Chemical Process Equipment Selection and Design

802 Pages·2014·17.65 MB·English

Chemical Process Equipment
Selection and Design
Third Edition
James R. Couper
W. Roy Penney
James R. Fair
Stanley M. Walas
AMSTERDAM•BOSTON•HEIDELBERG•LONDON
NEWYORK•OXFORD•PARIS•SANDIEGO
SANFRANCISCO•SINGAPORE•SYDNEY•TOKYO
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Preface to the Third Edition
Thiseditionofthebookcontainsrevisedandupdatedinformation extensively updated and revised compared to the second and
from both the second edition and the revised second edition, as revisedsecondeditionsofthebook.
wellasnewmaterialasofearly2010.Theauthorsandcollabora- DrWayneJ.Genck,PresidentofGenckInternational,aren-
torshaveincludedinformationessentialtothedesignandspecifi- owned international expert on crystallization has joined the con-
cation of equipment needed for the ultimate purchasing of tributors, replacing John H. Wolf, Retired President of Swenson
equipment.Thevastamountofliteraturehasbeenscreenedsothat ProcessEquipmentCompany.
onlytime-testedpracticalmethodsthatareusefulinthedesignand Oldermethodsandobsoleteequipmentforthemostparthave
specification of equipment are included. The authors and colla- beenremoved.Ifthereaderhasaninterestinoldermaterial,heor
borators have used their judgment about what to include based shemightconsultpreviouseditionsofthisbook.
upon their combined industrial and academic experience. The Thisbookisnotintendedasaclassroomtext,however,with
emphasis is on design techniques and practice as well as what is some modifications and addition of examples and problems, it
required to work with vendors in the selection and purchase of couldbeusedforteachingpurposes.
equipment.Thismaterialwouldbeespeciallyhelpfultotheyoung
engineer entering industry, thus bridging the gap between aca- JamesR.Couper
demia and industry. Chapters 10, 13, 14, 15, and 16 have been W.RoyPenney
ix
Contents
PREFACETOTHETHIRDEDITION ix 6.4. PipelineNetworks 88
6.5. OptimumPipeDiameter 92
PREFACETOTHESECONDEDITION x 6.6. Non-NewtonianLiquids 93
PREFACETOTHEFIRSTEDITION xi 6.7. Gases 99
6.8. Liquid-GasFlowinPipelines 103
CONTRIBUTORS xii 6.9. GranularandPackedBeds 106
6.10. Gas-SolidTransfer 110
CHAPTER0 RULESOFTHUMB:SUMMARY xiii 6.11. FluidizationofBedsofParticleswithGases 111
References 118
CHAPTER1 INTRODUCTION 1
1.1. ProcessDesign 1 CHAPTER7 FLUIDTRANSPORTEQUIPMENT 121
1.2. Equipment 1 7.1. Piping 121
1.3. CategoriesofEngineeringPractice 1 7.2. PumpTheory 123
1.4. SourcesofInformationforProcessDesign 2 7.3. PumpCharacteristics 126
1.5. Codes,Standards,andRecommendedPractices 2 7.4. CriteriaforSelectionofPumps 128
1.6. MaterialandEnergyBalances 3 7.5. EquipmentforGasTransport 130
1.7. EconomicBalance 4 7.6. TheoryandCalculationsofGasCompression 139
1.8. DesignSafetyFactors 6 7.7. EjectorandVacuumSystems 152
1.9. SafetyofPlantandEnvironment 7 References 159
1.10. SteamandPowerSupply 8
1.11. DesignBasis 10
1.12. LaboratoryandPilotPlantWork 12 CHAPTER8 HEATTRANSFERANDHEAT
OtherSourcesofInformation 15 EXCHANGERS 161
8.1. ConductionofHeat 161
8.2. MeanTemperatureDifference 163
CHAPTER2 FLOWSHEETS 17 8.3. HeatTransferCoefficients 165
2.1. BlockFlowsheets 17 8.4. DataofHeatTransferCoefficients 171
2.2. ProcessFlowsheets 17 8.5. PressureDropinHeatExchangers 183
2.3. ProcessandInstrumentationDiagrams(P&ID) 19 8.6. TypesofHeatExchangers 184
2.4. UtilityFlowsheets 19 8.7. Shell-and-TubeHeatExchangers 187
2.5. DrawingofFlowsheets 19 8.8. Condensers 195
References 29 8.9. Reboilers 199
8.10. Evaporators 201
8.11. FiredHeaters 202
CHAPTER3 PROCESSCONTROL 31 8.12. InsulationofEquipment 211
3.1. TheFeedbackControlLoop 31 8.13. Refrigeration 214
3.2. ControlLoopPerformanceandTuningProcedures 33 References 220
3.3. SingleStreamControl 34
3.4. UnitOperationControl 37 CHAPTER9 DRYERSANDCOOLINGTOWERS 223
Bibliography 51
9.1. InteractionofAirandWater 223
9.2. RateofDrying 226
CHAPTER4 DRIVERSFORMOVINGEQUIPMENT 53 9.3. ClassificationandGeneralCharacteristics
ofDryers 230
4.1. Motors 53 9.4. BatchDryers 234
4.2. SteamTurbinesandGasExpanders 54 9.5. ContinuousTrayandConveyorBeltDryers 236
4.3. CombustionGasTurbinesandEngines 57 9.6. RotaryCylindricalDryers 239
References 60 9.7. DrumDryersforSolutionsandSlurries 246
9.8. PneumaticConveyingDryers 247
CHAPTER5 TRANSFEROFSOLIDS 61 9.9. FlashandRingDryers 249
9.10. FluidizedBedDryers 253
5.1. SlurryTransport 61 9.11. SprayDryers 259
5.2. PneumaticConveying 63 9.12. CoolingTowers 266
5.3. MechanicalConveyorsandElevators 68 References 275
5.4. Chutes 76
5.5. SolidsFeeders 77
References 81 CHAPTER10 MIXINGANDAGITATION 277
10.1. ABasicStirredTankDesign 277
10.2. VesselFlowPatterns 279
CHAPTER6 FLOWOFFLUIDS 83 10.3. AgitatorPowerRequirements 281
6.1. PropertiesandUnits 83 10.4. ImpellerPumping 281
6.2. EnergyBalanceofaFlowingFluid 84 10.5. TankBlending 281
6.3. Liquids 86 10.6. HeatTransfer 287
v
vi CONTENTS
10.7. VortexDepth 288 14.3. CalculationofStageRequirements 494
10.8. SolidSuspension 289 14.4. CountercurrentOperation 497
10.9. SolidsDissolving 294 14.5. LeachingofSolids 501
10.10. Gas-LiquidDispersions 295 14.6. NumericalCalculationofMulticomponent
10.11. Liquid-Liquid(L-L)Dispersions 298 Extraction 503
10.12. PipelineMixers 303 14.7. EquipmentforExtraction 507
10.13. CompartmentedColumns 307 14.8. Pilot-Testing 526
10.14. FastCompetitive/Consecutive(C/C)Reactions 315 References 527
10.15. Scale-Up 321
References 326
CHAPTER15 ADSORPTIONANDIONEXCHANGE 529
CHAPTER11 SOLID-LIQUIDSEPARATION 329 15.1. AdsorptionProcesses 529
15.2. Adsorbents 529
11.1. ProcessesandEquipment 329 15.3. AdsorptionBehaviorinPackedBeds 536
11.2. Liquid-ParticleCharacteristics 330 15.4. Regeneration 537
11.3. TheoryofFiltration 330 15.5. GasAdsorptionCycles 543
11.4. ResistancetoFiltration 337 15.6. AdsorptionDesignandOperatingPractices 544
11.5. ThickeningandClarifying 341 15.7. ParametricPumping 547
11.6. LaboratoryTestingandScale-Up 342 15.8. IonExchangeProcesses 548
11.7. IllustrationsofEquipment 343 15.9. ProductionScaleChromatography 554
11.8. ApplicationsandPerformanceofEquipment 355 GeneralReferences 558
References 359
CHAPTER16 CRYSTALLIZATIONFROMSOLUTIONS
CHAPTER12 DISINTEGRATION,AGGLOMERATION, ANDMELTS 561
ANDSIZESEPARATIONOFPARTICULATESOLIDS 361
16.1. SomeGeneralCrystallizationConcepts 562
12.1. Screening 361
16.2. ImportanceoftheSolubilityCurveinCrystallizer
12.2. CommercialClassificationwithStreamsofAiror Design 563
Water 368 16.3. SolubilitiesandEquilibria 563
12.3. SizeReduction 368 16.4. CrystalSizeDistribution 566
12.4. EquipmentforSizeReduction 370 16.5. TheProcessofCrystallization 566
12.5. ParticleSizeEnlargement(Agglomeration) 378 16.6. TheIdealStirredTank 574
References 396 16.7. KindsofCrystallizers 577
Bibliography 397 16.8. MeltCrystallizationandPurification 584
References 589
CHAPTER13 DISTILLATIONANDGAS
ABSORPTION 399
CHAPTER17 CHEMICALREACTORS 591
13.0. Introduction 399
13.1. Vapor-LiquidEquilibria 400 17.1. DesignBasisandSpaceVelocity 591
13.2. Single-StageFlashCalculations 402 17.2. RateEquationsandOperatingModes 591
13.3. EvaporationorSimpleDistillation 406 17.3. MaterialandEnergyBalancesofReactions 596
13.4. BinaryDistillation 407 17.4. NonidealFlowPatterns 597
13.5. BatchDistillation 419 17.5. SelectionofCatalysts 602
17.6. TypesandExamplesofReactors 608
13.6. MulticomponentSeparation:General
Considerations 421 17.7. HeatTransferinReactors 623
17.8. ClassesofReactionProcessesandTheirEquipment 630
13.7. EstimationofRefluxandNumberofTrays
17.9. BiochemicalReactorsandProcesses 642
(Fenske-Underwood-GillilandMethod
(1932,1948,1940)) 423 References 652
13.8. AbsorptionFactorShortcutMethodofEdmister
(1947–1949) 426 CHAPTER18 PROCESSVESSELS 655
13.9. SeparationsinPackedTowers 427
13.10. BasisforComputerEvaluationofMulticomponent 18.1. Drums 655
Separations 433 18.2. FractionatorRefluxDrums 656
13.11. SpecialKindsofDistillationProcesses 439 18.3. Liquid-LiquidSeparators 657
13.12. TrayTowers 454 18.4. Gas-LiquidSeparators 657
13.13. PackedTowers 460 18.5. StorageTanks 664
13.14. EfficiencesofTraysandPackings 464 18.6. MechanicalDesignofProcessVessels 667
13.15. EnergyConsiderations 476 18.7. BinsandHoppers 669
References 485 References 675
CHAPTER14 EXTRACTIONANDLEACHING 487 CHAPTER19 MEMBRANESEPARATIONS 677
14.1. Introduction 487 19.1. MembraneProcesses 677
14.2. EquilibriumRelations 488 19.2. Liquid-PhaseSeparations 683
CONTENTS vii
19.3. GasPermeation 684 CHAPTER21 COSTSOFINDIVIDUAL
19.4. MembraneMaterialsandApplications 684 EQUIPMENT 731
19.5. MembraneCellsandEquipmentConfigurations 686
19.6. IndustrialApplications 687
19.7. SubqualityNaturalGas 687
19.8. TheEnhancementofSeparation 690
APPENDIXA UNITS,NOTATION,ANDGENERAL
19.9. PermeabilityUnits 693 DATA 743
19.10. DerivationsandCalculationsforSingle-StageMembrane
Separations 697
19.11. RepresentationofMultistageMembraneCalculations
foraBinarySystem 703
19.12. PotentialLarge-ScaleCommercialization 706 APPENDIXB EQUIPMENTSPECIFICATION
References 707 FORMS 753
CHAPTER20 GAS-SOLIDSEPARATIONS 709
20.1. Gas-SolidSeparations 709 APPENDIXC QUESTIONNAIRESOFEQUIPMENT
20.2. FoamSeparationandFrothFlotation 717 SUPPLIERS 799
20.3. SublimationandFreezeDrying 719
20.4. SeparationsbyThermalDiffusion 720
20.5. ElectrochemicalSyntheses 722
References 729 INDEX 819
Chapter 0
RULES OF THUMB: SUMMARY
Although experienced engineers know where to find information 11. Efficienciesofreciprocatingcompressors:65–70%atcompres-
and how to make accurate computations, they also keep a mini- sionratioof1.5,75–80%at2.0,and80–85%at3–6.
mumbodyofinformationreadilyavailable,madelargelyofshort- 12. Efficiencies of large centrifugal compressors, 6000–100,000
cuts and rules of thumb. This compilation is such a body of ACFMatsuction,are76–78%.
information from the material in this book and is, in a sense, a 13. Rotarycompressorshaveefficienciesof70–78%,exceptliquid-
digestofthebook. linertypewhichhave50%.
Rules of thumb, also known as heuristics, are statements of 14. Axialflowcompressorefficienciesareintherangeof81–83%.
known facts. The word heuristics is derived from Greek, to dis-
coverortoinvent,sotheserulesareknownordiscoveredthrough
CONVEYORSFORPARTICULATESOLIDS
useandpracticebutmaynotbeabletobetheoreticallyproven.In
practice,theyworkandaremostsafelyappliedbyengineerswho 1. Screwconveyorsareusedtotransportevenstickyandabrasive
are familiar with the topics. Such rules are of value for approxi- solidsupinclinesof20°orso.Theyarelimitedtodistancesof
mate design and preliminary cost estimation, and should provide 150ftorsobecauseofshafttorquestrength.A12in.diacon-
eventheinexperiencedengineerwithperspectiveandwherebythe veyor can handle 1000–3000 cuft/hr, at speeds ranging from
reasonableness of detailed and computer-aided design can be 40to60rpm.
appraisedquickly,especiallyonshortnotice,suchasaconference. 2. Beltconveyorsareforhighcapacityandlongdistances(amileor
Everydayactivitiesarefrequentlygovernedbyrulesofthumb. more,butonlyseveralhundredfeetinaplant),upinclinesof30°
Theyserveuswhenwewishtotakeacourseofactionbutwemay maximum.A24in.widebeltcancarry3000cuft/hrataspeedof
notbeinapositiontofindthebestcourseofaction. 100ft/min,butspeedsupto600ft/minaresuitedforsomemate-
Much more can be stated in adequate fashion about some rials.Thenumberofturnsislimitedandthemaximuminclineis
topics than others, which accounts, in part, for the spottiness of 30degrees.Powerconsumptionisrelativelylow.
the present coverage. Also, the spottiness is due to the ignorance 3. Bucket elevators are used for vertical transport of sticky and
and oversights on the part of the authors. Therefore, every engi- abrasivematerials.Withbuckets20×20in.capacitycanreach
neerundoubtedlywillsupplementormodifythismaterial(Walas, 1000cuft/hr at a speedof 100ft/min, but speedsto 300ft/min
1988). areused.
4. Drag-typeconveyors(Redler)aresuitedforshortdistancesinany
directionandarecompletelyenclosed.Unitsrangeinsizefrom
COMPRESSORSANDVACUUMPUMPS
3in.squareto19in.squareandmaytravelfrom30ft/min(fly
1. Fans are used to raise the pressure about 3% (12 in. water), ash)to250ft/min(grains).Powerrequirementsarehigh.
blowers raise to less than 40 psig, and compressors to higher 5. Pneumatic conveyors are for high capacity, short distance
pressures, although the blower range commonly is included (400ft)transportsimultaneouslyfromseveralsourcestoseveral
inthecompressorrange. destinations. Either vacuum or low pressure (6–12 psig) is
2. Vacuum pumps: reciprocating piston type decrease the pres- employed with a range of air velocities from 35 to 120 ft/sec
sure to 1 Torr; rotary piston down to 0.001 Torr, two-lobe depending on the material and pressure. Air requirements are
rotarydownto0.0001Torr;steamjetejectors,onestagedown from1to7cuft/cuftofsolidtransferred.
to 100 Torr, three stage down to 1 Torr, five stage down to
0.05Torr.
COOLINGTOWERS
3. A three-stage ejector needs 100 lb steam/lb air to maintain a
pressureof1Torr. 1. Water in contact with air under adiabatic conditions even-
4. In-leakageofairtoevacuatedequipmentdependsontheabso- tuallycoolstothewetbulbtemperature.
lutepressure,Torr,andthevolumeoftheequipment,Vcuft, 2. Incommercialunits,90%ofsaturationoftheairisfeasible.
accordingtow=kV2/3lb/hr,withk=0.2whenPismorethan 3. Relative cooling tower size is sensitive to the difference
90 Torr, 0.08 between 3 and 20 Torr, and 0.025 at less than betweentheexitandwetbulbtemperatures:
1Torr. ΔT(°F) 5 15 25
5. Theoreticaladiabatichorsepower ðTHPÞ=½ðSCFMÞT1=8130a(cid:1) Relativevolume 2.4 1.0 0.55
½ðP =P Þa−1(cid:1), where T is inlet temperature in °F + 460 and
2 1 1
a=(k−1)/k,k=C /C. 4. Towerfillisofahighlyopenstructuresoastominimizepressure
p v
6. OutlettemperatureT =T ðP =P Þa: drop,whichisinstandardpracticeamaximumof2in.ofwater.
2 1 2 1
7. Tocompress air from 100°F, k=1.4, compression ratio =3, 5. Watercirculationrateis1–4gpm/sqftandairratesare1300–
theoretical power required = 62 HP/million cuft/day, outlet 1800lb/(hr)(sqft)or300–400ft/min.
temperature306°F. 6. Chimney-assisted natural draft towers are of hyperboloidal
8. Exit temperature should not exceed 350–400°F; for diatomic shapes because they have greater strength for a given thick-
gases (C /C = 1.4) this corresponds to a compression ratio ness; a tower 250 ft high has concrete walls 5–6 in. thick.
p v
ofabout4. Theenlargedcrosssectionatthetopaidsindispersionofexit
9. Compressionratioshouldbeaboutthesameineachstageofa humidairintotheatmosphere.
multistageunit,ratio=(P /P )1/n,withnstages. 7. Countercurrentinduceddrafttowersarethemostcommonin
n 1
10. Efficiencies of fans vary from 60–80% and efficiencies of process industries. They are able to cool water within 2°F of
blowersareintherangeof70–85%. thewetbulb.
xiii
xiv RULESOFTHUMB: SUMMARY
8. Evaporationlossesare1%ofthecirculationforevery10°Fof 6. Hammermillsbeatthematerialuntilitissmallenoughtopass
cooling range. Windage or drift losses of mechanical draft through the screen at the bottom of the casing. Reduction
towersare0.1–0.3%.Blowdownof2.5–3.0%ofthecirculation ratiosof40arefeasible.Largeunitsoperateat900rpm,smal-
isnecessarytopreventexcessivesaltbuildup. leronesupto16,000rpm.Forfibrousmaterialsthescreenis
9. Towers that circulate cooling water to several process units providedwithcuttingedges.
andarevulnerabletoprocessintrusionshouldnotusefilmfill 7. Rod mills are capable of taking feed as large as 50 mm and
duetotheriskoffoulingandfillfailure(Huchler,2009). reducing it to 300 mesh, but normally the product range is
10. Sites with nearby obstructions or where there is the risk that 8–65 mesh. Rods are 25–150 mm dia. Ratio of rod length to
the tower plume or combustion exhaust may be entrained mill diameter is about 1.5. About 45% of the mill volume is
should use a couterflow configuration, and may need special occupiedbyrods.Rotationisat50–65%ofcritical.
airintakedesigns(Huchler,2009). 8. Ballmillsarebettersuitedthanrodmillstofinegrinding.The
11. If the facility, like a power plant, has very high heat loads chargeisofequalweightsof1.5,2,and3in.ballsforthefinest
requiring high recirculating water rates and large cooling grinding. Volume occupied by the balls is 50% of the mill
loads, it may require the use of natural-draft towers with volume. Rotation speed is 70–80% of critical. Ball mills have
hyperbolicconcreteshells(Huchler,2009). alengthtodiameterratiointherange1–1.5.Tubemillshave
12. Theuseofvariable-frequencyfandrivesincreasecapitalcosts a ratio of 4–5 and are capable of very fine grinding. Pebble
and provide operating flexibility for towers of two or more mills have ceramic grinding elements, used when contamina-
cells(Huchler,2009). tionwithmetalistobeavoided.
9. Roller mills employ cylindrical or tapered surfaces that roll
along flatter surfaces and crush nipped particles. Products of
CRYSTALLIZATIONFROMSOLUTION 20–200mesharemade.
10. Fluidenergymillsareusedtoproducefineorultrafine(submi-
1. Thefeedtoacrystallizershouldbeslightlyunsaturated.
cron)particles.
2. Completerecoveryofdissolvedsolidsisobtainablebyevapora-
tion,butonlytotheeutecticcompositionbychilling.Recovery
bymeltcrystallizationalsoislimitedbytheeutecticcomposition. DISTILLATIONANDGASABSORPTION
3. Growth rates and ultimate sizes of crystals are controlled by
1. Distillationusuallyisthemosteconomicalmethodofseparat-
limitingtheextentofsupersaturationatanytime.
ing liquids, superior to extraction, adsorption, crystallization,
4. Crystalgrowthratesarehigherathighertemperatures.
orothers.
5. TheratioS=C/C ofprevailingconcentrationtosaturation
concentrationiskespattneartherangeof1.02–1.05. 2. Foridealmixtures,relativevolatilityistheratioofvaporpres-
suresα =P /P .
6. Incrystallizationbychilling,thetemperatureofthesolutionis 12 2 1
kept at most 1–2°F below the saturation temperature at the 3. Foratwo-component,idealsystem,theMcCabe-Thielemethod
offersagoodapproximationofthenumberofequilibriumstages.
prevailingconcentration.
4. Toweroperatingpressureisdeterminedmostoftenbythetem-
7. Growth rates of crystals under satisfactory conditions are in
the range of 0.1–0.8 mm/hr. The growth rates are approxi- perature of the available condensing medium, 100–120°F if
coolingwater;orbythemaximumallowablereboilertempera-
matelythesameinalldirections.
ture,150psigsteam,366°F.
8. Growthratesareinfluencedgreatlybythepresenceofimpuri-
5. Sequencingofcolumnsforseparatingmulticomponentmixtures:
tiesandofcertainspecificadditivesthatvaryfromcasetocase.
(a) perform the easiest separation first, that is, the one least
9. Batchcrystallizerstendtohaveabroadercrystalsizedistribu-
demanding of trays and reflux, and leave the most difficult to
tionthancontinuouscrystallizers.
thelast;(b)whenneitherrelativevolatilitynorfeedconcentra-
10. Tonarrowthecrystalsizedistribution,coolslowlythroughthe
tionvarywidely,removethecomponentsonebyoneasoverhead
initialcrystallizationtemperatureorseedattheinitialcrystal-
products;(c)whentheadjacentorderedcomponentsinthefeed
lizationtemperature.
varywidelyinrelativevolatility,sequencethesplitsintheorder
ofdecreasingvolatility;(d)whentheconcentrationsinthefeed
DISINTEGRATION varywidelybuttherelativevolatilitiesdonot,removethecom-
ponentsintheorderofdecreasingconcentrationinthefeed.
1. Percentagesofmaterialgreaterthan50%ofthemaximumsize 6. Flashing may be more economical than conventional distilla-
are about 50% from rolls, 15% from tumbling mills, and 5% tionbutislimitedbythephysicalpropertiesofthemixture.
fromclosedcircuitballmills. 7. Economically optimum reflux ratio is about 1.25 times the
2. Closedcircuitgrindingemploysexternalsizeclassificationand minimumrefluxratioR .
m
returnofoversizeforregrinding.Therulesofpneumaticcon- 8. Theeconomicallyoptimumnumberoftraysisnearlytwicethe
veying are applied to design of air classifiers. Closed circuit minimumvalueN .
m
ismostcommonwithballandrollermills. 9. The minimum number of trays is found with the Fenske-
3. Jawandgyratorycrushersareusedforcoarsegrinding. Underwoodequation
4. Jaw crushers take lumps of several feet in diameter down to
4 in. Stroke rates are 100–300/min. The average feed is sub- N =logf½ðx=ð1−xÞ(cid:1) =½x=ð1−xÞ(cid:1) g=logα:
jected to 8–10 strokes before it becomes small enough to m ovhd btms
escape.Gyratorycrushersaresuitedforslabbyfeedsandmake 10. Minimumrefluxforbinaryorpseudobinarymixturesisgivenby
amoreroundedproduct. thefollowingwhenseparationisessentiallycompleteðx ≃1Þand
D
5. Roll crushers are made either smooth or with teeth. A 24 in. D/Fistheratioofoverheadproductandfeedrates:
toothed roll can accept lumps 14 in. dia. Smooth rolls effect
reductionratiosuptoabout4.Speedsare50–900rpm.Capa- R D=F =1=ðα−1Þ,when feed is at the bubblepoint,
m
cityisabout25%ofthemaximumcorrespondingtoacontin-
uousribbonofmaterialpassingthroughtherolls. ðRm+1ÞD=F =α=ðα−1Þ,when feed is at the dewpoint:
RULESOFTHUMB: SUMMARY XV
11. Asafetyfactorof10%ofthenumberoftrayscalculatedbythe 2. Forunder100HP,electricmotorsareusedalmostexclusively.
bestmeansisadvisable. Theyaremadeforupto20,000HP.
12. Refluxpumpsaremadeatleast25%oversize. 3. Induction motors are most popular. Synchronous motors are
13. Forreasonsofaccessibility,trayspacingsaremade20–30in. made for speeds as low as 150 rpm and are thus suited for
14. PFea=kupeffffiρffiifficffiffiiiennctyheorfantrgaey1s.0i–s1.a2t (vfta/sluece)s pofffilffibffiffit=ffiffihfficffieffiuffiffiffifffitvffiffi:aTpohrisfraacntgoer emxaadmepslmeafollrerlotwhansp5e0edHrPec.iAprovcaartieintygocfomenpcrleosssuorress,ibsuatvaarileabnloet,
s v
of F establishes the diameter of the tower. Roughly, linear fromweather-prooftoexplosion-proof.
s
velocities are 2 ft/sec at moderate pressures and 6 ft/sec in 4. Steam turbinesare competitive above 100HP. Theyare speed
vacuum. controllable. They are used in applications where speeds and
15. The optimum value of the Kremser-Brown absorption factor demandsarerelativelyconstant.Frequentlytheyareemployed
A=K(V/L)isintherange1.25–2.0. assparesincaseofpowerfailure.
16. Pressure drop per tray is of the order of 3 in. of water or 5. Combustion engines and turbines are restricted to mobile and
0.1psi. remotelocations.
17. Trayefficienciesfordistillationoflighthydrocarbonsandaqu- 6. Gasexpandersforpowerrecoverymaybejustifiedatcapacities
eous solutions are 60–90%; for gas absorption and stripping, of several hundred HP; otherwise any needed pressure reduc-
10–20%. tioninprocessiseffectedwiththrottlingvalves.
18. Sievetrayshaveholes0.25–0.50in.dia,holeareabeing10%of 7. Axial turbines are used for power recovery where flow rates,
theactivecrosssection. inlettemperaturesorpressuredropsarehigh.
19. Valvetrayshaveholes1.5in.diaeachprovidedwithaliftable 8. Turboexpanders are used to recover power in applications
cap,12–14caps/sqftofactivecrosssection.Valvetraysusually whereinlettemperaturesarelessthan1000°F.
arecheaperthansievetrays.
20. Bubblecap trays are used only when a liquid level must be DRYINGOFSOLIDS
maintained at low turndown ratio; they can be designed for
lowerpressuredropthaneithersieveorvalvetrays. 1. Dryingtimesrangefromafewsecondsinspraydryersto1hr
21. Weir heights are 2 in., weir lengths about 75% of tray dia- orlessinrotarydryersanduptoseveralhoursorevenseveral
meter,liquidrateamaximumofabout8gpm/in.ofweir;mul- daysintunnelshelforbeltdryers.
tipassarrangementsareusedathighliquidrates. 2. Continuoustrayandbeltdryersforgranularmaterialofnatural
22. Packingsof random and structuredcharacter aresuited espe- size or pelleted to 3–15 mm have drying times in the range of
cially to towers under 3 ft dia and where low pressure drop 10–200min.
isdesirable.Withproperinitialdistributionandperiodicredis- 3. Rotary cylindrical dryers operate with superficial air velocities
tribution, volumetric efficiencies can be made greater than of 5–10 ft/sec, sometimes up to 35 ft/sec when the material is
thoseoftraytowers.Packedinternalsareusedasreplacements coarse. Residence times are 5–90 min. Holdup of solid is
for achieving greater throughput or separation in existing 7–8%. An 85% free cross section is taken for design purposes.
towershells. Incountercurrentflow,theexitgasis10–20°Cabovethesolid;
23. For gas rates of 500 cfm, use 1 in. packing; for gas rates of in parallel flow, the temperature of the exit solid is 100°C.
2000cfmormore,use2in. Rotation speeds of about 4 rpm are used, but the product of
24. Theratioofdiametersoftowerandpackingshouldbeatleast rpmanddiameterinfeetistypicallybetween15and25.
15. 4. Drumdryersforpastesandslurriesoperatewithcontacttimes
25. Becauseofdeformability,plasticpackingislimitedtoa10–15ft of 3–12 sec, produce flakes 1–3 mm thick with evaporation
depthunsupported,metalto20–25ft. ratesof 15–30kg/m2hr.Diameters are1.5–5.0ft;therotation
26. Liquid redistributors are needed every 5–10 tower diameters rate is 2–10 rpm. The greatest evaporative capacity is of the
with pall rings but at least every 20 ft. The number of liquid orderof3000lb/hrincommercialunits.
streamsshouldbe3–5/sqftintowerslargerthan3ftdia(some 5. Pneumatic conveying dryers normally take particles 1–3 mm
expertssay9–12/sqft),andmorenumerousinsmallertowers. diabutupto10mmwhenthemoistureismostlyonthesurface.
27. Height equivalent to a theoretical plate (HETP) for vapor- Air velocities are 10–30 m/sec. Single pass residence times are
liquid contacting is 1.3–1.8 ft for 1 in. pall rings, 2.5–3.0 ft 0.5–3.0secbutwithnormalrecyclingtheaverageresidencetime
for2in.pallrings. is brought up to 60 sec. Units in use range from 0.2 m dia by
28. Packed towers should operate near 70% of the flooding rate 1mhighto0.3mdiaby38mlong.Airrequirementisseveral
givenbythecorrelationofSherwood,Lobo,etal. SCFM/lbofdryproduct/hr.
29. Reflux drums usually are horizontal, with a liquid holdup of 6. Fluidizedbeddryersworkbestonparticlesofafewtenthsofa
5min half full. A takeoff pot for asecond liquidphase, such mmdia,butupto4mmdiahavebeenprocessed.Gasvelocities
aswaterinhydrocarbonsystems,issizedforalinearvelocity of twice the minimum fluidization velocity are a safe prescrip-
ofthatphaseof0.5ft/sec,minimumdiameterof16in. tion. In continuous operation, drying times of 1–2 min are
30. Fortowersabout3ftdia,add4ftatthetopforvapordisen- enough, but batch drying of some pharmaceutical products
gagement and 6 ft at the bottom for liquid level and reboiler employsdryingtimesof2–3hr.
return. 7. Spray dryers are used for heat sensitive materials. Surface
31. Limit the tower height to about 175 ft max because of wind moisture is removed in about 5 sec, and most drying is com-
load and foundation considerations. An additional criterion pletedinlessthan60sec.Parallelflowofairandstockismost
isthatL/Dbelessthan30. common. Atomizing nozzles have openings 0.012–0.15 in. and
operate at pressures of 300–4000 psi. Atomizing spray wheels
rotate at speeds to 20,000 rpm with peripheral speeds of
DRIVERSANDPOWERRECOVERYEQUIPMENT
250–600 ft/sec. With nozzles, the length to diameter ratio of
1. Efficiency is greater for larger machines. Motors are 85–95%; the dryer is 4–5; with spray wheels, the ratio is 0.5–1.0. For
steam turbines are 42–78%; gas engines and turbines are the final design, the experts say, pilot tests in a unit of 2 m
28–38%. diashouldbemade.