Pillow plate heat exchanger applications in condensing paper pulp black liquor

Critical equipment for the chemical pulp mill

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

Mission:

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

MVR Technology

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.

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