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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #19 Oxygen Bleaching. Agenda. Process Overview Advantages/Disadvantages Reduction of Oxygen: Oxygen species Lignin Reactions Carbohydrate Reactions Effect of Process Variables. Oxygen Bleaching Process Overview.

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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 #19 Oxygen Bleaching PSE 476: Lecture 19

  2. Agenda • Process Overview • Advantages/Disadvantages • Reduction of Oxygen: Oxygen species • Lignin Reactions • Carbohydrate Reactions • Effect of Process Variables PSE 476: Lecture 19

  3. Oxygen BleachingProcess 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

  4. 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 capacityavailable). 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

  5. 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

  6. 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

  7. 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. PSE 476: Lecture 19

  8. 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

  9. Oxygen BleachingLignin 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

  10. Oxygen BleachingCarbohydrate 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

  11. Oxygen BleachingProcess Variables PSE 476: Lecture 19

  12. Oxygen BleachingConsistency • 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. PSE 476: Lecture 19

  13. Oxygen BleachingEffect of Temperature PSE 476: Lecture 19

  14. Oxygen BleachingEffect of NaOH on Lignin Removal PSE 476: Lecture 19

  15. Oxygen BleachingEffect of NaOH on Carbohydrates PSE 476: Lecture 19

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