Ultrasound-enhanced conversion of biomass to biofuels

38 Pages·2014·2.36 MB·English

by  Jia Luo

ProgressinEnergyandCombustionScience41(2014)56e93
ContentslistsavailableatScienceDirect
Progress in Energy and Combustion Science
journal homepage: www.elsevier.com/locate/pecs
Review
Ultrasound-enhanced conversion of biomass to biofuels
Jia Luoa, Zhen Fanga,*, Richard L. Smith Jr.b
aChineseAcademyofSciences,BiomassGroup,KeyLaboratoryofTropicalPlantResourceandSustainableUse,XishuangbannaTropicalBotanicalGarden,
88Xuefulu,Kunming,YunnanProvince650223,China
bTohokuUniversity,ResearchCenterofSupercriticalFluidTechnology,GraduateSchoolofEnvironmentalStudies,AramakiAzaAoba6-6-11,Aoba-ku,
Sendai9808579,Japan
a r t i c l e i n f o a b s t r a c t
Articlehistory: Twoimportantchallengesneedtobeaddressedtorealizeapracticalbiorefineryfortheconversionof
Received1February2013 biomass to fuels and chemicals: (i) effective methods for the degradation and fractionation of ligno-
Accepted31October2013 celluloses and (ii) efficient and robust chemical methods for the conversion of bio-feeds to target
Availableonline2December2013
products via highly selective catalytic reactions. Ultrasonic energy promotes the pretreatment and
conversionprocessthroughitsspecialcavitationaleffects.Inthisreview,recentprogressandmethods
Keywords: for combining and integrating sonication into biomass pretreatment and conversion for fuels and
Ultrasound
chemicalsarecriticallyassessed.Ultrasonicenergycombinedwithpropersolventsallowsdestructionof
Lignocellulose
the recalcitrantlignocellulosic structure,fractionation ofbiomass components, and then assists many
Pretreatment
Microalgae thermochemicalandbiochemicalreactions,withincreasedequilibriumyieldsofsugars,bio-ethanoland
Biodiesel gasproductsby10e300%.Sonicationpromoteshydrolysis,esterificationandtransesterificationinbio-
diesel synthesis and leads to reduced reaction time by 50e80%, lower reaction temperature, less
amounts of solvent and catalyst than comparable unsonicated reaction systems. For algal biomass,
sonicationbenefitsthedisruption,lysisandcontentreleaseofmacroandmicroalgaecells,andreduces
the time required for subsequent extraction and chemical/biochemical reactions, with efficiencies
typically being improved by 120e200%. High-frequency ultrasound of 1e3 MHz allows harvesting of
microalgae,liquidproductseparationandin-situprocessmonitoringofbiomassreactions,whilehigh-
intensityultrasoundat20e50kHzactivatesheterogeneousandenzymaticcatalysisofthebiomassre-
actions.Theuseofultrasoundinconversionofbiomasstobiofuelsprovidesapositiveprocessbenefit.
(cid:1)2013ElsevierLtd.Allrightsreserved.
Contents
1. Introduction................................................................ ........................................................57
2. Physicalmechanismsforultrasonicenergy.................................................. ...........................................57
2.1. Ultrasound,energytransformationandacousticcavitation ...................................... ..................................57
2.2. Basisforreactordesignandparameters ................................................ .........................................58
2.2.1. Designofultrasonicreactors................................................. ..........................................58
2.2.2. Ultrasonicfrequency .................................................... ..............................................60
2.2.3. Propertiesofhomogeneoussolvents ............................................. .......................................61
2.2.4. Heterogeneouspropertiesofultrasound-assistedreactions .................................................................61
2.2.5. Otherparameters ...................................................... ...............................................62
3. Ultrasonicpretreatmentandreactionsoflignocellulosicbiomass....................................... ................................. 62
3.1. Destructionandfractionationoflignocellulose ....................................................................................63
3.2. Conversionoflignocellulosetosaccharidesandbio-ethanol ...................................... .................................66
3.3. Fermentationoflignocellulosicwastestobiogasandbio-hydrogen .................................. ..............................68
3.4. Mechanismandresearchneeds .................................................................................................68
* Correspondingauthor.Tel.:þ8687165137468;fax:þ8687165160916.
E-mailaddress:[email protected](Z.Fang).
URL:http://brg.groups.xtbg.ac.cn/
0360-1285/$eseefrontmatter(cid:1)2013ElsevierLtd.Allrightsreserved.
http://dx.doi.org/10.1016/j.pecs.2013.11.001
J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93 57
4. Ultrasound-assistedsynthesisofbiodiesel ............................................................................................. 69
4.1. Esterificationoverhomogeneous/heterogeneouscatalysts ....................................... ..................................69
4.2. Transesterificationoverhomogeneousacids/bases........................................... .....................................69
4.3. Transesterificationoverheterogeneousacids/bases ................................................................................72
4.4. Enzyme-catalyzedhydrolysis/esterification/transesterification...................................... ................................73
4.5. Ultrasonicpretreatmentinsupercriticalsynthesisofbiodiesel ..................................... ................................74
4.6. Ultrasound-assistedconversionofglyceroleaby-productofbiodiesel................................. .............................74
4.7. Mechanismandresearchneeds .................................................................................................74
5. Ultrasonicenhancementofalgaepretreatmentandreactionsforbiofuels ................................................................. 75
5.1. Microalgaegrowthandnutrientaccumulation ....................................................................................76
5.2. Harvestingofmicroalgae ...................................................... ...............................................77
5.3. Pretreatment,extractionandconversionofalgae .................................................................................77
5.4. Mechanismandresearchneeds .................................................................................................78
6. Ultrasonicemulsificationanddemulsificationofcrudebio-oilandbiomaterials................................ ........................... 79
7. Biorefineryprocessmonitoringusinglow-intensityultrasound........................................................................... 79
7.1. Principlesofultrasoundmonitoring .............................................................................................79
7.2. Ultrasoundmeasurementinbiorefinery ................................................ .........................................80
8. Discussiononcriticalissuesandrecommendations.............................................. ....................................... 80
8.1. Selectionandoptimizationofultrasonicfieldparameters........................................ ..................................82
8.2. Impactonchemicalreactionsandheterogeneouscatalysis ...................................... ..................................82
8.3. Impactonenzymesandbiochemicalreactions....................................................................................83
8.4. Energyandprocessingbenefitassessmentincasestudies ....................................... ..................................84
9. Conclusions ................................................................ ....................................................... 87
Acknowledgments ..................................................................................................................87
References................................................................. ........................................................88
1. Introduction contributestoeasierpretreatment,fractionationandchemicalre-
actionsofbiomassmaterialsundermildconditions,andresultsin
Thelackofmethodstocirculatecarbonsustainablyisthoughtto increased reaction efficiency and higher catalytic activity over
be oneof the dominantreasonsfordeterioration ofourenviron- thermochemicalmethods(Challenges1and2).Theapplicationof
ment[1].Techniquesthatefficientlycirculaterenewablecarbonare ultrasoundenergyintensifiesmassandheattransferinreactions,
anurgentpriorityandiftheuseoffuel-baseddevicesiscontinued, enhances the contact and disengagement of heterogeneous re-
efficient methods to produce biofuels are needed. Available actants,intermediatesandproducts,andthereforeacceleratesthe
biomass resources that can be considered for use as biofuels are reactionrateorchangesthekinetics(Challenge3).
mainly sugars, polysaccharides, lignocelluloses, chitosan, lipids, Reviews on ultrasonic applications are related to biomass
algae and polyols [2]. Some of the possible conversion paths of treatment, such as sono-assisted lignocellulosic pretreatment
these materials to biofuels [2e7] are acid/base/enzyme-catalyzed [14,15], extraction ofnatural products [16], sonochemistryof car-
hydrolysis of biomass to fermentable sugars, fermentation of bohydrate compounds [17], catalytic esterification and trans-
cellulosic biomass and sugars to bio-ethanol, bio-hydrogen and esterificationoflipids[18e20],foodprocessing[21],pretreatment
othersmallmolecules,catalytichydrocracking,hydrogenationand and fermentation of organic wastes (e.g., bio-sludge) to gaseous
reformingoflignocellulosetopolyols,catalyticoxidationofligno- products (e.g., H2, CH4) [22,23] and biochemical engineering/bio-
cellulose to aromatic aldehydes, thermal gasification of cellulosic technologies[24e26]suchasbiologicalwastewatertreatmentand
biomass to hydrogen and methane, and chemical/enzymatic bioremediation. However, critical assessment of ultrasound-
esterification/transesterificationoflipidstomonoesters. assisted pretreatments and reactions of biomass for producing
Intheconversionofbiomassresourcestobiofuels,threechal- biofuelsisneededtoconsidertheroleandmechanismofultrasonic
lenges must be met. The first challenge is the recalcitrance of intensification and how it can be used advantageously in future
lignocellulose[8]andthereticularconnectionoflignin[9].Crude biorefineries.
ligninecarbohydrate requires harsh conditions and specialized This review examines ultrasound-assisted pretreatments and
solvents to degrade, which makes processing complicated, envi- reactions of biomass materials. Recent applications of ultrasonic
ronmentallyproblematicandeconomicallyunsustainable[3]. energy in the pretreatment and conversion of lignocelluloses,
The second challenge is the uncertain chemical constituents biodiesel and microalgae are described and critically evaluated.
contained in biomass. Raw biomass from different sources has Then, the potential of ultrasound for other related biomass pro-
variedcontentsofcellulose,hemicellulose,lignin,freesugars,wax, cessing (emulsification, demulsification and aggregation) and in-
proteins,alkaloids,traceorganiccompounds,andinorganics[10]. situprocessmonitoringareintroducedanddiscussed.Finally,key
Thethirdandthemostimportantchallengeistheheterogeneity issuesarediscussedandanalyzed.
ofbiomassreactionsystems.Biomassmaterialshavelowthermal
conductivitythatcreatesabarrierforheatandmasstransfer[11]. 2. Physicalmechanismsforultrasonicenergy
Conversionandproductselectivityinbiomasssystemstendtobe
poor due to insufficient catalystereactant contact. Immiscible 2.1. Ultrasound,energytransformationandacousticcavitation
alcoholelipidecatalyst systems are a fundamental problem in
biodieselsynthesis[12]. Ultrasoundismechanicalacousticwavewiththefrequencyrange
Ultrasonicenergyprovidesaspecialphysicochemicalenviron- fromroughly10kHzto20MHz[27].Itimpartshighenergytoreac-
mentforprocessingbiomasssystems[13].Thehigh-energyimpact tionmediumbycavitationandsecondaryeffects[13,28].Inatypical
and corrosion by high-intensity ultrasound to biomass system dynamic process of cavitational bubbles, numerous microbubbles
58 J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93
containingsolventvaporsaregeneratedthatgrowandundergoradial Various methods can be introduced to experimentally deter-
motion as acoustic energy propagates through the liquid medium minethevelocity,sizeandconcentrationofcavitationalbubblesin
[29].Thesemicrobubblesgrowtoamaximumofabout4e300mmin thereactor.Thesuddenchangeofacousticpressureinsoundfieldis
diameter[29],andcanbestableortransient.Withlowacousticin- regardedasthecharacteristicofcavitation,whilethefeaturedis-
tensity,theradiiofmicrobubblesperiodicallyandrepetitivelyexpand tributioninacousticemissionspectrum,namelythegenerationof
andshrink(radialoscillation)withinseveralacousticcycles.While subharmonic, ultraharmonic, or harmonic frequency, is used to
acousticenergyhassufficientintensity,somemicrobubblesareun- discriminate between transient cavitation from stable cavitation
stablewithinonlyoneor two acoustic cycles. Whenthe resonant [32,35].Cavitationalintensityisrelatedtothepopulationdensityof
frequency of bubbles exceeds that of ultrasonic field, the bubbles active cavitational bubbles and the released energy intensity in
collapsewithinseveralnanoseconds,whichcreatesspecialphysical bubblecollapse, and is normally quantified through thedetermi-
and chemical effects, and enhances thermochemical/biochemical nationofsecondaryeffects.Chemicalmethodshelpinthemapping
reactionsortreatment[29]. ofcavitationalintensitydistributionandactiveradicalformationin
Theunsymmetricalcollapseofbubblesatabroadsolid/solvent the sonochemical reactors [36,37], including iodine method
interface(>200mm)producesmicrojetsathighspeed(>100m/s) (H2O / (cid:3)OH þ (cid:3)H, I(cid:4) þ (cid:3)OH / I2(cid:4) / I3(cid:4)), Fricke method
towardsolidsurfaces[27,30].Theinstantaneouscollapseofbubbles (H2O/(cid:3)OHþ(cid:3)H,Fe2þþ(cid:3)OH/Fe3þþOH(cid:4)),salicylicacidmethod
also produces strong shockwaves that might be up to 103 MPa (the degree of salicylic acid hydroxylation under sonication), ter-
[30,31]. This violent movementof fluid towardorawayfromthe ephthalic acid method (terephthalic acid / hydroxyterephthalic
cavitational bubbles is defined as micro-convection, which in- acid),sonoluminescence(instantaneousflashduringthecollapseof
tensifies the transport of fluids and solid particles and results in cavitational bubbles), sonochemiluminescence (the reaction of
forcesthatcancauseemulsificationordispersiondependingonthe cavitation-induced radicals with chemical reagents such as lumi-
conditions, while the strong shockwaves and microjets generate nol),aluminumfoilerosionandelectrochemicalredoxprocess.
extremely strong shear forces over those of conventional me- However, these analytical methods have their limitations.
chanicalmethods,andareabletoscatterliquidintotinydropletsor Chemicalmethodsinvolvingradicaldeterminationdependonthe
crushsolidparticlesintofinepowders. number of radical release at the collapse of cavitational bubbles,
The chemical effect of ultrasonic comes from local hotspots while aluminum foil erosion is more suitable for quantitative
producedbycavitation.Atthemomentofbubblecollapse,ahuge analysisoftheintensityofsecondaryeffectssuchasshockwaveand
amount of energy is released that cannot be immediately trans- microjets.Chemiluminescencerelatesthebehaviorofbubblesthat
ferredtothesurroundings.Asaresult,localhotspotsaredeveloped achieve a certain energy level (bubble radius of 2.6e4.1 mm at
that have extremely high temperatures (ca. 5000 (cid:2)C), high pres- 575kHz),whilesonoluminescencegivesthemostviolentcharac-
sures (ca. 50 MPa) and high rates of heating and cooling in the teristicsinbubbledynamics(activebubbleradiusof5.4e5.5mmat
bubbles (>109 (cid:2)C/s) [27]. The extremely high temperature and 575 kHz) [29]. Methods that use advanced technologies such as
pressurecandestroythecrystallinestateofsolidmaterials,cause laser diffraction, pulsed multibubble sonoluminescence, laser
solids to meltorfuse solid particleswhen theycollidewith each phase-Doppler [38], methyl radical recombination and others
other[30].Ultrasonicenergycancausetheformationofshort-life- [29,33]areusednotonlyforanalysisofsinglecavitationalbubbles,
timereactiveradicalssuchasH(cid:3)andHO(cid:3)fromreactantsorsolvent but also for the acquisition of qualitative and quantitative infor-
moleculesatthemomentofbubblecollapse[27]. mationofbubbleclustersorstreamers[29].
Oneofthemostimportantaspectsofapplyingultrasonicenergy Cavitationisthemainmechanismforultrasoundintensification.
to systems is how its energy is transferred to reactant solutions, Thekeytoefficientapplicationofultrasoundiscontrolandselec-
whichhasthreesteps:1)thetransformationofelectricalinputto tionoftheenergyintensityandpopulationofactivecavitation.The
mechanical energy through a piezoelectric or piezomagnetic energy intensity of cavitation depends on the mean behavior of
transducer;2)thedeliveryofvibrationalenergy(acousticenergy) bubbles,whilethepopulationofactivecavitationdeterminesthe
fromtheemissiontipofthetransducertotheliquidmedium;3)the cavitational efficiency. Many experimental methods have been
conversionoftheenergyofultrasonicstreamingtotheenergythat developed to measure the population and secondary effects of
activatesreactantsbyacousticcavitation[32].Energylossesinany cavitation,however,thesemethodshavetheirlimitations. There-
stepwillinfluencethetotalenergyefficiency. fore,itisnecessarytousetheoreticalmethodtoestimatetheen-
Direction-selectivedeliveryofacousticenergyanditsattenua- ergylevelofcavitationalbubbles.
tion in homogeneous or heterogeneous system results in a
nonuniformenergyfield.Acousticcavitationefficiency,namelythe 2.2. Basisforreactordesignandparameters
conversion efficiency of ultrasonic energy to cavitational energy,
alsoplaysaroleinmechanisms.Onlycavitationalbubblesthatgrow Forprocessingofbiomassmaterialswithultrasound,theselection
toacertainsizecanaccumulatesufficientenergyforreactantacti- of ultrasonic parameters such as ultrasonic mode (continuous or
vation,sincethisenergyisrapidlyreleasedwiththeradialmotionof pulse),frequency,power,processingtemperature,solvent,aeration
thebubbleorasbubblecollapseintransientcavitation.Therefore, andthedesignofreactorswithpropergeometricconstructionde-
acousticcavitationisregardedasthemostimportantmechanismin terminesthelevelanddistributionofenergyintensityinthesystem,
interpretingtheeffectofultrasonicenergyonreactions. and thus influence efficiency and reliability of the results [13,39].
Manyinterestingstudieshavebeenreportedonthedescription Dependingonexperimentalobjectives,parameteroptimizationcan
and evaluation of cavitation behavior in sonochemical reactors beverydifferent.Nevertheless,theselectionofultrasonicreactors
qualitativelyandquantitatively,bothwiththeoreticalandexperi- andparameterscanbenefitfrommanystudiesonbubblecavitation
mentalmethods[29,33,34].Throughtheoreticalcalculationsatthe characteristics,includinggrowth/radialoscillation/collapse,popula-
thermodynamic and kinetic level, it is possible to estimate the tion/density,intensityandlifetime,aswellassecondaryeffectssuch
growth, size, pressure, temperature and lifetime of cavitational asshockwavesandmicro-convection.
bubbles, which are related to the energy level of bubbles [29].
Throughcomputersimulation,itispossibletoestimatethedistri- 2.2.1. Designofultrasonicreactors
bution of acoustic energy in an ultrasonic reactor with different Inthedesignofultrasonicreactors,someofthedesignparam-
geometriesandoperatingparameters[33]. eterstobeconsideredarereactortypes(e.g.,bath,probe,flatplate
Table1
Commonultrasonicreactortypes.
Reactors Briefdescription Frequencyandpower/ Advantages Disadvantages Typicalapplicationcases Scale-up
intensity
Liquidwhistle Ultrasoundgenerated 5e30kHz[13], 1)Lowcost; Lowfrequencyandpoweroutput 1)Vegetableoil Yes[41]
bymechanicaloscillation 1.5e2.5W/cm2 2)Suitableforcontinuous emulsification[44,45];
flowreactions 2)Liquidesolidmixture
homogenization[13];
3)Animalfathydrolysis[46]; J.
4)Industrialwastewater Luo
degradation[47] et
Ultrasonicbath Transduceratbottomor 15kHze>1MHz, 1)Lowcost; 1)Needadditionalmechanical 1)Cleaningofmetalpiece; Possible al.
sidepositionofbath; Normally1e2W/cm2 2)Commercialized stirring; 2)Simplechemicalreactions /P
reactionvesselfixedat 2)Dispersedandinhomogeneous rog
somepositionsinbath a3c)oLuoswticcainvtiteantsioitnyadliesftfiricbiuenticoyn.; ressin
Probereactor Directlydeliverofultrasonic Upto>100W/cm2 1)Highpoweroutput; 1)Easyerosionofhorntip[13]; 1)Intensivechemicalreactions; Possible En
energytoliquidreactant 2)Concentratedenergydeliver; 2)Concentratedandinhomogeneous 2)Stubbornsolidcrushing. erg
throughimmersiblehorn 3)Commercialized acousticintensitydistribution; y
a
3)Difficultcontrolofreaction nd
temperature; Co
4)Lowcavitationalefficiency. m
b
Cup-hornreactor Tthraencsudpucbeerinfigttedwithacup; 2109ee520740kWHaz[,48,50] 12))CAovnoicdenctornattaemdiennaetriognydoefliver; 1q)uaOnntliytysureitaacbtiloenfsorsmall Vegetableoilsaponification[49] No ustio
n
thereactor[48e50] reactantfromhorntiperosion S
Resonating Ultrasoundgenerated 20e35kHz, 1)Avoidpossible e Phenoloxidation[52] Possible[40] cien
tubularreactor byaxial Upto2kWa[48] contaminationofreactants; ce
(Sonitube) vibrationofstainless 2)Capableforhandling 41
steelresonating large-scalefeedstocks. (2
0
tubescontaining 14
flowingreactant )5
6
Reverberative Intensifiedacoustic 17e45kHz, Concentratedandintensified e Biodieselsynthesis[54] Yes e
9
flowreactor intensityby Upto3W/cm2[54] energydeliver,uniform 3
reflectionand ultrasonicfield
reverberation[54]
PolygonalReactor Transducer e Concentratedandintensified e Celluloseenzymatic Yes
arraysurrounds energydeliver,uniform hydrolysis[55,56]
thepolygonal ultrasonicfield
vesselcontaining
liquidreactant
a Valuesgivenforacousticenergyusedarepowerratings.
5
9
60 J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93
or tube), reactor geometry, transducer design and arrangement, activity in the reactor by theoretical and mapping techniques
andvolumeorscaleoffeedstocksforpracticalsystems. [34,58].
Table 1 lists and compares sonochemical reactors that are 3) Replacementofasinglehigh-outputtransducerwithanarrayof
commonlyusedinultrasound-assistedbiomassreactions[13,40e transducersthathavelowoutput[58].Byusingseveraltrans-
56].Commercializedbathandprobereactorsarecommonlyused ducers, the following are favored: i) the formation of more
inlaboratories,althoughtheyhaveaninhomogeneousdistribution uniform cavitational fields by reducing dead ends of acoustic
ofacousticfieldandlowenergyefficiency[33].Forexample,probe- streaming;ii)intensificationofreverberationandsuperposition
typereactorshavethehighestcavitationalintensitywithinadis- effects of ultrasonic energy; iii) reduction in the erosion of
tanceofonewavelength(about3e5cm)fromtheirradiatingsur- transducer tips due to the lower emission power of each
faceofthetransducerorhorntip.Asaresult,thegreatestbenefitof transducer.
physicalorchemicalprocessing suchasemulsificationisrealized 4) Superposition of ultrasonic field by changing reactor geome-
within the distance of one wavelength for probe-type reactors tries. In a reverberative flow reactor (Table 1), two emission
[34,57]. arraysmadeupofseveralultrasonictransducersare mounted
Forlarge-scaleorcommercialreactors,designsthatuseprobes on the upside and downside of a metal cuboid reactor, while
or transducers have to consider reactor geometry to be able to liquid reactant flows through the inside of the cuboid space
handle highand concentratedacoustic intensities [53,55,56].Key [40,54].Basedonthereflectionofultrasonicwaveattheinner
pointsinreactordesignandarrangementofultrasonicprobesand wallofthereactorchamberanditsreverberationinthecham-
transducersare: ber,theacousticintensityinthistypereactorwillbedoubledor
multiplied,anditwillhaveamorenonuniformcavitationfield
1) Positionofultrasonichorninasonochemicalreactor.Forahorn- thansingletransducersystems.Similarideashavebeenadopted
typeultrasonicdevice,thepositionoftheultrasonichorninthe insonochemicalpolygonalreactors[55,56,60]andinnercircular
reactionvesselinfluencesthefeatureofacousticstreamingand focusingreactors[63].
thusthedistributionofcavitationalactivities[58].Appropriate
hornpositiondependsonthereactorsize,processingscaleand A properly designed ultrasonic reactor on laboratory- or in-
eventheheightofliquidreactants[59].Conventionalultrasonic dustrialscaleprovidesauniformenergyfieldforbiomassmaterials,
hornsareverticallydippedintotheliquidsolutiontoadepthof with a suitable energy level for reactant activation. An advanced
only several centimeters, which provides a poor streaming of sonochemicalreactordesignisscalabletocontinuousflowopera-
acousticenergyinlarge-scalereactors.Acousticvelocityinthe tionandhasmatchingenergyefficiencyandprocessingbenefitof
radial direction is only 10% of the vertical velocity [58]. The thelaboratoryunit.
cavitationalactivityislimitedtoaverysmallzoneneartheul-
trasonichornthatisonly10%fora2.5Lreactorasanexample 2.2.2. Ultrasonicfrequency
[34].Kumaretal.[34,60]proposedthehorntobehorizontally Ultrasonic frequencyinfluencesthebehaviorofbubblecavita-
immersed into the sonochemical reactor and longitudinally tionthroughthechangeinthedurationtimeofacousticcycle[40],
vibrated,withtheimmersionlengthofthehorntobe82%ofthe whichisshortatrelativelyhighfrequencies(e.g.,2msfor500kHz)
horizontallengthofthereactionvesselbasedona7Lvolume. andlongforlowfrequencies(e.g.,50msfor20kHz).Highfrequency
Thismodificationchangestheacousticstreamingflowcharac- doesnotfavortheoccurrenceofactivecavitationasthetimeforthe
teristicsinthereactor,anditgivesgoodpowerdissipationover growth,radialmotionandcollapseofbubblesmaybeinsufficient.It
the reactor volume, and thus generates a nearly uniform cav- alsoleadstorapiddecayofacousticenergyintheliquidmedium.
itationalactivitycomparedtoconventionaldesigns. However,theinfluenceofultrasonicfrequencyonpowerdissipa-
2) Surfaceandshapeofthetipofthetransducerorhorn.Largetip tionlevelandenergyefficiencyincavitationreactorsseemstohave
diameters of the ultrasonic horn give low ultrasonic energy anumberoffactors.Somefreeradicalreactionsarereportedtobe
densities due to the large emitter surfaces, however, rapid acceleratedathighultrasonicfrequenciesofseveralhundredkHz
deactivationofa-amylasewasobservedforlargetipdiameters [64]. The relatively high frequencies shorten the duration of an
from1 to7 mm [61]. Kadkhodaee and Povey [61] interpreted ultrasoniccycle,whichisaccompaniedbyincreaseinthenumber
this unusual phenomenon occurred due to three reasons: i) of useful cavitation events in per unit of time [65]. The formed
reaction between (cid:3)OH radicals and enzymes not taking place short-life-timeradicalsmayhaveahigherprobabilitytoescapethe
insideofthebubbles;ii)highdensitiesofcavitationalbubbles cavitationalbubblesbeforebeingquenched[40].Ashokkumar[29]
around 1 mm sonotrode tip causing serious acoustic attenua- performed methyl radical recombination experiments in Ar-
tion; iii) high energy in individual cavitational bubbles being saturatedalcoholsolutionsatthreedifferentfrequencies(20,363
released when bubbles collapse, even though the delivered and 1056 kHz). The volume change of bubbles generated during
energy density was low. However, the distribution of acoustic collapseatrelativehighfrequencywasmuchsmallerthanthatin
fields is changed byenlarging of tip diameters. In the experi- the low-frequency ultrasonic field, which means that much less
ment with alterable tip diameters of the ultrasonic horn, the energy was released at lower collapse temperature. Surprisingly,
reactorgeometrywasflat-bottomedglasstubewithfixedinner the determined temperature of cavitational bubbles inwater fol-
diameter of 16 mm, with the rating ultrasonic power density lowedthe orderof1056 kHz(4730e5930(cid:2)C) >363 kHz(3930e
keeping at about 24.4 W/mL [61]. With an increase of tip 4430(cid:2)C)>20kHz(3430e3930(cid:2)C).Thiscanbeexplainedbythe
diameterfrom1to7mm,theproportionalscaleofthesurface waterevaporationthatwouldoccurinoneacousticcycle.Longer
areaofthehorntiptothecrosssectionalareaoftheglassinner durationandlargerbubblesizeexistingduringtheradialmotionof
tubeincreasesfrom0.4%to19.1%.Widedistributionofacoustic bubbles in the low-frequency ultrasonic field means that more
fieldsisgivenbylargeemittersurfacesthusthisinfluencesthe liquid molecules enter into high-energy bubbles, and then they
largebulkofenzymesolutions,andispossiblyoneoftherea- vaporize,whichsubstantiallylowerstheactualenergylevelwithin
sons for the unusual behavior of a-amylase under ultrasonic the bubbles [29]. Another report demonstrates that the rate of
irradiation [62]. Therefore, rational design of scaled-up ultra- energyreleaseofsinglebubblesandthecavitationalefficiencywas
sonic reactors requires the quantitative prediction of acoustic higherwithultrasoundathigherfrequenciesbyseveraltimes[62].
streaming, power dissipation, mass transfer and cavitational Therefore, there are optimal values for frequency choice in the
J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93 61
ultrasonic operation of specific reactions. For the oxidation of from 77.1 to 79.8 ppm and from 0.96e1.90 to 3.55e6.94 ppm,
ferrousion,KIandwater,thevalueswere130kHz,200e300kHz respectively[69].Themostactivezoneforsonochemicalreactions
and 358 kHz, respectively [33]. However, ultrasonic frequencies isnormallyregardedtobetheinnerofcavitationalbubblesandthe
higher than 200 kHz should be used with caution as they have thin liquid shell that surrounds the bubbles. As a result, the
adverseeffectsonenergybenefitoftheprocessandonthelifetime degradationefficiencyofphenolsincreasedtothree-fold.
ofultrasonictransducers. Theadditionofsurfactantsreducesthedissolutionandcoales-
Comparedwithsingle-frequencyultrasound,ultrasonicreactors cence of bubbles in multibubble systems [29,70]. The sizes of
withmultiple-frequencydesignscannotablyreduceenergylossin cavitationbubblesin1mMsodiumdodecylsulfatesolutionwere
the scattering, reflection and absorption of the acoustic waves. significantlyreducedcomparedwiththoseinwater[29].
Cavitation created withmultiple-frequency designsbenefitsfrom Thephysicalpropertiesoftheliquidmediuminfluencethede-
thelowcavitationthresholdoflow-frequencyultrasoundandthe livery and dissipation of acoustic power to the solution. Detri-
high collapse rate of high-frequency ultrasound [66]. Dual- mentalattenuationofacousticenergyoccursinviscousmediums
(25kHzþ40kHz)andtriple-frequency(20kHzþ30kHzþ50kHz) such as in plant oil [40], which decreases the process benefit of
flow cells with bath-type and horn-type devices have been re- ultrasonicoperation.
ported to have higher cavitational yields (iodine method) of Physicalpropertiesofhomogeneoussolventshavemanyeffects
7.06(cid:5)10(cid:4)7g/Jand5.67(cid:5)10(cid:4)6g/Jandhigherenergyefficienciesof on ultrasonic cavitation. Low viscosity, relative low volatility and
55%and76%,respectively,muchhigherthansingle-frequencyde- relative high surface tension of liquid solvents are regarded as
vice[39,67]. favorable for achieving efficient cavitation. Frequently, water is
used as solvent in most ultrasonic operations as it has a higher
2.2.3. Propertiesofhomogeneoussolvents cavitationalintensitythancommonorganicliquids[28].
Thephysicalandchemicalpropertiesofsolventsgreatlyinflu-
enceacousticpropagationandcavitation.Thephysicalproperties 2.2.4. Heterogeneouspropertiesofultrasound-assistedreactions
includetheviscosity[68],volatility[69],surfacetensionofsingle Active cavitation in ultrasound-assisted reactions is generally
and multicomponent solvents, while chemical properties include preferred to occur at the heterogeneous sites in liquids. These
the composition in completely miscible binary/ternary organic heterogeneoussitescanbeimpurities,freegasmicrobubblesinthe
mixtures, electrolyte solutions, or surfactant solutions, as well as liquid,newnucleiduetocollapseandfragmentationofbubblesin
thechemicalactivitiesassociatedwithradicalgeneration. the previous acoustic cycle, and crevices at the heterogeneous
Theformationofcavitiesinthebulkliquidphaseunderultra- interfacesuchasreactorwallornon-volatileadditives[28,71].In
sonicconditionsmeanstheinevitableweakeningandbreakdown theacousticemissionspectrumofpurewaterwithoutanyimpu-
ofintermolecularinteractions.Inthebulkliquid,theseinteractions rities or gas microbubbles, only one peak, corresponding to the
arereflectedinthephysicalpropertiesofsolventsthroughchanges fundamentalfrequencyoftheultrasoundisseen,whichmeansthat
in viscosity, vapor pressure, boiling point and surface tension. A nocavitationoccursforpurewater[32].Onthecontrary,forwater
solventthathashighsurface tensionandhighviscosity probably containingacertainconcentrationofgasnuclei (microbubblesin
hasanincreasedenergybarrierfortheformationofcavities,which thereactorcrevices),strongsubharmonic,ultraharmonicandhar-
isdetrimentalforcavitation. monicemissionsappearedintheacousticemissionspectrum.
Inthegrowthandoscillatingstageofcavitationalbubbles,the The existence of dissolved gas affects the behavior of bubbles
surfacetensionandvolatilityaffectthesizeandchemicalcontentof andthusthecavitationintensityinsonochemicalreactors.For(cid:3)OH
thebubbles.Relativehighsurfacetensionofsolventsleadstostable radicalreactions,theorderoftherateconstantof(cid:3)OHradicalfor-
bubble sizes that tend not to dissolve in the bulk liquid. High mation(mmol/(Lmin)atanultrasoundfrequencyof513kHzunder
volatilityofliquidallowsliquidmoleculestoflowintointensified different gas atmospheres was Kr (3.17) > Ar (2.90) > O2
bubbles and evaporate at negative pressure (rectified diffusion), (2.68) >He(0.58)[64].GaseousHehadthelowestrateconstant
which reduces the force of bubble collapse. Similar buffering of thatcouldbeattributedtoitshighthermalconductivitycompared
cavitational intensity occurs when chemical reactions involving with other gases [72]. When the dissolved gas has high thermal
volatilereactantsorintermediatestakeplacewithinthebubbles. conductivity,moreenergyinthecavitationalbubblesistransferred
Theadditionofasecondhomogeneouscomponentinamixture tothesurroundingsolution,whileconcurrentlylessenergyisob-
can have a remarkable influence on the cavitation behavior tained as the bubble collapse. The solubility of gas in water
[29,34,70], because it affects the volatility, surface tension and (Kr[ArzO2[He)isalsoafactorintheresults[72].Lowgas
viscosityofthebulksolution.Asecondhomogeneouscomponent solubility inwater does not provide enough nuclei for ultrasonic
changesboththeacousticstreamingandpowerdissipationinthe cavitation, while high gas solubility in water might weaken the
solution. The addition of 10% (w/v) electrolyte NaCl in water cavitationeffectsastheformedcavitationalbubblesmayredissolve
increasedthepowerdissipationby14.7%and57.9%,andcontrib- beforetheircollapse[40].
utedtoanincreaseincavitationalproductionby11.6%and60.8%at Forspecificbiologicalorchemicalprocessingsuchasemulsifica-
frequenciesof694and204kHz,respectively[34].Theadditionof tion[73],microbialcelldisruption[74],enzymeactivation[61]and
10% (w/v) NaCl also changes nucleation, growth and collapse of radicalformation[64],appropriateamountsofdissolvedgasprovide
eachbubble.Theexpandingratioofcavitationalbubblesizeduring nucleifortheproductionofcavitationalbubbles,andfavorultrasonic
itsgrowthincreasedfrom6.2to8.8atfrequenciesof204kHz,with cavitation.Foremulsification,nocleareffectofgascontentisobserved
thecollapsetimeofbubblesbeingprolongedfrom3.43to3.84ms onthesizeofemulsiondropletsforaconstantenergydensity[73].
[34].Theintroductionofanon-volatilesaltchangesthevolatility However, for processing of microbial cells, the rate constant of a-
and partitioning behavior of solvents. For the sonochemical amylasedeactivationwasrelativelylowerinpre-degassedwater[61].
degradationofvolatilechlorobenzeneandnon-volatilephenol,the Bubbling with CO2 or (CO2 þ H2) during sonication enhanced the
introductionof4%(w/v)NaClenrichedtheorganicsattheinterface releaseofpolysaccharidesfromPorphyridiumsp.cells[75].
betweenthebubbleandbulkliquid,withthepartitioncoefficient The presence of foreign additives, including volatile and non-
ofchlorobenzeneandphenolincreasedfrom29.2to48.0andfrom volatile substances, not only lowers the very high-energy
3.3 to11.7,respectively[69]. The molarconcentrations of chloro- threshold for cavitation in the pure solvents, but also seems to
benzene and phenol vapors entrapped in the bubbles increased induce chemical effects in acoustic cavitation through the
62 J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93
promotion or inhibition of radical formation. Katekhaye and actualvolumeofliquidreactantinthereactionvesselforprobe-type
Gogate[71]determinedtheeffectofsomeadditivesoncavitational reactors,whileitisthevolumeofthewaterbathinthereactorfor
yield with a horn-type reactor (20 kHz, power dissipation of sonication,asthesurroundingliquidinthetankisthefirstreceptorof
0.098 W/mL by calorimetric measurement) using the iodine ultrasonicirradiation.
method.Theyusedsolidparticles(CuO,TiO2),salts(NaCl,NaNO2) Factors that influence the energy level of cavitation and the
and radical promoters (H2O2, FeSO4, metal iron, CCl4 and tert- energyreleasedduringcavitationtothereactantsarerelatedtothe
butanol).Alladditivesshowedapositiveinfluenceoncavitational size change in bubble radial motion, the lifetime of bubbles, the
yieldwiththeimprovementbeing40e240%,whiletheadditionof collapse time of bubbles and the chemical compositions in the
radicalpromoterssuchastert-butanol,CCl4andFeSO4showedthe bubble. While higher acoustic intensity and lower threshold for
bestresults.Withpropercombinationofseveraladditives(suchas cavitation may result in more population of active bubbles. For
TiO2 þ H2O2), synergistic effects can be seen with much higher differentintensificationmechanism,theinfluenceoffactorsmaybe
cavitationalyieldsthanthoseforusingindividualadditives.How- different. For shockwaves and micro-convection induced intensi-
ever,forsomeadditivessuchasTiO2,H2O2andtert-butanol,high fication, the intensity of shockwaves and micro-convection is
concentrationscanbedetrimentaltocavitationalyield.Thismight related to the intensity and rate of energy release during bubble
be interpreted as the enhancement of the premature rupture of collapse,andthusisdependedontheviolenceoftheextendingand
cavitational bubbles or the recombination offreeradicals at high shrinking of bubble size, the evaporation of bubble contents, the
concentrations of these additives, while the detailed mechanism adiabaticpropertyofliquidandthetimeforbubblescollapse.For
needsfurtherinvestigation. chemicalradicalinducedintensification,itisimportanttoensure
As highlightedin Section 2.1, the level and efficiency forcavi- sufficientvolatilesubstratestrappinginthebubblesorstayingat
tationenergydependsnotonlyonthetransformationofacoustic the thin liquid shell that was veryclose to the bubbles. It is also
energytocavitationenergy,butalsoisrelatedtotheattenuation importanttoshortenthetimeforbubblescollapseandenhancethe
and energy absorption of the acoustic wave propagating in the escapeofradicalsfromthebubbles.Fornon-volatilereactants,the
system.Forinstance,highdensityofbubblecloudsmaybegener- absorptionofcavitationenergybyviscousliquidorheterogeneous
atedatthezoneswithhighultrasoundintensities,suchasthere- systemshouldbealsocarefullyconsidered.Therefore,theoptimi-
gionnearthetipofthetransducer.Thebubblecloudscouldabsorb zation of ultrasonic parameters should consider the specific re-
orscatteracousticwaves,andresultininefficientenergydelivery actionsandtherealintensificationmechanism.
withinthereactor[61].Cavitationenergyisalsoabsorbedbysol-
ventsandheterogeneoussystem.Theamplitudeofpressurepulse
measuredat 1mm fromthebubblecenterwasonlyone-in-one- 3. Ultrasonicpretreatmentandreactionsoflignocellulosic
thousands of its actualvalue [35]. Similarattenuation of acoustic biomass
energyandcavitationenergymightbeseeninslurriesorsuspen-
sions such as insoluble biomass particles, humin products and Lignocellulosic biomass is one of the most abundant bio-
nano-scalecatalysts.Therefore,additional care must betaken for resourcesintheworld,withanannualworldgrowthof170e200
treatingthesetypesystems[58].Therearetwoconflictingaspects billion tons [2]. The structure of lignocelluloses is complex and
regardingthepresenceofimpurities:(i)promotionofcavitationby recalcitrant,withcellulose(40e50%),hemicellulose(10e20%)and
impuritiesand(ii)acousticattenuationbythescatteringofacoustic lignin (20e30%) as the three most abundant components. The
energywhentheacousticwavebypassestheimpurities.Asaresult, primary requirements for the thermochemical and biochemical
theexistingandconcentrationofheterogeneousimpuritiesshould conversionoflignocellulosesare:
haveareasonablevalue,whichdependsonthesizeandproperties
of impurities. However, qualitative and quantitative information 1) Pre-fractionationofrawlignocellulosesbeforereactions.Lignin
aboutacousticattenuationinheterogeneoussystemsisstilllacking and hemicellulose should be firstly removed from cellulosic
andneedtobestudiedinfuture. materials,astherawlignocellulosiccomplexhinderstheaccess
ofenzymemoleculesandchemicalcatalysts.Thecriticalissueis
2.2.5. Otherparameters howtoimprovetheefficiencyandeconomicsinthedestruction
Foracousticcavitation,otherfactorsrelatedtothesonochemical oflignocellulosicstructureandtheseparationoftheconnected
environmentincludethetemperatureofmedium,thestaticpressure components at mild condition. The reduction in the reaction
ofthefluid[73,76]andthelevelofpowerdissipationinthesolution. severity also means less production of by-products (such as
Thedetailscanbeseeninthefollowingsectionsandotheravailable phenols,furfuralsandorganicacids)aswellaslessalterationof
literature [28,33,40]. However, the selection and optimization of theglucosidicstructureofcellulose.
theseparametersconsidersnotonlyacousticcavitation,butalsothe 2) Opening of the crystalline structure of raw cellulose at mild
requirementsforrawmaterialsandprocessing. conditions.Comparedwithligninandhemicellulose,theregu-
Inmuchofthecontentpresented,powerratingsofacousticlevels larity in the chemical and crystalline structure is the most
aregiventodescribethesystemsusedinthestudies.Theratingvalues prominent characteristic of cellulose. Hundreds to tens of
mightbeviewedasaspecificationofsonochemicalreactors.How- thousandsofD-glucopyranosesconstitutethemolecularchains
ever,itshouldbenotedthatthesepowerratingsarenotpractical throughb(1/4)glucosidiclinkageswithunifiedorientation,
powerlevels,sincetherearelossesintheenergytransformation.The whilemostchainsparticipateintherecalcitrantcrystallinere-
actualpowerlevelinthereactantsolutionsisnormally10e40%ofthe gion of raw cellulose by intra and intermolecular hydrogen
ratedvalues[71,77e80].Actualpowerlevelscanbedeterminedwith bonds.Thismakescellulosestronglyhydrophobic,anddifficult
calorimetricmeasurements.Forthesecases,thedeterminedvalues to be dissolved or degraded into common solvents. In the
arespecificallynotedas“bycalorimetricmethod”inthisreview.The degradationdynamicsofcellulose,theloweringanddestruction
powerlevelsforultrasonicprocessingarelargelyrelatedtothescale of its crystalline structure requires extremely high activation
orvolumeofprocessing.Therefore,thevaluesofmeanpowerdensity energy.Forexample,theenergyrequiredforremovingaglucan
(the ratio of acoustic power to the volume of liquid reactant) are fromcrystalline cellulose inroomtemperaturewateriscalcu-
preferentiallyconsideredandlistedinthisreviewasasubstitutefor lated to be about 8.1 kJ/mol [81], which influences the
rawpowervalues.The volume ofliquidreactantisdefinedasthe saccharificationefficiencyandhydrolysisselectivity.
J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93 63
3) Intensification of mass transfer in heterogeneous reactions Cellulosic materials are crushed to particles or crystalline grains
containing solid lignocelluloses. The surface of lignocelluloses havingmicro-orevennano-sizerangesafterbeingsubjectedtoa
needstobeincreasedthroughpretreatmentforbettercontact mean rating power of 3.0e10.0 W/mL [88,89]. Sonotreatment of
withsolventsforreactions. chemically-separated cellulose inwaterat 6.67 W/mL for 30min
gave nano-sized fibers with diameters of lower than 20 nm
To meet these requirements, pretreatment and reactions of [88,90,91]. Pinjari and Pandit [86] reported their work on the
lignocellulosesshouldbeperformedinsurroundingsthatprovide millingofnaturalcellulosetonanofibrilsusinghydrodynamicand
sufficient reaction intensity such as high-energy input to destroy ultrasonic cavitation in succession. Sonication at 3.0 W/mL for
lignocelluloses, but remain at relatively mild conditions and low 110minreducedthesizeofthecelluloseparticlesfrom1360nm
energyintensitiessuchasroomtemperature,shorttreatmenttime after hydrodynamic cavitation to 301 nm. High-intensity ultra-
andneutralpHvaluesofthesolutionforthepurposesofatomef- soundalsoincreasesthesurfaceareaofsolidbiomassbycavitation
ficiency. It seems to be contradictory, however, ultrasonic pre- erosion [56,92]. As a result, size reduction and erosion of ligno-
treatment can address these requirements, because ultrasonic cellulose particles benefits the extraction of chemicals from
treatment features high local strength with its intense cavitation biomassandthesaccharificationofcellulosicmaterials[85,87].
and overall moderate effects. However, much progress has been Applicationofultrasonicenergyaidsinthedissolutionorsol-
reported on ultrasound-assisted lignocellulose fractionation, vationofcellulose,hemicelluloseandlignininorganicsolventsand
component solvation, thermochemical/biochemical conversion ionic liquids. Ultrasound accelerates the dissolution of cellulose.
especiallyforsaccharificationandfermentation.Progressonthese Withultrasoundpre-irradiationfor20minandsubsequentheating
topicswillbereviewedinthissection. withoutultrasoundforanother60minat110(cid:2)C,thedissolutionof
cellulose in ionic liquid 1-butyl-3-methylimidazolium chloride
3.1. Destructionandfractionationoflignocellulose ([C4mim]Cl) was complete, as compared with 190 min for con-
ventionalheatingonly[93].Dissolutionorextractionofligninfrom
Ultrasonic energyapplied as pretreatment has special mecha- bagasseorbamboopowdersin1,4-butanediol/watermixtures[94]
nistic effects on the structural integrityoflignocelluloses (Fig.1). or95%ethanol[95]ismoreeffectivewithultrasonicenergythan
Ultrasonic treatment can destroy wax layers and silica bodies conventionalagitation.
depositedontothesurfaceoflignocellulosicstructuresandcanaid Sonication has been used to achieve favorable results in the
in their removal [82,83]. The size of biomass particles can be fractionationofrawlignocelluloses[14,96]andextractionofcom-
reduced when subjected to high power ultrasound [84e87]. pounds in raw biomasses [16] such as polysaccharides, phenolic
Fig.1. Mechanisticeffectsofultrasonicenergy(US)inlignocellulosepretreatmentandconversion[14,16,56,82e84,86,88,100,102](a:ReprintedwithpermissionfromRef.[56],
Copyright(cid:1)2011Springer.)
64 J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93
Table2
Ultrasound-assistedpretreatmentandreactionsofstarch,celluloseandlignocellulose.
Materials Ultrasonicprocessandparametersa Consequentprocessandparameters Keyfindings Ref.
Sugary-2maize Pretreatment: Starchhydrolysis: Starchconversionimprovedfrom13.7% [116]
Probe,20kHz,4.8e8.3W/mL, 32(cid:2)C3h, to39.2e56.8%.
5e40s, Stargen(cid:3)001
Concentrationof3g/32mL
Cornmeal Pretreatment: Starchliquefaction: 1) Glucoseyieldincreasedfrom29.6wt.% [115]
Sonicator,40kHz,about1.58W/mL, 85(cid:2)C,1h, to31.6wt.%afterliquefaction.
60/80(cid:2)C,3e10min, Solid/waterweightratioof1/3 2) Bio-ethanolconcentrationincreasedfrom
Sampleconcentrationof25wt.% TermamylSC,pH6.0 0.45g/gstarchto0.50g/gafterfermentation.
Simultaneoushydrolysisand
fermentation(SSF):
30(cid:2)C,32h,
Saccharomycescerevisiaevar.
ellipsoideus,SANExtraL,pH5.0
Microcrystalline Pretreatment: Microwave-assistedhydrolysis 1) Sizeofcelluloseparticlesreducedfrom [87]
cellulose Probe,20kHz,1.5W/mL, oversolidcatalyst: 38mmtoabout0.3mm.
80(cid:2)C,3e9h, Microwavepowerof350W, 2) Glucoseyieldpromotedfrom3.8e4.9%
Solid/liquidratioof5g/200mL 150(cid:2)C,1h, to11e21%,comparablewithballmillingor
Solid/waterratioof0.5g/5mL, ionicliquidpretreatment.
Sulfonatedcarbon 3) Highselectivityof64e71%forhydrolysis,
withoutby-productsuchasfurfurals.
4) Muchshortertimeof<1hforcellulose
hydrolysis.
Triticalemeal Pretreatment: Starchliquefaction: 1) Glucoseandmaltoseyieldsincreasedby [123]
Bath,40kHz,about0.05W/mL, 60(cid:2)C,65min 12.3e15.7%and46.7e52.6%afterliquefaction.
40e60(cid:2)C,5min, SSF: 2) SSFtimereducedfrom72hto<48h,
Sampleconcentrationof25wt.% 30(cid:2)C,72h, withbio-ethanolcontentincreasedby8.2e10.9%.
Saccharomycescerevisiaeyeast
Cassavachip Pretreatment: Simultaneousliquefaction-SSF: 1) Ethanolyieldincreasedfrom31.4%to [118]
Probe,20kHz,8.5W/mL, 32(cid:2)C,72h, 43.1%aftersonicationat8.5W/mLfor30s.
10e30s, Stargen(cid:3)001þSaccharomyces 2) Fermentationtimereducedfrom72hto36h.
Slurrieswith5%totalsolids. erevisiae(ATCC24859)
Oilpalmfruit Pretreatment: Hemicellulosehydrolysis: Xyloseyieldincreasedfrom22%to52%. [83]
bunch Probe,20kHz,about2e3W/mL, 100e140(cid:2)C,<200min,
25(cid:2)C,15e45min, Solid/liquidratioof1:25,
Solid/liquidratioof50g/500mL, 2%H2SO4
2%H2SO4
Switchgrass Pretreatment: Celluloseenzymatichydrolysis: Reducingsugaryieldincreasedfrom8wt.% [56]
Hexagonalreactor,20kHz, 50(cid:2)C,5h, ofcontrolgroupto9.75wt.%.
50(cid:2)C,5h, Solid/0.1MCH3COONabuffer
Solid/0.1MCH3COONa of10g/250mL
bufferratioof10g/250mL Accellerase1500(cid:4)
Ricehull Pretreatment: Cellulosefungalpretreatment: Netyieldsoftotalsolublesugarandglucose [121]
Bath,40kHz,about0.025W/mL, 28(cid:2)C,18days, were31.8%and32.2%comparedwithcontrol
25(cid:2)C,30min, Solid/liquidratioof35/65, groupvaluesof6.9%and7.6%(solefungal
Solidloadingof15%(w/v) P.ostreatus pretreatmentfor18d).
Enzymehydrolysis:
45(cid:2)C,48h,
Enzyme,pH4.8
Arecanuthusk Limepretreatment: SSF: 1) Delignificationratiosincreasedfrom50e54% [77]
(Arecacatechu), Probe,30kHz,100W 35(cid:2)C,72h, to64e68%,withtimereducedfrom>8
bonbogori (36W,calorimetry), Biomassloadingof10wt.%, weeksto1.5e3h.
(Ziziphus 35(cid:2)C,1e3h, Cellulase(Accellerase1500)þ 2) Ethanolyields,73.7%forarecanuthusk,
rugosa),moj Biomassloadingof3.2e9.0wt.%, Saccharomycescerevisiae,pH4.5 83.1%forbonbogoriand85.2%formoj.
(Albizialucida) Lime,0.5wt.%ofbiomass.
Kenafcorefiber Ionicliquidpretreatment: Celluloseenzymatichydrolysis: Saccharificationratioincreasedfrom47%to86%. [125]
Probe,24kHz,about8W/mL, 50(cid:2)C,48h,
25(cid:2)C,15e30min, Cellulase(fromTrichoderma
1-Ethyl-3-methylimidazolium viride),pH5.0
acetate(5g/0.25gmaterial)
Microcrystalline Ionicliquidpretreatment: Celluloseenzymatichydrolysis: Celluloseconversionimprovedfrom75.6%to [126]
cellulose Bath,45kHz,about0.01W/mL, 50(cid:2)C,24h, 95.5%withultrasound.
60(cid:2)C,30min, [Mmim][DMP]concentrationof20%,
Alkylimidazolium Cellulase(fromTrichoderma
dimethylphosphateionic reessei),pH4.8
liquid([Mmim][DMP])
Microcrystalline Ionicliquidpretreatment: Materialregenerationinwater Saccharificationratioimprovedto92.4% [127]
cellulose, Probe,24kHz,1.7W/mL, Enzymatichydrolysisof from55.0%withionicliquidpretreatmentonly.
bamboo 25(cid:2)C,60min, recoveredmaterial:
Cholineacetate 50(cid:2)C,48h,
Cellulase(fromTrichodermaviride),pH5.0
Kenafcorefiber Sonocatalytic-Fenton Celluloseenzymatichydrolysis: Saccharificationratiopromotedto22.4e24.3%of [124]
pretreatment: 50(cid:2)C,48h, sonocatalytic-Fentontreatmentfrom9.8to11.0%of
Probe,24kHz,0.7W/mL, Kenafpowderloadingof2%(w/v), controlgroup,13.0to14.0%of
25(cid:2)C,3e6h, Cellulase(fromTrichodermaviride),pH4.0 sonocatalytic
J.Luoetal./ProgressinEnergyandCombustionScience41(2014)56e93 65
Table2(continued)
Materials Ultrasonicprocessandparametersa Consequentprocessandparameters Keyfindings Ref.
TiO2amountof2g/L, treatmentand14.9to15.3%of
Fentonreagent(H2O2, Fentontreatment.
100mM,FeSO4,1mM)
Carboxymethyl Pretreatment: Enzymehydrolysis: 1) Maximumreactionrateover [120]
Cellulose(CMC) Probe,20kHz,0.72e1.35W/mL, 50(cid:2)C,CMCconcentrationof10g/L, mixedcellulase
50(cid:2)C,20e60min MixedcellulasesfromTrichoderma (70wt.%Trichoderma
pH4.8 virideandAspergillusniger,pH4.8 virideþ30wt.%Aspergillus
niger.)increasesfrom
6.94(cid:5)10(cid:4)7to9.86(cid:5)10(cid:4)7
mol/(Ls)withultrasound.
2) Maximumreactionrate
overAspergillusniger
didnotincreasewithultrasound.
Waxyricestarch Starchhydrolysis: e Number-averagemolecular [117]
Homogenizer,20kHz, weightincreased
upto0.12W/mL, from1.3(cid:5)10(cid:4)5to0.2(cid:5)10(cid:4)5.
60(cid:2)C,24h,
Concentrationof1%
Maizestarch Starchacidhydrolysis: e Glucoseyieldof32e97wt.%,22e100%higher [128]
Probe,25kHz,about4W/mL, thancontrolgroup.
90e100(cid:2)C,0.5e2h,
H2SO4(3e5wt.%)
Glucose Glucosefermentation: e Reactionrateconstant [129]
Bath,continuous,40kHz, increasedby1.3e1.5times
about0.027W/mL, comparedwithstirring.
20or30(cid:2)C,4e20h,
Glucose(20e40wt.%),
Saccharomycescerevisiae
Sugarcane Alkalinepretreatment: Hydrolyzatefermentation: 1) Celluloseandhemicellulose [100]
bagasse(SCB) Probe,24kHz,2.0W/mL, 30(cid:2)C,60h, recoveriesof99%and
50(cid:2)C,20min, Saccharomycescerevisiae,initialpHof5.0. 79%,ligninremovalof75%after
Solid/liquidratioof1g/20mL, alkalinepretreatment.
2%NaOH 2) Maximumethanolyieldof0.47g/gglucose,
Acidhydrolysis: 0.17g/gofpretreatedSCB.
Pulse,cyclecontrol(50%cycle),
50(cid:2)C,30e45min,
Solid/liquidratioof1g/20mL,
2%H2SO4
Newspaper, Celluloseenzymatichydrolysis: e Apparentrateconstantsincreasefrom11e13to [78]
officepaper, Probe,0.8e6.4Lreactor, 16e24h(cid:4)1for0.8Lreactor,from10e14to
pulp 20kHz,0.005e0.075 15e19h(cid:4)1for3.2Lreactor,from8.5e14to
W/mL(calorimetry), 14e18h(cid:4)1for6.4Lreactor.
45(cid:2)C,48h,
Substrateconcentrationof
7.5e25g/L,
Cellulase(fromTrichoderma
viride),pH4.8
Sugarcane Alkalinepretreatment: Hydrolyzatefermentation: 1) Afteralkalinepretreatment,80.8% [102]
bagasse Probe,continuous,24kHz, 30(cid:2)C48h, ofhemicellulose
upto2.0W/mL, Zymomonasmobilis(MTCC89),pH5.7. and90.6%ofligninremoved.
40(cid:2)C,30min, 2) Glucoseyieldafterenzymatic
Solid/liquidratioof1g/20mL, hydrolysisincreased
2%NaOH from34.5wt.%ofcontrolto57.6wt.%.
Enzymatichydrolysis: 3) Ethanolyieldincreasedfrom43.7g/g
Probe,interval,24kHz, glucoseto46.6g/g.
upto2.0W/mL,
40(cid:2)C,6h(ultrasonicfor3h),
Solid/liquidratioof1g/15mL,
Cellulomonasflavigena
(MTCC7450),pH6.0
Avicelcellulose, Pretreatment: Hydrolyzatefermentationto Conversionsofavicelcelluloseandsugarcane [132]
sugarcane Bath,45kHz,about0.01W/mL, biodiesel: bagasseathydrolysistimeof12h,
bagasse 80(cid:2)C,72h, NMMO,20%(w/v) 1) Untreatedsamples,28%and16%.
Substrateconcentration 30h, 2) NMMO-treatedsamples,72%and72%.
of3%(w/v), R.opacusACCC41043 3) NMMO-ultrasound-treatedsamples,96%and91%.
N-methylmorpholine-
N-oxide(NMMO)
Celluloseenzymatichydrolysis:
Bath,45kHz,about0.01W/mL,
50(cid:2)C,12e48h,
Substrateconcentration
of0.6%(w/v),
b-glucosidase(fromAspergillus
niger),pH4.8
a Powerlevelsadditionallymarkedwithword“calorimetry”isactuallydeterminedusingcalorimetrymethod,whileotherswithoutlabelsarethepowerratingofthedevice.