Pulp Mill Black Liquor Concentration: Evaporator Trains, Scaling Control and Energy Recovery

Black liquor is the single largest energy resource in a kraft pulp mill, and its concentration is the step that determines whether that energy can be recovered. Evaporating black liquor from weak wash solids to firing solids is the most energy-intensive unit operation in the mill, and the one where scaling control most directly affects uptime. A well-designed evaporator train will concentrate liquor from 15 percent to 75 or 80 percent solids while returning high-quality condensate for reuse.

The liquor contains dissolved lignin, hemicellulose degradation products, sodium salts and a substantial inorganic fraction, including sodium carbonate, sodium sulphate, sodium thiosulphate and sodium chloride. It arrives at the evaporators at 12 to 18 percent dry solids after brown stock washing, at a temperature near or above its boiling point, and it must be concentrated to 65 to 80 percent solids before it can be burned in the recovery boiler. The concentration and crystallisation engineering overlaps substantially with brine concentrator and crystallizer system design, although black liquor presents organic fouling that brine systems do not.

Weak Black Liquor Characteristics

Design begins with an accurate liquor analysis, because evaporation performance is governed by its physical properties rather than by its water content alone.

• Dry solids content: 12 to 18 percent entering the evaporator train

• Boiling point elevation: rises from about 2 degrees Celsius at 20 percent solids to 12 to 18 degrees Celsius at 70 percent solids

• Viscosity: increases steeply above 55 percent solids, reaching several hundred centipoise at 70 percent, which limits heat transfer

• Sodium and sulphur balance: determines the sulphidity and therefore the scaling behaviour

• Soap content: tall oil soap must be skimmed before evaporation, usually at tank storage, since it causes foaming and fouling

• Non-process elements: calcium, magnesium, aluminium, silicon, iron and chloride accumulate in the liquor circuit and drive much of the scaling

Multiple-Effect Evaporator Train Design

Kraft mills almost universally use multiple-effect falling film or forced circulation evaporators, typically five to seven effects, arranged to maximise steam economy while retaining the ability to clean individual vessels without stopping the train.

Effect Arrangement and Steam Economy

In a conventional forward-feed arrangement, weak liquor enters the coldest effect and flows toward the hottest, using the pressure gradient to drive liquid transfer without pumps. Steam economy improves with more effects, but each additional effect adds capital cost and occupies plot space. Six effects with liquor preheating and vapour recompression typically achieve an economy of 5 to 6 kilograms of water evaporated per kilogram of steam.

Vapour Recompression Options

• Thermal vapour recompression: a steam ejector uses high-pressure motive steam to compress vapour from an intermediate effect, reusing it in a hotter effect; low capital cost, moderate efficiency

• Mechanical vapour recompression: a compressor driven by steam turbine or motor raises vapour pressure; higher capital cost, considerably higher efficiency, and often the most economical choice where low-pressure steam is scarce

• Combined configurations: TVR on the first effect with MVR on a later effect gives flexibility across the operating range and is common in mills with variable production

Mechanical recompression on black liquor faces a limitation that water evaporators do not: the boiling point elevation at high solids reduces the compressor pressure ratio that can be economically achieved, and the vapour is contaminated with volatile organics and sulphur compounds that must be compatible with the compressor materials.

High-Solids Concentration

The final concentration step, from about 60 percent to 75 or 80 percent solids, is the most difficult part of the train. Viscosity is high, boiling point elevation is large, and the liquor is close to its solubility limits for several salts. Options for this stage include a dedicated high-solids falling film effect, a forced circulation crystalliser-type vessel, or a plate-and-frame or scraped surface heat exchanger arrangement. Superconcentrators using falling film with high circulation rates are the most widely adopted solution.

Scaling Control

Scaling is the dominant cause of evaporator downtime in pulp mills. Three scale types predominate.

Calcium Carbonate Scale

Calcium carbonate deposits when calcium reacts with carbonate in the liquor, particularly in the hotter effects. Control is by calcium removal upstream, typically through green liquor clarification and by limiting the calcium input from wood and from make-up chemicals. Where deposits still form, they are removed by acid cleaning at pH 3 to 4 with inhibited hydrochloric or sulphamic acid, or by the sequential acid and alkaline cleaning that mills commonly schedule on a rotating basis.

Calcium Sulphate and Sodium Salt Scale

Calcium sulphate is far more difficult to remove than carbonate, requiring mechanical or high-pressure water cleaning rather than acid alone. Its formation is controlled through the sodium-to-sulphur balance, by maintaining the sulphidity of the cooking liquor within the desired range, and by removing sulphate from the liquor circuit where possible. Sodium carbonate and sodium sulphate can also crystallise within the liquor at high solids content, which is why the high-solids stage is designed with sufficient circulation to keep salts in suspension.

Organic Fouling

Lignin and hemicellulose derivatives form tenacious organic deposits, especially in the coldest effects. Alkaline cleaning at pH 10 to 11 with caustic soda dissolves organic deposits effectively, and the sequence of alkaline followed by acid cleaning is standard. The cleaning chemistry, including the importance of correct temperature and circulation velocity, parallels the principles used in membrane fouling prevention and CIP protocols.

Condensate Segregation and Reuse

Evaporator condensate is a valuable water and heat resource, but it is contaminated to varying degrees depending on which effect it comes from.

• Foul condensate: from the effects handling the weakest liquor, contains methanol, reduced sulphur compounds, terpenes and ammonia with COD of 1,000 to 5,000 mg/L; must be steam stripped before reuse

• Intermediate condensate: from the middle effects, lower in volatiles, often reused as wash water after stripping

• Clean condensate: from the hottest effects downstream of the stripping section, low in contaminants, suitable as boiler feedwater make-up after polishing

Segregating condensate streams with independent collection headers is standard practice because mixing them destroys the ability to reuse the clean fraction. Foul condensate stripping with live steam removes 90 to 95 percent of the methanol and sulphur volatiles, and the stripped gas is routed to the recovery boiler or to a dedicated incinerator.

Energy Integration and Heat Recovery

Evaporation is the largest steam consumer in a kraft mill, typically 25 to 40 percent of total steam demand, so energy integration determines mill economics.

Heat Recovery Measures

• Liquor preheating: use hot condensate and flash vapour to preheat incoming weak liquor, recovering 5 to 10 percent of evaporator steam demand

• Vapour reuse: use evaporator vapour for other mill heating duties such as white water heating or hot water generation

• Condensate flash recovery: flash condensate from higher-pressure effects to generate low-pressure steam for the lowest effect

• Surface condenser optimisation: where the final effect vapour is condensed rather than reused, correct air removal and clean tube surfaces have a measurable effect on vacuum and therefore on capacity

Integration with the Recovery Boiler

The evaporator train sets the solids content of the recovery boiler fuel, which in turn governs boiler steam output and the mill's ability to generate power. Increasing firing solids from 65 to 75 percent reduces the flue gas and the associated heat loss and raises steam generation, which is why mills invest in high-solids concentration even when the evaporators themselves appear cost-neutral. The integration logic and the analysis of energy per unit of product are structurally similar to the recovery calculations used in cost estimation for large effluent treatment plants.

Operational Monitoring

Reliable operation depends on a modest but disciplined set of measurements.

• Solids content of feed, inter-effect and product liquor, measured by refractometer with periodic gravimetric validation

• Viscosity of the product liquor, since it governs achievable concentration and heat transfer

• Condensate COD and methanol, indicating carryover and stripping efficiency

• Steam economy, calculated daily as total water evaporated divided by steam consumed

• Effect-by-effect temperature profiles, where deviations signal fouling before capacity is lost

A falling steam economy or a widening temperature deviation across effects is the earliest warning of scaling, and responding at that point avoids unplanned shutdowns.

Conclusion

Black liquor evaporation is a mature technology where the competitive advantage lies in attention to chemistry and to energy integration rather than in new equipment. Accurate liquor characterisation, correct effect arrangement, disciplined cleaning cycles and full utilisation of condensate heat together determine the steam economy and the uptime of the train. Because the evaporator sits between the pulp line and the recovery boiler, its performance propagates through the entire mill, and its design should always be evaluated against mill-wide energy and chemical balances.

Frequently Asked Questions

How many effects should a black liquor evaporator train have?

Most modern kraft mills use five to seven effects, achieving a steam economy of 5 to 6. Adding a seventh effect raises capital cost significantly for a diminishing economy gain, unless vapour recompression is also applied. The correct number depends on steam price, liquor properties and available plot space, and the economics follow the same logic used when selecting between technologies for MVR and multi-effect evaporation.

Why does black liquor viscosity limit concentration?

Above about 55 percent solids, physical viscosity rises steeply as lignin polymers interact. High viscosity reduces the liquid-side heat transfer coefficient, makes pumping difficult, and can cause poor liquor distribution in falling film effects, leading to dry patches and rapid scaling. This is why the high-solids stage is engineered separately rather than by simply pushing the main train harder.

How is evaporator condensate reused?

Clean condensate from the hottest effects is polished and used as boiler feedwater make-up. Intermediate condensate is steam stripped and reused as pulp washing water. Foul condensate from the weakest effects requires stripping to remove methanol and reduced sulphur compounds before it can be reused, or it is routed to biological treatment. Segregating the streams with separate headers is what makes this reuse possible, and the treatment of the foul fraction has much in common with colour and COD removal in pulp and paper mill effluent.