Black liquor evaporation is an essential part of the chemical recovery process as it significantly concentrates the dry solid matter in black liquor so that the liquor can be effectively combusted in recovery boiler. The water removed from the liquor can be segregated and processed so that it can be reused in the mill – reducing fresh water requirements, effluents, and saving money.
Evaporators and concentrators, with their lamella heating surfaces, are adept at processing the spent cooking liquors or mill effluents using the minimum amount of energy while maximizing product dry solids, plant availability, and condensate quality. The lamella heating surfaces are inherently non-scaling. This allows the use of various heat sources inside the lamella, allowing the mill to select the most energy-efficient process connection.
Evaporation of weak black liquor to separate water and create a combustible product-strong black liquor
The three main processes that occur in the evaporator are:
Black liquor+heat to Strong Black liquor + Water + Steam
Condensate + Steam to Clean + Dirty condensate + NCG
Black liquor to Black liquor + Soap (only softwood)
Basic process requirements
Efficient use of energy
Efficient separation of water vapor from black liquor
Proper separation of methanol, tall oil soap
Concentration of black liquor to 75-85% dry solids
|Name||Pillow plate series||Shell and tube heat exchanger||Detachable plate heat exchanger||Spiral plate heat exchanger|
|Operating temperature range||<800℃||<800℃||<170℃||<350℃|
|Maximum pressure||<60 bar||<200 bar||<32 bar||<25 bar|
|Heat transfer coefficient to water[W/m2·℃]||3500||2700||5600||2000|
|Application of air and water heat exchange||fit||fit||not fit||Partial fit|
|Immersion in tank or water||fit||Partial fit||not fit||not fit|
|Welding of tank and reactor||Applicable||not applicable||not applicable||not applicable|
|Install into the existing reactor and other equipment||Flexible application||Partial applicable||not applicable||not applicable|
|All welded construction||Applicable||Applicable||not applicable||not applicable|
|Heavily contaminated liquids and other applications||Applicable||Applicable||Partial Applicable||Applicable|
|Weight per unit area||low||high||low||high|
|Falling film, condenser and evaporator||fit||fit||Partial fit||Partial fit|
MVR technology is to compress the secondary steam of the evaporator by mechanical method, increase its pressure and temperature, increase its enthalpy, and then send it back to the heating chamber of the evaporator to be used as heating steam, so as to keep the feed liquid in boiling state, while the heating steam itself condenses into water. Compared with the multi effect evaporation technology, MVR technology compresses and recycles all the secondary steam and recovers the latent heat, so it is more energy-saving than the multi effect evaporation technology.
Multiple effect evaporators
Multiple effect evaporators are always used in black liquor service. The term muliple effectcomes from the multiple effective use of energy to perform the evaporation task. In such configuration,live steam is condensed only in the first effect evaporator, generating vapors that are then sent to condense in a second effect where additional evaporation takes place. The process can then be repeated until reaching the last effect evaporator where generated vapors are condensed in a condenser using cooling water.
Steam generation in the Kraft mill is a significant operating expense and every effort must be made to conserve its use. The evaporation plant is by far the major consumer of that steam for the removal of water from the weak black liquor.Economic operation of the evaporator is therefore predicated upon the multiple effective use of the heat available from the steam and therefore on the number ofeffects in the MEE.
Application of technology in black liquor evaporation of pulp
Taking the evaporation of wheat straw pulp black liquor as an example, the combination evaporation process of MVR technology and multi effect evaporation has perfect effect.