basic concepts kraft process

7/30/2019 Basic Concepts Kraft Process

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Basic Training

Kraft Process



Wood



Production of Chips



Good Size of Chips

The size of the chips is very important for the impregnation of cook liquor (white liquor)

inside the chips.



Selection of Chips



Selection of chips

Mesh Discs



Chip Bin



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



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



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)



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



Digester Retention Time• Retention time (min) = Volume of digester

(m3) * Compaction Factor (1.9)/ Chip meter(rpm) * volume of screw (m3)



H Factor• Combination of time and temperature

• Temperature => Reaction rate

• Ex:

• Retention time = 1.5 h• Temperature = 170oC

•

How H Factor is calculated?



H FactorReaction Rates (based on temperature) – Calculation of H Factor

H Factor = 921.4 (Reaction Rate 170oC) x 1.5 (Retention Time) = 1382



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



Chip Meter



Top Separator

Liquor and chips – Feed Circulation

Liquor – Feed Circulation - Return

Liquor and chips – Digester - Cooking



Bottom of Digester

Importance of the cold

blow flow added at the

bottom of digester:

- Wash the pulp

- Controlling of blow

consistence



Feed Circulation



Digester

CD1 – Upper Extraction

CD3 – Lower Extraction

CD2 – Cook Circulation



Digester Screens



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



Brownstock Wash

• Aims

– Clean the pulp

– Save water

– Recovery alkali



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



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



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



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 ?



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



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



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



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



O2 Delignification



• 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



• 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



Screening Process• Aims

– Take knots out – Take shives out

– Take plastics out

– Take dirts out



Screen

Feed

AcceptReject



Screen



Rotors



Cylinder / Meshes



Knot Separator Cilinder



Mesh – Primary Screen



Screens

Screen Process



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



Screen Process

BlowFlow

Pulp to Bleaching.

Knoter

Primary

Washer

Equipment

Knots to Digester

Fiber to blow tank

Knot Wash

Pressed

Reject

Bleaching



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



Composition of Kappa #



Bleaching – Why do use many stages?

Bleaching – 1 stage

BrightnessPulp Resistence

Chemical Charge Chemical Charge

Bleaching – Many stages



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



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



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)



P Stage

• H2O2

• Focus on bleaching• Less selective than ClO2

A S



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



•

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



• 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



• 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



• 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



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



Brightness Reversion

• Sources of brightness rerversion

–

Wrong pH in the process – Lignin

– Extractives

– Hexenuronic Acid

Importance of last stage = P



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



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



Ca

Cycle Na

Cycle

Evaporation



Weak Liquor Concentration: 14-20%Strong Liquor Concentration: Aprox. 80%

Vacuum

Evaporation



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



Causticizing



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



Lime Cycle

basic concepts kraft process

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