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PRES 2: Pulp Mill Processes and Fiber Line

PRES 2 · Work through your exam, one topic at a time.

All guide sections

Pulp Mill Processes and Fiber Line

What this domain tests

PRES 2 candidates must connect kraft mill process chemistry to the pressure and corrosion duty of fiber-line equipment. Expect questions on white liquor composition, cooking intensity, washing efficiency and bleach-stage chemistry, not just equipment names.

Core rules and concepts

  • White liquor is NaOH plus Na2S. Effective alkali (EA) is NaOH + 1/2 Na2S as Na2O; active alkali (AA) is NaOH + Na2S as Na2O; sulfidity is Na2S/(NaOH+Na2S) x 100. EA and AA are not interchangeable.
  • Cooking targets kappa number, a lignin indicator. Kappa is not a pulp-strength test; viscosity or physical testing covers strength.
  • Continuous digesters use impregnation, heating, cooking and washing zones; batch digesters use relief and blow. Liquor-to-wood ratio, temperature and time set the delignification path.
  • Washing removes black liquor and carryover; poor washing raises bleach chemical demand and chloride load.
  • Bleach stages such as D0, Eop and D1 use ClO2 and alkali. Chloride, chlorate and low pH make 304L marginal; 316L, duplex or titanium may be required.

Worked example

A batch digester charges 25 t of oven-dry chips at 14% EA on wood. White liquor strength is 95 g/L EA as Na2O. Required liquor volume = (25,000 kg x 0.14) / (95 kg/m3) = 3,500/95 = 36.8 m3. If the operator used wet-chip mass, the chemical charge would be overstated because water is not wood.

Traps that cost marks

Using wet chip tonnes instead of oven-dry tonnes; mixing EA and AA; treating kappa as a strength result; assuming washing has no effect on bleaching; ignoring chloride in bleach filtrates.