Validation and Application of the Kinetic Model

_ Prediction of delignification in the case where [OH– ]is changed.

After an initial impregnation stage, the course of delignification during a constant

composition cook with a L/W ratio of 41:1 at a very low alkali concentration of

[OH– ]= 0.1 M and [HS– ]= 0.28 M was determined [33]. After a cooking time of

approximately 220 min, the cooking liquor is replaced with a high-alkalinity liquor

of [OH– ]= 0.9 M and [HS– ]= 0.28 M. In a second run, the cook was run at a high

[OH– ]concentration of 0.9 M prior to a change to a very low alkali concentration

of [OH– ]= 0.1 M. Figure 4.33 shows that the presented model developed by

Andersson et al. is able to predict both scenarios very precisely [7].

0 100 200 300 400

0,1

[OH-] = 0.1 M [OH-] 0.1 to 0.9 M

[OH-] = 0.9 M [OH-] 0.9 to 0.1 M

lignin trend (dashed) for step decrease in [OH-] from 0.9 to 0.1 M

lignin trend (solid) for step increase in [OH-] from 0.1 to 0.9 M

Lignin on wood [%]

time [min]

Fig. 4.33 Model predictions of an autoclave cooking scheme.

The effect of changing [OH– ]from 0.1 to 0.9 M and 0.9 to

0.1 M in the residual phase in two cooks at constant

[HS– ]= 0.28 M and maximum cooking temperature of 170 °C.

Data from Lindgren and Lindstrom [33].

_ Prediction of the unbleached pulp quality of softwood kraft pulping

and the course of EA-concentration using a conventional

batch process.

The wood raw material consisted of a mixture of industrial pine (Pinus sylvestris)

and spruce (Picea abies) chips in a ratio of about 50:50. The chips were screened in

a slot screen, and the fraction passing a plate having 7-mm round holes and

retained on a plate with 3-mm holes were used. Bark and knots were removed by

hand-sorting. The mean (± SD) thickness of the chips was 3.5 ± 1.5 mm; the corresponding

mean length of the chips was 25.4 ± 6.5 mm. The chips had a dry content

of 49.5% and were stored frozen. The cooking trials were carried out in a 10-L

4.2 Kraft Pulping Processes 223

digester with forced liquor circulation. The digester was connected to three pressurized

preheating tanks, which allowed precise simulation of a large-scale operation.

In addition, dosage volumes, temperatures and H-factors were monitored

and recorded on-line. The digester and pressurized tanks were heated by steam

injection and/or a heat exchanger in circulation and/or an oil-filled jacket. Dry

wood chips (1700 g) were charged, followed by a short steaming phase (7 min,

0.2 g water g–1 wood, final temperature 99 °C). Subsequently, the white liquor with

an average temperature of 90 °C was added to a total L/W ratio of 3.7–3.8:1. The

effective alkali charge (EA) of 19% was kept constant. The sulfidity varied slightly

in the range between 35 and 39% (see Tab. 4.24). The conventional batch cooking

procedure was characterized by a heating-up time of 90 min and a H-factor- controlled

cooking phase at constant temperature. The cooking phase was terminated

by cold displacement from bottom to top using a washing filtrate at 80 °C comprising

an EA concentration of 0.2 mol L–1, a sulfidity of 65% (equals to [HS– ]of

0.19 mol L–1) and a dry solid content (DS) of approximately 10%. The time–temperature

and time–pressure profiles are shown schematically in Fig. 4.34.

Finally, the pressure was released to fall to atmospheric by quenching with cold

water. Two series of H-factors in the range between 800 and 1400 were investigated

at two different cooking temperatures, 170 °C and 155 °C. Further details

concerning the experimental conditions and the results are summarized in

Tab. 4.24.

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

Pressure [bar]

temperature (top) temperature (bottom)

Temperature [. C]

time [hh:mm]

pressure

Fig. 4.34 Time–temperature (top/bottom) and time–pressure

profiles of a selected laboratory kraft cook using conventional

batch technology (cook labeled CB 414).

224 4 Chemical Pulping Processes

4.2 Kraft Pulping Processes 225

Tab. 4.24 Conditions and results of pine/spruce kraft cooking experiments.

Comparative evaluation of experimental and calculated values (according to [16]).