Multi-effect evaporator

A multi-effect evaporator concentrates kraft black liquor from weak liquor after pulping to high-solids liquor suitable for firing in the recovery boiler. A typical train uses several evaporator effects at descending pressure and temperature so vapour from one effect heats the next, reducing live-steam demand.

Weak black liquor may enter around 15% dry solids. Modern recovery boilers commonly need firing liquor around 70 to 80% dry solids, depending on mill design and boiler requirements. The evaporator train is therefore a central link between pulping and chemical recovery.

How the train works

Each effect boils water from the liquor. The vapour released in one effect becomes the heating medium for another effect at lower pressure. Surface condensers, separators, condensate handling, vacuum systems and non-condensable-gas collection are part of the system. Some trains include concentrators or crystallising effects for the highest-solids duty.

The design must manage viscosity, foaming, scaling, heat-transfer area, liquor carryover and condensate segregation. Poor evaporator performance reduces recovery-boiler firing capacity and can upset the mill chemical balance.

Scaling and fouling

Black-liquor evaporators foul through sodium salts, carbonate, sulphate, organic deposits, fibre carryover and high-solids scaling. Scaling lowers heat transfer, increases steam use and can force water washing or chemical cleaning. High-solids effects are usually the most challenging because viscosity and salt concentration are highest.

Operators track steam economy, solids concentration, effect temperatures, vacuum, condensate quality, differential pressure, heat-transfer coefficients and wash frequency.

Sonic-horn relevance

The evaporators themselves are primarily liquid-side heat-transfer equipment, so gas-side sonic horns are rarely the right cleaning method. Acoustic cleaning becomes relevant downstream, where the concentrated liquor is burned and the recovery boiler generates ash, carryover and fouling in gas passes.

The glossary connection matters because evaporator performance affects recovery-boiler fouling. Low solids can increase water load and disturb combustion; high solids can improve boiler thermal efficiency but may change carryover behaviour. Sylio-style acoustic cleaning engages most directly with the recovery boiler, ash hoppers and gas-cleaning equipment rather than the evaporator tubes.

Process and reliability detail

Multi-effect evaporators are governed by heat-transfer surface cleanliness, vapour economy and liquor chemistry. Scaling, foaming, entrainment, poor condensate segregation and vacuum instability can all reduce capacity. In pulp mills, the evaporator train also controls black-liquor solids to the recovery boiler, so its performance affects combustion stability, steam generation and carryover.

Operators monitor effect temperatures, pressures, liquor solids, steam flow, condensate conductivity, differential pressure, wash sequence, non-condensable-gas handling and heat-transfer coefficients. Maintenance focuses on tube scaling, gasket condition, liquor distributors, pumps, vapour separators, condensers and cleaning-in-place effectiveness. Safety concerns include hot liquor, pressure and vacuum hazards, odorous gases and the potential for concentrated organics to behave differently during upset conditions.

The acoustic-cleaning link is indirect but important. Poor evaporator performance can send liquor to the recovery boiler at the wrong solids content, changing combustion, fouling and carryover. A good acoustic-cleaning system on the boiler can help manage resulting ash deposits, but it does not solve evaporator scaling or liquor concentration problems. The glossary relationship helps engineers place each cleaning method in the correct part of the mill.

Related terms

• Black liquor
• Recovery boiler