pulping and bleaching pse 476 chem e 471 n.
Skip this Video
Loading SlideShow in 5 Seconds..
Pulping and Bleaching PSE 476/Chem E 471 PowerPoint Presentation
Download Presentation
Pulping and Bleaching PSE 476/Chem E 471

Loading in 2 Seconds...

play fullscreen
1 / 24

Pulping and Bleaching PSE 476/Chem E 471 - PowerPoint PPT Presentation

  • Uploaded on

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.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Pulping and Bleaching PSE 476/Chem E 471' - benedict-tyler

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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).
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 .

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 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.

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.

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).

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.

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

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.