Kraft Recovery

8/8/2019 Kraft Recovery

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Pulping and BleachingPSE 476/Chem E 471

Lecture #15

The Kraft Recovery Process

Lecture #15

The Kraft Recovery Process



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A genda

Overview of processBlack Liquor EvaporationRecovery FurnaceSmelt

CausticizingLime Kiln

Overview of processBlack Liquor EvaporationRecovery FurnaceSmelt

CausticizingLime Kiln



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Basics of the Kraft Recovery

SystemTh is diagram (takenfrom ³ Kraft Pulping:

A Compilation of Notes´ s h ows w h atwill be covered inth is lecture. Th eobject of t h is systemis to regenerate t h ecooking c h emicalsand to generate h eat= steam & power.

Th is diagram (takenfrom ³ Kraft Pulping:

A Compilation of Notes´ s h ows w h atwill be covered inth is lecture. Th eobject of t h is systemis to regenerate t h ecooking c h emicalsand to generate h eat= steam & power.

Cooking&

Washing

Evaporation&

Burning

Causticizing

BLACK LIQUOR Alkali Lignin

Hydrolysis SaltsSulfonation Products

GREEN LIQUOR Na 2CO 3

Na 2S

WHITE LIQUOR NaOH Na 2S

ChipsPulp

CaCO 3CaOMakeup 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

D igester

Chips

NaOH Na 2S

Pulp

Black Liquor

14-18% Solids pH 12+65% Organics35% Inorganics

Inorganics NaOH (7%) Na 2S (19%) Na 2CO 3 (36%) Na 2SO 3 (9%)

Na 2SO 4 (13%) Na 2S2O3 (16%)

Notes



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Evaporation I

In order to burn t h e black liquor, t h e solids content needs tobe raised to over 60% .» Th is means t h at over 8 0% of th e water needs to be removed.

Th is is accomplis h ed t h roug h th e use of multiple effectevaporators.

In order to burn t h e black liquor, t h e solids content needs tobe raised to over 60% .» Th is means t h at over 8 0% of th e water needs to be removed.

Th is is accomplis h ed t h roug h th e use of multiple effectevaporators.



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Evaporation II

Evaporation is accomplis h ed t h roug h:

» Direst contact wit h flue gasses (old)

» Indirect contact wit h steam- Falling film (new)- Raising film (old)

With indirect contact, steam is applied countercurrentto t h e liquor ( h igh temperature/pressure steam is applied toth ickest liquor). Th e steam produced from t h e liquor isused to h eat t h e next effect (evaporator) at a lower temperature and pressure. Th is continues to t h e firsteffect (most dilute liquor) w h ich is run under vacuum.( Please read the reading)

Evaporation is accomplis h ed t h roug h:

» Direst contact wit h flue gasses (old)

» Indirect contact wit h steam- Falling film (new)- Raising film (old)

With indirect contact, steam is applied countercurrentto t h e liquor ( h igh temperature/pressure steam is applied toth ickest liquor). Th e steam produced from t h e liquor isused to h eat t h e next effect (evaporator) at a lower temperature and pressure. Th is continues to t h e firsteffect (most dilute liquor) w h ich 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 arecovery furnace. Th e functions of t h e recovery

furnace are :» Remove remaining water from black liquor.» Burn t h e black liquor organic compounds

- Solves disposal issues.

- Produces steam (for h eating and energy).» Reduce sulfur species to sulfide.» Generate Na 2CO 3 wh ich will later be converted to

NaOH

Concentrated black liquor is burned in arecovery furnace. Th e functions of t h e recovery

furnace are :» Remove remaining water from black liquor.» Burn t h e black liquor organic compounds

- Solves disposal issues.

- Produces steam (for h eating and energy).» Reduce sulfur species to sulfide.» Generate Na 2CO 3 wh ich will later be converted to

NaOH



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Recovery Furnace III

Smelt (molten Na 2CO 3, Na 2S, etc)

Liquor

Air

Air Air

System for recoveryof heat to generate power

Reduction Zone

Combustion Zone

Gasses from Pyrolysis

(A)

Notes

(B)

(C)

(D )



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Recovery Furnace IV

Liquor is sprayedto c h ar bed (droplet

size 2- 3 mm)Particles s h ould be

dry before landingth e c h ar bed

Ch ar is required inbed for effectivereduction of sulfates



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Reduction of Inorganics

Formation of sodium carbonate (Na 2CO 3 )» Combustion of organics results in t h e formation of CO 2

and CO. Sodium compounds are converted intoNa 2O.» Na 2O + CO 2 p Na 2CO 3

» Na 2S + H 2O + CO 2 p Na 2CO 3 + H 2S

Reductive reactions in th

e ch

ar results in th

eformation of sodium sulfide (Na 2S)» Na 2SO 4 + 2C p Na 2S + 2CO 2

» Na 2SO 4 + 4 C p Na 2S + 4 CO

Formation of sodium carbonate (Na 2CO 3 )» Combustion of organics results in t h e formation of CO 2

and CO. Sodium compounds are converted intoNa 2O.» Na 2O + CO 2 p Na 2CO 3

» Na 2S + H 2O + CO 2 p Na 2CO 3 + H 2S

Reductive reactions in th

e ch

ar results in th

eformation of sodium sulfide (Na 2S)» Na 2SO 4 + 2C p Na 2S + 2CO 2

» Na 2SO 4 + 4 C p Na 2S + 4 CO



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Smelt

Th e molten inorganics at t h e bottom of t h erecovery furnace are known as t h e smelt. Th is

material at 1 500 ° F leaves t h e furnace and isadded to water in t h e smelt tank.» Th is results in t h e formation of green liquor » Th is is also a dangerous operation. Th e extremely h ot

smelt causes t h e water to instantly vaporize in a violentreaction.

» Green liquor composition : NaOH (8 % ), Na 2S (2 0% ),Na 2CO 3 (60% ), Na 2SO 3 ( 3 % ), Na 2SO 4 (6% ), Na 2S 2O 3

(3 %

).

Th e molten inorganics at t h e bottom of t h erecovery furnace are known as t h e smelt. Th is

material at 1 500 ° F leaves t h e furnace and isadded to water in t h e smelt tank.» Th is results in t h e formation of green liquor » Th is is also a dangerous operation. Th e extremely h ot

smelt causes t h e water to instantly vaporize in a violentreaction.

» Green liquor composition : NaOH (8 % ), Na 2S (2 0% ),Na 2CO 3 (60% ), Na 2SO 3 ( 3 % ), Na 2SO 4 (6% ), Na 2S 2O 3

(3 %

).



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Caustizing

Caustizing is t h e process in w h ich sodiumcarbonate is converted into sodiumh ydroxide.In th e first step, green liquor is clarified toremove insoluble materials.» Calcium compounds» Unburned carbon» Metals

Clarified green liquor is sent to a ³slaker´wh ere lime (CaO/Ca(OH)

2) is added .

Caustizing is t h e process in w h ich sodiumcarbonate is converted into sodiumh ydroxide.In th e first step, green liquor is clarified toremove insoluble materials.» Calcium compounds» Unburned carbon» Metals

Clarified green liquor is sent to a ³slaker´wh ere lime (CaO/Ca(OH)

2) is added .



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Slaker (Causticizing)

ChemistryCalcium carbonate (CaCO 3 ) h as extremely lowsolubility in water w h ile sodium carbonate (Na 2CO 3 )h as fairly h igh solubility. W h en lime (CaO) is added toa Na 2CO 3 solution, t h e following reaction occurs : ( important to note that CaO + H 2 O p Ca ( OH) 2 this is the slaking reaction ).» Ca(OH) 2 + Na 2CO 3 p CaCO 3 + 2 NaOH» Th e calcium carbonate precipitates out of solution driving t h e

reaction to t h e rig h t side of t h e above equation t h us formingsodium h ydroxide.

» Calcium carbonate is removed t h roug h clarification forming w h iteliquor. Th is step completes t h e circle as t h e w h ite liquor isreturned to t h e digester for pulping.

Calcium carbonate (CaCO 3 ) h as extremely lowsolubility in water w h ile sodium carbonate (Na 2CO 3 )h as fairly h igh solubility. W h en lime (CaO) is added toa Na 2CO 3 solution, t h e following reaction occurs : ( important to note that CaO + H 2 O p Ca ( OH) 2 this is the slaking reaction ).» Ca(OH) 2 + Na 2CO 3 p CaCO 3 + 2 NaOH» Th e calcium carbonate precipitates out of solution driving t h e

reaction to t h e rig h t side of t h e above equation t h us formingsodium h ydroxide.

» Calcium carbonate is removed t h roug h clarification forming w h iteliquor. Th is step completes t h e circle as t h e w h ite liquor isreturned to t h e digester for pulping.



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Lime Kiln

Th e wet calcium carbonate slurry is treated ina lime kiln.» Th is is a very long, refractory brick lined, slig h tly

tilted, rotating tube w h ich is extremely h ot (1 500 to21 00 ° F).

» Th e calcium carbonate slurry is dried in t h e firstsection of t h e kiln and t h en t h e calcium carbonate isconverted to CaO : CaCO 3 + h eat p CaO + CO 2

Th e wet calcium carbonate slurry is treated ina lime kiln.» Th is is a very long, refractory brick lined, slig h tly

tilted, rotating tube w h ich is extremely h ot (1 500 to21 00 ° F).

» Th e calcium carbonate slurry is dried in t h e firstsection of t h e kiln and t h en t h e calcium carbonate isconverted to CaO : CaCO 3 + h eat p CaO + CO 2



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Black versus green liquor

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Kraft Recovery