1-Advances-in-high-frequency-ultrasound-separation-of-particulates-from-biomass

Advances in high frequency ultrasound separation of particulates from biomass

Pablo Juliano ?,Mary Ann Augustin,Xin-Qing Xu,Raymond Mawson,Kai Knoerzer

CSIRO Food and Nutrition,671Sneydes Rd,Werribee,VIC 3030,Australia

a r t i c l e i n f o Article history:

Received 15December 2015

Received in revised form 3April 2016Accepted 28April 2016Available online xxxx Keywords:Ultrasound

High frequency Reactor Design Separation Oil Fat

Biomass

a b s t r a c t

In recent years the use of high frequency ultrasound standing waves (megasonics)for droplet or cell sep-aration from biomass has emerged beyond the micro?uidics scale into the litre to industrial scale appli-cations.The principle for this separation technology relies on the differential positioning of individual droplets or particles across an ultrasonic standing wave ?eld within the reactor and subsequent biomass material predisposition for separation via rapid droplet agglomeration or coalescence into larger https://www.sodocs.net/doc/3c13636027.html,rge scale transducers have been characterised with sonochemiluminescence and hydrophones to enable better reactor designs.High frequency enhanced separation technology has been demonstrated at industrial scale for oil recovery in the palm oil industry and at litre scale to assist olive oil,coconut oil and milk fat separation.Other applications include algal cell dewatering and milk fat globule fraction-ation.Frequency selection depends on the material properties and structure in the biomass mixture.Higher frequencies (1and 2MHz)have proven preferable for better separation of materials with smaller sized droplets such as milk fat globules.For palm oil and olive oil,separation has been demonstrated within the 400–600kHz region,which has high radical production,without detectable impact on product quality.

Crown Copyright ó2016Published by Elsevier B.V.All rights reserved.

1.Introduction

The use of ultrasound waves in the high frequency range (>0.4to several MHz)has been introduced as a method to initiate highly controllable label-free separation of particles.This principle has been successfully implemented in microchannel devices,where the acoustic resonator dimensions are designed to match a half wavelength (less than a millimetre for frequencies higher than 1MHz aqueous solutions)of the standing wave ?eld.Signi?cant attention has been paid to new lab-on-a-chip applications in a lam-inar ?ow regime to allow ef?cient acoustic trapping in continuous ?uid currents.Examples include separations for microbiological cell and droplet sorting,acoustic levitators,and cytometers [1–3].In recent years,the principle of applying acoustic standing waves as the driving force to separate droplets and/or particles suspended in ?uids has been further explored on a litre scale in view of new potential industrial applications.The positioning of individual droplets or particles on pressure antinodes or nodes,respectively,within a high frequency ultrasonic wave ?eld reactor may cause them to agglomerate or coalesce into larger entities

rapidly.Increased particle sizes promote ?otation or sedimenta-tion,and therefore enhance the predisposition of material mixtures for separation [4].In addition to the differential positioning of dro-plets to particles in antinodes to nodes,droplet separation due to streaming effects can be a complementary mechanism for predis-position to separation.

The design aspects for building high frequency ultrasound reac-tors for separation of food materials from liquid/liquid or solid/liq-uid mixtures have been reviewed by Leong et al.[4],describing key considerations for the design of megasonic reactors.Since then,further information has been generated to advance the under-standing of the sonochemistry and pressure distribution aspects in large scale reactors in relation to its impact on food quality,which will be discussed in this review.The potential application for more effective separation of cell biomass and in oil industries other than palm oil will be covered with new examples.

2.High frequency ultrasound separation principles 2.1.Fluid-mechanical principles

The acoustic separation technique is based on the action of acoustic radiation forces,resultant from a standing wave sound

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1350-4177/Crown Copyright ó2016Published by Elsevier B.V.All rights reserved.

?Corresponding author.

E-mail address:pablo.juliano@csiro.au (P.Juliano).

?eld,which cause particles suspended in a ?uid to move towards pressure nodal or antinodal planes or regions.The creation of a sta-tionary standing wave ?eld is achieved by the constructive inter-ference of the sound waves generated by the transducer and the waves re?ected by an opposite solid surface (e.g.vessel wall)located at a length that is an integer multiple of the half-wavelength of the propagating sound [3,5].Particles migrate towards either the pressure antinodal or nodal planes of the stand-ing wave ?eld,depending on the density and compressibility ratios of the particulates in the ?uid.The acoustic radiation force can be observed visibly as a banding effect that can cause particles to aggregate,clump together and sediment (or ?oat due to buoyancy)more rapidly than if gravity alone were to act on the individual particles themselves.

The time-averaged primary radiation force in an ideal standing wave ?eld in the direction of the propagating wave is given by Yosioka et al.[6]:

F ac ?à

4p R 3

kE ac /sin e2kx Te1T

where R is the particle radius,k ?2p k

the wave number,k is the wavelength of sound,E ac the speci?c energy density,x the distance from a nodal point of the standing wave and /is the acoustic con-trast factor [6].The acoustic contrast factor gives an indication of the separability of the suspended particle from the medium based on the density and compressibility,and is calculated using:

/?

5q p à2q m 2q p tq m à

b p

b m

e2T

where q is the density,b is the compressibility,and the subscripts m and p refer to the medium and the particles respectively [6].The compressibility of a particle or droplet can be estimated using:

b p ?

1q p c 2

e3T

where c is the speed of sound in the medium.The inverse of b p is called the bulk modulus (in GPa)which is known for many common substances.In simple terms,a liquid droplet like oil will have an acoustic contrast factor <0and will be driven towards the pressure antinodes.A solid particle with an acoustic contrast factor >0will be driven towards the pressure nodes.Provided that the driving ampli-tude or acoustic energy density of the sound wave is above a certain threshold [7,8],particle banding will occur.As particles are moved into either the nodal or antinodal planes of pressure,a secondary acoustic radiation force comes into action.This force is also known as the secondary Bjerknes [9]force,and tends to bring particles within a plane together.This force originates from the scattering of the sound ?eld off neighbouring particles in an oscillating sound ?eld.In the plane,particles may eventually ?occulate together into larger aggregates or,for liquid droplets possibly coalesce into larger droplets [10].Larger particles are generally easier to manipulate since the acoustic radiation force is strongly dependent on the par-ticle size.Further information on the secondary force theory can be found in Leong et al.[3].The gravitational force corrected for buoy-ancy is often used in addition to the acoustic radiation force to achieve separation.

In general,the theory applies when a particle is signi?cantly smaller than the half wavelength of the ultrasound wave.A particle that is too large (compared to a half-wavelength)would distort the ultrasound ?eld,and the distance it would be moved would be too small to achieve meaningful separation.Similarly,maximum yield of sonochemical products such as hydrogen peroxide the primary radiation force created around it will be too weak to enable its col-lection in the nodes or antinodes.For batch systems,the aggregates would then be extracted by some type of collection tool at the top

or bottom of the separation container.The aggregates formed may be compact or weak enough to fall apart,when the ultrasound is switched off [11].This behaviour is dependent on the type and strength of the inter-particle forces [12,13].Liquid droplets under suitable conditions can also undergo coalescence [14,15],which is one of the key principles for the separation of oils from vegetal biomass suspended in an aqueous phase.Another mechanism that may facilitate removal and extraction of oil droplets from non-oil solid matter is acoustic streaming,which will be discussed later in this manuscript.

2.2.Sonochemical considerations for standing wave systems

The sonochemical effects on food products in standing wave systems have received little attention in the literature.A number of publications have identi?ed that a frequency in the range between 300and 400kHz results in high production of sonochem-ical products,or 800kHz depending on the level of speci?c energy applied into the system or the types of transducers used [16–19].Within this frequency region,maximum yield of sonochemical products such as hydrogen peroxide,are produced from the radi-cals formed.Radical production is also dependent on the energy density of the applied ultrasound.Incidentally,the ultrasonic fre-quencies most suitable for use in separation applications range between 400kHz and 2000kHz.Further increase in the frequency beyond 800kHz diminishes the sonochemical yield of such prod-ucts,since the energy released by cavitation bubble collapse becomes small.At these frequencies,cavitation is described to be more of the stable and less transient cavitation occurs [20].Hence,there are less violent collapses and less extreme shear ?ows but still high temperatures (Temperature >2000–5000K)[22].In the case of a standing wave,there is an additional effect of a high con-centration of bubbles in the pressure anti-nodal planes.Cavitation of a bubble in the proximity of an additional bubble has been shown to occur by asymmetrical shape oscillation.Asymmetrical shape cavitation was found to occur at lower power levels than for single-bubble cavitation [21].Hence,the cavitation threshold in the standing wave condition might be lower than otherwise,and the cavitation regime in the case of a standing wave may be a more gentle type of cavitation [19].In any case,for acoustic sep-aration in the cavitation regime,the radical formation will occur at the same pressure in anti-nodal regions as the oil droplets have aligned to.Fig.1shows the additional sonoluminescence occurring at antinodes resulting from free radical production.

In the frequency range between 1and 4MHz,the cavitation effects become comparatively gentler.These frequencies are com-monly used for non-invasive analytical methods such as medical diagnostic imaging and industrial testing of materials,or for gentle cleaning of sensitive surfaces.Even though low frequency ultra-sound (<200kHz)decreases the formation of sonochemical prod-ucts in aqueous conditions,it is not recommendable for the separation of particulates due to the occurrence of strong cavita-tion effects that may either further mix or potentially damage the integrity of the particles being separated.Since the formation of radicals is usually at peak rates within the frequency range suit-able for separation,this may result in unwanted oxidation of food materials.

2.2.1.Preventive measures for standing wave systems

Operating near non-cavitation (stable cavitation)conditions ?rstly avoids exposure to cavitation generated radicals that oxidise free oil or fat,thereby avoiding the generation of off-?avours and reducing the antioxidant activity provided by other components in the sample.Secondly,high temperatures and shear stresses reached locally,near a cavitating bubble,can potentially also mod-ify molecules and structures in the product.Thirdly,operating in

2P.Juliano et al./Ultrasonics Sonochemistry xxx (2016)

xxx–xxx

the nearly non-cavitation/stable cavitation regime is also advanta-geous for avoiding the risk of generating micro and nano sized metal particles deposited in the sample from the direct exposure of transducer surfaces to the processed media [22].

Direct exposure of transducers to water may provoke cavitation pitting of the surface,ultrasonic peening,oxidation related corro-sion and enhanced corrosion by cavitation disruption of a surface passivation layer.However,transmission plates placed in front of the transducers have been developed to avoid direct contact of transducers with the media [4],thereby minimising or eliminating deleterious effects.These plates enable the application of cooling water for sonication at higher temperatures and non-aqueous (sol-vent based)extraction mixtures.

2.2.2.Luminol studies to follow free radical formation in standing wave systems

Johansson et al.[19]developed a method to study the cavitation threshold in situ in large scale (>1L)standing wave systems using luminol.Luminol imaging evaluates the entire operation volume using,for instance,a digital SLR camera.Johansson et https://www.sodocs.net/doc/3c13636027.html,ed this method for evaluation of cavitation threshold in acoustic standing wave systems,in contrast to most sonochemiluminescence stud-ies,which aimed at maximising cavitation activity.Luminol mea-surements at 0.5MHz have shown much higher number of radicals per cavitating bubble forming for A OH as compared to KI-dosimetry (measuring H 2O 2concentration)[23].Luminol mea-surements provide an instant indication of free radical formation while KI-dosimetry measures the cumulative concentration of per-oxide formed over time.

2.2.

3.Impact of standing wave free radical formation on fat oxidation Leong and Juliano [24]summarised the current knowledge of the effects of megasonic separation on the nutritional and physical properties of foods including dairy products and palm oil.The effects of high frequency ultrasound treatment of milk have been evaluated by Juliano et al.[25].The authors considered frequency effects (0.4–1MHz)speci?c energy effects (5and 20min operation

Juliano et al.[15].Pressed palm fruit feed was treated using ultra-sound at 400kHz for up 60min (13.4kJ/kg)without affecting the content of natural antioxidants and other phytonutrients present in the oil,particularly vitamin E (a -tocopherols and tocotrienols)and carotenoids.Other quality parameters such as free fatty acids and deterioration of bleachability index (DOBI)value for samples exposed to ultrasound were comparable to a commercial sample produced that same day in the conventional process without ultra-sound.The commercial ultrasound separation process is now in place producing crude oil that meets quality speci?cations.2.2.4.Physical ultrasound standing wave effects in milk structure The physical changes induced by high frequency ultrasound in milk have also been studied recently.Even though zeta-potential measurements have shown that milk fat globules remain intact after high frequency sonication [28],high frequency ultrasound at 0.4MHz (but not at 1.6MHz)was shown to be able to modify casein micelle size and form protein aggregates when using high power [29,30].This was mainly attributed to mechanical cavitation effects generating high local temperatures,high shear forces,microjets and shock waves that may induce bond breakage by mechanical effects.2.3.Streaming effects

Another mechanism that may facilitate removal and extraction of oil droplets from non-oil solid matter is acoustic streaming.Acoustic streaming is the non-linear generation of ?uid ?ow,which results from spatial or temporal variations in a pressure ?eld [31].While streaming sets the whole ?uid into motion,the particle may be carried around by this ?ow preventing it to be manipulated by the acoustic radiation force.In general,the higher the acoustic energy input into the system,the stronger these ?ows become.Depending on the characteristics of the agglomerated particles such as their size,density,compressibility,concentration and inter-particle forces,their banding can be disrupted due to motion produced by excessively strong streaming ?ows.The gradient in the velocity ?eld permitting acoustic streaming arises from various mechanisms,such as the spatial attenuation of the sound wave in

Sonochemiluminescence produced by bubbles aligned in a standing at 860kHz in a cylindrical transducer attached to a 500mL vertical cylinder with a re?ector placed on top of the liquid-air interphase [76].

the medium due to absorption of the energy,scattering of sound waves,or due to friction between a vibrating element and the sur-rounding medium.What is commonly observed is the develop-ment of a velocity?eld with a non-zero time average.Vorticity may also be observed,i.e.the tendency for?uid to‘spin’or‘curl’, through Rayleigh streaming[32]in a standing wave-?eld,which occurs between the nodes and antinodes or through Schlichting streaming[33]around an object placed in the sound?eld.

Further detail of the physical principles that govern acoustic particle/droplet separation and the mathematical modelling tech-niques developed to understand,predict,and design acoustic sep-aration processes,with particular emphasis on acoustic streaming are covered by Trujillo et al.[34].

3.Design aspects for ultrasound standing wave reactors

The design aspects for building a megasonic reactor for separa-tion of food materials from liquid/liquid or solid/liquid mixtures were recently reviewed[4].Key considerations for the design of megasonic reactors include the transducer selection,positioning and alignment,construction materials and geometry of transduc-ers and reactor.However,the pressure and sonochemistry distri-bution and transmission aspects of industrial scale high frequency transducers have not been widely addressed in the liter-ature.Furthermore,the high frequency transducer performance using transmission plates to enable indirect contact processing in terms of pressure distribution has not been discussed in a wider context.

3.1.Sound penetration and sound pressure distribution

Extensive research has been carried out in the characterisation of low frequency power ultrasonic reactors[35–38].Recent litera-ture focuses on the distribution of cavitational events throughout the reactor volume guided by other parameters such as sound pressure intensity,frequency of ultrasound and the physicochem-ical properties of the medium,along with the geometry of the reac-tor.The performance of high frequency standing wave systems is mainly guided by the level of sound pressure achieved in the med-ium.Moreover,sound pressure distributions are important to be understood to determine the active ultrasound volume of the ves-sel and thereby establish the most effective geometry and the transducer con?guration selection.However,until now most research has mainly considered measuring and predicting sound pressure distributions in laboratory scale reactors.

Leong et al.[39]evaluated sound pressure penetration and dis-tribution in water provided by industrial high frequency plate transducers operating at400kHz(120W)and2MHz(128W), installable in large separation vessels.This research showed that the choice of the transducer distance to the opposite reactor wall depends on the transducer plate frequency selected.Their effective operating distance was determined across the chamber’s vertical cross section with the use of hydrophones in a2m long reactor chamber.The2MHz transducer produced the highest pressure amplitude near the transducer surface,with a sharp decline of approximately40%of the sound pressure occurring in the range between55and155mm from the transducer(Fig.3a).However, the400kHz plate transducer was found to penetrate the?uid up to2m without signi?cant losses.This has important implications in the determination of workable transducer to wall distances and optimised reactor cross sections,which will dictate operating volumes and operational residence times.The placement of a re?ector plate500mm from the surface of the transducer was shown to improve the sound pressure uniformity of2MHz ultra-sound(Fig.3b).On the other hand,400kHz ultrasound plates were able to generate a more uniform sound pressure distribution regardless of the presence or absence of a re?ector plate.Therefore, large scale reactor designs with the400kHz large scale plate trans-ducers tested in this study can consider larger transducer to oppo-site wall distance and larger active cross-section,and therefore may treat larger volumes than when using2MHz transducer plates.

The limitation of the current pressure measurement work is that studies in large reactors have been carried out in water due to the limitations in the usage of hydrophones in biomass media, which affect the reliability of the measurements and hydrophone durability[40].In most cases,a biomass material will consist of a continuous phase(usually liquid)and a dispersed phase(solids,

characterisation of standing wave1MHz and2MHz transducer plate–re?ector systems:(a)photography of luminol intensity measurement; using Matlab software.A1L rectangular vessel holds one transducer(T)re?ected over a rectangular re?ector plate(L)[19].

liquids or both),both containing carbohydrates,proteins,lipids,?bres and minerals in different proportions and conformations as well as bubbles trapped in the media that may impact on the acoustic properties such as sound penetration[41].The current and limited knowledge on acoustic properties of food materials, namely speed of sound and sound attenuation coef?cient,has been reviewed recently[42,43].

The achievement of uniform sound pressure penetration and uniform standing waves inside the ultrasound separation reactors is important to be able to achieve adequate scale up of the process. The dispersed biomass material will be either i)less dense than the surrounding medium or,ii)denser than the surrounding medium. This will de?ne where the material will accumulate in the standing wave and also where it will be collected after separation.Less dense particulates are those such as lipid in water emulsions. Examples include palm oil and milk fat.These particulates collect at pressure antinodes[14].More dense particulates are composed of mostly solid type materials such as plant cell matter,bacterial and blood cells,or denser liquid phases.These will collect at pres-sure nodes and separate to the bottom of the reactor vessel by sed-imentation.Further research is required to understand sound pressure distributions in such complex materials in situ and the subsequent allocation of food components in standing wave?elds.

3.2.Non-contact sound transmission in large scale transducers

The development of non-contact transmission plates to be applied in the food industry has emanated from the need to adjust high frequency ultrasound reactors to the palm oil extraction pro-cess.Palm oil extraction generally operates with palm fruit feeds at temperatures between85and95°C.Because most commercial transducer plates are limited to operating at temperatures below 60°C,the transducer can be maintained by either placing the transducer externally with a cooling water layer between the transducer and a transmission plate mounted to the reactor wall (Fig.4a),and/or by?owing cool air inside the transducer.Indirect contact of transducers in an ultrasound reactor allows designing smooth internal surfaces in the reactor,thereby preventing

representations(relative to the pressure measured at55mm)in a large scale rectangular vessel:(a)pressure penetration re?ector plate;(b)pressure distributions in reactor cross sections as a function of hydrophone to2MHz transducer distance from the transducer.Adapted from[39].

Fig.4.Representation of the transmission plate for non-contact transmission in the

food medium:(a)concept schematic;(b)Luminol sonochemiluminescence inside

the experimental vessel(1MHz;3mm stainless steel transmission plate).Adapted

from Leong et al.2015and Michaud et al.).

internal product blockages and enabling the installation of clean-

in-place systems.In this case,the transducer is placed externally

to the reactor such that there is a cavity where cooling liquid can

be circulated between the ultrasonic transducer and the reactor

walls.This however means that the emitted sound wave has to

be transmitted through an additional layer of metal(i.e.,a trans-

mission plate).

Fig.5.Effect of ultrasonic frequency on transmission ratio as a function of distance from the surface of the transducer

will provide a bene?t in recovery of a high value material post-decantation or centrifugation.In this situation,the applied mega-sonics serves to initiate agglomeration and/or coalescence of sus-pended food material,as well as enhance the extraction of product that may be trapped in solid raw materials.This interven-tion enhances the rate at which the downstream process achieves separation,and also increases the recoverable yield of the separa-ble material.

4.Application examples

The implementation of ultrasound in separation and extraction processes should start with an evaluation of the critical points to be selected for ultrasound intervention at various steps of the pro-cesses.This section will highlight the potential points of ultra-sound intervention for aqueous based extraction processes such as palm oil,olive oil and coconut oil from oleaginous material. The technology readiness of the megasonics separation technology for these industries will be also discussed with new preliminary results.Secondly,the advances on megasonic applications for milk fat separation and fractionation as well as algal dewatering will be shown.Key results are summarised in Table1.

4.1.Enhanced palm oil recovery

Selection of the possible intervention points for the ultrasound application requires an appreciation of the conventional palm oil extraction process used in the industry.Fig.6shows the key steps in the palm oil milling process(Fig.6[49]).Fresh fruit bunches are sterilised at125–130°C to facilitate fruit removal from the bunch.Fruits are then placed in a hot water tank at85–90°C to soften the fruit material for pressing and enable further oil release.The ex-screw press feed is then conveyed into a vertical clari?cation tank, where oil is skimmed from the top of the tank and the deoiled sludge(or under?ow sludge feed)is pumped to a decanter or disc-bowl centrifuge.The centrifuge will separate the under?ow sludge feed into a high oil enriched stream,which is fed back into the clari?cation tank,and the palm oil mill ef?uent(POME).Three key points for megasonic treatment to predispose oil separation may include:(a)the ex-screw press feed;(b)the under?ow sludge feed;and(c)the POME.

The concept of ultrasound-assisted recovery of palm oil was tested on a laboratory scale on the ex-screw press and under?ow sludge feed streams[15]as well as on the palm oil mill ef?uent (POME)[50].The laboratory results demonstrated that megasonic sound waves at400kHz and1.6MHz(5min,34kJ/kg)on the ex-screw press or the under?ow sludge streams were more effective in assisting oil recovery than in the POME.This showed that larger oil droplets present in ex-screw press feed at initial stages of the palm oil process,which are present in higher concentration,are easiest to separate.Stronger primary forces are required at smaller droplet size[3]and larger concentrations of oil will promote antin-odal regions of more concentrated oil droplets that will promote further agglomeration or coalescence.

Trials were then carried out in a batch process at pilot-scale in two100L vessels(control and ultrasound),one of them carrying 400kHz transducers plates for treatment of the ex-screw press with standing waves at a speci?c energy input of13.4kJ/kg after 60min[14].Comparisons demonstrated the ability of ultrasound to increase the oil recovery as well as to reduce the time for oil sep-

Table1

Summary of recent publications where ultrasound assisted separation was demonstrated beyond the microliter scale.

Material stream Target

separation

material Volume Frequency

(kHz)

Energy input

(kJ/kg)

Effect Authors Refs.

Ex-screw press and under?ow sludge

feeds Palm oil7mL400and

1600

34Enhanced oil separation Juliano et al.

(2013)

[15]

Palm oil mill ef?uent Palm oil20L400and

160034Enhanced oil separation Augustin et al.

(2014)

[50]

Ex-screw press feed Palm oil100L40013.4Faster oil separation including unripe fruit

materials Juliano,et al. (2013)

[14]

Ex-screw press Palm oil200L vessel;5ton

palm fruit/h 600 2.2Higher oil recovery;reduced losses Augustin et al.

(2013)

[51]

Olive paste pre-malaxer Olive oil 4.25L3520kJ/kg Enhanced oil separation(heating effect)Clodoveo et al.

(2013)

[54]

olive paste pre-malaxation Olive oil0.7–0.8kg40150W(time

not reported)

Enhanced oil separation(heating effect)Bejaoui et al.

(2015)

[55]

Olive paste during malaxation Olive oil0.7–0.8kg24128Enhanced oil separation(heating effect)Jimenez et al.

(2007)

[57]

Olive paste post-malaxation Olive oil0.7–0.8kg60072Enhanced oil separation(non-thermal

effect)

Juliano and

Augustin,

(2015)

[59]

Coconut meat/water mixture Coconut oil/

cream

Lab scale200072Enhanced oil separation Juliano and

Augustin,

(2015)

[59]

Coarse milk emulsion Milk fat/

cream 7mL400and

1600

Enhanced emulsion splitting Juliano,et al.

(2011)

[69]

Coarse milk emulsion Milk fat/

cream 6L(more effective at

higher speci?c

energies)

400,1000,

and2000

Enhanced emulsion splitting at larger scale;

best separation at higher speci?c energy

Juliano,et al.

(2013)

[70]

Raw milk Milk fat/

cream 0.3–1.8L1000and/

or2000

<200More effective at reduced volumes(shorter

transducer-wall distance)

Leong et al.

(2014)

[29]

Raw milk Milk fat/

cream 1.8L(temperature

effects4,25,45°C)

1000and/

or2000

<200Better separation at$25°C Leong et al.

(2014)

[71]

Raw milk Milk fat/

cream 1.8L1000and/

or2000

$200Fat globule size fractionation achieved at

higher speci?c energies

Leong et al.

(2016)

[65]

Raw milk Milk fat/

cream Flow rate:11L/h1000

+2000

$200Separation achieved in a continuous system Leong et al.

(2015)

[72]

Algae stream Cell

concentrate Flow rate:4–6L/day2100N/A Concentration of algal cells achieved Bosma et al.

(2003)

[2]

P.Juliano et al./Ultrasonics Sonochemistry xxx(2016)xxx–xxx

7

Additional recovery has been attributed to a combination microstreaming effects,which promote oil body release tissue,and standing wave effects,which impart oil dro-

particle separation.

these batch experiments,it was concluded that the most point for intervention on a commercial scale was the the ex-screw press stream.The process was then from a pilot batch scale process(100L)to a semi-

continuous process(5ton fresh fruit bunch/h),which

kg extra oil/ton palm fruit bunch[51].Fig.7shows vessel design trialled in a continuous process in a palm Malaysia.The vessel was further redesigned with trans-plates(as opposed to the internal air cooling in the semi-system)for its implementation in a commercial oper-

tonnes fresh fruit bunch per hour(1–2min residence

nominal transducer power,operating at a frequency commercial recovery of palm oil assisted with megasonics in October2013and an extra2.00–2.50kg oil/MT bunch has been achieved[50,52].The plant has been

full scale since installed and further installations are other palm oil mills.As discussed earlier,the sound pen-matter,the level of pressure achieved and the unifor-achieved across the standing wave?eld dictates the spacing transducer and vessel wall.This will ultimately de?ne cross-section and required vessel length according to pressure capacity required inside the vessel.A larger may thereby reduce the amount of transducers achieve desired effects on oil recovery.We have shown

the knowledge of sound penetration and pressure dis-

water has been advanced,however improved sound

methods are required to determine the effects of

material on sound penetration,which will facilitate

developing more cost effective reactor vessel designs.

4.2.Enhanced olive oil separation

A simpli?ed version of the olive oil process is depicted in Fig.8. The olive paste is produced by crushing the entire olive fruit,most commonly with inclusion of the pits,using a hammer mill,a stone mill or a disc mill.The paste is then pumped into a malaxation ves-sel controlled at23–30°C where the paste is slowly macerated by a set of kneading blades rotating at20–30rpm,to enable further dis-ruption of cell tissues through the action of natural enzymes.The residence time and temperature of the paste during malaxation

Fig.6.Possible points of megasonic intervention in the palm oil extraction process.

Fig.7.Semi-commercial scale megasonic reactor prototype pre-disposing ex-screw

press feed into deoiling before reaching the vertical clari?cation tank.

amount of oil released,where a longer time and tem-exposure will release more oil[53].However,the limiting

consider is oil quality,which decreases with the increase time and temperature,promoting oxidative reac-increasing the solubilisation of antioxidant polyphenols

water phase and away from the oil phase.Certain proces-co-adjuvants to enable more rapid or further oil increase oil yield;these may include talc to reduce

content in the paste.Other examples include the enzymes into the process,which is permitted in certain

outside Europe.The?nal step of the olive oil separation

the malaxed paste into a decanter centrifuge to separate

the paste.Most commonly,for the production of virgin

virgin olive oil,there are two-phase or three-phase sys-different levels of water addition.

Megasonics can be applied to the olive paste at three stages of process:(a)before malaxation;(b)during malaxation;

malaxation(see Fig.8).Previous work at low frequency has considered sonication of the olives at35kHz and 150W(20kJ/kg)[54].However,the physical phenomena in a

low frequency cavitation regime points at disrupting cell tissue, which high frequency ultrasound would not be able to achieve due to changes in bubble size and bubble interactions[36].A num-ber of publications have focused on the application of high power low frequency ultrasound before malaxation[54–57].In this case, sonotrodes would provide cavitational strength to disrupt the cell wall and release further oil.The main advantage of low frequency ultrasound application has been its ability to heat the paste and therefore shorten preheating time to reach the malaxation target temperature of30°C.The combined temperature and physical effects from cavitation has led to increased oil yield(e.g.,additional 8g oil/kg olives or3%additional in2.5kg olives at36kJ/kg)after treating the olive fruits[56]and olive paste[54–57].The reported research has been carried out using an ultrasonic horn[55,57]or an ultrasonic bath(with a capacity of up to4.25L)[56,57],which have limitations in scalability due to the sound penetration in a process that is expected to operate at2.5–10tonnes per hour in commercial scale.There is however other low frequency equip-ment available using special arrangements of multiple transducers, which has demonstrated to be promising in providing uniform and scalable ultrasound treatment[58].

Our preliminary work has evaluated the application of mega-sonics to the olive paste before and after malaxation as shown in Fig.9and its effect in oil yield enhancement[59].The non-malaxed and malaxed paste(1kg)was placed in a ultrasonic standing wave?eld created in a rectangular vessel with a 600kHz transducer plate for5min(72kJ/kg;in a controlled tem-perature environment).Even though both interventions showed positive results,only the0.7%(paste weight basis)extra oil yield found on the paste treated after malaxation is statistically signi?-cant(P<0.05).The pre-malaxation intervention would be justi?ed in the case that ultrasound affects further access of cell wall deplet-ing enzymes such as pectin esterases within the cellular structure through enhanced tissue permeabilisation[60],while promoting further release of oil bodies.Microstreaming effects mentioned earlier would also contribute to the‘‘cleaning”of oil entrapped within the non-oil solid structure.However,megasonics may be considered preferable as an intervention on the paste post-malaxation,since there would be free oil readily available for separation.

Signi?cant attention has been paid to the effects of low fre-quency ultrasound on olive oil quality and extractability of other components such as chlorophyll,carotenoids,tocopherol com-pounds and phenolic compounds[56,61,62].Trials held in an ultra-sonic bath(2.5kg paste)at35kHz,150W up to10min and30°C showed improvements in the antioxidant content in virgin olive oils from two varieties as seen by increased tocopherols and caro-tenoids released.However,ultrasound enhanced further chloro-phyll release and decreased polyphenol concentration,with positive effect in sensory properties.Further research is required to understand extractability of components after megasonic treat-ments and its impact on sensory properties and nutrition.

4.3.Improved coconut oil extraction

Coconut oil is mostly produced by a solvent extraction process. The coconut endosperm is removed and dried to produce the dried coconut meat,also known as copra,and subsequently,applying

Fig.8.Possible points of megasonic intervention in the olive oil extraction process.

https://www.sodocs.net/doc/3c13636027.html,parison between oil yield after megasonic treatment pre-malaxation

and post-malaxation of olive paste.Different letters indicate signi?cant difference

(P<0.05)between average values[59].

solvents to maximise oil extractability.The use of solvents is expensive and carries safety risks,however,it achieves higher oil yield than the aqueous based extraction methods.Nevertheless,aqueous extraction provides the opportunity to fully utilise food and non-food materials in the coconut towards the production of value added bioproducts.The full utilisation and conversion of this material into high value products will not only provide greater returns,but also minimise contamination due to biological and sol-vent residues.

A simpli?ed example of an aqueous base extraction process is shown in Fig.10.Coconut milk is produced by separating the coco-nut meat from the coconut shell and husk.Coconut meat is then placed in a digester tank with hot water at 60°C to facilitate the release oil bodies,followed by wet pressing or grinding to reduce particle size and form an emulsion [63].Sequential ?lters and/or centrifuges can then be used to obtain the coconut milk.A third centrifuge separates the fat from the coconut milk to obtain coco-nut oil and skim milk.

Our preliminary work has evaluated the potential megasonic

clearly separated from the sediment as compared to the control (without megasonics)where separation was less evident.Further work on coconut milk extracted from coconut meat (following a thermal and ?ltering process)showed the ability of 2MHz stand-ing waves to increase oil yield.Fig.11b shows the enhancement of oil content in the free oil and emulsi?ed oil layer.

https://www.sodocs.net/doc/3c13636027.html,k fat globule separation and fractionation

Megasonic separation has been recognized as a technology that enhances the ‘‘natural”separation rate of fat globules from milk [28].Natural creaming is commonly used in the manufacture of Italian artisanal cheeses.However,natural separation of cream requires several hours,and thereby becomes a limiting factor for productivity [64].Furthermore,the separation of milk into size fractions has proven promising in the production of specialty cheeses and other dairy products [65–67].

The research of high frequency ultrasound waves applied for Fig.10.Possible points of megasonic intervention in the coconut oil process.

megasonic treatments (10min)on oil yield:(a)shredded coconut meat:water mixture (1:3ratio)using 600MHz or 2MHz transducers indicate signi?cant difference (P <0.05)between average values and (b)extracted coconut milk treated at 2MHz (230W,45°C –asterisks difference (P <0.05)by paired comparisons)[59].

10P.Juliano et al./Ultrasonics Sonochemistry xxx (2016)xxx–xxx

Leong et al.[28]have shown,at a litre scale,that1MHz(energy input<200kJ/kg)can better separate the smaller-sized fat glob-ules present in natural whole milk than lower frequencies.Small milk fat globules are within the micron size and require shorter transducer–re?ector separation distance(between30and 85mm),which will provide a reduced attenuation?eld for better performance.Leong et al.[71]have also shown that the application of1MHz ultrasound for5min at operating temperatures near 25°C provide better acceleration of milk fat separation that lower or higher temperatures.

Leong et al.[65]recently demonstrated the potential of ultra-sound standing waves to fractionate milk fat globule into sizes. Batch trials in a1.8L vessel enhanced fractionation of fat enriched fractions(up to13±1%w/v)with larger globules and semi-skim milk fractions(1.2±0.01%)with proportionally smaller sized fat globules.Particle size differentiation was enhanced at higher ultra-sound energy input(up to347W/L)and higher frequency of 2MHz.

This same vessel was then adjusted for continuous separation and size fractionation of milk fat utilising the set up shown in Fig.13[72].Fig.14shows the achievement of continuous milk fat separation at volumetric?ow rates scaled up to11L/h by using simultaneous1MHz+2MHz sonication at a temperature of approximately30°C and a resident time of10min.Continuous milk fat globule size fractionation into fat enriched streams (Fig.15)and fat depleted streams can be achieved by the use of multiple units or by multiple passing the fractionated milk into the vessel(Fig.15).The achievements made by this technology make it amenable for small scale sized cheese or yoghurt making facilities employing up to5000L milk per day.

4.5.Enhanced microalgae cell concentration and microbial cell retention

Algal stream dewatering was recently researched[1,73].Bosma et al.applied2.1MHz to algae?owing continuously through a small volume and achieved a separation ef?ciency of over90%, albeit at low?ow rates of only4–6L/day.The separation here is driven by gentle agglomeration at the pressure nodal points,fol-lowed by enhanced sedimentation as algae cells are aggregated together.

One signi?cant advantage of the use of ultrasound assisted sep-aration,as shown by numerous studies[3,74]is that microbial and yeast cell viability is not compromised by the sound intensity used in acoustophoretic separation processes at relatively short inter-vals and low energy levels.Studies have shown that selective retention of viable versus non-viable cells is possible[75]reducing the need to‘bleed-out’product during operation.Such systems are commercially available with the development of BioSepTM(Appli-Sens,Schiedam,The Netherlands),and could be readily imple-

coconut oil meat in water(1:3ratio)non-megasonic(left)and megasonic treated(right)at2MHz,10min and230W:(a)photograph of after10min;(b)micrograph of the top layer after settling for30min at60°C(oil stained with Nile Red).

mented into food applications such as fermentation tanks in the beer and wine industry.Such units are available from throughputs as small as10L/day up to200L/day.This particular system is a multi wavelength,semi-continuous system.For achieving even higher?ow rates,a further scale-out of the process by using sev-eral200L/day reactors was recommended,rather than a scale-up by using larger transducers with higher?https://www.sodocs.net/doc/3c13636027.html,mercial realiza-tion of these cell perfusion systems based on ultrasound separation has been achieved with the development of BioSep TM.

5.Final remarks and prospects of the technology

Megasonics is an emerging separation technology applied to predispose oil or fat globules as well as cells or solid particles for separation from biomass in a high frequency standing wave?eld. It has proven amenable for early ultrasound interventions in the oil extraction processes to decrease oil loss into the environment while increasing oil yield.The palm oil industry is bene?ting from the process with multiple commercial installations.Advantages are not limited to increased oil yields,but faster oil separation has allowed for the possibility of reducing the number of centrifuges, and reduced amount of oil in the ef?uent has further environmen-tal and economic bene?ts.Even though the technology has not

Fig.13.Schematic of inlet and outlet?ows into prototype milk separation vessel a)side on view b)front on view.Adapted from[72].

Fig.14.Change in fat concentration relative to initial of collected streams after one-

pass continuous?ow(non-megasonic and megasonic–1MHz)processing at

0.18L/min(10min residence time in megasonic?eld).Adapted from[72].

Fig.15.Schematic for the cream collection process after megasonic treatment into

second pass and subsequent cream collection(a);Increase in milk fat after

reprocessing the collected cream fraction for up to3passes(b).Adapted from[72]

been fully tested at large scale in the coconut and olive oil indus-tries,laboratory scale results show promising results when consid-ering various interventions.

The selection of megasonic separation parameters (e.g.,high frequency high intensity standing waves)can also be tuned to selectively remove particles of a particular size.It can therefore be a useful tool for the fractionation of materials such as milk fat globules from a product stream to provide fractionated streams with higher nutritional or structural bene?t.

Each extraction or separation process described creates differ-ent streams,which may be sonicated to predispose separation.These streams differ in their acoustic properties according to the original materials,water and fat content,and fat droplet and non-fat droplet sizes.It has been shown that higher frequencies near 2MHz provide better separation in shredded coconut poten-tially not only due to a standing wave ?eld droplet collection effect,but also due to a cleaning effect on the coconut surface resulting from microstreaming.Such high frequency has also shown more suitable for size fractionation of milk fat globules due to their smal-ler size range,which require higher primary forces provided by higher frequencies for globule collection at the antinodes.On the other hand,other tested streams such as a coarse milk emulsion or the palm oil ex-screw press feeds contain large (>5l m)droplets which are more easily separable in a standing wave ?eld at lower frequencies (400–600kHz),where there is greater space in between antinodal collection points.

Another important aspect to be considered to further the devel-opment of this technology,is the understanding of pressure distri-butions in situ ,which will enable better de?ning reactor requirements (e.g.type and number of transducers)for successful operation and capital cost minimisation.Acknowledgements

We would like to acknowledge the contributions of Minsik Seo and Fabian Bainczyk for carrying out the preliminary work on coconut and olive oil.References

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