Evaporation for the chemical recovery loop: Kraft black liquor, sulfite red liquor and biomass refinery streams. Falling-film and forced-circulation evaporators engineered as one cascaded train — sized to liquor viscosity, carbonate–sulfate–silica scaling and the mill’s steam economy.
The evaporator train sits at the heart of the chemical recovery loop — its performance defines the mill’s steam economy and alkali balance. Two liquor families, two different failure modes.
The recovery loop’s most demanding stream: viscosity climbs exponentially with solids while carbonate–sulfate–silica–calcium scaling species attack the same surfaces that must push the liquor to boiler-firing concentration.
Acidic spent sulfite liquor, where the base-salt chemistry — calcium, magnesium, sodium or ammonium sulfite — rather than raw viscosity drives corrosion allowance, scaling form and the by-product recovery route.
Black liquor is not the same liquid at the outlet as at the inlet. Viscosity, scaling propensity and boiling-point rise all climb with solids — so the configuration is cascaded, not chosen once.
In the mid-to-low solids region, falling-film evaporators deliver high heat-transfer coefficients with short residence time and low pumping power. In the high-solids, scaling-prone finishing zone, forced circulation with suppressed boiling in the tubes takes over. The cascade between them — where falling film hands off to forced circulation — is the core design decision, not an equipment preference.
Cascade configuration disclaimer: the falling-film → forced-circulation split point, effect count and MVR placement are not catalog choices — they depend on the target firing solids, the liquor’s scaling propensity and its boiling-point-rise profile. Final design requires black-liquor characterization (solids, viscosity–temperature curve, silica, soap content) and fouling test data.
Four duty families across the mill — each with its own driver, and each routed to the configuration the liquor chemistry demands.
The classic recovery-loop train: multi-effect falling-film body with MVR integration on the low-BPE front effects and a high-solids forced-circulation finishing effect.
Falling-film concentration of acidic spent sulfite liquor, engineered around the base-salt chemistry rather than against it.
The evaporator train’s neighbors in the recovery loop — soap handling, green-liquor slurry duty and a clean, declared boundary to the boiler island.
The non-pulp side of the fiber line: pretreatment liquors, fermentation-stage streams and lignin-recovery mother liquors — thermally sensitive and fouling-prone.
Calcium carbonate scaling in black liquor evaporators is the availability problem of the application: hard, adherent scale on heated surfaces that shuts effects down for acid cleaning. The public evidence
Bagasse-and-wood black liquor concentration at scale is delivered today by seven-effect trains of tube-based falling-film design with integrated condensate handling: the public reference is the evaporation system supplied to Century
How black liquor evaporation plants lift weak black liquor to firing solids for the recovery boiler, the four engineering challenges that decide performance, and how multi-effect and MVR configurations are
Concentrating black liquor above 65% solids is the point where conventional evaporation stops cooperating – viscosity outruns concentration and sodium salts precipitate onto heat surfaces as hard scale – and
Indicative routing for Kraft black liquor evaporation to recovery-boiler firing solids — the canonical sequence from washing to firing.
Simplified PFD for indicative routing only. Effect count, MVR integration point, soap-removal location and washing strategy are project-specific — they depend on the mill steam balance, recovery-boiler design and black-liquor composition.
Common feed streams across pulp, paper and biomass processing — with the technology route and material focus for first-pass screening.
| Stream | Source / Context | Key Behavior | Typical Route | Material Focus |
|---|---|---|---|---|
| Kraft black liquor | NaOH / Na₂S pulping of softwood / hardwood | 15–75% solids; viscosity above 500 cP at 65% solids; silica 100–500 mg/L (wood) | Falling Film + FC | 316L / Duplex 2205 |
| Sulfite red liquor | Acidic sulfite pulping — Ca / Mg / Na / NH₄ base | pH 2–4; residual SO₂; higher corrosion duty | Falling Film | Duplex 2205 / Titanium |
| Bleach plant effluent | ECF (ClO₂) / TCF bleaching stage effluent | Cl⁻ 500–5,000 mg/L; chlorate; pH 2–7; COD 1,000–5,000 mg/L | Pretreat + FC | Titanium throughout wetted parts |
| Biomass hydrolysate | Cellulosic sugar production for biofuels / bio-chemicals | C5/C6 sugars 5–20%; thermal sensitivity above 75 °C; fouling | Falling Film, low ΔT | 316L |
| Paper machine white water | Sheet-forming drainage; filler & fines | TDS <1,000 mg/L; TSS 500–5,000 mg/L; CaCO₃ filler | Membrane + Multi-Effect | 316L |
| Recausticizing green liquor | Na₂CO₃ → NaOH conversion in the causticizer | Na₂CO₃ + Na₂S 100–150 g/L; heavy CaCO₃ slurry scaling | FC Slurry Crystallizer | 316L |
Routes shown are for preliminary screening only — recovery-cycle integration requires a mill-wide mass and energy balance. Bleach-plant effluent with high chloride demands titanium throughout the wetted parts.
On black-liquor service the washing strategy is core engineering, not an afterthought — each scale species has its own location and its own removal chemistry.
| Scale Type | Typical Location | Cleaning & Control Strategy |
|---|---|---|
| “Burrito” scale — Na₂CO₃–Na₂SO₄ eutectic | High-solids effects and forced-circulation tubes | Alkaline boil-out on a scheduled cycle; deposition suppressed by FC design — 2–3 m/s tube velocity with suppressed boiling in the tubes |
| Calcium scale — CaCO₃ / CaSO₃ | Mid-train heat-transfer surfaces | Acid wash on cycle; calcium-based red liquors sized with longer washing allowance from the start |
| Sodium silicate (non-wood liquors) | Whole train — silica 1,000–5,000 mg/L in straw / bagasse vs 100–500 mg/L in wood | Upstream desilication: CaO at 5–15 g/L (or CO₂) precipitating silica as CaSiO₃ at pH 10–11 before the evaporator — formed silicate scale is very difficult to remove chemically |
| Soap / tall-oil foaming & carryover | Effect vapor spaces and condensers | Soap skimming between effects plus antifoam dosing; complete removal before firing is a recovery-boiler safety requirement, not a preference |
Without a matched washing strategy, heat-transfer coefficient drops 30–50% within 7–14 days on scaling black-liquor service. The standard cycle — acid wash plus alkaline boil-out every 5–15 days — is delivered through CIP (clean-in-place) routing and tuned to the measured scaling rate of the specific liquor.
The evaporator train is the mill’s largest steam consumer and the boiler’s feed-preparation unit — four integration points decide the economics.
The duty families this industry most often brings us — the full case index lives on the Projects page.
Multi-effect falling-film trains with high-solids forced-circulation finishing, engineered to boiler firing solids with matched washing cycles.
Front-effect MVR integration on existing 5–7-effect trains — cutting live-steam draw where the mill steam balance allows electrification.
Hydrolysate and lignin-recovery mother-liquor concentration in low-ΔT, short-residence falling-film duty.
Representative scopes for orientation — the full project index, with streams and capacities, is maintained on the Projects page.
Send your black-liquor analysis — solids %, viscosity profile, silica, soap — and your target firing solids. You will receive a feasibility assessment, a simplified PFD and indicative scope within two business days.
Send your feed characterization (TDS, COD, main salts, viscosity), throughput and product targets — we respond with a preliminary process route within two business days.
