Kraft process

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Kraft process

  1. 1. KRAFT PROCESS
  2. 2. • The kraft process also known as kraft pulping or sulfate process • Is an industrial process for conversion of wood into wood pulp consisting of almost pure cellulose fibers • Treatment of wood chips with a mixture of sodium hydroxide and sodium sulfide, known as liquor, • That breaks the bonds that link lignin to the cellulose
  3. 3. The problem Pulping Wood (this is what we have) Fibers (this is what we want)
  4. 4. History • German word Kraft meaning paper • invented by Carl F. Dahl in 1879 • a pulp mill using this technology started in Sweden in 1890. • The invention of the recovery boiler by G.H. Tomlinson in 1930s.
  5. 5. Kraft Pulp Making Process
  6. 6. RAW MATERIAL
  7. 7. STEPS INVOLVED IN PROCESS • • • • • • • • Impregnation Cooking Recovery process Blowing Screening Washing Bleaching Process chemicals
  8. 8. IMPREGNATION • Common wood chips used are 12–25 millimeters long • Prestreamed and wetted to impregnate the chips with white liquor • White liqour contain NaOH+NaSH
  9. 9. COOKING • enters the digester below 100 °C (212 °F). • Typically delignification requires several hours at 170 to 176 °C (338 to 349 °F). • The combined liquids, known as black liquor contain lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulfate and other inorganic salts.
  10. 10. 13
  11. 11. RECOVERY PROCESS Thermochemical sulfate reduction 1. Na2SO4 + 2 C → Na2S + 2 CO2 Calcium carbonate precipitates from white liquor 2. Na2S + Na2CO3 + Ca(OH)2 ←→ Na2S + 2 NaOH + CaCO3 Calcination 3. CaCO3 → CaO + CO2 Slaking 4. CaO + H2O → Ca(OH)2
  12. 12. Blowing cooked wood chips are blown by reducing the pressure to atmospheric by release of steam Screening Screening of the pulp after pulping is a process whereby the pulp is separated from large shives, knots, dirt and other debris Washing • • • • • Pressure diffusers Atmospheric diffusers Vacuum drum washers Drum displacers Wash presses
  13. 13. Bleaching In bleaching tower the pulp are bleached to a high brightness
  14. 14. Process chemicals Process chemicals are added to improve the production process: • Surfactants may be used to improve impregnation • Anthraquinone is used as a digester • An emulsion breaker can be added in the soap separation • Defoamers remove foam • Dispersing agents, detackifiers and complexing agents are keeping the system cleaner • Fixation agents are fixating finely dispersed potential deposits to the fibers

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