Rotary Kilns, Second Edition: Transport Phenomena and Transport Processes

370 Pages·2016·18.47 MB·English

by  Boateng

Rotary Kilns
Transport Phenomena and
Transport Processes
Second Edition
A. A. Boateng
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Dedication
Tomywife,Sita,mynumberonecheerleader,tothechildren,andtoall
the children of the underdeveloped world, particularly in Africa, who,
although faced with little or no technological resources, are still moti-
vatedanddeterminedtofulfillthedreamsofbecomingthescientistsand
engineersoftomorrowjustastheircounterpartsinthedevelopedworld.
.Inlovingmemoriesoftwofrontierswelostwhoinspiredmychemicalengineering
developmentandcareer,ProfessorL.T.FanofKansasStateUniversity(2014)and
J.R. Ferron emeritus professor of chemical engineering, University of Rochester
(2015).
Foreword to the First Edition
In contrast to the situation for other gasesolid reactors such as fluidized beds, the
rotarykilnreactorhassofarlackedanauthoritativetreatmentinasinglebook.Process
engineers,operators,designers,researchers,andstudentshavestruggledwithawide-
spreadcollectionofpapers,frommanyindustries,withkeyarticlesandclassicalcon-
tributions that are over 40years old and difficult to access. At the same time, new
applications for the kiln have arisen, new fuels are being used, and new tasksdsuch
as waste incinerationdare being applied to kilns originally designed for other uses.
Muchimprovedunderstandingofbedprocesseshasbeendevelopedinthepastdecade
orso,andtoolsfordesignandanalysisofexistingoperations,suchascomputational
fluiddynamics,haveevolvedfromraretowidespreaduse.Therehasbeenapressing
needforthesedevelopmentstobebroughttogetherandassessedsuchthatthekilnen-
gineer,forexample,canappreciatewhatcanbeachievedfromdifferenttypesofmath-
ematical modeling and so that the fluid particle researcher can understand what has
beenaccomplished byrecent research andwherefurther effort isneeded.Thetimely
publication of this new book, Rotary Kilns: Transport Phenomena and Transport
Processes, by Akwasi Boateng, admirably fills this gap.
Written by an engineer gifted in his understanding of the fundamentals of fluide
solidsystemsandtheirmathematicalmodeling,andhavingextensiveexperienceinro-
tarykilnpractice,thebookprovidesthereaderwithauniquevantagepointfromwhich
toanalyzerotarykilnprocesses.Thebookprovidesaclearexpositionofkilnbehavior
andofthebasesandassumptionsofthevariousmathematicalmodelsusedtodescribe
it. Examples areprovided for the purposesof illustration.
In terms of the content, Rotary Kilns: Transport Phenomena and Transport
Processesprovideschaptersdevotedtothekiln’sbasicdescriptionandtoitsnumerous
applications. Several chapters on freeboard processes, aerodynamics, combustion
and flames, and heat transfer are linked to a detailed exposition of bed processes,
including particulate behavior, granular flow models, mixing and segregation, and
heattransferandtheireffectsonkilnperformance.Overallmassandenergybalances,
measurements, and site surveys are presented in the penultimate chapter, after the
readerhasbeenwellpreparedbythepriortreatmentoftheindividualprocesses,rather
thanatthebeginningofthebook.Thefinalchaptercontainspracticalinformationon
several key rotary kiln processes in the minerals industry. An extensive appendix is
included.
Readerswillfindthatthisbookstartsmosttopicsataverybasiclevel.Someexam-
ples of this are the chapters on combustion and on mass and energy balances. Soon,
xiv ForewordtotheFirstEdition
however, the complexities become evident, such as in the chapters on granular flow
and segregation.
Thisbookwillbecomeabenchmarkforthestudyofrotarykilnphenomena,forthe
design of new rotary kiln processes, and for the modeling and analysis of existing
processes. It will be an indispensable addition to the bookshelves of engineers in
the cement, lime, pulp and paper, and mineral industries. It will prove invaluable to
researchers and academics studying the complexities of solidegas flows, mixing,
and transport processes.
Paul Watkinson, PhD
Vancouver, British Columbia
October 2007
Foreword to the Second Edition
Thefirsteditionofthisbookhasmadeasignificantcontributiontothestudyofrotary
kilns, providing all who work with or study these simple, but complex, devices a
wealthofknowledgeinonevolume.Thisneweditioncontainsanumberofadditions
and improvements, including two additional chapters that reflect the growing impor-
tance of petroleum coke and environmental applications for rotary kiln operations.
These additions are a significant enhancement to the value of this already excellent
book.
Andrew A. Shook, PhD
Perth, WesternAustralia
2015
Preface to the First Edition
The author was born in Ghana in a little village called Kentikrono, now a suburb of
Kumasi,theAshanticapital.Afterabriefstintasanengineeringcadetinthemerchant
marinecorps ofGhana,heacceptedascholarshiptostudymarineengineering inthe
formerSovietUnion.Changingfields,hepursuedstudiesinmechanicalengineeringin
Moscow specializing in thermodynamics and heat engines where he redesigned a
turboprop engine for hydrofoil application. He pursued graduate school in Canada,
thereafter, and completed an MS in thermofluids mechanics at the University of
New Brunswick with a thesis under J.E.S. Venart on energy conservation in green-
houses using thermal night curtains to prevent low-temperature nighttime infrared
radiation heat losses through polyethylene roofs. He accepted a faculty position at
theUniversityofGuyanainSouthAmerica,wherehiswifehailsfrom,andtaughtun-
dergraduate thermodynamics and heat transfer. Hisresearchinfluidizedbedthermo-
chemicalconversionofricehullstoprovidebioenergyandutilizationofthericehull
ashforcementapplicationsearnedhimafacultyFulbrightawardtotheUnitedStates,
wherehespentayearworkinginDr.L.T.Fan’slaboratoryatthedepartmentofchem-
ical engineering at Kansas State University. Upon an invitation to Canada, he joined
Dr. Brimacombe’s group at the Center for Metallurgical Process Engineering at the
University of British Columbia, where he pursued a PhD in rotary kilns under the
sponsorshipofAlcanCanada.Hisdissertation,RotaryKilnTransportPhenomenad
StudyoftheBedMotionandHeatTransfer,supervisedbyP.V.Barr,presentedsome
pioneeringworksontheapplicationofgranularflowtheoriesforthemodelingofpar-
ticlevelocitydistributioninmineralprocessingkilnsfromwhichheattransferwithin
the kiln bed could be adequatelyand sufficientlysolved.
AfterabriefstintasanAssistantProfessoratSwarthmoreCollegeinPennsylvania,
hejoinedSoliteCorporation,arotarykilnlightweightaggregatemanufacturingcom-
panyinVirginiafoundedbyJaneandJohnRoberts(Swarthmore1939)asaresearch
andproductionengineer.AtSolite,hedevelopedatwo-parttrainingmanualonrotary
kilntransportphenomenaforproject10-10-10,anoperationalcampaignpromotedby
JohnRobertstoincreaseproductionandproductqualityby10%andalsoreducefuel
consumption by 10%. After Solite restructured in 1997, the author returned toPenn-
sylvania and joined Fuel and Combustion Technology (FCT) founded by colleagues
from the United Kingdom, Peter Mullinger and Barrie Jenkins, who, having also
completedtheirPhDworksonkilncombustionhaddevelopedmethodsofoptimizing
turbulent diffusion flame burners to match cement and lime kiln processes. When
FCT’s owner, Adelaide Brighton Cement, was acquired by Blue Circle Cement, the
xviii PrefacetotheFirstEdition
authorjoinedtheprocessgroupofFuller-FLSchmidt(FFE)Minerals,nowFLSMin-
eralsinBethlehem,PA,whereheparticipatedinworksleadingtothedesignofseveral
large direct-fired mineral processing kilns including limestone calcination, vanadium
extraction,andsodaashproduction.HelaterworkedforHarperInternationalinLan-
caster, NY, a lead provider of indirectly heated, high-temperature, rotary kilns
employed for niche applications including inorganic materials. After Harper, he
became a consultant to the industry providing process expertise including training
totherotarykilncommunitywherehewasdubbed“thekilndoctor.”Heisnowase-
niorResearchScientistwiththeAgriculturalResearchServiceoftheUSDepartment
of Agriculture pursuingresearch inbiofuels and bioenergy.
RotaryKilns:TransportPhenomenaandTransportProcesses,isaculminationof
theauthor’sworkinrotarykilnsinbothacademicresearchandindustry.Itcapturesthe
author’s experiences in production, process design, and commissioning, and more
importantly, attempts tobridge theclassroom andtherotarykiln industry. The focus
of Rotary Kilns: Transport Phenomena and Transport Processes is to provide the
process engineer and the researcher in this field of work some of the quantitative
descriptionsoftherotarykilntransportphenomenonincludingfreeboardandbedpro-
cess interactions. The latter combines the transverse bed motion and segregation of
granularmaterialsandtheresultanteffectofthesephenomenaonthebedheattransfer.
Althoughotherbedphenomena,suchasaxialsegregation(sequentialbandingofsmall
andlargeparticlesalongthekilnlength)andaccretion(depositionorgrowthofmate-
rial onto the refractory wall forming unwanted dams) are also not well understood,
these are only qualitatively described. However, these phenomena can be better
explained after careful elucidation of the transverse bed motion, segregation, and
heat transfer. The work has been divided into sequential topics beginning with the
basic description of the rotary kiln operation followed by fluid flow in rotary kilns
where the freeboard phenomenon is presented. Here the similarities of fluid flow in
conduitsaredrawntodescribethecharacteristicsofconfinedflowsthatmanifestthem-
selves incombustion and flames typical of therotarykiln environment.
In Chapter 4 the granular flow phenomenon in rotary kilns is presented. In rotary
kilns,oftenthematerialbeingprocessediscomposedofgranules,hencetheunderly-
ingtheoriesforsuchflowsareimportanttothebedmotion,gasesolidsreactions,and
solidesolidreactionsthattakeplaceinthebed.Withtheknowledgeoftheseflows,it
isonlyprudenttocovermixingandsegregationastheydevelopinrotarykilns.Thisis
accomplishedinChapter5.Theseverityofmixingphenomenaimpactsgreatlyonthe
qualityoftheproductsinceitinfluencesthethermaltreatmentofanygranularmaterial.
Mixingandsegregationdeterminestheextenttowhichtherotarykilncanbeclassified
as a continuous stirred reactor. The flame is the heart of direct-fired kilns, thus com-
bustionandflameistreatedinChapter6.Thetypesofflamesdevelopedinrotarykilns
depend on the flow distribution in the freeboard, which, in turn, determines the heat
fluxes to the charged material and also emissions. Treatment of heat transfer in free-
board is therefore a logical sequence and this follows in Chapter 7 with a review
ofthefundamentals ofprocessheat transfer. Many mathematical models haveprevi-
ously been applied to describe freeboard heat transfer in rotary kilns including
one-dimensional zone models, and two- and three-dimensional computational fluid
PrefacetotheFirstEdition xix
dynamics. Some of these are presented, including recent developments. Freeboard
treatment is followed by bed heat transfer in Chapter 8. Like fixed-bed heat transfer,
rotary kiln bed heat transfer is composed of particle-to-particle conduction, convec-
tion,andradiation.However,superimposedonthisphenomenonisanadvectivetrans-
port component that is generated due to granular flow that sets apart rotary kiln heat
transfer from packed-bed heat transfer. Some existing packed-bed models and their
extension to rotary kilns are presented here. Following the bed heat transfer, the
massandenergybalanceisestablishedinChapter9byconsideringthekilnoperation
as a thermodynamic system that interacts with the atmosphere. A simple mass and
energy balance is presented for a lime kiln. Having established all the above, it is
only prudent to present some specific mineral processing applications for which the
rotary kiln has been the main workhorse in Chapter 10. Some of the processes
discussed include lime making, cement making, carbothermic reduction kilns, and
lightweight aggregate kilns.
Theauthorisindebtedtothemanystudentsbothinthecollegeshehastaughtandin
the industry where he has lectured. He is grateful to Solite Corporation, which gave
him an unprecedented opportunity to test his theories and mathematical models on
large rotary kiln processes in the early years. He is also indebted to FCT, FLS
Minerals, Harper, and all the many members of the family of rotary kiln operators
particularly Utelite Corporation,Graymont,Inc.,andotherswhogavehimanunpar-
alleled education beyond the classroom. Finally, the author is indebted to Dr. Gus
Nathan,Dr.PhillipShaw,andDr.PeterCookeforthecriticalfeedbacktheyprovided
on themanuscript for this book.
A. A.Boateng, PhD
Royersford, PA
2008
[email protected]
Preface to the Second Edition
Since the publication of the first edition of this book in 2008, feedback from our
numerous readers has been very positive but certainly not without suggestions for
improvement.Whilereviewersofthefirsteditiongenerallyagreedupontheadequacy
andsufficiencyofthematerial,theyrequestedthataddingacontentonpyrolysispro-
cesses withlow temperatures, adding a new material on thecalcinationof petroleum
coke,andprovidingenvironmentalcomplianceinformationontheuseofrotarykilns
asincinerators for chlorinatedhydrocarbonsandcarbon dioxidecapturewouldbring
thebookinlinewithrecentdevelopments.Morecomputationalfluiddynamics(CFD)
information was also requested.
We have attended to these requests by either expanding on existing chapters or by
providingcompletenewchapters.Specifically,wehaveexpandedonChapter10with
a section on rotary kiln pyrolysis/carbonization where we have provided information
on pyrolysis processes through a case study carried out at the UK Biochar Research
Centre(UKBRC)ofthe UniversityofEdinburghonproducing “standardbiochar” for
useinbiocarbonapplicationsexemplifyingfeaturesofarotarykilnforcarbonization.
Wehaveincludedtwonewchapters,Chapters11and12.Chapter11providesnew
material and information on the characteristics of petroleum coke calcination in the
rotary kiln and the use of this information as a design case for sizing a rotary kiln
forthesaidapplication,sothatthereader,aprocessengineerorthestudentinthepro-
cess engineering classcould identify withthedesignmethodologies underlying such
application.ThischapterisusedtohighlightnewdevelopmentsinCFDwhereCFDis
used as atool to optimizea new kiln design or toimprovean existing one.
FinallyChapter12,contributedwiththehelpofStephenP.Holt,addressesrotary
kilnenvironmentalapplicationsandintroducesthereadertothetechnicalbasicsofthe
US environmental regulations that are applicable when an industrial rotary kiln is
permitted to burn hazardous waste for energy and also for incineration of toxic
materials. The chapter also introduces new but preliminary developments in carbon
emissions and capture.
The author is grateful for the contributions made to this second edition by
Dr. Ondrez Masek of the UKBRC of the University of Edinburgh (www.biochar.ac.
uk) and to Mr. Stephen P. Holt, P.E., ([email protected]) Director, Environmental
Affairs, Giant CementHolding, Inc.,Summerville, South Carolina.
A. A.Boateng, PhD
Royersford, PA
2015
[email protected]