Parameter

Cooking #

Unit CBC

CONV

Yield 47.1 48.0

Screened yield 46.8 46.8

Max. temperature °c 155 155

Kappa number 25.6 25.2

Viscosity mL g–1 1234 1162

Cellulose % on pulp 77.5 75.4

GGM % on pulp 7.8 8.3

AX % on pulp 8.2 9.8

Residual (not analyzed) % on pulp 6.5 6.5

The ratio of cellulose to hemicellulose concentration increases further when the

EA concentration of the CBC cooking liquor is increased from 0.62 mol L–1 to

0.87 mol L–1; these data are in agreement with the observations of Jiang, who

investigated modified continuous cooking procedures [139]. The lower xylan content

of the CBC pulps may be attributed to the lower extent of xylan reprecipitation

during the late stage of the cook due to both a higher residual EA content and

a lower xylan concentration in the cooking liquor (long retention time at cooking

temperature favors fragmentation reactions). The more pronounced preservation

of the cellulose fraction of the CBC pulp, however, cannot be identified unambiguously

because the higher cellulose loss of conventionally kraft-cooked spruce

4.2 Kraft Pulping Processes 285

40 50 60 70 80 90 100

Cellulose (CBC) Cellulose (Conv) AX+GGM (CBC)

AX+GGM (Conv) Lignin (CBC) Lignin (Conv)

Wood component yield [rel%]

Wood yield [%]

Fig. 4.72 The removal of cellulose, lignin, and

hemicelluloses (AX + GGM) as a function of

total wood yield during CBC cooking of spruce

[55]and conventional kraft cooking of pine

[140]. CBC cooking: Impregnation liquor

[OH– ]= 0.38 mol L–1; [HS– ]= 0.30 mol L–1.

Cooking liquor [OH– ]= 0.62 mol L–1;

[HS– ]= 0.34 mol L–1. Cooking temperature =

160 °C. Conventional kraft cooking of pine:

liquor-to-wood ratio 4:1, EA-charge 20.3% on

o.d. wood; 25% sulfidity; 170 °C maximum

cooking temperature (see also Fig. 4.23)

remained, even after lowering the cooking temperature to 155 °C (see Tab. 4.36).

The course of the relative content of the main wood components as a function of

total wood yield (as studied by Aurell and Hartler for a conventional pine kraft

cooking procedure) was compared with corresponding data obtained from spruce

CBC cooking (Fig. 4.72) [140]. Although differences in the wood species (pine has

higher contents of GGM and lignin, but a lower cellulose content) and the analytical

methods applied may affect the results, it can be expected that the principal

degradation pattern of the two kraft cooking processes should be identified. Figure

4.72 displays the removal of lignin, cellulose, and hemicelluloses (sum of

GGM and AX) as a function of total wood yield.

The data illustrated in Fig. 4.72 confirm the better preservation of the cellulose

fraction during CBC cooking as compared to conventional kraft cooking. The

higher stability of the cellulose fraction during the initial CBC cooking phase (at

about 80% yield) may be led back to the lower [OH– ]and the higher [HS– ]as compared

to conventional kraft cooking. A further selectivity advantage for the CBC

pulp can be observed in the final bulk and beginning residual delignification

phases, where a higher [OH– ]combined with a lower content of dry solids causes

a significantly higher rate of delignification, thus improving delignification selectivity.

The significantly higher delignification selectivity throughout the whole

cooking process, with two maxima at approximately 80% and 60% yield, com-

286 4 Chemical Pulping Processes

prises the main difference between CBC and conventional kraft cooking. Again, it

may be speculated that the low but constant [OH– ]ion profile and the rather high

ratio of [HS– ]ion to [OH– ]ion throughout the whole process may be the main

reason for the higher delignification selectivity of CBC cooking. The pattern of

hemicellulose dissolution proceeds in parallel up to a wood yield of about 58%

(the lower values during CBC cooking in the initial delignification phase are due

to the bigger gap between the determined wood yield and the sum of the identified

single wood components than in the case of a conventional kraft cooking procedure).

During the final cooking phases, the CBC pulp retains less hemicelluloses

as compared to the conventional kraft pulp. As expected before, this behavior

most probably indicates a more intense sorption of dissolved xylan back onto the

fibers in the case of conventional kraft pulping.

Effect of [OH– ] ion in the cooking liquor

The effect of three different levels of [OH– ]ion in the cooking liquor (0.38, 0.63,

and 0.85 mol L–1) was investigated with respect to the processability of CBC cooking

and the quality of the resulting unbleached pulps. As the sulfidity of the cooking

liquor was kept constant at about 70%, the [HS– ]ion concentration changed

correspondingly from 0.22 mol L–1 to 0.34 mol L–1 and to 0.46 mol L–1, respectively.

Undoubtedly, the cooking intensity necessary to achieve a certain kappa number

target was most influenced by the [OH– ]ion, as seen in Fig. 4.73. Using a cooking

600 800 1000 1200 1400 1600

[OH-] = 0.38 mol/l [OH-] = 0.63 mol/l [OH-] = 0.85 mol/l

Kappa number

H-factor

Fig. 4.73 Influence of [OH– ]ion concentration in cooking

liquor on the course of kappa number as a function of the

H-factor during CBC cooking of spruce (according to [55]).

Impregnation liquor constant at [OH]= 0.39 mol L–1 and

[HS– ]= 0.25 mol L–1; cooking temperature constant at 155 °C.

4.2 Kraft Pulping Processes 287

10 20 30 40 50 60

CBC: [OH-]=0.38 mol/l;[HS-]= 0.22 mol/l; CBC: [OH-]=0.63 mol/l,[HS-]=0.34 mol/l

CBC: [OH-]=0.85 mol/l;[HS-]= 0.46 mol/l; Conventional reference

Viscosity [ml/g]

Kappa number

Fig. 4.74 Selectivity plot as viscosity–kappa

number relationship of CBC kraft cooking of

spruce wood as a function of the [OH– ]ion

concentration of the cooking liquor (according

to [55]). Constant CBC cooking conditions:

temperature and profile, cooking temperature

155 °C. Conventional reference cooking conditions

according to [8].

liquor with a [OH– ]ion of 0.85, 0.63, and 0.38 mol L–1, the corresponding H-factors

to reach a kappa number of 25 comprised 820, 1080, and 1620, respectively.

These H-factors translate to pure cooking times of 190 min (53), 250 min (70) and

375 min (105) at given cooking temperatures of, for example, 155 °C (170 °C),

respectively (Fig. 4.73). Thus, the change from an intermediate [OH– ]ion of

0.63 mol L–1 to 0.38 mol L–1 results in a significant prolongation of the cooking

time (to 125 min at 155 °C).

The delignification selectivity is only marginally influenced by the [OH– ]ion of

the cooking liquor, as shown in Fig. 4.74. When cooking to kappa numbers lower

than 25, the delignification selectivity tends to improve at a lower level of hydroxyl

ion. The viscosity advantage at a given kappa number is at most 50 units in case

of adjusting the cooking liquor to the low [OH– ]ion. Taking also the screened

yield into account, the application of the medium [OH– ]ion seems to be an optimum

choice (Fig. 4.74). Clearly, the amount of reject increases at higher kappa

number levels and decreasing [OH– ]ion.

Independent of the [OH– ]io n of the cooking liquor, CBC cooking technology

proved to be superior in delignification selectivity as compared to conventional kraft

cooking (seeFig. 4.73).Again, the higher ratio of [HS– ]ion to [OH– ]ion of the impregnation

liquor and the constant [OH– ]ion throughout the whole cooking process

in combination with a reduced concentration of the dry solids concentration during

the late stage of the cook, can be put forward as the main reasons for the selectivity

advantage of the CBC cooking process. The slightly higher screened

288 4 Chemical Pulping Processes

10 20 30 40 50 60

CBC: [OH-]=0.38 mol/l;[HS-]= 0.22 mol/l; CBC: [OH-]=0.63 mol/l,[HS-]=0.34 mol/l

CBC: [OH-]=0.85 mol/l;[HS-]= 0.46 mol/l; Conventional reference

Screened Yield [%]

Kappa number

Fig. 4.75 Selectivity plot as screened yield–

kappa number relationship of CBC kraft cooking

of spruce wood as a function of the [OH– ]

ion concentration of the cooking liquor

(according to [55]). Constant CBC cooking conditions:

temperature and profile, cooking temperature

155 °C. Conventional reference cooking

conditions according to [8].

yields observed for the CBC pulps are mainly due to the lower amount of rejects

as compared to the conventional kraft pulps (Fig. 4.75). The total yields are comparable

for both cooking technologies, despite the significantly higher delignification

selectivity of the CBC cooking technology. As mentioned above, this discrepancy

can be led back to the lower extent of xylan precipitation during the final

cooking phase in case of CBC cooking.

Influence of the cooking temperature

Cooking temperature is an important process parameter determining the demand

of steam, the whole cooking time (cover-to-cover time) and the cooking performance

(see Section 4.2.6.2.1, Principles of Modified Kraft Cooking). The effect of

cooking temperature on delignification selectivity was investigated in the range

between 155 and 170 °C. An increase from 155 °C to 160 °C showed no influence

on delignification selectivity, provided that the kappa number stays in the range

between 15 and 28 (Fig. 4.76). Beyond this kappa number range (at ca. kappa 38),

the application of the higher temperature level tends to reduce the viscosity at a

given kappa number. Raising the cooking temperature to 170 °C significantly

impairs the delignification selectivity over the whole kappa number range. The

viscosity of spruce CBC pulps at a kappa number level of 30 (40) is 50 (100) units

higher as compared to conventional kraft pulps using the same cooking temperature.

The selectivity advantage of the CBC pulps tends to decrease with decreasing

kappa numbers. Due to the much faster heating-up time, CBC cooking technolo-

4.2 Kraft Pulping Processes 289

10 20 30 40 50 60

CBC: 155.C CBC: 160.C

CBC: 170. C Conventional Reference

Viscosity [ml/g]

Kappa number

Fig. 4.76 Selectivity plot as viscosity–kappa

number relationship of CBC kraft cooking of

spruce wood as a function of cooking temperature

(according to [55]). Constant CBC cooking

conditions (cooking liquor):[OH– ]= 0.63molL–1,

[HS– ]= 0.34 mol L–1. Conventional reference

cooking conditions according to [8].

gy can be performed at a lower cooking temperature while maintaining the same

cover-to-cover time as compared to a conventional kraft cook.

Effect of adding polysulfide to the impregnation liquor

The CBC cooking technology combines the advantages of both batch and continuous

cooking technologies with respect to the homogeneity and selectivity of

delignification (see Figs. 4.74 and 4.76). The screened yield at a given kappa number

is also superior as compared to conventional kraft cooking due to the better

impregnation conditions.

A secure method to further increase the pulp yield is to add polysulfide solution to

stabilize the reducing end groups against alkaline peeling reactions (see Section

4.2.4.2.1, Polysulfide pulping). In some preliminary tests the effect of polysulfide on

the performance ofCBCcooking was investigated using spruce as a rawmaterial [55].

Pretreatment with polysulfide solution was carried out by dissolving elementary

sulfur in the impregnation liquor. The [OH– ]ion of the impregnation liquor was

increased from 0.38 mol L–1 to 0.50 mol L–1 to compensate for the additional consumption

of caustic during preparation of the polysulfide solution (Eq. 131).

Despite this additional charge of EA, the [OH]ion decreases to a minimum level

below 0.1 mol L– before increasing again to the target values. At the same time,

the [HS– ]ion rises to values above 0.3 mol L–1, thus increasing the ratio of [HS– ]

ion to [OH– ]ion to a level greater than 5 to 1, which essentially leads to an

improved sulfide sorption (Fig. 4.77).

290 4 Chemical Pulping Processes

00:00 01:00 02:00 03:00 04:00 05:00

0.0

0.2

0.4

0.6

Temperature,. C

[S

] [OH-] [HS-]

[S

], [OH-], [HS-], mol/l

Cooking Time [hh:min]

Fig. 4.77 Polysulfide CBC cooking of spruce

wood with 4% sulfur addition (according to

[55]). Concentration profile of [OH– ], [HS– ]and

[S0]throughout the cook. Polysulfide analysis:

HPLC; column Shandon Hypersil BDS C8; eluent

85% MeOH, 14.25% H2O, 0.75% AcOH;

flow rate 0.8 mL min–1 isocratic; detection UV

280 nm.

The polysulfide treatment must be carried out during the impregnation stage,

when the temperature is still below 120 °C, as polysulfide easily decomposes at

cooking temperature. The course of the concentrations of active species throughout

a typical CBC cook is illustrated graphically in Fig. 4.77. The molar polysulfide

concentration (as S0) decreases rapidly to values below 0.02 mol L–1 before a temperature

of 140 °C is reached.

Two polysulfide cooking series with 2% and 4% sulfur addition on wood were

conducted, respectively. As expected, the addition of polysulfide led to a substantial

increase in yield at a given kappa number (Fig. 4.78).

No additional yield gain can be observed by doubling the sulfur addition from

2% to 4% on wood. The yield advantage comprises about 1.0–1.2% over the whole

kappa number range investigated as compared to the CBC reference cooks. Based

on carbohydrate analysis of the resulting unbleached pulps, an even higher yield

gain of polysulfide CBC cooking can be assumed. The cellulose yield on wood

increases by more than 2% and the GGM yield by about 0.8% at a kappa number

level of about 25 (Tab. 4.37).

4.2 Kraft Pulping Processes 291

10 20 30 40 50 60

CBC: no Polysulfide CBC: 2 % S

CBC: 4% S

Conventional Reference

Screened Yield [%]

Kappa number

Fig. 4.78 Effect of polysulfide addition on the

screened yield kappa number relationship of

CBC kraft cooking of spruce wood (according

to [55]). Constant CBC cooking conditions:

cooking temperature 160 °C; impregnation

liquor of polysulfide cooks: [OH– ]= 0.50 mol L–1,

[HS– ]= 0.24 mol L–1; impregnation liquor for

reference CBC cooks: [OH– ]= 0.38 mol L–1,

[HS]= 0.20 mol L–1; cooking liquor for all CBC

cooks: [OH– ]= 0.63 mol L–1, [HS– ]= 0.34 mol L–1.

Conventional reference cooking conditions

according to [8].

Tab. 4.37 Effect of polysulfide addition an total and carbohydrate

yield of spruce CBC pulps (according to [55]). Each result is an

average of four or five cooking experiments, respectively.

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