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23-Chem-B5 Pulp and Paper Technology · Undated paper

Question 2 of 6: Oxygen Delignification & ClO 2 Dosing — Wood Supply, Washing & Fresh Water

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exam 16-Chem-B5, Pulp and Paper Technology — May 2019. 3 hours, CLOSED BOOK exam (Casio or Sharp approved calculators only). Per the exam notes, any FIVE of the six questions constitute a complete paper (only the first five as they appear in the candidate's answer book are marked); for completeness this solution answers all SIX questions in full. Most parts require an essay-format answer — clarity and organization of the answer are explicitly marked.

Every specific reconstruction is flagged inline at the point it is used; the underlying arithmetic for all boxed numbers.

Reference texts: Smook (rev. Kocurek), Handbook for Pulp & Paper Technologists, 4th ed.; Biermann, Handbook of Pulp and Paper Technology, 2nd ed.; Perry's Chemical Engineers' Handbook, 9th ed. (generic mass/energy-balance and heat-exchanger methods).

Question 2: Oxygen Delignification & ClO2 Dosing — Wood Supply, Washing & Fresh Water 20 marks

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

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The question's opening scenario is stated with slightly different wording/numbers before the numbered sub-parts — "28.0 Kappa Number, 48% yield, 46% delignification" is used as the working data set. The "chlorine dioxide substitution of 0.70" is read as: the Kappa-Factor relation first gives the TOTAL Cl2-equivalent charge needed to take the pulp from its Kappa Number entering D1 down toward the 2.0 target; the substitution fraction (0.70) is the proportion of that total active-chlorine charge supplied as ClO2 (vs. conventional Cl2), and the "C Factor" (1.008) is applied as a small actual-vs-theoretical dosing correction, consistent with typical mill practice.

Oxygen delignification & D1-stage ClO2 dosing

Given.

QuantityValue
Bleached pulp production1500 ADMT/day (softwood)
Digester yield48% (on O.D. wood)
Kappa Number leaving digester28.0
O2 delignification46%
D1 Kappa Factor0.180
ClO2 substitution0.70
C Factor1.008
Chip moisture44% (total mass)
Reject rate3.25 ADMT/day
Bleaching yield loss6%

Find. (1) ClO2 flow to D1 (kg/min); (2) wet wood supply required (t/day); (3) purpose/equipment for washing after O2 delignification; sources of mill fresh water, weak black-liquor pre-evaporation equipment, and the impact of un-washed soap.

Approach. Reduce the digester Kappa Number by the stated O2-delignification fraction to get the Kappa Number entering D1. Apply the Kappa-Factor relation and the C-Factor correction to get the total active-chlorine charge, take 70% of it as the ClO2 substitution portion, and convert to ClO2 mass via the 2.63 available-chlorine factor. For the wood supply, work BACKWARD from the 1500 ADMT/d bleached product through the bleaching yield loss and reject rate to the brown-stock (digester) O.D. output, then through the digester yield and chip moisture to wet wood.

  1. Kappa Number entering D1. $$\kappa_{D_1,in}=28.0\times(1-0.46)=\boxed{15.12}$$
  2. ClO2 dose. Total Cl2-equivalent charge (with the C-Factor correction): $$\%\text{Cl}_2\text{-eq,total}=0.180\times15.12\times1.008=2.744\%$$ ClO2's share, at 70% substitution: $$\%\text{Cl}_2\text{-eq,ClO}_2=0.70\times2.744=1.920\%=19.20\ \text{kg/t}\ \Rightarrow\ \text{ClO}_2=19.20/2.63=\boxed{7.30\ \text{kg/ODMT}}$$
  3. ClO2 mass flow. Pulp entering the bleach plant, before the stated 6% bleaching yield loss: $1500/(1-0.06)=1595.7\ \text{ADMT/d}$; O.D. production $=1595.7\times0.90=1436.2\ \text{ODMT/day}$: $$\dot m_{ClO_2}=7.30\times1436.2/1440=\boxed{7.28\ \text{kg/min}}\ (10{,}484\ \text{kg/day})$$
  4. (2) Wood supply (working backward from final production). Adding the 3.25 ADMT/d reject rate to the pulp entering the bleach plant: $$1595.7+3.25=1598.95\ \text{ADMT/d}=1598.95\times0.90=1439.1\ \text{O.D. t/d brown stock}$$ Dividing by the 48% digester yield gives O.D. wood, then dividing by (1−chip moisture) gives wet wood: $$\text{O.D. wood}=1439.1/0.48=2998.0\ \text{t/d}\qquad \text{Wet wood}=2998.0/(1-0.44)=\boxed{5354\ \text{wet t/day}}$$
QuantityValue
Kappa Number entering D115.12
ClO2 dose7.30 kg/ODMT
(1) ClO2 flow to D17.28 kg/min (10,484 kg/day)
(2) Wet wood supply required5354 t/day

(3) Washing after O2 delignification. Washing after this stage removes the lignin and organics solubilized during O2 delignification, keeping that lignin-rich filtrate on the brown-stock (Kraft recovery) side of the mill rather than letting it carry forward into the bleach plant proper, where it would otherwise inflate D/E-stage chemical consumption and effluent (BOD/AOX) load. Typical equipment is a rotary vacuum drum washer, a pressure diffusion washer, or (for better displacement efficiency at lower dilution) a wash press; the recovered filtrate is returned to the brown-stock washing/evaporator system for chemical recovery in the recovery boiler.

Fresh water sources in a pulp mill are, in order of decreasing quality demand: river/well intake water (treated for boiler feed and highest-purity uses), and increasingly, on-site treated effluent or recycled process water for lower-purity duties (shower water, seal water, general washdown) — modern mills aim to minimize raw fresh-water intake by cascading water from clean, high-purity uses down to progressively dirtier ones (a "closed white-water" philosophy).

Weak black-liquor pre-evaporation is carried out in direct-contact evaporators (cascade/cyclone evaporators) in older mills, or more commonly today in a dedicated low-pressure stage of the multiple-effect evaporator train itself; the weak liquor (12–18% DS from the brown-stock washers) is concentrated a first step (to roughly 20–25% DS) using low-grade waste heat (e.g. flash steam or hot condensate) before entering the main MEE train, reducing the live-steam load the main train must supply.

Impact of un-washed soap: if the sodium-soap fraction of the extractives (the tall-oil precursor) is not washed out of the pulp stock before the liquor reaches the evaporators, the soap carries into the black liquor and (a) causes foaming and scaling in the evaporator train, reducing heat transfer and steam economy, (b) is lost from the tall-oil recovery skimming step, forfeiting that by-product revenue, and (c) can redeposit on pulp fibres downstream, causing brightness/quality defects in the finished sheet.