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Pulping and Bleaching PSE 476/Chem E 471

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

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  1. Pulping and BleachingPSE 476/Chem E 471 Lecture #20 Bleaching: Ozone and H2O2

  2. 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

  3. 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

  4. 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.

  5. Ozone

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

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