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Pulping and BleachingPSE 476/Chem E 471

Lecture #15The Kraft Recovery Process

Lecture #15The Kraft Recovery Process

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Agenda

• Overview of process• Black Liquor• Evaporation• Recovery Furnace• Smelt• Causticizing• Lime Kiln

• Overview of process• Black Liquor• Evaporation• Recovery Furnace• Smelt• Causticizing• Lime Kiln

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Basics of the Kraft Recovery System

This diagram (taken from “Kraft Pulping: A Compilation of Notes” shows what will be covered in this lecture. The object of this system is to regenerate the cooking chemicals and to generate heat = steam & power.

This diagram (taken from “Kraft Pulping: A Compilation of Notes” shows what will be covered in this lecture. The object of this system is to regenerate the cooking chemicals and to generate heat = steam & power.

Cooking&

Washing

Evaporation&

Burning

Causticizing

BLACK LIQUORAlkali Lignin

Hydrolysis SaltsSulfonation Products

GREEN LIQUORNa2CO3

Na2S

WHITE LIQUORNaOHNa2S

ChipsPulp

CaCO3CaOMakeup Chemicals

Heat Water

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Black liquor separation

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Basics of the Kraft Recovery System II

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Black Liquor

Digester

Chips

NaOH

Na2SPulp

Black Liquor

14-18% SolidspH 12+65% Organics35% Inorganics

InorganicsNaOH (7%)Na2S (19%)Na2CO3 (36%)Na2SO3 (9%)Na2SO4 (13%)Na2S2O3 (16%)

Notes

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Evaporation I

• In order to burn the black liquor, the solids content needs to be raised to over 60%.» This means that over 80% of the water needs to be

removed.

• This is accomplished through the use of multiple effect evaporators.

• In order to burn the black liquor, the solids content needs to be raised to over 60%.» This means that over 80% of the water needs to be

removed.

• This is accomplished through the use of multiple effect evaporators.

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Evaporation II

• Evaporation is accomplished through:» Direst contact with flue gasses (old)» Indirect contact with steam

- Falling film (new)- Raising film (old)

• With indirect contact, steam is applied countercurrent to the liquor (high temperature/pressure steam is applied to thickest liquor). The steam produced from the liquor is used to heat the next effect (evaporator) at a lower temperature and pressure. This continues to the first effect (most dilute liquor) which is run under vacuum. (Please read the reading)

• Evaporation is accomplished through:» Direst contact with flue gasses (old)» Indirect contact with steam

- Falling film (new)- Raising film (old)

• With indirect contact, steam is applied countercurrent to the liquor (high temperature/pressure steam is applied to thickest liquor). The steam produced from the liquor is used to heat the next effect (evaporator) at a lower temperature and pressure. This continues to the first effect (most dilute liquor) which is run under vacuum. (Please read the reading)

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Evaporation III

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Recovery Furnace

• Concentrated black liquor is burned in a recovery furnace. The functions of the recovery furnace are:» Remove remaining water from black liquor.» Burn the black liquor organic compounds

- Solves disposal issues.- Produces steam (for heating and energy).

» Reduce sulfur species to sulfide.

» Generate Na2CO3 which will later be converted to NaOH

• Concentrated black liquor is burned in a recovery furnace. The functions of the recovery furnace are:» Remove remaining water from black liquor.» Burn the black liquor organic compounds

- Solves disposal issues.- Produces steam (for heating and energy).

» Reduce sulfur species to sulfide.

» Generate Na2CO3 which will later be converted to NaOH

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Recovery Furnace III

Smelt (molten Na2CO3, Na2S, etc)

Liquor

Air

Air

Air

System for recovery of heat to generate power

ReductionZone

Combustion Zone

Gasses from Pyrolysis

(A)

Notes

(B)

(C)

(D)

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Recovery Furnace IV

•Liquor is sprayedto char bed (droplet size 2-3mm)•Particles should bedry before landingthe char bed •Char is required inbed for effectivereduction ofsulfates

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Reduction of Inorganics

• Formation of sodium carbonate (Na2CO3)» Combustion of organics results in the formation of

CO2 and CO. Sodium compounds are converted into Na2O.

» Na2O + CO2 Na2CO3

» Na2S + H2O + CO2 Na2CO3 + H2S

• Reductive reactions in the char results in the formation of sodium sulfide (Na2S)» Na2SO4 + 2C Na2S + 2CO2

» Na2SO4 + 4C Na2S + 4CO

• Formation of sodium carbonate (Na2CO3)» Combustion of organics results in the formation of

CO2 and CO. Sodium compounds are converted into Na2O.

» Na2O + CO2 Na2CO3

» Na2S + H2O + CO2 Na2CO3 + H2S

• Reductive reactions in the char results in the formation of sodium sulfide (Na2S)» Na2SO4 + 2C Na2S + 2CO2

» Na2SO4 + 4C Na2S + 4CO

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Smelt

• The molten inorganics at the bottom of the recovery furnace are known as the smelt. This material at 1500°F leaves the furnace and is added to water in the smelt tank.» This results in the formation of green liquor» This is also a dangerous operation. The extremely hot

smelt causes the water to instantly vaporize in a violent reaction.

» Green liquor composition: NaOH (8%), Na2S (20%), Na2CO3 (60%), Na2SO3 ( 3%), Na2SO4 (6%), Na2S2O3 (3%).

• The molten inorganics at the bottom of the recovery furnace are known as the smelt. This material at 1500°F leaves the furnace and is added to water in the smelt tank.» This results in the formation of green liquor» This is also a dangerous operation. The extremely hot

smelt causes the water to instantly vaporize in a violent reaction.

» Green liquor composition: NaOH (8%), Na2S (20%), Na2CO3 (60%), Na2SO3 ( 3%), Na2SO4 (6%), Na2S2O3 (3%).

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Caustizing

• Caustizing is the process in which sodium carbonate is converted into sodium hydroxide.

• In the first step, green liquor is clarified to remove insoluble materials.» Calcium compounds» Unburned carbon» Metals

• Clarified green liquor is sent to a “slaker” where lime (CaO/Ca(OH)2) is added .

• Caustizing is the process in which sodium carbonate is converted into sodium hydroxide.

• In the first step, green liquor is clarified to remove insoluble materials.» Calcium compounds» Unburned carbon» Metals

• Clarified green liquor is sent to a “slaker” where lime (CaO/Ca(OH)2) is added .

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Slaker (Causticizing) Chemistry

• Calcium carbonate (CaCO3) has extremely low solubility in water while sodium carbonate (Na2CO3) has fairly high solubility. When lime (CaO) is added to a Na2CO3 solution, the following reaction occurs: (important to note that CaO + H2O Ca(OH)2 this is the slaking reaction).» Ca(OH)2 + Na2CO3 CaCO3 + 2 NaOH» The calcium carbonate precipitates out of solution

driving the reaction to the right side of the above equation thus forming sodium hydroxide.

» Calcium carbonate is removed through clarification forming white liquor. This step completes the circle as the white liquor is returned to the digester for pulping.

• Calcium carbonate (CaCO3) has extremely low solubility in water while sodium carbonate (Na2CO3) has fairly high solubility. When lime (CaO) is added to a Na2CO3 solution, the following reaction occurs: (important to note that CaO + H2O Ca(OH)2 this is the slaking reaction).» Ca(OH)2 + Na2CO3 CaCO3 + 2 NaOH» The calcium carbonate precipitates out of solution

driving the reaction to the right side of the above equation thus forming sodium hydroxide.

» Calcium carbonate is removed through clarification forming white liquor. This step completes the circle as the white liquor is returned to the digester for pulping.

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Lime Kiln

• The wet calcium carbonate slurry is treated in a lime kiln.» This is a very long, refractory brick lined,

slightly tilted, rotating tube which is extremely hot (1500 to 2100°F).

» The calcium carbonate slurry is dried in the first section of the kiln and then the calcium carbonate is converted to CaO: CaCO3 + heat CaO + CO2

• The wet calcium carbonate slurry is treated in a lime kiln.» This is a very long, refractory brick lined,

slightly tilted, rotating tube which is extremely hot (1500 to 2100°F).

» The calcium carbonate slurry is dried in the first section of the kiln and then the calcium carbonate is converted to CaO: CaCO3 + heat CaO + CO2

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Black versus green liquor