Dependence of the enthalpy and vapor pressure of Kraft black liquor on solids content

88 Pages·1992·3.1 MB·English

by  Stoy, Mark A

DEPENDENCEOFTHEENTHALPYANDVAPORPRESSURE
OFKRAFTBLACKLIQUORONSOLIDSCONTENT
By
MARKA.STOY
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OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1992
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TABLEOFCONTENTS
LISTOFTABLES
LISTOFFIGURES
ABSTRACT
INTRODUCTIONANDOVERVIEW
BACKGROUND
PDreesvcriioputsiWonorokfKraftBlackLiquor 95
MATERIALSANDMETHODS
13
LiquorSelection 13
SolutionCalorimetry 15
DifferentialScanningCalorimetry 20
Vapor-LiquidEquilibrium 24
PRESENTATIONANDANALYSISOFRESULTS 27
DataPresentation 27
Enthalpy-ConcentrationCharts 27
HeatofDilutionResults 28
HeatCapacityResults 34
VaporPressureResults 39
DataAnCaallycsuilsatedVaporPressures .. 4442
CONCLUSIONSANDRECOMMENDATIONS
51
APPENDIXA DETAILEDDISCUSSIONOFTHEENTHALPYWORK
. 54
DerivationoftheWorkingEquationfortheEnthalpyScale 54
EHConentashttarCluapcpytaiTcoainbtlyoefMsetahesuCrheamretnts ’’’’’’’ 5c7f.
SItmapionrletsasncSteeeolfPtahensStandardRunwithSapphire .’. ^61
SampleSize m.
11
MethodofDataAnalysis 65
UncertaintyAnalysis 67
APPENDIXB DETAILEDDISCUSSIONOFTHEVAPORPRESSURE
WORK
72
RelationshipBetweentheEnthalpyandVaporPressure 72
CalculationofVaporPressuresfromEnthalpyData 74
REFERENCELIST 76
BIOGRAPHICALSKETCH
78
in
1 1
LISTOFTABLES
Table Page
1 TypicalcompositionoforganicmaterialinpineKraftblackliquor 7
2 Typicalpropertyvaluesatdifferentstagesofconcentration 8
3 Pulpingconditionsforexperimentalliquors 14
4 Kappinumbersforexperimentalliquors 14
5 ComparisonofcalculatedandexperimentalvaporpressuresforL5 43
6 ComparisonofBPEestimateswithliteraturevalues 50
7 EnthalpyforliquorLI 58
8 EnthalpyforliquorL31 59
9 EnthalpyforliquorL51 60
10 Heatcapacityresultsforthreesamplesofthesamesolidscontent 70
1 Meanandstandarddeviationofsamplesforuncertaintyanalysis 71
IV
LISTOFFIGURES
Figure Page
1 Heatcapacitybehaviorathighsolidscontents 10
2 Mixingcellusedfordilutionexperiments 17
3 Powerresponsecurveforadilutionexperiment 19
4 SchematicofthemeasuringheadforapowercompensatedDSC .... 21
5 DSCscansusedtodeterminetheheatcapacityofasample 23
6 Schematicofvaporpressureapparatus 25
7 Enthalpy-concentrationchartforliquorLI 29
8 Enthalpy-concentrationchartforliquorL23 29
9 Enthalpy-concentrationchartforLiquorL31 30
10 Enthalpy-concentrationchartforliquorL51 30
11 Plotofthetotalheatofdilutionforaseriesofdilutions 31
12 Heatabsorbedtodiluteblackliquortoinfinitedilution 33
13 Baseisothermanddefinitionofinfinitedilutionreferencestate 33
14 Heatcapacityvs.temperaturedataforLiquorLI 36
15 Heatcapacityvs.temperaturedataforliquorL23 36
16 Heatcapacityvs.temperaturedataforliquorL31 37
17 Heatcapacityvs.temperaturedataforliquorL51 37
18 Heatcapacityvs.solidscontentdataforliquorL51 38
19 Vaporpressureversustemperatureatthreesolidscontentsforliquor
40
20 Vaporpressureversustemperaturebehaviorat30%solids 40
v
21 Vaporpressureversustemperaturebehaviorat50%solids 41
22 Vaporpressureversustemperaturebehaviorat70%solids 41
23 Comparisonofbaseisotherms 45
24 FittedvaluesoftheheatcapacityforliquorLI 47
25 FittedvaluesoftheheatcapacityforliquorL51 47
26 BoilingpointelevationplotforliquorL51 48
27 ComparisonofcalculatedandmeasuredvaporpressuresforliquorL51 .. 50
28 Methodforaligningendingisotherms 66
29 SamplecalculationforuncertaintyinCpmeasurementbyDSC 69
vi
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
DEPENDENCEOFTHEENTHALPYANDVAPORPRESSURE
OFKRAFTBLACKLIQUORONSOLIDSCONTENT
By
MarkA.Stoy
May,1992
Chairperson:A.L.Fricke
MajorDepartment:ChemicalEngineering
Datafortheequilibriumpropertiesofkraftblackliquorweredevelopedforfour
different liquors from laboratory cooks ofslash pine. Theenthalpy change for
concentratingblackliquorovertherangeof0-80%solidsand40-120°Ccannowbe
determinedeasilyfromenthalpyconcentrationcharts.Thechartsincorporate,forthe
firsttime,theheatofsolutioneffectforseparatingwaterfromblackliquorintothe
energybalancearoundtheevaporators. Vaporpressuredata,consistentwiththe
enthalpycharts,weredevelopedforthesamesetofliquors.
Sincethechartsareusefulforbinarymixturesonly,blackliquorwasassumed
tobeapseudo-binarymixtureofpuresolidsandwater.Oneisothermofthechartswas
determined from heats of dilution which were measured at 80°C by solution
calorimetry.Samplesofblackliquorweredilutedfrom80%solidsdowntoalowsolids
contentinaseriesofdilutions. Thetotalheatevolvedtodiluteblackliquorfroma
givensolidscontenttoinfinitedilutionwasdeterminedfromthesedata.Theremaining
Vll
isothermsweredeterminedfromheatcapacitydatawhichweremeasuredbydifferential
scanningcalorimetry.
Thevaporpressureofwateroverblackliquorwasmeasuredasafunctionof
solidscontentandpressurebyusinganequilibriumstill.Fromtheseresults,theboiling
pointelevationwasdeterminedasafunctionofsolidscontentandpressure.
Thedifference amongtheenthalpychartsforthefourliquorsstudiedwas
negligible. Thissuggeststhatallblackliquorsarethesameintermsoftheenthalpyand
thattheycanbetreatedasabinarymixtureofsolidsandwater. Therewasno
significantdifferenceintheheatsofdilutionforanyoftheliquors. Theheatcapacity
wasfoundtochangenon-linearlywithincreasingsolidscontentabove30%solids.This
behavior,however,wasnotobservedforalltheliquorsstudied. Theboilingpoint
elevationwassimilarforalltheliquorsstudiedandwasintherangeofvaluesinthe
literature.
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INTRODUCTIONANDOVERVIEW
TheKraftrecoverycyclerecoverstheinorganicchemicalsfromblackliquorand
disposesoftheorganicby-productsfrompulping. Itisalargecomplexprocess,which
isnecessaryforKraftpulpingtobeeconomicallyfeasible. Kraftrecoveryoperations
intheU.S.andCanadaprocessabout80x1 6tonsofblackliquorsolidsannually.The
industrial significanceofthiscapital andenergyintensiveprocessisgenerallynot
recognizedoutsideofthepaperindustry.
Theorganicby-productsfrompulpingaredisposedofbyburningtheminthe
Kraftrecoveryboiler. Theresultingenergyofcombustionisusedtoproducetohigh
pressuresteamforpowergeneration. Thespentcooking chemicalsarepartially
regeneratedintheboilerandarethenrecoveredforfurtherprocessing. Theoperation
oftherecoverycycleandthemilldependupontherecoveryboilerperformingthesetwo
functions.
AlthoughtheKraftrecoveryboilerservesitspurposewell,thethermalefficiency
oftheboilerislow.Alargeamountoftheenergyreleasedintheboilerisnotrecovered
becauseitisusedtoevaporatetheremainingwaterinthefiringliquor.Iftheliquorwas
concentrated to highersolidscontents in theevaporators, moreenergycould be
recoveredforpowergeneration. Theresultingenergysavingsofamoreconcentrated
firingliquorislargebecausetheevaporatorsremovewatermoreefficientlythanthe
recoveryboiler.Theevaporatorsoperateata4-to-1steameconomy,whichmeansthat
4poundsofwaterareevaporatedforeverypoundofsteamconsumed. Incontrast,the
steameconomyoftherecoveryboilerisabout1-to-l.
1
2
Amoreconcentratedfiringliquorwouldreducetheamountofwaterevaporated
inthefurnace,increasethecapacityofthefurnace,andimprovesafety. Priorto1980,
whenenergypriceswerelow,therewaslittleeconomicincentivetoimprovethethermal
efficiencyoftherecoverycycle.Thedesignoftherecoveryboilerhadmaturedwithout
aclearunderstandingofthecombustionprocessesandchemicalreactionsthattake
placeinsideit. Toincreasethesolidslimitofthefiringliquorwouldhaverequireda
largeexpensiveR&Dprogrambeyondtheresourcesofasinglepapercompany.
Whenenergypricesroseintheearly1980s,theU.S.DepartmentofEnergy
beganfundingandcoordinatingresearchtoadvanceblackliquorrecoverytechnology.
Thegoalofthisprogramwastoincreasethesolidscontentlimitfromthepresent65%
to75%orabove. Naturally,manyprogramswerestartedtodevelopafundamental
understandingoftheprocessesoccurringinsidethefurnace.Itwasalsorecognizedthat
withoutimproveddataforthephysicalpropertiesofblackliquorfuturedevelopment
ofrecoverytechnologywouldbehampered.Asaresult,acomprehensiveprogramwas
startedin1982tomeasurethephysicalpropertiesofKraftblackliquor. Thisprogram
isnowintheDepartmentofChemicalEngineeringattheUniversityofFlorida.
Theblackliquorphysicalpropertyprogramisdesignedtodeterminehowthe
pulpingconditionsandcompositionaffecttherheologicalandequilibriumproperties
ofblackliquor.Aseriesofwellcharacterizedliquors,pulpedundercarefullycontrolled
conditions, arebeingusedforthiswork. Therheologicalpropertiesarethemost
importantpropertiestounderstandforprocessingblackliquor. Theyaffectanumber
ofdifferentprocessesfromheattransferintheevaporatorstodropletformationand
combustioninthefurnace.
Accurateenthalpyandvaporpressuredataarenecessarytosizeandoperatethe
evaporatorsforblackliquorconcentration. Althoughtherearedataforthevapor
pressure,therearenocompleteenthalpydataforblackliquor.Theonlyenthalpicdata