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Pulping and Bleaching PSE 476/Chem E 471. Lecture #15 The Kraft Recovery Process. Agenda. Overview of process Black Liquor Evaporation Recovery Furnace Smelt Causticizing Lime Kiln. Pulp. Chips. Cooking & Washing. BLACK LIQUOR Alkali Lignin Hydrolysis Salts Sulfonation Products.

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Pulping and bleaching pse 476 chem e 471 l.jpg

Pulping and BleachingPSE 476/Chem E 471

Lecture #15

The Kraft Recovery Process


Agenda l.jpg
Agenda

  • Overview of process

  • Black Liquor

  • Evaporation

  • Recovery Furnace

  • Smelt

  • Causticizing

  • Lime Kiln


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Pulp

Chips

Cooking

&

Washing

BLACK LIQUOR

Alkali Lignin

Hydrolysis Salts

Sulfonation Products

GREEN LIQUOR

Na2CO3

Na2S

Evaporation

&

Burning

WHITE LIQUOR

NaOH

Na2S

Causticizing

Heat

Water

CaCO3

Makeup Chemicals

CaO

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.




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Chips

NaOH

Digester

Pulp

Na2S

Black Liquor

Black Liquor

Inorganics

NaOH (7%)

Na2S (19%)

Na2CO3 (36%)

Na2SO3 (9%)

Na2SO4 (13%)

Na2S2O3 (16%)

14-18% Solids

pH 12+

65% Organics

35% Inorganics

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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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

Recovery Furnace


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Air The functions of the recovery furnace are:

Air

Air

Recovery Furnace III

System for recovery

of heat to generate power

(A)

Combustion

Zone

(C)

Liquor

(B)

Reduction

Zone

Gasses from Pyrolysis

(D)

Smelt (molten Na2CO3, Na2S, etc)

Notes


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Recovery Furnace IV The functions of the recovery furnace are:

  • Liquor is sprayed

  • to char bed (droplet size 2-3mm)

  • Particles should be

  • dry before landing

  • the char bed

  • Char is required in

  • bed for effective

  • reduction of

  • sulfates


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Reduction of Inorganics The functions of the recovery furnace are:

  • 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 functions of the recovery furnace are:

  • 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 The functions of the recovery furnace are:

  • 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 The functions of the recovery furnace are:

  • 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 functions of the recovery furnace are:

  • 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 The functions of the recovery furnace are:


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