9.1.2.3 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

is about the same as for pure water.

9.1.2.4 Density

The density of black liquor is predominantly influenced by the concentration of

inorganic components; this is a near-linear function of the dry solids content. The

971

9 Recovery

density of black liquors at 25 °C can be predicted up to a dry solids content of 50%

by the following expression [10]:

_25 _ 997 _ 649 _ DS _3_

where DS is the weight fraction of dry solids in black liquor.

The influence of temperature on black liquor density can be estimated by

Eq. (4):

_T

_25 _ 1 _ 3_69 _ 10_4 __T _ 25__1_94 __T _ 25_2 _4_

where T is the temperature (in °C).

9.1.2.5 Thermal Conductivity

The capability of a material to transfer heat is described by its thermal conductivity.

As water shows the highest contribution to thermal conductivity, the latter

decreases with increasing dry solids content and increases with increasing temperature.

This relationship is expressed by the following empirical equation:

k _ 1_44 _ 10_3 _ T _ 0_335 _ DS _ 0_58 _5_

where K is thermal conductivity (in Wm–1 °C–1); and T is temperature (in °C).

9.1.2.5 Heat Capacity [8,11]

The specific heat capacity represents the heat necessary to raise the temperature

of 1 kg of a material by 1 °C. Enthalpy data for black liquor are essential for estimating

energy balances of kraft recovery boilers. The heat capacity of the black

liquor decreases along with the increase in dry solids content. It can be approximated

by a linear addition of the specific enthalpy contributions of water and

black liquor solids. Moreover, an excess heat capacity function is incorporated to

account for changes in black liquor heat capacity:

Cpbl __1 _ DS__Cpw _ DS _ CpDS _ CpE _6_

where Cpbl is the heat capacity of black liquor (in J kg–1 °C–1); Cpw is the heat capacity

of water (4216 J kg–1 °C–1); CpDS is the heat capacity of black liquor solids (in

J kg–1 °C–1); and CpE is the excess heat capacity (in J kg–1 °C–1).

The temperature-dependence of the heat capacity of dry black liquor solids is

expressed by Eq. (7):

CpDS _ 1684 _ 4_47 _ T _7_

972

9.2 Chemical Recovery Processes

The dependence of excess heat capacity on temperature and dry solids content

Is described by the empirical equation:

CpE __4930 _ 29 _ T___1 _ DS___DS_3_2 _8_

The dependency of the heat capacity of black liquor, Cp,bl, for different dry solids

contents is illustrated graphically in Fig. 9.2 (cp,w), as shown in Fig. 9.2.

50 100 150

2,0

2,5

3,0

3,5

4,0

4,5

18% DS 50% DS 75% DS

C

p

[kJ/kg.K]

Temperature [°C]

Fig. 9.2 Heat capacity of black liquor for different dry solids

contents as a function of temperature.

9.2

Chemical Recovery Processes

9.2.1

Overview

The chemical recovery processes contribute substantially to the economy of pulp

manufacture. On the one hand, chemicals are separated from dissolved wood substances

and recycled for repeated use in the fiberline. This limits the chemical

consumption to a make-up in the amount of losses from the cycle. On the other

hand, the organic material contained in the spent cooking liquor releases energy

for the generation of steam and electrical power when incinerated. A modern

pulp mill can, in fact, be self-sufficient in steam and electrical power.

The main stations in the cooking chemical cycle are illustrated in Fig. 9.3. The

digester plant is provided with fresh cooking chemicals, which are consumed during

the course of the pulping process. Spent cooking liquor contains, besides

973