Herbert Sixta 3 страница

(HW-S) [33]. Low-viscosity pulp: viscosity 490 mL g–1, kappa

number 6.2, xylan content 4.5%; Medium-viscosity pulp: viscosity

730 mL g–1, kappa number 6.2, xylan content 6.5%.

1 2 3 4 5 6

Unbleached (E/O)-treatment:

Medium viscosity Low viscosity Medium viscosity Low viscosity

R18 content [%]

Xylan content [%]

Fig. 8.21 R18 versus xylan content during (E/O)-treatment of

hardwood sulfite dissolving pulp (HW-S) [33]. Low-viscosity

pulp: viscosity 490 mL g–1, kappa number 6.2, xylan content

4.5%; Medium-viscosity pulp: viscosity 730 mL g–1, kappa

number 6.2, xylan content 6.5%.

958 8 Pulp Purification

The distinct difference in the residual xylan contents of low- and medium-viscosity

hardwood sulfite dissolving pulps at a given R18 content is clearly shown in

Fig. 8.21. The xylan content of the medium-viscosity pulp is approximately 1%

higher than that of the low-viscosity pulp when compared at a level of 95% R18

(2.8% versus 1.8% xylan).

8.4.1.3 Purification versus Viscosity

The removal of short-chain carbohydrates through HCE treatment results in a

slight increase in viscosity because stepwise degradation (peeling) has only a small

effect on the molecular weight of long-chain cellulose. The effect of HCE on viscosity

has been expressed as a negative change of chain scissions for both spruce

and beech dissolving pulps to consider different levels of initial viscosity.

The data in Fig. 8.22 reveal a clear relationship between the degree of purification

and viscosity increase, reflecting the removal of short-chain material. The

change in viscosity is more pronounced for beech dissolving pulp, indicating that

the molecular weight of the removed hemicelluloses is lower than that from

spruce dissolving pulp. Alternatively, a greater amount of low molecular-weight

material is removed from the beech dissolving pulp during HCE treatment; this

suggestion would be in line with the higher specific yield loss when compared to

spruce dissolving pulp.

91 93 95 97

-1,6

-1,2

-0,8

-0,4

0,0

HW-Sulfite SW-Sulfite

Medium viscosity Medium viscosity High viscosity

Chain scissions, 1/DP

HCE

-1/DP

untreated

[*10-4 mol/AGU]

R18 content [%]

Fig. 8.22 Change in pulp viscosity, expressed

as negative number of chain scissions, as a

function of the degree of purification, characterized

as R18 content, during E-treatment of

hardwood and softwood sulfite dissolving

pulps (HW-S, SW-S) [33]. Medium-viscosity

HW-S: viscosity 590 mL g–1, kappa number 4.6;

Medium-viscosity SW-S: viscosity 625 mL g–1,

kappa number 4.6; High-viscosity SW-S: viscosity

890 mL g–1, kappa number 12.2.

8.4Hot Caustic Extraction 959

8.4.1.4 Purification versus Kappa Number and Extractives

Hot caustic purification also removes other pulp impurities such as lignin and

extractives. Most of the kappa number reduction occurs already at low NaOH

charge, and this can be attributed to a readily available lignin (Fig. 8.23). The

more alkali-resistant lignin is gradually decreased with increasing NaOH charge.

It may be speculated that part of the removed lignin is associated with the

extracted (and degraded) xylan.

0 30 60 90 120

E-treatment of HW-Sulfite Pulp

Kappa number

NaOH charge [kg/odt]

Fig. 8.23 Course of kappa number as a function of NaOH

charge during E-treatment of hardwood sulfite dissolving

pulps (HW-S) [33]. HW-S: viscosity 580 mL g–1, kappa number

5.1; E conditions: 90 °C, 240 min.

Hot caustic extraction is a very efficient stage in the removal of resin constituents

of sulfite pulps [27]. The saponification of fats, waxes and other esters is the

key reaction responsible for the removal of extractives. The removal efficiency can

be further enhanced by the addition of surfactants (nonyl phenol with attached

polyoxyethylene chain), and this may also solubilize the nonsaponifiables. The deresination

of softwood pulps with large amounts of resin can be further improved

by subjecting the pulp to increased mechanical forces that allow removal of the

encapsulated resin from the ray cells. A process developed by Domsjo involves the

use of a Frotapulper, along with the addition of caustic for the de-resination of sulfite

pulps (Fig. 8.24) [35,36].

960 8 Pulp Purification

0 10 20 30 40 50

0.0

0.2

0.4

0.6

0.8

DCM extractives, %

Energy, kWh/odt

Fig. 8.24 Course of dichloromethane (DCM) extractives in

fully bleached softwood dissolving pulps as a function of

energy input in a Frotapulper [35,36].

8.4.1.5 Composition of Hot Caustic Extract

Hot caustic extract contains a large amount of low molecular-weight hydroxycarbonic

acids,with glucoisosaccharinate as the main component, derived frompurification of

softwood sulfite pulp [37]. A typical extract composition is shown in Tab. 8.4.

Tab. 8.4 Typical composition of hot caustic extract [37].

Compounds % of solids as sodium Salts

Chloridea 3.3

Formate 17.0

Acetate 3.4

Glucoisosaccharinate 27.0

Other Hydroxy acids 38.0

“Complex” acids 11.3

a. C stage preceding HCE treatment.

The combined saccharinic acids and other hydroxy acids constitute about 65%

of the hot caustic extract. These compounds are readily biodegradable in a wastewater

treatment plant. However, the COD load is significant and calculates to

about 180 kg odt–1, assuming an average yield loss of about 15% on bleached pulp

across the stage (150 kg carbohydrates/odt. 1.185 kg COD/kg carbohydrates). As

8.4Hot Caustic Extraction 961

a consequence, several sulfite dissolving pulp mills have recently installed evaporation

plants and recovery furnaces (soda boiler) to concentrate and burn the filtrates

from hot caustic extraction. To date, no products are prepared from the thick

liquor, mainly because of the high costs to isolate, purify, and modify the saccharinic

acids. Reintjes and Cooper have proposed a scheme to utilize these compounds

where the acids are lactonized and converted to amides; they may then be

further processed to anionic and nonionic surfactants by reaction with chlorosulfonic

acid or ethylene oxide [37].

8.4.2

MgO as an Alternative Alkali Source

The main disadvantage of using NaOHas an alkali source forHCE is that the evaporated

caustic extract cannot be recycled to the spent sulfite liquor (SSL) of a Mg-based

cooking process due to the formation of low-melting Na-Mg eutectic mixtures. Thus,

efforts were undertaken to investigate the possibility of using Mg(OH)2 as an alkali

source for hot caustic extraction, as this enables the combined recovery of hot caustic

extract and Mg-based SSL [38]. As known fromweak bases such as sodiumcarbonate,

sodium sulfite and others, a higher temperature than is used for NaOH is required

for the purification. The time and temperature of MgO-based HCE (EMgO)

are the two main parameters that determine the degree of purification, rather

than the Mg(OH)2 charge. Charges higher than 15 kg odt–1 have no effect on purification,

due mainly to the low solubility of Mg(OH)2 in aqueous solution.

130 140 150 160 170

120 min 240 min at temperature

R18 content [%]

Temperature [.C]

Fig. 8.25 R18 content of a hardwood sulfite

pulp (HW-S) as a function of temperature during

hot caustic extraction using MgO as a

base, at two different reaction times [39]. HW-S

for 120 min reaction time: kappa number 6.2,

viscosity 640 mL g–1, 91.0%R18; HW-S for

240 min reaction time: kappa number 9.9, viscosity

600 mL g–1, 90.9%R18.

962 8 Pulp Purification

Figure 8.25 illustrates the successful use of MgO to obtain degrees of purification

sufficiently high for the production of viscose staple fiber pulps. The main

drawback when using MgO is the high temperature needed to achieve the necessary

purification. Another problem may be to achieve homogeneous distribution

of Mg(OH)2 within the pulp suspension in order to obtain a uniform pulp quality.

The prolongation of retention time from 120 to 240 min may reduce the temperature

by almost 10 °C, while maintaining the same R18 content. Moreover, the

MgO-based hot caustic extraction appears to be more selective than the conventional

system, with a specific yield loss of only 2.4% per 1% increase in R18

(Fig. 8.26).

91 93 95 97

E

MgO

- 240 min E

NaOH

E

MgO

- 120 min

Purification Yield [%]

R18 content [%]

Fig. 8.26 Purification yield as a function of R18 content for

MgO- and NaOH-based hot extraction processes of a hardwood

sulfite pulp (HW-S) [39]. Pulp substrate and conditions:

EMgO according to Fig. 8.25; ENaOH according to Fig. 8.18.

References 963

References

1 Hermans, P.H., The analogy between

the mechanism of deformation of cellulose

and that of rubber. J. Phys. Chem.,

1941; 45: 827–836.

2 Avela, E., et al., Sulfite pulps for HWMfibres.

Pure Appl. Chem., 1967: 289–301.

3 Sixta, H., et al., Evaluation of new organosolv

dissolving pulps. Part I: Preparation,

analytical characterization and viscose

processability. Cellulose, 2004; 11:

73–83.

4 Rydholm, S.A., Pulping Processes. Malabar,

Florida: Robert E. Krieger Publishing

Co., Inc., 1965: 992–1023.

5 Richter, G.A., Production of high alphacellulose

wood pulps and their properties.

Tappi, 1955; 38(3): 129–150.

964 8 Pulp Purification

6 Hempel, K., Solubility of cellulose in

alkalies and its technical significance.

Przeglad Papierniczy, 1949; 5: 62–69,

73–81.

7 Shogenji, T., H. Takahasi, K. Akashi,

The cold alkaline purification of sulfite

pulp. Use of ion-exchange resin for the

analysis of waste liquor and some information

on alkali consumption. J. Jap.

Tech. Assoc. Pulp Paper Ind., 1952; 6:

201–211.

8 Wilson, K., E. Ringstrom, I. Hedlund,

The alkali solubility of pulp. Svensk. Papperstidn.,

1952; 55: 31–37.

9 Ranby, B.G., The mercerization of cellulose.

II. A phase-transition study with

X-ray diffraction. Acta Chim. Scand.,

1952; 6: 116–127.

10 Ranby, B.G., The physical characteristics

of alpha-, beta- and gamma-cellulose.

Svensk. Papperstidn., 1952; 55:

115–124.

11 Corbett,W.M., J. Kidd, Some aspects of

alkali refining of pulps. Tappi, 1958;

41(3).

12 Sixta, H., A. Schrittwieser, Alkalization

of hardwood dissolving pulps. R&D

Lenzing AG: Lenzing, 2004: 1–10.

13 Saito, G.-I., The behaviour of cellulose

in solutions of alkalies. Kolloid-Beihefte,

1939; 49: 365–366.

14 Saito, G.-I., The behaviour of cellulose

in solutions of alkalies. I. Cross-sectional

swelling of fibers of different celluloses

in sodium hydroxide solutions

at different temperatures. Kolloid-Beihefte,

1939; 49: 367–387.

15 Bartunek, R., The reactions, swelling

and solution of cellulose in solutions of

electrolytes. Das Papier, 1953; 7:

153–158.

16 Dobbins, R.J., Role of water in cellulosesolute

interactions. Tappi, 1970; 53(12):

2284–2290.

17 Sixta, H., et al., Characterization of

alkali-soluble pulp fractions by chromatography.

In 11th ISWPC. Nice, France,

2001.

18 Sartori, J., Investigations of alkaline

degradation reactions of cellulosic

model compounds. In Institute of

Chemistry. University of Natural

Resources and Applied Life Science:

Vienna, 2003: 134.

19 Mais, U., H. Sixta, Characterization of

alkali-soluble hemicelluloses of hardwood

dissolving pulps. In ACS Symposium

Series, 2004: 94–107.

20 Krassig, H.A., Cellulose: Structure, Accessibility

and Reactivity. Polymer Monographs.

M.B. Huglin, Ed. Vol. 11. Gordon

and Breach Science Publishers,

1993: 258–323.

21 Sixta, H., Comparative evaluation of

TCF bleached hardwood dissolving

pulps. Lenzinger Berichte, 1999; 79:

119–128.

22 Fink, H.-P., J. Kunze, Solid state 13C

NMR studies of alkalization of hardwood

dissolving pulps. Fraunhofer,

Institut fur Angewandte Polymerforschung:

Golm, 2003: 1–5.

23 Fink, H.-P., B. Philipp, Models of cellulose

physical structure from the viewpoint

of the cellulose I → cellulose II

transition. J. Appl. Polym. Sci., 1985;

30(9): 3779–3790.

24 Fink, H.-P., et al., The composition of

alkali cellulose: a new concept. Polymer,

1986; 27(6): 944–948.

25 Fink, H.-P., et al., The structure of

amorphous cellulose as revealed by

wide-angle X-ray scattering. Polymer,

1987; 28(8): 1265–1270.

26 Fink, H.-P., et al., 13C-NMR studies of

cellulose alkalization. Cellulose and Cellulose

Derivatives, Physico-chemical

Aspects and Industrial Applications.

J.F. Kennedy, G.O. Williams, L. Piculell,

Eds. Woodhead Publishing Ltd: Cambridge,

1995: 523–528.

27 Hinck, J.F., R.L. Casebier, J.K.Hamilton,

Dissolving pulp manufacturing. In Sulfite

Science & Technology. J.K.O. Ingruber,

P.E. Al Wong, Eds. TAPPI, CPPA:

Atlanta, 1985: 213–243.

28 Borgards, A., A. Lima, H. Sixta, Cold

caustic extraction of various hardwood

dissolving pulps. Internal Report, R&D

Lenzing AG, 1998.

29 Sears, K.D., J.F. Hinck, C.G. Sewell,

Highly reactive wood pulps for cellulose

acetate production. J. Appl. Polym. Sci.,

1982; 27(12): 4599–4610.

30 Leugering, H.-J., Zur Kenntnis der Zellstoffveredelung

durch Heissalkalisierung.

Das Papier, 1953; 7(3/4): 47–51.

References 965

31 Meller, A., Studies on modified cellulose.

I. The alkali stability of oxidized,

hydrolyzed, and methanolized cellulose.

Tappi, 1951; 34: 171–179.

32 Samuelson, O., C. Ramsel, Effect of

chlorine and chlorine dioxide bleaching

on the copper number, hot-alkali solubility,

and carboxyl content of sulfite cellulose.

Svensk. Papperstidn., 1950; 53:

155–163.

33 Yaldez, R., H. Sixta, Hot caustic extraction

of sulfite dissolving pulps. Internal

Report, R&D Lenzing AG, 1998.

34 MacLeod, J.M., L.R. Schroeder, b-d-(glucopyranosyl)-

d-glucose-3.6-anhydro-4-Omethyl-

d-glucose, and d-glucose.

J. Wood Chem. Technol., 1982; 2(2):

187–205.

35 Lindahl, J.A.I., Process and apparatus

for the deresination and brightness

improvement of cellulose pulp. Mo och

Domsjo Aktebolag: US Patent, 1981.

36 Assarsson, A., et al., Control of rosininduced

complications in pulp. Przeglad

Paperniczy, 1982; 38(2): 53–55.

37 Reintjes, M., G.K. Cooper, Polysaccharide

alkaline degradation products as a

source of organic chemicals. Ind. Eng.

Chem. Prod. Res. Dev., 1984; 23: 70–73.

38 Sixta, H., T. Gerzer, W. Muller, Verfahren

zur Veredelung von Zellstoffen.

Osterreichische Patentanmeldung,

2002.

39 Sixta, H., Hot caustic extraction of hardwood

sulfite pulp with MgO as a base.

Internal Report, R&D Lenzing AG,

2002.

Recovery

Andreas W. Krotscheck, Herbert Sixta

9.1

Characterization of Black Liquors

9.1.1

Chemical Composition

Kraft black liquor contains most of the organic compounds removed from the

wood during the cook and the inorganic chemicals charged, mainly in the form of

salts with organic acids. A major portion part of the extractives removed from the

wood during kraft pulping is, however, not included in the black liquor solids.

The volatile wood extractives such as low molecular-weight terpenes are recovered

from the digester relief condensates (turpentine). The resin and fatty acids, as

well as some neutral resins (e.g., b-sitosterol), are suspended in the diluted black

liquor (in the form of stable micelles). During the course of black liquor evaporation,

when a concentration of 25–28% of total solids is reached, these extractives

are separated from the aqueous phase as “soap skimmings”. Crude tall oil is

obtained from the soap skimmings after acidification with sulfuric acid. The composition

of the tall oil is described elsewhere [1].

The remaining kraft black liquor contains organic constituents in the form of

lignin and carbohydrate degradation products. The composition of the spent

liquor depends greatly on the wood species, the composition and amount of white

liquor charged, the unbleached pulp yield, and the amount of recycled bleach filtrates

(predominantly from the oxygen delignification stage). During kraft pulping,

lignin and a large part of carbohydrates mainly derived from hemicelluloses,

are degraded by alkali-catalyzed reactions. Thus, the organic material of the black

liquor consists primarily of lignin fragments (mainly high molecular-mass fragments)

and low molecular-weight aliphatic carboxylic acids originating from wood

carbohydrates. The approximate composition of a black liquor from birch and

pine kraft cooks is shown in Tab. 9.1.

Organic material also contains minor amounts of polysaccharides mainly derived

from xylan (part of “Other organics” in Tab. 9.1). Quite recently, it was

shown that black liquor from Eucalyptus globulus kraft cooking contains substantial

amounts of dissolved polysaccharides (BLPS = black liquor dissolved polysaccharides)

[4]. BLPS represent about 20% of the total dissolved and/or degraded

wood polysaccharides. The major component of BLPS is xylan, with a molecular

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

Tab. 9.1 Composition of the dry matter of pine (Pinus sylvestris)

and birch (Betula pendula) kraft black liquors. Values are % of

total dry matter [2,3].

Component Pine Birch

Lignin

Aliphatic carboxylic acids

Formic acid

Acetic acid

Glycolic acid

Lactic acid

2-Hydroxybutanoic acid

3,4-Dideoxypentonic acid

3-Deoxypentonic acid

Xyloisosaccharinic acid

Glucoisosacharinic acid

Others

Other Oganics 8 12

Inorganicsa

Sodium bound to organics

Inorganic compounds

Total 100 100

a. Including sodium bound to organic material.

weight in the range 17–19 kDa. During pulping, the black liquor xylans are progressively

enriched in hexenuronic acid.

Black liquor is concentrated by evaporation and then combusted in the recovery

furnace for the recovery of cooking chemicals and the generation of energy. The

heating value of black liquor has a major impact on the steam generation rate,

knowledge of which is essential in the design and operation of a recovery boiler.

The higher heating (gross calorific) value (HHV) is determined by oxidizing the

black liquor quantitatively, condensing the water vapor produced, and cooling the

products to 25 °C (TAPPI method T 684 om-90). The net heating (net calorific)

value (NHV), which better reflects the actual energy release, accounts for the fact

that the generated water is not condensed during combustion and steam generation.

NHV is obtained by subtracting the heat of vaporization of the water from

the HHV value. In addition, any sulfur is completely oxidized in the oxygen bomb

calorimeter, whereas with kraft black liquor it always appears as sodium sulfide

(Na2S). The reduction process of Na2S to Na2SO4 is endothermic by 13 090 kJ kg–1

of Na2S. The NHV can be calculated according to the following expression:

NHV _ HHV _ 2440 _

_2 _ H__ 13090 _

32 _ S _ g_ RED_ _1_

968 9 Recovery

where NHV is the net heating value of black liquor solids (BLS; in kJ kg–1 BLS);

HHV is the higher heating value of BLS (in kJ kg–1 BLS); H and S are the weight

fractions of hydrogen (H) and sulfur (S) in BLS; and gRED is the degree of reduction

given as a weight fraction.

Typical values for the HHV of kraft black liquor range between 13 MJ kg–1 BLS

(predominantly derived from hardwoods) and 15.5 MJ kg–1 BLS (predominantly

derived from softwoods), as indicated in Tab. 9.2.

It should be noted that the recycling of bleach (e.g., oxygen delignification) and

purification (e.g., cold caustic extraction) filtrates has an impact on the composition

and heating value of the BLS due to a generally lower content of organic compounds.

Tab. 9.2 Chemical analysis and heating values of black liquor solids [5–8].

Components A B C D E F G

Wood species Unit hardwood hardwod hardwood softwood softwood softwood softwood

Elemental analysis

C wt% on DS 32.3 33.3 33.4 35.8 37.8 35.8 38.0

H wt% on DS 3.8 3.6 3.9 3.5 4.2 3.6 3.8

N wt% on DS 0.2 0.1 0.1 0.2 0.1

O wt% on DS 35.8 33.6

S wt% on DS 3.0 5.4 4.4 4.1 4.8 4.6 3.7

Na wt% on DS 18.2 19.9 20.7 19.9 17.9 19.6 19.2

K wt% on DS 3.0 1.5 1.7 1.1 1.2 1.8 0.6

Cl wt% on DS 0.7 0.6 0.3 0.2 2.9 0.5 1.0

HHV MJ kg–1 13.2 13.2 13.2 14.1 15.4 15.1

NHV (calculateda) MJkg–1 11.5 10.9 11.1 12.2 13.1 13.2

Reference [5] [6] [6] [6] [6] [7] [8]

a. Assuming a degree of reduction of 90%.

9.1 Characterization of Black Liquors 969

9.1.2

Physical Properties

The most important physical properties which affect evaporator and recovery boiler

design and operation include liquor viscosity, boiling point rise, surface tension,

density, thermal conductivity and heat capacity.

9.1.2.1 Viscosity

The viscosity of the black liquor is determined by its composition, the dry solids

content, and temperature. At low shear rates, black liquor behaves as a Newtonian

fluid, and the macromolecular components such as lignin and polysaccharide

molecules control the rheological properties. At a dry solids content of about 15%,

the viscosity of the black liquor is only three-fold that of water at a given temperature.

At about 50% solids content, however, black liquor behaves as a polymer

blend with water as plasticizer, and the viscosity increases exponentially with solids

content. The relationship between dry solids content and viscosity at a low

shear rate is expressed in Eq. (2) [8]:

Log

lbl

lw _ __

DS _ 373

T

0_679 _ 0_656 _ DS _ 373

T

_2_

where lbl is the viscosity of black liquor (in Pa.s); lw is the viscosity of water (in Pa.s):

DS is the weight fraction of dry solids in black liquor; and T is temperature (in K).

According to Eq. (2), viscosity can change by five orders of magnitude over the

range of dry solids contents typical for kraft black liquor recovery.

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].

9.1.2.2 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].

970 9 Recovery

9.1 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].

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

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:

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