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Pulping and Bleaching PSE 476. Lecture #6 Kraft Pulping Chemicals. Chemical Pulping Agenda. Basic Description of Liquors & Process White, Black & Green Liquors Definition of Terms Total alkali, Effective Alkali, Sulfidity, etc. Why is everything on a Na 2 O basis?.

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Pulping and BleachingPSE 476

Lecture #6

Kraft Pulping Chemicals

PSE 476: Lecture 6


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Chemical PulpingAgenda

  • Basic Description of Liquors & Process

    • White, Black & Green Liquors

  • Definition of Terms

    • Total alkali, Effective Alkali, Sulfidity, etc.

  • Why is everything on a Na2O basis?

PSE 476: Lecture 6


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Kraft Pulping:Definition of Terms

  • White liquor.

    • Fresh pulping liquor for the kraft process containing NaOH, Na2S, and a variety of impurities.

  • Black liquor.

    • The waste liquor from the kraft pulping process. Contains most of the original inorganic components (most in different forms) and a high concentration of dissolved organics.

  • Green liquor.

    • Partially recovered kraft liquor (intermediate liquor in recovery sequence).

PSE 476: Lecture 6


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Lime

Kilm

White Liquor

Green Liquor

Digester

Recovery

Furnace

Black Liquor

Simplified Liquor Scheme

This is a very simplified diagram. There are several steps between each box. We will discuss this whole sequence in depth in a later lecture.

PSE 476: Lecture 6


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Typical Composition of Kraft Liquors

* Median concentrations as g/l as Na2O

PSE 476: Lecture 6


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Typical White Liquor Composition

Notes Page

PSE 476: Lecture 6


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Definition of Terms (US)

  • All chemicals are reported as concentrations in liquor (g/l) or as charge (%) on dry wood.

  • Total Chemical: All sodium salts (as Na2O).

  • Total Alkali: NaOH + Na2S + Na2CO3 + 1/2Na2SO3 (as Na2O).

    • This is the sum of the sodium salts that contribute to or are converted during kraft cooking to chemicals which contribute to active alkali.

  • Active Alkali: Na2S + NaOH (as Na2O) 100g/L

PSE 476: Lecture 6


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Na2S

Sulfidity =

* 100%

NaOH + Na2S

NaOH

Causticity =

* 100%

NaOH + Na2S

Definition of Terms (US)

  • Sulfidity: 24-28%

  • Causticity:

PSE 476: Lecture 6


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NaOH

Causticizing eff. =

* 100%

NaOH + Na2CO3

Na2S

Reduction eff. =

* 100%

Na2S + Na2SO4 + Na2SO3 + Na2S2O3

Definition of Terms (US)

  • Effective Alkali: NaOH + 1/2 Na2S (as Na2O) no more than 55 g/L

  • Activity: % ratio of Active to Total Alkali

  • Causticizing Efficiency: 78-80%

  • Reduction Efficiency: 95%

PSE 476: Lecture 6


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Why Na2O? (1)

  • Expressions such as sulfidity, causticity, effective alkali, etc “best” describe the conditions in a kraft cook.

    • These expressions contain information on the amounts (g/liter or %) of different chemicals such as NaOH, Na2S, etc which have different degrees of effectiveness

    • Reporting on a Na2O basis indicates the actual chemical relationship between these chemicals

PSE 476: Lecture 6


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Why Na2O? (2)

PSE 476: Lecture 6


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Why Na2O? (3)

PSE 476: Lecture 6


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Kraft Pulping Liquor

Sample Calculation

PSE 476: Lecture 6


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Kraft Pulping LiquorIn-Class Example Calculations (1)

  • 50 Tons Chips

  • 50% Moisture Content

  • Liquor Charge to Digester:

    • 1200 ft3 white liquor

      • EA = 13% (alkali charge on OD wood as Na2O)

      • Sulfidity = 25.2%

    • 1300 ft3 black liquor

  • Question: How many lbs./ft3 of NaOH and Na2S were charged to the digester in the white liquor? (assume no chemical contribution from black liquor)

PSE 476: Lecture 6


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Kraft Pulping LiquorIn-Class Example Calculations (2)

Step 1: Calculate the amount of oven dry wood

50 tons chips • 2000 lbs./ton • 0.5 (m.c.) = 50,000 lbs. o.d. wood

Step 2: Calculate the amount of NaOH and Na2S as Na2O in the

white liquor using the EA and Sulfidity numbers

EA = NaOH + 1/2 Na2S = 13% on od wood.

NaOH + 1/2 Na2S = 0.13 • 50,000 = 6500 lbs.

NaOH = 6500 lbs. - 1/2 Na2S

PSE 476: Lecture 6


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Na2S

0.5 Na2S + 6500 lbs.

Na2S

• 100 = 25.2%

Sulfidity =

Na2S + NaOH

Na2S

= 0.252 =

Na2S + (6500 - 1/2Na2S)

Kraft Pulping LiquorIn-Class Example Calculations (3)

Na2S = 0.126 Na2S + 1638 lbs.

0.874 Na2S = 1638 lbs.

Na2S = 1874 lbs. (Na2O)

NaOH = 6500 lbs. - (0.5)(1874 lbs.) = 5563 lbs. (Na2O)

PSE 476: Lecture 6


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Kraft Pulping LiquorIn-Class Example Calculations (4)

Step 3: Convert NaOH and Na2S values from Na2O

Na2O = 62 g/mole or lbs./mole for this exercise

NaOH = 40 g/mole

Na2S = 78.1 g/mole

As we discussed in class, these calculations are based on

an equivalence in sodium (Na). This means that Na2S and NaOH

are equivalents but that NaOH is equal to 1/2 Na2O.

Na2S = 1874 lbs. (Na2O) • 1mole/62 lbs. • 78.1 lbs./mole = 2360.6 lbs.

NaOH = 5563 lbs. • 1 mole/62 lbs. • 2.0 • 40 lbs./mole = 7178 lbs.

So: Na2S = 2360.6/1200 ft3 = 1.97 lbs./ft3

NaOH = 7178/1200 ft3 = 5.98 lbs./ft3

PSE 476: Lecture 6