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Pulping and BleachingPSE 476/Chem E 471

Lecture #15The Kraft Recovery Process

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Agenda

• 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.

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

NaOHNa2S

Pulp

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.

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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)

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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

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Recovery Furnace III

Smelt (molten Na2CO3, Na2S, etc)

Liquor

Air

AirAir

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

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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%).

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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 .

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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.

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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

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Black versus green liquor