Transcript No Slide Title

Pulping and Bleaching
PSE 476/Chem E 471
Lecture #19
Oxygen Bleaching and NaOH
extraction
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Agenda
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Process Overview
Advantages/Disadvantages
Reduction of Oxygen: Oxygen species
Lignin Reactions
Carbohydrate Reactions
Effect of Process Variables
NaOH extraction
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Oxygen Bleaching
Process Overview
• Feed: Unbleached pulp from brown stock
washer.
» Medium consistency 10-14%, High consistency 2028%.
• 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.
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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.
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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 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
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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.
-
O2
+
e,H
HO2
.
-
+
e,H
hydroperoxy radical
-
H2O2
+
e,H
hydrogen peroxide
.
HO
-
+
e,H
H2O
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 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 .
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Radical Reactions: Generalities
Ring Cleavage
Ortho Quinones
CO2H
CO2H
O
.
O
R=Aryl
OCH3
O
( )
O
R=H
X
OCH3
O
OCH3
O
R
Ring Substitution
Side Chain Cleavage
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Oxygen Bleaching
Carbohydrate 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.
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Oxygen Bleaching
Process Variables
Medium Consistency
High Consistency
Consistency (%)
10-14
25-28
Reaction Time (min)
50-60
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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 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
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)
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Oxygen Bleaching
Effect of NaOH on Lignin Removal
Kappa number
35
30
25
1% NaOH
2.5% NaOH
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3.5% NaOH
15
10
0
10
20
30
40
50
60
Reaction Time (hours)
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Oxygen Bleaching
Viscosity, mPa's
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)
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NaOH Extraction
Lignin 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.
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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).
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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.
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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
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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.
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