Label Maximum

Temperature

[°C]

Time at max.

Temperature

[min]

L/W

Ratio

Inititial

[OH– ]

[mol L–1]

Residual [OH– ] Initital

[HS– ]

[mol L–1]

Yield, unscr. Kappa number Intrinsic viscosity Carbohydrates

Exp.

[mol L–1]

Model

[mol L–1]

Exp.

[%]

Model

[%]

Exp. Model Exp.

[mL g–1]

Model

[mL g–1]

Exp.

[%]

Model

[%]

CB412 170 52 3.7 1.34 0.31 0.31 0.28 50.2 50.5 42.8 41.3 1173 1145 45.6 47.7

CB413 170 60 3.8 1.32 0.26 0.30 0.26 49.2 50.0 34.0 36.9 1140 1120 45.6 47.4

CB414 170 71 3.8 1.32 0.23 0.30 0.30 49.0 49.3 25.9 27.9 1102 1091 45.3 47.3

CB430 150 185 3.7 1.35 0.30 0.32 0.31 49.9 50.7 40.7 43.7 1264 1256 45.7 47.6

CB431 150 227 3.7 1.35 0.26 0.32 0.32 49.4 50.0 33.8 34.0 1237 1230 46.0 47.5

CB432 150 266 3.7 1.35 0.26 0.31 0.32 49.1 49.5 29.4 28.7 1190 1207 45.5 47.4

CB433 150 312 3.7 1.35 0.25 0.30 0.33 49.2 49.0 25.4 24.1 1162 1183 45.2 47.2

20 30 40 50

exp 170.C calc 170.C exp 155.C calc 155.C

Viscosity [ml/g]

Kappa number

Fig. 4.35 Selectivity plot of pine/spruce kraft cooking:

comparison of predicted and experimentally determined

values (see Tab. 4.24).

The most important parameters characterizing the results of a kraft cook –for

example, unscreened yield, kappa number, intrinsic viscosity and the content

of carbohydrates – have been predicted by applying the kinetic model introduced

in Chapter 4.2.5.3. The correspondence between the calculated and the experimentally

determined values is rather satisfactory for unscreened yield, kappa

number and intrinsic viscosity. The content of carbohydrates differ, however, significantly

(on average, by >2%) mainly due to the fact that the measured values

are based simply on the amounts of neutral sugars (calculated as polymers, e.g.,

xylan = xylose. 132/150). By considering the amounts of side chains (4- O -methylglucuronic

acid in the case of AX and acetyl in the case of GGM), the difference

between calculated and experimentally determined values would be greatly diminished.

Moreover, the experimental determination of carbohydrates in solid substrates

always shows a reduced yield due to losses in sample preparation (e.g.,

total hydrolysis). The predicted yield values are a little higher (average 0.5%) and

showamore pronounced dependency on cooking intensity as compared to the experimentally

obtained values. The selectivity, or the viscosity at a given kappa number, is

predicted rather precisely which is really remarkable because modeling of the

intrinsic viscosity is based on a very simple approach [see Eq. (90) and Fig. 4.35].

A detailed glance at the single values reveals that the calculated viscosity values

show a higher temperature dependency, especially at lower kappa numbers. This

might be due to several reasons, for example, a changed degradation behavior of

the residual carbohydrates (degree of order increases with an increasing removal

of the amorphous cellulose part and the hemicelluloses, etc.) and/or an altering

226 4 Chemical Pulping Processes

dependency on EA concentration at lower levels. The difference in predicted and

experimentally determined values is however very small, considering both the

model-based assumptions and the experimental errors.

Consequently, the model is appropriate for optimization studies to predict the

influence of the most important cooking parameters.

The precise calculation of the time course of EA concentration in the free and

entrapped liquor throughout the whole cook is an important prerequisite to reliable

model kraft pulping. For a selected cook (labeled CB 414), the concentration

profiles of the free effective alkali are compared for the calculated and experimentally

determined values as illustrated in Fig. 4.36. The agreement between model

and experiment is excellent.

The heterogeneous nature of the cooking process is clearly illustrated in

Fig. 4.36, where the concentration profiles for the effective alkali in both free and

entrapped cooking liquors are visualized. The difference between these profiles is

remarkable up to a temperature level of approximately 140 °C. The EA concentration

in the entrapped liquor has been calculated for two cases, the average value

in the chip (denoted bound EA) and the minimum value in the center of the chip

(denoted center EA). Interestingly, the EA concentration in the bound liquor (for

both calculated cases) experiences a minimum value after a reaction time of about

40 min at 127 °C, presumably due to augmented EA consumption caused by

extensive hemicellulose degradation reactions (peeling) in the initial phase.

According to the selected example, the EA concentration inside the chips approaches

that outside the chips only after reaching the cooking phase.

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

0,0

0,3

0,6

0,9

1,2

1,5

exper. free EA calc. free EA calc. bound EA calc. center EA

effective alkali [mol/l]

Time [hh:mm]

Temperature [° C]

Fig. 4.36 Course of the effective alkali concentration

in the free and entrapped cooking

liquor during a kraft cook (CB 414); the

entrapped liquor is differentiated in “bound

liquor”, which equals the average content of

entrapped EA concentration, and the “center

liquor”, which corresponds to the EA concentration

in the middle of the 3.5-mm chip. Model

and experimental values for free EA concentration.

Note that the initially ensued bound EA

value has been calculated according to

Eq. (116).

4.2 Kraft Pulping Processes 227

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