1 Pulping and Bleaching PSE 476/Chem E 471 Lecture #19 Oxygen Bleaching and NaOH extraction Lecture #19 Oxygen Bleaching and NaOH extraction.

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Pulping and BleachingPSE 476/Chem E 471

Lecture #19Oxygen Bleaching and NaOH

extraction

Lecture #19Oxygen Bleaching and NaOH

extraction

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Agenda

• Process Overview • Advantages/Disadvantages• Reduction of Oxygen: Oxygen species• Lignin Reactions• Carbohydrate Reactions• Effect of Process Variables• NaOH extraction

• Process Overview • Advantages/Disadvantages• Reduction of Oxygen: Oxygen species• Lignin Reactions• Carbohydrate Reactions• Effect of Process Variables• NaOH extraction

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Oxygen BleachingProcess Overview

• Feed: Unbleached pulp from brown stock washer.» Medium consistency 10-14%, High consistency 20-

28%.

• Alkali (NaOH) added in pump to feed tank.• Oxygen added in high shear mixer.• Pulp (oxygen) pass through up flow reactor

(1 hr).

• Pulp and gases (O2 & other) separated.

• Pulp thoroughly washed (twice).

• Feed: Unbleached pulp from brown stock washer.» Medium consistency 10-14%, High consistency 20-

28%.

• Alkali (NaOH) added in pump to feed tank.• Oxygen added in high shear mixer.• Pulp (oxygen) pass through up flow reactor

(1 hr).

• Pulp and gases (O2 & other) separated.

• Pulp thoroughly washed (twice).

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Oxygen Bleaching

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Sequence from last lecture

O

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Oxygen Bleaching

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Advantages of Oxygen Bleaching

• Environmental:» Less chlorinated organics in discharge (AOX).» Significantly lower BOD, COD, and color in

effluent.– This is because the effluent from oxygen bleaching

can be evaporated and burned in the recovery system (if capacity available). This means that the oxygen bleaching stage must be the first stage (before any ClO2 used).

• Chemical costs:- Oxygen much cheaper than ClO2.

• Lower corrosiveness.

• Environmental:» Less chlorinated organics in discharge (AOX).» Significantly lower BOD, COD, and color in

effluent.– This is because the effluent from oxygen bleaching

can be evaporated and burned in the recovery system (if capacity available). This means that the oxygen bleaching stage must be the first stage (before any ClO2 used).

• Chemical costs:- Oxygen much cheaper than ClO2.

• Lower corrosiveness.

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Disadvantages of Oxygen Bleaching

• High capital costs.» Low solubility of oxygen (75 times less soluble than

Cl2).

» Need equipment that can generate good oxygen gas/fiber contact. Economics dictate that this is done at a medium to high consistency.

• Loss of selectivity when delignification above 50%.» Oxygen bleaching is used to remove lignin.» Approximately 50+% lignin can be removed using

oxygen - no more.

• High capital costs.» Low solubility of oxygen (75 times less soluble than

Cl2).

» Need equipment that can generate good oxygen gas/fiber contact. Economics dictate that this is done at a medium to high consistency.

• Loss of selectivity when delignification above 50%.» Oxygen bleaching is used to remove lignin.» Approximately 50+% lignin can be removed using

oxygen - no more.

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Chemistry of Oxygen Bleaching

• Oxygen used in bleaching is applied as a gas (O2). In this state, most of the oxygen is in the triplet state which means there are 2 unpaired electrons in the outer shell with parallel spin.» Oxygen can also exist in the singlet state: 2 paired or

unpaired electrons with antiparallel spinexcited state.• Oxygen is not extremely reactive. It reacts in the

triplet state with ionized phenolic hydroxyl groups generating phenolic radicals. Therefore, the bleaching must be carried out under alkaline conditions (to generate phenolic hydroxyls)» Metals are needed to drive this reaction

• Oxygen used in bleaching is applied as a gas (O2). In this state, most of the oxygen is in the triplet state which means there are 2 unpaired electrons in the outer shell with parallel spin.» Oxygen can also exist in the singlet state: 2 paired or

unpaired electrons with antiparallel spinexcited state.• Oxygen is not extremely reactive. It reacts in the

triplet state with ionized phenolic hydroxyl groups generating phenolic radicals. Therefore, the bleaching must be carried out under alkaline conditions (to generate phenolic hydroxyls)» Metals are needed to drive this reaction

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Oxygen Species Generated During Bleaching

• Oxygen is reduced through the reaction with phenolic hydroxyl groups to superoxide radical (-O2•). A simplified version of what happens next is that through a variety of oxidation/reduction and interconversion reactions, a number of different oxygen species are generated (pH dependent). All of these different species have different degrees of reactivity. The scheme below shows the reduction steps of oxygen on the acid side.

O2e-, H+

HO2. e-, H+

H2O2e-, H+

HO. e-, H+

H2Ohydroperoxy radical hydrogen peroxide hydroxide radical

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Oxygen Species

• HO2• : hydroperoxy radical, pKa ~ 4.8

» Ionized form (- O2•) : Superoxide radical = weak oxidant.

• H2O2: hydrogen peroxide, pKa ~ 11.6

» ionized form (-HO2): hydroperoxy anion = weak oxidant

• HO. : hydroxide radical (strong oxidizer), pKa ~ 11.9» Ionized form (-O•): oxyl anion radical

• Radical species are very reactive: Unfortunately they are not selective and react with carbohydrates.

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Oxygen BleachingLignin Reactions

• Bleaching conducted under alkaline conditions.» Requires free phenolic hydroxyls on lignin.» Ionized form of oxygen species typically more

reactive.

• All oxygen species involved in process.• Reaction Mechanisms.

» Ring structures are cleaved and/or substituted with oxygen.

» Some cleavage of side chains/linkages.» Lignin/carbohydrate cleaved .

• Bleaching conducted under alkaline conditions.» Requires free phenolic hydroxyls on lignin.» Ionized form of oxygen species typically more

reactive.

• All oxygen species involved in process.• Reaction Mechanisms.

» Ring structures are cleaved and/or substituted with oxygen.

» Some cleavage of side chains/linkages.» Lignin/carbohydrate cleaved .

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Radical Reactions: Generalities

Ring Cleavage Ortho Quinones

Ring SubstitutionSide Chain Cleavage

O

OCO2HCO2H

OCH3

O

X

R

O

OCH3

(.)

R=Aryl

R=H

O

OCH3

O

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Oxygen BleachingCarbohydrate Reactions

• Carbohydrates degraded more in oxygen stage than in ClO2 or extraction stages.

• Two major degradation pathway:» Glycosidic cleavage by hydroxide radicals (OH•)-

major reaction.» Peeling induced through oxidation.

• Both pathways accelerated by metals (radical formation).» Selectivity improved through the addition of

magnesium.- Precipitates metals thus reducing radical formation.

• Carbohydrates degraded more in oxygen stage than in ClO2 or extraction stages.

• Two major degradation pathway:» Glycosidic cleavage by hydroxide radicals (OH•)-

major reaction.» Peeling induced through oxidation.

• Both pathways accelerated by metals (radical formation).» Selectivity improved through the addition of

magnesium.- Precipitates metals thus reducing radical formation.

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Oxygen BleachingProcess Variables

Medium Consistency High Consistency

Consistency (%) 10-14 25-28

Reaction Time (min) 50-60 30

Initial Temperature (C) 85-105 100-115

Inlet Pressure, kPa 700-800 415-600

Alkali Consumption kg/t 18-28 18-23

Delignification (%) 40-45 45-55

Medium Consistency High Consistency

Consistency (%) 10-14 25-28

Reaction Time (min) 50-60 30

Initial Temperature (C) 85-105 100-115

Inlet Pressure, kPa 700-800 415-600

Alkali Consumption kg/t 18-28 18-23

Delignification (%) 40-45 45-55

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Oxygen BleachingConsistency

• Oxygen has a low solubility in alkaline solutions.» In order to obtain reasonable rates of

delignification it is necessary to have good distribution of bubbles in solution.

» Originally this was accomplished by dewatering the pulp to very high consistency and fluffing it. This creates a slurry of fibers in a continuous gas phase.

» The development of shear mixing devices in the 1970s made it possible to produce very small gas bubbles in medium consistency pulp.

• Oxygen has a low solubility in alkaline solutions.» In order to obtain reasonable rates of

delignification it is necessary to have good distribution of bubbles in solution.

» Originally this was accomplished by dewatering the pulp to very high consistency and fluffing it. This creates a slurry of fibers in a continuous gas phase.

» The development of shear mixing devices in the 1970s made it possible to produce very small gas bubbles in medium consistency pulp.

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Oxygen BleachingEffect of Temperature

10

15

20

25

30

35

40

0 1 2 3

Reaction Time (hours)

Kap

pa

num

ber Temp. 85°C

Temp 100°CTemp 115°Temp 130°

10

15

20

25

30

35

40

0 1 2 3

Reaction Time (hours)

Kap

pa

num

ber Temp. 85°C

Temp 100°CTemp 115°Temp 130°

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Oxygen BleachingEffect of NaOH on Lignin Removal

10

15

20

25

30

35

0 10 20 30 40 50 60

Reaction Time (hours)

Kap

pa

num

ber

1% NaOH

2.5% NaOH

3.5% NaOH

10

15

20

25

30

35

0 10 20 30 40 50 60

Reaction Time (hours)

Kap

pa

num

ber

1% NaOH

2.5% NaOH

3.5% NaOH

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Oxygen BleachingEffect of NaOH on Carbohydrates

2527293133353739414345

0 10 20 30 40 50 60

Reaction Time (hours)

Vis

cosi

ty, m

Pa'

s

1% NaOH2.5% NaOH3.5% NaOH

2527293133353739414345

0 10 20 30 40 50 60

Reaction Time (hours)

Vis

cosi

ty, m

Pa'

s

1% NaOH2.5% NaOH3.5% NaOH

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NaOH ExtractionLignin Reactions

Purpose: to dissolve and then to remove compounds made alkali soluble in the proceeding acid delignification.

Lignin (2 functions)• Removal (solubilization) of reacted (modified) lignin• Reactivation of residual lignin (3 theories)

1. Chlorolignin prevents migration of Cl2 or ClO2. Removal by NaOH opens new sites.

2. Remaining lignin in LCC’s. Other oxidants beside chlorine are necessary to cleave.

3. Acid oxidation changes lignin to non reactive form. Alkali converts lignin to a reactive species.

Purpose: to dissolve and then to remove compounds made alkali soluble in the proceeding acid delignification.

Lignin (2 functions)• Removal (solubilization) of reacted (modified) lignin• Reactivation of residual lignin (3 theories)

1. Chlorolignin prevents migration of Cl2 or ClO2. Removal by NaOH opens new sites.

2. Remaining lignin in LCC’s. Other oxidants beside chlorine are necessary to cleave.

3. Acid oxidation changes lignin to non reactive form. Alkali converts lignin to a reactive species.

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NaOH Extraction Carbohydrate Reactions

• Peeling» Limited amount – somewhat dependent upon

previous stage.

• Stopping

• With Oxygen or H2O2

» Oxygen enhances the effect of extraction.

» Standard O2 reactions (oxidative peeling, etc).

• Peeling» Limited amount – somewhat dependent upon

previous stage.

• Stopping

• With Oxygen or H2O2

» Oxygen enhances the effect of extraction.

» Standard O2 reactions (oxidative peeling, etc).

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NaOH Extraction Conditions (1)

• NaOH Charge» The more NaOH used, the more residual lignin

removed.» Effect levels out at higher NaOH concentration.» Optimum charge leaves a residual pH = 10.5» NaOH charge affects the amount of oxidant

necessary in the next stage.

• Consistency» Medium (8-10%) most practical.

- Mixing, capital costs, operating costs, etc.

• NaOH Charge» The more NaOH used, the more residual lignin

removed.» Effect levels out at higher NaOH concentration.» Optimum charge leaves a residual pH = 10.5» NaOH charge affects the amount of oxidant

necessary in the next stage.

• Consistency» Medium (8-10%) most practical.

- Mixing, capital costs, operating costs, etc.

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NaOH Extraction Conditions (2)

• Temperature» Higher temperature = faster rate. Typical:

60-80°C.

• Time» Dependent on other variables. Typical: 60 –

90 minutes.

• pH: >10.5

• Temperature» Higher temperature = faster rate. Typical:

60-80°C.

• Time» Dependent on other variables. Typical: 60 –

90 minutes.

• pH: >10.5

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NaOH Extraction Conditions (3)

• Oxygen or H2O2 Addition» By 1980, it was discovered that addition of O2 or H2O2

to the E stage improved delignification.» Driven by the desire to get away from AOX out of plant.

» Less expensive O2.

» Reduce chlorinated hydrocarbons in pulp.» Activates lignin towards future stages.

• Alkalinity after Acidic Stage Improves O2/H2O2 Efficiency.

• Washing after-very important.

• Oxygen or H2O2 Addition» By 1980, it was discovered that addition of O2 or H2O2

to the E stage improved delignification.» Driven by the desire to get away from AOX out of plant.

» Less expensive O2.

» Reduce chlorinated hydrocarbons in pulp.» Activates lignin towards future stages.

• Alkalinity after Acidic Stage Improves O2/H2O2 Efficiency.

• Washing after-very important.