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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #17 Introduction to Bleaching. Agenda. Brightness General Bleaching Principles Chemistry Process Chemicals Description Advantages/Disadvantages. Why Bleach?. Improve brightness. Improve brightness stability.

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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 #17 Introduction to Bleaching PSE 476: Lecture 17

  2. Agenda • Brightness • General Bleaching Principles • Chemistry • Process • Chemicals • Description • Advantages/Disadvantages PSE 476: Lecture 17

  3. Why Bleach? • Improve brightness. • Improve brightness stability. • Clean up pulp (impurities). • Wood based (bark, resins, sand, shives). • Process based (carbon specs, rust, rubber). • External sources based (plastics, grease, ash). • Increase capacity of paper to accept printing. • Remove impurities from pure cellulose (rayon). PSE 476: Lecture 17

  4. Reflected Light Absorbed Transmitted Brightness Determination (1) Light shinning on a sheet of paper is either transmitted, adsorbed, or reflected. • Light is scattered by fibers at air/fiber interfaces • Light is adsorbed by certain chemicals in the fibers (lignin) PSE 476: Lecture 17

  5. Brightness Determination (2) • Brightness is measurement of how much light is reflected from a sheet of paper. • Whiteness does not mean brightness. • Whiteness is a physical phenomena related to how the eye views the paper. • A very white looking piece of paper may not have high brightness. • Example: blue dye added to a yellow tinged sheet of paper will give a white sheet of paper with low brightness. PSE 476: Lecture 17

  6. Brightness Determination (3) • Brightness determination method: • Light reflectance measured and compared to light reflectance from MgO. • MgO assumed to reflect 100% light. • Brightness is reported as % of MgO reflectance (85 brightness is equivalent to 85% of MgO). • Variables: • Angle of light: Light is applied to sheet at 45° angle. • Wavelength: 457 nm (blue light most sensitive). PSE 476: Lecture 17

  7. General Principles • Two types of bleaching: • Lignin removing: chemical pulps. • Lignin retaining: mechanical pulps. • Bleaching is used because at a certain point in the pulping process, carbohydrate degradation becoming greater than lignin removal. • Bleaching chemicals are more selective for lignin. • Bleaching chemicals much more expensive than pulping chemicals so they are not used in pulping. PSE 476: Lecture 17

  8. General Principles:Chemistry • Pulping • Pulping typically involves cleavage of ether linkages and some substitution (sulfonation). • Bleaching • Bleaching involves attacks on aromatic rings, olefinic structures, and carbonyl groups. • Substitution reactions play a big role. PSE 476: Lecture 17

  9. General Principles:Process • Bleaching uses a combination of chemicals in series. • One chemical alone will not remove residual lignin. • Each step reacts with material modified in previous step. NaOH NaOH ClO2 ClO2 O2 O2 Unbleached O D D EO Bleached Pulp Pulp PSE 476: Lecture 17

  10. General Principles:Chemicals (1) PSE 476: Lecture 17

  11. General Principles:Chemicals (2) PSE 476: Lecture 17

  12. General Principles:Chemicals (3a) PSE 476: Lecture 17

  13. General Principles:Chemicals (3b) PSE 476: Lecture 17

  14. General Principles:Chemicals (3c) PSE 476: Lecture 17

  15. Bleaching Generalities • It is important to note that when bleaching with a specific reagent, it will be converted into a number of different reactive species which will react with lignin and carbohydrates differently. A simple example is when chlorine gas is added to water; both hypochlorous acid and/or hypochorite is formed depending on the pH. PSE 476: Lecture 17

  16. Bleaching Generalities (2) • Often, radical species are generated from the bleaching chemicals. Let us use oxygen for an example. Molecular O2 reacts with ionized free phenolic groups generating a phenoxy radical and a superoxide radical (under alkaline conditions). The species is not terribly reactive with lignin but under a variety of other reactions can be reduced to hydroxide radical which reacts very very rapidly with lignin and carbohydrates. PSE 476: Lecture 17

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