Question 2 of 4: ECF Bleach Plant — DEopDEpD Sequence, Effluent Parameters, Oxygen Delignification, ClO 2 Generation & Heat Recovery

Question 2: ECF Bleach Plant — DEopDEpD Sequence, Effluent Parameters, Oxygen Delignification, ClO₂ Generation & Heat Recovery (a)–(c) 10 marks each, (d)–(f) 20 marks each

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) DEopDEpD bleach sequence

D = chlorine dioxide (ClO₂) delignification/brightening stage; E = alkaline extraction stage (NaOH washes out chlorinated/oxidized lignin fragments solubilized by the preceding D stage); the subscripts o and p denote that the extraction stage is reinforced with oxygen (o) and hydrogen peroxide (p) respectively — i.e. "Eop" is a single alkaline-extraction vessel dosed with both O₂ and H₂O₂ to add extra delignifying/brightening power to that stage without a separate tower. Reading the sequence left to right: D₀ (first ClO₂ stage) → E_op (O₂/H₂O₂-reinforced extraction) → D₁ → E_p (peroxide-reinforced extraction) → D₂ (final brightening stage) — a standard 5-stage ECF (elemental-chlorine-free) sequence.

The chemical dose in every D stage is set from the Kappa Factor relation, $$\%\text{Cl}_2\text{-equivalent charge (on O.D. pulp)}=\text{Kappa Factor}\times\text{Kappa Number entering that stage}$$ so for the D₀ stage on this 25-Kappa feed, a KF of 0.20 corresponds to a charge of about $0.20\times25=5.0\%$ Cl₂-equivalent, i.e. $5.0\times10/2.63\approx19$ kg ClO₂ per O.D. tonne of pulp (dividing by the 2.63 available-chlorine factor). D₁/D₂ doses are much smaller because the Kappa Number entering them has already been cut by D₀ and E_op: typically ≈5–10 kg ClO₂/ODMT in D₁ (with a little NaOH for pH control) and ≈2–5 kg ClO₂/ODMT in D₂.

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

BOD (biochemical oxygen demand) 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 the organic content of a sample onto activated carbon, then combusting the carbon and measuring the total halide released (predominantly chloride) by microcoulometric titration, reported as mg Cl/L. Toxicity is measured by standardized bioassays — acute 96-hour LC₅₀ tests (e.g. rainbow trout, Daphnia) and chronic (sub-lethal, growth/reproduction) tests on whole final effluent, per regulatory protocols (e.g. Environment Canada EPS methods). TSS (total suspended solids) is measured gravimetrically: a known volume is filtered through a pre-weighed glass-fibre filter, the residue dried at 103–105°C and weighed, TSS reported in mg/L.

A kraft bleach plant is historically the mill's largest contributor to all four parameters: chlorinated and oxidized lignin/carbohydrate fragments solubilized in the D and E filtrates add BOD (biodegradable organics) and, where any elemental chlorine or hypochlorite is used, AOX and chlorinated-phenolic toxicity; fibre fines lost in 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 loads remain significant simply from the volume of organics solubilized.

Before discharge, combined mill effluent (bleach plant plus other mill streams) is routed through primary clarification (settling/DAF to remove TSS and fibre), then secondary biological treatment — typically an aerated stabilization basin or activated-sludge system — where microorganisms consume the biodegradable organic load (reducing BOD by roughly 90%+ and further degrading a portion of the AOX and toxic compounds), before final discharge under a regulatory permit that sets 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 is passed through a dedicated high-shear MC mixer that mechanically disperses gaseous O₂ into fine bubbles throughout the fibre suspension before the mixture enters an upflow/downflow pressurized 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 in a reactor vessel — mixing/diffusion here relies on the pulp's own high porosity and large exposed fibre surface area rather than mechanical shear. Both processes achieve broadly comparable delignification, typically 35–50% Kappa reduction; the practical ceiling (rather than any equipment limitation) is set by cellulose/carbohydrate degradation — beyond about 50% delignification, O₂ radical attack increasingly targets cellulose rather than residual lignin, causing unacceptable pulp-strength (viscosity) loss, so mills deliberately stop short of that point and let the bleach plant proper (D/E stages) complete delignification to final brightness.

(d) Chlorine dioxide generation (SVP/HP-type generator)

1) Reducing agent: hydrogen peroxide (H₂O₂) is the reducing agent — it reduces chlorate (Cl(V)) to chlorine dioxide (Cl(IV)) under the strongly acidic conditions maintained by sulphuric acid, while itself being oxidized to O₂ (visible as the O₂ product in the stated net reaction).

2) Typical generator concentrations: NaClO₃ maintained near 600–700 g/L; H₂SO₄ maintained very strong, roughly 700–900 g/L (generator liquor operates below pH≈1, essential for reasonable ClO₂ generation rate and selectivity); Na₂SO₄ by-product is kept near its solubility limit in the strongly acidic, high-ionic-strength liquor so that it continuously crystallizes out as it forms.

3) Typical feed strengths: NaClO₃ solution fed at roughly 600–700 g/L; H₂SO₄ fed as concentrated (93–98%) acid; H₂O₂ fed as a 35–50% aqueous solution (50% in this problem).

4) By-product separation: because Na₂SO₄ has low solubility in the 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.

5) Typical use: the by-product sodium sulphate (saltcake) is fed directly to the kraft recovery boiler as makeup sodium and sulphur — it is reduced to Na₂S in the furnace exactly like the mill's normal Na₂SO₄→Na₂S recovery chemistry — recycling it back into the kraft chemical cycle to replace sodium/sulphur losses rather than discarding it.

6) Calculation — see below.

Given.

Find. ClO₂ solution concentration; 50% H₂O₂ flowrate (kg/min); 6.2 M NaClO₃ solution flowrate (L/min).

Approach. The 42 t/day rating is the ClO₂ the plant makes; dividing by the 2000 L/min absorber flow gives the concentration. "92.5% efficient" is the generator's chemical efficiency — the fraction of chlorate fed that ends up as ClO₂ (the rest is lost to side reactions) — so the chlorate feed is the stoichiometric 1:1 (2 NaClO₃ → 2 ClO₂) requirement divided by 0.925, and the peroxide follows the reaction's 2 NaClO₃ : 1 H₂O₂ ratio on the chlorate actually consumed.

1. ClO₂ production and solution concentration. $$\dot m_{ClO_2}=\frac{42{,}000}{1440}=29.17\ \text{kg/min}\qquad C_{ClO_2}=\frac{29.17\times1000}{2000}=\boxed{14.58\ \text{g/L}}$$
2. NaClO₃ solution flowrate. M(ClO₂)=67.45, M(NaClO₃)=106.45: $$n_{ClO_2}=\frac{29{,}167}{67.45}=432.4\ \text{mol/min}\ \Rightarrow\ n_{NaClO_3}=\frac{432.4}{0.925}=467.5\ \text{mol/min}$$ $$\dot V_{NaClO_3}=\frac{467.5}{6.2}=\boxed{75.4\ \text{L/min}}\qquad(\dot m=467.5\times106.45/1000=49.8\ \text{kg/min})$$
3. H₂O₂ flowrate. M(H₂O₂)=34.016; 1 mol H₂O₂ per 2 mol NaClO₃: $$n_{H_2O_2}=467.5/2=233.7\ \text{mol/min}\ \Rightarrow\ \dot m_{H_2O_2,100\%}=233.7\times34.016/1000=7.95\ \text{kg/min}\ \Rightarrow\ \dot m_{50\%\ soln}=7.95/0.50=\boxed{15.90\ \text{kg/min}}$$ (The bare stoichiometric minimum, on the ClO₂ made, would be 14.71 kg/min.)

(e) Oxygen delignification & D₀ stage sizing

Given.

Find. (1) ClO₂ solution flow to D₀ (L/min); (2) wet wood supply required (t/day); (3) purpose/equipment for washing before and after O₂ delignification.

Approach. The D₀ Kappa Factor applies to the Kappa Number the pulp carries INTO D₀, i.e. after the 42% O₂-stage delignification has already reduced the 27.5 digester Kappa. Convert the resulting Cl₂-equivalent dose to actual ClO₂ mass via the 2.63 available-chlorine factor, and scale by the O.D. production rate. For the wood supply, work BACKWARD from the 1500 ADMT/d bleached product through the stated bleaching yield loss and reject rate to the brown stock (digester) O.D. pulp output, then through the digester yield and chip moisture to wet wood.

1. Kappa Number entering D₀. $$\kappa_{D_0,in}=27.5\times(1-0.42)=\boxed{15.95}$$
2. ClO₂ dose and D₀ solution flow. Cl₂-equivalent charge $=\text{KF}\times\kappa=0.195\times15.95=3.110\%=31.10\ \text{kg Cl}_2\text{-eq/tonne}$; dividing by the 2.63 available-chlorine factor for ClO₂: $$\text{ClO}_2\ \text{dose}=31.10/2.63=11.83\ \text{kg/ODMT}$$ O.D. production $=1500\times0.90=1350\ \text{ODMT/day}$ (90% AD convention), so $$\dot m_{ClO_2}=11.83\times1350=15{,}965\ \text{kg/day}=11.09\ \text{kg/min}$$ $$\dot V_{D_0}=\frac{11.09\times1000}{10.5}=\boxed{1056\ \text{L/min}}$$
3. Wood supply (working backward from final production). Pulp entering the bleach plant, before the stated 5% bleaching yield loss: $$1500/(1-0.05)=1578.9\ \text{ADMT/d}$$ Adding the 3.25 ADMT/d reject rate (brown stock produced, including the material later screened out as rejects): $$1578.9+3.25=1582.2\ \text{ADMT/d}=1582.2\times0.90=1424.0\ \text{O.D. t/d brown stock}$$ Dividing by the 45% digester yield gives O.D. wood, then dividing by (1−chip moisture) gives wet wood: $$\text{O.D. wood}=1424.0/0.45=3164\ \text{t/d}\qquad \text{Wet wood}=3164/(1-0.44)=\boxed{5651\ \text{wet t/day}}$$

(3) Washing before and after O₂ delignification. Washing BEFORE the stage removes the dissolved lignin, spent cooking chemical and inorganic carryover from the digester/brown-stock system, protecting the O₂ stage's own chemical (NaOH) economy from being consumed by residual digester alkali demand. Washing AFTER the stage removes the additional lignin and organics solubilized during O₂ delignification itself, keeping that lignin-rich filtrate on the brown-stock (kraft recovery) side of the mill rather than allowing it to carry forward into the bleach plant proper, where it would otherwise inflate D/E-stage chemical consumption and effluent (BOD/AOX) load. Typical washing equipment is rotary vacuum drum washers, pressure diffusion washers, or (increasingly, for better displacement efficiency at lower dilution) wash presses; the removed component is dissolved organic solids (lignin fragments plus residual cooking/O₂-stage chemicals), and the recovered filtrate is returned to the brown-stock washing/evaporator system for chemical recovery in the recovery boiler rather than sent to effluent treatment.

(f) ClO₂ preheat heat exchanger & D₁-stage steam savings

1) Filtrate/water sources: the most practical source is hot washer filtrate already generated on-site — typically E-stage (extraction) filtrate, as used in this problem, since it is discharged at a useful temperature (70–80°C) and would otherwise be partly wasted to sewer; D₂-stage filtrate or general mill hot process water are secondary options.

2) Design criteria: size the exchanger for the design heat duty at the required flow/ΔT; select heat-transfer area/approach temperature to balance capital cost against recovered energy; ensure the ClO₂ side never exceeds a safe temperature (ClO₂ solution should stay well below the temperature at which dissolved ClO₂ gas evolution/decomposition becomes a hazard, which is exactly why the target outlet here is a modest 43°C rather than higher); design for fouling resistance (filtrate carries fines and scale-forming ions) with adequate cleaning access; and use corrosion-resistant construction (see metallurgy, below).

3) Safety parameters: ClO₂ is a toxic, thermally unstable gas that can decompose explosively if concentrated and/or heated beyond safe limits, so the exchanger and associated piping must be designed and interlocked to prevent local hot-spots, vapour pocket formation, or loss of dilution water flow; adequate ventilation, ClO₂ gas detection, and pressure-relief provisions are required in the exchanger's immediate area.

4) Metallurgy: yes — both process streams (ClO₂ solution and chlorinated E-stage filtrate) are highly corrosive (acidic, chloride/chlorine-dioxide-bearing); ordinary carbon steel or standard 300-series stainless steel will pit and corrode rapidly in this service, so the exchanger must be built from a corrosion-resistant material such as titanium or a high-nickel/chromium alloy suited to ClO₂/chloride duty.

Given.

Find. (vi) ClO₂ solution flow to D₁; (vii) E₁ filtrate flow; (viii) E₁ filtrate temperature out; (ix) steam savings.

Approach. Size the D₁ ClO₂ dose exactly as in part (e), using KF=0.76 on the 6.0 incoming Kappa Number. The E₁ filtrate doubles as the D₁ mixer's own dilution water (needed to bring the O.D. pulp to the stated 11% AD operating consistency) — its flow is therefore fixed by that mass balance, not a free design choice — and it is this same filtrate stream that is routed through the ClO₂ preheat exchanger before being added to the mixer. An energy balance on the exchanger (heat lost by filtrate = heat gained by ClO₂ solution, Cp≈4.18 kJ/kg·°C for both dilute streams) then gives the filtrate's outlet temperature, and the heat duty recovered is exactly the steam load that would otherwise have been needed to warm the ClO₂ stream from 4°C to 43°C by direct steam injection — converted to an equivalent steam mass using the latent heat of the mill's low-pressure steam supply.

1. (vi) D₁ ClO₂ dose and solution flow. $$\%\text{Cl}_2\text{-eq}=0.76\times6.0=4.56\%=45.6\ \text{kg/t}\ \Rightarrow\ \text{ClO}_2=45.6/2.63=17.34\ \text{kg/ODMT}$$ O.D. production $=800\times0.90=720\ \text{ODMT/day}=500.0\ \text{kg O.D. pulp/min}$: $$\dot m_{ClO_2}=17.34\times720/1440=8.669\ \text{kg/min}\ \Rightarrow\ \dot V_{D_1}=\frac{8.669\times1000}{10.5}=\boxed{825.6\ \text{L/min}}$$
2. (vii) E₁ filtrate flow (= D₁ dilution water at 11% AD). Total The consistency is stated on an air-dry basis, so it is applied to the air-dry pulp flow, $800\ \text{ADMT/day}=555.6\ \text{kg AD pulp/min}$. Total stock flow at 11% AD $=555.6/0.11=5050.5\ \text{kg/min}$, and the water (filtrate) portion is the difference: $$\dot m_{filtrate}=5050.5-555.6=\boxed{4495\ \text{kg/min}}\ (\approx\text{L/min})$$
3. HX heat duty. With the ClO₂ solution mass flow ≈825.6 kg/min (ρ≈1.0): $$Q=825.6\times4.18\times(43-4)=\boxed{134{,}600\ \text{kJ/min}}$$
4. (viii) E₁ filtrate outlet temperature. Energy balance on the exchanger, filtrate loses what the ClO₂ solution gains: $$\Delta T_{filtrate}=\frac{Q}{\dot m_{filtrate}\times Cp}=\frac{134{,}600}{4495\times4.18}=7.16\,{}^{\circ}\text{C}$$ $$T_{filtrate,out}=76-7.16=\boxed{68.8\,{}^{\circ}\text{C}}$$
5. (ix) Steam savings. The HX duty is heat that steam no longer has to supply directly at the D₁ mixer; converting to an equivalent mass of steam at h_fg=2257 kJ/kg: $$\dot m_{steam,saved}=\frac{134{,}600}{2257}=\boxed{59.6\ \text{kg/min}}$$