Pulp, Paper & Biomass
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.
What Makes Pulp Mill Evaporation Hard
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.
Kraft Black Liquor
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.
- High viscosity at high solids: above ~50% solids the film becomes the constraint — falling-film distribution turns unreliable past roughly 50 cP, forcing a configuration change before the finishing stage
- Scaling chemistry: Na₂CO₃–Na₂SO₄ eutectic (“burrito” scale), silica and calcium species depositing on heat-transfer surfaces
- Soap & tall-oil carryover: foaming inside the effects and a firing-safety hazard in the recovery boiler if soap is not removed first
- Recovery upside: tall oil, turpentine and lignin by-product routes coordinated with train operation
Sulfite Red Liquor
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.
- Base-salt chemistry: Ca / Mg / Na / NH₄ sulfite systems set the whole design — calcium-based liquors carry a CaSO₃ scaling duty that shapes washing-cycle design
- Acid service: pH 2–4 with residual SO₂ across the train pushes materials to Duplex 2205 / titanium
- By-product routes: lignosulfonate and wood-sugar recovery integrated with concentration instead of treated as waste
- Corrosion-led materials: wetted-part selection follows chloride and SO₂ profile, not a catalog default
Why Both Falling Film and Forced-Circulation Are Needed
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.
Typical Applications
Four duty families across the mill — each with its own driver, and each routed to the configuration the liquor chemistry demands.
Black Liquor Concentration
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.
- Multi-effect falling-film body — 5–7 effects, liquor sequenced weak-to-strong across the ΔT ladder
- MVR integration on the low-BPE front effects (effects 1–3 of a 5–7-effect train)
- High-solids forced-circulation finishing effect to firing concentration
- Engineered falling-film → forced-circulation cascade (see the logic above)
Red Liquor Concentration
Falling-film concentration of acidic spent sulfite liquor, engineered around the base-salt chemistry rather than against it.
- pH 2–4 service with residual SO₂ across the train
- Calcium-based liquors: CaSO₃ scaling duty managed through washing-cycle design
- Materials upgraded to Duplex 2205 / titanium per SO₂ and chloride profile
- SO₂ and heat recovery coordinated with the acid preparation island
Alkali Recovery Interface
The evaporator train’s neighbors in the recovery loop — soap handling, green-liquor slurry duty and a clean, declared boundary to the boiler island.
- Soap & tall-oil skimming integrated between effects; turpentine handled on the vapor side
- Green-liquor duty: Na₂CO₃ + Na₂S at 100–150 g/L with heavy CaCO₃ slurry scaling — forced-circulation slurry handling
- Clean scope boundary: EvapCryst delivers the evaporation train and its fluid interfaces; the recovery boiler, recausticizing plant and lime kiln come from specialized partners — the boundary is declared in the proposal
Biomass & Biorefinery Liquors
The non-pulp side of the fiber line: pretreatment liquors, fermentation-stage streams and lignin-recovery mother liquors — thermally sensitive and fouling-prone.
- Pretreatment / prehydrolysate liquors from cellulosic biomass
- Fermentation-stage liquor concentration — engineered interface with Food, Fermentation & Bioprocessing
- Lignin-recovery mother liquors: high-viscosity, fouling-prone finishing duty
- Low-ΔT, short-residence falling-film design for heat-sensitive sugar streams
Kraft Black Liquor Evaporation Train
Indicative routing for Kraft black liquor evaporation to recovery-boiler firing solids — the canonical sequence from washing to firing.
Weak Black Liquor Feed
15% solids from brown-stock washing — soap-bearing, low viscosity, entering the front effects of the train.
Soap / Tall-Oil Removal
Skimming and degassing between effects; turpentine vent management and foam control protect both the effects and the boiler.
Multi-Effect / MVR Evaporation
5–7-effect falling-film body concentrating to ~50%; MVR on the low-BPE front effects cuts live-steam draw 30–60% where the mill balance allows.
High-Solids Finishing & Firing
Forced-circulation concentrator (2–3 m/s tube velocity) to 65–75% — firing-solids target range 65–80% — feeding the recovery boiler.
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.
Typical Streams, Routes & Materials
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.
Scaling Management: What Deposits, Where, and How It Is Cleaned
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.
Integration with the Recovery Boiler
The evaporator train is the mill’s largest steam consumer and the boiler’s feed-preparation unit — four integration points decide the economics.
Representative Project Scopes
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.
Discuss Your Pulp, Paper or Biomass Project
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.