Advanced process control trims chlorine dioxide in real time, while xylanase “bio‑bleaching” knocks out chemical demand up to roughly one‑fifth—all without sacrificing brightness.
Modern bleach plants are getting a digital and biological upgrade. Online sensors and model‑based control are tightening brightness to within a hair of spec—freeing mills to cut chlorine dioxide (ClO₂), chlorine (Cl₂), peroxide and alkali. And upstream, xylanase enzymes are making lignin more removable, letting operators swap harsh oxidants for a short, hot enzyme stage.
The upshot: about 5–10% chemical reduction from advanced controls and ≈15–20% from xylanase pre‑treatment, backed by mill‑scale and pilot data. The savings compound alongside steadier pH and cleaner effluent, including big cuts in chemical oxygen demand (COD) and adsorbable organic halides (AOX).
Advanced process control in bleach stages
Modern mills employ online brightness and residual sensors to feed advanced control algorithms—model predictive control (MPC), fuzzy logic, and AI—that dynamically adjust ClO₂/Cl₂, peroxide and alkali dosages. A trained MPC‑based “bleach optimizer” at a Kraft mill used a total‑bleach‑load (BLT) sensor (measuring lignin in fiber plus filtrate) with inline brightness/residual analyzers to control ClO₂ in real time.
That tighter loop let operators trim the over‑dose margin: final ISO brightness standard deviation fell from about 0.80 to 0.39 over five months. At Sappi Somerset in the U.S., the total kappa factor (a proxy for all oxidant charges) dropped from 0.446 to 0.413—a 7.4% cut in total bleaching chemical usage versus conventional kappa‑factor control. A Metso APC retrofit at Burgo Ardennes in France smoothed bleach operations and yielded an 8.7% reduction in ClO₂/chemical usage (pulp-paperworld.com). And at Billerud in Sweden, a model‑based optimizer cut chemicals by ~10% (≈€2 million savings) while making brightness more uniform (researchgate.net).
These gains come with co‑benefits: more consistent effluent pH and stability. In the Capstone implementation, E1 stage pH variation (standard deviation) fell from 0.38 to 0.17, and better pH control improves extraction efficiency while reducing carryover of lignin. In the first chlorine stage, compensated‑brightness algorithms helped a plant hit target brightness more reliably—lowering average deviation from 62.9% to 60.7% ISO and cutting the ±α variability (σ) from 3.0 to 2.5 (bioresources.cnr.ncsu.edu).
In short, by closing the loop on brightness measurements, advanced APC systems let mills cut bleach doses ≈5–10% (in line with industry reports) while maintaining product quality; researchgate.net). In many plants, those algorithms drive the setpoints that feed accurate chemical dosing through the installed dosing pumps.
Xylanase pre‑treatment and bleach demand
Hydrolytic enzymes—especially xylanases—are proven pretreatments that selectively remove hemicellulose/xylan fragments and solubilize lignin‑carriers, improving bleachability. Treating eucalyptus kraft pulp with a purified bacterial xylanase stage at about pH 10 and ~60–90 °C enabled a 20% reduction in ClO₂ charge in the first chlorination stage D₀ and a 10% reduction in the alkaline extraction (Eₚ) caustic, with no loss of target brightness or strength (pmc.ncbi.nlm.nih.gov). A review notes xylanase pretreatment typically “reduce[s] the demand for oxidizing chemicals by ≈20% to achieve the same level of brightness” (researchgate.net), and an Indonesian dissolving‑pulp study likewise found that adding a xylanase stage could cut required bleaching chemicals by ~20% for equal brightness (researchgate.net).
Pilot‑scale data confirms these savings can accumulate. In a demonstration at industrial intensity (Acacia/Eucalyptus pulp), insertion of an extremophilic xylanase (Xyn11) enabled the mill to reach 91.1% ISO brightness using 15% less ClO₂ overall (mdpi.com). In fact, after a D₂ stage the enzyme‑treated pulp was already brighter (90.7% ISO after D₁) than the untreated reference, implying the final stage could be eliminated—omitting D₂ would cut another ≈25% of the ClO₂ load (mdpi.com). Hardwoods respond similarly: applying Xyn11 to hardwood pulp yielded ≈15% ClO₂ savings while still exceeding reference brightness (mdpi.com).
Reported yield penalties are modest (~1% pulp yield loss), and paper strength properties remain acceptable (pmc.ncbi.nlm.nih.gov). Biobleaching also improves effluent quality: the published pilot saw a 44% drop in total COD in the XD₀EₚD₁ sequence when Xyn11 was used (mdpi.com).
AOX, ECF context and cost implications
By lowering chlorine(oxide) charges, mills cut AOX (adsorbable organic halides) formation; a US EPA study notes that eliminating 20% of ClO₂ under ECF (elemental chlorine‑free) conditions typically reduces AOX nearly proportionally. Likewise, raised pH and less chloride usage in extractions (from xylanase‑assisted bleaching) yields fewer chlorinated organics. In practical terms, integrating xylanases can reduce bleaching chemical costs by one‑fifth or more (researchgate.net; pmc.ncbi.nlm.nih.gov), an especially attractive strategy in high‑target‑brightness lines or mills aiming for TCF/low‑AOX certification. Some producers project even higher savings by combining enzyme steps with outbound process controls.
Sources and further reading
Peer‑reviewed studies and industry reports document these outcomes: researchgate.net; pulp-paperworld.com; researchgate.net; pmc.ncbi.nlm.nih.gov; mdpi.com; researchgate.net; mdpi.com; bioresources.cnr.ncsu.edu.
