In pulp mills, black liquor, a by-product of the raw material digesting process, contains inorganic spent cooking materials which are typically recovered by evaporation and incineration. The black liquor exits the digestor at a concentration between 8 and 13% total solids. There is enough heating value in the organic portion to incinerate the liquor in a fluidized bed reactor at concentrations between 35 and 50% depending upon the type of organics.

Concentration of black liquor is economically achieved by use of multi-effect evaporators (either rising or falling film type). The energy required decreases as the number of effects increases. For example, a double effect will provide about 1.5 pounds of evaporation per pound of steam, while a seven effect evaporator will provide about 6 pounds of evaporation per pound of steam.

This is possible by condensing water vapor generated in one effect, in the shell side of the following effect's heating element. Saturated steam at 20 to 100 psig in the first effect heating element's shell side boils liquor in the tube side. This boiling liquor/vapor enters the vapor head where vapor is separated and channeled to the shell side of the second effect heating element. Likewise, vapor from the second effect goes to the third effect and so-on until as many as eight effects. Vacuum in the final effect is necessary to provide adequate temperature driving force in each effect. The final effect vapors are condensed with condensate pumped to the storage tank where it is mixed with condensate from other heating elements.

The liquor is typically fed to the tube side of the last two effects and progressively concentrated as it moves through the other effects in reverse order. This is known as backward feeding and has the advantage of the hottest effect in contact with the highest viscosity liquor. Since viscosity decreases as temperature increases, liquor remains pumpable at concentrations of 70 to 80%.