Steam side of the heating element and adversely affect heat transfer by reducing

Both the heat transfer coefficient and the effective heating surface. When NCG

Are present, the evaporators require continuous venting from the steam side. The

NCG are odorous and may be inflammable. Kraft NCG are typically forwarded to

Incineration, whereas sulphite NCG can be re-used for cooking acid preparation.

Evaporators are usually arranged in groups in order to improve the steam economy,

And to accommodate the large heat exchange surfaces. When black liquor is

Transferred from one evaporator body to the other, the thickened liquor may be

Separately extracted from the evaporator sump (see Fig. 9.4), or branched off after

The circulation liquor pump. The separate extraction of thick liquor before

Recovery

Droplet separator

CIRCULATION

LIQUOR

STEAM

CONDENSATE

VAPOUR

Liquor distribution

Heating elements

THIN LIQUOR

THICK LIQUOR

NCG VENT

Fig. 9.4 Example of a plate-type falling film evaporator.

Dilution with thin liquor keeps the concentration level in the evaporator comparatively

Low. This is especially helpful at high dry solids concentrations, where the

Boiling point rise can considerably reduce the evaporator performance.

The performance of an evaporator is determined by the heat transfer rate, Q

(W). The very basic equation of heat transfer relates the transfer rate to the overall

heat transfer coefficient, U (W m–2 K–1), the surface of the heating elements, A

(m2), and the effective temperature difference, DTeff (°C):

Q _ UADTeff _9_

The effective temperature difference which drives the evaporation is given by

The difference between the steam side condensing temperature and the vapor side

gas temperature, DT, minus the boiling point rise, BPR:

DTeff _ DT _ BPR _10_

The overall heat transfer coefficient U depends on evaporator design, on the

Physical properties of the liquor (especially its dry solids concentration and viscosity),

And on potential fouling of heat exchange surfaces. Typical heat transfer coefficients

for falling film evaporators are between 700 and 2000 Wm–2 K–1, with lowend

Values related to high dry solids concentrations. The heat transfer rate is pro-

Chemical Recovery Processes

Portional to the evaporation capacity. Thus, more surface area and a higher temperature

Difference result in increased capacity.

As concentrations rise during evaporation, fouling of the heat exchanger surfaces

On the liquor side can be caused by the precipitation of inorganic and organics

Liquor compounds. Inorganics with a tendency to scaling include calcium carbonate,

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