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FUNDAMENTALSOFCHEMICAL
ENGINEERING
COMPILEDBY:
MUHAMMADAFTABAMIN
(COURSEMATERIALFORDEPARTMENTALPROMOTIONEXAMINATION(DPE))
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2FUNDAMENTALS OF CHEMICAL ENGINEERING
Preface .................................................................................................................................................... 4WHAT IS CHEMICAL ENGINEERING? ........................................................................................ 5RAW MATERIALS FOR THE CHEMICAL INDUSTRY.............................................................. 9
2.1 MINERALS IN THE CHEMICAL INDUSTRY ......................................................... 102.2. MINING TECHNIQUES .......................................................................................... 142.3. TECHNIQUES OF PURIFICATION OR REFINING ............................................. 162.4 EXTRACTION TECHNIQUES FOR SOME MINERALS ......................................... 24
FUNDAMENTALS ............................................................................................................................. 283.1. THE SCOPE OF CHEMICAL ENGINEERING ..................................................... 303.2 UNITS - THE SI SYSTEM ...................................................................................... 323.3. THE BASIC RATE EQUATION ............................................................................. 353.4 DIMENSIONAL ANALYSIS ...................................................................................... 363.5 HEAT TRANSFER....................................................................................................... 373.6. FLUID MECHANICS .............................................................................................. 42MACHINERY ......................................................................................................................... 48
THERMODYNAMICS ...................................................................................................................... 524.1 THE THERMODYNAMIC FUNCTIONS................................................................... 534.2 THE FIRST LAW ......................................................................................................... 544.3 THE SECOND LAW .................................................................................................... 554.4 THERMODYNAMIC TEMPERATURE .................................................................... 574.5 ENTROPY .................................................................................................................... 594.6 HEAT ENGINES .......................................................................................................... 604.7 THERMODYNAMICS AND EQUILIBRIUM..................................................................... 64
REACTION ENGINEERING ........................................................................................................... 675.1 TYPES OF CHEMICAL REACTION ......................................................................... 675.2 REACTION KINETICS ............................................................................................... 715.3 CATALYSIS ................................................................................................................. 765.4 CONTINUOUS REACTION EQUIPMENT ............................................................... 785.5 REACTOR DESIGN .................................................................................................... 815.6 SPECIAL CONSIDERATIONS IN REACTOR DESIGN ...................................... 84
UNIT OPERATIONS ......................................................................................................................... 866.1 MATERIAL AND THERMAL BALANCES .............................................................. 866.2 MASS TRANSFER ...................................................................................................... 916.3 ABSORPTION ............................................................................................................. 94
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3FUNDAMENTALS OF CHEMICAL ENGINEERING
6.4 SOLVENT EXTRACTION .......................................................................................... 966.5 DISTILLATION ........................................................................................................... 996.6 CRYSTALLIZATION ................................................................................................ 1076.7 FILTRATION ............................................................................................................ 110
PLANT SERVICES AND PLANT CONTROL ............................................................................. 1127.1 STEAM ENERGY ...................................................................................................... 1127.2 ELECTRICAL ENERGY ........................................................................................... 1177.3 STEAM - WATER SYSTEMS ................................................................................... 1197.4 COOLING WATER SYSTEMS................................................................................. 1197.5 OTHER FACTORY SITE SERVICES ................................................................... 1217.6 INSTRUMENTATION AND CONTROL .............................................................. 121
DESIGNING AND BUILDING A CHEMICAL PLANT ............................................................. 1298.1 DESIGN INFORMATION ......................................................................................... 1318.2 PROJECT PROCEDURE ........................................................................................... 1328.3 PROJECTS INVOLVING A PROCESS UNDER DEVELOPMENT ....................... 1338.4 AMMONIA PLANT DESIGN ................................................................................... 139
THE CHEMICAL ENGINEERING PROFESSION .................................................................... 1509.1 THE INSTITUTION OF CHEMICAL ENGINEERS ................................................ 1519.2 THE CHEMICAL ENGINEER IN INDUSTRY ........................................................ 152
REFERENCE:................................................................................................................................... 161SUGGESTED READING MATERIAL FOR FURTHER READING: ...................................... 161Sample MCQs: ................................................................................................................................. 162
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4FUNDAMENTALS OF CHEMICAL ENGINEERING
PREFACE
Chemicalindustryisoneofourmajorgrowthandexportingindustries.Inspiteofinflation
many of its products have been steadily reduced in price, largely because of the
improvementsandadvancesachievedbytheindustry'stechnicalstaff.Chemicalengineersfindthattheirtrainingqualifiesthemforworknotonlyinthechemicalindustrybutalsointhe
wholerangeofprocessindustries.Theprocess industriesare thoseinwhichmaterialsare
continuouslychangedfromoneformintoanother,andincludeoilrefining,plasticsandfibers,
foodandwater,steel,glass,paper,drugsandotherchemicals.Theoperationsofmanyof
these industries are increasingly international in nature, involving engineers of both
producingcompaniesandtheplantconstructionindustryinfrequenttravel.
Chemical engineering ispartscientific andpartengineering. It ischemical engineerswho
translatethereactionsandprocessesdiscovered in the laboratory into thousandtonsper
dayprocessplants.For this is the scale ofoperationofmanyof today'splants-ethylene,
ammoniaandotherfertilizers,oilrefiningandmineralsandmetallurgicalprocessingplants.
Thecornerstoneofchemicalengineeringisitsconcernwithsizeandprocessingrates.The
chemicalengineermustdecidetheprocessingstepsandequipmentneededtopreparethe
reactants,carryoutreactioninoneormorestages,andseparatetherequiredproductfrom
thestreamleavingthereactorandpurifyit.Intheseprocessingstepsthetypesofequipment
usedincludedistillationcolumns,absorbers,heaters,driers,crushers,crystallizers,etc.The
processes occurring in these units are the unit operations of chemical engineering. The
chemicalengineeruseshisknowledgeoftheappropriateunitoperationtospecifythesize,
shape, internal design and operating temperature and pressure for each item of
equipment.
Inspiteofthemodernscaleoftheprocessindustries,themanwhoentersthemtodayhas
difficulty ingaining adequate knowledge of these industries at the stageofchoosing his
courseofprofessionalstudies.Hejoinstheselectedindustryasanactoffaithwithoutfully
realizingwhathewillfindinit. Itisoneoftheobjectsofthisseriestohelptoredressthis
situation. This book will provide the intending student with a picture of chemical
engineeringscienceanditsplaceinthechemicalindustry;itwillalsoenablehimtoforma
foundationonwhichtobasemoredetailedstudyofspecificsubjects.Thecontents,then,of
thebookaretwofold.First,therearesectionsconcerningthebasicprinciplesofchemical
engineeringandthetoolsofthechemicalengineer;second,therearesectionsdescribing
theroleofchemicalengineersintheindustry.
Mostofthebookisdescriptive,butchemicalengineeringisanumericalsubjectandafew
illustrative sections are included to outline the mathematical derivation of certain
techniques.Thesesectionscouldwellbejumpedoverinafirstreadingofthebookand
returnedtoatleisure'(bythosewhohaveany).Thebook,afterall,isintendedtoberead-
notstudied.WhenyouhavereadityouwillnotbeachemicalengineerbutIhopeyouwill
haveabetterideaoftheprinciplesandpracticeofchemicalengineeringinindustry.
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5FUNDAMENTALS OF CHEMICAL ENGINEERING
WHAT IS CHEMICAL ENGINEERING?
Chemicalengineeringisarelativelynewscience.TheInstitutionofChemicalEngineerswasfoundedin1922andthefirstundergraduateuniversitydepartmentin1933.Therearenow
26 universities and colleges with chemical engineering faculties, from which some 600
engineers graduateannually.UnlikemostofBritain'sothermajor industries, thechemical
industryproducesavastrangeofproducts.It isoftendifficulttorememberthatthemajority
of its productswouldnot be regardedaschemicals bythegeneralpublic atall -for them,
chemicalsareinthechemist'sshop.Pharmaceuticalsareimportantproductsofthechemical
industry, but the plastics, fibers, fertilizers, and detergents industries are giants by
comparison.And,onthesubjectofthegeneralpublicanditsviewofthechemicalindustry,it
seemsthatevenmanyaninformedarideducatedmemberofthepublicwouldconsiderthat
achemicalengineerwassomesortofcrossbetweenthewhite-coatedmaninthechemist's
and the overalledmechanicwho repairshis car; a few paragraphson the partplayed by
chemicalengineersinindustryarethereforeinorder.
The chemicalengineer is concernedwith the task of taking a chemical reaction that has
been established in laboratory experiments and then designing, building, and operating
large-scaleplantexploitingthereaction.Toamplifythepartplayedbythechemicalengineer
wecanlookatsomeoftheproblemsthathemustfaceandthetypeofinformationheneeds
tosolvethem.Thechemicalengineermustbeawareofthetechnologyinvolvedinallthese
problems, althoughsomeof themaregenerally tackledbymore specialist engineers and
scientists.
Areactionisknowntoproceedinthelaboratory,buthowfastisthereaction,i.e.
whatlengthofreactiontimeisneededinthereactionvessel?Thisproblem-andthemore
general one of choosing all reaction conditions including temperature, pressure, etc.is
usually solved by carrying out laboratory experiments over a range of conditions. The
chemicalengineerthenusesdatafromthese,togetherwiththephysicalpropertiesofthe
reactantsandotherinformationonthesystem,toobtainamathematicalequationorsetof
equationsdescribingthebehavioroftheprocessundervaryingconditions.Theseequations
canbeusedtocalculatetheconditions,includingreactiontime,forwhichtheplantis tobe
designed.
Typically,rawmaterialsavailableforthereactionwillnotbeintheformrequired
bythereaction;theymayhavetobedried,heated,purified,orcompressed.Likewise,the
productsofthereactionmayincludeunwantedbyproductsorunreactedrawmaterials.The
chemicalengineermustdesignacompleteplantincludingalloftherawmaterialpreparation
facilitiesandproducttreatmentsectionsrequired.Insizingtheequipmentforthesepartsof
theplant,thechemicalengineerreliesonmethodsdescribedinlaterchaptersofthebook.
Whereno information isavailable for a specific item ofplant equipment, laboratory tests
mustbecarriedouttoestablishthedesignofthatitem.
How is the plant to be operated? The reaction conditions and those for
purificationstages,etc.,havebeenchosen,butthedesignermustensurethatthestaffwhowill operate the plantwill beable tomaintain the compositions, temperatures, pressures,
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6FUNDAMENTALS OF CHEMICAL ENGINEERING
etc.,at the figureshehasselected.Themodernapproachis touse instrumentsfor these
typesofcontrolduty.Theinstrumentsystemsarehousedinacentralizedcontrolroomwhich
enables all process variables to be controlled and adjusted from a single point by one
operator. The designer must decide on the control methods required and specify the
instrumentsneeded.Thedetailedinstrumentationiscarriedoutbyaspecialistengineer.
Ofwhatmaterialsistheplanttobebuilt?Thespecificationofmaterialsforthe
plant equipment is the responsibility of the chemical engineer. For this, he uses his
knowledgeofthenatureofthechemicalsinvolved,backedupbylaboratorytestsinthecase
ofunusualmaterialsormixturesofchemicals.Thisproblemis,ofcourse,alsotherealmof
themetallurgistandmaterialsengineer.
Designoftanks,pumps,compressorsandgeneralequipment,buildings,etc.The
detaileddesignnowbecomesthetaskofmechanical,civil, electrical, andother specialist
engineers; the chemical engineer needs only sufficient knowledge of these spheres to
ensurethattherequirementsofthereactionprocessarebeingmet.
Itisnotnecessarytostressthatthislistisincomplete;itdoesindicatethebreadthofthefield
inwhichthechemicalengineeroperates.
In spite of recognizing the board area of the industry in which the chemical engineer
operates,itisstilldifficultforthestudentchemistorchemicalengineerto'cometogrips'with
achemicalplant.Oneofthedifficultiesisthesimilaritymanydifferentitemsofequipment
beartooneanother.Areactionvessellooksverymuchlikeaboilerdrumorastoragetank;
pumps, compressors, turbines look alike. There is an 'authentic' story of (he chemical
engineeringgraduatewho was being conducted round a new plant. After being told the
purposeofvariousreactors,columns,pipes,etc.,heenquired,'Andwhatdoesthatpipedo?"andreceivedthereply,'Oh!That'sapieceofscaffolding!'Afurtherdifficultyliesinthescale
oftheoutputofmodernchemicalplant,alliedto thefactthatoftenneitherproductsnorraw
materials can be seen; thereare fewmenabout and it is difficult for thevisitor to know
whethertheplantisrunningornot.Manymodernplantsyieldatrain-loadofproductaday.A
singlepumpinaplantmaydelivermorefuelinaminutethanadomesticheatingsystemwill
useinayear.Thesedifficultiesofvisualizationcannotbeovercomeatonce,butthisbook
shouldhelp intheprocessbyexplaining thecomplexityanddemonstrating itsbreakdown
intoconstituentbasictechnologies.
Thescopeofchemicalengineeringwasexaminedbrieflyabove,butitiswrongtothinkthatonlyqualifiedchemicalengineersworkinthisfield.Apartfromchemicalengineers,thereare
atleastfourothertypesofprofessionalstaffworkinginthisarea.Thereare,firstly,applied
chemists and chemical technologists; courses in their subjects are closely related to
chemicalengineering,althoughlesscomprehensive.Whiletheterm 'chemicalengineer* is
welldefined,theterm'chemicaltechnologist'or'appliedchemist'isratherlooseanddoesnot
immediately define the type of knowledge to be expected precisely. Thirdly, there are
physical chemists and, fourthly, chemists who havegained the necessary experience, or
training,inthechemicalindustrytobeabletopracticechemicalengineering.It isgenerally
agreed,however,thatacourseinchemicalengineeringprovidesthebestbasictrainingfor
aninterestingcareerinthechemicalindustry.
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7FUNDAMENTALS OF CHEMICAL ENGINEERING
Havingseenwherechemicalengineeringfitsintothechemicalindustry,onewishtoknow
where one finds chemical engineers in the organization of a chemical company. This is
discussedmore fullyin the last chapterofthebook.Atpresent, it issufficient tosay that
chemicalengineerscanbefoundinalmostanydepartment.Mainly,ofcourse,theyworkin
the technical departments concerned with process development, plant design, and plant
operation,butmayalsoworkinplanning,marketing,andrelatedcommercialdepartmentsandthroughoutthemanagementstructureoftheindustry.Thechemicalindustryisfarfrom
being the only home for the chemical engineer; chemical engineering is essentially the
engineeringof processes and the chemical industry isnot alone inoperating processes.
Thuschemicalengineersaremoreandmorebecomingassociatedwiththepowerindustry-
gas, of course, but electricity also-with the food industry, and with steel and othermetal
extraction industries. There is a place for chemical engineers in any industry where
materials are put through a series of operations which change their characternot
necessarily chemically. Many of the problems in the food industry, for example, lie in
carryingoutprocessesheating,freezing,drying,etc.withoutchemicalchange.
As indicatedearlier,chemicalengineering isabranchofsciencewhichoverlapswithabroad range of other sciences and engineering disciplines. The link with organic,
inorganic, and physical chemistry isobvious and, inpractice, themajority ofchemical
engineers initially study chemistry. A point to remember is that chemical engineering,
particularlytheoreticalchemicalengineering,hasastronglinkwithmathematics.Thisis
because it is essentially a quantitative science concerned always with methods for
determining numerical values. Chapters 3, 4, 6, and 7 have brief sections which
exemplifythetypeofmathematicalderivationusedinchemicalengineering.Nostudent
should be disturbed by the mathematics since their standard is similar to that
encountered in most scientific or engineering courses. In reading this book it is not
essential, or indeed expected, that all its more difficult passages are understoodimmediately.
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Achemicalplantisasynthesisofallengineeringdisciplines-mechanical,civil,electrical,
instrumentation, metallurgical, and even electronicand the chemical engineer
concerned with design, construction, or plant operation must understand the basic
principlesof theseengineeringdisciplines.Chemicalengineeringis thedisciplinewhich
embracesandcoordinates theactivities of the chemical industry. For this reason it is
probablythemostinterestingthekeytothedecision-makingsectoroftheindustry.
Theobjectofthispreamblehasbeentoshowtheplaceofchemicalengineeringinthe
industrynowtothelayoutofthebookitself.Thenextchapterisconcernedwiththeraw
materialsoftheindustry.Chemistrytextbookstendtodescribethemethodofpreparation
of chemicals, while the chemical industry is based on the flow of chemical materials
throughaprocessingnetworkwhichoriginateswiththeextractionofrawmineralsfrom
theearth.Thefollowingchapters,andtheserepresentthemajorproportionofthebook,
are an introduction to chemical engineeringboth theoretical and practical. We then
reviewenergy,theservicesrequiredbyachemicalplant,anditscontrol.Chapter8deals
withsomeoftheproblemsofdesigningandbuildingaplantwhichwereoutlinedatthestartofthisintroduction.Thebookcloseswithadiscussionofthechemicalengineering
profession.
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9FUNDAMENTALS OF CHEMICAL ENGINEERING
RAW MATERIALS FOR THE CHEMICAL INDUSTRY
Therawmaterialsofthechemicalindustryareenergy,air,water,andawiderangeofbasicmineralswhichcanbeextractedfromtheearth.Thischapterprovidesanintroductiontothe
processingmethodsusedformineralextractionandpurification.Miningistheprovinceofthe
mining engineer; in dealing with minerals purification processes we enter the area of
chemicalengineering.Webegintoseethesortofcontinuouslyoperatingprocesseswhich
the chemicalengineerdesignsand someof the equipment whichheuses. This chapter,
then,coversthestartingpointforchemicalprocessthebasicmaterialsandalsoprovidesa
leadintothesubjectofchemicalengineering.
The importanceofwaterasa rawmaterial inthechemical industryarisesmainlyfrom itsrelationshiptotheenergybalanceofachemicalplant.Mostchemicalprocessesresultin
the liberation orconsumption ofa significant quantityofenergy. If a supply ofenergy is
requiredbytheplantitcanbeobtainedbythecombustionoffuelsorbyuseofelectricity.
Energy considerations are so intimately connected with processes that energy is as
importantastherawmaterialsandproductsthemselves.Forthisreason,Chapter7deals
with the integrationof the energy systemwith the plant. It isvery frequently the need to
conserveenergy,inordertoachieveeconomyinproductionthatresultsinthecomplexityof
chemical plants. The most frequently used source of energy or absorbent of energy is
steam and steam-water systems represent a significant part of most chemical plants.
Natural water, however, is not suitable for use in steamsystems, nor as a process rawmaterial,nor,indeed,formanycoolingwaterapplications.Inaboilersystem,naturalwater
wouldcauseharddepositsasoccur inkettlesand thesereduce theboilerefficiency; the
salts innaturalwaterare frequentlyunacceptable toa chemical reaction,whileitsuse in
cooling systems will lead to corrosion. The chemical plantmust therefore include water
treatmentsystems.
Air,aswellaswater, isusedasacoolingmedium,airbeingdrawnby fansoverbanksof
finnedtubesthroughwhichpassestheprocessliquidtobecooledthedesignislikethatofa
car radiator. Air, however, also plays an increasingly important role as a process raw
material.Wemaydivideitsusesupintofivemainones:
1.asanoxidant;
2.asastrippingagentforremovalofunwantedvolatilecomponentsfromaliquid;
3.asasourceofnitrogenforthefertilizerindustry;
4.asfeedtoair-separationplantssupplyingoxygen;and
5.foroperationofpowertoolsandplantinstrumentsandvalves.
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Theapplicationsofairasadirectoxidantarenumerous.Insomecasesitisusedforgas
phasereactionsoverselective,oxidativecatalysts;frequentlyitisusedinliquidsystems,
being bubbled through the liquid or passing upwards through an absorption column in
whichtheliquidflowsdownward.Intheuseofairasa'strippingagent'theprocessequip-
mentissimilartothatforitsuseinliquidoxidationsystems.Theairpassesthroughthe
liquidandtransferofthevolatilecomponentsfromliquidtoairtakesplace.
Nitrogenisusedbythefarmersfortheircropstotheextentofabout750000tonsperyear.
The separation of this nitrogen from oxygen occurs in the process for preparation of
ammonia synthesis gas. Reaction of liquid or gaseous hydrocarbons with steam, and
subsequentlywith air, yieldsa gas streamcontainingnitrogen, hydrogen,andoxidesof
carbon; when the oxides of carbon are removed, a mixed hydrogen and nitrogen gas
streamforammoniasynthesis isproduced.Onemightexpectthatequipmentneededfor
theuseofairinsuchaprocesswouldbesimple.Infact,eventhissectionoftheplantisof
considerable complexity and requires careful chemical engineering. For an ammonia
synthesisplanttheairupto50tonsperhour-isfirstfilteredtoremovedustparticlesandthencompressed.Whenairiscompresseditbecomeshotandcompressionofhotgasesis
less efficient. This difficulty is overcome by compression in stages with intermediate
coolingoftheair.Coolingoftheaircausescondensationandagascompressorshouldnot
haveliquidsfedintoit.Thecondensedwatermust,therefore,beseparatedfromtheair
beforeitentersthenextcompressionstage.
The development of tonnage oxygenplantshas enabled those processeswhichbenefit
from the useofoxygen freeof nitrogen toobtainoxygenby the liquefactionofair.The
nitrogenalsofindsmanyindustrialusesasdotheothergases-argon,helium,krypton,and
xenon-whichcanberecoveredfromtheair.Themoderntrendwiththistypeofunitisto
siteindividualairseparationplantsattheplaceswherethereisademandforoxygenintonnagequantities.Inthisway,thehandlingandtransportationoflargeamountsofoxygen
areavoided.Thedesignandconstructionofairseparationplants isaspecializedfieldof
chemical engineering and has become the accepted province of a few specialist
companies. As well as building plants of a packaged nature, these companies often
produceanddistributeliquefiedgases.Forthereaderwishingtohavemoreinformationon
oxygen,nitrogen,and industrialgasesgenerally,referenceismadeto thevolumein this
seriesofindustrialgases.
Amostimportantgroupofmineralrawmaterialsisthatofgaseousandliquidhydrocarbons
which,apartfromtheiruseasfuels,arealsomajorrawmaterialsforchemicalproduction.
Hydrocarbons are of primary importance as rawmaterials for the organic sector of the
industryplastics,fibers,solvents,andthewiderangeoforganicandfinechemicals.Their
secondmajoroutletisintheproductionofammonia,nitricacid,andnitrogeneousfertilizers.
2.1 MINERALS IN THE CHEMICAL INDUSTRY
Mineralmaterialsareallsubjecttoessentiallythesameprocessingstages:
1. Theyareextractedfromtheearthbyminingtechniques.
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2. Theyaregroundtoafinepowder.
3. Theyarerefinedtoseparatethedesiredchemicalfromimpurities.
4. Theyareusedtoproducemetalsorchemicals.
Mineralsaregenerallyseparatedintotwocategories.Thoseusedprimarilyfortheirmetal
values are considered the province of the metallurgical industry, while the mining and
treatmentofthoseextractedfortheirchemicalvaluesaregenerallytheresponsibilityofthe
chemical industry. In both cases, however, the techniques used are within the scopeof
chemical engineering.Themetallurgicalmineralsandthemetals extracted fromthemare
supplied to the chemical industry in a reasonably pure state for use in the production of
chemicals.Becauseof thecloseapproachof themetalsandchemicalindustryinthisarea,
therearemanycasesofchemicalcompaniesproducingmetalsandviceversa.
Itshouldbenotedthatsomemineralsfindlargescaleusedintheboththechemicalsandmetalindustries.
1. CopperandIronPyritesareusedbothfortheirmetalvalueandasasourceof
sulfurforsulfuricacid.Thepyriteisproducedandpurifiedinthemetalindustryandusually
transported to a chemical works for production of sulfuric acid. The byproduct from the
roasting iscalledcalcineandconsistsofironandcopperoxides.This issoldbacktothe
metalsindustryforironorcoppermanufacture.
2. Bauxite is used as a source of aluminium and of aluminium oxide for the
chemicalandbuildingindustries.Thebauxiteisextractedandpurifiedbythemetalsindustry
andsuppliedtothechemicalsindustry.
3. Titanium ores are used as a source of metallic titanium and for titanium
dioxide,whichisusedasawhitepaintpigment.
4. Salt is used for the manufacture of sodium hydroxide, sodium chemicals,
sodiumitself,andchlorine.Sodiummetalisusedprimarilyforchemicalpurposesandhas
veryfewmetallurgicaluses;infact,thewholesalt-based,alkaliindustry,includingsodium
production,isapartofthechemicalindustry.
In the rest of
this chapter
CaSO4
Sulfuricacid,ammoniumsulphate.
barite
BaSO4
Paints, barium chemicals (carbonate, sulphide,
sulphate,chloride,oxide,hydroxide,peroxide).
borates
various
Enamels,glass,boronchemicals.
dolomite
mixed
MgCO3,CaCO3
Buildingindustry,catalysts,magnesiumchemicals.
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feldspars
K, Al, Ca, Na,
silicates
Glassmaking,pottery,porcelain.
fuller'searth
clay
Generally in the chemical industry for cleaningand
decolorizing.
graphite
carbon
Graphiteequipment,lubricants
Note: Theseminerals are widely distributed as indigenous ores inmany countries. The
crystallinecompositioncanvaryquitelargelybetweendeposits.
InTable 2.1 some of the other mineralsused by the chemical industry are listed.Some
mineralsare useddirectly with very little processing, for example sandand limestone forbuildingmaterials. Even the building industry, however, is using increasing quantities of
preformed building materials, whose manufacture is very much a part of the chemical
industry.Furthermore,claysandsands,limestone,etc.,evenwhenusedwithoutchemical
processing, must be taken through some mineral treatment stages if good structural
properties are to beobtained.Similarly, thediagramshows potassium-containingmineral
saltsgoingdirectlyintofertilizers.Apotassiumchlorideminemaycostaround$50million,
while the associated surface plant for extracting potassium chloride suitable for use in
fertilizersfromtherawmineralwilladduptoanother$30million.
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Thechemistrytextbookmaysay,'Takepotassiumchlorideandtreatitwith\butfroma
chemicalengineer'sviewpointthemajorproblemliesinextractingthepotassiumchloride-the
restiseasy!
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2.2. MINING TECHNIQUES
The first task is the location of suitable deposits of minerals for extraction and the
provingoftheirsizeandworkability.Thisistheworkofthemineralogistandgeologist,
whousetechniquesbasedontheirknowledgeofrocks,strata,etc.Theprovingofthesize of a deposit is usually carried out by drilling out core samples; these are
subsequently examined and analyzed to provide a three-dimensional map of the
regionfromwhichthecoresaretaken.Theanalysesenabledecisionstobemadeon
theminingmethod, the shaft location, and the processes tobe used inrefining the
mineral.
Therearethreemethodsforrecoveringtheorefromtheearth.Theseare:
1.Surfacemining,
2.Undergroundmining,and
3.Solutionmining.
The simplest typeofsurfacemining isdredging, which is typically used for recovering
gravelsfromriverbeds.Abucketelevatorgrabsthegravelfromthebedandthebuckets
carryittoahopperinthehullofthedredger.Herethematerialisscreenedandthentaken
ashorebybeltconveyortopile.
Thetechniqueusedforrecoveringmineralswhichlieinseamsjustbelowthesurfaceis
open-cutmining.Theseammaylieatalmostanyangletothesurfaceandoftenseams
cometothesurfaceinahillside.Thefirstactionistoremoveoverburden,etc.,toexpose
a cliff orworking facewhich is vertical, and fromwhichmineral can be extracted with
power machinery. In the case of seams parallel to the surface, the working face is
exposedbydiggingdownthroughtheseam.Material removedfrom thefacebypower-
drivenshovelsisloadedintowagonsortrucksandconveyedtothemineralprocessing
plant.Inthiscountrythelandisusuallyrestoredtoitsoriginalconditionorbetteroncethe
mineralhasbeenextracted.
Minerals frequently extracted by surface mining include clay, limestone, sand, gravel,
coal,phosphaterock,bauxite,ironore,andothermetallurgicalminerals.
Undergroundminingitselfcanbeconductedinmanydifferentways,dependingchieflyon
thenatureofthedepositanditsorientationrelativetothesurface.Thebasicelementsofthe
minearetheshaftdowntothedeposit,tunnelingalongthelineofthedeposit(unlessitis
verysteep),andremovaloforefromafacebyexplosiveand/ormechanicalmeans.Theore
istransportedtotheshaftbyconveyors,trucks,orwagonsandthenhoistedtothesurface.
Thechieffactordifferentiatingoneminingmethodfromanotheristhemethodoftreatingthespaceleftbytheextractedore,sothatdangeroussurfacesubsidencedoesnottakeplace
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15FUNDAMENTALS OF CHEMICAL ENGINEERING
whenmining isover.Thesimplestmethod is to leave anemptyspace; this can be done
whererockformationssurroundingtheorebodyremovedarestrongenough.Thealternative
proceduretothisisonlytoextractpartoftheoreperhaps60-70%leavingtheremainderas
pillarswhichsupporttheroof.
The 'open' methods above are mining methods in which no artificial roof supports areneeded.Itismoreusual,however,tosupporttheroof.Ifthisisdonethensomeactionmust
betakenwhentheorehasbeenremoved.Thefirstapproachistorefillthespacewithwaste
rockandore.Thesecondistocaveintheroofafteroreextraction.Therearemanyvariants
whichhavebeendevelopedforminesofthistype.
Mineral isfrequentlyextractedfromthe facebyblasting.Holes aredrilled inthe faceand
explosivechargesinserted.Ondetonation,thefacefallsinwards.Afterthedusthascleared,
mechanical pick-up-and-loading equipment is brought forward. The equipment has long
armswhichmoveintothebrokenoreandfeeditontoaconveyor;thisloadsthetrucksor
wagonswhichcarrytheoretotheshaft.
Inminingoperations,particularattentionmustbepaid toventilation.Airsupplyequipment
andductsmustbeprovidedtodeliverfreshairtothemenworkingattheface,andtoventair
byanotherrouteoutofthemine.Pocketsofgas,especiallymethane,areencounteredfrom
time to time andhigh ventilation rates are used to keep thepurityof theair as high as
possible.Inaddition,regularanalysesof theatmosphereintheminearecarriedout.Inall,
underground mining is exacting work and requires discipline and rigorous attention to
proceduresifsafeworkingistobeassured.
Rocksalt sodiumchloride-and alsopotassiumchloridemay berecovered byasolution
technique.Ashaftorwellissunkintothesaltstrataordomeandwaterispumpeddownto
dissolvethesalt.Asaturatedbrinesolutionisreturnedtothesurface.Thesolutionisthen
clarified, heated, and evaporated; as evaporation takes place the solution becomes
saturatedandthensaltisprecipitatedfromthesolutionasreasonablypurecrystals.Therate
ofproductionthatcanbeachievedfromasaltwellisdependentontherocksurfacearea
exposedtowater,whichinturnincreasesassaltisextracted.Itisnotuntilsomeyearsafter
extractionisstartedthatawellreachesfullproduction.Carefulforecastingoffutureneedsis
thereforenecessary,ifasetofwellsistohavetherightproductioncapacityattherighttime.
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2.3. TECHNIQUES OF PURIFICATION OR REFINING
Therawproductfromaminecancontainthreetypesofimpurity.
1. Gangueearth, clay, sand, etc.which is recovered together with the majorcrystallineaggregatesofmineral.
2.Similarimpuritiesmuchmoreintimatelymixedinthecrystalmassofthemineral.
3.Cocrystallineimpurities,whicharepresentaspartofthesamecrystalstructureas
thechemicalproductdesired.
Whilethefirsttwotypesofimpurityareinprincipleseparablebypurelyphysicalmethods,
thethirdtypeofimpurityusuallyrequirestheuseofchemicaltechniques.Inpractice,itisnot
usualtoseparateco-crystallineimpuritiesattheminesite.Thematerialwithitsco-crystalline
impuritiesistransportedtothechemicalplantandanyfurtherpurificationiscarriedoutthere
aspartofthechemicalprocessing.
Physicalpurificationtechniquesinvolvefindingapropertyofthedesiredmineralwhichdiffers
from that of thegangue. A typical example of such a property is themagnetic nature of
certain iron ores. This property allows iron ore to be extracted from earthymaterials in
magneticseparationequipment.Other typesofpropertycommonly usedarediscussed in
Section2.4.Thetermusedforrefiningofmineralsinthiswayisbeneficiation.
The first step in beneficiation is to grind the raw material finely. This is because the
propertiesusedforseparationarethoseofsmallparticles.Thesubsequentprocesseswillrequire that a particular size ofparticle isused for optimumoperation. The first stageof
mineral dressing, then, is to break up and classify the lumps recovered in the mining
procedure. The process of classification per se often results in some separation of the
desiredmineralfromthegangue.Aftergrindingandclassificationthemineralissubjectedto
thechosenseparationprocess.Abasicprinciplethroughoutchemicalengineeringistocarry
outprocessescontinuouslywithmaterialflowingatsteadyratesfromonestepintheprocess
tothenext.Thisisalsothecaseinmineralsprocessingandtheprocessesandequipment
discussedbelowalloperateonacontinuous-flowbasis.
Crushingisthetermappliedtothebreakingupoflargeparticlesofrawmineral,usuallyin
the size range from about 10mm to about 300mm.Grinding is the term applied to the
processoffurtherbreakingdownmaterialwhichisalreadyfairlysmall.Thebasicdifference
intheequipmentforthetwoprocessesresultsfromtheparticlesize.Largeparticlesrequire
the application ofa largeforce toa relatively small number ofparticles tocrusha ton of
material. Grindingsmall particles requires the application of a much smaller force to a large
numberofparticles.Inthecaseofcrushers,theactivecomponentofthemachinemustbe
able toapply a large forceover a rather small surface area which is in contactwith the
particles.Thegrinderprovidesmeansofapplyingasmallforceovera largesurfacearea
which comes into contact with the fine particles. Anadditional difference is that inmostcrushingmachinestheparticlesresultingfromfirstbreakingaredeliveredbythemachine,
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whileinagrinderfurthersubdivisionofthebrokenfragmentstakesplaceduringaprotracted
periodofresidenceofthematerialinthemachine.
Thejawcrusherconsistsoftwoplatesonestationaryandonemoving.Themovingplateismadetoundergoareciprocatingmotionalternatelytowardsandawayfromthestationary
plate.Asitapproachesthestationaryplatethematerialissqueezedbetweentheplatesand
broken.Typically,thefeedisverticalthroughthecrusherandtheplatesarepositionedtobe
nearertogetheratthebottomthanthetop.Thebrokenparticlesbecomesmallerastheyfall
betweentheplatesandarebroken,andtheyfinallyfalloutfromthebottomoftheplates.
Asimilartypeofcrusheristhegyratorycrusherinwhichtheplatesareconicalinshape,the
movingplaterotatinginsidethefixedone-usuallyeccentrically;thisrotationbringsaboutthe
reciprocatingcrushingmotion.
Therearetwotypesof rollcrusher-single-anddouble-rollmachines.Inthesingle-rolltype,
theparticlesarecrushedbetweentherotatingrollandafixedbreakerplate.Thedouble-roll
crusherhastworollsrotatinginoppositedirectionsandtheparticlesare'nipped'between
thetworolls.Therollsarenotusuallysmoothbutserratedorroughenedtoimprovethegrip
ontheparticlestobecracked.Therollsareheldbyspringswhichtakethefluctuatingloads.
Theuseofspringsisessentialtopreventdamagetothebearingsoftherolls.
In the hammer crusher, the particles are broken by impact rather than squeezing. A
horizontalshafthasanumberofbarsattachedtoitsothattheypivotastheshaftrotates;the
rotationcausesthebarstoflyroundinacircularplane.Particlestobecrushedarefedinto
thepathoftherotatinghammerbars.Thefragmentsandanyparticlesunbrokenintheinitial
impactarethrownagainstabreakerplate.
Theequipmentmostcommonlyusedforgrindingistheballmill.Thisconsistsofarotating
drumcontainingsteelballswhicharekeptinmotioninsidethedrumbyitsrotation.Theballs
breaktheparticlesastheyarenippedbetweenballsorbetweentheballsandthewallofthedrum.Materialisfedincontinuouslyatoneendofthedrumandremovedthroughagrating,
whichretainstheballs,attheotherend(Figure2.2a).
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18FUNDAMENTALS OF CHEMICAL ENGINEERING
Ifthefeedrateincreases,themillbecomesmorefullandmaterialpassesoutatagreaterratebecausealargerareaoftheexitgratingiscovered.
Themillsmaybeoperatedwetordry.Inearlierdaysitwasmorecommontooperatewet.
This overcame one of the serious problems of operating any crushing equipmentdust.
Thereareotheradvantagesapplicabletogrindingparticularminerals.Inwetgrindingitis
usual touse about30%waterbelow this themix isverysticky.With thedevelopment of
moresophisticatedequipmentfordustcontrolithasbecomemuchmorecommontocarry
outdrygrinding.
After crushing and grinding, the next step is to ensure that the desired size grading of
material is taken forward to the refining stage. This is done by a classification process.
Classificationisfrequentlyincorporatedinacircuitwiththemilling(andcrushing)step.The
productfromthemillisclassifiedandtheoversizematerialisreturnedtothemill.
Screening is a process which can segregate particles only according to size, while
classificationseparatesparticlesintogroupsorclassesidentifiedbyotherpropertiesaswell
assize. To this extent, classificationcan carryout some initial separation of the desired
mineralfromthegangue.Inbothcases,theparticleswhicharerequiredfortheseparation
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19FUNDAMENTALS OF CHEMICAL ENGINEERING
processingstepare segregated from the off-sizematerial, which is returned toan earlier
stageorpassedforwardtoanalternativeprocessingstream.
Theprincipleusedinscreeningisexactlythatofthesieve.Thematerialtobescreenedisbrought intocontactwithawiremeshwhichhasholeswhosesizedeterminesthesizeof
materialpassing through.Thematerial isdividedinto twostreams.Toensureseparation
andcontinuityofoperation,twotypesofmotionofthematerialarenecessary.
1.Thematerialmustflowacrossthescreen.
2.Theindividualparticlesmustbemaintainedinmotionsothattheyareconstantly
re-presentedtothescreensurfaceandgiventheopportunitytopassthrough.
Thewiremesh screen isusuallyvibratedupand downbymechanicalmeanstoensure
thatparticlesarekeptinmotionrelativetothemesh.Thevibrationmodemayalsoinclude
a horizontal component of motion to move the particles across the mesh surface.
Alternatively, the screenmaybe inclinedand theparticlesmade to flow across itunder
gravity(Figure2.2b).Screensizesandcapacityvaryoveraverywiderange-typically,a
screen6mlongby 2mwideslopingat20mayhandle40 tonsofmaterialperhour.If
separation into severalsize ranges isrequired then aseriesofscreenswithdecreasing
meshsizemaybeused;theseparatedproductstreamsaretakenfromtheuppersurfaces
oftheindividualmeshes.
Analternativemeansofsubdivisionofamaterialstreamintoseveralsizerangesistouse
atrommel.Thisisarotatingdrumformedofmesheswhoseaperturesizeincreasesfromonesectiontothenextalongthedrum.Theaxisofthedrumslopesandthisensuresthat
thematerialpassesforwardalongthedrum.Thematerialentersthedrumattheendwhere
theapertureissmallestandfractionsofincreasingparticlesizearetakeninsuccessionas
thematerialpassesfromsectiontosection.Thelargestparticlespassoutattheendofthe
trommel.Itisalsopossiblefortrommelstohaveasetofconcentricscreens;inthiscasethe
meshwithlargestapertureisattheinsideandallstreamsleavethedrumsattheendofthe
trommel,whichisagainslopedtogiveforwardmotion.
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20FUNDAMENTALS OF CHEMICAL ENGINEERING
Screeninghasavarietyofapplications inoredressing.Crushersandmillsusuallyoperate
mostefficientlyonafeedoffairlynarrowsizerangeandscreeningis,therefore,usedpriorto
theseoperationsaswellasforseparationoftheproductmaterialofthedesiredsize.
Airclassificationisanalternativemeansofseparatingparticlesintosetsofdifferentsize.It
relies on the different effects of air velocity onparticles of different sizes. If air isblown
across astreamof fallingparticles, the smallest particlesare deflected furthestand ifthe
stream consists of a range of sizes then thesearespread out in thedirectionof theair
stream.Theparticlescanthenbecollectedintostreamsofdifferentsizes.Asimpleclassifier
ofthistypeisshowninFigure2.3(a).Therearemanyotherwaysinwhichtheprinciplecan
beused.Forinstance,inmilling,anairstreamthroughthemillmaybeusedtocarryaway
particleswhentheybecomesufficientlyfine;theheavierparticlescannotbecarriedbytheairandremaintobegroundfurther.
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21FUNDAMENTALS OF CHEMICAL ENGINEERING
Particle feed
Productparticlestreams
decreasingsize
Airandfinestparticlestocycloneseparator
(a)
Theparticlesintheairstreamarethenseparatedfromtheairinacyclone(Figure2.3).The
gasesenterthecyclonetangentiallyandswirlroundinside.Therotationprovidescentrifugal
forcewhichisusedtoseparatetheparticlesfromtheair.Theparticlesareforcedtothewall
ofthecyclonewheretheyaresloweddownbyfrictionalcontactwiththewall.
Theydroptothebaseofthecycloneandfalloutthroughaflapvalve.Thegasespassout
throughthecentralholeatthetop.Correctdesigncanallowforparticlesofaparticularsize
to be carried on with thegas stream. The centrifugal force fieldacts in asimilarway togravityintheairclassifiershowninFigure2.3(a).
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22FUNDAMENTALS OF CHEMICAL ENGINEERING
Inairclassification,thedifferentdensityofmaterialscanalsohaveaneffectsince,generally,
thehigher thedensity the less theparticlewillbedeflectedby theair stream.Particlesof
highdensitywould,therefore,gointotheproductstreamwithlargerparticlesoflower-density
material.Airclassification,however,isnotusuallyusedasameansofseparatingmaterials
ofdifferentdensitiesbutonlyforsizing.
Inwaterclassification,thedensityofthemineralismuchclosertothatoftheclassifying
fluid and, although theprinciple ofclassification isagain that of interaction between the
fluid and theparticle, the results that can beachieved are different. Inair classification,
density has only a small effect compared with size, whereas in water classification the
effectofdensitydifferencesbetweenparticlesismoreimportant.Weknowthatifwestirup
apowderofdifferentparticlesizesinaliquid,theparticleswillbesuspendedintheliquid
owingtotheturbulentmotionoftheliquid.Ifstirringisstoppedtheparticleswillsettle;the
largeoneswillsettlefirstbecausetheyhavethehighestfallingvelocitythroughthefluid
medium.If wemixed the types of particles- one typehavingdensityclose to thatof the
waterandonebeingmuchheavierthanwaterthenevenquitefineparticlesoftheheavy
materialwouldsettletothebottombeforethelargerparticlesofthelightermaterialstarted
tosettle.Thisisaprinciplehavingverywideapplicationinmineralsprocessing.Theterm
classification,however,isusuallyappliedtotheseparationoflargeparticlesfromsmaller
ones,withdensityhavinglittleeffect.
Inacontinuouslyoperatingclassifier,liquidandmineralfeedentertheclassifiertogether
andmechanicalagitationisapplied.Thiskeepsthefinerparticlesinsuspensionwhilethe
larger ones settle through the liquid to the bottom.The finematerial overflowswith the
liquid, while the larger settles to the bottomand is raked along the sloping baseof theclassifieroutoftheliquid.Ifthematerialsarerequireddryforsubsequentoperationsthey
mustbefilteredfromthewateranddried.Avarietyofdifferenttypesofclassifierequipment
canbeusedaccordingtothenatureofthematerialsinvolved.Arecentdevelopmentisthe
hydro-cyclone.The principle of thehydro-cyclone orhydro-clone issimilarto that of the
cycloneinFigure2.3(b),exceptthatthereisabottomexitforfluidaswellasoneatthetop.
The topoverflowconveysthefinermaterial,while thebottomunderflowcarriesawaythe
largerparticles.
Thenextstageinmineralprocessingisseparationorconcentration,inwhichthepurityofthe
mineral is improvedbyseparating the impurities fromit.Theprior crushing, grinding, and
classificationwerenecessarytogivethefine,evenlysizedparticlesrequiredforthevarious
separationprocesses.Onemethodofseparation,asmentionedintheprevioussection,is
basedonthedensitydifferencesbetweenthemineralandgangueparticlesandaseparating
fluid.Separationbythismeansiscalledgravityseparation;theothercommonmethodsare
flotationandmagneticseparation.
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The simplestwayofusinggravityseparation is touse a liquidwhosespecificgravity lies
betweenthatofthetwomaterialstobeseparated;thelightermaterialfloatsandtheheavier
sinks.Mostmineralsandthegangueareheavierthanwaterandsuitablehigh-densityliquids
aredifficulttofind.Therearemethods,however,inwhichwatercanbeusedandseparation
isstillpossible.Thesemethodsrelyonthedifferencesinmotionofthelighterandheavier
particleswhenthewateritselfisinmotion.Ifthewater,forexample,ismadetoflowverticallyupwardsitwilltendtocarrythelighterparticleswithit,whileheavieronesofthesamesize
will sink.A further development is tomake use of the separating effectof the combined
liquid-particle mixture. The effect achieved is * one of hindered settling in which the
particlesintheliquidinterferewitheachother'smotion.Equipmentinwhichhinderedsettling
isexploitediscalledajigoratable.Botharenamesforbroad,shallowtroughstowhich
water and ore are fedbatch-wiseor continuously, and to which it is possible to apply a
vibratorymotion.Inthecaseofthefig,motionisvertical.Thejiggingactioncausesincreased
liquidmotionrelativetothesolids,andincreasedparticle-to-particleinterference;theaction
resultsinanequilibriumorientation,inwhichparticleswithdifferentpropertiesarestratified
in the jig. In jigging, the interaction of the various size,density and shape factors of theparticles iscomplicated.Theverticaljiggingmotionmaybeappliedbypulsingthewateror
feedflowtothe
jig-In tabling, themotion is applied directly to the table or trough and is a reciprocating,
horizontalmotion. The motion isnot simple-harmonic but rather with steady acceleration
from rest and an abrupt stop. This motion now produces stratification which becomes
graduallymoredefinedasthewaterandoreprogressalongthetablefromthefeedpointto
theofftakeregion.
Arecentdevelopmentalsohasbeentheuseofafluidizedbedofparticlesastheseparating
medium.Thebedismadeupofparticlesofappropriatedensityandsizewhichareheldinafluidizedconditionbyairblownupwardsthroughagridatthebaseofthebed.
Oneofthemostwidelyusedbeneficiationtechniquesisthatoffrothflotation.Theprincipleof
thisprocessisthatthemineralparticlesarenotwettedbytheliquidandthusremainonthe
surfaceoftheliquidwhentheliquidusuallywateroraqueoussolutionismixedwiththeraw
mineral.The capacityofthequiescent surfaceofthe liquid for holdingmineral is not very
large.Itcanbevastlyincreasedbystirringorbubblingtheliquidtoformafroth,inwhichthe
poorlywettedmineralparticlesareheldinthebubblefilms.Themajorproblemintheuseofthistechniqueisthatmostmineralsarerelativelyeasilywettedanddonotfloat.Thisdifficulty
is overcome by the use of anti-wetting agents. These agents are used to treat the raw
mineralaftergrindingandtheybecomeselectivelyattachedtothemineralparticles,which
arethenheldinthesurfaceratherthanthebulkliquid.
Anti-wetting agents (or surf ace-active agents) are frequently long-chain hydrocarbon
moleculeswithpolarradicalsatoneendforexample,octadecylamine.Theaminoendofthe
molecule ispolarand isattracted to themineral salt particlewhich isapolarmaterial.A
loosebondisformedbetweenthesaltandtheaminoendoftheorganicmolecule.Thenon-
polarhydrocarbontailof themoleculenowrepelspolarliquidssuchaswaterandprevents
the mineral particle becoming wetted. Because the agent is only required to form a
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24FUNDAMENTALS OF CHEMICAL ENGINEERING
monomolecular layer on the mineral particle, quite small amounts are required; and this
smallimpuritydoesnotgenerallyinterferewithsubsequentprocessingofthemineral.Flota-
tionagentscanalsoshowahighdegreeofspecificityforonemineralratherthananother.If
the rightagentis used, flotation can be employedto separate the desiredmineral froma
crystallineimpurity.
Frothfromtheflotationcelloverflowsandiscollectedinatankwherethefoambreakseither
naturallyorwiththeaidoffoam-breakingchemicals.Therecoveredparticlescanthensettle
and be separated from the liquid phaseprior to filtration and drying or further stages of
beneficiation.
Magnetic separation relies on the mineral and the gangue having significantly different
magnetic permeabilities. Themotionofparticles inamagnetic field is influencedby their
permeability. If thepermeabilitydifference issufficiently large,a significant segregationofmaterialsintotwostreamscanbeachieved.Themineralstowhichthistypeofseparationis
usuallyappliedare those having some degreeof ferromagnetism, especially iron-bearing
ores. As the material passes through the magnetic field, the ferromagnetic material is
attractedtothemagneticpolecollectingsurface.Variousmechanicalarrangementsarethen
usedtoremovethecollectedmaterial,keepthecollectingsurfaceclear,anddistributethe
materialshavingdifferentpropertiestoappropriatefollowingstagesofprocessing.
2.4 EXTRACTION TECHNIQUES FOR SOME MINERALS
Inthissectiontheprocessesforextractionandbeneficiationofthreemineralmaterialsare
describedtoshowhowthevarioustechniquesdiscussedintheprevioussectionarefitted
intoaprocessingscheme.Itshouldberememberedthatrarelyaretwomineraldepositsof
thesamechemicalidenticalandthataprocessingtrainsuitableforonedepositmayrequire
tobeconsiderablymodifiedforprocessinganotherdeposit.Inparticular,depositsvarywith
respect to the nature of the impurities present and different impurities require different
separationtechniques.
Native sulphur or brimstone occurs in associationwith other sulphur-containingminerals
suchasgypsum(CaSO4.2H2O)andanhydrite(CaSO4),andalsowithcalciteordolomite.
Theprocessbywhichnativesulphur isusuallyextracted istheFraschprocess.A hole is
drilleddownintothedepositanda150mmpipeconsistingofthreeconcentricpipesisput
downintothewell,asshowninFigure2.4.Hotwater,which isunderpressureso that its
temperature isabove themelting pointof sulphur, goes down the outer annulus and out
throughholesintothesulphur-bearingrockmatrix.Thesulphurismeltedbythehotwater
andsinksbeneaththewaterlevel.Themoltensulphurthenflowsintothecentral/oneofthe
pipe,andcompressedair,whichhasbeenbroughtindownthecentralpipe,forcestheliquidsulphurwiththeairHowtothesurfacethroughthemiddleannulusinatwo-phasemixture.At
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25FUNDAMENTALS OF CHEMICAL ENGINEERING
the surface the sulphur is disengaged from the air and allowed to solidify in large vats.
Underground,thewaterreplacesthesulphurthathasbeenremoved.
Thesulphurproducedinthiswayisofhighpurityandmaybebrokenfromthesolidification
vatsfortransportation.Asinglewellofthistypeiscapableofproducingamilliontonsayear.
An increasing proportion of world sulphur requirements is now being met by extracting
hydrogen sulphide from 'sour1 natural gas. The H2S is removed byabsorptionand then
stripping of the solvent.Part of theH2S is converted to SO2 which then reacts with the
remainderoftheH2SintheClanskilnprocess.
2H2S+SO2______________3S+2H2O
Infact,inCanadaatpresent,naturalgasisbeingextracted,thesulphurrecovered,andthe
gasthenreturnedintothegroundasitisnotneeded.
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26FUNDAMENTALS OF CHEMICAL ENGINEERING
Silviniteisamixedorecontainingseparatefinecrystalsofsodiumchlorideandpotassium
chloride. One method bywhich thesemay be separated is fractional crystallization. The
solubility of the KCl increases quitemarkedly with solution temperaturewhile that of salt
changes little. By making up a hot saturated solution and then cooling, it is possible toseparateKClcrystals.
It isnowmoreusualto carryout themajorseparationbyflotation.A typicalflow-sheet is
showninFigure2.5.Theoremustfirstbecrushedtoreducetheparticlesizetothatofthe
individualKClandNaClcrystals.
Rawore(largelypotassiumandsodiumchlorides)
To optimize crushing, the ore is screened to three sizes in the first screen which has a
coarse mesh with fine mesh below. The 'fines' passing bothmeshes are already small
enoughforthenextstepintheprocess.Theremainderpassesthroughthecrushingcircuit
asshown.Clayandotherfineimpuritiesintheorearenextwashedoffthecrystalswithbrine
and separated from themas 'slimes' in the classifiers.The brine -slimesmixture goes to
thickeningtanksinwhichthebrineisrecoveredforreuse.Thebrinecontainingthecrystalsis
treatedwithaconditioningagent,whichcausestheKCltobefloated.Aliphaticaminesare
used;twostagesofflotationarerequired.Inthefirststage,theobjectistoensurethatallKClisfloatedandtheunfloatedimpuritycanbediscarded;someNaClalsofloatsandthesecond
stageofflotationremovesmostofthis.Inthesecondstage,theobjectiveistoremoveall
NaClfromtheKClmaterialwhichispassedforward.AgreaterfractionofKClisrejectedand
so the unfloatedmaterial is recycled to the first stage. The KCl product from the second
stageisusually9798%KCl.Itisrecoveredfromthefoaminthecentrifugeanddriedina
drumdrierwithacounter-currenthotairflow.Brineisrecoveredfromallwastestreamsand
recycled;theclayandNaClwastearedumped.Aproductionrateofamilliontonsperyear
maybeexpectedfromsuchanoperation.
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Phosphate rock isabroadtermapplied tovariousphosphate-containingminerals. The
phosphateisusuallycombinedastricalciumphosphateCa3(P04)2.Thecalciumphosphate
isalsooftenassociatedwithcalciumfluorideasfluoroapatite[Ca3(P04)2]3.CaF2,andother
commonimpuritiesarealuminiumandironphosphates,calciumcarbonate,sulphate,and
silicate.Depositsareminedbybothopen-pitandundergroundminingtechniques.
TheproductfromtheminingoperationsinFloridaistypicallyone-thirdeachofclay,sand,
andphosphatedepositsintheformof'pebble".Pebbleisthenamegiventothistypeof
particulatephosphatedepositmixedwithclayandsand;thepebblesizemaybefrom2to
20mm.Thefirststageofbeneficiationistoseparatethepebblefromtheclay,sand,and
matrix. This is done by a washing operation and screening. The fine material, still
containing fine phosphate particles, then goes into a classification stage fromwhich it
passestofrothflotationforfinalseparationofthefinephosphatefromsand.Useofafatty
acid flotation agent allows the phosphate mineral to be separated. The phosphate
mineralsrecoveredfromthewashing,classification,andflotationstagesare,ofcourse,of
differing purity as well as differing size grading. Each phosphate mineral producingcompany therefore markets a range of products and in addition the compositions of
minerals from different mines all vary. This variation in composition, size range, etc.
presentsconsiderableproblemsforthedesignerof theplantconsumingphosphaterock.
Thedesignerofaphosphoricacidplantmustcharacterizetherockgradeorgradeswhich
willbeusedintheplantanddesigntheacidplantaccordingly.
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28FUNDAMENTALS OF CHEMICAL ENGINEERING
FUNDAMENTALS
Thischapterbeginsthe chemicalengineering sectionof thebook.Letusstartwitha fewthoughtsonthecontributionmadebyindustryandthevalueofengineeringitself.Industryis
concerned with meeting human needs, whether it makes fertilizers to meet food
requirements,washingmachines tohelp provide time for leisure pursuits,or golf balls to
meetrelaxationneeds.Theconsumertellsindustryinaverysimplewaywhetherhisneeds
are being catered for by his willingness to pay the price the manufacturer asks for his
product.Inafreesocietynootherindicationisnecessary.Likewise,industryknowsthatitis
successfullymeetinganeedif,atthepriceitcanobtain,itisabletosellsufficientofitspro-
duct to cover costs and preferably expand its production facilities. The major concern of
engineersistheminimizationofproductioncosts.Thesuccessoftheengineerinthisallows
prices to be lowered and sales and production to be expanded which leads to higher
standardsofliving.
Thecostofproductioninthefactoryismadeupofseveralcomponents:
1.Costofrawmaterialsandpower.
2.Costofman-hoursneeded.
3.Costofresearchanddevelopment.
4. Interestorreturnoncapitalemployed.
5.Depreciationandmaintenancecosts.
6.Administrativeandsellingexpenses.
If the engineer, chemical orotherwise, findsaway to reduce a basiccost ofproduction,
severalsmallchangescanbeexpectedinduecourse:
1.Sellingpricecanbereduced,allowingmorecustomerstobuytheproduct.
2.Themenworkinginthefactorycanbepaidhigherwages.
3.Theexpenditureonresearchanddevelopmenttofindthenextimprovement
canbeincreased.
4. The interest and dividends paid on capital can be increased so that more
capitalwillbecomeavailabletobuildmorefactories.
Thesechangesoccurgraduallyratherthanatonce.Overtheyears,however,progressiveimprovementinlivingstandardsresultsfromdevelopmentsmadebyengineersinanindustry
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29FUNDAMENTALS OF CHEMICAL ENGINEERING
whichoperateswithin theconstraintsof themonetary,socialandlegal requirements.The
chemicalengineer,then,isassistinginprogress,improvingthestandardsofhisfellowmen
customersandco-workers-andsojustifyinghisownexistence.Thesuccessoftheindividual
canonlycontributeto,andoccurwithinthecontextof,improvedstandardsforeveryone.
Thecostoftheproduct,togetherwithanyreductionwhichcanbemade,isthustheessentialyardstick with which the engineer must measure his work. The chemical engineer must
understand the rudiments of company and project finance and be able to evaluate the
financialviabilityofprojectsanddevelopmentwork.(Formoreinformationonthisreference
shouldbemadetothevolumeinthisseriesonsocialandeconomicaspects.)
Thedivisionbetweenengineeringandscience is frequently anarrowone,andthe career
choicebetweenthetwoisadifficultonetomake.Atthetimethechoicemustbemadethe
studentgenerallyhasseveralyearsofsciencebehindhimand'engineering'isnotaterm
conveyingmuchmeaning.Perhapsthesimplestwaytoseparatescienceandengineeringis
toaskoneselfwhatquestionstheyareseekingtoanswer.Generallyspeaking,scienceisconcernedwiththequestion 'why?' andengineeringwith 'how?Science isprobingwhy a
subatomic particle reacts ina given way; engineeringasks how this fact canbe used to
producepower. Engineering isalways quantitative: the answersmust appear as sizesof
buildings,vessels,catalystpellets,pumps,conveyors,etc.
Allengineeringisconcernedwiththeestimationofthequantitativevalueswhichspecifythe
structureandoperationofman'smaterialenvironment.Intheengineeringofchemicalplants,
the chemical engineer is concerned with flow rates and physical properties of materials
within the plant and with dimensions of equipment used to contain and convey those
materialsandtoaltertheirphysicalproperties.Oncethebasicdataofaplantareestablished
the details of how vessels are to be constructed, how equipment is to be sited, howfoundationsaretobebuilt,etc.,etc.,becometheprovinceofspecialistengineers.Chemical
engineeringisparticularlyconcernedwiththestudyoftheconditionsunderwhichprocesses,
reactions, changesinphysical conditionsofmaterials takeplace,andwith thepracticeof
usingthisinformationtodeterminephysicaldesignvalues.Thatmayappearratherabstract
but,asanexample,considerthequestionofthefallinpressureasafluidflowsthrougha
pipe.Inordertobeabletocalculatethisfallinpressureitisnecessarytoknowarelationship
betweenthepressuredropPandtheviscosityofthefluid ,itsvelocityvanddensityp,
andthedimensionsofthepipe,lengthl,diameterd;thatis:
P=f(,v,p,l,d) (3.1)
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By study of the theory of fluids flowing in smooth pipes it is possible to derive such a
relationship.Inpractice,thewallofthepipemayberough,dirty,orrustyanditisnecessary
toknowhowtoallowforsuchdeviationsfromtheidealsystemforwhichtherelationshipwas
derived.Practicalexperienceisaresultofexperimentandfromthismethodsforallowingfor
roughness,dirtiness,etc.,areworkedout.Thusthetoolsusedbyachemicalengineerarein
parttheresultoftheoreticalstudyoftheprocesseswithwhichheisconcerned,andinpart
empiricalfactorswhichtheengineerusestoallowfor thedeviationoftherealsystem,with
which he deals, from the ideal system to which his theoretical model applies. This is a
recurrentphenomenonofallbranchesofchemicalengineering.It isalsointerestingtonote
that there is some tendency to division of chemical engineers into two types-thosewho
enjoy,andareprimarilyconcernedwith,thetheoreticalaspectsandthosewhoareprimarily
practicalandworkintheempiricalarea.Anobjectiveofchemicalengineeringresearchisto
extend the areaof the subject for which theory isapplicable and empiricism isno longer
necessary.Infact,onewayof lookingattheempiricalapproachisthatitisnecessaryforaparticular problem,because thatsector is socomplex that the theoretical engineers have
beenunabletodeveloptheappropriatetheory.ComingbacktoA/;inprinciplethereisno
reasonwhythetheoreticalengineer shouldnotdevelopa theorywhichcoversentirelyall
possiblepipes,includingdirty,rough,rustyones;thentherewouldbenoneedforempiricism
-itisjustthatthetheoryhasnotyetbeendevelopedfarenough.
Theimportanceofnumericalquantitiesinchemicalengineeringgivesgreatemphasistothe
systemofunitsusedbythechemicalengineertoexpresshisvariables,hisflows,pressures,
heat fluxes, etc. Acommon system ofunits isshortly tobe introduced throughoutBritish
industryand,itishoped,onaworld-widebasis.Inviewofthis,asectionofthischapteris
devotedtothissystemanditisusedthroughoutthebook,withtheexceptionthattheuseof
the commaasa decimal marker is not adopted.Another factor, related to the numerical
natureofchemicalengineeringwork,istheavailabilityofdataonthephysicalpropertiesof
chemicals. The chemical engineer must be aware of the sources of such information.
Publishedliteraturebooks,journals,proceedingsofconferencesisgenerallythesourceof
the data required aswell asmethodsdeveloped for carryingoutdesigncalculations.The
chemicalengineer soon becomesexperienced inusing the index and referencesystems
availabletohelphiminseekingtheinformationherequires.
3.1. THE SCOPE OF CHEMICAL ENGINEERING
Justaschemistryissubdividedintoorganic,inorganic,andphysicalchemistry,sochemical
engineeringmaybesubdividedintotopics.Therearetwotypesoftopicwemightcallthese
'general'and'specific'.The'general'areasprovideinformationwhichisusedthroughouta
chemicalplant,while'specific'areasareconcernedwithonetypeofequipmentorstepina
plant.Generalareasinchemicalengineeringare:
Chemistry-knowledgeofchemicalsandtheirinteractions Physicsofsolids,gasesandliquids:diffusion,kinetictheory,etc. Mechanicsoffluidflow,heatflow
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Thermodynamics
Mathematics-calculus,vectors,statistics,economics Principlesofengineeringmechanical,electrical,civil,control
Withthebriefdescriptionofeachsubject,theyarelargelyself-explanatory.InChapter2we
havealreadymetsomeofthebasicprocessingstepsinaplant-sizereduction,classifying,
etc.Ifweexaminethebasicstepsinachemicalplantmorecarefullywewillbeabletolistthe
'specific'areasofchemicalengineering.
ReactantPreparation
Compression Heating Mixing Crushing Agglomerating Dissolution Classification
Reaction
Catalytic- HeterogeneousHomogeneous
Non-catalytic-Homogeneousormulti-phaseProductSeparation
Absorption Distillation Solventextraction Crystallization Filtration Evaporation Drying
Chemicalreactionisnormallytreatedasaseparatesubject.Topicslistedundertheothertwo
headingsare frequentlyreferredtoas'unitoperations'by chemicalengineers. (Unitoperations
listedunderreactantpreparationmaybeusedforproductseparation,andviceversa.)
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These,then,arethesubjectsmakingupacourseinchemicalengineering.Inthisbookparticular
aspects of some of the subjects will bedealtwith using thosewhichare, so far aspossible,
representativeofthewholesubject.Thischapterisconcernedwithfundamentals,especially
fundamental subjects normally regarded as falling within the scope of a chemical
engineering syllabus. The three topics to which this applies are thermodynamics, heat
transfer,andfluidflow.ThewholeofChapter4isdevotedtothermodynamics.Thischaptercontinueswithdiscussionofunits,thebasicrateequation,anddimensionalanalysis,since
theseformthefoundationforheattransferandfluidflowwithwhichthechapterends.
3.2 UNITS - THE SI SYSTEM
Threesystemsofmeasurementunitshavebeenincommonuseinthiscountry:
Foot,Pound,Second (FPS)
Centimeter,Gram,Second (CGS)
Meter,Kilogram,Second (MKS)
The conversion problem between FPS and CGS has been a major difficulty in the
dealingsoftheinternationalengineeringcompanies.
TheeleventhGeneralConferenceofWeightsandMeasuresataninternationalmeeting
in1960adoptedtheInternationalSystemofUnits(SI).Mostcountriesusingthemetric
systemwilladopttheSI.Theperiodofconversiontothissystemis1968-72.Infact,SIis
MKSwithcertainadjustmentshavingchangesinvariousconventionalusesofsymbols.
It is possible to base all units on four independent units of mass, length, time and
temperature.(TheinterrelationoftheunitsisshowninFigure3.1.)SIinfacthassixbasic
unitsbuttwoofthesetheunitsofcurrentandluminousintensity-canbederivedfrom
theotherfour.
Measurement UnitSymbol
Length meter m
Mass kilogram kg
Time second s
Current ampere A
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Thermodynamictemperature Kelvin K
Luminousintensity candela cd
AchangehasrecentlybeenmadeinthetemperaturescaledefinitionTemperaturesarenow
basedontheabsoluteorKelvinscaleinwhichthetriplepointofwaterisdefinedas273.16K,sincethetriplepointcanbereproducedwithgreateraccuracythaneitherboilingpointor
freezingpoint. In fact, it ispermissible to use for temperatureeither the thermo-dynamic
temperaturescaleortheInternationalPracticalTemperatureScale.Thecomparisonisgiven
belowthedegreeintervalisthesameinboth.
AbsoluteZero 0.00 -273.15
Triplepointofwater 273.16 0.01
BoilingPointofwater 373.15 100.0
AllotherunitsinSIarederivedfromthebasicsixunits.Asystemofunitsofthissort is
called a coherent system; the systems we used previously were, of course, non-
coherent-pressureon theFPSsystemwas typicallyquoted inpoundsper squareinch.
Table3.1listssomeofthemoreimportantderivedunitsusedbytheengineer.
ItcanbeseenthatsomeofthenewunitsofSIhavebeengivennames,othersnot.The
namesPascal,PoiseandHertzhavenotyetbeenadoptedbyallcountries.Thereare,in
addition,otherderivedunitsofelectromagneticandlight.
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Frequency Hertz Hz s-
Force Newton N kg.m/s
Pressure Pascal Pa(N/m ) kg/m.s
Viscosity poise PI kg/m.s
Density _ _ kg/m
Velocity m/s
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SurfaceTension N/m kg/s
diffusivity m /s
work(heat) joule J(N.m) kg.m /s
Power Watt W(J/s) kg.m /s
Heattransfer
coefficient
W/m .K kg/s .K
Enthalpy J/kg m /s
Specificheat J/kg.K m /s K
Thermal
conductivity
W/mK kg.m/s3.k
Electricalcharge Coulomb C A.s
Electricalvoltage Volt V kg.m /A.s
Forthepracticingengineer,thechangetotheSIsystemwillinvolvemanyadjustments
changesinstandardsizesandthreads,andeliminationoffamiliarunits.
3.3. THE BASIC RATE EQUATION
Processes proceed. The chemical engineer is concerned with the rate at which they
proceedandtheinfluenceofconditionswithintheplantonthatrate.Becauseofthisinterest
inrates,astandardformofrateequationarisesinalmosteverybranchofthesubject.This
standardexpressionisverysimpleandtakestheform
Rateofprocess=(processrateconstant)X(drivingforce)
Averytypicalexampleistheequationfortherateofheattransferfromafluidmaintainedat
temperature T1 through a tube wall to another fluid maintained at temperature T2. The
expressionis
Q=(UA)X(T1-T2) (3.2)
WhereQ=heatflow(J/s), (UA)=processrateconstant,
U=heattransfercoefficient(J/sKm2), A=areaoftubewallconsidered(m2),and(T1-
T2)=drivingforce(K).
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Inmostofthese rateequations, thechemicalengineer'sproblem lies inestimatingthe
rate constant (UA in heat transfer). It is not possible to set up an experiment and
determineUAbymeasurementoneachoccasionwhenaheatflowmustbecalculated.
The procedure is therefore to develop methods for calculating the constant, U will
obviouslydependonmanyvariablessuchas
1.Thicknessandthermalconductivityofthetubewall,and
2.Velocityandphysicalpropertiesofthetwofluids.
Equationsfor calculatingUhavebeenbuiltup. Theseare partly theoreticaland partly
derived from experimental studies carried out by many workers over awide range of
conditions. The chemical engineer thus uses recognized procedures to determineUA
andhenceQ.
Inmanyoftheprocesseswithwhichchemicalengineeringdeals,thedrivingforceisthesimpledifferenceoftwovaluesofthesamephysicalpropertyliketemperature,pressure,
concentration, or electrical potential; the chemical engineer's task then lies with the
evaluationoftherateconstantonly.Inchemicalreaction,however,theexpressionwhich
isthedrivingforceismorecomplexandgenerallyinvolvesthepartialpressuresorcon-
centrationsofallthemolecularcomponentstakingpartinthereaction.Chemicalreaction
isdiscussedindetailinChapter5.
Afurtherconceptwhichthechemicalengineerneedstograspconcerningratesisthatof
continuity.Thechemistinhislaboratoryworksinbatches;aprimaryobjectinallchemical
processindustryistoachievecontinuousoperation;rawmaterialsarefedcontinuouslyat
oneendfromabulkstoreandproductsleavetheplantcontinuouslyandarefedtothe
product storage.The emphasis isoncarryingout the necessaryprocesses ofmixing,
heating,reacting,andpurifyingduringthesteadyprogressofthematerialsthroughthe
plantequipment.Batchprocessingisnowusedinfrequently.
3.4 DIMENSIONAL ANALYSIS
Inchemicalengineeringitisnotunusualtodealwithaprocessorsysteminwhichaprocess
variabletobecalculateddependsonalargenumberofothervariables.Inordertocalculate
thedesiredvariable,anequationisrequiredtoexpressthatvariableintermsofthevariables
onwhichitdepends.Tolookattheproblemfromanotherangle,letussupposethatinthe
laboratorytheexperimenterhasdeterminedalargenumberofvaluesofthedesiredvariable
(sayboilingpointofasolution)fordifferentsetsofvaluesoftheothervariables(pressure,
concentration);hethenwishestodotwothings.Firstly,hewouldliketoplotgraphsofhis
resultsusingappropriategroupingsofthevariablesand,secondly,hewouldliketoproduce
anequationorcorrelationofthevariables.Thiscorrelationisasimpleformoftheresultswhich
other chemical engineers canuse readily.Dimensionalanalysis enables thegroupingsand the
formofthecorrelation tobedefined.
Dimensionalanalysisisbasedonasimpleprinciplethat,inanyequation,theunitsofthefunctions
oneachsidemustbethesame,i.e.avelocitycannotbeequatedwithaforce.Thedimensionsofaquantityarethetypesofmeasurementneeded,andthewaytheyareused,todefinethequantity.
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Velocity,forexample,requirestheuseofthe dimensionsoflength(ordistance)andtimeinfact
lengthdividedbytime.Thedimensionsofaquantityarecloselyrelatedtoitsunitssincetheunits
used todefine a quantity aremade up from the units used tomeasure the dimensions of the
quantity. The unitoflength isthemeter,of time the second,and of velocitym/ssimilarly, the
dimensionscanbeexpressed(length)/(time).
Thedimensionsofthequantitieswithwhichthechemicalengineerdealsare
massm,lengthl,timeT,temperaturet
AspointedoutinSection3.2,allunitscanbederivedfromthesefour.However,itissometimes
convenienttouseotherbasicdimensionsandthisispermissible.Thedimensionofheat(H)may
beusedinproblemsnotinvolvinginterchangeofmechanicalandthermalenergy.Redundancyin
thedefinitionofdimensionsmustbeavoided.Thismeansthatitmustnotbepossibletoequate
one dimension to a function of the other dimensions. For example, having specified the
dimensions length and time, one must not specify velocity (ratio of length to time) as adimension.
3.5 HEAT TRANSFER
The presenceofheat energy inagas, liquid,orsolid is recognizedby themotion of the
atomsormoleculeswithinthesubstance.
Therearethreebasicmechanismsforthemovementofheat.
Inconductioninsolidsandliquids,heatistransferredfromthewarmerregionofthe
substance to the cooler by the interaction of individual molecules one with its neighbor,
duringwhichtheenergyofonemoleculeissharedwiththeneighbor.Ingases,conductive
transferisduetokineticmovementofindividualmolecules.Thekineticmotionallowsheatto
be conducted in two ways. Firstly, hotter molecules can move into cooler regions and,
second,interactionsbetweenmoleculesenableenergytobeconductedfromhottertocooler
regions.
Convectionheattransfercanoccurinfluidswhichareinmotion.Heatmovesfrom
thewarmer region of the fluid to the cooler by means of bulk motion of the fluid which
interchangessmallpocketsofthefluid.Eachtimeahotpocketoffluidmovesintothecolder
region,andviceversa,bulktransferofheataswellasmaterialtakesplace.
Radiativeheat transferoccursbytheemissionof electromagneticradiationatinfra-
redwavelengths fromonematerialanditsabsorptionbyanothermaterial.Solids, liquids,
andsomegasescanexchangeheatinthiswayatthetemperaturespertaininginsystems,withwhichachemicalengineernormallydeals.
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Whileconductionandconvectiononlytakeplaceinacontinuoussystem,radiativeexchange
canoccurbetweenbodieswellapart.
The problem which frequently confronts us is shown in Figure 3.2(a). Heat is beingtransferredfromahotfluidflowinginsidethetubetocoldfluidoutsidethetube.Cisthetube
wallandflowiscountercurrent.Thetemperatureprofileissketchedbesidethediagramof
thetubeandthetemperaturesaredescribedinmoredetailshortly.Heattransferwithinthe
regionsisasfollows:
RegionA.Whena fluidisflowingwithinatubethetubewallcausesdragonthefluidandflowisfastestatthecentreofthetube.WithinA,convectiveheattransferoccursowingtothemotionofthefluid.
RegionB.InregionB,theboundarylayer,flowvelocityismuchslowerandinsufficientfor
convection.Heattransferisconductiveinthisregion.
Region C. This is the solid material of the tube wall through which heat passes byconduction.
RegionsD,E.TheseregionsfortheouterfluidcorrespondtoBandArespectively,forthefluidinsidethetube.
Fromthetemperaturediagram,itwillbeseenthatmostofthetemperaturedifferenceisin
the two conductivetransfer zones. This isbecauseconduction in these zonesprovidesa
slow rate of heat transfer. (For the moment we are neglecting radiationit is usually not
significantbelowabout500C.)
Heat transfer problems of this type are solved by using heat transfer coefficients. To
demonstrate such a coefficient we can consider heat flow through the tube wall by
conduction.TheheatflowrateQisgivenby
Q=conductivityXareaXtemperaturegradient=k.A.T
where T= temperature difference across the tube wall, = thickness of tube wall, k -
thermalconductivityofwallmaterial.Thus,forthattube
Q=A.T.k/=h.A.T
(Thisformulaisonlystrictlytrueforheatflowthroughaflatplatebutisagoodapproximation
forthin-walltubes.)histheheattransfercoefficientforthattubewallandistheheatflowing
perunittimeperunitareaperunittemperaturedifference.Itwasseenearlierthatheattrans-
ferratesintheliquidzonesarealsolimitedbyconductionthroughaliquidlayerandasimilar
equationcanbewrittenfor flowofheat fromthebulkfluidtothe inner tubewallandfrom
outertubewalltoouterfluid.
Fortransferofheat,anexpressioncanbewritten
Q=h.A.T (3.4)
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whereQ-heatflowrate(J/s) h=heattransfercoefficient(J/sKm2)
A=areaforheatflow(m2)
T=differenceintemperatures(K)betweenwhichheattransferisbeingconsidered.
ThetemperaturegraphinFigure3.2(a)showsthatthetemperatureofthefluidinthetubeis
not uniform, but the fluiddoeshaveamean temperature or 'cup temperature'; this is the
uniform temperaturewhich the whole liquid volume would attain if heat transfer from the
liquidweresuddenlystopped.It isthismeantemperature(T1inthegraph)whichisusedin
theheattransferequationtocalculateT.
The discussion here has been concerned with tubes because the majority of heat
exchangers aremade up from a large number of tubes, which provide the surface area
neededfortransferofheatbetweentwoprocessfluids.Thesameheattransferrelationship
isused for thewhole set of tubesasfor the single tube.One of the process fluids flows
throughthetubesandtheotherflowsoutsidethetubes,thetubesbeingsurroundedbya
shellwhichcontains the outer fluid. This iscalled a 'shell-and-tube' heatexchanger (see
Figure3.4)andthefluidinthetubesiscalledthetube-sidefluid,thatoutsidetheshell-sidefluid.Itisusualtoputbafflesintheshellwhichmaketheshell-sidefluidflowbackandforth
acrossthetubesasitgoesfromendoftheshelltotheother.
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Thiscross-flowfactorspeedsuptheflowoverthetubesandincreasestheshell-sideheat
transfercoefficient.Allowance ismade for this,bystandard techniques, incalculating thecoefficient.
Afewfurthercommonformsofexchangerusedforcoolingdutiesshouldberecognizedby
thereader.
The fluid tobe cooled passes throughaset ofhorizontal zigzag tubes; the set of
tubesliesinaverticalplane.Thecoolingwaterisdistributedoverthelengthofthetoptube
section,runsroundit,andthendropsoffontothenexttubesectionandsoon.
Heat transfercoefficients for air and gasesaremuchsmaller than for liquids,and
largesurfaceareaisthereforenecessary.Itiscommonlyachievedinaircoolers,whereairis
usedtocoolagasorliquid,byputtingfinsontotheoutsideofthetubes.Thiscangiveupto
tentimesthesurfacearea.Thefluidtobecooledpassesthroughthetubesandthecooling
air isdrawnthroughabankof closelypacked, finned tubes bya fanplaced inthespace
abovethetubes.
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Solids are frequently cooled by air. This must be done by keeping the solids
continuously inmotion inthe air.Astandardmethod is tousea rotatingdrumwhich has
platesprotrudingashortwayfromtheinnersurfaceofthedrum.Theserepeatedlyliftthe
solidparticlesupandthendropthemthroughtheair.Aslopeofthedrumoranglingofthe
lifterscausesthesolidstomovealongthedrumcounter-currenttotheairflow.Inventiveness
isstillvitaltochemicalengineeringandarecentinventionisanewtypeofsolidscooler thewaterfallcooler.Thesolidsfalldownwardsthroughup-flowingairinaseriesofcascades.
The effect issimilar to the drum,but the equipment ischeaper incost and requires less
power.
Heattransferisaprocessinwhichthefollowingfactorsareinvolved:
Liquidvelocity v
Liquiddensity p
Liquidviscosity u
Liquidspecificheat CpLiquidthermalconductivityk
Tubediameter d
Tubelength l
The importanceofmostofthese isobvious,ofothersperhapsnot soobvious.Velocity isimportant because as velocity increases so also does the rate of interchange of liquid
'pockets' causing convection transfer; p and Cp measure the heat-carrying capacity of a
pocket, and the viscosity affects the "mobility' of the liquid and rate of movement of the
'pockets'.Itispossibletousedimensionalanalysistoderiveausefulrelationshipbetween
thevariables:
hd/k(pvd/u)aX(uCp/k)bX(l/d)c
Theleft-handsideincludesonlythecoefficient//whichwewishtodetermineandkandd
whicharephysicalpropertiesofthesystem.
Firstly,whatis thesignificanceof l/dinthisrelationship?Itsappearanceisindicativeofthe
factthattheflowpatternoftheliquidinatubechangesalongthelengthofthetube.Infact,
the liquid must be 20-40 diameters along the tube before a constant flow pattern is
established-more about this in the next section of this chapter. In heat transfer design
problems,this"entryregion'effectisusuallyignoredandonlythefirsttwotermsoftheright-
handsideareusedinpracticalcorrelations.The threedimensionlessgroupsaregiventhe
namesoftheiroriginators.
hd/k =Nusseltnumber=(Nu)
pvd/u=Reynoldsnumber=(Re)
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uCp/k=Prandtlnumber=(Pr)
Thus
(Nu) (Re)ax(Pr)b
Byfurtherdimensionalanalysis,otherdimensionlessgroupscanbederived,but thesecan
beexpressedintermsofthethreeabove.
Pecletnumber-(Pe)=(Re).(Pr)
Stantonnumber-(St)=(Nu)/(Re).(Pr)
Therelationshipof(Nu)with(Re),(Pr)isusedbyexperimenterstocorrelatetheirresultsand
establishequationsforcommonusebychemicalengineersindesignwork.Experimentson
tubeswithfullyestablished,turb