Question 1 of 6: ECF Bleach Chemistry, Oxygen Delignification & Chlorine Dioxide Generation/Dosing

Question 1: ECF Bleach Chemistry, Oxygen Delignification & Chlorine Dioxide Generation/Dosing (a)–(c) 10 marks each, (d)–(f) 20 marks each (2 of 3 required)

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.

(a) D–E_op–D₁–D₂ bleach sequence

D = chlorine dioxide (ClO₂) delignification/brightening stage; E = alkaline extraction stage (NaOH washes out the chlorinated/oxidized lignin fragments solubilized by the preceding D stage); the subscript o denotes that this extraction stage is reinforced with oxygen — i.e. "E_op" (or, where peroxide is also dosed, "E_op") is a single alkaline extraction vessel given extra delignifying/brightening power without a separate tower. Reading the sequence left to right: D₀ (first ClO₂ stage) → E_op (O₂-reinforced extraction) → D₁ → D₂ (final brightening stage) — a standard 4-stage ECF (elemental-chlorine-free) sequence for a softwood Kraft pulp.

Every D-stage dose is set from the same Kappa Factor relation, $$\%\text{Cl}_2\text{-equivalent charge (on O.D. pulp)}=\text{Kappa Factor}\times\text{Kappa Number entering that stage},$$ applied to the Kappa Number the pulp actually carries INTO the stage being dosed — this is the single relation used throughout parts (d)–(f) below and in Question 2.

(b) Effluent parameters: BOD, AOX, Toxicity, TSS

BOD is measured by the standard 5-day BOD₅ test: a diluted, microbially-seeded sample is incubated in the dark at 20°C and BOD is the drop in dissolved oxygen over 5 days (mg O₂/L). AOX (adsorbable organic halogens) is measured by adsorbing a sample's organic content onto activated carbon, combusting the carbon, and measuring the total halide released (overwhelmingly chloride) by microcoulometric titration, reported as mg Cl/L. Toxicity is measured with standardized acute (96-h LC₅₀, e.g. rainbow trout, Daphnia) and chronic (sub-lethal growth/reproduction) bioassays on whole final effluent. TSS is measured gravimetrically: a known volume is filtered through a pre-weighed glass-fibre filter, the residue dried at 103–105°C and weighed, reported in mg/L.

A Kraft bleach plant is historically the mill's largest single contributor to all four parameters: chlorinated/oxidized lignin and carbohydrate fragments solubilized in the D and E filtrates add BOD and (wherever elemental chlorine or hypochlorite is used) AOX and chlorophenolic toxicity; fibre fines lost during washing add TSS. Modern ECF sequences (ClO₂ only, no Cl₂/hypochlorite) cut AOX and toxicity dramatically relative to older chlorine-bleaching technology, though BOD and TSS remain significant simply from the sheer volume of organics solubilized.

Before discharge, combined mill effluent passes through primary clarification (settling/DAF removing TSS and fibre) and then secondary biological treatment — typically an aerated stabilization basin or activated-sludge system — where micro-organisms consume the biodegradable organic load (cutting BOD by roughly 90%+ and further degrading a portion of the AOX/toxic compounds), before discharge under a permit setting BOD, TSS, AOX, toxicity, pH and temperature limits.

(c) Oxygen delignification technologies

Two reactor configurations are in commercial use, distinguished by pulp consistency: (1) medium-consistency (MC) O₂ delignification at 10–14% consistency, where the pumpable pulp slurry passes through a dedicated high-shear MC mixer that mechanically disperses gaseous O₂ into fine bubbles throughout the fibre suspension before the mixture enters a pressurized upflow/downflow tower reactor (85–100°C, 600–700 kPa O₂ partial pressure, NaOH charge); and (2) high-consistency (HC) O₂ delignification at 25–30% consistency, where pulp is first fluffed (defibrated to a low-density, high-surface-area mass) and O₂ gas plus steam are then blown directly into the fluffed pulp — mixing/diffusion here relies on the pulp's own porosity and exposed fibre surface area rather than mechanical shear. Both processes achieve broadly comparable delignification, typically 35–50% Kappa reduction; the practical ceiling is set by cellulose/carbohydrate degradation rather than any equipment limit — beyond about 50% delignification, O₂-radical attack increasingly targets cellulose instead of residual lignin, causing unacceptable pulp-strength (viscosity) loss, so mills deliberately stop short of that point and let the D/E stages finish delignification to final brightness.

(d) SVP-Lite chlorine dioxide generation

1) Reducing agent: methanol (CH₃OH) is the reducing agent in the R8/SVP-Lite process — it reduces chlorate (Cl(V)) to chlorine dioxide (Cl(IV)) under the strongly acidic conditions maintained by sulphuric acid, itself being partially oxidized to formic acid and CO₂.

2) Role of chloride ion: Cl⁻ is the catalytic intermediate that actually carries out the chlorate reduction — NaClO₃ first reacts with Cl⁻ (present from a small deliberate chloride charge or generated in situ) to form ClO₂ and Cl₂/HOCl, with the methanol then re-reducing any Cl₂/HOCl back to Cl⁻, so the chloride ion is recycled rather than consumed, and the process runs at a much faster rate than direct chlorate/methanol reaction alone.

3) Typical generator concentrations: NaClO₃ maintained near 550–650 g/L; H₂SO₄ maintained very strong, roughly 6–7 M (generator liquor operates below pH≈1, essential for good ClO₂ yield and selectivity); sodium sesquisulphate (Na₃H(SO₄)₂) is kept near its solubility limit in the hot, strongly acidic, high-ionic-strength liquor so that it continuously crystallizes out as it forms.

4) Typical feed strengths: NaClO₃ solution fed at roughly 600–700 g/L; H₂SO₄ fed as concentrated (93–98%) acid; methanol fed as a dilute (typically 5–15%) aqueous solution, as in this problem (10%).

5) By-product separation: because sodium sesquisulphate has low solubility in the hot, strongly acidic, high-salinity generator liquor, it precipitates continuously as fine crystals ("saltcake") and is removed from the recirculating generator liquor by a vacuum filter or centrifuge.

6) Typical use: the sesquisulphate saltcake is fed to the Kraft recovery boiler as makeup sodium and sulphur — reduced to Na₂S in the furnace exactly like the mill's normal Na₂SO₄→Na₂S recovery chemistry — recycling it into the Kraft chemical cycle rather than discarding it.

7) Calculation — see below.

Given.

Find. 10% methanol solution flowrate (kg/min); 8.2 M sodium chlorate solution flowrate (L/min).

Approach. Convert nameplate×efficiency to an actual ClO₂ mass rate, then apply the reaction's molar ratios (3 NaClO₃ : 0.85 CH₃OH : 3 ClO₂, i.e. 1:1 NaClO₃:ClO₂ and 0.2833:1 CH₃OH:ClO₂) to size the two reagent feeds.

1. Actual ClO₂ production. $$\dot m_{ClO_2}=45\times0.98=44.1\ \text{t/day}=\boxed{30.63\ \text{kg/min}}$$ M(ClO₂)=35.45+2(16.0)=67.45, so $n_{ClO_2}=30{,}625/67.45=454.0\ \text{mol/min}$.
2. Methanol solution flowrate. M(CH₃OH)=12.01+4(1.008)+16.0=32.04. The reaction needs 0.85 mol CH₃OH per 3 mol ClO₂: $$n_{CH_3OH}=454.0\times\frac{0.85}{3}=128.6\ \text{mol/min}\ \Rightarrow\ \dot m_{100\%}=128.6\times32.04/1000=4.123\ \text{kg/min}$$ $$\dot m_{10\%\ soln}=4.123/0.10=\boxed{41.2\ \text{kg/min}}$$
3. Sodium chlorate solution flowrate. M(NaClO₃)=23.0+35.45+3(16.0)=106.45; the reaction needs 1 mol NaClO₃ per mol ClO₂ (3:3 ratio), so $n_{NaClO_3}=454.0$ mol/min. At 8.2 mol/L: $$\dot V_{NaClO_3}=\frac{454.0}{8.2}=\boxed{55.4\ \text{L/min}}\qquad(\dot m=454.0\times106.45/1000=48.3\ \text{kg/min})$$

(e)+(f) D₁ Kappa-Factor dosing, ClO₂ preheat heat exchanger & steam savings

Given.

Find. (i) ClO₂ solution flow to D₁; (ii) D₁ filtrate chemicals content; (iii) D₁ filtrate temperature out of the heat exchanger; (iv) steam savings achieved by the heat exchanger relative to heating the stock directly with steam.

Approach. Size the D₁ ClO₂ dose from the Kappa-Factor relation, then find the total D₁ stock/filtrate flow from the stated 10% operating consistency (dilution-water mass balance). An energy balance on the ClO₂ preheat exchanger (heat lost by hot D₂ filtrate = heat gained by cold ClO₂ solution) gives the filtrate outlet temperature and the recovered heat duty; comparing that duty against the duty needed to heat the FULL stock flow directly with steam (60→75°C) gives the steam savings.

1. (i) D₁ ClO₂ dose and solution flow. $$\%\text{Cl}_2\text{-eq}=0.20\times4.0=0.80\%=8.0\ \text{kg/t}\ \Rightarrow\ \text{ClO}_2=8.0/2.63=3.04\ \text{kg/ODMT}$$ O.D. production $=1000\times0.90=900\ \text{ODMT/day}$ (90% AD convention) $=625.0\ \text{kg O.D. pulp/min}$: $$\dot m_{ClO_2}=3.04\times900/1440=1.901\ \text{kg/min}\ \Rightarrow\ \dot V_{D_1}=\frac{1.901\times1000}{12.0}=\boxed{158.4\ \text{L/min}}$$
2. D₁ total stock/filtrate flow (10% consistency). Total flow at 10% AD $=625.0/0.10=6250\ \text{kg/min}$; the filtrate (water) portion is the difference from the O.D. pulp flow: $$\dot m_{filtrate}=6250-625.0=\boxed{5625\ \text{kg/min}}$$
3. (ii) D₁ filtrate chemicals content. The dosed ClO₂ diluted into the full filtrate/stock flow: $$C_{filtrate}=\frac{1.901\times1000}{5625}=\boxed{0.338\ \text{g/L}}$$
4. HX heat duty. ClO₂ solution mass flow ≈158.4 kg/min (ρ≈1.0 kg/L), heated 4→46°C: $$Q=158.4\times4.18\times(46-4)=\boxed{27{,}810\ \text{kJ/min}}$$
5. (iii) D₂-filtrate outlet temperature. Energy balance on the exchanger, filtrate loses what the ClO₂ solution gains: $$\Delta T_{filtrate}=\frac{27{,}810}{5625\times4.18}=1.18^{\circ}\text{C}\ \Rightarrow\ T_{filtrate,out}=75-1.18=\boxed{73.8^{\circ}\text{C}}$$
6. (iv) Steam savings. Direct steam heating of the FULL stock flow (6250 kg/min) from 60°C to the D₁ mixer's 75°C target would need $$Q_{direct}=6250\times4.18\times(75-60)=391{,}875\ \text{kJ/min}\ \Rightarrow\ \dot m_{steam,direct}=391{,}875/2257=173.6\ \text{kg/min}.$$ Using the HX to preheat only the ClO₂ makeup stream instead recovers $$\dot m_{steam,saved}=\frac{27{,}810}{2257}=\boxed{12.3\ \text{kg/min}}$$ of that direct-steam duty — a modest but "free" saving, since the D₂ filtrate would otherwise be discharged at 75°C with its heat unused.