Cavitationally Driven Transformations: A Technique of Process

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Cavitationally Driven Transformations: A technique of Process Intensification Chandrakant R. Holkar, Ananda J. Jadhav, Dipak V Pinjari, and Aniruddha Bhalchandra Pandit Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04524 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Industrial & Engineering Chemistry Research

Cavitationally Driven Transformations: A technique of Process Intensification Chandrakant R. Holkar 1, Ananda J. Jadhav 1, Dipak V. Pinjari 2*, Aniruddha B. Pandit 1*

1

Chemical Engineering Department, Institute of Chemical Technology, Nathalal Parekh Road, Matunga (E), Mumbai-400019, Maharashtra, India. 2

National Centre for Nano Sciences and Nanotechnology, University of Mumbai, Kalina Campus, Kalina, Santacruz (E), Mumbai-400098, Maharashtra, India

*Author to whom correspondence should be addressed Email:

[email protected] (Aniruddha B. Pandit) [email protected] [email protected] (Dr. Dipak V. Pinjari)

Tel:

+91-22-3361 2012;

Fax:

+91-22-33611020

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The process intensification (PI) can significantly improve energy and process efficiency by enhancing mixing, mass and heat transfer as well as driving forces. There are several benefits of such improvements, which includes energy and cost savings, enhanced safety and smaller reactor size, less waste generation and higher product quality. This review article focuses on the PI, discussion about its dimensions and structure, what it involves and recent developments in PI which can be achieved using the technique of cavitation. Recommendations for optimum operating parameters needed for process intensification using cavitation phenomena which has been reported in the literature have been presented along with some of our own work in the area. Some experimental case studies have been presented which highlight the degree of intensification achieved when cavitation is used for different physico-chemical transformations. These physicochemical transformations include crystallization, emulsification, extraction, wastewater treatment, depolymerisation and water disinfection. Keyword: Process Intensification, Cavitation, Chemical Processes, Wastewater, Extraction, Emulsion, Crystallization, Depolymerisation, Delignification.


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1. Introduction Today, world is witnessing the replacement of traditional chemical engineering processes with the newly developed modern chemical engineering processes. Researchers from academics and industries are working on the development of novel equipment and techniques that are energyefficient, compact, safe and environmentally-friendly which can be further utilized in the chemical plants. Process intensification (PI) is a way to revolutionize the chemical process industry in terms of its approach and addresses the issue of sustainability. Recently, PI as a concept has been receiving considerable attention. This concept was first defined, way back in 1970, which was sparked by the need to reduce the plant volume (capital cost) without sacrificing its production rate and the quality. PI is an idea which is expected to professionally direct the future journey of chemical process industry to be sustainable. Various researchers have put in significant efforts to define the PI. Variety of efforts made by the pioneers to convey the ideas of PI are summarized in Table 1. Table 1: Different concept of process intensification. Ramshaw (1983) 1

Developing compact plant to reduce both the plant item along with its installation costs

Cross and Ramshaw

It is the strategy to reduce the size of a chemical plant giving the same

(1986) 2

production objective.

Stankiewicz and

It is the development of cheaper, safer, viable technologies that

Moulijn (2000) 3

decrease the equipment volume, energy consumption and waste generation.

Tsouris and Porcelli

It is the development of technologies that combine many operations

(2003)4

into fewer devices resulting into less costly and efficient processes

European Roadmap

It is the development of process along with equipment design which

for Process

can benefit in terms of capital and operating expenses, quality, waste generation and process safety.



Intensification (ERPI) (2008) 5 Bech (2009) 6

It is an approach for the innovation in process and product in order to sustain profitability.

Jadhav (2017) 7

It is the development in the process that leads to cleaner and energy efficient technology.

It is difficult to define an exact concept of PI in precise words because it addresses various aspects and issues simultaneously. To simplify this, Gerven and Stankiewicz (2009) provided the four guiding principles for PI (as shown in Figure 1).8

Process Intensification Guidelines

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Improve the effectiveness of intramolecular as well as intermolecular events

Provide the same process experience to all molecules

Maximize the specific area along with the optimization of driving forces at all scales

Maximize the synergistic effects of different processes

Figure 1: Guidelines for process intensification. Using PI, it is possible to reduce the energy usage, cost of the equipment. However, it must be recognized that significant efforts (resource and intellectual) are necessary to implement the PI methodology and confirm the use of the new technologies which are expected to be sustainable.


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Klemes (2014) also suggested the footsteps (as shown in Figure 2) for effective management of PI process.9

Identification of business and process drivers Make decision on implementation

Review the entire process

PI Process Management

Identify ratelimiting steps

Select equipment

Analyze design alternatives

Generate design concepts

Figure 2: Steps to manage process intensification.

The process intensification can be achieved with a combination of skilled process engineers and the above steps. It involves the scanning of intensive literature work, technology assessments, accurate heat and mass balance and systematic process flow diagrams including thermodynamic limitation and finally the cost estimation. Once the PI study is over, it is essentials to identify that the superiority of the intensified process over the conventional one. The determination of optimal process conditions at bench and pilot scale is important and that can be further utilized in the scaleup of the intensified process. 5 ACS Paragon Plus Environment


According to Stankiewicz and Moulijn (2000), PI mainly deals with engineering equipment and method.1 Process intensifying methods are the combination of multiple processing steps and the use of energy sources efficiently. Process intensifying equipment significantly enhances the transport processes and hence improves the kinetics of reaction, yield and specificity of the product. These improvements result into the reductions in equipment size, the cost and the risk in the process. In short, Process intensifying equipment such as spinning disk reactors, static mixers and micro-reactors are characterized by designs that optimize mass, heat and momentum transfer. Thus, to start with, an identification of the rate controlling step, which could physical or chemical or biological in nature, which can be targeted for process intensification is essential. Since, it is the rate controlling step, its intensification is expected to have a maximum impact on the overall process efficiency. In recent years, Cavitation has been used effectively for process intensification.7 The cavitation phenomena involve the generation of free radicals, local hot spots and microturbulence. These spectacular effects of cavitation result in an intensification of homogenous and heterogeneous reactions.10 In this article, a closer look at process intensification by the phenomena of cavitation has been attempted. This paper aims to discuss the fundamentals of process intensification. Also, various approaches of PI have been discussed in the current review paper. Cavitation induced PI, fundamentals, mechanism, advantages, disadvantages or limitations and case studies have also been reported. In this article, different physico-chemical transformations which have been successfully intensified by cavitation, such as crystallization, emulsification, extraction, wastewater treatment, depolymerisation and water disinfection have been included. The discussion ends with the conclusions and the future work on the role of cavitationally induced PI.


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2. Principles of Cavitation Cavitation is a physico-chemical phenomenon of sequential generation, development and collapse of the huge amount of microscopic cavitities in liquid medium. These collapse of cavities release large amount of energy (energy densities of the order of 1 to 1018 kW/m3) over an extremely small interval of time i.e.(millisecond to microseconds. (Figure 3). The energy release is in the format of high temperature and high pressure. Cavitation is categorized into four types on the basis of way of generation of cavity: acoustic, hydrodynamic, optic and particle. Among all these techniques, hydrodynamic and acoustic cavitation have proven to be effective for the desired physico-chemical transformation on a commercial scale.11

Figure 3: Cavitation phenomenon 2.1.

Acoustic cavitation:

Acoustic cavitation is a result of pressure variations in the liquid when ultrasonic sound waves (16 kHz–100 MHz) propagate through it (Figure 4a) which consist of compression and rarefaction phase. In rarefaction cycles, the negative acoustic pressure pulls liquid molecules apart from each other and creates the void in the liquid after excedding the critical molecular distance. These voids cause the formation of cavities. In the compression cycle, acoustic positive pressure pushes the molecules together and compresses the cavities which collapse in fraction of time under near adiabatic condition. These adiabatic collapse produces high local temperature and pressure condition for small interval of time (millisecond to microsecond). Acoustic cavitation is difficult to use in large scale operation due to higher operating cost and low energy efficiency as compared to hydrodynamic cavitation.11 In recent years, hydrodynamic cavitation is found to be a good alternative to the acoustic cavitation. Because of its high energy efficiency, high cavitation 7 ACS Paragon Plus Environment


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activities, easy to scale-up and cost effectiveness as compared to acoustic cavitation though, with some limitations.11–13

Figure 4: Method of generating cavitation 2.2.

Hydrodynamic Cavitation:

In hydrodynamic cavitation, liquid passes through the constriction such as an orifice or a venturi (Figure 4b) at which kinetic velocity increases at the cost of pressure. The cavity forms at the throat of the constriction because of pressure drops below vapor pressure. These vaporous cavities get imploded during the pressure recovery in the downstream of the constriction. The cavity implosion results in to very high local energy density also known as hot spot contain high temperature about 10,000 K and pressures of 1000 atm.14 This extreme conditions produces the highly reactive free radicals and enhances mass transfer due to turbulence by cavity collapse. In HC, cavitation number (Cv) is used to describe the cavitating condition inside a cavitating device.15 It is expressed as a ratio of difference in local absolute pressure from the vapor pressure to the kinetic energy per volume. It is given by the following Eq. (1): Cv =

( ) P2 ― Pv

(1)

1 2 2ρv0

Cavitation occurs when Cv is equal to one. There are significant cavitational effects and better cavitational yield when Cv is less than 1. If the cavitation number is very small, the cavitation is 8 ACS Paragon Plus Environment

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known as supercavitation which is chocked and has no practical use. To avoid this, the cavitating device must be operated at optimum conditions.14,15 In many cases, cavitation inception takes place when Cv >1 due to the presence of a small amount of dissolved gases along some suspended particles in the liquid which act as nuclei in the presence of liquid phase turbulent pressure fluctuations.

3. Effects of cavitation and their role in process intensification 3.1. Cavitational collapse and its effects Cavitational collapse affects the physicochemical transformations depending upon the systems in which it occurs. The symmetric collapse and asymmetric collapse occurs in homogeneous liquid and heterogeneous system respectively (Figure 5).

Figure 5: Cavity collapse and its shape In homogeneous liquid reactions, two major effects occur. First, the cavity encloses a volume containing the vapor from the liquid medium. During the collapse, enclosed vapor is exposed to extreme conditions of temperatures and pressures, resulting into dissociation and generation of highly reactive radical species. Secondly, the immediate collapse of the bubble also produces shear forces and a subsequent spherical shock wave in the surrounding bulk liquid. These shear forces 9 ACS Paragon Plus Environment


and shock waves can break the chemical bonds of many a material in the fluid and facilitating the transport due to boudry layer disturbance. 14,16 In a heterogeneous system, the asymmetric collapse of the bubble brings out structural and mechanical defects. High-pressure/ high-velocity liquid jets are generated when bubble collapses near the solid surface. These jets can clean the solid surface and activate the solid catalyst by desorbing the adsorbed intermediates and also increase the mass transfer by disturbing the interfacial boundary layers. Asymmetric collapse on the solid surface also cause fragmentation and surface roughening, particularly in powders. Thus, ultrasound can increase the surface, micro/molecular level mixing and hence mass transport.16–18 3.2. Optimum operating conditions for cavitation and its intensification: The solvent, bulk operating temperature and the type of the equipment are significant factors and are often interrelated for cavitationally induced transformation. The efficiency of cavitation reactors depends on geometric as well as operating parameters. The following useful guidelines presented in Table 2 can be used for their optimum selection.14,16,18,19


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Table 2: Factors for selecting operating conditions for cavitation Equipment Properties : Broad Guidelines Properties

Affects

Favorable conditions

Intensity of irradiation ( 1 to 300 W/cm2)

Number of cavities and collapse pressure of the cavity

 Use an optimum power dissipation to avoid large cloud formation

Irradiation frequency( 20 to 200 kHz)

Cavity collapse time, pressure and temperature

 Use higher frequencies until an optimum value as threshold power required increases with increasing frequency

Cavitational intensity

 An orifice cavitating device for intense chemical reactions.  A venturi cavitating device for milder chemical reactions

Cavitational intensity

 Higher discharge pressure (higher cavitational intensity) for stubborn reactions  Lower discharge pressure (lower cavitational intensity and higher cavitational active volume )for milder reactions,

Type of cavitating device in HC

Discharge pressure in HC

Liquid Phase Properties Properties

Affects

Favorable Conditions

Liquid-Vapor pressure: 40 to 100 Cavitation threshold, Cavitational Intensity and the rate of chemical reaction rate. mm of Hg at 30C

 Liquids with low to medium vapor pressures in the operating temperatures i.e. water

Viscosity: 1 to 6 cP

 Low viscosity

Transient threshold 11 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Surface tension: 0.03 to 0.072 N/m Bulk liquid temperature: 30 to 70C

Dissolved gas a) Solubility b) Polytropic constant and thermal conductivity

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the nuclei size

 Low surface tension

Collapse intensity, the reaction rate, threshold/ nucleation

 Optimum value and lower temperatures are desirable

Dissolved gas content, nucleation, the cavitational intensity

 Low solubility  Gases having lower thermal conductivity and higher polytropic constant


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To intensify any process, it is crucial to identify the rate controlling step and select the operating parameters accordingly. The possible ways by which the cavitational activity that can also be intensified are summarized in Table 3.14,19,20



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Table 3: Possible ways for intensification of cavitational intensity and activity Possible ways

 Aeration

References

Principle behind the intensification Large number of cavitation events due to additional

21,22

nuclei for the generation of cavities 

Lower magnitude of pressure pulse due to the cushioning effect caused by the presence of air bubble and a compressible fluid medium

 Presence of solids

More number of cavitational events due to solid particles

23

which act as additional nuclei 

Too many particles can also attenuate the propagation of the pressure sound wave

Combination of two or more transducers



More cavitation intensity due to more turbulence



More cavitationally active volume and mixing of

24

reactants due to acoustic streaming by two or more transducers

Hybrid Methods



Increases the heat and mass transfer rate



Combination of ultrasound and hydrogen peroxide o More number of free radicals



Combination of ultrasound and ultraviolet irradiation


25–27

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o Catalyst reactivation, higher surface area and mass transfer rates and generate a large number of free radicals 

Combination of ultrasound + ultraviolet irradiation + hydrogen peroxide o Combination of the above two



Thus, Cavitation can be effectively used for applications where greener routes are required. 3.3. Role of cavitation in process intensification To achieve chemical, physical and biological transformation using conventional approaches involved various limitations such as higher time, temperature and pressure, required huge amount of toxic solvents, catalyst and unsatisfactory yields (from commercial angle). In the case of a heterogeneous system, the conventional approaches have drawback such low heat and mass transfer due to aggregation of the solid phase. Whereas in the case of a homogeneous system, the conventional approaches have a drawback such as a low mixing rate. To overcome these drawbacks of conventional approaches, a suitable modern and an efficient process intensification approach is needed. It has been proved that, use of cavitation can process the reactions that are naturally slow.16 The significant advantages of cavitationally assisted physico-chemical transformations include improved selectivity, without or nonhazardous solvents, less energy consumption and the reaction time.28 A significant degree of PI can be achieved by acoustic or hydrodynamic cavitation.17,18 Thus, the concept as described in the process intensification guidelines, i.e., delivery/removal of required quantity and form of energy at/from the actual location of the targeted transformation can be employed using the concept of Acoustic (ultrasound based) or Hydrodynamic (liquid flow) cavitation. If this is achieved, then the process tries to mimic the biological or natural processes, which are far more sustainable. The use of cavitation for PI in different application have been summarized in Table 4.


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Table 4. Process intensification effects of cavitation Source of energy

Application

Examples

Reported intensification

1. Diel-Alder cyclization 29 Reduction in reaction time by 25 times Ultrasound irradiation

100% yield of the product in case of acoustic cavitation-assisted synthesis 5 times enhancement in gas liquid mass

2. Selective reduction of , -unsaturated carbonyl compounds in the presence of ZnNiCl2 30 CO2-water system 31

transfer

Acoustic field

20 times enhancement in the liquid-solid

Low-frequency acoustics

1. Synthesis of dibenzyl sulfide 32

mass transfer

2. Extraction of protein from brewer’s yeast 33

3 times enhancement in liquid- liquid

Liquid-liquid extraction (toluene–acetic acid–

mass transfer

water) 34

2 times enhancement in gas solid mass

Wood vacuum drying 35

transfer 10 times higher cavitational yield in case Flow

Hydrodynamic

of hydrodynamic cavitation as compared

cavitation

to ultrasound for same overall energy

1. Potassium iodide Degradation 36 2. p-nitrophenol degradation 37 3. Microbial cell disruption 38,39

input 17 ACS Paragon Plus Environment



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4. Application of Cavitation Phenomena for PI 4.1. Ultrasonic Extraction The extraction of metals and phytochemicals from the related sources using conventional solvent extraction methods are dependant on a correct combination of solvent and the source of heat and/or agitation. It is reported that the use of cavitation-assisted solvent extraction is much superior to the conventional methods in terms of yield, time, energy and ultimately the cost. The schematic of mechanism of acoustic cavitation extraction is expected to be as shown in Figure 6.

Cavity Cavity Collapse

Oil

Seed Cracks Solvent Layer

Figure 6 Mechanism of ultrasonic extraction In acoustic cavitation extraction, larger amplitude (higher power) high frequency (ultrasonic) pressure waves are passed through the liquid media (solvent), which creates acoustic cavitating conditions. When these cavities collapse or implode symmetrically, it creates high localized temperatures roughly of about 10000 K and a pressure of about 1000 atm and associated spherical shock wave. This results into short-lived, localized hot-spot in cold liquid, without affecting the overall environment which remains at ambient or atmospheric conditions. Whereas, asymmetric collapse of these cavities creates the violent shock wave and high-speed jet respectively, which could improve the penetration of the solvent into the material from which an active component is to be extracted from and facilitate the release of intracellular product into the solvent by physically disrupting the respective base material boundaries. Bubble collapse in the vicinity of material may cause strong shear forces and high-speed liquid jets that can cause micro-fractures in the material. Additionally, the violent shock wave and high-speed jet may cause better solid-liquid mixing,



enhancing the solute mass transfer rate and hence the extraction process.40 The various literature related to ultrasonic extraction are summarized in Table 5.


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Table 5: Various application of ultrasound in the extraction Compound and its source for extraction

Optimum Process Condition

Reported possible process intensification Total anthocyanin: 10.26 ± 0.39 mg/100 g Phenolics: 552.64 ± 1.57 mg GAE/100 g Flavonoid (104 ± 1.13 mg RE/100 g Advantages: Enabling automation, reduction in extraction time and organic solvent consumption

Temperature: 50oC Bioactive compounds from Time: 20 min Nephelium lappaceum L. Power: 20 W fruit peel Solid-liquid ratio: 1:18.6 g/ml

Reference

41

Pectin yield: 14.5% of the total.

Pectin from waste Artocarpus heterophyllus (Jackfruit) peel

Temperature: 60oC Time: 24 min Solid-liquid ratio: 1:15 g/ml pH: 1.6

Anthocyanins from haskap berries (Lonicera caerulea L.)

Temperature: 35oC Time: 20 min Solid-liquid ratio: 1:25 g/ml pH: 1.6 Solvent composition: 80% ethanol and the addition of 0.5% formic acid.

Advantages: faster, highly efficient, solvent saving technique, and biocompatible.

42

Anthocyanin: 22.73 mg (Cyanidin 3-glucoside equivalents (C3G)/g dry weight) 43

Advantages: Extraction at low temperature and in a reasonable time.

For anthocyanins: Anthocyanins and total phenolic compounds in mulberry (Morus nigra) pulp

Temperature: 48oC Time: 10 min Solid-liquid ratio: 1.5:12 g/ml pH: 3 Solvent composition: 76% Methanol in water For total phenolic: Temperature: 64oC

Advantages: Rapid, simple, low solvent and low-cost.


44


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Time: 10 min Solid-liquid ratio: 1.5:11 g/ml pH: 7 Solvent composition: 61% Methanol in water. Polyphenols from the spruce wood bark

Temperature: 54oC Time: 60 min Solvent composition: 70% Ethanol in water.

Total polyphenols: 13.23 mg Gallic acid equivalents (GAE)/g of spruce bark tested)

45

Oil from waste date seeds

Temperature: 20oC Time: 45 min Power: 2.98 W Solid-liquid ratio: 1:5 g/ml Solvent composition: 100% Hexane.

Advantages: Short time, high yield,76.69% less energy (0.299 kJ per gram of oil obtained) as compared to Soxhlet method.

40

Carotenoids from pomegranate wastes using vegetable oils

Temperature: 51.5oC Time: 30 min Power: 2.98 W Solid-liquid ratio: 1:10 g/ml Solvent composition: 100% refined Sunflower oil.

Carotenoids: 0.3255 mg /100 g of dry peels. Advantages: High extraction rate and yield, minimum detrimental effect on the extracted compounds, use of green solvents.

46

Optimized conditions Phenolic compounds and anthocyanins from blueberry wine pomace (BWP).

Temperature: 61.03 °C Efficient and economic extraction

Time: 23.67 min

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Solid-liquid ratio: 0.46:10 g/ml,


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Leaching of nickel sulfate in sulfuric acid and hydrogen peroxide media

Temperature: 60oC Time: 4 hrs. Power: 200 W Solid-liquid ratio: 1:10 g/ml Solvent composition: Sulfuric acid concentration 30%, hydrogen peroxide 10% in water.

Pectin: 29.43% of total.

Pectin from sisal waste.

Temperature: 50oC Time: 26 min Power: 61 W Solid–liquid ratio: 1:28 g/ml

Phenolic compounds from the mandarin peel

Temperature: 48oC Time: 40 min Power: 56.71 W

Total Phenolic: 15,263.32 mg Eq. gallic/100 g DW Hesperidin: 6,435.53 mg/100 g DW.

Oil from papaya seed

Temperature: 62.5oC Time: 38.5 min Power: 700 W Solid-liquid ratio: 1:7 g/ml.

Leaching recovery: 60.41%.

Advantages: lower extraction temperatures and time and lower energy consumption.

Oil recovery: 73%


48

49

50

51


Overall, acoustic cavitation-assisted extraction is a well accepted extraction technique that reduces extraction times and energy as well as improves extraction yield and often, the quality of the extracted product due to lower operating temperature. In food industry, Acoustic cavitation has been widely applied to aid the extraction of components of interest from plant sources.52 Pilotscale use of acoustic cavitation has been reported for number of food systems. Pingret et al. (2012) showed a 30% increase in the extraction yield of polyphenols from apple pomace in a 30 L tank using acoustic cavitation (20 kHz, 40 oC, 40 min) over conventional extraction.53 A study focused on waste stream valorization which involved extraction of phenolic compounds from maritime sawdust waste reported 30% increase in phenolic yield using acoustic cavitation (25 kHz, 40 min) compared to conventional maceration on a pilot-scale.54 4.2. Cavitation based PI technology for Emulsification The emulsions were formed by passing the mechanical energy to immiscible liquids to create an additional interfacial area, and simultaneous adsorption of the surfactant on the newly formed interface, stabilizes the two phases. Although high-speed stirring, high shear mixing, and highpressure homogenizer can be used to supply mechanical energy, these devices are energy intensive with little control over the droplet size distribution as majority of energy is used for the creation and motion in the continuous phase which is not essential for the creation of the new surface. Cavitation is an alternative and yet an effective technique to create a stable emulsion. The mechanical effect of cavity collapse in both acoustic and hydrodynamic cavitation is the creation of local zones of microturbulence which enhance the heat, mass and momentum transfer rates. The pressure shock waves, generated by cavity collapse, can be utilized to disrupt the oil droplets which are stabilized by the Laplace pressure (2g/r, thus as r, the drop radius, reduces, higher pressures are required to disrupt these smaller drops). When cavitation is applied at the interface of two immiscible liquids, tiny droplets of one liquid (dispersed phase) are dispersed into the other liquid (continuous phase) i.e. emulsion is formed. Excess energy provided by cavitation is mostly used for the creation of new interface leading to the formation of emulsions even in the absence of surfactants (emulsifiers).12,55

For relatively stable emulsion, there is a maximum (limiting)

concentration of emulsion (percentage of the dispersed phase hold-up) corresponding to energy intensity above the threshold. As cavitational intensity increases, the limiting concentration of


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emulsion increases. Different types of emulsions prepared by cavitation are summarized in Table 6:



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Table 6: Cavitation assisted preparation of different types of emulsion Type of emulsion

Optimum Process Condition

Reported Process Intensification

human milk fat analog emulsion

Lecithin = 0.50% ; Ultrasonic power = 60W to 600 W.

Stable emulsion over range of optimized conditions pH of 6.4–8.0 temperatures of 50 °C–90 °C sugar levels (0-140 mg/mL) sodium ion strengths (0-2 mg/mL) Also stable under storage below 25 °C for 7 days

56

Citronella oil in water

Surfactant type: Span 80 and Tween 80 Oil to Surfactant Ratio:1 HLB: 14 Sonication Time: 5 min

Droplet size: 60 nm PDI: 0.3 Advantages: Minimum time, low energy, high quality of emulsion stability and droplet size.

12

Palmolein based Nanoemulsions

Oil to surfactant Ratio: 3 Sonication time: 5 min Power: 700 W

Droplet size: 263 nm PDI: 0.244 Advantages: Emulsion stable for 3 months, minimum time, low energy

57

Droplet size: 87.38 nm PDI: 0.244 Advantages: Emulsion stable for 3 months, minimum surfactant required, minimum time, low energy.

58

Droplet size: 87 nm PDI: 0.244 Advantages: Emulsion kinetically stable, energy efficient method (1092 kJ/nm per m3

59

Mustard oil in water Nanoemulsion

Oil Concentration: 10% v/v Surfactant Concentration: 8 % Sonication time: 30 min

Low-pressure hydrodynamic cavitation-assisted Mustard oil in water emulsion

Process time: 90 min Optimum CV:0.19 (at 10 bar optimum operating pressure) 26

ACS Paragon Plus Environment

References

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Optimum cavitation number: 0.17-0.20

Paraffin Wax in water Nanoemulsions

Oil Concentration: 20% w/v Surfactant Concentration: 10% w/v Sonication time: 15 min Power density : 0.61 W/ml

Turmeric oil in skimmed milk emulsion

Oil Concentration: 4% w/v Surfactant Concentration: 5 mg/mL Sonication time: 15 min Power density : 0.61 W/ml

of processing volume) for large-scale production. Droplet size: 160.9 nm PDI: 0.244 Advantages: Emulsion stable for more than 1 year, energy efficient method (0.05 kJ per gram of processed material) as compared to conventional method (0.9 kJ per gram of processed material), suitable for oil phase which is solid at room temperature. Droplet size: 232.2 nm PDI: 0.12 Advantages: Stable emulsion, low energy consumption


55

60


The key advantages of cavitationally assisted emulsification over the conventional emulsification methods are low energy consumption, less or no use of the surfactant, excellent control on droplet size distribution and the production of the rear mono-disperse emulsion. The efficacy of cavitationassisted emulsification mainly depends on the optimization of the process parameters (irradiation frequency, power and time, oil/water ratio) on the basis of stability and droplet size of the disperse phase and physicochemical properties of the oil following the guidelines provided in Table 1.57 It is reported that, as the irradiation time or power increases, dispersed phase volume increases (up to a maximum), while dispersed phase droplet size decreases. When both liquids, continuous and dispersed phase, have low viscosity, smaller droplet size is generally observed, possibly due to higher cavitational effects (lower attenuation) in these systems. No or less surfactant is required to obtain the same mean droplet size when ultrasound is used. Also, very less amount of energy is wasted in generation of continuous phase motion.55,60 No visible phase separation is observed when an emulsion is prepared using ultrasound (US) and hydrodynamic cavitation (HC) as compared to mechanical agitation. However, the emulsions prepared by HC are generally more monodisperse ( having lower polydispersity index) relative to the US. This is due to the more uniform distribution of energy to emulsion in HC than in US, because in HC, the whole volume of the emulsion passes through the cavitating device while in US, significant cavitation events are restricted in the vicinity of sonotrode.12 Barbell Horn Ultrasonic Technology (BHUT) have been utilized to scale-up the ultrasound assisted emulsification process, which enables horn designs with independent amplification and output surface and permits the construction of large-scale industrial processors with high amplitudes.61 BHUT-based processors have been used for the manufacturing of high-quality products such as nanocrystals, liposomes and nanoemulsions etc.62 But for industrial scale emulsification, hydrodynamic cavitation reactors are preferred over acoustic cavitation reactors, as hydrodynamic cavitation reactors are easy to scale up due to the availability of information about the fluid dynamics and downstream of a physical constriction of different shapes. Further, the operating efficiency of the circulating pumps, which is the only energy dissipating device in the hydrodynamic cavitation system, is always higher at large scales of operation.63 4.3. Ultrasonic Crystallization


Page 28 of 75

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Researchers have tried to explore the possible mechanism by which ultrasound (US) affect the crystallization process, the precise mechanism of sonocrystallization is yet unknown. Various mechanisms proposed are as follows: 1) Nucleation because of local hot spots, created by the collapsing cavity and rapid quenching effect afterward.64 2) The pressure shockwave produced by cavity collapse generates locally high-pressure pulse. For substances for which the solubility decreases with pressure, this increases the local super saturation and could induce nucleation.65 3) The high velocity of the shockwave generated after the collapse of the cavity throws solute molecules together with great force, overcoming the energy barrier normally associated with nucleation and phase change. This also suggests that the shockwaves could shatter existing crystals, creating new nuclei.66 4) Another hypothesis is that, nucleation initiates due to the segregation of the solute and solvent near the bubble wall.67 5) Another mechanism postulates that the nucleation caused is due to the heterogeneous mechanism. This suggests that the cavities or bubble act as the heterogeneous sites for nucleation.68 6) Solvent evaporating into a cavitating bubble or cooling of the liquid interface layer increases the super local saturation, which could lead to nucleation around the collapsing cavity.69 It is possible that instead of a single mechanism, multiple mechanisms are acting simultaneously and are responsible for the process of sonocrystallization. The physical effect of acoustic cavitation, which is the improvement in mixing and homogenization; influences the principal variables of crystallization, namely, the induction period, super-saturation condition and Metastable Zone Width (MZW). The source and the location of cavitation vary these effects in terms of its strength and magnitude. Also, their influence is a function of the specific physicochemical properties of the medium. Application of US to various systems results in smaller crystal size. The US as an additional variable provides better control of the crystal size distribution; as in the case of sonocrystallization, the obtained particle size distribution is narrow. There have also been reports related to the morphological changes due to the local high energetic environment in the formed crystals. Although, numerous literature on sonocrystallization is available, some of



the latest work reporting distinguishing significant observations and conclusions in the area are presented in Table 7.


Page 30 of 75

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Table 7: Different studies on sono-crystallization (ultrasonic crystallization)

Different studies

Process Condition

Reported process intensification

Crystallization with the gassing of Adipic acid.

Sonocrystallization of Adipic acid using different high-frequency ultrasound.



  Micrometer-Scale Adipic Acid Crystals.

Sonocrystallization of Adipic acid using aqueous solution in Batch and continuous cooling mode

 



Anti-solvent crystallization of salicylic acid

Investigation of the effects of following parameters on the antisolvent sonocrystallization of salicylic acid - process parameters like irradiation time, power and frequency of ultrasound and - type of reactor

  


References

355.5 kHz irradiation yielded a narrower MZW and CSD than those with 204 and 610 kHz.

70

Spherical particle by sonocrystallization Rod-shaped crystals in case of micronization and hammer milling Rounded-shaped elongated particles in case of high shear wet milling. No need of size reduction operation after crystallization in case of sonocrystallization.

71

Reduced particle size with increasing solution concentration, sonication time, power. Reduced particle size for an ultrasonic horn than an ultrasonic bath. Constant particle size over the temperature range of 25-35oC Larger crystal size for an increase in the solution injection rate.

72


Cooling Crystallization of Adipic Acid

Use of a short ultrasonic burst at the beginning of a cooling crystallization.



 Antisolvent Crystallization of Benzoic Acid

parameters of ultrasound, agitation speed of stirrer and antisolvent addition rate.

Growth and size control in anti-solvent crystallization of glycine

ultrasonic irradiation of 1.6 MHz

Antisolvent crystallization of sodium chloride.

High-frequency ultrasound (i.e., 44–645 kHz) from a plate transducer

Reactive crystallization of strontium sulfate

parameters such as concentration, temperature, additive (i.e., ethylene disodium salt dihydrate (EDTA)), and power of ultrasound on induction time

 










Page 32 of 75

Regular hexagonal particle shapes with reduced elongation in case ultrasonic than that after conventional seeding

73

Fine crystals of benzoic acid with high purity and morphology. A narrow crystal size distribution. The mean crystal size of 80 μm and 340 μm in the case of sonication and the conventional stirring process respectively.

74

Irradiation enhanced the growth of glycine crystal with narrow size distribution.

75

High-frequency ultrasound produced sodium chloride crystals of similar size distribution as an ultrasonic horn.

76

The decrease in the induction time of strontium sulfate with an increase in the reactant concentration, ultrasonic power input and temperature. Maximum crystal size range of 0.40– 1.10 μm for conventional crystallization

77

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Antisolvent Crystallization of Lactose from Whey

Ultrasonic horn frequency of22 kHz power levels (40 W to 120 W) ultrasonic exposure time: 10 min and 20 min.



Minimum of the crystal size range of 0.19–0.53 μm for 50% amplitude of the ultrasound.



Increased lactose recovery and purity with an increase in ultrasonic power for 100% duty cycle and 10 min treatment time Increased lactose recovery as well as purity only until an optimum for a 20 min treatment.

78

Reduced the mean crystal size from 170 μm to 13 μm Lowered the induction time from 360 sec to 30 sec Narrow the size distribution

79

enhancement in the yield and reduction in the induction time for crystallization Optimum conditions of power dissipation and ultrasonic irradiation time were 30 W and 10 min respectively. Needle-shaped crystals for conventional crystallization and obtained plate-shaped crystals for the ultrasound-assisted crystallization.

80



 Crystallization of Paracetamol

various ultrasonic frequencies

 



Crystallization of Mefenamic Acid

cooling crystallization of Mefenamic Acid 



Crystallization of lactose from whey proteins and κ-carrageenan

Diphenyl oxide and dimethyl phenyl carbinol crystallization

Sonication of 25% lactose solutions in a continuous flow energy densities: 9 and 50 J mL−1 κ- carrageenan (0, 150 and 300 mg L−1) whey proteins (0.64%)

Crystallization from a crude melt by using an ultrasonic horn having power 240 W and frequency of 22 kHz(DAKSHIN).

Page 34 of 75

 

Formed amorphous lactose. reduced amorphous lactose formation at ultrasound energy density of 9 J mL−1 in case of addition of whey proteins.

81



Lower/reduction in crystallization temperature. Lower subcooling needed. Higher yield and better purity

82

 


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It has been well proven that the sonocrystallization is a non-invasive way to improve and control the crystal properties such as size distribution, habit and morphology. The following benefits can be achieved using cavitation: 1. Improved product and process consistency and control over the process; 2. Excellent crystal purity; 3.

Enhanced secondary physical properties (flowability, packing density etc.) of the product;

4.

Reduced crystallization cycle times;

5.

Fewerand reliable downstream processes such as centrifuging, filtration and drying through manipulated crystal size distribution (CSD).

By varying sonication power, the crystal size distribution can be tuned to ease the downstream processing. Though nucleation by sonication shows a marked increase in the mean crystal size, prolonged insonation reduces the mean crystal size dramatically. The manipulation of size and habit of the crystals can achieve the following benefits: 1. Rapid filtration: uniform crystal size and compact habit can allow rapid filtration. 2. Improved washing and drying speed: better access to the inter-crystal voids can reduce washing and drying time. 3. Reduced contamination: Elimination of the risk of mutual contamination of the product and environment involved in milling step as well as reduction in solvent entrapment. 4. Flowability:

Improved crystal flowability rendering powder filling operations much more

reliable and unproblematic.



Page 36 of 75

4.4. Cavitation based PI technology for wastewater treatment The water is getting severely polluted by the hazardous chemicals discharged from chemical industries such as pesticides, dyes and textiles etc. Their existence in the effluent wastewater even in very small concentrations is harmful to animals and human beings. The conventional methods are not capable of complete degradation of bio-refractory pollutants.10 Over the ages, various techniques such as biological methods, membrane-based processes, and advanced oxidation processes (AOP’s) such as photocatalysis, cavitation have been used especially for the destruction of the bio-refractory pollutants. AOP’s can form highly oxidative hydroxyl radicals (●OH) (it has high oxidation potential about 2.80 eV) which can easily oxidize inorganic as well as organic toxic pollutant. It has been reported that the among all the AOP’s, cavitation is most energy efficient technique.10,11 When wastewater is exposed to the cavitation, the cavities will form and collapse which creates local hot spots containing extremely high temperature and pressure. Due to extremely high conditions water molecule dissociate into hydroxyl radicals (●OH), which has very high oxidation potential and able to oxidize pollutant molecule present in the wastewater. Cavitation assisted destruction of the organic pollutant is mainly occurring through two mechanisms, first, the thermal decomposition of the volatile pollutant molecule and secondly, the reaction of pollutants with ●OH radicals. This is called the chemical effect of the cavitation, which can occur inside the cavity, at the interface and/or in the bulk of the liquid. Sometimes the high intensity of shockwaves form due to the asymmetric collapse of the cavity can easily break big pollutant molecule into small (intermediate) molecules know as physical effect of the cavitation. Thus, cavitation can also be used as a pretreatment method. The following series of reactions may occur during the oxidation of organic pollutant molecules using cavitation in Eq. (1) – (4). H2O +))) H + ●H

●

OH + ●OH

●

●

H + ●OH

(1)

H2

(2) H2O2

OH + organic molecules

●

(3) CO2 + H2O + some intermediates

(4)

The importance and advantage of cavitation reactor for real industrial wastewater treatment have been discussed in Table 8. The synergetic effects of cavitation and other AOPs in terms of improved efficiency have also been discussed in Table 8.

The level of PI achieved, over and


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above the normal effluent treatment schemes is highlighted along with the possible quantification of the same.



Page 38 of 75

Table 8: Recent findings on the cavitation process for wastewater treatment

Types of

Cavitating

pollutants

Device

Removal of

Hydrodyna

acetone, methyl

mic

ethyl ketone

cavitation

(MEK) and

(vortex

toluene

diode)

Experimental details

Optimized parameters

Important Findings



References

80% degradation of

Pilot plant with

Inlet

pressure = 0.5 bar

capacity of 1 m3/h.

Time: 180 min.

toluene (cavitational yield 32.23 mg/J).


83

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Inlet pressure = 5 bar; pH of solution = 2.0; HC reactor: 15L tank

Acid Red 88 (Bio refractory in

Venturi

nature)

capacity, control valves, power of pump 1.1kW, cooling jacket



92% decolorization and

Cavitation number = 0.3;

35% reduction in TOC was

Concentration of dye = 100μM; concentration of H2O2 : 4000μM; molar

obtained. 

obtained.

Process time = 120 min  Reactive Red 120 (Bio refractory in

Venturi

nature)

volume of tank

Cavitation number =

capacity, power of

0.15; pH = 2.0;

pump 1.1kW, cooling

Optimum concentration

jacket

of H2O2 = 2040 μM

In the case of HC/H2O2 TOC reduction was

1:40

Inlet

84

100% decolorization, 72%

ratio of dye to H2O2 =

HC reactor: 15L

In case of HC only, about

Almost 60% decolorization and 28% TOC removal

pressure = 5 bar;

was obtained in HC treatment; 

85

100% decolorization and 60% TOC removal was obtained in HC/H2O2 combined process.



Orange-G (Bio

Circular

HC reactor: 15L tank

Inlet pressure for slit

refractory in

Venturi

size, control valves,

venturi = 3bar; circular

rate in case of slit venturi

nature)

Slit Venturi

power of pump

venturi and orifice plate

while 76% and 45%


Around 92% decolorization

86


Page 40 of 75

and Orifice

1.1kW, cooling jacket

= 5bar; pH of solution =

decolorization rate for

Plate

pump power of 1.1kW

2.0; Orange-G initial

circular venturi and orifice

concentration = 50 μM

plate was obtained

Process time = 120 min

respectively 

Almost 37% TOC reduction for slit venturi while 28% and 14% TOC reduction for circular and orifice plate was obtained.


Page 41 of 75 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47




HC only: 37.23% decolorisation, 22.22% TOC reduction;

Reactive Orange 4 (Bio refractory

Venturi

in nature)


Inlet pressure of fluid = 5

capacity, control

bar; pH of solution = 2.0;



HC/H2O2: Nearby 99.56% decolorization and 50.73%

valves, pump power of Initial dye concentration

TOC reduction, Synergetic

1.1kW, cooling jacket,

= 40 ppm; Molar ratio of

coefficient of 3.87;

variable frequency

dye to H2O2 = 1:30

drive to control the

Feed rate of ozone = 3g/h



87

HC/ozonation: around 76.25% TOC reduction in

rpm of motor

60 min of treatment, Synergetic coefficient of 3.03;

C.I. Reactive Red 2 (Bio refractory in nature)

Venturi

Water jet cavitation:

TiO2 loading = 100mg/L;

reactor having a

pH of solution = 6.7;

capacity of 5L, an

Initial concentration of

electric power of

dye = 20 mg/L



Processing time = 120 min



HC only: 76.6% decolorization;



degradation; 

0.75kW and speed of 2900rpm


HC/TiO2: Around 98.8% Treatment time = 90 min,

88


Page 42 of 75



59.3% degradation, 30% reduction in TOC in HC treatment

Inlet pressure = 4.84 atm; Temperature = 350C;

Rhodamine B (Bio refractory in nature)



99.9 % degradation and

Pollutants concentration

55% reduction in TOC for

HC reactor: tank

= 10 ppm; pH = 2.5;

HC/H2O2 combined

Venturi and

capacity of 15L,

Concentration of H2O2 =

process

orifice

reciprocating pump

200 mg/L;

with power of 1.1kW

FeSO4:H2O2 = 1:5

TOC reduction for

Amount of CCl4 = 1g/L

HC/Fenton combine

Process time = 120 min;

process





89

100% degradation and 57%

82% degradation, 34% reduction in TOC for HC/CCl4 combine process

Concentration of dye = 20mg/L; Fluid pressure = Reactive brilliant

Swirling jet cavitation

0.6 MPa; pH =5.5;

red K-2BP (Bio

reactor with

Temperature= 323K;

centrifugal pump

H2O2 concentration =

(power 3.5kW )

300mg/L

refractory in nature)

Venturi





Around 14% removal of dye by HC only



Maximum degradation of 98% by combined HC/H2O2 process

90

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Inlet pressure = 15 bar; pH = 2.0; Cavitation Imidacloprid


number = 0.067;

(neonicotinoid

capacity, positive

Imidacloprid

displacement pump

concentration = 25mg/L;

insecticide)

with power of 1.1kW,

Ratio of imidacloprid to

Bio-disinfectant

cooling jacket

H2O2 = 1:40

class of

Venturi



treatment. 

disinfectant

91

TOC reduction in 45 min for the combined HC/H2O2 process.



Orifice

Around 100% degradation of imidacloprid and 9.65%


Triazophos Bio-

26.5% degradation in HC

Around 50% degradation

HC reactor:

Inlet pressure = 5bar; pH

and 30% removal in TOC

reciprocating pump of

of solution = 3; initial

by HC only

power rating 1.1 kW,

concentration of

orifice plate of

Triazophos = 20mg/L;

and 56% TOC removal by

diameter 25 mm with

ratio of triazophos:

HC/Fenton

a 2 mm hole inside,

FeSO4:H2O2 = 1:4:4;

control valves,

Ozone Feed rate=1.95

90min of reaction time and

pressure gauges,

g/h

96% TOC reduction by





Almost 83% degradation 92

100% degradation in

HC/ozonation Carbamazepine Bio-insecticide

Orifice

Hydrodynamic –

Temperature = 250C,

Acoustic-Cavitation

concentration of 43 ACS Paragon Plus Environment



27 % conversion of carbamazepine in HC

93


(HAC) reactor:

Page 44 of 75

pollutants = 48.8µg/L,

treatment. 

frequency about 24kHz and power of

33% conversion in AC treatment.

200W



Maximum conversion of more than 96% in 15 min for HAC



Hydrodynamic – Chloroform

Acoustic-Cavitation

Orifice plate diameter =

(Halogenated and

reactor (HAC):

2.8mm; power = 40W;

centrifugal pump with

Temperature = 250C

power of 1.1 kW,


Bio refractory in

Orifice

nature)

HC treatment 

of HAC

Bio refractory in nature

temperature = 350C; pH

Orifice

centrifugal pump with power of 370W and 2900rpm

Around 12.4% degradation in HC treatment.

Inlet pressure = 4 bar;

capacity of 15L, a



21.3% degradation in

= 4.0; ratio of 2,4-

HC/H2O2 combined

dinitrophenol: H2O2

process 

=1:5; H2O2:FeSO4 ratio = 1:6

100% degradation in HC/Fenton combine process


94

conversion per pass in case



2,4-dinitrophenol

90% conversion of chloroform and 0.5%

frequency of 850 kHz

HC reactor: having

Total conversions of 7% in

95

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Thus, the pretreatment with cavitation, followed by the conventional methods can improve the degradation efficiency. Cavitation has been effectively utilized for wastewater treatment containing pharmaceutical discharges, pesticides, dyes and other complex organic compounds. HC approach has been tested at an industrial scale to degrade various organic and inorganic pollutants.11 Furthermore, the combination of HC with AOPs such as H2O2, O3, Fenton process and photocatalysis, etc. provide the synergetic effect and gives the desirable outputs, also overcoming the weaknesses of single AOP techniques. The efficiency of the degradation is proportional to the number of hydroxyl radicals and its effective utilization. It has been confirmed that the HC and other AOP’s synergistically work i.e. •OH radicals generation will improve.92,95 This combination effect can be better utilized for wastewater treatment than individual operation. The reported values of the synergetic coefficient have been in the range of 1.5 to 10, though there exists a small window of operating parameters for this synergy. It can be inferred that the waste streams containing bio-refractory pollutants can be treated successfully using cavitation and considering the energy efficiency of HC, HC can be scaled up as per industrial requirements.11 4.5. Process intensification of chemical transformation/chemical synthesis by cavitation Chemical synthesis using conventional methods do suffer with some limitations such as longer reaction time, low yield, poor quality, the requirement of expensive and toxic solvents or reaction media and catalysts, require very high or very low reaction temperature and pressure which is unsafe and uneconomical. In case of heterogeneous systems, the agglomeration of the catalyst or adsorption of impurities reduces the active surface area decreasing the reaction rate significantly. The physical as well as chemical effect produced by the cavitation offers an attractive tactic to intensify physicochemical processes.13 Cavitation can enhance reaction rate and product yield without using expensive and hazardous chemicals or solvents. It has been reported and proved that the cavitation is useful for all types of chemical reactions as shown in Table 9. The governing mechanisms vary with reaction systems and are facilitated by the chemical and/or the physical effects of the cavitation. Homogeneous reaction systems involved the radical formation, which is accelerated by cavitation that follows via radical or radical-ion intermediates. Typically, chemical effects of cavitation intensify radical reactions, however, physical effects also remain significant.16 The heterogeneous reactions with ionic intermediates, are affected primarily by the mechanical 45 ACS Paragon Plus Environment


effects of cavitation such as catalyst size reduction which increases the active surface area and catalyst surface renewal which enhances the heat and mass transfer rate. Heterogeneous reactions are generally intensified by the physical/mechanical effects of cavitation. Table 9 focuses on the intensification of the chemical synthesis in a variety of reactions using ultrasonic reactors, also highlighting the greener processing aspects of the same. By no means, this is a complete list but just highlights the typical transformation which is amenable to PI using cavitation.


Page 46 of 75

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Table 9: Recent literature on the chemical synthesis by cavitation based process intensification technology Chemical process Process Parameter’s Cyclohexanol Oxidant: H2O2 along with H2WO4 oxidation to cycloCatalyst: PTC hexanone Ultrasonic horn type: 3 mm diameter having frequency of 20 kHz Calorimetric power dissipation: 1.26 W mL−1. Oxidative Ultrasonic bath esterification of 4- frequency 37 kHz with 100% power output Nitrobenzaldehyde temperature 60°C.

Process Intensification  More conversion in less time Less energy as compared to the conventional method

References

96



Ultrasound decreased the reaction time and enhanced yields. 97



Acetylation of alcohol Ultrasonic bath in ionic liquids frequency 50 kHz



power 120 W

Synthesis of biaryls via Suzuki–Miyaura cross-coupling Ultrasound-assisted aza-Michael reaction

 

Ultrasonic bath frequency 47 kHz power 250 W. Ultrasonic bath



frequency 42 kHz


Reduced reaction time of 5 min. 95% yield at ambient conditions without any catalyst. 91% yield in 60 min for conventional stirring under silent conditions The required reaction time decreased from 18h to 2 h at 80°C in case of ultrasound 92% reaction yield in 30 min under conventional stirring.

98

99

100


power 135 W.

Synthesis of lutein di- Ultrasonic bath succinate Frequencies 45 kHz, 80 kHz, and 100 kHz



98% reaction yield in 5 min under the application of sonication.



High temperature for reaction propagation in case of conventional approach 60% conversion in conventional approach with 18 h of reaction time Ultrasound irradiation resulted in an 80% conversion in only 2 h

 

Lipase-catalyzed the Ultrasonic bath conversion of glycerol to glycerol carbonate frequencies 25 kHz and 40 kHz (GlyC) power of 200 W



Esterification butyric acid methanol



Synthesis of chloro-aziridines

of Ultrasound (145 W power dissipation and 22 with kHz frequency) Acid amberlyst-15 as a catalyst

di- Ultrasound-assisted addition reaction of dichloro-carbene in the presence of Mg/CCl4 with the Schiff base compounds



Reduction in reaction time by 10 hrs as in ultrasonic method than the conventional stirring.

In the presence of an ultrasonic approach, The reduction in time for equilibrium conversion of 91.64% from 180 min to 120 min

Page 48 of 75

101

102

103

30 % yield after 960 min for conventional stirring 104




96% yield under ultrasonic irradiation with 75 W power dissipation in 12 min

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Desulphurization of fuels or organics (noctane, toluene and noctanol)

Hydrodynamic cavitation (pressure drop: 0.5 and 2 bar, flow rate: 330 and 680 LPH)



Triglyceride transesterification for biodiesel production

Base-catalyzed transesterification (methanol/sodium hydroxide) of refined and bleached palm oil and waste vegetable cooking oil using hydrodynamic cavitation



Pretreatment of

Hydrodynamic cavitation (venturi type, throat diameter: 1.8 and 1.4 mm, flow rate: 3.2 and 2.4 L/min, number of circulations: 480 and 360, inlet pressure: 220 and 340 kPa, cavitation number: 0.44 and 0.29 respectively).



Lignocellulosic Biomass

100 % removal of Sulphur. 105




Electrical energy consumption of the HC reactor: 0.015 kW h per liter of produced crude biodiesel.

106

Pretreatment conducted at 30 °C Pretreatment method was evaluated by measuring lignin removal and glucose and xylose formation.

107


Overall, use of cavitation can result in the intensification of process in terms of energy and material consumption and its productivity, low level of waste generation and safe operation; all these aspects conform to the green process principles and demonstrate the potential of ultrasound in process intensification. The better knowledge of the effect of operating and geometric parameters of cavitation and its optimization will help to achieve significant process intensification reducing the cost of the operation. The most important intensification parameters with the economic incentive are the ultrasonic power, frequency, duty cycle and the temperature and it is important to optimize these parameters based on the specific system. Chemical Reactions, which are governed by physical effects such as mass transfer limited reactions, can be intensified using lower frequencies (< 250 kHz), while the chemical reactions, which are limited by intrinsic kinetics or free radical mechanisms, can be intensified using higher frequencies (>300 kHz). Optimization of ultrasonic power, duty cycle, agitation speed and operating temperature can be carried out through actual lab-scale experiments. This is particularly crucial for the enzymatic reactions where a significant reduction in the conversion may be onbserved beyond the optimum value (power delivered by ultrasound) due to the possible deactivation of the enzymes. The future of harnessing the cavitational energy at industrial scale, making process greener and environmentally benign, looks to be promising. The guidelines related to the optimization of the reactor configuration and designs for large scale operations can be found in the review by Sancheti and Gogate, 2017.16 4.6. Delignification of Biomass by cavitation based PI technology Delignification is a process of removal of lignin from cellulose. The methods of delignification can be categorized as alkaline cooking methods, aqueous-organic solvent based oxidation methods and biological treatment methods. All these methods are energy intensive and utilize various chemicals in large quantity (non-green route of processing).108 In the field of papermaking, over the last decades, the reduction in consumption of chemicals and energy for delignification has been a subject of great interest. Considering a recent emphasis on process intensification using non-conventional energy approaches, hydrodynamic or acoustic cavitational reactors for the process of delignification hold promise. The physical as well as chemical effects of cavitation makes it a novel technique for the intensification of the delignification.109 A detailed review of the work related to the use of


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hydrodynamic or acoustic cavitational reactors for delignification is summarized in Table 10. It highlights the details of reactor, systems investigated and important outcomes of the work.



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Table 10: Recent literature on the use of cavitational reactors for delignification Type of Wood/Raw Material

Process Details

Important results

Sugarcane bagasse

100 W Power, 20 kHz frequency, 40 min at 55oC Solvent: Water



 Sunflower husk

230–460 W/cm2, Frequency 30 kHz, 0–35 min, Solvent: Water

Wood wastes and wheat straw

400W between 20–100% amplitude, Frequency of 24 kHz Solvent: Diethylene glycol/glycerol Mixtures

Coconut (Cocos nucifera)

Ultrasonic bath, Power of 150 W, Frequency of 25 kHz, 20–60 min at 30oC Solvent: Ethanol

Sawdust

Horn type: 240 W Power, 22 kHz Frequency Ultrasonic bath:120 W Power, 20 kHz Frequency, Hydrodynamic cavitation: 750W pump Solvent: Aqueous soda solution

2.2% and 1.3% increase in the yield of total lignin and hemicellulose respectively when treated with ultrasound Reduced extraction time

References

110



The yield of hydrolyzed polysaccharides increased from 24% to 38% in the presence of ultrasound.



The shortened liquefaction time in the presence of ultrasound



More extraction of Phenolic groups by ultrasound process

113



Improved delignification at ambient conditions

114


111

112

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 Wheat straw

Raw-material (Waste Newspaper)

Wheat straw retted with 0.3 M KOH. rotor rotational speed of 2200 rpm and 2700 rpm.

NaOH concentration, biomass loading, temperature, ultrasonic power and duty cycle





 Groundnut shells, coconut coir, Delignification and subsequent and pistachio shells enzymatic hydrolysis





Sugarcane bagasse

Hydrodynamic cavitation Pretreatment parameters: NaOH concentration (0.1–0.5 M), solid/liquid ratio (S/L, 3–10%) and HC time varied from 15 to 45 min




Increase in the tensile index of the synthesized paper sheets by about 50– 55% in case of hydrodynamic cavitation treatment for 10-15 min. Less treatment time and energy consumption for hydrodynamic cavitation reactor than conventional techniques.

115

Reduction in the pretreatment time, temperature and alkali requirement with increased efficiency

116

Enhanced yields, reduced extremity of the process conditions. Reduced processing time and the requirement of the enzymes

117

52.1% of glucan, 60.4% of lignin removal and 97.2% of enzymatic digestibility under an optimal HC condition (0.48 M of NaOH, 4.27% of S/L ratio and 44.48 min). For pretreated SCB, enzymatic hydrolysis resulted in 82% and 30% higher yield than the untreated and alkaline-treated controls, respectively.

118


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Based on the literature reported in Table 10, the important guidelines to optimize the hydrodynamic or acoustic cavitational reactors for the process of delignification are given as follow: 1. The physical effects of ultrasound-induced cavitation have been found to enhance the reactants diffusivity, mass transfer rates and hence the efficacy of delignification. These also reduce the size of the solid particles and weaken the plant cell walls facilitating the extraction of its components.116 2. It should be noted that optimum levels of power dissipation enhance the physical effects such as acoustic microstreaming and turbulence.112,117 3. It has been established that the level of cavitational intensity is more in than in hydrodynamic cavitation reactors. As a result of this, under similar levels of energy dissipation, ultrasound-based reactors give higher delignification as compared to hydrodynamic cavitation.114 4. Presence of noble gases (Argon, Krypton and Xenon gases) in the cavitational system should lead to more delignification due to enhanced the cavitational intensity.108 5. Combination of cavitation with ozonolysis and microwave is also expected to synergize the delignification though not much work has been reported. Generally speaking, cavitational reactors shorten the extraction time from 60-240 min to 10–60 min, reduces the consumption of the reagent and the extraction temperature from 150–180oC to ambient conditions. Hydrodynamic cavitation has been found to be an effective way to increase the biomass digestibility for pulping.115 In hydrodynamic cavitation, delignification of biomass mainly occurs through hydrolysis. However, for the better understanding of the effect of cavitation on the molecular structure of the biomass further study is required. 4.7. Cavitation based PI technology for depolymerization A Polymer is a large molecule consisting of repeating units of monomer. Polymers can be natural (cellulose, de-soxyribo-nucleic acid (DNA) etc.) or synthetic (polyethylene, nylon etc.)

In

biomedical devices and drug delivery systems, depolymerization step is required to control the polymer properties which depend on their molecular weight. Various techniques available for depolymerization include photolytic, ultrasonic, microwave, thermal, mechanical, biodegradation, oxidative and hydrolytic. The conventional depolymerization methods have major drawbacks such as higher treatment cost and uncontrolled molecular weight reduction.119 The method based on 54 ACS Paragon Plus Environment

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cavitation has a good potential for efficient depolymerization.120 Cavitation based depolymerization is a convenient approach to reduce the molecular weight of the polymers. In cavitation based approach, no chemicals are required which offers an alternative green processing technique. In the initial period of sonication, the shear force disaggregates the polymer clusters by breaking the non-covalent intra and inter-molecular bonds. The prolonged ultrasonic irradiation splits the polymeric chains into the shorter random coiled chains. As the ultrasonic treatment time increases, the relatively high molecular weight fragments get degraded further into low molecular weight fragments. Yan et al. reported such an ultrasound assisted depolymerization mechanism of an aqueous solution of carboxylic curdlan (as shown in Figure 7).121

Figure 7: Schematic representation of ultrasound-assisted degradation mechanism of aqueous carboxylic curdlan solution 121 The recent works on the use of cavitation based approaches for depolymerization are summarized in Table 11. All these work reported in Table 11, analyze different aspects related to polymer degradation mechanism, reactor designs and also present guidelines for operating parameters and the possible optimum design.



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Table 11: Overview of polymer degradation (depolymerization) by cavitation Type of work/Polymer

Process Details

Process Intensification in the depolymerization

Chitosan and starch

Ultrasonic (Frequency 360 kHz power 100W) Time 90 min pH 3 temperature 22 ± 2oC



molecular weight reduction of polysaccharides

122



Degraded 1% guar, 1% xanthan and 2% pectin Significant reductions in apparent viscosity of guar, xanthan and pectin dispersions after sonication. Ultrasound seems to be a promising technique where degradation of hydrocolloid dispersions is desirable.

123

 Guar, xanthan, and pectin

Polyethylene oxide

Ultrasonic intensity levels 3.7, 6.3, 8.1, and 10.1 W/cm2) 



Solvent: water, benzene and polystyrene in benzene Ultrasonic frequency ( 20 kHz to 1 MHz)

References

Reduction in viscosity for frequency above 100 kHz.


124

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 

Poly-acrylic (PAA)

0.2% (w/v) aqueous solution of PAA Different treatment (Solar irradiation (SI), Ultrasound(US), Ozone (O3), acid Combination of US + SI, Combination of US+ O3, a combination of O3 + SI, Combination of SI+ US + O3

 

98.97% viscosity reduction in 35 min for US (60 W) + SI + H2O2 (0.01%) combination for 98.08%, 90.13%, 8.91% and 90.77% reduction in intrinsic viscosity in 60 min for SI + H2O2 (0.01%), US (60 W), H2O2 (0.3%) and SI respectively. 99.47% viscosity reduction in 35 min for US (60 W) + SI + O3 (400 mg/h flow rate). 69.04%, 98.97% and 98.51% reduction in viscosity for O3 (400 mg/h flow rate), O3 (400 mg/h flow rate) + the US (60 W) and O3 (400 mg/h flow rate) + SI in 60 min, 55 min, and 55 min respectively.


120






Polyacrylamide

Combination of Hydrodynamic cavitation (HC) with H2O2, O3, and UV irradiation.







 Carboxy-methyl cellulose

Combined ultrasonic–ultraviolet enzymatic method 

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97.27% viscosity reduction in 180 min for HC (3 bar inlet pressure) + UV (8 W power) + H2O2 (0.2% loading). 35.38% and 40.83% reduction in intrinsic viscosity for HC (3 bar inlet pressure) and UV (8 W power) in 180 min respectively. 89% viscosity reduction for HC (3 bar inlet pressure) + UV (8 W power) + O3 (400 mg/h flow rate) approach 50.34%, 60.65% and 75.31% viscosity reduction for O3 (400 mg/h flow rate) + HC (3 bar inlet pressure) and ozone (400 mg/h flow rate) + UV (8 W power) respectively.

119

Increase in depolymerization from 20% to 58.5% with an increase in the UV power from 8 W to 32 W with an irradiation time of 180 min Increase in depolymerization from 74% to 87% with an increase in ultrasonic power from 60 W to 120 W with an irradiation time of 120 min. 99% depolymerization of CMC for US (60 W) + UV (16 W) + enzyme (0.05%) combination with the treatment time of 18 min.

125


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Based on the literature reported in Table 11, it is observed that cavitation is a novel approach for the depolymerization. The effect of various parameters like initial polymer concentration, the presence of different functional groups in the polymer, reaction volume, molecular weight of polymer, operating frequency, power dissipation, temperature and process intensifying additives needs to be investigated to get the maximum and the quantification of the extent of depolymerization 125. The low concentrations of polymer and higher power dissipation are found to be favorable for the higher extents of depolymerization. Typically, low frequency is preferred for the depolymerization of water-soluble polymers. However, The higher frequencies can also give same results due to the chemical effects of cavitation 124. Overall, it has been observed that an efficient depolymerization can be achieved using a combination of process as compared to individual treatment. 4.8. Water disinfection by cavitation based PI technology The quality of drinking water is of principal importance in life. The quality has been found to be deteriorating over the years due to the contamination from industries and agricultural activities. The quality of drinking water can be restored by controlling the contamination and using effective methodologies for water treatment. Microbial disinfection is one of the important steps of the water treatment process as it is aimed at inflicting the damage to pathogenic microbes that are harmful to human. Over the years, chemical and physical means have been used to disinfect the water. However, the drawbacks and limitations of all these conventional techniques compensate for their efficacy. For example, the use of chlorine as a disinfectant also gives rise to mutagenic and carcinogenic agents such as tri-halo-methane (THM) in water. The chemical methods have lower disinfection rates due to limited mass transfer.126,127 Thus, it is a need of the hour to find alternative techniques for process intensification in water disinfection. Cavitation has a potential of being an effective water disinfection technique as it generates hot spots, highly reactive free radicals and microturbulence associated with acoustic streaming. For a pathogenic cell disruption to take place, rendering it unviable following conditions (as shown in Figure 8) need to be satisfied.



Figure 8: Conditions for cell disruption during disinfection by cavitation In the condition I, Cell should physically be present near the collapsing cavity, so that the energy released by the cavity on collapse is not dissipated in the surrounding liquid but is intercepted by the cell. In condition II, the energy dissipated by the cavities should reach the cell and should not be intercepted by another bubble in the vicinity and should be sufficient in magnitude. In condition III, energy delivered by a cavity should be high enough to disrupt the cell, i.e., to overcome the cell wall strength. The exact mechanism of the role cavitation in water disinfection (the inactivation of the microorganisms) has not been conclusively established. It has been thought to be a combination of mechanisms described below.128 1. Mechanical effects (generation of turbulence, liquid circulation currents, and high magnitude shock waves) 2. Chemical effects (generation of active free radicals). 3. Heat effects (generation of local hot spots) Thus, Cavitation is a promising technique for water disinfection. As it is a non-chemical method, its use does not form any toxic byproducts (DBP, disinfection by-products) as it is the case with the use of chlorine as a disinfectant. Cavitation is an energy efficient process and can be considered as a process intensification technique for a large scale water disinfection. The recent reports on different cavitating devices for water disinfection are summarized in Table 12: 60 ACS Paragon Plus Environment

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Table 12: Different cavitating devices for water disinfection Type of water

Process Conditions/ Details

Main Results 

References

Higher disinfection rates with increasing inlet pressure and using orifice plates with more and bigger holes

Combination of hydrodynamic



short disinfection time When

cavitation with chlorine dioxide:

hydrodynamic cavitation was

(2 mg/L) sodium hypochlorite: (2 bore well water

mg/L)

combined with chemicals 

Process time: 30 min

Less dose of the chemicals and

Hydrodynamic cavitation improves the disinfection rate and

Operating pressure range: 1 to 5

degrades natural organic matter

bar

(NOM) 

The pretreatment with hydrodynamic cavitation enhances the disinfection rate by 32 %


127




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40 % less energy consumption for Slit type of venturi geometry as compared to cylindrical geometry for the similar extent of seawater disinfection.

Combination of hydrodynamic 

cavitation with

seawater

Two times increase in the

Hypochlorite (5 mg/L)

disinfection rate for 5 ppm of

Cavitating devices: orifice,

hypochlorite disinfectant with the

cylindrical venturi, and rectangular

combination of cavitation

slit venture

compared to only 5 ppm of

Operating pressure range: 1 to 5

hypochlorite. 

bar

126

2.5 times increase in the rate of disinfection at 50oC in combination with cavitation compared to disinfection at only 50oC

 E. Coli contaminated water

2 log and 3 log reduction in

Vortex Diode as a hydrodynamic

CFU/mL count for inlet pressure

cavitation reactor

of 100 kPa and 140 kPa

Process Time: 20 min

respectively 


129

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 Inactivation of MS-2 virus in

Hydrodynamic cavitation with

water

venturi type constriction

more than 4 logs reductions of viral infectivity



.OH

130

radicals and high shear are

responsible for inactivation. Rotational hydrodynamic E. Coli contaminated effluent



cavitation reactor (HCR) flow rates: 8, 11, and 14 L/min

100% disinfection with a 4.3 L/min treatment rate using HCR

131

A pump pressure = 0.5 bar a rotational speed = 3600 rpm. 

93% reduction in the number of live cells

when treated with O3

alone (1mg/l) after 16 minutes  E coli in saline suspension

24% reduction in the number of

Combination of ultrasound (US)

live cells when treated with

with ozone (O3) system

Ultrasound 

99% reduction in the number of live cells for a combination of O3 and ultrasound within 4 minutes of treatment.


132


Various methods, involving cavitation in combination with chemicals, have been used for the treatment of bore well water.126,127 Here, the combination of hydrogen peroxide or ozone with cavitation is found to increase the efficacy of the destruction of bacteria. The increase in efficacy of destruction is attributed to the increased bacterial cell wall permeability of H2O2/O3 and to large number of free radicals generated which also reduced the O3 requirement by 70 to 80%.132 Pretreatment with hydrodynamic cavitation can reduce the consumption of chemical disinfectants substantially in the case of water disinfection. Overall, combination of cavitation with chemical methods is more efficient compared to individual methods. It reduces treatment times and the chemical consumption under optimized conditions. It also reduces the formation of the harmful disinfection by-products (DBPs). 5. Conclusions: This paper outlines the fundamentals of cavitation and its role in process intensification. The concept of process intensification and its drivers for applying process intensification using cavitation have been explained. The roles of cavitation in PI are discussed. The questions related to when to and how to employ cavitation for process intensification are answered with the help of illustrative examples. The main needs for the generation of more sustainable process alternatives through the cavitation as process intensification technique are discussed. PI characterized and defined in a variety of ways presents a quick and optimized solution to chemical engineers. Emerging cavitation based PI technology promises spectacular improvements in the performance of different processes like crystallization, emulsification, extraction, wastewater treatment, depolymerisation and water disinfection if employed judiciously. These reported improvements in the performance for different processes are summarized as follow: 

In case of extraction, cavitation reduces the time of extraction and improve the quality and quantity of the extracted product due to lower operating temperature.



Cavitationally assisted emulsification results into the controlled (narrow) droplet size distribution with no or lower use of the surfactants as compared to conventional methods



The Sonocrystallization improves the crystal purity and reduces the crystallization cycle times with tunable crystal properties such as size distribution and morphology.



The cavitation as a pretreatment and its combination with other AOPs improves the degradation efficiency of a variety of molecules, including toxic pollutants 64 ACS Paragon Plus Environment

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

Cavitation based chemical synthesis results in material and energy saving with an increase in the productivity along with little or no waste generation.



Hydrodynamic cavitation is an effective way to increase the digestibility for wood pulping as it reduces the extraction time, temperature and the reagent consumption.



A combination of processes based on cavitation, as compared to individual treatment gives efficient depolymerisation.



The use of cavitation in combination with other chemical methods of water treatment reduces the formation of the harmful disinfection by-products.

The implementations of cavitation for process intensifications can transform conventional chemical engineering unit operations into a revolutionary process technology by altering intrinsic chemical process elements to eliminate avoidable process bottlenecks. Changes can be usually be assigned by a substantial cost reduction, progress in the delivery/processing time. Cavitation technology is very effective for the intensification of chemical processing operations. The case study depicts the work clearly illustrating the efficacy of this novel technology based on cavitation, quantitatively. Overall, it can be said that cavitation is a well-established technology at laboratory as well as pilot scale and in some cases, even or on an industrial scale. Combined efforts of chemical engineers, chemists and physicists are required to harness and extend this technology to an industrial scale of operation to achieve a significant degree of PI. 6. References (1)

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