Sodium salts, gypsum, silicates, or oxalates. Scaling worsens with higher

Concentrations and higher temperatures. A high fiber content in the feed liquor,

As well as insufficiently removed soap, also accelerate fouling. Scales reduce the

Heat transfer, and by that the capacity of the evaporation plant. Hence, scales must

be removed periodically by, in order of increasing operational disturbance: switching

The evaporator body to liquor of a lower concentration; rinsing with clean condensate;

Cleaning with chemicals (mostly acids); or hydroblasting. High-temperature,

High-concentration stages may require daily cleaning, whereas low-temperature

Low-concentration stages may continue for several months without cleaning.

Multiple-Effect Evaporation

The basic idea of multiple-effect evaporation is the repeated use of vapor to

Achieve a given evaporation task. Compared to single-stage evaporation, only a

Fraction of the fresh steam is required for the same amount of water evaporated.

The principle is illustrated in Fig. 9.5. Multiple-effect evaporation plants consist

Of a number of evaporators connected in series, with countercurrent flow of vapor

And liquor. Live steam is passed to the heating elements of effect I and is condensed

There, evaporating water from the liquor and producing thick liquor. The

Liquor temperature in this effect depends on the low-pressure steam level available

at the mill, and is usually in the range of 125–135 °C. The vapor released in the

First effect is condensed in the heating elements of the second effect at a somewhat

Lower temperature. The vapor released in turn on the liquor side of the second

Effect proceeds to the third effect and so forth, until the vapor from the last

effect is condensed in a surface condenser at 55–65 °C. The vacuum needed at the

Last effect is most favorably provided by a liquid-ring vacuum pump.

LIVE STEAM

THIN LIQUOR

THICK LIQUOR

LIVE STEAM

CONDENSATE

CONDENSATE

SURFACE

CONDENSER

COOLING

WATER

CONDENSATE

EFFECT

I

EFFECT

II

EFFECT

III

EFFECT

IV

EFFECT

V

VACUUM

PUMP

WARM

WATER

VAPOUR NCG

Fig. 9.5 The principle of multiple-effect evaporation demonstrated

On a five-effects system.

Recovery

Vapor condensates of different degrees of contamination come from the surface

Condenser, and from all the effects but the first. The live steam condensate from

The first effect is collected separately from the vapor condensate for re-use as boiler

Feedwater.

In Fig. 9.5, the thin liquor is fed to the last effect. The actual feed position of

Thin liquor in a multiple-effect system depends on the temperature of the weak

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