Transcript No Slide Title

Pulping
and
Bleaching
Pulping and Bleaching
PSE 476/Chem E 471
Lecture #19
Oxygen Bleaching
PSE 476: Lecture 19
1
Pulping
and
Bleaching
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Agenda
Process Overview
Advantages/Disadvantages
Reduction of Oxygen: Oxygen species
Lignin Reactions
Carbohydrate Reactions
Effect of Process Variables
PSE 476: Lecture 19
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Pulping
and
Bleaching
Oxygen Bleaching
Process Overview
• Feed: Unbleached pulp from brown stock washer.
» Medium consistency 10-14%, High consistency 20-28%.
• Alkali (NaOH or white liquor) 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).
PSE 476: Lecture 19
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Pulping
and
Bleaching
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.
PSE 476: Lecture 19
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Pulping
and
Bleaching
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.
PSE 476: Lecture 19
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Pulping
and
Bleaching
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 spinexcited 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
PSE 476: Lecture 19
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Oxygen Species Generated
During Bleaching
Pulping
and
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.
-
O2
+
e,H
HO2
.
-
+
e,H
-
H2O2
+
e,H
PSE 476: Lecture 19
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HO
-
+
e,H
H2O
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Pulping
and
Bleaching
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.
PSE 476: Lecture 19
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Pulping
and
Bleaching
Oxygen Bleaching
Lignin 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 .
PSE 476: Lecture 19
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Pulping
and
Bleaching
Oxygen Bleaching
Carbohydrate Reactions
• Carbohydrates degraded more in oxygen stage
than in ClO2 or extraction stages.
• Two major degradation pathway:
» Glycosidic cleavage by radicals (OH•).
» Peeling induced through oxidation.
• Both pathways accelerated by metals (radical
formation).
» Selectivity improved through the addition of magnesium.
- Precipitates metals thus reducing radical formation.
PSE 476: Lecture 19
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Pulping
and
Bleaching
Oxygen Bleaching
Process 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
PSE 476: Lecture 19
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Pulping
and
Bleaching
Oxygen Bleaching
Consistency
• 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 Bleaching
Pulping
and
Bleaching
Effect of Temperature
Kappa number
40
35
Temp. 85°C
Temp 100°C
Temp 115°
Temp 130°
30
25
20
15
10
0
1
2
3
Reaction Time (hours)
PSE 476: Lecture 19
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Oxygen Bleaching
Pulping
and
Bleaching
Effect of NaOH on Lignin Removal
Kappa number
35
30
25
1% NaOH
2.5% NaOH
20
3.5% NaOH
15
10
0
10
20
30
40
50
60
Reaction Time (hours)
PSE 476: Lecture 19
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Oxygen Bleaching
Viscosity, mPa's
Pulping
and
Bleaching
Effect of NaOH on Carbohydrates
45
43
41
39
37
35
33
31
29
27
25
1% NaOH
2.5% NaOH
3.5% NaOH
0
10
20
30
40
50
60
Reaction Time (hours)
PSE 476: Lecture 19
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