Transcript No Slide Title

Pulping
and
Bleaching
Pulping and Bleaching
PSE 476/Chem E 471
Lecture #18
Bleaching Fundamentals and Bleaching
Sequences
PSE 476: Lecture 18
1
Pulping
and
Bleaching
Bleaching Sequences
Agenda
• Bleaching chemistry fundamentals
» Cationic mechanisms
» Radical mechanisms
» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Bleaching Conditions
• Typical Sequences
PSE 476: Lecture 18
2
Pulping
and
Bleaching
Bleaching: Basic Chemical
Principles
• Bleaching reactions can be divided into anionic,
cationic (?) and radical reactions.
• There are a multitude of different reactions that
occur during bleaching.
• We are going to cover the chemistry in very
generic terms.
PSE 476: Lecture 18
3
Pulping
and
Bleaching
Bleaching Mechanisms
Joseph Gierer, a
Radical
Cationic
well known lignin
Acidic or Basic
Acidic
chemist, has taken
the very
+.
Cl , O3, OH+ ClO ., O ., Cl.,
2
2
complicated
.
bleaching reactions
HO , HO2.
and classified them
Electrophiles
by a number of
Aromatic and Olefinic
different
categories.
PSE 476: Lecture 18
Anionic
Basic
-
-
ClO2 , HO2
Nucleophiles
Carbonyl and
Conjugated
Carbonyl
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Pulping
and
Bleaching
Cationic Reactions
• Cationic reactions occur under acidic conditions.
• Reactive species:
» Chlorine: Cl+
» Peroxy acids: OH+
» Ozone: O3
• How can these species be cations???????
» Electrophillic addition reaction:
C

Cl
-
+
Cl
C
C
PSE 476: Lecture 18
Cl
-
Cl
5
Pulping
and
Bleaching
Cationic Reactions: Generalities
(2)
The most important reaction is
electophillic substitution
onto the aromatic ring.
» There can be multiple
substitutions.
» Substitution at #1 ring position
can later cause side chain
elimination.
» Substitution at #3 or #4 position
can lead to the formation of an
ortho quinone.
» There is no ring cleavage.
+ Cl
+
Cl
+
OCH3
OCH3
O
O
R
R
Cl
Cl
Cl
Cl
OCH3
OCH3
Cl
O
R
PSE 476: Lecture 18
Cl
O
O
O
R
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Pulping
and
Bleaching
Radical Reactions: Generalities
• Nobody adds free radicals to a bleaching reaction;
they are formed from other additives.
• Examples:
» Chlorine: Cl• (chlorine radical)
» Oxygen: -O2• (superoxide radical)
HO• (hydroxyl radical)
• Many free radicals react very rapidly with lignin
so they are good except that they degrade
carbohydrates very rapidly so they are also bad.
PSE 476: Lecture 18
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Pulping
and
Bleaching
Radical Reactions: Generalities (2)
• Pathways are pH dependent.
• Free radicals couple with aromatic rings.
» This and further mechanisms affected by free versus etherified
phenolic hydroxyls.
» Results in generation of free radical structures.
• These structures can undergo additional substitutions
and oxidation reactions.
»
»
»
»
Generation of ortho quinones.
Ring opening reactions.
Ring substitution by bleaching agent.
Some side chain cleavage.
• Carbohydrates will react and are degraded.
PSE 476: Lecture 18
8
Pulping
and
Bleaching
Radical Reactions: Generalities
Ring Cleavage
Ortho Quinones
CO2H
CO2H
O
.
O
R=Aryl
OCH3
O
( )
O
R=H
X
OCH3
OCH3
O
O
R
Side Chain Cleavage
Ring Substitution
PSE 476: Lecture 18
9
Pulping
and
Bleaching
Anionic Reactions:
Generalities
• These are alkaline bleaching reactions.
• ClO -, HOO - added reagents
.
.
» Other species generated: O2-, O-, etc.
» These chemicals are involved in nucleophilic attacks.
• Target of attacks: carbonyls and conjugated
carbonyls.
• Products of reactions:
» Ring opening: Dicarboxylic acids
» Side chain cleavage
PSE 476: Lecture 18
10
Anionic Reactions:
Generalities
Pulping
and
Bleaching
-
HC O
HC O
CH
CH
HC O
CH
O
CH
CH
XO CH
OCH3
OCH3
O
HC O
XO
O
CH
CH
OCH3
O
CHO
OCH3
OCH3
O
O
O
X = OH, Cl
(-)
+ OX
(-)
O
O X
O
O
O
(-)
CO2
CO2H
X = OH, Cl, ClO
PSE 476: Lecture 18
11
Pulping
and
Bleaching
Bleaching Sequences
Nomenclature Rules (1)
• Nomenclature rules found in TAPPI Information Sheet TIS
0606-12.
• Bleaching sequences are described by capital letters which
describe the chemicals used.
» C = chlorine, H = hypochlorite, D = chlorine dioxide, O =
oxygen (with NaOH), P = peroxide, Z = ozone, Y =
hydrosulfite, X = enzymes, E = NaOH, Q = chelants, Paa =
peracetic acid
• Letter represents each chemical followed by a washing
step.
» CED = Cl2/wash/NaOH extraction/wash/ ClO2/wash
PSE 476: Lecture 18
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Pulping
and
Bleaching
Bleaching Sequences
Nomenclature Rules (2)
• If washing is omitted, a parenthesis is added around the steps.
» C(EO)D = Cl2 /wash/NaOH/Oxygen/wash/ClO2/wash
• If chemicals are added simultaneously, the letters are placed
in parenthesis separated by a + sign. C(E+O)D = Cl2
/wash/NaOH+Oxygen/wash/ClO2/wash
» People will also list this as CE/OD
• Sometimes small amounts of chemicals are added to fortify a
step. The additives are typically listed as subscripts: EO or
EP or EOP
• Subscripts are also used to designate differences in conditions
(ie temp, pH, etc): D0, D1, D2
PSE 476: Lecture 18
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Pulping
and
Bleaching
Differences in Chlorine
Dioxide Stages
Condition
D0
D1
D2
Final pH
1.5-2.5
3.5-5
3.5 - 5
Temp °C
40°C - 60°C
55°C - 75°C
60°C - 85°C
Consistency
10-15%
10-15%
10-15%
Time
30-80min
2-4 hours
2-4 hours
Charge
1-2 times
kappa #
4-6 times
kappa #
4-6 times
kappa #
PSE 476: Lecture 18
14
Pulping
and
Bleaching
Bleaching Sequences
Typical Bleaching Sequence (1990)
Stage
Chemicals
Charge
Temp.
(kg/ton pulp) ( C)
15 + 20
100
O
O2 + NaOH
D+C
ClO2 + Cl2
12 + 18
70
45
EO
NaOH + O2
15 + 5
70
120
D
ClO2
17
70
180
EP
NaOH + H2O2
8+3
70
90
D
ClO2
5
70
180
PSE 476: Lecture 18
Time
(min)
60
15
Pulping
and
Bleaching
Bleaching Sequences
Traditional Sequences
Sequence
CEH
(C+D)EODED
Comments
Classical sequence for semi-bleached
pulp
Market pulp sequence of 1980’s
DEODED
ECF for kraft pulp
OZEOD
First sequence using ozone in North
America
First commercial TCF sequence for
kraft pulp
Two stage for mechanical pulp
OQP
YP
PSE 476: Lecture 18
16
Pulping
and
Bleaching
Bleaching Sequences
Some Current (2001) Sequences
Sequence
OD0EOPD1E(p)D2
Comments
Don't know why (p)????
OD0EOPD1D2
OD0EOPD1P
Oq(OP)(ZE)DD
Z-ECF Bleaching
(Z(EO))DnD
Dn designates the addition of
NaOH to bring up the pH
Oq(OP)(ZQ)(PO)
TCF Bleaching
PSE 476: Lecture 18
17