Liquor, on the course of temperatures over the effects, and on other unit operations

Which may be combined with evaporation such as stripping of foul condensate,

Soap skimming or black liquor heat treatment. The vapor from the stage,

Into which the thin liquor is fed, contains most of the volatile compounds from

The black liquor. The place where this vapor is condensed delivers the foul condensate.

In Fig. 9.5, this would be the surface condenser. The condensate from the

Other effects is less contaminated. Foul condensate is usually subjected to stripping

For the removal of volatile substances such as methanol and organic sulfur

Compounds. Cleaner condensates can be used elsewhere in the mill instead of

Fresh water, for example for pulp washing or in the causticizing plant.

If the thick liquor concentration needs to be raised to above approximately 75%,

The associated boiling point rise may require the use of medium-pressure steam

in the first effect. The first effect – often called the “concentrator” – usually incorporates

Two or three bodies due to the more frequent cleaning required at the

High-end temperature and concentration levels. Other effects may also need to be

Cleaned during operation from time to time. Depending on the cleaning procedure,

Arrangements may need to be provided for switching feed liquor between

The bodies of an effect, or for by-passing a body when it is in cleaning mode.

In the last stages of a multiple-effect evaporation plant, the dry solids concentration

Of the black liquor changes only slightly. This is due to the large quantities of

Water to be evaporated at low concentrations. The amount of water in black liquor

As a function of the dry solids concentration, together with calculated concentration

levels in a five-effect plant starting with a 15% feed and thickening to 75%, is

Shown graphically in Fig. 9.6. Note that the rise in dry solids concentration is just

7% over effects 4 and 5, but more than 30% over effect 1 alone.

The steam economy of a multiple-effect evaporation plant depends mainly on

The number of effects and on the temperature of the thin liquor. Other factors influencing

The economy are, for example, the use of residual energy contained in

Condensates by flashing, venting practices, and cleaning procedures for scale

Removal. Typical multiple-effect evaporation plants in the pulp industry comprise

Five to seven effects, and have a gross specific steam consumption of between 0.17

And 0.25 tons of steam per ton of water evaporated. The specific consumption is

Roughly calculated by dividing 1.2 through the number of effects.

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