Post on 18-Dec-2015
PSE 476: Lecture 18 1
Pulping and BleachingPSE 476/Chem E 471
Lecture #18Bleaching Fundamentals and
Bleaching Sequences
Lecture #18Bleaching Fundamentals and
Bleaching Sequences
PSE 476: Lecture 18 2
Bleaching Sequences Agenda
• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Typical Sequences• Practice
• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Typical Sequences• Practice
PSE 476: Lecture 18 3
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.
• 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 4
Bleaching Mechanisms
Cationic Radical Anionic
Acidic Acidic or Basic Basic
Cl+.
, O3, OH+ ClO2
., O2
., Cl
.,
HO., HO2
.
ClO2
-, HO2
-
Electrophiles Nucleophiles
Aromatic and Olefinic Carbonyl and Conjugated
Carbonyl
PSE 476: Lecture 18 5
Cationic Reactions
• Cationic reactions occur under acidic conditions.• Reactive species:
» Chlorine: Cl+
» Peroxy acids: OH+
» Ozone: O3
• How can these species be cations???????» Electrophilic substitution reaction:
C
CCl Cl
-
C Cl
Cl -+
PSE 476: Lecture 18 6
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.
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.
OCH3
OR
OCH3
OR
+ Cl+Cl+
OCH3
OR
OCH3
O
Cl
R
O
O
Cl
Cl
Cl
Cl
Cl
PSE 476: Lecture 18 7
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.
• 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 8
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 9
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
PSE 476: Lecture 18 10
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 11
Anionic Reactions: Generalities
O
OCH3
CH
CH
HC O
O
OCH3
CH
CH
HC O-
XO
O
OCH3
CHXO
HC O
O- CH
O
OCH3
HC O
OCH
CH
O
OCH3
CHO
X = OH, Cl
O
O
OOO
X
(-)+ OX
(-)
CO2HCO2
(-)
X = OH, Cl, ClO
oxirane
PSE 476: Lecture 18 12
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
• 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 13
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
• 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 14
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 15
Bleaching Sequences Typical Bleaching Sequence (1990)
Stage Chemicals Charge (kg/ton pulp)
Temp. ( C)
Time (min)
O O2 + NaOH 15 + 20 100 60
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 16
Bleaching Sequences Traditional Sequences
Sequence Comments CEH Classical sequence for semi-bleached
pulp (C+D)EODED Market pulp sequence of 1980’s
DEODED ECF for kraft pulp
OZEOD First sequence using ozone in North America
YP Two stage for mechanical pulp
Sequence Comments CEH Classical sequence for semi-bleached
pulp (C+D)EODED Market pulp sequence of 1980’s
DEODED ECF for kraft pulp
OZEOD First sequence using ozone in North America
YP Two stage for mechanical pulp
PSE 476: Lecture 18 17
Bleaching Sequences Some Current (2001) Sequences
Sequence Comments OD0EOPD1E(p)D2 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
Sequence Comments OD0EOPD1E(p)D2 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 18
Elemental Chlorine Free
• ECF pulping signifies that there is no elemental chlorine (Cl2) used in the bleaching sequence.» Most often oxygen or ClO2 is used for lignin
removal.» Brightening is most often accomplished by
using DED.» Peroxide is used mainly to reinforce oxygen
or in the extraction stages.
• ECF pulping signifies that there is no elemental chlorine (Cl2) used in the bleaching sequence.» Most often oxygen or ClO2 is used for lignin
removal.» Brightening is most often accomplished by
using DED.» Peroxide is used mainly to reinforce oxygen
or in the extraction stages.
PSE 476: Lecture 18 19
Total Chlorine Free
• TCF means that there is no chlorine of any kind used in the bleaching process.
• There has been significant effort made in this direction for the following reasons:» There is no possibility of the generation of AOX (AOX
= Absorbable Organically Bound Halogens).» Chlorine free bleach plant effluent can be burned
thus allowing a pulp mill to have basically no effluent.• The problem to date with TCF is that it has not
been possible to obtain desired brightness without significant strength loss.
• TCF means that there is no chlorine of any kind used in the bleaching process.
• There has been significant effort made in this direction for the following reasons:» There is no possibility of the generation of AOX (AOX
= Absorbable Organically Bound Halogens).» Chlorine free bleach plant effluent can be burned
thus allowing a pulp mill to have basically no effluent.• The problem to date with TCF is that it has not
been possible to obtain desired brightness without significant strength loss.
PSE 476: Lecture 18 20
Bleaching Sequences Name the Sequence (1)
1. O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2
2. Cl2/wash/NaOH/wash/NaOCl
3. Cl2 & ClO2
/wash/NaOH/wash/NaOCl/wash/ClO2
/wash/NaOH/wash/ClO2
4. O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2
5. O2/wash/EDTA/wash/NaOH then H2O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2
1. O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2
2. Cl2/wash/NaOH/wash/NaOCl
3. Cl2 & ClO2
/wash/NaOH/wash/NaOCl/wash/ClO2
/wash/NaOH/wash/ClO2
4. O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2
5. O2/wash/EDTA/wash/NaOH then H2O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2
PSE 476: Lecture 18 21
Bleaching Sequences Name the Sequence (2)
• Can you name this sequence?• Hint: Oxygen bleaching (O) is typically carried
out in only the first phase. This allows the effluent to be sent to the recovery system (no possibility of Cl compounds)
• Can you name this sequence?• Hint: Oxygen bleaching (O) is typically carried
out in only the first phase. This allows the effluent to be sent to the recovery system (no possibility of Cl compounds)
PSE 476: Lecture 18 22
Bleaching Sequences Agenda
• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Typical Sequences• Practice
• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Typical Sequences• Practice