Examples and Results

A selection of the results published by Kazi and Chornet [56]were used to evaluate

the impregnation model and to calculate D at the given impregnation conditions.

Axial impregnation with a 2.3 mol L–1 NaOH solution was calculated for two different

conditions:

_ Conditions A: Pressure 200 kPa, Temperature 75 °C, time 15 min.

_ Conditions B: Pressure 1480 kPa, Temperature 100 °C, time 15 min.

The concentration profiles obtained from model and experimental results are

depicted in Fig. 4.15.

Figure 4.16 shows the radial profiles for the same impregnation conditions as

in Fig. 4.15, under conditions B.

The calculated diffusion coefficients, D, at the given conditions are summarized

in Tab. 4.14.

0 10 20 30 40

0,0

0,2

0,4

0,6

0,8

1,0 Hardwood Axial Impregnation

P = 200 kPa; T = 348 K, t = 15 min

P = 1480 kPa; T = 373 K, t = 15 min

Experiment

Model

NaOH concentration ratio

Axial Position, Z, mm

Fig. 4.15 Profiles of concentration ratio for axial impregnation

with 2.3 mol L–1 NaOH at specified conditions A and B

(see text). Wood sample length Z = 150 mm; radius a = 25 mm

(data from Kazi and Chornet [56]).

4.2 Kraft Pulping Processes 155

0,0 0,1 0,2 0,3 0,4 0,5

0,0

0,2

0,4

0,6

0,8

1,0

Hardwood Radial Impregnation

P = 1480 kPa; T = 373 K, t = 60 min

Experiment

Model

NaOH Concentration ratio

Radial Position, (R-r)/R

Fig. 4.16 Profile of concentration ratio for radial impregnation

with 2.3 mol L–1 NaOH; impregnation pressure = 1480 kPa;

temperature = 100 °C. Wood sample length Z = 150 mm;

radius a = 25 mm (data from Kazi and Chornet [56]).

Tab. 4.14 Conditions of selected impregnation experiments [56]

and calculated diffusion coefficients, D, by numerical solution

(see Section 4.2.3.8).

Parameter unit Axial Radial

Temperature °C 75 100 100

Pressure kPa 200 1480 1480

Time min 15 15 60

D (calculated) 10–5 cm2 s–1 293.6 503.3 0.98

According to the results listed in Tab. 4.14, the D-value for axial diffusion is

more than 500-fold higher than that for radial diffusion under the same impregnation

conditions. This ratio of diffusion coefficients is thus more than 10-fold

higher than that reported by Christensen [38]and Behr et al. [37].

Based on extensive impregnation experiments, Kazi and Chornet have calculated

the coefficients D0, m and EA from Eq. (66) for the radial and axial directions.

The results are shown in Tab. 4.15.

The diffusion coefficients calculated directly by the impregnation model, and

summarized in Tab. 4.14, correspond quite well with those calculated with Eq. (66)

using the average diffusion coefficient parameters listed in Tab. 4.15. The activa-

156 4 Chemical Pulping Processes

Tab. 4.15 Diffusion parameters from Eq. (66) calculated by Kazi

and Chomet [56]. These values were obtained from experiments

using 14 samples impregnated with a 2.3 mol L-1 NaOH

aqueous solution at different pressures, temperatures and times.

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