Gas mixing in anaerobic digestion

282 Pages·2016·18.27 MB·English

by  Davide Dapelo

Gas mixing in anaerobic digestion
Davide Dapelo
First supervisor: Prof J. Bridgeman
Second supervisor: Prof M. Sterling
June, 2016
University of Birmingham Research Archive

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“Merelyseeingeachother’sfacewouldinitselfbeinsignificant. Itistheheart
thatisimportant. SomedayletusmeetatEaglePeak,whereShakyamuni
Buddhadwells. Nam-myoho-renge-kyo,Nam-myoho-renge-kyo.”
NichirenDaishonin(1278).
TheDrumatTheGateofThunder
Tomymentor,DaisakuIkeda
andtoallmycomradesinfaith
iii
iv
Abstract
Mesophilicanaerobicdigestionisoneofthemostusedandsuccessfultechnologiesto
treatthesludgesresultingfromwastewatertreatment. However,traditionalapproachesto
digester design are firmly rooted inempiricismand rule ofthumb ratherthan science. In
particular, whilst mixing has been recognised to be a key element for the success of the
digestionprocess,thedegreeofmixingnecessaryforastableprocess,aswellasaclear
pictureofitseffectonbiogasproductionarestillunclear. Inparticular,theliteratureongas
mixingisstillparticularlypoor. Mixingisanenergy-intensiveoperation,andthereforethe
needtolowerthewastewaterprocesscarbonfootprintrequiressearchinghowtolowerthe
inputmixingenergywithoutcompromising—andindeedenhancing—biogasproduction.
Computational Fluid Dynamics was used to produce recommendations on how to
enhancemixinginthespecificcaseofgas-mixedanaerobicdigesters. Forthefirsttime,an
Euler-Lagrangian multiphase model was developed to simulate gas mixing in anaerobic
digestion. ThemodelwasvalidatedagainstlaboratoryexperimentswiththeParticleImage
Velocimetry and the Positron Emission Particle Tracking techniques, and gave predictions
inagreementwiththeexperimentalresults.
Full-scalesimulationsreproducingarealdigesterwereperformedwiththevalidated
model,andscenarioswithdifferentmixinginputpower,bubblesizeandsludgerheologies
werereproduced. Inallthescenarios,itwaspossibletoshowthatinputmixingpowercan
beloweredbythesame,significantamount,whilemaintainingthedegreeofmixingunal-
tered. Thesimulationsalsoshowedtheformationoflow-viscosityflowpatternslocalized
inthezoneswheretheliquidphasevelocitymagnitudewashigher. Thisphenomenonis
intrinsically linked to the non-Newtonian nature of the sludge, and is problematic as it
v
leadstoshort-circuitedmixing.
Finally, recommendations on how to mitigate the issue of the low-viscosity flow
patterns were given. Explanations on how to understand the mixing in laboratory and
full-scalevessels,inparticulartherespectiverolesofdiffusion,advectionandturbulence,
weretraced,aswellasonhowtodevelopamorecomprehensivecriterionfordefiningand
assessinga satisfactorydegreeof mixing. Recommendations forfurtherworks comprise
experimentalresearch onbiogasbubble sizeinsidea gas-mixedindustrialdigester, further
validation conducted on pilot-scale vessels, computational simulations on different geome-
tries,couplingcomputationalfluiddynamicswithabiochemicalmodel,andresearchon
the effectof differentsources ofnutrients uniformization(e.g., turbulenceor diffusion)on
biogasproduction.
Ajournalpaper publishedinWater Researchandaconferencepaper presentedatthe
Fifteenth International Conference on Civil, Structural and Environmental Engineering
Computing(Civil-Comp)wereproducedasaresultofthiswork. Theyarebothreported
inAppendixF.Twootherpapersarecurrentlyinpreparation.
vi
Acknowledgements
Iwouldliketothankmyfirstsupervisor,ProfessorJohnBridgeman. Hissupporthas
not been limited solely to the field of research, as he has been also—and especially—a
referencepointforthehumanaspect. Additionally,Iamgratefultomysecondsupervisor,
ProfessorMarkSterling: hisinsighthasbeeninvaluable. Ioweadebtofgratitudetowards
bothforthetrusttheygrantedtome.
APhDisquiteawidejob,anditwouldhavenotbeenpossiblewithouttheinvaluable
helpofmanyandmanypeopleworkingwith,supportingandadvising. Iamgratefultothe
BlueBear high performance computing facility, in particular with Paul Hatton, for their
fundamentalsupport. IalsowishtothankSevernTrentWater,Inc.,inparticularPeteVale,
for the precious information they shared with me. The experimental part of this work
would not have been possible without the knowledge, the support and the capabilities of
Federico Alberini and Mark Simmons for the PIV part, and of Thomas Leadbeater and
DavidParkerforthePEPT.Iamdeeplygratefultoallofthem. MarkCarter’shelpwasalso
determinant torealising the experimental apparatus; hiscontribution isdeeply appreciated.
ThesupportandtheknowledgeofmyPostgraduateandresearchfellowscolleagueshas
beeninvaluable. IwishtothankRebeccaSindall,my“predecessor”,forallheradviceand
the work we undertook together; Nainesh, Dominic and Justin for their insight in CFD;
Roger,Sally,Ashley,Simon,Giulio,Valeria,Carlo,Martin,Marianna,Michaela,Anna,
AryanandMehranfortheirwarmsupport. OutsidetheUniversity,Iwouldliketodeeply
acknowledgeDoctorMassimod’EliaandDoctorAndreaMazzinofortheconstantsupport
through these years. Without Euroforma and the European Social Fund, I would ignore
even what anaerobic digestion means. Thank you to Mladen Ciglar and Luca Marigo,
vii
amongtheothers.
Myfamily—itisimpossibletoexpressinwordshowIamgratefulforyourconstant
support. Thankyoutomywife,Cinzia,foryourloveandsupport. Iamdeeplyhonouredto
be your husband, Thank you to Michele, as you are making our lives much busier and full
ofjoyandblissaswell—especiallynowthatyouhavefinallydecidedtogetoutofyour
mother’s belly! Still I am convinced that some fundamental principles such as the mass
conservationdonotholdforchildren,especiallyasregardswhattheyleaveinthenappy.
Thank you to my parents, Gabry and Guido. It is said that the debt of gratitude towards
one’sparents ishigher thanthe heaven anddeeper thanthe earth: I hopeto becomeable
to repay mydebt towards you. Thank you tomy parents in law, Luisa and Franco: I am
deeplyhonouredtobepartofyourfamily,eventhoughIhavenothadtheopportunityto
personally meet you, Luisa,if wedo notconsider themirror ofyour daughter. Thank you
to my brothers in law, Daniele and Claudia, your spouses Giulia and Daniele, and your
children Pietro and Veronica. Thank you to my grandparents, who is still in live, Silvia,
and whois not,Mari, Luisito andMino. You haveall been areference pointand amodel,
inparticularyou,Mino: Icannotwaitformeetingyouagain!
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Contents
Abstract v
Acknowledgements vii
Contents ix
ListofFigures xv
ListofTables xix
1 Introduction 1
1.1 ResearchRelevance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 LayoutofThisWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 AnaerobicDigestionandMixing 5
2.1 AnaerobicDigestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1 BiochemicalReactions . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.2 BacteriaConsortiaandSyntrophicRelationships . . . . . . . . . 14
2.2 FactorsAffectingAnaerobicDigestion . . . . . . . . . . . . . . . . . . . 15
2.3 Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.1 MethodsofMixing . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.2 IndustrialMixing . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.4 PreviousStudiesontheEffectsofMixing . . . . . . . . . . . . . . . . . 21
2.4.1 EffectsofMixingIntensity . . . . . . . . . . . . . . . . . . . . . 22
2.4.2 EffectsofMixingMethod . . . . . . . . . . . . . . . . . . . . . 24
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Contents
2.4.3 EffectsofMixingonBacteria . . . . . . . . . . . . . . . . . . . 24
2.5 SummaryofthePreviousWorksandResearchGaps . . . . . . . . . . . . 26
3 FluidDynamicsModelling 29
3.1 Navier-StokesEquations . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 FluidRheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.1 TheKolmogorovDescriptionofTurbulence . . . . . . . . . . . . 36
3.3.2 ReynoldsEquations . . . . . . . . . . . . . . . . . . . . . . . . 40
3.3.3 BoundaryLayer . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3.4 TurbulenceModels . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.4 ComputationalFluidDynamics(CFD) . . . . . . . . . . . . . . . . . . . 46
3.4.1 Pre-Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.4.2 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.3 Post-Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.5 MultiphaseModels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.5.1 Euler-LagrangeModel . . . . . . . . . . . . . . . . . . . . . . . 51
3.5.2 Volume-Of-FluidModel . . . . . . . . . . . . . . . . . . . . . . 53
3.5.3 Euler-EulerModel . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.6 LiteratureReviewonSludgeViscosity . . . . . . . . . . . . . . . . . . . 55
3.7 PreviousCFDStudiesofAnaerobicDigestion . . . . . . . . . . . . . . . 56
3.7.1 Laboratory-ScaleLiterature . . . . . . . . . . . . . . . . . . . . 57
3.7.2 Pilot-ScaleLiterature . . . . . . . . . . . . . . . . . . . . . . . . 60
3.7.3 Full-ScaleLiterature . . . . . . . . . . . . . . . . . . . . . . . . 60
3.8 PreviousMultiphaseCFDStudiesofAnaerobicDigestion . . . . . . . . 62
3.9 SummaryofthePreviousWorksandResearchGaps . . . . . . . . . . . . 65
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