Advanced process controls are tightening brightness and trimming chlorine dioxide, while xylanase pretreatments unlock double‑digit chemical cuts and lower AOX.
Modern kraft bleach plants are quietly becoming optimization labs. Advanced process control (APC) systems are shaving 3–10% off chemical costs while stabilizing final brightness, and enzyme stages are pushing chlorine dioxide (ClO₂) demand down by 10–25%—with standout trials hitting roughly half the usual dose.
These are not hand‑wavy promises. ABB’s “Expert Optimizer Bleach” reports 30–50% tighter brightness control and 3–10% lower chemical costs (new.abb.com) (new.abb.com), and a North American retrofit halved day‑to‑day brightness variability (standard deviation fell from 0.80 to 0.39 ISO) while cutting total ClO₂ “kappa factor” dosage ~7.4% (papertoexport.com) (papertoexport.com).
Enzyme stages, especially xylanases (enzymes targeting hemicelluloses), are showing double‑digit reductions in ClO₂ and lower adsorbable organic halogens (AOX), the regulated organochlorine indicator in effluent. In Indonesia, regulators are nudging mills to move faster toward this kind of cleaner tech under the government’s PROPER “Hijau” program (kemenlh.go.id) (kemenlh.go.id).
Advanced process control in bleach plants
APC systems—often model‑predictive controllers (MPC, a control approach that uses a process model to forecast and optimize future behavior)—monitor in‑line Kappa number (a lignin proxy), pH, and brightness to feed a cost optimizer that automatically trims reagent charges while holding targets steady (new.abb.com) (new.abb.com). Mills report single‑digit to ~10% reductions in chlorine dioxide alongside cleaner runnability and quality gains.
The North American case using a Tapio/Olympus‑based platform is illustrative: brightness variability halved (SD from 0.80 to 0.39 ISO), and the total ClO₂ dosage per unit “kappa factor” fell ~7.4% relative to legacy controls (papertoexport.com) (papertoexport.com). Gains come from stabilizing front‑end conditions (wood quality and incoming pulp Kappa) and shifting load smoothly between stages to avoid overshoot. In practice, control signals that adjust chemical charges rely on accurate metering via equipment such as a dosing pump to enact the optimizer’s setpoints.
Measured savings and quality impacts
Brightness stability: final ISO brightness variation drops by 30–50% with APC (new.abb.com), allowing lower setpoints and less overshoot.
Chemical reduction: typical 3–10% less bleaching agent (ClO₂, NaOH) (new.abb.com) (papertoexport.com). One MPC study cut total ClO₂ per unit kappa factor ~7.4% (papertoexport.com).
Yield and quality: tighter Kappa control preserves fiber viscosity/yield while meeting whiteness targets; pulp strength variability also narrows (new.abb.com). Online analyzers (pH, Kappa analyzers, spectral “Brightometers”) and multivariable optimization help trim charges and reduce carryover, yielding more stable runs—and percent‑level chemical savings for a small controls investment (new.abb.com) (papertoexport.com).
Enzymatic xylanase pretreatment outcomes
Enzymes are now proven to shrink bleach chemical use. Xylanases, which act on hemicellulose, can increase the efficiency of subsequent ClO₂ stages. In hardwood kraft trials, pretreating an oxygen‑delignified (oxygen stage removes residual lignin before bleaching) eucalyptus pulp with a heat‑stable xylanase at 90 °C and pH 10.5 enabled the mill to meet brightness using 15–25% less ClO₂ (mdpi.com). At pilot scale, the same endoxylanase stage cut effluent AOX by ~18% while maintaining pulp quality (mdpi.com).
Matos et al. (2024) reported that xylanase on eucalyptus kraft saved ~20% of ClO₂ plus ~10% NaOH and improved brightness stability (pmc.ncbi.nlm.nih.gov). In blended enzyme trials, Bajpai et al. showed a laccase–xylanase system (with a mild acid post‑treatment) could halve ClO₂ demand—down to ~45–58% of normal (researchgate.net) (researchgate.net). Ajith et al. add that ~25% ClO₂ savings were enough to lower AOX by ~20% (researchgate.net).
Indonesia context and dissolving pulp evidence
In Indonesian settings, similar patterns appear. A Jurnal Selulosa study on Acacia crassicarpa dissolving pulp (rayon‑grade) found xylanase pretreatment produced higher‑quality pulp meeting SNI specs (mendeley.com). Under optimal conditions—A. niger xylanase, 60 °C, pH 6, ~0.5 L/T—one trial reduced Kappa from 9 to 4.04 and achieved roughly 23.7% chlorine savings (researchgate.net).
Several studies note a possible rise in effluent COD (soluble sugars) from enzyme stages, but significantly lower AOX (researchgate.net) (researchgate.net). Indonesian officials have explicitly encouraged pulp makers such as Riau Andalan Pulp & Paper (RAPP) to pursue PROPER “Hijau” performance via cleaner tech and better waste/emissions management (kemenlh.go.id) (kemenlh.go.id).
Comparative outcomes and pairing strategy
Side‑by‑side, APC implementations typically deliver a few percent—3–10%—chemical savings and much tighter brightness control (new.abb.com) (papertoexport.com). Enzyme pretreatment commonly brings double‑digit ClO₂ cuts—on the order of 10–25%—often with lower AOX (mdpi.com) (pmc.ncbi.nlm.nih.gov), and in exceptional lab/pilot work, combined enzyme systems have pushed ClO₂ down by ~50% (researchgate.net) (researchgate.net).
The strategies complement each other: APC tightens operation around existing sequences, squeezing incremental savings and stabilizing quality, while xylanase stages reduce the load required from the outset. By reducing bleed and overshoot, APC both meets tight brightness specs and trims excess ClO₂ use (new.abb.com) (papertoexport.com). In short, modern bleach APCs trade a small investment in controls for percent‑level chemical savings and much more consistent pulp quality.
Sources and technical references
Peer‑reviewed journals (e.g., Appl. Microbiol. Biotechnol. 2024; Cellulose 2022), industry case studies, and technical reports (ABB Inc., Capstone Technology) underpin these results: new.abb.com; papertoexport.com; mdpi.com; pmc.ncbi.nlm.nih.gov. Each numeric claim above is drawn directly from these sources.
