Application of Photo-Fenton Process for the Treatment of Kraft Pulp ...
Kraft process
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Transcript of Kraft process
KRAFT PROCESS
• 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
PulpingThe problem
Wood (this is what we have) Fibers (this is what we want)
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.
Kraft Pulp Making Process
RAW MATERIAL
• Impregnation• Cooking• Recovery process• Blowing• Screening• Washing• Bleaching• Process chemicals
STEPS INVOLVED IN PROCESS
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
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.
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Thermochemical sulfate reduction1. Na2SO4 + 2 C → Na2S + 2 CO2
Calcium carbonate precipitates from white liquor2. Na2S + Na2CO3 + Ca(OH)2 ←→ Na2S + 2 NaOH + CaCO3
Calcination
3. CaCO3 → CaO + CO2
Slaking
4. CaO + H2O → Ca(OH)2
RECOVERY PROCESS
Blowing
Screening
Washing
Screening of the pulp after pulping is a process whereby the pulp is separated from large shives, knots, dirt and other debris
• Pressure diffusers• Atmospheric diffusers• Vacuum drum washers• Drum displacers• Wash presses
cooked wood chips are blown by reducing the pressure to atmospheric by release of steam
In bleaching tower the pulp are bleached to a high brightness
Bleaching
Process chemicalsProcess 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