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

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Pulping and Bleaching PSE 476/Chem E 471

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Pulping and bleaching pse 476 chem e 471

Pulping and BleachingPSE 476/Chem E 471

Lecture #19

Oxygen Bleaching and NaOH extraction



  • Process Overview

  • Advantages/Disadvantages

  • Reduction of Oxygen: Oxygen species

  • Lignin Reactions

  • Carbohydrate Reactions

  • Effect of Process Variables

  • NaOH extraction

Oxygen bleaching process overview

Oxygen BleachingProcess Overview

  • Feed: Unbleached pulp from brown stock washer.

    • Medium consistency 10-14%, High consistency 20-28%.

  • 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).

Oxygen bleaching

Oxygen Bleaching

Sequence from last lecture

Sequence from last lecture


Oxygen bleaching1

Oxygen Bleaching

Advantages of oxygen bleaching

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.

  • Disadvantages of oxygen bleaching

    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.

    Chemistry of oxygen bleaching

    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

    Oxygen species generated during bleaching

    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.

    hydroperoxy radical

    hydrogen peroxide

    hydroxide radical

    Oxygen species

    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.

    Oxygen bleaching lignin reactions

    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 .

    Pulping and bleaching pse 476 chem e 471

    Radical Reactions: Generalities

    Ring Cleavage

    Ortho Quinones

    Side Chain Cleavage

    Ring Substitution

    Oxygen bleaching carbohydrate reactions

    Oxygen BleachingCarbohydrate 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.

    Oxygen bleaching process variables

    Oxygen BleachingProcess Variables

    Oxygen bleaching consistency

    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.

    Oxygen bleaching effect of temperature

    Oxygen BleachingEffect of Temperature

    Oxygen bleaching effect of naoh on lignin removal

    Oxygen BleachingEffect of NaOH on Lignin Removal

    Pulping and bleaching pse 476 chem e 471

    Oxygen BleachingEffect of NaOH on Carbohydrates

    Pulping and bleaching pse 476 chem e 471

    NaOH ExtractionLignin 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)

      • Chlorolignin prevents migration of Cl2 or ClO2. Removal by NaOH opens new sites.

      • Remaining lignin in LCC’s. Other oxidants beside chlorine are necessary to cleave.

      • Acid oxidation changes lignin to non reactive form. Alkali converts lignin to a reactive species.

    Pulping and bleaching pse 476 chem e 471

    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).

    Pulping and bleaching pse 476 chem e 471

    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.

    Pulping and bleaching pse 476 chem e 471

    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

    Pulping and bleaching pse 476 chem e 471

    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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