Parameter Unit Conditions

A B

Chip length mm 25 25

Chip thickness mm 6 6

Pressure kPa 200 2000

Temperature °C 80 80

0 5 10 15

0,0

0,2

0,4

0,6

0,8

1,0

P = 200 kPa, 80 °C: average minimum

P = 2000 kPa, 80 °C; average minimum

NaOH concentration ratio

Impregnation time, min

Fig. 4.17 Course of average and minimum concentration of

NaOH in a hardwood chip of thickness 6 mm and length

25 mm with impregnation time at two different pressure

conditions. Impregnation model based on Kazi and Chornet

[56](numerical solution).

The simulation of point concentrations across the axial and radial directions in

a chip enables determination of progression of the impregnation front for a given

impregnation time. The two-dimensional concentration profile across chip length

and chip thickness after an impregnation time of only 1 min (80 °C and 2000 kPa)

is shown in Fig. 4.18.

158 4 Chemical Pulping Processes

0 5 10 15 20 25

0,0

0,2

0,4

0,6

0,8

1,0

1,0 mm thickness

3,0 mm thickness

0,5 mm thickness

0,24 mm thickness

0,12 mm thickness

NaOH concentration ratio

Chip Length, mm

Fig. 4.18 Concentration distribution after 1 min of impregnation.

Chip dimensions: thickness 6 mm, length 25 mm.

Model simulation using the following impregnation conditions:

pressure 2000 kPa, temperature 80 °C. Impregnation

model based on Kazi and Chornet [56](numerical solution).

As expected, the concentration gradient in both directions – axial and radial –

remains very pronounced after 1 min of impregnation. In this particular case, the

minimum concentration of the diffusing species, NaOH, is reached at only 1 mm

beneath the chip surface (radial direction).

Chemical impregnation into wood chips having a moisture content above the

fiber saturation point can be modeled by diffusion mechanisms. The introduced

model considers both radial and axial diffusion processes, and is able to predict

the required time to impregnate wood chips to achieve a uniform distribution of

active cooking chemicals.