The shape of the dissolved lignin molecules is influenced by the content of residual

Effective alkali of the black liquor. With decreasing pH, the volume occupied

By the lignin molecules increases. The larger spheres can entangle more easily,

And this contributes to a higher viscosity. Dissolved polysaccharides such as

Xylan tend to form expanded random coils which greatly influence the viscosity of

Black liquor. The viscosity of black liquor can be reduced by a heat treatment. The

black liquor is heated up to 180–190 °C to further degrade the polymeric material

In the presence of residual alkali. The resulting reduced viscosity allows the black

liquor to be concentrated up to 80% dry solids in order to maximize the benefits

of high dry solids in black liquor combustion [9].

Boiling Point Rise (BPR)

According to Raoult’s law, the vapor pressure of the solvent decreases proportionally

To themolal concentration of the solute. Thus, the boiling point of the black liquor

Increases with increasing dry solids content. The BRP can increase up to values of

close to 30 °C for black liquors leaving the concentrator (BLS about 80%) [8]. The

dependency of BRP on dry solids content is illustrated graphically in Fig. 9.1[8].

Recovery

Characterization of Black Liquors

20 40 60 80

Boiling Point Rise [.C]

Solids Content [%]

Fig. 9.1 Boiling point rise (BPR) as a function of dry solids

content (according to Frederick [8]).

The inorganic compounds (sodium, potassium, etc.) constitute more than 90%

Of the solute on a molar basis. Therefore, the BPR is mainly influenced by the salt

Concentration in the black liquor. The BPR is an important parameter for evaluating

The efficiency of black liquor evaporators. Heat transfer is dependent upon the

Temperature difference between the condensing steam and the evaporating black

Liquor. More detailed information regarding the calculation of BPR as a function

of pressure and molal concentration are provided in Ref. [8].

Surface Tension

The surface tension of black liquor is influenced by the temperature, as well as by

The nature and concentration of the dissolved components. Inorganic compounds

Such as sodium salts increase the surface tension, whereas some organic substances

(e.g., extractives, lignin, etc.), which are known as surface-active agents,

Reduce the surface tension of water. It has been shown that the latter effect outweighs

That of the inorganic compounds. The surface tension comprises a value

of 40–60% of the value for pure water (72.8 mN m–1 at 20 °C) in the range between

15% and 40% dry solids content. The effect of temperature on the surface tension

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