fundamentals of chemical engineering

7/30/2019 Fundamentals of Chemical Engineering

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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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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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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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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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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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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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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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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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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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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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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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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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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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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


25/164


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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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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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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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

fundamentals of chemical engineering

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