Experimental and numerical investigation of the hydrodynamics of mixed anaerobic digester

394 Pages·2014·5.11 MB·English

Experimental and numerical investigation of the
hydrodynamics of mixed anaerobic digester

by

Daniel Asrat Balcha

A Thesis Submitted to the Graduate Studies of
The University of Manitoba
In Partial Fulfillment of the Requirements for the degree of

DOCTOR OF PHILOSOPHY

Department of Mechanical and Manufacturing Engineering
University of Manitoba
Winnipeg, MB
Canada

© Copyright 2014 by Daniel Asrat Balcha
ABSTRACT

A review of the literature indicates that the understanding of the mixing phenomena in
anaerobic digesters is limited and the ability to measure digester mixing characteristics is
lacking. Moreover, rheological characteristics of the sludge are largely ignored. The
need for a more thorough understanding of fundamental mixing relationships and the
ability to measure these relationships in the anaerobic digester is recognized. To this
end, investigations using experimental and numerical methods to visualize flow patterns
and quantify mixing that impact biogas yields is reported. Results from this research
identifies optimum mixing regimes for digesters depending on their sludge rheology,
operational regimes, digester dimensions and mixing systems, and also produces an
industrial guide to improve digester design.

The study uses lab-scaled mixed anaerobic digester and a candidate transparent fluid
with a minimum power law index, n, of 0.55 and the power law constant range 0.014 to
1.18 that is rheologically characterized and parameters affecting the viscosity are
investigated at concentrations of 0.05–1.00% w/w HEC solution corresponding to 2-
10% TS manure and temperatures of 5–45oC. The experimental tools used to analyze the
systems under investigation are: particle image velocimetry for experimental
determination of velocity profiles; a conductivity technique to quantify mixing time; a
strain gage-based rotary torque transducer system to measure the power dissipated by the
impeller; and an acid-base reaction coupled with image processing technique to
understand the formation and degeneration of segregated flow regions that exist at low
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Re and a visual observation method to determine the minimum agitation speed for
complete solids suspension, Njs. A computational fluid dynamic model is developed to
predict the behavior of the system in terms of its velocity profile, power number,
pumping number, and mixing time. For a Re based on impeller diameter and blade
rotational speed, the following flow regimes are identified and investigated in detail:
laminar, Re = 1, 6.9, and 15.5, transient, Re ≈ 3.103 and turbulent, Re ≈ 3.104. Numerical
predictions are obtained using MRF/SM model coupled, when needed, with a volume of
fluid model, in order to study systems in which a vortex could be expected to form over
a range of Re covering laminar, transitional and turbulent flow regimes.

The results show variation of mixing intensities and duration eliminate dead zones in
digester up to 28% faster and eliminate scum layer, maximize use of digester volume
and efficiently mix incoming feed with digester contents. The results also indicated
critical parameters that keep solids and heavier sediment in suspension for uniform
liquid and temperature distribution and prevent sludge buildup leading to reduction
and/or elimination of digester cleaning cost and digester downtime.

In addition, the experimental study on the effects of mixing intensity and mixing
duration on the anaerobic digestion performance shows how biogas output is dependent
upon, and can be influenced by flow patterns in an anaerobic digester. The mixing
intensities studied had a significant effect on soluble chemical oxygen demand (SCOD)
utilization rate. The Biogas yield showed an increase ranging from 2.5 to 14.6% for
digesters operated under various intermittent mixing compared to biogas yield from
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continuous mixing. Hence, it is possible to tailor mixing regimes to produce flow
patterns that could potentially increase biogas production and/or reduce mixing energy
input of an anaerobic digester.

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ACKNOWLEDGMENT

This research was funded by the NSERC/Manitoba Hydro Industrial Research Chair and
I wish to relate much appreciation for the opportunity for the creation of such a training
program in renewable energy. Many thanks go to Mr. Tom Molinski for working on the
Chair program.

I would like to thank my supervisor, Dr. Eric L. Bibeau, who has been of a great
intellectual support. He shared with me a lot of his expertise and research insight and
gave me complete academic freedom to explore and heighten my research interests in
alternative energy field. That freedom was instrumental in helping me develop into an
independent professional engineer.

I would also like to show my genuine appreciation to the other members of my PhD
committee who monitored my work and took effort in reading and providing me with
valuable comments on earlier versions of this thesis: Dr. Nazim Cicek, Dr. Robert
Derksen (University of Manitoba) and Dr. Oladiran Fasina (Auburn University). I thank
all of you to the bottom of my heart.

Many thanks to my wife Ribka Melesse, for her unconditional love, patience, assistance,
support and faith in me. She has endured this long process with me, always offering me
love and support. I deeply thank my Family: brothers, sisters, aunts and cousins for
believing in me and for being proud of me. Most importantly, I would like to thank and
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dedicate this work to my loving parents, Asrat Balcha and Aberash Dejene who are the
angels that constantly watch over my head and whose simple voice gives me the necessary
strength to hold on and persevere.

I am tempted to individually thank all of my friends but as the list might be long and for
fear I might omit someone, I will simply and genuinely say: Thank you to you all for
your love, care and trust. In particular, I would like to cite Kwadwo Owusu, Bafoor,
Jonathan, Godwin (Ghana); Amir, Mohamed (Iran); Moftah Mohamed (Libya) and
David Gaden and Jeremy Langner (Canada) for their constant presence, care, instant
moral support and intellectual discussions.

I cannot finish without acknowledging how eternally grateful and thankful I am to The
One, The Everlasting, The All Determiner, The Trustee, The Dependable and The
Protecting Friend that guides me, teaches me to be patient and to never give up. Thank
you Dear Lord!

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የጥበብ መጀመሪያ እግዚአብሔርን መፍራት ነው፤ /ምሳ.1፥7/ አቤቱ አምላኬ እግዚሀብሔር ሆይ ለእኔ ያላደረከዉ ምን አለ?
ዉለታህና ቸርነትህ ፍቅርህና ምግባርህ ከአዕምሮዬ በላይ ነዉና ፈጣሪ ከፀሐይ መውጫ ጀምሮ እስከ መግቢያው ድረስ
ስምህ ይመስገን፡፡
አምላከ ቅዱሳን የኃያላን ኃያል የድንግል ማርያም ልጅ ጌታ መድኃኒዓለም ሀገራችንን ይጠብቅልን!!!

መመታሰቢያ (Dedication)
ስንሰደድ የምናፈቅራቸውን ወላጆቻችንን የምንወዳቸውን እና የሚወዱንን ቤተሰቦቻችንን፣ ዘመድ አዝማዶቻችንን
የኛ የምንለውን ማንነት የባለ ሃገርነትን ስሜታችንን ባህላችንን ፣ወጋችንን ስለራሳችን ያለንን ግምት ክብራችንን ተስማማህ/ሽ
አማረብህ/ሽ የሚለንን፣የሚያስብልን የሚጨነቅልልን የሚያበረታታንን ብቸኝነት እንዳይሰማን እና ሙሉነት እንዲሰማን
የሚያደረጉንን ነገሮች ትተን አዲስ ህልም ሰንቀን የተሻለ ህይወትን ፣ ብሩህ ነገን አስበን ወጥተናል። ሁላችንም በስደት ያለን
በተለያየ ስራ ላይ ተሰማርተን ህልማችንን እና ግባችንን እውን ለማድረግ ደፋ ቀና በማለት ላይ እንገኛለን፡፡ አብዛኞቻችን
ለኢትዮጵያ መስራት የምንችለውን እና መስራት የሚገባንን ሳይሆን ኑኖ ያስገደደን ስራ እየሰራን ሙሉ ችሎታችንን
እውቀታችንን እና አቅማችንን ለኢትዮጵያ ሳንጠቀም እርካታም ሳይሰማን ቀን ይወጣል ቀን ይመጣል እያልን በተስፋ
እንኖራለን ፡፡
እያንዳንዷ ቀን ባለፈች ቁጥር ግን ኢትዮጵያ አስከፊና አማራሪ ቦታ አየሆነች ነው፡፡ አገር ሲቆረስ ድንበር ሲፈርስ
ማንነት እና ታሪክ ሲሻር መሬት ሲገመስ አገር ያለ ባህር በር ስትቀር የዜጋ ማንነት በጎሳ ሲቀየር ድንበር ተቆርሶ ለባዕድ
ሲለገስ ወገን ሲጨቆን መኖሪያ ቤቱ ሲፈርስ ሀብት ንብረቱ ተዘርፎ ከቀዬው ሲሰደድ የዜግነት ክብሩ ሲጣስ ሰብእናው
ሲዋረድ በአረብ ምድር ሲገደል እንደ እንስሳ ሲታረድ ህይወቱ ሲቀጠፍ በገመድ ሲታነቅ ዜግነት ክብሩ ሲዋረድ ሰብአዊ
መብቱ ሲናድ የመኖር ህልሙ ባዶ ሲሆን ፊቱ በእንባ ሲታጠብ በሀዘን ሲዋጥ ለስቃይ ችግር ሲዳረግ በረሀብ አለንጋ
ሲገረፍ ጠኔ ሲመታው እና በከባድ ህመም ሕይወቱ ሲቀጠፍ ማየት የተለመደባት አገር ሆናለች፡፡
ይህ መመረቂያ ፅሁፍ መታሰቢያነቱ ሰብአዊ መብቶች በሚደፈጠጡባት ስልጣኔ ወደኋላቀርነት በሚዘወርባት
ፍትህ በሚጣስባት፣ የዘር ማጥፋት በሚፈጸምባት፣ ህዝቦች በሚደኸዩባት እና በሚራቡባት፣ ወጣቶች አፋቸው
በሚሸበቡባት፣ በሚለጎሙባት እና በሚታፈኑባት፣ የአካባቢ ተፈጥሮ ሀብት በሚወድምባት፣ ግድቦች ቋሚ ዝርያ ያላቸውን
ህዝቦች መብት በሚገድቡባት እና ህዝቦች ወደ አስከፊ ሁኔታ እየወረዱ ባሉባት ሀገሬ ለሚኖሩ ኢትዮጵያዊ ወገኖቼ
ይሁንልኝ፡፡ ኢትዮጵያ በልጆቿ የመንፈስ ጽናት ተመልሳ በእግሯ ትቆማለች።

ኢትዮጵያ በነጻነቷና በአንድነቷ ለዘላለም ኮርታ ትኑር።
ዳንኤል አስራት ባልቻ

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TABLE OF CONTENTS
ABSTRACT ...................................................................................................................... ii
ACKNOWLEDGMENT ................................................................................................... v
TABLE OF CONTENTS ............................................................................................... viii
LIST OF TABLES ........................................................................................................... xii
LIST OF FIGURES ........................................................................................................ xiv
NOMENCLATURE AND ABBREVIATIONS .......................................................... xxvi
1. Introduction .................................................................................................................. 1
1.1 Feedstock for anaerobic digestion plants ........................................................... 4
1.2 Anaerobic digestion of livestock manure ........................................................... 5
1.3 Research motivation ........................................................................................... 7
1.4 Research objectives, procedures and outcomes ............................................... 11
1.5 Outline of thesis ............................................................................................... 15
1.6 Publications ...................................................................................................... 16
2. Summary of literature review .................................................................................... 19
2.1 Anaerobic digestion process ............................................................................ 20
2.2 Non-Newtonian fluids ...................................................................................... 22
2.3 Experimental and numerical studies in mixing ................................................ 24
2.4 Anaerobic digestion modeling ......................................................................... 27
3. Experiments on viscosity, mixing time, power and suspension using lab-scale AD 30
3.1 Laboratory scale experimental set up ............................................................... 31
3.2 Methodology .................................................................................................... 43
3.2.1 Viscosities measurement ...................................................................... 43
3.2.2 Mixing time and power measurement .................................................. 47
3.2.3 Experimental flow measurement ......................................................... 48
3.2.4 Determination of impeller pumping flow rate and pumping number .. 52
3.2.5 Determination of impeller discharge angle .......................................... 53
viii
3.2.6 Determination of the impeller power dissipation and power number .. 54
3.3 Data analysis and results .................................................................................. 55
3.3.1 Non-Newtonian viscosities measurement ............................................ 55
3.3.2 Mixing time and power measurement for Newtonian fluids ............... 59
3.3.3 Mixing time and power measurement for non-Newtonian fluids ........ 81
3.3.4 Suspension of high concentration non-Newtonian fluid in agitated AD
.............................................................................................................. 98
3.4 Conclusion ..................................................................................................... 110
4. Investigation of unmixed zone in mixed anaerobic digester ................................... 113
4.1 Scaled laboratory experimental set up ........................................................... 114
4.2 Methodology .................................................................................................. 116
4.3 Results and discussions .................................................................................. 120
4.4 Effect of perturbation on segregated mixed regions ...................................... 122
4.5 Effect of temperature change on segregated mixing regions ......................... 144
4.6 Conclusion ..................................................................................................... 147
5. Numerical modeling of hydrodynamics of mixed AD ............................................ 149
5.1 Model geometry for lab scale digester ........................................................... 151
5.2 Governing equations ...................................................................................... 152
5.3 CFD modeling of mixing in a lab scale digester ............................................ 157
5.3.1 Modeling viscosity ............................................................................. 157
5.3.2 Volume of Fluid (VOF) method ........................................................ 158
5.3.3 Computational prediction of impeller pumping flow and pumping
number................................................................................................ 161
5.3.4 Computational prediction of impeller discharge angle ...................... 161
5.3.5 Computational prediction of impeller power dissipation and power
number................................................................................................ 162
5.3.6 Computational prediction of mixing time .......................................... 163
5.4 Results and discussions .................................................................................. 165
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5.4.1 Laminar hydrodynamics..................................................................... 167
5.4.2 Transitional hydrodynamics ............................................................... 176
5.5 Conclusion ..................................................................................................... 195
6. Effect of geometrical and operating parameters on the performance of mixing in
turbulent mixed AD ................................................................................................. 198
6.1 Methodology .................................................................................................. 199
6.2 Validation ....................................................................................................... 200
6.3 Results and discussion (single phase) ............................................................ 202
6.3.1 Effect of impeller submergence ratio on flow pattern with H/T < 1 .. 202
6.3.2 Effect of off-bottom clearance (C /T) ................................................ 209
b
6.3.3 Effect of D/T and Sb/D Ratios ........................................................... 215
6.4 Results and discussion (two phase) ................................................................ 217
6.4.1 Investigation of sedimentation in mixed AD ..................................... 217
6.4.2 Investigation of suspension of solid in mixed AD ............................. 221
6.5 Conclusion ..................................................................................................... 251
7. Effect of mixing strategy on methane production on lab and pilot scaled AD ........ 254
7.1 Experimental setup, materials and methods ................................................... 254
7.1.1 Lab scale studies ................................................................................ 254
7.1.2 Pilot–scale studies .............................................................................. 257
7.2 Experimental results ....................................................................................... 259
7.2.1 Lab-scaled anaerobic digester ............................................................ 259
7.2.2 Effect of mixing on the performance of pilot-scale AD .................... 275
7.3 Conclusions .................................................................................................... 280
8. Conclusion, contributions and recommendations .................................................... 282
8.1 Conclusions and contributions ....................................................................... 282
8.1.1 Significance of experimental work .................................................... 282
8.1.2 Significance of numerical work ......................................................... 285
x