In the first set, 30W power was provided using power generator.
4) Sonication was done for one hour and sample was withdrawn at the end of the
sonication.
5) Sample of potassium iodide was analyzed on UV-spectrophotometer and sample
of salicylic acid was analyzed on HPLC.
6) Same procedure was carried out at different power such as 60W, 90W and 120W.
Study of intensification of sonochemical reactions using gaseous additives Page 50
3.6 Experimental procedure for sonolysis of reaction solution in the presence of
gases using horn reactor:
1) 300 ml of reaction solution was taken in the glass reactor having capacity of 500
ml.
2) 1 cm length of the horn was dipped in the reaction solution.
3) The output power of the reactor was adjusted at 60 W. 60% duty cycle was
provided by adjusting the on and off time of the reactor.
4) First sonication was done for 1 hour without using any gases additives. Sample
was taken out after every 10 minutes.
5) Another 300 ml reaction solution was prepared and taken in the glass reactor. L-
shaped sparger was inserted in the reaction solution.
6) 1 hour sonolysis was carried out with continuous sparging of air in it at
atmospheric pressure. Flow rate of air was 5 liter per hour measured using
rotameter. Sample was taken out after every 10 minutes.
7) Same procedure was followed for other gaseous additives such as carbon dioxide,
oxygen and nitrogen.
8) Samples of potassium iodide were analyzed on UV-spectrophotometer and
samples of salicylic acid were analyzed on HPLC.
3.7 Experimental Set-up of ultrasonic longitudinal horn reactor:
Second reactor used was ultrasonic longitudinal horn. The experimental set up consists of
single large transducer having longitudinal vibrations and was procured from Roop
Telsonic Ultrasonics, Mumbai, India which had an operating frequency of 36 kHz and
rated power output of 150 W. The bath was divided into two sections. The upper section
had dimensions like 33 cm length, 20 cm width and 15 cm height. The lower section was
a V shaped channel of height 3 cm. The total body was made up of stainless steel. A
drainage valve was also provided at the bottom of the bath. The internal of the bath
consists of a horn which was fitted at the bottom of the bath horizontally along the length
of bath. A transducer was attached to one end of the horn and the energy to this
transducer is provided by a generator which is a separate unit. The reactor had a
Study of intensification of sonochemical reactions using gaseous additives Page 51
maximum capacity of 9.5 liter. L-shaped sparger was inserted in to the reactor for
sparging of gaseous additives as shown in the figure 3.4.
Figure 3.4: The schematic view of experimental setup for ultrasonic longitudinal
horn of 36 kHz and 25 kHz
Third reactor used was ultrasonic longitudinal horn. The experimental set up similar to
the second reactorand was also procured from Roop Telsonic Ultrasonic, India. The
ultrasonic bath had an operating frequency of 25 kHz and rated power output of 1 kW.
The bath was divided into two sections. The upper section had dimensions as 35cm
length, 12cm width and 17cm height. The lower section was a V shaped channel of 3 cm
height. The reactor body was made up of stainless steel. A drainage valve was also
provided at the bottom of the bath. The internal section is similar to the second reactor.
Study of intensification of sonochemical reactions using gaseous additives Page 52
3.8 Experimental procedure for sonolysis of reaction solution using longitudinal
horn reactors of different capacity:
1) Firstly the reactors were filled up with the reaction solution. Volume required for
the longitudinal horn reactor having capacity of 1 kW was 7 liters and that for 150
W capacity reactor was 9 liters.
2) First sonication was done for 1 hour without using any additives. Sample was
taken out after every 10 minutes.
3) Again reaction solutions were prepared and taken in to the reactor. L-shaped
sparger was inserted in to the reaction solution.
4) 1 hour sonolysis was carried out with continuous sparging of air at the
atmospheric pressure. Flow rate of air was maintained at 20 liters per hour for the
sonolysis in the reactor having 1 kW capacity and 25 liters per hour for reactor
having 150 W capacity. Sample was taken out after every 10 minutes.
5) Same procedure was followed for other gaseous additives such as carbon dioxide,
oxygen and nitrogen.
6) Samples of potassium iodide were analyzed using UV-spectrophotometer and
samples of salicylic acid were analyzed using HPLC.
Study of intensification of sonochemical reactions using gaseous additives Page 53
4. Result and Discussion
4.1 Effect of Temperature
It was observed that cavitational yield decreases with an increase in the temperature. It is
mainly due to the fact that vapours pressure of the liquid medium increases with the
temperature. Also vapors which enters the bubble during its formation cushions the
collapse of the bubble. This 'vaporous' or 'transient' cavitation is expected to be the
predominant effect when little dissolved gas is present. This predicts that as the bulk
temperature increases, the temperature of the 'hot spot' formed by the cavity collapse
decreases (Entezari and Kruus, 1996).
Results of sonolysis of potassium iodide have been shown in the table 4.1 and that for
salicylic acid have been shown in the table 4.2.
Table 4.1: Effect of reaction temperature on cavitational yield of sonolysis of
potassium iodide.
Temperature ( 0 C) Iodine Liberated (ppm)
10 198.25
15 185.00
20 163.25
25 141.25
30 115.00
Table 4.2: Effect of reaction temperature on cavitational yield of sonolysis of
salicylic acid.
Temperature % degradation
10 32.01
15 32.67
20 31.83
25 10.85
30 7.3
Study of intensification of sonochemical reactions using gaseous additives Page 54
For further experiments 150 C temperature was chosen as the optimum for the sonolysis
of the potassium iodide and 20 0C for the sonolysis of the salicylic acid, because below
this reaction temperature, only marginal variation in the cavitational yield is obtained.
4.2 Effect of duty cycle
It was observed that sonication yield increases with an increase in the percentages of duty
cycle. It is attributed to the fact that more the duty cycle more is the power dessipatation,
results in the increased cavitational yield. Results of sonolysis of potassium iodide have
been given in the table 4.3 and for salicylic acid have been given shown in the table 4.4.
Table 4.3: Effect duty cycle percentage on cavitation yield of potassium iodide.
% duty
cycle
On time
(sec.)
Off time
(sec.)
Iodine
liberated
(ppm)
20 10 40 78.75
40 20 30 108.25
50 25 25 141.25
60 30 20 171.00
80 40 10 200.25
Table 4.4: Effect duty cycle percentage on cavitation yield of salicylic acid.
% duty
cycle
On time
(sec.)
Off time
(sec.)
Degradation obtained (%)
20 10 40 9.2
40 20 30 19.42
50 25 25 23.56
60 30 20 28.83
80 40 10 36.76
Study of intensification of sonochemical reactions using gaseous additives Page 55
For the further experiments 60 % duty cycle has been selected for the both the cases,
because at this value percentage a sufficient cavitation yield was observed than that
obtained at lower percentages of duty cycle. Also energy conservation and instrument
maintenance will be better at 60 % compared to that at higher percentage of duty cycle.
4.3. Effect of power
It was observed that sonication yield increases with an increase in the power supply. It
can be attributed to the enhanced mixing and circulation currents with an increase in the
ultrasound power (Toukoniitty et al.2006; Hingu et al. 2010). Also Merouani S. et al.,
(2010) explain the iodine liberation on the basis of increase in the number of active
cavitationa bubbles with power. That is when power is increased, transmittance of
ultrasonic energy into the reactor increases. Due to this energy, the pulsation and collapse
of bubble occur more rapidly, the number of cavitation bubbles increases and realizing a
higher concentration of OH radicals into the aqueous solution of potassium iodide. Thus,
an increase in ultrasonic power results in an increase in acoustic amplitude, which favors
more violent cavitation bubble collapse because the bubble collapse time, the transient
temperature, and the internal pressure in the cavitation bubble during collapse are all
dependent on the acoustic amplitude.
Results of sonolysis of potassium iodide have been given in the table 4.5 and of salicylic
acid were shown in the table 4.6.
Table 4.5: Effect of power supply on sonolysis of potassium iodide.
Power supplied (W) Iodine Liberated (ppm)
30 46.25
60 93.75
90 172.00
120 210.00
Study of intensification of sonochemical reactions using gaseous additives Page 56
Table 4.6: Effect of power supply on sonolysis of salicylic acid.
Power supplied (W) Degradation obtained (%)
30 12.34
60 21.46
90 30.32
120 34.65
For further experiments 90W power supply has been selected for both the cases, because
at this value a sufficient cavitation yield was observed than that at lower amount of power
supply. Also energy conservation and instrument maintenance will be better at 60W
compared to that at higher powers.
4.4 Comparison of different sonochemical reactors:
Three low frequency (20 kHz, 25 kHz and 36 kHz) reactors were used in the present
work. Sonication of three different concentrations of potassium iodide (100 ppm, 300
ppm and 500 ppm) and 100 ppm of salicylic acid were studied in all three reactors.
Sonication was performed in the presence of different gases such as air, oxygen, nitrogen
and carbon dioxide.
Firstly sonolysis of potassium iodide of concentration 100 ppm was investigated using
three sonochemical reactors in the absence of any gases. Reactions were carried out at
optimized conditions for one hour. Iodine liberated with time was plotted for all three
reactors, as shown in the figure 4.1. It was observed that iodine liberated in the 20 kHz,
25 kHz and 36 kHz sonochemical reactors at the end of one hour was 61.25 ppm, 50
ppm, and 20 ppm respectively. Similar study was done but in the presence of different
gases.
In the presence of air it was observed that iodine liberation at the end of 60 minutes
sonication in each case was larger than the sonication in the absence of air. The reason of
that is discussed in later section. But it was also observed that the order of the increase in
iodine liberation with respect to type of reactor remain the same. The obtained results are
given in the figure 4.2. It was seen that 20 kHz reactor gave 70 ppm of iodine liberation
Study of intensification of sonochemical reactions using gaseous additives Page 57
in the presence of air whereas 25 kHz and 36 kHz had gave 60 ppm and 30 ppm of iodine
liberation respectively. This trend is attributed to the fact that in the 20 kHz reactor power
dessipatition per liter of volume is larger followed by 25 kHz and 36 kHz reactor. And
more power dessipatation results in more cavitational yield.
Figure 4.1 : Sonication of 100 ppm KI in different reactors in the absence of any
gases
Similar effect was observed when 100 ppm of potassium iodide was sonicated in these
three reactors in the presence of oxygen, nitrogen and carbon dioxide. Figure 4.3 shows
that in the presence of oxygen, maximum iodine liberation was given by 20 kHz reactor
(80 ppm) followed by 25 kHz (70 ppm) and 36 kHz reactor (40 ppm). Similarly figure
4.4 and 4.5 shows the comparison of the three reactors on the basis of cavitational yield
using nitrogen and carbon dioxide respectively. It was observed that in the presence of
nitrogen 20 kHz, 25 kHz and 36 kHz reactors gave 63 ppm, 58 ppm and 27 ppm of iodine
liberation from 100 ppm of potassium iodide respectively. In the presence of carbon
dioxide the yields are 85 ppm, 80 ppm and 46 ppm of iodine liberation for the 20 kHz, 25
kHz and 36 kHz reactors respectively.
0
10
20
30
40
50
60
70
0 20 40 60 80
Iod
ine
Lib
erat
ed (p
pm
)
Time (min)
on 36 KHz longitudnal horn
on 20 khz horn
On 25kHZ longitudnal horn
Study of intensification of sonochemical reactions using gaseous additives Page 58
Figure 4. 2 : Sonication of 100 ppm KI in different reactors in the presence of air.
Figure 4.3: Sonication of 100 ppm KI in different reactors in the presence of oxygen.
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
0 20 40 60 80
Iod
ine
Lib
era
ted
(pp
m)
Time (min)
on 20 KHz horn
on 25 kHz longitudnal horn
36 KHz longitudnal horn
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
0 20 40 60 80
Iod
ine
liber
ate
d (p
pm
)
Time (min)
on 20 kHz horn
on 25 kHz longitudnal horn
on 36 kHz longitudnal horn
Study of intensification of sonochemical reactions using gaseous additives Page 59
Figure 4.4 : Sonication of 100 ppm KI in different reactors in the presence of
nitrogen.
Figure 4.5: Sonication of 100 ppm KI in different reactors in the presence of carbon
dioxide.
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
0 20 40 60 80
Iod
ine
Lib
era
ted
(pp
m)
Time (min)
Using 20 KHZ horn
Using 25 kHz longitudnal horn
Using 36 KHz longitudnal horn
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
0 20 40 60 80
Iod
ine
Lin
era
ted
(pp
m)
Time (min)
Using 20 KHz horn
Using 25 kHz longitudnal horn
Using 36 KHz longitudnal horn
Study of intensification of sonochemical reactions using gaseous additives Page 60
Salicylic acid with 100 ppm concentration was also examined to study the effect additives
in of different reactors. Figure 4.6 shows the comparison of the three reactors on the basis
of salicylic acid degradation with respect to time. It was observed that percent
degradation was larger in 20 kHz reactor. 28 % degradation was observed in this reactor
in absence of any gases. The 25 kHz reactor yields lesser cavitational yield and 22 % of
degradation was achieved using this reactor in the same operating time. The 36 kHz
reactor gave the lowest cavitational degradation (13 %) of 100 ppm salicylic acid in
absence of any gases in 60 minutes.
Figure 4.6: Sonication of 100 ppm S.A. in different reactors in the absence of any
gases.
Cavitational degradation of salicylic acid was also examined in the three reactors in the
presence of gases. Nitrogen, oxygen, air and carbon dioxide were the gases used in the
study. It was observed that with the effect of gases, was similar as compared to liberation
of iodine. 20 kHz reactor gave the largest percent of degradation followed by 25 kHz and
36 kHz reactor. This means that 20 kHz was more efficient in generating higher intensity
of cavitation activity than other two. 36 kHz reactor had shown the lowest efficiency in
terms of cavitational yield.
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
0 20 40 60 80
% c
han
ge in
SA
Time (min)
using 36 KHz longitudnal horn
using 25 kHz longitudnal horn
usig 20 kHz horn
Study of intensification of sonochemical reactions using gaseous additives Page 61
Percent degradation of 100 ppm salicylic acid in these 3 reactors in the presence of
different gases, have been presented in figure 4.7 to 4.10.
Air- Figure 4.7 shows that in the presence of air, 20 kHz reactor gave 40 % of
degradation of 100 ppm salicylic acid. The percent degradation was 29.5 in the
case of 25 kHz reactor and 18 % in the case of 36 kHz reactor.
Oxygen- Figure 4.8 shows that in the presence of oxygen, 20 kHz reactor gave 44
% of degradation of 100 ppm salicylic acid. The percent degradation was 27% in
the case of 25 kHz reactor and 27 % in the case of 36 kHz reactor.
Nitrogen- Comparison has been shown in the figure 4.9 and it was observed that
20 kHz reactor gave 30 % degradation of salicylic acid in the presence of
nitrogen. The degradation observed was 25.5 % and 17 % in the case of 25 kHz
and 36 kHZ reactor respectively.
Carbon dioxide- From figure 4.10, it was seen that 48 % of degradation of
salicylic acid was given by 20 kHz reactor in the presence of carbon dioxide. This
yield was 42% in the case of 25 kHz and 31 % in the case of 36 kHz reactor.
Figure 4.7: Sonication of 100 ppm S.A. in different reactors in the presence of air.
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
0 20 40 60 80
% c
han
ge in
SA
co
nc.
Time (min)
Using 36 KHz Longitudnal horn
Using 25 kHz longitudnal horn
Using 20 kHz horn
Study of intensification of sonochemical reactions using gaseous additives Page 62
Figure 4.8: Sonication of 100 ppm S.A. in different reactors in the presence of
oxygen.
Figure 4.9: Sonication of 100 ppm S.A. in different reactors in the presence of
nitrogen.
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
0 20 40 60 80
% c
han
ge in
SA
co
nc.
Time (min)
Using 36 KHz longitudnal horn Using 25 kHz longitudnal horn using 20 KHz horn
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
0 20 40 60 80
% c
han
ge in
SA
co
nc.
Time (min)
using 36 KHz longitudnal horn
Using 25 kHz longitudnal horn
Using 20 KHz horn
Study of intensification of sonochemical reactions using gaseous additives Page 63
Figure 4.10: Sonication of 100 ppm S.A. in different reactors in the presence of
carbon dioxide.
Studies were also carried out using 300 ppm and 500 ppm of potassium iodide.
All results of sonication using 300 ppm and 500 ppm potassium iodide in all the reactors
and in presence of different gases are shown in the table 4.7. All values in table are
showing the ppm concentration of iodine liberated in respective system.
Though it was reported that (Rooze J. et al., 2011) sonochemical reactors efficiency
should increase with the frequency, but it was observed in the study that the maximum
iodine liberation was obtained in lower frequency reactor that is in the 20 kHz reactor
followed by 25 kHz and 36 kHz reactor. That is iodine liberation increased from lower
frequency reactor to higher. So it is the fact that the sonochemical reactors were
influenced by some other reactor parameter like power than the frequency of the reactor.
Hence the observed trend of iodine liberated can be explained on the basis of power
dissipate per liter of reaction solution in the different reactors.
0.00
10.00
20.00
30.00
40.00
50.00
60.00
0 20 40 60 80
% c
han
ge in
SA
co
nc.
Time (min)
Using 36 kHz longitudnal horn
Using 25 kHz longitudnal horn
Using 20 KHz horn
Study of intensification of sonochemical reactions using gaseous additives Page 64
Table 4.7: Comparison study of three different reactors using different initial
concentrations of potassium iodide.
Reactor > 20 kHz 25 kHz 36 kHz
Gas 300
ppm KI
500
ppm KI
300
ppm KI
500
ppm KI
300
ppm KI
500
ppm KI
No gas 170 230 118 185 31 43
Air 250 190 150 205 41 60
Oxygen 198 275 168 232 54 82
Nitrogen 177 250 132 193 36 52
Carbon
Dioxide 221 295 190 252 68 107
20 kHz sonochemical horn was operated at 90 W i.e. 90 J/sec. Horn was operated for one
hour with 60 % duty cycle. Therefore total working time of the reactor was 36 minutes
i.e. 2160 seconds. Therefore total power supplied to the reaction solution was 194400
Joules (supplied power X time). Volume of reaction solution was 300 ml. Hence total
energy supplied per liter was 648 kJ.
In case of 25 kHz sonochemical reactor which worked at 1 kW i.e. 1000 J/sec and having
capacity of 7.5 liters was operated for one hour. Therefore total energy supplied to the
reaction solution was 36,00,000 Joules. Total volume of reaction solution was 7.5 liters
which was the maximum volume capacity of the reactor. Hence total energy supplied per
liter of reaction solution was 480 kJ.
Similarly, in the case of 36 kHz sonochemical reactor which worked at 150 W i.e. 150
J/sec and having capacity of 7.5 liters was operated for one hour. Hence total energy
supplied per liter of reaction solution was 56.8 kJ.
Study of intensification of sonochemical reactions using gaseous additives Page 65
It is cleared from the above calculation that due to comparatively larger power
dissipatation in the 20 kHz reactor, it shows the higher cavitational yield. In other words
the performance of the reactor is directly proportional to the power dissipated per liter of
reaction solution. It is attributed to the fact that as the power dissipatation increases
mixing of the reaction solution and circulation current increases which results in
enhanced cavitational yield (Hingu et al., 2010).
4.5 Effect of initial concentration on sonochemical reactions:
For study of effect of concentration, 3 different concentrations were used (100, 300 and
500 ppm). Study was carried out in all three reactors using different gases additives such
as air, nitrogen, oxygen and carbon dioxide. It was observed that in all cases iodine
liberation increased with initial concentration of reaction solution. Iodine liberation is
highest in the case of 500 ppm initial concentration followed by 300 ppm and 100 ppm
respectively. For comparison study, iodine liberation was plotted against the time of
treatment in each case.
In figure 4.11 results of sonication of 100 ppm, 300 ppm and 500 ppm potassium iodide
using the 20 kHz horn reactor have been given. Sonication was done in absence of any
gases and at optimized conditions. The figure shows that iodine liberation in the case of
100 ppm initial concentration of potassium iodide was 60 ppm and in the case of 300
ppm it was 160 ppm where as in the case of 500 ppm, 220 ppm of iodine liberated.
Similar results were obtained (with additional effect of gases) when sonication was done
in the presence of different gases like air, oxygen, nitrogen and carbon dioxide. In the
presence of air, iodine liberation was around 70 ppm, 190 ppm and 260 ppm for 100, 300
and 500 ppm of initial concentration respectively, as shown in the figure 4.12. Similar
increment was observed for other gases. For oxygen it was 80 ppm to 175 ppm to 265
ppm, as shown in the figure 4.13. For nitrogen it was 77 ppm to 168 ppm to 250 ppm, as
shown in the figure 4.14. For carbon dioxide the increment was as 85 ppm to 220 ppm to
300 ppm which is shown in the figure 4.15. All increments discussed above were for 100
ppm, 300 ppm and 500 ppm of initial concentrations respectively.
Study of intensification of sonochemical reactions using gaseous additives Page 66
In the case of 25 kHz reactor (reactor volume 7.5 liters) similar studies were carried out
on effect of initial concentration of reaction solution. Initially sonication of potassium
iodide of different concentrations, as discussed above was carried out in absence of any
gases. It was observed that iodine liberated was lower than that of 20 kHz ultrasonic horn.
It is due to difference in the power dissipatition of the different reactors as discussed in
earlier section. But the increasing trend of iodine liberation with concentration remained
same. Results have been given shown in the figure 4.16. It shows that for 100 ppm of
initial concentration, 50 ppm of iodine was obtained at the end of 60 minutes. In the case
of 300 ppm of initial concentration 115 ppm and in case of 500 ppm of initial
concentration, 180 ppm of iodine was obtained at the end of 60 minutes under same
conditions. It was clearly indicating that iodine liberation was maximum for 500 ppm of
initial concentration followed by 300 ppm and 100 ppm. Studies of effect of initial
concentration were also carried out in this reactor in the presence of gases additives.
Iodine liberation for 100 ppm, 300 ppm and 500 ppm of initial concentration of
potassium iodide in the presence of gases has been shown as per following description:
For air iodine liberation was increased as 60 ppm to 155 ppm to 200 ppm, (figure
4.17).
For oxygen iodine liberation was increased as 70 ppm to 165 ppm to 230 ppm,
(figure 4.18).
For nitrogen iodine liberation was increased as 55 ppm to 150 ppm to 185 ppm,
(figure 4.19).
For carbon dioxide iodine liberation was increased as 80 ppm to 190 ppm to 245
ppm, (figure 4.20).
Third reactor that is 36 kHz longitudinal horn which has reactor capacity of 9.5 liters was
also used to study the effect of initial concentration on iodine liberation. Iodine liberat ion
in this reactor was much lesser than above two reactors. But the trend of change in
amount of iodine liberation with initial concentration was same as the above two reactors.
The results on this reactor have been given in the table 4.8.
Table 4.8: Study of effect of initial concentration on iodine liberation using 36 kHz
reactor in presence of different gases. (all values are in ppm)
Study of intensification of sonochemical reactions using gaseous additives Page 67
Gas additive\
reactor 100 ppm 300 ppm 500 ppm
No gas 20 31 43
Air 30 40 60
Oxygen 40 55 80
Nitrogen 34 45 70
Carbon
Dioxide
50 70 110
This enhancement in iodine liberation with initial concentration can be attributed to the
fact that, increase in initial concentration of potassium iodide may increase the surface
tension and ionic strength of the reaction medium (Merouani et al, (2010), Kirpalani &
McQuinn, (2006)). Also, as concentration of potassium iodide increases the vapor
pressure of the medium decreases. Due to the changes in the physical property collapse
conditions are also changed. As the surface tension and ionic strength increases
interfacial energy of the bubble and medium also increased and hence given more energy
release at the cavity collapse. Also due to decrease in vapor pressure cavity formation is
easier. Both factors results in the collapsing of the bubbles more violently. It means that
temperature and pressure at cavity collapse increase with an increase in initial
concentration of potassium iodide. This helps in producing more hydroxyl radicals
available for the oxidation of potassium iodide.
Increase in cavitation yield can also be attributed to the fact that as concentration of KI
increases more molecules of potassium iodide are available for the oxidation reaction
with increased hydroxyl radicals. Hence the probability of recombination of reaction
radicals gets reduced Naidu et al., (1993) and Seymour et al., (1997). As possibility of
recombination of radicals get reduced, less peroxide formation takes place. Hence
maximum hydroxyl radicals are consumed by iodide ions which increase with potassium
iodide concentration. This helps in improvement in amount of iodine liberation.
Study of intensification of sonochemical reactions using gaseous additives Page 68
Figure 4.11: Effect of initial concentration of potassium iodide on iodine liberation
using 20 KHz horn in absence of any gases.
Figure 4.12: Effect of initial concentration of potassium iodide on iodine liberation
using 20 KHz horn in the presence of air.
0.00
50.00
100.00
150.00
200.00
250.00
0 20 40 60 80
Iod
ine
Lib
erat
ed (p
pm
)
Time (min)
wiyhout using any gases-300 ppm
wihout any gases-100 ppm
without any gases - 500 ppm
0.00
50.00
100.00
150.00
200.00
250.00
300.00
0 20 40 60 80
Iod
ine
Lib
erat
ed (p
pm
)
Time (min)
Using air horn- 300ppm
Using air horn - 100 ppm
Using air - horn- 500 ppm
Study of intensification of sonochemical reactions using gaseous additives Page 69
Figure 4.13: Effect of initial concentration of potassium iodide on iodine liberation
using 20 KHz horn in the presence of oxygen.
Figure 4.14: Effect of initial concentration of potassium iodide on iodine liberation
using 20 KHz horn in the presence of nitrogen.
0.00
50.00
100.00
150.00
200.00
250.00
300.00
0 20 40 60 80
Iod
ine
Lib
era
ted
(pp
m)
Time (min)
Horn- Using Oxygen 300 ppm
Horn- Using Oxygen- 100 ppm
Horn- Using air- 500 ppm
0.00
50.00
100.00
150.00
200.00
250.00
300.00
0 20 40 60 80
Iod
ine
Lib
era
ted
(pp
m)
Time (min)
Horn- using nitrogen - 300 ppm
Horn- Using nitrogen- 100 ppm
Horn- Using Nitrogen- 500 ppm
Study of intensification of sonochemical reactions using gaseous additives Page 70
Figure 4.15: Effect of initial concentration of potassium iodide on iodine liberation
using 20 KHz horn in the presence of carbon dioxide.
Figure 4.16: Effect of initial concentration of potassium iodide on iodine liberation
using 25 KHz longitudinal horn (1 kW) in the absence of any gases.
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
0 20 40 60 80
Iod
ine
Lib
era
ted
(pp
m)
Time (min)
Horn- Using CO2- 300 ppm
Horn- Using CO2- 100 ppm
Horn- Using CO2- 500 ppm
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80
Iod
ine
libe
rate
d (p
pm
)
Time (min)
1 Kw- Nogase- 100ppm
1 Kw- no gas- 300 ppm
1 Kw- no gase- 500 ppm
Study of intensification of sonochemical reactions using gaseous additives Page 71
Figure 4.17: Effect of initial concentration of potassium iodide on iodine liberation
using 25 KHz longitudinal horn (1 kW) in the presence of air.
Figure 4.18: Effect of initial concentration of potassium iodide on iodine liberation
using 25 KHz longitudinal horn (1 kW) in the presence of oxygen.
0
50
100
150
200
250
0 20 40 60 80
Iod
ine
lib
era
ted
(pp
m)
Time (min)
1 Kw- Using air- 100 ppm
1 Kw- using air- 300 ppm
1 kw- using air- 500 ppm
0
50
100
150
200
250
0 20 40 60 80
Iod
ine
libe
rate
d (p
pm
)
Time (min)
1kw- using oxygen- 100ppm
1 Kw- using oxygen- 300 ppm
1 Kw- using oxygen- 500 ppm
Study of intensification of sonochemical reactions using gaseous additives Page 72
Figure 4.19: Effect of initial concentration of potassium iodide on iodine liberation
using 25 KHz longitudinal horn (1 kW) in the presence of nitrogen.
Figure 4.20: Effect of initial concentration of potassium iodide on iodine liberation
using 25 KHz longitudinal horn (1 kW) in the presence of carbon dioxide.
0
50
100
150
200
250
0 10 20 30 40 50 60 70
Iod
ine
lib
era
ted
(pp
m)
Time (min)
1 Kw- using nitrogen- 100 ppm
1 Kw- using nitrogen- 300 ppm
1 Kw- using nitrogen- 500 ppm
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70
Iod
ine
lib
era
ted
(pp
m)
Time (min)
1 Kw- using CO2- 100 ppm
1 Kw- using CO2- 300 ppm
1 Kw- Using CO2- 500 ppm
Study of intensification of sonochemical reactions using gaseous additives Page 73
4.6 Effect of different gaseous additives on sonochemical reactions:
Gases such as air, oxygen, nitrogen and carbon dioxide were used to study the effect as
an additive on the selected sonochemical reactions, potassium iodide oxidation and
salicylic acid degradation. Studies were carried out in three different reactors (20 kHz, 25
kHz and 36 kHz). It was observed that cavitational yield that is iodine liberation by
oxidation of potassium iodide and percent degradation of salicylic acid increases due to
the use of these gases.
In 20 kHz reactor, to study the effect of gases as an additive, initially 300 ml of potassium
iodide solution of concentration 100 ppm was taken. Flow rate of each gas was kept
constant at 5 liter per hour (lph). Results of sonication in the presence of gas at the end of
60 minutes were shown in the figure 4.21. It was observed that in the absence of any
gases one hour sonication liberates around 60 ppm of iodine. After the initial
experiments, air was passed at given flow rate through reaction solution simultaneously
during sonication. It was observed that because of presence of air, amount of iodine
liberation at the end of one hour sonication increased up to 71 ppm. It means that air
increased the amount of iodine liberation by 16.6 %. Nitrogen also showed similar effect
on the amount of iodine liberation, and iodine liberation increased up to 65 ppm. Thus
use of nitrogen increased iodine liberation increased by 7 %. Similarly increased iodine
liberation was observed in the case of oxygen and carbon dioxide. In the presence of
oxygen, 77 ppm of iodine was obtained and in the presence of carbon dioxide, 80 ppm of
iodine was obtained. Iodine liberation observed to be increase by 28 % due to oxygen and
33 % due to carbon dioxide. Similar study was carried out for the 300 ppm and 500 ppm
of potassium iodide keeping other operating parameters constant. It was observed that the
trend of effect of gaseous additives is same but with additional concentration effect (The
relation of iodine liberation with initial concentration has been already discussed in the
previous section.). Sonication in the presence of air gave the increment of 12 % in 300
ppm initial concentration and 13 % in 500 ppm initial concentration of potassium iodide.
Similar increment of iodine liberation was observed in the presence of oxygen (16.5 %
and 18 %), Nitrogen (5 % and 7.5 %) and carbon dioxide (20.5% and 24 %) for 300 ppm
Study of intensification of sonochemical reactions using gaseous additives Page 74
and 500 ppm of initial concentration of potassium iodide, as shown in the figure 4.22 (for
300 ppm KI) and 4.23 (for 500 ppm KI).
Studies were also carried out using 100 ppm salicylic acid using same reactor. Same
gases were used to study the effect at optimized conditions. It was observed that similar
to iodine liberation in the case of potassium iodide, extent of salicylic acid degradation
also increased in the presence of gases. Order of increment was observed to be same as
that observed for iodine liberation, i.e. extent of degradation was in the following order
for different gases as CO2 > O2 > Air > N2. Results have been shown in the figure 4.24.
From this figure, it can be established that increase in degradation of salicylic acid in the
presence of air was 17 %, for oxygen it was 39 % and for nitrogen it was 8.5 %. It shows
maximum increase in the presence of carbon dioxide as 50 %.
Figure 4.21: Effect of gaseous additives on KI (100 ppm) oxidation (using 20
KHz horn)
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
0 20 40 60 80
iod
ine
liber
ati
on
(pp
m)
Time (min)
Without any gases
Using Air
Using Oxygen
using Nitrogen
Using Carbon Dioxide
Study of intensification of sonochemical reactions using gaseous additives Page 75
Figure 4.22: Effect of gaseous additives on KI (300 ppm) oxidation (using 20 KHz
horn)
Figure 4.23: Effect of gaseous additives on KI (500 ppm) oxidation (using 20 KHz
horn)
0.00
50.00
100.00
150.00
200.00
250.00
0 20 40 60 80
iod
ine
lib
era
tio
n (p
pm
)
Time (min)
Using Air
Using oxygen
Using Nitrogen
Using Carbon Dioxide
Without any gases
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
0 20 40 60 80
iod
ine
liber
ati
on
(pp
m)
Time (min)
without any gases
Using Air
Using Oxygen
Using Nitogen
Using Carbon Dioxide
Study of intensification of sonochemical reactions using gaseous additives Page 76
Figure 4.24: Effect of gaseous additives on S.A. (100 ppm) oxidation (using 20 KHz
horn)
In the case of 25 kHz reactor (reactor capacity 7.5 liters) studies were carried out to study
the effect of various gases as an additives. All gases were passed through the reaction
solution at constant flow rate i.e. at 15 liter per hour. Initially sonication of 100 ppm of
potassium iodide was carried out in the absence of any gases. Iodine liberated at the end
of 60 minutes was 50 ppm. But when air was passed simultaneously during the sonication
it was seen that iodine liberation increased up to 63 ppm i.e. by 26 %. Sonication of 100
ppm of potassium iodide was also done in the presence of oxygen, nitrogen and carbon
dioxide using same reactor. Results have been given in the figure 4.25. It cab be seen
from figure that iodine liberation increased in the presence of other gases also. But the
extent of increase was different for different gases. For oxygen 40 % increment was
observed. Similarly iodine liberation was increased by 15 % in the presence of nitrogen
and by 60 % in the presence of carbon dioxide.
300 ppm and 500 ppm potassium iodide were also sonicated using same reactor in the
presence of above mentioned gases. Results for these two were plotted against the time
and shown in the figure 4.26 and 4.27 respectively. It was observed that gases affect the
sonication of these two solutions. The trend of effect of different gases was same as that
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
0 20 40 60 80
% d
egr
adat
ion
Time (min)
Sonication without gases Using Air
Using Oxygen
Using Nitrogen Using Carbon Dioxide
Study of intensification of sonochemical reactions using gaseous additives Page 77
for the sonication of 100 ppm potassium iodide. It means that maximum increment was
observed in the case of carbon dioxide followed by oxygen, air and nitrogen. But if
compared quantitatively i.e. on the basis of amount of iodine liberated then results
showed some difference. The attributes of this difference were already discussed in
previous session.
In the case of sonication of 300 ppm of potassium iodide, 118 ppm of iodine liberation
was observed in absence of any gases. Using air iodine liberation increased by 26.5%,
whereas by using oxygen it went up by 41 % and for nitrogen by 13%. In the case of
carbon dioxide, the extent of increase in the iodine liberation was maximum as 55 %, as
shown in the figure 4.26. Similarly in the case of sonication of 500 ppm of potassium
iodide 14 %, 24 %, 9 % and 42 % increase of iodine liberation was observed in presence
of air, oxygen, nitrogen and carbon dioxide respectively which has been shown in the
figure 4.27.
In same reactor sonication was also done on 100 ppm salicylic acid using all above
mentioned gases. It was noticed that effect of gases showed the same trend as that in the
case of sonication of potassium iodide. Carbon dioxide resulted in maximum benefits and
gave the largest increase in the percent degradation. Extent of effect of carbon dioxide
was followed by oxygen, air and nitrogen respectively. In absence of any gases,
sonication of salicylic acid gave 23.5% degradation. Carbon dioxide increased this
degradation by 52 %. Similarly due to the presence of oxygen, air and nitrogen
degradation increased by 41 %, 28.5% and 9 % respectively. Results has been given in
the figure 4.28.
Study of intensification of sonochemical reactions using gaseous additives Page 78
Figure 4.25: Effect of gaseous additives on KI (100 ppm) oxidation (25 KHz reactor)
Figure 4.26: Effect of gaseous additives on KI (300 ppm) oxidation (25 KHz reactor)
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70
Iod
ine
lib
era
ted
(pp
m)
Time (min)
Without any gases
Using Air
Using Oxygen
Using Nitrogen
Using CO2
0
50
100
150
200
250
0 20 40 60 80
Iod
ine
Lib
era
ted
(pp
m)
Time (min)
Without any gases
Using Air
Using Oxygen
Using Nitrogen
Using CO2
Study of intensification of sonochemical reactions using gaseous additives Page 79
Figure 4.27: Effect of gaseous additives on KI (500 ppm) oxidation (25 KHzreactor)
Figure 4.28: Effect of gaseous additives on S.A. (100 ppm) oxidation (25 KHz
reactor)
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70
iod
ine
lib
era
ted
(pp
m)
Time (min)
Using CO2
Using Nitrogen
Using Oxygen
Using Air
Without any gases
-5.00
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
0 10 20 30 40 50 60 70
% c
ha
nge
Time (min)
Without any gases
Using Air
Using Oxygen
Using Nitrogen
Using Carbon Dioxide
Study of intensification of sonochemical reactions using gaseous additives Page 80
Effect of gases was also investigated in the 36 kHz reactor. Similar trends as in earlier
two reactors were observed. Only quantitatively, it was different as the reactors
parameters were different than the previous ones. The reasons for that already discussed
in previous sections.
Same set of reaction solutions (100 ppm KI, 300 ppm KI, 500 ppm KI and 100 ppm
salicylic acid) were used. Studies were carried out using same gases (Air, O2, N2 and
CO2). Flow rate of each gas was maintained at 20 lph. All the results of this set are shown
in the table 4.9. All values in the table indicate the percent increase in cavitational yield.
Table 4.9: Effect of gaseous additives on sonochemical reactions ( using 36 kHz
reactor).
System\gases Air Oxygen Nitrogen Carbon
dioxide
100 ppm KI 30 41.5 12.5 51
300 ppm KI 28.5 36.5 8 50
500 ppm KI 26 30 10 49.5
100 ppm
Salicylic
acid
34 42 11.5 55.5
This observed increase in cavitational yield is attributed to the fact that, presence of gases
increases the heterogeneity of the medium. Due to this increased heterogeneity, additional
nuclei are supplied for the cavitation events. So with an increase in nuclei, cavitational
events get increased (Chakinala A. G. et al., (2007), Katekhaye and Gogate (2011), Pang
Y. L. et al., (2011)). As every cavity works as a micro-reactor, additional available nuclei
increase the number of microreactors. Each microreactor produces a thousand fold
temperature and pressure at a local point resulting in the reactive radical formation.
Hence with an increase in number of microreactors, amount of radical formation
Study of intensification of sonochemical reactions using gaseous additives Page 81
automatically get increased. This additional radical helps in increasing the cavitational
yield to further level.
Another possible reason for the increased cavitational yield is that, presence of gases
changes the physical properties of the medium like density, Cp/Cv value (polytropic
constant), viscosity etc. The gases decrease the overall viscosity of the solution. Gogate
and Pandit (2004) in their study of scale up of sonochemical reactor mentioned that at
low viscosity cavitational yield would be greater. Also Gogate and Pandit (2004) and
Entezari and Kruus (1996) mentioned that final temperature and pressure at the cavity
collapse is depends on the polytropic index of the gases. Both temperature and pressure at
cavity collapse are directly proportional to the polytropic index of the present gases.
Diatomic species like oxygen nad nitrogen has more polytropic index (~1.4) than the
carbon dioxide (~1.2). As polytropic index of water is lower than all the gases used.
Hence when gases passed through the reaction solution it increases the overall polytropic
index of the medium. Hence the temperature and pressure of cavity collapse ia increased
as more violent cavity collapse takes place. This more violent cavitation yields in
increased number of reaction radicals in the medium. These reactive radicals further
increase the cavitational yield of the reaction.
Another possible means of raising the cavitational yield is to scavenge the radicals in the
bulk medium as well as inside the bubble. It means reacting the radicals with other
species (present in relatively large quantities in the bubble or in the liquid medium) to
generate new species. This species might or might not take a direct part in the reactions
but this prevents radical recombination. Hence the loss of oxidation potential of
cavitation events gets prevented. Scavenging of radicals inside the bubble by other
species present in the bubble (such as oxygen molecules) could result in greater release of
radical species in the bulk medium. Moreover, scavenging of the radicals in the bulk
medium results in penetration (or diffusion due to concentration gradient) of the radicals
in the bulk medium to greater distances from the location of the collapse of cavitation
bubble. This helps in minimizing the dead zones in the reactor as due to diffusion a
raction molecule in dead zone also get a chance to react with the oxidative radical and
hence resulting cavitational yield increases (Sivasankar and Moholkar (2009).
Study of intensification of sonochemical reactions using gaseous additives Page 82
Also one more probable reason for the increased cavitational yield is that due to
continuous sparging of the gases gives an additional mixing effect. Due to this agitation,
more and more reaction solution gets exposed to the active cavitational zone. Hence the
dead zone effect of the sonochemical reactor decreases, leading to better cavitational
yield.
Also Entezari and Kruus (1996) had mentioned that the effect of each gas is depends on
the nature of the gases and the involvement of the gas in sonochemical reaction. Nature
and involvement of gas molecule of each gas has been discussed below (Sivasankar and
Moholkar (2009).
Nitogen: The presence of nitrogen increases the cavitational yield. It is attributed to the
fact that it gives the extra nuclei for cavitation by virtue of heterogeneity and also lowers
the vapor pressure to improve the cavitation effect. Also it increases the polytropic index
of the medium, which results in the more violent cavitational activity.
Oxygen: Similar to nitrogen, oxygen also supply nuclei for the cavitation effect. Also the
polytropic index of medium gets increased due to presence of oxygen which helps in
more efficient cavitation. With lowering the vapor pressure it also acts as a scavenging
agent which further improves the cavitational activity. In addition, oxygen can also
participate in the reaction by forming 2 OH. radicals per molecule of oxygen. Formation
of hydroxyl radical from oxygen takes place by following reaction.
O2 2O•
O• + H2O 2•OH
This hydroxyl radical helps in further increase in the cavitation yield. Also some
oxidative species like peroxide and ozone are formed from the recombination of oxygen
radical with water molecule or HO2 radical and oxygen radical with oxygen molecule
respectively. These oxidative species further helps in improvement of both the reaction
species under study.
Study of intensification of sonochemical reactions using gaseous additives Page 83
Air: The major part of air is nitrogen and air (nearly 99.9%). Thus it gives the effect of
both the species in the sonochemical reaction. Due to presence of oxygen gas, air gives
more cavitational yield than nitrogen. But at the same time 70 percent of nitrogen pull
down the overall cavitational yield lesser than that of pure oxygen.
In air, nitrogen as a scavenging agent produces the species likes NO, N2O, NO2, HNO
and HNO2. These species recombine with the oxygen molecule and produce the reactive
radicals likes •OH followed by O•, HO2• and H• (Sivasankar and Moholkar (2009)).
Carbon Dixide: Polytropic index of the carbon dioxide is less than that of oxygen and
nitrogen. (Cp/ Cv for CO2 is 1.29 and that for O2 and N2 is nearly 1.4). Also Rooze J. et
al. (2011) mentioned that there is no significant radical production in the presence of this
gas. But still the presence of the carbon dioxide during sonication yields largest
efficiency. This might be because of bubble size is comparatively large in the presence of
carbon dioxide because of its higher solubility in the water. This makes more nuclei
available for cavitation, and it makes bubble growth easier. This helps in increase in
cavitations rate of the system.
Also carbon dioxide may form some carbanium ions by interacting with radicals present
in the system. This ions lowers the pH of the system. Chakinala A. G. et al. (2007)
mentioned that lower pH, i.e. acidic conditions helps to increase the concentration of the
hydrophobic species at the bubble interface leading to exposure of a higher quantum of
the reaction molecule to the cavitating conditions. Thus, higher rates of reactions can be
achieved.
In addition to that it might possible that carbon dioxide also some scavenging effect like
nitrogen and oxygen. Also it might take the direct part in sonochemical reactions by
producing some oxidative species or hydroxyl radicals.
4.7 Effect of air flow rate on sonochemical reactions:
Effect of air flow rate was studied on sonolysis of KI and degradation of salicylic acid
using ultrasound reactors of frequencies 20, 25 and 36 kHz. It was observed that the
amount of iodine liberated after sonolysis of KI increases when air is passed through the
Study of intensification of sonochemical reactions using gaseous additives Page 84
solution. The amount of iodine liberated increases with an increase in air flow rate; goes
through a maxima and then reduces again, as shown in Fig. 4.29- 4.31. Similar trend is
observed for degradation of salicylic acid as well, as shown in fig. 4.32-4.34.
In the case of the 20 kHz reactor (reactor volume = 300 mL), iodine liberation from
sonolysis of KI was maximum for air flow rate of 6 lph. The amount of iodine liberated at
this flow rate was 81.25 ppm. Similarly, maximum degradation of salicylic acid using
this reactor was observed for air flow rate of 6 lph, and the percent degradation observed
at this flow rate was about 39 %. The effect of air flow rate in case of 20 kHz reactor is
as shown in fig. 4.32.
For the 25 kHz reactor, the reaction volume used was 7.5 liters. Similar to the 20 kHz
reactor, degradation studies were carried out for various air flow rates for degradation of
KI and Salicylic Acid. It was observed that maximum amount of iodine is liberated from
KI when air is passed through the reactor at a flow rate of 30 lph. The amount of iodine
liberated at this air flow rate was approximately 80 ppm. Similarly maximum degradation
of Salicylic acid was observed for an air flow rate of 40 lph, and the percent degradation
of salicylic acid at this flow rate was observed to be about 30%.
Similar study was carried out using 36 kHz reactor with reaction volume of 9.5 liters. In
this case maximum amount of iodine was liberated for air flow rate of 60 lph, and it was
observed to be 30 ppm. In case of degradation of salicylic acid as well, the maximum
degradation was obtained at 60 lph air flow rate and the degradation at this flow rate was
about 30%
This enhancement in the yield can be attributed to reason that the presence of air bubbles
in the system provides additional nuclei by virtue of heterogeneity, resulting in an
increase in the intensity of cavitation by increasing the number of cavitation events as
compared to the absence of bubbling. This was also visually observed during the
experimentation. The increase in intensity of cavitation should in turn lead to an increase
in the amount of hydroxyl radicals generated in the reactor. (Chakinala A. G. et al., 2007)
Study of intensification of sonochemical reactions using gaseous additives Page 85
Another factor that contributes to enhancement in the yield is that air contains
approximately 30 % of oxygen, which can take part in sonochemical reactions. Oxygen
promotes the generation of additional oxidizing species in the system, which would
increase the extent of degradation.
It can be observed that as the air flow rate increases, iodine liberation and salicylic acid
degradation increases only up to certain point. After that it decreased with a further
increase in air flow rate. This may be because of the presence of excess of gaseous
species in the liquid, which reduces the intensity of cavitation at collapse. Air penetrates
into the cavity to a greater extent and reduces the intensity of the shockwave produced
upon the collapse. Also, presence of air in large quantum might interfere with the passage
of ultrasound into the system thereby decreasing the available energy for the cavitation
events. (Sivshakanr T. et al., 2009). Furthermore presence of more amount of gas
produces cushioning effect, i.e. using higher flow rates of air possibly also could result in
the formation of a blanket of bubbles in the immediate vicinity of the transducer surface,
thereby minimizing the transfer of energy into the system, and hence results in decrease
in the cavitational yield.
It was also observed that the optimum value of air flow rate was different for all three
reactors. This is probably due to the fact that as the reaction volume increases, the amount
of air required to achieve the desire heterogeneity in the reaction medium also increases.
As already discussed due to heterogeneity additional nuclei are get provided for the
cavitational events. Hence for a reactor having lower volume capacity like 20 kHz reactor
(300 ml), a less amount of air is required to achieve the heterogeneity compare to the 25
kHz (7.5 liters) and 36 kHz (9.5 liter) reactors. Hence to achieve a desired heterogeneity
and so the desire amount of nuclei per unit volume in large volume reactors, amount of
air required is also larger.
Also if the reaction solution is saturated with the air, then sufficient gas is get entrapped
in each cavity. So at the collapse more radicals can be formed. With increase in the
reaction volume, amount of air required to saturate the reaction mixture is also increases.
Hence a low air flow rate is sufficient only for low volume reactors like 20 kHz reactors
Study of intensification of sonochemical reactions using gaseous additives Page 86
but not in 25 kHz and 36 kHz reactors. Hence larger reactors require higher air flow rates
to achieve similar enhancements in the yield.
Figure 4.29: Effect of air flow rate on iodine liberation of 300 ppm KI (20 KHz
horn)
Figure 4.30: Effect of air flow rate on iodine liberation of 300 ppm KI (36 KHz
longitudnal horn)
40
50
60
70
80
90
100
0 2 4 6 8 10 12
iod
ine
lib
era
ted
(p
pm
)
Air flow rate (lph)
0
5
10
15
20
25
30
35
0 20 40 60 80 100
Iod
ine
liber
ate
d (p
pm
)
Flow rate (lph)
Study of intensification of sonochemical reactions using gaseous additives Page 87
Figure 4.31: Effect of air flow rate on iodine liberation of 300 ppm KI (25 KHz
reactor)
Figure 4.32: Effect of air flow rate on iodine liberation of 100 ppm S.A. (20 KHz
horn)
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70
Iod
ine
lib
era
ted
(pp
m)
Air Flow rate (lph)
25
27
29
31
33
35
37
39
41
0 2 4 6 8 10 12
% c
han
ge
Air flow rate (lph)
Study of intensification of sonochemical reactions using gaseous additives Page 88
Figure 4.33: Effect of air flow rate on iodine liberation of 100 ppm S.A. (36 KHz
longitudnal horn)
Figure 4.34: Effect of air flow rate on iodine liberation of 100 ppm S.A. (25 KHz
longitudnal horn).
10
15
20
25
30
35
0 10 20 30 40 50 60 70 80
% c
han
ge in
SA
co
nce
ntr
ati
on
Air flow rate (lph)
15
17
19
21
23
25
27
29
31
0 10 20 30 40 50 60 70 80
% d
egr
adat
ion
Air Flow rate (lph)
Study of intensification of sonochemical reactions using gaseous additives Page 89
5. Conclusion:
The present work has shown that both the standard sonochemical reactions, oxidation of
potassium iodide and salicylic acid dosimetry were strongly influenced by operating
parameters. Optimization studies with different operating parameters indicated that lower
temperature, optimized power supply and duty cycle result in maximum extent of
degradation. Study on effect of initial concentration on cavitation yield has shown that
cavitational activity shows the direct relation with the initial concentration. As initial
concentration of potassium iodide increases, iodine liberation at the end of sonication
increases.
Also the comparison study between three reactors shown that the cavitational activity
using ultrasonic irradiation was the maximum in the case of 20 kHz followed by 25 kHz
longitudinal horn and 36 kHz longitudinal horn respectively.
Gaseous additives have increased the cavitational yield of the reaction. But the extent of
increase in cavitational yield was vary with the gases. In the case of oxidation of
potassium iodide, amount of iodine liberation at the end of sonication was increased by
around 20-30 % in the presence of air in all three reactors. Increase in the iodine
liberation was maximum in the presence of carbon dioxide (45 – 55 %) followed the
sonication in presence of oxygen (30-40 %). Presence of nitrogen gave the lowest
increase in iodine liberation (8-14%). Similarly extent of degradation of salicylic acid
increased in the presence of gases additives in all three reactors. Percent increase in the
degradation of salicylic acid was nearly same as that obtained in the case of oxidation of
potassium iodide.
Study of effect of air flow rate on sonochemical reaction has shown that the cavitational
yield increases in the presence of air. Cavitational yield increased with the air flow rate
up to a certain optimum point. After that peak, cavitational yield decreased with increase
in the flow rate of air. Also the optimum value of air flow rate was different for all the
three reactors.
Study of intensification of sonochemical reactions using gaseous additives Page 90
6. Future Scope:
In the present work effect of operating parameters, comparison of different reactors,
effect of gaseous additives, and effect of air flow rate on the sonochemical reaction has
been studied. However there are some suggestions for future work to analyze same study
in detail. Also it can help in obtaining enhanced intensification of sonochemical reaction.
Use different configuration of sonochemical reactors like high frequency and
low frequency bath, hexagonal sonochemical reactor to find out the suitable
geometry and specifications required for the design, feasible for the large scale
operations.
Study the effect of noble mono-atomic gases as additives and compare those
with present additives. Also study a combination of different gases to get more
cavitational yield. Try different methods of sparging and different types of
reactor to make it more suitable for improving the cavitational yield. This will
help you to find out most suitable gas or combination of gases for the desired
intensification of sonochemical reactions.
Also try to conduct whole study on hydrodynamic reactor which is more suitable
for the industrial scale operation.
Modeling study can be performed with the help of available software like
COMSOL or CFD.
Study of intensification of sonochemical reactions using gaseous additives Page 91
7. References:
Adewuyi Y. G., Oyenekan B. A. Optimization of a Sonochemical Process Using a
Novel Reactor and Taguchi Statistical Experimental Design Methodology.
Industrial & Engineering Chemistry Research 2007, 46, 411-420.
Ando, T., Sumi, S., Kawate, T., Ichihara, J. and Terukiyp, H. Sonochemical
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