basic concepts kraft process
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Basic Training
Kraft Process
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Wood
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Production of Chips
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Production of Chips
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Good Size of Chips
The size of the chips is very important for the impregnation of cook liquor (white liquor)
inside the chips.
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Selection of Chips
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Selection of chips
Mesh Discs
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Chip Bin
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Chip Bin Level ControlThe retention time is very important for the pre-heating of the
chips => take the air out of the chips for a good impregnation of
the liquor inside the chips
a) Bad impregnation
b) Good impregnation
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Basic Concepts
• Kappa - Measurement of the delignification of
the wood• Viscosity – Measurement of the quality of the
fiber. “Fiber length”
• White Liquor: NaOH+Na2S+Na2CO3
– Total Titratable Alkali (g/L): NaOH+Na2S+Na2CO3
– Active Alkali (g/L): NaOH+Na2S
– Efective Alkali (g/L): NaOH+1/2Na2S
– Sulfidity (%) : Na2S/TTA
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Alkali Charge
• Alkali Charge: % of alkali (EA) added in the digester
relative to the wood. Alkali charge = 20% for
100 kg of wood is added 20 kg of alkali
• Kg of alkali / h = concentration of white liquor (EA)
(g/L or kg/m3) x flow of white liquor (m3 / h)• Kg of wood / h = chip meter (rpm) x 60 x vol of
screw (m3) x chip density (kg/m3)
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Digester Production• Digester Production (adt/h) = chip meter
(rpm) x 60 x vol of screw (m3) x chip density(kg/m3) x yield / 0.9
• Adt – air dry ton – 10% of water
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Digester Retention Time• Retention time (min) = Volume of digester
(m3) * Compaction Factor (1.9)/ Chip meter(rpm) * volume of screw (m3)
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H Factor• Combination of time and temperature
• Temperature => Reaction rate
• Ex:
• Retention time = 1.5 h• Temperature = 170oC
•
How H Factor is calculated?
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H FactorReaction Rates (based on temperature) – Calculation of H Factor
H Factor = 921.4 (Reaction Rate 170oC) x 1.5 (Retention Time) = 1382
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H Factor
• H Factor = Retention Time (Hours) x Reaction
Rate for a given temperature (T)
• Reaction Rate = e^(43,2-16113/T)
• T – Temperature in Kelvin
RT (hours) 1,5
T(oC) 170
T(K) 443
RR 922,9172
H Factor 1384,376
Example
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Chip Meter
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Chip Meter
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Top Separator
Liquor and chips – Feed Circulation
Liquor – Feed Circulation - Return
Liquor and chips – Digester - Cooking
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Bottom of Digester
Importance of the cold
blow flow added at the
bottom of digester:
- Wash the pulp
- Controlling of blow
consistence
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Feed Circulation
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Digester
CD1 – Upper Extraction
CD3 – Lower Extraction
CD2 – Cook Circulation
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Digester Screens
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Brownstock Wash
DIG DDW DELIG SCREEN DDW BLEACHEVA
IndustrialWater
Filtrate
Tank
Filtrate
Tank
Weak
Liquor
TankLC
By passBy pass
Factors that affect BSW:
- High DDW Pressure / Low consistency in outlet
- High level of WLT / Low solids content in WL
Counter-current wash
Reuse of waterCOD
Condutivity
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Brownstock Wash
• Aims
– Clean the pulp
– Save water
– Recovery alkali
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Brownstock Wash
• Dirty pulp $$$
– Less efficiency in delignification process
– Greater consumption of chemicals in bleaching
• Water consumption $$$
– Greater consumption of steam in evaporation
• Recovery alkali $$$
– Less lost of soda
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Dilution Factor• Amount of wash liquor in excess in relation to
the dirty liquor mixed with the pulp• DF (m3/adt) = (Flow of wash liquor – liquor in the
pulp) (m3/h) /production (adt/h)
•
Ex1: – Production = 100 adt/h
– Flow of wash liquor = 200 m3/h
– Liquor in the pulp = 100 m3/h – DF = 1 m3/adt
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Dilution Factor• Ex2:
– Production = 100 adt/h
– Flow of wash liquor = 300 m3/h – Liquor in the pulp = 100 m3/h
– DF = 2 m3/adt
• Ex3: – Production = 100 adt/h
– Flow of wash liquor = 400 m3/h
– Liquor in the pulp = 100 m3/h – DF = 3 m3/adt
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Dilution Factor• Conclusions
– DF = 1 2 x more wash liquor than dirty liquor in
pulp – DF = 2 3 x more wash liquor than dirty liquor in
pulp
–
DF = 3
4 X more wash liquor than dirty liquor inpulp
– Can DF be 0 (zero) ?
• What does it mean ?
– Can DF be – (negative) ?• What does it mean ?
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Dilution Factor• DF = (Flow of wash liquor – liquor in the pulp) /production
• Flow of wash liquor is measured
• Production is measured
• How can be measured the amount of liquor in the pulp ? – By the consistency!!!
• We have 2 ways of improving dilution factor: – Increasing flow of wash liquor
– Increasing consistency in the outlet of the filter
• Same concept of wash and wring clothes!!!
∗
−−
= 9,01100Pr
WLof Flow
Out Consist oduction
DF
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Alkaline Loss
• The alkaline loss is usually measured after the lastDDW (before bleach process)
• It indicates the amount of soda (NaOH) presents inthe pulp that was “transfered” to the next process
• If this soda had been recovered with the washliquor it would go to recovery area and could berecovered
• The alkaline loss is an indirect measurent of thewashing process:
– High alkaline loss Bad wash
– Low alakline loss Good wash
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COD• Chemical oxygen demand – Indicates the
“dirt” rate of a pulp. It’s measured in lab. – Pulp dirty High COD Bad wash
– Pulp clean Low COD Good wash
• In the process, the condutivity is as indirect
measurement of the COD
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O2 Delignification
NaOH or OWL
O2 MPS O2 MPS
Pulp
R e a c t o
r 2
R e a c t o
r 1
O2 Delignification
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O2 Delignification
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• Delignification efficiency: 30-40% – Kappa in: 15-17
– Kappa out: 8-11
• It causes improvement on the brightness
• Filtrate free of ions Cl
• It isn't very selective. O2 attacks lignin and pulp.
• But it’s more selective than extend the cookingprocess until kappa 10 is reached in digester.
• If the process isn’t well controlled, viscosity canbe a problem!
O2 Delignification
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• Main variables of the process
– O2 Dosage: + 18 kg/adt
– Alkali Charge(15-20 kg/adt): Can be soda or oxidizedwhite liquor (OWL) or both. It’s important to thereaction, but in excess, can cause problems in theviscosity.
– pH Control: + 11
– Temperature: High temperature is important to thedelignification kinetics, however, it could bedangerous to viscosity. About 100 oC.
– Pressure is very important to the delignificationkinetics
O2 Delignification
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Screening Process• Aims
– Take knots out – Take shives out
– Take plastics out
– Take dirts out
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Screen
Feed
AcceptReject
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Screen
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Rotors
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Cylinder / Meshes
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Knot Separator Cilinder
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Mesh – Primary Screen
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Screens
Screen Process
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Screen Process
Feed
AccK
RejK
Knotter PrimaryAcc1
Rej1
Secondary
Acc2
Rej2
Tertiary
Acc3
Feed1
Feed2
Feed3
Rej3
Knot
Wash
Reject
WashDIG
DDW BLEACH
Screen Process
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Screen Process
BlowFlow
Pulp to Bleaching.
Knoter
Primary
Washer
Equipment
Knots to Digester
Fiber to blow tank
Knot Wash
Pressed
Reject
Bleaching
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g
Kappa Brite:
- Inlet of A Tower (Kappa & Brightness)
- Inlet of D0 (Kappa & Brightness)
- Outlet of EOP (Kappa & Brightness)
- Outlet of D1 (Brightness)- Outlet of P Bri htness
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Composition of Kappa #
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Bleaching – Why do use many stages?
Bleaching – 1 stage
BrightnessPulp Resistence
Chemical Charge Chemical Charge
Bleaching – Many stages
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Bleaching – Why do use many stages?
• Softer conditions
– Lower temperature
– Less chemical charge
• Pulp is washed between stages (Filters /
Presses)
– Wash the pulp
–
Remove the lignin
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D Stage
• ClO2
• Oxidize and degrade the lignin• The lignin bocomes soluble
•
Selective – Little drop of the viscosity• D0 STAGE – Focus on removing the lignin
• D1 – D2 STAGE – Focus on bleaching the pulp
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E Stage
• Stage of extraction (of lignin)
• NaOH• Can be considered part of the previous stage
•
In this stage, the lignin that became soluble inD stage, is dissolved in the pulp and then it is
extracted
• Usually E stage is combined with O (oxygen)and P (peroxide)
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P Stage
• H2O2
• Focus on bleaching• Less selective than ClO2
A S
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A Stage
• Remove Hex,A
• High Temperature – 90 oC
• Dosification of H2SO4
• Low pH – 2,5 - 3
•
Long retention time: usually more than 3 hours• Main controls: pH and Temperature
• Kappa in: 10-11
•
Kappa out: 5-6• Brightness out: 60 – 65
D0
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•
Dosification of ClO2• Attack lignin and gain of brightness
• High Temperature – 90 oC
• pH – The same of A Stage –
Low pH – 2,5 - 3• Short retention time: 20 minutes
• Main controls: ClO2 dosage and Temperature – Kappa Factor Concept: Dosification based on kappa number
–
Conversion of ClO2 to Active Chlorine = 2,63 – 1 kg of ClO2 = 2,63 kg of Active Clorine
– Kappa Factor= 2 2 kg of active clorine per kappa # in the inlet of D0
• Common kappa factor= 2 – 3,5 (depends on the desired brightness in theoutlet of EOP)
• Kappa in: 5-6
• Kappa out: No measurement on line
• Brightness out: No measurement on line
• ClO2 at low pH – Good for remove lignin
EOP
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• Dosification of O2 and H2O2• In some plants – EP – There isn’t O2 dosification
• NaOH dosage is importante to increase pH
– 6 – 8 kg of soda/adt
– pH: 10,5 – 11
• High pH is necessary to H2O2 develops his bleachingpower
• H2O2 dosage: 3 – 5 kg/adt
•
Retention time: 1,5 – 2 hours• Main controls: H2O2 dosage and pH
• Kappa out: 2 - 3
• Brightness out: 80 - 85
D1
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• Dosification of ClO2 (2 – 5 kg/adt)
• For controlling pH can be used NaOH or H2SO4
–
NaOH – pH: 5-5,5 – Good for gain brightness – H2SO4 – pH: 2,5 – 3,5 – Good for delignification (in case
of problems with shives)
• Retention time: 1,5 – 2 hours• Main controls: ClO2 dosage and pH
•
Kappa out: (very low)• Brightness out: 86 - 90
P
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• Dosification of H2O2 (3kg/adt)
• For controlling pH is used NaOH
–
2,5-4 kg NaOH/adt – pH: 10,5 - 11
• Retention time: 1,5 – 2 hours
• Main controls: H2O2 dosage and pH
• Kappa out: (very low)
• Brightness out: 89 - 93
Calculation of Total Active Chlorine
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Calculation of Total Active Chlorine
• ClO2 – 2.63
• H2O2 – 2.09• TAC (kg/atd) = ClO2 dosage x 2.63 + H2O2
dosage x 2.09
Brightness Reversion
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Brightness Reversion
• Sources of brightness rerversion
–
Wrong pH in the process – Lignin
– Extractives
– Hexenuronic Acid
Importance of last stage = P
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Importance of last stage = P
0.5
1
1.5
2
2.5
3
3.5
4 D(PO)DD
DHT(PO)DD
A/D(PO)DD
R
e v e r s i o n
, % I S O
D(PO)DP
DHT(PO)DP A/D(PO)DP
Avoiding Brightness Reversion
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Avoiding Brightness Reversion
• Remove the lignin
• Minimizing HexA’s• Final kappa # < 1
•
Finish the bleaching sequence in P• Final pH (after dried) > 7
Recovery Cycle
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Ca
Cycle Na
Cycle
Evaporation
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Weak Liquor Concentration: 14-20%Strong Liquor Concentration: Aprox. 80%
Vacuum
Evaporation
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Evaporation
1 – Live Steam
2 - Condensed
3 – Diluted Liquor
4 – Concentrated Liquor
5 - Evaporated
“ The evaporated of one effect is condensed in the next effect”
Recovery Boiler
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Causticizing
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Causticizing
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Causticizing
• Green Liquor => H2O + Na2CO3 + Na2S
•Slaking Reaction (Exotermic Reaction) – CaO + H2O (green liquor) -> Ca(OH)2
• Causticizing Reaction (Equilibrium Reaction)
– Ca(OH)2 + Na2CO3 (green liquor) 2 Na(OH)
(white liquor) + CaCO3 (lime mud)
•
Calcination Reaction – CaCO3 + heat -> CaO + CO2
• White Liquor => NaOH + Na2S + Na2CO3
Lime Cycle
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Lime Cycle