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Integration of a sulfite pulp mill and a viscose plant
Hans Magnusson, Gunnar Henriksson, Ulf Germgård
Karlstad University, Karlstad, Sweden
COST 1205 Budapest 1
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Hypothesis
• There are several benefits obtained if a dissolving pulp mill and a viscose fiber plant are built on the same site and ”connected”. – Lower chemical consumption (less waste) – Lower polluting emissions – Lower energy consumption – Lower production cost
COST 1205 Budapest 2
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The project is divided into the following cases:
• Prehydrolysis kraft pulping and viscose • Sodium sulfite pulping and viscose • Magnesium sulfite pulping and viscose • Multi stage sulfite pulping and viscose • Dissolving pulping and other fiber processes
COST 1205 Budapest 3
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Our project is divided into the following cases:
• Prehydrolysis kraft pulping and viscose – Appita 69(2016)3, 264-272
• Sodium sulfite pulping and viscose • Magnesium sulfite pulping and viscose • Multi stage sulfite pulping and viscose • Dissolving pulping and other fiber processes
COST 1205 Budapest 4
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A sulfite dissolving pulp mill
Na+/Mg2+
pH 4,5/1,5 Na+/Mg2+
pH 1,5 HCE Bleach Wood
Diss pulp
SO2 NaOH SO2 NaOH (O2) Bleaching chem.
Lignin, Hemi, Na2SO4 COD
Hemi NaOH COD
H+ OH-
COD
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A sulfite dissolving pulp mill
Na+/Mg2+
pH 4,5/1,5 Na+/Mg2+
pH 1,5 HCE Bleach Wood
Diss pulp
SO2 NaOH SO2 NaOH (O2) Bleaching chem.
Lignin, Hemi, Na2SO4
Hemi NaOH
H+ OH-
COD • The cooking base is either sodium or magnesium
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A sulfite dissolving pulp mill
Na+/Mg2+
pH 4,5/1,5 Na+/Mg2+
pH 1,5 HCE Bleach Wood
Diss pulp
SO2 NaOH SO2 NaOH (O2) Bleaching chem.
Lignin, Hemi Na2SO4
Hemi NaOH
H+ OH-
COD • The cooking base is sodium or magnesium • Initial pH is 4,5 or 1,5. The rest of the cook is pH 1,5. •
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A sulfite dissolving pulp mill
Na+/Mg2+
pH 4,5/1,5 Na+/Mg2+
pH 1,5 HCE Bleach Wood
Diss pulp
SO2 NaOH SO2 NaOH (O2) Bleaching chem.
Lignin, Hemi Na2SO4
Hemi NaOH
H+ OH-
COD • The cooking base is sodium or magnesium • Initial pH is 4,5 or 1,5. The rest of the cook is pH 1,5. • Initial L/W ratio is high but reduced to 2,5/1 after puffing
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A sulfite dissolving pulp mill
Na+/Mg2+
pH 4,5/1,5 Na+/Mg2+
pH 1,5 HCE Bleach Wood
Diss pulp
SO2 NaOH SO2 NaOH (O2) Bleaching chem.
Lignin, Hemi Na2SO4
Hemi NaOH
H+ OH-
COD • The cooking base is sodium or magnesium • Initial pH is 4,5 or 1,5. The rest of the cook is pH 1,5. • Initial L/W ratio is high but reduced to 2,5/1 after puffing • Bleaching is done in acidic and alkaline stages
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Mercer. Xanth. Spinning
Na2SO4 H2SO4
Textile fibers
Wash Drying
H2SO4 H2O CS2
CS2 NaOH NaOH
A viscose textile fiber plant
Dry pulp
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Sulfur based compounds in the two integrated plants
• SO2
• CS2
• H2SO4
• Na2SO4
• H2S • Na2S
• S
COST 1205 Budapest 11
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Sulfur ”Mass balance”
SO2
CS2
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Integration alternatives
• Wet pulp vs dry pulp • On site production of CS2 • On site production of H2SO4 • Reuse of Na2SO4 from viscose plant as make-up in
the pulp mill. • Avoid SO2 and H2S emissions and landfill of by-
products
• (1 t wood =1/3 t pulp =1/3 t viscose) • (Swedish Rayon viscose plant)
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Steam saving in the pulp mill
• The dissolving pulp is only semi-dried after the pulp mill i.e. to about 40% dry solids.
• Steam saving (3 bar) about 3,3 GJ/ton of pulp
• (Based on GROT biofuel = 15€ /t)
COST 1205 Budapest 14
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Chemical Recovery in Na based sulfite pulp mills
• Stora Kopparberg method • SCA Billerud method
• General
– Spent liquor Sulfur compounds are incinerated and reduced to Sulfide.
– Oxidized to SO2 or reduced to elemental S and then oxidized to SO2.
– Dissolved in Na2CO3 and add SO2.
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Stora Kopparberg Sulfite recovey process
• Recovery boiler delivers: – Na2CO3 + Na2S (recovery boiler similar to kraft)
– Na2S + CO2(g) H2S (g) – 2 H2S (g) + O2 2 S + 2 H2O – S + O2 SO2 (g) – SO2 +Na2CO3 solution new cooking acid
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SCA Billerud Sulfite recovery process
• Recovery Boiler delivers: – Na2CO3 + Na2S (reducing conditions in boiler) – Cooler generates dust (Na2CO3 + C)
– Na2S + CO2 (g) H2S (g) + Na2CO3 – H2S + 3/2 O2 SO2 (g) + H2O – SO2 dissolved in Na2CO3 solution new cooking
acid
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Claus process
COST 1205 Budapest 18
2 H2S + O2 2 S + 2 H2O ( over all reaction) Step 1 Thermal step (> 850 oC) 2 H2S + O2 2 SO2 + 2 H2O Step 2 Catalytic step ( heat, cat reaction (Al (III) or Ti (IV), cooling) 2 H2S + SO2 3 S + 2 H2O
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Production of CS2 (bp 46oC)
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Modern, high efficiency process:
CH4 + S4 CS2 + 2 H2S (methane) Older process using charcoal: C + 2S CS2
600 °C
To viscose plant To sulfite pulp mill (Demand 150 kg CS2/t) (Claus process)
(850 °C)
20 oC
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H2SO4 process
• S + O2 SO2
• SO2 + O2 (V2O5, 1-2 bar, 450 C) SO3 • SO3 + H2SO4 H2S2O7 • H2S2O7 + H2O 2 H2SO4
• Over all reaction: • 2 SO2 + O2 2 SO3 (450 oC, cat V2O5)
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S-balance for an integrated dissolving pulp mill and a viscose plant (kg S/t)
COST 1205 Budapest 21
Sulfite pulp mill
Viscose plant
Na2SO4,CS 2 250 kg S/t
CS2 125 kg S/t
H2SO4 250 kg S/t
Na2SO4 125 kg S/t
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S-balance for an integrated dissolving pulp mill and a viscose plant (kg S/t)
COST 1205 Budapest 22
CS2 production Sulfite pulp mill
Viscose plant
Na2SO4,CS 2 250 kg S/t
CH4 SO2 + Na2CO3
S 125 kg S/t
CS2 125 kg S/t
360 kg S/t
H2SO4 250 kg S/t
H2SO4 production
Claus process
O2
H2S 125 kg/t
360 kg S/t
Na2SO4 125 kg S/t
H2SO4 110 kg S/t
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S-balance for an integrated dissolving pulp mill and a viscose plant (kg S/t)
COST 1205 Budapest 23
CS2 production Sulfite pulp mill
Viscose plant
Na2SO4,CS 2 250 kg S/t
CH4 SO2 + Na2CO3
S 125 kg S/t
CS2 125 kg S/t
110 kg S/t
H2SO4 250 kg S/t
H2SO4 production
Claus process
O2
H2S 125 kg/t
360 kg S/t
Na2SO4 125 kg S/t
H2SO4 110 kg S/t
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Conclusions
• Wet pulp can replace dried pulp in the viscose plant which saves steam.
• Internal CS2 can be prepared from CH4 and elemental S at high temperature (600 oC)
• The viscose by-product Na2SO4 can be reused as make up in the pulp mill.
• On site production of H2SO4 and CS2 is beneficial
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Basic idea.
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=€
Pulp Mill
Viscose Mill
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Steam saving in the pulp mill
• The dissolving pulp is only semi-dried after the pulp mill i.e. to about 40% dry solids. Is it possible? Overflow in viscose mill?
• Steam saving (3 bar) about 3,3 GJ/ton of pulp
• (Based on GROT biofuel = 15€ /t)
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Claus process
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2 H2S + O2 2 S + 2 H2O ( over all reaction) Step 1a Thermal step (> 850 oC) 2 H2S + O2 2 SO2 + 2 H2O Step 1b 2 H2S + SO2 3 S + 2 H2O Step 2 Catalytic step ( heat, cat reaction (Al (III) or Ti (IV), cooling) 2 H2S + SO2 3 S + 2 H2O
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Reuse of by-product Na2SO4
28
The Na2SO4 by-product from the viscose plant can be concentrated by multi stage evaporation and then crystallization at low temp, centrifuged and finally semi or fully dried. This salt can be used as sulfur make-up in the sulfite pulp mill. Cf the Swedish Vargön sulfite pulp mill. Consumed in Si-mill: 24% tot SO2
* 360 kg S/t pulp Available in viscose plant 450-600 kg Na2SO4 /t pulp or 145 – 200 kg S/t pulp.** *No puffing is performed **50% in spinn bath/50% with wet string
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The Stora Kopparberg chemical recovery process
In the recovery boiler spent cooking acid is converted to Na2CO3 and Na2S. The sulfides are then converted to H2S with the help of CO2 .The H2S is then either reacting with SO2 to elemental S in a Claus process or oxidized with O2 to SO2. Elemental sulfur is oxidized to SO2 with air and then dissolved in Na2CO3. After addition of more SO2 the cooking acid is ready for use.
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The SCA Billerud chemical recovery process
• The recovery boiler is burning concentrated spent acid under reducing conditions to ”dust” (Na2CO3 and carbon) and H2S. The H2S is then incinerated with the undissolved dust and the generated SO2 is dissolved in the Na2CO3 solution. The cooking acid is ready for use.
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S mass balance II (kg S/ton pulp).
Into/ out of the viscose mill • CS2 30% & 50% recycle 150 kg CS2/ t
• H2S04 750 kg/ t
• Tot S 125 + 250 = 375 kg S / t
• Na2SO4 550 kg/ t pulp = 125 kg S/ t • CS2 to air 70 kg S/ t • H2S to air 5 kg S/ t • SO2 to air 5 kg S/ t • S to landfill 30 kg S/ t • To effluent treatment 140 kg S/ t • Tot S 125+70+4+4+30 + 140 = 375 kg S/ t
Into the pulp mill • (6% Tot SO2 & 4/1 & 35% yield) • Tot S 60 x 4 x 3 x 32/64 = 360 kg S/ t pulp
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Carbon disulfide, CS2
• Boiling point : 46,3 °C • Flash point: - 43 °C • Lower explosion limit , LEL : 0,6 vol%; 19 mg/l • Upper explosion limit, UEL : 60 vol%; 1900 mg/l