Intensification of Sonochemical Reactions

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 reaction using

gaseous additives

Thesis submitted to


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


Maharashtra, India

May 2012

© 2012, Suraj S. Shaha, All rights reserved


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:


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:


Dedicated to my family and friends..!!


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.


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


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


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


nitrogen

62



carbon dioxide

63

4.11 Effect of initial concentration of potassium iodide on sonolysis using

20 KHz horn in absence of any gases

68


20 KHz horn in presence of air

68


20 KHz horn in presence of oxygen

69


20 KHz horn in presence of nitrogen

69


20 KHz horn in presence of carbon dioxide

70


25 KHz longitudinal horn (1 Kw) in the absence of any gases

70


25 KHz longitudinal horn (1 Kw) in presence of air

71


25 KHz longitudinal horn (1 Kw) in presence of oxygen

71


25 KHz longitudinal horn (1 Kw) in presence of nitrogen

72


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


KHz horn)

75


KHz horn)

75

4.24 Effect of gaseous additives on S.A. (100 ppm) oxidation (using 20

KHz horn)

76


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


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


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


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.


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


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


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


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


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


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


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.


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.


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.


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).


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


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


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)


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.


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


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


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.


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.


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.,


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)


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)


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


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


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


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


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,


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.


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


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.


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


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)


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)


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.


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.


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.


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.


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.


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


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.


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.


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


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


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


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


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


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




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 36 KHz longitudnal horn


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


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 20 kHz horn


Figure 4.8: Sonication of 100 ppm S.A. in different reactors in the presence of

oxygen.


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 20 KHz horn



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 20 KHz horn


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.


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.


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)


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.


Figure 4.11: Effect of initial concentration of potassium iodide on iodine liberation

using 20 KHz horn in absence of any gases.


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



using 20 KHz horn in the presence of oxygen.


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



using 20 KHz horn in the presence of carbon dioxide.


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



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



using 25 KHz longitudinal horn (1 kW) in the presence of air.


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



using 25 KHz longitudinal horn (1 kW) in the presence of nitrogen.


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



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


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


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


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


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



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


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.


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


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



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


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).


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.


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


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)


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


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)


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)



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)



longitudnal horn)


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)


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.


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.


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Intensification of Sonochemical Reactions