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Pulping and Bleaching PSE 476/Chem E 471. Lecture #20 Bleaching: Ozone and H 2 O 2. Ozone/Hydrogen Peroxide Bleaching: Agenda. Ozone Advantages/Disadvantages Lignin & Carbohydrate Reactions Effect of Process Variables Hydrogen Peroxide Advantages/Disadvantages

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pulping and bleaching pse 476 chem e 471

Pulping and BleachingPSE 476/Chem E 471

Lecture #20

Bleaching: Ozone and H2O2

ozone hydrogen peroxide bleaching agenda
Ozone/Hydrogen Peroxide Bleaching: Agenda
  • Ozone
    • Advantages/Disadvantages
    • Lignin & Carbohydrate Reactions
    • Effect of Process Variables
  • Hydrogen Peroxide
    • Advantages/Disadvantages
    • Lignin & Carbohydrate Reactions
    • Effect of Process Variables
ozone advantages disadvantages
OzoneAdvantages/Disadvantages
  • Advantages:
    • Powerful oxidizer
    • No chlorinated hydrocarbons generated
  • Disadvantages
    • Very low water solubility (0.98 mg/liter @ 25°C)
    • Toxic at low levels but readily detectable at very low levels
    • Less selective than Cl2 or ClO2
ozone conditions
OzoneConditions
  • Ozone generated from oxygen or air through an ozone generator (electric discharge).
  • Ozone applied with air or oxygen (4-14%).
  • Ozone is used as a delignifier to replace Cl2 and ClO2.
    • Does not achieve the same level of delignification.
    • Requires additional other stages.
ozone bleaching lignin reactions
Ozone BleachingLignin Reactions

Methanol is a VOC which is monitored

coming off the bleaching sequence

  • Ozone functions as a electrophile
  • under acidic conditions
  • Aliphatic double bonds are cleaved.
  • Aromatic ring is cleaved forming muconic acids.
  • Hydroxyl groups are introduced into the ring
  • Radical oxygen species -O2•, HO• generated.

Source: Wood Chemistry, Sjostrom page 188

ozone bleaching carbohydrate reactions
Ozone BleachingCarbohydrate Reactions
  • Ozone is 106 times more reactive towards lignin than towards carbohydrates.
    • Unfortunately, the reactions products of ozone (superoxide, hydroxyl radical) are very reactive towards carbohydrates.
    • Therefore, ozone treatments are limited because of cellulose degradation.
  • Reactions are the same as described under oxygen bleaching conditions.
ozone bleaching conditions 1
Ozone BleachingConditions (1)
  • Ozone Charge
    • Low solubility of O3 means that the rate determining step in oxidation is the accessibility of fiber to gas.
    • Rate is increased by increasing concentration of O3 in gas as well as increasing the pressure of the system.
  • Consistency
    • Low (3.5%): Low viscosity allows good mixing. Diffusion rate determining step.
    • Medium: Vigorous mixing required.
    • High (>25%) Pulp “fluffed” to achieve gas contact. Very little free water.
ozone bleaching conditions 2
Ozone BleachingConditions (2)
  • Temperature/Time
    • Low temperature improves selectivity by reducing ozone degradation to radicals.
      • 25-35°C for high/medium consistency.
      • As low as 0°C for low consistency.
    • Time: Very short (4 minutes ?).
  • pH
    • Most efficient at a pH of approximately 2.
h 2 o 2 bleaching conditions
H2O2 BleachingConditions
  • Alkaline Conditions
    • For brightening, HO2- is the reactive species: requires alkaline conditions (>pH 9).
  • Chelants
    • Chelating agents used prior to bleaching to remove metals that decompose H2O2 to radical species.
  • Three methods of use
    • Alone for brightening, the end of sequence.
    • With oxygen for lignin removal (0.2-0.5%)-booster, beginning of the sequence.
    • With NaOH in extraction stage.
h 2 o 2 bleaching lignin reactions
H2O2 BleachingLignin Reactions
  • Delignification
    • H2O2 does not degrade (remove) lignin.
    • At elevated temperatures (90°C), H2O2 is degraded to superoxide and hydroxide radicals which degrade the lignin.
      • Ring opening reactions.
  • Brightening
    • Lower temperature reaction to eliminate radical formation.
    • HO2- reacts mainly with carbonyl compounds.
h 2 o 2 bleaching carbohydrate reactions
H2O2 BleachingCarbohydrate Reactions
  • Under delignification conditions (90°C), H2O2 less selective than Cl2, ClO2, and oxygen.
    • Reduction in pulp viscosity (strength).
  • Two major degradation pathways.
    • Glycosidic cleavage by radicals (OH•).
    • Peeling induced through oxidation.
h 2 o 2 bleaching conditions 1
H2O2 BleachingConditions (1)
  • Charge
    • Higher levels of H2O2 and NaOH increase delignification but also cellulose degradation.
  • Consistency
    • 10% typical although increases to 25% improve delignification.
  • Temperature
    • Delignification: Typical temperature of 90°C although temperatures as high as 120°C work.
    • Brightening: 50-70°C.
h 2 o 2 bleaching conditions 2
H2O2 BleachingConditions (2)
  • Time: typically 120 minutes @ 90°C
  • pH: 9-12, typically >10.5
  • Metals
    • Removal of metals using chelants (EDTA) is important to reduce cellulose loss.
      • Even though radical formation is necessary for lignin removal, selectivity is better if the generation of radicals is slow.
    • Silicates do not work, magnesium sulfate does provide some protection.