Pulping and Bleaching PSE 476/Chem E 471

Pulping and BleachingPSE 476/Chem E 471

Lecture #15

Kraft Pulping

Review of Reactions/Kinetics

Lecture #15

Kraft Pulping

Review of Reactions/Kinetics

Kraft Pulping

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Yield of Lignin (%)

Eff

ecti

ve A

lkal

i (g

/l N

aOH

)

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Yield of Lignin (%)

Eff

ecti

ve A

lkal

i (g

/l N

aOH

)

Bulk Phase

Initial PhaseImpregnation zone

Residual Phase

70°C

70°C137°C

170° C

Kraft Pulping Kinetics:Initial Phase

ExtractivesReactions1) Saponification of

Extractivesa) Fatsb) Waxesc) Lignans (Lactones)

2) Neutralizationa) Fatty Acidsb) Resin Acidsc) Monoterpene Acidsd) Lignanse) Phenolic Acids

3) Solubilizationa) Nearly all

extractives areremoved during theinitial phase of thecook.

Kinetics1) Effective Alkali

a) These reactions areso rapid that thelevel of alkali haslittle effect on therate

2) Sulfiditya) Sulfidity has no

effect on thesereactions

ExtractivesReactions1) Saponification of

Extractivesa) Fatsb) Waxesc) Lignans (Lactones)

2) Neutralizationa) Fatty Acidsb) Resin Acidsc) Monoterpene Acidsd) Lignanse) Phenolic Acids

3) Solubilizationa) Nearly all

extractives areremoved during theinitial phase of thecook.


a) These reactions areso rapid that thelevel of alkali haslittle effect on therate


effect on thesereactions

GlucomannansReactions1) Dissolution

a) A certain small %of Glucommans aresoluble in alkaliand will diffusefrom the wood.

2) Peelinga) Peeling reaction

starts during thisphase @ ~ 100C

3) Deacetylation


a) No effect on rate ofdegradation

2) Sulfide Chargea) No effect on reaction

GlucomannansReactions1) Dissolution

a) A certain small %of Glucommans aresoluble in alkaliand will diffusefrom the wood.

2) Peelinga) Peeling reaction

starts during thisphase @ ~ 100C

3) Deacetylation


a) No effect on rate ofdegradation


Compound % RemovedExtractivesCelluloseGlucomannansXylansLignin

Majority<1%60-70%20-30%20-30%

Compound % RemovedExtractivesCelluloseGlucomannansXylansLignin

Majority<1%60-70%20-30%20-30%

Initial Reactions: Low Temperature

• Carbohydrates» Alkaline hydrolysis of acetyl groups on xylan (see next slide).

» Removal of certain soluble carbohydrates.- Certain galactoglucomannans.

- Arabinogalactans.

• Extractives» Alkaline hydrolysis of fats (saponification), waxes, and other

esters.

» Neutralization of extractives.- There are a number of acidic extractives which consume NaOH.

• Carbohydrates» Alkaline hydrolysis of acetyl groups on xylan (see next slide).

» Removal of certain soluble carbohydrates.- Certain galactoglucomannans.

- Arabinogalactans.

• Extractives» Alkaline hydrolysis of fats (saponification), waxes, and other

esters.

» Neutralization of extractives.- There are a number of acidic extractives which consume NaOH.

Alkaline Hydrolysis:Example Using Acetyl Groups

• Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.

• Hydrolysis of acetyl groups occurs readily in alkaline solutions.» Reaction occurs rapidly even at room temperature.

• Reaction consumes alkali.

• Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.

• Hydrolysis of acetyl groups occurs readily in alkaline solutions.» Reaction occurs rapidly even at room temperature.

• Reaction consumes alkali.

O

OH

OHHO

CH2OH

O CO-

CH3

OH

O

OH

OHHO

CH2OH

O CO

CH3

HO-O

OH

OH

OHHO

CH2OH

CO

CH3HO

+

Saponification of Fats(Review slide from PSE 406)

C

O

OH2C

R

OH-

C

O-

OH2C

R OHH2O

C

O

O-

R

H2C OH

• Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.

• Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.

HOCH2CHCH2OH

OH

Glycerol (glycerine)

Once again this reaction consumes part of the alkalicharge.

Removal of Glucomannans:Effect of Effective Alkali

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Time (minutes)

Glu

co

ma

nn

an

Yie

ld (

%)

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Te

mp

era

ture

(C

)

25% EA

15.8% EA

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Time (minutes)

Glu

co

ma

nn

an

Yie

ld (

%)

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150

200

Te

mp

era

ture

(C

)

25% EA

15.8% EA

Why doesn’t EA affect glucomannan removal?•Peeling reaction not sensitive to OH concentration•Rapid removal at the beginning of the cook (dissolving)•Over 100C greater rate of peeling•70% removed rate drops again

Kraft Pulping Kinetics:Initial Phase

LigninReactions1) Cleavage of -O-4

linkagesa) Units with free

phenolic hydroxylb) Very fast

2) Cleavage of -O-4linkages

a) Units with freephenolic hydroxylor carbonyl.

b) Rapid reaction3) Solubilization

a) Small amount oflignin alkali soluble

4) Condensationa) Limited


a) No affect onrate during thisphase

b) Rate diffusionlimited in thisphase


effect duringthis phase

LigninReactions1) Cleavage of -O-4

linkagesa) Units with free

phenolic hydroxylb) Very fast

2) Cleavage of -O-4linkages

a) Units with freephenolic hydroxylor carbonyl.

b) Rapid reaction3) Solubilization

a) Small amount oflignin alkali soluble

4) Condensationa) Limited


a) No affect onrate during thisphase

b) Rate diffusionlimited in thisphase


effect duringthis phase

XylansReactions1) Dissolution

a) Major reaction ofXylans during kraftpulping

2) Peelinga) Very minor reaction

starts @ ~ 100C3) Stopping

a) See 2a4) Deacetylation


a) Higher EA increases therate of dissolution



a) Major reaction ofXylans during kraftpulping

2) Peelinga) Very minor reaction

starts @ ~ 100C3) Stopping

a) See 2a4) Deacetylation




CelluloseReactions1) Peeling

a) Loss of cellulose begins@ ~ 120C - 130 C

2) Minor reaction as only 10%of all cellulose lost duringcook.


a) ?2) Sulfide Charge

a) No effect on reaction


a) Loss of cellulose begins@ ~ 120C - 130 C

2) Minor reaction as only 10%of all cellulose lost duringcook.




Lignin Removal:Effect of Sulfidity

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Sulfidity

KA

PPA

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Sulfidity

KA

PPA

• Sulfidity effect zero order in initial delignification (this means that changing sulfidity has no effect on initial rate)

» Sulfidity is important during bulk delignification» Effects final lignin/carbohydrate ratio

- HS- does not react with carbohydrates

• Sulfidity effect zero order in initial delignification (this means that changing sulfidity has no effect on initial rate)

» Sulfidity is important during bulk delignification» Effects final lignin/carbohydrate ratio

- HS- does not react with carbohydrates

It depends on:•Wood species•EA•Cooking temp.•Hardwoods 20%•Softwoods 25%

Removal of Xylans:Effect of Effective Alkali

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Time (minutes)

Xyl

an Y

ield

(%

)

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Tem

per

atu

re (

C)

25% EA

15.8% EA0

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Time (minutes)

Xyl

an Y

ield

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25% EA

15.8% EA

Why does EA affect xylan removal?•Stable at the beginning•Hydrolysis and 2nd peeling

Kraft Pulping Kinetics:Bulk Phase

LigninReactions1) Cleavage of non

phenolic -O-4followed by:

a) Cleavage of -O-4linkages in unitswith free phenolichydroxyl

b) Cleavage of -O-4linkages in unitswith free phenolichydroxyl Rapidreaction

c) Condensationd) Cleavage of C-C

bonds (double)


a) Higher EA =faster rate ofdegradation

2) Sulfide Chargea) Higher sulfide

charge increasesthe rate of ligninremoval.

b) Higher ligninremoval rateresults in alower lig/carb atend of cook

LigninReactions1) Cleavage of non

phenolic -O-4followed by:

a) Cleavage of -O-4linkages in unitswith free phenolichydroxyl

b) Cleavage of -O-4linkages in unitswith free phenolichydroxyl Rapidreaction

c) Condensationd) Cleavage of C-C

bonds (double)


a) Higher EA =faster rate ofdegradation

2) Sulfide Chargea) Higher sulfide

charge increasesthe rate of ligninremoval.

b) Higher ligninremoval rateresults in alower lig/carb atend of cook


a) Loss of celluloseaccelerates at temperaturedue to glycosidic cleavage

2) Stoppinga) Stopping reactions slow

loss of cellulose3) Glycosidic Cleavage

a) See 1ab) Starts to reduce

cellulose viscosity





a) Loss of celluloseaccelerates at temperaturedue to glycosidic cleavage

2) Stoppinga) Stopping reactions slow

loss of cellulose3) Glycosidic Cleavage

a) See 1ab) Starts to reduce

cellulose viscosity




Compound % Removed (total)ExtractivesCelluloseGlucomannansXylansLignin

Majority<10%60-70%30-50%80-85%


Majority<10%60-70%30-50%80-85%

Kraft Pulping Kinetics:Bulk Phase

GlucomannansReactions1) Peeling

a) Peeling isextensive untilabout 70% of xylanis lost and thenalmost stops

2) Gylcosidic cleavagea) Increases the rate

of peeling3) Stopping

a) Stops losses due topeeling


b) No effect on rate ofdegradation



a) Peeling isextensive untilabout 70% of xylanis lost and thenalmost stops

2) Gylcosidic cleavagea) Increases the rate

of peeling3) Stopping

a) Stops losses due topeeling





a) Major reaction ofXylans during thisphase

2) Peelinga) Minor reaction

3) Glycosidic Cleavagea) Will increase rate of

peeling reaction4) Precipitation on fibers

a) Starts as alkaliconsummed





a) Major reaction ofXylans during thisphase

2) Peelinga) Minor reaction

3) Glycosidic Cleavagea) Will increase rate of

peeling reaction4) Precipitation on fibers

a) Starts as alkaliconsummed




Kraft Pulping Kinetics:Residual Phase

LigninReactions1) Majority of ether

linkages that willcleave are gone:

2) Cleavage of C-Cbonds (double)

3) Condensationa) Increase the Mw

and lower reactivityof remaining lignin


a) The rate slowsrapidly:

b) amount of ligninremaining afunction of theEA and sulfidecharge duringthe cook

LigninReactions1) Majority of ether

linkages that willcleave are gone:

2) Cleavage of C-Cbonds (double)

3) Condensationa) Increase the Mw

and lower reactivityof remaining lignin


a) The rate slowsrapidly:

b) amount of ligninremaining afunction of theEA and sulfidecharge duringthe cook

CelluloseReactions1) Glycosidic Cleavage

a) Major reaction resulting inlower molecular weightand strength loss

2) Peeling/stoppinga) Minor reactions




CelluloseReactions1) Glycosidic Cleavage

a) Major reaction resulting inlower molecular weightand strength loss

2) Peeling/stoppinga) Minor reactions





Majority~10%70-80%40-50%%85-95%


Majority~10%70-80%40-50%%85-95%

Kraft Pulping Kinetics:Residual Phase


a) Limited2) Gylcosidic cleavage

a) Limited3) Stopping

a) Limited





a) Limited2) Gylcosidic cleavage

a) Limited3) Stopping

a) Limited




XylansReactions1) Peeling/stopping/

a) negligible2) Precipitation on fibers

a) Significant amountdeposited as alkalicontinues to beconsummed


a) Lower levels of alkali at endof the cook casusedeposition of xylans


XylansReactions1) Peeling/stopping/

a) negligible2) Precipitation on fibers

a) Significant amountdeposited as alkalicontinues to beconsummed


a) Lower levels of alkali at endof the cook casusedeposition of xylans


Pulping and Bleaching PSE 476/Chem E 471

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