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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #18 Bleaching Fundamentals and Bleaching Sequences. Bleaching Sequences Agenda. Bleaching chemistry fundamentals Cationic mechanisms Radical mechanisms Anionic mechanisms Bleaching chemicals nomenclature rules Typical Sequences Practice.

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Pulping and Bleaching PSE 476/Chem E 471

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  1. Pulping and BleachingPSE 476/Chem E 471 Lecture #18 Bleaching Fundamentals and Bleaching Sequences PSE 476: Lecture 18

  2. Bleaching SequencesAgenda • 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. PSE 476: Lecture 18

  4. Bleaching Mechanisms 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: 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. 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. 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 Side Chain Cleavage Ring Substitution 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 oxirane PSE 476: Lecture 18

  12. Bleaching SequencesNomenclature 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

  13. Bleaching SequencesNomenclature 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

  14. Differences in Chlorine Dioxide Stages PSE 476: Lecture 18

  15. Bleaching SequencesTypical Bleaching Sequence (1990) PSE 476: Lecture 18

  16. Bleaching SequencesTraditional Sequences PSE 476: Lecture 18

  17. Bleaching SequencesSome Current (2001) Sequences 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. 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. PSE 476: Lecture 18

  20. Bleaching SequencesName the Sequence (1) • O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2 • Cl2/wash/NaOH/wash/NaOCl • Cl2 & ClO2 /wash/NaOH/wash/NaOCl/wash/ClO2 /wash/NaOH/wash/ClO2 • O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2 • O2/wash/EDTA/wash/NaOH then H2O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2 PSE 476: Lecture 18

  21. Bleaching SequencesName 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) PSE 476: Lecture 18

  22. Bleaching SequencesAgenda • Bleaching chemistry fundamentals • Cationic mechanisms • Radical mechanisms • Anionic mechanisms • Bleaching chemicals nomenclature rules • Typical Sequences • Practice PSE 476: Lecture 18

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