Effect of Cooking Temperature

The rate of carbohydrate degradation during alkaline pulping is affected by both

EA concentration and cooking temperature (see Section 4.2.5.2.1, Kinetics of carbohydrate

degradation). Kubes et al. determined an activation energy of

179 ± 4 kJ mol–1 for the chain scissions of the carbohydrates by applying the

Arrhenius equation that describes the temperature dependence in Soda-anthraquinone

(AQ) and kraft pulping [21]. The corresponding activation energy for

bulk delignification is known to be about 134 kJ mol–1 (see Tab. 4.19 in) [46]. The

selectivity of kraft cook with respect to the intrinsic pulp viscosity is defined as the

ratio of the rate of delignification (kL) to the rate of carbohydrate degradation, determined

as chain scissions (kC). Pulping selectivity improves by decreasing the

cooking temperature due to a significantly higher activation energy for the chain

scissions (see Tab. 4.18). Laboratory and industrial cooking experiments according

12 14 16 18 20 22

Bulk-T 155. C / Resid-T 155. C Bulk-T 175.C / Resid-T 155. C

Bulk-T 155. C / Resid-T 175.C Bulk-T 165.C / Resid-T 165.C

Bulk-T 175.C / Resid-T 175. C

Intrinsic Viscosity [ml/g]

Kappa number

Fig. 4.51 EMCC laboratory kraft cooks of pine/

spruce mixture. Pulp viscosity versus kappa

number (according to [47]). Total EA-charge

18% on wood; EA-split 80%/20%; sulfidity

40%; beginning of counter-current cooking

after H-factor of 600; residual delignification is

assumed to start beyond H-factor 1450.

248 4 Chemical Pulping Processes

to the isothermal cooking (ITC) and extended modified cooking concept (EMCC)

confirmed the predictions from kinetic investigations [47,48]. Figure 4.51 demonstrates

that lowering the temperature during bulk delignification is preferable

with respect to selectivity as compared to a decrease in temperature in the residual

phase. When translated to the EMCC cooking procedure, this means that a low

temperature during the co-current and the first counter-current cooking zones is

more efficient for a selective kraft cook than a low temperature in the “HiHeat”

cooking zone.

Figure 4.51 shows that a decrease in cooking temperature of 10 °C results in an

increase in pulp viscosity by 80 units. Isothermal conditions at 165 °C yield pulps

of equal selectivity as compared to those being produced at 155 °C during bulk

and 175 °C in the course of final phase delignification. The gain in pulp yield with

decreasing temperature is not clear. The results indicate that the pulp yield is

increased by 0.5% on wood when the cooking temperature is decreased by 10 °C

(Fig. 4.52).

In contrast to the results from industrial isothermal cooking (ITC), the strength

properties of the laboratory-cooked pulps are not affected by the cooking temperatures

[47,48]. A decrease in temperature was also unsuccessful in increasing the

tear strength of Eucalyptus pulps, though a small improvement in pulp yield was

reported [23].

12 14 16 18 20 22

Bulk-T 155. C / Resid-T 155. C Bulk-T 175. C / Resid-T 155.C

Bulk-T 155. C / Resid-T 175.C Bulk-T 165. C / Resid-T 165.C

Bulk-T 175. C / Resid-T 175. C

Total Yield [%]

Kappa number

Fig. 4.52 EMCC laboratory kraft cooks of pine/

spruce mixture. Total yield versus kappa number

(according to [47]). Total EA-charge 18% on

wood; EA-split 80%/20%; sulfidity 40%;

beginning of counter-current cooking after Hfactor

of 600; final delignification is assumed

to start beyond H-factor 1450.

4.2 Kraft Pulping Processes 249

0 100 200 300

0,0

0,5

1,0

1,5

2,0

[OH-]free170 °C, low EA-charge

[OH-]free 160 °C, high EA-charge

[OH-]free 160 °C, low EA-charge

OH- concentraion [mol/l]

Cooking time [min]

Intrinsic Viscosity [ml/g]

Viscosity 170 °C, low EA-charge

Viscosity 160 °C, high EA-charge

Viscosity 160 °C, low EA-charge

Fig. 4.53 Prediction of the course of effective

alkali (EA) concentrations and intrinsic viscosities

of three model cases through conventional

softwood kraft pulping to kappa number

25. Case 1, high temperature, low EA charge;

Case 2, low temperature, high EA charge; Case

3, low temperature, low EA charge. Kinetic

model based on Ref. [50]

It is clear that to compensate for the lowering of the cooking temperature, either

the cooking time or the EA charge must be increased. Prolonging the cooking

time would clearly reduce the digester capacity, which would hardly be accepted

in an existing digester plant. To compensate for decreasing the temperature from

170 °C to 160 °C on the kraft pulping of hardwood to a given kappa number of

21 ± 1, the EA charge (as NaOH) was increased from 16.3% to 23.1% on o.d.

wood. Simultaneously, the H-factor was reduced from 1021 to 441 [49]. In this particular

case, the total yield remained almost unaffected, whereas the ratio cellulose

to pentosan content shifted in favor of the cellulose content. The effect of decreasing

the cooking temperature on the conventional kraft pulping of softwood was

investigated by using the kinetic model introduced in Section 4.2.5.3 (Fig. 4.35).

The applied reaction conditions and the calculated results are summarized in

Tab. 4.27 and Fig. 4.53.

According to the predicted results, cooking at low temperature and applying a

high EA charge to reach the target kappa number without extending the cooking

time leads to pulps with low yield and poor properties (low viscosity) compared to

the high-temperature reference (case 1). If cooking time is prolonged while maintaining

a low EA charge, the viscosity of the resulting pulp increases as expected,

whereas the pulp yield remains unaffected. Thus, it can be concluded that the

only way to improve kraft pulping selectivity with respect to viscosity is to compensate

for the lowering of the cooking temperature by increasing the cooking

time. For an economic optimization, a compromise between temperature, EA

charge and cooking time must be found.

250 4 Chemical Pulping Processes

Tab. 4.27 Effect of the interdependence of temperature, cooking

time and effective alkali (EA) charge on process and pulp

parameters of softwood kraft pulping. Values are predicted for a

kappa number 25-pulp by using a kinetic model based on an

extended model of Andersson (see Section 4.2.5.3, Reaction

kinetics) [50]. Case 1, high temperature, low EA charge; Case 2,

low temperature, high EA charge; Case 3, low temperature, low

EA charge.

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